Recent advancements in the development of bifunctional electrocatalysts for oxygen electrodes in unitized regenerative fuel cells (URFCs)

Accepted Manuscript Recent advancements in the development of bifunctional electrocatalysts for oxygen electrodes in unitized regenerative fuel cells (URFCs) Yan-Jie Wang, Baizeng Fang, Xiaomin Wang, Anna Ignaszak, Yuyu Liu, Aijun Li, Lei Zhang, Jiujun Zhang PII: DOI: Reference:

S0079-6425(18)30067-7 https://doi.org/10.1016/j.pmatsci.2018.06.001 JPMS 521

To appear in:

Progress in Materials Science

Received Date: Revised Date: Accepted Date:

14 July 2016 18 June 2018 26 June 2018

Please cite this article as: Wang, Y-J., Fang, B., Wang, X., Ignaszak, A., Liu, Y., Li, A., Zhang, L., Zhang, J., Recent advancements in the development of bifunctional electrocatalysts for oxygen electrodes in unitized regenerative fuel cells (URFCs), Progress in Materials Science (2018), doi: https://doi.org/10.1016/j.pmatsci.2018.06.001

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Recent advancements in the development of bifunctional electrocatalysts for oxygen electrodes in unitized regenerative fuel cells (URFCs)

Yan-Jie Wang,a* Baizeng Fang, b,c Xiaomin Wang,d* Anna Ignaszak,e Yuyu Liu,b Aijun Li, b Lei Zhang,b,f Jiujun Zhang,b,f*

a

School of Environment and Civil Engineering, Dongguan University of Technology,

No. 1, Daxue Rd, Songshan Lake, Dongguan, Guangdong Province, P.R. China b

Institute for Sustainable Energy, Shanghai University, 99 Shangda Rd, Baoshan,

Shanghai, P.R. China c

Department of Chemical and Biological Engineering, University of British

Columbia, 2360 East Mall, Vancouver, BC, V6T 1Z3, Canada d

College of Materials Science and Engineering, Taiyuan University of Technology,

Taiyuan 030024, China e

Department of Chemistry, University of New Brunswick, 30 Dineen Drive,

Fredericton, NB, E3B 5A3, Canada f

Energy, Mining and Environment, National Research Council Canada, 4250

Wesbrook Mall, Vancouver, BC, V6T 1W5, Canada

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Abstract An ever-increasing energy demand has stimulated intense research into electrochemical technologies for both energy storage and conversion such as unitized regenerative fuel cells (URFCs). Due to its high efficiency, low cost, and low environmental impact, the URFC, which combines a polymer electrolyte membrane fuel cell (PEMFC) with a polymer electrolyte water electrolyzer (PEWE), has been regarded as an important energy technology that can offer new opportunities to not only further reduce investment costs but also to open doors to the mass production of domestic applications. Despite this, URFCs still have to be further improved and optimized to reach a level of maturity of both fuel cells and electrolyzers in terms of energy efficiency and long-term performance. Currently, the major challenge of URFC technology has been the insufficient performance of the bifunctional electrocatalyst. Based on the most recent research trends and progresses of bifunctional oxygen catalyst materials, this review will provide a systematic introduction and a comprehensive assessment of various bifunctional oxygen catalysts; their science and technology, including material selection, synthesis, and characterization, as well as their applications in URFCs. This review aims to correlate the physicochemical characteristics of URFCs with the catalytic activity/stability of these materials.

Keywords: unitized regenerative fuel cells, polymer electrolyte fuel cell, polymer electrolyte water electrolyzer, bifunctional electrocatalyst, catalytic activity, stability

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Contents 1. Introduction 2. Working mechanism of bifunctional catalysts in URFC 3. Supported bifunctional catalysts 3.1 Carbon-supported catalysts 3.1.1 Carbon-supported metal(s) 3.1.2 Carbon-supported metal oxides 3.1.3 Other carbon-supported catalysts 3.2 Modified carbon-supported catalysts 3.2.1 Modified carbon-supported metals 3.2.2 Modified carbon-supported metal oxides 3.2.3 Modified carbon-supported sulfides 3.2.4 Other modified carbon-supported catalysts. 3.3 Noncarbon-supported catalysts 3.3.1 Metal oxide-supported metal(s) 3.3.1.1 IrO2-supported metal(s) 3.3.1.2 Other oxide-supported metal 3.3.2 Carbide-supported metal(s) 3.3.3 Other material-supported catalysts 4. Unsupported bifunctional catalysts 4.1 Metal-based catalysts 4.1.1 Pt-based catalysts

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4.1.2 NonPt-based catalysts 4.2 Free metal catalysts 4.2.1 Doped carbon-based catalysts 4.2.1.1 Nitrogen-doped carbon-based catalysts 4.2.1.2 Double elements doped carbon-based catalysts 4.2.2 Oxide-based catalysts 5. Evaluation of bifunctional oxygen catalysts for URFCs 6. Summary, challenges and future research directions 6.1 Summary 6.2 Challenges 6.3 Future research directions Author Information Corresponding Authors Notes Acknowledgements References Vitae

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Nomenclature/Acronyms AA

Ascorbic acid

AB

Acetylene black

ACN

Acetonitrile

AEM

Anion exchange membrane

APS

Ammonium peroxodisulphate

APS

Ammonium peroxydisulfate

BSCF

Ba0.5Sr0.5Co0.8Fe0.2O3-δ

BCFO

Bi0.6M0.4 FeO3

BFO

BiFeO3

BJH

Barrett-Joyner-Halenda

cCo3O4

Cubic Co3O4

CNTs

Carbon nanotubes

CNFs

Carbon nanofibers

CP

Carbon paper

CTAB

Cetyltrimethylammonium bromide

CTAC

Cetyltrimethylammonium chloride

CV

Cyclic voltammetry

CVD

Chemical vapor deposition

DBSA

Dodecyl benzene sulfonic acid

DEA

Diethyl amine

-5-

DMF

Dimethylformamide

DMFCs

Direct methanol fuel cells

ECSAs

Electrochemical surface areas

EDX

Energy-dispersive X-ray

EDTA

Ethylenediaminetetraacetic acid

EXAFS

Extended X-ray absorption fine structure

EG

Ethylene glycol

FC

Fuel cell

FePc

Iron phthalocyanine

FTIR

Fourier transform infrared spectrometer

GDL

Gas diffusion layer

GNR

Graphene nanoribbon

GO

Graphene oxide

GSH

SWCNT/graphene hybrid

HAADF

High-angle annular dark field

HDC

Heteroatom doped carbon

HMTA

Hexamethylenetetramine

HPA

H3Mo12O40P

HRTEM

High-resolution transmission electron microscopy

ICP-AES

Inductively coupled plasma-atomic emission spectrometer

ICP/MS

Inductively coupled plasma/mass spectrometry

ICP-OES

Inductively coupled plasma optical emission spectroscopy

-6-

LCMO

La(Co0.55Mn0.45)0.99O3-δ

LDH

Layered Double Hydroxides

LDO

Layered double oxides

LN

LaNiO3-δ

L60SF

La0.6Sr0.4FeO3

L76SCF

La0.76Sr0.2Co0.2Fe0.8O3

LCaMC

La0.83Ca0.15Mn0.5Co0.4O3

L58SCF

La0.58Sr0.4Co0.2Fe0.8O3

LSV

Linear sweep voltammetry

MEA

Membrane electrode assembly

MWCNTs

Multi-walled carbon nanotubes

MOFs

Metal-organic frameworks

MOF

Metal-organic framework

NC

N-doped carbon

NGL

N-doped graphitic layer

NGSH

N-doped SWCNT/graphene hybrid

NCNT

N-doped carbon nanotube

N-CNFs

N-doped carbon nanofibers

N-rGO

N-doped reduced graphene oxides

N,P-CNS

N,P-doped carbon nanospheres

N-CNP

N-doped carbon nanoparticles

NGC

Nitrogen-doped grapheme-carbon nanotubes

-7-

NG

N-doped grapheme

N,P-GCNS

Nitrogen

NS-rGO

nanosheet

N-MC

N, S co-doped reduced graphene oxide

PEDOT

N-doped mesoporous graphitic carbon

PEMFC

Poly(3,4-ethylenedioxythiophene)

PEWE

Polymer electrolyte membrane fuel cell

PEM

Polymer electrolyte water electrolyzer

ppy

Polymer electrolyte membrane

PVC

polypyrrole

ORR

Polyvinyl chloride

OER

Oxygen reduction reaction

Pm

Oxygen evolution reaction

PBC

Physical mixture

PANi

PrBaCo2O5+δ

PA

Polyaniline

rGO

Phytic acid

RDE

Reduced grapheme oxide

RHE

Rotation disk electrode

RHE

Reference hydrogen electrode

RRDE

Reversible hydrogen electrodes

SCE

Rotating ring-disk electrode

and

phosphorus

-8-

dual-doped

graphene/carbon

STEM

Saturated calomel electrode

SEM

Scanning transmission electron microscopy

S-GC

Scanning electron microscope

S-N-C

S-doped graphitized carbon

SWCNTs

S,N co-doped carbon nanodot

TEA

Single-walled carbon nanotubes

TEM

Trimethylamine

TGA

Transimission electron microscopy

TU

Thermogravimetric analysis

URFCs

Thiourea

VC

Unitized regenerative fuel cells

WE

Vulcan XC-72

WE

Water electrolysis

XANEs

Water electrolyzer

XRD

X-ray absorption near edge structure

XPS

X-ray diffraction

3D

X-ray photoelectron spectroscopy Three-dimensional

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1. Introduction With growing concerns in energy security and climate change, numerous efforts have been made to develop efficient, renewable and eco-friendly energy conversion and storage technologies for next-generation devices. Cutting edge renewable energy generation and storage technologies have attracted tremendous interest because of their technological ability to convert natural resources (e.g., sunlight, wind, hydroelectricity, tides, biomass, and geothermal heat) into electricity and heat which in turn can be utilized in our homes and businesses. In the pursuit of new and innovative ways to store energy using hydrogen; the most plentiful element on Earth as well as in the universe, regenerative fuel cells, especially unitized regenerative fuel cells (URFCs), have the potential to be the answer that hydrogen economists are looking for. Interestingly, as one type of electrochemical energy storage and conversion device, the URFC is often considered to be a “battery” that includes a discharge/recharge process whereby hydrogen and oxygen gases in the fuel stack can be converted into electricity and water during discharge while water can be split back into reactants in the fuel stack such as hydrogen and oxygen during recharge. In reality, URFCs are a combined unit that includes a fuel cell and an electrolyzer; of which only one of the two modes can be operated at one time [1, 2]. It has been recognized that URFCs, as well as metal-air batteries, can meet the driving distance requirements of electric vehicles [3]. However, URFCs need to be improved and optimized to reach a level of maturity that is currently obtained with fuel cells and

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electrolyzers based on hydrogen polymer electrolyte membrane (PEM) fuel cells. For certain, the electrochemical performance of URFCs needs to be optimized both in terms of efficiency and in terms of long-term performance. In particular, the commercialization of URFC technology is hindered by the intrinsically sluggish kinetics at the oxygen electrode towards oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) for the discharging and charging process, which involves a complicated four electron transfer process [4-7]. A stable and highly active bifunctional oxygen catalyst is well known to be one key issue for ORR and OER at the oxygen electrode of URFCs. To date, the research and development of ideal bifunctional electrocatalysts that offer simultaneously high activity for both ORR and OER has become the main obstacle in the realization of URFCs as efficient electrochemical devices. The overall charge-discharge efficiency of a URFC is found to be dependent on the reversibility of the electrode reaction, which in turn is strongly related to the activity of bifunctional catalysts. By investigating and comparing material selections, synthesis methods and structural features in recent progresses of experiments and characterizations, this review; for the first time, provides a systematic introduction and comprehensive assessment of various bifunctional oxygen electrocatalysts in relation to different types of catalysts and support materials in the application of URFCs, with an emphasis on the relationship between the physicochemical characterization and the catalytic performance of catalysts.

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(Fig. 1)

2. Working mechanism of bifunctional catalysts in URFCs The use of electrocatalysts, especially bifunctional oxygen electrocatalysts, is vital to the future development of URFCs as a source of advanced energy. Fig. 1 presents a schematic for a bifunctional electrode’s catalyst layer in a designed URFC that unitizes both electrolyzer and fuel cell functions with a PEM [8, 9]. It is known that the catalyst layer, which has direct contact with the PEM and the gas diffusion layer, is designed and fabricated to ensure that the catalyst particles are as close as possible to the PEM so as to achieve high efficiency in terms of catalyst performance [10]. The research and development of catalysts and catalyst layers have become crucial in efficient URFCs. In a fuel cell assembly, the URFC produces electricity from hydrogen and oxygen and generates by-products such as heat and water. The electrochemical reactions that occurs at the anode and cathode in the fuel cell system are: (Anode): 2H2  4H+ + 4e-

(1)

(Cathode): O2 + 4H+ + 4e-  2H2O

(2)

The protons in the reaction can react with the adsorbed oxygen to produce water at the cathode through ORR. The overall reaction in the fuel cell system is Overall reaction: 2H2 + O2  2H2O

(3)

In contrast to a traditional fuel cell, the electrolyzer mode in a URFC can absorb electricity from a power source (solar, wind, turbine…) or heat from the system

- 12 -

environment and/or from the loss of heat from the system itself to divide water into oxygen and hydrogen fuel. Water is oxidized to form oxygen and releases protons and electrons at the anode through OER. The electrochemical reactions at the anode and cathode are: Anode: 2H2O  O2 + 4H+ + 4e-

(4)

Cathode: 4H+ + 4e-  2H2

(5)

The protons can travel through the electrolyte and combine with electrons to form hydrogen at the cathode. The overall reaction in the electrolyzer system is Overall reaction: 2H2O  2H2 + O2

(6)

So the total reaction of a full URFC system is presented as

2H2 + O2

Fuel Cell Mode Electrolyzer Mode

2H2O (7)

The efficiency of the URFC is dependent on two systems; the fuel cell and the electrolyzer. Oxygen is more important in URFCs than hydrogen because its electrochemical process at the interface between the electrolyte and the electrode is generally slow and complicated due to the strong irreversibility of oxygen [11, 12]. Additionally, the electrochemical reactions of oxygen are strongly dependent on many factors such as electrolyte type and the surface chemistry of the electrocatalyst used [11]. It is evident [13] that oxygen tends to form strong chemical bonds in both alkali and acid electrolytes, and therefore results in a lower open circuit potential for the oxygen electrode than that of an equilibrium oxygen potential. In alkaline media, most electrochemical systems are often ran using non-noble metal-based bifunctional - 13 -

catalysts for the oxygen/air electrodes. Such electrodes have lower equilibrium potentials at high pH than that when using noble metal-based catalysts [14, 15]. When compared to acidic media, alkaline media also tends to present higher exchange current densities in oxygen evolution/reduction. The ORR in alkaline systems is actually increasingly inclined to the utilization of non-noble metal-based catalysts [16] even though oxygen/air electrodes are sensitive to carbon dioxide which can react with OH- ions in alkaline electrolytes to form carbonates as contaminants [17, 18]. The presence of carbon dioxide causes a detrimental effect to the life span of bifunctional oxygen/air electrodes [17]. In acidic media, when using hydrogen peroxide as an indicator of the overall reaction, the four-electron electrochemical reaction was evident at the oxygen/air electrode, which can produce considerable operational current densities that is comparable to other catalysts such as noble metal-based catalysts when used in the oxygen electrode. For the ORR process in fuel cell systems, oxygen is first adsorbed on the surface layer of a catalyst in the oxygen electrode. Then the adsorbed oxygen can be reduced into water in a four-electron transfer in which hydrogen peroxide is formed as intermediate with the assistance of two electrons [13, 19] (see Equations 8-10). In contrast to acidic media, alkaline media provides a high-pH media in which the direct four-electron electrochemical reaction is found to cause the adsorbed oxygen to reduce to peroxide rather than OH- or H2O [19] (see Equations 11-13). O2 + 4H+ + 4e-  2H2O

(8)

O2 + 2H+ + 2e-  2H2O2

(9)

- 14 -

H2O2 + 2H+ + 2e-  2H2O

(10)

O2 + 2H2O + 4e-  2OH-

(11)

O2 + H2O + 2e-  HO2- + OH-

(12)

HO2- + H2O + 2e-  3OH-

(13)

In the electrolyzer system, water electrolysis is performed to obtain hydrogen rather than oxygen in a technical process. The OER at the oxygen electrode is however more important than the hydrogen evolution reaction at the hydrogen electrode due to its slower kinetics and higher overpotential. Usually, the electrochemical reaction mechanism in oxygen evolution is very complex and significantly depends on the electrode potential. In acidic media, a proposed mechanism for the OER was found on the active oxide electrode [20] (see Equations 14-17). With the assist of active sites (S) at the oxygen electrode, the charge-transfer goes through the formation of an adsorbed hydroxyl species, resulting in the formation of O2. Compared to the OER mechanism in acidic media, the OER mechanism in alkaline media involves the formation of OH radicals at the electrode surface [21] (see Equations 18-21). The rate-determining step was found to be dependent on the catalyst at the oxygen electrode. S + H2O  S-OH + H+ + e-

(14)

S-OH  S-O + H+ + e-

(15)

S-OH + S-OH  S-O + H2O

(16)

S-O + S-O  2S + O2

(17)

OH-

(18)

OHads + e-

- 15 -

OHads + OHOads

Oads + H2O

Oads + e-

Oads + Oads

O2

(19) (20) (21)

3. Supported bifunctional catalysts In regards to supported bifunctional catalysts, the use of supporting materials is crucial in providing high surface areas and distributing small catalyst particles thus resulting in high electrochemical performances. In fuel cells and electrolyzers, the physicochemical properties of support materials such as surface area, electronic conduction, wettability, and electrochemical resistance has important effects on the performance efficiency of URFCs. To date, various supported bifunctional catalysts have been researched in their application of URFCs. These can be basically divided into three categories, such as carbon-, modified carbon- and noncarbon- supported bifunctional catalysts.

3.1 Carbon-supported catalysts 3.1.1 Carbon-supported metal(s) As a traditional catalyst, carbon-supported metals, especially carbon supported Platinum, is promising in the application of fuel cell electrocatalysts due to carbon’s high surface area, good electronic conductivity, porous structure, and low cost [22, 23]. However, it is found that carbon materials often suffer from heavy oxidation/corrosion with rising oxidation rates proportional to the potential under high

- 16 -

potential (about 1.0V) at the oxygen electrode during water electrolysis in URFCs [24]. This subsequently results in the loss of mass transport and therefore negatively affects URFC performance [2, 25]. Having known that carbon graphitization can improve electrochemical and thermal stability [26], Pai and Tseng [27] investigated a bifunctional graphitized carbon-supported Pt catalyst as the oxygen electrode for the measurement of the round-trip energy conversion efficiency in URFC applications. In their experimental preparation of Pt/graphitized carbon, they first utilized high-temperature techniques [28] to graphitize carbon materials (i.e., Vulcan XC72) and obtain graphitized carbon. Then the graphitized carbon is fabricated as a catalyst support layer in a URFC via a montmorillonite-assisted dispersion. After the reduction of H2PtCl6·6H2O, the final Pt nano-dots with different amounts of 10wt%, 15wt%, 20wt% and 30wt% were collected and installed on support as the bifunctional oxygen electrode catalysts. In the evaluation of the graphitization degree

(Fig. 2)

of carbon, the results of Raman spectra demonstrated a decreasing ID/IG ratio from 1.43 to 0.81 after heat treatment, indicating a high degree of graphitization [29], where ID is the intensity of D-band peak related to the disorder in the graphitic sp2 network typical of carbonaceous impurities while IG is the intensity of normal graphite structures [30, 31]. In the morphology examination of support and catalyst, the

- 17 -

combination

of

scanning

transmission

electron

microscopy

(STEM)

and

high-resolution transmission electron microscopy (HRTEM) in Fig. 2 shows that in the presence of montmorillonite, the average particle size of the graphitized carbon support was in the range of 10-20 nm while the Pt particles were found to be in fine dispersion with an average particle diameter of 3.48±0.12, 3.62±0.16, and 4.16±0.25 nm for 10wt%Pt/graphitized carbon, 20wt%Pt/graphitized carbon, and 30wt%Pt/ graphitized carbon, respectively. Based on CV results tested in 1M H2SO4 solution, the calculated quantification of Pt utility demonstrated that the Pt/graphite carbon had the optimum weight ratio and utilization of 20wt% and 64%, respectively. In the measurement of round-trip energy conversion efficiency (7min of electrolyzer and 7 min of fuel cell mode each), it was found that as the oxygen electrode, 20wt%Pt/graphitized carbon presented an initial energy conversion efficiency of 37.5% at 100 mA cm-2, a little higher than 20wt%Pt/Vulcan XC72 (~37.2%). Furthermore, the 20wt%Pt/Vulcan XC72 ceased to work for more than 3 cycles due to obvious degradation in both water electrolysis and fuel cell operation [32]. This suggests an active effect of graphitization to improving the stability of carbon and catalyst. Meanwhile, for the electrolyzer mode in the URFC, the 20wt%Pt/graphitized carbon resulted in a higher hydrogen production rate (0.34 ml min -1 at 105 mA cm-2) than the 20wt%Pt/Vulcan XC72 (0.23 ml min-1 at 105 mA cm-2). When the generated hydrogen was used to produce electricity, a total energy transfer efficiency of the cell measurement system can reach 50.6%. As a popular carbon-supported metal(s) catalyst, Pt/C alone is not a suitable

- 18 -

catalyst for water oxidation and oxygen evolution due to its poor activity towards OER. Due to this, it cannot be used as an efficient bifunctional oxygen catalyst in URFCs [2, 33, 34] even if it currently act as a good electrocatalyst for oxygen reduction in fuel cell applications. To solve this problem, Hari et al. [35] used a dual metal-based catalyst (i.e., PtIr/C) as a bifunctional oxygen catalyst and checked the catalyst’s utility in a designed URFC. In their fabrication of the oxygen electrode, iridium was deposited onto Pt/C with an iridium content of 20wt% of the Pt after hexachloro iridium was reduced. The resulting data in the electrochemical measurements of URFC demonstrated the feasibility of the fuel cell and the electrolyzer in a unit when PtIr/C was used as the bifunctional oxygen electrode. For the electrolyzer, the use of PtIr/C resulted in a higher electrolysis current (350 mA cm-2 at 2.2 V) than that when using Pt/C (250 mA cm-2 at 2.4 V). However, the use of PtIr/C slightly lowered the performance of the fuel cell when compared to Pt/C. Similar to their researches, Kim et al. [36] used a a simple spray pyrolysis to develope Pt-Ir alloy (Pt-Ir/rGO) with a catalyst support of three-dimensional (3D) crumpled reduced graphene oxide (rGO) as a bifunctional oxygen catalyst in URFCs. Their results showed that with a limited degree of alloy, the produced crystalline Pt-Ir nanoparticles decorated on the 3D crumpled rGO sheets could control the adsorption strength of molecular oxygen for ORR and hydroxyl species from water molecule during OER, thus affecting overall ORR and OER processes. Particularly, the Pt-Ir/rGO heat-treated at 600 oC could provide the higher ORR activity and stability than the commercial Pt/C, and present the comparable OER activity and durability to

- 19 -

the commercial unsupported Ir black. In spite of this, in order to develop and find optimum bifunctional oxygen catalysts, Pt/C has been and is being modified and substituted by other catalysts materials. This will be discussed in detail and reviewed below.

3.1.2 Carbon-supported metal oxides There is tremendous interest in earth-abundant 3d metal (Mn, Fe, Co, Ni) oxides due to these oxides’ high ORR activity, low cost, and excellent stability, as well as their widespread application on earth [37-39]. As a result of this interest, spinel oxides have been developed as a potential bifunctional catalyst candidate for both ORR and OER in alkaline environments. Generally, the formula of spinel oxides can be described as AB2O4, with A being a tetrahedral site and B an octahedral site [40, 41]. Considering that spinel oxides such as transition-metal oxides generally exhibit limited electrocatalytic performance due to their large overpotential and low specific activity, Du Et al. [40] reported a novel graphene-supported ultrasmall (1.7-3 nm) and monodispersed CoMn2O4 catalyst (CoMn2O4/rGO) and its highly efficient ORR/OER in 0.1 M KOH solution. In a rapid synthesis, they used a facile and low-temperature reaction (120oC) in xylene to obtain CoMn2O4 nanodots using cobalt dichloride and Manganese (II) acetate as a source of Co and Mn. The CoMn2O4 nanodots were then assembled on a self-made reduced graphene oxide (rGO) and then annealed in air at

(Fig. 3)

- 20 -

170oC for 12h. Transmission electron microscopy (TEM) measurements demonstrated a relatively narrow size dispersion (1.7 – 3 nm) of CoMn2O4 on the rGO sheet (see Fig. 3), while Powder X-ray diffraction (XRD) and Raman spectra revealed the presence of the tetragonal spinel phase of CoMn2O4, suggesting that Mn and Co cations can occupy different sites [42]. In the investigation of the surface chemical state of the CoMn2O4/rGO by X-ray photoelectron spectroscopy (XPS), it was found that Mn3+ and Co2+ are the two dominant valences on the oxide surface while the C 1s spectrum confirmed the presence of abundant oxygen functional groups, favoring the interfacial interaction between graphene and metal oxide through a C-O=metal bridge and thereby electrochemical activity [43]. To examine the electrochemical activity, a rotation disk electrode (RDE) technique was used in the preparation of catalyst samples. When three different concentrations (20wt%, 30wt%, and 40wt%) of CoMn2O4 in the composite of CoMn2O4/rGO were synthesized and tested, 30wt% ultrasmall CoMn2O4/rGO was found to show the best ORR performance due to a balance of particle density, surface area, and electron conductivity in the composite. In comparison with 30wt%CoMn2O4/rGO, the other four samples of rGO, CoMn2O4/Vulcan XC72, 10 nm CoMn2O4/rGO, and commercial Pt/C (10wt%Pt) were also tested in the electrochemical test. When ORR measurements were carried out in O2-saturated 0.1M KOH solution at 5 mV s -1 and 400 rpm, the calculated ORR mass activities of the catalyst samples at 0.85 V were found in the following decreasing order: ultrasmall CoMn2O4/rGO (254.7 A g-1 metal) > 10 nm CoMn2O4/rGO

- 21 -

(16.5 A g-1metal) > bulk CoMn2O4 (3.34 A g-1metal). This apparently results from the effect of smaller particle sizes on the active sites. In addition, the rGO substrate was shown to be electrochemically active but showed only low ORR current and potential. When a comparison of kinetic current was performed for the CoMn2O4/rGO and the commercial Pt/C, it is shown that at 0.90V, the CoMn2O4/rGO catalyst had a kinetic current of 3.33 mA cm-2, which is twice that of Pt/C (1.61 mA cm-2). This suggests that the former has superior ORR catalytic activity as compared to the benchmark Pt/C. Durability examinations showed that the ORR current density of the CoMn2O4/rGO catalyst held at more than 92% over 46h of continuous polarization at a constant potential of 0.7 V. This is superior to commercial Pt/C (76%) and is due to the porous rGO support being firmly anchored to the CoMn2O4 with a uniform dispersion. In addition to the ORR results, the corrected polarization curves in the OER test of the CoMn2O4/rGO catalyst showed a smaller Tafel slope (56 mV dec -1) than that of rGO (156 mV dec -1) and Pt/C (280 mV dec-1), indicating a high OER activity for CoMn2O4/rGO. Interestingly, when a potential separation between ORR (at the current of 1 mA cm-2) and OER (at 10 mA cm-2) was selected as a metric to evaluate the performance of bifunctional oxygen electrocatalysts, the value for CoMn2O4/rGO was found to be ~0.65 V which is lower than that of other recently reported oxygen electrodes such as Co xO6y/N-doped carbon (0.86 V) [44] and CoO/N-doped graphene (0.76 V) [45]. These research results depended heavily on the composition, shape, size, and structure of nanoparticle catalysts [23, 46], suggesting that CoMn2O4/rGO is a promising bifunctional ORR/OER catalyst candidate.

- 22 -

Besides spinel oxides, Co 3O4 is another important non-precious metal oxide used to

(Fig. 4)

prepare novel composite bifunctional catalysts. The composite of Co3O4 and its support materials (e.g., graphene, or multi-walled carbon nanotubes (MWCNTs)) have been found to demonstrate strong synergistic coupling interactions, favoring both ORR and OER in alkaline solutions [33, 47, 48] even though Co3O4, by its self, presents little catalytic activity. Chen’s group [48] studied cubic Co3O4 and MWCNT composite materials (cCo3O4/MWCNT) for ORR and OER in alkaline environments. Based on the active impact of morphology on bi-functionality for Co3O4 nanoparticles [47, 49], they employed a facile hydrothermal route to prepare a cubic Co3O4/MWCNT

composite

using

acid-functionalized

MWCNT

and

Co(CH3COO)2·4H2O as starting materials. The content of cCo3O4 was tested to be ~54wt% in the measurement of thermogravimetric analysis. In a further examination of morphology and structure, XRD patterns of cCo3O4 and cCo3O4/MWCNT in Fig. 4(a) not only confirmed the formation of cCo 3O4 but also show a slight angle shift of cCo3O4 in the composite material, indicating a slight lattice contraction due to crystal size or interactions between cCo 3O4 and MWCNTs [48, 50, 51]. The scanning electron microscope (SEM) and TEM images (see Fig. 4(b) and (c)) demonstrated the successful synthesis of cubic Co 3O4 with an effective distribution on the MWCNT

- 23 -

support. The acid-functionalized MWCNTs was thought to be the oxidizing/structure directing agents, which can oxidize cobalt ions resulting in the formation of cubic cobalt oxide with functionalized carboxylate groups during the chemical reaction. However, there were no particle size information provided in the characterization even though size is an important factor that affects catalytic performance [23, 46]. After ORR/OER measurements were performed in 0.1M KOH using a RDE technique, the ORR/OER performance of cCo3O4/MWCNT was compared with pure cCo 3O4, MWCNT, and a physical mixture of cCo 3O4-MWCNT (pm-cCo3O4/MWCNT with an identical weight ratio of 54:46 as the composite material). It was demonstrated in all measured

ORR/OER

polarization

curves

that

cCo3O4/MWCNT

>

pm-cCo3O4/MWCNT > MWCNT > cCo3O4. The cCo3O4 showed very poor ORR activity while MWCNT alone showed weak ORR activity. It should be especially noted that the interaction between cCo 3O4 and MWCNT through a simple physical mixture resulted in the higher catalytic performance of cCo 3O4/MWCNT than that of pm-cCo3O4/MWCNT. Also, the cCo3O4/MWCNT produced excellent OER activities with the highest current density (16.0 mA cm-2 at 0.7 V) among all tested samples. Additionally,

a

comparison

test

for

the

stability

of

cCo3O4/MWCNT,

pm-cCo3O4/MWCNT, cCo3O4, and MWCNT was done by using continuous 500 CV cycles. It was found that the current density of cCo 3O4/MWCNT decreased by only 25% while the current densities of the others had decreased by over 50%. Moreover, cCo3O4/MWCNT demonstrated an OER degradation of 48%, which is better than MWCNT (97%), cCo3O4 (98%), and pm-cCo3O4/MWCNT (49%). All these

- 24 -

electrochemical results indicate that synergistic coupling effects between cCo3O4 and MWCNT effectively enhanced bi-functionality and stability. This suggests that cCo3O-MWCNT composites are a promising bifunctional catalyst candidate for the oxygen electrode in URFCs. Like this work, the durable and efficient Co 3O4 nanocrystals supported on carbon nanotubes (Co3O4/CNTs) have been explored in the development advanced bifunctional electrocatalysts for URFCs by Zhao et al. [52]. The hybrid structure in the supported catalyst was achieved by nucleating cobalt oxides on functionalized CNTs through hydrolysis and hydrothermal treatment. The Co3O4/CNTs catalyst showed both ORR and OER activities and 2000 cycles of durabilityin 0.1 M KOH solution, suggesting its perspectives as advanced bifunctional electrocatalysts for URFCs applications.

(Fig. 5)

Among various non-precious metal bifunctional electrocatalysts, perovskite oxides are an important class of transitional metal oxides with perovskite-type crystal structures [53-55]. These usually consisting of a general formula of ABO3, consisting of a corner-shared BO6 octahedra together with A cations at the corners of its unit cell, as seen in Fig. 5 [56]. Due to the substitution of A-site and/or B-site metal cations [38, 57, 58] and the generation of oxygen deficiency/vacancy [59, 60], advanced perovskite oxides (e.g., AaA1-aBbB1-bO3-x) have been progressively developed and modified to produce characteristic effects on the electronic structures and

- 25 -

coordination chemistries, resulting in the enhancement of ORR/OER activity. In recent years, perovskite oxides have been studied and proposed as an oxygen electrode material due to their defective structures, low cost, excellent oxygen mobility, and outstanding activities for ORR and OER [61-64]. Considering that carbon can act as an activity booster for the electrocatalytic activity of select perovskite oxides through the electronic effect of perovskite/carbon composites, Fabbri et al. [65] developed a novel Ba0.5Sr0.5Co0.8Fe0.2O3-δ/carbon composite and studied its bi-functionality on ORR/OER. In their experiment, they used a modified sol-gel method coupled with a calcination process at 1000oC to obtain Ba0.5Sr0.5Co0.8Fe0.2O3-δ

(BSCF)

powder

with

Ba(NO3)2,

(CH3CO2)2Sr,

Co(NO3)2·6H2O, and Fe(NO3)3·9H2O precursors. After acetylene black (AB) carbon was treated in nitric acid at 80oC overnight to create surface oxygen-containing functional groups, the BSCF and AB were incorporated via an ultra-sonication procedure to form the BSCF/AB composite electrode material with a BSCF to AB weight ratio of 5:1. When ORR and OER tests of the BSCF/AB composite electrode in 0.1 M KOH was conducted in a three-electrode system consisting of a catalyst sample on glassy carbon as the working electrode, a reference hydrogen electrode (RHE), and a gold counter electrode, it was found that when compared to single BSFC, the BSCF/AB composite showed a decrease in the overpotential (130 ± 10mV at -0.025 mA cmgeo-2) and an increase in the reduction current over the whole ORR potential range. This indicates an improved ORR performance. For the OER, the BSCF/AB composite was also found to have a lower quasi steady-state potential (1.55

- 26 -

± 0.012 V at 10 Agmetal-1) that is 100 mV lower than the single BSCF, which indicates a significant enhancement of OER activity. However, the carbon was also found to undergo an inevitable corrosion (oxidation) at high potential during a tested 15 potential cyclings. To effectively understand the interaction between BSCF and carbon (i.e., AB) as well as its active effect on ORR/OER, a shift toward lower energy and lower intensity of the pre-edge peak in the Co K-edge was found in the XANES spectrum of BSCF/AB as compared to that of BSCF, indicating an increase of electron density of the Co cations in the BSCF perovskite due to the surface functionality of AB that resulted from it. The reduction of the Co oxidation state was evident in the XRD to modify the lattice parameter and thus make a shift of the BSCF/AB diffraction peaks to smaller angles in comparison with those of pure BSCF. Even though preparation methods can play effective role in electronic structures and electrochemical properties [22, 23], it has been significantly evidenced that the nature of carbon materials such as carbon type (i.e., carbon black or graphite carbon) [54, 65, 66] and surface chemistry produces possible electronic interactions between perovskites and carbon and can lead to improvements in electrochemical activity [67, 68]. Nishio et al. [54, 69] examined and compared ORR and OER properties using perovskite oxides with different B-site elements, such as LaMnO3, La0.6Sr0.4FeO3, and LaNiO3, as well as their previous La0.5Sr0.5CoO3, and their composites with carbon. In their experimental preparation, La2O3, SrCO3, MnCO3, Iron(III) citrate n-hydrate, and NiCO3 were corresponded to the perovskite compositions as starting materials while C-NERGY Super C65® was the carbon material. Three perovskite oxides were

- 27 -

synthesized by the malic acid precursor method and several calcination processes at different temperatures (e.g, 600oC, 900oC, or 1200oC). The perovskite oxide-C65 composite was obtained with a layered structure for the evaluation of the effects of carbon (i.e., C65). The analysis of XRD patterns confirmed the perovskite structures for LaMnO3, La0.6Sr0.4FeO3, LaNiO3, and La0.5Sr0.5CoO3, with an average particle size of 1.2, 1.0, 1.6, and 1.0 µm, respectively. Minor NiO impurities were found in LaNiO3 however. After electrochemical measurements were carried out in 8M KOH, cyclic voltammetry studies revealed a very low oxygen reduction current density (see Table 1) for pure perovskite oxides, while perovskite oxide-carbon composites resulted in an increase of ORR current density by

(Table 1)

about two orders. The poor ORR activities of the carbon-free perovskite oxides were attributed to insufficient gas permeability and low electric conductivity in the active layer. At the same time, the OER activities of the perovskite oxides were also improved after the addition of carbon. Despite this, the current density values of LaMnO3-C65 and La0.6Sr0.4FeO3-C65 seemed much lower than those of LaNiO3-C65 and La0.5Sr0.5CoO3-C65, possibly due to the presence of NiO impurities [70, 71]. To further investigate perovskite oxides, composite oxides of MnO2 and perovskite oxides (i.e., LaNiO3 and LaCoO3), along with Vulcan XC-72 (VC) were fabricated in Gyenge’s group [72] in the development of highly active, durable

- 28 -

MnO2-based catalysts for ORR/OER as MnOx attracts increasing attention for its electrocatalytic properties [73-75]. A co-precipitation method was performed using La(NO3)3·6H2O, Ni(NO3)2·6H2O and Co(NO3)2·6H2O as precursors to obtain doped MnO2 with two perovskite oxides such as LaNiO3 and LaCoO3. The final composites (i.e., MnO2-LaNiO3-VC and MnO2-LaCoO3-VC) were then prepared using a sonication method. The weight ratio of MnO2, perovskite oxide, and Vulcan XC-72 was fixed at 1:1:1. After XRD spectra confirmed the presence of MnO2, LaNiO3 and LaCoO3, electrochemical properties such as ORR and OER were measured in 6M KOH via a RDE set-up with a scan rate of 5 mV s -1. In the analysis of the OER activity (see Fig. 6(a)), the comparison of the three pure species suggest an increasing order of activity: LaNiO3 > LaCoO3 > MnO2. The combination of MnO2 with perovskite (LaNiO3 or LaCoO3) also

(Fig. 6)

resulted in an improvement of the initial OER activity when compared to pure perovskite

(LaNiO3

or

LaCoO3).

The

calculated

OER

Tafel

slope

of

MnO2-LaNiO3-VC is 90 mV dec-1. This is lower than pure LaNiO3 (121 mV dec-1), demonstrating the active effect of perovskite doping. When compared with MnO2-LaCoO3-VC and with individual metal oxide catalysts, MnO2-LaNiO3-VC generated the highest oxygen evolution current densities. At 650 mV, the oxygen evolution current density of MnO2-LaNiO3-VC was about seven times higher than

- 29 -

that of pure MnO2. As for the ORR activity (see Fig. 6(b)), the combination of MnO2 with LaNiO3 was found to induce a synergistic effect on the improvement of ORR. This effect is better than the effect obtained from the combination of MnO2 and LaCoO3. No significant ORR activity was observed for the two pure perovskite, LaNiO3 and LaCoO3. This indicated that both perovskites were not active towards ORR. Interestingly, when the electrocatalytic durability of MnO2-LaNiO3-VC and MnO2-LaCoO3-VC catalysts was investigated through 100 continuous potential cycles between -300 mV to +700 mV, the measured results demonstrated a significant performance drop in OER/ORR activity for the combination of MnO2 and LaNiO3 despite their promising initial activity. After an examination of the sample’s structures before and after the durability test using a combination of XRD and XPS, a mechanism to enhance the catalytic activity was proposed in that K+ ions be intercalated into the MnO2-based electrocatalyst structure to potentially act as a promoter of OER and ORR reactions [72, 76].

3.1.3 Other carbon-supported catalysts Since Pt/C alone is recognized to have poor activity for OER, oxides such as IrO2 [77] and perovskite oxide [3]; due to their excellent OER activity, has been employed to modify Pt/C and obtain increased activity towards both ORR and OER in URFC applications. Shao et al. [77] used H2PtCl6 and iridium salt [78] to prepare a bifunctional catalyst mixture of Pt and IrO2 with a weight ratio of 1:1. They used the mixture of Pt-IrO2 as a catalyst with carbon paper as a support layer in an assembled

- 30 -

single cell for electrochemical measurements. The combination of TEM and SEM images revealed that Pt particles (15 - 20 nm); when mixed with IrO2 particles (~40nm), exhibited a uniform distribution in the electrocatalyst layer. The resulting fuel cell performance demonstrated that the Pt-IrO2/C can effectively run the fuel cell and the electrolysis of the URFC and that it was a better bifunctional oxygen catalyst than Pt/C. For the fuel cell performance, the cell voltage was 0.7 V at 80 oC and the H2/O2 pressure was 0.3 MPa when the current density was at 400 mA cm-2. For the electrolysis performance, the cell voltage was 1.71 V at 80 oC and the H2/O2 pressure was ambient when the current density was at 400 mA cm-2. Lee et al. [79] also fabricated Pt/IrO2/carbon-paper (Pt/IrO2/CP) electrocatalyst for oxygen electrode in URFC, using sequential formation of IrO2 layer (loading 0.1 mg cm-2) and porous Pt layer (0-0.3 mg cm-2) on CP by electrodeposition and spraying techniques. They found that the fuel cell (FC) performance was increased linearly up to 690 mA cm-2 with increasing Pt loading (up to ~0.3 mg cm-2) at 0.6 V, while the water electrolysis (WE) activity was the highest at Pt loading of 0.2 mg cm-2. The current densities in the FC and WE modes and round-trip efficiency of the developed Pt/IrO2/CP electrodes with the oxygen electrocatalyst loadings of 0.3 and 0.4 mg cm-2 were higher or comparable to those previously reported with higher loading (1.5-4.0 mg cm-2). It was observed that the high performance of oxygen electrode in this study was resulted from efficient mass transport in the electrode with the open structure of Pt/IrO2/CP layer. Based on the idea that the successful composite of Pt-CaMnO3 showed favorable

- 31 -

ORR/OER activity in alkaline medium [80], Zhu et al. [3] prepared a mixture of Pt/C-perovskite oxide as a bifunctional oxygen catalyst in which the presence of carbon favors electronic conduction and improved the surface area of the support for Pt deposition, while the perovskite oxide has an outstanding OER activity. The as-obtained composite of Pt/C-perovskite oxide with a wide range of Pt to oxide combinations were studied for ORR/OER activity. In the preparation of Pt/C-perovskite

oxide,

three

compounds,

Ba0.5Sr0.5Co0.8Fe0.2O3-δ

(BSCF),

PrBaCo2O5+δ (PBC), and LaNiO3-δ (LN) were first synthesized through a standard combined ethylenediaminetetraacetic acid (EDTA)/citrate complexing sol-gel process [81] using different series of metal precursors (i.e., Ba(NO3)2, Sr(NO3)2, Co(NO3)2·6H2O, and Fe(NO3)3·9H2O for BSCF; Pr(NO3)2·6H2O, Ba(NO3)2, and Co(NO3)2·6H2O for PBC; La(NO3)3·6 H2O and Ni(NO3)2·6H2O for LN). These prepared perovskite oxides (i.e., BSCF, PBC, and LN) were distributed on a conductive carbon (Super P Li) to obtain carbon-supported perovskite oxides (i.e., BSCF/C, PBC/C, and LN/C) with a mass ratio of perovskite oxide to conductive carbon at 4:6. A commercial 40wt%Pt/C was then mixed with these carbon supported perovskite oxides to form three different final composites: Pt/C-BSCF/C, Pt/C-PBC/C, and Pt/C-LN/C. The weight ratio of Pt to perovskite oxide was fixed at 4:1, 1:1 and 1:4 respectively to determine the effect of oxide content on physicochemical properties. Based on XRD patterns, the three perovskite oxides, BSCF, PBC, and LN were indexed to pure cubic, tetragonal, and rhombohedral perovskite structure respectively, without other impurity phases. The XRD patterns of the three typical

- 32 -

composites with a fixed Pt to perovskite oxide mass ratio of 1:1 showed that in addition to the characteristic peaks of Pt/C, other peaks can be detected and assignable to perovskite oxide phases. The main peaks in the Pt/C-BSCF/C and Pt/C-PBC composites seemed relatively weak however, which is probably caused by an interaction between the oxide phase and the Pt/C [82-84]. In order to detect the interaction, XPS analysis was used and this revealed that the characteristic peaks of Pt 4f7/2 for Pt/C-BSCF/C and Pt/C-LN/C were at 71.42 and 71.43 eV respectively. These peaks were slightly shifted towards low binding energy than those of Pt/C (71.62 eV), indicating a change in the electronic structure of Pt after the incorporation of oxides [85, 86]. In addition, the average Co valence in the Pt/C-BSCF/C turned out to be close to 3+, which is higher than that in BSCF. This suggests a change in the electronic structure of Co in the Pt/C-BSCF/C composite due to the presence of Pt. It was deduced that an electron transferred from the oxide to Pt, caused by interactions in the Pt/C-oxide composite, resulted in the improvement of the ORR/OER activity. In order to further investigate the electronic interaction between PtC and oxides on ORR/OER activity, electrochemical tests were conducted in 0.1 M KOH using a RDE technique. All ORR and OER activities (see Table 2) showed that the three Pt/C-oxides composites; Pt/C-BSCF/C, Pt/C-PBC/C, and Pt/C-LN/C, exhibited better ORR activity than a single Pt/C catalyst and better OER activity than a single oxide. This demonstrates the improvement of bi-functionality.

(Table 2)

- 33 -

Furthermore, Pt/C-BSCF/C at 0.65 V displayed higher OER activities (>17.49 A g-1total) than the others, while with a Pt:oxide ratio of 1:4, Pt/C-PBC/C at -0.2 V exhibited the highest ORR activity (292 A g-1Pt). The three carbon-supported perovskite oxides (BSCF/C, PBC/C, and LN/C) were confirmed to be highly active on OER rather than ORR. It was concluded that a synergistic effect between Pt and oxide can be created using an electronic transfer mechanism and a RDS and spillover mechanism which will then enhance bifunctional OER/ORR activity. Duarte et al. [87] developed a set of five Layered Double Hydroxides (LDH) with a C oand Mn ratio of 4 using oxidized carbon nanotubes as the support material. Due to the possible incorporation of metal ions into layers with several compostions, the LDH structure could be converted into the corresponding mixed metal oxides with a uniform M2+ and M3+ distribution after thermal treatment. According to the measurements of X-ray diffraction (XRD), the LDH hybrid supported on carbon nanotubes was composed of a mixture of LDH and MnCo 2O4. A detailed ORR study revealed that the LDH structure mixed with CoMn oxides could play a role in catalyzed ORR, which exhibited an onset potential of -0.274 V and a similar four-electron ORR mechanism to the Pt/C catalyst. The best OER potential (0.636 V) was obtained with the CNT-supported LDH catalyst. This work demonstrated the significant role of the LDH structure in the electron transfer ORR mechanism and OER performance by reducing electrode overpotentials

- 34 -

3.2 Modified carbon-supported catalysts As carbon-supported catalysts are being continuously developed as bifunctional oxygen catalysts, progress has been made to prevent typical stability/durability issues due to the carbon corrosion under harsh electrochemical oxidation circumstances through the modification of carbon [22, 88, 89]. These modification methods involve the modification of physical and/or chemical characteristics of pure carbon, resulting in a novel series of modified carbon support materials [90-92]. To date, a wide range of modified carbon materials have been and are being explored as exciting and promising support candidates for supported catalysts in the application of URFC oxygen electrode.

3.2.1 Modified carbon-supported metals To reduce catalyst costs and obtain suitable substitutions for precious metals such as Pt, Ir, and Ru, considerable efforts have been focused on non-precious metal-based bifunctional electrocatalysts towards ORR and OER [93, 94]. As candidates of supported catalysts, the combination of non-precious metals such as Co [95, 96], Fe [96-98] and modified carbon (e.g., N-doped carbon) have recently been examined to develop novel bifunctional electrocatalysts for both ORR and OER in URFCs. Based on the positive effect of N-doped carbon materials on bifunctional catalytic activity in oxide-based composites [33, 45, 99], Su et al. [95] fabricated a hybrid composite of Co nanoparticles and N-doped carbon and studied the synergistic effect of the two components on ORR/OER activity. In their experiment, Co(Ac)2·4H2O (Co

- 35 -

resource) was reacted with dopamine in 2.0 ml NH4OH solution based on a solvothermal carbonization strategy in which dopamine was self-polymerized to poly dopamine as a source of N and C. 700oC, 800oC, 900oC were selected as the three carbonization temperatures under argon to prepare Co/N-C-700, Co/N-C-800, and Co/N-C-900. The Co/N-C-800 sample was further treated at 250oC and 450oC under ambient air to obtain Co/N-C-800-250 and Co/N-C-800-450 samples. This was done for more in-depth comparisons of structure and property. In the results of a Raman spectra, the increasing IG/ID ratios indicated the increasing graphitization degree of the carbon in Co/N-C samples when the temperature was elevated from 700oC to 900oC during carbonization. The XRD patterns of the Co/N-C-800 sample gave not only a broad C (002) diffraction peak, but also well-defined peaks at 44.3o, 51.6o, and 75.8o, indicating the presence of metallic Co. When thermal treatment was processed at 250oC and 450oC, the XRD results suggests the complete oxidization of Co into Co3O4 for Co/N-C-800-450, while Co/N-C-800-250 was composed of Co 3O4, CoO, and traces of Co metal. This suggests a partial oxidization of metallic Co. In the examination of the morphology, TEM images detected the uniform distribution of Co metal nanoparticles in the carbon matrix. The size of Co was also observed to increase with increasing temperature from 700oC to 900oC in carbonization, as confirmed with the decreasing BET surface area for Co/N-C-700 (246 m2 g-1), Co/N-C-800 (220 m2 g-1), and Co/N-C-900 (199 m2 g-1) in BET measurements. When the analysis of SEM and HRTEM images was applied, most of the Co nanoparticles (20-50 nm) were uniformly embedded in a granular-like carbon matrix, while some smaller Co

- 36 -

nanoparticles (5-20 nm) were incorporated into the carbon layers. This distribution of Co nanoparticles in the N-doped carbon matrix was identified to effectively prevent the agglomeration and oxidation of Co metal nanoparticles, and to

(Fig. 7)

enhance the rapid electron transport between N-doped carbon matrix and Co, resulting in efficient electrochemical performances [100]. Importantly, in the XPS measurement of Co/N-C-800 (see Fig. 7), a set of peaks at C 1s (284.6 eV), N 1s (401.0 eV), O 1s (531.6), and Co 2p (780.0 eV) corresponded to the presence of C, N, O, and Co. Specifically, the high-resolution XPS spectrum of the N 1s peak revealed that the graphitic N dominates the majority of the species among the pyridinic, graphitic, and oxidized N species in Co/N-C-800. The presence of N species was found to play an important role in electrochemical properties [101]. Furthermore, the deconvolution of the C 1s spectrum suggest three types of C species: 66.5% C=C, 12.6% C=N & C-O, 8.3% C-O-C & C-N, and 12.6% -O-C=O. This suggests the existence of carbon atoms connected to N and O heteroatoms. Co 2p3/2 and 2p1/2 high-resolution spectras were fitted with Co(0), Co(II) and shake-up (satellites) peaks, suggesting the oxidization of Co nanoparticles on the surface of the Co/N-C-800 in air, as well as the formation of a thin CoO shell due to the sensitivity of cobalt (0) nanoclusters in aerobic atmosphere [102-104]. The high-resolution O 1s spectrum also demonstrated a relationship to cobalt(II) oxides, along with hydroxyls and carboxyls. The structural characterizations

- 37 -

indicates a synergistic interaction in the hybrid composite, enhancing electrochemical properties. When the ORR/OER activities were evaluated based on electrochemical measurements in KOH solution using RDE, three pyrolyzed samples (i.e., Co/N-C-700, Co/N-C-800, and Co/N-C-900) were analyzed to test the effect of carbon graphitization. Co/N-C-800 with 20wt% Pt/C was also compared for ORR/OER, including the two heat-treated samples of Co/N-C-800-250 and Co/N-C-800-450. The ORR/OER activity in O2-saturated 0.1 M KOH at 1600 rpm demonstrated a higher activity for Co/N-C-800 than Co/N-C-700 and Co/N-C-900, suggesting that an optimal graphitization degree was achieved at 800oC. Further ORR tests in O2-saturated 0.1M KOH at 1600 rpm revealed that according to a four-electron transfer process, the ORR catalysis in the polarization curve displays a smaller Tafel slope of 61 mV dec -1 at lower overpotentials for Co/N-C-800 than those for Co/N-C-800-250 (68 mV dec-1), Co/N-C-800-450 (80 mV dec-1), and 20wt% Pt/C (68 mV dec-1), demonstrating the excellent ORR activity of Co/N-C-800. Furthermore, such a slope is close to the Nernstian Tafel slope (59 mV dec-1), suggesting that the rate-determining step of ORR might be the splitting of O-O bonds when two electrons are transferred from the active sites to the adsorbed O2 molecules [105]. In the OER tested in an O2-saturated 0.1 M KOH, the resulting Tafel slopes were calculated as 61.4, 74.1, 116.1 and 169.6 mV dec -1 for Co/N-C-800, Co/N-C-800-250, Co/N-C-800-450, and 20wt% Pt/C, respectively. This suggests that Co/N-C-800 is the most efficient OER catalyst among all the samples, and has a better OER kinetic than other reported N-doped carbon supported cobalt oxides [33, 45, 106]. An accelerated

- 38 -

stability test with 500 continuous cycles also demonstrated the superior durability of Co/N-C-800 as compared to other measured samples, confirming the predicted synergistic interaction of the Co embedded in the N-doped carbon. To further study the hybrid composites of non-precious-metals and N-doped carbon as bifunctional electrocatalysts, Zhao et al. [96] prepared and compared Co/N-C and Fe/N-C catalysts in their research of OER/ORR. In their experiments, Co(NO3)2·6H2O and Fe(NO3)2·9H2O were used as Co and Fe precursors, while melamine was used as the N source. Shortly after the polymerization of melamine, the metal precursor and carbon (Ketjenblack EC-300) were incorporated into a polymer gel paste. Then, a pyrolysis procedure at 700oC was conducted to obtain the final hybrid composites of Fe/N-C and Co/N-C. 700oC was used because their previous study showed that 700oC provided optimal graphitization for catalyst conductivity and catalytic performance [107]. A synthesized N-C without the metal, 20wt% Pt/C, and IrO2/C were used as reference samples in all measurements, and the metal-C/N hybrid composites are assigned as the metal doped carbon/N materials. In the examination of the structure and morphology, the combination of XRD and Raman spectroscopy confirmed similar graphitization degrees for Fe/N-C, Co/N-C and N-C samples. The XPS analysis showed that among the three N species (pyridinic-N, quaternary-N, and quaternary-N-O in Table 3), both pyridinic-N and

(Table 3)

- 39 -

quaternary-N significantly dominated in the Fe/N-C, Co/N-C and N-C samples. The contents of Fe and Co were detected as 2.8 wt% and 1.4 wt%, respectively. OER/ORR measurement were then carried out in KOH electrolyte (pH=13) at 25 oC using a bipotentiostat equipped with a rotating ring-disk electrode (RRDE). The data obtained for ORR/OER revealed that the Fe/N-C, Co/N-C, and N-C catalysts produced a high OER activity with small overpotentials, resulting in a current density of 10 mA cm-2 at 1.59 ± 0.01 VRHE, 1.61 ± 0.01 VRHE, and 1.60 ± 0.01 VRHE respectively. These results are almost the same as for IrO2/C (10 mA cm−2, 1.60 ± 0.01 VRHE). No difference in OER overpotential was observed for the Fe/N-C and Co/N-C catalysts, indicating that the metal species (i.e., Co and Fe) have little influence on OER. Additionally, the comparison of ORR activities showed that the ORR onset potential and the half-wave potential of Fe/N-C were 0.94 ± 0.02 VRHE and 0.83 ± 0.01 VRHE. These are lower than those of 20wt% Pt/C (0.98 ± 0.02 VRHE, 0.86 ± 0.01 VRHE), and higher than those of Co/N-C (0.89 ± 0.02 VRHE, 0.80 ± 0.01 VRHE) and N-C (0.87 ± 0.02 VRHE, 0.78 ± 0.01 VRHE). This suggests that the ORR activity of Fe/N-C is better than that of Co/N-C and N-C. Significantly, based on the oxygen electrode activity, the Fe/C/N catalyst can offer a potential gap of 0.76 V between the OER potential (at 10 mA cm−2) and the ORR half-wave potential, suggesting that Fe/N-C is a more promising bifunctional oxygen catalyst with superior activity than other reported oxide materials such as Mn3O4 (~1.04 V) [108], Co3O4/N-graphene (~0.69 V) [33], and perovskite oxides (~1.0 V) [24, 61]. Since metal-containing N-doped carbons, especially Fe/N-C composites,

- 40 -

exhibited the most promising activities towards ORR/OER, interesting modifications [97, 98, 109] have been designed and fabricated into the Fe/N-C composite to further enhance the catalytic performance of the oxygen electrode in URFCs. Following the strategy of using non-precious metal bifunctional catalysts, Schuhmann’s group [98] created Fe/Nx-C/perovskite oxide composites using four perovskites (La0.6Sr0.4FeO3 (L60SF), La0.76Sr0.2Co0.2Fe0.8O3 (L76SCF), La0.83Ca0.15Mn0.5Co0.4O3 (LCaMC), and La0.58Sr0.4Co0.2Fe0.8O3 (L58SCF)), and studied their structural characteristics and their catalytic performance. These perovskites have shown good ORR/OER activities, depending on their composition and stoichiometry [38, 62]. In their experiments, pyrrole was polymerized in a solution system containing Vulcan XC72 to form polypyrrole/Vulcan composite. A pyrolysis procedure under He gas was done to this composite to obtain nitrogen functionalized carbon (Nx-C). Iron phthalocyanine (FePc) was then mixed with Nx-C (Mass ratio of FePc:Nx-C is 3:7) with the aid of acetonitrile. This mixture was further heat-treated in a fume hood under air to prepare Fe/Nx-C. For the preparation of Fe/N x-C/perovskites, the perovskites, L60SF, L76SCF, LCaMC, and L58SCF were added to Nx-C, along with FePc, and underwent the same heat treatment to form new Fe/Nx-C/L60SF, Fe/Nx-C/L76SCF, Fe/Nx-C/LCaMC, and Fe/Nx-C/L58SCF composites. Among the four pure perovskite oxides, L60SF, L76SCF, LCaMC showed high OER but poor ORR activities, therefore focus was given to L58SCF in structural and electrochemical characterizations. According to XRD results, pure perovskite oxide is highly-crystalline and the presence of Fe/N x-C in the Fe/Nx-C/perovskite oxide composite caused shifts in some characteristic peaks

- 41 -

of the perovskite oxide, indicating a lattice distortion resulting from strong interactions between the perovskite oxide and the Fe/Nx-C. According to SEM images, Nx-C (100 - 300 nm) was distributed on the perovskite oxide (1 - 10 µm). When electrochemical measurements were conducted in a conventional three-electrode cell with 0.1 M KOH using RDE method, the ORR activities of the L58SCF were clearly improved after the combination of Fe/Nx-C and L58SCF since Fe/Nx-C can act as a conductive additive to boost the conductivity of the L58SCF. Furthermore, the Fe/Nx-C/L58SCF exhibited higher OER activity than Fe/Nx-C and L58SCF individually. Taking into account the potentials corresponding to currents measured at 10 mA cm-2 and -1 mA cm-2 for OER and ORR, respectively, the Fe/Nx-C/L58SCF shows better bifunctional properties with a voltage gap of only 0.86 V when compared to Fe/Nx-C/L60SF (1.02 V), Fe/Nx-C/L76SCF (0.95 V), Fe/Nx-C/LCaMCF (0.94 V), Fe/Nx-C (0.97 V), Pt/C (1.22 V), IrO2 (1.32 V), and RuO2 (1.10 V). This composite represents a new family of highly efficient bifunctional catalysts in the development of oxygen electrodes in URFCs. Metal-organic frameworks (MOFs) were also used to prepare a novel modified Fe/N-C composite in Lan’s group [109], which has the characteristic of porous crystalline materials [110] and has been utilized in the electrocatalytic field [111, 112]. In their experiment, MIL-101(Fe) crystals were prepared [113] using FeCl3·6H2O as the Fe precursor, and then mixed with melamine by ball milling. This mixture was then pyrolyzed under N2 at 700oC for 5 h to form the final Fe/Fe3C@N-doped

graphitic

layer/

N-doped

- 42 -

carbon

nanotube

hybrid

(Fe/Fe3C@NGL-NCNT) sample. For comparison purposes, MIL-101(Fe) and MIL-101(Fe) soaked in a melamine/DMF solution were also pyrolyzed under N2 at 700oC for 5h to obtain Fe/Fe3C@C and Fe/Fe3C@N-doped carbon (Fe/Fe3C@NC), respectively. In the detailed experiment, 700oC was selected as the optimal pyrolysis temperature as the temperature resulted in optimal graphitization and therefore the best ORR activity as compared to other temperatures (i.e., 500oC, 900oC, and 1100oC). The analysis of XRD patterns not only revealed two characteristic peaks at 44.8o and 65.1o, which correspond to the (110) and (200) reflections of α-Fe, respectively, it also confirmed the presence of Fe3C species. Importantly, it was found in the examination of a Raman spectrum that the intensity ratio of the G-band (1590 cm-1) and D-band (1350 cm-1) of Fe/Fe3C@NGL-NCNT demonstrated a higher graphitization degree (~0.83) than those of Fe/Fe3C@NC (~0.49) and Fe/Fe3C@C (~0.46), as confirmed by XRD studies. This suggests that higher graphitization can enhance the electric conductivity of Fe/Fe3C@NGL-NCNT, promoting charge transfer and favoring ORR activity. Three N species, graphitic N (~401.2 eV), pyrrolic N (~399.8 eV) and pyridinic

N (~398.6 eV)

were

also

observed

in the XPS

spectra of

Fe/Fe3C@NGL-NCNT. The presence of graphitic and pyridinic N were reported to be more active in comparison to their pyrrolic counterpart which benefits ORR activity [114]. SEM and HRTEM observations demonstrated that some of the Fe/Fe3C nanoparticles were wrapped in a 5 nm think graphitic carbon layer with a spacing of 0.34 nm while others were encapsulated within the graphitic carbon layers. This was also confirmed by corresponding elemental mapping. The graphitic carbon layers can

- 43 -

efficiently prevent the aggregation of Fe/Fe3C species. In order to investigate the ORR/OER activities of the Fe/Fe3C@NGL-NCNT hybrid synthesized using MOFs as solid precursors, the corresponding electrochemical experiments were performed on a CHI 760D electrochemical station in a standard three electrode cell with 0.1 M KOH at room temperature. In the detailed examination of ORR with a scan rate of 10 mV s-1 and 1600 rpm, the Fe/Fe3C@NGL-NCNT catalyst exhibited a higher onset potential (0.04 V) than that of Fe/Fe3C@NC (-0.19 V) and Fe/Fe3C@C (-0.2 V) and was very close to commercial 20wt% Pt/C catalyst (0.045 V), proving the active effect of N and Fe species for the enhancement of electrocatalytic activity. The electrochemical studies correlates well with the XRD, SEM, TEM, and Raman spectroscopy results. Based on further comparison of the ORR tests carried out from 400 to 1600 rpm (see Fig. 8(a-c)), the current density is distinctly observed with increasing rotation rates, while the calculated electron transfer number of the Fe/Fe3C@NGL-NCNT was 3.6, which is higher than those of the Fe/Fe3C@NC (3.4) and the Fe/Fe3C@C (3.5), and comparable to that of 20wt% Pt/C (3.7). The corresponding current density of the Fe/Fe3C@NGL-NCNT was

(Fig. 8)

slightly lower than that of the commercial 20wt%Pt/C, but it was greater than the Fe/Fe3C@NC and the Fe/Fe3C@C. This highlights the excellent ORR activity of Fe/Fe3C@NGL-NCNT in alkaline electrolytes. Interestingly, after methanol was

- 44 -

added to the KOH solution, no clear changes in CV curves were observed. This demonstrates a high tolerance to methanol crossover at the cathode for Fe/Fe3C@NGL-NCNT as compared to commercial 20wt% Pt/C. This property which can play a vital role in fuel cell commercialization. Regarding OER activity, the potentials at current densities of 5 mA cm-2 and 10 mA cm-2 are generally used as key parameters for OER [107]. Compared to commercial 20wt%Pt/C, the potentials of Fe/Fe3C@NGL-NCNT at these two current densities are much lower, proving that Fe/Fe3C@NGL-NCNT is an efficient OER catalyst. In the current-time (i-t) chronoamperometric response (See Fig. 8(d) and (e)), at -0.4 V and 0.7 V (vs. Ag/AgCl)

in

O2-saturated

0.1

M

KOH

at

1600

rpm

for

ORR/OER,

Fe/Fe3C@NGL-NCNT displayed significantly superior stability as compared to commercial 20wt% Pt/C, demonstrating that Fe/Fe3C@NGL-NCNT is an efficient bifunctional oxygen catalyst for URFCs.

3.2.2 Modified carbon-supported metal oxides Owing to its variable valence states and its structural flexibility, spinel oxides have been combined with pure carbon materials to form bifunctional oxygen catalysts, offering exciting opportunities to fine-tune their catalytic properties [40, 115, 116]. Since modified carbon material supports can provide active sites and interact with metal-based catalysts that benefits the ORR/OER [95-98, 109], spinel oxides have been and are being used in the development of novel modified carbon supported bifunctional oxygen catalysts for URFCs.

- 45 -

Using N-doped carbon nanofibers (N-CNFs) as a modified carbon support, Lee’s group [117] fabricated a bifunctional MnCo2O4-NCNFs composite using a solvothermal method in which polypyrrole nanowires were used to produce N-C-N boding motifs as active ORR sites which stem from the formation of graphitic carbon lattices (graphitic-N) [118-121]. In the preparation of polypyrrole nanowires, the monomer

(i.e.,

pyrrole)

was

polymerized

in

a

mixture

solution

of

cetyltrimethylammonium bromide (CTAB), HCl, and ammonium persulfate through a simply synthesis route. The formed polypyrrole nanowires were carbonized in N2 at 800oC for 2h to obtain nitrogen-enriched carbon nanofibers (N-CNFs). Co(OAc)2 and Mn(OAc)2 were used as the Co and Mn precursors. This resulted in an aged solution with N-CNFs. After a heat treatment at 150oC, the final composite was formed as MnCo2O4-NCNFs with 35 wt% MnCo 2O4. XRD results confirmed the MnCo 2O4 spinel structure, with SEM and TEM images revealing a uniform deposition of MnCo2O4 in the size range of 2-4 nm on the surface of the N-CNFs, while the N-CNFs was about 40-60 nm in diameter and several µm in length. Importantly, the deconvolution of the N1s signal in the XPS spectra suggests the presence of pyridinic-N (397.8eV) and graphitic-N (400.4 eV). The latter has a calculated content of 54.6%, which favors charge transfer and electron conduction [118, 119]. The molar ratio of Co-Mn was 2 and in accordance with the starting precursor ratio, as measured by XPS. After the formation of the MnCo2O4/N-CNFs composite, the binding energies of O1s, Mn2p3/2, and Co3p3/2 were shifted to lower BE in comparison with pure MnCo2O4, while the binding energy of C1s signal slightly decreased in

- 46 -

comparison with N-CNFs. The binding energies of graphitic-N1s and pyridinic-N1s in the MnCo2O4/N-CNFs composite were especially higher than those in the N-CNFs. XPS results for all these compounds suggested that there is a strong interaction between MnCo2O4 and N-CNFs, resulting in an enhancement of catalytic performance. These chemical shifts are generated by electron migrations not only from -CNFs to MnCo2O4, but also from the nitrogen lone pair to the sp2 carbon skeleton [42]. To investigate the effect of MnCo2O4 and CNFs on catalytic performance, electrochemical measurements were ran in 0.1 M KOH at 900 rpm using a standard three-electrode cell and a RDE method. The MnCo 2O4/N-CNFs composite in ORR performance showed a more positive ORR onset potential (-0.08 V vs. Ag/AgCl); close to the commercial 20wt% Pt/C (-0.06V), and a higher cathodic peak current than the components alone. This suggests a synergistic effect between the support and the catalyst. Moreover, detailed comparisons of linear sweep voltammetry (LSV) curves demonstrated that the MnCo2O4/N-CNFs composite provided higher current density and greater positive half-wave potential than 20wt% Pt/C. After 30,000 seconds of continuous operation at -0.4 V, it was also found that the MnCo2O4/N-CNFs composite experienced less activity decrease (17%) than that of 20wt%Pt/C (34%). This shows that MnCo2O4/N-CNFs composites surpassed Pt/C in both activity and stability in ORR. Regarding the OER performance, the MnCo2O4/NCNFs composite displayed catalytic activities with both less positive potential and higher overall current densities, suggesting that this composite can outperform conventional Pt/C systems in the OER process. However, in comparison

- 47 -

with commercial precious metal oxides (RuO2 or IrO2); which are still the best OER catalysts for industrial electrolysis [122, 123], the OER catalytic activity for MnCo2O4/N-CNFs composites are still lower. In evaluating the overall suitability of MnCo2O4/N-CNFs composites as a bifunctional oxygen electrocatalyst, the difference between the half-wave potential of ORR and the potential for water electrolysis at 10 mA cm-2 was 1.04 V. after 400 cycles. The current density also decreased to 15 mA cm-2 with a retention of 83.3% of its initial current density. This indicates that MnCo2O4/N-CNFs composites are a good alternative bifunctional oxygen catalyst for URFCs. MnCo2O4 was also combined with other modified carbon materials in Hor’s group [124]. In their proposed synthesis procedure, N-doped reduced graphene oxides (N-rGO) and carbon nanotubes (CNTs) were used as the starting materials. The covalent coupling phenomenon, which results from both the interactions between MnCo2O4 and N-rGO and also between MnCo2O4 and CNTs, effectively enhanced ORR/OER activities of the MnCo2O4/NrGO-CNTs composite. In contrast to MnCo 2O4, other oxides such as Mn, and/or Co-based oxides [44, 99, 125, 126]; which have a well-crystalline spinel structure, were also researched in combination with carbon materials as potential bifunctional oxygen catalysts in the development of URFCs. Zhao et al. [125] designed and developed a simple, scalable, and novel method for the synthesis of low-cost, spinel Mn-Co oxide particles, partially embedded in N-doped CNTs by oxidative thermal scission. Experimentally, the N-CNTs were synthesized by catalytic chemical vapour deposition using spinel-type cobalt-manganese mixed oxide (Co-Mn-Mg-Al oxide) as

- 48 -

the catalyst and pyridine as the precursor. An acid-washing procedure was processed for these as-prepared N-CNTs, followed by heat treatment at different temperatures (i.e., 300oC, 400oC, 500oC, and

(Table 4)

600oC) for 5 min. The thermal oxidative cutting of N-CNTs was achieved by repeating this two-step procedure three times at a flow rate of air. The final composite samples were denoted as Co-Mn-oxide/N-CNT-300, Co-Mn- oxide/N-CNT-400, Co-Mn-oxide/N-CNT-500, and Co-Mn-oxide/N-CNT-600. The Co-Mn-oxide/N-CNT composite before heat treatment was used as a reference sample. The content of Mn and Co presented in Table 4 increased with increased temperature as confirmed by inductively coupled plasma optical emission spectroscopy (ICP-OES). In XRD patterns, no changes were found for the N-CNTs at the oxidative temperature of 300oC, suggesting spinel oxides were not formed. At temperatures higher than 400oC, the oxidative treatment resulted in the formation of a cubic spinel structure, with a Mn/Co ratio of 1:2 as confirmed by ICP-OES measurements. At the same time, the intensity of C reflections diminished with increasing temperatures. This could indicate either structural changes or the formation of defects, confirmed also by surface XPS analysis. Interestingly, the ratio of D-band to G band; investigated by Raman spectroscopy, increased from 1.0 to 1.1, as compared to the sample before oxidative annealing. This suggests an increasing concentration of structural defects, which

- 49 -

affects the catalytic performance [33, 127]. These studies revealed the importance of heat treatment on the activity of this type of compound. Furthermore, the ORR/OER activities were investigated by running electrochemical measurements in 0.1 M KOH via a RDE technique. The onset potential and the reduction current in tested linear sweep voltammograms at 100 rpm and 5 mV s -1 showed an increasing order for all samples:

Co-Mn-oxide/N-CNT

<

Co-Mn-oxide/N-CNT-300

<

Co-Mn-oxide/N-CNT-400 < Co-Mn-oxide/N-CNT-500 < Co-Mn-oxide/N-CNT-600. This indicates that thermal oxidative cutting has a strong effect on ORR activity and thus confirming the importance of spinel oxides in the catalytic process. The investigated

average

electron

transfer

numbers

were

3.8

and

3.4

for

Co-Mn-oxide/N-CNT-500 and Co-Mn-oxide/N-CNT, respectively, over a potential range from 0.8 to 0.2 V, suggesting the prevalence of both the four-electron and two-electron ORR reduction pathways in both cases. When using a combination of OER and ORR to evaluate all samples in comparison with other samples (i.e., 20wt% Pt/C, RuO2, and IrO2), Co-Mn-oxide/N-CNT-500 exhibited the highest OER activity among all prepared samples and also presented a higher ORR activity than IrO2 and RuO2 as well as a remarkable OER activity similar to that of RuO2. Furthermore, the Co-Mn-oxide/N-CNT-500 bifunctional catalyst showed a degradation tendency similar to that of RuO2 over the same time scale, indicating that the durability of this catalyst is comparable to that of RuO2. All electrochemical results underline the high potential of Co-Mn-oxide/N-CNT composites, resulting from thermal oxidative cutting. Due to the direct contact between the metal oxide and the graphene walls,

- 50 -

electronic interactions are possible, especially when defects are present in both the metal oxide and the graphene structures. This composite can be an excellent bifunctional oxygen catalyst under alkaline conditions. Similar to spinel oxides, perovskite oxides were also combined with modified carbon support materials and examined as a bifunctional oxygen catalyst in URFCs. Ge et al. [55] used lanthanum nitrate hexahydrate, cobalt nitrate hexahydrate and manganese nitrate hydrate as the precursors of La, Co, and Mn, and prepared a spinel La(Co0.55Mn0.45)0.99O3-δ (LCMO) material. This cost-effective synthesis route has several phases involving hydrothermal reaction at 160oC, freeze drying, heat treatment from 700oC to 1000oC, and calcination at 810oC. In a further step, a reduced graphene oxide (GO) solution was added with urea and the LCMO, followed by hydrothermal treatment at 170oC, resulting in a LCMO/N-rGO composite catalyst with a LCMO content of 20wt% and a yield of 90%. It was recognized that the urea in the hydrothermal treatment acted not only as a nitrogen source but also as a reducing agent for the formation of N-rGO. The structural and morphology characterizations by XRD, SEM, energy-dispersive X-ray (EDX), HRTEM, TEM, and XPS revealed that the LCMO particles had the shape of nanorods with an average diameter of 48.5 ± 7.2 nm and the aspect molar ratio of LCMO/N-rGO ranging from 3 to 10. The highly crystalline LCMO was composed of low-index and relatively isometric nanocrystals with grain sizes in the range of 19 - 28 nm, while the XRD patterns showed a pure perovskite phase in LCMO after calcination at temperatures as low as 710oC. When the LCMO/N-rGO composite was formed, the LCMO still retained the characteristic

- 51 -

morphology of nanorods, uniformly distributed in the LCMO/N-rGO. The incorporation of N-rGO also resulted in the molecular orbital sharing between the B-site of perovskite and the N-rGO, giving rise to the binding energies of both Mn and Co in the XPS spectra. Moreover, the separation of binding energies between the satellite and the main peak of La 3d5/2 (ΔBE) and the satellite-to-main-peak intensity (S/M) ratio of La 3d5/2 corresponding to La-O covalent bonding were 4.4 eV and 0.87 for the LCMO. This decreased for the LCMO/N-rGO, with figures of 4.2 eV and 0.74. Both chemical shifts indicated on the changes in the covalent La-O bonding are due to the electronic interaction (electron sharing) between the La cation and the N-rGO. Binding energies of O 1s, C 1s and N 1s in the LCMO/N-rGO were also 0.1 – 0.3 eV smaller than those of LCMO and N-rGO. These BE shifts were correlated with the changes in the XPS signals (peak position) for La, Co and Mn cations for the LCMO/N-rGO, suggesting the molecular orbital sharing and the disorder of the electronic structure of the cations. XPS analysis conformed covalent coupling between LCMO and N-rGO in the LCMO/N-rGO sample. It is noted that based on Raman spectra, the intensity ratio of D-band and G-band of N-rGO is 0.99, indicating a high graphitization degree due to N doping. In addition, as a complementary analysis, the electron spin resonance (ESR) detected unpaired electrons of Co and Mn cations. Their presence is beneficial to the electrocatalytic activity for ORR and OER [38, 62]. To investigate the electrocatalytic performance of the LCMO/N-rGO catalyst, LCMO, N-rGO, Pt/C and Ir/C control samples as well as a mechanical mixture of LCMO and N-rGO (LCMO+N-rGO) were used as references for electrochemical

- 52 -

measurements in 0.1 M KOH using RDE technique. The oxygen reduction polarization curves in O2-saturated 0.1 M KOH with 2000 rpm reveal that LCMO/N-rGO exhibited the best ORR activity among all tested samples. The oxygen reduction peak potential and the peak current of LCMO/N-rGO were 136 mV more positive and 6% larger than those of the Pt/C reference. In a comparison of ORR activity between the LCMO/N-rGO and the LCMO+N-rGO catalysts, the diffusion-limiting current density at -0.8 V were -0.184 V for the former and -0.269 V for the latter. This test demonstrates better ORR activity for LCMO/N-rGO than that of LCMO+N-rGO, indicating stronger hybrid coupling and interaction between the support and catalyst for LCMO/N-rGO compared to that of LCMO+N-rGO. Analysis of Koutecky-Levich plots revealed a four-electron reaction pathway for the ORR in LCMO/N-rGO rather than a two-electron reaction pathway. When RDE polarization curves were used to check OER, the OER activity of LCMO/NrGO is comparable to that of Ir/C and is higher than other samples. The OER onset potential of LCMO/N-rGO was around 0.45 V. This is only 50 mV smaller than for Ir/C. At 10 mA cm-2 (a standard current density required to achieve a water splitting efficiency of 10% with one-sun illumination for solar-to-fuel conversion [108, 128]), the potential of LCMO/N-rGO is 0.787 V. This is close to that of Ir/C (0.737 V). Generally, the potential gap (ΔE) between ORR and OER potentials at current densities of practical significance is used to evaluate the electrocatalytic activity of a bifunctional catalyst. When the ORR current density and OER current density were selected as -3 mA cm-2 and 10 mA cm-2 respectively, the potential gap of LCMO/N-rGO was 0.960 V, which

- 53 -

is slightly lower than Ir/C (1.086 V). Even with this in mind, LCMO/N-rGO still seemed to be a good alternative bifunctional catalyst compared to other samples. One main reason being its long term stability. The tested durability showed that the degradation rate of the electrical potential of LCMO/N-rGO was 14.8 mV h-1 under polarization with a load of 5 mA cm-2 in OER mode. That is 34% lower than that of Ir/C. This analysis proves that LCMO/N-rGO is a durable bifunctional catalyst for both ORR and OER. This investigation provides a novel design with modified carbon materials and perovskite structures that rely on electronic interactions between the support and the catalyst, resulting in improved ORR/OER activities. Different from spinel oxides and pervskite oxides, other common oxides such as Co3O4 [33, 129, 130] and MnOx [131] were also reported to show a synergistic effect with modified carbon materials (e.g., doped carbon) and beneficial bifunctional activity toward both ORR and OER. For instance, Co3O4 nanocrystals were grown on N-doped graphene and tested as a bifunctional catalyst for ORR and OER by Fellinger’s group [33]. N-doped reduced graphene oxide (N-rGO) was used as the functionalized carbon support, and demonstrated a strong electronic interaction with Co3O4 nanocrystals. For the preparation of Co3O4/N-rGO composite, GO was synthetized through an oxidization-associated route. Co(Ac)2 aqueous solution was mixed with GO/EtOH, followed by the addition of NH4OH solution and water. After the hydrolysis and oxidation of Co(Ac)2, a hydrothermal reaction was performed at 150oC for 3h, followed by purification which led to the final Co3O4/N-rGO product. Thermal-gravimetric analysis revealed that the combined material had ~70 wt% of

- 54 -

Co3O4. References samples such as rGO, N-rGO, and Co3O4 particles were also prepared using the same experimental procedures. According to TEM images, Co3O4/NrGO showed smaller particles (4-8 nm) than Co3O4/rGO (12-25 nm), highlighting the importance of NH4OH in the synthesis route as it can tune the hydrolysis of Co2+ and its oxidation, leading to the controlled particle nucleation on the N-rGO, and thus to the decrease in Co3O4 particle sizes [132, 133]. HRTEM combined with XRD analysis identified a crystalline spinel structure for Co 3O4. The surface characterization examined by XPS showed the presence of pyridinic and pyrrolic nitrogen species with a total of 4% in the Co3O4/NrGO, while no N content was found in the Co3O4/rGO system. Furthermore, the electrocatalytic activity studied by cyclic voltammetry (CV) (see Fig. 9) in O2 and Ar-saturated 0.1 M KOH indicated that pure Co3O4 and rGO exhibited very weak ORR activity, while Co3O4/rGO showed more positive ORR onset potential and higher cathodic currents. This indicates

(Fig. 9)

the synergistic ORR activity of Co 3O4 and rGO in a Co3O4/rGO catalyst. Comparing Co3O4/N-rGO and Co3O4/rGO, the N-doped sample showed more positive ORR peak potential and higher peak current. This suggests that N-doping into the graphene oxide had additional positive effects, such as improved electronic interactions between both components, and presumably, improved ORR activity at the N-doped

- 55 -

catalytic centers. Importantly, RDE measurements revealed an electron transfer number of ~4.0 at 0.60-0.75 V for the Co 3O4/N-rGO catalyst. The half-wave potential at 1600 rpm was 0.83 V, which is similar to that of Pt/C (0.86 V) and more positive than that of the Co 3O4/rGO hybrid (0.79 V). At 0.7 V vs. RHE, the Co3O4/N-rGO sample offered an ORR current density of 52.6 mA cm-2. This was higher than for the Co3O4/rGO system (23.3 mA cm-2) and close to Pt/C catalysts (68.0 mA cm-2). The oxygen reduction currents of Co 3O4/N-rGO were 1 – 3 orders of magnitude higher than that of Co3O4 (0.012 mA cm-2), rGO (0.19 mA cm-2) or N-rGO alone (3.5 mA cm-2), suggesting that the strong coupling effect between the Co3O4 and N-rGO results in the best ORR activity of Co 3O4/N-rGO catalysts. After 10 000 – 25 000s of continuous operation in 0.1 – 6M KOH solution, results showed the superior durability of Co3O4/N-rGO in comparison to 20wt% Pt/C, 50wt% Pt/C, Fe-N/C [134, 135], and commercial 10%Pd/C. The OER results in 0.1 M KOH showed that at the current density of 10 mA cm-2, Co3O4/N-rGO hybrid exhibited a small overpotential of ~0.31 V and a low Tafel slope that is 67 mV/decade. These are promising characteristics for Co3O4-based bifunctional oxygen catalysts. In regards to the N-doping effect on graphene oxide, results show that Co3O4/N-rGO had slightly higher OER activity than Co3O4/rGO. OER stability tests over 1500 cycles revealed that both Co3O4/N-rGO and Co3O4/rGO were inherently stable. The combined results of ORR and OER indicates that with the synergistic effect between the support and the catalyst, Co3O4/N-rGO hybrids can be a powerful non-precious metal-based bifunctional catalysts for URFCs. Together with Co3O4/N-rGO, Co3O4 was also

- 56 -

fabricated and combined with other modified carbon materials such as heteroatom doped carbon (HDC) and N-doped carbon nanotube (CNT) and graphene nanoribbon (GNR). The samples were assigned as Co3O4/HDC [129] and Co3O4/N-(CNT-GNR) [130] composite bifunctional catalysts, respectively. These were tested for both ORR and OER. The structure and property relationship studies for these systems revealed the interaction between the catalyst and the modified carbon support; resulting in the enhancement of electrocatalytic activities towards ORR/OER, provides future research ideas for further optimization of bifunctional oxygen catalysts in URFCs. Similar to N-doping that can result in the interaction between N-doped carbons and metal oxides, S-doping was also found to result in the interaction between S-doped carbons with mesoporous manganese oxides (MnOx) [131]. In the reported synthesis procedure, commercial polyvinyl chloride (PVC) beads were mixed with (NH4)2Fe(SO4)2 and KMnO4 solutions. Pyrolysis was then carried out at 1100oC under N2 for 24 min resulting in a composite of S-doped graphitized carbon (S-GC) and MnOx (i.e. MnOx/S-GC composite). In this simple method, the PVC, (NH4)2Fe(SO4)2, and KMnO4 were used as sources of carbon, sulfur, and Mn while (NH4)2Fe(SO4)2 also acts as the graphitization catalyst. In this process the reduced Fe facilitates both the carbon precipitation and the multilayer graphitization [136]. For comparison, pure MnOx was prepared as a reference sample using the same synthesis method without the addition of PVC and (NH4)2Fe(SO4)2. The Raman spectroscopy of MnOx/S-GC shows that the intensity ratio of D band to G band peaks (ID/IG) is ~0.55, indicating the presence of structural disorders and defects [137] due to the formation

- 57 -

of multilayered graphitized carbon (this was also observed in TEM images). The small signal at 650 cm-1 representing MnOx includes Mn3O4, MnO2, and MnO [138, 139]. However, the characteristic peaks of MnOx were not identified by XRD since the concentration of metal oxide was too low for XRD analysis (MnOx content of 8 wt% as confirmed by Thermogravimetric analysis (TGA)). The combination of Raman spectroscopy, XRD, TGA, and TEM measurements confirmed that the composite consists of multilayered graphitized carbon with defects and small amounts of embedded MnOx. The interaction between S-doped graphitized carbon and MnOx was then studied by XPS. This showed negative shifts for the MnOx signal indicating the electronic effects between the carbon and the metal oxide [140]. Furthermore, the XPD elemental composition of the composite surface showed C (80.1%), O (8.1%), S (7.3%), and Mn (4.5%). This was also confirmed by energy dispersive X-ray spectroscopy (EDX). The ORR activity in O2-saturated 0.1 M KOH solution revealed that compared to S-GC, MnOx/S-GC exhibited higher positive onset potential and half wave potential as well as larger limiting current density corresponding to higher catalytic activity. The calculated electron transfer number of MnO x/S-GC varied from 3.7 to 4.0 for potentials ranging from 0.50 V to 0.75 V, demonstrating the 4e- oxygen reduction process. These results indicate that S-doping into graphitized carbon can enhance the electrocatalytic activity by changing electronic structures such as charge density and electron distribution around the catalytic active sites [141]. The d-orbitals of S are also easily polarized, facilitating the adsorption and interaction with O2 and H2O2 molecules and thus resulting in the improvement of ORR/OER activity [142].

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The OER analysis showed that the OER catalytic activity of the MnOx/S-GC composite was higher than those of S-GC and Pt/C and was comparable to other reported OER catalysts such as Mn oxide and Ir/C [108]. This suggests that the addition of MnOx plays an important role in its interaction with S-GC, leading to a more active system. The bifunctional activity was assessed by the difference between the potentials of OER (10 mA cm-2) and ORR (-3 mA cm-2). For the MnOx/S-GC composite, this was 0.81 V, demonstrating that this system is a promising bifunctional oxygen electrode in URFCs.

3.2.3 Modified carbon-supported sulfides In order to avoid the use of precious metal-based catalysts, various nonprecious metal-based materials supported on carbon have been found to exhibited excellent catalytic activity for ORR and OER. In the research and development of low-cost and high-efficient bifunctional catalysts, it has been reported that among transition metal chalcogenides, sulfides have shown great promise and evoked intensive interest as an advanced catalyst towards both ORR and OER because they exhibited noble-metal-like catalytic properties in energy storage [143, 144]. Considering not only the fact that the conjunction of sulfur- and nitrogen-doped carbon materials can create more active sites for O2 absorption and activation in alkaline medium [145, 146], but also that the Ni doped Co-S of thiospinel tryp (i.e., NiCo2S4) displays higher ORR activity than the binary Co-S system [147], Zhang’s group [148], for the first time, constructed a NiCo2S4/N, S co-doped reduced graphene

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oxid (NiCo2S4/NS-rGO) composite as an effective bifunctional nonprecious electrocatalyst for ORR and OER in alkaline medium through a one-pot solvothermal strategy. Experimentally, Co(OAc)2, Ni(OAc)2, and thiourea; a metal precursor and a precursor for N and S, respectively, was mixed with a suspension of graphene oxide (GO) and ethylene glycol (EG). After a solvothermal reaction coupled with a post-treatment procedure including filtration, washing, and lyophilization, the obtained product was labelled as NiCo2S4/NS-rGO. For comparison purposes, Ni3S4/NS-rGO, Co3S4/NS-rGO, rGO, NS-rGO, and NiCo2S4 were also prepared under the same conditions for the characterization of structure and property. In the experimental strategy, EG was used as a mild reductant to obtain rGO. It was also used as an effective solvent to avoid the agglomeration of nanoparticles. TEM images showed a selective and uniform distribution of metal sulfide (4-8 nm) on graphene sheets without detachment and aggregation, confirming the efficient synthesis strategy. The combination of HRTEM and XRD showed the NiCo2S4 nanocrystals in crystallite phases of (440), (511), (400), and (311). The elemental analysis in both EDS and XPS further confirmed the as-prepared hybrid material has a CO/N molar ratio of 2. In addition to the compositional information, XPS analysis revealed that after the doping of N and S, the NS-rGO existed in the forms of pyridinic-N (398.5 eV), pyrrolic-N (400.0 eV), and thiophenic-S (aromatic C-S-C, 164.3 eV). These are often desirable species in doped graphene as they enhance catalytic activity [142, 149]. In the deconvolution of Co and Ni spectra in XPS, the spectral Co 3+/Co2+ ratio also decreased from 1.85 to 1.22 after the incorporation of NS-rGO to NiCo2S4, while the

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spectral Ni2+/Ni3+ ratio of NiCo2S4/NS-rGO was ~2.32, higher than NiCo 2S4. Furthermore, the O/C value decreased from 0.268 to 0.153 after the incorporation of NS-rGO. These XPS results suggested a significant electronic interaction exists between the catalyst and the support, resulting from the chemical coupling between NiCo2S4 and NS-rGO. The electrocatalytic properties of NiCo 2S4/NS-rGO were investigated

through

electrochemical

measurements

using

a three-electrode

electrochemical cell coupled with a RDE technique. After an ORR examination using cyclic voltammograms in O2- and Ar-saturated 0.1 M KOH at room temperature, the ORR onset potential and peak potential of NiCo 2S4/NS-rGO was -0.11 and -0.22 V, respectively. This was more positive than those of Co3S4/NS-rGO (-0.12 V, -0.27 V) and Ni3S4/NS-rGO (-0.11 V, -0.38 V), suggesting not only that Co3S4 is more active toward ORR and Ni3S4, but also that the addition of Ni resulted in the improvement of ORR activity. A further investigation based on LSV curves in 0.05-0.6 V showed that NiCo2S4 exhibited higher intrinsic ORR activities than Co3S4 and CoNi2S4, confirming that the Ni doping content is a crucial factor to optimize the catalyst’s ORR performance. With the assistance of ORR kinetics, the detected electronic transfer number of Co3S4/NS-rGO was calculated to be in the range of 3.6 – 3.8 throughout the tested potential range, suggesting a four electron ORR pathway. Compared to commercial 20wt% Pt/C (26.82 mA cm-2 at -0.45 V) however, Co3S4/NS-rGO presented a lower calculated Jk (kinetic-limiting current density), indicating an inferior ORR activity. To check tolerance ability to the crossover effect for the application of direct methanol fuel cells (DMFCs), current-time

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chronoamperometric responses were tested in O2-saturated 0.1 M KOH with the addition of methanol. Here, Co3S4/NS-rGO showed a better methanol tolerance and a more stable current response than commercial Pt/C. When ORR durability was investigated, the 18% current decay for Co 3S4/NS-rGO is better than the 50% current decay of commercial Pt/C. When OER was estimated by polarization experiments in 0.1 M KOH (pH~13) and 0.1 M phosphate buffer at pH 7.0 with a sweep rate of 10 mV s-1, it was found that Co 3S4/NS-rGO displayed greater OER current than other samples such as Co 3S4/NS-rGO, Ni3S4/NS-rGO, or commercial 20wt% Pt/C. To further evaluate the OER activity, the small overpotential of ~0.47 V for Co3S4/NS-rGO at a key current density of 10 mA cm-2, is close to those of top performing, well-investigated Co3O4/graphene hybrids [33] and CoSe2/Mn3O4 [150] catalysts at similar loadings. The calculated difference between potentials for OER (10 mA cm-2) and the half-wave potential (E1/2) for ORR was 0.94 V for NiCo2S4/NS-rGO, demonstrating that it has potential to be a bifunctional oxygen catalyst. To investigate the effect of coupling interactions between NiCo2S4 and NS-rGO, a physical mixture of NiCo 2S4 and NS-rGO (i.e., p-NiCo2S4/NS-rGO) with an approximate catalyst composition of NiCo2S4/NS-rGO was used for comparison in electrochemical

measurements.

p-NiCo2S4/NS-rGO

showed

Compared

improved

ORR

with

each

activity

but

component was

alone,

inferior

to

NiCo2S4/NS-rGO, suggesting that NiCo 2S4/NS-rGO has a higher ORR activity due to the

synergistic

effect

between

NiCo2S4

and

NS-rGO.

Furthermore,

p-NiCo2S4/NS-rGO produced a lower OER activity than NiCo 2S4 and NS-rGO while

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NiCo2S4/NS-rGO exhibited the best OER activity. It can be inferred from this that the OER activity of NiCo 2S4/NS-rGO is generated from the synergistic effect between NiCo2S4 and NS-rGO. This research concludes that the high ORR/OER performance of NiCo2S4/NS-rGO is closely attributed to three factors, the doping effect, the nanostructure of NiCo2S4, as well as the strong synergetic coupling interaction at the interfaces of NiCo 2S4 and NS-rGO. Using N and S co-doped graphene as a modified carbon support material, Ganesan et al. [151] also developed a series of hybrids (i.e., CoS2(T)/NSG; T=400, 500, and 600), consisting of CoS2 phases and N,S co-doped graphene through a solid-state thermolysis approach at different temperatures of 400oC, 500oC, and 600oC. At temperatures of 400oC and 500oC, it resulted in a hybrid with a pure CoS2 phase. They also confirmed the strong coupling interaction between CoS2 and N,S-doped graphene, improving the oxygen electrode potential and resulting in the best CoS2(400)/NSG oxygen electrode catalyst, having a potential of ~0.82 V against a reversible hydrogen electrode in alkaline medium. This is far better than the performance of precious catalysts such as Pt/C (1.16 V), Ru/C (1.01 V) and Ir/C (0.92 V). Different from N,S co-doped graphene, N-doped carbon materials like N-doped mesoporous graphitic carbon (N-MC) was also used as a modified carbon support in the conjunction of sulfides to present bi-functionality towards ORR and OER in Ai’s group [152]. In their design of Co0.5Fe0.5S/N-MC composites, the incorporation of FeS in the composite can benefit the formation of Co 9S8 cubic phases as a higher active center for ORR [153]. Moderate substitution of well-dispersion of iron in

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bimetallic Co-S sulfide composites can also result in favorable adsorption of oxygen-containing species and catalytic enhancement compared to their monometallic counterparts. In addition, covalent assembling with mesoporous graphitic carbon can facilitate charge transport and mass diffusion during catalysis. Following a successful, facile, and scalable soft-template mediated route (see Fig. 10(a)), they mixed cobalt (II) acetate and iron(III) nitrate aqueous solution with thiourea (TU) and pluronic F127

(Fig. 10)

((EO)106-(PO)70-(EO)106) and then conducted an evaporation at 80oC and a heating process at 110oC. TU was the carbon and sulfur sources while F127 acts as a soft template precursor that promoted assembly during evaporation and assisted the abundant functional groups C=S and –NH2 to trap cobalt and iron ions through the hydrogen bond interactions between TU and F127. After annealing was carried out at different temperatures (600oC, 700oC, 800oC, 900oC, and 1000oC) for 1 h under Ar, the obtained product was purified to form Co0.5Fe0.5S/N-MC composites. In the analysis of structure and morphology using a combination of TGA, XRD, SEM, TEM, HRTEM, energy-dispersive X-ray spectroscopy (EDX), Brunauer-Emmett-Teller (BET), X-ray absorption near edge structure (XANEs), and XPS, it was found that according to XRD patterns, few cobalt-iron double sulfides were generated below 800oC while a high annealing temperature such as 900oC or 1000oC results in

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well-crystallized

cobalt-iron

bimetallic

sulfides.

In

the

XRD

pattern

of

Co0.5Fe0.5S/N-MC composites prepared at an annealing temperature of 900oC, a detailed analysis revealed three different groups with diffraction peaks corresponding to hexagonal FeS, cubic Co9S8 and hexagonal CoS, as confirmed in lattice parameters seen in the HRTEM images (see Fig. 10(e)). This indicates the successful fabrication of complicated and well-mixed bimetallic sulfides. As confirmed in XPS, Co 2p at 778.4 and 793.5 eV also indicates a mixed composition of Co 9S8 and CoS [155-156] while the Fe 2p spectrum at 710.1 and 711.9 eV were assigned to Fe2+ oxidation in FeS [157]. Calculated from Rietveled refinement results, the mass fraction of Co 9S8 in the cobalt sulfide was 64% higher than that of CoS, which was shown to be active toward O2 reduction [153]. SEM and TEM images (see Fig. 10(b), (c) and (d)) revealed that the relatively uniform and well-dispersed sulfide nanocrystals with diameters of 20 - 40 nm were observed to be anchored inside folded carbon film fragments with low aggregation while the well-defined graphitic carbon shell structure after heat-treatment had a lattice spacing ranging from 0.34 to 0.36 nm, favoring the in situ incorporation of N [158] and thereby optimizing the electronic conductivity of the composite [159]. EDX and XPS analysis confirmed the existence of C, N, S, Fe, Co, and O elements in the Co 0.5Fe0.5S/N-MC composite. In particular, the atomic ratio of S to the overall metal was nearly 1, suggesting the formation of Co0.5Fe0.5S and a high level of N doping (~6.89 at%). Based on XPS analysis, C 1s peak at 284.6, 285.8, and 288.8 eV were assigned to the graphitic structure of the combination of both C=N and C-S [142, 160], and C-N respectively, while the N 1s

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profiles at 398.3 and 400.6 eV were associated with pyridinic N and graphitic N respectively. The pyridinic N was reported to improve the onset potential and the graphitic N determined the limiting current for ORR [101]. For the S 2p spectrum, the first two peaks at 163.5 and 162.5 eV resulted from the spin-orbit coupling in metal sulfide [148, 161], while the peak at 163.5 eV corresponded to the covalent binding of S to C in the Co0.5Fe0.5S/N-MC composite [137, 142], indicating the covalent assembling of sulfide nanoparticles in N-doped mesoporous graphitic carbon. The XANES results also showed covalent bonding of sulfides to carbon because the peak intensity of Co and Fe K-edge XANES spectra increased with the formation of electron coupling between sulfide and carbon. With a standard three-electrode cell, cyclic voltammetry measurements in O-saturated 0.1M KOH showed an appreciable cathodic peak at 0.82 V for Co0.5Fe0.5S/N-MC, coupled with a higher peak current of 1.39 mA cm-2 than that of 20wt%Pt/C, indicating superb ORR catalytic activity for Co0.5Fe0.5S/N-MC. When RDE measurement was employed to gain further ORR information, the compared polarization curves at 1600 rpm in O2-saturated 0.1M KOH displayed a smaller Tafel slope (67 mV/decade) for Co0.5Fe0.5S/N-MC than that of 20wt% Pt/C (69 mV/decade), revealing high kinetic specific activity in Co0.5Fe0.5S/N-MC. The calculated electron transfer number of Co0.5Fe0.5S/N-MC is 3.8 – 4.0 from 0.4 to 0.7 V, suggesting an ideal quasi-four-electron pathway towards ORR. Interestingly, an investigation of the relationship between structural features and ORR activities revealed that Co0.5Fe0.5S/N-MC not only included an optimum Co/Fe ratio of 1.0 but also offered the best sulfide loading with 72 wt% in the

- 66 -

TGA-analyzed composite, resulting in the best electronic conductivity, the highest activity of ORR, the steepest slope in the kinetic region, the earliest reach to the diffusion-limiting current density and the most positive half-wave potential (E1/2). Moreover, Co0.5Fe0.5S/N-MC at 900oC exhibited a higher BET specific surface area (124.5 m2 g-1) than composites prepared without Pluronic F127 (17.4 m2 g-1). Further polarization curves tested in 0.1 M KOH with the addition of 0.5M methanol indicates that Co0.5Fe0.5S/N-MC exhibited a better resistance toward possible methanol crossover poisoning as compared to 20wt% Pt/C. The OER activity of Co1-xFexS/N-MC measured in Ar-saturated 1M KOH in an anodic direction showed that Co0.5Fe0.5S/N-MC presented the earliest onset potential (~1.57 V) and an overpotential of ~0.41 V at a current density of 10 mA cm-2. When the OER comparison of Co0.5Fe0.5S/N-MC and IrO2 catalysts was processed, it was found that the former exhibited a much lower slope (159 mV/decade) than the latter (267 mV/decade), indicating a higher OER activity for Co0.5Fe0.5S/N-MC. A long-lasting viability, as shown in the tested chronopotentiometry response with 20000 sec, confirms Co0.5Fe0.5S/N-MC as a potential OER catalyst. All the electrochemical results including ORR and OER demonstrates the strong coupling and synergistic electronic effect between sulfides and carbon, making them favorable to bi-functionality for both ORR and OER. Considering that the sulfides supported reduced graphene [162,163] can paly an important role in the catalyzed ORR, and that the active sites of Ni/NiOOH [164] can enhance the OER activity, Ma and He [165] simultaneously encapsuled Co 4S3, NixS6

- 67 -

(7≥x≥6) and NiOOH nanocrystals on 3D nitrogen-doped graphene-carbon nanotubes (NGC) to fabricate an advanced NGC@Co 4S3/NixS6(7≥x≥6)/NiOOH composite catalyst, and used as an oxygen electrode catalyst in URFCs. It wass found that based on XPS results, the integration of Co4S3, NixS6 (7≥x≥6), NiOOH and NGC could produce a synergistic effect, favoring quick electron and mass transfer. The ORR study showed that the NGC@Co 4S3/NixS6(7≥x≥6)/NiOOH catalyst has similar kinetic parameters to and a better durability than 20wt%Pt/C. Moreover, the catalyst displayed considerable OER activity in comparison with RuO2, and a good stability of 200 cycles of testing. This work confirmed that the multiple-contained metallic omposites played a vital role in catalyzing oxygen reaction since some of the included composites took effect in ORR and some promoted OER activity. This work also suggested a way to design bifunctional catalysts using a model of multiple-metallic composites on carbon materials.

3.2.4 Other modified carbon-supported catalysts. Differing from metal(s), oxides, and sulfides, catalysts such as iron carbide [97] and cobalt oxalate [166] have been supported on doped-carbon materials and have shown to displayed active physicochemical interactions between the catalyst and the support. These interactions could provide improvements towards the bifunctional activity of ORR/OER due to the doped carbon material being able to act as a main support material. The properties of these catalysts have attracted considerable attention in the development of advanced bifunctional oxygen catalysts for URFC

- 68 -

applications. Based on growing interests towards the active activity for both ORR and OER of composites of transition-metal (e.g., Fe, Co, or Ni), N-doped carbon [45, 168-170], and in particularly, metal encapsulated inside N-doped carbon [171, 172], Jiang et al. [97] designed and prepared a Fe3C/N-doped graphene (Fe3C/NG) hybrid in which Fe3C nanoparticles were made chemically stable due to their encapsulation in graphitic layers and dispersed on N-dope graphene. In a reported one-step solid-state thermal reaction, three precursors, Fe(NO3)3·6H2O, glucose, and urea were dissolved into ultrapure water. Then after being evaporated at 80oC, the dried mixture was annealed at three temperatures (700oC, 800oC, and 900oC) to obtain the as-prepared products (i.e., Fe3C/NG700, Fe3C/NG800, and Fe3C/NG900). To remove any ORR unstable phases, an acidic leaching using 0.5 M H2SO4 solution was carried out at 80oC for the as-prepared products. For comparison purposes, NG800 and FeC/C800 were prepared as the two reference samples, using annealing temperature of 800oC in the same experimental route with Fe(NO3)3·6H2O and urea. The Fe3C/NG800 showed a higher BET surface area of 755.6 m2 g-1 than the Fe3C/NG700 (576.4 m2 g-1) and the Fe3C/NG900 (401.2 m2 g-1), indicating that 800oC is the optimal temperature to produce maximum surface area, which increases active site exposure and thus promote rapid ORR (or OER) species transport. The intensity ratio of D- and G bands in Raman spectra of the sample slightly decreased however with increasing annealing temperatures: Fe3C/NG700 (1.06) > Fe3C/NG800 (1.01) > Fe3C/NG900 (0.99), suggesting the formation of graphitization at higher temperatures [173]. XRD patterns

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of Fe3C/NG800 presented a diffraction peak at 26.1o, corresponding to the formed graphitic carbon after annealing, while other characteristic peaks were associated with the crystalline F3C phase. In further structural characterizations, SEM and TEM images not only confirmed a typical graphene-like sheet morphology but also revealed a uniform distribution of Fe3C of an average size of ~11.1 nm. In particular, HRTEM images demonstrated well-defined crystalline lattice spacings of 0.21 and 0.36 nm, corresponding to the presence of (211) planes of the Fe3C phase and (002) planes of the graphite phase, respectively. When elemental compositions were investigated, the analysis of energy-dispersive X-ray spectroscopy (EDS) revealed that Fe3C/NG800 consisted of 29.4 wt% Fe, 56.3 wt% C, 9.7 wt% O, and 4.6 wt% N, which correlates with TGA results. To examine the doped element and bonding configurations in the interaction between Fe3C and NG, the XPS spectrum of Fe3C/NG800 demonstrated the presence of C, N, O, and Fe elements without other elements detected. In the detailed analysis of the high-resolution spectrum, the spectrum of C 1s was deconvoluted to several peaks of C species (i.e., O-C=O, C-N and C=O, C=N and C-O, and C=C), indicating the existence of heteroatoms in the NG-based hybrids [174]. The deconvolution of N 1s spectra showed three peaks at 398.3, 400.1, and 401.2 eV, corresponding to pyridinic-N, pyrrolic-N, and graphitic-N, respectively, suggesting successful N-doping in the carbon framework [175]. The former two Ns were found to be the dominant species, acting as efficient ORR active sites [175, 176]. Interestingly, the content of N decreased with increasing annealing temperatures due to the instability of N dopant at elevated temperatures. Notably, no

- 70 -

signals of Fe0 at 707 eV was observed in XPS, suggesting the encapsulation of Fe3C nanoparticles in carbon layers, while the peak at 711.4 eV in the Fe 2p3/2 XPS spectrum indicated the existence of Fe-Nx bonding [177]. To verify the effects of annealing temperatures on catalytic activity for ORR/OER, a comparison of electrochemical measurements were ran in O2-saturated 0.1M KOH solutions through RRDE and RDE. It was found that compared to Fe3C/NG700 and Fe3C/NG900, Fe3C/NG800 not only exhibited a more positive onset potential and larger diffusion-limited current density in tested linear sweep voltammetry (LSV) curves, it also had a better OER activity due to the lower overpotential at a current density of 10 mA cm-2, coupled with a higher electron transfer number of 3.94. These results were attributed to an optimum balance of porosity, electrical conductivity, type and density of active sites at an optimal temperature of 800oC [170, 178]. The ORR and OER performance of Fe3C/NG800 was also compared with other samples such as NG800, 20wt%Pt/C, and RuO2 using further electrochemical measurements tested in O2-saturated 0.1 M KOH. In the comparison of ORR, Fe3C/NG800 displayed a well-defined cathodic peak centered at +0.811 V in CV measurements, suggesting superior ORR catalytic activity to NG800 but similar to 20wt% Pt/C. After 3000 continuous cycles in O2-saturated 0.1 M KOH, no changes were found for the ORR performance of Fe3C/NG800 while 20wt% Pt/C displayed obvious decay, indicating the better stability of Fe3C/NG800 to 20wt% Pt/C. In the evaluation of OER, the polarization curves showed earlier onset potential and greater current density for Fe3C/NG800 than those of Pt/C and NG800, and is comparable to that of

- 71 -

state-of-the-art RuO2 catalysts. In particular, the comparison of overpotantial at current densities of 10 mA cm-2 revealed a smaller overpotential of ~0.361 V. This value is approaching the overpotential value of RuO2, and smaller than that of NG800 or 20wt% Pt/C. The calculated Tafel slope of Fe3C/NG800 was 62 mV dec-1. This is lower than those of RuO2 (69 mV dec-1), NG800 (65 mV dec-1), and 20wt% Pt/C (168 mV dec-1) samples, demonstrating the superior intrinsic OER kinetics of Fe3C/NG800 and suggesting that Fe-related species play an important role in high OER activity. In a key evaluation of bi-functionality using the calculated difference between the potential for OER (10 mA cm-2) and the potential (E1/2) for ORR (3 mA cm-2), Fe3C/NG800 gave the smallest value of 0.78 V, suggesting that it has the best bifunctional catalytic activity and the most potential for practical applications among iron carbides. Among modified carbon-supported bifunctional oxygen catalysts, cobalt oxalate (CoC2O4); which is simply made from an organic acid (e.g., oxalic acid) and Co(NO3)2·6H2O, was used to produce a synergistic effect with N-doped graphene supports in Lee’s group [166]. Based on a simple, facile and cost-effective route, they dissolved self-made graphene oxide (GO) flakes into DI water. After a gentle reduction, reduced graphene oxide (rGO) was formed in a reduction process using NH3 solution and heat-treatment from room temperature to 100oC. Oxalic acid and Co(NO3)2·6H2O were subsequently added into the rGO solution under sonication to fabricate the CoC2O4/rGO hybrid. In the experiment, the mild reduction process played an important role in controlling the high dispersion of GO even after reduction.

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As observed in SEM and TEM images, CoC2O4 rods were clearly seen wrapped around layers of rGO and the thickness of the rGO coating varied along the length of the rod, probably due to the interaction between CoC2O4 and rGO. The combination of XRD and XANES suggested the presence of CoC2O4 in the CoC2O4/rGO hybrid while TGA data showed the mass loading of CoC2O4 to be around 66 wt%. This is close to the composition of 63 wt% based on the amount of the precursors in the synthesis. At the same time, CoC2O4/rGO was observed at two small peaks at 810 cm-1 and 1310 cm-1 in the Fourier transform infrared spectrometer (FT-IR) results. These peaks corresponded to N-functionalization, which is favorable to OER performance, where as a physical mixture of CoC2O4 and gRGO (pm-CoC2O4-rGO) had no N-functionalization. XPS spectra also confirmed the presence of N1s, consisting of quaternary-N, graphitic-N, and pyridinic-N. When electrochemical properties were investigated using a three-electrode cell, the measured LSV data in 0.1 M KOH showed that rGO almost had no OER activity while CoC2O4 exhibited higher OER activity than CoC2O4/rGO. This is probably due to the covering of rGO on CoC2O4. After a continuous 1800s, the OER of CoC2O4 based on tested chronoamperometric responses degraded more dramatically than compared to CoC2O4/rGO, indicating that CoC2O4/rGO is a durable catalyst for OER. In further comparisons of OER, CoC2O4/rGO presented an onset potential of 0.71 V. This is higher than Ir and lower than pm-CoC2O4-rGO, indicating that CoC2O4/gRGO has a lower OER activity than Ir black and higher OER activity than pm-CoC2O4-gRGO. The tested chronoamperometric responses after 1800s also showed superior durability

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for CoC2O4/rGO. Moreover, when a long stability test was ran for up to almost 6 hr at an operating potential of 0.8V, CoC2O4/rGO also exhibited better durability than Ir black, 20wt% Ir/C, and pm-CoC2O4-rGO. These OER results confirmed the fact that the interaction between CoC2O4 and rGO plays a significantly active role in OER performance. Considering that rGO had no OER activity, the OER mechanism for CoC2O4/rGO was attributed to CoC2O4 being able to transform into Co(OH)2 under KOH solution at the beginning of the OER reaction and then transform into OER-active Co(III)OOH and Co(IV)O2 [179, 180], as evident in XRD patterns of CoC2O4/rGO after the 1st voltammetric scan in KOH. Interestingly, after the 240th scan, Co2O3 and CoO2 species were found as the dominant Co species in the tested CoC2O4/rGO [181]. As confirmed in FT-IR, formed N-functional groups during GO reduction is critical to providing active ORR sites on the surface of rGO and results in minor ORR activity of rGO in measurements of ORR using RDE while CoC2O4 showed almost no ORR activity. The comparable onset potential and slightly higher limiting current density (0.5 mA cm-2 difference) in LSV showed that CoC2O4/rGO exhibited higher ORR activity, demonstrating a strong synergistic interaction between CoC2O4 and rGO favoring ORR. However, the tested durability of CoC2O4/rGO showed a considerable decay of ORR activity when compared to rGO and 20wt% Pt/C catalysts, which contradicts the results of OER. Based on the analysis of Koutecky-Levich on CVs, the calculated electron transfer number of CoC2O4/rGO is 3.6, suggesting a quasi-four-electron pathway in the ORR kinetics. Therefore, although a synergistic interaction between CoC2O4 and rGO was found in

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CoC2O4/rGO that benefits OER/ORR activity, the durability of CoC2O4/rGO still needs to be improved for ORR. Since Co-based materials [182-184] have been reported to exhibit the catalytic activity for ORR and/or OER, Liu et al. [185] developed CoO@Co nanoparticles with a core-shell structure immobilized on N-doped reduced graphene oxide (rGO) (CoO@Co/N-rGO), and used it as the oxygen electrode in URFCs. They combined a rapid microwave-polyol method with a vacuum thermal treatment for the synthesis. According to XPS results, the dominant existence of Co 2+ oxides suggested the oxidation of Co nanoparticles on the surface of garphene, which was consistent with the results of TEM and XRD for the formation of Co-N moieties. It was found that the coordination between the N and Co atom on carbon surface could lead to the formation of interacted heterostructure, favoring a highly multi-functional catalytic activity. The ORR test in 0.1 M KOH showed that the CoO@Co/N-rGO catalyst had a comparable ORR activity to the commercial Pt/C with a similar four-electron transfer pathway. Furthermore, a good OER performance could be observed for this catalyst, achieving a current density of 10 mA cm-2 with a small overpotential in 0.1 M KOH, which was comparable to that of a commercial RuO2 catalyst. It was concluded that the unusual catalytic activities of the CoO@Co/N-rGO catalyst were induced by the synergetic chemical coupling effects of metallic, resulting in a multi-functional catalyst for URFCs.

3.3 Noncarbon-supported catalysts

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Since noncarbon materials were discussed as developed supports for Pt-based catalysts in our previous review [22], noncarbon support materials have become increasingly attractive for use in energy storage and conversion devices such as fuel cells, photoelectrochemical water splitting cells, solar cell, biosensors, Li-ion batteries and supercapacitors [186-189]. Undoubtedly, nanocarbon materials; particularly metal oxides and carbides, have been chosen and studied as supports for bifunctional catalysts toward both ORR and OER in URFCs due to their stable physical and chemical properties, corrosion resistance, and favourable interactions with catalysts.

3.3.1 Metal oxide-supported metal(s) 3.3.1.1 IrO2-supported metal(s) Before other oxides, iridium oxide (IrO2) has been used as the support material for metals, especially Pt catalysts, in the development of advanced bifunctional oxygen catalysts in URFCs due to IrO2‘s high OER activity [190, 191]. Up to now, two different types of methods have been used to synthesize IrO2 supported Pt bifunctional oxygen catalysts. One method is the physical mixing of Pt and IrO2 particles with or without media while the other is through the physicochemical deposition of Pt onto the surface of IrO2 support. Although physical mixing is a simple method, it cannot produce ideal synergistic interactions between the support and the catalyst. Even so, some researchers continue to use this method to prepare Pt/IrO2 for the research and development of bifunctional oxygen catalysts in URFCs [192-196]. Friedrich’s group [192, 193] studied and used

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different methods of physical mixing of Pt and IrO2 to form bifunctional oxygen electrodes in the membrane electrode assembly (MEA) of a single cell for URFC tests. In their experiment, three options were used in the preparation of bifunctional catalysts at the bifunctional oxygen electrode where platinum was fixed as the catalyst at the hydrogen electrode. As shown in Table 5, option I was a simple mixture of Pt and IrO2 used as the bifunctional oxygen catalyst while option II was multilayered and option III was segmented. In option II, there were two type of multilayered catalyst fabrication of catalysts in the tested MEA: Pt(inside)/IrO2(outside) and Pt(outside)/IrO2(inside). In option III, a square design and a stripe design for the segmented catalyst was fabricated at the oxygen electrode to compare with a regular segmented configuration. Before the MEA was fabricated and tested, the membrane (i.e., Nafion®1135) was pretreated in H2O2 and H2SO4 and 30wt% Nafion was added to increase the proton conductivity of the electrodes. Except option 3.12, other options were measured in corresponding MEA-assembled single cells. The resulting polarization curves (Ej) (see Fig. 11(a))

(Table 5)

(Fig. 11)

obtained potentiostatically in fuel cell mode (beginning at the open cell output potential difference) and galyanostatically in electrolysis mode (starting at 0 A)

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reveals that option 2.1 exhibited comparable performance in fuel cell mode to option 1 while having higher overpotentials than option 1, indicating a poorer performance in electrolysis mode. Conversely, the performance in electrolysis mode for option 2.2 is close to that of option 1 and its performance in fuel cell mode is lower than that of option 1. These two different results were mainly attributed to the choice of inner and outer layers in terms of Pt and IrO2, resulting in different charge transfer resistances. When options 3.11, 3.12 and 3.2 were measured and investigated, option 3.23 was operated from electrolysis mode to fuel cell mode, which was different from the other options. The E(j) curves (see Fig.11(b)) showed that superior fuel cell performance was obtained for option 3.11, whereas option 3.12 showed superior performance in electrolysis mode. This distinct behavior results mainly from the changed properties of the gas diffusion medium (e.g., the hydrophobicity of the layer) under different operation conditions. Option 3.2 exhibited similar behaviors in performance to option 3.12. Interestingly, in further detailed comparisons of select samples in the combined E(j) curves (see Fig. 11(c)) that had optimum performance in fuel cell mode in each option, option 3 showed lower performance in both modes due to it having the highest cell impedance as compared to the other two options while option 2 obtained the best performance in fuel cell mode due to it having the smallest cell impedance throughout the fuel cell modes. Importantly, in electrolysis mode, option 1 was found to have the smallest impedance resulting in the highest performance. However, all MEA was tested in short-term stability. Some further challenges in the development of advanced bifunctional oxygen catalysts are remain.

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Compared to the physical mixing method of synthesising and preparing bifunctional oxygen catalysts, physicochemical deposition of metals onto a support material is accepted as the better method for synthesising and preparing bifunctional oxygen catalysts. Not only does this method control the size and distribution of catalyst particles on the surface of the support, it can also induce the interaction between the catalyst and the support, resulting in the enhancement of catalytic performance. In regards to the deposition of Pt onto IrO2 for bifunctional oxygen catalysts in URFCs [197, 198], Kong’s group displayed their rich experience in literature [199-201] by using different self-made IrO2 for the deposition of Pt nanoparticles during a microwave-assisted polyol process. Considering that two main preparation methods (direct thermal decomposition [200, 202, 203] and modified Adams method [199, 204, 205]) are used in the fabrication of the iridium precursors to obtain IrO2 support with poor distribution and irregularity, they designed and utilized a SBA-15 template to prepare monodispersed s-IrO2 for the deposition of Pt nanoparticles [201]. In the first step, in order to synthesize s-IrO2 support, SBA-15 was mixed with H2IrCl6 solution using ethanol as a solvent through stirring. Then after consecutive vaporization at 40oC, a dried mixture of H2IrCl6/SBA-15 was obtained and moved into a furnace for calcination at 500oC for 2h. Finally, 15wt% HF solution was used to remove the SBA-15 template to prepare the s-IrO2 support. When the second step was ran to prepare the Pt/s-IrO2 catalyst, H2PtCl6 was used as a Pt precursor and dispersed Pt particles were formed using a microwave-assisted polyol process [206]. The molar ratio of Pt to IrO2 was 1:1. For comparison purposes,

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commercial IrO2 (i.e., c-IrO2) was used to prepare a referenced Pt/c-IrO2 catalyst using the same polyol method. Using a combination of XRD, TEM and HRTEM to examine the structure and morphology of s-IrO2 support, the diffraction peaks in the XRD pattern of s-IrO2 were composed of two features: sharp diffraction peaks and halo-like lines with a certain width. The former was related to the crystalline phase, stability and conductivity, indicating a useful Pt support, while the latter was associated with an amorphous phase favoring the dispersion of active sites to improve OER activity [201]. According to the Scherrer Equation, the particle size of s-IrO2 was calculated to be 6.4 nm, as confirmed in TEM measurements. TEM images revealed a uniform and extremely ordered distribution of s-IrO2 nanoparticles, suggesting that thin side-supports existed between the nanoparticles preventing agglomeration of s-IrO2 and demonstrating the effect of the SBA-15 template in the synthesis procedure. Compared to s-IrO2 supports, Pt/s-IrO2 exhibited the coexistence of the Pt phase and the s-IrO2 phase in the measured XRD patterns while HRTEM confirmed a Pt particle size ranging from 5 to 7 nm. To analyze the bifunctional activity of ORR and OER, electrochemical measurements were conducted in 0.5 M H2SO4 solution using an electrochemical cell where platinum, Hg/Hg2SO4, and GCE coated with catalyst samples were used as the counter electrode, reference electrode, and working electrode, respectively. For the evaluation of ORR, CV polarization curves in Ar-saturated 0.5M H2SO4 showed that Pt/s-IrO2 and Pt/c-IrO2 catalysts exhibited characteristic

hydrogen adsorption/desorption peaks of Pt

catalyst,

whose

electrochemical surface areas (ECSAs) were 34.4 and 21.8 m2 g-1, respectively,

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indicating that Pt/s-IrO2 had a higher ECSA value and higher electrochemical activity than that of Pt/c-IrO2. The LSV results obtained in O2-saturated 0.5M H2SO4 revealed a kinetic current densities of 38.9 and 30.7mA mg -1 for Pt/s-IrO2 and Pt/c-IrO2, respectively, suggesting that Pt/s-IrO2 has a higher ORR activity, which corresponds to its higher ECSA. Essentially, the better ORR performance of Pt/s-IrO2 is mainly attributed to the spatial arrangement of Pt on the geometric surface of the self-made s-IrO2 which is associated with the potential interaction between Pt and s-IrO2. Before OER was assessed for both Pt/s-IrO2 and Pt/c-IrO2 in O2-saturated 0.5 M H2SO4 using RDE, a comparison of OER for s-IrO2 and c-IrO2 supports was analyzed by LSV technique. Compared to the c-IrO2 support, the s-IrO2 support displayed a more rapid polarization process at high potential but a slightly higher onset potential at around 1.52 V. In detail, the current densities of c-IrO2 and s-IrO2 supports are 16.2 and 9.3 mA mg-1 at 1.55 V, respectively, while 45.2 and 54.8 mA mg -1 at 1.60 V, respectively. The improved OER activity of s-IrO2 at higher potentials can be attributed to the spatially ordered distribution and uniform regularity of s-IrO2 particles with sufficient porosity favoring oxygen transfer. Based on factual operations to evaluate bifunctional oxygen catalysts in URFC at 1.60 V, the as-prepared s-IrO2 was found to be more feasible in applications due to its higher OER activity at operating potentials. In a further comparison study of OER, both Pt/c-IrO2 and Pt/s-IrO2 had higher OER activities than pure IrO2 since the deposition of Pt onto IrO2 support resulted in the formation of conductive networks that benefits the improvement of OER even though Pt exhibited negligible performance compared to IrO2 catalysts in the applied voltage

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region. The current density of Pt/c-IrO2 varied from 17.1 mA mg -1 at 1.55 V to 69.7 mA mg-1 at 1.60 V while those of Pt/s-IrO2 were 10.6 mA mg-1 at 1.55 and 73.3 mA mg-1 at 1.60 V. In particular, when the measured potential increased up to 1.65 V, Pt/s-IrO2 presented a current density of 215.9 mA mg -1, 42.5% higher than that of Pt/c-IrO2, indicating a factual OER behavior for URFC applications. This further demonstrated the potential effect between Pt and self-made s-IrO2 supports, resulting from the spatial dispersion framework of Pt supported on s-IrO2. To further improve the bi-functionality of Pt/IrO2 toward both ORR and OER in acidic media, the incorporation of oxide (e.g., RuO2 [207]) or metal (e.g., Pt [208] or Ir [209, 210]) was used to modify the Pt/IrO2 catalyst. Compared to the incorporation of RuO2, the incorporation of metal was reported to result in an active effect on the interaction between catalyst and support. Interestingly, compared to the incorporation of Pt, the incorporation of Ir not only enhanced the interaction between Pt and IrO2 [198], but also produce an interaction between Pt and Ir [209, 210]. Both of the interactions were found to benefit the improvement of bi-functionality towards ORR/OER activities. In a fast and convenient way to synthesize a combination of Ir and PtIrO2, Irx(IrO2)10-x was firstly synthesized in Yin’s group [209] as a support material by reducing IrCl3·nH2O and depositing Ir nanoparticles onto self-made IrO2 nanoparticles using a microwave-assisted polyol process. The molar ratios of Ir to IrO2 were controlled at 0:10, 1:9, 2:8, 3:7, and 4:6. Then, another similar microwave-assisted polyol was carried out to achieve a distribution of Pt nanoparticles onto Irx(IrO2)10-x support after the reduction of H2PtCl6. In the obtained

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Pt/ Irx(IrO2)10-x catalyst, Pt had the same molar ratio of 50 mol% as Ir. In XRD patterns, 50 mol% Pt gave identical characteristic peaks in all the catalysts. With increasing x, the peak intensity of IrO2 gradually reduced while the peak intensities of Ir and Pt increased due to the overlapped diffraction peaks of Pt and Ir. Typical TEM images of Ir2(IrO2)8 and Ir3(IrO2)7 revealed a uniform distribution and crystalline texture of Ir and Pt particles in the two catalysts. Moreover, Ir2(IrO2)8 showed a little smaller average particle size of 2.64 than Ir 3(IrO2)7, indicating that an increasing Ir content should result in increasing particle size. When electrochemical measurements were performed in 0.5 M H2SO4 solution with glassy carbon RDE in a three-electrode cell, the tested LSVs showed increasing kinetic currents at 0.85 V with increasing Ir content, confirming the increasing ECSA result and an increasing ORR activity. When x increased, the increasing content of Ir, coupled with the increasing utilization of Pt nanoparticles in ORR test, determined an increasing electronic conduction and thus ORR activity. Regard OER, however, the obtained current densities for Pt/Ir0(IrO2)10, Pt/Ir1(IrO2)9, Pt/Ir2(IrO2)8, Pt/Ir3(IrO2)7, and Pt/Ir4(IrO2)6 at 1.55 V are 18.8, 31.24, 45.69, 42.35, and 31.68 mA mg -1, respectively, suggesting that the OER behavior of Pt/Irx(IrO2)10-x is not consistent with ORR behavior. In particular, Pt/Ir2(IrO2)8 exhibited the best OER activity because the OER activity was mainly dependent on both Ir and IrO2 at the same amount of Pt. Therefore, the incorporation and presence of Ir was thought to optimize the ratio interaction between Pt and IrO 2 as well as to improve both OER and ORR at the same time.

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3.3.1.2 Other oxide-supported metal Based on findings in recent research [115, 211, 212] that calcium-manganese oxide compounds are active for oxygen electrocatalysis, Han et al. [80] developed and studied Pt/CaMnO3 catalysts for ORR/OER activities in which Pt was deposited onto interconnected porous self-made CaMnO3 supports. In the first step to synthesizing porous CaMnO3 support material, Ca(NO3)2·4H2O and Mn(NO3)2 as metal precursors were dissolved into a mixture solvent of water and ethylene glycol, followed by the addition of citric acid. After evaporation and drying, the obtained dry gel was treated at 400oC for 2h and 900oC for 3h in Air to form porous CaMnO3 powder as a support material. In the second step, the support material was dispersed in an ethanol solution containing H2PtCl6·6H2O. The ethanol was then removed through evaporation and the mixture was calcined at 450oC for 5h in air to produce a Pt/CaMnO3 nanocomposite. Finally, a heat treatment at 50oC was conducted to produce the final porous hydrogenated H-Pt/CaMnO3 product. To investigate the structure and morphology of the composite, a combination of XRD, EDS, inductively coupled plasma-atomic emission spectrometer (ICP-AES), SEM, TEM, HRTEM, and XPS was used to obtain measurements. The XRD

(Fig. 12)

patterns (see Fig. 12(a)) of Pt/CaMnO3 and H-Pt/CaMnO3 samples displayed the characteristic peaks for Pt and CaMnO3 while the combination of EDS (see Fig. 12(e))

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and ICP-AES determined the Pt content to be 8.06 wt% in the H-Pt/CaMnO3 nanocomposite. SEM, TEM, and HRTEM images (see Fig. 12(b-d)) of H-Pt/CaMnO3 revealed not only that the CaMnO3 maintains an interconnected porous microstructure but also that Pt nanoparticles with average size of 1.0 nm were uniformly distributed on the CaMnO3 support. Importantly, based on the analysis of XPS spectra, the relative ratio of the Peak intensity of surface oxygen to lattice oxygen increased in the order: CaMnO3 < Pt/CaMnO3 < H-Pt/CaMnO3, indicating a stronger interaction between the hybrids and the adsorbed oxygen-containing species due to the effect of hydrogenation. When electrochemical measurements were carried out in 0.1 M KOH solution with a three-electrode electrochemical cell, the tested ORR performance revealed that in comparison with CaMnO3, Pt/CaMnO3 and H-Pt/CaMnO3 seemed to be more active in term of higher half-wave potential. At 0.85 V, the mass activity and specific activity of hydrogenated H-Pt/CaMnO3 were 0.35 A mg-1Pt and 1.06 mA cm-2Pt, which were 5 and 11 folds higher than those of Pt/C (0.075 A mg -1Pt and 0.097 mA cm-2Pt,) [213], while Pt/CaMnO3 showed comparable mass activity and specific activity to Pt/C. These ORR results were also confirmed by Tafel plots related to slope and kinetic current density. After a tested polarization was continued for 27.8 h, it was found in the chronoamperometric response that the initial ORR current remained at 95% for H-Pt/CaMnO3. This is higher than that (~70%) of Pt/C, suggesting that H-Pt/CaMnO3 had a more durable ORR activity. Furthermore, H-Pt/CaMnO3 was seen to have remarkable immunity against methanol crossover as compared to Pt/C. Meanwhile, the OER measurements in 0.1 M KOH solution revealed that compared to

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other samples such as CaMnO3, Pt/CaMnO3 and Pt/C, H-Pt/CaMnO3 displayed a lower overpotential and a greater OER current due to the stronger interactions between the catalyst and the support. The calculated difference between the OER potential (at 10 mA cm−2) and the ORR half-wave potential was 1.01 V [33, 108], suggesting that H-Pt/CaMnO3 is an efficient bifunctional oxygen catalyst due to the interactions between Pt and calcium-manganese oxide. Different from oxides such as IrO2 and CaMnO3 with active bifunctional activity on ORR and/or OER, Ti-based oxides without catalytic activities were also used as support materials to develop novel bifunctional oxygen catalysts for URFC applications [214-216]. To improve performance and stability of the bifunctional oxygen electrode and to solve the problem of the corrosion of carbon, Popov’s group [214] studied platinum and iridium catalysts supported by TiO2 (Pt/TiO2 and Ir/TiO2) and prepared a novel bifunctional oxygen electrode for URFC applications using TiO2 supported electrocatalysts. In the experimental synthesis, titanium isopropoxide was first used as a Ti precursor to be mixed with Pluronic P123. After hydrolysis and aging, the P123 was removed with hot water to prepare the mesoporous TiO 2 support. Then, the TiO2 support was mixed with sodium dodecyl sulfate and H2PtCl6 to form a suspension. When a colloidal reaction was done, the 60wt%Pt/TiO 2 product was formed. Ir/TiO2 was also prepared using a similar method at an elevated temperature. The two catalysts were treated at 200oC under Ar to increase the interaction between the catalyst particles and the TiO2 support. The physical mixtures consisted of Pt/TiO2 and Ir/TiO2 with different ratios of Pt to Ir: 100:0, 95:5, 90:10, 85:15, 80:20, 75:25,

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and 70:30. When TEM, SEM, and HRTEM were used in the physical characterization of TiO2, Pt/TiO2, and Ir/TiO2, Pt (4.2 nm) and Ir (2.0 nm) showed a uniform distribution on the TiO2, while the size of the self-made TiO2 support was in the range of 7 - 15 nm. Importantly, BET measurements showed that the TiO2 support possessed enough surface area (250 m2 g-1) for the dispersion of Pt or Ir particles [22]. When electrochemical measurements were performed in 0.5M H2SO4, LSV results revealed that as a preferred oxygen reduction catalyst, the Pt/TiO2 exhibited poor OER activity while as a preferred oxygen evolution catalyst, the Ir/TiO 2 displayed poor ORR activity. This indicated that an optimized ratio of Pt/TiO2 and Ir/TiO2 is needed for a comparable ORR and OER performance. Based on the calculated catalyst efficiency (ε, see Eq(22)): ε (%) = 100  VORR/VOER

(22)

where VORR and VOER are the voltages for the oxygen reduction and oxygen evolution reaction, it was found that among all mixtures, Pt 85Ir15/TiO2 delivered the best catalytic efficiency (~15%) at a select current density of 1.5 mA cm-2. To further evaluate the performance of Pt85Ir15/TiO2 as a bifunctional oxygen catalyst in URFCs, a 5 cm2 single cell was fabricated and used in the testing. According to the comparison of a round-trip energy conversion efficiency (εRT, see Eq(23)): εRT (%) = 100  VFC/VWE

(23)

where VFC and VWE are the voltages for the fuel cell and water electrolysis at a given current density. At a catalyst loading of 1.0 mgPt-Ir cm-2, the URFC efficiencies of Pt85Ir15/TiO2 at 0.5 and 1.0 A cm-2 were 52.0% and 45.9%. This is higher than those of

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unsupported Pt-Ir (45.3%, 33.5%), indicating better URFC performance for Pt85Ir15/TiO2. Furthermore, by employing gas diffusion layer (GDL) with a protective micro-porous layer made of iridium–titanium, the tested unit cell with Pt 85Ir15/TiO2 as the bifunctional oxygen catalyst exhibited better stability after a continuous 20 cycles under URFC operation. Interestingly, Chen et al. [216] used conductive Ebonex (mainly Ti4O7 and Ti5O9) as a catalyst support to obtain a series of Pt xRuyIrz/Ebonex catalysts with 20wt% metal loading and compared their bifunctionality in ORR and OER. The experimental ratios of x/y/z were listed as 3/4/2, 4/4/1, 4.5/3/1.5, 3.5/4/1.5, 2.5/5/1.5, and 4.5/4/0.5. Under the half-cell testing, the tested polarization curves revealed that among all catalysts, Pt4Ru4Ir1/Ebonex catalysts exhibited the optimal bifunctional performance due to the active effect of Pt and Ir on ORR and OER. As an inert support, Ebonex was thought to increase the surface exposure of the catalyst and also produce the interactions between the catalyst and the support through the change of electronic structures. Won et al. [217] fabricated and studied PtIr alloy supported on Ti4O7 (PtIr/Ti4O7) as a bifunctional electrocatalyst for improving both ORR and OER catalytic activities. Compared to Pt/C and Pt/Ti4O7, PtIr/Ti4O7 with a metal loading of 60wt/% showed an enhanced ORR/OER activities and stability due to the formation of PtIr alloy phase, high stability of Ti4O7 support in an acid medium, and the interaction between catalyst and support. Instead of Ti-based oxides, sulfonated silica (SiO2-SO3H) was used by Roh et al. [218] to support 80wt% Pt catalyst in an investgation of electrochemical performance of Pt/SiO2-SO3H, Pt/SiO2 and commercial Pt/C in URFCs. It was found that the sulfonic acid group on the support

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(SiO2-SO3H) could enhance proton conduction in the catalyst layer, and that the silica support did not prohibit the electrical conductivity of Pt in Pt/SiO 2-SO3H electrocatalyst due to its high loading. Under a tested URFC system, the current density (~800 mA cm-2) catalyzed by Pt/SiO2-SO3H catalyst at 0.70 V in FC mode was significantly higher than those (600, 650 mA cm-2, respectively) of the Pt/SiO2 and Pt/C catalysts. Furthermore, the round-trip energy efficiency (46.1%) of Pt/SiO2-SO3H is better than those (44.60%, 45.32%, respectively) of the Pt/SiO2 and Pt/C catalysts. In water electrolyzer (WE) mode, the cyclic stability of the Pt/SiO2-SO3H exhibited remarkably better than that of the Pt/C, confirming that there was no corrosion or degradation of Pt/SiO2-SO3H catalyst due to the carbon-free and corrosion-resistant SiO2-SO3H support. It was concluded that the enhancement of URFC performance could be attributed to the use of Pt/SiO2-SO3H support with a rapid proton transportation in the catalyst layer facilitated by the sulfonic acid group in the silica. This research provided a novel approach to fabricate a promising electrocatalyst in the development of URFCs. To increase both the activity and stability of noble-metal based OER catalysts, Ru-Ir mixed oxides have been studied in the development of advanced bifunctional catalysts for URFCs. For example, Gutsche et al. [219] decorated RuO2 nanoparticles by Ir nanodots, and Pt nanoparticles by Ir nanodots, respectively. Their stepwise synthesis could control the morphology of nanoparticles and thus enhance the activity and stability. They employed cyclic voltammtry (CV), X-ray diffraction (XRD), transmission electron microscopy (TEM), and energy-dispersive X-ray (EDX)

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spectroscopy for material and performance chraterization. Ir nanodots and Ir-decorated RuO2 nanoparticles were found to be less stable toward the OER than pure RuO2 nanoparticles. The bifunctional catalysts made from Ir deposited on Pt nanorods showed a different behavior from the individual components. For OER, Pt/Ir-RuO2 was less stable than that of unsupported Ir (monometallic OER catalyst) The improved stablilty of Pt/Ir-RuO2 in ORR could be observed to be better than pure Pt nanorods (monometallic ORR catalyst).

3.3.2 Carbide-supported metal(s) As oxides were delivered as substitute candidates to resist the corrosion from carbon support; carbides, especially TiC, have been also explored and investigated as support materials to develop advanced bifunctional oxygen catalysts for URFCs. This is due to their high corrosion resistance, high electrical conductivity and relatively high surface area when compared to carbon supports for OER [220]. Selecting a low surface area TiC support material, Fuentes et al. [221] made a co-deposition of Pt-Ir nanoparticles on TiC with 40wt% metal loading, and optimized and evaluated the performance of PtIr/TiC catalysts in the application of bifunctional oxygen catalysts for URFCs. In a simple polyol synthesis procedure [222], hexachloroiridate acid and chloroplatinic acid hexahydrate were dissolved into ethylene glycol, followed by the addition of commercial TiC powder (30-50 nm). After the co-deposition of Pt and Ir, refluxing, filtering, washing, and drying were utilized to purify the sample. Finally, the sample was heat-treated from room temperature to 200oC under a gas mixture of

- 90 -

96%Ar and 4%H2. The nominal metal loading was fixed at 40 wt% while the molar ratio of Pt to Ir was 1:1. Reference Pt/C and Ir/C samples were also prepared using the same method. Pt black and Ir black were also used as the two other samples. Due to the effect of EG in the reduction process [223, 224], the reduced metal particles were uniformly distributed on the surface of the TiC support, as confirmed by TEM measurements. The HRTEM images revealed that the Ir/TiC, Pt/TiC, and Pt-Ir/TiC catalysts had sized of 1.7, 4.7, and 1.8 nm, respectively. To evaluate the catalytic activity of both ORR and OER, electrochemical measurements were conducted in 0.1 M HClO4 with a standard three-electrode electrochemical cell. The measured ORR polarization curves demonstrated a decreasing order: Pt black > Pt-Ir/TiC > Pt/TiC > Ir black < Ir/TiC, suggesting a large performance gap between Pt containing catalysts and Ir containing catalysts. This is attributed to Ir being less active for ORR than Pt. Furthermore, Pt black showed the best ORR activity due to more Pt loading on the tested RDE while the ORR activity of Pt-Ir/TiC was better than that of Pt/TiC due to higher Pt surface area. Meanwhile, the measured OER polarization curves revealed an increasing order: Pt/TiC < Pt black < Pt-Ir/TiC < Ir black < Ir/TiC, suggesting that Pt was less active for ORR than Ir. Ir black exhibited better OER activity than Ir/TiC because the TiC support provided an increased active surface area, favoring catalyst utilization and therefore OER performance. Sui’s group [225] also prepared two different Pt-Ir/TiC bifunctional oxygen catalysts using a chemical reduction method and a plasma reduction process. Their studies revealed that compared to the chemical reduction method, the plasma reduction method resulted in a relatively uniform

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distribution of Pt-Ir particles (< 5 nm) [214] and a significant improvement in electrocatalytic activities for both OER and ORR. In the same scope, Martínez-Huerta’s group [226] prepared three Pt3Ir1 catalysts supported on three different support materials; TiC, TiCN and TiN, using an ethylene glycol-polyol method [227], and compared the structure and properties of Pt3Ir1/TiC with Pt3Ir1/TiCN and Pt3Ir1/TiN as bifunctional oxygen catalysts. In the experimental synthesis, two metal precursors, IrCl3 and PtCl4, were first dissolved in EG and mixed with a support suspension in the same solvent. The molar ratio of Pt to Ir was 3:1 while the three support materials used were commercial TiC (23 m2 g-1), TiC0.7N0.3 (22 m2 g-1), and TiN (50 m2 g-1). After reduction, the three products obtained were Pt3Ir1/TiC, Pt3Ir1/TiCN, and Pt3Ir1/TiN. The nominal metal loading was 20wt%. Before the support effect on the catalytic activity for ORR and OER was examined, structure and morphology was characterized using inductively coupled plasma-optical emission spectrometry (ICP-OES), XRD, SEM, TEM, and XPS. When Pt, Ir, TiC, TiCN, and TiN phases were confirmed in XRD patterns without peaks of impurities, SEM and TEM (see Fig. 13)

(Fig. 13)

images clearly revealed similar particle size and distribution for Pt3Ir1/TiC (2.8 ± 0.6 nm), Pt3Ir1/TiCN (2.6 ± 0.7 nm), and Pt 3Ir1/TiN (2.6 ± 0.7 nm). In the analysis of XPS spectra of all the catalysts, Ti binding energies appears at higher values when nitrogen

- 92 -

existed in the support, indicating the higher electropositivity of Ti in TiN than that in TiC. However, Pt/Ti ratios decreased with the increasing content of N in the support while the content of deposited Ir was not dependent on N content in the support. The combination of XPS and ICP-OES revealed a lower metal loading for Pt and Ir in TiCN. This is due to electronic factors of TiN resulting in decreased surface areas for the deposition of metal during reduction, even though TiCN had the largest BET surface area of 50 m2 g-1. Electrochemical measurements were then performed in 0.5 M H2SO4 with a three-electrode cell at room temperature. The comparison of onset potential and limiting current in tested ORR polarization curves revealed a decreasing order toward ORR: Pt 3Ir1/TiCN > Pt3Ir1/TiN > Pt3Ir1/TiC, indicating the effect of the support on ORR activity. All three catalysts were disclosed to have a 4-electron path mechanism. On the other hand, the OER polarization curves showed the following order: Pt3Ir1/TiCN > Pt3Ir1/TiN >> Pt3Ir1/TiC. Interestingly, an anodic peak was observed at around 1.1 V for Pt3Ir1/TiC and Pt3Ir1/TiCN catalysts rather than for Pt3Ir1/TiN. After 1.4 V, Pt3Ir1/TiN gave a high rise of anodic currents in its OER polarization curve. Based on FTIR spectra, it was found that the anodic peak at 1.1 V resulted from the formation of CO2 and TiO2 from the TiC and TiCN supports, suggesting the better stability of TiN supports. An accelerated test of electrochemical stability confirmed the lower stability of Pt3Ir1/TiC and Pt3Ir1/TiCN due to the electrochemical oxidation of their supports. Among all the catalysts, Pt3Ir1/TiC had the worst bifunctional activity toward ORR and OER. Pt3Ir1/TiN displayed good activities of ORR and OER that were a little lower than those of Pt 3Ir1/TiCN but did

- 93 -

not suffer from corrosion in the research. In fact, TiN was reported to form Ti(IV) hydroxide ions that passivates the surface by absorbing oppositely charged anion in the electrolyte solution [228]. Therefore, TiCN can be proposed as a feasible support material for bifunctional oxygen catalyst in URFC due to Pt3Ir1/TiCN displaying high and stable catalytic activities. In their previous study [1], when the molar ratio of Pt to Ir was fixed at 1:1, Pt1Ir1/TiCN, Pt1Ir1/TiN, Pt1Ir1/TiC were also synthesized as catalysts and their structural and electrochemical properties were examined. The investigated bifunctional activity toward ORR and OER further demonstrated that TiCN is a suitable support material for bi-operational oxygen catalysts in URFC applications.

3.3.3 Other material-supported catalysts To obtain optimum noncarbon support materials, some researchers have also devoted themselves in the exploration and development of new nanocarbon support materials for bifunctional oxygen electrodes in URFCs. Differing from previously discussed

noncarbon

supports,

conducting

poly(3,4-ethylenedioxythiophene)(PEDOT)

[229]

were

polymers also

such

studied

in

as the

development of novel bifunctional oxygen catalysts. Considering that nanocomposites of metal spinel with a conducting matrix favors catalytic performance [230-233], Chowdhury et al. [229] studied PEDOT supported CoMn2O4 catalysts toward OER and ORR in alkaline media. In their experiment, α-MnO2 was first synthesized using MnSO4 and KMnO4. Then, after α-MnO2 was mixed with CoCl2·6H2O using water as

- 94 -

a solvent, a reduction reaction was processed by the addition of NaBH4 and a further procedure including purification and drying was used to prepare nanocrystalline CoMn2O4 spinel. Finally, the CoMn2O4 was added to a mixture solution of EDOT monomer and dodecyl benzene sulfonic acid (DBSA) with water as a solvent. As ammonium peroxodisulphate (APS) was added as an oxidant, polymerization was induced to form a mixture of PEDOT and CoMn2O4. When purification was finished, the CoMn2O4/PEDOT product was produced. As a referenced sample, pure PEDOT was also synthesized with the same procedure. In the examination of structure and morphology, it was found in FTIR spectra (see Fig. 14(a)) that for CoMn2O4/PEDOT, the characterization peaks at 1519, 1350, 1141, 1088, 983, 840, and 691 cm-1 can be assigned to C=C, C-C, C-O-C, and C-S bonds, suggesting the presence of PEDOT, while the peaks of the CoMn2O4 spinel at 1630, 1405, 628, and 508 cm-1 was not observed for CoMn2O4/PEDOT, suggesting a thorough incorporation of the CoMn2O4 spinel into the PEDOT. At the same time, TEM images showed that CoMn2O4 particles (15-20 nm) were bounded within the polymer chain. When ORR/OER activities were investigated by electrochemical measurements with a three electrode cell, the tested LSVs in O2-saturated KOH solution showed that PEDOT and α-MnO2 did not exhibited

(Fig. 14)

any ORR activity. After the addition of CoMn2O4, the CoMn2O4/PEDOT had a

- 95 -

comparable ORR activity to commercial Pt/C. According to the Koutecky-Levich equation, the calculated electron transfer number for the CoMn2O4/PEDOT was 3.9. This is similar to ORR catalyzed by commercial Pt/C catalyst. For OER, a quasi-steady polarization curve tested in 0.1M KOH in the potential range of 0.5-0.8 V revealed that CoMn2O4/PEDOT was sufficient to evolve H2 in the higher potential, suggesting a higher OER activity than α-MnO2 and CoMn2O4. A further stability test in Ar-saturated 0.1 M KOH (see Fig. 14(b) and (c)) showed that the CoMn2O4/PEDOT catalyst presented a similar decay to that of Pt/C after a continuous 500 CV cycles. Therefore, CoMn2O4/PEDOT catalysts prove to have potential as a bifunctional oxygen catalyst in URFCs. With the development of bifunctional oxygen catalysts containing various support materials, a novel metal-organic framework (MOF) has shown the effects of a support for the dispersion of catalyst particles with high bifunctional activities toward ORR and OER. Yin’s group [234] routinely employed MIL-101(Cr) as a support to design and prepare Co-based bifunctional electrocatalysts in which the surface Co III and CoII contents can be tuned to produce optimized activities for ORR and OER using KBH4 and H2O2 as the reducing and oxidizing agent. In their routine synthesis, a self-made MIL-101(Cr) [235] was dissolved into ethanol and mixed with Co(NO3)2·6H2O. After the addition of KBH4 or H2O2, the product were centrifuged, washed and dried to form the as-prepared electrolyst Co/MIL-101(Cr)-R or Co/MIL-101(Cr)-O. As a referenced sample, Co/MIL-101(Cr) was prepared in the same method without the addition of KBH4 and H2O2. MIL-101(Cr) has a stable MOF structure due to its strong

- 96 -

interaction between the hard ions (i.e. Cr 3+) and the carboxylate liners [236, 237]. The XRD patterns of MIL-101(Cr) before and after soaking in 0.1 M KOH showed the same crystallinity of MIL-101(Cr), indicating the stability of MIL-101(Cr) in alkaline media. In contrast, after the incorporation of Co, the XRD patterns of all the catalyst samples demonstrated no crystallized Co species (e.g., metallic O, Co oxides and Co hydroxides) or crystallized Cr species, coupled with the remained crystallinity of the support MIL-101(Cr). In FTIR measurements, the evaluated coordination mode between the carbonxylate and the metal ions [238] suggested that the dicarboxylate may bridge with metal ions such as Co ions. The comparison of BET surface area showed a smaller surface area

for Co/MIL-101(Cr)-R (~986.1 m2 g-1),

Co/MIL-101(Cr) (~967.6 m2 g-1), and Co/MIL-101(Cr)-O (~941.6 m2 g-1) than that of the MIL-101(Cr) support (~2214.9 m2 g-1), confirmed the effect of Co incorporation into MIL-101(Cr). TEM and HRTEM images not only revealed absolutely no visible heterogeneous nanoparticles, but also showed that the pristine morphologies of MIL-101(Cr) remained in all three catalyst samples, confirming the high stability of MIL-101(Cr) and the good dispersion of Co species inside of the MIL-101(Cr) support. In terms of XPS measurements, Co/MIL-101(Cr)-R had a higher Co contents of 2.45% than Co/MIL-101(Cr) (2.41%) and Co/MIL-101(Cr)-O (2.38%). In the investigation of Co oxidation states by XPS, the calculated CoIII/CoII ratios from Co 2p3/2 main peaks for Co/MIL-101(Cr)-O was 0.89. This is higher than those of Co/MIL-101(Cr)-R (0.19) and Co/MIL-101(Cr) (0.33). When electrochemical measurements were carried out to evaluate the catalytic activity via a RDE technique,

- 97 -

the LSV curves obtained in 0.1M KOH showed that at 10 mA cm-2, the Co/MIL-101(Cr)-R, Co/MIL-101(Cr), and Co/MIL-101(Cr)-O catalysts reached 0.94, 0.85, and 0.75 V, respectively. Notably, Co/MIL-101(Cr)-O displayed a Tafel slope of 122 mV dec-1, lower than that of Co/MIL-101(Cr) (131 mV dec-1) and Co/MIL-101(Cr)-R (156 mV dec-1). Both of these results indicate better OER activity for Co/MIL-101(Cr)-O. In regards to ORR, the tested LSV curves in O2-saturated 0.1M KOH revealed that the onset potential and half-wave potential (E1/2) of Co/MIL-101(Cr)-R were -0.05 V and -0.03 V. This is more positive than those of Co/MIL-101(Cr) (-0.08 V, -0.12 V) and Co/MIL-101(Cr)-O (-0.38 V, -0.41 V), and more negative than Pt/C (0.07 V, -0.11 V). This suggests that Co/MIL-101(Cr)-R has a higher ORR activity than the other two catalysts, but lower ORR activity than Pt/C. During different rotating speeds ranging from 400 to 2500 rpm, the calculated electron transfer numbers were 3.9, 3.8, and 3.4 for Co/MIL-101(Cr)-R, Co/MIL-101(Cr)-O, and Co/MIL-101(Cr). According to the Koutecky-Levich equation, this indicates that Co/MIL-101(Cr)-R has a better Orr kinetics than the others. Based on the comparison of ORR and OER in electrochemical measurements, it was shown in the measurements that a higher content of CoIII in Co/MIL-101(Cr)-O resulted in a higher OER while a higher content of CoII in Co/MIL-101(Cr)-R resulted in a better ORR. When the gap between ORR (half-wave potential) and OER (at 10 mA cm-2) was used to evaluate the overall bifunctional activity, Co/MIL-101(Cr)-O displayed a value of 1.16 V. This is lower than Co/MIL-101(Cr) (1.23 V) and Co/MIL-101(Cr)-R (1.27 V), suggesting that Co/MIL-101(Cr)-O has more

- 98 -

bifunctional activity. ORR stabilities was also tested and it was found that after 50,000 s, Co/MIL-101(Cr)-O and Co/MIL-101(Cr)-R remain at 32% and 28% of the initial current density. This is better than Co/MIL-101(Cr) (48%), suggesting that compared to Co/MIL-101(Cr), Co/MIL-101(Cr)-O and Co/MIL-101(Cr)-R improves the interaction between MIL-101(Cr) and active Co species. In contrast to these metal-organic-frameworks (MOF) containing Co, another metal-organic framework containing Fe (MOF-Fe) was previously studied and developed as a good bifunctional activities of ORR and OER in their group [239]. Using metal-organic-frameworks (MOF) as a novel support material, their studies contributed to the development of advanced bifunctional oxygen catalysts.

4. Unsupported bifunctional catalysts In contrast to supported bifunctional oxygen catalysts, materials such as metal-based materials and oxides doped carbon, etc. have also been found to be active toward both ORR and OER due to their inherent reactivity in acidic and/or alkaline media under electrochemical circumstances and thus can act as catalysts by themselves without any supporting materials. It is beneficial and vital to review these unsupported catalysts by examining their catalytic activity for ORR and OER and detect the relationships between their structures and properties.

4.1 Metal-based catalysts 4.1.1 Pt-based catalysts

- 99 -

As a typical type of metal-based catalyst, Pt-based catalysts without any support materials have been welcomed into the catalytic field due to their excellent physicochemical properties despite the high cost of Pt. Specifically, unsupported Pt-based catalysts have garnered special attention from researchers due to their relatively high efficiency in ORR in the research and development of advanced bifunctional oxygen catalysts for URFCs. To avoid carbon corrosion and thus dissolution of metal catalysts, Popov’s group [190, 240, 241] studied in detail the electrochemical performance of unsupported Pt xIry catalysts as a bifunctional oxygen electrode in URFC. In their experiment, physical mixtures of commercially available Pt black and Ir black were prepared as the catalyst using de-ionized water as a solvent. The catalyst was then used to form the membrane and electrode assembly (MEA) in a 5 cm2 single cell for a unit cell test. The ratios of Pt to Ir were listed as 100:0, 85:15, 70:30, and 40:60. When the electrochemical measurements were ran in 0.5 M H2SO4 solution, the desorption peak of hydrogen in the tested CV decreased with the increasing content of Ir in the Pt xIry catalyst. At 85:15, the Pt 85Ir15 catalyst showed a comparable ECSA to Pt black. The LSV curves revealed that when Ir increased, the PtxIry catalyst displayed a decreasing ORR activity. At 40:60, the Pt 40Ir60 catalyst delivered the lowest onset potential for oxygen reduction due to its lowest ORR activity while the polarization curves at different rotation speeds indicated a four electron transfer for the Pt xIry catalyst. For OER activity, the onset potential for Pt black is close to 1.6 V, more positive than that of unsupported Pt xIry catalysts, indicating that the Pt xIry catalyst was more effective than Pt. Furthermore, by

- 100 -

increasing the ratio of Pt to Ir content from 100:0 to 40:60, the OER activity of the PtxIry catalyst rose up to 30 wt%. When round-trip energy conversion efficiencies (εRT, see Eq(24)) was used to evaluate the cell performance using different Pt xIry catalysts, εRT (%) = VFC/VWE

(24)

it was found that the Pt85Ir15 catalyst gave the highest efficiency at an applied current density of 0.5 and 1.0 A cm-2. In particular, at a current of 0.5 A cm-2, the Pt85Ir15 catalyst maintained its stability for 120 h.

(Fig. 15)

To investigate the effects of structure and morphology of the catalyst on the bifunctional activity of ORR/OER, Zhang et al. [242] utilized a one-pot synthesis method to form a core-shell structured Ir@Pt dendritic bifunctional oxygen catalyst in which Ir was the core while Pt acted as the shell. In a typical synthesis route, Ir nanodenrites were first obtained after reducing H2IrCl6 using NaBH4 as the reducing agent and cetyltrimethylammonium chloride (CTAC) as the template. Metallic Pt was then deposited on Ir nanodenrites to form core-shell structured Irx@Pty catalysts when H2PtCl6 was reduced by ascorbic acid (AA). The nominal ratios of x to y were controlled as 50:50 and 67:33, corresponding to Ir 50@Pt50 and Ir67@Pt33 catalyst samples. A physical mixture of commercial Pt and Ir black with an Ir/Pt molar ratio of 57:43 was used as a reference catalyst (Ir57/Pt43). In the morphology examined by TEM images (see Fig. 15(a)), Ir nanodenrites were observed in well dispersion with

- 101 -

an average size of 15 nm, which is significantly smaller than that of Ir black in Fig. 15(d). Moreover, Ir67@Pt33 nanodenrites (see Fig. 15(b)) showed a similar dendritic nanostructure to Ir nanodenrites. Specifically, the elemental maps and line profiles in the High-angle annular dark field

(HAADF)-STEM-EDX analysis in Fig. 15(c)

indicated not only that the randomly selected nanodenrite was composed of Ir and Pt, but also that Pt was well deposited on the surface of Ir, confirming the core-shell structure. When electrochemical properties were measured with a conventional three electrode cell, the CV obtained in N2-purged 0.5 M H2SO4 showed that the ECSAs of Ir50@Pt50 and Ir67@Pt33 were 54.1 and 72.0 m2 g-1 Pt, respectively. These were 2.7 and 3.6 times that of Ir57/Pt43 (19.6 m2 g-1Pt). This suggests that cores-shell structured Irx@Pty had a better dispersion of Pt and higher Pt surface area than that of the physical mixture of Ir57/Pt43. In the two core-shell structured catalysts, Ir67@Pt33 seemed to have the smaller size and better dispersion of Pt than Ir 50@Pt50. It was also noted that the clear peak at 0.9 V resulted from Ir(III)/Ir(IV) redox couple, especially for increasing Ir/Pt molar ratio, while no peaks were observed for Ir in the Hupd region. The polarization curves for ORR tested in O2-saturated 0.5 M H2SO4 revealed that at 0.85 V, the Ir67@Pt33 and Ir50@Pt50 catalysts had higher a ORR activity of 73 and 53 mA mg-1Pt than the Ir57/Pt43 (30 mA mg-1 Pt), suggesting that for IrxPty catalysts, the core-shell structure results in a better ORR activity than the physical structure. Meanwhile, on the basis of LSVs for OER, it was found that the content of Ir determined the OER, resulting in the highest OER activity for Ir67@Pt33 and demonstrating the strong effect of Ir. The core-shell structured IrxPy proved to be

- 102 -

useful in the development of highly active bifunctional oxygen electrocatalysts for URFCs. Along with binary Pt-based catalysts, ternary Pt-based catalysts have also attracted considerable attention from researchers in the catalytic field. In their research of unsupported Pt-based bifunctional oxygen catalysts, Morales and Fernández [243] focused on unsupported PtxRuyIrz catalysts in the binfunctional activities of ORR/OER in comparison with Pt xIrz, Pt, Ir, and Ru. In a typical synthesis of unsupported Pt xRuyIrz catalysts, H2PtCl6, RuCl3, and IrBr3 were dissolved into deionizer water. With the addition of NaBH4, a reduction reaction was processed and then purified to form Pt xRuyIrz products. Samples such as Pt xIrz, Pt, Ir and Ru were also synthesized with the same method. In the physical characterization and analysis, Energy-dispersive X-ray spectroscopy (EDX) results revealed that the molar ratio of x:y:z in the Pt xRuyIrz samples was 43.9 : 38.92 : 17.17 while the Pt xIrz sample had a molar ratio of x:z of 81.16 : 18.84 . In XRD patterns, the PtxRuyIrz and PtxIrz samples showed similar diffraction peaks to Pt. Specifically, these characterization peaks were associated to (111), (200), (220), (311) and (222) planes,. Detailed comparison show that the interplanar distance value for the Pt xRuyIrz sample was less than Pt, indicating that the incorporation of Ru caused a reduction in crystal size [244, 245]. When TEM and HRTEM were used in the characterization of morphology, the Pt xRuyIrz sample was found with a size range of 3 – 10 nm, along with some aggregation in the particle distribution. The average size of Pt was around 14 nm while Ir or Ru had

- 103 -

agglomerated particles around 10 nm. To evaluate the electrochemical behavior of all the synthesized catalyst samples, electrochemical measurements were ran in 0.5 M H2SO4 solution with a three-electrode cell containing a Pt mesh counter electrode, an Ag/AgCl/KCl reference electrode, and a 7 mm diameter glassy carbon covered with the electrocatalyst deposit. In the analysis and comparison of ORR and OER for PtxRuyIrz and PtxIrz catalyst samples, the PtxRuyIrz catalyst had a better OER activity while the PtxIrz catalyst had a higher ORR activity. The reason as discussed was that the presence of Ru increased OER activity, resulting in the formation of non-conductive RuO4 material and thus lowered ORR activity after OER was done [122, 246, 247]. The comparison of CVs for Pt, Ir, and Pt xIrz samples showed that an increment in the current was evident before 1.4 V in the anodic regions and after 0.9 V in the cathodic region for Pt xIrz samples due to the presence of Ir. Moreover, Ru was shown to produce an increment of current after 1.3 V for Pt xRuyIrz after the addition of Ru into Pt-Ir. In the measurement for stability, PtxRuyIrz was unstable in OER performance due to the decay of the Ru phase while PtxIrz was also not stable for OER. This results in a decreased current. In contrast, PtxIrz had adequate ORR without any changes after several CVs while PtxRuyIrz was not suitable for ORR due to the presence of the Ru phase. Using a pulse electrodeposition method, Ye et al. [248] synthesized a PtRuIr (Pt:Ru = 1:1) nanoclusters and investigated their ORR/OER catalytic activities. The electrochemical measurement showed that the PtRuIr catalyst with 10 mol% Ir presented significantly higher OER and ORR catalytic current densities than both PtRu and PtRuIr catalysts. The result was also

- 104 -

supported by the tested electrochemical impedance data. Particularly, with PtRuIr catalysts containing 10 mol% Ir for oxygen electrode, a 5 cm2 MEA showed higher round-trip efficiency and better energy efficiency in the water electrolysis mode, and in H2/O2 fuel cell mode, the MEA exhibited higher ORR current density (55.0 mA cm-2 at 0.6 V) and higher power density (53.68 mW cm-2) compared to those (45.0 mA cm-2 at 0.6 V, 47.2 mW cm-2) of the PtRu catalyst. This work provided a way for the fabricating catalyst and the improvement of catalytic activity.

4.1.2 NonPt-based catalysts To reduce the cost of catalysts, other non-Pt metals such as Ag are tested as possible unsupported bifunctional oxygen catalysts in URFCs. Saikala et al. [249] developed and studied silver nano-powder as a bifunctional catalyst for both ORR and OER in alkaline

(Fig. 16)

medium. Using a wet chemical method, they mixed silver nitrate with glucose to form a homogeneous solution. Then after the addition of diethyl amine (DEA) solution; the reaction accelerated, resulting in the precipitation of Ag. After purification, Ag nano-powders were obtained as the final product. In all the experiments, glucose acted as a reducing agent for silver nitrate while the addition of amine was used to tune the reaction rate [250]. XRD patterns clearly showed the presence of cubic Ag due to

- 105 -

characteristic peaks at 38.1o, 44.2o, 64.4o and 77.5o. These were assigned to the (111), (200), (220), and (311) planes of the Ag nanoparticles. In SEM images (see Fig. 16), Ag particles were observed to have partially agglomerated, resulting from drying in the purification stage, while the particle size of Ag dropped to a range of 80-100 nm. When electrochemical experiments were conducted in a conventional three-electrode cell in O2-saturated 6M KOH solution, LSV curves for OER activity obtained were in the range of 100 – 900 mV at 10 mV s-1 showing that the onset potential of Ag nano powders was 700 mV. This is close to that of Pt/C (65 mV) but more positive than those of other reported OER catalysts such as Cu-Mn-Co (600 mV) [251] and Ni (300 mV) [252]. Based on LSV curves for ORR activity tested from -0.8 to 0.4 V and the Koutecky-Levich law, the calculated electron transfer number for Ag nano powders is 3.7, matching a four-electron reaction mechanism of ORR. It was reported however that the use of carbons supports would improve electron transfer [253, 254]. For ORR and OER reactions, silver obtained a current density of 50 mA cm-2 at a potential of -0.3 V and 0.725 V, respectively, suggesting that silver can effectively replace Pt/C for both ORR and OER. To further explore novel cost-effective electrode materials, Ni nano powders were studied in Wills’s group [255] as they tested Ni-H3Mo12O40P (Ni-HPA) bifunctional activity toward both ORR and OER. In their experiment, the mixture catalyst was composed of commercial HPAs and commercial Ni with a weight ratio of 1:1. To investigate the electrochemical properties, cyclic voltammetry and galvanic potentiometry measurements were ran in 4 M KOH solution, using a three-electrode

- 106 -

cell that included a 1.13 cm2 working electrode, a Pt mesh (1.5 cm2) counter electrode and a saturated calomel electrode (SCE, a reference electrode). At a constant current density of 44 mA cm-2, Ni/HPA presented an ORR potential of 0.52 V and an OER potential of -0.53 V while Ni showed an ORR potential of 0.97 V and an OER potential of -0.75 V. the potential difference for Ni/HPA was 1.05 V. This is lower than that of Ni (1.72 V), suggesting that the addition of HPA significantly improved the bifunctional performance. At the same time, they also found that the C/HPA catalyst which consisted of HPA and Vulcan carbon nano powder had better bifunctional activity than the Vulcan carbon nano powder. Their results suggested that based on the use of active HPA, modified Ni exhibited an improved bifunctional activity of ORR/OER in alkaline media.

4.2 Free metal catalysts Among unsupported bifunctional oxygen catalysts, another major category is free metal catalysts without any catalytic metal catalyst particles. These can be classified as heteroatom doped carbon materials [107, 256-258], oxides [259-261], and hydroxides [262, 263]. These catalysts are active for both ORR and OER by themselves in contrast to supported catalysts.

4.2.1 Doped carbon-based catalysts Recently, it has become increasingly attractive to explore non-precious metal catalysts and metal-free catalysts as alternatives to Pt-based catalysts. Considerable

- 107 -

efforts have been placed on various carbon-based nanomaterials in the development of inexpensive metal-free electrocatalysts with both high performance and durability. Carbon-based catalysts show longer stability, higher selectivity and more competitive electroactivity when compared to noble-metal based electrocatalysts [264-266]. In particular, doping carbon materials with one or two elements can tailor and tune their electronic structures, resulting in good catalytic activities for both ORR and OER [107, 256, 257].

4.2.1.1 Nitrogen-doped carbon-based catalysts Nitrogen, which has a higher electronegativity than carbon, is an only element used in single element-doped carbon-based bifunctional catalysts for both ORR and OER because the addition of N induces active sites; breaking the O-O bonds of oxygen molecules and improving catalytic performance [258]. To date, it is well known that carbon materials such as carbon nanotubes [267, 268] and graphene [269] have received increasing attention in N-doped carbon bifunctional oxygen catalysts in URFCs. To explain the origin of catalytic activity toward ORR/OER after N doping into carbon nanotubes (N-CNTs) and to find the optimal synthesis conditions for the preparation of electrocatalytic N-doped CNTs, Yadav et al. [267] investigated and analyzed the effects of nitrogen functionality in diameter-tailored carbon nitrogen nanotubes on the electrocatalytic activity of N-CNTs toward ORR and OER after N-CNTs were prepared by varying the precursors and growth conditions. In a liquid

- 108 -

chemical

vapor

deposition

(CVD)

route

[270],

acetonitrile

(ACN),

dimethylformamide (DMF), trimethylamine (TEA), and hexamethylenetetramine (HMTA) were used as C/N hydrocarbon precursors to prepare different N-CNTs with ferrocene in Ar/H2 atmosphere at elevated temperatures. Some synthesis conditions was fixed: (1) main tuning temperature range of 650oC - 1000oC, (2) quartz tube diameter from 5 cm to 1 m, (3) heating rate of 40 oC/min from room temperature under Ar, (4) Injection of precursor at an optimum temperature with a flow rate of 12 mL/h in 80%Ar/20%H2 atmosphere, and (5) post-treatment at 360oC for 3 h in air to remove other carbonaceous species. Based on the use of different precursors, the products were listed as N-CNT-ACN, N-CNT-DMF, N-CNT-TEA, and N-CNT-HMTA. In the examination of structural and microstructural characterizations, typical TEM images of CNNT samples revealed the presence of multi-walled nanotubes with bamboo-shaped morphology. Without other carbonaceous materials, according to a microscopic investigation, the diameters of the N-CNTs were 20, 31, 45, and 66 nm. These were associated with the use of TEA, HMTA, DMF, and CAN, respectively. When the crystallinity and disorder were analyzed in the carbon nanostructures using Raman spectroscopy, the intensity ratio of the D-band and G-band, which are considered as an approximate measure of nitrogen hybridization and thus defect fraction [270, 271], was found not to be dependent on the diameter variation of N-CNT but to structural defects, resulting from nitrogen doping and concentration, as

(Fig. 17)

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seen in Figs. 17(a) and (b). In the other measurement using fwhmD/fwhmG (D band full width at half-maximum normalized with respect to G band full width at half-maximum) to detect the defects in N-CNTs however, the normalized fwhm of D band depended strongly on the modification of microstructures from strains produced by the curvature of CNTs [272] rather than from the structural defects induced by N doping even though the fwhmD/fwhmG of N-CNTs decreased with increasing CNT diameter in Fig. 17(c). As XPS techniques were used to examine the stoichiometric composition and chemical bonding states of N-CNTs, N 1s peaks around 398.5, 400.8, and 401.5 eV in high resolution XPS spectra were assigned to piridinic, pyrrolic and graphitic nitrogen, respectively for all four CNNTs while the total nitrogen percentages were 2.4, 2.9, 2.4, and 0.3 at.%, respectively, for N-CNT-CAN, N-CNT-DMF, N-CNT-TEA, and N-CNT-HMTA that were synthesized at 850oC. Interestingly, based on the correlation between the diameter of CNTs and the type of N content in terms of the combination of TEM and XPS, it was found that with increasing N-CNT diameters, both pyridinic N increased while pyrrolic N decreased. Furthermore, graphitic N only slightly increased as the N-CNT diameter increased. When electrochemical measurements was carried out in 0.1 M KOH solution to evaluate the catalytic performance using a typical three-electrode configuration with Ag/AgCl as the reference electrode, Pt wire as the counter electrode, and carbon electrode covered by catalyst sample as the working electrode, it was confirmed in O2-saturated 0.1 M KOH that an optimal temperature of 850oC resulted in more active

- 110 -

N-CNTs on ORR. For all N-CNT samples synthesized at 850oC, N-CNT-CAN, with the largest diffusion-limiting current density, showed higher ORR performances in terms of its more positive onset potential of 0.93 V than N-CNT-DMF (0.89 V), N-CNT-TEA (0.83 V), and N-CNT-HMTA (0.78 V). In the analysis of the role of N moieties as the catalytic active site to determine whether it was graphitic-N [101, 119, 273-275] or pyridinic-N [276-279] that results in the enhancement of ORR, it was found that the ORR activity of N-CNTs was enhanced with decreasing pyrrolic-N, suggesting that pyrrolic-N did not have an active effect. However, both graphitic-N and pyridinic-N in CNNTs were confirmed to play an active effect on ORR activity. Furthermore, ORR activity increased with increasing atomic content of both graphitic-N and pyridinic-N in N-CNT-DMF, N-CNT-TEA, and N-CNT-HMTA samples (Figs. 17 (c) and (d)). The comparison of N-CNT-CAN and N-CNT-DMF samples revealed that N-CNT-CAN was slightly more positive, relating to higher ORR activity, than N-CNT-DMF even though the atomic content of graphitic-N and pyridinic-N is slightly lower in N-CNT-CAN than that in N-CNT-DMF. According to literature, the reason for this is that compared to N-CNT-DMF, N-CNT-ACN has a larger diameter (see Figs. 17 (d)), resulting in a lower O2 adsorption energy and thus contributing to the positive shift of onset potential [280]. To summarize, the ORR activity followed a decreasing order for all N-CNTs: N-CNT-ACN > N-CNT-DMF > N-CNT-TEA > N-CNT-HMTA. A further electrochemical measurement showed that CNNT-CAN has an electron transfer number of 3.7-3.9, indicating its 4 electronic pathway on ORR. At 900 rpm, the obtained polarization curve revealed a Tafel slope

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of ~110 mV/dec in the low overpotential region, which was associated with a chemical rate-determining step (e.g., O2 adsorption onto the surface of the N-CNTs), while Pt/C catalysts have a Tafel slope of 79 mV/dec [281, 282]. In particular, a stability test was conducted with the addition of methanol to 0.1 M KOH and after running for 500 continuous cycles, N-CNT-CAN exhibited excellent durability and tolerance to methanol. To evaluate the electrocatalysis of OER on N-CNTs, the polarization curves tested in 0.1 M KOH solution showed that N-CNT-CAN and N-CNT-DMF had a similar OER activity and delivered much higher OER currents than N-CNT-TEA. At a current density of 10 mA cm-2, N-CNT-CAN produced a potential of 1.68 V, this is close to that of the highest reported active IrO2/C (1.60V) [107] and lower than that of N-CNT-DMF (1.70 V),

N-CNT-TEA (1.80 V),

N-CNT-HTMA (1.89 V), and 20wt%Pt/C (1.78 V). According to the tested polarization curves, N-CNT-ACA needs to be improved in the development of OER catalysts even though it had a smaller Tafel slope (330 mV/dec) than N-CNT-DMF (437 mV/dec), N-CNT-TEA (588 mV/dec), N-CNT-HTMA (621 mV/dec), and 20wt%Pt/C (338 mV/dec). A large reported range of 658 to 83 mV/dec [283, 284] suggests a variety of reaction mechanisms in the presence of complicated N functionality in dope carbon nanostructures. OER results of the four N-CNTs suggests that the overpotential for OER and Tafel slopes decreases with the increase of N-CNT diameters, which confirmed the active effect of higher diameter nanotubes on OER activity, similar to ORR. N-CNT-CAN is shown to be an optimum bifunctional oxygen catalyst with the largest diameter nanotubes. The smaller diameter of

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CNNT-HMTA and CNNT-TEA also had worse bifunctional activity despite N-CNT-TEA giving a similar N content to N-CNT-CAN. This further suggests that the choice of suitable precursors can result in a powerful CNNT bifunctional catalyst for both ORR and OER. At the same time, Tian et al. [268] carried out a parallel study on N doping in CNTs for ORR and OER. Interestingly, they built a CNT@N-CNT model in which the pristine CNTs acted as a core while the NCNTs acted as a shell. Their study conclusively showed that N doping on the surface played an important role in the production of active sites and thus the enhancement of ORR and OER. At 10 mA cm-2 (an important parameter to evaluate the OER catalyst) the CNT@N-CNT reached a potential of -0.75 V, showing that their as-prepared N-doped CNTs is a promising bifunctional oxygen catalyst. Acting as an important N-doped carbon material in modified carbon materials, N-doped graphene has drawn significant attention in recent years because graphene exhibits intriguing properties such as high mechanical strength, large surface area, and excellent electrical conductivity [285]. To obtain enriched graphitic N and to study the structure-property relationships of N-doped graphene (NG), Wong’s group [269] utilized polypyrrole (ppy) as an N source to gain N-doped graphene with a high percentage of graphitic N and examined the electrocatalytic activities of both ORR and OER. In a typical pyrolysis synthesis, a self-made graphene oxide (GO) was dissolved into water then followed by the addition of concentrated HCl and pyrrole. (NH4)2S2O4 was next put in the mixture solution which resulted in the polymerization of pyrrole. After vacuum filtration, washing and drying

- 113 -

were finished, a GO-ppy composite was formed and pyrolyzed at 900oC for 30 min under Ar. The product was labelled as NG-900. Un-doped graphene was prepared as a reference sample using the same conditions without ppy. In the structural and morphological characterization, it was observed in TEM images that the ppy was formed as a thin layer on the GO, indicating that the GO acted as a structuring agent, while the incorporation of ppy resulted in a morphological change from a 2D closely stacked layer to a 3D porous structure. After pyrolysis, clean multi-layer graphene sheets were observed with an open 3D porous structure, facilitating the diffusion of gaseous species. BET measurements revealed a high portion of meso- and micro-pores in the 3D NG-900 which had a surface area of ~370 m2/g and an average pore diameter of ~5.3 nm. Not only could N-doping be visualized from the N elemental mapping results in EDX measurements but also evidenced by XPS characterization. In particular, the survey spectrum of NG-900 in XPS revealed the presence of C, O, and N without other impurities. In the detailed analysis of high resolution XPS, C 1s peak indicated a large portion of C connected with N and O heteroatoms and according to the O 1s spectrum, O atoms generally occurred in the form of C=O and C-OH. To probe the nature of N functionalities in NG-900, N 1s spectrum was deconvoluted and then assigned to four N species: pyridinic N (298.3 eV), pyrrolic N (400.2 eV), graphitic N (401.2 eV), and oxidized N (403.0 eV). Moreover, pyrolysis temperatures significantly determined the contents of the different N species. At pyrolysis temperatures of above 500oC, pyrrolic N may directly transform to into graphitic N. Based on XPS measurements, the graphitic N in

- 114 -

NG-900 was ~44%, this is higher than those regularly reported in literature [174, 286]. To examine the electrochemical properties of NG-900, electrochemical measurements were tested in 0.1 M KOH solution with a three-electrode system. 900oC was fixed as the optimal pyrolysis temperature at which the synthesized NG produced the highest ORR performance among all samples prepared at the tested temperatures. When RDE measurements were used to investigate the ORR kinetics of NG-900, the calculated number of electron transfer was 3.8-3.9 and between -0.3 and 0.6 V from the slope of the Koutecky-Levich plots, indicating a predominant four-electron ORR of the NG-900. Following the reported results that N-doped carbon materials had superior stability and tolerance to methanol crossover than commercial Pt/C catalysts [258, 287], NG-900 exhibited no change in its CV curve after 2000 continuous cycles while Pt/C had a significant decay. At the same time, the addition of 3M methanol to 0.1 M KOH did not hinder the catalytic ORR of NG-900 whereas Pt/C presented methanol oxidation. Based on an ORR comparison of NG-900 and reported N-doped graphene catalysts [174, 286-291], it was proposed that for highly catalytic N doped carbon materials, a graphitic N content of 2-3 at% would be optimal for ORR activity. The tested LSV for OER also revealed that NG-900 had a superior OER activity as compared to un-doped graphene and commercial Pt/C, demonstrating the potential application of NG-900 as a bifunctional oxygen catalyst in URFCs.

(Fig. 18)

- 115 -

As N-doped graphene (NG) and N-doped CNTs (N-CNT) were explored as the two unsupported catalysts for both ORR and OER, an interesting hybrid bifunctional catalyst (see Fig. 18) was designed and prepared by N doping in a mixture of graphene and single-walled carbon nanotubes (SWCNTs) in Zhang’s group [283]. This catalyst not only possessed a three dimensional interconnected network built simultaneously by graphene and SWCNTs, but also presented an intrinsic dispersion of graphene and CNTs with the dispersion of N-containing functional groups with a highly conductive scaffold. In a high-temperature catalytic CVD growth method with in situ N-doping, self-made FeMoMgAl (Layered Double Hydroxides) LDH flakes were used as the catalyst precursor in a reactor with heat treatment at 950oC. A mixed gas (ethylene/ammonia/H2), coupled with Ar, was then introduced into the reactor. After the CVD growth was kept for 15 min at 950 oC, the temperature was cooled down to room temperature under Ar. With the removal of FeMoMgAl LDO flakes using NaOH and HCl solution, a further procedure of CO2 oxidation at 900oC and post-acid treatment [292] was used to remove residual Fe. Finally, filtering/washing/drying was carried out to obtain the final hybrid product of N-doped SWCNT/graphene hybrid (NGSH). In all the experimental procedures, ethylene and ammonia were used as C and N precursors. For a reference sample, un-doped GSH was prepared using the same method without the use of ammonia. Also, to detect the effects of CNT introduction, a control sample of N-doped graphene (NG) with an N content of 2.31wt% was synthesized by the same method without CNTs. In SEM and TEM images, the removal of the FeMoMgAl LDH catalyst resulted in a flake

- 116 -

morphology during the unstacking of the double-layer graphene while the graphene protuberances acted as spacers to help the stacking of the graphene layers on top of the flakes rather than on both sides of the flakes. The N2 sorption isotherm revealed micropores of ca. 1 nm and mesopores of ca. 4.5 nm on the NGSH. The detected BET surface area of the NGSH was 812.9 m2 g-1. This is much larger than that of the undoped GSH (565.6 m2 g-1). The N 1s in XPS indicated that N atoms were bonded to the surrounding C atoms in pyridinic N (0.13 at.%), pyrrolic N (0.15 at.%), quaternary N (0.15 at.%), oxidized N (0.07 at.%), and chemsorbed N (0.08 atl.%), with corresponding bonding energies of 398.7, 400.0, 401.1, 403.1, and 404.8 eV [293, 294], respectively. The total N content in the NGSHs was 0.58 at.%. To evaluate the electrocatalytic activity towards ORR and OER, a three-electrode chemical station was used in measurements with a piece of Pt foil as the counter electrode, a saturated calomel electrode as the reference electrode, and a glass carbon electrode covered by catalyst sample as the working electrode. Based on the tested LSV curves for OER in 0.1 M KOH, it was found that NGSH produced much higher OER currents than un-doped GSH and commercial 20wt% Pt/C. At a current density of 10 mA cm-2, NGSH delivered a potential of ~1.63 V, which was similar to that of IrO 2/C materials [107] and lower than undoped GSH (~1.74 V). The calculated Tafel slope for NGSH was ~83 mV/decade, which is much smaller than those for undoped GSH (~97 mV/decade) and Pt/C (~288 mV/decade), demonstrating the superior OER activity of NGSH. When the ORR performance was examined, the tested LSV curves in O2-saturated 0.1 M KOH showed that compared to undoped GSH, NGSH presented a

- 117 -

more positive onset potential and higher current density, indicating that N-doping delivered an enhancement of ORR activity, even if 20wt% P/C was better in ORR activity than NGSH. The calculated electronic transfer numbers were 3.3 for NGSH and 2.5 for GSH, confirming the greatly enhanced electrocatalytic performance of NGSH after N-doping. At the same time, a comparison of NG and NGSH revealed that at 0.47 V, NG had a much lower current density than NGSH, demonstrating the active effect of SWCNT introduction on ORR. Furthermore, NGSH exhibited a high electrical conductivity of 53.8 S cm-1, which was higher than GSH (38.4 S cm-1), and much higher than N-doped multi-walled CNTs (2 - 60  10-3 S cm-1) [295] and reduced graphene oxides (2.48  10-5 S cm-1) [296]. This electrical conduction in NGSH can guarantee the full collection of the current, enabling a fairly high ORR reactivity. The measurements of electrochemical stability and the methanol crossover effect further confirmed the effects of N-doping, in that it results in better stability and tolerance towards methanol in NGSH as compared to those of 20wt% Pt/C. NGSH was demonstrated to have potential for application in bifunctional oxygen catalysts.

4.2.1.2 Double elements doped carbon-based catalysts Although N doping in carbon nanomaterials (e.g., CNTs and graphene) can play an active effect on catalytic performance, the improvement of catalytic activity is still limited. More recent studies have shown that co-doping N-doped carbon nanomaterials with a second heteroatom, such as B, S, or P, can be an ideal alternative to improving the electrocatalytic activity by modulating electronic properties and

- 118 -

surface polarities [141, 297-299]. Significantly, the reported synergistic effect arising from co-doping of heteroatoms benefited the enhancement of electrocatalytic performance [300-304]. In particular, co-doping N-doped carbon nanomaterials with S or P have been explored as an unsupported metal-free bifunctional oxygen catalyst for both ORR and OER [302-304].

(Fig. 19)

To improve the catalytic activity of ORR and OER using dual element-doped nanocarbon and to increase available active sites, Li et al. [304] designed and synthesized a nitrogen and phosphorus dual-doped graphene/carbon nanosheet (N,P-GCNS) catalyst by a facile and cost-effective pyrolysis procedure in which graphene oxide (GO) sheets were used as the precursor of graphene and the structure directing agent for conformal coating of polyaniline (PANi) and phytic acid (PA) molecules during the polymerization of aniline monomers. According to their experimental route (see Fig. 19(a)), a self-made GO was mixed with aniline using alcohol as the solvent. With the addition of an aqueous solution containing PA and ammonium peroxydisulfate (APS), polymerization was carried out in an ice bath. After 24 h, a gel was obtained and heated at 850oC for 2 h under N2. The final product was labelled as N,P-GCNS. For a comparison sample, N,P-doped carbon nanospheres (N,P-CNS) were prepared via the same procedure without the addition of GO. N-doped carbon nanoparticles (N-CNP) were also synthesized by the pyrolysis of

- 119 -

pristine PANi obtained from using hydrochloride acid instead of PA as the catalyst to exclude P-doping in the final product. N-doped graphene (NG) was obtained through the same procedure of N-CNP in the presence of GO during polymerization. P-doped graphene (P-G) was prepared by pyrolyzing a dried mixture of PA-GO. In the characterization of structure and morphology, SEM and TEM images (see Fig. 19(b-d)) clearly showed that N,P-GCNS presented a sandwich-like structure with porous N,P-doped carbon conformal coating on few-layers-thick graphene nanosheet. The thickness of the nanosheets was in the range of 5 – 10 nm, while the BET measurements revealed a surface area of 900.2 m2 g-1 with broad pore size distribution in the micro- and mesoranges. This structural distribution and characterization indicated that more active sites (i.e., N,P-doped carbon) can be completely exposed to reactant molecules, and that graphene nanosheets can facilitate electron transfer during the redox process and thus enhance electrocatalytic activity and kinetics. The combination of XRD and Raman measurements suggested that N,P-doping did not effectively inhibit the orderly restacking of graphene nanosheets but created more defect sites [305], benefiting catalytic activity. In the XPS characterization, the high resolution N 1s spectra revealed three N species such as pyridinic-N (398.3 eV), quaternary-N (400.9 eV), and chemsorbed-N (404.9 eV). According to literature [101, 268, 283], the content of the first two N species determines ORR activity for N-doped carbon materials where pyridinic-N can improve the onset potential for ORR and quaternary-N can control the limiting current density. In contrast to the N 1s spectra, the P 2p spectrum was deconvoluted into two peaks at 133.1 and 134.1 eV,

- 120 -

corresponding to P-C and P-O bonding, respectively [306]. Furthermore, the peak area of P-C was 2 times that of P-O, indicating that P atoms were well incorporated into the C framework, creating more active sites and trigger synergistic effects for ORR [306, 307]. When electrochemical measurements were ran in 0.1 M KOH using a standard three-electrode glass cell, the CV comparison for N,P-GCNS, N,P-CNS, and N-CNP samples showed that all samples pronounced an oxygen reduction peak in the O2-saturated KOH solution rather than in the O2-free KOH solution, demonstrating that all three samples had catalytic activity toward ORR. According to the presented potential and current density at the oxygen reduction peak, their ORR activity followed in an order: N,P-GCNS (0.85V, 2.24 mA cm-2) > N,P-CNS (0.78V, 0.80 mA cm-2) > N-CNP (0.69V, 0.55 mA cm-2), indicating that in nanocarbon materials, co-doping using N and P resulted in higher ORR activity than N doping, and that GO was a good structure-directing agent resulting in a combination of unique hierarchically porous structures and sandwich-like graphene sheets with high electronic conductivity. The tested LSV curves in O2-saturated 0.1 M KOH further proved this order of ORR activity for these three samples. On the one hand, the onset potential increased from 0.84 V for N-CNP to 0.95 V for N,P-CNS and 1.01 V for N,P-GCNS. On the other hand, the oxygen reduction current density at 0.6 V increased from 1.70 to 2.07 and 5.56 mA cm-2. The high difference in the onset potential and current density (at 0.6V) between N,P-GCNS and N-CNP confirmed an large enhancement in ORR activity for co-doping using N and P. This is due to the additional P-doping into N-doped carbon enhancing charge delocalization and the

- 121 -

asymmetric spin density of carbon atoms [300] as well as promote N-doping at the edges of the graphene, resulting in the further improvement of ORR activity [308]. This also suggested that the incorporation of P created more active sites, triggering synergistic effect on ORR [306]. Compared to commercial 20wt% Pt/C, N,P-GCNS exhibited a higher half-wave potential and cathodic current density over the entire potential range, and a smaller Tafel slope (51 mV dec -1) at low overpotentials, demonstrating that N,P-GCNS had greater ORR activity than 20wt% Pt/C. When LSV measurements were continuously performed at different rotating rates in the range of 900-3600 rpm, the electron transfer numbers for N,P-CNS and N-CNP were found to change in the ranges of 2.87-3.16 and 2.05-2.49, respectively. These are lower than 3.96 for N,P-GCNS, suggesting a higher ORR kinetic and catalytic efficiency for N,P-GCNS than N,P-CNS and N-CNP. Importantly, N,P-GCNS followed a four-electron oxygen reduction process in ORR, which is highly desirable for efficient energy conversion and storage. To further investigate the possibility of N,P-GCNS as a bifunctional oxygen catalyst, obtained LSV curves for OER from 1.0 to 1.95 V clearly showed that OER onset potential followed an increasing order: N,P-GCNS (1.32V) < RuO2 (1.39V) < N,P-CNS (1.41V) < Pt/C (1.67V) < N-CNP (1.78V) while the current density at 1.9 V gave an decreasing order: N,P-GCNS (70.75 mA cm-2) > RuO2 (32.41 mA cm-2) > N,P-CNS (15.57 mA cm-2) > Pt/C (6.50 mA cm-2) > N-CNP (.78 mA cm-2). N,P-GCNS delivered the earliest onset potential and the greatest current density, demonstrating the highest OER activity, with better OER activity than even RuO2 (the best OER catalyst at present). Particularly, at 10 mA cm-2, N,P-GCNS,

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RuO2, N,P-CNS, Pt/C, and N-CNP samples delivered current densities of 1.57, 1.59, 1.79, 1.94, >2, respectively. The OER results indicate that N,P-GCNS is a better OER catalyst in comparison with reported RuO2, IrO2/C and other doped carbon catalysts [107]. When an evaluation of bifunctional catalyst ability was ran using the difference in potentials between the OER current density of 10 mA cm-2 and the ORR current density of -3 mA cm-2, N,P-GCNS showed a small value of 0.71 V. This is lower than most of the reported catalysts listed in literature [304], demonstrating N,P-GCNS as a promising bifunctional oxygen catalysts in URFCs. Based on co-doping with N and P, a mesoporous carbon foam co-doped with N and P was also studied in Dai’s group [302] as an effective bifunctional catalyst in Zn-air batteries. To enlarge and further study the use of co-doping in nanocarbon materials for bifunctional oxygen catalysts, Xu’s group [303] worked on a novel hybrid material consisting of cable-like multiwall carbon nanotubes and S,N co-doped carbon nanodots, and examined its electrochemical properties for both ORR and OER. In the design of the hybrid material, MWCNT can act as a high-surface-area substrate for the dispersion of polythiophene. Particularly, after the pyrolysis of polythiophene, the achieved S,N-doped carbon nanodots is coated on the MWCNTs. MWCNT can also provide porous structures and high electron transfer rates favoring electrocatalytic performance. In a typical experimental route, a mixture solution was prepared from MWCNT, FeCl3, CHCl3, and thiophene monomer using CHCl3 as a solvent. At 0 oC, the thiophene monomer in the mixture solution was polymerized for 24 h to form MWCNT@polythiophene. After washing and drying, a pyrolysis procedure was

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carried out at 900oC for 1 h under NH3 to obtain a one-dimensional (1D) cable-like multiwall carbon nanotube/S,N co-doped carbon nanodot (MWCNT@S-N-C) hybrid. As a reference sample, MWCNT@S-C was prepared with the same synthesis route using Ar in the pyrolysis process. TEM and SEM images clearly revealed carbon nanodots with a size of 2 nm on the rough and worn surface of MWCNTs, indicating the successful synthesis of the MWCNT@N-S-C hybrid. Elemental mapping verified the presence of C, S. and N for the surface of the MWCNT@N-S-C hybrid and that S and N were homogeneously distributed in N-S-C. Owing to the cable-like structure and the highly porous shell, the pore size and BET surface area of the MWCNT@N-S-C hybrid were found to be 6.4 nm and 121 m2 g-1. This is good for electron transfer and also increases the surface exposure of the active sites for the enhancement of electrocatalytic activity. Importantly, in the XPS characterization of the MWCNT@N-S-C hybrid, sulfur doping was found in the forms of –C-S-C- and –C-SOx-C- (x=2-4) [309] while N 1s spectrum revealed four kinds of N (pyridinic-N, pyrrolic-N, quaternary-N, and N-oxide) [310, 311]. The pyridinic–N sites and the many open edge sites were reported to be favorable towards ORR activity [312, 313]. The combination of XPS and elemental mapping confirmed the successful co-doping of S and N atoms in the carbon framework via covalent bonds. When electrochemical experiments were ran in 0.1 M KOH solution using a three-electrode cell with Pt wire as the counter electrode, Ag/AgCl as the reference electrode and a glassy carbon electrode loaded with catalyst sample as the working electrode, the CV curves for ORR in O2-saturated 0.1 M KOH showed that MWCNT@S-N-C exhibited a more

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positive potential (-0.27 V) for the cathodic reduction peak than MWCNT@S-C. Moreover, MWCNT@S-N-C and MWCNT@S-C had no obvious redox peaks in terms of the CVs tested in N2-saturated 0.1 M KOH solution. These results indicates that S and N dual doping played a more effective role in the enhancement of ORR activity and the creation of more active sites than that of N-doping. When the comparison of ORR was ran for MWCNT@S-N-C and commercial 20wt% Pt/C using a typical LSV curves obtained in O2-saturated 0.1 M KOH at 1600 rpm, MWCNT@S-N-C presented a more negative onset potential but a higher limiting current density, reaching 7.62 mA cm-2 at -0.8 V vs. Ag/AgCl. Based on RDE measurements at various rotation speeds from 400 to 1600 rpm, the calculated electronic transfer number for MWCNT@S-N-C was in the range of 3.9 – 4, suggesting a favored 4e reduction reaction process. Also, after the addition of 3M methanol, the chronoamperometric responses revealed a better tolerance for MWCNT@S-N-C than for 20wt% Pt/C. While at -0.40 V after 20 000s in an O2-saturated 0.1 M KOH solution, MWCNT@S-N-C retained 76.8% of its initial current, indicating a higher durability than that of 20wt% Pt/C (39.8%). This also confirmed that MWCNT@S-N-C has potential application in direct methanol fuel cells. For OER, the LSV curves in N2-saturated 0.1 M KOH showed that MWCNT@S-N-C exhibited a more positive onset potential than IrO 2. At a current density of 10 mA cm-2, the overpotential of MWCNT@S-N-C was 0.44 V, which was about 26 mV more negative than that of IrO2. The calculated Tafel slope also followed an increasing order: MWCNT@S-N-C (245 mV/decade) < IrO2 (255 mV/decade) <

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MWCNT@S-N-C (275 mV/decade). At a current density of 10 mA cm-2, MWCNT@S-N-C exhibited a comparable OER value to those reported for Co-based catalysts [95, 314-317], but higher than those of many other materials such as Mn or Ni-based catalyst composites [74, 318-323], spinel or perovskite oxide catalysts [64, 324, 325] and carbon-based catalysts [268, 283, 326]. These results indicated that MWCNT@S-N-C has an excellent OER activity, confirming the advantages of S,N co-doping on both ORR and OER. Interestingly, a scanning of i-t tested in N2-saturated 0.1 M KOH at 1600 rpm [45] was ran to evaluate the durability of MWCNT@S-N-C.

After

5000s, the chronoamperometric test

demonstrated

outstandingly high activity and strong durability even though S and N were measured to be 0.8% and 0.78% in MWCNT@S-N-C. This research importantly indicates that for MWCNT@S-N-C, the high catalytic performance resulted from synergistic chemical coupling effects between N and S after the incorporation of N and S. The N and S co-doping was very effective for the enhancement of electrocatalytic activity and the creation of much more active sites for both ORR and OER.

4.2.2 Oxide-based catalysts In comparison to noble metals, spinel structured cobalt and manganese oxides such as Co3O4, MxCo3-xO4 (M = Cu, Ni) and CuxMn3-xO4, have been considered as potential candidates for bifunctional oxygen catalysts because they themselves are active for both ORR and OER [327-330]. Gyroid structured materials include not only mesoporous structures that provides highly accessible, catalytically active surface

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sites for catalytic performance, but also interconnected networks that are favorable to the enhancement of stability under harsh catalytic or electrocatalytic reaction conditions. Based on these properties, Sa et al. [331] prepared an ordered mesoporous spinel structured Co3O4 (meso-Co3O4) using KIT-6 mesoporous silica as a template to produce two different pore structures for both OER and ORR. Using a template-assisted nanocasting method, Co(NO3)2·6H2O and a self-made KIT-6-100 were first dissolved into absolute ethanol to form a purple-colored mixture. After evaporation at 80oC, decomposition of the nitrate species was done at 200oC for 3 h, followed by impregnation, drying, and heating at 200oC again. A further heat treatment was then done at 450oC for 6h under air to completely form the cobalt oxide. After the removal of the template using 2 M NaOH, the final product was obtained as meso-Co3O4-100. A reference sample denoted as meso-Co3O4-35 was prepared by using a self-made KIT-6-35 template in the same synthesis process, coupled with only half of the cobalt precursor and ethanol during impregnation. Co 3O4 nanoparticles and 20wt% Ir/C were also used as reference samples. Based on Brunauer-Emmett-Teller (BET) measurements, meso-Co3O4-100, with a BET surface area of 114 m2 g-1, was lower than that of meso-Co3O4-35. However, both of these two samples had higher BET surfaces than the referenced Co 3O4 sample (58 m2 g-1), which means they can provide higher surface areas for active sites. The Barrett-Joyner-Halenda (BJH) measurement for the examination of pore sizes revealed that meso-Co3O4-100 exhibited a maximum value of 3.8 nm while meso-Co3O4-35 had two maximum values of 9.4 and 21.3 nm, suggesting that meso-Co3O4-35 had significantly more

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pores and bigger pore sizes, thus benefiting the transfer of electrons and masses. This was also confirmed in SEM and TEM images. Furthermore, compared to meso-Co3O4-35, meso-Co3O4-100 showed uncoupled sub-frameworks with larger mesopores in the edge areas, resulting from the disconnection of two mesoporous areas and the non-uniform presence of Co 3O4 nanoparticles phases. The referenced Co3O4 nanoparticles was found to be ~20 nm with a predominant cubic morphology. XRD clearly showed a face-centered cubic spinel phase for meso-Co3O4-100, meso-Co3O4-35, and the referenced Co3O4 samples with crystallite sizes of 11.5 (±0.3) nm, 12.7 (±0.4) nm, 21.2 (±0.5) nm according to the Debye-Scherer equation, respectively. In the additional structure characterization of XANES and EXAFS spectra, the characterization peaks at 7719 eV and 7723 eV showed that meso-Co3O4-100 had the same features as Co3O4 spinel samples [332-335] but at a higher oxidation state. To investigate the catalyst’s bifunctional activity for both OER and ORR, electrochemical measurements were performed in 0.1 M KOH with a three-electrode cell. In the LSV polarization curves for OER tested from 1.0 to 1.7 V (vs. RHE), it was found that meso-Co3O4-35 exhibited lower onset potential and higher current density. In particular, at 10 mA cm-2, meso-Co3O4-35 reached 1.64 V, corresponding to an overpotential of 411 mV, while meso-Co3O4-100, self-made Co3O4, Pt/C, and Ir/C samples had overpotentials of 426, 449, 634, and 409 mV, respectively, indicating that meso-Co3O4-35 has a comparable OER activity to Ir/C and higher OER activity than the other samples. Interestingly, when a quantitative analysis was ran for meso-Co3O4-35 and the other catalysts, the mass activity of the

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two meso-Co3O4 samples at 1.6 V was 53-63 A g-1. This is higher than those of self-made Co3O4 (31 A g-1) as well as previously reported catalysts (22-43 A g-1) [336-338]. The meso-Co3O4 sample also had a higher OER activity than MnOx-based catalysts (450-610 mV at 10 mA cm-2) [74, 108], demonstrating the high catalytic activity of meso-Co3O4 catalyst among other noble metal-free catalysts in the same class for OER. When cycling 1500 times, meso-Co3O4-35 presented a 29% loss of OER activity. This is lower than self-made Co3O4 nanoparticle catalysts, confirming the active effect of 3D interconnected networks on the improvement of durability. In regards to ORR, the tested LSVs in O2-saturated 0.1 M KOH showed that meso-Co3O4-35 and meso-Co3O4-100 had nearly identical polarization curves, which presented onset and half-wave potentials of 0.75 and 0.62 V, respectively. These were lower than that of 20wt% Pt/C (0.98V, 0.85V). The calculated electronic transfer numbers for the two meso-Co3O4 samples were around 3.5-3.9, close to the electronic transfer number of 20wt% Pt/C catalysts, suggested that the two meso-Co3O4 samples were quite active for ORR. In the presence of methanol during the test, the ORR polarization curves revealed a good tolerance for the two meso-Co3O4 samples as compared to 20wt% Pt/C. When the sum of the overpotentials for OER (at 10 mA cm-2) and ORR (at -3 mA cm-2 ; approximate half-wave potential) was used to evaluate the bifunctional activity as a catalyst, meso-Co3O4-35 (411 mV for OER and 623 mV for ORR) delivered a value of 1.03 V, which is comparable to Ir/C (409 mV for OER and 516 mV for ORR) and 20wt% Pt/C catalyst (636 mV for OER and 375 mV for ORR), indicating that meso-Co3O4-35 has the potential to be a bifunctional oxygen

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catalyst for URFCs. To study and improve the bifunctional activity of Co3O4, Lambert et al. [339] designed a template-less electrode position approach to obtain Co 3O4 and Ni-doped Co3O4 (NixCo3-xO4) films for both ORR and OER. Especially for NixCo3-xO4, they thought that the inclusion of an optimized amount of Ni could lead to the improvement of activity. In a typical synthesis route, Co(NO3)2·6H2O and Ni(NO3)2·6H2O were used as precursors of Co and Ni, and aqueous NaNO3 was employed as the supporting electrolyte to deposit metal hydroxide (i.e., Co1-zNiz(OH)2-x(NO3)x·y(H2O) ) films. After annealing at 300oC, NixCo3-xO4 was formed. The ratio of Co(NO3)2·6H2O to Ni(NO3)2·6H2O was used to determine the x of NixCo3-xO4. Without Ni(NO3)2·6H2O, the prepared Co3O4 product was used as a reference sample. When inductively coupled plasma/mass spectrometry (ICP/MS) was used in the elemental analysis, various NixCo3-xO4 such as Ni0.44Co2.56O4, Ni0.59Co2.41O4, and Ni0.89Co2.11O4 were synthesis at initial Co/Ni ratios of 1:0.25, 1:0.5, and 1:1, respectively, indicating only partial incorporation of Ni in the final NixCo3-xO4 product. XRD evidenced the spinel phase in both Co 3O4 and NiCo2O4 without other impurity phases, while SEM images revealed a nano-textured surface and mesoporous nature of the electrodeposited film that remained even after heating. To evaluate and compare the bifunctional activity for Co3O4 and NixCo3-xO4 catalysts, electrochemical studies were conducted in 0.1 M KOH with a three-electrode cell. In LSVs tested in O2-saturated 0.1 M KOH, it was found that the half-wave potential, half-wave current density, and terminal current density for Ni0.6Co2.4O4 were at 0.768

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V, -3.29 mA cm-2, and -6.57 mA cm-2, respectively. These were significantly better than Co3O4 (0.692 V, -2.82 mA cm-2, -5.63 mA cm-2), suggesting that Ni-doping played an active effect on the improvement of ORR activity. However, 20wt% Pt/C still exhibited higher ORR activity than Ni0.6Co2.4O4. With the addition of methanol, Ni0.6Co2.4O4 exhibited higher tolerance to methanol and higher stability than 20wt% Pt/C. When the LSVs was ran for OER, Ni0.6Co2.4O4 and Co3O4 samples at 10 mA cm-2 delivered the same potential of 1.76 V, which is a little lower than that of 20wt% Ir/C (1.85 V). When the comparison of ORR and OER for NixCo3-xO4 and Co3O4 was carried out, the potential at -3 mA cm-2 as the ORR figure of merit [108] revealed a decreasing order of: Ni0.4Co2.6O4 > Ni0.6Co2.4O4 > Ni0.9Co2.1O4 > Co3O4, while according to measurements of voltage at 10 mA cm-2 [108, 128, 340], the OER activity followed the same order: Ni0.4Co2.6O4 > Ni0.6Co2.4O4 > Ni0.9Co2.1O4 > Co3O4. The difference of potentials for OER (at 10 mA cm-2) and ORR (at -3 mA cm-2 ; approximate half-wave potential) for Ni0.4Co2.6O4 was 0.96 V, which is competitive with 20wt% Ir/C (0.92 V) and 20wt% Pt/C (1.16 V) reported in literature [108]. This suggests that the least amount of Ni in Ni-doped Co3O4 resulted in the most active bifunctional activity as a potential bifunctional oxygen catalyst candidate in URFCs.

(Fig. 20)

Aside from doping methods, another approach where the mixed metal oxides are modified to enhance the bifunctional activity of Co3O4 toward both OER and ORR

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was tested. Wang et al. [341] synthesized binary Co3O4/Co2MnO4 metal oxides with an ORR-active Co2MnO4 component (see Fig. 20). At an optimum ratio of 1:2.7 for Co3O4/2.7Co2MnO4, the difference of potentials for OER (at 10 mA cm-2) and ORR (at -3 mA cm-2 ; approximate half-wave potential) was 1.09 V, indicating a lower bifunctional activity than that of Ni0.4Co2.6O4 (0.96 V). In addition to Ni doping, Cu-doping into Co3O4 was also synthesized to compare with an oxide composite of CoxOy/ZrO2 in Vitanov’s group [342]. They used both thermal decomposition and electrochemical deposition to prepare CuxCo3-xO4 and CoxOy/ZrO2 catalysts. Their electrochemical results showed that the bifunctional activity of ORR and OER in 3.5 M KOH followed the same increasing order of: CoxOy/ZrO2 < Cu0.3Co2.7O4 < Cu0.2Co2.8O4, suggesting that Cu-doped Co3O4 had higher bifunctional activity than CoxOy/ZrO2 composites. This also suggested that lower amounts of Cu in Cu-doped Co3O4 correlates to higher bifunctional activity. With the exploration of M-doping into Co3O4 (M = Nb or Cu), a series of dual metal-doped Co3O4 (e.g., CuxMn0.9-xCo2.1O4, x=0, 0.3, 0.6, and 0.9) was synthesized and measured for their bifunctional activities for both ORR and OER in 1 M KOH by Wu and Scott [252]. The electrochemical measurements revealed that the gap between ORR half wave potentials of Pt/C and Cu xMn0.9-xCo2.1O4 was only 50 mV while the onset potentials for OER in CuxMn0.9-xCo2.1O4 catalysts was more than 100 mV more negative than Pt/C. On the one hand, Mn content was shown to play an important role in the improvement of ORR. On the other hand, the ratio of Cu to Mn can be optimized for the best ORR activity. These research results suggests that an optimized dual metal

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co-doped Co3O4 material (e.g. Cu, Mn co-doped Co3O4) could be a good bifunctional oxygen catalyst in URFCs. Similar to Co-based spinel structured oxides, Mn-based spinel structured oxides have demonstrated highly active activity towards OER and ORR. Kumar’s group [261] controlled a α-crystallographic form via different reaction conditions to prepare three shape-controlled α-MnO2 materials (i.e., nano-wires, nano-tubes, and nano-particles) and studied the effects of shape on bi-functionality of bifunctional oxygen catalysts in URFCs. In the characterization of structure and morphology, XRD patterns clearly revealed a body-centered tetragonal α-MnO2 phase in all three samples (i.e., α-MnO2 nano-wires, nano-tubes, and nano-particles) without any other impure phases. In TEM images, the nano-wires were close to 20 nm in diameter for the α-MnO2 nano-wires while the nano-tubes had a moderate wall thickness of 10 nm for the α-MnO2 nano-tubes, coupled with a diameter of 20 nm. As electrochemical measurements were ran in 0.1 M KOH using a three-electrode cell, the LSVs for OER showed that the α-MnO2 nano-wire catalyst gave the lowest onset potential with significant activity in the high potential region. At 0.8 V, the α-MnO2 nano-wire catalyst displayed a current density of 0.25 mA cm-2. This was almost double that of the other two shaped catalysts, indicating that the α-MnO2 nano-wire catalyst had the highest activity. This was confirmed by a lower Tafel value (110 mV dec-1) of the α-MnO2 nano-wire catalyst than those of the α-MnO2 nano-tube catalyst (125 mV dec-1) and the α-MnO2 nano-particle catalyst (150 mV dec-1). It was also found in the LSV data obtained in O2-saturated 0.1 M KOH that the α-MnO2 nano-wire and the

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α-MnO2 nano-tube catalysts followed 4-electron oxygen reduction while the α-MnO2 nano-particle catalyst had a lower electronic transfer (~3) in ORR. Compared to the other two shapes, the shape of the wire resulted in the highest ORR activity for α-MnO2. According to density functional theory (DFT) calculations, the best bifunctional activity of both ORR and OER of the α-MnO2 nano-wire catalyst was attributed to the affinity of (310) crystallographic planes towards water molecules and its exposure to reaction interfaces favorable to the adsorption of the reactant on the catalyst surface. MnOx oxides differs from spinel structured MnO2 materials, significantly displaying crystal phases from Mn2O3 and MnO instead of α-MnO2 spinels [74, 343, 344], resulting in the high bifunctional activity for ORR and OER. For instance, Mosa et al. [343] designed and prepared a mesoporous MnOx catalyst in a heat treatment route promoted by 0.16% Cs ions. Their studies revealed that the surface and bulk active Mn3+ can play a critical role in the enhancement of catalytic activity for ORR and OER. Based on a heat treatment temperature of 450oC, the synthesized MnOx material not only exhibited a high ORR activity (0.87 V vs. RHE at -3 mA cm-2) that was comparable to state-of-the-art Pt/C catalysts, it also showed a comparable OER performance with highly active Ir/C and RuO2 catalysts. For the MnOx material, the evaluated difference in potential between ORR (at -3 mA cm-2) and OER (at 10 mA cm-2) reached 0.78 V, which is a smallest value among all manganese oxide catalysts, confirming Cs-promoted MnOx materials as a promising bifunctional oxygen catalyst in URFCs. In the development of high-performance non-noble metal-based catalysts,

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perovskite oxides have emerged as one of the most promising bifunctional oxygen catalysts in alkaline media. Malkhandi et al. [345] used different annealing temperatures to influence crystallite sizes and oxidation states of cobalt and thus oxygen evolution and oxygen reduction in typical quaternary perovskite oxides. Experimentally, lanthanum nitrate, calcium nitrate, and cobalt nitrate were used as metal precursors. They were dissolved into water and combined with 1% solution of citric acid in water to form a sol-gel material. After being heated at 350oC for 30 min, the gel was changed into a black oxide which was then grounded. The grounded oxide was then separately heated at 600oC, 650oC, 700oC, and 750oC to form the final products. Annealing temperatures from 600oC to 750oC resulted in calculated crystallite sizes ranging from 14 to 26 nm in XRD, matching the crystallite growth of nanoparticles in the heat-treatment [346]. XPS of two asymmetric peaks for 2p1/2 and 2p3/2 states were assigned to CoO and Co 3O4, respectively, confirming the partial site substitution of lanthanum (III) by calcium (II). Furthermore, the binding energy of the 2p3/2 state of cobalt shifted to higher values by about 1 eV with heat treatment from 600oC to 750oC, suggesting that the oxidation state of cobalt increases with annealing temperatures. The tested electrocatalytic activities for both ORR and OER significantly increased with annealing temperatures even though the Tafel slope remained constant, indicating the role of the cobalt center in oxygen evolution and oxygen reduction. Alkaline earth metal-doped bismuth iron oxides (Bi0.6M0.4 FeO3, M = Ba, Sr, Ca, and Mg) have been explored as the perovskite-based bifunctional catalysts for URFCs.

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For example, Afzal et al. [347] found that compared to other catalysts, the Bi0.6M0.4 FeO3 (BCFO) could give remarkable OER and ORR catalytic performances with better long-term stability than that of apristine BiFeO3 (BFO) catalyst in alkaline media. The onset and half-wave potentials at -3 mA cm-2 of BCFO were 0.705 and 0.619 V, respectively for ORR, while those of BFO were 0.633 and 0.480 V. the measured ORR Tafel slopes of BFO and BCFO catalysts were -99 and -76 mV dec-1, respectively. The catalytic OER current density of BCFO was 6.93 mA cm-2 at a fixed overpotential of 0.42 V (1.65 V vs. RHE), which was approximately 2 times higher than those of baseline catalysts. It was showed that the enhanced performance of BCFO for both OER and ORR was resulted from the doping of CaO into BFO, increasing of transport rates of charge species for the electrochemical reactions through the change of electronic structure as confirmed by EIS measurement. In addition, the less performance decay could be observed for BCFO catalyst seen by the cycling tests for OER and ORR than that of the pristine BFO in an alkaline medium. To obtain high catalytic performance, recent efforts in the modification of general ABO3 perovskite oxides have been focused on the creation of oxygen-deficiency and the substitution of A or B sites [3, 38, 57-60, 81, 348-351]. Having said this, most perovskite oxides are however still mixed with some support material, especially carbon materials, for the measurement of bifunctional activity toward ORR and OER due to their low surface areas and electronic conductivities.

5. Evaluation of bifunctional oxygen catalysts for URFCs

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Due to their ability to operate both as a fuel cell, generating power and as an electrolyzer, generating its own fuel, URFCs have great potential in a wide range of energy storage applications [344, 352-354]. A critical roadblock in the development and commercialization of URFCs lies in its bifunctional oxygen catalysts. These bifunctional oxygen catalysts are crucial in enhancing ORR and OER but produces irreversible oxygen redox reactions generating complex oxygen species such as O2-, O22-, O, O-, and O2- at the same time [200, 355]. Therefore, properties such as high catalytic

activity,

long-term

stability,

and

strong

corrosion

resistance

in

physicochemical and electrochemical circumstances such as acidic/alkaline mediums and/or high potentials are vital for viable advanced bifunctional oxygen catalysts in the application of URFCs [198, 208, 356]. It is imperative that these properties be effectively and thoroughly evaluated in bifunctional oxygen catalysts for URFCs before their overall performance as a catalyst can be validated. Before a bifunctional oxygen catalyst can be used in the fabrication of MEAs for the examination of URFC performance, the oxygen electrode activity is often used to evaluate the bi-functionality of the catalyst towards both ORR and OER [33, 108]. This is defined as the difference (ΔE) between the half-wave potential of the ORR and the OER potential required to oxidize water at a current density of 10 mA cm-2. 10 mA cm-2 is the standard current density required to achieve a water splitting efficiency of 10% with one-sun illumination for the solar-to-fuel conversion [108, 128]. In the calculation of the difference (ΔE), the approximate potential of the ORR at -3 mA cm-2 is often the same as the half-wave potential of the ORR [331]. The difference

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(ΔE) will often act as an indicator for the bi-functionality of a bifunctional oxygen catalyst. The smaller the difference, the closer the catalyst is to being truly bifunctional [117]. Tested potentials

(Table 6) (Table 7)

and current densities are determined by the preparation and measurement of catalysts that are strongly related to its starting materials, conditions, apparatus, electrode types, electrolytes, and so on. Table 6 summarizes the synthesis methods, electrolytes, ORR activities, OER activities, and the potential differences (ΔE) of typical bifunctional oxygen catalysts for URFC applications [3, 33, 55, 74, 95, 96, 106, 108, 117, 129-131, 148, 151, 172, 234, 283, 304, 331, 339, 341, 343, 346]. It can be seen that Co3O4/NrGO catalysts [33, 304] exhibited the lowest ΔE, indicating that it has the great potential as a bifunctional oxygen catalyst. However, no data has been reported for URFC performance when this catalyst is fabricated in the MEA of the URFC. Even with increasing research, the technology for the combination of fuel cells and electrolyzers in URFCs is far from mature. Up until now, significant research have been given to Pt-based bifunctional oxygen catalysts in URFCs despite their high material and fabrication costs. Typical research results of URFC performance are collected and tableted in Table 7 for Pt-based bifunctional oxygen catalysts in URFCs [27, 35, 77, 192-195, 197, 207, 208, 214, 215, 240, 358]. Based on the collected data

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in Table 7, it is evident that the overall efficiency and long-term usage of these catalysts have to be improved and optimized to enhance electrochemical performances; whether it be through the preparation of the catalyst or the fabrication of the MEA for URFC applications. More importantly, the parameters and research results obtained from these URFC performance tests using Pt-based bifunctional catalysts can be used as important references for the research and development of non-precious metal bifunctional catalysts in URFC applications.

(Table 8)

In regards to non-precious metal bifunctional catalysts for URFCs that contain an anion exchange membrane (AEM) fuel cell and a water electrolyzer, the US DOE 2015 targets [359-361] indicate that at 0.8 V, the MEA containing the catalyst is required to produce at least 350 mW cm-2 vs. reversible hydrogen electrodes (RHE) while the voltage loss must remain at less than 10% over hundreds of cycles. Table 8 presents the URFC performance for two typical non-precious metal bifunctional catalysts [252, 344]. It is evident that improvements and optimizations are needed in order for non-precious metal bifunctional oxygen catalysts to be viable for utilization in MEA for URFC operations. However, having said that, non-precious metal bifunctional oxygen catalysts have great potential.

6. Summary, challenges and future research directions

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6.1 Summary URFCs are a type of rapidly advancing electrochemical energy device technology that can be constructed though the combination of PEMFCs and water electrolyzers. This technology permits the uncoupling of power and energy aspects of the storage system and has been recognized as a new, highly efficient and non-polluting energy conversion and storage system that can meet the challenges of global warming and finite natural sources of fossil fuels. In the past several decades, various research efforts ranging from active catalyst particles to support materials have demonstrated their benefits in terms of their advanced nanostructures, their poison tolerance, and their physicochemical and electrochemical stability, providing different avenues for the research and development of high performance bifunctional oxygen catalysts. In this review, bifunctional catalysts for URFCs are comprehensively reviewed in terms of their design, synthesis, characterization, and performance validation. With respect to experimental studies, this review provides a systematic and comparative evaluation

on

catalyst

material

selection,

synthesis

methods,

structural

characterization and catalytic performance, as well as device validation. It is shown that before the exploration of next-generation advanced bifunctional oxygen catalysts, the relationship between structure and catalytic performance needs to be thoroughly explored. A more profound understanding of this relationship will lead to the synthesis of more efficient catalysts and significantly better catalytic performance. Based on various summarized and reviewed bifunctional catalysts that are classified as

carbon-supported

catalysts,

modified

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carbon-supported

catalysts,

noncarbon-supported catalysts, metal-based catalysts and metal-free catalysts, important points in this review can be emphasized as follows: (1) Based on the physicochemical interaction between support and catalyst and its effect on the bifunctionality of ORR/OER, modified carbon materials (i.e. doped carbon materials) and non-carbon materials (e.g., oxides and carbides) have been explored as a support substitution of pure carbon materials for bifunctional catalysts in URFCs. This is due to the fact that their advanced nanostructures and physicochemical properties can influence the size and electronic structure of the distributed catalyst particles and thus the catalyst performance. (2) Metal-based alloy catalysts, especially Pt-based alloy catalysts, tends to have an efficient bifunctionality for both ORR and OER under URFC operations. The high bifunctionality is related to important factors such as particle size and distribution, as well as their interactions with support materials. (3) Non-precious metal(s)-based catalysts and metal-free catalysts (e.g., doped carbon materials and oxide-based materials) have been recognized as promising

alternatives

to

Pt-based

bifunctional

catalysts

due

to

their

cost-effectiveness and comparable bifunctionality as well as their acceptable stability. (4) Modified carbon materials, particularly doped carbon materials with N, B, S, and/or P, can be used as support materials as well as good metal-free bifunctional catalysts. This is because the dopant, as a secondary phase, can induce structural defects therefore enhance electronic conduction and transfer, and produce a synergistic effect in the modified carbon framework, benefiting ORR/OER and the

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interaction between support and catalyst particles. (5) Among all non-carbon materials, oxide materials have emerged as promising bifunctional catalysts due to their unique physicochemical structure. Oxide materials as typical ceramic materials can be composited with carbon to form carbon-ceramic composites. These composites have shown some enhancements in the support’s stability and improvement in its electronic conduction, thus favoring the interaction between the oxide and the catalyst particles for catalytic activity.

6.2 Challenges In the past two decades, although considerable research efforts have been undertaken in the research and development of novel bifunctional catalysts, several major technological challenges still exists in the exploration of a new generation of bifunctional catalysts for URFC technology: (1) low catalyst activities for both ORR and OER, that can determine the energy power, density, and efficiency of URFCs; (2) insufficient stability/durability of catalysts, with low resistance to physicochemical corrosion, resulting in performance degradation; (3) poor product selectivity that is related to the design and operation of novel bifunctional catalysts and its URFCs; (4) low cost-effectiveness, which is a key step to the commercialization of bifunctional catalysts and URFCs; and (5) un-established scale-up capabilities for catalyst production, where practical large scale production of catalysts are required to produce the same high quality samples as on a laboratory scale.

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6.3 Future research directions In the research and development of advanced bifunctional catalysts for URFCs, a number of novel bifunctional catalysts have been explored, reported and achieved in literature. However, it seems that the level of maturity required of bifunctional catalysts for the commercialization of URFCs is still far from complete. This is due to the major technological challenges as described above. To overcome these challenges, future research directions may be proposed to design and synthesize next-generation of highly active bifunctional oxygen catalysts for URFCs as follows: (1) Enhancing catalytic activity and stability by creating innovative bifunctional catalysts. Whether support materials are included or not, breakthroughs in catalysis must be obtained for both ORR and OER. Withrespect to this, new material synthesis technologies are required to produce innovative new bifunctional catalysts with optimal performance for both ORR and OER. Support materials in supported catalysts should not only have some regular advantages such as large surface area (>100 m2 g-1), sufficient electrical conductivity (>0.1 S cm-1), high resistance to physicochemical and electrochemical corrosion, suitable porosity and reasonable proton conductivity, but also produce strong interactions between the support and the catalyst, favoring catalytic activities of both ORR and OER. Unsupported catalysts such as metal catalysts and metal-free catalysts should have more active sites in terms of catalytic performance, along with a tailored nanostructure to produce good synergistic effects. (2) Establishing a

more advanced

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fundamental understanding of

bifunctional oxygen catalysts for URFCs by combining pertinent experimental characterization with theoretical models and/or simulation calculations. This is because URFCs are the combination of a PEMFC and a water electrolyzer, in which ORR has the same importance as OER. For down-selecting bifunctional catalysts, designing and optimizing new catalyst structures related to the improvement of catalytic activity and stability/durability through a combination of experiments and theoretical modeling is critical. For instance, it is necessary to fundamentally understand the mechanisms of catalytic performance and their relationships with the catalyst’s active site structure and composition using both theoretical calculations (e.g., molecular/atomic modeling) and experimental characterizations to guide new catalyst research and development. To improve stability/durability, it is also important to understand the various catalysts’ degradation mechanisms and failure modes through a combination of experimental and theoretical approaches. Typically, both SEM and TEM should be used to investigate the morphology of catalysts before and after lifetime tests. With greater fundamental understanding, it is very possible to develop new durable catalysts with high activities for both ORR and OER. (3) Optimizing the design of electrodes and URFCs for practical application. Currently, the commercialization of URFC technology is hindered by major barriers such as low catalytic performance (i.e. low ORR/OER and stability/durability) which can lead to poor energy efficiency and the high cost of electrodes. It is understood that the catalyst’s activity and durability are related to not only the catalyst materials itself but also to its operational environment. So it is important to optimize the electrode

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design and the corresponding operating conditions. For example, the fabrication of MEAs should be effectively improved at an optimal status. It can be proposed that a creative strategy for thoughtful integration linking directly from the advanced catalyst synthesis to the MEA fabrication should be built in a scientific route where the catalyst and its URFC are optimized.

Author Information Corresponding Authors *

Dr. Yan-Jie Wang, School of Environment and Civil Engineering, Dongguan

University of Technology, No. 1, Daxue Rd, Songshan Lake, Dongguan, Guangdong Province, P.R. China. Tel: 86-769-22861199. E-mail: [email protected] *

Dr. Xiaomin Wang, College of Materials Science and Engineering, Taiyuan

University of Technology, Taiyuan 030024, China. Tel: 86-351-6018639. E-mail: [email protected] *

Dr. Jiujun Zhang, Institute for Sustainable Energy, Shanghai University, 99 Shangda

Rd, Baoshan, Shanghai, P.R. China & National Research Council of Canada, Vancouver, BC, V6T 1W5, Canada. Tel: 778-952-1663. E-mail: [email protected] Notes The authors declare no competing financial interests.

Acknowledgements

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The authors would like to acknowledge the contributions of all the members of Prof. Anna Ignaszak’s catalyst group in the Department of Chemistry at the University of New Brunswick.

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Vitae

Dr. Yan-Jie Wang obtained his M.S. in Materials from North University of China in 2002 and his Ph.D. in Materials Science & Engineering from Zhejiang University, China, in 2005. Subsequently, he conducted two and a half years of postdoctoral research at Sungkyunkwan University, Korea, followed by two years as a research scholar at Pennsylvania State University, USA, studying advanced functional materials. In April of 2009, he was co-hired by the University of British Columbia, Canada, and the National Research Council of Canada to research advanced materials. During November of 2012 and August of 2017, he worked as a senior research scientist for the University of British Columbia and Vancouver International Clean-Tech Research Institute Inc. (VICTRII), researching core–shell structured materials. From September of 2017, He started as a full professor in the School of Environment and Civil Engineering at the Dongguan University of Technology. Also, he is an adjunct professor at Fuzhou University in China. Dr. Wang has published 60 papers in peer-reviewed journals, conference proceedings and industry reports. His research interests include electrochemistry, electrocatalysis, polymer materials and nanostructured material synthesis, characterization, and application in energy storage and conversion, biomass engineering, and medical areas.

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Dr. Baizeng Fang received his Ph.D. in Materials Science from the University of Science and Technology Beijing in 1997. He worked as a postdoctoral fellow (PDF) at the Energy Research Centre of the Netherlands in 1998; as a JSPS (Japan Society for the Promotion of Science) research fellow and guest researcher from 2000 to 2004 at the Institute of Research and Innovation, Japan; as a PDF from 2004 to 2005 at the National Institute of Advanced Industrial Science and Technology Japan, and as a Lise Meitner Scientist at the Graz University of Technology, Austria from 2005 to 2006. He worked as a PDF at Hannam University, Korea from September 2006 to February 2008, and as a research professor from March 2008 to March 2010 at Korea University. Since July 2010, he has been working as a senior research scientist at the University of British Columbia, Canada. Dr. Fang has published around 100 research papers in peer-reviewed journals including J. Am. Chem. Soc., Acc. Chem. Res. and Chem. Rev. His research interests include the development of novel nanostructured materials for energy conversion and storage, and photo/electrocatalysis. He also serves as an associate editor for RSC Advances.

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Dr. Xiaomin Wang is currently a professor in the College of Materials Science and Engineering and director of Development Planning Department, Taiyuan University of Technology. She received her Ph.D. (2005) in Materials Processing from Taiyuan University of Technology and she was the nominee of the “National Excellent 100 Doctoral Dissertation”. After her Ph.D. she conducted research functional carbon materials as a carrier in the field of energy and biological applications and further her research in Japan, German and British. Her current research are nanomaterials synthesis and applications in energy storage.

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Dr. Anna Ignaszak is an assistant professor at the Department of Chemistry, University of New Brunswick and the adjunct assistant professor in the Institute of Organic and Macromolecular Chemistry at the Friedrich-Schiller University (Jena, Germany, Carl-Zeiss junior professorship holder since May 2012), after completing her appointment as a research associate at the Clean Energy Research Center (CERC), The University of British Columbia (Vancouver, Canada), and as a research associate at the National Research Council of Canada, NRC – Institute for Fuel Cell Innovation (Vancouver, Canada). She has a diverse background in materials for electrochemical energy storage and conversion, electrochemical sensors, functional materials (carbons, composites, metal clusters) and heterogeneous catalysis. The research conducted in her labs in Canada and Germany aim to synthesize

morphology-controlled

nano-catalysts,

understanding the

structure-reactivity interplay for the optimum redox activity, electrochemical characterization, and electrode engineering. She spent also several years working in industrial R&D (ABB Corporate Research, Pliva Pharmaceutical Company, Ballard Power Systems Inc.).

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Dr. Yuyu Liu is a Professor at Institute for Sustainable Energy Storage and Conversion / College of Science, Shanghai University, China (Since July, 2016) and also a Distinguished Professor (Hundred Talent Program of Shanxi Province, China) at College of Environmental Science and Engineering, Taiyuan University of Technology, Taiyuan, China (Since December, 2015). Dr. Liu received his Ph.D. in Environmental Engineering from Yamaguchi University, Japan in 2003. He then worked at Kyushu Environmental Evaluation Association, Osaka Institute of Technology, Tokyo University of Agriculture and Technology, Yokohama National University, Tohoku University, Japan, as Postdoctoral, Research Fellow, Assistant Professor and Associate Professor. Dr. Liu has more than 10 years of experience in the environment science and technology, particularly in the areas of air quality monitoring (e.g. PM 2.5), wastewater treatment and soil pollution analysis and remediation, and their associated instrument development. Recently, his research interest is moving to the electrochemical, photochemical and photoelectrochemcial reduction of CO2 to low-carbon fuels.

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Dr. Aijun Li is currently a Professor at the School of Materials Science and Engineering at the Shanghai University (SHU). He received his Ph.D. in Materials Science from Northwestern Polytechnical University, Xi’an China in 2004. Dr. Li worked as a senior scientist and then a group leader of carbon materials at the Karlsruhe Institute of Technology (KIT), Germany from 2015 to 2010, involved with the research, development and application of composites. He has over 15 years of research experience in the areas of chemically reacting flows. He is now the Vice-President of Shanghai Society of Composite Materials and a Member of Youth Committee of Chinese Society for Composite

aterials. Dr. Li’s main research interests are in the complex

interaction of multi-physical and chemical phenomena involved in chemically reacting flows, mainly focusing on modeling, simulation and synthesis of composites by chemical vapor infiltration/deposition processes.

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Dr. Lei Zhang is a Research Council Officer at National Research Council of Canada (NRC), an adjunct Professor of Federal University of Maranhao and Zhengzhou University, and a vice president of International Academy of Electrochemical Energy Science. Prof. Zhang received her Bachelor’s degree in

aterials Science and Engineering (1990) from Wuhan University of

Technology in China, first in China and her second

aster’s degree in

aterials Chemistry (1993) from Wuhan University

aster’s degree in Physical Chemistry (2000) from Simon Fraser

University in Canada. Following her second Master degree, Prof. Zhang was appointed as a principle investigator at Molecular Membrane Technologies Inc., Vancouver (2001-2004). In 2004, Prof. Zhang joined NRC to help initiate the PEM Fuel Cell program. As a key technical player she helped NRC develop novel PEM Fuel Cell catalysts and testing capabilities. She has furthered fundamental understanding of PEM Fuel Cells. Prof. Zhang has also carried out R&D of other electrochemical energy technologies, such as supercapacitors, metal-air batteries, and hybrid batteries.

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Dr. Jiujun Zhang is a Principal Research Officer at the National Research Council of Canada (NRC), and a Guest Professor at Shanghai University. Dr. Zhang’s expertise areas of Dr. Zhang are Electrochemistry, Photoelectrochemistry, Spectroelectrochemistry, Electrocatalysis, Fuel cells (PEMFC, SOFC, and DMFC), Batteries, and Supercapacitors. Dr. Zhang received his B.S. and M.Sc. in Electrochemistry from Peking University in 1982 and 1985, respectively, and his Ph.D. in Electrochemistry from Wuhan University in 1988. Starting in 1990, he carried out three terms of postdoctoral research at the California Institute of Technology, York University, and the University of British Columbia. Dr. Zhang holds more than 14 adjunct professorships, including one at the University of Waterloo, and one at the University of British Columbia.

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Figures Fig. 1. Schematic of the unitized regenerative fuel cell (URFC) and the corresponding bifuncitonal catalyst layer electrodes. Fig. 2. TEM micrograph (left) and Pt size (right) distribution of catalysts with weight ratio of (a) 10 wt%, (b) 20wt%, and (c) 30wt%, respectively. Reprint permission from Ref. [27]. Fig. 3. (a) TEM image of CoMn2O4/rGO. (inset) Particle size distribution histogram. (b) High-resolution TEM image of CoMn2O4 NDs. Reprint permission from Ref. [40]. Fig. 4. (a) XRD patterns of cCo3O4 and cCo3O4/MWCNT. (b) SEM and (c) TEM images of cCo3O4/MWCNT. Modified with permission from Ref. [48]. Fig. 5. The crystal structure of perovskite oxides with ABO3 stoichiometry (a). The same structure as in (a) but showing the BO6 octahedral network (b). Reprint permission from Ref. [56]. Fig. 6. OER (a) and ORR (b) polarizaiton curves for MnO2, LaCoO3, LaNiO3, MnO2-LaCoO3, and MnO2-LaNiO3 materials supported on VC support. (note: Hg/HgO/0.1M KOH is abbreviated as MOE). Reprint permission from Ref. [72]. Fig. 7. (a) XPS spectrum of Co/N-C-800, the inset is the high-resolution spectrum of N 1s; high- resolution spectra of (b) C 1s, (c) Co 2p, and (d) O 1s. Reprint permission from Ref. [95]. Fig. 8. (a) LSVs and (b) corresponding K–L plots of Fe/Fe3C@NGL-NCNT at various speeds. (c) Kinetic limiting current densities and corresponding electron-transfer numbers of different samples at -0.6 V. (d)(e) Current-time (i-t) chronoamperometric

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response of Fe/Fe3C@NGL-NCNT and commercial 20wt% Pt/C electrodes at -0.4 V and 0.7 V (vs Ag/AgCl) in O2-saturated 0.1 M KOH at a rotation rate of 1600 rpm for the ORR and OER, respectively. Modified with permission from Ref. [109]. Fig. 9. Cyclic voltammograms (CVs) of Co3O4 nanocrystal, rGO, NrGO, Co 3O4/rGO and Co3O4NrGO in oxygen (solid) or argon (dash) saturated 0.1M KOH. Reprint permission from Ref. [33]. Fig. 10. (a) A soft-template mediated synthesis process, (b) SEM, (c,d) TEM images, and (e) high-resolution TEM images for Co 0.5Fe0.5S/N-MC composite annealed at 900oC . Modified with permission from Ref. [152]. Fig. 11. (a) E(j) curves of Option 2 and Option 1 membrane electrode assemblies. (fuel cell mode: cell temperature 358 K, ambient pressure, gases fully humidified, hydrogen flow 0.4 l min−1, oxygen flow 0.4 l min−1; electrolysis mode: cell temperature 368 K, ambient pressure; no flows); (b) E(j) curves of Option 3 membrane electrode assemblies: Option 3.1× with squares and Option 3.2 with stripes (black: Pt, grey: IrO2). (fuel cell mode: cell tem-perature 358 K, ambient pressure, gases fully humidified, hydrogen flow 0.4 l min−1, oxygen flow 0.4 l min−1; electrolysis mode: cell temperature 368 K, ambient pressure; no flows); (c) E(j) curves of best performing membrane electrode assemblies in fuel cell mod for the investigated electrode configurations. (Fuel cell mode: cell temperature 358K, ambient pressure, gases fully humidified, hydrogen flow 0.41 min -1, oxygen flow 0.41 min-1; electrolysis mod: cell temperature 368K, ambient pressure; no flows). Reprint permission from Ref. [192].

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Fig. 12. (a) XRD patterns of synthsized CaMnO3, Pt/CaMnO3, and H-Pt/CaMnO3 samples. (b-d) SEM image (b) and TEM images (c,d) of H-Pt/CaMnO3. The insets of (b) and (d) show the size distribution of CaMnO3 nanoparticles and Pt clusters, respectively. The circles in (d) mark some of Pt clusters. (e) Elemental mapping and EDS spectra of H-Pt/CaMnO3. Reprint permission from Ref. [80]. Fig. 13. Transmission electron micrographs of three catalysts: (a) Pt3Ir1/TiC, (b) Pt3Ir1/TiCN, and (c) Pt3Ir1/TiN. Modified with permission from Ref. [226]. Fig. 14. (a) FTIR spectra of CoMn2O4, CoMn2O4/PEDOT, PEDOT. Repetitive cycling voltammograms of (b) CoMn2O4/PEDOT and (c) Pt/C in Ar-saturated 0.1 M KOH solution at 100 mV s-1 scan rate. Modified with permission from Ref. [229]. Fig. 15. (a) TEM image of Ir nanodendrites, (b) TEM and (inset) HRTEM images of Ir67@Pt33 nanodendrites, (c) HAADF-STEM image and elemental maps and line profiles of Ir67@Pt33, (d) TEM images of Ir57@Pt43 physical mixture and (inset) Pt and Ir blacks. The scale bars unmarked are 50 nm. Reprint permission from Ref. [242]. Fig. 16. SEM image of nano-silver powder. Reprint permission from Ref. [249]. Fig. 17. Structural characterization of N-CNTs: (a) Raman spectra of as synthesized N-CNTs at 850 oC, and (b) Variation of Id/Ig and fwhmD/fwhmG with diameter and precursors. Comparison of electrochemical activity of four N-CNTs synthesized by different precursors: (c) steady-state polarization curves, and (d) the dependence of onset potential on N distribution and tube diameter. N1, N2, and N3 stand for pyridinic, pyrrolic, and graphitic nitrogen, respectively. Modified with permission from Ref. [267].

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Fig. 18. One step synthesis of N-doped SWCNT/graphene hybrids (NGSHs). Reprint permission from Ref. [283]. Fig. 19. (a) Illustration of the fabrication process and structure, SEM (b) and TEM (c, d) images of N,P-GCNS electrocatalyst. Modified with permission from Ref. [304]. Fig. 20. Illustration of Co 3O4/Co2MnO4 nanocomposites derived from a CoMn-LDH single-source precursor and their electrocatalytic oxygen reactions. Reprint permission from Ref. [341].

Fig. 1. Schematic of the unitized regenerative fuel cell (URFC) and the corresponding bifuncitonal catalyst layer electrodes.

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Fig. 2. TEM micrograph (left) and Pt size (right) distribution of catalyst with weight ratio of (a) 10 wt%, (b) 20wt%, and (c) 30wt%. Reprint permission from Ref. [27].

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(a)

(b)

Fig. 3. (a) TEM image of CoMn2O4/rGO. (inset) Particle size distribution histogram. (b) High-resolution TEM image of CoMn2O4 NDs. Reprint permission from Ref. [40].

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(a)

(b)

(c)

Fig. 4. (a) XRD patterns of cCo 3O4 and cCo3O4/MWCNT. (b) SEM and (c) TEM images of cCo3O4/MWCNT. Modified with permission from Ref. [48].

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Fig. 5. The crystal structure of perovskite oxides with the ABO3 stoichiometry (a). the same structure as in (a) but showing the BO6 octahedral network (b). Reprint permission from Ref. [56].

- 209 -

(a)

(b)

Fig. 6. OER (a) and ORR (b) polarizaiton curves for MnO2, LaCoO3, LaNiO3, MnO2-LaCoO3, and MnO2-LaNiO3 materials supported on VC support. (note: Hg/HgO/0.1M KOH is abbreviated as MOE). Reprint permission from Ref. [72].

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Fig. 7. (a) XPS spectrum of Co/N-C-800, the inset is the high resolution spectrum of N 1s; high- resolution spectra of (b) C 1s, (c) Co 2p, and (d) O 1s. Reprint permission from Ref. [95].

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Fig. 8. (a) LSVs and (b) corresponding K–L plots of Fe/Fe3C@NGL-NCNT at various speeds. (c) Kinetic limiting current densities and corresponding electron-transfer numbers of different samples at -0.6 V. (d)(e) Current-time (i-t) chronoamperometric response of Fe/Fe3C@NGL-NCNT and commercial 20wt% Pt/C electrodes at -0.4 V and 0.7 V (vs Ag/AgCl) in O2-saturated 0.1 M KOH at a rotation rate of 1600 rpm for the ORR and OER, respectively. Modified with permission from Ref. [109].

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Fig. 9. CVs of Co3O4 nanocrystal, rGO, NrGO, Co 3O4/rGO and Co3O4NrGO in oxygen(solid) or argon (dash) saturated 0.1M KOH. Reprint permission from Ref. [33].

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Fig. 10. (a) A soft-template mediated synthesis process, (b) SEM, (c,d) TEM images, and (e) high-resolution TEM images for Co 0.5Fe0.5S/N-MC composite annealed at 900oC . Modified with permission from Ref. [152].

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(a)

(b)

- 215 -

(c)

Fig. 11. (a) E(j) curves of Option 2 and Option 1 membrane electrode assemblies. (fuel cell mode: cell temperature 358 K, ambient pressure, gases fully humidified, hydrogen flow 0.4 l min−1, oxygen flow 0.4 l min−1; electrolysis mode: cell temperature 368 K, ambient pressure; no flows); (b) E(j) curves of Option 3 membrane electrode assemblies: Option 3.1× with squares and Option 3.2 with stripes (black: Pt, grey: IrO2). (fuel cell mode: cell tem-perature 358 K, ambient pressure, gases fully humidified, hydrogen flow 0.4 l min−1, oxygen flow 0.4 l min−1; electrolysis mode: cell temperature 368 K, ambient pressure; no flows); (c) E(j) curves of best performing membrane electrode assemblies in fuel cell mod for the investigated electrode configurations. (Fuel cell mode: cell temperature 358K, ambient pressure, gases fully humidified, hydrogen flow 0.41 min-1, oxygen flow 0.41 min-1; electrolysis mod: cell temperature 368K, ambient pressure; no flows). Reprint permission from Ref. [192].

- 216 -

- 217 -

Fig. 12. (a) XRD patterns of synthsized CaMnO3, Pt/CaMnO3, and H-Pt/CaMnO3 samples. (b-d) SEM image (b) and TEM images (c,d) of H-Pt/CaMnO3. The insets of (b) and (d) show the size distribution of CaMnO3 nanoparticles and Pt clusters, respectively. The circles in (d) mark some of Pt clusters. (e) Elemental mapping and EDS spectra of H-Pt/CaMnO3. Reprint permission from Ref. [80].

- 218 -

(a)

(b)

(c)

Fig. 13. Transmission electron micrographs of three catalysts: (a) Pt3Ir1/TiC, (b) Pt3Ir1/TiCN, and (c) Pt3Ir1/TiN. Modified with permission from Ref. [226].

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Fig. 14. (a) FTIR spectra of CoMn2O4, CoMn2O4/PEDOT, PEDOT. Repetitive cycling voltammograms of (b) CoMn2O4/PEDOT and (c) Pt/C in Ar-saturated 0.1 M KOH solution at 100 mV s-1 scan rate. Modified with permission from Ref. [229].

- 220 -

- 221 -

Fig. 15. (a) TEM image of Ir nanodendrites, (b) TEM and (inset) HRTEM images of Ir67@Pt33 nanodendrites, (c) HAADF-STEM image and elemental maps and line profiles of Ir67@Pt33, (d) TEM images of Ir57@Pt43 physical mixture and (inset) Pt and Ir blacks. The scale bars unmarked are 50 nm. Reprint permission from Ref. [242].

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Fig. 16. SEM image of nano-silver powder. Reprint permission from Ref. [249].

- 223 -

- 224 -

Fig. 17. Structural characterization of N-CNTs: (a) Raman spectra of as synthesized N-CNTs at 850 oC, and (b) Variation of Id/Ig and fwhmD/fwhmG with diameter and precursors. Comparison of electrochemical activity of four N-CNTs synthesized by different precursors: (c) steady-state polarization curves, and (d) the dependence of onset potential on N distribution and tube diameter. N1, N2, and N3 stand for pyridinic, pyrrolic, and graphitic nitrogen, respectively. Modified with permission from Ref. [267].

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Fig. 18. One step synthesis of N-doped SWCNT/graphene hybrids (NGSHs). Reprint permission from Ref. [283].

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Fig. 19. (a) Illustration of the fabrication process and structure, SEM (b) and TEM (c, d) images of N,P-GCNS electrocatalyst. Modified with permission from Ref. [304].

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Fig. 20. Illustration of Co 3O4/Co2MnO4 nanocomposites derived from a CoMn-LDH single-source precursor and their electrocatalytic oxygen reactions. Reprint permission from Ref. [341].

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7Tables

Table 1. ORR and OER values of perovskite oxides with and without carbon. Modified with permission from Ref. [54]. Table 2. Electrochemical parameters of OER and ORR activities (e.g., onset potential, mass activity, Tafel slope, ΔE) for Pt/C, BSCF/C, PBC/C, LN/C, and Pt/C-oxide composites. Modified with permission from Ref. [3]. Table 3. Different nitrogen species in Fe/N-C, Co/N-C, and N-C catalysts obtained by the deconvoluted N1s XPS spectra. Modified with permission from Ref. [96]. Table 4. Composition of metals in Co-Mn-oxide/N-CNT, Co-Mn-oxide/N-CNT-300, Co-Mn- oxide/N-CNT-400, Co-Mn-oxide/N-CNT-500, and Co-Mn-oxide/N-CNT-600 composites obtained by ICP-OES. Modified with permission from Ref. [125]. Table 5. Three fabricated catalyst options at the oxygen electrode for the tested membrane electrode assembles. Modified with permission from Ref. [192]. Table 6. Synthesis method, electrolyte, ORR potential, Half-wave ORR potential, ORR potential at -3 mA cm-2, OER potential at 10 mA cm-2) and the potential difference (ΔE) of typical bifunctional oxygen catalysts for URFC application. Table 7. Synthesis method, catalyst loading at the oxygen electrode, and URFC performance for typical Pt-based bifunctional oxygen catalysts. Table 8. Synthesis method, catalyst loading at the oxygen electrode, and URFC performance for two typical non-precious metal bifunctional oxygen catalysts.

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Table 1. ORR and OER values of perovskite oxides with and without carbon. Modified with permission from Ref. [54].

Oxide

ORR current density at 0.426 V

OER current density at 1.626 V vs.

vs. RHE (mA cm-2)

RHE (mA cm-2)

Without carbona

With carbonb

Without carbona

With carbonb

LaMnO3

0.5

150

0.15

5

La0.6Sr0.4FeO3

0.3

125

0.65

10

LaNiO3

1.0

120

22.5

110

La0.5Sr0.5CoO3

1.0

165

40.0

110

a

Slurr-coated electrocatalyst layer; Perovskite: PTFE = 90:10 (wt%).

b

Rolled-sheet electrocatalyst layer; Perovskite: C65: PTFE = 40:40:20 (wt%).

- 230 -

Table 2. Electrochemical parameters of OER and ORR activities (e.g., onset potential, mass activity, Tafel slope, ΔE) for Pt/C, BSCF/C, PBC/C, LN/C, and Pt/C-oxide composites. Modified with permission from Ref. [3].

Samples Pt/C BSCF/C PBC/C LN/C Pt/C to BSCF/C = 4:1 Pt/C to BSCF/C = 1:1 Pt/C to BSCF/C = 1:4 Pt/C to PBC/C = 4:1 Pt/C to PBC/C = 1:1 Pt/C to PBC/C = 1:4 Pt/C to LN/C = 4:1 Pt/C to LN/C = 1:1 Pt/C to LN/C = 1:4

Onset potential (V vs. AgCl/Ag) -0.022 -0.251 -0.205 -0.273 -0.066 -0.069 -0.099 -0.053 -0.071 -0.091 -0.068 -0.098 -0.114

ORR Tafel slope (mV dec-1) 69 NA NA NA 41 59 75 39 72 70 37 46 55

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-1

Jk (Ag Pt) at -0.2 V 134 NA NA NA 130 161 191 128 240 292 71 165 192

OER Tafel slope Jk (Ag-1total) (mV dec-1) at 0.65 V 184 1.60 79 12.15 120 3.31 184 1.07 70 17.49 67 31.77 70 24.89 106 5.52 109 4.90 116 3.68 167 1.39 147 2.54 161 1.56

ΔE (V) 1.02 1.14 1.21 1.33 0.80 0.80 0.83 0.85 0.88 0.91 1.03 0.99 1.06

Table 3. Different nitrogen species in Fe/N-C, Co/N-C, and N-C catalysts obtained from the deconvoluted N1s XPS spectra. Modified with permission from Ref. [96].

Catalyst

N1 (Pyridinic-N)

N2 (Quaterary-N)

N3 (Quaterary-N-O)

LaMnO3

52.2

36.8

5.3

La0.6Sr0.4FeO3

64.4

35.5

0.0

La0.5Sr0.5CoO3

54.8

36.9

8.3

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Table 4. Composition of metals in Co-Mn-oxide/N-CNT, Co-Mn-oxide/N-CNT-300, Co-Mn- oxide/N-CNT-400, Co-Mn-oxide/N-CNT-500, and Co-Mn-oxide/N-CNT-600 composites obtained by ICP-OES. Modified with permission from Ref. [125].

Catalyst

Mn (wt%)

Co (wt%)

Mg (wt%)

Al (wt%)

Co-Mn-oxide/N-CNT

0.59

1.12

0.12

0.73

Co-Mn-oxide/N-CNT-300

0.62

1.16

0.12

0.80

Co-Mn- oxide/N-CNT-400

1.90

3.44

0.32

1.65

Co-Mn- oxide/N-CNT-500

2.73

4.50

0.56

2.37

Co-Mn-oxide/N-CNT-600

3.98

6.52

0.71

3.21

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Table 5. Three fabricated catalyst options at the oxygen electrode for the tested membrane electrode assembles. Modified with permission from Ref. [192].

Name Option 1 Option 2.1 Option 2.2 Option 3.11 Option 3.12 Option 3.2

Hydrogen Loading Catalyst (mg cm-2) Pt black 1.56 Pt black 0.70 Pt black 0.70

Oxygen Structure

IrO2 + Pt (1:1) Pt and IrO2 IrO2 and Pt

Loading (mg cm-2) 1.61 0.93/0.78 0.78/0.87

Mixed Multilayer (Pt inside) Multilayer (IrO2 inside)

Catalyst

Pt black

0.87

IrO2/Pt

0.73/0.73

Segmented (square)

Pt black

0.87

IrO2/Pt

0.73/0.73

Segmented(stripes)

- 234 -

Table 6. Synthesis method, electrolyte, Half-wave ORR potential, ORR potential at -3 mA cm-2, OER potential at 10 mA cm-2) and the potential difference (ΔE) of typical bifunctional oxygen catalysts for URFC application.

No.

Sample

1

Pt/C to BSCF/C=4:1 Pt/C to BSCF/C=1:1 Pt/C to BSCF/C=1:4 BSCF/C Pt/C Co3O4/NrGO LCMO/NrGO 20wt% Ir/C Mn2O3 20wt% Ru/C 20wt% Pt/C Co/N-C-800 20wt% Pt/C Fe/C/N Co/C/N C/N 20wt% IrO2/C 20wt% Pt/C CoMn2O4/PDDA-C NTs Co3O4/MWCNT Mn-oxide

2 3 4

5 6

7 8 9

Synthesis method UM UM UM HR HR ALD SC

Electrolyte

SC SC SC SC

0.1 M KOH 0.1 M KOH 0.1 M KOH 0.1 M KOH 0.1 M KOH 0.1 M KOH 0.1 M KOH 0.1 M KOH 0.1 M KOH 0.1 M KOH 0.1 M KOH 0.1 M KOH 0.1 M KOH KOH (pH=13) KOH (pH=13) KOH (pH=13) KOH (pH=13) KOH (pH=13) 0.1 M KOH

HR PE

0.1 M KOH 0.1 M KOH

ORR: Half-wave potential (V) -

0.83 0.80 0.78 0.73 0.86 -

ORR: potential (V) OER: potential (V) (at -3 mA cm-2) (at 10 mA cm-2) 0.81 1.61 0.79 1.59 0.76 1.59 0.49 1.63 0.83 1.85 0.85 1.54 0.782 1.742 0.612 1.698 0.71 1.81 0.62 1.62 0.83 2.02 (1.88) 0.74 1.599 0.784 1.857 1.59 1.61 1.60 1.60 1.76 -0.133 0.716 0.58 0.73

-

- 235 -

1.62 1.77

ΔE (V) (OER-ORR) 0.80 0.80 0.83 1.14 1.02 0.69 0.960 1.086 1.10 1.00 1.19 (1.05) 0.859 1.073 0.76 0.80 0.80 0.76 0.90 0.849 1.04 1.04

Year, References 2014, 3

2011, 33/304 2015, 55 2012, 74

2014, 95 2015, 96

2013, 99 2013, 106 2010, 108

10 11

MnCo2O4/N-CNFs Co3O4@BDHC3

12

Co3O4/N-csCNT-GN R MnOx/S-GC NiCo2S4/NS-rGO CoS2(400)/NSG 20wt% PtRu/C 20wt% Pt/C N-graphene/CNT

13 14 15

16 17

18 19 20 21 22

23

Co/MIL-101(Cr)-R Co/MIL-101(Cr) Co/MIL-101(Cr)-O NGSHs N,P-GCNS meso-Co3O4-35 Ni0.4Co2.56O4 20wt% Ir/C Co3O4/2.7Co2MnO4 Co3O4 Co2MnO4 20wt% Pt/C Cs-MnOx-450 MnOx-450 20wt% Pt/C

SC Template-assis ted SC Microwave-as sisted HR SC SR SC One-pot synthesis SR SR SR CVD SC NC In situ preparation TDE Template-assis ted IMA Template-assis ted IMA -

0.1 M KOH 0.1 M KOH

0.83

-

1.59

1.04 0.76

2014, 117 2014, 129

0.1 M KOH

-

0.79

1.59

0.80

2015, 130

0.1 M KOH 0.1 M KOH 0.1 M KOH 0.1 M KOH 0.1 M KOH 0.1 M KOH

-

0.81 0.72 0.79 0.74 0.86 0.69

1.62 1.70 1.61 1.62 0.86 1.65

0.81 0.98 0.82 0.88 1.16 0.96

2015, 131 2013, 148 2015, 151

0.1 M KOH 0.1 M KOH 0.1 M KOH 0.1 M KOH 0.1 M KOH 0.1 M KOH 0.1 M KOH 0.1 M KOH 0.1 M KOH 0.1 M KOH 0.1 M KOH 0.1 M KOH 0.1 M KOH

-0.33 -0.38 -0.41 -

0.70 0.86 0.60 0.80 0.93 0.68 0.49 0.59 0.80 0.87

0.94 0.85 0.75 1.63 1.57 1.634 1.76 1.85 1.77 1.83 1.92 2.08 1.65

1.27 1.23 1.16 1.06 0.71 1.034 0.96 0.92 1.09 1.34 1.33 1.28 0.78

2015, 234

0.1 M KOH

-

0.64

1.72

1.08

0.1 M KOH

-

0.84

2.01

1.17

- 236 -

2014, 172

2014, 283 2015, 304 2013, 331 2015, 339 2013, 341

2016, 343

24

CCH-2/C HR 0.1 M KOH 0.82 1.74 0.92 2014, 357 Note: UM = Ultrasonic mixing, ALD = Atomic layer deposition; SC = Solvothermal carbonization, HR = Hydrothermal reaction, SR = Solvothermal reaction, PE = Potentiostatic electrodeposition, NC = Nanocasting, TDE = Thermal decomposition and electrodeposition, IMA = Inverse micelle approach, CVD = Chemical vapor deposition,

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Table 7. Synthesis method, catalyst loading at the oxygen electrode, and URFC performance for typical Pt-based bifunctional oxygen catalysts.

No.

Sample

Synthesis method

Oxygen electrode (loading, mg cm-2) 1 (Pt loading)

1

PtIr/C (Ir=20wt% Pt)

Colloid deposition

2

Pt/C (20wt% Pt)

Ultrasonic technique

0.25 (catalyst loading)

3

Pt/IrO2 (50wt% Pt)

Mixture

0.2 (catalyst loading)

4

Pt85Ir15/TiO2, Pt85Ir15

Physical mixture -

1.0 (metal loading)

5

Pt/IrO2 (50wt% Pt)

Physical mixture

0.2 (catalyst loading)

URFC performance Five cycles under operation: (1) Electrolysis mode: best current density of 600 mA cm-2 at 2.2 V when temperature was fixed at 65 oC; (2) Fuel cell mode: current was very stable at temperature of 30 – 80 oC; (3) During cycling, performance of fuel cell and electrolyzer declined. Three cycles under operation: (1) Electrolysis mode: 100 mA cm-2 at 1.67V, 300 mA cm-2 at 1.8 V; (2) Fuel cell mode: optimal power output of 190 mW cm-2 at 404 mA cm-2; (3) Initial energy conversion efficiency of the round-trip operation about 37.5% at 100 mA cm-2. Cycle performance showed an obvious degradation. (1) Fuel cell mode: 400 mA cm-2 at 0.7 V at 80 oC with a H2/O2 pressure of 0.3 MPa; (2) Electrolysis mode: 400 mA cm-2 at 1.71 V at 80 oC with an ambient pressure; (3) in tested time, the URFC performance was reproducible and stable. (1) Fuel cell mode: for Pt 85Ir25/TiO2, a maximum power density of 0.93 W cm-2 at 0.60 V, coupled with a current density of 1.38 A cm-2; (2) Electrolysis mode: the performance of supported Pt 85Ir25/TiO2 was higher than the values for Pt85Ir25; (3) At 1.0 mg cm-2, the round-trip energy conversion efficiency of Pt 85Ir25/TiO2 was 42.2%, higher than that of unsupported Pt 85Ir25 (30%); Cycle life of both fuel cell mode (at 300 mA cm-2 and 400 mA cm-2) and electrolysis mode showed a fairly performance constant over 25

- 238 -

Year, References 1993, 35

2012, 27

1999, 77

2012, 214; 2010, 215

2004, 195

6

Pt/IrO2 (70wt% Pt)

Physical mixture

3 (catalyst loading)

7

IrO2+Pt (1:1) Pt and IrO2 IrO2 and Pt IrO2/Pt IrO2/Pt IrO2/Pt Pt/IrO2 (7/93, 11/89, 14/86)

Physical mixture Multilayer (Pt inside) Multilayer (IrO2 inside) Segmented Segmented (square) Segmented (stripes) Incipient wetness technique

1:1 0.93/0.78 0.78/0.87 0.73/0.73 0.73/0.73 0.73/0.73 6 (catalyst loading)

9

Pt+5wt%Pt/IrO2 (100:100)

Chemical reduction

1 (catalyst loading)

10

(RuO2-IrO2)/Pt (25:25:50)

Colloid deposition

2 (catalyst loading)

11

Pt balck, PtIr, PtRuOx, PtRu, PtRuIr, PtIrOx

Commercial Colloid deposition Mixing Commercial Colloid deposition Mixing

4 (metal loading)

8

cycles. (1) Electrolysis mode: 1.62 V at 1 A cm-2 and 1.8 V at 2 A cm-2 ; (2) Fuel cell mode: 900 mA cm-2 at 0.52 V; (3) During 20 cycles, cycle performance showed a slight performance loss. (1) Electrolysis mode: the IrO2+Pt catalyst showed the best performance (e.g., at 1.6 V, current density of 930 mA cm-2); (2) Fuel cell mode: the IrO2/Pt catalyst (segmented-stripes) exhibited the best performance (e.g., at 0.4 V, current density of 900 mA cm-2).

(1) The IrO2/Pt (14/86) had the highest performance for water electrolysis (e.g., at 1.6V, the density current of 350 mA cm-2). (2) The IrO2/Pt (7/93) presented the highest performance for fuel cell (e.g., at 0.4 V, the density current of 350 mA cm-2). (1) Fuel cell mode: the highest power density was 1160 mW cm-2 at 2600 mA cm-2, coupled with a potential of 0.45 V; (2) Electrolysis mode: 1000 mA cm-2 at 1.6 V. (1) At 80 oC, the terminal voltages of fuel cell/electrolysis of URFC were 0.76 and 1.52 V at 400 mA cm-2. (2) At 60 and 70 oC, the the terminal voltages of fuel cell/electrolysis of URFC at 400 mA cm-2 were 0.71V/1.56 and 0.73V/1.53 V, respectively. (3) During 10 cyclic tests, the average terminal voltages of fuel cell/electrolysis of URFC were 0.73/1.53 V at 400 mA cm-2 and 0.67/1.56 V at 500 mA cm-2. (1) Fuel cell performance: Pt black > PtIr > PtRuOx > PtRu ~ PtRuIr > PtIrOx; (2) Electrolysis performance: PtIr ~ PtIrOx > PtRu > PtRuIr > PtRuOx ~ Pt black; (3) PtIr exhibited the best URFC performance with the highest round-trip efficiency and stability during the cycling operation of the URFC system. For instance, at 200 and 500 mA cm-2, the PtIr delivered the round-trip efficiency of 53% and 46%,

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2006, 194

2009, 193; 2011, 192

2011, 197

2010, 208

2007, 207

2004, 358

12

PtxIry (x:y = 100:0, 85:15, 70:30, and 40:60)

Physical mixture

4 (catalyst loading)

respectively. (1) At the current densities of 500 and 1000 mA cm-2, the Pt85Ir15 catalyst showed the highest round-trip energy conversion efficiency of 49% and 41%, respectively. (2) At 500 mA cm-2, the cycle performance of the URFC with Pt 85Ir15 catalyst was stable for 120 hrs.

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2009, 240

Table 8. Synthesis method, catalyst loading at the oxygen electrode, and URFC performance for two typical non-precious metal bifunctional oxygen catalysts.

No.

Sample

Synthesis method

1

MnOx on a stainless steel sbustrate

Electrodeposition

2

Cu0.6Mn0.3Co2.1O4

Heat treatment method

Oxygen electrode (loading, mg cm-2) 0.3 (catalyst loading)

3 (catalyst loading)

URFC performance For the MnOx catalyst, (1) the peak power density at first cycle was 27 mW cm-2. After 4 cycles, it dropped to 26 mW cm-2 ; (2) after 10 cycles, the current density of 60 mA cm-2 at 1.75 V was maintained at the electrolyzer mode while the current density dropped from 57 mA cm-2 to 43 mA cm-2 at the fuel cell mode (at 0.45 V); (3) the round-trip efficiency at 20 mA cm-2 in both modes decreased from 45% to 42% over 10 cycles. (1) Fuel cell mode: the peak power density was over 80 mW cm-2; (2) Electrolyzer mode: the onset voltage was about 1.55 V. (3) At 100 mA cm-2, the voltages of fuel cell mode and electrolyzer mode were 0.58 and 1.82 V, respectively. The fuel cell to electrolyzer voltage ration at 100 mA cm-2 achieved ca. 31.8%.

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Year, References 2014, 344

2012, 252