Recent developments in bifunctional air electrodes for unitized regenerative proton exchange membrane fuel cells – A review

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 3 ( 2 0 1 8 ) 2 1 4 7 8 e2 1 5 0 1

Available online at www.sciencedirect.com

ScienceDirect journal homepage: www.elsevier.com/locate/he

Review Article

Recent developments in bifunctional air electrodes for unitized regenerative proton exchange membrane fuel cells e A review M. Hunsom a,b,*, D. Kaewsai a, A.M. Kannan c a

Fuels Research Center, Department of Chemical Technology, Faculty of Science, Chulalongkorn University, 254 Phayathai Road, Bangkok 10330, Thailand b Center of Excellence on Petrochemical and Materials Technology (PETRO-MAT), Chulalongkorn University, 254 Phayathai Road, Bangkok 10330, Thailand c The Polytechnic School, Ira. A. Fulton Schools of Engineering, Arizona State University, Mesa, AZ 85212, USA

article info

abstract

Article history:

Unitized regenerative proton exchange membrane fuel cell (UR-PEMFC) technology has

Received 6 August 2018

progressed in the recent past and has started appearing towards few applications. How-

Received in revised form

ever, the UR-PEMFC viability is limited by its lower round-trip efficiency mainly due to

18 September 2018

several reasons such as sluggish air electrode reactions, lower performance/stability,

Accepted 20 September 2018

higher materials cost etc. In this context, many approaches are being implemented for

Available online 19 October 2018

efficiency enhancement including design and development of effective bifunctional air electrodes (oxygen reduction and evolution reactions) materials both for fuel cell and

Keywords:

electrolyzer modes as well as for optimization of operating condition for performance

Unitized regenerative PEM fuel cell

stability in real life applications. This review focusses on the recent developments of air

Bifunctional air electrode

electrode active materials design/development for performance improvement in UR-

Electrocatalyst design

PEMFC. Among all developed electrode materials, the catalysts with Pt- and Ir-based

Catalyst support

metals still provided the maximum round-trip efficiency of about 50% at 500 mA cm
2 in the unit cell. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21479 Development of air electrode for UR-PEMFCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21482 Bifunctional air electrode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21482 Pt-M catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21483 Pt-MO2 catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21493 Non-noble catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21494 * Corresponding author. Fuels Research Center, Department of Chemical Technology, Faculty of Science, Chulalongkorn University, 254 Phayathai Road, Bangkok 10330, Thailand. E-mail address: [email protected] (M. Hunsom). https://doi.org/10.1016/j.ijhydene.2018.09.152 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 3 ( 2 0 1 8 ) 2 1 4 7 8 e2 1 5 0 1

21479

Electrode configurations and designs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21495 Catalyst supports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21498 Conclusion and future outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21499 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21499

Introduction The unitized regenerative fuel cell (URFC) is a reversible energy conversion device that has a dual-function operation as a fuel cell and water electrolyzer (WE) in a unitized stack. The regenerative fuel cell (RFC) can be considered as an energy conversion/storage device similar to a rechargeable battery with H2 as the storage medium [1]. The difference in operating

principles between RFC and URFC can be clearly understood from the Fig. 1(a) and (b). Very importantly, the reactants of the RFC are stored separately from the reactor as well as the storage tanks of the reactant gases and reaction products, providing the flexibility of sizing the electrochemical reactor needed for the WE and FC operations [2]. The major benefits of the RFCs compared to typical rechargeable batteries are; (a) invariant electrode chemical composition during charge and discharge cycles, (b) freedom from self-discharge on long-

Fig. 1 e Schematic diagram of (a) conventional discrete RFC, and (b) URFC [2].

21480

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 3 ( 2 0 1 8 ) 2 1 4 7 8 e2 1 5 0 1

term storage, (c) higher specific packaged (up to 1.0 kWh.kg
1) and theoretical (3.66 kWh.kg
1) energy densities, (d) rapid start-up/shutdown cycle, especially for a low-temperature RFC, and (e) use of water as a fuel carrier with benign electrode reactions [2e7]. Overall, the RFC is considered as an energy storage system for large scale energy storage such as remote off-grid power sources, renewable energy sources (solar and wind), emergency or backup power generation systems, unmanned underwater vehicles, high-altitude/longendurance solar rechargeable aircrafts, stratospheric platform airships and a hybrid energy storage/propulsion system for spacecraft [6,8e12]. Currently, various URFC technologies are being developed with proton exchange membrane, alkaline and solid oxide electrolytes as well as utilized regenerative microfluidic fuel cell (UR-MFC) [4,13]. The use of different electrolytes causes the transfer of different ions to proceed the oxidation/reduction reactions, leading to the difference in their main feature, working temperature, efficiency, advantages as well as the issue to be improved, as summarized in Table 1. Among various systems, the UR-PEMFC technology is well developed with the round-trip efficiency, the energy efficiency (εRT) calculated from the ratio between obtained voltage in FC mode (VFC) and in the WE mode (VWE) at a given current density as in Eq. (1) [5], of 40e50% at 20e100  C. εRT ¼

VFC  100 FWE

(1)

As evident from the literature, the most research progress has been achieved for UR-PEMFCs, which has led to their early application stages in many areas, such as aerospace and aviation, renewable energy, power supply and transportation [4,16e19]. In addition, UR-PEMFCs work at relatively lower temperatures with low pollution/emission and high power density, making them attractive for both mobile and stationary applications [18]. The basic components of a UR-PEMFC are two electrodes and the solid electrolyte or membrane that allow the transfer of positive ions in the direction corresponding to the flow of electrons in an external circuit [20,21]. During the FC mode, the electrical energy is produced by feeding H2 and O2 (from air) gases to the anode and cathode, respectively. In brief, protons generated at the anode move to the cathode through Nafion electrolyte and combine with oxidant to form H2O in the presence of catalysts (Anode: 2H2 / 4Hþ þ 4e
and Cathode: O2 þ 4e
þ 4Hþ / 2H2O). On the other hand, in the WE mode, the opposite reactions occur in the electrochemical cell, known as the water-splitting process. In brief, H2O fed to the anode oxidizes to generate protons, which move to the cathode through Nafion electrolyte and reduces to form H2 gas in the presence of catalysts, at the expense external electrical energy (Anode: 2H2O / O2 þ 4e
þ 4Hþ and Cathode: 4Hþ þ 4e
/ 2H2). The H2 and O2 gases produced in the WE mode are stored and used as feed gases in the FC mode for energy generation [20,21]. The URFCs could be constructed based on the specific electrodes (Fig. 2(a)) or the specific reactions (Fig. 2(b)) configurations. In the first approach, each electrode deals with the same gas in both the WE and FC modes with simplicity in management of gases. However, carbon-based materials employed as the gas diffusion layers (GDL) and catalyst supports in FC mode are

not stable under the WE mode [16]. In the second approach, the same electrode deals with the same redox reaction, which can alleviate the stability issues (corrosion) of the conventional GDLs and electrocatalysts. However, this configuration requires the gas purging system to eliminate the residual gas before switching from one mode of operation to the other [4,22]. Even though the UR-PEMFCs demonstrate the highest performance among various systems, they still exhibit a lower efficiency than the PEM water electrolyzer at any operating conditions/current density (see Fig. 3). This is attributed to a slow kinetic rate of ORR together with mass transport limitations of gaseous reactants to the reaction sites [19], resulting to a decreased cell efficiency to < 50% at the current density higher than 400 mAcm
2. As reported, the H2 oxidation and reduction (evolution) reactions are performed best with noble metal catalysts, with minimum mass transport limitation under normal operation [23]. The performance and stability of the H2 evolving electrode is restricted only by poisoning agents or fuel impurities [24]. However, this is not the case for the reaction at the oxygen electrode, where Pt is the effective catalyst for the oxygen reduction reaction (ORR) in the FC mode, but it is ineffective for the oxygen evolving reaction (OER) in the WE mode as Pt could oxidize leading to stability issues. In this context, the most effective for the OER catalysts are Ir based [25], but does not work for the ORR. In addition, both Pt and Ir are expensive and this restricts their worldwide application. The breakdown of the projected FC stack costs in 2015 for 500,000 systems/y was mostly associated with the catalyst and application costs, which accounted for 45% of the total cost (Fig. 4). As demonstrated in Table 2 [26], the US-DOE Hydrogen and Fuel Cells Program Record expected to reduce the a cost of the URFC from ~ $116/kWnet in 2015 to ~ $91/kWnet in 2020 by reducing the cost of structural materials and composites using new technologies for advanced developments in the URFC unit cell components. As a consequence, to overcome these limitations, many efforts have been performed (i) to enhance the round-trip energy conversion efficiency of the UR-PEMFCs by optimizing the operating condition and/or using effective catalysts that are capable of enhancing the ORR during a FC mode operation and the OER in the WE mode, so-called bifunctional catalyst, and (ii) to sustain the cyclability/durability of the URPEMFC by alleviating the loss of materials and components, such as the catalyst support, Nafion binder, electrolyte membrane, GDL and bipolar plate, and to reduce the component cost in the URFC unit cell [19,27]. The present review attempts to present the research advances and development of bifunctional catalysts and electrode materials for the oxygen electrode of UR-PEMFCs from the past to present in order to overcome the limitation of UR-PEMFCs owing to their low efficiency and stability as well as high material cost. Furthermore, future prospects for the advanced development of oxygen electrodes for UR-PEMFCs are discussed, which will be helpful for researchers who develop oxygen electrode materials for UR-PEMCs to further advance their development.

Type/Electrolyte

Main feature

Operating temperature ( C)

Efficiency (%)

1

UR-PEMFC/Nafion

Noble metal catalyst

20e10

40e50

2

UR-AFC/AEM (ex. porous PTFE filled with PDTB-OH-, FAA-3, A201, A3PE) UR-SOFC/Oxygen ion-conducting ceramics (ex. BCY, BCZY, CZYbCo)

Non-noble metal catalyst

20e120

30e40

High temperature operation, high energy efficiency, non-noble metal catalyst

500e1000

60e80

3

4

UR-SOFC/Proton-conducting ceramics (ex. YSZ, ScSZ, Mg-doped LaGaO3, LSGM, SDC, GDC)

High temperature operation, high energy efficiency, non-noble metal catalyst

500e700

60e80

5

UR-MFC/Acid/alkaline solution

Membraneless structure; low capital cost

20e80

60 for vanadium-species

Advantages

Issue to be improved

High specific energy, deep cycling ability, decoupled energy storage capacity with rated power, long-term energy storage and power output Simpler structure and no electrolyte leakage Superior efficiency comparable with secondary batteries, high possibility for reverse mode operation Great potential for energy conversion applications under intermediate temperatures, low activation resistance Carbon neutral energy conversion cycle, costeffectiveness, easy fabrication, no membrane-related problems, flexible pH adjustment

Cost effective components (ex. catalysts, membrane, bipolar plats), high round-trip efficiency and cycle stability Performance optimization and stability improvement Material optimization and longterm stability

Electrode optimization and long-term stability

Power output, fuel utilization, stacking, fuel cycling and modelling works to better understand the electrochemical mechanism

Where BCY is yttrium-doped barium cerate, BCZY is yttrium-doped barium cerate zirconate, GDC is gadolinium-doped ceria, LSGM is lanthanum strontium gallate magnesite, ScSZ is scandiastabilized zirconia, SDC is samaria-doped ceria and YSZ is yttria-stabilized zirconia.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 3 ( 2 0 1 8 ) 2 1 4 7 8 e2 1 5 0 1

Table 1 e Technology status of URFC technologies [4,13e15]. No.

21481

21482

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 3 ( 2 0 1 8 ) 2 1 4 7 8 e2 1 5 0 1

Fig. 2 e UR-PEMFC showing (a) H2 and O2 electrodes, and (b) reduction and oxidation electrodes' configurations [4].

Development of air electrode for UR-PEMFCs Bifunctional air electrode In general, the bifunctional catalysts for the ORR and OER in UR-PEMFCs can be categorized as unsupported- and

supported Pt-M, Pt-MO2 and Pt-free catalysts (where M is cometal), as discussed in detail below. Table 3 consolidates the major features of various air electrode materials and designs. The following sections bring out the recent development and the status of catalysts and catalyst supports of the bifunctional air electrodes.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 3 ( 2 0 1 8 ) 2 1 4 7 8 e2 1 5 0 1

21483

Table 2 e Projected cost status at 500,000 systems per year compared with 2020 cost targets [26]. Component System Stack MEAa Fuel cell membrane Bipolar platec Air compressor (CEM)d Humidifier system Humidifier membraneg a

Fig. 3 e Efficiency of PEM water electrolysis cell, FC and URFC at 80  C [19].

b

Pt-M catalysts

c

Various single metals, such as Ir, Ru, Os, Rh and Ta have frequently been added on the surface of Pt in order to improve the ORR and OER activities of the bifunctional catalyst for the UR-PEMFCs. In the past, it was reported that the addition of Ir into the Pt black clearly enhanced the WE performance, but not the FC performance. The WE performance was significantly increased as the Ir composition was increased from 0 to 50 wt% in PtIr [29,30]. The incorporation of a physically mixed commercial Pt black fine powder (Johnson Matthey) and Ir black fine powder (Johnson Matthey) onto polytetrafluoroethylene (PTFE) sheets improved the efficiency of the URPEMFC during the WE mode, and the improvement was greater as the Ir content increased from 0 to 50 wt% [31], but this decreased the efficiency of the FC mode. Similar results on the effect of the Ir content on the ORR and OER activity were also observed when commercial PtIr catalysts were loaded onto the carbon-based GDL (LT140W ELAT®, E-TEK) [21]. The electrochemical surface area (ESA) and ORR of the Pt85Ir15 catalyst were comparable to those of the unsupported Pt black catalyst. The onset potential for the OER for the PtIr black catalyst was about 150 mV more negative than that of the Pt black catalyst, indicating that the unsupported PtIr black catalyst is more effective than the Pt black catalyst for the OER. The Pt85Ir15 catalyst also showed a good initial performance and durability in a 120-h cycle test at an applied current density of 0.5 A cm
2 [21]. At an identical Pt: Ir atomic ratio (ex. 3:1), increasing the catalyst loading from 0.32 to 0.80 mg cm
2 significantly affected the ORR activity, but not affected the OER activity in the UR-PEMFC. A loading of 0.20 mg Ir and 0.60 mg Pt per unit area (PtIr at 0.8 mg cm
2)

d

e f g

Cost status ($)

2020 cost target ($)

53/kWnet 26/kWnet 17/kWnet 17/m2,b 7/kWnet 750/systeme 81/systemf 20/m2

40/kWnet 20/kWnet 14/kWnet 20/m2 3/kWnet 500/systeme 100/systemf 10/m2

Includes membrane, catalyst, GDL, gaskets, hot pressing, and cutting/slitting into cells. Based on 11.8 m2 of active area per stack, which does not include cost of membrane lost in stack fabrication or membrane that is outside the cell active area. Equivalent to $2.60/kWnet. Includes the plates and the coating. Includes the compressor/expander/motor (CEM) unit and CEM controller. Equivalent to $9.40/kWnet. Equivalent to $1.00/kWnet. The humidifier uses 1.48 m2 of membrane, so the humidification membrane cost is $30/system or $0.37/kWnet.

exhibited a comparatively higher performance in both operating modes [32]. To facilitate catalyst utilization, either carbonaceous or non-carbonaceous supports were employed. A crumpled reduced graphene oxide (rGO) supported PtIr alloy catalyst (PtIr/rGO) prepared by spray pyrolysis was further subjected to heat treatment at 600  C with (PtIr/rGO_P600) or without (Pt-Ir/ rGO_NP) additional slow pyrolysis. The PtIr/rGO_P600 catalyst exhibited a higher ORR and OER activity and stability than the PtIr/rGO and Pt-Ir/rGO_NP catalysts, although the metal NPs decorated on the support were relatively large [33]. This is because the slow pyrolysis treatment after the fast pyrolysis of the aerosol particles enhanced their metallic states and the degree of alloying between Pt and Ir by lattice contraction, characterized by the extent of overlapping d orbitals of the adjacent PtIr atoms. This then controlled the adsorption strength of oxygen molecules for the ORR and hydroxyl species from water molecule during the OER. The TiO2-supported PtIr catalyst (PtIr/TiO2) exhibited a higher round-trip efficiency than the unsupported PtIr black catalyst [7]. Dispersion of single Pt or Ir nanoparticles (NPs) on a metal oxide support, such as TiO2, can enhance the ORR and

Fig. 4 e Schematic illustration of the breakdown of the projected 2015 FC cost [28].

No.

Air electrode Catalyst

H2 electrocatalyst

Electrolyte

Major findings

Support

50 wt% Pt þ 50 wt% IrO2

Carbon paper

Pt black

Nafion 115

2

Mixed Pt/IrO2 and Pt black catalysts

PTFE sheet

Pt black

Nafion 115

3

Mixed Pt/IrO2 and deposited Pt/IrO2 catalysts

PTFE sheet

Pt black

Nafion 115

4

Mixed metal catalysts (Pt, Ru, Os, Ir, Rh)

Carbon sheet

Pt gauze

0.5 M H2SO4

5

Pt þ Ir with Ir

PTFE sheet

Pt black

Nafion 115

Transfer printing technique provided a good adhesion of the electrode to the membrane which can reduce the catalyst loading and minimize the mass transport and ohmic limitations. Mixed IrO2/Pt increased the overpotential slightly during FC operation, but exhibited a considerably high OER activity compared with Pt black. Appropriate amount of IrO2 added to the Pt surface was found to be at 10e30 mol %. The deposited IrO2/Pt catalyst (20 wt% Ir) exhibited a similar FC performance to the mixed IrO2/Pt electrode with a high Pt content (10 wt% Ir) and maintained the WE performance, since it maintained the conduction path of electrons along the Pt agglomeration as a result of a good dispersion of IrO2 particles. The Pt-Ru rich region of the Pt-Ru-Ir ternary exhibited a high ORR and OER activity and a good resistance to anodic corrosion. The Pt4.5Ru4Ir0.5 catalyst was more active than the Pt1Ir1 bifunctional catalyst. The addition of oxophilic Ru enhanced the reaction rate by stabilizing S-O bonds (S ¼ surface atom) and accelerated the oxidative deprotonation of S-OH groups. The addition of Ir metal to the oxygen electrode improved the WE efficiency but decreased the FC efficiency. The PTFE content (0e12 wt%) affected only the ORR activity in the FC, while the Nafion content (1.5e14 wt%) affected both the FC and WE performance. Electrodes with 5e7 wt.% PTFE and 7e9 wt.% Nafion were appropriate for the UR-PEMFC.

Reference

~33% at 500 mA cm
2

Zhigang et al. [64]

42% at 500 mA cm
2

Ioroi et al. [46]

44% at 500 mA cm
2

Ioroi et al. [47]

Chen et al. [38]

50% at 200 mA cm
2

Ioroi at al [31].

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 3 ( 2 0 1 8 ) 2 1 4 7 8 e2 1 5 0 1

1

Performance and Efficiency

21484

Table 3 e Bifunctional oxygen catalysts, electrode configurations/designs and supports.

Pt

IrO2

Pt black

0.5 M H2SO4

7

Pt

s-IrO2

Pt foil

0.5 M H2SO4

8

PtIr

Ti4O7

Pt wire

0.5 M H2SO4

9

Mixed metal catalysts (Pt, Ru, Os, Ir, Rh)

Ebonex, Ti4O7, Ti0.9Nb0.1O2

Pt gauze

0.5 M H2SO4

10

Pt-Ir mixed catalyst

Ti-GDB

Pt catalyst

Nafion 115

11

Pt

Deposited on PPy/Nafion composite membrane

Pt

Nafion 117

12

Pt black, PtIr, PtRuOx, PtRu, PtRuIr, PtIrOx

Carbon paper

Pt black

Nafion 1135

The Pt/IrO2 1:9 electrocatalyst exhibited better balance between the OER and ORR mass activities. Its OER activity decreased when cycling in both the OER and ORROER regions, due to the accumulation of O2 bubbles at catalyst surface, resulting in catalyst dissolution via the particles growth, shrinking, and detachment. The spatially arranged framework of Pt/sIrO2 can facilitate a high mass transfer rate of the resolved O2 in the electrolyte solution and till hold a high catalytic activity even at drastic O2 evolution condition. The PtIr alloy phase, highly stable Ti4O7 support in an acid electrolyte, and interaction between catalyst and support improved the electrocatalytic properties of PtIr alloy supported by Ti4O7. The Ti0.9Nb0.1O2 exhibited a higher electrochemical and thermal resistances than Ebonex and Ti4O7. The UR-PEMFC performance was ~46.6% at 500 mA cm
2 independent of the amount of PTFE loaded on the hydrogen side of the GDB, but strongly dependent on the oxygen side of the GDB. Appropriate PTFE loading on the GDB achieved and enhanced the URPEMFC performance and stability Polypyrrole impregnated into Nafion ~24% at 400 mA/cm2 membrane provided the electronic conduction pathways necessary to sink electrons generated from the chemical reduction of platinic chloride and so increased the UR-PEMFC performance. The addition of Ir, IrOx and Ru into the Pt 46% at 500 mA cm
2 black clearly enhanced the WE but not FC performance. The PtIr catalyst exhibited the highest UR-PEMFC performance (1100 mA cm
2 at 0.6 V for FC mode and 250 mA cm
2 at 1.55 V for WE) and a round-trip efficiency of 53 and 46% at a current density of 200 and 500 mA cm
2, respectively.

da Silva et al. [53]

Kong et al. [52]

Won et al. [37]

Chen et al. [39]

Ioroi et al. [75]

Lee et al. [65]

Yim et al. [29]

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 3 ( 2 0 1 8 ) 2 1 4 7 8 e2 1 5 0 1

6

(continued on next page)

21485

No.

H2 electrocatalyst

Air electrode Catalyst

Electrolyte

Major findings

Support

Pt black þ Ir black

Deposited on membrane

Pt black

Nafion 117

14

Pt black, PtIr, PtRuOx, PtRu, PtRuIr, PtIrOx

Carbon paper

Pt black

Nafion 1135

15

Pt black, PtIr, PtRu and PtRuIr

Deposited on membrane

16

70 wt% Pt þ 30 wt% IrO2

Deposited on membrane

Pt/C

Nafion 112

17

Pt/IrO2 catalysts

Acetylene black

Pt black

0.1 M HClO4

18

Deposited RuO2-IrO2/Pt and mixed RuO2-IrO2/Pt

Carbon paper

Pt/C

Nafion 115

Nafion 115

The MEA with an addition layers of sputtered Pt on the O2 electrode exhibited the best performance in comparison with the standard MEA (polished membrane), the standard MEA with hydrophobic O2 electrode and the standard MEA with the Pt sputtered on membrane (O2 side) The FC performance decreased while the WE performance significantly increased with increasing Ir or IrOx from 0 to 50 wt% in PtIr and PtIrOx catalysts. The optimized Ir content in the PtIr catalyst was found to be 1 wt% with a 2.0 mg cm
2 loading at the oxygen electrode. The PtIr exhibited the highest energy conversion efficiency compared with Pt black, PtRu and PtRuIr electrocatalysts. A corrosion-resistive GDL exhibited a slightly lower FC performance than a conventional GDL, but a much higher WE performance (1.62 V at 1000 mA cm
2 and 1.8 V at 2000 mA cm
2) and durability during the FC/WE cycle. The deposited Pt/IrO2 presented a slightly lower ORR electrocatalytic activity than the mixed Pt/IrO2 due to the formation of Pt-OH at a more negative potential and the downshift of the d-band center (2d) through d-band coupling. However, it showed higher OER activity than the mixed Pt/IrO2 due to the electronic and surface-structural effect. A deposited RuO2-IrO2/Pt exhibited a better performance than a mixed RuO2IrO2/Pt catalyst due to the high dispersion of the former catalyst.

Reference

~48% at 200 mA cm
2

Wittstadt et al. [67]

46% at 500 mA cm
2

Yim et al. [30]

35% at 100 mA cm
2

Pettersson et al. [40]

~47% at 300 mA cm
2

Song et al. [73]

Yao et al. [48]

~42% at 500 mA cm
2

Zhang et al. [57]

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 3 ( 2 0 1 8 ) 2 1 4 7 8 e2 1 5 0 1

13

Performance and Efficiency

21486

Table 3 e (continued )

Pt (2.1 mg/cm2) þ IrO2

Deposited on membrane

Pt/C

Nafion 115

20

PtIr

Carbon cloth

No data

Nafion 112

21

Pt black þ IrO2

Deposited on membrane

Pt/C

Nafion 1035

22

IrO2, RuO2, Pt-IrO2, PtRuO2

Ebonex (mixture of Ti4O7 and other phases)

Pt-mesh

0.5 M H2SO4

23

Pt black þ Pt/IrO2, Pt black þ IrO2

Carbon sheet

Pt/C

Nafion 212

24

PtIr black

TiO2

Pt catalyst

Nafion 112

A novel MEA prepared by a Nafionpyrolyzed method exhibited a slight and marked improvement in the WE and FC modes, respectively, particularly at a high current density, due to the increased number of pores formed in the catalyst layer, making the transfer of reaction gas easy and preventing flooding of the catalyst layer. Increased Ir contents in the PtIr catalyst decreased the UR-PEMFC ORR activity and increased the OER activity. The Pt85Ir15 catalyst showed a good initial performance and durability in the 120-h cycle test at an applied current density of 500 mA cm
2. The unit cell with bifunctional catalyst deposited on GDL exhibited a higher performance than those deposited onto the membrane in the FC mode. The catalyst layer obtained by direct mixing of Pt black and IrO2 and separated layer as a Pt catalyst layer and an IrO2 catalyst layer fabricated on GDL provided the comparable high round-trip efficiency of the URFC. Unsupported and supported RuO2 showed a good OER catalytic activity but a bad electrochemically stability for longperiods at high anodic potentials, while IrO2 had a high overpotential to OER, but a long-term stability to corrosion. Both metal oxide catalysts showed no ORR catalytic activity, but the addition of Pt enhanced the ORR activity. The Pt/IrO2 þ Pt black electrode provided a higher ORR and OER activity than the IrO2 þ Pt electrode, due to its lower interparticle catalyst resistance. The ORR and OER preferred the Pt/TiO2 and IrO2/TiO2 catalysts, respectively. The Pt85Ir15/TiO2 showed the highest catalyst efficiency towards the ORR and OER compared with the unsupported PtIr black.

44.2% at 500 mA cm
2

Chen et al. [68]

49% at 500 mA cm
2

Jung et al. [21]

42.1% at 800 mA cm
2

Chen et al. [69]

Escalante-Garcı´a et al. [58]

~49.8% at 500 mA cm
2

Zhang et al. [51]

50.0% at 500 mA cm
2

Huang et al. [34]

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 3 ( 2 0 1 8 ) 2 1 4 7 8 e2 1 5 0 1

19

(continued on next page)

21487

No.

H2 electrocatalyst

Air electrode Catalyst

Electrolyte

Support

Pt black þ IrO2

Carbon powder, TiC, TiC þ IrTiOx

Pt/C catalyst

Nafion 1035

26

Pt þ ItO2

Deposited on membrane

Pt black

Nafion 1135

27

PtIr/C

TiC

Platinized Pt foil

0.5 M H2SO4

28

IrO2/Pt

Deposited on membrane

Pt/C

Nafion 117

29

Pt-IrO2 mixed catalyst

Carbon paper

Pt catalyst

Nafion 115

30

Pt-IrO2 mixed catalyst

Carbon paper

Pt catalyst

Nafion 115

Performance and Efficiency

The URFC with the novel GDL (80 wt% TiC ~47% at 200 mA cm
2 and 20 wt% IrTiOx) exhibited a comparable performance under both FC and WE modes to the traditional GDL, but was more stable than the traditional GDL. The mixed catalysts structure provided ~40% at 600 mA cm
2 the best performance for a technical application for the present stage of development. The plasma process gave a significant improvement in the electrocatalytic activity for both the OER and ORR since it resulted in fine metal crystals and a high metal dispersion due to changes in the electronic valence states of the Pt and Ir NPs. The FC operation required an excess of Pt ~32.25% at 300 mA cm
2 catalyst, while the WE operation favored the IrO2 catalyst. PTFE treatment for the oxygen-electrode GDL did not significantly affect the FC operation under a dry operation, but negatively affected the mass transport of liquid and gas under a wet condition. A Tifelt GDL with large and uniformly distributed pores gave a similar effect as the small holes in MPL, allowing the liquid water transport through the larger pore sites and a specific ratio of small pore sites (inaccessible to water) kept dry and suitable for the gas transport path. The Ti-felt GDLs with high PTFE contents performed better in dry conditions in the FC mode, but accelerated the flooding problem in a wet condition. The electrolyzer performances were almost the same at different PTFE contents in the GDL.

Reference

Chen et al. [79]

Altmann et al. [70]

Sui et al. [35]

Baglio et al. [25]

Hwang et al. [76]

Hwang et al. [77]

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 3 ( 2 0 1 8 ) 2 1 4 7 8 e2 1 5 0 1

25

Major findings

21488

Table 3 e (continued )

Pt/IrO2

Deposited on membrane

Pt/C

Nafion 115

32

Pt85Ir15, Pt4.5Ru4Ir0.5, Pt3.5Ru4Ir1.5, Pt2.5Ru4Ir2.5 Pt1.5Ru4Ir3.5

Deposited on membrane

Pt/C

Nafion 115

33

Pt/IrO2

IrO2

Pt foil

0.5 M H2SO4

34

Pt/IrO2, Pt/Ir-IrO2

IrO2

Pt foil

0.5 M H2SO4

35

Pt/Irx(IrO2)10-x

IrO2

Pt foil

0.5 M H2SO4

36

Pt85Ir15/TiO2

TiO2

Pt/C

Nafion NRE-212

Novel ultrasonic polyol method allowed 30% at 200 mA cm
2 the deposition of metallic Pt NPs on the IrO2 substrate without any metallic Ir and gave a high catalyst dispersion compared with the conventional mechanical mixing of catalysts. The Pt85Ir15 catalyst exhibited a better ~47.45% at 100 mA cm
2 behavior than the unsupported PtRuIr for use as a bifunctional catalyst in a URPEMFC. Porous IrO2 NPs allowed the deposition of Pt NPs on both internal and external sites, leading to a high degree of Pt dispersion as well as an enhanced ORR and OER activity of the Pt/porous-IrO2 catalyst. The Pt/Ir-IrO2 exhibited a comparable OER activity to Pt/IrO2, but a higher ORR activity, attributed to a uniform dispersion of ultrafine Ir NPs on the surface of IrO2, providing an improved electronic conductivity for Pt NPs. It also exhibited an excellent stability compared with the Pt/IrO2 catalyst, due to its special structure providing interactions between Pt NPs and Ir NPs that prevented the Pt agglomeration. The presence of Ir NPs improved the electronic conductivity of Pt/IrO2 nanocomposite, while IrO2 NPs enhanced the Pt dispersion and so improved the Pt utilization and ORR activity. Individual IrO2 had a slightly higher OER activity than Ir, but deposition of the appropriate amount of Ir on the IrO2 surface enhanced the OER activity. The Pt/Ir3(IrO2)7 catalyst possessed the highest ORR (21.71 mA mg
1 at 0.85 V) and OER (42.35 mA mg
1 at 1.55 V) activities due to its high ESA and electrical conductivity. A TiO2 support enhanced the utilization of 50.3% at 500 mA cm
2 Pt85Ir15. Addition of Ir/TiN as a protective MPL alleviated the GDL corrosion during WE.

Cruz et al. [49]

Rivas et al. [42]

Kong et al. [50]

Kong et al. [55]

Kong et al. [56]

Huang et al. [7]

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 3 ( 2 0 1 8 ) 2 1 4 7 8 e2 1 5 0 1

31

(continued on next page)

21489

No.

H2 electrocatalyst

Air electrode Catalyst

Electrolyte

Major findings

Support

Pt/C and Pt/GC

Carbon powder

Pt/C, Pt/GC

Nafion 212

38

Pt-IrO2/ATO

Sb-doped SnO2 (ATO)

Pt/C

Nafion 115

39

PtIr black

Pt black

Nafion 112

40

PtIr

Deposited on membrane

Pt/C

Nafion 212

41

PtIr

C, TiC, TiCN

Carbon row

0.5 M H2SO4

42

PtxIry, PtxRuyIrz

No data

Pt mesh

0.5 M H2SO4

The Pt/GC provided a higher ORR and OER activity and stability than the Pt/C catalyst, because the appropriate montmorillonite content enhanced the fine dispersion of Pt and improved the anode wettability and hydrophilic property. The Pt-IrO2/ATO provided a maximum mass current activity of 1118 mA mg
1 at 1.8 V in the WE mode due to the contribution of the Pt with surface oxide, but provided a low ORR activity of 565 mA mg
1 at 0.3 V for the FC mode, since IrO2 and ATO were not active in the ORR and may block active Pt sites. The dimethylacetamide-modified Nafion binder showed a stronger persistence in the MEA than that with the commercial Nafion binder. A low PTFE content induced a flooding phenomenon, while a high (excess) PTFE content blocked water access through the GDLs and reach three-phase boundaries near catalyst particles. Feeding modes, PTFE content showed the same performance effect order as 20.68% > 26.95% > 29.97%. Low heat treatment (250  C) of PtIr/C achieved a high ORR activity due to the superior ESA and conductivity of the carbon support. High heat treatment (ex. 400  C) increased the ORR and stability due to the change in the electronic valence states of Ir and Ti metals, Pt surface enrichment and the passivation of the TiCN surface after the ORR test. The PtxIrz catalyst was more active and stable for the ORR and OER than PtxRuyIrz.

Reference

37.5% at 100 mA cm
2

Pai and Tseng [81]

48% at 250 mA cm
2

Cruz et al. [83]

~46.3% at 400 mA cm
2

Jung and Choi [71]

~40.6% at 500 mA cm
2

Zhuo et al. [32]

Garcı´a et al. [89]

Morales et al. [44][

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 3 ( 2 0 1 8 ) 2 1 4 7 8 e2 1 5 0 1

37

Performance and Efficiency

21490

Table 3 e (continued )

PtIr catalyst

C, TiC, TiCN

Carbon row

0.5 M H2SO4

44

PtIrO2

Ti-powder on the Ti-felt GDL

Pt

Nafion 115

45

PtIr

46

Pt/IrO2/CP

Loaded on a homemade micro porous layer (80% carbon black with 20% PTFE) Carbon paper

47

48

Nafion 112

Pt/C

Nafion 212

N-doped bifunctional carbon

Pt or graphite electrode

1.0 M HClO4 and 0.1 M KOH

Tobacco-derived Ncontaining ordered mesoporous carbon (NOMC)

Commercial Pt/C

1.0 M HClO4

The nitrogen content in the catalyst support affected the catalyst performance and noble metal anchoring, with increased particle agglomeration at higher nitrogen loadings. The Pt3Ir/TiCN catalyst exhibited a higher ORR and OER activity and stability than the Pt3Ir/TiN and Pt3Ir/ TiC catalysts. A relatively uniform PTFE distribution was obtained by drying under vacuum. The presence of the Ti-felt with uniform PTFE distribution improved the cell performance of URFCs under a fully wet condition. The MEA with Gr-C exhibited a higher round trip energy efficiency and stability than a non-MPL and typical amorphouscarbon MPL containing MEAs. Facile oxygen and water transport through well-developed macropores, originated from the open CP structures enhanced the catalyst utilization and performance. The porous Pt/IrO2/CP electrode with a catalyst loading of 0.3 mg cm
2 provided the highest current densities in both the FC (0.89 mA cm
2 at 0.6 V) and WE (1.5 A cm
2 at 1.7 V) modes as well as the round-trip efficiency. A large surface area, high N doping content as well as the increased amount of pyridinic N and Fe-N-C species affected positively the ORR and OER in both acidic and alkaline media. The N-OMC catalyst with high pyrrolic and pyridinic Ns content and low graphitic N content displayed higher ORR and OER activities than that with high graphitic N and low pyrrolic and pyridinic Ns content. It showed that the pyrrolic and pyridinic Ns were more beneficial to the improvement of ORR and OER activities than graphitic N.

e

Roca-Ayats et al. [86]

~41% at 500 mA cm
2

Ito et al. [74]

43.8% at 1000 mA cm
2

Sadhasivam et al. [5]

46% at 500 mA cm
2

Lee et al. [54]

Xuan et al. [59]

Li et al. [60]

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 3 ( 2 0 1 8 ) 2 1 4 7 8 e2 1 5 0 1

43

(continued on next page)

21491

No.

H2 electrocatalyst

Air electrode Catalyst

Electrolyte

Support

(Ir0.8Sn0.2)O2:F

Pt

0.5 M H2SO4

50

IrO2, Cu1.5Mn1.5O4:F

Pt wire

0.5 M H2SO4

51

Pt black

C, SiO2, SiO2-SO3H

52

Pt/IrO2, Pd/IrO2, Pt-Ru/ IrO2

Carbon paper

Pt/C

Nafion 212

53

PtRuIr

Carbon paper

Pt/C

0.1 M H2SO4

Nafion 112

Performance and Efficiency

Incorporation of F on (Sn0.80Ir0.20)O2 exhibited a promising candidate as OER electrocatalyst for PEM based WE, causing by the fact that addition of appropriate F content induced the return back of Ir dband center to the position corresponding to pure IrO2, resulting in a high catalytic activity comparable to that of pure IrO2. The F-doping in Cu1.5Mn1.5O4 led to modify the electronic structure, causing a shift in the d-band center position in particular, thereby improving the overall electrochemical catalytic activity of Cu1.5Mn1.5O4. A 10 wt% F (Cu1.5Mn1.5O4: 10F) gave the best electrochemical catalytic activity and provided a similar onset potential (1.43 V for OER and 1 V for ORR vs RHE) to that of IrO2 and Pt/C. The quantity of transferred protons in the 46.1% at 200 mA cm
2 FC mode should be high in order to generate a high current density, but it did not need to be high for the WE mode due to the operation of the cell at low current density. The Pt/SiO2-SO3H electrocatalyst exhibited a better round-trip efficiency than that of the Pt/C and Pt/SiO2 catalysts due to the enhanced proton conductivity in the catalyst layer in the presence of the sulfonic acid functional group on the support material (SiO2-SO3H). The roughness factor of GDL mainly 42% at 400 mA cm
2 affected the electrode performance in both the WE and FC modes. The porosity and substrate material had very little impact. The Pt-Ru/IrO2 was a promising catalyst for UR-PEMFC operation in terms of cost-effectiveness and roundtrip efficiency. The PtRuIr catalyst with 10 mol.% of Ir demonstrated higher OER and ORR currents compared to the PtRu and other PtRuIr catalysts due to the uniform catalyst dispersion on the graphite substrates.

Reference

Datta et al. [61]

Patel et al. [62]

Roh et al. [88]

Choe et al. [66]

Ye et al. [41]

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 3 ( 2 0 1 8 ) 2 1 4 7 8 e2 1 5 0 1

49

Major findings

21492

Table 3 e (continued )

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 3 ( 2 0 1 8 ) 2 1 4 7 8 e2 1 5 0 1

OER catalytic activity in comparison with an unsupported single metal, which was caused by a better catalyst utilization on the TiO2 support [34]. Varying the Pt:Ir (w/w) compositions from 100:0 to 70:30 revealed that the Pt85Ir15/TiO2 catalyst showed the highest efficiency for the ORR and OER. The PtIr/ TiC catalyst synthesized by the chemical reduction and plasma reduction processes significantly improved the electrocatalytic activity for both the OER and ORR in an acid electrolyte (0.5 M H2SO4), since this preparation method gave finer metal crystals (<5 nm) and a higher metal dispersion on the TiC support, and also induced some changes in the electronic valence states of the Pt and Ir NPs [35]. The Pt3M (M ¼ Ru, Ir or Ta) NPs supported on titanium carbonitride (TiCN), synthesized by the ethylene glycol method, exhibited a similar efficiency for the ORR, while the Pt3Ru/TiCN appeared to be the best catalyst by far due to the interesting promotion effect of the TiCN support [36]. The Ti4O7 supported Pt-based catalysts (PtIr/Ti4O7) exhibited superior electrocatalytic activity and stability for both ORR and OER in comparison with Pt/C and Pt/Ti4O7 [37]. It provided the round-trip efficiency of 59.3% at 1.5 mA cm
2, which were higher than that of with Pt/C and Pt/Ti4O7 of around 1.09 and 1.06-fold, respectively. This is because the PtIr alloy phase, highly stable Ti4O7 support in an acid electrolyte, and interaction between catalyst and support improved the electrocatalytic properties of PtIr alloy supported by Ti4O7. Besides the bimetallic Pt-based catalysts, various ternary catalysts have frequently been investigated and developed for use as a bifunctional catalyst in UR-PEMFCs. A combinatorial approach was initially used to screen the ORR and OER activities of electrode arrays containing 715 unique combinations of five elements (Pt, Ru, Os, Ir and Rh) [38]. The PtRu-rich region of the PtRuIr ternary catalysts exhibited a high activity for both reactions and a good resistance to anodic corrosion. The Pt4.5Ru4Ir0.5 catalyst was more active than the PtIr bifunctional catalyst in both the OER and ORR. The addition of the oxophilic Ru enhanced the reaction rate by stabilizing S-O bonds (S is the surface atom) and accelerated the oxidative deprotonation of S-OH groups. Supporting the PtRuIr catalyst on a doped rutile compound (Ti0.9Nb0.1O2) resulted in a higher electrochemical and thermal resistance than when supported on Ebonex (a mixture of Ti4O7 and other phases) or phase-pure microcrystalline Ti4O7 [39], which was due to the significant electronic interaction between the catalyst and support, and a substantial increase in catalyst utilization. The addition of Ir and/or Ru into the printed layer of Pt black deposited on a Nafion membrane increased the ORR performance of the Pt black in the order of PtRuIr > PtRu > PtIr > Pt black, while the WE performance was in the order PtIr > PtRu > PtRuIr > Pt black [40]. However, the presence of too low or too high Ir content on the PtRu nanoclusters did not enhance the catalytic performance for ORR and OER, which was attributed to the change in the morphology and specific surface area of the PtRuIr catalysts after deposition of Ir [41]. The optimum content of Ir enhanced the ORR and OER, considered in terms of the round-trip efficiency, because the catalyst surface consisted of very small particles that were connected to each other to form a heterogeneous structure that was well dispersed on the substrates. Although the Pt4.5Ru4Ir0.5 catalyst exhibited a higher electrocatalytic activity in the WE operation

21493

than the Pt3.5Ru4Ir1.5, Pt2.5Ru4Ir2.5 and Pt1.5Ru4Ir3.5 catalysts [42]. It was unstable in UR-PEMFCs due to the corrosion of Ru at a potential of 1.4e1.5 V at an acidic pH of 0 to
2 [43]. The presences of Ru in the PtxRuyIrz catalyst decreased an overpotential and the current increment for OER and showed an improvement to ORR after potential for interest reaction was applied due to the formation of PtxRuyIrz alloy [44]. However, the catalyst is not durable/stable under OER conditions with several cycles.

Pt-MO2 catalysts The direct mixing of Pt and IrO2 catalysts yielded a low ORR activity in the UR-PEMFC because the IrO2 was less active in the ORR than Pt [45], resulting in an increased high overpotential during the FC operation. However, the mixed Pt/IrO2 catalysts had a considerably higher activity for the WE mode than Pt black due to the high ESA of the IrO2 powder [46]. Thus, the FC operation required an excess of the Pt catalyst, while the WE operation favored the IrO2 catalyst [25]. Overall, the appropriate amount of IrO2 to add to the Pt surface was found to be low, in the range of 10e30 mol. %. To reduce the Pt loading and maintain the WE performance, instead of direct mixing, the IrO2 particles were deposited on the surface of Pt black via a colloidal iridium hydroxide hydrate precursor. At 20 wt% Ir, the IrO2/Pt catalyst exhibited a comparable FC performance with the mixed IrO2/Pt electrode with a higher Pt content (10% Ir) and still maintained a similar WE performance [47]. This is because the deposited IrO2/Pt catalyst can maintain the conduction path of electrons along the Pt agglomeration due to the well-dispersed IrO2 particles, while the electron conduction path tended to be hindered by the IrO2 agglomeration in the mixed IrO2/Pt catalyst. A selective heterogeneous Pt nucleation and growth on the IrO2 surface with a relatively uniform Pt dispersion and high Pt loading in a nanostructured Pt/IrO2 composite was achieved by the addition of a dispersant, such as sodium dodecylbenzenesulfonate, during the deposition process [48]. The preparation procedure enhanced the electronic connection and interaction between Pt and IrO2, and exhibited a slightly lower ORR activity but a markedly higher OER activity than the mixture of Pt and IrO2. This was explained by the downshift of the d-band center (2d) through the d-band coupling model, resulting in a weak O2 molecule adsorption on the Pt catalysts supported by IrO2. This was also supported by the observation that the Pt/IrO2 catalyst prepared by the modified polyol method exhibited a better activity than that prepared by mechanical mixing of metallic Pt and IrO2 [49], as the former method led to highly homogenous distribution of catalysts particles without any metallic Ir. The Pt/porous-IrO2 catalyst gave a higher ORR and OER activity than the Pt/commercial-IrO2 catalyst [50], where the Pt/porous-IrO2 catalyst provided an OER activity of 28% (at 1.55 V) with > two-fold higher OER activity at 0.85 V than the Pt/commercial-IrO2 catalyst. This is because the porous structure of the porous IrO2 NPs allowed the deposition of Pt NPs on both the internal and external sites, leading to high degree of Pt dispersion. The cell performance in the WE mode was also slightly improved, as observed by the decreased potential of 16 mV at 1 A cm
2 when using the directly mixed Pt black with Pt/IrO2 catalyst [51]. In addition, it provided the

21494

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 3 ( 2 0 1 8 ) 2 1 4 7 8 e2 1 5 0 1

highest FC power density of 1.16 W cm
2 at 2.6 A cm
2, which was close to that with the commercial Pt/C and about two times higher than that with the conventional mixed Pt black and IrO2 catalyst (Pt black þ IrO2). This was attributed to the good interaction between Pt and IrO2, resulting in a low interparticle catalyst resistance [51]. The s-IrO2 supported Pt nanocomposite (Pt/s-IrO2) exhibited more remarkable ORR/OER performance than the Pt/ commercial IrO2 [52]. The Pt/s-IrO2 displayed higher ORR activity than the commercial one due to its higher ESA. This is because s-IrO2, prepared with SBA-15 template, displayed excellent uniformity and regularity in particle shape as well as the ordered distribution in geometric space, which can provide a uniform Pt dispersion on its surface of the s-IrO2 particles. In addition, the spatially arranged framework of this catalyst can facilitate a high mass transfer rate of the resolved O2 in the electrolyte solution and still hold a high catalytic activity even at drastic O2 evolution condition. The Pt/IrO2 (1:9) electrocatalyst exhibited better balance between the OER and ORR mass activities [53]. Its ORR activity can be maintained during cycling in the ORR and OER regions, but dropped significantly when it was cycled in the ORR-OER potential window. While its OER activity decreased when cycling in both the OER and ORR-OER regions, due to the accumulation of O2 bubbles at catalyst surface, resulting in catalyst dissolution via the particles growth, shrinking, and detachment. The porous Pt/IrO2/carbon paper (CP) electrode prepared by the sequential formation of IrO2 and porous Pt layers by electrodeposition and spraying exhibited a linearly increase in FC performance up to 0.69 A cm
2 with increasing Pt loading up to ~0.3 mg cm
2 at 0.6 V, whereas the WE activity was highest with 0.2 mg Pt per cm2 [54]. The porous Pt/IrO2/CP electrode with a catalyst loading of 0.3 mg cm
2 provided higher current density in the FC (0.89 A cm
2 at 0.6 V) and WE (1.5 A cm
2 at 1.7 V) modes with a round-trip efficiency of 46% at 500 mA cm
2. These high performances with a low catalyst loading were probably due to the facile oxygen and water transport through the well-developed macropores that originated from the open CP structures, leading to an effective utilization of the IrO2 and Pt electrocatalysts towards the OER and ORR. The Pt/Ir-IrO2 catalyst prepared by depositing the metallic Ir NPs by a microwave-assisted polyol process exhibited a comparable OER activity to the Pt/IrO2 catalyst (29.30 and 31.44 mA mg
1 for Pt/Ir-IrO2 and Pt/IrO2, respectively, at 1.52 V), but a higher ORR activity (10.67 and 7.62 mA mg
1 for Pt/Ir-IrO2 and Pt/IrO2, respectively, at 0.85 V) [55]. This was attributed to the uniform dispersion of ultrafine Ir NPs on the IrO2 surface, providing an improved electronic conductivity for the Pt NPs. In addition, it still exhibited an excellent stability compared with the Pt/IrO2 catalyst, due to its special structure that provided the interaction between the Pt NPs and Ir NPs and so prevented Pt agglomeration. The presence of Ir NPs improved the electronic conductivity of the Pt/IrO2 nanocomposite, while the presence of IrO2 NPs enhanced the dispersion of Pt and so improved the Pt utilization as well as the ORR activity [56]. Individual IrO2 had only a slightly higher OER activity than Ir, but deposition of the appropriate amount of Ir on the IrO2 surface enhanced the OER activity to a greater

extent. Overall, the Pt/Ir3(IrO2)7 catalyst possessed the highest ORR (~22 mA mg
1 at 0.85 V) and OER (~42 mA mg
1 at 1.55 V) activities among the series of catalysts studied, which was due to it having the highest ESA (24.74 m2 g
1) as well as electrical conductivity. The addition of different composite metal oxides on the Pt catalyst has also been studied, where it was reported that RuO2-IrO2/Pt prepared by colloidal deposition exhibited a better performance than a mixed RuO2-IrO2/Pt catalyst, due to the high catalyst dispersion level of the former catalyst [57]. The unsupported RuO2 and the RuO2 supported on Ebonex, when mixed with the Pt NPs, showed a good OER catalytic activity but a poor electrochemical stability in long-period tests at high anodic potentials, while IrO2 showed a high overpotential to the OER, but a long-term stability to corrosion. However, both the RuO2 and IrO2 catalysts did not show any catalytic activity for the ORR. The addition of Pt to both metal oxide catalysts enhanced their ORR activity, revealing the possibility of using of IrO2-Pt and RuO2-Pt supported on Ebonex as a bifunctional catalyst in URFCs [58].

Non-noble catalysts To reduce the usage of highly expensive Pt and lower the cost of the generated electrical energy, many attempts to develop the Pt-free bifunctional catalysts have been reported. A N-doped bifunctional carbon (N-BCE3) electrocatalysts exhibited comparable ORR and OER activities to the commercial Pt/C and IrO2 electrocatalysts for ORR and OER in both acidic (1.0 M HClO4) and alkaline media (0.1 KOH) [59]. This is ascribed to its large surface area, high N doping content as well as the increased amount of pyridinic N and Fe-N-C species. Besides, it provided a superior stability compared to the commercial Pt/C catalyst. That is, it exhibited the loss of initial current density only 3.7 and 8.8% after 30,000 s in alkaline and acidic media, respectively, while were lower than those of Pt/C catalyst (13.0 and 20.4%). A similar influence of N species on the ORR and OER activity was also observed via the use of tobacco-derived N-containing ordered mesoporous carbon (N-OMC) electrocatalysts [60]. The N-OMC catalyst with high pyrrolic and pyridinic Ns content and low graphitic N content displayed higher ORR and OER activities than that with high graphitic N and low pyrrolic and pyridinic Ns content. It showed that the pyrrolic and pyridinic Ns were more beneficial to the improvement of ORR and OER activities than graphitic N. The N-OMC2, extracted from a physical mixture of dicyandiamide and pure tobacco, displayed a loss of peak current density of 5.8% after 6000 cycles LSV in 1.0 M HClO4 which was lower than that of Pt/C for 5.5-fold in the same media after the same cycle. A solid solution of (Ir0.8Sn0.2)O2 exhibited lower electrochemical activity for OER for PEM based WE in comparison with IrO2 catalyst [61]. This is because the complex hybridization of the Ir d-states and corresponding s and p-states of Sn and O during formation of the binary oxide caused a positive shift of the Ir d-band center of (Sn0.80Ir0.20)O2 with respect to pure IrO2 arises. However, incorporation of F on (Sn0.80Ir0.20)O2 exhibited a promising candidate as OER electrocatalyst for PEM based WE, causing by the fact that addition of appropriate F content induced the return back of Ir d-band center to the

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 3 ( 2 0 1 8 ) 2 1 4 7 8 e2 1 5 0 1

position corresponding to pure IrO2, resulting in a high catalytic activity comparable to that of pure IrO2. A copper/manganese oxide based electro-catalyst system was the first studied as bifunctional catalyst for UR-PEMFCs [62]. Based on theoretical calculations of the total energies and electronic structures, Cu1.5Mn1.5O4 was predicted to be a highly active and durable catalyst due to its unique electronic structure leading to adsorption and desorption of the reaction intermediates, similar to that of Pt for the ORR and IrO2 for the OER. Doping of fluorine (F) into the Cu1.5Mn1.5O4 structure modified the MnOx electronic structure in general by shifting the d-band center position to match the characteristics of the noble metal and noble metal oxide catalyst systems, thereby improving the overall electrochemical catalytic activity of Cu1.5Mn1.5O4. In summary, the Cu1.5Mn1.5O4 catalyst with 10 wt% F (Cu1.5Mn1.5O4:10F) showed the best electrochemical catalytic activity in comparison to those with other F contents, and provided similar onset potential (1.43 V for the OER and 1 V for the ORR vs RHE) to that of IrO2 and Pt/C with a long term stability for 6000 cycles in acidic media.

Electrode configurations and designs Because the reactant gas (H2/O2) is fed to the UR-PEMFC during the FC mode, while water is supplied during the WE mode, the porous structure of the GDL must allow an effective diffusion of each reactant gas and water to the reaction zone on the membrane electrode assembly (MEA). Typically, the FC operation requires a hydrophobic GDL material, such as hydrophobic CP or carbon cloth, to prevent water flooding. However, the WE operation requires the GDL to have a hydrophilic property to supply water to the oxygen electrode. Thus, the conventional GDL used in the FC mode typically does not work during the WE operation because it limits the diffusion of reactants and products [63]. In addition, carbon materials tend to corrode at high potentials on the oxygen electrode side during the WE operation. Therefore, the balance between the hydrophobic and hydrophilic properties of the material used to prepare the GDL is required for the FC and WE operations. Several strategies have been developed to overcome these problems, such as using a special electrode configuration, balancing the hydrophilic/hydrophobic property of the GDL by optimizing the chemical content in the GDL or minimizing the electrode thickness. A good adhesion of the electrode to the membrane in the MEA can reduce the required catalyst loading and minimize the mass transport and ohmic limitations [64]. Polypyrrole impregnated Nafion membranes provide the electronic conduction pathways necessary to sink the electrons generated from the chemical reduction of platinic chloride and enhance the direct loading of Pt NPs onto the Nafion membrane for performance improvement of the URPEMFC [65]. The addition of graphitized carbon (Gr-C) as a mesoporous layer (MPL) in MEAs for URFCs improved the round trip energy efficiency of the UR-PEMFC (43.8%) compared to that with a non-MPL (36.6%) and a typical amorphous-carbon MPL (41.8%) [5]. In addition, it improved the system stability, even after 20 cycles (42.3%), owing to the increased electrical conductivity and high crystallinity of the Gr-C after thermal treatment. The roughness factor of

21495

substrates/diffusion layers mainly affected the electrode performance in both the WE and FC modes, while the porosity and substrate material had a relatively insignificant impact [66]. The MEA with additional layers of sputtered Pt on the O2 electrode exhibited a better performance than the standard MEA (polished membrane) with traditional hydrophobic porous O2 electrode or with the Pt sputtered O2 electrode [67]. This is because the sputtered Pt layer on the O2 electrode tended to reduce the gas permeation through the membrane. A MEA prepared by a Nafion-pyrolyzed method exhibited a slight and marked improvement in the WE and FC modes, respectively, particularly at a high current, when compared with that of the conventional MEA [68]. This is because the developed method enhanced the formation, and so the number, of pores in the catalyst layer, making an easier transfer of the reactant gases and preventing flooding of the catalyst layer. Different bifunctional catalyst layers (Fig. 5(a)), fabricated by coating the catalyst on the GDL (C1, C3, C5 and C7) and membrane (C2, C4, C6 and C8), affecting the FC performance significantly [69]. The unit cell using Pt sprayed onto the GDL exhibited a cell voltage of ~0.10 V higher than those with Pt sprayed onto the membrane in the FC mode due to the presence of a greater homogeneity and more porous catalyst layer on the GDL, which prevented the unit cell from water flooding at a high current density. The electron conduction path was also found to be hindered by IrO2 agglomerates, which led to a decreased cell performance. The catalyst layer obtained by the direct mixing of Pt black and IrO2 (C1) and a separated layer of Pt catalyst layer and IrO2 catalyst layer (C5) fabricated on the GDL provided the two highest and comparable URFC round-trip efficiencies of 42.1% at 800 mA cm
2 (Fig. 5(b)). The oxygen electrode configurations and characteristics control most importantly the unit cell performance and reliability [60]. Among the three oxygen electrode configurations evaluated for UR-PEMFC; (i) mixture of catalysts (Pt and IrO2), (ii) multilayer and (iii) segmented electrode for UR-PEMFC (Fig. 6(a)), the unit cell with segmented electrode showed the lowest performance in both the FC and WE modes (Fig. 6(b)). This was probably due to the reduced active area and the resulting cross currents due to activity heterogeneity and a non-optimized gas diffusion medium. The unit cell with a mixture of Pt and IrO2 catalysts exhibited the best performance in the WE mode, while the cell with multilayer electrodes provided the best performance in the FC mode and their overall performance was equivalent at a cell potential of 0.7 V. The performance of the multilayer electrodes was improved with higher Nafion content in the inner layer and possibly by improving the mass transport by introducing a pore former (to reduce the diffusion limitation) in the outer electrode layer. To avoid water flooding during the FC operation and to supply water to the oxygen electrode during the WE operation, the hydrophilic/hydrophobic property of the GDL in the URPEMFC has to be balanced, which can be done by optimizing the PTFE and Nafion contents in electrode layer. Nafion contents in the range of 1.5e14 wt% affected both the FC and WE performances [31]. The presence of lower or higher Nafion content degraded the FC performance, probably due to the

21496

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 3 ( 2 0 1 8 ) 2 1 4 7 8 e2 1 5 0 1

Fig. 5 e (a) Schematic diagram of the different bifunctional catalyst layers for URFCs in which the Pt was sprayed onto the GDL (C1, C3, C5 and C7) or applied onto the membrane (C2, C4, C6 and C8), and (b) Efficiencies of URFCs at 100 and 800 mA cm¡2 [69].

lack of effective reaction sites from the low interface between Pt and the Nafion electrolyte at a low PTFE content and the high resistance of gas diffusion into the electrode or gas desorption from the electrode in the presence of a high PTFE content. The modification of the commercial Nafion solution using dimethylacetamide as a dispersion solvent gave a stronger persistence of the MEA than that of the commercial Nafion dispersion [71]. This is because the home-made Nafion increased the interfacial adhesion between the membrane and the electrode layer.

A PTFE content in the range of 0e12 wt% in the catalyst layer only affected the ORR activity in the FC [32]. The FC performance was poor in the presence of too low a PTFE content, due to catalyst wetting in the electrode. However, too high a PTFE content in the CP blocked the water/gas channels during the WE mode and so increased the resistance to water transfer, leading to the cell drying out around the three-phase boundary when a large amount of water was consumed [32]. The choice of the water feeding mode on the WE affected the appropriate PTFE content. The performance was ranked as

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 3 ( 2 0 1 8 ) 2 1 4 7 8 e2 1 5 0 1

21497

Fig. 6 e (a) Oxygen electrode configurations. (Left) mixture of both catalysts (Middle) multilayer electrode and (Right) segmented electrode, (b) oxygen electrode performance in a UR-PEMFC [70].

oxygen electrode feeding > dual feeding > hydrogen electrode feeding in the presence of 21% PTFE, and dual feeding > hydrogen electrode feeding > oxygen electrode feeding in the presence of 30% PTFE [32]. To improve the corrosion resistance and electrical conductivity of the GDL, TiO2 has been incorporated as the current supplier material [72]. A corrosion-resistive GDL, prepared by loading IrO2/Ti powder on the wet-proofed CP (TGP-H-060, Toray), exhibited a slightly lower FC performance than the conventional carbon black loaded-GDL, but a much higher WE performance [73]. The performance drop of the corrosion-resistive GDL in the FC mode at a medium current density was probably due to its relatively high hydrophilicity, while its high WE performance was attributed to a good contact between the catalyst and microporous layers. In addition,

the cell with a corrosion-resistive GDL exhibited a greater stability than that with a conventional GDL during the FC/WE cycling, because the decline in the hydrophobicity of the corrosion-resistive GDL induced a partial removal of PTFE from the GDL. The properties of Ti-felt GDLs did not significantly affect the concentration overpotential under a dry operation of the FC mode. Cells with no PTFE-coated Ti-felt GDLs showed a superior performance during the WE mode, due to the smooth supply of water, but a very poor performance in the FC operation mode, due to the water flooding of gas diffusion backing (GDB) and catalyst layer [74]. However, under a wet condition, coating different amounts of PTFE on the Ti-felt GDL in the oxygen side significantly affected the UR-PEMFC performance [75]. This is because the hydrophobic mesopores produced by

21498

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 3 ( 2 0 1 8 ) 2 1 4 7 8 e2 1 5 0 1

the Ti-powder must be kept free from water and suitable for gas transport under fully humidified conditions [76]. A slight change in the PTFE contents in the oxygen side of the GDL did not noticeably affect the WE mode although the porosity and hydrophobicity of GDL were different because enough water was supplied for all the cells [77]. However, too high a PTFE content caused an unstable FC and reduced the WE performance due to the decreased water supply with drying out of the catalyst layers. The efficiency of the URFC decreased with increasing cycles when the PTFE loading on the oxygen side of the GDB layer was too low. A stable unit cell operation can be achieved with the use of (i) an optimum PTFE loading level on the GDB for balanced water management during the FC operation [76], and (ii) Tifelts with large fiber diameter with a low porosity [78]. The latter case will give a similar effect as the small holes in the MPL that allow the preferential movement of liquid water through the larger pore sites and a specific ratio of small pore sites that is inaccessible to water and kept dry and suitable for the gas transport. On the other hand, when the GDL substrate has a relatively uniform distribution, water percolation occurred uniformly over the entire pore network and so gas transport to the electrode was significantly hindered by the percolation. In addition to TiO2, the metallic ceramic TiC was also incorporated into the GDL of a UR-PEMFC and reported exceptionally higher performance than the conventional GDL (Vulcan XC-72 on CP) [79]. However, the TiC GDL surface can still be oxidized to form a low conductive oxide layer, resulting in an increased cell resistance during the cell stability test. The addition of 20 wt% IrTiOx within the TiC layer enhanced the high cycle performance of the URFC compared to that with the conventional carbon GDL, as the IrTiOx can rapidly catalyze the active oxygen species to form the oxygen molecule.

Catalyst supports The catalyst support material plays a vital role in the catalytic performance because it (i) provides a physical surface to disperse the catalyst particles leading to a larger catalyst surface area and so a higher electrocatalysis rate and (ii) helps to control the wettability and provide better electronic

conductivity for the electrode [4]. Thus, the utilized catalyst support should possess (i) a large surface area per unit volume, (ii) high electrical conductivity, (iii) corrosion resistance and be of a (iv) low cost [4,27]. Commercial carbon powder is usually used as the anode and cathode catalyst supports in the FC, but in the UR-PEMFC, the high anodic potential at the oxygen electrode during the WE mode will cause severe carbon corrosion [80]. Thus, several approaches have been performed to develop an effective support for the oxygen electrode in the UR-PEMFC by either modifying the structure of the carbonaceous support or developing a new type of noncarbonaceous support. Graphitized carbon (GC) supports with montmorilloniteassisted dispersion promote the ORR and OER activity and the stability of Pt catalysts compared to popular carbon supports (Vulcan XC-72) without dispersion agents [81]. This is because the addition of the appropriate amount of montmorillonite improved the fine dispersion of the Pt catalyst and the wettability and hydrophilic property of the anode [82]. The Pt/ GC catalyst with a Pt/GC concentration of 20 wt% and a loading of 0.25 mg cm
2 exhibited a cell voltage of only 1.67 and 1.8 V at 100 and 300 mA cm
2, respectively for the WE mode, and a power output of 190 mW cm
2 with a current density of 404 mA cm
2 for the FC mode, giving an energy conversion efficiency of 37.5%. The Sb-doped SnO2 (ATO) showed promising properties as a support for Pt catalysts in the WE mode in UR-PEMFC compared with an Ebonex® commercial support [83] due to its high electrical conductivity, high surface area and high stability in acidic media [84,85]. The Pt-IrO2/ATO provided a maximum mass current activity of 1118 mA mg
1 at 1.8 V in the WE mode, which was due to the contribution of the Pt NPs with the surface oxide, while it still provided a low ORR activity of 565 mA mg
1 at 0.3 V for the FC mode, since IrO2 and ATO are not active in the ORR and may block active Pt sites. The addition of Ir/TiN as a protective MPL enhanced the stability of the URFC cycle performance, mainly due to the reduced corrosion of the GDL, especially during WE. The heat treatment at 400  C can promote the ORR activity as well as stability of the PtIr/TiCN, which were higher than that for PtIr/TiC This is because the heat treatment induced a change in the electronic valence states of the Ir and Ti

Fig. 7 e Round-trip efficiency of various UR-PEMFCs.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 3 ( 2 0 1 8 ) 2 1 4 7 8 e2 1 5 0 1

metals, Pt surface enrichment and the passivation of the TiCN surface after the ORR test. Whilst the Pt3Ir/TiN catalyst showed a good activity towards the ORR and OER, close to those observed with a Pt3Ir/TiCN catalyst [86]. It had a high ESA loss, probably due to the passivation of the TiN surface after repeated potential cycling in acidic media [87]. Although the Pt3Ir/TiC and Pt3Ir/TiCN catalysts showed similar structural properties (like particle size, support surface area, particle agglomeration and total metal loading), Pt3Ir/TiCN displayed a higher activity towards the ORR and the OER than PtIr/TiC, due to its high higher proportion of reduced species (Ir0, Pt0 and TiC) on the Pt3Ir/TiCN as well as the nitrogen content in the TiCN support. However, the Pt/ SiO2-SO3H electrocatalyst exhibited the highest round trip energy efficiency in the unit cell (46.13%) compared with that of the Pt/C (45.32%) and Pt/SiO2 (44.60%) catalysts. This reflects that the sulfonic acid functional group on the support material (SiO2-SO3H) increased the proton conduction in the catalyst layer and so reduced the ohmic resistance in the unit cell [88]. The Table 3 summarized the recent developments of bifunctional catalysts and electrode materials for the oxygen electrode of the UR-PEMFC along with the round-trip efficiency as summarized in Fig. 7. Although many strategies have been performed in order to improve the oxygen reaction rate, catalysts with Pt- and Ir-based metals still provided the maximum round-trip efficiency (50.3% at 500 mA cm
2) in the unit cell [7].

Conclusion and future outlook In this review, recent developments of bifunctional catalysts and electrode materials in the oxygen electrode for URPEMFCs are critically analyzed and highlighted. Although the noble metal catalysts are not stable for the OER during the WE mode, most catalyst developments have still been based on noble metals, but with the addition of various other metals, such as Ir, Ru, Os, Rh and Ta both in metallic and/or oxide forms in order to improve the OER performance and durability. To enhance the catalyst utilization and stability of the oxygen electrode, both carbonaceous (ex. carbon powder, reduced graphene oxide) and non-carbonaceous supports (TiO2, TiC, TiN, SiO2-SO3H and ATO) are being employed. To ensure an effective diffusion of the reactant gas during the FC and WE modes, the hydrophobic/hydrophilic property of GDL is balanced by optimizing the PTFE and Nafion content as well as the electrode configuration. In summary, to develop commercially viable UR-PEMFC, future research should focus on finding non-precious cost-effective and durable catalysts, which will work on both ORR and OER modes with lower polarization losses. For the electrodesupport matrix, both carbonaceous and non-carbonaceous materials with structural modification are required to exhibit better corrosion resistance. It is strongly suggested that the conductive polymers such as polypyrrole or polyaniline, composited with carbonaceous and/or noncarbonaceous support structure be investigated and developed for durable UR-PEMFC systems.

21499

references

[1] Barbir F. Chapter 9 e fuel cell system design. In: Barbir F, editor. PEM fuel cells. Burlington: Academic Press; 2005. p. 271e336. [2] Barsukov V, Beck F. New promising electrochemical systems for rechargeable batteries. Springer Science & Business Media; 2013. [3] Savinell RF, Ota K-I, Kreysa G. Encyclopedia of applied electrochemistry. Springer; 2014. [4] Wang Y, Leung DY, Xuan J, Wang H. A review on unitized regenerative fuel cell technologies, part-A: unitized regenerative proton exchange membrane fuel cells. Renew Sustain Energy Rev 2016;65:961e77. [5] Sadhasivam T, Roh S-H, Kim T-H, Park K-W, Jung H-Y. Graphitized carbon as an efficient mesoporous layer for unitized regenerative fuel cells. Int J Hydrogen Energy 2016;41:18226e30. [6] Mitlitsky F, Myers B, Weisberg AH. Regenerative fuel cell systems. Energy Fuels 1998;12:56e71. [7] Huang S-Y, Ganesan P, Jung H-Y, Popov BN. Development of supported bifunctional oxygen electrocatalysts and corrosionresistant gas diffusion layer for unitized regenerative fuel cell applications. J Power Sources 2012;198:23e9. [8] Eguchi K, Fujihara T. Research progress in solar powered technology for SPF airship. NAL Res Prog 2002;2003:6e9. [9] Agbossou K, Chahine R, Hamelin J, Laurencelle F, Anouar A, St-Arnaud J-M, et al. Renewable energy systems based on hydrogen for remote applications. J Power Sources 2001;96:168e72. [10] Barbir F, Molter T, Dalton L. Efficiency and weight trade-off analysis of regenerative fuel cells as energy storage for aerospace applications. Int J Hydrogen Energy 2005;30:351e7. [11] Maclay JD, Brouwer J, Samuelsen GS. Dynamic analyses of regenerative fuel cell power for potential use in renewable residential applications. Int J Hydrogen Energy 2006;31:994e1009. [12] Sone Y. A 100-W class regenerative fuel cell system for lunar and planetary missions. J Power Sources 2011;196:9076e80. [13] Wang Y, Leung DY, Xuan J, Wang H. A review on unitized regenerative fuel cell technologies, part B: unitized regenerative alkaline fuel cell, solid oxide fuel cell, and microfluidic fuel cell. Renew Sustain Energy Rev 2017;75:775e95. [14] Xu L, Leung DY, Huizhi W, Leung KH, Hong X, Jin X. Microfluidic reversible fuel cell for carbon-neutral energy conversion cycle. Fuel Cells Batter 2015;10:1e481. [15] Kjeang E, Djilali N, Sinton D. Microfluidic fuel cells: a review. J Power Sources 2009;186:353e69. [16] Dihrab SS, Sopian K, Alghoul M, Sulaiman M. Review of the membrane and bipolar plates materials for conventional and unitized regenerative fuel cells. Renew Sustain Energy Rev 2009;13:1663e8. [17] Stolten D, Emonts B. Fuel cell science and engineering, 2 volume set: materials, processes, systems and technology. John Wiley & Sons; 2012. [18] Gabbasa M, Sopian K, Fudholi A, Asim N. A review of unitized regenerative fuel cell stack: material, design and research achievements. Int J Hydrogen Energy 2014;39:17765e78. [19] Millet P. Unitized regenerative systems. Weinheim, Germany: Wiley-VCH; 2015. [20] Sørensen B. Chapter 3 e fuel cells. In: Sørensen B, editor. Hydrogen and fuel cells. 2nd ed. Boston: Academic Press; 2012. p. 95e200. [21] Jung H-Y, Park S, Ganesan P, Popov BN. Electrochemical studies of unsupported PtIr electrocatalyst as Bi-functional

21500

[22]

[23]

[24]

[25]

[26]

[27]

[28] [29]

[30]

[31]

[32]

[33]

[34]

[35]

[36]

[37]

[38]

[39]

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 3 ( 2 0 1 8 ) 2 1 4 7 8 e2 1 5 0 1

oxygen electrode in unitized regenerative fuel cells (URFCs). ECS Trans 2008;16:1117e21. Grigoriev S, Millet P, Dzhus K, Middleton H, Saetre T, Fateev V. Design and characterization of bi-functional electrocatalytic layers for application in PEM unitized regenerative fuel cells. Int J Hydrogen Energy 2010;35:5070e6. Rasten E, Hagen G, Tunold R. Electrocatalysis in water electrolysis with solid polymer electrolyte. Electrochim Acta 2003;48:3945e52. Marshall A, Borresen B, Hagen G, Tunold R, Tsypkin M. Development of oxygen evolution electrocatalysts for proton exchange membrane water electrolysis. In: Proceedings of the first European hydrogen energy conference, Grenoble, France; 2003. p. 45e9. Baglio V, D'Urso C, Di Blasi A, Ornelas R, Arriaga LG, Antonucci V, et al. Investigation of IrO2/Pt electrocatalysts in unitized regenerative fuel cells. Int J Electrochem 2011;2011:5. Marcinkoski J, Spendelow J, Wilson A, Papageorgopoulos D. DOE hydrogen and fuel cells program record#: 15015. 2015. Fuel Cell System Cost. Sadhasivam T, Dhanabalan K, Roh S-H, Kim T-H, Park K-W, Jung S, et al. A comprehensive review on unitized regenerative fuel cells: crucial challenges and developments. Int J Hydrogen Energy 2017;42:4415e33. Fuel cell system cost e 2015. US Department of Energy Hydrogen and Fuel Cells Program; 2015. Yim S-D, Lee W-Y, Yoon Y-G, Sohn Y-J, Park G-G, Yang T-H, et al. Optimization of bifunctional electrocatalyst for PEM unitized regenerative fuel cell. Electrochim Acta 2004;50:713e8. Yim S-D, Park G-G, Sohn Y-J, Lee W-Y, Yoon Y-G, Yang T-H, et al. Optimization of PtIr electrocatalyst for PEM URFC. Int J Hydrogen Energy 2005;30:1345e50. Ioroi T, Yasuda K, Siroma Z, Fujiwara N, Miyazaki Y. Thin film electrocatalyst layer for unitized regenerative polymer electrolyte fuel cells. J Power Sources 2002;112:583e7. Zhuo X, Sui S, Zhang J. Electrode structure optimization combined with water feeding modes for bi-functional unitized regenerative fuel cells. Int J Hydrogen Energy 2013;38:4792e7. Kim IG, Nah IW, Oh I-H, Park S. Crumpled rGO-supported PtIr bifunctional catalyst prepared by spray pyrolysis for unitized regenerative fuel cells. J Power Sources 2017;364:215e25. Huang S-Y, Ganesan P, Jung WS, Cadirov N, Popov BN. Development of supported bifunctional oxygen electrocatalysts with high performance for unitized regenerative fuel cell applications. ECS Trans 2010;33:1979e87. Sui S, Ma L, Zhai Y. TiC supported PteIr electrocatalyst prepared by a plasma process for the oxygen electrode in unitized regenerative fuel cells. J Power Sources 2011;196:5416e22. ~ a M, Martı´nezRoca-Ayats M, Herreros E, Garcı´a G, Pen Huerta M. Promotion of oxygen reduction and water oxidation at Pt-based electrocatalysts by titanium carbonitride. Appl Catal B Environ 2016;183:53e60. Won J-E, Kwak D-H, Han S-B, Park H-S, Park J-Y, Ma K-B, et al. PtIr/Ti4O7 as a bifunctional electrocatalyst for improved oxygen reduction and oxygen evolution reactions. J Catal 2018;358:287e94. Chen G, Delafuente DA, Sarangapani S, Mallouk TE. Combinatorial discovery of bifunctional oxygen reductiondwater oxidation electrocatalysts for regenerative fuel cells. Catal Today 2001;67:341e55. Chen G, Bare SR, Mallouk TE. Development of supported bifunctional electrocatalysts for unitized regenerative fuel cells. J Electrochem Soc 2002;149:A1092e9.

[40] Pettersson J, Ramsey B, Harrison D. Fabrication of bifunctional membrane electrode assemblies for unitised regenerative polymer electrolyte fuel cells. Electron Lett 2006;42:1444e6. [41] Ye F, Xu C, Liu G, Li J, Wang X, Du X, et al. A novel PtRuIr nanoclusters synthesized by selectively electrodepositing Ir on PtRu as highly active bifunctional electrocatalysts for oxygen evolution and reduction. Energy Convers Manag 2018;155:182e7.  ndez A. Evaluation of Pt[42] Rivas S, Arriaga L, Morales L, Ferna Ru-Ir as bifunctional electrocatalysts for the oxygen electrode in a unitized regenerative fuel cell. Int J Electrochem Sci 2012;7:3601e9. [43] Pourbaix M, Van Muylder J, De Zoubov N. Electrochemical properties of the platinum metals. Platin Met Rev 1959;3:47e53. [44] Morales S, Fernandez A. Unsupported PtxRuyIrz and PtxIry as bi-functional catalyst for oxygen reduction and oxygen evolution reactions in acid media, for unitized regenerative fuel cell. Int J Electrochem Sci 2013;8:12692e706. [45] Kinoshita K. Electrochemical oxygen technology. John Wiley & Sons; 1992. [46] Ioroi T, Kitazawa N, Yasuda K, Yamamoto Y, Takenaka H. Iridium oxide/platinum electrocatalysts for unitized regenerative polymer electrolyte fuel cells. J Electrochem Soc 2000;147:2018e22. [47] Ioroi T, Kitazawa N, Yasuda K, Yamamoto Y, Takenaka H. IrO2-deposited Pt electrocatalysts for unitized regenerative polymer electrolyte fuel cells. J Appl Electrochem 2001;31:1179e83. [48] Yao W, Yang J, Wang J, Nuli Y. Chemical deposition of platinum nanoparticles on iridium oxide for oxygen electrode of unitized regenerative fuel cell. Electrochem Commun 2007;9:1029e34. [49] Cruz J, Baglio V, Siracusano S, Ornelas R, Arriaga L, Antonucci V, et al. Nanosized Pt/IrO2 electrocatalyst prepared by modified polyol method for application as dual function oxygen electrode in unitized regenerative fuel cells. Int J Hydrogen Energy 2012;37:5508e17. [50] Kong F-D, Zhang S, Yin G-P, Zhang N, Wang Z-B, Du C-Y. Pt/ porous-IrO2 nanocomposite as promising electrocatalyst for unitized regenerative fuel cell. Electrochem Commun 2012;14:63e6. [51] Zhang Y, Zhang H, Ma Y, Cheng J, Zhong H, Song S, et al. A novel bifunctional electrocatalyst for unitized regenerative fuel cell. J Power Sources 2010;195:142e5. [52] Kong F-D, Liu J, Ling A-X, Xu Z-Q, Wang H-Y, Kong Q-S. Preparation of IrO2 nanoparticles with SBA-15 template and its supported Pt nanocomposite as bifunctional oxygen catalyst. J Power Sources 2015;299:170e5. [53] da Silva GC, Fernandes MR, Ticianelli EA. Activity and stability of Pt/IrO2 bifunctional materials as catalysts for the oxygen evolution/reduction reactions. ACS Catal 2018;8:2081e92. [54] Lee B-S, Park H-Y, Cho MK, Jung JW, Kim H-J, Henkensmeier D, et al. Development of porous Pt/IrO2/ carbon paper electrocatalysts with enhanced mass transport as oxygen electrodes in unitized regenerative fuel cells. Electrochem Commun 2016;64:14e7. [55] Kong F-D, Zhang S, Yin G-P, Wang Z-B, Du C-Y, Chen G-Y, et al. Electrochemical studies of Pt/IreIrO2 electrocatalyst as a bifunctional oxygen electrode. Int J Hydrogen Energy 2012;37:59e67. [56] Kong F-D, Zhang S, Yin G-P, Zhang N, Wang Z-B, Du C-Y. Preparation of Pt/Irx(IrO2)10
x bifunctional oxygen catalyst for unitized regenerative fuel cell. J Power Sources 2012;210:321e6. [57] Zhang Y, Wang C, Wan N, Mao Z. Deposited RuO2eIrO2/Pt electrocatalyst for the regenerative fuel cell. Int J Hydrogen Energy 2007;32:400e4.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 3 ( 2 0 1 8 ) 2 1 4 7 8 e2 1 5 0 1

[58] Escalante-Garcı´a I, Duron-Torres S, Cruz J, ArriagaHurtado L. Electrochemical characterization of IrO2-Pt and RuO2-Pt mixtures as bifunctional electrodes for unitized regenerative fuel cells. J New Mat Electrochem Sys 2010;13:227e33. [59] Xuan J, Liu Z. High-performance N-doped bifunctional carbon electrocatalysts derived from polymer waste for oxygen reduction and evolution reaction. Int J Electrochem Sci 2017;12:10471e83. [60] Li M, Liu Z, Wang F, Xuan J. The influence of the type of N dopping on the performance of bifunctional N-doped ordered mesoporous carbon electrocatalysts in oxygen reduction and evolution reaction. J Energy Chem 2017;26:422e7. [61] Datta MK, Kadakia K, Velikokhatnyi OI, Jampani PH, Chung SJ, Poston JA, et al. High performance robust F-doped tin oxide based oxygen evolution electro-catalysts for PEM based water electrolysis. J Mater Chem 2013;1:4026e37. [62] Patel PP, Datta MK, Velikokhatnyi OI, Kuruba R, Damodaran K, Jampani P, et al. Noble metal-free bifunctional oxygen evolution and oxygen reduction acidic media electrocatalysts. Sci Rep 2016;6:28367. [63] Pettersson J, Ramsey B, Harrison D. A review of the latest developments in electrodes for unitised regenerative polymer electrolyte fuel cells. J Power Sources 2006;157:28e34. [64] Zhigang S, Baolian Y, Ming H. Bifunctional electrodes with a thin catalyst layer for unitized'proton exchange membrane regenerative fuel cell. J Power Sources 1999;79:82e5. [65] Lee H, Kim J, Park J, Joe Y, Lee T. Performance of polypyrroleimpregnated composite electrode for unitized regenerative fuel cell. J Power Sources 2004;131:188e93. [66] Choe S, Lee B-S, Jang JH. Effects of diffusion layer (DL) and ORR catalyst (M ORR) on the performance of M ORR/IrO2/DL electrodes for PEM-type unitized regenerative fuel cells. J Electrochem Sci Technol 2017;8:7e14. [67] Wittstadt U, Wagner E, Jungmann T. Membrane electrode assemblies for unitised regenerative polymer electrolyte fuel cells. J Power Sources 2005;145:555e62. [68] Chen G, Zhang H, Cheng J, Ma Y, Zhong H. A novel membrane electrode assembly for improving the efficiency of the unitized regenerative fuel cell. Electrochem Commun 2008;10:1373e6. [69] Chen G, Zhang H, Ma H, Zhong H. Effect of fabrication methods of bifunctional catalyst layers on unitized regenerative fuel cell performance. Electrochim Acta 2009;54:5454e62. [70] Altmann S, Kaz T, Friedrich KA. Bifunctional electrodes for unitised regenerative fuel cells. Electrochim Acta 2011;56:4287e93. [71] Jung H-Y, Choi J-H. The effect of a modified Nafion binder on the performance of a unitized regenerative fuel cell (URFC). J Solid State Electrochem 2012;16:1571e6. [72] Takenaka H, Torikai E, Kawami Y, Wakabayashi N. Solid polymer electrolyte water electrolysis. Int J Hydrogen Energy 1982;7:397e403. [73] Song S, Zhang H, Ma X, Shao Z-G, Zhang Y, Yi B. Bifunctional oxygen electrode with corrosion-resistive gas diffusion layer

[74]

[75]

[76]

[77]

[78]

[79]

[80] [81]

[82]

[83]

[84]

[85]

[86]

[87]

[88]

[89]

21501

for unitized regenerative fuel cell. Electrochem Commun 2006;8:399e405. Ito H, Abe K, Ishida M, Hwang CM, Nakano A. Effect of through-plane polytetrafluoroethylene distribution in a gas diffusion layer on a polymer electrolyte unitized reversible fuel cell. Int J Hydrogen Energy 2015;40:16556e65. Ioroi T, Oku T, Yasuda K, Kumagai N, Miyazaki Y. Influence of PTFE coating on gas diffusion backing for unitized regenerative polymer electrolyte fuel cells. J Power Sources 2003;124:385e9. Hwang CM, Ishida M, Ito H, Maeda T, Nakano A, Kato A, et al. Effect of titanium powder loading in gas diffusion layer of a polymer electrolyte unitized reversible fuel cell. J Power Sources 2012;202:108e13. Hwang C-M, Ishida M, Ito H, Maeda T, Nakano A, Kato A, et al. Effect of PTFE contents in the gas diffusion layers of polymer electrolyte-based unitized reversible fuel cells. J Int Counc Electr Eng 2012;2:171e7. Hwang CM, Ishida M, Ito H, Maeda T, Nakano A, Hasegawa Y, et al. Influence of properties of gas diffusion layers on the performance of polymer electrolyte-based unitized reversible fuel cells. Int J Hydrogen Energy 2011;36:1740e53. Chen G, Zhang H, Zhong H, Ma H. Gas diffusion layer with titanium carbide for a unitized regenerative fuel cell. Electrochim Acta 2010;55:8801e7. Gruver GA. The corrosion of carbon black in phosphoric acid. J Electrochem Soc 1978;125:1719e20. Pai Y-H, Tseng C-W. Preparation and characterization of bifunctional graphitized carbon-supported Pt composite electrode for unitized regenerative fuel cell. J Power Sources 2012;202:28e34. Pai Y-H, Ke J-H, Chou C-C, Lin J-J, Zen J-M, Shieu F-S. Clay as a dispersion agent in anode catalyst layer for PEMFC. J Power Sources 2006;163:398e402. Cruz J, Rivas S, Beltran D, Meas Y, Ornelas R, OsorioMonreal G, et al. Synthesis and evaluation of ATO as a support for PteIrO2 in a unitized regenerative fuel cell. Int J Hydrogen Energy 2012;37:13522e8. Vicent F, Morallo E, Quijada C, Va J, Aldaz A, Cases F. Characterization and stability of doped SnO2 anodes. J Appl Electrochem 1998;28:607e12. Antolini E, Gonzalez E. Ceramic materials as supports for low-temperature fuel cell catalysts. Solid State Ionics 2009;180:746e63. ~ a M, Martı´nezRoca-Ayats M, Garcı´a G, Galante J, Pen Huerta M. Electrocatalytic stability of Ti based-supported Pt3Ir nanoparticles for unitized regenerative fuel cells. Int J Hydrogen Energy 2014;39:5477e84. Avasarala B, Haldar P. Electrochemical oxidation behavior of titanium nitride based electrocatalysts under PEM fuel cell conditions. Electrochim Acta 2010;55:9024e34. Roh S-H, Sadhasivam T, Kim H, Park J-H, Jung H-Y. Carbon free SiO2eSO3H supported Pt bifunctional electrocatalyst for unitized regenerative fuel cells. Int J Hydrogen Energy 2016;41:20650e9. ~ a M, Martı´nezGarcı´a G, Roca-Ayats M, Lillo A, Galante J, Pen Huerta M. Catalyst support effects at the oxygen electrode of unitized regenerative fuel cells. Catal Today 2013;210:67e74.