Recent progress of anode catalysts and their support materials for methanol electrooxidation reaction

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Review Article

Recent progress of anode catalysts and their support materials for methanol electrooxidation reaction Muliani Mansor a, Sharifah Najiha Timmiati a,*, Kean Long Lim a, Wai Yin Wong a, Siti Kartom Kamarudin a,b, Nur Hidayatul Nazirah Kamarudin b a

Fuel Cell Institute, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia Department of Chemical and Process Engineering, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia

b

article info

abstract

Article history:

This review paper summarizes the recent progress of anode catalysts for methanol

Received 18 January 2019

oxidation reaction (MOR) in direct methanol fuel cells (DMFCs). The electrocatalytic ac-

Received in revised form

tivities of the noble and noble-free catalysts in different electrolyte media are compared

8 April 2019

and discussed. Noble-free catalysts exhibit high activity in alkaline medium, whereas Pt-

Accepted 10 April 2019

based catalysts are the most active MOR catalysts in acidic medium. The types of cata-

Available online 11 May 2019

lyst support materials for DMFC anodes are also discussed and further divided into carbonaceous and non-carbonaceous materials. The ion and electron transport through

Keywords:

the support materials and their effects on the overall performance are elaborated. Lastly,

Methanol oxidation reaction

this paper highlights the major challenges in achieving the optimum DMFC performance

Direct methanol fuel cell

from the aspect of tailoring the properties of MOR electrocatalysts to pave its way for

Catalyst support

commercialisation.

Electrocatalyst

© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14745 Anode catalyst . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14746 Catalysts for methanol oxidation in acidic media . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14746 Pt and Pt-based alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14746 Pt-free . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14750 Catalysts for methanol oxidation in alkaline media . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14750 Pt and Pt alloy-based catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14751 Pt-free metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14752

* Corresponding author. E-mail address: [email protected] (S.N. Timmiati). https://doi.org/10.1016/j.ijhydene.2019.04.100 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

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Structure of catalyst support materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14755 Mesoporous silicas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14755 Mobil Crystalline Material (MCM-41) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14755 Santa barbara amorphous 15 (SBA-15) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14756 Santa barbara amorphous 16 (SBA-16) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14756 Metal oxide-based materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14757 TiO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14757 SnO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14758 Carbon-based materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14758 Functionalised mesoporous carbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14758 Activated carbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14759 Carbon nanotube . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14760 Graphene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14761 Carbon nanosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14761 Challenges and future perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14762 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14762 Author contribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14763 Funding sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14763 Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14763 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14763

Introduction Direct methanol fuel cells (DMFCs) have attracted considerable attention because of their less complex configuration and suitability for small and portable applications, such as chargers for mobile phones, laptops and cameras. DMFCs have the potential to replace batteries because methanol has a remarkably higher specific energy density than the best rechargeable batteries, lithiumepolymer, and lithiumeion polymer systems. DMFCs use liquid and renewable fuels, which are relatively cheap, easily stored and handled, and simplify the fuel cell system [1]. It has a similar configuration to proton exchange membrane fuel cells (PEMFCs), in which a polymer electrolyte membrane is used to separate the anode and cathode compartments [2,3]. However, DMFCs differ from PEMFCs in terms of fuel type utilization. The elimination of fuel reforming to produce hydrogen fuel in PEMFCs using liquid methanol fuel as a feed to the system make DMFCs an attractive option because of their simplicity. However, commercialization of DMFCs still confronts four major challenges: cost, heat and water management, methanol crossover through the membrane and stability, and durability of the electrocatalyst. The proton exchange membrane used in the system has significantly resulted in power densities and efficiency loss due to high methanol crossover, which lowers the cell performance and wastes fuel [4,5]. The selection of anode electrocatalysts that are suitable for MOR is limited. Thus, anode electrocatalysts that enhance methanol oxidation activity and minimize CO poisoning must be considered to improve DMFC performance. Then, anode catalysts must be active for both oxidation and hydrogenation reactions [4]. A pure Pt catalyst is the most active metal for the dissociative adsorption of methanol. However, Pt is easily poisoned by the CO produced by methanol electro-oxidation

at low temperatures [5]. Alloying Pt with another metal, such as Ru, Pd, Ni, Fe or Co, can improve the performance of a catalyst [6e12]. Given their relatively high activity in MOR, Ptbased catalysts are suitable anode electrocatalysts for DMFCs. These metals can change the CO adsorption site and improve the anti-CO poisoning ability. The kinetics of methanol oxidation improves significantly upon the addition of a second metal, such as Ru alloys with Pt [13]. The performance of DMFCs can be improved by embedding Pt on metal oxides, such as RuO2, MnO2, MoO2, and IrO2; the hydration of metal oxides can essentially change the transfer condition of electrons and protons. This process alters the electrochemical feature of Pt [14]. Therefore, alloying Pt with another metal and embedding Pt using metal oxides can improve methanol oxidation and DMFC efficiency. However, Pt-based catalysts are too expensive in practical applications. In addition, the global shortage and high cost of Pt have urged researchers to search for another material that can enhance the performance of DMFCs. Nevertheless, studies on possible alternatives to Pt and Pt alloys as anode catalysts are still lacking. Several works focused on using nonplatinum anode catalysts with alkaline electrolytes to oxidize liquid fuels, such as methanol and ethanol [15e18]. The catalytic activity of materials depends on their size and shape; hence, immobilisation of nano-sized catalysts has recently attracted the attention of researchers. Therefore, the development of efficient DMFC anode catalysts by combining catalyst and catalyst support materials is still needed. Catalyst supports commonly used in DMFC are mesoporous and conductive. A practical support should exhibit the following: (i) good electrical conductivity, (ii) strong catalystesupport interaction, (iii) large surface area, (iv) mesoporous structure, (v) good water handling, (vi) good corrosion tolerance and (vii) easy retrieval of the catalyst [19]. Catalyst efficiency and

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durability can also be enhanced by improving catalystesupport interaction by reducing catalyst poisoning. Thus, the best selection of support materials is crucial and dominant in deciding the behaviour, efficiency, stability and the expense of the catalyst and all-inclusive fuel cell [20]. This paper discusses the progress on anode catalysts in DMFCs and the types of catalyst supports that have been investigated thus far. In the interest of anode catalysts, the effect of Pte and Ptfree catalysts in acidic and alkaline electrolyte will be reviewed. For catalyst support materials, carbonaceous and non-carbonaceous materials (e.g. functionalised mesoporous carbon, activated carbon, carbon nanotubes (CNTs) and graphene) and non-carbonaceous materials (mesoporous silica and metal oxides) will be reviewed. We shall discuss the different role of the catalyst and its effect towards the catalytic activities and fuel cell performance. The challenges of finding suitable anode catalyst for DMFC are also described in this review. Finally, the briefly discussion of future perspectives anticipated to determine a suitable route for an effective electrocatalyst.

Anode catalyst DMFC technology achievement relies on several factors, such as membrane, anode and cathode electrocatalysts. Anode electrocatalysts experience sluggish reaction kinetics that can only surmount through the search for new electrocatalysts [21]. The main considerations when designing a new electrocatalyst are as follows: (1) cost reduction and (2) catalyst performance (e.g. activity, durability, and reliability). Currently, acidic and alkaline electrolytes are mainly used in DMFC. The following section summarises the recent progress in anode electrocatalyst development in DMFC in both acidic and alkaline electrolytes.

Catalysts for methanol oxidation in acidic media The use of acid electrolyte can provide high power density while operating at a low temperature and starting up easily, but its main problem was that an expensive noble metal catalyst was required for the reaction [22].

Pt and Pt-based alloys In recent years, the development of efficient anode catalysts for DMFC has centred on Pt and its alloys. Pt is the most active metal for the dissociative adsorption of methanol, which is widely used as a catalyst in fuel cells and has demonstrated good results in DMFC systems in both acidic and alkaline media [23e28]. However, Pt is vulnerable to CO poisoning in acidic media. Thus, methanol-tolerant cathode and anode catalysts and substitutes to the expensive Pt-type catalysts should be developed. Table 1 summarizes the performance of different types of catalysts in MOR in acid and alkaline electrolytes. In 2010, Li et al. [29] used Pt nanoparticles (NPs) as electrocatalysts for methanol electro-oxidation in acidic medium. Pt deposited onto the surfaces of multi-walled carbon nanotubes (MWCNTs) and graphite oxide (GO), which was removed during the deposition of Pt and resulted as

chemically converted graphene (CCG), were synthesized in this research. The electrochemical catalytic performance of the Pt/CCG hybrids shows that Pt/CCG has higher electrocatalytic activity compared with Pt/MWCNT, which agrees with the result on electrochemical active surface area (ECSA). The ECSA value of Pt/CCG is 36.27 m2/g, whereas that of Pt/ MWCNT is 33.43 m2/g. From the electrochemical measurements, the ratio of forward anodic peak current (If) to the backward anodic peak current (Ib) was computed to analyse the intermediate carbonaceous species tolerance of the catalysts. Pt/CCG acquires a ratio of 0.83, which is higher than that of Pt/MWCNT. This result shows that Pt/CCG is more tolerant than Pt/MWCNT to CO poisoning, implying that the former has less carbonaceous build-up than the latter. Zhao et al. [30] compared the electrocatalytic activities of Pt/N-graphene and Pt/graphene in DMFCs, in which N-graphene was synthesised via the functional doping of graphene with nitrogen. They found that Pt/N-graphene can yield a peak current density value that is almost twice that of Pt/graphene (on a Pt-mass normalized basis) at a similar scan rate. Fig. 1 shows that Pt/ N-graphene has a smaller average particle size and enhanced dispersion than Pt/graphene and thus ought to offer more active catalytic sites for upgraded MOR. PteRu alloy catalysts have promising applications in DMFCs in acidic media because they can reduce CO poisoning and increase current densities [31e35]. The kinetics at low overpotential are much slower in acidic media than in alkaline media [36]. To promote the electrocatalytic activity of methanol electro-oxidation, previous works focused on developing electrocatalysts. Pt anodes are promptly poisoned in DMFCs because of the strong CO adsorption on the metal surface, thereby considerably decreasing the power output of the cell [37]. Therefore, Pt at room temperature could never be a good electrocatalyst for methanol electro-oxidation [37]. Ru further improves the oxidation of CO into CO2 by the bi-functional mechanism and/or a “ligand effect” in electrocatalysts and forms oxygenated species at lower potential than Pt [38]. The adsorption of oxygen having species on Ru atoms at low potentials is involved in the bi-functional mechanism, which can be summarized as follows: Pt þ CH3OH /PtCOads þ 4Hþ þ 4e

(1)

Ru þ H2O /Ru(OH)ads þ Hþ þ e

(2)

PtCOads þ Ru(OH)ads /CO2 þ Pt þ Ru þ Hþ þ e

(3)

The broadly acknowledged catalytic mechanism of the Pte Ru complex electrode is that Ru facilitates the adsorption of dissociative water and thus promotes complete methanol oxidation on the Pt surface [39]. Thus, the ratio surface of Pte Ru can be controlled through understanding the solution chemistry entailed is vital to enhance the catalytic activity for DMFC applications. Pt completes the dissociative chemisorption of methanol and Ru forms a surface oxy-hydroxide that is then utilized to oxidize the carbonaceous residues to CO2. Thus, RuO2 phases at the catalyst surface possibly play a vital role in oxidizing adsorbed CO-species on Pt catalysts based on the mechanism of methanol oxidation [40]. The performance

Table 1 e Comparison between acid and alkali electrolyte in terms of electrochemical active surface area (ECSA), MOR activity, performance and durability on catalysts. Electrolyte Acid

Catalyst sample PtRu/TiO2eCNF

PtRunanodendrites Cu2O/PPy-GO

Metal ECSA, MOR activity loading (wt.%) (m2/gmetal)

Ref.

with

20

10.4

345.6 mA/mg

Maximum power density of 3.8 mW/cm2

[112]

with

50

~59

~10.2 mA/cm2

[113]

with

30

5

8.75 mA/mg

Maximum power density of ~32.6 mW/cm2, limiting current of 0.65 V; ending current of ~5.46 mA/cm2 after 24 h. Maximum power density of 49 mW/cm2

with

20

0.3

38.02 mA/cm2

Maximum power density of 118.4 mW/cm2, limiting current of 0.7 V; ending current of 22.43 mA/cm2 after 1000 s

with

43.4

80.6

410 mA/mg-

NA

65

476 mA/mg

CO stripping test: onset potential of CO oxidation (potential at which 5% of [115] the maximum current was reached); 0.45 V, limiting current of 0.4 V; ending current of 70 mA/mgPt after 1200 s For ADT test, after 4000 cycles, initial ECSA of PteZrO2/NGN retained about [116] 50.2%

100

40

10.08 mA/cm2

After 2000 cycles, initial ECSA of PtRu nanodendrites retained about 91%

[117]

80

NA

7.9 mA/g

Maximum power density of 31 mW/cm2 at 80  C, The ratio of forward anodic peak current density (IF) to the reverse anodic peak current density (IB) of Cu2O/PPy-GO is 1.51, limiting current of 0.4 V; the initial current density of 197 mA/cm2 decreased to 22 mA/cm2 after 900 s Maximum power density of 27 mW/cm2 at 60  C, IF/IB ratio of Ni/sPANI is 1.44, limiting current of 0.4 V; initial current density of ~280 mA/cm2 decreased to 121 mA/cm2 after 700 s After 50th cycle to 375th cycle, the current remained almost constant, and then the peak current decreased gradually until 600th and retained about 91% Limiting current of 0.5 V; the initial current density of ~300 A/g decreased to 165 A/g after 20 000 s The Tafel slope of Pt/p-Ni was recorded (18.6 mV/dec) was less than that of the commercial Pt/C (96.0 mV/dec) at low overpotentials, implying the faster kinetics on the Pt/p-Ni for MOR. After 120 cycles, the high mass activity of Pt/p-Ni remained with a retention of 95%, limiting current of 0.25 V; the Pt/p-Ni electrode mass current decayed gradually and still retained a large value of 168 mA/mg after 20 000 s The IF/IB ratio of PdNPs/R3DNG was 5.0, indicating that methanol can be electro-oxidised almost completely on PdNPs/R3DNG, limiting current of 0.2 V; PdNPs/R3DNG can retain the highest oxidation current and display the lowest current decay over the testing duration, showing that has the lowest poison of CO produced throughout the oxidation of methanol molecules, PdNPs/R3DNG forward peak current was maintained with 66.5% after 1000 cycles, signifying the tremendous long-term stability Limiting current of e 0.05 V; The current density initially had a sharp decay then subsequently a slow decay for 25 min, owing to the initial deactivation of the catalyst by the chemisorbed carbonaceous species

[65]

1.0 M CH3OH in 0.5 M H2SO4 with 50 mV/s scan rate, room temperature 1.0 M CH3OH in 0.1 M HClO4 with 50 mV/s scan rate 2.0 M CH3OH in 0.05 M H2SO4 with 100 mV/s scan rate

Ni/sPANI

2.0 M CH3OH in 0.05 M H2SO4 with 100 mV/s scan rate

80

NA

6.1 mA/g

PtRu/PPDAMWCNT

1.0 M CH3OH in 0.5 M H2SO4 with 50 mV/s scan rate

NA

73.5

731.6 mA/mg

EP-M

0.5 M CH3OH in 1.0 M KOH with 50 mV/s scan rate 0.5 M CH3OH in 1.0 M NaOH with 100 mV/s scan rate, N2 purged

NA

NA

28.56 mA/cm2

20 wt%

96.9

1383.9 mA/mg

Pt/p-Ni

Performance and durability

0.5 M CH3OH in 1.0 M NaOH with 50 mV/s scan rate, N2 saturated

20%

55.8

2.71 A/mg

Pd-pDAN

1.0 M CH3OH in 0.5 M KOH with 50 mV/s scan rate, 30  C

NA

6.04

1.7 mA/cm2

[114]

[64]

[118]

[59] [119]

[120]

[121]

(continued on next page)

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PdNPs/R3NDG

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2.0 M CH3OH in 0.5 M H2SO4 20 mV/s scan rate, Room temperature CSG-Pt(Ru)/TiN 1.0 M CH3OH in 0.5 M H2SO4 10 mV/s scan rate, 40  C PtRuMo/C 2.0 M CH3OH in 0.5 M H2SO4 10 mV/s scan rate, 75  C PtCo/RGO 2.0 M CH3OH in 0.5 M H2SO4 20 mV/s scan rate, room temperature Pt/OMC-WO3-500 1.0 M CH3OH in 0.5 M H2SO4 20 mV/s scan rate PteZrO2/NGN

Alkaline

Measurement conditions

**CSG-complexed sol-gel, RGO-reduced graphene oxide, OMC-ordered mesoporous carbon, NGN-nitrogen-doped graphene nanosheets, PPy-polypyrrole, SPani-sulfonated polyaniline, PPDA-p-phenylenediamine, EP-M-NieP decorated with microspherical NiO, R3DNG-robust 3D N-doped porous graphene, pDAN-poly-(diaminonaphthalene), MNCS-mesoporous nitrogen-doped carbon spheres, ADT-accelerated durability test, NA-not available.

100.1 mA/cm2 NA 1.0 M CH3OH in 1.0 M KOH with 50 mV/s scan rate 50 NieCu/TiN

NA

1007 mA/mg 84.55 1.0 M CH3OH in 1.0 M NaOH with 50 mV/s scan rate Pt/MNCS

20

1.8 A/mg 1.0 M CH3OH in 6.0 M KOH with 60 mV/s scan rate, 400  C

10

NA

during methanol oxidation. Based on the forward peak potential value recorded, the current retention value sustained with 83.8% for the first 100 cycles Limiting current of 0.6 V; NiO/NeCNF had an adequate tolerance towards [81] adsorption of methanol oxidation intermediates and products and also exhibited outstanding long-term stability for more than an hour The onset potential on Pt/MNCS (0.53 V) was more negative than that of [122] Pt/C (0.42 V), limiting current of e 0.2 V; the current densities gradually decreased until 1800 s as the electrode was poisoned by reaction intermediates. The durability of Pt/MNCS was analysed continuously until the 1000th cycle; the peak current reached nearly 88.6% in the 1000th scan after decreasing slowly in each successive scan from that evaluated in the first scan Limiting current of 0.7 V; the initial period of time, the oxidation current [123] declined, followed by a slower decline until it reached a steady state (3500 s) of 6.67 mA/cm2

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NiO/NeCNF

Catalyst sample Electrolyte

Table 1 e (continued )

Measurement conditions

Metal ECSA, MOR activity loading (wt.%) (m2/gmetal)

Performance and durability

Ref.

14748

of PteRu electrocatalysts is greatly dependent on the morphology, structure, composition, particle size and alloyed degree. Sui et al. [41] investigated electrocatalytic activity and stability on PtRu/CeTiN electrocatalysts for MOR. MOR results show that PtRu/CeTiN-15% has about 0.81 A/mgPt of the forward peak current density. This result implies that an electronic interaction exists between Pt and TiN and therefore deteriorates the adsorption force of COads on Pt NPs. As a result, the increase in catalytic activity due to Pt NPs deteriorates from less COads accumulation. The longtime durability of the catalyst was studied for 1000 cycles. The forward peak current density for PtRu/CeTiN-15% decreased after 1000 cycles. This result can be ascribed to the anti-corrosion and the anchoring effect of TiN. In recent decades, an effective way to improve catalytic performance is by introducing a third metal, allowing the properties of different elements to combine synergistically. Examples of a third metal that were used in previous studies are Fe [42], Ni [43,44], Cu [45,46], Co [47] and Mo [48]. A previous study evaluating the low-cost transition metal Mo as the third metal observed that Mo adsorbs and activates water at potentials much lower than Pt [35]. Mo has also been extensively studied because it can improve CO tolerance in MOR, displaying that Mo assists to break CeC bonding [35,49]. Moreover, Mo does not demonstrate affinity towards CO; hence, additional sites on the Mo surface can be accessible over a wide potential range to electro-dissociate water, leading to the generation of OH-species which oxidize organic intermediates adsorbed at adjacent Pt sites [50]. The mechanism by which Mo increases the oxidation of the CO adsorbed (COads) on the Pt surface remains unclear, but it is normally considered to be of a bifunctional nature such that Mo provides OH species to the system. These alternative steps can be written as follows: Pt-CHOads þ MoOx-OHads / CO2 þ 2Hþ þ 2e

(4)

Pt-COads þ MoOx-OHads / CO2 þ Hþ þ e

(5)

An effective technique to increase the electro-oxidation of CO surface is use Mo species in oxidized state. In addition, Mo should be easily reduced and competent to undergo oxygen transfer. A small percentage of the whole CO adsorbed onto the catalyst is weakly bonded at low potentials when CO is oxidized by Mo [51,52]. The activity of PtMo catalysts in methanol oxidation is poorer than that of PtRu catalysts because most of the CO produced and adsorbed onto the Pt surface during methanol dehydrogenation is strongly bound onto the surface [35]. However, the electrochemical role of Mo in these materials is still poorly understood. Whether Mo can work according to the double catalytic effect or whether methanol can be oxidized directly on the molybdenum surface has yet to be clarified [52]. Efforts have been exerted to increase the catalytic activity for methanol electro-oxidation and to reduce the cost of electrocatalysts. Ternary catalysts show higher catalytic activity compared with PtRu catalysts. Jeon et al. reported that PtRuFe is a promising candidate for methanol electrooxidation [53]. Fe as a third metal of PtRu can lower the activation energy of reaction. Alloying Pt and Fe may weaken

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Fig. 1 e SEM images (aeb) and TEM images (ced) of Pt/graphene and Pt/N-graphene [30].

PteCO bonding by prohibiting electron back-donation from Pt to CO [54]. PteCO bonding can be attenuated through the orbital mixing of electron-rich Pt and electron-poor Fe. The orbital mixing decreases Pt electron density, which prevents electron back-donation from Pt to CO, thereby weakening Pte CO bonding. In another study, Jeon et al. [55] reported that PtRuFe/C is a better catalyst than PtRuCo/C and PtRuNi/C. In this study, the PtRuFe/C catalyst exhibited the lowest particle size of approximately 4.6 nm (Fig. 2) and achieved the onset voltage of 0.46 V for CO electro-oxidation. The lowered CO electro-oxidation onset voltage of PtRuFe/C might be attributed to PteFe alloy formation, which is good compared with that in the study by Poh et al., who reported an onset potential of ~0.6 V. Low onset potential significantly boosts the kinetics of methanol electro-oxidation. In the measurement of methanol electro-oxidation activity, PtRuFe/C exhibited a mass of 2.6 A/g catal., which was 70% higher than that of commercial PtRu/C catalyst.

Ribeiro et al. [34] studied PtRuNi/C as an electrocatalyst for methanol electro-oxidation. They found that PtRuNi has a greater transformation number and lesser activation energy for methanol oxidation than PtRu. The combination of Ru and Ni promotes water activation, leading to the formation of eOHads species that assists the oxidation of COlike species intermediates produced from methanol decomposition, releasing the sites of Pt. Thus, the existence of Ni along with Ru would enhance the electrocatalytic activity and stability for methanol oxidation. Then, the modification of electronic structure can improve the oxidation of CO-like species on Pt sites by electron transfer from Ni to Pt [56]. The hydrogen spillover effect of Ni hydroxides and the electron effect of metallic can be explained the effect of Ni on PtRuNi [57]. Zhao et al. [58] deposited PtRuNi onto MWCNTs through step process. This study explained the bifunctional mechanism of PtRuNi/MWCNTs (Fig. 3):

Fig. 2 e TEM images of PtRuFe/C catalyst [55].

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Fig. 3 e Schematic of bifunctional mechanism of PtRuNi/MWCNT electrode surface in methanol solution [58].

Pt-free Several studies have utilized other metals rather than Pt as a catalyst for MOR [15e17,59e62]. Nowadays, researchers have explored alternative low-cost non-noble metal-based anode catalysts. However, non-Pt based catalysts have limited applications because of their poor stability in acidic media. Therefore, less research focused on non-Pt based catalysts in acidic media compared with alkaline media. Shantosh et al. [63] synthesized Au NPs onto polyaniline (PANI)-grafted multiwall CNTSs (MWNT-g-PANIs) using a two-step electrochemical process. They found that the MWNT-g-PANI-Au catalysts in acidic medium have advantages in DMFC applications, including enhanced electrocatalytic activities, high tolerance to poisoning from adsorbed carbon monoxide, high oxidation kinetics and performance at elevated temperatures. In another study, Das et al. [64] developed an anodic catalyst with modified PANI (partially sulfonated PANI) and Ni compared its performance to those of Ni/PANI, Ni/C and commercial PtRu/C catalysts in acidic medium. The partially sulfonated PANI (sPANI) showed better electrochemical performance compared with Ni/PANI. The presence of sPANI enhanced the dispersion of Ni particles due to its high accessible surface area and high stability, which increased the catalytic activity. This combination produced a peak current density of 306 mA/cm2 at þ0.57 V, which was higher by 25.6 mA/ cm2 than the corresponding values acquired for PteRu/C. The stability test results are presented in Fig. 4. Ni/C has the lowest initial current density of 250 mA/cm2, which decreased to 42 mA/cm2 compared with PtRu/C of 280 mA/cm2, which decreased further to 62 mA/cm2 due to Ni dissolution as a base metal in acidic medium. Pattanayak et al. [65] found that cuprous oxide NP-supported polypyrrole-graphene oxide (Cu2O/PPy-GO) is a promising anode catalyst for methanol oxidation in acidic medium as a non-noble metal catalyst. Cu2O/PPy-GO exhibits improved electrochemical performance

in acidic medium with a peak current density of 300 mA/cm2 at þ0.68 V, confirming its high electrocatalytic activity. This result can be attributed to the small and uniform size of NPs and the high dispersion and distribution of NPs on the support material.

Catalysts for methanol oxidation in alkaline media Recently, alkaline electrolytes have attracted the interest of many researchers to improve methanol oxidation because the kinetics of alcohol deprotonation is favoured in alkaline media. Therefore, the poisoning effect is weaker in the reaction in alkaline media compared with acidic media [66]. Alkaline DMFCs have a few advantages over acidic DMFCs. In

Fig. 4 e Chronoamperometric curves of the different catalystesupport combinations in 2 M methanol and 0.05 M H2SO4 at potential 0.4 V vs. SCE.

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brief, the favourable features of alkaline media for MOR include facile polarisation at low anodic overpotential, reduced electrode poisoning and fast oxidation kinetics of methanol [67]. Alkaline media also has a high coverage of OHads at low potential, which is necessary in methanol oxidation [68]. In addition, the selection of possible electrode materials with practically no sensitivity to surface surfaces has become wider [69]. Nevertheless, an issue with the alkaline medium is the carbonation of the solution due to CO2 production of the fuel oxidation from air: 2OH þ CO2 /CO2 3 þ H2O.

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This reaction causes the pH reduction and solid precipitation of carbonate salts on the electrode in the alkaline electrolyte solution. Thus, the reactivity for fuel oxidation in the system is reduced [70]. The utilization of alkaline electrolytes compared with acidic electrolytes not only enhances polarization features on Pt but also widens the usage of non-Pt catalysts which are noticeably cheaper than Pt-based catalysts.

Pt and Pt alloy-based catalysts Pt exhibits higher catalytic activity than other pure metals in both acidic and alkaline media for the MOR. Spendelow and Wieckowski reported a review on Pt and Pt-based catalysts in alkaline media for MOR [71]. Pt is easily poisoned by CO during MOR in acidic media, but the poisoning effect in alkaline media is very weak [72]. However, in alkaline media, the output of MOR is carbonate or formate, which is an issue of disputation. The rate-determining step during the MOR on Pt in alkaline media is a chemical step involving the reaction of adsorbed intermediates, namely, CHO and adsorbed CO. In alkaline media, methanol oxidation on Pt surface occurs through a set of reaction steps involving successive electron transfer, i.e., partial oxidation, to form adsorbed species. The adsorbed OH reacts with the previous adsorbed species to potentially form CO2. The reaction mechanism has been written as follows [70]: Pt þ OH/e Pt e (OH)ads þ e,

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Pt þ (CH3OH)sol /Pt e (CH3OH)ads,

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Pt e (CH3OH)ads þ OHe /Pt e (CH3O)ads þ H2O þ e,

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Pt e (CH3O)ads þ OHe /Pt e (CH2O)ads þ H2O þ e,

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Pt e (CH2O)ads þ OHe /Pt e (CHO)ads þ H2O þ e,

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Pt e (CHO)ads þ OHe /Pt e (CO)ads þ H2O þ e,

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Pt e (CHO)ads þ Pt e (OH)ads þ 2OHe /2 Pt þ CO2 þ 2H2O (13) þ2e, Pt e (CHO)ads þ Pt e (OH)ads þ OHe /Pt þ Pt e (COOH)ads þ H2O þ e,

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Pt e (CO)ads þ Pt e (OH)ads þ OHe /2 Pt þ CO2 þ H2O þ e,(15)

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Pt e (CO)ads þ Pt e (OH)ads /Pt þ Pt e (COOH)ads,

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Pt e (COOH)ads þ OHe /Pt e (OH)ads þ HCOO,

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Pt e (COOH)ads þ Pt e (OH)ads /2 Pt þ CO2 þ H2O.

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Tripkovic et al. conducted a relative investigation of methanol oxidation on a Pt(1 0 0) surface in different alkaline solutions [73]. Pt (1 0 0) in alkaline media during the MOR adsorbs OH and ‘poisoning species’ in the reaction. The formation of ‘poisoning species’ in the methanol oxidation blocks the surface partially at low potentials and participates in the reaction at high potentials. This study assumed that HCO is a reactive intermediate and that a formate is a reaction product in the main path and CO2 is a product of ‘poisoning species’ oxidation in a parallel reaction path. In another study, Tripkovic et al. [36] studied the effect of electrolyte on Pt and PtRu in methanol electro-oxidation. He found that the kinetics of Pt and PtRu at low overpotential are much faster in alkaline (0.1 M NaOH) than in acidic (0.5 M H2SO4) solution at 0.5 V. The effect of pH is ascribed from the pH competitive adsorption of OHads with bisulfate anions. Nagashree and Ahmed evaluated Pt/PANI electrocatalysts in three alkaline media, namely, NaOH, Na2CO3 and NaHCO3 [74]. Pt/PANI has a higher electrocatalytic activity than Pt and PANI, which can be ascribed to the large surface area amenable to the reaction on the electrode because of the cumulative effect of the dispersed Pt microparticles and the polymer matrix. Interestingly, Pt/PANI is less vulnerable to intermediate poisoning in alkaline media, and the poisoning effect of Pt/PANI is weaker in NaOH than in the two other media. The deterioration of the electrocatalytic activity in Na2CO3 and NaHCO3 is due to inefficient adsorption of methanol. Zhou et al. [75] studied a composite catalyst of Pt NPs supported on poly(5-nitroindole) (Pt/PNI) for methanol electro-oxidation in alkaline media (1.0 M KOH). They found that Pt/PNI provides enhanced catalytic activity and stronger poisoning-tolerance compared with the common Pt electrode. Although the position of the peak currents is almost the same for all the Pt-based catalysts, the current density of Pt/PNI is higher than that of the common Pt electrode. The onset oxidation potential of Pt/PNI is e 0.53 V versus SCE at scan rate of 50 mV/s with a current density of 14.29 mA/cm2, whereas that of the common Pt electrode is e 0.42 V versus SCE with a current density 3.94 mA/cm2. Chronopotentiometry was used to study the poisoning tolerance of catalysts for methanol oxidation. The potential increases with reaction time, and the sustained time is longer for Pt/PNI compared with the common Pt electrode at the same current density of 3 mA/cm2. This result verifies that Pt/PNI has greater poisoning tolerance than the common Pt electrode in alkaline media. A previous study also proved that Pt (111) in methanol oxidation has weaker poisoning effect and faster kinetics in alkaline electrolytes (NaOH) compared with acidic electrolytes [73]. Lastly, Wang et al. [76] studied the synergetic effect between Pt and Au in alkaline media for MOR. These findings suggest that the Pt entities on Au surface are sensitive to the upper potential limit during CV sweeping. The peak current of MOR on Pt-on-

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Au/SnO2-CNTs at ~ 0.2 V would descend quickly with CV sweeping, with the upper limit at 0.3 V, as shown in Fig. 5. These results are probably due to the fact that Pt entities placed on Au are highly dispersed as small domains and might accumulate under severe oxidation/reduction reactions. Previous studies reported that Pt and Au in alkaline and acidic media are expected to have a similar mechanism as the synergistic interaction between Pt and Au [77,78]. AueOH catalyzes the oxidation of CO-like species on Pt and Au surfaces. Overall, Pt-based catalysts are promising anode catalysts for MOR in alkaline media.

Pt-free metals Over the past few decades, the utilization of non-noble metals as electrocatalysts has attracted the attention of many researchers because it can lessen the adsorption of CO intermediates that might limit electrocatalysts and the enhanced kinetics in a low anodic overvoltage for methanol oxidation [59]. Non-noble metals are cheap but great alternatives to Pt-based catalysts.

Nickel-based catalyst. Nickel is one of the most explored among non-noble metals in alkaline media as an alternative to Pt anode catalysts for methanol oxidation. Nickel can be used as a catalyst because of its surface oxidation properties. Nickel surface in contact with a solution of aqueous alkaline becomes concealed with a layer of nickel hydroxide. Thus, the transformation on the surface is usually expressed as: Ni(OH)2 /NiOOH þ Hþ þ e.

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However, the oxidation state of nickel in the oxide layer may switch continually between two and four over a range of potentials [79]. A general mechanism for the oxidation of primary alcohol in alkaline medium was schemed by Fleischmann et al. [79]: Ni(OH)2 þ OHe /NiOOH þ H2O þ e,

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NiOOH þ RCH2OH /RC$HOH þ Ni(OH)2,

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Fig. 5 e Cyclic voltammogram of Pt-on-Au/SnO2-CNTs in 1 M NaOH þ1 M CH3OH deposition at 0.65 V for 20 s.

R$CHOH þ 3OH/ RCOOH þ 2H2O þ3e.

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This mechanism shows that the oxidation of primary alcohols at Ni/NiOOH causes the formation of organic acids in alkaline medium. Rahim et al. [61] studied the electrocatalytic oxidation of methanol on graphite electrodes modified with nickel in alkaline electrolytes, KOH. They found that nickel-modified graphite electrodes are good catalysts for the reaction in alkaline electrolytes because high current densities (over 150 mA/cm2) can be attained. In the electrooxidation of methanol in alkaline medium, Ni is dispersed on graphite electrodes by charge transfer with the electrode for thick oxides and then by direct chemical reaction for thin nickel oxides on activation-controlled proceeding with NiOOH. Basing from the result, we can conclude that this nickel electrode has high electrocatalytic activity for methanol oxidation. By contrast, S. N. Azizi et al. [80] suggested that transition metal ions can transport to the electrode surface by exchanging with Naþ in the electrolytes in alkaline solution (NaOH). In the Ni/SBACPE catalyst, the transport of Ni2þ to the electrode surface results from the ion exchange process, where they can contribute to the electrocatalytic oxidation of methanol [80]. Moreover, the presence of Ni2þ ions in CPE is necessary in oxidation process. Recently, Wang et al. have prepared a novel ordered mesoporous alumina-supported nickel (Ni/Al2O3) with a modified carbon electrode [67]. Cyclic voltammograms in alkaline solution show that the oxidation current of methanol is higher on Ni/Al2O3-5 (11.1 mA/cm2) than on Ni/Al2O3-2 (7.3 mA/cm2) and Ni/Al2O310 (4.2 mA/cm2). Compared with Ni/Al2O3-5, Ni/Al2O3-10 shows a lower current density because of its lower Ni content in the catalyst. Nevertheless, the Ni/Al2O3-2 catalyst with the highest nickel content does not exhibit the highest activity in methanol electro-oxidation because its mesoporous structure and nickel dispersion have been sacrificed. Aside from pure nickel electrode, other nickel-based electrodes, such as nickel alloy and nickel oxide, have been explored for methanol oxidation in basic media. Nickel oxide is a novel electrocatalyst for MOR. Enizi et al. [81] found that the NiO/NeCNF catalyst prepared using chemical precipitation for MOR in an alkaline medium has greater specific and mass activities for methanol oxidation than other nickelbased catalysts. They also studied the effect of loading NiO/ N-CNFs on MOR. Cyclic voltammetry (CV) shows that the methanol oxidation current linearly improves as the loading amount of NiO/N-CNFs is increased. This result can be ascribed to the mass transport of effective ions into the NiO catalyst owing to the presence of a highly nitrogen-doped macro and mesoporous CNF network. The onset of oxidation potential also slightly shifts to a more negative potential (from 380 mV to 300 mV vs. SCE) when the catalyst loading amount is increased from 2.0 mg to 12.0 mg, indicating an improved kinetic performance for MOR. In addition, the specific and mass catalytic activities of NiO/N-CNFs for methanol oxidation can reach up to 0.3 A/cm2 and 1.8 A/mg, respectively. Tong et al. [59] synthesized a NieP framework with microspherical NiO (EP-M) and nanoflake NiO (EP-F). Between both samples, EP-M possesses a high specific surface area of

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210.03 m2/g. Thus, EP-M indirectly exhibits a high current density of ~467 A/g and long-term chronoamperometry (CA) stability (20 000 s) in alkaline media. This catalyst outperforms many state-of-arts NiO-based catalysts. The improved catalytic activity of the EP-M catalyst is attributed to the strong electronic interaction between NiO and NieP components. Aside from pure nickel and nickel oxide, nickel alloy is one of the most studied nickel catalysts. In 2010, Danaee et al. [82] produced nickel-manganese alloy modified graphite electrodes (G/NiMn) using galvanostatic deposition. Cyclic voltammograms in alkaline solution indicate that methanol G/ NiMn shows a significantly higher oxidation current density compared with nickel-modified graphite electrodes (G/Ni). This result is due to the pre-adsorption of methanol molecules at the Mn sites by interaction of the partially vacant d-orbital of the Mn species within the oxide film with non-bonded electron pairs of the O-atom of methanol. Ahmed [83] evaluated the performance of a nickel phosphate (NiPh)-modified platinum (Pt) electrode synthesized by a simple reflux-based method in an alkaline medium direct methanol fuel cell. This electrode shows a fuel cell current density ranging from 2.5 mA/cm to 5.8 mA/cm (a ~50% increase), which is higher than that of the bare Pt electrode. These works expect improve fuel cell performance.

Palladium-based catalyst. Many studies have focused on searching for materials cheaper than Pt. Pd is at least 50 times more abundant on earth than Pt. Thus, the use of Pd has attracted interest to reduce the cost of catalysts. Recent studies have investigated Pt-free electrocatalysts, such as Pd, and found that Pd is a good electrocatalyst for ethanol and methanol oxidation in alkaline media [84,85]. One study reported that Pd is an unsuitable electrocatalyst in methanol oxidation but shows distinctively excellent steady-state and superior activity for ethanol electro-oxidation in alkaline media [86]. Pd has a considerably lower activity than Pt but is active for methanol oxidation in alkaline media [87]. The MOR activity of Pd can be improved by adding a second metal, such as Au or Ni. In 2010, Wang et al. [88] described the electrocatalysis of PdeAg supported on carbon black towards methanol oxidation in alkaline media. All the PdeAg/C catalysts are relatively active for MOR in alkaline media, and their electrocatalytic activities are higher compared with that of Pd/C. Noticeably, the onset potentials of methanol oxidation are more negative on PdeAg/C than on Pd/C, and the peak current densities increase. This result indicates that methanol electro-oxidation is more active on PdeAg/C than on Pd/C. In addition, the second metal activates water at lower potentials than Pd, and the activated water can oxidize the adsorbed CO or other poisoning intermediates via the bifunctional mechanism and therefore release Pd active sites. As mentioned before, the presence of a second metal is important to enhance the MOR activity of Pd. Thus, Amin et al. [89] used Ni as a second metal to improve the MOR activity of Pd. Adding Ni in PdeNi/C electrocatalysts generates another oxidation peak at 0 mV and shifts the potential of the oxidation peak in the positive direction by 90 mV (þ860 mV). This new peak of Pd results from the oxidation of freshly chemisorbed methanol species at the Pd surface. The current density of this oxidation

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peak at the PdeNi/C electrocatalyst is 1.92 higher than that at Pd/C. This result indicates that Ni improves MOR at this potential region because of the change in the electronic properties of Pd. It also reflects that the presence of Ni at a low potential value facilitates the removal of incompletely oxidized carbonaceous species formed in the forward scan. According the bifunctional mechanism, Ni is an oxopholic element similar to Ru. Ni facilitates the oxidative desorption of intermediate products and generates OHads at a low potential value, thus increasing the stability of PdeNi electrocatalysts [90]. Wang et al. [91] fabricated nanoporous palladium (npPd) for methanol electro-oxidation in alkaline media (KOH). The results show that the CV curve only reveals the oxidation and reduction of the surface Pd atoms in alkaline solutions and that electrocatalytic oxidation current signals are absent in the methanol-free KOH solution. A complex process involving ethoxyl ((CH3CO)ads) and hydroxyl (OHads) competing adsorption on the surface of the Pd electrode occurs during ethanol oxidation on the polycrystalline Pd electrode in alkaline media [92]. Thus, an analogous competing adsorption of methoxyl ((CH3O)ads) and hydroxyl (OHads) is expected to occur on the Pd electrode surface during methanol oxidation in alkaline media [93]. Furthermore, the inadequate adsorption of OHads can shift the oxidation of Pd surface atoms at a more positive potential with methanol in the electrolyte. Calderon et al. [94] synthesized PdeNi catalysts supported on carbon blacks consisting of metal contents (25 wt%) and Pd:Ni atomic ratios (1:1 and 1:2). The methanol oxidation on PdeNi catalysts recommends that the presence of Ni improves the activity of the materials, indicating that Pd/CB exhibits the lowest methanol oxidation current densities. Investigation of the catalyst activity in the supporting electrolyte (0.1 M KOH) shows that the current densities increase between 0.6 and 1.0 V vs. RHE in the forward scan, which is ascribed to the formation of PdO. In the backward scan, this oxide is reduced, producing a cathodic current peak between 0.6 and 0.7 V. The area bounded by this reduction peak was used to define the electroactive area. The presence of surface functional groups elevates the increase in the electroactive area. This result agrees with the metal surface areas calculated from the average crystallite size obtained from the TEM result. The NPs supported on the chemically modified carbon blacks show the largest size. Hence, small NPs and high-metal and electroactive surface areas were achieved.

Rhodium-based catalyst. Rhodium (Rh) is a silver-white metallic element that is highly reflective and resistant to corrosion. Beforehand, Rh was almost not considered as an electrocatalyst for methanol oxidation reaction in alkaline media despite it is already employed in numerous nonelectrochemical and electrochemical applications as a catalyst. But, Rh shows an interesting property as its exhibit oxophilicity in alkaline medium. The oxophilicity of Rh in alkaline media for MOR suggested that it is helpful for e OH adsorption, which further help with the removal of the adsorbed CO [95,96]. In previous, there were studies focused on Rh as an electrocatalysts for ethanol oxidation reaction (EOR). Suo et al. [97] reported the synthesis of rhodium/carbon (Rh/C)

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electrocatalyst for ethanol oxidation. The result shows that in an alkaline medium, Rh/C exhibit much higher catalytic activity at low potential region than Pd/C. The higher onset potential of ethanol oxidation of Pd/C resulted from a higher onset potential of oxygen desorption. In this study, also reported that there is no activity for Pd/C and Rh/C for ethanol oxidation in acidic medium. This is because of the difficulty of dehydrogenation of ethanol results in nearly no ethanol oxidation on Pd in acidic medium, which the explanation also applicable for Rh [98]. Although recent work has demonstrated that in alkaline medium, Rh-based catalysts show high electrocatalytic activity for EOR than Pd-based catalysts [97,99], the MOR using Rh-based catalysts has been neglected. In 2016, Kang et al. [100] synthesized Rh nanodendrites using a facile diethylene glycol reduction method. In particular, 3-D branched Rh nanodendrites shown improved electrocatalytic activity compared with Rh nanoaggregates. From this study, Rh nanodendrites gave a larger hydrogen adsorption/desorption peak, indicating that Rh nanodendrites (43.35 m2/g) had a larger ECSA compared with Rh nanoaggregates (23.09 m2/g) due to their highly branched structure and the 2-D feature of the nanosheet subunits [100]. The electrocatalytic activity depends on structural of metal nanostructures electrocatalysts. Currently, 2-D nanosheets can present an atomic level bridge between that active center and catalytic activity because of its flat facet, low coordination facets, and ultrahigh surface area [101,102]. In the recent studies, Kang et al. [103] developed Pt-alternative anode electrocatalyst, Rh nanosheets (Rh-NSs) on the reduced graphene oxide (RGO). In their work, they obtained the onset oxidation potential of the MOR using Rh-NS/RGO negatively shift ca. 120 mV compared to the commercial Pt/C electrocatalyst. In the meantime, CV measurements show that the MOR mass activity of Rh-NS/RGO(41 mA/gRh) electrocatalyst is 3.6 times bigger than that at Rh-NPs/RGO (27 mA/gRh). Due to the ordinary 2D structure, the RheRh coordination number in Rh nanosheets is much lower than that in conventional Rh nanoparticles. These Rh atoms with low coordination number generally are regarded as the highly active sites for electrocatalytic reactions [103]. Fu et al. [104] showed that ultrathin wavy Rh nanowires can be robustly synthesized with 2e3 nm diameters. The electrocatalyst exhibited a current peak at the potential of 0.61 V vs. RHE, considerably lower than that of Pt based catalysts (~0.8e0.9 V vs. RHE). This research demonstrated that the IF/IR ratio of the ultrathin Rh wavy nanowires is ~2.3, which is higher compared with some of the previously reported Pt-based catalysts. The high IF/IR represent high oxophilicity of the catalyst, in which Rh binds more easily to OH and oxygen, which also indicates that the Rh is considered to be more oxophilic than Pt. Kang et al. [105] discovered hollow Rh nanospheres (Rh HNSs) were capable of improving activity for the methanol oxidation reaction (MOR) compared to the state-of-the-art Pt nanoparticles in alkaline media. This Rh H-NSs catalyst displayed a much smaller activation energy (Ea) value (29.81 kJ/ mol) of the MOR than Ea value (50.74 kJ/mol) of commercial Pt nanoparticles for the MOR. Apparently, smaller Ea leads to the high electrocatalytic activity suggested by the Thermodynamics theory [106]. Besides, MOR kinetics study of Rh H-NSs catalyst reveals that it has lower Tafel slope (111 mV/dec) of

MOR compared with the Tafel slope (223 mV/dec) of MOR commercial Pt nanoparticles. These indicating that Rh H-NSs catalyst has the fastest charge-transfer kinetics than commercial Pt nanoparticles which could result to faster transport of electron and other active species from the MOR [107].

Other metal-based catalysts. Compared with precious metals, transition metals are more economical and abundant. Transition metals such as Cu and Co are likely used as catalysts for MOR. Cu-based catalysts are well-studied catalysts from transition metals because of their highly regarded performance as electrocatalysts [108]. Pesika et al. [109] studied the MOR activity in alkaline solution of platelike copper crystals and found that the platelike copper crystals are active for MOR in alkaline media. This study discovered that copper is easily oxidizable, which does not allow enough time to mark the prominence between different characteristic regions of hydrogen and oxygen adsorption. In addition, Barakat et al. [110] studied the effect of Cu NPs as novel non-precious catalysts for methanol oxidation in alkaline media. They observed that the peaks in the voltammogram (Fig. 6) of the pristine Cu NPs equivalent to Ni are an activation of the surface anode to form the active CuOOH layer as follows [61]: Cu þ 2OHe 4Cu(OH)2 þ 2e

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Cu(OH)2 þ OHe 4CuOOH þ H2O þ e

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The current density of the cathodic peak increases with the number of potential sweeps because the entry of OH into the Cu(OH)2 surface layer leads to the progressive formation of a thick CuOOH layer corresponding to the Cu(OH)2/CuOOH transition [110]. Barakat et al. [111] investigated the influence of Cu content on the electrocatalytic activity towards methanol oxidation of CoxCuy-CNF in alkaline media. The Cu content strongly influences electrocatalytic activity; in specific, redox peaks

Fig. 6 e Cyclic voltammograms of electrocatalyst pristine copper nanoparticles in 1 M KOH for 50 cycles at a scan rate of 100 mV/s.

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almost completely become depleted with increasing Cu content in electrospun solutions.

Structure of catalyst support materials The support for the metal NPs affects NP dispersion and stability. The catalyst requires the highest surface area as possible because catalysis is a surface effect [61]. In the past, numerous materials have been tested as catalyst supports for DMFCs. The requirement for electrical conductivity has ruled out conventional catalyst supports such as alumina. Hydrophobicity, surface area, morphology, porosity and corrosion resistance are also essential factors in selecting a good catalyst support. The characteristics of the support materials have distinct effects on the preparation procedures and performance of synthesized, supported catalysts. A considerable amount of literature has focused on understanding the effects of non-carbon and carbon-based materials as a support for DMFC.

Mesoporous silicas Synthesis of mesoporous silica was patented around 1970. In 1990, researchers in Japan individually synthesized mesoporous silica NPs (MSNs). MSNs are a common class of solid silica materials with periodic mesopores of hexagonal, cubic or lamellar structures with 2e50 nm of tunable pores [20,21]. MSNs have served as robust inorganic supports with soughtafter features for flexible applications [124]. The first discovery of mesoporous materials by scientists at Mobil Corporation belonged to the M41S family in 1992 [125]. The most common types of mesoporous NPs are MCM-41, SBA-15 and SBA-16. MCM-41 is obtained in basic media, whereas both SBA-15 and SBA-16 are obtained in acidic media [125].

Fig. 7 e Cyclic voltammograms of Ni-MCM-41 powder mixed with carbon black in the ratios of 1:1 and 1:3 in 1.0 M NaOH at 10 mV/s [128].

the peak potential in the anodic direction. This result indicates that the modification of the Ni-MCM-41 electrode by immersion in Ni2þ solution boosts its catalytic activity towards methanol electro-oxidation 2.7 times greater than that of the unmodified electrode. The presence of MCM-41 in the catalysts is vital as it can offer a large surface area for metal dispersion and contains many channels and pores as shown in Fig. 8. This structure allows the diffusion of electrolytes inside the electrode and increases the efficiency of oxidation process [128]. Furthermore, the resistance to deactivation caused by the poisoning species can be improved by adding MCM-41 as a catalyst support for Ni.

Mobil Crystalline Material (MCM-41) Mobil Crystalline Material (MCM-41) is a mesoporous material from a family of silicate and alumosilicate solids with a hierarchical structure consisting of an ordered hexagonal arrangement of cylindrical mesopores. Since its discovery in 1992, MCM-41 has been synthesized in many types of morphologies and different pore sizes via the Stoeber method. This method is effective for the synthesis of mono-dispersed silica particles. Studies using the Stoeber method obtained mesopore sizes in the range of 2e50 nm [126]. MCM-41 is characterized by an adjustable pore diameter, a large surface area, a sharp pore size distribution and a large pore volume [127]. Pore size distribution can also be easily altered, and the pores are larger than with zeolites. Hassan et al. [128] prepared Ni-MCM-41 (5 wt% Ni) via impregnation and mixed it with conducting carbon black in an appropriate ratio in different concentrations of NaOH. Results show that the current density values of Ni-MCM-41 increase with increasing NaOH concentration. As a result, a high concentration of OH ions in the solution would increase Ni(OH)2 formation and Ni(OH)2/NiOOH transformation. Cyclic voltammograms (Fig. 7) at the potential value of about þ500 mV (MMO) indicate the start of oxidation process for methanol with NiOOH formation, and it is also the maximum value at

Fig. 8 e TEM micrograph of Ni-MCM-41 powder [128].

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Santa barbara amorphous 15 (SBA-15) In 1998, Zhao et al. [129] developed SBA-15, which has attracted aroused interest due to its large surface area inert framework, high biocompatibility [130], well-defined pore structure, thermal and hydrothermal stability and nontoxicity. SBA-15 is a mesoporous silica (SiO2) synthesised with Pluronic triblock-copolymer P123, which has parallel pores and highly ordered hexagonal arrangement. SBA-15 has a 2-D hexagonal arrangement of uniform cylindrical mesopores, p6mm mesostructures [13,17]. These structures allow SBA-15 to be used in chemical sensing, separation by chromatographic techniques [19,20], adsorption [135], immobilization, catalysis [22,23] and drug delivery systems [138]. As a new class of synthetic materials, SBA-15 mesoporous silica exhibits a large surface area, a large pore size and thick walls. Compared with mesoporous MCM-41 and related silicas, SBA15 has thick walls to provide high thermal and mechanical stability [138]. Azizi et al. [80] studied the modified electrode of Ni/mesoporous silica (SBA-15) for methanol electro-oxidation. In this study, mesoporous SBA-15 was synthesized using amorphous silica with approximately 80% purity with average diameter of 82 nm and have a spherical and rod-like shape. The modified electrode was prepared by incorporating Ni/ SBA-15 with carbon paste (Ni/SBA-15CPE). The electrocatalytic behaviour of Ni/SBA-15CPE on methanol oxidation was investigated in alkaline solution by CV and CA. From the CV (Fig. 9) of Ni/SBA-15CPE in 0.1 M NaOH solution at a potential sweep rate of 25 mV/s, the pair of redox peaks appearing at 0.56 and 0.29 V is assigned to Ni2þ/Ni3þ redox couple. SBA-15 is a highly porous material, and the large number of pores in Ni/SBA-15CPE offers a large surface area for NiOOH as active sites to contribute with methanol oxidation. Thus, Ni/SBA-15CPE can enhance methanol oxidation in alkaline solution.

material which is potentially applicable in many areas of science and engineering materials [139]. SBA-16 is synthesized in acidic media in the presence of triblock copolymer surfactants Pluronic F127 (PEO106PPO70PEO106) [140]. Ideally, SBA-16 has a structure of spherical body-centred nanocages with a cubic arrangement, Im3m, wherein each sphere is connected to eight neighbouring spheres through smaller pore apertures [141]. The structure of SBA-16 renders it attractive for many applications, such as catalysis [13,16], functionalization, metal incorporation [142] and templating [143]. However, limited synthesis methods have been reported because SBA-16 can only be produced in a narrow window of synthesis parameters of cage-like SBA-16 mesostructured silica such as temperature [144e146]. In recent years, SBA-16 has been prepared using cetrimonium bromide (CTAB) as a co-template in which CTAB regulates the morphology and controls the shape of SBA-16 mesostructure [144,145]. Azizi et al. [131] synthesized SBA-16 hydrothermally under an acidic medium using SiO/F127/ BuOH/HCl/H2O gel and modified with Ni(II) by dispersion in a 0.1 M nickel chloride solution. The SBA-16 NPs obtained have spherical shapes in the range 30e60 nm, which agglomerate to each other as shown in Fig. 10. Then, Ni/ SBA-16 is modified by mixing with carbon paste (Ni/SBA16CPE). The results obtained show an increase in anodic oxidation current on Ni/SBA-16CPE. Noticeable characteristics of SBA-16 include high porosity and large pore number in Ni/SBA-16CPE, which offers a large surface area for Ni2þ ion uptake. The large surface area acts as active sites during conversion to NiOOH and accelerates methanol oxidation. The presence of Ni2þ ions in CPE is mandatory for methanol oxidation because the small oxidation peak at 0.6 V represents the oxidation peak of Ni.

Santa barbara amorphous 16 (SBA-16) Among the reported cubic mesoporous silica materials, SBA-16 is one of the best candidates for catalytic support because of the good thermal stability of its thick wall, economical synthesis with inexpensive silica sources and 3D large pores [131]. In general, SBA-16 features easy preparation method, orderly structure and control over the size and shapes of their pores. Thus, SBA-16 is a versatile

Fig. 9 e Cyclic voltammograms of Ni/SBA-15 and CPE (A) in the (a) absence and (b) presence of (B) 0.03 M methanol in 0.1 M NaOH at a scan rate of 25 mV/s [80].

Fig. 10 e (A) SEM and (B) TEM images of synthesised mesoporous SBA-16 nanoparticles [131].

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Metal oxide-based materials Metal oxide supports have been investigated to verify whether they can be a good catalyst supports in DMFC. Many oxide materials can act as co-catalysts, due its good corrosionresistant and catalytic properties [147]. Unfortunately, metal oxides are not favourable in electrocatalyst application because of conductivity considerations. However, some metal oxides, can offer electrical conductivities comparable to that of graphite. Hence, they can be regarded as appropriate supports.

TiO2 Titanium dioxide (TiO2) is also known as titania, exists in three crystalline form; anatase, rutile and brookite. Of the three crystalline form, anatase and rutile are the most typical and to that refer to the number of researches that has been conducted on anatase and rutile as support materials. The crystallite size of the rutile is larger than the anatase phase. The TiO2 rutile structure has a tetragonal structure and 6 atoms per unit cell with shape of octahedron TiO6 is slightly distorted as presented in Fig. 11. While, anatase has the same structure as a rutile, but have a slightly larger distortion of TiO6 octahedron (Fig. 10). The structure of TiO2 crystals of rutile and anatase have a good thermodynamic stability and contributes to good results for thermal stability and electrochemical composite materials [148]. Anatase is frequently utilized as a catalyst supports due to its strong interaction with metal nanoparticles and electron transfer is about 10

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times faster in anatase than in rutile [149,150]. Among different metal oxide materials such as SnO2, WO3, and perovskite, TiO2 has drew enormous interest in electrochemistry because of its catalytic properties, high chemical and thermal stability in both acidic and alkaline electrolyte, non-toxic, low cost, and a superior corrosion resistance in the fuel cell environment [151,152]. However, the same limitation in SiO2 was also notice in TiO2 materials, where these materials have low electrocatalytic activity because of its relatively low electrical conductivity and low surface area. To overcome this hindrance, numerous approaches have been conducted [112,151,152]. Sui et al. [151] presented the preparation of Pt deposited on the mixture of carbon black and TiO2 nanotubes (TNT) by a microwave-assisted polyol process. TiO2 nanoparticles (TNPs) are used as a reference. The Pt/C-TNTs catalyst demonstrate the highest ECSA (89.7 m2/gPt) and the highest MOR activity (~600 mA/mgPt at 0.05 V) compared with Pt/C-TNPs. The improved performance of the Pt/C-TNTs catalyst was obviously contributed from the superior composite carrier of CTNTs: (1) TNTs has strong corrosion resistance in acidic and oxidative environment and a metal support interaction between Pt and TNTs; (2) Compared to TNPs, TNTs is more suitable for electrocatalytic field on account of its better electronic conductivity; (3) The distribution of carbon enhances the poor conductivity of TNTs. The addition of carbon materials not only can enhance its conductivity, it also will help increase the surface area of the catalysts and thus can prevent the agglomeration of Pt [153].

Fig. 11 e Crystal structures of the rutile and anatase phases of TiO2 [154].

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SnO2 Among metal oxide materials, SnO2 is believed to be one the most promising candidate, because of its low cost, can adsorb OH species at low potentials, and have ability to induce electronic effect with Pt [20]. Unlike other metal oxides, SnO2 can work as an independent catalyst support without involving an alteration (e.g. addition with carbon materials). As in work by Matsui et al. [155] displayed that when SnO2 supported on Pt catalysts without any modification, their catalytic activity for electrochemical oxidation of CO were improved compared with Pt/C. This is due to the metal-support interaction between SnO2 and Pt metal (electronic effect) [156]. Although, Pt/ SnO2 catalysts already displayed a good result, using only SnO2 with Pt cannot surpass the performance of PteRu catalysts. These can be improved by modification its structure. Normally, SnO2 has a surface area (<100 m2 g1) lower than that of reference Vulcan XC-72. Considering the importance of a high surface area of the support material for anode catalysts, addition of carbon materials into SnO2 will help to enhance the electrocatalytic activity. Cui et al. [157] prepared PteSnO2 supported on graphitized mesoporous carbon (GMC). They proposed that GMC with high surface area (585 m2/g) and high conductance increased the electrochemical catalytic activity which make the Pt nanoparticles highly dispersed and therefore easily accessible by methanol molecules. Also, the formation of oxygen-containing groups Sn-OHads from SnO2, can lower the methanol oxidation potential by removal of COads species from the Pt surface [157]. The durability of support materials for anode metal catalysts has been identified as one of the significant factors in DMFC. Despite the fact that carbon materials have high electronic conductivity, carbon is suffered with corrosion in aqueous solution which leading to agglomeration of nanoparticles and produce CO2, thus lowered the cell performance [158,159]. The addition of SnO2 into carbon materials can hinder the carbon from electrochemical corrosion. Hence, SnO2 can be ascribed as an effective catalyst support to Pt in direct methanol fuel cell applications.

Carbon-based materials In past research, carbon black supports (especially Vulcan XC72R, black pearls 2000, etc.) are the most frequently utilized in Pt and Pt-based alloy catalysts for fuel cells. Fig. 12 summarizes the approximate percentage of different types of carbonaceous materials in fuel cell applications. Carbon black materials are <50 nm in diameter with a BET surface area of ~250 m2/g, a mesopore and macropore percentage of 54% and an electric conductivity of 2.77 S/cm, which can satisfy the requirements of a support for an electrocatalyst. Compared with other widely used supports, carbon particles are more suitable catalyst supports because of their relative stability in both acidic and basic media, good electric conductivity and high specific surface area [160]. The characteristics of supported metal catalysts, such as metal particle size, stability, alloyed degree, size distribution, dispersion and morphology, are strongly influenced by the support material. Meanwhile, the performance of supported catalysts in fuel cells is influenced by carbon material in terms of electrochemical surface

Fig. 12 e Approximation percentages of different types of carbonaceous materials in fuel cell applications from 2011 to 2016 [166].

area, mass transport and catalyst layer electronic conductivity, and the stability of metal NPs during the operation [161]. Carbon supports needed for electrocatalytic reactions have good electronic conductivity (graphitization degree) to prevent oxidation and to lessen electrode resistance [162]. In fuel cells, the conductivity of electronic properties of carbon with significant performance benefits represents that the threephase boundary (electrodeeelectrodeereactant) can be extended into the electrode. The characteristics of the carbon support materials strongly influence the performance preparation and procedures of synthesized supported catalysts. Carbon black is the state-of the-art catalyst support for fuel cell electrodes because of its high electrical conductivity [163]. However, some of its properties, such as surface area, pore size distribution and surface chemistry, prevent satisfactory performance in fuel cell reactions [164]. Thus, carbon black supports lead to poor stability during long-term operation and low utilization of the metal catalysts deposited on their surface [165]. Consequently, various new carbon materials have been explored as electrocatalyst supports in DMFC, and some remarkable results have been achieved.

Functionalised mesoporous carbon In recent years, the interest in mesoporous carbon with tailored structured for catalyst support in DMFC anode catalysis has increased [13,37,38,167,168]. Catia et al. [13] prepared PtRu supported on mesoporous carbon with different pore size distributions as anode and reported the catalytic activity in passive and active DMFCs. Salgado et al. [37] synthesized PteRu electrocatalysts supported on ordered mesoporous carbon (CMK-3) by formic acid method and compared their electrocatalytic activity towards CO oxidation with those of Vulcan XC-72 and commercial catalyst from E-TEK. However, Bruno et al. [167] found that mesoporous carbon as support improves catalyst dispersion, obtained by the impregnation and reduction method NaBH4 as the reducing agent, as compared with the catalyst supported on Vulcan carbon. The structural parameters of surface area, pore size and particle

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morphology should be controlled for the extra fine tuning of the catalyst electroactivity [168]. This technique allows a higher degree of dispersion of the catalysts and facilitates an efficient diffusion of the reactants and by products [169]. Mesoporous carbon also has a small amount of oxygen surface groups that can cause drawback for many applications. Carbon supports which interact with the metal precursor by means of surface oxygen groups increase dispersion [37]. Carbon support materials must strongly affect the efficiency of synthesized supported catalysts and preparation methods. The structure of the carbon support depends on the removal of the products and the accessibility of the reactants to the catalytic site, which affect the performance of electrocatalysts. Mesoporous carbon also allows efficient diffusion of hydrogen to the active catalyst sites. Chai et al. [169] synthesised ordered uniform porous carbon with pore sizes in the range of 10e1000 nm by carbonization of phenol and formaldehyde against removable colloidal silica crystalline templates (Fig. 13) and studied the anodic performance of porous carbon-supported PtRu catalysts under DMFC state. Their result demonstrated that the one with about 25 nm in pore diameter has the maximum performance, equivalent to a ~40% increase in activity in comparison with the commercial existing PteRu alloy catalyst (E-TEK). The performance not only relies on high surface areas and large pore volumes that allow a high degree of dispersion of the catalysts but also on good pore interconnection systems with periodic order that permits effective diffusion of reagents. This approach allows the pore sizes of the carbon structure to be varied in the range of several nanometres to several micrometres by altering the mesoporous silica in the colloidal crystal template. The main limitation of this approach is the difficulty to synthesize uniform mesoporous silica with small particle sizes, particularly as the particle diameters turn into smaller than 50 nm [169]. Kim et al. [170] synthesized different types of ordered mesoporous carbons (CMK-3, CMK-3G and CMK-5) by a nano casting method using the ordered mesoporous silica SBA-15 as a template and supported on Pt. In this study, they evaluated electrochemical performance and durability. Pt/CMK-3G has the highest electrochemically active surface area, kinetic current density, mass activity and half-wave potential among the electrocatalysts. The formation of highly crystallite Pt

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particles as well as its highly graphitic, crystalline carbon structure also results the highest oxygen reduction reaction (ORR) activity. This result is probably because of the strong interaction and weak adsorption of surface oxide between the Pt particles and the support, respectively. Bruno et al. [167] studied the effect of Pt supported on mesoporous carbon based on synthesis and performance. Mesoporous carbon synthesized the carbonization of a resorcinol-formaldehyde with a cationic polyelectrolyte as a soft template to enlarge the specific surface area. The obtained result indicates that mesoporous carbon-supported Pt catalyst utilized as a cathode in DMFC is 30% higher in power density compared with Pt supported on Vulcan carbon under the same conditions. From the results obtained in this study, we can conclude that mesoporous carbon can improve catalyst dispersion. Thus, mesoporous carbon is a good candidate for catalyst support in DMFC anode catalysis because it can reduce catalyst loading and minimize the cost by improving catalyst utilization.

Activated carbon Activated carbon has an amazingly high-surface-area per unit volume and a complex structure composed originally of carbon atoms that are a highly porous adsorptive medium. Activated carbon has all the required characteristics to be used as a catalyst support. Moreover, it has unique features, such as stability in both acidic and basic media, the possibility of tailoring both its textural and surface chemical properties and the possibility of easy recovery of valued metals supported on it according to the targeted aims of the catalyst producers [171]. Activated carbon is manufactured from carbonaceous source materials, such as coconut shell, peat, hard and soft wood, lignite and coal. The physical and activity characteristics of activated carbon are affected by the base raw material and pre-treatment steps prior to activation. These different properties make some carbons more suitable than others for applications. Several researches have investigated activated carbon as support for DMFC catalysts in recent times. In other research, Yan et al. [172] found that the gold NPs supported on activated carbon (Au/C) demonstrate good catalytic activity towards methanol electro-oxidation and the inadequately adsorbed OH on the surface of Au NPs has supplementary catalysis for

Fig. 13 e Procedure for uniform porous carbons of tunable pore sizes through the colloidal crystal template approach [169].

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Fig. 14 e Schematic of structure (a) single-walled carbon nanotubes (SWCNTs) and (b) multi-walled carbon nanotubes (MWCNTs).

methanol electro-oxidation. The results indicate that few inventive ways may be analysed to understand catalytic activity and performance enhancement and optimization.

Carbon nanotube CNTs have been of extreme attraction in materials science after their discovery by Iijima (NEC, Japan) in 1991. From the discovery, it confirmed that the structure provides a seminal breakthrough in the science of nanomaterials. CNTs are allotropes of carbon with a 2D cylindrical nanostructure, commonly tubes formed by rolled-up single sheets of hexagonally aligned carbon atoms which can be synthesized in the laboratory and show remarkable properties. These incredible structures have unusual properties, such as electronic, magnetic and mechanical, which are beneficial for nanotechnology, optics and other fields of materials science and technology. Another fascinating property of CNTs is their capability to capture atoms of other elements within their molecular structure [173]. CNTs can be categorized in many different types, but they are typically categorized as either single-walled carbon nanotubes (SWCNTs) or MWCNT, as shown in Fig. 14. SWCNT is a structure composed from one cylinder like a regular straw. It has a smaller diameter, as low as 0.4 nm, and can be many millions of times longer. MWCNT contains a concentric set of nested tubes with a constant interlayer of continuously increasing diameters. The diameter of MWCNT starts from a few nanometres to tens of nanometres and are conducting materials. Recently, CNTs are the most broadly studied and leading carbon nanostructures for utilisation as catalyst support in fuel cells, especially DMFC. Both SWCNTs and MWCNTs have been widely investigated as catalyst supports in DMFC application [174e177]. SWCNTs provide large surface areas, whereas MWCNTs are more conductive. Chetty et al. [31] used plasma treatment to synthesize nitrogen-doped carbon nanotubes (N-CNT) and deposited NCNT on PtRu by impregnationereduction. The PtRu/N-CNT electrocatalyst has achieved particles larger than those of commercial carbon-supported catalysts (typically 2e3 nm), whereas the particle size-dependent catalytic activity. Their result showed that PtRu/N-CNT has higher activity for

methanol oxidation compared with PtRu/O-CNT because nitrogen plasma treatment developed pyridinic and pyrrolic species on the CNT surface. The species can behave as specific anchoring sites for the dispersion and deposition of PtRu particles. Thus, nitrogen-doped CNTs are a promising catalyst support for DMFC. Wang et al. [88] synthesized activated CNTs (aCNTs) using alkali treatment and loaded them on Pde Ag using a reduction method. Their result showed that the mass activity of the PdeAg(1:1)/aCNT catalyst at an onset potential of 0.59 V is higher compared with those of other catalysts. This result can be ascribed to the large electroactive surface area and improved intrinsic catalytic activity. Wang et al. [178] activated MWCNTs through alkaline treatment using activate agent (solid KOH) and prepared a simple reduction reaction with NaBH4 to load the Pd with La2O3 on chitosan (CS)-functionalized aCNTs. The obtained result showed that the PdeLa2O3/C catalyst has a high catalytic activity. This finding is ascribed to the increase in metallic Pd and good dispersion status of metal particles owing to the presence of CS. Additionally, Jha et al. [19] prepared MWNTs by chemical vapour deposition and Pt-supported MWNT (Pt/ MWNT) and PteRu-supported MWNT (PteRu/MWNT) electrocatalysts by chemical reduction. In this research, the performances of DMFC were evaluated at different temperatures. Results show that the power density increases with increasing temperature. The maximum power density of 39.3 mW/cm2 is gained at a temperature of 80  C and a current density of 130 mA/cm2 probably because of the dispersion and accessibility of MWNT support and PteRu in the electrocatalyst mixture for MOR. Thus, MWNT is a potential support material in DMFC. Table 2 summarizes the literature results for the CNT supports with different amounts of metal loading used in DMFC. Regardless of all the benefits provided by CNT, its utilization to DMFC still confronts several hindrances. All these works reported that the use of CNT materials as support improves electrocatalytic activity. CNT synthesis techniques are inappropriate for massive production and experience expenditure constraints. Further development is needed for the massive production of CNTs to fit the demands of all potential applications, especially in the fuel cell industry.

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Table 2 e Summary of reported carbon nanotube supports with different metal loading amounts used in DMFC. Electrocatalyst Pt/P-MCNT PtNi/FCNT PtRu/CNT PtRuNi/MWCNT 20% PtRu/PEI-MWCNT Pt10-x-Fex/CNT Pt/IrO2/CNT Ni@Pd/MWCNT

Particle size (nm)

Electrochemical surface area, ECSA (m2/g)

Activity

Reference

1.86 2.7 2.28 2e4 2.42 1.5e3 3.5 6e8

78.9 47.2 89.3 720 113.2 NA 78 176.2

970 A/g at 100 mV/s (vs. Ag/AgCl) 474.2 mA/mg at 20 mV/s (vs. Ag/AgCl) 160 mA/mg at 20 mV/s (vs. RHE) 5400 mA/mg at 100 mV/s (vs. Ag/AgCl) 625 mA/mg at 50 mV/s (vs. Ag/AgCl) 37.6 mA/cm2 at 20 mV/s (vs. Ag/AgCl) 873.1 A/g at 100 mV/s (vs. Ag/AgCl) 770.7 mA/mg at 50 mV/s (vs. SCE)

[175] [25] [179] [58] [180] [181] [182] [183]

Graphene In 2004, graphene, a mono-layer graphite with a hexagonal packed lattice was intensively studied since its discovery. The name ‘graphene’ was first mentioned in 1987 to visualize the layers of graphite with numerous compounds inserted between them. Graphene has similar physicochemical properties to CNTs but features larger surface areas and can be regarded as an unrolled SWNT [184]. In recent years, researchers have explored graphene, a new and efficient catalyst support material for fuel cell electrocatalysis because of its superior electrical conductivity, high surface-to-volume ratio, ultrathin thickness, unique porous architecture, high structural flexibility and chemical stability [30,185,186]. Graphene also possesses high anti-poisoning ability. In fuel cells, graphene is not particularly constrained to that of catalyst supports (graphene and GO) but also being investigated as a material for (i) conducting membranes as a composite with polymers and as well as (ii) a bipolar plate material [187]. In this review, we only focus on the use of graphene as a catalyst support. Jang et al. [188] synthesized high-performance 3D graphene (GR) decorated with platinumegold alloy NPs (3D-GR/ PtAu) from a colloidal mixture of GR and PteAu alloy NPs with aerosol spray drying, producing a morphology similar to a crumpled paper ball. Compared with the commercial Ptcarbon black, 3D-GR/PtAu has a larger ECSA (325 m2/g (Pt)) and thus greater number of active reaction sites and superior catalytic ability in electrochemical methanol reaction. In another study, Lu et al. [189] used boron- and nitrogen-doped graphene as PtRu NP support for methanol electro-oxidation studies. Boron- and nitrogen-doped graphene-supported PtRu electrocatalysts were synthesized by a single-step heat treatment approach. The introduction of boron or nitrogen containing functional groups into graphene sheets could finetune the particle size and dispersion on PtRu NPs and thus

improve the electrocatalytic performance of MOR. Cyclic voltammograms revealed that the peak current density on PtRu/ NG at the potential of about 0.7 V is 328.1 A/g with a scan rate of 50 mV/s, which corresponds to the dehydrogenation oxidation of methanol and is greater than the methanol electro-oxidation activity of the commercial PtRu-C-HS catalyst (307.8 A/g) [190,191]. However, the CV curves for PtRu/BG display a higher forward current density than those for PtRu/ NG and commercial PtRu-C-HS catalysts. Nevertheless, a few issues, such as the restacking of graphene layers due to van der Waal forces and the severe aggregation of Pt NPs on graphene surfaces, must be fixed before graphene can be used as a catalyst support [192]. The utilization efficiency of noble metal catalysts is affected because of these issues. Depositing metal on graphene surfaces is complicated because it is chemically inert and relatively insoluble in organic solvents or aqueous solutions. Liu et al. [193] found that 3D graphene aerogel (3DGA) support material, where 3DGA was successfully prepared by a facile and efficient hydrothermal method without a surfactant and template of Pd NP catalyst, enables a 3.4 times higher ECSA (425 m2/g) compared with commercial Pd/C in an alkaline DMFC. Thus, 3DGA confirms that the utilization ratio of Pd could improve because of the high surface area for large interfaces on Pd/3DGA nanocomposites. Table 3 shows a comparison of the electrocatalytic activities of graphene as a catalyst support. Results show that the electrocatalytic activity increases with increasing particle size, emphasising that the particle size of graphene augments fuel cell performance.

Carbon nanosphere The choice of a suitable catalyst support is vital to enhance catalytic activity and reduce the use of noble metal catalysts. Carbon-based materials, considered as one of the most ideal catalyst supports for fuel cells, have been

Table 3 e Comparison of the electrocatalytic activities of graphene as a catalyst support. Electrocatalyst

Particle size (nm)

Onset potential (V vs. SCE)

Activity

Reference

3.0 4.8 15.2 2.46 ~2e10

0.17 0.27 0.65 0.34 0.4

443 mA/mg at 50 mV/s (vs. Ag/AgCl) 642 mA/mg at 50 mV/s (vs. SCE) 610 mA/mg at 50 mV/s (vs. SCE) 525.08 mA/mg at 50 mV/s (vs. SCE) 351.7 mA/mg at 50 mV/s (vs. Ag/AgCl)

[194] [195] [196] [197] [198]

Pt/3D-G PtCu2/rGO Pt-16.5%MoO3/rGO PtCo/EG Pt/N-G-800 *EG e Expanded graphite. *GA e Graphene aerogel.

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extensively used. Among them, carbon nanospheres (CS) have the advantages of high surface area, excellent electronic conductivity, good chemical durability and suitable porosity for mass transport, and they have been proven as promising catalyst supports for DMFCs [199,200]. Wang et al. synthesised CS with diameter ~200 nm from glucose by using a novel composite-molten-salt (CMS) method. Pt NPs supported on those CS are used for methanol electrooxidation in alkaline media. Compared with Pt/C, this composite shows a higher efficiency for methanol electrooxidation in terms of electrode conductivity, electrochemically active surface, oxidation peak current density and onset potential. The efficiency of the Pt/CMS catalyst for MOR is influenced by the formation of a porous structure by CS, which significantly reduces the liquid sealing effect allowing efficient gas diffusion. High carbonization of the CMS-synthesised carbon nanospheres also improves its efficiency. Liu et al. [201] reported that the presence of CS in Pt leads to a peak current density value 2.6 times higher compared with that of Pt/C. It also causes the peak current density to shift toward a 100 mV more negative, which shows a better electro-catalytic activity. This result can be attributed to the higher electrochemical active surface area of Pt/CS (56 m2/g) as compared with Pt/C (28 m2/g). Zhang et al. [122] also discovered that honeycomb-like mesoporous nitrogen-doped carbon nanospheres (MNCS) on Pt display a higher peak current density (1007 mA/mg) and are more stable during methanol oxidation than Pt/C. Pt/MNCS also has a larger electrochemical active surface area (89.2 m2/g) than a commercial Pt/C. The improved performance of Pt/ MNCS is largely attributed to the honeycomb-like porous structure and nitrogen doping that induce changes in Pt nucleation, growth behaviour, electronic structure and support/catalyst binding, which result in uniform, highly dispersed, and high-activity Pt NPs for MOR.

Challenges and future perspectives Finding suitable catalysts is the main challenge in determining the optimum condition for DMFC. The catalyst is a key factor in determining the efficiency, activity, stability, durability and cost of fuel cell devices. Thus, this paper highlighted the most significant anode catalysts and supports that are suitable and mostly used in DMFC. The optimum condition of DMFC depends on the kinetics of the anode reactions [202]. In a DMFC system, methanol oxidizes to carbon dioxide at the anode, but the formation of carbon monoxide as an intermediate that is adsorbed on the Pt catalyst surface proceeds at the oxidation reaction [203]. These intermediates may contaminate the catalyst surfaces and likely increase the deterioration of the catalyst. Catalysts with good poisoning tolerance against the methanol oxidation process that contains oxygen intermediates facilitate better electrocatalytic activity in terms of methanol oxidation [204]. Good poisoning tolerance signifies that less pores in the catalyst are poisoned and more catalytic sites are available for methanol oxidation. Thus, the catalyst activity for the redox reactions in the fuel cells increases obliquely.

Numerous effective catalysts support has been studied, mostly focusing on carbon materials, with limited studies on non-carbon materials. However, several degrees of corrosion still persevere for oxidised (and/or functionalised) carbon supports [20]. The use of mesoporous materials as supports has recently undergone developments. Considering their anticorrosive ability, good pore distributions, high surface area and stability in both acid and alkaline media, these may even come to prominence as ideal supports in the near future. In addition, the durability of supports is one of the vital factors affecting the performance of a fuel cell. The corrosion rate is mostly influenced by the structure and composition of the catalyst support layer and also the operational conditions [166]. Future works to determine a suitable route for an effective electrocatalyst with a modified catalyst support should consider four aspects: 1) Low cost. Materials which are cost effective while offering enhanced benefits will help to accelerate the commercialization of fuel cells. 2) The development of new catalyst systems is more likely in alkaline than acidic media. In comparison with acidic media, alkaline media offer a wider range of selections for the catalyst and support materials. Pt catalysts are more likely vulnerable to CO poisoning compared with other metal catalysts. Therefore, it is recommended to use nonPt catalysts as anode materials, but it has very low performance. So, the more research of non-Pt catalyst is required to improve catalytic activity. 3) The performance of catalyst such as electrochemical active surface area, mass transport, electronic conductivity and long-term stability depend on catalyst support. The development of catalysts with supporting materials modifies their physical and chemical properties, thus enhancing the electrocatalytic performance. So, the more development of support materials must be expected in the future. 4) Durability of catalysts plays an indispensable role in the practical implementation of DMFCs for commercial applications. The improved durability of catalysts is expected to be without loss in catalytic activity when the interaction between support and metal is increased.

Conclusion Anode catalysts and their supports play a crucial role in determining the durability, performance and price of DMFC. The use of nanostructured materials for catalyst support has given high dispersion of metal, high surface area and good electrical conductivity to improve the existing and also develop novel DMFC catalyst supports. Combination of metal loading with excellent supports could produce a new generation of DMFC anode catalysts in the near future. In this report, we have reviewed the recent progress in the developments and studies reported on miscellaneous anode catalysts and their support materials in DMFC. Existing combination of a catalyst with its support allows great

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improvement in catalytic support activity but has not yet sufficiently empowered fuel cell commercialization. Enhancement in durability, stability, reactivity and cost efficiency will bring a trustworthy technology that can be widely used in daily applications.

Author contribution The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. These authors contributed equally.

Funding sources FRGS/1/2016/STG07/UKM/03/2.

Acknowledgement The authors acknowledge the financial support provided by Ministry of Higher Education, Malaysia through Fundamental Research Grant Scheme No. FRGS/1/2016/STG07/UKM/03/2.

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