Anode Catalysts for Direct Methanol Fuel Cells in Acidic Media: Do We

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Anode Catalysts for Direct Methanol Fuel Cells in Acidic Media: Do We Have Any Alternative for Pt or Pt−Ru? Nitul Kakati,† Jatindranath Maiti,† Seok Hee Lee,† Seung Hyun Jee,† Balasubramanian Viswanathan,*,‡ and Young Soo Yoon*,† †

Department of Chemical Engineering, Gachon University, 1342 Seongnamdaero, Sujeong-gu, Seongnam-si, Gyeonggi-do 461-701, Republic of Korea ‡ National Centre for Catalysis Research, Department of Chemistry, Indian Institute of Technology Madras, Chennai, Tamil Nadu 600036, India 6. Conducting Polymer Catalyst Supports 6.1. Polyaniline 6.2. Polypyrrole 6.3. Sulfur-Containing Polymers 7. Composite Materials as Supports 8. Core−Shell Nanostructures and Other Architectures for Methanol Oxidation 8.1. Bimetallic Core−Shell Nanoparticles 8.2. Tertiary Core−Shell Nanoparticles 8.3. Quaternary Core−Shell Nanoparticles 9. Concluding Remarks Author Information Corresponding Authors Notes Biographies Acknowledgments References

CONTENTS 1. Introduction 1.1. Catalyst Loading and Electrode Configuration in Direct Methanol Fuel Cells (DMFCs) 1.2. Noble Metal Issue 1.3. Slow Electro-Oxidation Kinetics 1.4. Methanol Crossover 1.5. Electro-Oxidation of Methanol 1.6. Mechanism of Methanol Oxidation 2. Platinum-Based Nanostructured Catalysts for Methanol Oxidation 2.1. Platinum-Based Nanowires 2.2. Platinum-Based Nanotubes 2.3. Platinum-Based Nanoflowers 2.4. Platinum-Based Nanorods 2.5. Platinum-Based Nanocubes 3. Methanol Oxidation on Selected Pt-Based Alloys 3.1. Pt−Ru System 3.2. Pt−Sn Catalysts 4. Metal Nanoparticle Catalysts 4.1. Binary Metal Nanoparticle Catalysts 4.2. Ternary and Quaternary Metal Nanoparticle Catalysts 5. Catalysts Based on Metal Oxide Supports 5.1. Effect of RuO2 5.2. Effect of SnO2 5.3. Effect of CeO2 5.4. Effect of WO3 5.5. Effect of TiO2 5.6. Effect of MnO2 5.7. Effect of IrO2 5.8. Effect of Other Metal Oxides © 2014 American Chemical Society

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1. INTRODUCTION Although fuel cells have been known for over 150 years, it is only in recent years that they have been viewed as candidates for the dominant energy conversion devices in a variety of future applications. This is because fuel cells are adaptable in terms of operating temperatures, fuel types, electrolytes, membranes, and electrodes. This versatility is illustrated in Figure 1. However, few fuels have been examined for use in fuel cells; many fuels have been ignored because of their lower energy densities compared to that of hydrogen, as shown in Table 1. Table 1 shows that, after hydrogen, the next best fuel in terms of energy density is methanol. In addition, methanol offers certain specific advantages over hydrogen; it is cheap, plentiful, and renewable from wood alcohol, and since it is a liquid, it is easily stored, transported, and distributed (the existing infrastructure can be exploited). Direct use of methanol as an electrochemically active fuel enormously decreases the difficulty in constructing an energy conversion system, thereby reducing the complexity and cost.

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1.1. Catalyst Loading and Electrode Configuration in Direct Methanol Fuel Cells (DMFCs)

Catalyst loading and membrane−electrode assembly (MEA) preparation are two main aspects in DMFC manufacturing for Received: July 17, 2013 Published: December 24, 2014 12397

dx.doi.org/10.1021/cr400389f | Chem. Rev. 2014, 114, 12397−12429

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Figure 1. Schematic representation showing the versatile nature of fuel cells.

DMFCs. However, there are many other factors that influence the properties of the MEA, including the amount of ionomer, catalyst layer morphology, DMFC operating mode (active or passive), etc. Generally, the ionomer content balances the ionic conduction and mass transfer in the catalyst layer.8 A lower ionomer content limits the ionic conduction in the catalyst layer, while a higher ionomer content reduces the mass transport. Although the ionomer content controls the DMFC performance, the amount of ionomer used depends on the preparation method of the MEA. Abdelkareem et al. listed various studies using an active DMFC structure with different ionomer amounts and different MEA preparation methods.9 In their study the optimum ionomer content was 20 wt % under passive conditions and was considered to be lower than that under active conditions. You et al. studied the MEA produced by the CCM method following the decal transfer method (DTM) and direct coating method (DCM).10 In their study it was found that an increase in the thickness and porosity of the cathode catalyst layer enhanced the cell performance, whereas for the anode layer little difference was observed with increasing thickness. Reshetenko et al. studied the influence of the Pt−Ru catalyst loading and studied two different anode layers prepared by the CCM method and catalyst-coated substrate (CCS) method and a cathode layer prepared by the CCS method.11 The maximum power density was found with the CCManode/CCScathode MEA with a Pt−Ru loading of 4.52 mg cm−2 at 343 K. In their study it was found that homogeneous removal of CO2 could be achieved by the combination of the CCM anode and gas diffusion layer, while CO2 holdup occurs in the CCS anode. In another study Reshetenko et al. found that the optimal Nafion and Pt loadings for the CCM cathode in the CCMcathode/CCSanode configuration were 10.7 wt % and 6.0 mg cm−2, respectively.12 However, the cathode with a CCS configuration provided better performance in DMFCs due to enhanced mass transfer through the macropores.

Table 1. Chemical and Electrochemical Data of Various Fuelsa fuel

ΔG° (kcal/mol)

E°theor (V)

E°max (V)

energy density [(kW h)/kg]

hydrogen methanol ammonia hydrazine formaldehyde carbon monoxide formic acid methane propane

−56.69 −166.80 −80.80 −143.90 −124.70 −61.60 −68.20 −195.50 −503.20

1.23 1.21 1.17 1.56 1.35 1.33 1.48 1.06 1.08

1.15 0.98 0.62 0.28 1.15 1.22 1.14 0.58 0.65

32.67 6.13 5.52 5.22 4.82 2.04 1.72

a

Adapted with permission from ref 1. Copyright 2006 Universities Press (India) Private Ltd.

optimum performance. Till now, several catalyst loading processes have been adopted that include spraying, sputtering, screen printing, decaling, electrodeposition, etc. An overview of different catalyst loading processes can be found in ref 2. Generally, two types of MEA preparation techniques are followed: the catalyzed diffusion medium (CDM) method and catalyst-coated membrane (CCM) method. In a CDM method catalysts are mixed with an ionomer and then coated onto gas diffusion layers (GDLs) as the anode and cathode followed by hot pressing on both sides of a membrane to give a sandwich structure.3,4 On the other hand, in the CCM method a catalyst ink is directly coated onto the membrane or transferred through a PTFE support to the membrane to make the MEA.5,6 Tang et al. found that the CCM method for MEA preparation provides better performance for DMFC than the hot-pressing CDM method.7 Lindermeir et al. studied several preparation conditions for DMFC’s MEA, including CDM and CCM methods.4 They have also found that the CCM method is more beneficial than the CDM method for DMFC application. A more favorable proton-conducting network and a higher catalyst efficiency make the CCM method more beneficial for

1.2. Noble Metal Issue

Since platinum is a noble metal, it offers certain advantages in fuel cell applications, such as passivity in acid electrolytes. 12398


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even though the kinetics can be faster. The reaction of methanol oxidation

However, it is expensive; therefore, much effort has been expended on finding alternatives. Non-noble metals such as perovskites and organometallic chelates are promising candidates as air electrodes.13 Nevertheless, Appleby estimated that, if all road vehicles were operated by polymeric electrolyte fuel cells, the total amount of Pt required would be approximately 5 times the amount mined today.14 However, if all Pt could be recovered, platinum costs would be limited to the initial investment and recovery.15 Furthermore, the following are other possibilities for improvement: (i) The activity of the anode electrode could be improved even if it were still based on Pt. (ii) The required loading levels could be reduced by an order of magnitude or two by adopting appropriate synthetic strategies while maintaining the activity at the same or improved levels. Therefore, fears about limited supplies of Pt or its overall cost seem to be exaggerated.

CH3OH + H 2O → CO2 + 6H+ + 6e− (E° = 0.04 V)

has a thermodynamic equilibrium potential close to that of the hydrogen reaction. However, this overall reaction involves a six-electron transfer with many surface intermediates, while the hydrogen reaction is neat with a two-electron transfer. Therefore, the catalyst surface in the DMFC will be poisoned by many surface intermediates. Consequently, a suitable electrocatalyst must be developed that can resist poisoning by the surface intermediates created by partial oxidation. 1.6. Mechanism of Methanol Oxidation

Electrochemical oxidation of methanol on the best available catalyst, namely, Pt, proceeds through the elementary steps of sorption of methanol and subsequent insertion of oxygen, leading to the formation of carbon dioxide.22 In bimetallic systems (Pt−M),23−26 the second substrate, namely, water, gives rise to oxophilic species at low potentials on the metal surfaces (M). This facilitates the removal of partial oxidation intermediates from the surface, thus preventing the poisoning of the surface. Figure 3 illustrates the possible reaction sequence. As stated above, the essential drawbacks or limitations of Pt-based electrodes are the slow kinetics and other associated surface poisoning effects. Overcoming these limitations requires a high-purity fuel or high catalyst loading. The drawbacks of Pt catalysts, such as slow kinetics, low efficiency, and scarcity and high cost of Pt, hinder speedy commercialization of these fuel cells. Therefore, current research is oriented toward development of alternate catalysts that have high performance, high stability, and durability and are inexpensive. Fortunately, present research indicates that it is possible to use ultralow amounts of platinum catalyst for low-temperature fuel cells.27−32 Alternatively, non-Pt catalysts are being studied to replace Pt.33 Recent studies have shown that when direct alcohol fuel cells are operated in an alkaline medium rather than in an acid electrolyte, non-Pt electrocatalysts can be advantageously employed because of their significantly improved reaction kinetics in the alkaline medium.34 However, the critical issue for alkaline fuel cells is the anion exchange membrane, which has a low chemical stability. Over the past few decades, anode materials for DMFCs operating in acidic media have been extensively studied. Research has shown that the use of Pt is inevitable if DMFCs are to be extensively used in practical applications. To achieve the desired performance by Pt-based catalysts for methanol oxidation, Pt has been modified by other metals34−44,91,92 or forming core−shell,45−51 metal oxide,52−78 and other79−89 nanostructured particles. To increase the dispersion of the noble metal as well as facilitate charge mobility in the external circuit, carbon supports have been conventionally employed in fuel cells. Recent advancements on the architectures of carbon nanomaterials, including carbon nanotubes (CNTs) and layered configurations such as graphene, have widened the scope of supporting materials for catalysts. These developments have provided alternate directions for the development of efficient catalyst systems in fuel cells. High efficiencies may be achieved because of the unique properties of nanomaterials, particularly vectorial transfer of charge in dimensionally modulated supports;90 additional benefits include the possibility of high surface areas, sufficient chemical resistivity, and superior mechanical strength.

1.3. Slow Electro-Oxidation Kinetics

Electrochemical oxidation of methanol can give rise to various partial oxidation intermediates such as COHads, HCHOads, and COads; hence, these can contribute to the reduction in efficiency of the oxidation process and can block sites from fresh adsorption of methanol.16 Therefore, these surface impedances, such as inhibition by surface intermediates and slow oxidation kinetics, must be surmounted. In addition, new catalyst formulations should be capable of promoting complete combustion to carbon dioxide. 1.4. Methanol Crossover

In DMFCs, the anode and cathode compartments are separated by membranes. However, functionalities in the membrane, such as hydroxyl groups and hydrophilic properties, can induce interactions with the substrate, and these interactions can facilitate migration of methanol to the cathode compartment by the inherent concentration gradient. This situation can lead to undesirable internal short circuiting, resulting in a loss of current (Figure 2). In addition, the cathode, which is usually made of pure Pt, may be poisoned by the partial oxidation products of methanol.17

Figure 2. Cathodic oxygen reduction and undesired methanol oxidation with an internal short circuit created by crossover. Adapted with permission from ref 17. Copyright 2000 Royal Society of Chemistry.

1.5. Electro-Oxidation of Methanol

Electrochemical oxidation of methanol studied because of its relevance for DMFCs.18−22 The reaction is usually solutions because carbonates can form

(1)

has been extensively the development of performed in acidic in alkaline solutions 12399


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Figure 3. Schematic presentation of the different reaction steps during methanol oxidation on a model catalyst surface. Adapted with permission from ref 26. Copyright 2003 Wiley-VCH.

necessary active sites or specific exposed planes of the dispersed species (metal) on the catalyst surface. Various synthesis strategies have evolved to generate active species on supported systems. It is now well-known that the shape and size of supported metals and the nature of the support profoundly influence the surface properties of supported metallic systems. In addition, since the supported metals are in nanostates, dimensionality becomes a variable that can be exploited for charge transfer processes. Depending on the size, shape, morphology, dimensionality, and nature of the support material, supported Pt nanostructures show versatile catalytic performance.19 In spite of these advantages for the nanostate dispersion of metals on supports, there are also limitations associated with the use of nanoparticle catalysts. First, because of aggregation of nanoparticles, the effective electrochemical surface area (ECSA) can be reduced,79 and the large number of nanoparticle− nanoparticle interfaces can hinder mass diffusion and efficient charge transfer.80 However, it has been demonstrated that a catalyst having a one-dimensional nanostructure, such as a nanowire (NW) or nanotube (NT), can be more efficient than three-dimensional nanoparticles, thereby possibly overcoming these limitations by providing a high catalyst surface area without the need for a large surface area support.79−81 In short, there are a number of methods for producing Pt nanostructures; for example, wires can be prepared by electrochemical deposition into polycarbonate membranes.80,82 Alternatively, metallic NWs can be used as a template to create Pt NTs via galvanic replacement reactions.79,83 Since the dimensionality of the nanostate of Pt has considerable influence on catalytic behavior, brief discussions on the various forms of Pt nanoparticles, particularly the active ones, are presented in the following subsections.

A decade ago, a critical evaluation of the possible anode materials for DMFCs was reported.22 The authors examined conducting polymer supports and the state of metallic particles on them, and they stated that the data existing at that time did not permit direct comparisons that would enable the formulation of governing principles for future development. Against this background, the authors of this study feel that the knowledge acquired over the past decade needs careful evaluation for its predictive capability that could indicate directions for future developments in this important area. Hence, the strategy adopted for this presentation includes in its scope the examination of various unsupported or supported Pt-based catalyst systems used for methanol oxidation in acidic media. This self-imposed limitation is intended to gain attention. Accordingly, information on the activity of single Pt nanostructures and binary or ternary alloy analogues is assembled to arrive at the appropriate composition of the anodes used in methanol oxidation. Then the current research trend of alloying Pt with other metals is examined for enhancing oxidation activity with a view to facilitate catalyst selection. Platinum-based mono- or bimetallic systems supported on oxophilic metallic oxides showed enhanced activities for methanol oxidation, and these effects were rationalized on the basis of metal−support interactions (MSIs) or in terms of alterations in the interface geometric (ligand effect) or electronic properties. In the past decade, core−shell configured anodes have been examined, but many other studies have not given attention to these developments. The use of core−shell-type nanostructures can have far-reaching consequences, particularly for cost to performance ratios since the possible alterations in the density of states of frontier wave functions, particularly at interface sites, can be expected to give rise to new and possibly active sites for methanol oxidation. This could pave the way for the selection of materials for anodes in DMFCs. It has to be realized that any catalyst surface, even if it were a one-component system, differs from the bulk, and in this sense, almost all solid catalyst systems might be interpreted by a model having a core−shell architecture. Hence, all surface catalysis results should be reexamined from this point of view.

2.1. Platinum-Based Nanowires

Platinum NWs have been synthesized for methanol oxidation using polymer templates and a modified electrodeposition method.80 The Pt NWs were successfully separated from the polymer template by applying a CH2Cl2 solution and washing with ethanol. These unsupported Pt NWs exhibit better electrochemical mass activity for methanol electro-oxidation than supported or unsupported highly loaded Pt nanoparticles that are typically needed for DMFCs. Platinum NWs have also been synthesized on Pt and W gauzes using the polyol process. These Pt NWs supported on a Pt gauze show excellent

2. PLATINUM-BASED NANOSTRUCTURED CATALYSTS FOR METHANOL OXIDATION It is conventional to disperse an active metal on a support to increase the active metal area and preferentially obtain the 12400


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in terms of microroughness, electron conductivity, and specific activity; therefore, significant enhancement in catalytic properties for methanol oxidation can be achieved by tuning the diameter and roughness of the NWs. Platinum NWs can be formed through the hydrophilic channels of a Nafion 115 membrane by an electrodeposition technique.94 In this way, ECSA of the Pt NWs can be increased because of the formation of separate conduction paths for electrons and protons through the membrane. Above all, the crystallinity of the hydrophobic region increases because of Pt incorporation; this makes it difficult for methanol molecules to permeate through the hydrophobic region of the Nafion membrane. As a result, this new electrode showed higher performance than a commercial (E-TEK) Pt catalyst. However, at high current densities, slightly lower voltages were obtained at the Pt-integrated Nafion electrode than those at the E-TEK electrode; this was mainly attributed to the higher cell contact resistance of the Pt-integrated Nafion electrode than that of the latter one. Platinum NWs have also been synthesized by using a soft template formed by cetyltrimethylammonium bromide (CTAB) and adopting a phase transfer method.95 The resulting Pt NWs, supported on high surface area carbon, were reported to be an efficient catalyst for methanol and CO oxidation.96 The surface mobility of OHads and COads greatly influences the oxidation of CO to CO2 on Pt sites.97 Reduced oxophilicity, which increases the mobility of COads and OHads on Pt, large numbers of grain boundaries, and a downshift of the d-band center were thought to contribute to the higher electrocatalytic activity of the Pt NW networks toward methanol oxidation relative to that of the (E-TEK) Pt−C catalyst. Zhao et al. reported that a Pt NW array electrode on a Ti−Si substrate can be synthesized by dc electrodeposition of Pt using porous anodic aluminum oxide (AAO) as a template; the resulting electrode performed better for methanol oxidation than Pt thin films on a Ti substrate.98 They reported the synthesis of a Pt−Ru NW using the same procedure, and they compared its methanol oxidation activity with that of the Pt NW; the Pt−Ru NW performed better than the Pt NW.99 Platinum NWs have been formed on the surface of Vulcan XC-72 carbon powder to be used as a catalyst for methanol oxidation.100 To form the desired structure, the reaction rate was controlled by the pH value of the system. The NWs were formed at pH 2.5, and the optimum loading on carbon powder was found to be 40 wt %. A loose NW network is preferred over a compact one because of improved mass transfer for methanol oxidation due to formation of channels inside the NW networks. In addition to Pt and Pt−Ru NWs, researchers have also synthesized other Pt-based multimetallic NWs for methanol oxidation; the choice of additional metals takes into consideration the electronic effects of candidate metals. In a recent study, Carmo et al. proposed the possibility of using Pt−bulk metallic glass (BMG) NWs composed of Pt, Ni, Cu, and P as a promising catalyst for methanol, ethanol, and CO oxidation.101 The amorphous nature of BMGs, down to the atomic scale, possessed high strength, elasticity, and good corrosion resistance because of the absence of grain boundaries and dislocations in their bulk structures.102 These Pt−BMG NWs exhibited excellent catalytic performance toward methanol, ethanol, and CO oxidation because of inherent electronic properties and the strain effects exhibited by these systems.

Figure 4. SEM images of (A) Pt and (B) W gauzes prior to Pt NW growth, with insets showing the detailed surface morphology. The scale bars in the insets are 200 nm. (C, D) SEM images of Pt NWs on Pt and W gauzes, respectively, obtained as the final products of an iron-mediated polyol process by using a 1 mL solution of H2PtCl6 (80 mM). (E, F) SEM images of Pt NWs grown on the surface of Pt and W gauzes, respectively, with the concentration of H2PtCl6 reduced to 40 mM. Adapted from ref 81. Copyright 2008 American Chemical Society.

activity for methanol oxidation because of greater exposure of Pt{110} facets associated with Pt NWs.81 Figure 4 shows scanning electron microscopy (SEM) images of Pt NWs on Pt and W gauzes. Formo et al. reported an electrocatalytic study of electrospun anatase nanofibers decorated with Pt catalysts in the form of nanoparticles or NWs.86 They found that the supported nanostructures show improved electrochemical activity and durability compared to the commercial Pt−C catalyst. Titania as a support is potentially useful for the deposition of Pt to be used in DMFCs. The TiO2 support can lower the adsorption energy of CO intermediates, which increases the mobility of CO groups on Pt nanostructures. The effect of metal oxides in anode catalysis of DMFCs will be discussed in a later section. Utilizing the consequences of degradation of poly(vinylpyrrolidone) (PVP) and reduction of the Pt precursor, Kim et al. synthesized Pt NWs by hightemperature treatment of electrospun PVP−Pt composite fibers in an air atmosphere.93 A two-step heat treatment procedure was applied to increase the catalytic properties of the one-step synthesized NWs. Approximately a 2-fold increase was observed in ECSA of the two-step heat-treated NWs. Although the mass activity of these NWs was lower than that of commercial Pt catalysts, an enhancement in the specific activity of the catalysts toward methanol oxidation was observed. Platinum NWs have advantages over spherical Pt nanoparticles 12401


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NWs of Fe−Pt−Pd have been synthesized by thermal decomposition of Fe(CO)5 and sequential reduction of Pt(acac)2 and Pd(acac)2.103 For an efficient methanol oxidation reaction (MOR), the optimized composition for the Fe−Pt−Pd NWs was found to be 28:38:34. Pt−Co NWs have been synthesized by electrodeposition through a porous AAO membrane.104 A significant increase in methanol oxidation current was observed for these NWs compared to their corresponding thin film structure. 2.2. Platinum-Based Nanotubes

Górzny et al. used tobacco mosaic virus (TMV) as a template for synthesizing Pt NTs.84 They found that, for the same Pt loading, the electrochemically active surface area of Pt−TMV NTs was 4−8 times larger than that of similarly sized Pt nanoparticles. The increased electrocatalytic activity of Pt−TMV was assumed to be associated with transfer of oxidative electrons through a few hopping−tunneling steps. In contrast, for nanoparticle films, transfer of oxidative electrons becomes less efficient because of the presence of large numbers of hopping− tunneling steps. Platinum NTs have also been synthesized by galvanic replacement of Ag NWs.105 These catalysts showed improved tolerance to intermediates and more peak specific activity for methanol oxidation than Pt−C and bulk polycrystalline Pt (BP-Pt) electrocatalysts. Figure 5 shows SEM and transmission electron microscopy (TEM) images of Ag NWs and Pt NTs along with their high-resolution TEM (HRTEM) images; it also shows the morphology of the nanostates of these catalysts. Porous Pt−Ni nanoparticle tubes have been synthesized using an AAO template with an electrodeposition technique for methanol oxidation.106 These NTs were found to be active after 2000 potential cycles were performed in an Ar-saturated 0.1 M HClO4 solution. The potential cycles restructure the catalyst, thereby activating the catalyst surface. This behavior makes this catalyst a potential candidate for a new-generation restorable fuel cell catalyst. Bimetallic NTs composed of Sn−Pt (Figure 6) have been synthesized by a combined evaporation and electrodeposition method using an AAO template.107 In Figure 6e, the HRTEM image (inset) of the Sn−Pt NTs shows Pt dispersion in the inner wall of the Sn NTs. The catalytic activity of the Sn−Pt bimetallic NTs was found to be 2 times higher than that of the Sn−Pt film. Owing to the nanochannel structure of the Sn NTs, Pt nanoparticles were well dispersed with particle sizes smaller than those on the Sn−Pt plane surfaces with the same geometric area. These smaller particles provided a higher electroactive surface area for methanol oxidation. Shin et al. synthesized smooth and porous structured Au NTs to be used as a template for thin Pt layer deposition using AAO and polyaniline nanorod templates.108 The electrocatalytic activity for methanol oxidation obtained using nanoporous gold-containing NTs was found to be higher than that obtained using commercial smooth Au NTs. At this point, it appears that one-dimensional architectures are better suited for electrode applications because of the possibility of vectorial transfer of electrons from these architectured materials.

Figure 5. (a) SEM image of Ag NWs, (b) TEM image of Ag NWs, (c) SEM image of Pt NTs, (d) TEM image of Pt NTs, (e) HRTEM image of Pt NTs, and (f) selected area diffraction pattern of Pt NTs. This pattern contained an interpenetrated set of two individual diffraction patterns, one in square symmetry corresponding to the [001] zone axis and the other in rectangular symmetry corresponding to the [−112] zone axis. Adapted with permission from ref 105. Copyright 2010 Wiley-VCH.

compared with Pt nanoparticle aggregates and commercial Pt. Platinum nanoflower-decorated multiwall CNTs (MWCNTs) synthesized by the wet chemical hydrogen reduction route showed pronounced electrocatalytic activity in the reduction of oxygen and oxidation of methanol.87 The 3D flowerlike Pt nanoparticle clusters can be electrodeposited onto MWCNTs by a three-step protocol, which is completely electrochemical and involves a key second step: a potential pulse sequence.88 This structure is capable of achieving a higher electrochemical activity toward methanol oxidation and a higher order of magnitude of ECSA than 2D Pt nanoparticle dispersions. Recently, multilayer films containing polyoxometalates (POMs) and poly(amidoamine) were fabricated using the layer-by-layer technique. The resulting films were used as matrixes for electrodeposition of novel flowerlike Pt micro−nano clusters.89 This electrode material showed good electrocatalytic properties for methanol oxidation because of its unique morphologies of Pt micro−nano clusters. Superior CO tolerance and transport

2.3. Platinum-Based Nanoflowers

He et al. developed a facile electrochemical procedure to synthesize 3D Pt superstructures in a large-scale assembly of flowerlike nanorod aggregates at room temperature without any template.85 These Pt nanorod aggregates showed highly improved electrochemical activity toward methanol oxidation 12402


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Figure 6. (a, b) Typical SEM images of Sn NT arrays prepared by evaporating tin onto a porous AAO template. (c) Typical TEM image of a Sn NT. The inset shows a high-magnification TEM image of the Sn NT. (d) Typical SEM images of a Sn−Pt bimetallic NT array prepared by electrodeposition of platinum into Sn NTs. (e) Typical TEM image of a Sn−Pt bimetallic NT. The inset high-magnification TEM image in (e) shows the features of Pt nanoparticles inlaid in the inner wall surface of the Sn NT. (f) Energy-dispersive X-ray profile of the Sn−Pt NT shown in (e). Cu peaks are from the supporting copper grid. Adapted with permission from ref 107. Copyright 2005 Wiley-VCH.

characteristics contributed to the unique “pseudo-liquid-phase” reaction environment of POMs. This opens a new avenue for applications of POMs in electrocatalyzing alcohol oxidation. In these structures, POM may itself be active for methanol oxidation; therefore, the observed effect may be cumulative or synergistic. A facile synthesis of Pt nanoflowers has been reported by a simple electrodeposition on an indium tin oxide (ITO) substrate in a solution containing H2PtCl6 and H2SO4.109 For comparison, Pt nanoparticles were also synthesized by just changing the solution to H2PtCl6 and KCl. Figure 7 shows SEM images of Pt nanostructures at various electrodeposition conditions. The synthesized Pt nanoflowers showed enhanced catalytic activity toward methanol oxidation, and the peak current density on Pt nanoflowers was approximately 4.4 times higher than that on Pt nanoparticles. Bovine serum albumin has been used as a template to synthesize Pt and Pt−Ag mesoflowers.110 Synthesized Ag mesoflowers were also used as a sacrificial template for the synthesis of porous Pt catalysts and Pt−Ag alloy nanocatalysts. Figure 8 shows the morphology of the Pt−Ag mesoflowers obtained from SEM, TEM, and HRTEM studies. The unique 3D extended channels provide facile transport of methanol and other reactants to the surface of Pt; those channels also enhanced the electron conductivity, which improved the catalytic activity of these mesoflower structures. The catalytic activity and antipoisoning ability of these mesoflowers were found to be higher than those of a commercial Pt black catalyst. The electronic effects of Ag in Pt−Ag alloys make the catalyst more tolerant to CO poisoning; in addition, reducing the amount of Ag in the alloy by dealloying leads to a porous catalyst morphology that increases the methanol oxidation activity. Recently, Yao et al. used a facile electrochemical method to synthesize reduced graphene oxide modified carbon cloth for the deposition of Pt nanoflowers (Pt nanoflowers−RGO−CCE); the resulting nanoflowers were used as an anode material for

formic acid and methanol oxidation.111 The graphene sheets completely wrapped the fibers of carbon cloth, forming many wrinkles that later served as nucleating sites for Pt nanoflowers. Compared to Pt nanoflowers supported on bare carbon cloth and commercial Pt nanoparticles on carbon cloth, those on graphene-modified carbon cloth gave enhanced catalytic performance toward formic acid and methanol oxidation. 2.4. Platinum-Based Nanorods

In general, the interface between platinum and an oxophilic metal provides important sites for the catalysis of methanol electro-oxidation. This has motivated the synthesis of multisegmented Pt−Ru, Pt−Ni, and Pt−Ru−Ni nanorods having customizable lengths of the individual metal or alloy. The syntheses were performed by sequential deposition of the metal/alloy into the pores of AAO membranes.112−115 The resulting Pt−Ru−Ni multisegmented nanorods were catalytically more active than multisegmented Pt−Ru and Pt−Ni nanorods.115 The optimum Ru:Ni ratio in Pt−Ru−Ni nanorods was found to be 4.24:1 for the highest catalytic activity toward methanol oxidation.114 An increase in catalytic activity for methanol oxidation with the number of interfaces, with the length of the nanorods kept constant, clearly demonstrated the existence of a bimetallic mechanism in DMFC anode reactions. Yoo et al. reported a similar study by synthesizing multilayered Pt−Ru nanorods with controllable bimetallic sites via the oblique angle deposition method.116 From their study, it was evident that heat treatment of Pt−Ru nanorods in Ar produced the best results for methanol oxidation; this was attributed to consequences of the bifunctional mechanism and electronic effects from Ru atoms. Some intermetallic compounds, such as Pt−Pb, Pt−In, and Pt−Sn have been found to be active for methanol oxidation.117 The current density of methanol oxidation for Pt−In was 4 times greater than that for pure Pt, while Pt−Pb showed a current density 40 times higher than that for pure Pt. Maksimuk et al. synthesized intermetallic Pt−Pb nanorods by reducing Pt2+ and Pb2+ salts 12403


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Figure 7. SEM images of Pt nanostructures electrodeposited on ITO at −0.2 V under various conditions: (1) in 3.0 mM H2PtCl6 + 0.5 M H2SO4 electrolyte for (A) 25 s, (B) 400 s, (C) 1000 s, (D) 4000 s, and (E) 10 000 s; (2) in 3.0 mM H2PtCl6 + 0.1 M KCl electrolyte for (F) 2000 s. The insets are at higher magnifications. Adapted with permission from ref 109. Copyright 2010 Elsevier.

using tert-butylamine−borane, a strong reducing agent.118 The formation of intermetallic Pt−Pb nanorods proceeds via adatom incorporation into growing surfaces with a strong binding energy. To test the methanol oxidation activity, the synthesized Pt−Pb nanorods were deposited on carbon black and then treated by air plasma. The nanorods were then annealed at 600 °C for 2 h in a H2−Ar atmosphere, which changed the morphology of the nanorods. The peak current density for Pt−Pb−C was found to be higher than that for a commercial Pt−Ru−C catalyst. Figure 9 shows TEM images and cyclic votammograms of these Pt−Pb nanorods.

electrolytes, Pt{100} facets show higher catalytic activity for oxygen reduction than Pt{111} facets.119,120 Han et al. synthesized Pt NCs by the PVP and Fe3+ ion assisted polyol process.121 Compared to a Pt nanocatalyst with a polycrystalline phase, Pt NCs with {100} facets showed improved catalytic properties for methanol and ethanol oxidation in terms of onset potential and current density. In another study, they synthesized Pt NCs by thermal decomposition in the presence of PVP.122 These Pt NCs also exhibited dominant {100} facets with improved electro-oxidation activity toward methanol, ethanol, and formic acid compared to spherical Pt nanoparticles. In the absence of PVP, polydispersed Pt nanoparticles tend to form with slight agglomeration, implying that micelle formation by PVP leads to monodispersed nanoparticles with {100} facets. Peng et al. demonstrated an electrochemical restructuring approach to form Pt nanoboxes or nanospheres using truncated octahedral Ag nanotemplates.123 Figure 10 shows TEM images with catalytic activity for these nanostructures. The formation of nanoboxes can be understood as follows: (1) in H2SO4 solutions, the relative stability of the low-index

2.5. Platinum-Based Nanocubes

Another widely used nanostructure of Pt and Pt-based alloys for methanol oxidation is the nanocube (NC). We know that the electrochemical activity of catalysts increases with the surface area. Because of the importance of the shape of the nanoparticles for electrocatalytic activity and selectivity, great attention has been paid to manipulating the structure and shape of Pt and Pt-based nanoparticles. For example, in acidic 12404


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Figure 8. Representative (a) SEM, (b) TEM, and (d) HRTEM images of Pt45Ag55 and (e) SEM, (f) TEM, and (h) HRTEM images of Pt72Ag28. The insets in (a) and (e) are enlarged SEM images of Pt45Ag55 and Pt72Ag28, respectively. The insets in (b) and (f) are the corresponding selected area electron diffraction patterns. Adapted with permission from ref 110. Copyright 2012 Elsevier.

Figure 9. (a) TEM image and (b) selected area electron diffraction pattern of a single Pt−Pb nanorod and (c) powder X-ray diffraction pattern of an ensemble of Pt−Pb nanorods. The inset of panel a shows an HRTEM image of the tip region of a Pt−Pb nanorod. The inset of panel b shows a simulated electron diffraction pattern of a single Pt−Pb nanorod with the (110) zone axis. (d) Cyclic voltammograms of electrocatalytic oxidation of methanol with Pt−Pb−C and a commercial Pt−Ru−C catalyst. Adapted from ref 118. Copyright 2007 American Chemical Society.

1,2-tetradecanediol and 1-dodecanethiol.131 The as-synthesized Pt−Cu NCs were characterized by dominant {100} facets because of the selectivity of the bromide ions of TOAB toward {100} facets. The methanol oxidation activity for these cubic Pt−Cu nanoparticles was found to be higher than that for Pt−Cu and Pt nanospheres (Figure 11). Yin et al. reported the synthesis of Pt−Cu NCs and concave NCs (CNCs) along with Pt−Pd−Cu CNCs by a one-pot hydrothermal synthesis process.132 They prepared Pt−Cu NCs and Pt−Cu CNCs through the simultaneous reduction of Pt(II) and Cu(II) species by PVP molecules in aqueous solutions in the presence of Br− and H+ ions. Increases in the concentrations of Cu2+, H+, and Br− led to the formation of Pt−Cu CNCs while simultaneously increasing the particle size and agglomeration. These Pt−Cu and Pt−Pd−Cu NCs and CNCs had higher specific activities than a commercial Pt−C for methanol oxidation, although they had lower mass activities than Pt−C because of their smaller particle size.

facets of Pt is high, and (2) restructuring of Pt{100} facets is more difficult than that of other low-index facets when a linear potential cycle is used.124,125 Owing to the better activity of the Pt{100} surface than that of the Pt{111} surface for MORs,126−129 Pt nanoboxes with {100} facets yield an improved activity for MOR compared to Pt nanospheres and commercial Pt nanoparticles. Few studies have been reported on Pt-containing bimetallic or trimetallic NCs for methanol oxidation. However, a few reports on Pt−Cu, Pt−Pd, Pt−Co, Pt−Fe−Co, Pt−Pd−Cu, etc. are available. Nanoparticles of Pt−Fe−Co with varying shapes and compositions have been studied for methanol oxidation.130 In that study, Pt−Fe−Co branched NCs, in particular, showed the best activity and durability for electrocatalytic methanol oxidation. In a typical synthesis, Pt−Cu NCs were synthesized by tuning the reaction parameters and optimizing the amounts of the capping ligands tetraoctylammonium bromide (TOAB) and oleylamine (OLA) and the reducing agents 12405


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Figure 10. Representative TEM images of Pt hollow nanocubes at (a) low and (b) high magnifications and (c) individual cubes imaged under various tilting angles with respect to the direction of imaging beam. Inset in (a) shows the corresponding potential cycling profile. (d) i vs E for MOR catalyzed by Pt cubic nanobox, hollow nanosphere, and the reference catalysts, and (e) their turnover frequencies (TOF) at the peak potentials. Adapted from ref 123. Copyright 2010 American Chemical Society.

Nanocubes of Pt3Co with the {100} orientation were obtained using an OLA−oleic acid (solvent−capping agent) system as the shape control agent.133 These NCs showed much higher methanol oxidation activity than Pt NCs. The bifunctional effect was absent in this catalyst system, and the enhanced catalytic activity was attributed solely to the electronic effects of Co. In another study, Co−Pt NCs and spherical nanoparticles and their methanol oxidation behaviors were explored.134 Adamantanecarboxylic acid, hexadecylamine, and a Co precursor were used as the capping agent, heterogeneous species, and supporting metal for alloying with Pt; this promoted the growth of cubic nanoparticles predominantly along the {111} directions. In their study, spherical Co−Pt showed better activity for methanol oxidation than the cubic nanoparticles. Considering the facet-selective nature of small ions and capping agents, it is possible to synthesize Pt−Pd NCs or nanotetrahedrons (NTHs) by a hydrothermal method (see Scheme 1).135 Yin et al.135 synthesized Pt−Pd NCs using Br−−I− with a PVP system and used Na2C2O4 and formaldehyde with PVP to synthesize Pt−Pd NTHs. The Pt−Pd NCs were enclosed by {100} facets, whereas Pt−Pd NTHs were enclosed by {111} facets; this was because the Br−−I− system is {100}-facet-selective, while N2C2O4−HCHO is {111}-facetselective. The order of catalytic activity toward methanol oxidation was found to be Pt−Pd NCs > Pt−Pd NTHs > Pt−C (commercial), as shown in Figure 12. However, Pt−Pd NTHs possessed higher durability than Pt−Pd NCs owing to the highly durable nature of the {111} facets of Pt-based catalysts, as seen in the case of formic acid oxidation on Pt catalysts.136 The difference in catalytic activity of Pt−Pd NCs and Pt−Pd NTHs was attributed to the adoption of different reaction paths by the {111} and {100} facets.137−139

Figure 11. (a) Low-magnification TEM image of the overall morphology of Pt−Cu NCs. (b) TEM selected area electron diffraction pattern of the Pt−Cu NCs. (c) High-resolution TEM image of a selected Pt−Cu NC. (d) Cyclic voltammograms of MeOH oxidation on Pt−Cu NCs (), Pt−Cu nanospheres (---), and Pt nanospheres (-·-) in 0.1 M HClO4−1 M MeOH (scan rate 0.02 V s−1). The electrode potentials are reported versus a reversible hydrogen electrode (RHE). (e) Chronoamperometric results of MeOH oxidation at 0.8 V on Pt−Cu NCs, Pt−Cu nanospheres, and Pt nanospheres in 0.1 M HClO4−1 M MeOH. Adapted with permission from ref 131. Copyright 2009 Wiley-VCH. 12406


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Scheme 1. Shape-Selective Synthesis of Pt−Pd NTHs and NCsa

a

Adapted from ref 135. Copyright 2011 American Chemical Society.

to the observed activity,146,147 and this activity depends on the potential. In addition to binary systems, other systems with Ru and W24,148,149 have been explored for methanol oxidation. In general, the combination of other metals such as Cr, Fe, Sn, Mo, Os, Ti, Re, and Ta with Pt can promote the reactivity of Pt for oxidation of methanol.

4. METAL NANOPARTICLE CATALYSTS 4.1. Binary Metal Nanoparticle Catalysts Figure 12. (a) Stable cyclic voltammograms obtained for the Pt−Pd NCs and NTHs and Pt−C in an electrolyte of 0.1 M HClO4 and 1 M CH3OH at a sweep rate of 50 mV/s. (b) Cyclic voltammograms obtained after 4000 additional cycles. Adapted from ref 135. Copyright 2011 American Chemical Society.

The most common solution adopted to control poisoning of Pt during methanol oxidation is to alloy it with one or more oxophilic metals to form bimetallic or ternary catalyst systems. On the basis of a bifunctional catalysis model, a reaction mechanism has been developed to elucidate the resistance to CO poisoning by Pt-based alloy catalysts.91,92 Since the Pt−Ru catalyst system is the primary bimetallic catalyst, it has been studied for decades. Water activation on Ru sites occurs at lower potentials (0.2−0.3 V) than that on the pure Pt surface. Then surface poisoning species migrate to these sites from adsorbed sites.91 It is believed that the oxygenated species on Pt−Ru pair sites are more active than those on Ru−Ru pair sites or on Ru clusters.91 Platinum may be modified by adding other oxophilic metals such as Sn, Mo, W, Co, Os, Ni, Rh, Pd, and Ag.37−44 Wang and Hammer used density functional theory to investigate the adsorption and catalytic oxidation of CO by H2O on a completely water-covered Pt2Mo (111) surface. Their study revealed the following two effects: (1) a ligand effect in which CO coverage is reduced through decreased binding energy and (2) a bifunctional mechanism associated with H2O dissociation into OH, which in turn oxidizes adsorbed CO into CO2 on the Pt2Mo surface.38 The Pt−Sn catalyst has been studied extensively because of its promising activity toward methanol and ethanol oxidation. Ishikawa et al. reported a relativistic density functional study of dehydrogenation of CH3OH and H2O on Pt, Ru, and mixed metals of Pt−M (M = Ru and Sn) bimetallic catalysts.37 They found that the activity of Pt−M binary catalysts for CH3OH dissociation varies with the Pt:M atomic ratio. With more Sn, the electrocatalytic activity for methanol oxidation decreases because of the impedance property of Sn toward methanol adsorption. In contrast, methanol dissociation is facile on a ruthenium-rich surface. In Pt−Ru systems, a ligand effect plays some role in methanol oxidation; however, in Pt−Sn systems, the ligand effect is insignificant because the CO oxidation reaction is exothermic in this catalyst system. In a single-cell test using Pt−Ru−C and Pt−Sn−C as anodes, the Pt−Ru−C catalyst showed better activity for methanol oxidation, while the Pt−Sn−C catalyst showed better performance for ethanol oxidation.150 Wei et al. found that methanol oxidation on underpotential-deposited (upd) Ru or Sn on Pt surfaces is

3. METHANOL OXIDATION ON SELECTED Pt-BASED ALLOYS 3.1. Pt−Ru System

Among the bimetallic systems that have been studied, Pt−Ru assumes a unique place since this system has been shown to overcome the poisoning of active sites by CO; this occurs via OH groups generated on Ru sites. The optimum activity for a Pt−Ru catalyst can be observed when ruthenium remains in a solid solution with Pt.140 Other surface species such as Ru(Pt)2COH by successive dehydrogenation of methanol have also been proposed. A stripping voltammetric study of CO has provided evidence that Pt−Ru domain sites are responsible for the observed higher activity of this system.141 As stated earlier, the possibility of surface oxidic species on Ru could be one of the reasons for the observed higher activity.142 The enhanced activity of the Pt−Ru system has also been attributed to mixed electronic and proton conductivity.143 Whatever the reason, the Pt−Ru bimetallic system proves to be a good anode catalyst material for DMFC applications. 3.2. Pt−Sn Catalysts

Catalysts of Pt−Sn have the distinction of exhibiting both poor25 and good144 activities. The controversy could be because tin can be adsorbed on Pt, which can show good activity for methanol oxidation, while a well-formed alloy can exhibit poor activity.18 Shukla et al.145 used X-ray photoelectron spectroscopy (XPS) to study the effects of Sn and Ru additions to platinized carbons to elucidate differences in mechanistic behavior. They argued that Sn modified the electronic environment around Pt sites (through a charge transfer in the Pt−Sn alloy), while Ru sites in Pt−Ru alloys promoted the formation of lattice-bonded oxygenated species in the vicinity of methanolic residues adsorbed on Pt sites. The studies on the system generated by electrodeposition of Sn on Pt showed that the surface concentration of tin definitely contributes 12407


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enhanced compared to that on Pt electrodes.151 They found that upd-Sn−Pt electrodes showed better activity than updRu−Pt electrodes for methanol oxidation from 0 to 0.22 V. However, the enhancement of methanol oxidation disappeared beyond a certain potential. In a recent review on Pt−Sn systems as catalysts for methanol and ethanol oxidation, Antolini and Gonzalez summarized that the catalytic activity of the Pt−Sn system for methanol oxidation is lower than that of the Pt−Ru system, but its activity can be increased compared to that of Pt by optimizing the composition of the catalyst system.152 However, in a recent study by Habibi et al., an electrode modified with poly(o-aminophenol) and then decorated with Pt−Sn catalysts for methanol oxidation showed better electrocatalytic activity compared to catalysts decorated with Pt−Ru or Pt−Ir.153 In a theoretical study from DFT calculations, it was predicted that Pt−Ru and Pt−Sn alloys should have good catalytic properties toward methanol oxidation and that a Pt−Cu alloy could be a promising alternative.154,155 After verification of this prediction, it was found that the methanol oxidation activity increases at low potentials when Cu is added to a Pt{111} surface. Surface alloys of Cu3Pt1−Pt have been calculated to have properties similar to those of Pt−Ru alloys.155 Recently, Chiou et al. synthesized Pt−Cu nanoparticles for methanol oxidation, which formed a better solid solution with 5−20 wt % Cu.156 Beyond 20 wt % Cu, some crystalline Cu formed separately from the Pt−Cu solid solution, which subsequently blocked the active sites of Pt, thereby decreasing the activity of the catalyst. Nanoparticles of Pt−Ti are promising candidates that have been investigated for methanol oxidation.157−160 Abe et al.157 investigated a Pt3Ti catalyst that exhibited higher MOR activity than Pt−Ru and Pt catalysts. They also found that atomically ordered Pt3Ti nanoparticles exhibited higher MOR activity than their disordered phase (Figure 13). Furthermore, they found that Pt3Ti catalysts (atomically ordered or disordered) had a lower affinity for CO adsorption than Pt and Pt−Ru systems. However, a large particle size limits the use of Pt3Ti atomically ordered catalysts in direct liquid fuel cells. This study has opened up new avenues for the study of Pt-based intermetallic compounds containing early transition metals such as Zr, Hf, V, Nb, and Ta. In another study by Jeon and Mcginn, the Pt3Ti phases of Pt50Ti50−C and Pt25Ti75−C were investigated when annealed at 900 °C in a H2−Ar atmosphere.158 These catalysts showed enhanced electrocatalytic activity toward methanol oxidation compared to the Pt−C catalyst annealed at 900 °C. Li et al. studied the Pt−Bi alloy, another promising Ptcontaining binary alloy excluding Ru for methanol oxidation.161 They found that Pt and Bi formed a Pt face-centered cubic solid solution phase in the presence of phosphorus. The onset potential for the Pt−Bi system, which is related to breakage of C−H bonds and oxidation of intermediates through OHads formation, is lower than that for the state-of-the-art Pt−Ru−C system. The methanol oxidation current was found to be higher for the Pt−Bi−XC-72 system than that for the Pt−Ru system. The enhanced catalytic activity of the Pt−Bi alloy nanoparticles was attributed to the combined effects of the ligand and bifunctional mechanism. Up to now, Pt−Ru alloy nanoparticles have been considered to be the best binary metal catalysts for methanol oxidation. The optimum Ru content in Pt−Ru alloys for anodic methanol oxidation varies from 10% at room temperature to 50% at 60 °C.162 However, since Ru is a precious metal that is even less abundant

Figure 13. Full curves in the inset represent the cyclic voltammetry profiles of atomically disordered (green) and ordered (red) Pt3Ti nanoparticles for methanol oxidation. Dashed curves in the inset represent the profiles of the blank test for atomically disordered (green) and ordered (red) Pt3Ti nanoparticles. The green and red curves correspond to the right and left axes of the inset, respectively. The green, red, black, and blue curves on the main panel represent the intrinsic current densities of atomically disordered Pt3Ti, atomically ordered Pt3Ti, pure Pt, and Pt−Ru nanoparticles, respectively, for the oxidation of methanol. Adapted from ref 157. Copyright 2008 American Chemical Society.

than Pt,163 use of Ru can hardly lead to a reduction in catalyst cost. In addition, the toxicity of Ru remains a concern.164 Therefore, searches for tertiary, quaternary, or multimetallic catalysts have continued for decades. 4.2. Ternary and Quaternary Metal Nanoparticle Catalysts

To minimize the amount of Pt−Ru and amplify catalytic activity, researchers have searched for a third or even a fourth metal. To date, many ternary systems for methanol oxidation have been studied, such as Pt−Ru−Ni, Pt−Ru−Mo, Pt−Ru−Ir, Pt−Ru−W, Pt−Ru−Os, Pt−Ru−Co, and Pt−Ru−Sn.35,84,165−169 These ternary catalyst systems have been found to be more effective than the Pt−Ru binary alloy. Carbon-supported Pt−Co−W delivered better performance than Pt−Ru−C for methanol oxidation.43 Cobalt promoted the initiation of methanol dehydrogenation, while W contributed to CO removal through the hydrogen spillover effect made possible by the tungsten bronze system. This study might have been inspired by a combinatorial study of multimetallic array catalysts composed of Pt (Co, Ni, W), considering three metals at a time; 64 tertiary catalyst systems were studied via computational and experimental work.170 Of the 64 catalysts tested, unsupported Pt−Co−W was placed fifth, and its activity was better than that of the Pt−Ru alloy. Therefore, combinatorial studies can be considered as a successful method for finding new and advanced catalyst systems. Long before this study, a combinatorial electrochemistry study was reported on 12408


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the following five metals: Pt, Ru, Ir, Os, and Rh.171 Out of 645 catalysts, the best was found to contain Pt, Ru, Os, and Rh in an atomic ratio of 44:41:10:5. This catalyst was found to be more active than Pt−Ru (50:50) in DMFCs operating at 60 °C, even though it had a surface area approximately half that of the latter. Excluding Ru, there are few alloy catalysts that show better activity than the Pt−Ru alloy. Poh et al. took a different approach to synthesize Pt−Ni−Ru, Pt−Fe−Ru, and Pt−Co− Ru on ordered mesoporous carbon, unlike conventional alloy nanoparticles.172 Composites of Ni−Ru−C, Fe−Ru−C, and Co−Ru−C were first synthesized via sucrose impregnation and chemical vapor deposition (CVD) methods. Then Pt was deposited over the composite by an impregnation method, followed by mild-temperature H2 reduction. The Pt−Co−Ru system showed MOR performance that was comparable to that of a commercial Pt−Ru−C catalyst. Compared to a commercial catalyst, these catalysts did not show better performance for MOR because of the lack of any bifunctional effects and a proper ligand effect from the underlying supporting metals. Nevertheless, these catalysts had some advantages, which allowed higher loadings of the multimetallic catalyst system into the ordered mesoporous carbon material without decreasing the surface area of Pt. Nanoparticles of Pt−Sn−Ni synthesized on a porous matrix of chitosan possess higher catalytic activity for methanol oxidation than Pt−chitosan, Pt−Sn−chitosan, and Pt−Ni−chitosan catalysts.173 In a recent fluorescence-based high-throughput combinatorial study of methanol oxidation, it was found that Pt−Ti−Zn, Pt30Co10Ni60, Pt30Mn20Ni50, Pt30Mn30Ni40, and Pt30Fe40Ni30 ternary systems offer significant improvements relative to Pt−Ru systems.174 In Figure 14 the relevant results are summarized. To further minimize the use of noble metals and enhance activity for methanol oxidation, quaternary catalysts such as Pt−Ru−Mo−W, Pt−Ru−Rh−Sn, Pt−Ru−Ir−Os, and Pt−Ru−Rh−Ni have been studied.171,175−177

approximately 70 mV vs RHE (reversible hydrogen electrode) was more negative compared with that realized on Pt−In + Pb mixed oxide−GC without Au at 95 mV vs RHE. These were close to the theoretical oxidation potential in acidic solutions. Perovskite oxides have been studied as potential materials because they offer surface basic character, which can facilitate oxidative dehydrogenation steps in methanol oxidation, and are stable, particularly in alkaline media.180 In recent years, certain metal oxides such as RuO2,65,181 WO3,182,183 ZrO2,184 MgO,185 and CeO2186,187 have been found to enhance the catalytic activity for ethanol or methanol electro-oxidation through synergetic interactions with Pt. The dehydrogenation capability of WO3 has been effectively used for electro-oxidation of methanol. It is well-known that oxides such as WO3, MoO3, and V2O5 can form their respective bronzes in reducing atmospheres. This situation not only favors electrical conduction but also provides alternate sites for electro-oxidation. Among these oxides, WO3 has been used along with Pt and Pt−Ru for electro-oxidation of methanol.18,188 Nanoporous TiO2 has also been found to be an effective support for the oxidation of methanol.75 It has been shown that carbon, with basic surface oxides, is the most effective type of support. Electrochemical oxidation of methanol, ethanol, glycerol, and ethylene glycol (EG) on novel Pt−CeO2−C catalysts in an alkaline medium has been studied; these studies show improved performance in terms of electrode activity and poisoning resistance.189 Rare-earth-metal oxides LnOx (Ln = Sc, Y, La, Ce, Pr, and Nd) have also been used as catalyst modifiers for methanol electro-oxidation in acid electrolytes,190 and VOx NTs have been used as catalyst-supporting materials.191 Well-dispersed Pd nanoparticles supported on VOx NTs were successfully prepared through a simple reductive process. Compared with other supporting materials, VOx NTs can be easily synthesized as a pure product in gram quantities by low-temperature hydrothermal synthesis, and they have reactive defects on their surfaces. The prepared Pd−VOx NT composites showed an excellent electrocatalytic activity and long-term stability for methanol oxidation in alkaline media. It is possible that more oxide supports will be employed in electrode applications with possible advantages with respect to reactions and tolerance to experimental conditions. Although the catalytic activity toward methanol oxidation increased with the addition of a third metal to Pt−Ru, it was difficult to differentiate the exact roles of alloyed and oxide forms of the third metal. Therefore, a different approach based on adding a metal oxide instead of a third metal is also under consideration.52−62 The effects of a metal oxide, instead of Ru or a second metal, are also being explored.63−71

5. CATALYSTS BASED ON METAL OXIDE SUPPORTS Metal oxide supports are generally not preferred in electrode applications because of conductivity considerations. However, the concept of dimensionality has changed this situation, and many oxide materials can provide electrical conductivities comparable to that of graphite; hence, they can be considered as appropriate supports. In addition, because of support−metal interactions, the electronic properties of the metal species and oxide support can be appropriately altered in demanding electrode applications. These aspects are emerging, and hence, it is expected that, sooner or later, oxide supports will also be examined for electrode applications since they will be able to withstand adverse experimental conditions compared to carbon materials, which can undergo corrosion. At this stage, it may be appropriate to assess the situation so that future developments can be focused on these postulates. A variety of metal oxides such as Al2O3, Cu2O, ZnO2, TiO2, Fe2O3, SiO2, and RuO2 have been employed with a commercial Pt−Ru catalyst for methanol oxidation in a phosphoric acid medium.178 The catalyst using Fe2O3 (at a loading of 0.15 mg cm−2) showed better performance in terms of reduction polarization voltages compared to Pt−Ru alone. Such overvoltage alterations have to be exploited in the future. Biswas et al.179 studied the electrocatalytic activity of a graphite-based Pt electrode modified with In + Pb mixed oxide for methanol oxidation in 0.5 M H2SO4. The zero-current potential realized on Pt−In + Pb mixed oxide−GC with Au at

5.1. Effect of RuO2

The influence of RuO2 on methanol oxidation on Pt surfaces has been studied in the past few decades.143,181,192−207 The effects of RuO2 have been studied either by partially reducing RuO2 to form a metallic Ru phase on RuO2 or by partially oxidizing metallic Ru to RuO2.65,72,73 Huang et al. suggested that the presence of crystalline RuO2 is essential to have a better methanol oxidation from Pt nanoparticles.73 In this study, the improvement is attributed to a higher activity of the crystalline RuO2 phase than that of metallic Ru toward oxidization of CO by forming Ru−OH on the surface. Several reports explain that hydrous ruthenium oxide (RuO2·xH2O) is the actual Ru species in Pt−Ru alloys that promotes CO oxidation.143,181,198−200,204−207 Figure 15 shows representative 12409


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Figure 14. (a−h) Comparative values of I0.6 and I0.8 (from the 20th forward scan) of the materials validated in cyclic voltammetry experiments identified in high-throughput experiments according to a decreasing I0.6 value. Adapted from ref 174. Copyright 2011 American Chemical Society.

matrix.208 RuO2·xH2O can dissociate water molecules. The adsorption strength of CO on a RuO2 surface is similar to that of a bond of O bridging atoms, but it differs on a Ru metallic surface. The bonding of O atoms on a Ru metallic surface is too strong to allow bonded O atoms to react with CO.209

TEM images of hydrous ruthenium oxide and Pt-decorated hydrous ruthenium oxide on MWCNTs.199 Hydrous ruthenium(IV) oxide appears to be a promising material because it reportedly contains water along grain boundaries that can conduct protons within an electron-conducting 12410


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Figure 15. Representative TEM images of (a) RuOxHy−MWCNTs (Ru loading 1.3 wt %), (b) RuOxHy−MWCNTs (Ru loading 5.1 wt %), (c) Pt−RuOxHy (0.10)−MWCNTs, (d) Pt−RuOx Hy (0.40)− MWCNTs, and (e) Pt−MWCNTs. Samples c and d were obtained, respectively, by loading samples a and b with Pt. Adapted with permission from ref 199. Copyright 2010 Elsevier.

Figure 16. V−I characteristics of an MEA with a 2.3 mg cm−2 90% Pt−RuO2·xH2O−C anode, a 4 mg cm−2 Pt black cathode catalyst, and Nafion NE-105 as the electrolyte, recorded at 50 °C with a 1 M methanol solution feed of ca. 6 mL min−1 and an air feed of 500 mL min−1 (STP). Adapted with permission from ref 204. Copyright 2006 Wiley-VCH.

Therefore, hydrous RuO2 can easily provide the required hydroxide species to facilitate CO oxidation on Pt sites, thereby enhancing the catalytic properties. Rolison et al. found that a commercial Pt−Ru catalyst comprised of oxides of Pt and Ru could deliberately control the chemical state of Ru to form RuOxHy rather than Ru metal or particularly anhydrous RuO2 because of poor proton conduction.143 Later, by removing the structural water content via thermal treatment from hydrous RuO2 to form RuO2, Long et al. proved the importance of RuOxHy in enhancing the MOR activity in Pt−Ru systems.198 They also showed that the presence of RuOxHy is necessary not only on the surface but also in the bulk of the Pt−Ru system. In contrast to these reports, Sirk et al. found that the MOR activity of the Pt−Ru commercial catalyst can be enhanced by further reducing the catalyst and decreased by oxidizing it.194 They have found that reduced Pt−Ru showed better MOR activity than Pt−Ru catalysts produced by mild oxidation; the latter contains hydrous RuO2. Dinh et al. concluded that the catalytic activity for methanol oxidation was adversely affected because of blockage of active metal sites by such oxide materials.141 Using a simple in situ electrochemical method, Bock et al. found that formation of ruthenium oxides during methanol oxidation is insignificant and very little or no supportable ruthenium oxide appeared at methanol concentrations larger than 0.1 M.210 Furthermore, they found that RuO2 formed at a potential larger than 0.6 V inhibited methanol oxidation and that metallic Ru is the active state in the Pt−Ru catalyst to form −OH species for complete oxidation of methanol to CO2. Kim et al. previously summarized these results and studied Ru-decorated Pt(111) substrates for methanol oxidation by XPS.211 Proton conductivity can be increased by controlling the electrical resistance of the MEA and vice versa. Scheiba et al. reported that the presence of hydrous RuO2 increases proton conductivity in the MEA.204 In half-cell experiments, they found a lower MOR activity for the RuO2-supported catalyst than that for Pt−Ru−Vulcan XC-72; however, in a full-cell experiment, the reverse results were found (Figure 16). The superior activity is due to a lower electrical resistance of the MEA containing a RuO2-supported Pt anode. Later, Zhu et al. and Wang and Zheng identified the enhancement effect of hydrous RuO2 in a DMFC single-cell measurement.212,213 Therefore, the supporting effect of hydrous RuO2 in the DMFC can be attributed to its pseudocapacitive behavior, which can enhance the dynamic

response of the DMFC without sacrificing quasi-steady-state performance. 5.2. Effect of SnO2

Katayama214 asserted that, in the presence of SnOx, platinum oxide is stabilized by the formation of a redox pair, Pt0−Pt2+ or Pt2+−Pt4+; subsequently, SnOx contributes to activity enhancement for MOR. Aramata et al.215 later confirmed this assertion when they studied rhodium−tin, iridium−tin, and platinum− tin oxides for methanol oxidation and found that tin oxide has a negative effect on the catalytic activity of Rh and no effect on Ir for MOR. This confirmed that oxidative states of Pt are stabilized by tin oxide in an acidic medium. In the vicinity of Pt nanoparticles, tin oxide could enable oxygen species to conveniently remove CO-like residues by oxidation of methanol. More elaborately, Wang et al. described the effects of SnO2 on Pt−Ru nanoparticles for oxidation of methanol.54 When SnO2 and Pt−Ru are simultaneously deposited or when SnO2 is deposited previously on the surface of a carbon support, metal dispersions are improved and the fraction of alloyed Ru decreases. In an XPS study, it was observed that the chemical-state distribution of Pt and Ru was not affected by the deposition sequence of SnO2; however, the sequence does influence the surface atomic concentrations of Pt, Ru, and SnO2. In SnO2-modified Pt−Ru−C catalysts, COads oxidation is promoted by the presence of SnO2. Unlike Pt and Ru sites, CO adsorption does not occur on the surface of SnO2 particles; therefore, OHads formation can take place on SnO2 sites without interruption. The potential of the formation of OHads on SnO2 sites was lower than that on Ru sites; this made it feasible to oxidize COads on Ru sites at a faster rate after cleansing with SnO2. As a result, the onset potential of the catalyst is also reduced. Considering the importance of a high surface area of the support material for electrode materials, Saha et al. synthesized SnO2 NWs on carbon paper62,71 to deposit Pt or Pt−Ru nanoparticles for methanol oxidation. They proposed that the ligand effect of SnO2 reduces the CO adsorption energy on Pt. In addition, a 3D structure and the electronic properties of SnO2 NWs probably contribute to its high catalytic activity. 12411


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Sb doping; this facilitated COads oxidation at higher rates than those of a Pt−SnO2−C catalyst. Pang et al. synthesized Ru-doped SnO2 nanoparticles to be used as a support for Pt and second catalyst for methanol oxidation.221 The MOR activity of the synthesized catalyst was found to be higher compared to that of the undoped SnO2-supported Pt catalyst. From a long time cycle stability test, the Ru-doped SnO2-supported Pt catalyst was found to have better stability than the undoped SnO2-supported Pt catalyst. Hydrogen-reduced SnO2 NWs were synthesized on carbon paper to form Sn−SnO2 NWs (54% Sn) as a support for electrochemically deposited Pt nanoparticles.222 Although the Pt−Sn−SnO2 catalyst showed better activity toward ethanol oxidation, it showed a slightly inferior activity for MOR compared to a commercial carbon-supported Pt−Ru catalyst. As suggested in section 5.1, if catalytic activity is to be enhanced, proton conductivity is as important as electron conductivity in the support material. Utilizing the higher electronic conductivity of CNTs and proton conductivity of sulfated SnO2, Guo and You synthesized a S−SnO2−MWCNT composite material as a support for Pt nanoparticles to be used as a catalyst for methanol oxidation.223 This catalyst was found to be more active toward MOR compared to Pt−SnO2− MWCNT and commercial Pt−C and Pt−Ru−C catalysts. In Figure 18 cyclic voltammograms of various catalysts are

The effects of different nanostructures were further demonstrated by comparing SnO2 nanoflowers and nanorods on which Pt nanoparticles were deposited.63 The improved diffusion of fuel into the catalyst layer in a flowerlike nanostructure significantly reduces liquid sealing effects, which in turn increases the active surface area for electrochemical reactions. Yang et al. synthesized SnO2@C core−shell nanochain 3D superstructures to be used as a support for Pt−Ru nanoparticles.57 Since dissolution of SnO2 creates tin ions, which catalyze redox coupling of Pt species, it is necessary to prevent or control the dissolution of SnO2 in acidic media. A core−shell structure of SnO2@C and a sandwich structure of Pt−Ru− SnO2−C are beneficial for methanol oxidation in acidic media. The stability of support materials for metal catalysts is of supreme importance for high catalytic applications. Carbon is vulnerable to electrochemical oxidation in aqueous solutions, producing CO2.216 Platinum also speeds up carbon corrosion, leading to agglomeration of nanoparticles and deceleration of cell performance.217,218 Tin oxide can protect carbon from electrochemical corrosion. However, because of its poor electrical conductivity compared to that of a carbon support, it is clear that using only SnO2 with Pt cannot exceed the performance of Pt−Ru systems. Therefore, researchers are attempting to modify the electrical conductivity of SnO2 by doping it with metals. One common practice is to use Ru with Pt to further enhance MOR activity. Recently, it has been found that SnO2 is doped with Sb to provide higher electrochemical stability of SnO2 with increased conductivity.219,220 Lee et al. synthesized Pt nanoparticles supported on Sb-doped SnO2 (ATO) and found that the MOR activity was higher than that of Pt−C.219 In addition, the ATO support was found to have higher corrosion resistance than the C support, and there was a strong interaction between the ATO support and redeposited Pt because of the electrochemical dissolution process. After potential cycling, the growth rate of Pt nanoparticles was observed to be slower compared to that of Pt nanoparticles deposited on the C support. Pan et al. also synthesized an Sb-doped SnO2 support by a coprecipitation method to deposit Pt nanoparticles by a polyol synthesis method for methanol oxidation.220 They found a higher activity of the Pt−ATO−C catalyst for MOR than that of both the Pt−SnO2−C catalysit and a commercial Pt−C catalyst (Figure 17). The electrical conductivity of SnO2 increased because of Sb doping, and the optimum content of Sb was found to be 4 wt %. A large number of oxygen vacancies were created by

Figure 18. Cyclic voltammograms of various electrodes at a scan rate of 50 mV s−1 in 1.0 M HClO4 + 1.0 M CH3OH aqueous solution. Adapted with permission from ref 223. Copyright 2011 Elsevier.

presented. Since OH can be easily formed on Ru surfaces, the onset potential and peak potential of CO stripping for Pt−S− SnO2−MWCNTs were found to be higher than those for a Pt−Ru−C catalyst. However, during methanol oxidation, Ru can be easily dissolved. Hence, Pt−S−SnO2−MWCNTs can be considered as a promising anode catalyst for methanol oxidation. Nitrogen-doped CNTs (CNx) were directly synthesized on carbon paper to deposit SnO2 nanoparticles by atomic layer deposition, followed by Pt nanoparticles deposited by EG reduction.224 Because of N doping, the SnO2 and Pt nanoparticles were well dispersed on the CNx material. A higher ECSA was found for Pt−SnO2−CNx than for Pt−CNx because of the OH removal nature of SnO2 from Pt. The Pt−SnO2−CNx catalyst showed higher activity for MOR and oxygen reduction reaction (ORR) than the Pt−CNx catalyst.

Figure 17. Cyclic voltammograms of various electrodes at 50 mV s−1 in 1.0 M H2SO4 + 1.0 M CH3OH aqueous solution: (a) Pt−ATO−C, (b) Pt−SnO2−C, and (c) Pt−C. Adapted with permission from ref 220. Copyright 2011 Elsevier. 12412


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5.3. Effect of CeO2

Cerium oxide (CeO2) has been studied as a cocatalyst for methanol oxidation.60,61,69,74 The role of CeO2 in the catalyst with respect to methanol oxidation is not well established. However, switching of cerium ions between Ce4+ and Ce3+ in CeO2 is assumed to lead to oxygen vacancies on the oxide surface. This can provide additional active sites for the adsorption of CO species on the CeO2 surface, thereby cleansing the Pt surface to promote electrocatalytic reactions.61,74 In a recent study, Anderson et al. found that the major cause of electrocatalytic activity variation in ceria-containing Pt nanoparticles for methanol and ethanol oxidation is the shifting of the redox states of Ce.225 In alkaline media, Ru can be replaced by CeO2 because it allows OH ions to adsorb on its surface at a lower potential.69 Hence, at lower potentials, formation of OHads species can lead to the oxidation of COads adsorbed on Pt to produce CO2, thereby releasing the active sites of Pt for further reaction. A Pt−CeO2−MWCNT catalyst synthesized by a coprecipitation method showed better methanol oxidation activity compared to unmodified Pt−MWCNTs.226 The activity enhancement was explained on the basis of the bifunctional effect of ceria as well as an intrinsic mechanism by facilitating CO oxidation on Pt sites through adsorbed OH on ceria or by inhibiting CO adsorption on Pt sites. Scibioh et al. also proposed a tentative mechanism involving the bifunctional effect and an intrinsic mechanism for a CeO2-modified Pt catalyst on a C support.227 In their study, the optimum catalyst composition was found to be 40 wt % Pt−9 wt % CeO2, which showed better activity for methanol oxidation than a 40 wt % Pt−C (E-TEK) catalyst. Due to f−p hybridization in CeO2, electron transitions to the 3d orbital of Pt become easy; this may facilitate CO removal from Pt sites.228 Because of interactions between Pt nanoparticles and the CeO2 support, microstructural changes occur229 that can lead to the promoting effect for methanol oxidation. In their study, Ou et al.229 found that interactions between Pt and CeO2 lead to the formation of a Ce1−yPtyO2−ztype solution and subsequently generate different defects at Pt−ceria interfaces, i.e., Pt cations, Ce3+ cations, and oxygen vacancies. On increasing the Pt content, these defects can diffuse into the ceria support and influence the microstructure because of decreasing stability of the ceria lattice. In this way, some activated ions decorated the Pt particles and created triplephase interfaces among Pt, ceria, and the atmosphere−solution. This type of triple-phase interface, which contains high concentrations of defects, can promote methanol oxidation by generating OH groups on the ceria support that oxidize CO adsorbed on Pt sites. Interestingly, these catalysts showed better electrocatalytic activity for methanol oxidation than Pt−Ru catalysts. Gu et al. studied the effects of mixture supports composed of CeO2 and Vulcan XC-72 carbon black on Pt nanoparticles for methanol oxidation in acidic media.230 Since ceria is insoluble in dilute acidic solutions, it provides an anticorrosion property to the carbon material; therefore, catalysts on this mixture support possessed higher corrosion resistance than those on a carbon support alone. Apart from the corrosion resistance of CeO2, the oxide serves as an anchor to prevent agglomeration of Pt nanoparticles, thereby enhancing methanol oxidation activity. The best methanol electro-oxidation property was exhibited by Pt−CeO2−C with 20 wt % CeO2. Zhao et al. found that Pt supported on CeO2− hollow carbon spheres (HCSs) showed a higher methanol

Figure 19. TEM images of (a) Pt−HCSs and (b) 20Pt−35CeO2− HCSs. (c) Cyclic voltammograms of methanol electro-oxidation of (a) 20Pt−35CeO2−HCSs, (b) Pt−Ru−HCSs, (c) Pt−Ru−XC-72, and (d) Pt−XC-72 catalysts in a solution of 1.0 M CH3OH and 0.5 M H2SO4. Adapted with permission from ref 231. Copyright 2010 Elsevier.

oxidation current peak than a Pt−Ru−HCS catalyst with the same Pt mass.231 Figure 19 shows TEM images of Pt−HCSs and compares their catalytic activity with that of commercial catalysts. HCSs play an important role in enhancing catalytic activity by contributing to the dispersion of catalyst particles to possess high catalytically active surface areas. They proposed a bifunctional mechanism similar to that for hydrous RuO2 discussed in section 4.1; i.e., ceria can donate OH species for the oxidation of adsorbed COads species on Pt sites. Several other studies also show that the Pt−CeO2 catalyst is better than Pt in oxidizing methanol.232,233 The catalyst system containing CeO2 should be necessarily designed for good interactions with Pt nanoparticles to minimize side effects generated from the low electron conductivity of CeO2. Coating CeO2 by carbonization of β-cyclodextrin was found to greatly enhance the electron conductivity.234 The carbon generated from carbonization of β-cyclodextrin possesses more oxygen-containing species than XC-72; in particular, the number of COOR groups was found to be 3 times that in XC-72. Large numbers of functional groups obviously facilitated anchoring of Pt nanoparticles. Consequently, Pt deposited on carbon-coated CeO2 was found to exhibit higher methanol oxidation activity and stability compared to Pt deposited on bare CeO2. Zhou et al. synthesized a Pt−CeO2−CNT composite using AAO templates for glucose polymerization in its inner pores and carbonization at high temperatures, followed by Pt and CeO2 deposition.235 In this way, both Pt and CeO2 were simultaneously deposited on the inner and outer walls of the CNTs. They found that the catalytic activity of Pt−CeO2−CNTs toward methanol oxidation was higher than that of Pt−CNTs. They proposed several possible mechanisms for the promotion of methanol oxidation by CeO2, probably because surface oxygen of ceria oxidizes adsorbed COads on Pt sites. Alternatively, it may function similarly to Ru 12413


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in a Pt−Ru catalyst in which OHads species are formed at a lower potential and react with surface-adsorbed COads on Pt, forming CO2 and liberating Pt active sites. Another alternative is that Pt and ceria interactions may lead to large numbers of oxygen vacancies, which in turn facilitate adsorption sites for CO, thereby transforming CO to CO2. Although a few studies have succeeded in finding catalytic activity toward methanol oxidation that is higher than that of a Pt−Ru catalyst, the performance of Pt with a ceria catalyst was found to be less than that of a Pt−Ru catalyst in most studies. There are various ways to add other supporting metals to this catalyst to enhance its catalytic activity toward methanol oxidation. Various research groups have studied Pt−Ru with ceria as a catalyst for methanol oxidation.60,61,236−238 Guo et al.60 reported that CeO2 may induce the oxidation of unalloyed surface Ru in Pt−Ru (CeO2)−C catalysts, leading to additional oxygen atoms for high methanol oxidation. However, an increase in poisoning rate has been observed because of the consumption of oxygen from RuOx species, leading to its depletion. Sun et al. reported a rapid sonication process to simultaneously deposit Pt, Ru, and ceria on MWCNTs.61 In their study, Pt−Ru did not remain as an alloy but, instead, formed a Ru@Pt core−shell or another linked structure. The nanoparticles were found to be well dispersed on MWCNTs because of the tip sonication. In their study, it was assumed that the promotional effect of ceria might be due to a bifunctional mechanism that resembles that of Ru. Alternatively, it might be due to an electronic effect similar to Ce4+ to Ce3+ switching, which creates oxygen vacancies to form active sites for CO adsorption, and electron transfer from Pt to ceria, which can lead to Pt−CO bond weakening. In addition, ceria promotion of the catalytic activity of Pt−Ru for methanol oxidation was found to be proportional to the dispersion of the catalyst.236,237 Besides Pt−Ru with ceria, Pt−Au with ceria has also been studied for methanol oxidation.239 The higher methanol oxidation activity of Pt−Au−ceria than that of Pt− ceria was assumed to be attributed to either a higher poison tolerance or the synergistic effect of Au.

Figure 20. Schematic diagram for the preparation of the Pt−WO3−C catalyst. Adapted with permission from ref 250. Copyright 2010 Elsevier.

increasing the catalytic activity. Maiyalagan and Viswanathan66 synthesized WO3 nanorods using an AAO template to support Pt nanoparticles. The Pt−WO3 catalyst exhibited better catalytic activity than a Pt−C (Johnson Matthey) catalyst. Although several reports explain the activity enhancement of the Pt catalyst due to WO3 promotion, it is hard to find a report in which the activity of Pt exceeds that of a Pt−Ru commercial catalyst. From close observations, it has been found that, when WO3 is used as a high surface area catalyst support, the catalyst can show better activity than a commercial catalyst. A WO3 support for Pt nanoparticles, when reduced from bulk to 1D nanorods by pyrolysis of surfactant-encapsulated WO3 clusters, gave better electrocatalytic activity toward methanol oxidation than a commercial Pt−Ru−C catalyst.249 Ganesan and Lee reported a catalyst based on WO3 microspheres and Pt nanoparticles for methanol oxidation.242 The electrocatalytic performance of this catalyst exceeds those of a commercial Pt−Ru−C catalyst as well as Pt nanoparticles supported on C microspheres. The dissolution of semiconductor metal oxide can be controlled by modifying band bending at the electrode−electrolyte interface, thereby controlling electron−hole tunneling. Incorporation of Ti4+ into a WO3 framework helps to suppress dissolution of WO3.241

5.4. Effect of WO3

Platinum and Pt−Ru catalysts supported on tungsten oxide have been extensively studied for electro-oxidation of methanol.64,66,78,183,240−249 In these catalysts, enhancement of electrocatalytic activity is due to a spillover effect in methanol oxidation. The most popular mechanism for WO3 promotion is believed to be via tungsten bronze (HxWO3) formation in the acid medium. This, in turn, facilitates dehydrogenation of methanol. Hydrogen adsorbed on the surface of Pt moves to the WO3 surface where it forms HxWO3. In a cyclic process, HxWO3 can be split into hydrogen ions, electrons, and WO3, thereby enhancing methanol oxidation on the surface of Pt. In most studies, a Pt−WO3 composite anode showed better methanol oxidation performance than an anode of bare Pt. Recently, Cui et al.250 studied Pt supported on a WO3−C composite material using different precursors for WO3. They found that, compared to using sodium tungstate as a precursor for WO3, using phosphotungstic acid (PWA) provides better dispersion of WO3 nanoparticles on carbon because of the acid−base reaction of PWA and oxidized carbon. A schematic diagram is provided in Figure 20. This could lead to a better MSI for deposited Pt nanoparticles, thereby modifying the electronic and catalytic properties of the Pt metal nanoparticles. An increase in the surface area of the WO3 support can also provide better dispersion of metal nanoparticles, thereby

5.5. Effect of TiO2

Titanium dioxide is another metal oxide that can play some role in methanol oxidation on Pt and Pt−Ru nanoparticle surfaces.55,56,70 This oxide is stable in acidic solutions and can be easily prepared from hydrolysis of titanium organic or inorganic salts. Electrodes using TiO2 as a base for platinum change the electronic properties of the Pt surface; therefore, chemical adsorption on the surface is weak because of strong interactions between TiO2 and Pt.75 Furthermore, the presence of TiO2 enhances dispersion of the wetting process,76 which can enlarge the solid−liquid interfacial area and increase the concentration of methanol confined around Pt or Pt−Ru catalysts. Hirakawa et al. introduced a polygonal barrelsputtering method to deposit TiO2, followed by Pt−Ru nanoparticle deposition over carbon.55 This dry synthesis was found to be effective and better than wet synthesis methods. The resulting activity of the Pt−Ru−TiO2−C catalyst was found to be higher toward methanol oxidation than that of the Pt−Ru−C catalyst. Owing to the corrosion resistance properties of TiO2−C hybrid supports, various studies have been reported to enhance 12414


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containing TiO2 NTs had a self-cleaning ability; i.e., after the UV treatment, the electrode had the ability to recover from CO poisoning during methanol oxidation. In addition, the methanol oxidation current when the Pt−TiO2 NT catalyst was illuminated by UV light doubled during methanol oxidation compared to that without UV illumination. Song et al. also studied UV treatments for refreshing Pt−TiO2 NT electrodes.263 Rettew et al. studied MSIs via the formation of Cu-mediated Pt deposition on TiO2 NTs.265 Platinum was deposited by replacing Cu on TiO2 NTs, and the metallic states of Pt were studied at different distances from the TiO2 support. At a certain distance from the TiO2 support, Pt nanoparticles exhibited metallic characteristics; otherwise, below that threshold distance, they behaved as cationic species. Metal-like Pt nanoparticles deposited on a TiO2 support possess higher catalytic activity toward methanol oxidation than cation-like Pt. Yang et al.264 reported a carbon-modified TiO2 NT support for Pt nanoparticles in which carbon material was deposited by low-temperature CVD. The carbon material was characterized by disordered and graphitized structures that provided strong adhesion to Pt nanoparticles. The disordered carbons possessed a high number of OH groups, which oxidize CO on Pt sites, and the graphitized carbons provided electrical conductivity to the support material. The anode material was primarily studied in alkaline media; the peak current density for methanol oxidation was found to be 7.1 times higher in an alkaline medium than that in an acidic medium. Although forming TiO2 NTs on a Ti sheet by anodization seems to be a very convenient and cost-effective process, there are other ways to synthesize TiO2 NTs. Abida et al. and Ju et al. synthesized TiO2 NTs by hydrothermal treatment of TiO2 powder.260,261 Abida et al.260 reported formation of hydrogentitanate NTs as a support for Pt nanoparticles that showed better catalytic activity for methanol than Pt−C and Pt−TiO2 powder catalysts. Ju et al.261 reported a non-Pt catalyst consisting of Pd−Ni nanoparticles deposited on hydrothermally grown TiO2 NTs. In their study, they explained the activity enhancement of the catalyst through the photocatalytic effect of TiO2. In Figure 21, TEM images are shown, and the performance of the Pd−Ni−TiO2 catalyst is compared with that of the standard Pt−Ru−C catalyst. In the case of photocatalysis, rapid recombination of electrons and holes makes the process inefficient. The Pd−Ni alloy on the surface of TiO2 can act as an electron trap, thereby inhibiting rapid recombination of photogenerated carriers. This, in turn, liberates more positive holes to oxidize methanol. Therefore, the additional oxidation process of methanol by TiO2 and catalytic effect of the Pd−Ni catalyst make this catalyst a promising candidate for DMFCs. Although the Pt−Ru−C catalyst is regarded as the most prominent for methanol oxidation, its catalytic activity is reduced by carbon corrosion under fuel-cell operating conditions. Hence, a stable support such as TiO2 instead of carbon has also been studied for Pt−Ru catalysts.56,266−272 Wang et al. also studied a Pt−Au alloy catalyst supported on [email protected] The electrical conductivity of TiO2, as a catalyst support, is very low below 200 °C. Various attempts have been made to overcome the low electrical conductivity of titania. For example, TiO2 has been doped with Nb for use as a catalyst support in DMFCs.274−276 In addition, the anatase crystal structure of TiO2 is more stable for oxidizing organic compounds than the rutile and brookite structures. Therefore, while trying to increase the electrical conductivity, it is important to preserve the anatase structure along with its high surface area for the catalyst support.

the catalytic activity of these Pt-based catalysts toward methanol oxidation.70,251−256 Shanmugam and Gedanken251 reported, for the first time, on carbon-coated anatase TiO2 used as a support for Pt nanoparticles; they found better catalytic activity toward methanol oxidation than that of a commercial Pt−C catalyst. Recently, studies have been reported on supports of TiO2@carbon core−shell nanostructures for Pt nanoparticles used as a catalyst for methanol oxidation.70 The catalytic activities of these systems were found to be better than that of a Vulcan XC-72-supported Pt system; the increased activity was due to corrosion resistance of the TiO2@C core− shell nanostructure supports. Xia et al. synthesized various hybrid materials consisting of TiO2 and graphene or CNTs; these catalysts possess enhanced catalytic activity and stability toward methanol oxidation.253−255 The presence of TiO2 provides anticorrosion and antipoisoning functions, while the carbon material provides electronic conductivity that can lead to enhanced catalytic activity compared to Pt deposited on carbon material without TiO2. Li et al. synthesized core−shell TiO2−C nanofibers to support Pt nanoparticles252 and found the methanol oxidation activity of this system to be higher compared to those of the Pt−C and Pt−TiO2 systems. When illuminated with UV light, the methanol oxidation peak current density was found to be 2.5 times higher than that in the dark. This enhancement was attributed to the synergistic effect of the electrocatalyst and photocatalyst (TiO2) present in the catalyst system. Shen et al. reported Pt NWs grown on a TiO2 surface by a thermally assisted photoreduction process in which electrons and holes were produced by UV radiation on the surface of TiO2 and metal ions were reduced from the solution to form NWs.257 Although these Pt NWs on the TiO2 surface showed a lower methanol oxidation current than Pt microparticles, they showed the highest tolerance to CO poisoning. The high aspect ratio of Pt NWs provides a large surface area of TiO2, enabling the bifunctional effect to oxidize CO to CO2 and thereby releasing Pt active sites. Recently, Pt−C mixed/doped with TiO2 nanostructures and a TiO2−SWCNT composite for methanol oxidation have been studied.258,259 Xiao et al.259 suggested the importance of high porosity and proton conduction of the TiO2 cocatalyst. The structural and adsorbed water contents in the cocatalyst also affect oxidation of intermediates during methanol oxidation.259 Lv et al. created a hybrid support material by physically mixing TiO2 and Vulcan XC-72 carbon black to hydrothermally deposit Pt nanoparticles for methanol oxidation.256 Because of the presence of TiO2, their catalyst showed better catalytic activity toward methanol oxidation than the conventional Pt−C catalyst. The Pt−TiO2−C catalyst was characterized by lower COads formation on its surface than that on the Pt−C catalyst surface. Furthermore, the Pt−TiO2−C catalyst was found to be less susceptible to corrosion and Pt agglomeration than the Pt−C catalyst. Owing to their high surface area as a catalyst support, TiO2 NTs have been extensively studied as supporting materials of metal nanoparticles for methanol oxidation.260−266 The common route to TiO2 NT formation is through anodization of a Ti sheet.262−265 Another advantage of using TiO2 as a catalyst support is that it can yield a methanol oxidation current under UV light owing to its photocatalytic property. Recently, Hosseini and Momeni.262 used TiO2 NTs as a support for Pt nanoparticles and studied their methanol oxidation properties with and without UV exposure. It was found that, after UV exposure, the electrode 12415


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Figure 21. TEM images of (a) TiO2 NTs and (b) the Pd−Ni−TiO2 NT catalyst. (c) Cyclic voltammograms of methanol electro-oxidation of (1) Pd−Ni−TiO2 NTs and (2) TiO2 NTs. (d) Cyclic voltammograms of methanol electro-oxidation of the standard Pt−Ru−C catalyst. Adapted with permission from ref 261. Copyright 2012 Elsevier.

Gojković et al.275 concluded that Nb-doped TiO2 can replace a carbon support without influencing the reaction kinetics for methanol oxidation by the Pt or Pt−Ru catalyst. Fuentes et al.274,276 also studied Nb-doped TiO2 supports for Pt−Ru nanoparticles for methanol oxidation. Sulfated TiO2 and TiO2@N-doped C have also been found to be promising catalyst supports for methanol oxidation.277,278

5.7. Effect of IrO2

Iridium oxide has been studied as a support for Pt−Ru nanoparticles in catalysts for methanol oxidation.77 The presence of Ru in the catalyst decreases the oxidation potential of COads, which enhances the activity of the catalyst toward methanol oxidation. Most importantly, the activity of catalysts using Ir−IrO2 as the catalyst support exceed that of the commercial Pt−Ru catalyst. Baglio et al. investigated the effect of IrO2 as a support material for Pt−Ru nanoparticles.280 They used the multifunctional behavior of Pt, RuO2, and IrO2. Water displacement reaction occurs on RuO2 and IrO2 sites at lower overpotentials, while methanol dehydrogenation occurs on Pt sites. This ternary catalyst using IrO2 as the support material exhibited better performance for methanol oxidation than a benchmark unsupported Pt−Ru catalyst. In another study, Baglio et al. observed that a 20 wt % IrO2 addition to 50 wt % Pt−Ru−C showed a performance increase of 54% and 75% of that of the Pt−Ru−C anode for a 5 M methanol concentration at 60 and 90 °C, respectively.281 Wang et al. synthesized IrO2− CNTs and studied methanol oxidation by a Pt-deposited IrO2− CNT catalyst.282 In their study the Pt−IrO2−CNT catalyst showed better CO oxidation activity than the Pt−CNT catalyst due to the ease of formation of hydroxyl groups on IrO2 at lower potential. As a result, the Pt−IrO2−CNT catalyst offers better methanol oxidation activity than the Pt−CNT catalyst. Amin et al. investigated various metal oxides, including IrO2, as promoters for methanol oxidation with Pt.283 They observed a mass activity order of Pt−IrOx > Pt−RuOx > Pt−SnOx > Pt− VOx. Higher coverage of OH species on the surface of IrO2 and high mobility were related to enhanced activity for methanol oxidation.

5.6. Effect of MnO2

Recently, manganese dioxide (MnO2) has been studied as a catalyst support for DMFCs owing to its excellent proton conductivity, low cost, and environmentally benign nature. However, its poor electrical conductivity, lower Pt nanoparticle dispersion ability, and tedious preparation processes hinder its application as a catalyst support. To overcome those problems, MnO2 can be added to carbon support materials such as CNTs and graphene to realize their synergistic effect.52,279 Zhou et al. synthesized a CNT and hydrous MnO2 composite material as a support for Pt and Pt−Ru catalysts for methanol oxidation.52 In their study the methanol electro-oxidation activity was found to be higher on hydrous MnO2−CNT-supported Pt−Ru nanoparticles than that on Pt−Ru−CNTs and Pt−MnO2−CNTs. The promoting effect of MnO2 is due to fast and barrier-free channels in hydrous MnO2 that enable efficient ion transfer; this, in turn, helps adsorption−desorption of hydrogen and electro-oxidation of COads. In addition, migration of protons and CO may occur from Pt sites to MnO2, which can lead to high methanol oxidation activity for these catalysts. Huang et al. recently reported Pt nanoparticles deposited on MnO2functionalized graphene sheets (GSs) as a catalyst for methanol oxidation.279 Their catalyst showed a much higher electrocatalytic activity than Pt nanoparticles deposited on GS, XC-72, and MnO2−XC-72 supports. 12416


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5.8. Effect of Other Metal Oxides

was found to be better than that of a commercial Pt−C catalyst. However, the catalyst performance decreased as the annealing temperature increased to 600 and 800 °C due to aggregation of Pt nanoparticles. It was found that NiO was reduced to metallic Ni, which, in turn, alloyed with Pt, leading to a change in the electronic structure of the catalyst. NiO flower microspheres were used as a support for Pt nanoparticles.293 It was reported that the catalyst showed a better onset potential than a bulk Pt catalyst for methanol oxidation. Co3O4 has mostly been studied for CO oxidation.294−296 Zhao et al. studied Pt deposited on Co3O4 as a catalyst for methanol oxidation in an acidic medium.297 In their study the catalyst showed a better electrochemical performance for methanol oxidation than a bare nanostructured Pt electrode.

MoOx is one of the potential promoters for methanol oxidation and was studied recently as a cocatalyst for methanol oxidation with Pt and/or Pt−Ru.284−287 Elezović et al. synthesized a MoOx-doped Pt−C catalyst for methanol oxidation and oxygen reduction.284 A higher electrocatalytic activity toward methanol oxidation for the MoOx-doped Pt−C catalyst than the Pt−C catalyst was attributed to the presence of a Mo(VI)/Mo(IV) redox couple and a possible spillover of oxygen from MoOx to Pt sites to oxidize the methanol residues. Justin et al. also reported a MoO3−C composite as a support for Pt nanoparticles produced by intermittent microwave heating.285 In their study they have found that MoO3 formed a redox couple, HyMoO3/HxMoO3, during the electrochemical process for methanol oxidation in an acidic medium. Hydrogen molybdenum bronze is an electrically conductive material and is believed to be responsible for CO oxidation adsorbed on a Pt surface during methanol oxidation. More importantly, in their study it was observed that the activity and stability of the Pt−MoO3 (2:2)−C catalyst toward methanol oxidation were higher than those of a Pt−Ru (2:1)−C catalyst. Similarly, Zhang et al. studied the Pt−MoOx−C electrode and found it to exhibit better activity compared to Pt−C electrodes; this is attributed to the possible redox mechanism in the composite electrode as well as to hydrogen spillover, which could generate molybdenum bronzes.286 Villafuerte et al. synthesized a molybdenum-based core−shell substrate with a reduced Mo core (Mo2C, MoO2, and/or Mo0) and a MoO3 shell and used it as a support material for Pt.287,288 The stability and performance order of the binary catalysts at 20 °C was found to be Pt−MoC−C > Pt−Momix−C > Pt−MoO2−C. Although the catalyst with the MoO2 core (Pt−MoO2−C) showed a better methanol oxidation activity at 60 °C, its stability was found to be lower than that of the catalyst with the MoC core (Pt−MoC−C). In their study, all the catalysts with a Mo-based core−shell support showed a better methanol oxidation current than the Pt−C catalyst; however, only the catalyst with the MoC core showed better performance and stability than the Pt−C catalyst. In conclusion, the better performance was attributed to the better dispersion of active sites, a bifunctional effect, and an electronic effect from the Mo-based core−shell support. Tsiouvaras et al. synthesized a MoO3-modified Pt−Ru on carbon nanofiber (CNF) catalyst for direct methanol fuel cells.289 The catalyst containing MoO3 had higher tolerance to CO than the Pt−Ru−C catalyst. In their study it was found that the aged Pt−Ru−MoO3−CNF catalysts exhibited higher activity and stability than commercial Pt−Ru−C even though the MoO3-containing catalysts showed lower performance than the commercial Pt−Ru−C catalyst before electrochemical aging. Also acid functionalization of the CNF had an adverse effect on stabilization of the MoOx species over the CNF, thus affecting the electrochemical performances. Nickel oxide is a rarely studied metal oxide for methanol oxidation in an acidic medium. Kim et al. studied a catalyst made of Pt nanoparticles on a NiO-deposited C support (Pt−NiO−C).290 The catalyst showed a better methanol oxidation activity than a Pt−C catalyst owing to better dispersion of Pt nanoparticles and the higher electrochemical surface area of the catalyst. It was confirmed that nickel hydroxides were present with NiO, which might play some role in enhancing methanol oxidation activity by Pt catalysts.291,292 As the Pt−NiO−C catalyst was treated at 400 °C, its performance

6. CONDUCTING POLYMER CATALYST SUPPORTS For electrode applications, the basic support employed (conventionally carbon) should be an electrical conductor. It may also possess functional groups that anchor the active metallic species in a dispersed condition. One way to realize these features is by using conducting polymers.298,299 Polyaniline,300−302 polypyrrole,303,304 poly(3-methylthiophene),305 poly(N-methylpyrrole),306 poly(2-hydroxy-3-aminophenazine),307 and copolymers of pyrrole and dithiophene308 are some of the conducting polymers that have been employed for dispersing Pt. Such polymers could give rise to the Pt species nearly mimicking the situation in homogeneous systems. The conductivities of these conducting polymer supports have been examined by employing conventional conductivity measurements.309,310 These studies reveal changes in the electrical resistance of the polymer supports and also provide information on the charge transfer resistance.311−314 6.1. Polyaniline

This conducting polymer can be synthesized chemically315−317 as well as electrochemically.318 Microscopic studies show that this polymer possesses a rough surface morphology, but its conductivity is less than that of polypyrrole and poly(3methylthiophene).319 The conductivity of the polymer can be increased to 200−300 S/cm by doping it with mineral acids.320 The conducting polymer prepared by potential cycling321 yields a good-quality film. The favorable properties of these conducting polymer films include charge transfer properties.322,323 The methanol oxidation activity of Pt on these polymer films is improved because of reduced poisoning by CO.324 Use of these systems for oxidation of other substrates such as HCHO325 has also been studied. 6.2. Polypyrrole

This polymer has been examined for generating new, chemically modified electrodes.326 The electrochemical properties of this conducting polymer have been attributed to a number of factors such as steric interactions, changes in the planarity of the polymer backbone, effective orbital overlap, and the nature of the substituents.327−329 The electropolymerization of pyrrole has been shown to give rise to structural diversity.330 A variety of modified electrodes such as a polymer-coated Au electrode with Pt dispersed on the polymer showed better electroactivity and increased resistance to poisoning.303 It may be possible to generate new electrode systems based on this polymer. 12417


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6.3. Sulfur-Containing Polymers

Pt surface by altering the electronic configuration or providing geometrically suitable sites for weak CO chemisorption. On the other hand, the base non-noble metal could be protected from corrosion by the shell or dispersed Pt.

One sulfur-containing system that has been studied is poly(3methylthiophene), which is predominantly linked between thiophene rings; the sulfur functionality does not contribute to the electronic properties of this material. However, electrooxidation of methanol on Pt -Sn and Pt -Pd systems supported by poly(3-methylthiophene) have been studied.305,331 The copolymer pyrrole−thiophene has been examined for oxidation of methanol and other organic substrates. These studies primarily aimed only to extend the potential range of applicability. Copolymers such as poly(o-phenelenediamine) and polymer fibers of 1,5-dihydroxynaphthalene have also been investigated for increasing the potential window for oxidation.332,333

8.1. Bimetallic Core−Shell Nanoparticles

Three-dimensional Pd@Pt core−shell nanostructures with controllable shapes and compositions were synthesized by using a one-step microwave heating method.46 A Pd@Pt electrocatalyst with a Pd−Pt molar ratio of 1:3 exhibits excellent electrocatalytic properties toward ORR and MOR. The significant enhancement of the methanol oxidation current for Pd@Pt is due to modification of the electronic structure of the catalyst. Palladium is more oxophilic than platinum. As a result, Pd possesses a large number of oxygenated species, which greatly enhance the oxidative removal of COads from the Pt shell. However, a lack of sufficient neighboring Pt atoms reduces methanol adsorption and hinders methanol oxidation. Kristian and Wang used a reduction strategy to synthesize a Ptshell−Aucore−C electrocatalyst that had a higher specific activity than the conventional Pt−C catalyst toward MOR.47 A core−shell catalyst with a low Pt content, Pd−Pt@Pt−C, demonstrated high performance toward methanol oxidation and oxygen reduction.48 For the anodic oxidation of methanol, the catalyst shows an activity 3 times higher than that of the commercial Tanaka 50 wt % Pt−C catalyst. To obtain high dispersion, they used Pt−Pd alloy nanoparticles as the core rather than pure Pd nanoparticles because pure Pd nanoparticles aggregate easily. The methanol oxidation current of Pd−Pt@Pt is higher than that of the pure Pt or alloyed Pt−Pd catalyst because of the presence of a large number of active oxygen-containing species on the Pd core. Two other causes are associated with the high utilization rate of Pt and interactions between the outer Pt shell and inner Pd core. Wang et al. successfully synthesized dendritic Au@Pt using CTAB, a directing agent, and ascorbic acid, a reducing agent.49 The electrochemically active surface area was high because of the loose structure of the Pt shell, which is beneficial for DMFC catalysts. They also studied the effects of the size of the Au core by synthesizing Au nanoparticles of different sizes and Au nanorods. Better activity was found for a gold nanoparticle core with a smaller size; however, a thicker Pt shell limited the promotional effect of the Au core and utilization of Pt. Zeng et al. also synthesized Au@Pt−C and Ag@Pt−C catalysts and observed a higher methanol oxidation activity than that of Pt−C catalysts.32 Au@Pt showed a higher activity than Ag@Pt because of an effective interface formed by the porous Pt layer and Au core with a monolayer-like thickness. In addition, enhanced oxygen on the Au@Pt surface was observed, which increased the methanol oxidation activity of Au@Pt catalysts. Recently, Cui et al. reported Au−Pt core−shell nanoparticles on reduced graphene oxide synthesized by the upd redox replacement technique.342 This core−shell structure with Pt monolayer/submonolayer atoms on the Au nanoparticle surface exhibits excellent catalytic activity toward methanol oxidation. The special structure of Au−Pt core−shell nanoparticles and the presence of oxygen-containing functional groups on the graphene surface were thought to provide high catalytic activity to this catalyst system. Core−shell structures of Ru@Pt and Pt@Ru have been examined side-by-side for their interesting catalytic behavior toward methanol oxidation.28,343,344 Maillard et al. extensively reviewed Ru-decorated Pt surfaces for CO electro-oxidation,

7. COMPOSITE MATERIALS AS SUPPORTS In the context of conducting polymers, the role of Nafion, a proton-conducting polymer, has to be examined. Pt particles dispersed in Nafion polymer material have been reported to show enhanced activity compared to platinized platinum (pt-Pt).334,335 Composite electrodes of Pt−Ru−C−Au−TiO2 have been shown to perform better because of preferential oxidation of CO to CO2.336 Recently, Wang et al. presented a strategy to prepare a Mo2C−graphitic carbon (GC) composite by carbonization of a resin−MoO42−−[Fe(CN)6]4− precursor at 1100 °C in N2.337 Owing to the synergistic effect of Pt and Mo2C, proper utilization of Mo2C, excellent interaction between Mo2C and graphitic carbon, and mesoporous morphology of the composite, Pt−Mo2C−graphitic carbon showed higher mass activity and stability than the Pt−Ru−C catalyst for methanol oxidation. Mo2N−C also showed promising results as a hybrid support for methanol and formic acid oxidation.338 It was found that the Pt−Mo2N−C catalyst possessed higher catalytic activity and stability than the Pt−C catalyst. The higher catalytic activity of Pt−Mo2N−C was attributed to bond activation of C−H and O−H by Mo2N. A Pt−RuO2 composite on boron-doped diamond showed better performance compared to a glassy carbon-based electrode possibly because of its increased electronic and ionic conductivity.196 These composite electrodes, particularly the polymer-based electrodes, are suitable for the acidic atmospheres in working DMFCs. Various composite electrodes such as polyaniline−V2O5,339 PbO2−polyaniline,340 and Pt−Ru alloy on poly(N-vinylcarbazole) have also been examined for increasing oxidation activity.341 These studies indicate that there is ample scope for formulating new electrode systems that can provide enhanced electrocatalytic properties. 8. CORE−SHELL NANOSTRUCTURES AND OTHER ARCHITECTURES FOR METHANOL OXIDATION Because of the cost of the noble metal Pt, research has been performed on constructing core−shell structures that place platinum on the surface and non-noble metals in the core. Alternative dispersed architectures such as layer-by-layer model systems have also been explored. With these structures, it is possible to exploit the full potential of Pt atoms toward methanol oxidation while reducing the loading of the noble metal. The supporting system that forms the core provides the ligand effect and decreases the adsorption energy of CO on the 12418


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in which the shell thickness was controlled by the replacement reaction time. The catalyst synthesized with a 60 min reaction time showed the highest catalytic activity for methanol oxidation; its activity was found to be higher than those of commercial Pt−C and Pt−Ru−C catalysts. A bicontinuous interconnected porous structure of Pt−Cu facilitates excellent electronic conductivity with mass transfer, as well as strain, ligand, and bifunctional effects.

considering CO as the main poisoning species for methanol oxidation.345 Ru-decorated Pt nanoparticles, produced by various synthesis processes, have proven to be efficient for methanol oxidation.346,347 Jones et al. studied the promotion of methanol oxidation by Ru terraces on Pt.344 They found that the terraces provide a non-CO path that follows the direct oxidation of methanol to CO2 via formate on Ru at a lower potential than that on Pt surfaces. In Scheme 2, an indirect CO path to CO2 is shown on Pt sites, and a non-CO path through formate is shown on Pt and Ru sites.

8.2. Tertiary Core−Shell Nanoparticles

To decrease the Pt loading of Pt-based catalysts and increase their activity, PdCo@Pt−C, a pseudo-core−shell structure catalyst, was synthesized through a replacement reaction.45 On the basis of the Pt mass, the electrochemical results demonstrate that PdCo@Pt−C exhibits enhanced catalytic activity for methanol oxidation and better stability compared to pure Pt and Pt−Ru catalysts. The core−shell catalyst Co@Pt− Ru−MWCNTs was synthesized by surface displacement between Co and Pt−Ru alloy on the surface of MWCNTs.50 This catalyst exhibited high methanol oxidation performance. In Figure 22, an HRTEM image of the catalyst is provided, and a cyclic voltammogram of Co@Pt−Ru−MWCNTs for

Scheme 2. (a) Non-CO Path through Formate on Pt, (b) Direct Non-CO Pathway through Formate on Ru through Reverse Spillover, and (c) Indirect CO Pathway to CO2 on Pta

a

Adapted with permission from ref 344. Copyright 2010 Wiley-VCH.

Chen et al.28 studied the effect of the thickness of the Pt shell on Pt−Ru core−shell nanoparticles. They found a maximum 3.2-fold enhancement in CO tolerance and a 2.4-fold improvement in current density with a Pt shell thickness of 1.5 monolayers. A dominating role of the ligand effect was considered to be the cause for this enhancement; for Pt thicknesses over 2.7 monolayers, a combination of ligand and bifunctional effects was expected. Muthuswamy et al.343 found that an alloy shell of Pt−Ru in a Ru@Pt core−shell structure is more beneficial than a Pt-enriched shell. In their study, they found the activity of the alloy shell to be 2 times higher than that of a Pt-rich shell. These studies of Pt−Ru core−shell nanostructures suggest that dominant bifunctional effects with an electronically modified surface for core−shell nanostructures are beneficial for efficient methanol oxidation. Zhang et al. synthesized Co@Pt core−shell nanoparticles using a modified two-step method.348 They found the methanol oxidation activity of a Co@Pt−C catalyst with 12% Pt to be higher than that of a commercial Pt−Ru−C (30 wt %) catalyst; this enhanced activity was attributed to a strain effect on the Pt surface due to the Co core. Xu et al.349 reported a nanoporous core−shell structure of Cu−Pt synthesized by applying a Cu nanoporous structure produced by dealloying Cu−Al alloy. Platinum was deposited on the nanoporous Cu core by a galvanic displacement reaction

Figure 22. (a) HRTEM image of the Co@Pt−Ru−MWCNT catalyst. (b) Cyclic voltammograms of Co@Pt−Ru−MWCNT and Pt−Ru− XC72 catalysts on a glassy carbon electrode in 0.5 M H2SO4 + 1 M CH3OH. The inset shows cyclic voltammograms of Co@Pt−Ru− MWCNTs in 0.5 M H2SO4. The scan rate is 50 mV s−1, and the temperature is 25 °C. The Pt loading is approximately 2 mg cm−2. Adapted with permission from ref 50. Copyright 2008 Elsevier. 12419


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Figure 23. Schematic drawing for the overall synthesis processes and XRD patterns of the Pt−Cu−CNT and Pb−Pt−Cu−CNT catalysts. Adapted with permission from ref 352. Copyright 2012 Elsevier.

higher catalytically active surface area as well as electronic effects of the Pd−Cu core. Later, they studied the effects of the Pt:Ru ratio on the methanol oxidation activity for the same catalyst.354 A volcano curve was obtained, which gave a Pt:Ru ratio of 2:1 for the maximum methanol oxidation activity.

methanol oxidation is compared with that of a commercial Pt−Ru− XC72 catalyst. The higher catalytic activity is attributed to higher utilization of the active metal alloy on the surface of the Co core. Using a metathesis reaction procedure, Wang et al. synthesized Pt-decorated Pd−Fe nanoparticles on a C support.350 Their catalyst showed a very high utilization of Pt, which led to a higher activity for methanol oxidation than those of Pt−Ru−C and commercial Pt−C catalysts. Triple-layered core−shell nanoparticles of Au@Pd@Pt with a dendritic Pt shell were synthesized by a PVP-assisted method.351 This catalyst showed higher methanol oxidation performance in terms of specific and mass activities than Au@Pt, dendritic Pt nanoparticles, platinum black, and E-TEK Pt−C catalysts. In another study, Pb−Pt− Cu−CNTs, a hierarchically structured trimetallic nanocatalyst, was reported.352 In that study, Pt was deposited over Cu nanoparticles by a galvanic displacement reaction, and then Pb was deposited over the nanoparticles with a microwave synthesis method. In Figure 23 the schematic drawing of the synthesis procedure and XRD pattern of the catalysts are shown. The specific activity of the synthesized catalyst was approximately 2 times higher than that of a commercial Pt−Ru−C catalyst for methanol oxidation. Recently, He et al. reported a highly stable core−shell Pt−Au@Ru−C catalyst for methanol oxidation synthesized by the microemulsion method.353 The catalyst showed an insignificant ECSA loss compared to a more than 10% loss of ECSA by the Pt−Ru−C catalyst after an accelerated stability test. Surface stabilization by alloying with Au accounted for the enhanced electrochemical activity of Pt−Au@Ru− C over commercial Pt−Ru−C (E-TEK, 60%).

9. CONCLUDING REMARKS Research conducted over the past few decades has shown that Pt appears to be inevitable to bring DMFCs to the center of practical applications. The anodes in DMFCs are vulnerable to slow reaction kinetics, which is a prime issue in DMFCs. From this review, it is clear that Pt is being modified structurally or metal−metal oxides are being used as supporting materials for Pt to obtain better performance for practical applications. The following are some of the possible future directions for research in development of DMFC anodes. (1) Nanostructured Pt is being studied comprehensively for methanol oxidation catalysts. The Pt{100} facet dominates over other facets in methanol oxidation, and the Pt{111} facet exhibits higher durability than other facets. Facet-directed surfactants or ions are being used for preferential faceted nanostructure synthesis. These developments have now opened up an avenue for new research toward active catalysis of methanol oxidation. (2) When synthesizing Pt-based multimetallic nanoparticles, the aim should be minimization of the Pt and Ru amounts without compromising the catalytic activity. In this regard, highthroughput combinatorial studies appear to be promising for finding new catalysts that have optimum activity for practical applications. We have raised one question in the title of the paper, and possibly the best answer for that question will be that unless a combinatorial search is exhausted, there will always be a possibility of earth-abundant alternatives for Pt or Pt−Ru catalysts. (3) For metal oxide supports, the main concerns include proton conductivity and corrosion resistance in acidic media.

8.3. Quaternary Core−Shell Nanoparticles

A core−shell structure of Pd−Cu@Pt−Ru−C has been reported as a methanol oxidation catalyst synthesized by a two-step replacement reaction.51 The mass activity of this catalyst was found to be higher than those of Pt−C and Pt−Ru−C catalysts. The higher performance of this catalyst was attributed to the 12420


12421

1 M methanol, 1 M HClO4, scan rate 20 mV s−1, 25 °C

0.5 M methanol, 0.5 M HClO4, scan rate 50 mV s−1, 28 °C

1 M methanol, 0.5 M H2SO4, scan rate 50 mV s−1, 25 °C

Pt−CeO2−C229

20Pt−35CeO2−HCSs231

SCE

RHE

SCE

DHE

Pt−S−SnO2−MWCNTs223

Pt−RuO2·xH2O−Vulcan XC-72R204

SCE

anode: 2.3 mg cm−2 90% Pt− RuO2·xH2O−C, methanol feed of ca. 6 mL min−1 cathode: 4 mg cm−2 Pt black, air feed of 500 mL min−1 cell: 50 °C, 1 M methanol, Nafion NE-105

1 M methanol, 0.5 M H2SO4, scan rate 20 mV s−1, room temperature

Pt−Co−W−C43

NHE

SHE

0.5 M methanol, 0.5 M H2SO4, scan rate 50 mV s−1, room temperature

Pt−Bi−XC-72161

sodium chloride-saturated Ag/AgCl electrode

SCE

Ag/AgCl (3 M KCl)

ref electrodea (half-cell)

1 M methanol, 0.5 M H2SO4, scan rate 20 mV s−1, room temperature

0.5 M methanol, 0.1 M H2SO4, scan rate 10 mVs−1

Pt3Ti nanoparticles157

Pt−Ti−Zn174 Pt30Co10Ni60174 Pt30Mn20Ni50174 Pt30 Mn30Ni40174 Pt30Fe40Ni30174

0.1 M methanol, 0.1 M H2SO4, scan rate 20 mV s−1, 25 °C

0.5 M methanol, 0.1 M H2SO4, scan rate 50 mV s−1

methanol oxid. conditions

Pt−Sn−PoAP−p-Al153

Pt−Pb−C nanorods

118

cat. syst

the onset potentials of methanol oxidation for Pt−Ru−HCSs, Pt−Ru−XC-72, and 20Pt−35CeO2−HCSs were 0.20, 0.22, and 0.30 V, respectively; the methanol oxidation current densities for Pt−Ru−HCSs, Pt−Ru−XC-72, and 20Pt−35CeO2−HCSs were 327.24, 298.12, and 489.76 mA mg−1 of Pt, respectively

the peak current densities of the Pt−CeO2−C catalysts were 1.96 and 2.80 mA cm−2 of Pt at 0.8 V for samples with 10 and 30 wt % Pt contents, respectively; on the other hand, the peak current density of the 30 wt % Pt−Ru−C catalyst was 1.63 mA cm−2 of Pt

the current density at 0.7 V for Pt−S−SnO2−MWCNTs was 13.16 A cm−2, and that for Pt−Ru−C was 12.6 A cm−2; the onset potential on Pt−S−SnO2−MWCNTs was 0.24 V, and that on Pt−Ru−C was 0.25 V

Pt−Ru−Vulcan XC-72R showed a higher mass activity than Pt−RuO2·xH2O−Vulcan XC-72Rb

current densities at 0.6 and 0.8 V, respectively 2.60 and 21.66 mA mg−1 of Pt 6.35 and 50.65 mA mg−1 of Pt 2.41 and 23.61 mA mg−1 of Pt 2.88 and 26.98 mA mg−1 of Pt 3.80 and 32.22 mA mg−1 of Pt Pt−Ru: 10.37 and 32.11 mA mg−1 of Pt174 These catalysts showed significant improvements in stability for methanol oxidation relative to Pt−Ru catalysts

the peak current density at ∼0.62 V was 23 mA cm−2 for Pt−Co−W−C and 20 mA cm−2 for 20 wt % commercial PtRu (1:1)−C from E-TEK

Pt−Bi−XC-72 exhibited a higher methanol oxidation current density of 0.73 A mg−1 at 0.92 V and an onset potential of 0.40 V; on the other hand, 30 wt % commercial Pt−Ru showed a peak current density of 0.51 A mg−1 at 1.04 V and an onset potential of 0.43 V

atomically ordered Pt3Ti showed a higher current density and lower onset potential than Pt−Ru (1:1) nanoparticles

35% increase in the case of Pt−Sn−PoAP−p-Al and 18% increase in the case of Pt−Ru−PoAP−p-Al in methanol oxidation current compared with that of Pt− PoAP−p-Al

mass current density ∼700 mA mg−1 of Pt at ∼0.61 V for Pt−Pb−C and ∼400 mA mg−1 of Pt at ∼0.71 V for Pt−Ru−C (Johnson Matthey, 30 wt % Pt and 15 wt % Ru)

methanol oxid. perform. wrt Pt−Ru cat. (half-cell)

Table 2. Summary of the Available Data on Possible Alternative Candidates for Pt−Ru Catalysts in DMFC: Activity Data under Specified Conditions

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1 M methanol, 0.5 M H2SO4, scan rate 20 mV s−1

1 M methanol, 1 M H2SO4, scan rate 50 mV s−1 0.5 M methanol, 0.5 M H2SO4, scan rate 50 mV s−1, room temperature

Pt−MoO3 (2:2)−C285

Pt−Mo2C−GC337

Co@Pt−C348

12422

Hg/Hg2SO4 (0.680 V versus SHE)

Ag/AgCl

0.5 M methanol, 0.5 M H2SO4, scan rate 50 mV s−1, rotation speed 300 rpm

1 M methanol, 0.5 M H2SO4, scan rate 50 mV s−1

Ag/AgCl (3 M Cl−)

1 M methanol, 0.5 M H2SO4, scan rate 20 mV s−1 0.5 M methanol, 0.5 M H2SO4, scan rate 50 mV s−1, rotation speed 300 rpm

RHE

RHE

saturated Ag/AgCl

Ag/AgCl (BAS Instruments, United States)

methanol oxid. perform. wrt Pt−Ru cat. (half-cell)

the specific activity was 94.4 mA cm−2 for Pb−Pt−Cu−CNTs, and that for Pt−Ru−C (Johnson Matthey) was 50.0 mA cm−2 the forward to backward peak ratio (If/Ib) of Pb−Pt−Cu−CNTs was 2.14, and that for Pt−Ru−C (Johnson Matthey) was 0.99

the onset potential for Pt-decorated Pd−Fe−C was ∼0.2 V, while that for Pt−Ru−C was ∼0.27 V the mass activities were 1.01 A mg−1 of Pt for Pt-decorated Pd−Fe−C and 0.29 A mg−1 of Pt for Pt−Ru−C

the peak potential for Cu−Pt (60 min) was ∼0.84 V, while that for Pt−Ru−C was ∼0.86 V; the specific activity of Cu−Pt (60 min) was higher than that of Pt−Ru−C the peak current density of PdCo@Pt−C was 2.189 A mg−1 of Pt at 0.58 V, while that for Pt−Ru−C was 0.428 A mg−1 of Pt at 0.59 V

the mass activities of Pt were higher for Co@Pt−C with 12% and 14% Pt than for 30 wt % Pt−Ru−C (Johnson Matthey), and the onset potentials of Co@Pt−C with 12% and 14% Pt were lower than that for 30 wt % Pt−Ru−C (Johnson Matthey)

the onset potential for Pt−Mo2C−GC was ∼0.2 V, while that for Pt−Ru−C was ∼0.15 V; the mass activity at 0.7 V was 1.19 times higher for Pt−Mo2C−GC than for Pt−Ru−C

Pt−MoO3 (2:2)−C exhibited a current density of 490 mA mg−1 of Pt at 0.7 V; on the other hand, the current density exhibited by Pt−Ru (2:1)−C was 325 mA mg−1 of Pt at 0.7 V

a higher methanol oxidation current density was observed for Pd−Ni−TiO2 at ∼0.94 V than 20 wt % Pt−Ru−C (E-TEK) at ∼0.7 V

the onset potential was 0.2 V for 7.5 wt % Pt−WO3 microspheres and 0.3 V for 20 wt % Pt−Ru−C (E-Tek) and 20 wt % Pt−Ru−carbon microspheres the mass activity was 584 mA mg−1 of Pt for 7.5 wt % Pt−WO3 at 0.75 V and 280 and 307 mA mg−1 of Pt for 20 wt % Pt−Ru−carbon microspheres and 20 wt % Pt−Ru−C (E-Tek), respectively, at 0.80 V

the mass activity of Pt−WO3 nanorods was 295 mA mg−1 of Pt, and that of Pt−Ru−C (Johnson Matthey) was 268 mA mg−1 of Pt

a SCE = saturated calomel electrode. NHE = normal hydrogen electrode. SHE = standard hydrogen electrode. DHE = dynamic hydrogen electrode. RHE = reversible hydrogen electrode. bThe full cell results for this catalyst system are as follows: Pt−RuO2·xH2O−Vulcan XC-72R performed better at a high current density than Pt−Ru−Vulcan XC-72R.

Pb−Pt−Cu−CNTs

352

Pt-decorated Pd−Fe−C350

PdCo@Pt−C45

Cu−Pt core−shell nanocomposite349

1 M methanol, 1 M H2SO4, scan rate 50 mV s−1, room temperature

Pd−Ni−TiO2261 nanotube SCE

Ag/AgCl (3 M NaCl)

1 M methanol, 1 M H2SO4, scan rate 50 mV s−1, 25 °C

ref electrodea (half-cell)

Pt−WO3242 microspheres

methanol oxid. conditions Ag/AgCl, saturated KCl electrode

nanorods

cat. syst 1 M methanol, 1 M H2SO4, scan rate 25 mV s−1

Pt−WO3249

Table 2. continued

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In recent studies, the presence of hydrous metal oxides is found to enhance methanol oxidation activity for metal nanoparticles. This is because, during MEA formation, the proton conductivity of the anode catalysts becomes as important as their electronic conductivity. The electronic conductivity of metal oxide supports can be improved by doping them with conducting materials. Small surface areas are another important issue that should be addressed in synthesizing metal oxide supports. A high surface area metal oxide support can lead to enhanced catalytic activity. (4) Catalysts for oxidation or reduction generally possess a bifunctional mechanism or an electronic ligand effect or both. For methanol oxidation, bifunctional effects are 4 times more effective than ligand effects. When both bifunctional and ligand effects are present, the increase in catalytic activity is generally larger than when only a single effect dominates. A core−shell nanostructure with a core partially covered by an active metal (e.g., Pt) possesses both bifunctional and ligand effects for electrocatalysis. Recently, core−shell nanostructures with a monolayer surface of a noble metal have been found to offer an enormous possibility for minimizing the amount of Pt needed for catalysis. (5) The use of non-Pt catalysts in alkaline media is another cost-effective way for DMFC commercialization. Palladium has been the metal that is most tested in catalysts for methanol oxidation in alkaline media. Core−shell nanostructures using Pd on the surface and a supporting metal in the core could be an effective way to synthesize low-cost catalysts for use in alkaline DMFCs. From Table 2 it is observed that until now only a few catalysts have been detected as potential candidates for DMFC anode catalysts avoiding Ru with Pt as the state of the art material. Even though enormous studies have been carried out in this field, the lack of study of these potential candidates in DMFC operating conditions makes us realize that the search has not been exhausted. Unless we try these catalyst systems in a DMFC working environment, we cannot be sure about their potential as an alternative candidate to state of the art Pt−Ru catalysts.

then obtained his Ph.D. in Materials Science and Engineering from Yonsei University, South Korea, in 2013 under the supervision of Professor Young Soo Yoon. The research for his doctoral dissertation was on the synthesis, characterization, and application of novel nanostructured electrocatalysts for DMFC anodes. He was a Visiting Scientist in the Fuel Cell Research Center, Korea Institute of Science and Technology, South Korea. He is now an Assistant Professor in the Department of Chemical Engineering, Gachon University, South Korea. His research interests are catalysis and polymer membranes for PEMFC/direct alcohol fuel cells, nanomaterial synthesis and characterization, gas sensors, Li ion batteries, and thin film technology.

Dr. Jatindranath Maiti was born in 1980 in West Bengal, India. He received his B.Sc. in Chemistry from Vidyasagar University, West Bengal, in 2001. He obtained his M.Sc. in 2004 and his Ph.D. in 2010 in Polymer Science from Tezpur University, India. He then moved to Yonsei University, South Korea, and was a Postdoctoral Fellow in the laboratory of Professor Young Soo Yoon from 2010 to 2011. He was also a Postdoctoral Fellow at the Leibniz Institute of Polymer Research, Dresden, Germany. Then he was a Research Scientist at the King Abdulaziz City for Science and Technology, Saudi Arabia. Presently he is a Research Professor in the Department of Chemical Engineering at Gachon University, South Korea. His research interests involve polymer membranes, polymer nanocomposites, conjugated polymer/Block copolymer, emulsion polymerization, direct methanol fuel cells, and CNT functionalization.

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. Phone: +91-4422574241. *E-mail: [email protected]. Phone: +82-317505596. Fax: +82-317508839. Notes

The authors declare no competing financial interest. Biographies

Dr. Seok Hee Lee received his M.S. (2010) and Ph.D. (2013) degrees in Materials Science and Engineering from Yonsei University, South Korea. His main research field was the investigation of the electrochemical properties and fabrication of catalysts for lowtemperature fuel cells. His current research interests are focused on the synthesis and characterization of new alternative electrode and electrolyte materials for all-solid-state batteries.

Dr. Nitul Kakati received his B.Sc. in Chemistry from Gauhati University, India, in 2005. In 2007, he received his M.Sc. in Nanoscience and Technology from Tezpur University, India. He 12423


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Department of Advanced Technology Fusion at Konkuk University (2004−2008) and the Department of Materials Science and Engineering at Yonsei University (2008−2012). He is now a Professor in Chemical Engineering at Gachon University. His current research interests are (i) all-solid Li-based secondary battery materials and systems, (ii) anode and cathode materials and systems for DMFCs, and (iii) process and device development based on two-dimensional structures. He has expertise in the thin film process and measurement and synthesis of nanotailored ceramic−metal composite powder for electrodes and solid electrolytes of Li batteries and catalysts for fuel cells.

ACKNOWLEDGMENTS We acknowledge the financial support from a grant of MEMS Research Center for National Defense funded by Defense Acquisition Program Administration, South Korea. We are also thankful to Defense Nano Technology (Center), South Korea for the financial assistance for this work. Nitul Kakati would like to acknowledge the support from Gachon University for this research.

Dr. Seung Hyun Jee received his Ph.D. degree (2012) in Materials Science and Engineering from Yonsei University, South Korea. He was a Student Researcher at Auburn University in 2011. He was also a Postdoctoral Researcher at the Korea Atomic Energy Research Institute, South Korea, in 2012. Currently, he is a Research Professor at Gachon University, South Korea. His research interests are focused on the synthesis and investigation of new energy storage and conversion materials for all-solid-state batteries and fuel cells.

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Professor Balasubramanian Viswanathan is currently a faculty member of the National Centre for Catalysis Research after four decades of teaching and research at the Indian Institute of Technology, Madras. His research interests are catalysis, hydrogen energy, and solid-state materials.

Professor Young Soo Yoon received his Ph.D. from the Korea Advanced Institute of Science and Technology, South Korea. He was a Research Fellow (Academic Staff) at the University of Minnesota and Principal Research Scientist at the Korea Institute of Science and Technology (1997−2003). He was an Associate Professor in the 12424


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