Electrocatalyst For Methanol Oxidation Research Articles

Rationally Designed L12-Pt2RhFe Intermetallic Catalyst with High CO-Tolerance for Alkaline Methanol Electrooxidation.

It is a grand challenge to deep understanding of and precise control over functional sites for the rational design of highly efficient catalysts for methanol electrooxidation. Here, an L12-Pt2RhFe intermetallic catalyst with integrated functional components is demonstrated, which exhibits exceptional CO tolerance. The Pt2RhFe/C achieves a superior mass activity of 6.43 A mgPt -1, which is 2.23-fold and 3.53-fold higher than those of PtRu/C and Pt/C. Impressively, the Pt2RhFe/C exhibits a significant enhancement in durability owing to its high CO-tolerance and stability. Density functional theory calculations reveal that high performance of Pt2RhFe intermetallic catalyst arises from the synergistic effect: the strong OH binding energy (OHBE) at Fe sites induce stably adsorbed OH species and thus facilitate the dehydrogenation step of methanol via rapid hydrogen transfer, while moderate OHBE at Rh sites promote the formation of the transition state (Pt-CO···OH-Rh) with a low activation barrier for CO removal. This work provides new insights into the role of OH binding strength in the removal of CO species, which is beneficial for the rational design of highly efficient catalysts.

Palladium nanocrystals immobilized on boron and nitrogen codoped mesoporous carbon spheres as efficient methanol oxidation electrocatalysts

The development of cost-effective and high-performance electrocatalysts is crucial for the widespread application of direct methanol fuel cell. Herein, a convenient and robust method is developed to the controllable fabrication of Pd nanocrystals in situ immobilized on boron and nitrogen codoped mesoporous carbon spheres (Pd/BNMCS) as anode catalysts for the methanol oxidation reaction. The as-derived Pd/BNMCS hybrids are equipped with a series of useful structural features, such as large specific surface area, abundant mesoporous channels, presence of plentiful B and N atoms, well-dispersive Pd nanoparticles, and good electrical conductivity. As a consequence, the optimized Pd/BNMCS catalyst demonstrates superior electrocatalytic methanol oxidation properties in terms of a large electrochemically active surface area, high mass/specific activities, and reliable long-term stability, which are significantly better than those of traditional carbon black and undoped mesoporous carbon sphere supported Pd catalysts. This study highlights the potential of heteroatom codoped mesoporous carbon matrixes in the construction of advanced noble metal electrocatalysts for practical fuel cell applications.

Metal-organic framework-derived nanoflower and nanoflake metal oxides as electrocatalysts for methanol oxidation.

The energy crisis is the most urgent issue facing contemporary society and needs to be given top priority. As energy consumption rises, environmental pollution is becoming a serious issue. Direct methanol fuel cells (DMFCs) have emerged as the most promising energy source for a variety of applications such as electric vehicles and portable devices. Unfortunately, the kinetics of methanol oxidation is slow and needs an electrocatalyst to improve the reaction kinetics and the overall fuel cell efficiency. Herein, a straightforward hydrothermal procedure was utilized to prepare copper, nickel, and cobalt-based MOF composites by altering the elemental molar ratios. Cu-MOF (MOFP1), Cu/Ni-MOF (MOFP2), and Cu/Ni/Co-MOF (MOFP3) were prepared after carbonization and characterized using several key techniques such as FTIR, XRD, SEM, and EDX. The SEM analysis reveals that the morphology of MOFP1 is spherical aggregated particles, while that of MOFP2 or MOFP3 is in the form of nanoflakes and nanoflowers. Moreover, upon application of these composites as electrocatalysts for methanol electro-oxidation in an alkaline medium of 1 M NaOH using cyclic voltammetry (CV) and chronoamperometry (CA) tests, the electrochemical performance of MOFP2 in 1 M methanol exhibits the best performance for methanol oxidation with a current density reaching 38.77 mA cm-2 at a scan rate of 60 mV s-1. This can be attributed to the unique porous open flower structure and the synergistic effect between copper, nickel, and 2-aminoterephthalic acid which develop its catalytic activity.

Construction of high-efficiency Fe-MnO@Fe electrocatalyst for methanol and ethanol oxidation in alkaline medium

Abstract Acquiring effective, durable and economical non-precious metal electrocatalysts is crucial for the widespread utilization of fuel cells. In this work, an iron/manganese oxide nanocomposite (Fe-MnO@Fe) is synthesized as a novel electrocatalyst by direct-current (DC) arc plasma technology integrating transition metal and transition metal oxide. The Fe-MnO@Fe catalyst demonstrates excellent catalytic performance. By modulating the percentage of transition metal and transition metal oxide in the composite, the largest current density values of methanol and ethanol oxidation of the composite reach 20.18 and 7.83 mA/cm2, respectively. In addition, the composite catalyst exhibits excellent stability and conductivity, and the catalyst greatly reduces the experimental cost relative to the noble metal catalysts, indicating that the composite is a potential fuel cell catalyst candidate and provides an innovative concept for creation non-precious metal catalysts.

Self-Assembly of a Hierarchical Metal-Organic Framework at the Liquid/Liquid Interface via π-π Stacking Manipulations in Organoplatinum(IV) Complexes for Methanol Fuel Cells.

This study focuses on the facile synthesis of the hierarchical architecture of zeolitic imidazolate framework-8 (ZIF-8) films containing an ultrasmall amount of Pt(0) by investigating the synthesis of different organoplatinum complexes and manipulating the π-π stacking effect in these complexes at the liquid/liquid interface. The organometallic Pt(IV) precursors were complexes with a formula of [PtXMe2(R)(bpy)] (bpy = 2,2'-bipyridine; for complex 2, R = CH2CH═CHC6H5 and X = Br; for complex 3, R = CH2CH═CH2 and X = Br; for complex 4, R = Me and X = I) prepared by oxidative addition of cinnamyl bromide, allyl bromide, or methyl iodide to [PtMe2(bpy)] (complex 1). Different thin films were synthesized starting from three organometallic Pt(IV) precursors (i) by reduction of the Pt complexes at the toluene/water interface (TF2-TF4), (ii) by encapsulation of the Pt precursors in a ZIF-8 (TF5-TF7), and (iii) by reduction of the Pt precursors onto a ZIF-8 (TF8-TF10). The self-assembly of ZIF-8 and different organoplatinum precursors at the interface of two immiscible liquids leads to the preparation of films with well-engineered structures such as rhombic dodecahedra, nanorods, hierarchical architectures, and nanowires, which are very difficult and complicated to synthesize under normal conditions. The ultralow loading of platinum complexes with different degrees of π-π stacking of dangling moieties has a great impact on the structure and morphology (directing agent), which in turn drastically changes the catalytic properties. The obtained films were applied as electrocatalysts for methanol oxidation in fuel cells. The electrocatalytic performance of organoplatinum containing a cinnamyl group in hierarchical architecture TF8 was found to be superior to those of nonhierarchical structures.

One-step hydrothermal preparation of CeMo-codoped Ni3S2 as robust bifunctional electrocatalyst for urea oxidation and methanol oxidation

The production of hydrogen via electrolytic water splitting is mainly limited by the high overpotential of the oxygen evolution reaction (OER). Thus, the replacement of OER with more thermodynamically favorable urea oxidation (UOR) and methanol oxidation (MOR) reactions shows good promise. In this paper, a CeMo-Ni3S2/NF bifunctional electrocatalyst for UOR and MOR electrolysis was prepared by a one-step hydrothermal method. The experiments reveal that the doped Ce and Mo cations significant modulate the morphology of Ni3S2, and thus significant boost the electrochemically active surface area and increase the UOR and MOR performance. At a current density of 100 mA cm−2, UOR requires a potential of 1.35 V (vs. RHE) and MOR requires 1.38 V (vs. RHE), which are both significantly lower than the potential of 1.59 V (vs. RHE) required for OER. The UOR reaction produces NO2- with a Faradaic efficiency of 69.0 %, while the MOR reaction produces formate with a Faradaic efficiency of 81.5 %. These results provide strategies for the development of advanced MOR and UOR electrode materials.

Nickel-modified poly(aniline-co-pyrrole) as electrocatalyst for electrochemical oxidation of methanol in direct methanol fuel cell application

Nickel-modified poly(aniline-co-pyrrole) as electrocatalyst for electrochemical oxidation of methanol in direct methanol fuel cell application

Interconnected Pd Nanowire Networks Stereoassembled on Biomass-Derived Porous Carbon Skeletons as Bifunctional Electrocatalysts for Efficient Methanol and Formic Acid Oxidation

Interconnected Pd Nanowire Networks Stereoassembled on Biomass-Derived Porous Carbon Skeletons as Bifunctional Electrocatalysts for Efficient Methanol and Formic Acid Oxidation

Three‐Dimensionally Arranged NiSe2 Nanosheets as an Efficient Electrocatalyst for Methanol Electrooxidation Reaction

Methanol oxidation stands out as a pivotal solution in addressing the global energy crisis and environmental pollution, owing to its practical applicability, high current density, and the ready availability of methanol as a fuel source. To effectively catalyze methanol oxidation, an electrocatalyst is imperious to overcome the activation energy barrier. Herein, a three‐dimensionally arranged NiSe2 nanosheet‐based electrocatalyst is synthesized through a facile solvothermal followed by an annealing method. The catalyst's porous structure enhances catalytic efficiency by providing a substantial electrochemical surface area (ECSA) equivalent to 0.121 mF cm−2. Notably, the electrocatalyst exhibits a remarkable response of 21.58 mA cm−2 at an overpotential of 1.70 V vs RHE, accompanied by the lowest Tafel slope recorded at 39.14 mV dec−1. The electronic circuit, represented by Rs(Qf(RfW(QdlRct)), aligns well with electrochemical impedance spectroscopy data, elucidating the reaction path and intrinsic properties. Furthermore, the catalytic performance is elucidated concerning ECSA and weight, revealing current densities of 5.60 mA cm−2 and 71.34 mA mg−1, respectively. Impressively, the catalyst demonstrates exceptional resistance to poisoning and sustained stability over a continuous 3600‐s operation. This comprehensive study underscores the promising potential of the NiSe2 nanosheet‐based electrocatalyst for efficient methanol oxidation, providing valuable insights for advancing clean energy technologies.

Constructing Gradient Orbital Coupling to Induce Reactive Metal-Support Interaction in Pt-Carbide Electrocatalysts for Efficient Methanol Oxidation.

Reactive metal-support interaction (RMSI) is an emerging way to regulate the catalytic performance for supported metal catalysts. However, the induction of RMSI by the thermal reduction is often accompanied by the encapsulation effect on metals, which limits the mechanism research and applications of RMSI. In this work, a gradient orbital coupling construction strategy was successfully developed to induce RMSI in Pt-carbide system without a reductant, leading to the formation of L12-PtxM-MCy (M = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and W) intermetallic electrocatalysts. Density functional theory (DFT) calculations suggest that the gradient coupling of the d(M)-2p(C)-5d(Pt) orbital would induce the electron transfer from M to C covalent bonds to Pt NPs, which facilitates the formation of C vacancy (Cv) and the subsequent M migration (occurrence of RMSI). Moreover, the good correlation between the formation energy of Cv and the onset temperature of RMSI in Pt-MCx systems proves the key role of nonmetallic atomic vacancy formation for inducing RMSI. The developed L12-Pt3Ti-TiC catalyst exhibits excellent acidic methanol oxidation reaction activity, with mass activity of 2.36 A mgPt-1 in half-cell and a peak power density of 187.9 mW mgPt-1 in a direct methanol fuel cell, which is one of the best catalysts ever reported. DFT calculations reveal that L12-Pt3Ti-TiC favorably weakens *CO absorption compared to Pt-TiC due to the change of the absorption site from Pt to Ti, which accounts for the enhanced MOR performance.

PtPd @CeO2 nanoparticles highly dispersed on carbon nanotubes as efficient electrocatalysts for methanol oxidation

Pt-based alloys have garnered significant attention due to its high electro-catalytic activity and stability. In this study, CeO2 nanoparticles were uniformly loaded onto carbon nanotube. Pt-Pd alloy particles were electrodeposited onto CeO2 nanoparticles to form Pt-Pd@CeO2/carbon nanotubes composites (Pt-Pd@CeO2/CNTs). Performance characterization results showed that Pt1Pd1.8@CeO2/CNTs electrocatalyst exhibited outstanding mass activity (2521 mA·mg Pt+Pd−1) for MOR, which was 7.9 times higher than that of Pt/C catalyst. The synergistic effect of Pt-Pd substantially enhanced the activity and the active oxygen-containing substances of CeO2 effectively improved the anti-CO poisoning ability. This work has provided a new pathway for developing high-performance catalytic catalysts for MOR.

Enhanced methanol electrooxidation catalysis via dual modulation of PtCu alloy and oxygen vacancies

Achieved by introducing defect sites, enhances both the catalytic activity and stability of catalysts. This is crucial for advancing key applications in energy conversion and environmental protection. Here, we introduce a novel Pt-Cu@CeO2/MWCNTs electrocatalyst prepared by dual modulation of the oxygen vacancy anchoring point and the center of the alloying d-band. This catalyst significantly boosts the rate of methanol oxidation. The Pt-Cu@CeO2/MWCNTs catalyst demonstrates a mass activity 3.7 times higher than that of commercial Pt/C, along with exceptional durability. The electron transfer between the PtCu alloy and the oxygen vacancies effectively modulates the d-band center of Pt, leading to a “volcano” relationship between the catalyst’s mass activity and the d-band center. This study advances the development of methanol electrooxidation catalysts by introducing a novel “double enhancement effect” model.

Ni-Pt-Pd ternary alloy nanoparticles supported on carboxyl-modified carbon nanotubes as highly efficient catalyst for methanol and ethanol electrooxidation

Pt-based catalysts are currently the most efficient anode catalysts in alcohol fuel cells. However, their widespread commercialization is hindered by their high cost. In this study, a straightforward method was employed to deposit Ni-Pt-Pd ternary alloy nanoparticles onto carboxyl-modified multi-walled carbon nanotubes (CNTs) and subsequently annealed the synthesized catalysts at different temperatures to adjust the surface composition and alloying degree. The Ni-Pt-Pd alloy exhibits a synergistic effect between Pt and Ni. The presence of Ni effectively modulates the d-band center of Pt, thereby enhancing the electrochemical performance of for alcohol oxidation. The synergistic effect of Pt-Pd alloy is harnessed to enhance both catalytic activity and stability of the catalyst for oxidation of methanol or ethanol. Density functional theory (DFT) calculations demonstrated that alloying Pt with Pd and Ni effectively reduced the Gibbs free energy during EOR and MOR reactions. The resulting catalysts, denoted as Ni6Pt6Pd6 NPs/CNTs-M2, exhibited exceptional catalytic oxidation performance for both ethanol and methanol. Specifically, their electrochemical mass activity for ethanol oxidation reaction (EOR) reached 2345 mA mg-1Pt+Pd, which was 15.5-fold and 2.56-fold higher than that of commercial Pt/C and Pd/C catalysts respectively. Moreover, their mass activity for methanol oxidation reaction (MOR) was measured at 2995 mA mg-1Pt+Pd, which was 8.2-fold and 5.3-fold higher than that of Pt/C and Pd/C catalysts respectively.

Unveiling the Pivotal Role of dx2-y2 Electronic States in Nickel-Based Hydroxide Electrocatalysts for Methanol Oxidation.

The anodic methanol oxidation reaction (MOR) plays a crucial role in coupling with the cathodic hydrogen evolution reaction (HER) and enables the sustainable production of the high-valued formate. Nickel-based hydroxide (Ni(OH)2) as MOR electrocatalyst has attracted enormous attention. However, the key factor determining the intrinsic catalytic activity remains unknown, which significantly hinders the further development of Ni(OH)2 electrocatalyst. Here, we found that the electronic state within antibonding bands plays a decisive role in the whole MOR process. The onset potential depends on the deprotonation ability (Ni2+ to Ni3+), which was closely related to the band center of orbital. The closer of orbital to the Fermi level showed the stronger the deprotonation ability. Meanwhile, in the high potential region, the broadening of orbital would facilitate the electron transfer from methanol to catalysts (Ni3+ to Ni2+), further enhancing the catalytic properties. Our work for the first time clarifies the intrinsic relationship between electronic state and the MOR activities, which adds a new layer of understanding to the methanol electrooxidation research scene.

Unveiling the Pivotal Role of dx2−y2 Electronic States in Nickel‐Based Hydroxide Electrocatalysts for Methanol Oxidation

AbstractThe anodic methanol oxidation reaction (MOR) plays a crucial role in coupling with the cathodic hydrogen evolution reaction (HER) and enables the sustainable production of the high‐valued formate. Nickel‐based hydroxide (Ni(OH)2) as MOR electrocatalyst has attracted enormous attention. However, the key factor determining the intrinsic catalytic activity remains unknown, which significantly hinders the further development of Ni(OH)2 electrocatalyst. Here, we found that the electronic state within antibonding bands plays a decisive role in the whole MOR process. The onset potential depends on the deprotonation ability (Ni2+ to Ni3+), which was closely related to the band center of orbital. The closer of orbital to the Fermi level showed the stronger the deprotonation ability. Meanwhile, in the high potential region, the broadening of orbital would facilitate the electron transfer from methanol to catalysts (Ni3+ to Ni2+), further enhancing the catalytic properties. Our work for the first time clarifies the intrinsic relationship between electronic state and the MOR activities, which adds a new layer of understanding to the methanol electrooxidation research scene.

New type nanocomposite based on metal-organic frameworks decorated with nickel nanoparticles as a potent electrocatalyst for methanol oxidation in alkaline media

Methanol oxidation reaction (MOR) is used to provide electrical energy via direct methanol fuel cell (DMFC), which requires a practical design of electrocatalysts to accelerate the sluggish MOR. In this study, we successfully fabricated the carbon black nanoparticles/graphite ceramic electrode (CBNPs/GCE) supported metal-organic frameworks (MOFs) and decorated with nickel nanoparticles (NiNPs) as a new type nanocomposite modified electrode (NiNPs/MOFs/CBNPs/GCE) via a chemical and an electrochemical process, respectively. The surface morphology and structure of the synthesized nanocomposite materials and modified electrodes were characterized using field emission scanning electron microscopy, energy dispersive X-ray analysis, Fourier transform infrared spectroscopy, X-ray diffraction and thermogravimetric analysis. Then, the NiNPs/MOFs/CBNPs/GCE was used as a non-platinum electrocatalyst for electrocatalytic oxidation of methanol in alkaline media. The obtained results show the efficient electrocatalytic activity of the NiNPs/MOFs/CBNPs/GCE toward the MOR with high anodic peak current density (Jpa = 34.8 mA cm−2) compared with the other prepared electrocatalysts; MOFs/CCE, NiNPs/CCE, MOFs/CBNPs/GCE, NiNPs/CBNPs/GCE, and NiNPs/MOFs/CCE, due to the synergistic effect of nanocomposite components; NiNPs, MOFs and CBNPs. The prepared electrocatalyst; NiNPs/MOFs/CBNPs/GCE, not only shows a high electrochemically active surface area (ECSA = 2.15 cm2) but also has long-term durability in MOR. In general, it can be said the NiNPs/MOFs/CBNPs/GCE is a promising candidate for application in DMFC.

MoO3/WO3/rGO as electrode material for supercapacitor and catalyst for methanol and ethanol electrooxidation.

The potential of metal oxides in electrochemical energy storage encouraged our research team to synthesize molybdenum oxide/tungsten oxide nanocomposites (MoO3/WO3) and their hybrid with reduced graphene oxide (rGO), in the form of MoO3/WO3/rGO as a substrate with relatively good electrical conductivity and suitable electrochemical active surface. In this context, we presented the electrochemical behavior of these nanocomposites as an electrode for supercapacitors and as a catalyst in the oxidation process of methanol/ethanol. Our engineered samples were characterized by X-ray diffraction pattern and scanning electron microscopy. As a result, MoO3/WO3 and MoO3/WO3/rGO indicated specific capacitances of 452 and 583 F/g and stability of 88.9% and 92.6% after 2000 consecutive GCD cycles, respectively. Also, MoO3/WO3 and MoO3/WO3/rGO nanocatalysts showed oxidation current densities of 117 and 170 mA/cm2 at scan rate of 50 mV/s, and stability of 71 and 89%, respectively in chronoamperometry analysis, in the MOR process. Interestingly, in the ethanol oxidation process, corresponding oxidation current densities of 42 and 106 mA/cm2 and stability values of 70 and 82% were achieved. MoO3/WO3 and MoO3/WO3/rGO can be attractive options paving the way for prospective alcohol-based fuel cells.

Facile Synthesis of PtP2 Nanocrystals as Highly Active Electrocatalysts for Methanol Oxidation.

Although significant efforts have been made in the past few decades, the development of affordable, durable, and effective electrocatalysts for direct methanol fuel cells (DMFCs) remains a formidable challenge. Herein, we present a facile and efficient phosphorization approach for synthesizing PtP2 intermetallic nanocrystals and utilize them as electrocatalysts in the methanol oxidation reaction (MOR). Impressively, the synthesized PtP2 nanocatalysts exhibit a mass activity of 2.14 mA μg-1 and a specific activity of 6.28 mA cm-2, which are 5.1 and 9.5 times higher than those achieved by the current state-of-the-art commercial Pt/C catalyst, respectively. Moreover, the PtP2 nanocatalysts demonstrate improved stability toward acidic MOR by retaining 92.1% of its initial mass activity after undergoing 5000 potential cycles, far surpassing that of the commercial Pt/C (38%). Further DMFC tests present a 2.7 times higher power density than that of the commercial Pt/C, underscoring their potential for application in methanol fuel cells. Density functional theory calculations suggest that the accelerated MOR kinetics and improved CO tolerance on PtP2 can be attributed to the attenuated binding strength of CO intermediates and the enhanced stability due to strong Pt-P interaction. To our knowledge, this is the first report identifying the MOR performance on PtP2 intermetallic nanocrystals, highlighting their potential as highly active and stable nanocatalysts for DMFCs.

Chrysanthemum‐Flowers‐Like Bimetal Oxide Based Electrocatalyst for Methanol Oxidation Reaction

Transition metal oxides have garnered attention for the methanol electro‐oxidation reaction owing to their easy availability, high catalytic applicability, and multiple oxidation capabilities. Bimetal‐based catalysts further enhance the performance of the methanol oxidation reaction. In this context, a chrysanthemum‐flower‐like MoO2/WO3 is synthesized through a solvothermal and annealing strategy. The morphology attests to the rough surface and pores in the catalyst, augmenting its surface area for reactions and enhancing catalytic applicability. Electrochemical oxidation of methanol yields an optimal current density of 0.99 mA cm−2 and 2.07 mA mg−1 at a potential of 0.57 V versus Ag/AgCl. The catalyst exhibits a remarkably low Tafel slope of 37.45 mV dec−1, affirming its rapid reaction kinetics. Electrochemical impedance spectroscopy (EIS) reveals an equivalent electronic circuit of R(Q(R(QR)Q(RW))), confirming low charge transfer resistance (Rct) and diffusion‐controlled kinetics attributable to the pores in the sample. The sample also demonstrates an electrochemically active surface area (ECSA) value equal to 0.122 mF cm−2 and long‐term stability over continuous 5000 s, exhibiting no change and high resistance to CO poisoning. All these characterizations and obtained results collectively affirm that the chrysanthemum‐flower‐like MoO2/WO3 serves as a highly suitable electrocatalyst for the electrochemical oxidation of methanol.

Microwave Assisted Synthesis of PtxRu1−x-NiTiO3: A Novel Hybrid Nanostructured Electrocatalyst for Direct Methanol Oxidation

In the present investigation, we have explored the preparation of PtxRu1-x-NiTiO3 hybrid electrocatalysts for the application of methanol oxidization. The XRD and Raman analysis of PtxRu1-x-NiTiO3 confirmed the rhombohedral crystal structure of NiTiO3 and cubic-structured Pt(Ru) nanoparticles. TEM analysis signifies that Pt(Ru) nanoparticles is decorated on the surface of the NiTiO3. The mean size of the Pt(Ru) nanoparticles calculated from the TEM profile is around 2 nm which corroborates with the crystallite size estimated from XRD. The XPS analysis confirms the presence of metallic Pt along with its oxidized components (Pt2+ and Pt4+). The cyclic voltammetry analysis evidence that Pt0.5Ru0.5-NiTiO3 has shown better activity in methanol oxidation compared with the other compositions of PtxRu1−x-NiTiO3 along with the maximum current density of 65.41 mA cm−2 with less carbon poisoning. Chronoamperometry and polarization studies represent the stability and electrochemical activity of the Pt0.5Ru0.5-NiTiO3 electrocatalysts towards methanol oxidation. Based on the research carried out in this work, it is concluded that the PtxRu1−x-NiTiO3 would be a promising candidate as an electrocatalyst for methanol oxidation in direct methanol fuel cells.