Kinetic study of methanol oxidation on carbon-supported PtRu electrocatalyst

Methanol electrooxidation was investigated on the carbon-supported PtRu electrocatalyst (1:1 atomic ratio) in acid media. X-ray diffraction measurement indicated alloying of Pt and Ru. Cyclic voltammetry of the sample reflects the amount of Ru in the catalyst and its ability to adsorb OH radicals. Tafel plots for the oxidation of 0.02-3 M methanol in the solutions containing 0.05-1 M HClO₄ and in the temperature range 27-40 °C showed reasonably well-defined linear region with the slope of about 115 mV dec⁻¹ at the low currents, irrespective of the experimental conditions employed. Reaction order with respect to methanol was found to be 0.5. A correlation between methanol oxidation rate and pseudocapacitive current of OH adsorption on Ru sites was established. It was proposed that bifunctional mechanism is operative with the reaction between methanol residues adsorbed on Pt sites and OH radicals adsorbed on Ru sites as the rate-determining step.     

Preparation of methanol oxidation electrocatalysts: ruthenium deposition on carbon-supported platinum nanoparticles

Methanol oxidation electrocatalysts were prepared from Ru electrochemical or spontaneous deposition on commercial-grade carbon-supported Pt nanoparticles (Pt-Vulcan XC72, E-TEK). The resulting Ru coverage was estimated by cyclic voltammetry in supporting electrolyte. The maximum electrocatalytic activity for methanol oxidation at room temperature was observed at lower Ru coverage for spontaneous deposition than for electrodeposition; _Ru 10% vs 20%, respectively. On the other hand, higher current densities for methanol oxidation were obtained in the case of electrodeposited Ru. These two results were related to the presence of non-reducible ruthenium oxides in the spontaneous deposit. The present work provides evidence that (i) efficient DMFC electrocatalysts can be achieved by Ru deposition on Pt nanoparticles, and (ii) formation of a PtRu alloy is not a required condition for effective methanol electrooxidation.     

Electrochemical and spontaneous deposition of ruthenium at platinum electrodes for methanol oxidation: an electrochemical quartz crystal microbalance study

PtRu electrodes with Ru surface concentration ranging from 20 to 50% were prepared by electrolysis of Ru(NO)(NO₃)₃ at a constant potential and/or by spontaneous Ru deposition performed at open circuit potential from a RuCl₃ solution. The amount of either spontaneously or electrochemically deposited ruthenium on the platinum electrode was determined by means of an electrochemical quartz crystal microbalance (EQCM). The effect of the Ru surface concentration on the rate of methanol electrooxidation was also investigated and correlated to the EQCM measurements.     

Quaternary Pt-based electrocatalyst for methanol oxidation by combinatorial electrochemistry

The demonstration to apply the combinatorial method using a repeated cyclic voltammetry is reported to find the anodic material for DMFC that shows a higher electrocatalytic activity and that can replace a portion of precious metals with cheap ones. The activity of newly found electrocatalyst whose composition was determined through high-throughput screening was compared with that of commercially available Johnson–Matthey Pt(50)Ru(50). It was found Pt(77)Ru(17)Mo(4)W(2) was more active and stable than Pt(50)Ru(50) in methanol electro-oxidation. The repeated cyclic voltammetry makes the combinatorial method expand into a screening tool to find the electrocatalyst that not only showing an initial excellent performance but also being active in the long-run reaction.     

Cobalt/copper-decorated carbon nanofibers as novel non-precious electrocatalyst for methanol electrooxidation

In this study, Co/Cu-decorated carbon nanofibers are introduced as novel electrocatalyst for methanol oxidation. The introduced nanofibers have been prepared based on graphitization of poly(vinyl alcohol) which has high carbon content compared to many polymer precursors for carbon nanofiber synthesis. Typically, calcination in argon atmosphere of electrospun nanofibers composed of cobalt acetate tetrahydrate, copper acetate monohydrate, and poly(vinyl alcohol) leads to form carbon nanofibers decorated by CoCu nanoparticles. The graphitization of the poly(vinyl alcohol) has been enhanced due to presence of cobalt which acts as effective catalyst. The physicochemical characterization affirmed that the metallic nanoparticles are sheathed by thin crystalline graphite layer. Investigation of the electrocatalytic activity of the introduced nanofibers toward methanol oxidation indicates good performance, as the corresponding onset potential was small compared to many reported materials; 310 mV (vs. Ag/AgCl electrode) and a current density of 12 mA/cm² was obtained. Moreover, due to the graphite shield, good stability was observed. Overall, the introduced study opens new avenue for cheap and stable transition metals-based nanostructures as non-precious catalysts for fuel cell applications.     

Electrocatalytic activity of carbon-supported Pt-Au nanoparticles for methanol electro-oxidation

Pt-modified Au nanoparticles on carbon support were prepared and analyzed as electrocatalysts for methanol electro-oxidation. In this paper, a novel chemical strategy is described for the preparation and characterization of carbon-supported and Pt-modified Au nanoparticles, which were prepared by using a successive reduction process. After preparing Au colloid nanoparticles (∼3.5 nm diameter), Au nanoparticles were supported spontaneously on the surface of carbon black in the aqueous solution. Then a nanoscaled Pt layer was deposited on the surface of carbon-supported Au nanoparticles by the chemical reduction. The structural information and electrocatalytic activities of the Pt-modified Au nanoparticles were confirmed by transmission electron microscopy (TEM), X-ray diffractometry (XRD) and cyclic voltammetry (CV). The results indicate that carbon-supported Au nanoparticles were modified with the reduced Pt atoms selectively. The Pt-modified Au nanoparticles showed the higher electrocatalytic activity for methanol electro-oxidation reaction than the commercial one (Johnson-Matthey). The increased electrocatalytic activity might be attributed to the effective surface structure of Pt-modified Au nanoparticles, which have a high utilization of Pt for surface reaction of methanol electro-oxidation.     

Investigations of Compositions and Performance of PtRuMo/C Ternary Catalysts for Methanol Electrooxidation

PtRuMo/C catalyst was prepared by impregnation reduction method and characterised. Comparison is made between a home‐made PtRu/C prepared by similar method and Pt/C (E‐Tek Co., Pt/C‐ET) catalysts. One glassy carbon disc electrode for ternary alloy catalyst was used to evaluate the catalytic performances by cyclic voltammetric, chronoamperometric, amperometric i–t curves, and electrochemical impedance spectra (EIS). The electrochemical measurement results indicated that the performance of PtRuMo/C with a molar ratio of 6:3:1 was the highest among 15 Pt_xRu_yMo_10–x–y/C catalysts with different molar ratios. The composition, particle size, lattice parameter and morphology of the PtRuMo(6:3:1)/C catalyst were determined by means of X‐ray energy dispersive analysis, X‐ray diffraction (XRD) and transmission electron micrographs (TEM). The result of XRD analysis exhibits that PtRuMo(6:3:1)/C has the fcc structure with the smaller lattice parameter than the home‐made PtRu/C and Pt/C‐ET. Its typical particle sizes is only about 5 nm. With respect to the catalytic activity and stability, the PtRuMo(6:3:1)/C catalyst is superior to PtRu/C despite their comparable active areas. Though the electrochemically active surface area of Pt/C‐ET is the biggest, its performance is the lowest. EIS results also indicate that the reaction resistances for methanol electrooxidation on the PtRuMo(6:3:1)/C catalyst are smaller than those of PtRu/C at different polarisation potentials.     

Optimum Pt and Ru atomic composition of carbon-supported Pt–Ru alloy electrocatalyst for methanol oxidation studied by the polygonal barrel-sputtering method

The optimum Pt and Ru atomic composition of a carbon-supported Pt–Ru alloy (Pt–Ru/C) used in a practical direct methanol fuel cell (DMFC) anode was investigated. The samples were prepared by the polygonal barrel-sputtering method. Based on the physical properties of the prepared Pt–Ru/C samples, the Pt–Ru alloy was found to be deposited on a carbon support. The microscopic characterization showed that the deposited alloy forms nanoparticles, of which the atomic ratios of Pt and Ru (Pt:Ru ratios) are uniform and are in accordance with the overall Pt:Ru ratios of the samples. The formation of the Pt–Ru alloy is also supported by the electrochemical characterization. Based on these results, methanol oxidation on the Pt–Ru/C samples was measured by cyclic voltammetry and chronoamperometry. The results indicated that the methanol oxidation activities of the prepared samples depended on the Pt:Ru ratios, of which the optimum Pt:Ru ratio is 58:42 at.% at 25 °C and 50:50 at.% at 40 and 60 °C. This temperature dependence of the optimum Pt:Ru ratio is well explained by the relationship between the methanol oxidation reaction process and the temperature, which is reflected in the rate-determining steps considered from the activation energies. It should be noted that at 25–60 °C, the Pt–Ru/C with Pt:Ru = 50:50 at.% prepared by our sputtering method has the higher methanol oxidation activity than that of a commercially available sample with the identical overall Pt:Ru ratio. Consequently, the polygonal barrel-sputtering method is useful to prepare the practical DMFC anode catalysts with the high methanol oxidation activity.     

Alternative platinum electrocatalyst supporter with micro/nanostructured polyaniline for direct methanol fuel cell applications

In this study, a series of micro/nanostructured polyanilines were synthesized and their morphology-dependent electrochemical properties for acting as a catalyst supporter for direct methanol fuel cell (DMFC) applications were investigated. These micro/nanostructures include submicron spheres, hollow microspheres, nanotubes, and nanofibers. Among the four micro/nanostructures, polyaniline nanofibers (PANF) manifest their superiority in high electrochemical active surface. Accordingly, PANF is adopted as the catalyst supporter thereafter. To couple with the use of the alternative catalyst supporter, this study also investigates the effect of reductant type on morphology and electrocatalytic properties of the PANF-supported Pt particles through a chemical reduction reaction. TEM images indicate that formic acid as a reductant results in well-dispersed Pt particles on the PANF surface. On the other hand, aggregations of Pt are observable when NaBH₄ is selected as a reductant. Moreover, the methanol oxidation current density measured with the Pt/PANF electrode being prepared by using formic acid is double that by using NaBH₄. Compared with Pt/XC-72, the Pt/PANF electrode possesses higher electrocatalytic activity and exhibits double power density. Moreover, Pt/PANF is superior to Pt/XC-72 in the aspect of operation stability based on a continuous discharge for 5 h.     

Effect of cations in solution on the oxidation of methanol on the surface of platinum electrode

The effect of metal cations in solution on the oxidation of methanol on the electrode surface of platinum is a neglected aspect to direct methanol fuel cell (DMFC). In this paper, a smooth platinum electrode absorbing metal cations as the working electrode was applied to investigate the methanol oxidation with the cyclic voltammetry (CV) in 1.0 mol L⁻¹ H₂SO₄. From the analysis of experiment, it is found that the cations, Li⁺, Ce⁴⁺, Mn²⁺, Ni²⁺, Cu²⁺, have some negative effect on the catalytic oxidation of methanol on the surface of platinum. The degree of the effect from different cations was analyzed.     

Oxygen electroreduction on carbon-supported platinum catalysts. Particle-size effect on the tolerance to methanol competition

The kinetics of oxygen reduction in methanol-containing acid electrolyte was investigated at platinum-based electrodes using the porous rotating disk electrodes (RDE) technique. Utilization of commercial-grade (E-TEK) carbon-supported Pt particles with narrow size distribution provided evidences for a particle size effect on the tolerance of oxygen reduction electrocatalysts to methanol competition. In methanol-containing perchloric acid electrolyte, the mass activity (MA, A g⁻¹ Pt) for oxygen reduction increases continuously with a decrease in particle size from d=4.6 to 2.3 nm, whereas in methanol-free electrolyte MA is roughly independent of the size, when d≤3.5 nm. Effects of addition of a second metal to Pt were also investigated. Based on particle size considerations Pt:Cr-C appears to be a more active catalyst than Pt-C for oxygen reduction in methanol-containing electrolyte.     

Preparation and characterization of successively deposited Pt/Ru bimetallic electrocatalysts for the methanol oxidation

Two types of Pt/Ru electrocatalysts, which have different structural characteristics, were prepared with different synthetic routes. That is, Pt/Ru electrocatalysts were synthesized by the coreduction and successive deposition methods, respectively. The structural and catalytic properties of Pt/Ru electrocatalysts were characterized by XRD, TEM, voltammetry and chronoamperometry. From the XRD analysis, coreduced and successively deposited Pt/Ru electrocatalysts had an alloyed structure. TEM analyses showed that all the electrocatalysts had a highly dispersed state on the Vulcan XC-72R substrate. From the voltammetry, the coreduced electrocatalysts displayed higher catalytic activity than the successively deposited electrocatalysts for the electrooxidation of methanol. These results explain why coreduced catalysts are better able to dehydrogenate methanol and have a greater CO tolerance than the successively deposited ones. But chronoamperometry showed that successively deposited Pt/Ru electrocatalysts had stability similar to that of the coreduced ones. Although the successively deposited electrocatalysts showed lower catalytic activity than the coreduced ones, their enhanced catalytic activity was obtained by the successive deposition method in the comparison of methanol oxidation current density with pure platinum electrocatalyst.     

Electrochemical effect of gold nanoparticles on Pt/α-Fe2O3/C for use in methanol oxidation in alkaline solution

A catalyst containing gold nanoparticles with Pt/α-Fe₂O₃/C was prepared by a co-precipitation method and its catalytic activity for the oxidation of methanol, formaldehyde, and formic acid in alkaline solutions was evaluated by an electrochemical method and high-performance liquid chromatography (HPLC). The addition of gold nanoparticles improved catalytic activity only for the oxidation of methanol and formaldehyde, and not for the oxidation of formic acid. HPLC analysis was performed for methanol oxidation to detect the oxidative products. In HPLC analysis, only formate anion could be detected in the electrolyte solution and the ratio of formate anion obtained to the total passed charge in Pt/nano-Au/α-Fe₂O₃/C was less than that in Pt/C, indicating that formic acid is not the final product of methanol oxidation. These results show that gold nanoparticles promoted methanol oxidation up to CO₂.     

Electrocatalytic oxidation of methanol: carbon-supported gold–platinum nanoparticle catalysts prepared by two-phase protocol

This paper describes recent results of an investigation of the electrocatalytic oxidation of methanol at carbon-supported gold and gold–platinum nanoparticle catalysts. The exploration of the bimetallic composition on carbon black support is aimed at modifying the catalytic properties for methanol oxidation reaction (MOR) at the anode in methanol oxidation fuel cells. Gold and gold–platinum nanoparticles of 2–3 nm core sizes with organic monolayer encapsulation were prepared by two-phase protocol. The nanoparticles were assembled on carbon black materials and thermally treated. The electrocatalytic MOR activities were characterized using voltammetric techniques, and were compared with commercial catalysts under several conditions. The results have revealed some initial insights into the catalytic activity of gold–platinum nanoparticle catalysts. Implications of our findings to the design and manipulation of highly-active gold–platinum nanoparticle catalysts for fuel cell applications are also discussed.     

Fast methanol oxidation on polycrystalline Pt

Fast cyclic voltammetry and a combined chronoamperometry/cyclic voltammetry pulse-sweep technique are used to study methanol oxidation on platinum in sulphuric acid. Measurement of suppression of hydrogen desorption charges simultaneously with charges for stripping of the CO poison allows for accurate baseline correction of the CO stripping peaks. Saturation coverages of CO formed from methanol, formic acid and dissolved CO are not all the same and differ in their potential dependence. Only dissociative adsorption of methanol to give adsorbed CO and subsequent oxidation of CO to CO₂ occurs during continuous cycling, but in the pulse-sweep method evidence was found for the existence of a parallel pathway occurring independently of adsorbed CO production, provided there is some surface not covered by CO or oxide.     

Do magnetically modified PtFe/C catalysts perform better in methanol electrooxidation?

Motivated by the demonstrated magnetic field effect on the oxygen reduction reaction in polymer electrolyte fuel cells (PEMFC), a number of PtFe/C catalysts with different magnetic characteristics were prepared and tested for methanol electrooxidation in acidic solutions at room temperature. The catalysts were characterized by transmission electron microscopy (TEM), powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), energy dispersive X-ray microanalysis (EDX) and vibrating sample magnetometry (VSM). The activity and CO tolerance of the catalysts in the methanol oxidation reaction (MOR) was measured by cyclic voltammetry and anodic CO stripping voltammetry, respectively. Heat treatment transformed the as-prepared PtFe/C from a face-centered cubic (fcc) structure into a face-centered tetragonal (fct) structure. Vibrating sample magnetometer (VSM) measurements confirmed the as-prepared PtFe/C as superparamagnetic, and the heat-treated catalysts as ferromagnetic. The heat-treated ferromagnetic catalysts were, however, low in specific mass activity and showed no improvement in CO tolerance relative to the as-prepared one. These results led us to conclude that the magnetic modification of catalysts through heat treatment have no practical contributions to the catalysis of MOR.     

Composite electrocatalysts for anodic methanol and methanol-reformate oxidation

Binary anode electrocatalyst formulations were prepared by adsorption of phthalocyanine and tetraphenylporphyrin complexes of different transition metals on a commercial carbon supported platinum catalyst. Only after pyrolyzing the complexes at 700 °C under nitrogen were catalysts of some activity obtained. A binary Pt/Ni electrocatalyst prepared by this procedure exhibits considerable anodic catalytic activity in the acidic environment of the Nafion^® electrolyte for reformate and direct methanol oxidation for more than 400 h without deterioration. Ternary electrocatalyst formulations Pt/Ru/W = 1/1/y were produced according to the Bönnemann method. The Pt/Ru/W catalyst of 1/1/1.5 (mol/mol/mol) composition is optimal. Compared to the Pt/Ru catalyst, it enhances the performance of reformate (H₂ + 150 ppm CO) fuel cells by 50% and of direct methanol fuel cells (steam/methanol vapour = 50:1 mol/mol) by 80%. Attached to a GC electrode by a thin Nafion^® film, the catalysts were also tested for methanol oxidation in aqueous methanol solutions in half cells by slow potential stepping. This procedure is useful for fast initial screening.     

Nano-composite of PtRu alloy electrocatalyst and electronically conducting polymer for use as the anode in a direct methanol fuel cell

Nano-composites comprised of PtRu alloy nanoparticles and an electronically conducting polymer for the anode electrode in direct methanol fuel cell (DMFC) were prepared. Two conducting polymers of poly(N-vinyl carbazole) and poly(9-(4-vinyl-phenyl)carbazole) were used for the nano-composite electrodes. Structural analyses were carried out using Fourier transform nuclear magnetic resonance spectroscopy, AC impedance spectroscopy, X-ray diffraction (XRD), and transmission electron microscopy (TEM). Electrocatalytic activities were investigated by voltammetry and chronoamperometry in a 2 M CH₃OH/0.5 M H₂SO₄ solution and the data compared with a carbon-supported PtRu electrode. XRD patterns indicated good alloy formation and nano-composite formation was confirmed by TEM. Electrochemical measurements and DMFC unit-cell tests indicate that the nano-composites could be useful in a DMFC, but its performance would be slightly lower than that of a carbon-supported electrode. The interfacial property between the PtRu-polymer nano-composite anode and the polymer electrolyte was good, as evidenced by scanning electron microscopy. For better performance in a DMFC, a higher electric conductivity of the polymer and a lower catalyst loss are needed in nano-composite electrodes.     

Polyaniline/carbon black composite as Pt electrocatalyst supports for methanol oxidation: Synthesis and characterization

Core/shell composites of polyaniline (PANI) and Vulcan XC‐72 Carbon (VC), in which the carbon represents the core and PANI forms the shell, were synthesized by in situ chemical oxidation polymerization. Platinum (Pt) particles were then deposited on the PANI/VC composites by chemical reduction method. The highest conductivity is obtained when a mass ratio of PANI/VC equals to 0.28, as proved by Fourier transform infrared spectra. And it is also proved that there are some reactions happened between PANI and VC. Scanning electron microscope, transmission electron microscope, and X‐ray diffraction measurements were performed to analyze their structure and surface morphology. It has been observed that the Pt particles are smaller in size and more uniformly distributed on these composite supports than on pure VC supports, considered as a reference. Methanol oxidation performed on the electrode modified by such a composite catalyst has been measured by cyclic voltammogram focusing on the attenuation of methanol oxidation current after 200 cycles. The attenuation degree for the composite catalyst is only one‐third of the one measured for a simple Pt/VC catalyst. It is proved that the composite catalyst better resist carbon monoxide poisoning in comparison with the Pt/VC catalyst, which may be due to the synergetic effects between the composite support and the Pt catalyst. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010