Aspects of the anodic oxidation of methanol

This paper describes some aspects of recent investigations into the anodic oxidation of methanol. Methanol has long been proposed as an anode fuel for a fuel cell, chiefly because of its ease of carriage, distribution and manipulation. However, methanol is very much more difficult to oxidise anodically than hydrogen, the more conventional anode fuel, and this has hampered development of commercial direct methanol fuel cells. Platinum-ruthenium catalysts are the most active discovered to date. Some advances in electrocatalysis of the methanol reaction by non-noble materials are discussed.     

Electrochemical activation of nanostructured carbon-supported PtRuMo electrocatalyst for methanol oxidation

The factors controlling the behavior and the stability of electrocatalysts based on Pt, Ru and Mo nanoparticles during exhaustive electrochemical treatment are examined. Along this treatment, it has been observed that in the case of ternary catalysts there are pronounced changes in the structure of their surface resulting in electrode activation for methanol and CO electrooxidation, whereas the activity of binary PtRu/C and PtMo/C catalysts decreases. Therefore, the role of both Ru and Mo is crucial for the electrochemical activation of the catalyst, though metal losses do occur during electrochemical process. For the first time a detailed study of this phenomenon is presented, including characterization by HRTEM, TXRF, XRD, electrochemical measurements and in situ Fourier transform infrared spectroscopy (FTIR). In order to get a deeper insight into the surface structure, chemical state, and stability of the electrocatalyst under reaction conditions, a combination of cyclic voltammetry, chronoamperometry and X-ray photoelectron spectroscopy (XPS) has been used. By comparing bulk and surface composition, our results point out to the key role of the geometric effect enhanced by previous reduction of the nanoparticles. At the end of the electrochemical treatment, Mo-PtRu/C catalysts surface was restructured with substantial enrichment in Pt and a less pronounced Mo surface enrichment, while Ru is incorporated into the Pt-Mo overlayer. These results underline the possibility of further optimization of the surface structure and composition producing PtRuMo nanoparticles with high methanol and CO oxidation activity.     

Surfactant-stabilized PtRu colloidal catalysts with good control of composition and size for methanol oxidation

Well-dispersed PtRu/C catalysts were prepared by supporting surfactant-stabilized PtRu hydrosol on carbon followed by a heat-treatment at elevated temperature. The effect of the synthesis conditions and the heat-treatment on the composition and electrcatalytic properties of PtRu/C towards methanol oxidation was systematically investigated. It was found that the pH environment and the reaction temperature could greatly affect the final compositions of Pt and Ru in the PtRu catalysts. Moreover, after a post-heat-treatment process, the electrocatalytic activity of PtRu colloidal catalysts can be much improved, the enhancement of which can be largely explained by the improved alloying formation and the removal of surfactant from catalyst demonstrated by the XRD and XPS analyses, respectively.     

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.     

The efficiency of methanol conversion to CO2 on thin films of Pt and PtRu fuel cell catalysts

Yields were determined for the CO₂ produced upon the electrochemical oxidation of 1.0 M methanol in 0.1 M HClO₄ at the following four fuel cell catalyst systems: Pt black, Pt at 10 wt.% metal loading on Vulcan XC-72R carbon (C/Pt, 10%), PtRu black at 50 at.% Pt, 50 at.% Ru (PtRu (50:50) black), and PtRu at 30 wt.% Pt, 15 wt.% Ru loading on Vulcan XC-72R carbon (C/PtRu, 30 wt.% Pt, 15 wt.% Ru). Samples were electrolyzed in a small volume (50 μl) arrangement for a period of 180 s keeping the reactant depletion in the cell below 1%. The dissolved CO₂ produced was determined ex situ by infrared spectroscopy in a micro-volume transmission flow cell. For the PtRu materials, the efficiencies for CO₂ formation were near 100% at reaction potentials in the range between 0.4 V (versus the reversible hydrogen electrode (RHE), V_RHE ) and 0.9 V_RHE. At the Pt catalysts, the yields of CO₂ approached 80% between 0.8 and 1.1 V_RHE and declined rapidly below 0.8 V_RHE.     

PtRu colloidal catalysts: Characterisation and determination of kinetics for methanol oxidation

The activities of three 30 wt.% PtRu/C catalysts of the same nominal composition (Pt:Ru, 1:1 a/o) were investigated with regard to methanol oxidation. The catalysts were synthesized by Bönnemann's colloidal precursor method using different metal salt precursors and reducing agents. The catalysts were characterized ex situ by energy dispersive X-ray analysis (EDX) and X-ray diffraction (XRD) and in situ CO stripping and cyclic voltammetry. The activity towards methanol electro-oxidation was checked in steady-state experiments at 22 and 60 °C. The experimental rate data can be described well by a kinetic model, which includes methanol adsorption on the Pt-sites, formation of C-containing adsorbed species, OH_ads formation on the Ru sites and heterogeneous surface reaction between C-adsorbate and OH_ads. The kinetic model parameters were identified from the experimental data and were used to explain the differences in catalytic activity.     

MXene supported rhodium nanocrystals for efficient electrocatalysts towards methanol oxidation

Since conventional Pt/carbon catalysts usually suffer from CO poisoning as well as carbon corrosion issues during the methanol oxidation reaction, it is essential to explore high-efficiency Pt-alternative electrocatalysts supported by a robust matrix in the direct methanol fuel cells. Herein, we report a convenient low-temperature approach to the controllable fabrication of well-dispersive Rh nanocrystals in situ grown on Ti₃C₂T_x MXene nanosheets. The ultrathin lamellar MXene structure reveals unique superiorities on the construction of advanced Rh-based hybrid catalysts, which can not only provide a large number of efficient anchoring sites for immobilizing small-sized Rh nanocrystals with abundant exposed catalytic crystal planes, but also enable direct electronic interaction with Rh for strong synergistic effects and facilitate the fast charge transportation during the catalytic process. As a consequence, the resulting Rh/Ti₃C₂T_x hybrid exhibits prominent electrocatalytic properties towards methanol oxidation reaction, such as a large electrochemical active surface area of 71.6 m² g^?1, a high mass activity of 600.2 mA mg^?1, and good long-term stability, all of which are much better than those of conventional carbon-supported Rh as well as Pt/C and Pd/C catalysts.     

Mechanistic study of ethanol oxidation on Pd-NiO/C electrocatalyst

NiO supported Pt/C and Pd/C electrocatalysts have been prepared by IMH method. The ethanol oxidation on Pd-NiO/C electrocatalyst is tested in an ethanol solution with or without the presence of CO. The results show that the oxidation of CO on Pd-NiO/C electrocatalyst needs higher overpotential than that on Pt-NiO/C electrocatalyst. However, the peak area of the CO oxidation on the Pd-NiO/C electrocatalyst is much lower and the influence of CO on the performance of ethanol oxidation is small. The higher activity and less poisoning of the Pd-NiO/C electrocatalyst than that of the Pt-NiO/C electrocatalyst for ethanol oxidation is possibly due to the lower CO coverage on the Pd-NiO/C electrocatalyst that leads to the favorable adsorption of ethanol instead of CO and consequently the less poisoning effect.     

Hollow graphitic carbon spheres for Pt electrocatalyst support in direct methanol fuel cell

Hollow graphitic carbon spheres (HGCSs) with a high surface area are produced by the carbonization of hollow polymer spheres obtained by the polymerization of core/shell-structured pyrrole micelles. HGCSs are employed as a carbon support material in a direct methanol fuel cell catalyst, and their effect on the electrocatalytic activity toward methanol oxidation is investigated. Pt catalyst supported on HGCSs shows a better electrocatalytic activity compared to that on Vulcan XC-72, which has been commonly used in fuel cell catalysts. The observed enhancement in the electrocatalytic activity is attributed to the improved electronic conductivity and high surface area of HGCSs.     

Effect of electrochemical polarization of PtRu/C catalysts on methanol electrooxidation

To determine the influence of electrochemical polarization of PtRu/C catalysts on methanol electrooxidation, this work investigated methanol electrooxidation on as received and different electrochemically polarized PtRu/C catalysts. Thermogravimetric analysis (TGA) and X-ray diffraction (XRD) were used to characterize the redox state of PtRu/C after different electrochemical polarization. The methanol electrooxidation activity was measured by cyclic voltammetry (CV), Tafel steady state plot and electrochemical impedance spectroscopy (EIS). The results indicate that the metallic state Pt⁰Ru⁰ can be formed during cathodic polarization and contribute to electrooxidation of methanol, while the formation of inactive ruthenium oxides during anodic polarization cause the negative effect on methanol electrooxidation. Different Tafel slopes and impedance behaviors in different potential regions also reveal a change of the mechanism and rate-determining step in methanol electrooxidation with increasing potentials. The kinetic analysis from Tafel plots and EIS reveal that at low potentials indicate the splitting of the first CH bond of CH₃OH molecule with the first electron transfer is rate-determining step. However, at higher potentials, the oxidation reaction of adsorbed intermediate CO_ads becomes rate-determining step.     

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.     

High methanol oxidation activity of electrocatalysts supported by directly grown nitrogen-containing carbon nanotubes on carbon cloth

The microstructure and electrochemical activity of the Pt-Ru supported by nitrogen-containing carbon nanotubes (CN_x NTs) directly grown on the carbon cloth have been investigated. The CN_x NTs directly grown on the carbon cloth (CN_x NTs-carbon cloth composite electrode) were synthesized using microwave-plasma-enhanced chemical vapour deposition first and then use as the template to support the Pt-Ru nanoclusters subsequently sputtered on. The ferricyanide/ferrocyanide redox reaction in cyclic voltammetry (CV) measurements showed a faster electron transfer on the CN_x NTs-carbon cloth composite electrode than the one with carbon cloth alone. Comparing their methanol oxidation abilities, it is found that the Pt-Ru nanoclusters supported by the CN_x NTs-carbon cloth composite electrode have considerably higher electrocatalytic activity than the carbon cloth counterpart. This result suggests high performance of the CN_x NTs-carbon cloth composite electrode, and demonstrates its suitability for direct methanol fuel cell applications.     

Investigation of electrochemical oxidation of methanol in a cyclone flow cell

Electrochemical oxidation of methanol on carbon supported Pt/Ru gas diffusion electrodes was investigated in a cyclone flow cell at room temperature using chronoamperometry, cyclic voltammetry and electrochemical impedance spectroscopy. The influence of the flow rate was checked. It was proved that the cyclone cell is suitable for the investigation of methanol electrooxidation and provides additional information on the mass transfer limitations in the electrode assembly. Chronoamperometric measurements showed slow, but constant current decay at all investigated potentials. Impedance measurements in water and methanol containing solutions were performed and the experimental data were fitted to an appropriate equivalent circuit.     

Preparation of high loading Pt nanoparticles on ordered mesoporous carbon with a controlled Pt size and its effects on oxygen reduction and methanol oxidation reactions

A series of ordered mesoporous carbon (OMC) supported Pt (Pt/OMC) catalysts with a controlled Pt size from 2.7 to 6.7 nm at high Pt loading around 60 wt.% have been prepared and their electrocatalytic activities for the electrode reactions relevant to the direct methanol fuel cells have been investigated. The Pt/OMC catalysts with a high dispersion (Pt size around 3 nm) could be prepared by the use of a modified, sequential impregnation–reduction method. The Pt/OMC catalysts containing larger Pt particles were obtained by increasing reduction temperature under hydrogen flow and Pt loading, and by performing impregnation–reduction in a single cycle. The oxygen reduction reaction (ORR) and methanol oxidation reaction (MOR) activities of Pt/OMC catalysts as a function of Pt size were investigated at room temperature in 0.1 M HClO₄ and (0.1 M HClO₄ + 0.5 M methanol), respectively. The specific activity of Pt/OMC for ORR steeply increased up to 3.3 nm and became independent of Pt size from 3.3 to 6.7 nm, and the mass activity curve exhibited maximum activity at 3.3 nm. The MOR activity of Pt/OMC also exhibited the similar trend with the ORR activity, as the maximum of mass activity was also found at 3.3 nm. The results of the present work indicate that the Pt catalysts of ca. 3 nm is an optimum particle size for both ORR and MOR, and this information may be translated into design of high performance membrane electrode assembly.     

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.     

Methanol partial oxidation on carbon-supported Pt and Pd catalysts

Different Pt and Pd catalysts supported on an activated carbon were prepared by using different metal precursors. Prepared catalysts were pretreated at 400 °C under different atmospheres to decompose the precursor compound and reduce the metal. After pretreatments, the supported catalysts were characterized by H₂ chemisorption, X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy to know their metal dispersion, particle size, distribution and oxidation state. Afterwards, the catalysts were tested in methanol partial oxidation with two different O₂/CH₃OH molar ratios. Results obtained in this reaction were compared with those obtained for methanol decomposition in inert atmosphere. For Pt catalysts, there was an increase in methanol conversion and hydrogen production and a decrease in carbon monoxide production under oxidizing conditions. Both methanol conversion and partial oxidation reactions appear to be sensitive to Pt particle structure in the particle size range studied. Results obtained under oxidizing conditions differed between Pd and Pt catalysts. Finally, catalytic activity in methanol partial oxidation was more affected by Pt than Pd particle size in the size range studied.     

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.     

A study of PtRuO2 catalysts thermally formed on titanium mesh for methanol oxidation

Platinum-based catalysts, for the electro-oxidation of methanol, have been made by thermal decomposition of chloride precursors onto titanium mesh. The catalysed electrodes were successfully operated in acidic methanol electrolytes. Electrochemical characterisation has been carried out using cyclic voltammetry, electrochemical impedance spectroscopy and galvanostatic polarisations. A complete analysis of the electrochemical results showed that the preliminary performance of the catalysed titanium mesh was comparable to that achieved with carbon-supported PtRu catalysts. The catalysts formed on titanium mesh by thermal decomposition also exhibited dimensional stability. Catalysed titanium mesh therefore appears to be a promising alternative to carbon-supported catalysts for certain fuel cell applications.     

Impedance study of methanol oxidation on platinum electrodes

Methanol oxidation on Pt electrodes is studied by ac voltammetry. Data from voltammograms at frequencies from 0.5 Hz to 20 kHz are assembled into electrochemical impedance spectra and analysed using equivalent circuits. Inductive behavior and negative relaxation times are attributed to nucleation and growth behavior. The rate-determining step is proposed to be the reaction of adsorbed CO and OH at the edge of islands of OH, with competition between OH and CO adsorption for the released reaction sites.     

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.