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One of the significant drawbacks of the Direct Methanol Fuel Cells is the overpotential for the methanol oxidation reaction, which strongly limits the efficiency of this fuel cell. Furthermore, CO poisoning of Pt is considered to be a severe problem for these devices. In this work, electrocatalysts with suitable properties and outstanding performance have been developed for the oxidation reaction of methanol. PtCu and PtCuRu type materials have been synthesized through a relatively uncomplex, fast, highly reproducible, and easily scalable method. In turn, these materials have been subjected to a selective chemical dissolution process to enrich their surface with a higher density of active sites and surface defects. Evidence of alloy formation was found, specifically through XPS measurements on Ru 3d level. Besides, it was observed that with the trimetallic catalysts, which contain not only Cu but also Ru, the onset of CO electrooxidation is substantially reduced. Moreover, the leaching treatment further improves this value. This same trend was observed in the evaluation of methanol oxidation. Regarding the specific and mass activities, PtCuRu-le exhibits almost 240% and 195% higher SA and MA than commercial Pt, respectively. [ABSTRACT FROM AUTHOR]

Direct methanol fuel cells (DMFCs) generate electricity in a clean and efficient way, so they are a valuable alternative to traditional environmentally harmful technologies. Portable power sources are one of the applications of passive DMFCs. One of the requirements in these devices is the use of high alcohol concentration. Methanol permeation across the polymer electrolyte membrane (methanol crossover) causes a loss of fuel cell efficiency as the oxygen reduction reaction (ORR) and the methanol oxidation reaction (MOR) occur simultaneously at the cathode. To develop methanol-tolerant catalysts with suitable activity, different PtM/C and PtMRu/C combinations with M = Co or Fe were prepared by a modified impregnation method. The synthesized catalysts were studied to determine the role of the components in enhancing the ORR and simultaneously discouraging the MOR. The materials were characterized by TEM, XPS and EDS. Well-distributed particles for all the catalysts were shown by TEM. XPS spectra revealed that the method produces a great amount of metallic Pt. The electrochemical characterization was carried out by linear sweep voltammetry and cyclic voltammetry, in a three-electrode electrochemical cell with a glassy carbon rotating disk electrode covered with a thin catalytic layer and a Nafion film as the working electrode. Binary and ternary catalysts have a good activity for the ORR. However, the enhanced activity of binary catalysts is lost when the ORR is studied in the presence of methanol. Ternary catalysts containing Ru showed higher methanol-tolerance, regardless of the composition. [ABSTRACT FROM AUTHOR]

Direct methanol fuel cells (DMFCs) represent an interesting alternative in obtaining electricity in a clean and efficient way. Portable power sources are one of the most promising applications of passive DMFCs. One of the requirements in these devices is to use high alcohol concentration, which due to methanol crossover causes a considerable loss of fuel cell efficiency. In order to develop methanol tolerant cathodes with suitable activity, different supported catalysts namely PtCo/C and PtCoRu/C, were prepared either via ethylene glycol reduction (EG) with or without microwave heating assistance (MW) or via the alloy method, the latter followed by a thermal treatment in a reducing atmosphere (N/H). All cathode-catalysts were tested to determine the role of the components in simultaneously enhancing the oxygen reduction reaction (ORR) and discouraging the methanol oxidation reaction. According to the synthesis methodology, X-ray photoelectron spectra showed that the amount of metal oxides on the surface varies, being higher on the PtCo/C EG and PtCoRu/C EG catalysts. The electrochemical characterization of the catalysts was accomplished in a three electrodes electrochemical cell with a glassy carbon rotating disk electrode covered with a thin catalytic film as working electrode. To study the ORR and the influence of different methanol concentrations, linear sweep voltammetry and cyclic voltammetry were employed. The PtCo/C EG, with an important metal oxide amount on the surface, and the PtCoRu/C MW and EG electrodes, both with RuO on their surfaces, were the most tolerant to methanol presence. [ABSTRACT FROM AUTHOR]

An experimental analysis of platinum utilization in a DMFC cathode is performed. The chief objective of this work is to elucidate the effect of Nafion and polytetrafluoroethylene (PTFE) on platinum utilization values on the cathode. Polarization curves indicate that the performance of a DMFC is sensitive to the Carbon/Nafion (C/N) and Nafion:PTFE (N/P) ratio in the catalyst layer. Our analysis shows that supported Pt/C catalysts have a much higher platinum utilization value than unsupported catalysts. Platinum utilization is a maximum at a C/N ratio of 2.5 and at a N/P ratio of six. Platinum utilization is more sensitive to the Nafion content in the catalyst layer than the Teflon content. [ABSTRACT FROM AUTHOR]

A method that involves stenciling electrodes using dry powders for fuel cells is described and compared to anodes and cathodes prepared by the traditional spraying method using catalyst inks. Methods to determine the proton conductivity of the DMFC anode layer are also discussed. The stenciling method allows for the preparation of highly reproducible membrane electrode assemblies (MEAs) utilizing little waste material. MEAs can be prepared in a controlled manner using the stenciling technique. The resulting morphology of the as-prepared electrodes is observed to be dependent on the preparation method, while the thickness of the once hot-pressed catalyst layers appears to be independent of the preparation method. Stenciled anodes of the same catalyst loading were found to show a lower proton resistance (Rp) than sprayed anodes. However, the lower Rp value was not sufficient to result in a measurable increase in the performance of a direct methanol fuel cell (DMFC); as in fact, the average steady-state DMFC performance was found to be the same using sprayed or stenciled electrodes. The DMFC performance was found to be strongly dependent on the Nafion content and large increases in the Nafion content were needed to increase the DMFC performance measurably. Even though thick electrodes were prepared in this work, the Rp values of the stenciled anodes were found to be comparable to results reported in the literature for much thinner electrodes made using high metal catalyst loadings on carbon. This observation is most probably due to the higher Nafion content used in this work. [ABSTRACT FROM AUTHOR]

This work presents a new concept for realising a reference electrode configuration in a PEM fuel cell by means of laser ablation. The laser beam is used to evaporate a small part of the electrode of a catalyst-coated membrane (CCM) to isolate the reference electrode from the active catalyst layer. This method enables the simultaneous ablation of the electrodes on both sides of the CCM because the membrane is transparent for the laser beam. Therefore, a smooth electrode edge without electrode misalignment can be realised. A test fuel cell was constructed which together with the ablated CCM enables the separation of the total cell losses during operation into the cathode, anode and membrane overpotentials in PEFC as well as in DMFC mode. The methanol tolerance of a selenium-modified ruthenium-based catalyst (RuSe x ) was investigated under real fuel cell conditions by measuring polarisation curves, electrochemical impedance spectroscopy (EIS) and current interrupt measurements (CI). [ABSTRACT FROM AUTHOR]

The feasibility of using poly(3,4-ethylenedioxythiophene) (PEDOT) as Pt catalyst support for direct methanol fuel cell (DMFC) anodes was investigated. Measurements with freshly prepared Pt-PEDOT/C electrodes showed poor activity for methanol oxidation in a half-cell and a DMFC. A substantial enhancement in that activity was evident after either electrochemical over-oxidation of PEDOT or long-time storage of the Pt-PEDOT/C gas diffusion electrode (GDE) in air. Both procedures led to a reorganization and increase in porosity of the reaction layer, which obviously contributed to better methanol accessibility to Pt catalyst active centres. The effects of electrochemical activation and long-time storage in air on the morphology and elementary composition of the Pt-PEDOT layer were investigated by means of Hg porosimetry and SEM/EDAX. It was found that the increase in porosity was due to degradation of PEDOT characterized by a significant depletion of sulphur and oxygen in the conducting polymer matrix. [ABSTRACT FROM AUTHOR]

A good understanding of the fundamental transport phenomena of water and methanol in PSA (Perfluoro Sulfonic Acid) membranes is one of the keys in developing efficient direct methanol fuel cells (DMFCs). Electro-osmosis, being one of the most important water and methanol transport mechanisms in DMFCs, has been investigated with a number of different proton exchange membranes. Two aspects were of particular interest: firstly, the interdependencies between the electro-osmotic drag coefficient and methanol concentration and temperature, and secondly the interdependence between the electro-osmotic drag coefficient and current density. The results of these investigations are compared to published data. [ABSTRACT FROM AUTHOR]

A 60 wt% Pt-Fe/C and a 60 wt% Pt-Cu/C catalysts with Fe and Cu content of 5 wt% were prepared by using a combination of colloidal and incipient wetness methods; this has allowed synthesis of small nanostructured crystalline bimetallic catalysts with particle size less than 3 nm and with a suitable degree of alloying. These materials were studied in terms of structure, morphology and composition using XRD, XRF and TEM techniques. The electrocatalytic behaviour for ORR of the catalysts was investigated using the rotating disk technique and compared to that of a pure Pt catalyst with similar particle size. No improvement in performance was recorded with the Pt-Cu compared to Pt catalyst, whereas, a promoting effect in enhancing the ORR was observed for the Pt-Fe catalyst both with and without methanol in the oxygen-saturated electrolyte solution. [ABSTRACT FROM AUTHOR]

There is currently increased interest in small-size direct methanol fuel cells for portable applications. This work presents results of the influence of operational parameters on the performance of a mini-direct methanol fuel cell. The effects of methanol concentration, Pt load, membrane thickness and PTFE content in the cathode diffusion layer on the performance were studied. Two anodic materials were prepared, PtRu 75:25 at.% and PtRu 90:10 at.%, as nanoparticles supported on Vulcan XC-72 carbon, while for the cathodes Pt/C E-TEK catalysts were used. The materials were characterized physically by EDX and DRX and electrochemically in a half-cell. The results with single cells showed better performances with cells operating with 3 mg Pt cm−2, 5 mol l−1 methanol solution, Nafion® 112 membrane and with 30 wt.% PTFE in the cathode diffusion layer deposited on only one face of the electrode support. [ABSTRACT FROM AUTHOR]

Different Pt/Me/Pc and Pt/Me/Complex catalysts (with Me=Metal: Co, Ni, and Pc=phthalocyanine) were synthesized by an impregnation method. A commercially available platinum catalyst purchased from E-TEK. was impregnated with solutions of cobalt phthalocyanine (CoPc) and nickel phthalocyanine tetrasulfonic salt (NiPc). After the reaction, part of the catalyst was heat treated at 700?°C under nitrogen atmosphere. The resulting catalysts were structurally and electrochemically characterized before (Pt/Me/Pc) and after heat treatment (Pt/Me/Complex). The Pt/Me/Pc had an average particle size of about 3?nm, while the average size after heat treatment increased to about 7nm. The composition of the different catalysts was about 80at platinum and 20at of the second metal (Co or Ni), and was verified with EDXS. In single fuel cell tests the best electrocatalytic activity was observed for the Pt/Ni/Complex system. [ABSTRACT FROM AUTHOR]

The electrodeposition of platinum and ruthenium was carried out on carbon electrodes to prepare methanol anodes with different Pt/Ru atomic ratios using a galvanostatic pulse technique. Characterizations by XRD, TEM, EDX and atomic absorption spectroscopy indicated that most of the electrocatalytic anodes consisted of 2 mg cm-2 of Pt–Ru alloy particles with the desired composition and with particle sizes ranging from 5 to 8 nm. Electrochemical tests in a single DMFC show that these electrodes are very active for methanol oxidation and that the best Pt/Ru atomic ratio in the temperature range used (50–110 °C) is 80:20. The influence of the relaxation time toff was also studied and it appeared that a low toff led to smaller particle sizes and higher performances in terms of current density and power density. [ABSTRACT FROM AUTHOR]

The electrocatalytic oxidation of methanol was investigated on PtRu electrodes of different atomic compositions at several temperatures (from 25 to 110 °C). Very active catalyst nanoparticles supported on active carbon (Vulcan XC 72) were obtained using the colloidal synthesis developed by Bönnemann et al. [11], allowing easy variation of the atomic composition. These electrocatalysts were characterized by TEM, EDX and XRD; results indicate that they consist of platinum nanoparticles decorated by ruthenium. Methanol oxidation was studied as a function of composition, temperature and methanol concentration. Two effects were investigated: the effect of the working temperature and the effect of the atomic composition. It appeared that for lower methanol electrooxidation overvoltages, the best catalysts are ruthenium-rich, whereas at higher overvoltages the best one is the Pt + Ru (80:20)/C composition, irrespective of the working temperature, either in half-cell or in a single DMFC. [ABSTRACT FROM AUTHOR]

For low concentrations of methanol, mass transfer in the electrode is a limiting parameter for the direct methanol fuel cell (DMFC). To improve mass transfer, it is possible to induce convection in the gas backing layer or even in the porous electrode. In this study electrodes with different amounts of PTFE were compared to observe the influence of morphology on the anode performance. The hypothesis was that adding PTFE to the anode may make the morphology more favourable for carbon dioxide to evolve as a gas by creating the necessary pore sizes. Electrode performance was characterized electrochemically and the anode layer structure was studied using SEM, Hg-porosimetry and the van der Pauw method for measuring electric conductivity. Pores smaller than 0.04 μm were unaffected by adding PTFE while the volume fraction of pores of 0.04–1.0 μm diameter increased. Electrodes with 50% PTFE also performed as nonhydrophobized, despite the much higher ohmic losses and thickness. This implies that, above a certain amount, adding PTFE has a positive effect and that optimizing the electrode with PTFE may give better performance than electrodes without PTFE. The results suggest that gas evolves within the electrode, giving improved mass transfer in the liquid phase. [ABSTRACT FROM AUTHOR]

The cell performance of direct methanol fuel cells (DMFC) is 0.5 V at 0.5 A cm−2 under high pressure oxygen operation (3 bar abs.) at 110 °C. However, high oxygen pressure operation at high temperatures is only useful in special market niches. Therefore, our work has now focused on air operation of a DMFC under low pressure (up to 1.5 bar abs.). At present, a power density of more than 100 mW cm−2 can be achieved at 0.5 V on air operation at 110 °C. These measurements were carried out in single cells with an electrode area of 3 cm2 and the air stoichiometry only amounted to 10. The effects of methanol concentration and temperature on the anode performance were studied by pseudo half cell measurements and the results are presented together with their impact on the cell voltage. A cell design with an electrode area of 550 cm2, which is appropriate for assembling a DMFC stack, was tested. A three-celled stack based on this design revealed nearly the same power densities as in the small experimental cells at low air excess pressure and the voltage–current curves for the three cells were almost identical. At 110 °C a power output of 165 W at a stack voltage of 1.5 V can be obtained in the air mode. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The results of a numerical simulation of a direct methanol fuel cell (DMFC) with liquid methanol feed are presented. A two-dimensional numerical model of a DMFC is developed based on mass and current conservation equations. The velocity of the liquid is governed by gradients of membrane phase potential (electroosmotic effect) and pressure. The results show that, near the fuel channel, transport of methanol is determined mainly by the pressure gradient, whereas in the active layers, and in the membrane, diffusion transport dominates. ‘Shaded’ zones, where there is a lack of methanol, are formed in front of the current collectors. The results reveal a strong influence of the hydraulic permeability of the backing layer K p BL on methanol crossover through the membrane. If the value of K p BL is comparable to that of the membrane and active layers, electroosmotic effects lead to the formation of an inverse pressure gradient. The flux of liquid driven by this pressure gradient is directed towards the anode and reduces methanol crossover. [ABSTRACT FROM AUTHOR]