Ernesto R. Gonzalez

Development and electrochemical studies of gas diffusion electrodes for polymer electrolyte fuel cells

Electrochemical studies on low catalyst loading gas diffusion electrodes for polymer electrolyte fuel cells are reported. The best performance is obtained with an electrode formed from 20 wt% Pt/C, 0.4 mg Pt cm−2 and 1.1 mg Nafion® cm−2 in the catalyst layer and 15% PTFE in a diffusion layer of 50 µm thickness, for both the cathode and the anode. However, it is also observed that the platinum requirement can be diminished to values close to 0.2 mg Pt cm−2 in the cathode and 0.1 mg pt cm−2 in the anode, without appreciably affecting the good characteristics of the fuel cell response. The experimental fuel cell data were analysed using theoretical models of the electrode structure and of the fuel cell system. It is seen that most of the electrode systems present limiting currents and some also show linear diffusion components arising from diffusion limitations in the gas channels and/or in the thin film of electrolyte covering the catalyst particles.

Oxygen electrocatalysis on thin porous coating rotating platinum electrodes

Abstract This work discusses the electrocatalysis of the oxygen reduction reaction on platinum on carbon thin porous coating rotating disk electrodes in alkaline and acid media. The electrochemical techniques considered are cyclic voltammetry, steady state polarization and impedance spectroscopy. Both the dc and ac polarization results are analyzed and simulated with the thin film/flooded agglomerate model. Cyclic voltammetry allows an inspection of the crystal facet structure of the Pt particles and the determination of effective surface areas which are lower than those determined from X-ray diffraction techniques or transmission electron microscopy. The dc polarization and impedance results show clearly a duplication of the Tafel slope due to structural effects on the porous electrode. Experiments in alkaline solutions show that in this medium there is an important contribution from the carbon to the kinetics of the reaction, which is negligible in acid medium. The effect of Pt particle size on the oxygen reduction electrocatalysis in both electrolytes is correlated with the predominant facets of the platinum crystallites.

Effect of membrane characteristics and humidification conditions on the impedance response of polymer electrolyte fuel cells

In this work, an experimental study of the impedance response of H2/O2 polymer electrolyte membrane fuel cells (PEMFC) was carried out with single cells with four Nafion® membranes (117, 115, 1135 and 112) of different thicknesses, at four temperatures in the range 25–80°C, with reactant gases humidified under different conditions. The impedance results were analysed in terms of the high frequency resistance, the contribution of the charge transfer due to the oxygen reduction reaction and a low frequency relaxation process. With respect to the system, the results are analysed in terms of the membrane thickness and history, the working temperature of the cell and the humidification conditions of the reactant gases, and show clearly that thinner membranes present better characteristics of water management, being much less sensitive to temperature and current density changes.

The Electro-oxidation of Ethanol on Pt-Ru and Pt-Mo Particles Supported on High-Surface-Area Carbon

The oxidation of ethanol was studied on Pt-Ru and Pt-Mo electrocatalysts, supported on high-surface-area carbon, prepared by reduction with formic acid. The catalysts, containing up to 40 atom % of the second metal, were characterized by X-ray diffraction, energy-dispersive X-ray analyses and X-ray absorption experiments. The oxidation of ethanol was studied by cyclic voltammetry, chronoamperometry, and differential electrochemical mass spectroscopy experiments. Within the conditions used in this work, it was found that the activity for ethanol oxidation increased with the contents of the second metal and with the concentration of ethanol.

Electro-oxidation of ethanol on gold : analysis of the reaction products and mechanism

Although the electrocatalytic properties of gold are not as good as those of platinum, the oxidation of small alcohol molecules, such as ethanol, is possible at gold both in acid and alkaline media. By using programmed potential electrolysis coupled with chromatographic analysis of the products formed, the nature and the variation of their concentrations as a function of pH and time have been determined. In alkaline medium, the only product detected was acetic acid. On the other hand, in perchloric acid medium, the reaction products are highly dependent on the value of the oxidation potential, with the following successive steps: ethanol→acetaldehyde→acetic acid. A general tentative mechanism is proposed and discussed.

Methanol electro-oxidation on gas diffusion electrodes prepared with PtRu/C catalysts

This work presents results of the study of methanol electro-oxidation on gas diffusion electrodes with PtRu catalysts supported on carbon. The catalysts were prepared by reduction with formic acid and their performance compared with Pt/C and Pt50Ru50/C E-TEK commercial catalysts, in half cell and single DMFC experiments. The results show that: (i) the activity of the catalysts in a half-cell depends on the concentration of methanol, the maximum being observed close to 2 mol l−1; (ii) atomic contents of Ru of 25% and above increase the tolerance of the catalyst to larger methanol concentrations; (iii) the catalyst Pt75Ru25/C is the one that exhibits the larger activity for methanol oxidation. Gas diffusion electrodes built with this catalyst showed potentials 0.14 V higher than with Pt/C E-TEK when tested in single DMFC.

Surface structure effects on the electrochemical oxidation of ethanol on platinum single crystal electrodes

Ethanol oxidation has been studied on Pt(111), Pt(100) and Pt(110) electrodes in order to investigate the effect of the surface structure and adsorbing anions using electrochemical and FTIR techniques. The results indicate that the surface structure and anion adsorption affect significantly the reactivity of the electrode. Thus, the main product of the oxidation of ethanol on the Pt(111) electrode is acetic acid, and acetaldehyde is formed as secondary product. Moreover, the amount of CO formed is very small, and probably associated with the defects present on the electrode surface. For that reason, the amount of CO2 is also small. This electrode has the highest catalytic activity for the formation of acetic acid in perchloric acid. However, the formation of acetic acid is inhibited by the presence of specifically adsorbed anions, such as (bi)sulfate or acetate, which is the result of the formation of acetic acid. On the other hand, CO is readily formed at low potentials on the Pt(100) electrode, blocking completely the surface. Between 0.65 and 0.80 V, the CO layer is oxidized and the production of acetaldehyde and acetic acid is detected. The Pt(110) electrode displays the highest catalytic activity for the splitting of the C-C bond. Reactions giving rise to CO formation, from either ethanol or acetaldehyde, occur at high rate at any potential. On the other hand, the oxidation of acetaldehyde to acetic acid has probably the lower reaction rate of the three basal planes.

Electro-oxidation of methanol and ethanol on Pt-Ru/C and Pt-Ru-Mo/C electrocatalysts prepared by Bonnemann's method

Abstract Pt–Ru/C and Pt–Ru–Mo/C electrocatalysts prepared by Bonnemanns method have been studied as porous thin films on high surface area carbon electrodes, in order to evaluate their electroactivity on CO desorption in PEM fuel cells. Electrode precursor powders with and without thermal treatment were considered for comparison. The morphology of these precursor powders was characterized in a high-resolution transmission electron microscope equipped with a nano-EDX device and also by XPS measurements. Cyclic voltammetry showed that addition of Mo in the well-established Pt/Ru system is very promising for methanol and ethanol oxidation. Infrared spectroscopic measurements revealed that a thermal treatment in a hydrogen atmosphere of the electrode precursor material was necessary to enhance the efficiency. In order to compare the electroactivity of different catalysts a normalization procedure based on the amount of Pt was used.

Catalysts for DMFC: relation between morphology and electrochemical performance

Abstract In this work Pt–Ru/C alloys were prepared by chemical reduction using various modifications of the formic acid method with the objective of improving the performance. The supported alloys were characterized by XRD, backscattering and TEM analyses, and the active areas were determined by CO adsorption. The materials showed similar particle sizes and different morphological characteristics. The performance of the supported alloys for methanol electrooxidation was evaluated by linear sweep voltammetry experiments and curves of cell potential vs. current density in a single direct methanol fuel cell. It is shown that the difference in the morphology of the catalysts in nano- and microscale is the main effect in the different performances.

Effect of water transport in a PEFC at low temperatures operating with dry hydrogen

In this work, an experimental study of the polarisation response of a H2/O2 PEFC was carried out at low temperatures as a function of the membrane thickness. Results were analysed using a simplified model describing the transport of water in the membrane. It is seen that the model equations lead to good fittings of experimental current–potential data for polymer electrolyte fuel cells (PEFC) working with dry hydrogen and with Nafion® 115 and 117 membranes. For these membranes, limiting effects due to oxygen diffusion are minimal and the water transport through the membrane is a limiting effect for the cell behaviour at high current densities. With the Nafion® 112 membrane, oxygen diffusion effects dominate the characteristics of the current–potential curve.