Der gegenwärtige stand der wissenschaftlichen arbeiten über elektrodenvorgänge in brennstoffzellen

Abstract

Several important types of electrodes used in low temperature fuel cells are discussed. Porous gas-diffusion electrodes made of carbon or of suitable sintered metals are most commonly used. More recent developments are a pore free palladium membrane electrode for hydrogen as fuel and a slurry electrode consisting of the catalyst suspended in electrolyte.Some aspects of the kinetics of the anodic oxidation of hydrogen and of methanol (as an example of the organic fuels), as well as of the cathodic reduction of oxygen are discussed.In the case of the anodic oxidation of hydrogen two reaction paths are essentially considered as experimentally proved: the Tafel-Volmer and the Heyrovsky-Volmer mechanisms. Which mechanism operates depends strongly on the conditions of the experiment, especially on the metal of the electrode. In the case of “active” metals generally the transport processes are rate-determining. Transport processes are particularly important at the porous hydrogen electrodes in fuel cells.Much less is known about the kinetics of the oxidation of methanol as compared with that of hydrogen. The reaction occurs at the electrodes usually employed for this purpose as for instance Ni or on the metals of Pt group, with sufficient rate only there, when the metal surface is not covered by an oxide layer or by chemisorbed oxygen. It seems also that the adsorbed intermediates inhibit the oxidation reaction.Also the kinetics of the reduction of oxygen may not be treated as clearly elucidated. The fact that the thermodynamic reversible potential of the oxygen electrode generally cannot be obtained, may be connected with the reduction of oxygen to hydrogen peroxide and not to water. This reduction reaction is then followed by a catalytic decomposition of H2O2, into oxygen and water. The mechanism of this reaction is not discussed. It is possible that the anodic oxidation of H2O2 to oxygen and cathodic reduction to water occur along with its catalytic decomposition. hodique en H2O.