Fundamental equations of electrochemical kinetics at porous gas-diffusion electrodes

The kinetics of a charge-transfer reaction at a porous gas-diffusion electrode are presented using the simple pore model. The rate of diffusion of the reactant gas through the electrolyte-free part of the pore is sufficiently fast for pore radii of 10 -4 cm or higher. The dissolution of gas at the gas-electrolyte interface causes no limitations. For the processes occurring in the electrolyte occupied part of the pore, all forms of polarization are taken into account and expressions are derived for the current distribution and overpotential-current-density relations. By using numerical values of the various parameters (diffusion coefficient, solubility of reactant gas in electrolyte, specific conductance of electrolyte), it is shown that the case of all forms of polarization reduces to one of activation and concentration overpotential. A one-dimensional treatment is valid up to about 25% of the limiting current for this case. The dependence of current-distribution and overpotential-current-density relations on the various kinetic parameters is shown graphically for these cases. It is shown that most of the current (over 90%) is generated over a small fraction of the pore length (10 -2-10 -4) in the practical overpotential range (i.e. for reasonable current densities) when all forms of polarization are taken into account. The case of activation and ohmic overpotential shows the interesting result that current densities can be increased by a factor of up to 10 3 times if concentration overpotential is eliminated, as for example by using very soluble fuels or by circulating the electrolyte saturated with reactant through the porous electrode.SCOPUS: ar.j