Gas-generating porous electrodes at low overvoltages: Allowance for the outside-electrode limitations

Abstract

Characteristics of gas-generating porous electrodes (GPE) are calculated and analyzed at low over-voltages, when all the electrode pores are still filled with electrolyte. The calculations assume the existence of limitations outside the electrode, specifically, the diffusion of gas molecules dissolved in electrolyte and their conglomeration into bubbles. Separate solutions are found and then sewn for GPE and the electrolyte chamber outside it, where the generated gas is collected. An important parameter is revealed, namely, the ratio of a characteristic gas-generation current inside GPE to a characteristic gas-removal current inside the outside-electrode region. The parameter determines both the net current density in GPE and the depth of the electrochemical process penetration into the electrode’s porous space. The two limiting cases studied are the hydrogen-generating water electrolysis on a porous platinum electrode and the chlorine generation on dimensionally stable anodes (DSA). A way to estimate all quantities that characterize the gas removal into the outside-electrode region is shown. It is established that only a narrow (no greater than a micrometer) region adjacent to the front surface of GPE takes part in the chlorine generation process on DSA of standard thickness (5 μm).