The Influence of Attached Bubbles on Potential Drop and Current Distribution at Gas‐Evolving Electrodes

Abstract

A theoretical study is presented of the effects of bubbles attached to the surface of a gas‐evolving electrode, with emphasis on their influence on the local current distribution and on the potential drop at the electrode. The mathematical model accounts for the combined influence of ( i ) ohmic obstruction within the electrolyte, ( ii ) area masking on the electrode surface, which raises surface overpotential by increasing the effective current density, and ( iii ) decreased local supersaturation, which lowers the concentration overpotential. The electrolytic transport is described by potential theory, and the dissolved gas is assumed to obey steady‐state diffusion within a concentration boundary layer. The coupled field equations are solved numerically using the boundary‐element method. The model is applied to hydrogen evolution in potassium‐hydroxide solution. For gas evolution in the Tafel kinetic regime, the current distribution is nearly uniform over the unmasked electrode area, and the increase in surface overpotential is the dominant voltage effect. However, outside the Tafel regime ( e.g. on cathodes of greater catalytic activity) the current density is strongly enhanced near the bubble‐contact zone, and the supersaturation‐lowering effect is quite strong, largely offsetting the ohmic and surface‐overpotential effects. Proceeding from a set of base conditions, we perform a systematic examination of attached‐bubble effects, their relative importance, and their dependence on system variables.