ROLE OF MASS-TRANSFER ON HYDROGEN EVOLUTION IN AQUEOUS-MEDIA

The process of hydrogen evolution during alkaline electrolysis of aque ous solutions is governed by mass transfer, growth of hydrogen bubbles and removal of hydrogen from the cathode. Two mechanisms are decisive for hydrogen removal: (i) hydrogen dissolved in the solution is carri ed off from the cathode surface by diffusion and convection, and (ii) gas bubbles are transported by a two-phase flow. The paper describes e xperiments to determine the local concentration of dissolved hydrogen and the void fraction of hydrogen bubbles in aqueous solutions. Measur ements were performed in a flow channel by varying the height of the c athode (40-400 mm), the current density (up to 6250 A m(-2)) and the m ean velocity of the electrolyte (up to 0.95 m s(-1)). Two operating re gimes of the electrolyser are found. At high current densities a back flow is observed leading to an increase in the electrolyte resistance. Traces of dissolved oxygen are detected at high current densities. At low current densities the two-phase flow is confined to a thin layer along the cathode surface, the concentration of dissolved hydrogen bei ng small.