Ionic Transport Aspects of Water Electrolysis in Alkaline Media

Water electrolysis is a key industrial process for producing green hydrogen. To avoid the use of noble metals and fluorinated polymer membranes, liquid water electrolysis is often carried out in alkaline conditions. It is common to distinguish between 3 processes: alkaline electrolysis at high electrolyte concentrations (≥7 M) with porous membranes, alkaline electrolysis at high electrolyte concentrations (≥7 M) with ion-solvating membranes, and alkaline electrolysis at moderate electrolyte concentrations (<2 M) with anion-exchange membranes. Here, we consider the fundamental aspects of water and ion fluxes and of conductivity across the 3 types of membranes. We discuss ionic currents governed by ion-ion interactions and those resulting from a Grotthuss mechanism. Furthermore, in the case of porous membranes made of a polymeric fabric with compressed inorganic fillers such as zirconia, which are negatively charged in the presence of KOH, and of ion-solvating membranes such as polybenzimidazole, which also become negatively charged by deprotonation at high pH, those membranes should be a weak cation exchanger. We here address this dichotomy. All in all, we show that in all 3 cases, the membrane is an anion exchanger where hydroxide ion mobility differs from that of adjacent aqueous solution due to confinement favoring a Grotthuss-type transport and a jump mechanism.