Poly(vinyl alcohol-co-vinyl acetal) gel electrolytes for alkaline water electrolysis

A series of poly(vinyl alcohol-co-vinyl acetal) gel electrolytes was synthesized, characterized and assessed as electrode separators in alkaline water electrolysis. The copolymers were prepared by reacting poly(vinyl alcohol) with benzaldehyde or 4-formylbenzoic acid under acidic conditions at different ratios, and visually homogenous and water-insoluble membranes were subsequently obtained by solution casting. The physicochemical characteristics in terms of electrolyte uptake, swelling behavior, and ion conductivity could be tuned by varying the degree of functionalization. At a moderate vinyl acetal content of 5%, the membrane combined mechanical robustness with ion conductivity reaching 36 mS cm−1 in 30 wt% aqueous KOH at room temperature. Current densities of up to 1000 mA cm−2 were reached with uncatalyzed Ni-foam electrodes at a cell voltage of less than 2.6 V in alkaline water electrolysis tests, while the membrane effectively prevented hydrogen crossover. Although apparent membrane degradation was observed after a few days of electrolysis operation, the strategies presented in this work to tune membrane properties are of general relevance to the field towards the development of new ion-solvating membrane systems based on more alkaline stable and robust backbone chemistries.