Carboxylated Polystyrene-b-poly(ethylene-ran-butylene)-b-polystyrene Membranes for Alkaline Water Electrolysis

A series of membranes based on carboxylated polystyrene-b-poly(ethylene-ran-butylene)-b-polystyrene (SEBS) were synthesized via Friedel-Crafts bromoalkylation, substitution with malonate esters, and heterogeneous ester hydrolysis. The physicochemical properties of the obtained membranes, including electrolyte uptake, swelling ratio, and ionic conductivity, were highly affected by the degree of functionalization and the concentration of the aqueous KOH solution. Membranes with degrees of functionalization higher than 100% suffered from excessive swelling and poor mechanical stability. However, membranes with degrees of functionalization in the range of 50-75% combined high electrolyte uptake and ion conductivity with mechanical robustness. The area specific resistance during electrolysis testing in 15 wt % aqueous KOH at 80 °C was 0.18 Ω cm² after 1 week, with no signs of degradation. The H₂ in the O₂ level at 50 mA cm^-2 was about 0.5%, which was considerably lower than that of Zirfon despite being four times thinner.