Stable overall water splitting in an asymmetric acid/alkaline electrolyzer comprising a bipolar membrane sandwiched by bifunctional cobalt‐nickel phosphide nanowire electrodes

Abstract Water splitting has been proposed to be a promising approach to producing clean hydrogen fuel. The two half‐reactions of water splitting, that is, the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), take place kinetically fast in solutions with completely different pH values. Enabling HER and OER to simultaneously occur under kinetically favorable conditions while using exclusively low‐cost, earth‐abundant electrocatalysts is highly desirable but remains a challenge. Herein, we demonstrate that using a bipolar membrane (BPM) we can accomplish HER in a strongly acidic solution and OER in a strongly basic solution, with bifunctional self‐supported cobalt‐nickel phosphide nanowire electrodes to catalyze both reactions. Such asymmetric acid/alkaline water electrolysis can be achieved at 1.567 V to deliver a current density of 10 mA/cm² with ca. 100% Faradaic efficiency. Moreover, using an “irregular” BPM with unintentional crossover the voltage needed to afford 10 mA/cm² can be reduced to 0.847 V, due to the assistance of electrochemical neutralization between acid and alkaline. Furthermore, we show that BPM‐based asymmetric water electrolysis can be accomplished in a circulated single‐cell electrolyzer delivering 10 mA/cm² at 1.550 V and splitting water very stably for at least 25 hours, and that water electrolysis is enabled by a solar panel operating at 0.908 V (@13 mA/cm²), using an “irregular” BPM. BPM‐based asymmetric water electrolysis is a promising alternative to conventional proton and anion exchange membrane water electrolysis.