A reversible alkaline water electrolyser for load-flexible power-H₂ interconversion enabled by bifunctional catalyst

A safe, efficient and affordable electrochemical energy-storage system at the grid scale is necessary for the high-level integration of renewable energy. As an ideal energy carrier, hydrogen can be generated via water electrolysis and converted back into electricity via fuel cells. Although conventional alkaline water electrolysers are robust and relatively cost-effective, membrane-based fuel cells are often costly in terms of both capital and operational expenses. Herein, we developed a single-compartment, membrane-free alkaline water electrolyser, enabling reversible power-to-H₂ conversion. The NiOOH redox mediator, sandwiched by two bifunctional electrodes (one for hydrogen oxidation and oxygen reduction reactions, and the other for hydrogen evolution and oxygen evolution reactions), circumvented the use of vulnerable ion-conducting membranes while preventing the formation of H₂-O₂ gas mixtures. Owing to the decoupled anodic and cathodic reactions, high-purity H₂ (>99%) was produced at both high (>500 mA cm^-2) and low (<50 mA cm^-2) current densities, offering excellent load flexibility. Importantly, it effectively works reversibly as a fuel cell, with a peak power density of 0.23 W cm^-2. In addition, it was also capable of working as a Ni-H₂ battery for short-duration energy storage, delivering a peak power density of 1.4 W cm^-2 and a round-trip efficiency of 83.6%. The electrolyser was easily switched between different operational modes while retaining excellent performance in a 350-h endurance test.