Low-temperature water electrolysis: fundamentals, progress, and new strategies

Water electrolysis is a promising technology for sustainable energy conversion and storage of intermittent and fluctuating renewable energy sources and production of high-purity hydrogen for fuel cells and various industrial applications. Low-temperature electrochemical water splitting technologies include alkaline, proton exchange membrane, and anion exchange membrane water electrolyses, which normally consist of two coupled half reactions: the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). Despite the advances over decades, formidable challenges still exist and hinder the practical application of large-scale, energy-efficient, and economically viable water electrolysis, including large energy penalty, sluggish kinetics, high cost of precious metal based electrocatalysts, possible H-2/O-2 gas crossover, difficulty in storage, and distribution of H-2. Herein, we first briefly introduce the fundamentals of water electrolysis, summarize the recommended standardized electrochemical characterization protocols, and demonstrate the metrics and key performance indicators that are used to evaluate the performances of HER and OER electrocatalysts and electrolyser cells. Then, we present six new strategies to mitigate the technical challenges in conventional water electrolysis. These emerging strategies for disruptive innovation of water electrolysis technology include overall water electrolysis based on bifunctional nonprecious electrocatalysts (or pre-catalysts), magnetic field-assisted water electrolysis, decoupled water electrolysis, hybrid water electrolysis, acid/alkaline asymmetric electrolyte electrolysis, and tandem water electrolysis. Finally, the remaining challenges, perspectives and future directions are discussed. This review will provide guidance and inspire more endeavours to deepen the mechanistic understanding and advance the development of water electrolysis.