Recent advances in electrocatalysts for neutral and large-current-density water electrolysis

Hydrogen as a clean energy resource is considered as one of the most promising alternatives to alleviate energy crisis and environmental pollution, which induces the urgent requirement for electrochemical water splitting that contains the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Compared with alkaline or acidic electrolytes, the water splitting in neutral media is more daunting and less explored, which is mainly attributed to the additional challenge in improving the sluggish kinetics of water dissociation in neutral pH. However, if the neutral water splitting system could be realized, it would bring about the opportunity to direct splitting of less corrosive saline water that is the most abundant resource, which allows a massive and sustainable hydrogen fuel production. Here we review recent research in the development of both HER and OER electrocatalysts that are operated in neutral conditions from the perspective of the reaction mechanisms, rate-determining steps, and component-structure-activity correlations. Single-atom catalysts as an emerging field are also highlighted for their high atom utilization efficiency and high activity. In view of large-current-density operating conditions, three-dimensional freestanding electrodes where catalysts grow directly on the conductive substrates become the main tendency. In addition, superhydrophilic and superaerophobic electrode surfaces are summarized with regard to rapid ion diffusions and gas bubbles release, especially under large current densities. Finally, we address the challenges associated with neutral water electrolysis and future pathways in the hope of guiding the design of the electrocatalysts and electrode materials to make large-scale hydrogen production easier and more efficient.