Noble-metal-free catalysts for electrochemical water splitting: Recent progress and perspectives on application

Advanced water electrolysis powered by renewable energy is the most ideal and environmentally friendly approach to hydrogen production, which is believed to be the technological foundation for the large-scale application of hydrogen energy. It can effectively alleviate the pollution of the environment caused by energy consumption and promote the realization of China's carbon peak carbon neutral goal. However, the enormous electricity consumption and the high cost of noble metals hinder the scaling-up of hydrogen production from water electrolysis. The key to improving hydrogen production efficiency is to develop low-cost yet highly efficient noble metal-free electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Moreover, replacing the energy-intensive OER with other energy-efficient reactions is another promising way to promote hydrogen production. Herein, the transition metal-based electrocatalysts developed for water electrolysis in recent years are comprehensively summarized. Furthermore, recent progress in exploiting OER-substituting electrooxidation reactions for hydrogen production is also involved. This review will provide theoretical guidance for the development of advanced transition-metal-based electrocatalysts for water splitting, and shed light on the opportunity for energy-efficient hybrid water splitting applications.