Recent progress in ligand-centered homogeneous electrocatalysts for hydrogen evolution reaction

Abstract

Electrocatalytic hydrogen evolution reaction (HER) is a critical process in a sustainable energy conversion system. Many metal complexes as small-molecule catalysts for electrochemical H₂ evolution have been reported over recent decades, which have made a great contribution to a detailed understanding of their catalytic mechanisms. In most reported mechanistic pathways, metal hydride species are always considered as key intermediates during H₂ formation, with the metal centre at the active site docking both electrons and protons. These metal-centred HER routes need to employ metal ions with open coordination sites, which can accommodate multiple redox states to involve two-electron hydride transfer for H₂ production. By comparison, an alternative approach, specifically focusing on redox-active ligand-centred electrocatalytic HER, has garnered increasing attention over the last six years. This review highlights recent progress in metal and metal-free ligand-centred homogeneous electrocatalytic HER since 2012 and discusses it according to two categories: organic ligand-centred and inorganic ligand-centred HER. In addition, some existing issues and suggestions for future directions in this catalytic area have been provided.