Metal-free photo- and electro-catalysts for hydrogen evolution reaction

Hydrogen production as very attractive clean energy technology has sparked the accelerated development of catalysts for the hydrogen evolution reaction (HER) towards efficient photo- and electrolytic water splitting. In particular, metal-free catalysts are intensively investigated as good substitutes for metal-based catalysts towards the HER due to their earth-abundance, good conductivity, low toxicity and low cost. Although some reviews have surveyed metal-free catalysts for water splitting, the tremendous recent advances require an update as a guide for further development. This review mainly summarizes the recent progress on theoretical calculations, experimental approaches and fundamental insights for metal-free catalysts. In particular, the Gibbs free energy of the intermediate state, Delta G(H*), is an inclusive and much more influential descriptor for HER catalysts considering advanced methods beyond density functional theory (DFT) calculations, and a fine structure and high doping content can greatly improve the catalytic activity for the HER, but make the preparation process increasingly complicated. Therefore, simple and rapid preparation routes are highly desirable and urgently required. Thus, theoretical calculations have been applied to predict the structure-activity relationship of catalysts as a guide for good design. Non-carbonaceous metal-free catalysts are also included in this survey. All these developments and accomplishments are systematically summarized and thoughtfully discussed, and the overall perspectives for the current challenges and future development in this field are offered. It is our intention that this comprehensive review will be very informative for the future design and synthesis of metal-free catalysts for the HER and attract increasing interest to explore deep scientific insights for broad important practical applications and critical advances in the field of catalysis.