Recent advances in metal-free electrocatalysts for the hydrogen evolution reaction

Hydrogen energy, with the advantages of being pollution-free and having high combustion efficiency and a high utilization rate, has been widely developed as a substitute for traditional fossil fuels. The hydrogen evolution reaction (HER) plays an important role in water electrolysis. Metal-free electrocatalysts show the advantages of abundant reserves, tunable molecular structure and high tolerance to extreme pH conditions compared with metal-based catalysts. With the significant progress made in the discovery of novel metal-free HER catalysts, some of them can break through our knowledge limitations and demonstrate enormous industrial potential. In this review, the fabrication methods of metal-free catalysts are first introduced to provide readers with a general understanding of the synthesis procedures. Then, some important types of metal-free catalysts are discussed in detail. Finally, clues for obtaining high-performance HER metal-free catalysts and possible future research directions and prospects are provided. This review can accelerate the development of novel metal-free HER catalysts in related applications. The review outlines a comprehensive overview of recent research on metal-free HER catalysts, including synthesis procedures, important types of metal-free catalysts and future challenges and opportunities.