Fundamentals and Perspectives on Materials for Bifunctional Electrocatalysis.

Electrocatalysts are the core material for water electrolysis, as a promising technology for green hydrogen production coupled with renewable energy resources. Among various electrocatalyst development strategies, bifunctional electrocatalysts have attracted fast-growing efforts, due to their abilities for catalyzing both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) involved in the electrocatalysis process for reducing the cost in fabrication, operation, and systems. In this study, a comprehensive review of the latest advancements and ongoing research for the bifunctional electrocatalysts uniquely based on their families is presented, aiming to gain deep insights into the field and provide an outlook for the direction of future developments. First, an in-depth discussion is conducted on the mechanisms for both reactions involved in the electrocatalysis process, followed by examining their critical affecting factors. Then, as the heart of this work, a wide range of bifunctional electrocatalyst families are reviewed, including metal-organic frameworks, metal oxide-based catalysts, hydrides and hydroxides, carbon-based materials, phosphides, and chalcogenides. The synergistic effects are also explored for combining different materials through the formation of nanocomposites and heterojunctions. Finally, the challenges encountered with current technologies and perspectives are explored to provide practical and scalable electrocatalyst solutions for water electrolysis.