Challenges in photocatalytic hydrogen evolution: Importance of photocatalysts and photocatalytic reactors

Photocatalytic hydrogen evolution has attracted tremendous interest as it offers a process for generation of green hydrogen based on solar radiation. But the process is extremely complex as many factors influence photocatalytic hydrogen evolution. While novel photocatalysts such as MOFs, perovskites, conjugated polymers, carbon-based materials, quantum dots, and others face challenges such as stability and scalability, semiconductor photocatalysts and cocatalysts such as TiO₂/Pt, Ni–CdS, CdS/ZnO, CdS/ZnS have the advantages of simplicity in synthesis, stability, and scalability. Despite decades of research, challenges such as charge separation, spectrum usage, low hydrogen generation efficiency, photocatalyst stability and scalability persist. This review discusses what has been achieved so far in the domain of photocatalytic hydrogen evolution and presents a critical analysis of challenges to be addressed in developing a robust and practical photocatalytic hydrogen generation system based on efficiency of photocatalysts and their stability and the extreme importance of efficient reactor designs.