A Review of the Recent Advances in Development of Noble Metal-Free Materials as Electrocatalysts for Hydrogen and Oxygen Evolution Reactions

Sustainable energy development can no longer be met by fossil fuels alone. Hence, electrochemical water splitting containing oxygen and hydrogen evolution reactions is appealing as a clean energy pathway. As respects the water splitting efficiency which is largely determined by the selectivity, durability, and intrinsic activity of the electrocatalysts, one of the most challenging questions when studying these materials is "which category of electrocatalysts will show the best performance in this issue?" The best electrocatalysts for water splitting still come from noble metals. Although these materials show particularly good efficiency, but due to the scarce resources their massive use is limited. Therefore, the noble metal-free materials due to their stability, efficiency, abundance and variety of reaction sites were introduced as an interesting candidate for electrochemical water splitting reactions. In this review, based on the important above-mentioned points, our attention was focused on key categories based on transition metals (TMs), metal organic framework derived (MOF-derived), carbon-based hybrids, graphitic carbon nitride (g-C3N4) hybrids, and bio-assisted electrocatalysts. These compounds have shown significant activity and stability for broad electrocatalysis applications in water splitting reactions and displaying remarkable potential to replace with noble metal-based catalysts. This comprehensive review identifies rational strategies for designing and synthesizing high-performance novel noble metal-free electrocatalysts for water splitting.