Transition metal single-atom catalysts for water splitting: Unravelling coordination strategies and catalytic mechanisms for sustainable hydrogen generation

Single-atom catalysts (SACs) lead the field of electrocatalysis water splitting, providing critical benefits like high atomic efficiency, adjustable electronic properties, and metal-support solid binding. These characteristics collectively enhance catalytic performance and minimise metal consumption. Earth-abundant transition metals like iron (Fe), cobalt (Co), and nickel (Ni) have emerged as cost-effective, yet promising alternatives to precious metals, demonstrating comparable activity attributed to their substantially optimised coordination environments and electronic structures. A comprehensive review of advancements in transition metal single-atom catalysis (TMSACs) is indispensable in summarising mechanisms and strategies targeting performance enhancements, therefore guiding rational future design and facilitating industrial-scale water-splitting applications. This review showcases an in-depth analysis of significant synthesis methodology, structure-activity relationships, and the impact of metal coordination interactions on the reaction efficiency and structural integrity of single-atom catalysts (SACs). Here, it aims to guide future TMSAC research by highlighting opportunities to enhance electrocatalytic performance through coordination energy. A detailed analysis of surface coordination, covering coordination sites, atom types, coordination numbers, and structural configurations—We offer insights into their influence on the electrochemical properties and inherent catalytic of SACs. Furthermore, the review explores future directions for improving SAC performance through defect engineering, heteroatom doping, and bimetallic site formation, focusing on scaling up hydrogen production and advancing sustainable energy technologies.