Single atom catalysts: Ushering an era for revolutionizing the heterogeneous Electrocatalysis

Heterogeneous catalytic processes play a crucial role in global energy conversion, with nearly 25% of the world's energy consumption directly or indirectly reliant on heterogeneous catalysis. Recently, the emergence of single-atom catalysts (SACs) has marked a paradigm shift in this field. Owing to the unique characteristics of SACs such as atomic dispersion onto the surface of catalyst, tailored electronic structures, and precise geometric configurations, they exhibit excellent performance across a wide range of energy-related applications. Carbon-matrix supported single atoms offer tunable catalytic behaviour through rational modulation of their coordination environment and neighbouring active sites. However, clearly defining the electronic and geometric structure of single atoms and elucidating their relationships between structure and activity remains a grand challenge. Recently, various strategies have been developed for the microenvironment engineering of SACs including multi-heteroatom doping, introduction of heteroatom in different coordination shells, axial ligand coordination, construction of dual metal sites etc. In the present review, we critically examine recent trends in transition metal-based SACs aimed at improving the electrocatalytic performances. We focus on key electrochemical processes such as hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), CO₂ reduction reaction (CO₂RR), and nitrogen reduction reaction (NRR). Lastly, we highlight the prevailing challenges, propose general design principles, and discuss future directions for the progress of stable and effective SACs in electrocatalysis.