Atomically dispersed metal catalysts for the oxygen reduction reaction: synthesis, characterization, reaction mechanisms and electrochemical energy applications

In recent years, atomically dispersed metal catalysts (ADMCs) with well-defined structures have attracted great interest from researchers for electrocatalytic applications due to their maximum atom utilization efficiency (100%), distinct active sites and high catalytic activity, stability and selectivity. Based on this, this review will comprehensively discuss the recent developments in advanced single-atom and dual-atom ADMCs for the oxygen reduction reaction (ORR), including synthesis and characterization, reaction mechanisms and energy applications such as in fuel cells and metal–air batteries. In addition, challenges will be summarized and analyzed, including the rational design and fabrication of ADMCs and a deeper understanding of their geometric configuration, electronic structure and reaction dynamics towards the ORR. Furthermore, to facilitate further development, future research directions are proposed to overcome associated challenges, such as (1) the exploration of new/advanced materials including metal precursors and supporting substrates for the fabrication of ADMCs; (2) the optimization of rational design and synthesis techniques for single- and dual-atom catalysts to significantly enhance catalytic ORR activity and stability based on modern characterization techniques; (3) a deeper understanding of ADMC structures, reactive active sites, interactions between metal atoms and support surfaces and corresponding electrocatalytic ORR mechanisms at the atomic level using a combination of density functional theory (DFT) calculations and advanced experimental techniques; (4) the optimization of ADMC-based catalyst layers and membrane electrode assemblies to achieve high performance fuel cells and metal–air batteries using advanced electrochemical testing strategies.