Atomically Dispersed Metal Catalysts for Oxygen Reduction Reaction: Two‐Electron vs. Four‐Electron Pathways

Developing eco‐friendly electrochemical devices for electrosynthesis, fuel cells (FCs), and metal‐air batteries (MABs) requires precisely designing the electronic pathway in the oxygen reduction reaction (ORR) process. Understanding the principle of designing low‐cost, highly active, and stable catalysts helps to reduce the usage of noble metals in ORR. Atomically dispersed metal catalysts (ADMCs) emerge as promising alternatives to replace commercial noble metals due to their high utilization of active metal atoms, high intrinsic activity, and controllable coordination environments. In this review, the research tendency and reaction mechanisms in ORR are first summarized. The basic principles concerning the geometric size of the catalysts for both two‐electron ORR (2e ORR) and four‐electron ORR (4e ORR) were then discussed, aiming to outline the material design differences for 2e ORR and 4e ORR. Subsequently, the recent advances concentrated on the ADMCs, including coordination numbers, light heteroatom doping, dual‐metal atoms‐based coordination, and interaction between nanoparticle (NPs)/nanoclusters (NCs) and ADMs are documented. Finally, the key challenges and opportunities in the future design of ADMCs for the ORR are highlighted.