Recent Design Strategies for M-N-C Single-Atom Catalysts in Oxygen Reduction: An Entropy Increase Perspective

Abstract

Recently, a diverse array of novel metal-nitrogen-carbon (M-N-C) single-atom catalysts (SACs) have rapidly evolve, particularly in the realm of oxygen reduction reaction (ORR). Despite the plethora of proposed design and improvement strategies for SACs, a comprehensive review systematically compiling the components in M-N-C from a unified perspective is notably absent. For the first time, a thorough examination of each component in M-N-C is conducted, focusing on the perspective of entropy increase in the active sites of SACs. For the single M-N₄ sites and the whole M-N-C system, an increase in entropy implies an elevated degree of disorder and chaos. Broadly, the entropy-increasing modification of M (single mental sites) and guest groups entails an augmentation of chaos, with the most effective co-catalytic synergy achieved by establishing multiple active sites through a “cocktail effect”. Concerning N (nitrogen and other heteroatoms) and C (carbon supports), the entropy increase modification induces heightened disorder, with symmetry breaking more likely to drive M-N₄ toward adsorbing oxygen molecules to attain an equilibrium symmetric structure. All these innovative design strategies have led to a remarkable improvement in the ORR activity and stability and offer a guiding criterion for the future preparation of SACs.