Design principles for tunable electrocatalysis via electronic structure and orbital interactions in single-atom catalysts.

Well-defined active sites and tunable electronic features of single-atom catalysts (SACs) enable great versatility in modulating their electrocatalytic activity. However, an understanding of how the electronic structure governs the catalytic efficiency and product selectivity for CO2 and O2 reduction reactions (CO2RR and ORR) remains incomplete. This feature article highlights orbital-based design principles to unify the catalytic behavior of porphyrin, phthalocyanine, and non-heme macrocyclic complexes with that of graphene-based SACs. We show that metal-ligand interactions and ligand-field splitting define activation principles and preferred binding motifs of the reaction intermediates. The roles of the local coordination environment, axial ligation, and secondary coordination-sphere effects in tuning catalytic activity and stability are discussed. The discussion combines conventional thermodynamic reaction descriptors to assess scaling relations and activity volcano plots, and to define the electrochemical stability of the SACs. Finally, the review outlines opportunities to integrate orbital interactions and electronic structure principles with machine learning to develop catalyst design principles for CO2RR and ORR.