Tailoring the selectivity and activity of oxygen reduction by regulating the coordination environments of carbon-supported atomically dispersed metal sites

The oxygen reduction reaction (ORR) is an important half reaction in fuel cells and metal–air batteries as well as in the cost-effective electrosynthesis of H₂O₂. However, the issues of low selectivity and activity still call for solutions. The key to rationally optimizing the activity and selectivity of electrocatalysts for the ORR is to control their electronic structures. Carbon-supported atomically dispersed metal sites (C-ADMSs) feature isolated metal atoms with well-defined local coordination structures in a carbon host matrix, in which the coordination bonds, bond lengths and coordination number can be modulated to alter the selectivity/activity for the ORR. Benefiting from their metalloenzyme-like properties, C-ADMSs have been serving as an ideal and adjustable platform to controllably regulate the selectivity and activity for the ORR. Therefore, an in-depth summary of the regulation of coordination structures for the ORR is of great significance and a pressing need. In this review, we focus on the manipulation of the coordination number in the first coordination shell and the coordination atoms in the first, second and even outer coordination shells. Then, we summarize the influences of coordination numbers and coordination atoms in different coordination shells on the selectivity for 2e^- ORR and the activity for 4e^-ORR, respectively. In addition, we attempt to figure out the general principle of the regulating mechanism between the coordination environment and the catalytic activity/selectivity for the ORR. Finally, we present a brief conclusion, and discuss the challenges and opportunities with respect to the active microenvironment of central metals in C-ADMSs for the ORR.