Boosting the catalytic activity of water splitting and oxygen reduction reactions through axial coordination to MN4–C model catalysts

Abstract

Single atom catalysts (SACs) are currently widely recognized as the frontier and important prototypes for various electrochemical reactions, especially the four-nitrogen coordinated metal atom in a carbon sheet (MN₄–C). However, symmetric local geometry and electron distribution impose limitations on boosting their performance; therefore, modulating the coordination microenvironment and local electronic structure of the active center turns out to be the key. Accordingly, introducing an axial coordination ligand over the metal sites to form MN₄X–C SACs is an effective strategy. In this work, the combinations between 23 transition metals and seven axial ligands (C, N, O, F, P, S and Cl) were considered to determine the optimal metal–ligand match for achieving efficient HER, OER and ORR catalyst candidates; in particular, RhN₄Cl–C, FeN₄O–C and CoN₄Cl–C were screened with ultralow overpotentials of η^HER = 0.01 V, η^OER = 0.20 V and η^ORR = 0.32 V, respectively. We conclude that the axial X atom causes asymmetric electron distribution, hybridizes with the states of the metal active site and breaks the standard scaling relationship. Therefore, the adsorption of the oxygen-containing species was weakened, and the catalytic activity was ultimately improved. In particular, we found that either electron extraction from or electron injection into the central M atom can passivate the reactivity of d_xz/d_yz/d_z² orbitals of the oxygen-containing species. We further revealed that the hybridization between the M-d and X-p orbitals was a simple indicator to characterize the catalytic OER/ORR activity, which can be quantitatively described by the energy difference between the X p- and metal d-band center (Δ_d–p). In addition, we argue that axial coordination affects 4e⁻ OER/ORR selectivity. This study provides a new perspective and useful guidelines for designing advanced SACs.