Theoretical insights into dual-atom catalysts for the oxygen reduction reaction: the crucial role of orbital polarization

Abstract

Dual-atom catalysts (DACs) hold great potential for improving the catalytic activity towards the oxygen reduction reaction (ORR). However, the design of highly effective DACs is still limited by the understanding of ORR mechanisms on DACs due to their complex coordination environment and the electronic interplay of metal sites. Herein, we applied density functional theory (DFT) calculations to systematically investigate 42 types of DACs for the ORR, including homo- and hetero-nuclear DACs with two different coordination patterns, which are denoted as M₁M₂NC-i (i = 3 or 4, where i stands for the number of coordinated nitrogen atoms of each metal atom). Among them, FeFeNC-4 exhibits the best catalytic activity with the lowest overpotential of 0.15 V. To further elucidate the synergistic effect of DACs, the electronic properties of bimetallic sites with different nitrogen-coordinated configurations were analyzed in terms of d orbital interactions and spin states. We found that the occupancy of d_z²-derived molecular orbitals is the main origin of moderate spin polarization of dual-metal sites in FeFeNC-4. In addition, a key descriptor is defined by using the linear relationship between adsorption energy and the p-band center of adsorbed *OH, which can accurately estimate the catalytic activities of the DAC system towards the ORR. This work has established the correlation among activity, spin configurations, and orbital polarization, providing intrinsic understandings for the rational design of DACs.