Theoretical insight on the delicate regulation of coordination environments for carbon-nitrogen-supported single-atom metals for OER and ORR

Abstract

Carbon-nitrogen-supported single-atom catalysts have significant potential in electrochemistry; however, the real active site TM-C_x-N_y structures are typically complex and varied due to the high-temperature preparation conditions. In order to systematically identify the activity and intrinsic mechanism of these reaction sites, the density-functional theory was employed, whereby different TM-C_x-N_y structures were constructed by quantitative N-atom doping using graphene as a substrate. The adsorption strength of the intermediates and the catalyst activity were effectively regulated by precisely controlling the coordination of the single-atom metals. The findings indicate that the majority of TM-C_x-N_y exhibit robust thermodynamic and electrochemical stability. Among these, Ni-C₄, Co-N₄, and Rh-N₄ demonstrate the most promising OER and ORR activities. By calculating the modulation interval of intermediate adsorption energies from TM-C₄ to TM-N₄, it was found that metals with different valence electrons exhibit disparate sensitivities to coordination modulation with Group VIII(II) elements (N_e = 9) showing the highest modulation efficiency, which is further substantiated by charge density difference and Bader charge analyses employing Co and Au as illustrative examples. Furthermore, a desirable descriptor, termed φ, has been identified, which correlates well with the intermediate adsorption energy and η^OER/ORR, thus allowing an accurate representation of the activity of various TM-C_x-N_y structures. The catalyst exhibits optimum activity when the φ value is within the range of 120–130. However, when the value of φ exceeds 150, it is no longer reliable for evaluating the activity of the catalyst.