A Systematic Study of Cobalt Dual-Atom Catalysts Supported on N-Doped Carbon for the Hydrogen Evolution Reaction

Abstract

The industrial application of water splitting largely relies on developing high-performance and low-cost catalysts for the hydrogen evolution reaction (HER). As a representative cobalt single-atom catalyst, the CoN4 moiety embedded in an Ndoped carbon matrix exhibits significant HER potential. The Co2Nx (x = 4–8) dual‑atom catalysts (DACs) were systematically investigated for HER by means of density functional theory as implemented in the Vienna Ab initio Simulation Package. Most Co2N7/8 DACs are more favorable for the Volmer-Heyrovsky mechanism than CoN4. Furthermore, Co2N7 structures are better suited for the Volmer-Tafel mechanism than CoN4, and the Co2N8 structures also serve as promising candidates for this mechanism. The bond strength (-ICOHP) and bond order (ICOBI) values of the Co2N7/8 structures cluster around 0 – 0.13 eV and 0 – 0.2, implying weak Co-Co bonding and a low degree of electron overlap between Co atoms. Given the similar bond strengths and Ncoordination environments in the Co2N7/8 structures, combined with significant Bader charges (~ 0.90 e), the Co atoms adopt an electron-deficient, high-valence state, which consequently induces a low-spin state. Owing to this electron deficiency, less charge is transferred to H* during the interaction, resulting in a weaker interaction between them; consequently, their ΔGH* values approach zero. Therefore, the high valence state of Co plays a critical role in the HER performance by optimizing the hydrogen adsorption free energy (ΔGH*). The atomic configurations of Co2Nx (x = 4–8) moieties on N-doped carbon supports are the key to determining their HER activity. The Co2N7 and Co2N8 DACs are promising HER catalysts featuring a high N/Co ratio. Consequently, experimentally realizing these active sites requires nitrogen-rich carbon supports, precise control of Co loading, and spatial confinement during synthesis.