Uncovering the True Active Sites in Ni-N-C Catalysts for CO₂ Electroreduction

Understanding and designing active sites in single-atom catalysts (SACs) requires going beyond static models to capture their dynamic evolution under realistic electrochemical conditions. Here, we develop an integrated theoretical framework that accounts for operational conditions, by combining grand canonical density functional theory (GC-DFT) with machine-learning-accelerated sampling, to uncover structure-activity-stability relationships in Ni-N-C SACs for the CO₂ reduction reaction (CO₂RR). A library of NiN_xC_4-x (x = 0-4) motifs─representing coordination defects likely formed during high-temperature synthesis─was systematically evaluated. Under working conditions, these sites were found to undergo hydrogenation, and NiN₃C_{1_}H₁ was identified as the most probable active site. At reducing potentials, hydrogen adsorbs spontaneously at C-Ni bridge sites rather than Ni top sites, while subsurface hydrogen facilitates bent CO₂ adsorption crucial for activation. High CO₂RR selectivity toward CO arises from site separation: Ni centers drive CO₂RR, while the hydrogen evolution reaction (HER) occurs at the C-Ni bridge or N sites and from thermodynamic suppression of HER at moderate hydrogen coverage. At more negative potentials, a shift in the CO₂RR rate-determining process (RDP) and Ni out-of-surface displacement induced by coadsorption of H and H₂O jointly reduce activity and selectivity. Thus, both the high CO₂RR selectivity of Ni-N-C catalysts and its reversal with more negative potentials can be rationalized by accounting for hydrogenated surfaces. This highlights the necessity of modeling realistic; in situ conditions. This framework provides generalizable insights into the dynamic behavior of active sites in SACs, offering guidance for the rational design of active and robust catalysts for a wide range of electrochemical reactions.