Origin of the N-coordinated single-atom Ni sites in heterogeneous electrocatalysts for CO2 reduction reaction

Heterogeneous Ni-N-C single-atom catalysts (SACs) have attracted great research interest regarding their capability in facilitating the CO2 reduction reaction (CO2RR), with CO accounting for the major product. However, the fundamental nature of their active Ni sites remains controversial, since the typically proposed pyridinic-type Ni configurations are inactive, display low selectivity, and/or possess an unfavorable formation energy. Herein, we present a constant-potential first-principles and microkinetic model to study the CO2RR at a solid-water interface, which shows that the electrode potential is crucial for governing CO2 activation. A formation energy analysis on several NiNxC4-x (x = 1-4) moieties indicates that the predominant Ni moieties of Ni-N-C SACs are expected to have a formula of NiN4. After determining the potential-dependent thermodynamic and kinetic energy of these Ni moieties, we discover that the energetically favorable pyrrolic-type NiN4 moiety displays high activity for facilitating the selective CO2RR over the competing H-2 evolution. Moreover, model polarization curves and Tafel analysis results exhibit reasonable agreement with existing experimental data. This work highlights the intrinsic tetrapyrrolic coordination of Ni for facilitating the CO2RR and offers practical guidance for the rational improvement of SACs, and this model can be expanded to explore mechanisms of other electrocatalysis in aqueous solutions.