Unveiling the Potential Effects in CO 2 Electroreduction: Electronic Structure Modulation of Active Sites

In electrocatalysis, the applied potential plays an important role in tuning the activity and selectivity; however, the underlying factors that govern the potential-dependent reactivity are not well-understood yet. Herein, the potential-governed CO2 electrochemical reduction to CO on Fe, Co, and Ni supported on nitrogen-doped graphene was used as a probe reaction to investigate how the applied potential modulates the reactivity. We found that the reactivity was highly dependent on the applied potential, and under the proper potentials, the pyrrolic-NiN4C exhibited the highest CO selectivity, while pyrrolic-CoN4C exhibited the lowest onset potential. The detailed analysis of the electronic structure and bonding process between the active metal center and reaction intermediates indicated that the potential can directly affect the orbital hybridization during bonding. As the reduction potential shifted negatively, the Fermi level of the active center rose, while the energy of the dz2 orbital decreased, which is the primary contributor to bonding with reaction intermediates. Concurrently, the electron occupancy of the frontier orbitals underwent significant rearrangement. These findings underscore the critical importance of understanding potential effects in elucidating electrocatalytic reaction mechanisms, providing insights for accurately assessing the behavior of catalytic active sites under operational electrochemical conditions.