Direct Observation of Electron Donation onto the Reactants and a Transient Poisoning Mechanism During CO2 Electroreduction on Ni Single Atom Catalysts

Abstract

Single atom catalysts (SACs) are an important class of materials that mediate chemical reduction reactions, a key subset of which is Ni within a carbon support for the electrochemical CO₂ reduction reaction (CO₂RR). However, how the metal atom/clusters and the carbon-based support act in concert to catalyze CO₂RR is not well understood, with most reports attributing activity solely to the Ni-N_x/C moieties. To address this gap, we have undertaken a mechanistic investigation, employing in situ X-ray absorption spectroscopy (XAS) coupled with electrochemical studies and density functional theory (DFT) calculations to further understand how Ni single atoms work in conjunction with the nitrogen-doped carbon matrix to promote CO₂RR to CO, and how the presence of impurities such as those present in CO₂-containing waste flue gases (including NO_x, and CN⁻) changes the catalyst upon reduction. In contrast to previous works, we do not find strong evidence for a purely metal-based reduction upon application of negative reductive potentials. Instead, we present evidence for an increase in the equatorial vs. axial splitting of Ni, consistent with electrons moving onto the reactants via the Ni single atom 3d_z² orbital. In addition, we demonstrate a transient poisoning mechanism of the Ni SAC by nitrite and thiocyanate, explaining the recovery of activity during CO₂RR. These insights can aid the design of practical CO₂ valorization technologies.