Leverett, J.Baghestani, G.Tran-Phu, T.Yuwono, J.A.Kumar, P.Johannessen, B.Simondson, D.Wen, H.Chang, S.L.Y.Tricoli, A.Simonov, A.N.Dai, L.Amal, R.Daiyan, R.Hocking, R.K.2025-07-292025-07-292025Angewandte Chemie International Edition, 2025; 64(18):e202424087-1-e202424087-111433-78511521-3773https://hdl.handle.net/2440/146397Single 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-Nₓ/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 NOₓ, 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 3dz² 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.en© 2025 The Author(s). Angewandte Chemie International Editionpublished by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution Non-Commercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not usedfor commercial purposes.CO₂RR; single atom catalyst; XAS; catalyst poisoning; impurity toleranceJournal article10.1002/anie.202424087731841Yuwono, J.A. [0000-0002-0915-0756]