Tuning Transition Metal 3d Spin state on Single-atom Catalysts for Selective Electrochemical CO2 Reduction.

Tuning transition metal spin states potentially offers a powerful means to control electrocatalyst activity. However, implementing such a strategy in electrochemical CO2 reduction (CO2R) is challenging since rational design rules have yet to be elucidated. Here we show how the addition of P dopants to a ferromagnetic element (Fe, Co, and Ni) single-atom catalyst (SAC) can shift its spin state. For instance, with Fe SAC, P dopants enable a switch from low spin state (dx2- y2 0, dz2 0, dxz 2, dyz 1, dxy 2) in Fe-N4 to high spin state (dx2-y2 0, dxz 1, dyz 1, dz2 1, dxy 2) in Fe-N3-P. This is studied using a suite of characterization efforts, including X-ray absorption spectroscopy (XAS), electron spin resonance (ESR) spectroscopy, and superconducting quantum interference device (SQUID) measurements. When used for CO2R, the SAC with Fe-N3-P active sites yields > 90% Faradaic efficiency to CO over a wide potential window of ≈530 mV and a maximum CO partial current density of ≈600 mA cm-2. Density functional theory calculations reveal that high spin state Fe3+ exhibits enhanced electron back donation via the dxz/dyz-π* bond, which enhances *COOH adsorption and promotes CO formation. Taken together, the results show how the SAC spin state can be intentionally tuned to boost CO2R performance.