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

Abstract Tuning transition metal spin states potentially offers a powerful means to control electrocatalyst activity. However, implementing such a strategy in electrochemical CO 2 reduction (CO 2 R) 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 ( d x2‐ y2 0 , d z2 0 , d xz 2 , d yz 1 , d xy 2 ) in Fe‐N 4 to high spin state ( d x2‐y2 0 , d xz 1 , d yz 1 , d z2 1 , d xy 2 ) in Fe‐N 3 ‐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 CO 2 R, the SAC with Fe‐N 3 ‐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 Fe 3+ exhibits enhanced electron back donation via the d xz / d yz ‐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 CO 2 R performance.