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.