Tuning Transition Metal 3d Spin state on Single-atom Catalysts for Selective Electrochemical CO<sub>2</sub> Reduction.

Abstract

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 (d_{x2- y2} ⁰, d_z2 ⁰, d_xz ², d_yz ¹, d_xy ²) in Fe-N₄ to high spin state (d_x2-y2 ⁰, d_xz ¹, d_yz ¹, d_z2 ¹, d_xy 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 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 CO₂R performance.