Source of magnetic field effects on the electrocatalytic reduction of CO2

We present an analysis of reported magnetic field effects (MFEs) on the yield of formic acid produced by electrocatalytic reduction of carbon dioxide at a nanoparticle tin electrode [H. P. Pan et al., J. Phys. Chem. Lett. 11, 48-53 (2020)]. Radical pair spin dynamics simulations are used to show that (1) the Δg mechanism favored by Pan et al. is not sufficient to explain the observed magneto-current, (2) field-dependent spin relaxation, resulting from the anisotropy of the g-tensor of CO₂ ^•-, combined with the coherent singlet-triplet interconversion arising from isotropic hyperfine and Zeeman interactions, can quantitatively account for the observed MFE, and (3) modification of hyperfine interactions by isotopic substitution (¹H → ²H and/or ¹²C → ¹³C) could be used to test both the proposed reaction mechanism and the interpretation presented here.