Surface Hydrides on Fe₂P Electrocatalyst Reduce CO₂ at Low Overpotential: Steering Selectivity to Ethylene Glycol

Abstract

Development of efficient electrocatalysts for the CO₂ reduction reaction (CO₂RR) to multicarbon products has been constrained by high overpotentials and poor selectivity. Here, we introduce iron phosphide (Fe₂P) as an earth-abundant catalyst for the CO₂RR to mainly C₂-C₄ products with a total CO₂RR Faradaic efficiency of 53% at 0 V vs RHE. Carbon product selectivity is tuned in favor of ethylene glycol formation with increasing negative bias at the expense of C₃-C₄ products. Both Grand Canonical-DFT (GC-DFT) calculations and experiments reveal that *formate, not *CO, is the initial intermediate formed from surface phosphino-hydrides and that the latter form ionic hydrides at both surface phosphorus atoms (H@P_s) and P-reconstructed Fe₃ hollow sites (H@P*). Binding of these surface hydrides weakens with negative bias (reactivity increases), which accounts for both the shift to C₂ products over higher C-C coupling products and the increase in the H₂ evolution reaction (HER) rate. GC-DFT predicts that phosphino-hydrides convert *formate to *formaldehyde, the key intermediate for C-C coupling, whereas hydrogen atoms on Fe generate tightly bound *CO via sequential PCET reactions to H₂O. GC-DFT predicts the peak in CO₂RR current density near -0.1 V is due to a local maximum in the binding affinity of *formate and *formaldehyde at this bias, which together with the more labile C₂ product affinity, accounts for the shift to ethylene glycol and away from C₃-C₄ products. Consistent with these predictions, addition of exogenous CO is shown to block all carbon product formation and lower the HER rate. These results demonstrate that the formation of ionic hydrides and their binding affinity, as modulated by the applied potential, controls the carbon product distribution. This knowledge provides new insight into the influence of hydride speciation and applied bias on the chemical reaction mechanism of CO₂RR that is relevant to all transition metal phosphides.