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@Ps) 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.

Modeling Operando Electrochemical CO2 Reduction

“…Thus, Cu sits at the sweet spot because it exhibits strong enough CO binding, but thermoneutral H adsorption. Finally, oxygen binding energy was proposed as a descriptor for alcohol vs hydrocarbon competition, and this prediction was later confirmed by experimental reports on eCO 2 R to methanol or methane on Au, Cu, and Ag electrodes. ,, As a crucial remark, we highlight that the above-mentioned descriptors are specifically tailored to single metals, thus alloys or oxide-derived materials may have completely different reactivity, see, for instance, eCO 2 R to C 2 products (specifically ethylene glycol) on Fe 2 P and nickel oxygenates …”

Advancing the Electrochemistry of Gas‐Involved Reactions through Theoretical Calculations and Simulations from Microscopic to Macroscopic

“…Dismukes and his team investigated the process of CO 2 RR on the surface of Fe 2 P at low potential by Grand Canonical-DFT (GC-DFT). [98] The results of GC-DFT showed that *CO mainly formed ion hydrides at surface phosphorus atoms (H@P s ) and P-reconstructed Fe 3 hollow sites (H@P*) and *formate is the initial intermediate of surface phosphine hydride formation (Figure 4b). The more negative the applied potential was, the weaker the binding of these surface hydrides was, indicating the enhancement of HER and the transformation of multicarbon products (C 3 products or C 4 products) to C 2 products.…”

“…Reproduced with permission. [98] Copyright 2021, American Chemical Society. d) Summary of limiting potentials for the six most promising DACs.…”

The Role of Hydrogen Adsorption Site Diversity in Catalysis on Transition Metal Phosphide Surfaces

“…This phenomenon has been used to rationalize electrochemical CO2 reduction on Fe2P and Ni2P nanocrystals, where it was proposed that in addition to creating local active site motifs for CO2 hydrogenation, the binary surface also dictates the selectivity between various multiple carbon products. 94,95 DFT studies have shown that C-C coupling on Ni2P via *HCOO self-condensation is exergonic due to the presence of many types of H-binding sites, which results in selectivity toward multicarbon products. 103 It may be intuitive to think that the many H-adsorption sites promote higher activity for HER; however, it instead provides facile hydrogenation of CO2 intermediates.…”