Mechanistic Explanation of the pH Dependence and Onset Potentials for Hydrocarbon Products from Electrochemical Reduction of CO on Cu (111)

Abstract

Energy and environmental concerns demand development of more efficient and selective electrodes for electrochemical reduction of CO2 to form fuels and chemicals. Since Cu is the only pure metal exhibiting reduction to form hydrocarbon chemicals, we focus here on the Cu (111) electrode. We present a methodology for density functional theory calculations to obtain accurate onset electrochemical potentials with explicit constant electrochemical potential and pH effects using implicit solvation. We predict the atomistic mechanisms underlying electrochemical reduction of CO, finding that (1) at acidic pH, the C1 pathway proceeds through COH to CHOH to form CH4 while C2 (C3) pathways are kinetically blocked; (2) at neutral pH, the C1 and C2 (C3) pathways share the COH common intermediate, where the branch to C-C coupling is realized by a novel CO-COH pathway; and (3) at high pH, early C-C coupling through adsorbed CO dimerization dominates, suppressing the C1 pathways by kinetics, thereby boosting selectivity for multi-carbon products.

Atomistic Mechanisms Underlying Selectivities in C1 and C2 Products from Electrochemical Reduction of CO on Cu(111)

“…In our previous work, 18 we showed that our GC-QM methodology leads to onset potentials within 0.06 eV of experiment for pH 1, 7, and 12, validating the accuracy of the level of DFT theory, the CANDLE solvation model, and the GC-QM method. In order to provide additional experimental tests of our very detailed mechanisms for subsequent product formation during CORR on Cu(111), we propose here several specific probe molecules that could be introduced to access new pathways at each pH while avoiding the constraint in CORR whereby pH selects C 1 or C 2 pathways.…”

“…18 We show here that the C 2 pathway leads to formation of HOCCOH ad via H 2 O ad (2d) just as for pH 13 but with a lower uphill pH dependent ΔG of 0.47 eV, which then blocks kinetically the branch to OCC ad formation (2e). Just as for pH 13, the C 2 pathway is followed by production of HOCC ad via H 2 O ad (2f).…”

“…22 The algorithm employed by JDFTx variationally minimizes the grand free energy at fixed electron chemical potential with respect to Kohn−Sham orbitals, 39 fluid bound charge, and an auxiliary Hamiltonian for the occupations. 40 Previously, we have found that the relative free energies (barriers ΔG ⧧ and reaction energies ΔG) are linearly dependent on the applied potential U for |U| < ∼2 V (vs standard hydrogen electrode (SHE)), 18 so the Udependence of all ΔG ⧧ and ΔG was calculated assuming a linear relationship between U = 0.0 and −1.2 V. Note that here all U's are referenced to SHE.…”

“…18 We show here that the next step has two competing branches with comparable kinetics: formation of HOCCOH ad (2d) by acquiring a proton from a "hot" surface H 2 O with a pH independent ΔG ⧧ of 0.01 eV but an uphill pH dependent ΔG of 0.83 eV and formation of OCC ad (2e) by the H 2 O ad mechanism with ΔG ⧧ of 0.83 eV. Thus, the "hot" surface water plays an essential role in both the dehydration and the protonation of the CO group.…”

The Reaction Mechanism with Free Energy Barriers at Constant Potentials for the Oxygen Evolution Reaction at the IrO2 (110) Surface

“…Indeed most quantum mechanics (QM) calculations have been performed with constant total number of electrons (constant charge), instead of the constant potential conditions of the experiments. The difficulty of QM for treating charged systems in solution has made such constant potential calculations a formidable task until the recent implementation of the constant potential DFT in conjunction with the CANDLE implicit solvent method by Sundararamen et al 8 and applied recently to CO 2 reduction at Cu surfaces 9 .…”