First-principles microkinetics simulations of electrochemical reduction of CO2 over Cu catalysts

Electrochemical reduction of CO2 can contribute to the storage of excess renewable electricity in chemical bonds. Here we incorporate reaction energetics for CO2 reduction on Cu(111) and Cu(211) determined by DFT calculations in microkinetics simulations to predict the influence of surface topology, the presence of water and possible diffusion limitations on current density-potential curves and Faradaic efficiencies. A reaction-diffusion model was used that takes into account the effect of electrochemical potential on the stability of intermediates and associated activation barriers in proton-coupled electron transfer steps as well as diffusion of protons and CO2 from the bulk electrolyte to the electrode surface. The basic model can well reproduce hydrogen evolution including the effect of proton diffusion limitations and a shift of proton reduction (low potential) to water reduction (high potential). Considering CO2 electro-reduction, the stepped Cu(211) surface is more active than the Cu(111) terrace towards HCOO(H), CO and CH4. The presence of a catalytic H2O molecule increases the overall rate and selectivity to products (CO and CH4) derived from dissociated CO2. A catalytic H2O molecule facilitates the difficult electrochemical CO2 activation step to COOH and suppresses the competing activation step towards HCOO, which mainly yields HCOO(H). In general, the current densities increase at higher negative potential and the products follow the sequence CO2 -> CO -> CH4. That is to say, CO2 is converted to CO via COOH dissociation, followed by CO hydrogenation. Trendwise, the simulated product distribution follows the potential-dependent distribution observed in experiment. The low selectivity to CH3OH can be understood from the fast electrochemical steps that lead to CHx-OH dissociation. At high overpotentials the hydrogenation step from CO2 to COOH controls both activity and selectivity towards CH4. At high potential CO2 reduction becomes increasingly diffusion-limited, thus limiting the selectivity of CO2 reduction vs. hydrogen evolution. This aspect supports the need for better design of mass transfer in electrochemical reactors, which operate at high current density. (C) 2020 The Authors. Published by Elsevier Ltd.