(651d) Determining Effects of Interfacial Potential, pH and High Coverages on in-Situ Structure and Kinetics for CO2 Reduction on Cu(100)

Electrochemical CO₂ reduction (CO₂R) on copper (Cu) catalysts has shown potential for producing profitable C2 products, along with meeting carbon neutrality goals. The reaction is challenging to study, however, due to the complex reaction network, the unknown effects of reactant coverages, pH effects, and the impact of the competing hydrogen evolution reaction (HER), which causes a loss in faradaic efficiency. To provide molecular-level insights into these effects, we use density functional theory and probe the in-situ structure of Cu(100) under acidic and alkaline media as a function of electrode potential. A combined graph theory-simulated annealing approach is used to rigorously sample mixed coverages of CO^* and H^*, which are the most abundant intermediates for CO₂R and HER, on (100) facets. Thermodynamic phase diagram analysis shows that the surface is covered with a monolayer of H^* below potentials of -0.34 V_RHE. However, while H^* formation from protons may be kinetically feasible under acidic conditions, in alkaline conditions, the formation of H^* can become increasingly activated at higher H^* and CO^* coverages. We probe this by determining activation barriers for water dissociation considering the Volmer and Volmer-Heyrovsky mechanism, at several coverages of CO^* and H^*, using constrained ab-initio molecular dynamics methods. The activation energy is, indeed, observed to become prohibitively large beyond a coverage of 0.5 ML H^*. In contrast, H₂ formation barriers decrease at these higher coverages, which in turn, maintains a coverage below 0.5 ML H^*. Interestingly, water dissociation is unaffected by a CO^* coverage as high as 0.5 ML. We close by assessing the impact of these coverage effects on the competition between HER and CO₂R in both acidic and alkaline conditions.