Computational electrocatalysis beyond conventional hydrogen electrode model: CO₂ reduction to C₂ species on copper facilitated by dynamically formed solvent halide ions at the solid-liquid interface

The reduction of CO₂ into value-added chemicals and fuels has been actively studied as a promising strategy for mitigating carbon dioxide emissions. However, the dilemma for the experimentalist in choosing an appropriate reaction medium and neglecting the effect of solvent ions when using a simple thermochemical model, normally leads to the disagreement between experimental observations and theoretical calculations. In this work, by considering the effects of both the anion and cation, a more realistic CO₂ reduction environment at the solid-liquid interface between copper and solvent ions has been systematically studied by using ab initio molecular dynamics and density functional theory. We revealed that the co-occurrence of alkali ions (K⁺) and halide ions (F⁻, Cl⁻, Br⁻, and I⁻) in the electric double layer (EDL) can enhance the adsorption of CO₂ by more than 0.45 eV compared to that in pure water, and the calculated energy barrier for CO-CO coupling also decreases 0.32 eV in the presence of I ion on a negatively charged copper electrode. The hydrated ions can modulate the distribution of the charge near the solid-liquid interface, which significantly promotes CO₂ reduction and meanwhile impedes the hydrogen evolution reaction. Therefore, our work unveils the significant role of halide ions at the electrode-electrolyte interface for promoting CO₂ reduction on copper electrode.