Elucidating the Proton Source for CO₂ Electro-Reduction on Cu(100) Using Many-Body Perturbation Theory

The protonation of CO is recognized as the rate-determining step in the generation of C1 products during the electrochemical CO₂ reduction reaction (CO₂RR) on Cu surfaces. However, the detailed mechanism and the precise proton source remain elusive. While density functional theory (DFT) calculations at the GGA level have been widely used, they struggle to accurately describe adsorbate-metal interactions and surface stability. Here, we employed the Random Phase Approximation (RPA), a method based on many-body perturbation theory, to overcome these limitations. We coupled the RPA framework with the linearized Poisson-Boltzmann equation to model solvation effects and a surface charging method to account for the influence of the electrochemical potential. Our study reveals that in neutral or alkaline electrolytes, adsorbed surface water acts as the proton source for *CO reduction to *COH over a broad potential range via the Grotthuss mechanism. At highly negative potentials, solvent water becomes the primary proton donor, with multiple competing mechanisms observed. In contrast, DFT-GGA functionals significantly underestimate the reaction barriers for *COH formation and consistently predict solvent water as the proton source across all potentials of interest. Additionally, RPA offers distinct insights into H₂O adsorption and highlights the significant range of reducing potentials within which surface *OH can exist, which is crucial for accurate CO₂RR modeling. These insights illustrate a pronounced divergence between RPA and DFT-GGA results. Our findings offer a fresh perspective on proton transfer in CO₂RR and establish a framework for future theoretical studies on electrochemical processes.