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. Despite its importance, the detailed mechanism and the precise proton source for this elementary step remain elusive and are under intense debate. Density Functional Theory (DFT) calculations have been used to investigate such a mechanism. However, semi-local functionals at the generalized gradient approximation (GGA) level face significant challenges in accurately describing adsorbate-metal interactions and surface stability. In this work, 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 incorporated a surface charging method to account for the influence of the electrochemical potential. Our study reveals that, in neutral or alkaline electrolytes, adsorbed water at the surface acts as the proton source for the reduction of *CO to *COH over a wide range of potentials 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 not only significantly underestimate the reaction barriers for *COH formation but also consistently predict solvent water as the proton source across all the potentials of interest. Additionally, RPA offers distinct insights into H2O adsorption and highlights the significant range of reducing potential within which surface *OH can exist, which is crucial for accurate CO2RR modeling. These potential-dependent thermodynamic and kinetic data illustrate a pronounced divergence between the mechanistic insights offered by RPA and those derived from DFT-GGA functionals. Our findings offer a fresh perspective on proton transfer in CO2RR and establish a framework for future theoretical studies of electrochemical processes.