Electrochemical CO₂ reduction is a potential route to the sustainable production of valuable fuels and chemicals. Here, we perform CO₂ reduction experiments on Gold at neutral to acidic pH values to elucidate the long-standing controversy surrounding the rate-limiting step. We find the CO production rate to be invariant with pH on a Standard Hydrogen Electrode scale and conclude that it is limited by the CO₂ adsorption step. We present a new multi-scale modeling scheme that integrates ab initio reaction kinetics with mass transport simulations, explicitly considering the charged electric double layer. The model reproduces the experimental CO polarization curve and reveals the rate-limiting step to be *COOH to *CO at low overpotentials, CO₂ adsorption at intermediate ones, and CO₂ mass transport at high overpotentials. Finally, we show the Tafel slope to arise from the electrostatic interaction between the dipole of *CO₂ and the interfacial field. This work highlights the importance of surface charging for electrochemical kinetics and mass transport.

Stefan Ringe, Carlos G. Morales-Guio, Leanne D. Chen, Meredith Fields, Thomas F. Jaramillo, Christopher Hahn, Karen Chan