Electrochemical CO 2 reduction is a potential route to the sustainable production of valuable fuels and chemicals. Here, we perform CO 2 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 CO 2 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, CO 2 adsorption at intermediate ones, and CO 2 mass transport at high overpotentials. Finally, we show the Tafel slope to arise from the electrostatic interaction between the dipole of *CO 2 and the interfacial field. This work highlights the importance of surface charging for electrochemical kinetics and mass transport.

Under ambient conditions, copper with oxygen near the surface displays strengthened CO2 and CO adsorption energies. This finding is often used to rationalize differences seen in product distributions between Cu-oxide and pure Cu electrodes during electrochemical CO2 reduction. However, little evidence exists to confirm the presence of oxygen within first few layers of the Cu matrix under relevant experimental reducing conditions. Using density functional theory calculations, we discuss the stability of subsurface oxygen from thermodynamic and kinetic perspectives and show that under reducing potentials subsurface oxygen alone should have negligible effects on the activity of crystalline Cu.