Concerted Proton and Electron Transfer in Heterogeneous Electrocatalytic CO₂ Reduction

Understanding how protons influence heterogeneous electrocatalytic CO₂ reduction on electrode surfaces is critical for advancing energy-efficient carbon conversion technologies. Here, we show that on Ag, Au, and Zn surfaces, a concerted proton-electron transfer (CPET) pathway enables CO₂-to-CO conversion at up to ∼400 mV lower overpotential than commonly proposed cation-stabilized mechanisms, which dominate reactivity at higher overpotentials. Using both positively charged and neutral proton donors, we show that CPET operates at low overpotentials but is limited by the rate of proton supply. Kinetic isotope effect measurements and infrared adsorption spectroscopy support the involvement of protons in the rate-determining step. Furthermore, our data shed light on the complex competition for proton donors between CO₂ reduction, carbonate acidification, and hydrogen evolution, explaining commonly reported product trends. Our findings suggest that enhancing proton flux and suppressing the hydrogen evolution reaction can further promote CPET-based CO₂ reduction, offering a pathway to more efficient electrocatalytic processes.