Driving force dependence of inner-sphere electron transfer for the reduction of CO₂ on a gold electrode

The kinetics of the inner-sphere electron transfer reaction between a gold electrode and CO₂ was measured as a function of the applied potential in an aqueous environment. Extraction of the electron transfer rate constant requires deconvolution of the current associated with CO₂ reduction from the competing hydrogen evolution reaction and mass transport. Analysis of the inner-sphere electron transfer reaction reveals a driving force dependence of the rate constant that has similar characteristics to that of a Marcus-Hush-Levich outer-sphere electron transfer model. Consideration of simple assumptions for CO₂ adsorption on the electrode surface allows for the evaluation of a CO_2,ads/CO₂ ^•- _ads standard potential of ∼-0.75 ± 0.05 V vs Standard Hydrogen Electrode (SHE) and a reorganization energy on the order of 0.75 ± 0.10 eV. This standard potential is considerably lower than that observed for CO₂ reduction on planar metal electrodes (∼>-1.4 V vs SHE for >10 mA/cm²), thus indicating that CO₂ reduction occurs at a significant overpotential and thus provides an imperative for the design of better CO₂ reduction electrocatalysts.