Efficient Aqueous Electroreduction of CO₂ to Formate at Low Overpotential on Indium Tin Oxide Nanocrystals

Electroreduction of CO2 to formate powered by renewable energy offers an alternative pathway to producing carbon fuels that are traditionally manufactured using fossil fuels. However, achieving simultaneously high partial current density (jHCOO–), high product selectivity (Faradaic efficiency (FEHCOO–)), and low overpotentials (η) remains difficult due to the lack of suitable catalysts. Here, we report the electroreduction of CO2 on Sn-doped indium oxide (ITO) nanocrystal catalysts in an alkaline flow electrolyzer. Colloidally synthesized monodisperse 20 nm ITO nanocrystals (NCs) with various Sn-doping levels (0, 1, 5, 6.5, 8, and 12 atom %) were studied. We find that ITO NC catalysts exhibit a high selectivity for production of HCOO– over CO and H2 (approximately 87% HCOO–, 1–4% CO, and 2–6% H2 at −0.85 V vs RHE), an onset potential for HCOO– as early as −0.21 V vs RHE, and a high partial current density for HCOO– up to 171 mA/cm2 at a cathode potential of −1.08 V vs RHE. The main difference between the catalysts’ performances resides in the onset potential for formate production. The onset of formate production occurred at cell and cathode overpotentials of only −440 and −143 mV, respectively, by the 12% ITO. Analysis of the ITO electrodes before and after electrolysis suggests that no changes in surface composition, crystal structure, or particle size occur under the reduction conditions. Tafel slopes of ITO NC catalysts range from 27 to 52 mV per decade, suggesting that the rate-determining step is likely the proton-coupled electron transfer to CO2●–* to form HCOO–*.