CO2 reduction to alcohols in a polymer electrolyte membrane co-electrolysis cell operating at low potentials

Abstract

The electrocatalytic reduction of CO2 to alcohols was investigated in a co-electrolysis cell based on a solid polymer electrolyte. PtRu/C and Ru/C catalysts were used in gas diffusion cathodes for the reduction of humidified CO2. An IrRuOx catalyst was used for the oxygen evolution from liquid water at the anode. This electrochemical reactor employed a perfluorosulfonic acid membrane electrolyte separator (Nafion®) and it was operated in a temperature range from 30°C to 95°C. The cathode catalysts were characterized by well-dispersed metal nanoparticles (2nm mean particle size) on a carbon black support. Electrochemical polarization tests were carried out in the presence of CO2 or with humidified inert gas for comparison. The results evidenced that direct CO2 reaction on the catalysts surface was essentially occurring at low cell voltages (<1.6V), whereas the occurrence of water splitting (hydrogen evolution) was largely prevailing at high cell voltages. Methanol for the PtRu/C catalyst, or a mixture of alcohols (methanol, ethanol and iso-propanol) for the Ru/C catalyst were produced under steady-state chrono-amperometric tests at low cell potentials (1.6V). The productivity of methanol at the PtRu electrode increased significantly with temperature and showed maximum production rate at 1.25V corresponding to a voltage efficiency better than 90%. A critical issue is represented by the parallel hydrogen evolution process, which occurs at significant rates in the overall range of potentials reducing the current efficiency for CO2 conversion.