CO₂ Electroreduction with Enhanced Ethylene and Ethanol Selectivity by Nanostructuring Polycrystalline Copper

There are a number of recent reports on the use of oxidation/reduction cycling of Cu surfaces to improve their selectivity for ethylene formation in the aqueous CO₂ reduction reaction. Here, the oxidation/reduction process is examined in detail. It is found that the faradaic efficiencies for both ethylene and ethanol are enhanced after oxidation/reduction cycling in the presence of halide anions. Specifically, cycling of the electrode in the presence of chloride, bromide, or fluoride anions allows for an ethylene faradaic efficiency of approximately 15.2 %, a factor of 1.5 higher than that for polycrystalline copper (at −1.0 V vs. RHE). The faradaic efficiency for ethanol is also enhanced from 2.65 to approximately 7.6 %. The effects of electrochemical oxidation/reduction with the chloride anion were investigated by using in situ Raman spectroscopy, and the changes in the surface morphology of copper were monitored by using SEM. Consistent with prior reports, it is observed that during the oxidation part of the cycle, anodic corrosion forms a Cu₂O layer, which consists of cubical crystals of about 150 nm. During the reduction sweep, it is converted to metallic copper, which forms irregular Cu nanoparticles of around 20 nm in diameter. The enhancement in ethylene formation is presumably attributed to the formation of grain boundaries, which may serve as active sites.