Abstract
The electrosynthesis of ethylene (C2H4) in membrane-electrode-assembly (MEA) cells with high selectivity and large current density is a promising flow process strategy for the conversion of CO2 or CO into valuable products. Although considerable progress has been made in meeting industrial requirements in terms of Faradaic efficiency (FE) and formation rate, selectivity towards the formation of a single type of multi-carbon (C2+) product has not been demonstrated to date. Here, we evaluated a library of aryl diazonium salts to functionalize Cu catalysts in order to elucidate the influence of Cu valence on the formation of multi-carbon products during the CO2RR. By combining density functional theory (DFT) calculations with operando Raman and X-ray absorption spectroscopy (XAS), we identified the role of the surface oxidation state of Cu δ+ with 0<δ<1 on the selectivity and the formation rate of C2H4. As a result, we report a FE and a specific current density for C2H4 as large as 83±2% and 212 mA cm -2 , respectively on partially oxidized Cu 0.26+ . This corresponds to an energy efficiency of 26.9% and an electrical power consumption (EPC) of 61.4 kWh N -1 m -3 . The conversion performance was further improved by using a perfluorinated sulfonic acid (PFSA) ionomer to reach a record-high FE for C2H4 of 89±3% at a specific current density of 536 mA cm -2 . When coupled with an Ag-based MEA cell to generate CO from CO2 in a cascade flow process, an energy efficiency of ~40 % with a FEC2H4 of 86± 2% was achieved, corresponding to a record low EPC of 25.6 kWh N -1 m -3 .