Promoting Cu-catalysed CO2 electroreduction to multicarbon products by tuning the activity of H2O

The electrochemical reduction of CO2 to valuable C2+ feedstocks is hindered by the competitive formation of C1 products and H2 evolution. Here we tuned the H2O thermodynamic activity between 0.97 and 0.47 using water-in-salt electrolytes to obtain mechanistic insights into the role of H2O in controlling C–C coupling versus C1 product formation on Cu electrodes. By lowering the thermodynamic H2O activity to 0.66, we obtained a Faradaic efficiency of ~73% at a partial current density of −110 mA cm−2 for C2+ products, at modest overpotentials. The adjustment of the thermodynamic H2O activity provided fine control over C2+/C1 ratios, spanning a range from 1 to 20. The trends support the pivotal role of the thermodynamic H2O activity in increasing the CO surface coverages and promoting C–C coupling to C2 products. These findings highlight the potential of tuning thermodynamic H2O activity as a guiding principle to maximize CO2 reduction into highly desirable C2+ products. Copper-based electrocatalysts promote the formation of high-value multicarbon products from CO2, but the process competes with C1 product formation. Now a strategy is presented to tune the activity of water by using water-in-salt electrolytes to increase the C2+/C1 ratio.