Promoting Cu-catalysed CO 2 electroreduction to multicarbon products by tuning the activity of H 2 O

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