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.