Promoting Cu Catalyzed CO2 electroreduction to multi-carbon products by tuning the activity of water

The electrochemical reduction of CO2 to multi-carbon products is a sustainable route for the synthesis of energy dense chemical feedstocks. Cu is the only material known to produce multi-carbon (C2+) products with appreciable selectivity. However, the generation of C2+ products compete with the formation of C1 products and the reduction of water to hydrogen. Here, we tuned the activity of water by using a NaClO4-based water-in-salt-electrolyte with concentrations ranging from 1m to 17m, enabling the activity of water to be tuned from 0.97 to 0.47. Commercial Cu-nanoparticle electrodes evaluated in pH 9 NaClO4 with a water activity of 0.66 achieved a Faradaic Efficiency of ~ 73% for C2+ products (ethylene, ethanol, and propanol) with a C2+ partial current density of −110 mA cm-2 at –0.88 vs. Reversible Hydrogen Electrode. Furthermore, we were able to tune the C2+/C1 ratios between 1 to 20 by altering only the water activity, demonstrating unrivalled tunability between C1 and C2+ products. Analysis of the Tafel slopes and reaction orders on model Cu electrodes revealed that the mechanism for forming C2 products was unchanged across a wide range of water activities, while C1 products and H2 had mechanisms which changed as the activity of water was lowered. We have demonstrated that tuning the activity of water in an aqueous solvent is a powerful new guiding principle for improving the reduction of CO2 to C2 and C3 products.