Promoting Cu Catalyzed CO2 electroreduction to multi-carbon products by tuning the activity of H2O

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 H2O to hydrogen. Here, we tuned the activity of H2O from 0.97 to 0.47 by using a NaClO4-based H2O-in-salt-electrolyte. Commercial Cu-nanoparticle electrodes evaluated in pH 9 NaClO4 with a H2O 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 modulate the C2+/C1 ratios between 1 to 20 by altering only the H2O activity, demonstrating unrivaled 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 H2O activities, while C1 products and H2 had mechanisms which changed as the activity of H2O was lowered. Therefore, we can conclude that protons are part of the rate-determining step for the formation of C1 products, but not C2 products. We have demonstrated that tuning the activity of H2O in an aqueous solvent is a powerful new guiding principle for improving the reduction of CO2 to C2 and C3 products.