Electroreduction of CO₂ to Formate on a Copper-Based Electrocatalyst at High Pressures with High Energy Conversion Efficiency

Electrocatalytic CO₂ reduction (CO₂RR) to valuable fuels is a promising approach to mitigate energy and environmental problems, but controlling the reaction pathways and products remains challenging. Here a novel Cu₂O nanoparticle film was synthesized by square-wave (SW) electrochemical redox cycling of high-purity Cu foils. The cathode afforded up to 98% Faradaic efficiency for electroreduction of CO₂ to nearly pure formate under ≥45 atm CO₂ in bicarbonate catholytes. When this cathode was paired with a newly developed NiFe hydroxide carbonate anode in KOH/borate anolyte, the resulting two-electrode high-pressure electrolysis cell achieved high energy conversion efficiencies of up to 55.8% stably for long-term formate production. While the high-pressure conditions drastically increased the solubility of CO₂ to enhance CO₂ reduction and suppress hydrogen evolution, the (111)-oriented Cu₂O film was found to be important to afford nearly 100% CO₂ reduction to formate. The results have implications for CO₂ reduction to a single liquid product with high energy conversion efficiency.