• Energy Research

This work was supported by Rice University. This research used the 8-ID (ISS) beamline of the National Synchrotron Light Source II and the Center for Functional Nanomaterials, US Department of Energy Office of Science User Facilities operated for the DOE Office of Science by Brookhaven National Laboratory under contract no. DE-SC0012704. Q.J. and H.N.A. acknowledge the support from King Abdullah University of Science and Technology (KAUST).

Electrocatalytic CO2 reduction is often carried out in a solution electrolyte such as KHCO3(aq), which allows for ion conduction between electrodes. Therefore, liquid products that form are in a mixture with the dissolved salts, requiring energy-intensive downstream separation. Here, we report continuous electrocatalytic conversion of CO2 to pure liquid fuel solutions in cells that utilize solid electrolytes, where electrochemically generated cations (such as H+) and anions (such as HCOO−) are combined to form pure product solutions without mixing with other ions. Using a HCOOH-selective (Faradaic efficiencies > 90%) and easily scaled Bi catalyst at the cathode, we demonstrate production of pure HCOOH solutions with concentrations up to 12 M. We also show 100 h continuous and stable generation of 0.1 M HCOOH with negligible degradation in selectivity and activity. Production of other electrolyte-free C2+ liquid oxygenate solutions, including acetic acid, ethanol and n-propanol, are also demonstrated using a Cu catalyst. Finally, we show that our CO2 reduction cell with solid electrolytes can be modified to suit other, more complex practical applications. Liquid products from electrocatalytic CO2 reduction are often mixed with additional solutes in the electrolyte, meaning that downstream separation is required. Here, the authors design cells that use solid electrolytes to generate flows of CO2-derived liquid fuels with high concentrations that are free of extraneous ions.