Electrochemical CO2 reduction is an attractive option for storing renewable electricity and for the sustainable production of valuable chemicals and fuels. In this roadmap, we review recent progress in fundamental understanding, catalyst development, and in engineering and scale-up. We discuss the outstanding challenges towards commercialization of electrochemical CO2 reduction technology: energy efficiencies, selectivities, low current densities, and stability. We highlight the opportunities in establishing rigorous standards for benchmarking performance, advances in in operando characterization, the discovery of new materials towards high value products, the investigation of phenomena across multiple-length scales and the application of data science towards doing so. We hope that this collective perspective sparks new research activities that ultimately bring us a step closer towards establishing a low- or zero-emission carbon cycle.

CO2 electrolyzer designed to operate on dilute CO2 feed and low stoichiometric ratio would alleviate the separation costs for CO2 purification and electrolyzer exit gas processing, respectively. The effect of CO2 concentration, CO2 flow rate, and CO2 pressure on current density and faradaic efficiency of a solid polymer electrolyte membrane CO2 electrolyzer was quantified. An approach for estimating voltage breakdown into activation overpotential for CO2 reduction reaction as well as oxygen evolution reaction, ohmic losses, and concentration overpotential is introduced. No enhancement in current density (~160 mA/cm2) was observed above stoichiometry ratio of 4 whereas reducing the stoichiometric ratio to 2.7 still yielded a current density of ~100 mA/cm2. Dilution of CO2 in the feed from 100 mol% to 30mol%, at ~90kPa of cell pressure, resulted in a monotonically decreasing current density. A square root dependency on CO2 partial pressure was observed under these conditions. Operation with pure CO2 at different total pressure yielded only a minor increase in current density indicating some form of saturation-limited behavior. Long-term potentiostatic operation over 85 hours revealed continuous drop in current density and a corresponding increase in electrode resistance, observed in electrochemical impedance response.