Efficient Electroreduction of CO2 in an Ultra-Slim Pressurized Electrolyzer

Abstract

Society’s consumption of fossil fuels has caused a surge in atmospheric carbon dioxide (CO2) concentrations which, if left unchecked, will have tremendous implications for all life on our planet. Consequently, new approaches are being investigated to utilize CO2 instead of releasing it into the atmosphere. One of these approaches is the electrochemical CO2 reduction reaction (CO2RR) which can store intermittent renewable energy in the form of valuable chemicals and fuels. To encourage the implementation of industrial CO2RR systems, such systems must demonstrate economic benefits in conjunction with environmental benefits. To make such systems economical, electricity inputs, the single largest operating cost in these systems, must be minimized. A great deal of recent work has been devoted to optimizing the cathode catalyst and electrolyte to attain high selectivity towards specific products at low overpotentials. However, few studies optimize the full cell design, which includes the cathode, anode, ion exchange membranes, and electrolytes to demonstrate an efficient CO2RR process. These additional full cell elements require some consideration when designing industrial systems as improved cathode energetics may be overshadowed by increased losses in other areas of the system resulting from the cathode optimization. Here we present an ultra-slim pressurized electrolyzer whose membrane-free operation with minimized electrode spacing yields negligible cell resistances. To showcase this cell design, we employ a silver catalyst for carbon monoxide (CO) generation at the cathode coupled with a nickel anode for oxygen evolution on the anode. In conjunction with the low cell resistances, we demonstrate further improvements in the uncorrected full cell energetic efficiency. First, we show that operation at elevated pressures, up to 51 bar, offers benefits to the reaction rates and helps to maintain high CO selectivities, resulting in energetic efficiency improvements. We then couple these high pressures with high alkalinity to demonstrate even greater improvements in energetic efficiencies due to improved kinetics. The system presented demonstrates substantial improvements in full cell energetic efficiency, enabling efficiencies > 65% to be attained while operating at industrially relevant current densities above 100 mA/cm2.