CO2 Electrolysis System under Industrially Relevant Conditions.

Abstract

ConspectusCarbon dioxide emissions from consumption of fossil fuels have caused serious climate issues. Rapid deployment of new energies makes renewable energy driven CO2 electroreduction to chemical feedstocks and carbon-neutral fuels a feasible and cost-effective pathway for achieving net-zero emission. With the urgency of the net-zero goal, we initiated our research on CO2 electrolysis with emphasis on industrial relevance.The CO2 molecules are thermodynamically stable due to high activation energy of the two C═O bonds, and efficient electrocatalysts are required to overcome the sluggish dynamics and competitive hydrogen evolution reaction. The CO2 electrocatalysts that we have explored include molecular catalysts and nanostructured catalysts. Molecular catalysts are centered on earth abundant elements such as Fe and Co for catalyzing CO2 reduction, and using Fe catalysts, we proposed an amidation strategy for reduction of CO2 to methanol, bypassing the inactive formate pathway. For nanostructured catalysts, we developed a carbon enrichment strategy using nitrogen-rich nanomaterials for selective CO2 reduction.Direct CO2 electroreduction from the flue gas stream represents the "holy grail" in the field, because typical CO2 concentration in flue gas is only 6-15%, posing a significant challenge for CO2 electrolysis. On the other hand, direct electroreduction of CO2 in the flue gas eliminates the carbon capture process and simplifies the overall carbon capture and utilization (CCU) scheme. However, direct flue gas reduction is frustrated by the reactive oxygen (5-8%), low CO2 concentration (6-15%), and potentially toxic impurities. Surface CO2 enrichment catalysts with high O2 tolerance could be viable for achieving direct CO2 electroreduction for decarbonization of flue gas.In addition to the electrocatalysts, the incorporation of catalysts into the electrolyzer and development of a suitable process was also investigated to meet industrial demands. A membrane electrode assembly (MEA) is a zero-gap configuration with cathode and anode catalysts coated on either side of an ion exchange membrane. We adopted the MEA configuration due to the structural simplicity, low ohmic resistance, and high efficiency. The electrode factors (for example, membrane type, catalyst layer porosity, and MEA fabrication method) and the electrolyzer factors (for example, flow channels, gas diffusion layer) are critical to highly efficient operation. We separately developed an anion-exchange membrane-based system for CO production and cation-exchange membrane-based system for formate production. The optimized electrolyzer configuration can generate uniform current and voltage distribution in a large-area electrolyzer and operate using an industrial CO2 stream. The optimized process was developed with the targets of long-term continuous operation and no electrolyte consumption.