Benchmarking Performance Indices of Electrochemical CO₂ Reduction to Ethylene Based on Prospective Life Cycle Assessment for Negative Emissions

To mitigate global warming to the most ambitious targets, it is necessary to remove CO₂ from the atmosphere and reduce fossil fuels use. The electrochemical conversion of CO₂ to ethylene (C₂H₄) as a basic chemical is a promising technology that meets both requirements; however, its life cycle CO₂ emissions remain inconclusive because of varying assumptions in the performance indices. This study aimed to set benchmarks for the four most sensitive indices to achieve -0.5 t-CO₂/t-C₂H₄ by calculating net greenhouse gas (GHG) emissions through a prospective life cycle assessment of a model system including CO₂ capture, CO₂ enrichment, electrochemical conversion, CO₂ recycling, and cryogenic separation. As a result, the electrochemical conversion process was the hotspot of life cycle emissions, and representative benchmarks were determined as follows: cell voltage, 3.5 V; C₂H₄ Faraday efficiency, 70 %; conversion rate, 20 %; and electrochemical CO₂ recycling energy, 2.2 GJ/t-CO₂. The gaps between the benchmarks and current top data of cell voltage and Faraday efficiency were <10 %, and suppressing the performance degradation for up to one year was found to be a critical requirement. These results can direct research towards the development of a year-round stable system, rather than further improving the performance indices.