Electrochemical CO₂ conversion technologies: state-of-the-art and future perspectives

Electrochemical reduction of CO2 to produce chemicals or fuels may contribute to the zero-emission goal of the chemical industry. Here, we report the state-of-the-art and future perspective of electrochemical CO2 conversion processes to produce CO, syngas, formic acid and ethylene. We selected and explored six routes: low-temperature CO production, low-temperature formic acid production, low-temperature ethylene production, high-temperature CO production, high-temperature syngas production, and a tandem approach to produce ethylene. For these routes, we describe the current level of development, performance indicators, and costs. The state-of-the-art of the chlor-alkali process is included as an example of a commercially applied electrochemical process. We calculate the economic performance of the various pathways in terms of levelized production costs and we use a learning curve method to project costs up to 2050. The greenhouse gas performance for all routes is determined and compared to the current reference of production from fossil-based resources. We conclude that high-temperature solid-oxide electrolysis to produce CO and syngas is the most developed and closest to reaching break-even levelized production cost in comparison to the fossil reference. Low-temperature electrolysis processes are at a lower technology readiness level and still need a substantial reduction in investment costs and improvements in process efficiency to achieve break-even with incumbent technology. The most promising of the low-temperature processes is formic acid production. Electrochemical production of formic acid, CO, and syngas results or can soon result in substantial GHG savings compared to their fossil-based alternatives. The extent to which savings can be achieved depends merely on the carbon intensity of the local power grid, or more generally, the supplied electricity. Electrochemical CO2 conversion to produce ethylene would require a very low emission factor of electricity (<50 gCO2 per kW h) to be competitive with current production methods and is therefore not likely to contribute significantly to the zero-emission goal of the petrochemical industry in the foreseeable future. Research gaps are identified at various levels: improvement of the performance of the various components, such as catalysts and electrodes, and of purification of feedstock and product streams. Pilot and demonstration projects of the entire value chain from the CO2 stream to the final product are needed to more accurately determine the performance, total investment costs, and operating and maintenance costs in an industrial environment.