(Invited) Electrochemical CO2 Reduction to Chemical Feedstocks: System Capacity and Specifications for Carbon Neutrality
Authors: Sugiyama Masakazu , Yamaguchi Shingi , Ebe Hiroji , Kumagai Hiromu , Minegishi Tsutomu 
Published: February 2024

The transition of an energy system that realizes net-zero CO2 emission requires both energy saving and electrification of energy demands, coupled with the use of CO2-free fuel such as hydrogen and synthetic hydrocarbons derived from captured CO2. In addition, carbon feedstocks for chemical products must originate from biomass, recycled plastics, and captured carbon from both factory effluent and air. In this regard, the electrochemical reduction of CO2 to chemical feedstocks is a promising and indispensable technology for the realization of carbon neutrality.

To secure the social implementation of this technology, it is important to quantify the amount of CO2 that needs to be processed by a back-casting approach from an entire picture of a carbon-neutral energy system. Furthermore, the electricity input for CO2 reduction must be counted in reference to the total electricity demand of the energy system. The improvement in the energy efficiency of CO2 reduction is of crucial importance because the large amount of CO2 that needs to be processed may require electricity that exceeds the total electricity demand for direct use. When the CO2 footprint on electricity is not zero, the intensive use of electricity for CO2 reduction will result in the system with positive CO2 emission: the amount of CO2 converted into chemical feedstocks will be smaller than the emitted CO2 upon the generation of electricity which is necessary for electrochemical reduction.

Therefore, it is significant to think about the role of electrochemical CO2 reduction from the viewpoint of the energy transition scenario towards net-zero CO2 emission and to analyze the process in terms of life-cycle assessment (LCA). In this presentation, we will focus on the net-zero energy scenario in Japan as an example and will quantify the amount of CO2 that needs to be either sequestered or, more hopefully, converted to chemical feedstocks. Our scenario analysis indicates that the electricity demand in Japan that realizes net-zero CO2 emission will be ca. 1500 TWh, which is ca. 1.5 times larger than the existing value due to the electrification of the existing fossil fuel usage. The amount of CO2 emission will be 84 million tons, which is ca. 8 % of the existing value. As an ideal case, the total amount, 84 million tons of CO2, is assumed to be converted to ethylene by electrochemical reduction rather than being sequestered into the ground. Then, ca. 700 TWh of electricity is required in addition to the electricity demand for direct use, 1500 TWh. The electricity demand for CO2 electrochemical reduction depends on system specifications. Here, the operation voltage of 3 V is assumed.

The LCA of a CO2 electrochemical reduction system clarifies the specifications for negative CO2 emission from the system, which is a prerequisite of CO2 recycling. The life-cycle CO2 emission is dependent on the system configuration, and we found that CO2 recycling from a CO2 electrolysis reactor is of crucial importance for the realization of negative CO2 emission. With such a configuration, the most impactful factors on the CO2 balance (fixation versus emission) are the operation voltage and the Faradaic efficiency of CO2 reduction. For negative CO2 emission, they must be below 4 V and above 50%, respectively, assuming the CO2 footprint of electricity as 32 g/kWh corresponding to the existing level for renewable electricity. For 1.5 tons of CO2 fixation upon 1 ton of ethylene production, they must be below 3 V and above 80%, respectively. This analysis highlights the importance of setting targets for research and development from the viewpoint of LCA.