Highly efficient CO₂ utilization in high-temperature, high-pressure water environment: Focusing on unused waste heat as a step toward realizing a carbon circulation society

To achieve carbon neutrality in the chemical industry, efforts are underway to use raw materials other than fossil resources. In particular, the electrochemical CO₂ reduction reaction (CO₂RR), which directly converts CO₂ into chemical raw materials using electricity derived from renewable energy sources, has attracted attention as a radical method for decreasing the CO₂ concentration in the atmosphere. However, the energy efficiency of CO₂RR is currently low, and increasing this efficiency has been a major challenge for its practical applications.

A research group including Dr. Takaaki Tomai, Professor of the Frontier Research Institute for Interdisciplinary Sciences at Tohoku University, increased the efficiency of the CO₂RR process utilizing a high-temperature, high-pressure water environment (hydrothermal reaction field). In general, industrial water electrolysis is performed at high temperatures of up to 100℃ because electrochemical reactions accelerate at higher temperatures. However, CO₂ solubility in water and energy efficiency of the CO₂RR process decrease under excessively high-temperature conditions. The research group overcame this problem by using a high-pressure environment of 100 atm and higher. They demonstrated that a hydrothermal environment of 150℃ temperature and 100-atm pressure notably increases the energy efficiency of the CO₂RR process by increasing CO₂ supply to the electrodes via increasing CO₂ solubility and diffusion coefficient value.

The research group also focused on the use of unused low-temperature waste heat released from factories and power plants as an energy source to create a high-temperature environment. Through a technology assessment, they also demonstrated the possibility of "carbon-negative" methanol production, in which CO₂ absorption exceeded its emission. This result, achieved by incorporating a chemical engineering approach into the electrochemical process, will drive the chemical industry toward realizing a circulation society in which CO₂ is converted into a highly efficient resource. Electrolysis technology utilizing hydrothermal reaction fields will be important in the next-generation sustainable, recycling-oriented society.