Researchers at the Tokyo Institute of Technology have developed CASB (secondary carbon / air battery), an alternative electrical energy storage system that uses carbon (C) instead of hydrogen.
Detailed in Journal of energy sources, CASB consists of a solid oxide fuel and an electrolysis cell (SOFC / EC) where carbon is generated by electrolysis of CO2 is oxidized with air to produce energy. SOFC / EC can be supplied with compressed liquefied CO2 to constitute the energy storage system, the team said.
âLike a battery, the CASB is charged using energy generated by renewable sources to reduce CO2 to C. During the next discharge phase, the C is oxidized to generate energy, âsaid Professor Manabu Ihara of Tokyo Tech.
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As carbon is stored in a confined space in SOFC / EC, the energy density of CASB is limited by the amount of carbon it can contain. Despite this, the researchers found that CASB had a higher volumetric energy density than hydrogen storage systems.
During charge-discharge experiments, the team observed that the transformations between C and CO2 were due to Boudouard reactions characterized by a redox reaction of a mixture of carbon monoxide (CO), CO2 and C. More precisely, during the charging phase, C was deposited on the electrode via the electrochemical reduction of CO2 and the reduction of CO via the Boudouard decomposition. During the discharge phase, the C was oxidized to CO and CO2 via the Boudouard gasification reaction and electrochemical oxidation, respectively. The researchers found that the use of C for the energy production of the CASB depended on the balance between C, CO2, CO, which is known as Boudouard’s equilibrium.
According to Tokyo Tech, the CASB system used most of the carbon deposited on the electrode for power generation, demonstrating a Coulomb efficiency of 84%, indicating that most of the stored energy can be obtained during the phase discharge. It also showed a power density of 80mW / cm2 and a charge-discharge efficiency of 38% which was maintained over 10 charge-discharge cycles. This suggests that no degradation of the fuel electrode has occurred.
âCompared to hydrogen storage systems, the CASB system is expected to have a smaller system size and higher system efficiency,â Professor Ihara said. Their new system could lay the groundwork for compact and efficient carbon energy storage systems that could work with renewable energy sources, the team added.