CASB system uses carbon as energy source

Detailed in Journal of Power Sources, the CASB is said to consist of a solid-oxide fuel and electrolysis cell (SOFC/ECs) where carbon generated via electrolysis of CO₂ is oxidsed with air to produce energy. The SOFC/ECs can be supplied with compressed liquefied CO₂ to make up the energy storage system, the team said.

“Similar to a battery, the CASB is charged using the energy generated by the renewable sources to reduce CO₂ to C. During the subsequent discharge phase, the C is oxidised to generate energy,” said Prof. Manabu Ihara from Tokyo Tech.

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As the carbon is stored in a confined space in the SOFCs/ECs, the energy density of the CASB is limited by the amount of carbon it can hold. Despite this, the researchers found that the CASB had a higher volumetric energy density compared to hydrogen storage systems.

During charge-discharge experiments the team observed that the transformations between C and CO₂ were due to Boudouard reactions characterised by a redox reaction of a mixture of carbon monoxide (CO), CO₂ and C. Specifically, 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 oxidised to CO and CO₂ via the Boudouard gasification reaction and electrochemical oxidation respectively. The researchers found that the C utilisation for energy generation of the CASB depended on the equilibrium between C, CO₂, CO, which is known as the Boudouard equilibrium.