An alternative to the complex, low efficiency hydrogen storage systems under development to use electrolytic hydrogen for clean power production has been proposed by Tokyo Institute of Technology researchers. A carbon/air secondary battery (CASB) is being advanced as an electric energy storage system that consumes C instead of hydrogen.

C generated via electrolysis of carbon dioxide in the solid-oxide fuel and electrolysis cell is oxidized with air to produce energy. The system can be supplied with compressed liquefied CO₂ to make up the energy storage system. According to its developers, the CASB is charged using energy generated by renewable sources to reduce CO₂ to C. During the subsequent discharge phase, C is oxidized to generate energy. The energy density of the CASB is limited by the amount of C it can hold in a confined space, but the system offers a higher volumetric energy density compared to hydrogen storage systems.

Source: Tokyo Institute of Technology

During charge-discharge tests, the transformations between C and CO₂ were observed to be due to Boudouard reactions characterized by a redox reaction of a mixture of carbon monoxide (CO), CO₂ and C. C was deposited on the electrode via the electrochemical reduction of CO₂ and the reduction of CO via the Boudouard decomposition during the charging phase. C was then oxidized to CO and CO₂ during discharge via the Boudouard gasification reaction and electrochemical oxidation respectively.

The CASB system exploited most of the C deposited on the electrode for energy generation, demonstrating a high Coulombic efficiency of 84% and indicating that most of the stored energy can be obtained during the discharge phase. Results reported in the Journal of Power Sources confirm a power density of 80 mW/cm² and a charge-discharge efficiency of 38% that was sustained over 10 charge-discharge cycles, indicating no degradation of the fuel electrode.