One of the barriers to generating electricity from wind and solar energy is their intermittent nature. A promising alternative to accommodate the fluctuations in power output during unfavorable environmental conditions are hydrogen storage systems, which use hydrogen produced from water splitting to generate clean electricity. On the flip side of the coin, such systems suffer from poor efficiency and often need to be large in size to compensate for it. This, in turn, makes for complex thermal management and a lowered energy and power density.
With this in mind, researchers from Japan’s Tokyo Institute of Technology (Tokyo Tech) have proposed an alternative electric energy storage system that utilizes carbon as an energy source instead of hydrogen. The new “carbon/air secondary battery (CASB)” consists of a solid-oxide fuel and electrolysis cell (SOFC/ECs) where carbon generated via electrolysis of carbon dioxide (CO2) is oxidized with air to produce energy. The SOFC/ECs can be supplied with compressed liquefied CO2 to make up the energy storage system.
“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 oxidized to generate energy,” explains Prof. Manabu Ihara from Tokyo Tech.
The researchers tested both the energy density and charge-discharge efficiency of the system. While the energy density of the CASB is limited by the amount of carbon it can hold, the researchers found that the CASB had a higher volumetric energy density compared to hydrogen storage systems.
In addition, the researchers performed a charge-discharge experiment. They 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.
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. And 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 C utilization for energy generation of the CASB depended on the equilibrium between the three different carbon species (C, CO2, CO), also known as the “Boudouard equilibrium.”
The CASB system was able to utilize most of the carbon deposited on the electrode for energy generation, demonstrating a high Coulombic efficiency of 84%. Additionally, it showed a superior power density of 80 mW/cm2 and a charge-discharge efficiency of 38% that was sustained over ten charge-discharge cycles. Compared to hydrogen storage systems, the CASB system is expected to have a smaller system size and higher system efficiency.
The researchers will continue to improve and develop the system to increase efficiency. Their new system could lay the foundation for compact and efficient carbon energy storage systems that could work alongside renewable energy sources for a fossil-fuel-free future.
