Carbon/air secondary battery (CASB): utilizes carbon as energy source instead of hydrogen; consists of a solid-oxide fuel and electrolysis cell

https://www.eurekalert.org/news-releases/938342

"high Coulombic efficiency of 84%, indicating that most of the stored energy can be obtained during the discharge phase. Furthermore, it showed a superior power density of 80 mW/cm2 and a charge-discharge efficiency of 38% that was sustained over 10 charge-discharge cycles. This suggested that no degradation of the fuel electrode occurred. Compared to hydrogen storage systems, the CASB system is expected to have a smaller system size and higher system efficiency"