Thermodynamic Evaluation of Energy Storage and Recovery in Capacitive Deionization with Synergistic Faradaic Double Layer Electrodes

Capacitive deionization (CDI) is increasingly regarded as an energy-efficient desalination technology, and numerous studies have explored transition-metal oxide-carbon composites as pseudocapacitive CDI electrodes, typically reporting salt adsorption capacities of about 8–15 mg/g and energy-recovery efficiencies of 60–80%. However, most previous studies provide limited insight into how the supplied electrical energy is partitioned into recoverable and dissipated components during CDI operation, and a unified thermodynamic framework remains lacking. In this work, Mn3O4@C, Co3O4@C, and Fe3O4@C core-shell nanoparticles were incorporated into activated carbon to fabricate Mn3O4@C/AC, Co3O4@C/AC, and Fe3O4@C/AC composite electrodes. Under identical CDI conditions of 3000 mg/L NaCl and an applied charging voltage of 0.6 V, AC, Mn3O4@C/AC, Co3O4@C/AC, and Fe3O4@C/AC exhibited salt adsorption capacities of 6.7, 12.5, 10.8, and 9.3 mg/g, respectively. Under the same operating conditions, the three composite electrodes exhibited energy storage efficiencies of approximately 84 to 89%, compared with 54% for AC, with Mn3O4@C/AC showing the highest energy storage performance. Representative GCD measurements of Mn3O4@C/AC and open-circuit voltage-retention measurements of Mn3O4@C/AC and Co3O4@C/AC provided supporting operational information, whereas the cross-electrode energy metrics were derived from a common CDI charging and discharging protocol applied to all electrodes. Energy analysis showed that increasing the charging voltage systematically decreased energy storage efficiency because a smaller fraction of the supplied electrical energy was recovered during discharge. These findings provide a thermodynamic basis for designing energy-recoverable CDI systems with potentially lower net electrical energy demand.