Energy Recovery in Capacitive Deionization and the Role of Biomass- and Waste-Derived Carbon Electrodes: A Mini Review

Capacitive deionization (CDI) is an electrochemical water treatment technology in which cyclic charging and regeneration create opportunities for electrical energy recovery. In parallel, biomass- and waste-derived carbons are increasingly investigated as sustainable CDI electrodes because their pore structure, conductivity, capacitance, and surface chemistry can be tailored while valorizing low-value feedstocks. This mini-review integrates these two research directions by linking energy-accounting definitions, recovery architectures, system boundaries, and material descriptors. Fundamental energy metrics and loss pathways are summarized, followed by direct cell-to-cell transfer, converter-assisted recovery, intermediate storage, and configuration-specific strategies. Sustainable carbon electrodes are then assessed in terms of ion-accessible porosity, conductivity, internal resistance, reversible charge storage, electrode thickness, and preparation burden. Direct measurements of electrical energy recovery remain scarce for biomass- and waste-derived CDI electrodes; therefore, BET area, capacitance, salt adsorption capacity, and impedance should be treated as indirect descriptors rather than proof of recovery performance. Future progress requires standardized cell- and system-level energy accounting, lower-energy electrode preparation, long-term reusability, renewable-power integration, intelligent process control, selective resource recovery, and combined techno-economic and life-cycle assessment. These priorities can connect waste valorization, water treatment, resource recovery, and electrical energy reuse within a more rigorous sustainable CDI framework.