How should flow electrode capacitive deionization (FCDI) be operated to achieve efficient desalination and scalability?

Flow electrode capacitive deionization (FCDI) is an emerging desalination technology that utilises flowable electrodes and can be operated in diverse configuration modes. This study provides a systematic assessment of the three main configuration arrangements under a voltage range between 0.8 and 2.0 V: isolated closed-cycle (ICC), short-circuited closed-cycle (SCC), and single-cycle with separate concentrate chamber (SCSC). The ICC mode shows the highest specific energy consumption (up to 72.02 Wh/mol of NaCl at 2.0 V) and low operational stability manifested by extreme alteration of pH in the electrode compartments (anode compartment pH down to 2.17; cathode compartment pH up to 12.08), which leads to the need for frequent electrode regeneration or replacement. In comparison to the ICC mode, the SCC mode exhibited superior performance, with a 44.3 % increase in salt removal and up to 3.95 % higher current efficiency at 2.0 V, due to the regeneration of electrodes through short-circuiting, as it reduces the electrical resistance and minimises the side reactions. The SCSC mode emerged as the most stable and reliable among the three, with uniform current and conductivity profiles, as well as minimal pH fluctuations, which is critical to produce treated water within desired quality standards. These findings highlight the promising potential of SCSC mode as an optimal configuration for scalable, continuous and energy-efficient FCDI systems, providing a balanced solution for long-term desalination with reduced operational complexity and costs.