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How should flow electrode capacitive deionization (FCDI) be operated to achieve efficient desalination and scalability?
Publication . Saif, H. M.; Crespo, J. G.; Pawlowski, S.; DQ - Departamento de Química; LAQV@REQUIMTE; Instituto de Tecnologia Química e Biológica António Xavier (ITQB); Elsevier
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.
Deep eutectic solvent flow electrodes for high-voltage desalination via flow electrode capacitive deionisation
Publication . Gabirondo, Elena; Saif, Hafiz M.; Alves, Vitor D.; Crespo, João G.; Tomé, Liliana C.; Pawlowski, Sylwin; LAQV@REQUIMTE; DQ - Departamento de Química; Instituto de Tecnologia Química e Biológica António Xavier (ITQB); Elsevier
This study pioneers the application of deep eutectic solvents (DES) as electrolytes in flow electrode capacitive deionisation (FCDI) desalination systems, providing a novel and improved alternative to aqueous flow electrodes. The deep eutectic solvent, choline chloride-urea (ChCl-U), was selected for its wide electrochemical stability window, allowing voltages exceeding 1.23 V, which is the limit for aqueous flow electrodes. The effect of water doping on the viscosity and performance of the DES flow electrodes was also investigated. Cyclic voltammetry confirmed the electrochemical stability, while rheological and electrochemical impedance spectroscopy revealed that the addition of water reduced the viscosity and enhanced the conductivity of ChCl-U, making it suitable for use as an electrolyte in FCDI. Desalination experiments were performed within a potential range of up to 2.2 V. The ChCl-U flow electrode, containing 20 wt% water and 10 wt% activated carbon, achieved the best balance between desalination efficiency (83 %), desalination rate (0.17 mg/cm2.min), and effluent quality. Furthermore, 1H NMR analysis confirmed the absence of traces of the deep eutectic solvent in the dilute stream. The results highlight the potential of DES flow electrodes to enhance desalination processes by enabling higher operational voltages and improved performance, thereby paving the way for more efficient FCDI desalination systems.
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Fundação para a Ciência e a Tecnologia
Programa de financiamento
3599-PPCDT
Número da atribuição
PTDC/EQU-EQU/6193/2020