New insights into the mechanism of flow-electrode capacitive deionization

- Reversible Faradaic reactions result in acid and hydroxide production in flowing anode and cathode.
- Significant amount of ions are found in aqueous phase of the flow electrodes.
- H+ and OH- generation is hampered at low and high pH that reduces the desalination rate.
- Neutralization of pH in flow-electrodes enhances the desalination rate.
- Electrical energy can be produced by reacting acid and hydroxide in FCDI cell.

We report on Faradaic reactions producing H+ (anode) and OH- (cathode) in flow-electrode capacitive deionization (FCDI) operated at 1.2 V. These reactions underline an additional electrodialytical desalination mechanism within capacitive deionization, which proceeds in parallel to the known electrosorption mechanism. Examination of flow-electrodes (100 ml each, 5% (wt) activated carbon) during FCDI (121 cm2 effective membrane area) of 150 ml, 4 g/l NaCl solution revealed that significant amounts of Na+ and Cl− ions (up to 50% and 30% of Cl− and Na+, respectively) were not adsorbed in the activated carbon particles but were rather dissolved in the aqueous phase of the flow-electrodes. Production of acid (resulting in pH ≈ 1.5) and base (pH ≈ 12.5) in the flow-anode and -cathode solutions was observed during the operation. Reverse pH behaviors were obtained during the regeneration of the flow-electrodes by potential reversal. pH neutralization of the flow-electrode solutions resulted in a sharp increase in both the desalination rate and the electric current of the FCDI cell. Reacting NaOH and HCl in a short-circuited FCDI cell resulted in NaCl production in the water compartment and pH neutralization of both flow-electrodes.Apparently reversible Faradaic reactions that occur on the flow electrodes in the FCDI can be dependent on the properties of the carbon material, electrolyte composition and applied operational parameters (e.g. cell potential) and need to be studied in further detailed investigations.