Enhanced operating voltage and desalination performance using ionic liquids as electrolyte for flow electrode capacitive deionisation

Abstract

Flow-electrode capacitive deionisation (FCDI) technology is a promising approach for desalinating brackish water. Selecting the electrolyte with a high voltage window is essential for improving the FCDI performance. In this study, ionic liquids were used for the first time as a new electrolyte for FCDI systems. The water-desalting efficiency was selected as the evaluation index, and single-factor experiments were conducted on the activated carbon content of electrode slurry, brine flow rate, initial brine concentration, and electrolyte volume ratio, respectively. Box-Behnken design response surface experiments were used and models were constructed to analyse the desalting results of the single factor influence and interaction effects for the aqueous and ionic liquid systems respectively, to determine further the key influencing factors and optimal conditions for the brine desalination in the FCDI system. The results showed that the brine flow rate was the most significant factor affecting the desalination efficiency of the aqueous and ionic liquid FCDI systems (p < 0.0001). The optimal process conditions for the ionic liquid system fitted by the model were as follows: 6.6 wt% activated carbon content in the electrode slurry, a brine flow rate of 45 mL/min, an initial brine concentration of 1 g/L, and an electrolyte ratio of N, N-Dimethylformamide/1-ethyl-3-methylimidazolium tetrafluoroborate = 3. The desalination effect of FCDI under the above optimal process conditions was up to 99.739% at a voltage of 3.5 V, and the water-desalting efficiency was still maintained at over 95% after 20 cycles. Compared with the aqueous electrolyte, using ionic liquid electrolytes significantly improves the desalination rate and cycling stability, and this study provides new ideas and methods for developing and applying high-performance electrolytes in FCDI systems.