Flow electrode capacitive deionization with iron-based redox electrolyte

Abstract

Flow electrode capacitive deionization (FCDI) employs carbon-based materials for electrochemical separation. Redox additives like vanadium, ferricyanide, and organic molecules have boosted desalination rates in FCDI. However, these additives must be low-cost, have chemical stability in electrolytes, and have low toxicity. We investigate two iron-based redox couples, iron chloride, and iron citrate, for their cost-effectiveness and reduced health risks. We incorporate them with activated carbon (AC) in flow electrodes, resulting in a significant enhancement of the salt removal rate, with iron chloride increasing it by 100 % and iron citrate by 23 % compared to standard AC slurries. In our system, the AC exhibits a salt removal rate of 7.8μmolcm−2min−1, while the iron chloride system reaches 15.6μmolcm−2min−1. Iron citrate flow electrodes achieve a desalination rate of 9.6μmolcm−2min−1, maintaining a pH suitable for potable water. We observe that iron chloride concentration in the flow electrode influences iron crossover and pH in the water streams, leading to acidity. Conversely, the iron citrate-based slurry maintains minimal iron crossover and a neutral pH. This study highlights iron-based redox additives as promising for FCDI slurries and the need for tailored flow electrode composition based on specific ion removal requirements.