Capacitive Deionization for Brackish Water Purification Using Asymmetric Charge-Immobilized Activated Carbon With...

Capacitive deionization (CDI) is a promising technology for purifying brackish water into drinking fresh water based on reversible electrosorption of ions into the electrical double layer (EDL). Here, we demonstrate a facile approach to immobilizing charges on activated carbon (AC) electrodes made of a safe, highly hydrophilic binder via spontaneous adsorption of ionic surfactants. The amount of surfactant adsorption is carefully quantified and the pore structure is characterized with Brunauer-Emmett-Teller surface analysis. Electrochemical characterization shows that a large portion of the AC surface is accessible by ions in the aqueous solution, facilitating ion adsorption in CDI processes. The immobilized ionic surfactants regulate the counterions and co-ions in the EDL, behaving like ion-exchange membranes. The stability, charge efficiency, and energy consumption during CDI are substantially improved compared to bare AC films. Importantly, it enables an "inverted CDI mode" with ion adsorption at the open-circuit condition to neutralize the immobilized charges on the electrode surface and ion desorption by an inverted voltage bias. After optimization, a high average salt adsorption capacity of 14.6 mg_NaCl/g_{AC film} was obtained with surfactant-immobilized asymmetric cells in constant current mode for 50 cycles. These results provide new insights toward developing future CDI technologies for water purification.