Role of embedding choline chloride-urea deep eutectic solvent on biomass-derived porous activated carbon in its capacitive deionization performance

Capacitive deionization (CDI) is a propitious desalination technique for medium and large-scale applications. The CDI cell performance is primarily controlled by the electrode material characteristic. Beyond the classic electro-sorption phenomenon, assistive separation mechanisms may be incorporated through the surface functionality of the electrode technology to build on the salt removal efficiency of the CDI system. Herein, we report on the impact of modifying biomass-derived porous activated carbon (AC-FB) electrodes with a hydrophilic deep eutectic solvent (DES), such as choline chloride-urea (ChCl-U) to introduce extra ion-dipole interaction sites through its rich hydrogen bonding network. The HR-transmission electron microscopy evidences the presence of DES on the sample surface. The thermogravimetric analysis and water contact angle measurements were done on the AC-FB and AC-FB-DES electrode to confirm the successful embedding of DES into the activated carbon (AC). In this symmetric configuration, the Na⁺ and Cl⁻ ions are accommodated by the modified layer at the AC-FB-DES electrode. The electrochemical properties of the electrode were analyzed using CV and EIS analysis. The AC-FB-DES showed an 85% enhancement of removal percent compared to the plain AC-FB at 1.2 V in a 600 mg L⁻¹ NaCl solution, despite that it shows a 40% drop in the specific capacitance compared to that of the AC-FB at a scan rate of 10 mV s⁻¹. It is believed that upon applying potential, the rearrangement in the layered structure of urea–choline chloride at the electrified interface of the AC-FB electrode provides electro-induced active sites which enhance both CDI kinetics and overall ions removal. It is also believed that embedding DES on the AC-FB enhances the electro-adsorption performance of the AC by modifying the electrical double layer structure at the interfaces by introducing extra intermolecular interactions and surface polarization that play a synergistic role in the salt removal process.