Solar reduced graphene oxide decorated with manganese dioxide nanostructures for brackish water desalination using asymmetric capacitive deionization

Abstract

Capacitive deionization (CDI) has emerged as a low-cost, reagent-free technique for the desalination of water. This technique is based on the immobilization of dissolved ions on the electrically charged electrodes, by the electrosorption phenomenon. The electrosorption of dissolved ions by using CDI is limited for feed water having a low concentration of salts. To address this problem, we employ an asymmetric capacitive deionization (Asy-CDI) architecture having solar reduced graphene oxide decorated with manganese dioxide nanostructures (SRGO–MnO₂ composite). The Asy-CDI possesses an SRGO–MnO₂ composite as the cathode and SRGO as the anode with an anion exchange membrane. The cathode formed from the SRGO–MnO₂ composite serves the purpose of immobilization of cations, whereas the anode formed from SRGO is responsible for anion removal. The crystal structure, chemical composition and morphology of the as-synthesized SRGO–MnO₂ composite electrode materials are characterized by several techniques, confirming that the surface of SRGO is successfully loaded with α-MnO₂ nanostructures. The electrochemical characterization reveals a high specific capacitance of the as-synthesized SRGO–MnO₂ composite (419.9 F g⁻¹) at 100 mV s⁻¹. The Asy-CDI provides a higher salt adsorption capacity (40.2 mg g⁻¹) compared to Sy-CDI (28.3 mg g⁻¹) with feed water containing a salt concentration of 2000 mg L⁻¹. These results indicate that the Asy-CDI may be employed as an efficient technique for the desalination of high concentration salt water.