Nanostructured MXene and Graphene as Electrodes in Capacitive Deionization

Abstract

In order to deal with water scarcity, wastewater treatment, and resource recovery, ion separation and removal techniques from water/wastewater are needed. Conventional ion removal techniques such as adsorption, coagulation/flocculation, and membrane technologies have their own disadvantages, including sludge disposal, cost, energy-intensive, and membrane fouling. On the other hand, ion removal using capacitive deionization (CDI) has advantages over conventional techniques, such as lower energy consumption, cost-effectiveness, selective ion removal, and resource recovery. However, traditional porous carbon electrodes used in CDI are less effective in terms of selectivity and ion storage capacity. In this regard, MXene and graphene-based electrodes are more suitable for CDI as they are equipped with tremendous properties required for CDI electrodes, for instance, high conductivity, high specific capacitance, higher specific adsorption capacity (SAC), and stability for multiple cycles. Along with this, their selectivity toward a particular ion can be enhanced by tailoring electrode properties. In this work, different nanostructured MXene nanocomposite electrodes, including hybrid MXene/rGO nanocomposite and functionalized MXene-PSS, are synthesized and studied for salt adsorption capacity and selectivity under mixed ions solution. The result showed higher SAC for MXene/rGO nanocomposite compared to carbon-based electrodes with a significant selectivity towards multivalent atoms. Along with this, NiFe2O4 (NFO) & CoFe2O4 (CFO) spinel nanocrystals were synthesized using hydrothermal method and hybridized with reduced graphene oxide (rGO) to improve their specific adsorption capacity, stability, and selectivity toward a particular ion. Synthesized materials for this work include MXene/rGO, PSS-MXene, NTO, NTO/rGO, NFO, NFO/rGO, CFO, and CFO/rGO, which are mostly pseudocapacitive electrodes and deliver remarkable electrode properties. These advanced nanomaterials are employed for different ion separation applications, including desalination, selective ion separation, heavy metal removal, positive ion selectivity, negative ion selectivity, and lithium recovery. The outcome of this work contributes to the existing CDI's knowledge regarding the use of advanced nanocomposite electrodes in CDI, their selectivity toward a specific ion in a multi-ionic environment, electrode tailoring & engineering, and possible ways to improve, SAC, selectivity, and cyclic durability.