Hierarchical MXene/Polypyrrole-Decorated Carbon Nanofibers for Asymmetrical Capacitive Deionization

Abstract

Membrane capacitive deionization (MCDI) has emerged as a promising electric-field-driven technology for brackish water desalination and specific salt or charged ion separation. The use of carbon-based or pseudocapacitive materials alone for MCDI usually suffers from the drawbacks of low desalination capacity and poor cycling stability due to their limited accessible adsorption sites and obstructed charge-carrier diffusion pathways. Therefore, developing a hybrid electrode material with multiple charge storage mechanisms and continuous electron/ion transport pathways that can synergistically improve its specific capacitance and cycling durability has currently become one of the most critical technical demands. Herein, we developed a novel hierarchically architectured hybrid electrode by first spinning MXene into polyacrylonitrile (PAN)-based carbon nanofibers (MCNFs) to obtain a highly conductive carbon nanocomposite framework. The excellent spatial support structure can effectively prevent the dense packing of Cl–- and DBS–-doped polypyrrole (PPy) molecular chains during the following electrodeposition process, which not only ensures the efficient transport of electrons in the continuous hybrid carbon nanofibrous skeleton but also provides abundant accessible sites for ion adsorption and insertion. The obtained self-supporting membrane electrodes (MCNF@PPy+Cl– and MCNF@PPy+DBS–) have the advantages of outstanding specific capacitance (318.4 and 153.9 F/g, respectively), low charge transfer resistance (10.0 and 5.3 Ω, respectively), and excellent cycling performance (78% and 90% capacitance retention ratios, respectively, after 250 electrochemical cycles). Furthermore, the asymmetrical membrane electrodes showed a superior desalination capacity of 91.2 mg g–1 in 500 mg/L NaCl aqueous solution and obvious divalent ion (Ca2+, Mg2+) selective adsorption properties in high-salt water from the cooling towers of thermal power plants.

MXene‐Based Capacitive Deionization–Strong Competitor Among Faradaic Electrode Systems: Current Status and Future Perspectives

“…Therefore, further optimization/alteration of the operational parameters and desalination test in the RCDI/DEDI cell configuration are desired. Moreover, redox‐active components in the MXene‐based electrodes may be utilized for flow‐electrode CDI, which may be a continuous and energy efficient process due to the regeneration of electrodes in the separate container. Specific ions selectivity (Zn 2+ , Ca 2+ and Mg 2+ ) under an experimental condition has been demonstrated in the MXene‐based CDI, [159,179,183] which need to be extended for many other ions/toxic ions, such as Co 2+ , Pb 2+ , Cd 2+ , Fe 3+ , As 3+ , Cr 6+ , etc.commonly present in the real water system. Moreover, a deep understanding of ions dynamics and mechanistic insights for specific selectivity may be an important domain of fundamental research.…”

“…Specific ions selectivity (Zn 2+ , Ca 2+ and Mg 2+ ) under an experimental condition has been demonstrated in the MXene‐based CDI, [159,179,183] which need to be extended for many other ions/toxic ions, such as Co 2+ , Pb 2+ , Cd 2+ , Fe 3+ , As 3+ , Cr 6+ , etc.commonly present in the real water system. Moreover, a deep understanding of ions dynamics and mechanistic insights for specific selectivity may be an important domain of fundamental research.…”

“…Benefitted with the EDL capacitance and intercalation pseudocapacitance all‐together along with the micro‐structural feature, the S‐Ti 3 C 2 T x /PANI/F‐Ti 3 C 2 T x //AC device responded to an excellent desalination metrics (SAC/ASAR: 72 mg g −1 /2.40 mg g −1 min −1 , 100 % SAC retention after 30 cycles, charge efficiency: 0.82, and energy consumption: 0.35 kW h kg −1 ). Wang et al [183] . configured two freestanding membrane electrodes, namely, MCNF@PPy + Cl − and MCNF@PPy + DBS − , via electro‐deposition (ED, 3000 s) of Cl − and DBS − (dodecylbenzenesulfonate anion) doped PPy, respectively, onto Ti 3 C 2 T x MXene‐CNF hybrid membrane (MCNF: formed by ES of MXene and polyacrylonitrile (PAN) followed by carbonization, Table S1).…”

Unraveling the Ion Uptake Capacitive Deionization of Sea- and Highly Saline-Water by Sulfur and Nitrogen Co-Doped Porous Carbon Modified with Molybdenum Sulfide

“…Receiving benefits from energy efficiency, environmental friendliness with zero secondary pollution, reduced operating cost, facile electrode regeneration, long-term durability, its ability to remove both mono- and divalent ions under a low operating voltage, and also selective removal of anions and cations, CDI has drawn more attention compared to other alternatives, including membrane separation systems (e.g., reverse osmosis (RO), electrodialysis (ED), and thermal separations system at room temperature. , In comparison with the RO and distillation-based technologies, there is no need for special prerequisites such as high-pressure pumps or heat sources; also, it does not suffer from ion-selective membrane and membrane fouling. Thus, the CDI process is highly scalable . Despite these effective advantages, operating with low salt adsorption capacity (SAC) and in low saline influents is the main limitation of highly saline and seawater studies in CDI application.…”

“…Thus, the CDI process is highly scalable. 10 Despite these effective advantages, operating with low salt adsorption capacity (SAC) and in low saline influents is the main limitation of highly saline and seawater studies in CDI application. Recently, RO/ED/RED-CDI (RED = reverse electrodialysis) coupled systems have been proposed to reduce the barriers to CDI application in seawater studies.…”