Fabrication Of Anion And Cation Insertion-deinsertion Nano-composites Electrode And Their Hybrid Capacitive Deionization Performance

With the rapid population growth as well as worsening environmental pollution,freshwater shortages and energy crises have become major threats to humanity.Desalination technology is the preferred solution to freshwater supply.There are various conventional desalination technologies,wherein reverse osmosis,thermal processes and electrodialysis face challenges of high cost,high energy requirement and secondary pollution.Capacitive deionization(CDI)has attracted the attention of many researchers due to its low energy consumption and environmental friendliness.Since the desalination performance of CDI is closely related to electrode materials,so it is important to develop high-performance capacitor desalination electrodes.Traditional carbon-based materials have the advantages of excellent electrical conductivity,high specific surface area and chemical stability,but also have drawbacks such as low intrinsic desalination capacity and low charge efficiency(common ion effect),which severely restricted the practical application of CDI technology.New electrochemical technologies that use faradaic electrodes are being developed to achieve the high adsorption and low energy desalination.In this study,a series of faradaic electrodes,including anion and cation insertion-deinsertion nanocomposite electrode,was prepared by in-situ synthesis method,which can store anion and cation ion within crystal structure.The insertion-deinsertion electrodes were integrated with activated carbon to assemble asymmetric hybrid capacitive desalination devices.Two types of ion storage mechanism,electric double layer capacitor and faradaic capacitor,were introduced simultaneously at the positive electrode(or the negative electrode)to improve the CDI performance and charge efficiency.The paper systematically studied the relationship between electrode preparation process,structure and desalination performance.The main research contents are as follows:UiO-66 nanocube was prepared by solvothermal method using terephthalic acid as organic ligand,zirconium chloride as zirconium source and N-N dimethylformamide as solvent.Hierarchically porous carbon-coated zirconium Oxide nanocubes(HCZ)derived from metal-organic framework(Zr-UiO-66)have been developed as electrode for hybrid capacitive deionization.The obtained HCZ exhibits a hierarchically porous structure and provides excellent electrical connectivity between metal Oxides and carbon nanostructures.The XRD patterns showed that the in-situ synthesis method can effectively inhibit the rapid growth of crystal size and stabilized the tetragonal phase of ZrO2 at room temperature.The sample specific surface area of annealing at 900?(HCZ-900)was 52.61 m2 g-1 and the average pore diameter is 18.78 nm.Electrochemical tests showed that the HCZ-900 exhibited a high specific capacitance(125.84 F g-1,5 mV s-1)in 1 M NaCl solution.An asymmetric capacitive deionization made with HCZ//activated carbon shows a remarkable salt adsorption capacity of 55.17 mg g-1 in 250 mg L-1 aqueous sodium chloride solution at 1.4 V.The HCZ-900 electrode showed good cycling stability in 6 consecutive electrosorption-desorption cycles.MXene@NTO composites were prepared by in-situ growth of sodium titanate(NTO)by solvothermal method using MXene(Ti3C2Tx)as the precursor.The results showed that NTO was better grown in the interlayer and surface of MXene,consisting of wirelike Na4Ti5O12 and flaky Na4Ti9O20.Electrochemical tests showed that the composite exhibited a wider electrochemical window.The ion storage mechanism was based on the deintercalation reaction of MXene and NTO.The sodium ion deintercalation process was accompanied by the redOx reaction of metal Ti.When the reaction time was 2 h,MXene@NTO(2 h)had a high specific capacitance(108.29 F g-1,5 mV s-1),and the specific capacitance could reach 28.73 F g-1 at current density of 1 A g-1.MXene@NTO(2 h)was used as the negative electrode,and activated carbon(AC)was used as the positive electrode to assemble the asymmetric hybrid capacitor desalination device(H-CDI).The results showed that the AC//MXene@NTO(2 h)adsorption capacity was 70.2 mg g-1 in 250 mg L-1 aqueous sodium chloride solution at 1.4 V.At the same time,MXene@NTO(2 h)also had a good desalination performance in high concentration sodium chloride solution(87.3 mg g-1,1000 mg L-1).In addition,the MXene@NTO(2 h)electrode showed good cycle regeneration in 6 consecutive electrosorption-desorption cycles.Firstly,nickel-cobalt hydrOxide/graphene(NiC o-LDH/rGO)nanocomposite with nanosheet self-assembly micro-spherical morphology were synthesized by one-step solvothermal method,and then a nickel cobalt Oxide/graphene(NiCo-Ox/rGO)composite with sheet structure was obtained by high temperature annealing.It has been proved by electrochemical studies that NiCo-Ox realizes the storage of Cl-by intercalation reaction.When the amount of graphene Oxide was 2 mL,the prepared NiCo-Ox/rGO(2 ml)composite exhibited high specific capacitance(83.12 F g-1,5 mV s-1)and still maintained 26.39 F g-1 when the scan rate reached 100 mV s-1.Electrochemical impedance spectroscopy(EIS)showed that the electrical resistance of NiCo-Ox/rGO(2 ml)composites was significantly reduced,which was more conductive to the rapid migratio of charges on the electrode surface.With AC as the negative electrode and NiCo-Ox/rGO(2 ml)as the positive electrode to assemble the asymmetric hybrid capacitor desalination device(H-CDI).The results of CDI study showed that NiCo-Ox/rGO(2 ml)//AC adsorption capacity was 51.54 mg g-1 in 250 mg L-1 aqueous sodium chloride solution at 1.4 V.In 5 consecutive electrosorption-desorption cycles,the salt adsorption capacity was 51.53 mg g-1(the retention rate was 96%),indicating that the NiCo-Ox/rGO(2 ml)electrode showed good cycling stability.