Preparation Of Metal Ion Pre-inserted Vanadium-based Oxides Negative Electrodes And Their Capacitive Deionization Performance

Energy and environmental issues continue to plague human life,with problems such as water scarcity and water pollution having a serious impact on human survival and development With its low cost,low energy consumption,easy operation,and environmental friendliness,capacitive deionization(CDI)is a potentially effective technology for simultaneously addressing the challenges of energy and water constraint The performance of CDI desalination has been significantly impacted by electrode materials as CDI research has advanced in the areas of saltwater desalination,wastewater purification,hard water softening,and extraction of high value-added ions.Vanadium oxides are widely used in energy storage and conversion because of their variable chemical valence and their typical layered structure which provides fast channels for ion diffusion.However,the limited layer spacing of V2O5’s layered structure limits ion transfer rates and is prone to collapse during charging and discharging and ion absorption and de-embedding,which seriously affects its performance.Pre-embedding metal ions in the V2O5 structure and compounding with conductive materials are effective ways to improve the performance of V2O5.On the one hand,embedding metal ions between the layers can serve to support the laminar structure and widen the layer spacing,contributing to fast ion transport and reducing electrostatic repulsions between ions.On the other hand the formation of composites with conductive materials such as MXene helps to improve electrical conductivity,increase ion transfer rates and thus enhance electrochemical properties.In this paper,the design and preparation of vanadium-based metal oxide electrode materials involved two methods:metal ion pre-embedding and the construction of composite structures.The structures and desalination properties of materials were analysed by structural characterization,including X-ray photoelectron spectroscopy(XPS)and X-ray diffractometry(XRD)and electrochemical and desalination performance tests.The following three papers serve as a summary of the research in this thesis.1.AlV3O9 was prepared by a two-step hydrothermal method combined with calcination_The end result of the hydrothermal synthesis was H11Al2V6O23.2.As the calcination temperature was below 420℃,the corresponding layer spacing gradually reduced,suggesting that some of the crystalline water had been removed from the layers.AIV3O9 was produced by calcination over 420℃.Electrochemical tests showed that the specific capacitances were 78.2 F g-1 and 71.8 F g-1 for AlVO-350 and AlVO-550,respectively,at a current density of 1 mV s-1.The substance used as a negative electrode in the construction of an activated carbon and hybrid desalination device(HCDI).The desalination quantities of AlVO-350 and AlVO-550 at 1.2 V were 92.97 mg g-1 and 101.7 mg g-1,respectively,in a NaCl solution with an initial conductivity of 2000 μS cm-1.The energy losses were 0.35 kWh kg-1 and 0.32 kWh kg-1.Both the charge efficiency were 82.6%and 88.6%.This demonstrates the benefits of AlVO-550 for desalination,energy loss,and charge efficiency.It was further found that the presence of partial crystalline water in AlVO-350 with a lamellar structure effectively shielded the electrostatic gravitational force between sodium ions and laminate cations,reduced the electrostatic repulsion between metal ions and contributed to sodium ion de-embedding in the interlayer.In addition,the formation of hydrogen bonds between the bonded water and the laminate stabilizes the laminate structure.The improved performance of AlVO-550 is due to the larger proportion of V5+/V4+,where the higher amount of V5+providing the ion storage active site.2.The precursors were first prepared hydrothermally and then futher combined with MXene to abtain MXene@Na0.33V2O5 composites,optimize the electrochemical and CDI properties by adjusting different MXene doping ratios.According to test results,the nanosheets of Na0.33V2O5 were distributed on the surface of accordion-like MXene.It was found that the composite had the best electrochemical and desalination properties when 10%of MXene was compounded,with a specific capacitance of 88.49 F g1 at a current density of 1 mV s-1.The HCDI system assembled with AC using 10%MXene@NaVO as the negative electrode had a desalination of 94.63 mg g-1 at an initial conductivity of 1000 μS cm-1 and a voltage of 1.2 V.The energy loss was 0.331 kWh kg-1 and the charge efficiency was 94.6%.The enhanced performance of the MXene@Na0.33V2O5 composite CDI was attributed to the Na+ embedding increases the V2O5 layer gap,increasing its ability to store ions.The presence of MXene enhanced the overall electrical conductivity of the sample and stabilized the structure of Na0.33V2O5 by forming a Ti-O bond with Na0.33V2O5.3.Preparation of Fe5V5O39(OH)19·9H2O by chemical precipitation.The result show that FeVO is in the form of nanosheets,which has superior electrochemical properties,with a specific capacity of 185.4 F g-1 at a scan rate of 1 mV s-1.When FeVO was used as a capacitive deionisation anode material assembled with AC to form HCDI,the desalination amount is 95.1 mg g-1 at 2000 μS cm-1 and 1.2 V Its adsorption of Na+ results in the formation of NaV3O8 within the FeVO structure and the decomposition of NaV3O8 after ion desorption,thus forming a reversible Na+ de-embedding system.In addition the adsorption characteristics of FeVO on Na+,Ca2+ and Mg2+and Na+&Ca2+were tested separately to investigate its ion selectivity,with better adsorption of higher valence ions at higher voltages,in the order of Ca2+>Mg2+>Na+.The desalination performance of the FeVO desalination cell was further investigated.An asymmetric two-electrode desalination cell was formed using FeVO as the negative electrode and CoAl LDH as the positive electrode.The desalination performance and charge efficiency of the desalination cell module were improved and energy losses were reduced compared to the hybrid capacitive deionization.The two-electrode desalination cell achieving a desalination of 112.59 mg g-1 at 2000 pS cm-1 initial conductivity,1.2 V,87.5%charge efficiency and 0.33 kWh kg-1 energy loss.The improved performance of the desalination cell is due to the fact that both the positive and negative electrodes have a higher ion adsorption capacity through the Faraday reaction.