Study On Carbon Based Transition Metal Composite In Capacitor Energy Storage And Deionization

The shortage of energy and fresh water resources has become the most urgent global problem hindering the sustainable development and social progress of human beings.Traditional fossil fuels can not meet the requirements of sustainable development of the earth any more.Due to the discontinuity of clean energy such as solar energy,wind energy,tidal energy and geothermal energy,an effective energy storage device is needed to overcome this shortcoming.Supercapacitors?SC?have draw much attention of researchers in the field of energy storage due to its advantages of high power density,long cycle life and ultra-super high charge-discharge rate.There are abundant sea water on the earth.It is becoming more and more important to effectively convert sea water or brackish water into fresh water which can be directly used in human production and life.Compared with other traditional deionization technologies,capacitance deionization?CDI?as one of the most promising desalination technologies,has attracted more and more researchers'attention,since it was first proposed in 1960s.As an important part of SC and CDI system,electrode plays an important role in SC and CDI system.The morphology and structure,surface characteristics,chemical composition of electrode materials are closely related to the final performance of SC and CDI.According to the working principle of CDI and SC,The electrode materials can be divided into two kinds:the double layer electrode materials basing on the electrochemical double layer adsorption theory and the Faradaic electrode materials based on the oxidation-reduction reaction.The double-layer electrode material has the advantages of good electronic conductivity,high desorption rate,simple preparation and long cycle life.At the same time,it has the disadvantages of low adsorption capacity.Faradaic electrode material has the characteristics of high deionization amount,but its disadvantages are also obvious,such as low deionization rate,poor electronic conductivity,weak cycle stability and the complicated preparation route.It becomes of great significance to prepare a kind of electrode material which has both the advantages of electric double layer material and Faradaic electrode material and overcome each other's disadvantages simultaneously.Activated porous carbon and transition metal oxides are the most common materials in the field of double-layer electrode and Faradaic electrode respectively,which are ideal candidates for the synthesis of composite materials.In this study,firstly,the porous carbon materials with high electronic conductivity were prepared using the raw material of biomass waste;secondly,zinc/activated carbon composite materials with large specific surface area and rich porous structure were prepared with ZnCl₂ as activator,which were suitable for CDI anode;thirdly,the manganese/carbon nanocomposites with B atom as doping atom were prepared by deposition of MnO_x;Finally,metal organic frameworks?MOFs?were synthesized firstly,and Cu-1,3,5-benzoic acid MOFs?Cu-BTC nanocomposites?and polymer raw fiber composites were prepared by electrospinning.After high temperature carbonization,Cu-BTC derived Cu/carbon fiber composites were obtained.By exploring the composite and the optimal composite ratio of carbon based electrode materials and transition metal oxide electrode materials,the synergistic effect of the electric double layer characteristics and Faradaic oxidation-reduction characteristics is optimized,and finally the electrochemical and CDI performance of the electrode materials is improved.The main contents and significance of this study are as follows:?1?The in-situ graphite nitrogen doped porous carbon materials were prepared by one-step high-temperature method with common municipal waste ginkgo leaf as raw materials,in which the carbonization and activation of the materials occurred in one-step high-temperature process at the same time.The morphology,structure,surface functional groups,chemical composition and porous properties of the materials were systematically investigated.Compared with the conventional two-step high-temperature process including carbonization and a high-temperature activation in the presence of KOH,the material exhibits large specific surface area(1228.7m² g^-1),high graphite nitrogen content,good electronic conductivity and excellent electrochemical performance,with the specific capacitance of 176.1 F g^-1,the energy density of 24.86 Wh kg^-1 and the power density of440 W kg^-1 with 1 M NaCl solution as electrolyte.When the material was used as the CDI electrode,the deionization amount was 16.5 mg g^-1 in 10 mM NaCl solution at a constant charge voltage of1.2 V,and the average deionization rate was as high as 1.5 mg^-11 min^-1.The structure and properties of carbon materials prepared by one-step high-temperature method have been greatly improved,mainly because the lignin and cellulose in biomass materials can be fully exposed to alkaline conditions before high-temperature treatment,which leads to the alkaline hydrolysis of long-chain polymer polysaccharides into short-chain small molecules.In the following high-temperature treatment,abundant pores were formed due to the aromatization of short-chain small molecules.At the same time,the rich porous structure is more conducive to the formation of graphite nitrogen by in-situ doping of N atoms from biomass raw materials,further improving the electronic conductivity of the materials.?2?Porous carbon electrode materials suitable for CDI anode were prepared by hydrothermal method with biomass waste citrus peel as raw material and a small amount of ZnCl₂ as activate reagent.Field emission scanning electron microscope?FESEM?,X-ray diffraction?XRD?and N₂adsorption desorption isotherm were used to characterize the morphology and porous properties of the material.The electrochemical properties of the materials were measured by cyclic voltammetry?CV?,electrochemical impedance spectroscopy?EIS?and galvanostatic charge discharge?GCD?.Compared with the electrode materials prepared by using KOH and H₃PO₄ as activate reagents,the carbon materials prepared by ZnCl₂ activation have the best electric double layer capacitance characteristics,and the specific capacitance is 120 F g^-1,the energy density of 34.0 Wh kg^-1 and the power density of 81.7 W kg^-1,in 1 M NaCl solution.The deionization amount of CDI in 500 mg L^-1 NaCl solution was 16 mg g^-1,the average deionization rate was 0.67 mg g^-1 min^-1,the deionization retention ratio was 80%after 35 charging and discharging cycles,and the CDI performance was much higher than that of the electrode material prepared with KOH and H₃PO₄ as activate reagents.Finally,the related mechanism was revealed.The materials using ZnCl₂ as activation reagent exhibits the best dispersion and no obvious agglomeration due to the electrostatic repulsion between the surfaces adsorbed Zn²⁺.In the carbon materials prepared by ZnCl₂ activation,due to the existence of trice O-Zn on the surface of the material after carbonization,and the electronegativity of the O atom is greater than that of the Zn atom,so that the shared electron pairs between the O atom and the Zn atom are biased to the O atom,which leads to the electron deficiency of the Zn atom.Therefore,ZnCl₂ activated materials are more suitable for CDI positive adsorption solution of anions.?3?B,N Co-doped 3D interconnect carbon nanosheets were prepared by through freeze-drying chitosan hydrogel and H₃BO₃ mixture.The thickness of the carbon nanotubes is about 30nm and they are connected into a network structure.XPS results show that B,N atoms are successfully doped.The ion diffusion and storage properties of the materials were further improved after manganese oxide was modified on the surface of B,N co-doped 3D interconnected carbon nano sheets.The results show that when the reaction time with KMnO₄ solution is 4 h,the amount of modified manganese oxide is optimum.When the material was performed as the electrode of CDI,the specific capacitance in 1 M NaCl solution is 248.5 F g^-1,with the energy density of 34.52Wh kg^-1 and the power density of 621.25 W kg^-1.When the constant voltage of 1.2 V,the deionization amount of B,N co-doped 3D interconnect carbon nanosheets in 10 mM NaCl solution reaches 20.3 mg g^-1,and the average deionization rate is 1.5 mg g^-1 min^-1,and it also exhibits high cycle stability.After 50 charge-discharge cycles,the deionization retention ratio is still 90.3%.Due to the successful doping of B atom in the graphite layer,the repulsion force between B atoms overcomes the Vanderwaals force between the graphite layers,resulting in the formation of thin sheet with a thickness of about 30 nm.In addition,because some B atoms are shared by different graphite layers,the carbon nano sheets are connected to form a 3D interconnection structure.Due to the difference of electronegativity??B=2.04,?n=3.04,?C=2.55?between the N atom and B atoms,the electron cloud near the carbon atom which makes up the six membered ring of graphite is strongly disturbed,and the electronic conductivity of the material is further improved.The 3D interconnection structure has contributed to the improvement of the electronic conductivity of the whole electrode.In addition,the mesoporous dominant porous structure in the carbon nano sheets provides a convenient tunnel path for the rapid transportion of ions in the electrode material.?4?In this part of the research work,firstly,the uniform Cu-BTC nanoparticle was synthesized,and then Cu-BTC nanoparticle-containing polyacrylonitrile?PAN?fiber was fabricated by electrospining.After air pre-oxidation and high temperature carbonization in nitrogen atmosphere,the Cu-BTC derived CuO_x@CNF self-supporting electrode material is successfully synthesized.The results show that the self-supporting composite electrode material with 5%Cu BTC has the best electrochemical and deionization performance.The specific capacitance in the 1 M NaCl solution is 258.2 F g^-1,the energy density of 35.81 Wh kg^-1 and the power density of 99.31 W kg^-1.The deionization amount was 6.17 mg g^-1 and the average deionization rate was 1.12 mg g^-1 min^-1in 10 mM NaCl solution,when applying the materials in CDI systerm.The deionization retention ratio was 83.8%in 50 charge-discharge cycles.The Cu-BTC derived CuO_x@CNF possesses rich porosity.BTC ligand can also act as a carbon source to improve the electronic conductivity of the material after carbonization.CNF,as the self-supporting carrier and conductive network of CuO_x,contributes to the charge and ion transportation between CuO_x particles.The uniform distribution of CuO_x in CNF can also contribute to the electrochemical and CDI properties of CNF.CuO_x content was optimizaed by varying the Cu-BTC amount in the electrospining solution.It was found that the electrochemical performance and CDI performance of CuO_x@CNF reachs the best when Cu BTC of 5%in the electrospining solution was applied.