Synthesis Of Iron-System Phosphate And Its Application In Supercapacitor

Due to the advantages of fast charging and discharging,excellent cycling stability,high power density and high energy density,supercapacitors can partly replace the traditional chemical batteries.Electrode material is an important element to improve the performance of supercapacitors.In recent years,ferrous metal phosphates,which have stable structure,rich electrochemical behavior,low cost and rich natural resources,are widely concerned as electrode materials.In this thesis,hydrothermal and alcohol-water mixed solvothermal process were used to study the growth mechanism of ferrous metal phosphates with micro/nano structures.On the basis of monometallic ferrous phosphates,the controllable synthesis of bimetallic ferrous phosphates composites were realized.This paper will provide a theoretical basis for the development of controllable synthesis of ferrous metal phosphates with micro/nano structures and their application in supercapacitors.The main contents are summarized as follows:1.Two dimensional Fe(PO3)3 flakes for supercapacitorsTwo dimensional Fe(PO3)3 flakes were prepared by a one-step hydrothermal process.The effects of the amount of sodium tartrate,the mass ratio of phosphorus source and iron source,and the solvent volume on the composition and morphology of the samples were studied respectively.The electrochemical performance of these electrodes with different compositions and morphologies were studied under the traditional three electrode system,and the as-prepared Fe(PO3)3 flakes showed the best supercapacitor performance.The Fe(PO3)3 flakes were used as electrode materials to construct Fe(PO3)3//ACs supercapacitor.The voltage window of the supercapacitor can reach 1.4 V.After 3000 cycles of galvanostatic charging-discharging,89.9%of the specific capacitance kept,which shows good cycle stability.2.Nickel/Cobalt doped Fe(PO3)3 for supercapacitorUsing the as-prepared Fe(PO3)3 flakes as the precursor,nickel phosphate coated Fe(PO3)3(abbreviated to NiPO@Fe(PO3)3)and cobalt phosphate coated Fe(PO3)3(abbreviated to CoPO@Fe(PO3)3)were successfully prepared respectively by hydrothermal method.Through the synergistic effect between nickel-iron and cobalt-iron,sufficient pseudocapacitance reaction can be provided for the electrode.In the traditional three electrode system,the capacitance of nickel phosphate coated Fe(PO3)3 and cobalt phosphate coated Fe(PO3)3 was larger than that of pure Fe(PO3).The asymmetric supercapacitors were constructed by using nickel phosphate coated Fe(PO3)3 and cobalt phosphate coated Fe(PO3)3 as the positive active material respectively,and active carbons as the negative active material.Compared with the Fe(PO3)3//ACs aqueous supercapacitor,the specific capacity,energy density and power density of NiPO@Fe(PO3)3//ACs and CoPO@Fe(PO3)3//ACs aqueous supercapacitor were increased3.Ultrathin cobalt pyrophosphate nanosheets for supercapacitorUltrathin cobalt pyrophosphate nanosheets with thickness of 3.0～5.0 nm were successfully synthesized by a simple one-step hydrothermal process.The as-prepared ultrathin cobalt pyrophosphate nanosheets have large specific surface area and good conductivity.Under the traditional three electrode system,the ultrathin cobalt pyrophosphate nanosheets electrode showed high specific capacitance and excellent cycle stability.The aqueous/quasi-solid state supercapacitors were assembled by using the cobalt pyrophosphate nanosheets and activated carbon as positive and negative electrode materials respectively The excellent cycle stabilities,high power densities and energy densities exhibit the ultrathin cobalt pyrophosphate nanosheets are potential electrode materials for supercapacitor.4.Ultrathin nickel-cobalt phosphate nanosheets for supercapacitorsBased on the synthesis of two-dimensional ultrathin cobalt pyrophosphate,2D nickel-cobalt phosphate nanocomposites with a thickness of about 5 nm were successfully synthesized by introducing nickel source under mild hydrothermal conditions.The structural influence on the pseudocapacitive performance of the obtained nickel-cobalt phosphate was studied via electrochemical measurement.The specific capacitance of the 2D nickel-cobalt phosphate nanocomposites is much higher than that of Co2P2O7 and Ni doped C02P2O7 under the traditional three electrode system.More importantly,the aqueous and quasi-solid state flexible electrochemical energy storage device were assembled.The supercapacitors showed high energy density and excellent cycle stabilities,which confirm that the 2D nickel-cobalt phosphate is a promising electrode material for supercapacitor.5.Cobalt doped nickel phosphite for supercapacitorBouquet-like cobalt doped nickel phosphite(Ni11(HPO3)8(OH)6)was successfully synthesized by using the mixture of glycol and deionized water as the reaction solvent under mild and simple one-step solvothermal process.The effects of Ni/Co ratio,reaction time/temperature and glycol/deionized water ratio on crystal growth were discussed.The effect of the structure on the pseudocapacitive performance of the as-prepared cobalt doped Ni11(HPO3)8(OH)6 was investigated via a series of electrochemical measurements.The uniform cobalt doped Ni11(HPO3)8(OH)6 electrode presented a maximum specific capacitance of 714.8 F g-1 at a curret density of 1.0 A g-1.More significantly,aqueous and quasi-solid state supercapacitors were assembled,which show excellent cycling stabilities.