Preparation And Modification Of Polyaniline@FePO₄ Electrode Based On Flexible Graphite Felt For Supercapacitor

Supercapacitor is a new kind of energy storage device with energy density superior to that of conventional electrolytic capacitor and higher power density than secondary battery.With the increasingly quick development of intelligent,compact and flexible electrochemical energy storage devices,it is imperative to develop new generation of energy storage devices with small size,flexible light weight,high energy density and environmental friendliness.At present,the relatively low energy density of all-solid flexible supercapacitors is the main bottleneck for its practical application.Therefore,rational design and construction of key electrode materials with high energy density is the focus of research in this field.In this thesis,graphite felt（GF）with high conductivity is selected as flexible matrix,iron phosphate（FePO₄）and polyaniline（PANI）are used as dual active components.FePO₄ is hydrothermally grown and combined with subsequent PANI electrodeposition on GF substrate.A new hybrid electrode material GF@FePO₄@PANI was thereby obtained.It is desirable to increase the specific capacity and energy density of the composite electrode.The specific research contents and results obtained are as follows:（1）PreparationofGF@FePO₄electrodematerial.FePO₄microspheres were directly grown on a conductive GF substrate by a simple one-step hydrothermal method using iron nitrate（Fe（NO₃）₃·9H₂O）as the iron source and ammonium dihydrogen phosphate（NH₄H₂PO₄）as the phosphorus source.The FePO₄ microspheres with bead-like structure were successfully synthesized on GF by SEM,XPS and XRD characterization techniques.The electrochemical performance of GF@FePO₄ was optimized by adjusting the hydrothermal temperature,the concentration of the reactants and the hydrothermal reaction time.The electrochemical tests were carried out in a 2 mol L^-11 phosphoric acid electrolyte using a three-electrode system.The results show that the specific capacity of the prepared electrode is 832.5 mF cm^-22 at 1 mA cm^-2;and the specific capacity was retained only 6.0%when the current density is increased by 20 times.However its initial capacity can be kept¹00.0%after 2000 charge-recharge cycles at an elevated current density of 20 mA cm^-2,indicating excellent cycle stability.The above results indicate that GF@FePO₄ is still a promising candidate for supercapacitor electrode material.（2）Preparation of GF@FePO₄@PANI electrode material.The electrochemical performance of the composite electrode was improved by electrodeposition of PANI on GF@FePO₄.SEM,HRTEM and XPS were used to prove that the prepared GF@FePO₄@PANI exhibited a 3D network structure with GF carbon fibers as the connecting linkers and walnut-like microspheres as beads.The amorphous PANI was uniformly covered or embedded into the FePO₄ microspheres.The optimized GF@FePO₄@PANI electrode has a specific capacity of 2606.8 mF cm^-2at a current density of 1 mA cm^-2,which is 313.1%of the GF@FePO₄electrode.When the current density is increased to 20 mA cm^-2,the composite electrode can keep 51.0%of its initial capacitance,which is 45percentage points higher than that before PANI modification.After 2000consecutive charging/discharging cycles at a current density of 20 mA cm^-2,the capacity can retain 90.1%of its initial capacitance,indicating that the composite electrode exhibited good cycle stability.It could be attributed to the rich 3D conductive network generated by the close contact among GF,FePO₄ and PANI species,and the synergistic effect between the active components FePO₄ and PANI.The prepared GF@FePO₄@PANI hybrid electrode exhibited superior supercapacitor performance in the phosphoric acid electrolyte system over GF@FePO₄and GF@PANI alone.（3）Assembly of GF@FePO₄@PANI for flexible symmetrical supercapacitors.A symmetrical all solid state supercapacitor device GF@FePO₄@PANI//H₃PO₄-PVA//GF@FePO₄@PANI was assembled using two identical pieces of GF@FePO₄@PANI as positive and negative electrode,and PVA-H₃PO₄ gel as solid electrolyte.Electrochemical tests showthatthespecificcapacityofthe GF@FePO₄@PANI//H₃PO₄-PVA//GF@FePO₄@PANI device is 2274.0mF cm^-2(649.71 F g^-1)at 1 mA cm^-2.At a power density of 142.9 W kg^-1,the energy density is 90.2 Wh kg^-1,and at a high power density of 5714.6W kg^-1,the energy density is still 29.0 Wh kg^-1.After 2000 cycles of charge and discharge at a high current density(20 mA cm^-2),the device capacity remains about 80.6%of its initial capacitance,and the Coulomb efficiency can still maintain about 100%.Theaboveresultsindicatethatthecompositematerial GF@FePO₄@PANI is expected to be one of the electrode materials for all-solid flexible supercapacitors.The orderly combination of conductive PANI and FePO₄ produces a strong synergistic effect.It is expected to be used in the design of other similar electrochemical energy storage devices.