Preparation Of Graphene-based Transition Metal Compounds Or Polypyrrole Composites And Their Capacitive Properties

Supercapacitor,a new type of electrochemical energy storage device with fast charging/discharging rate,high power density,long cycle life,environmental protection and so on,has a broad application prospects.However,the energy density of supercapacitors based on storing charge via electrode surface reaction is much lower than that of batteries.Usually,the weakness of low energy density for supercapacitors can be solved by increasing the specific capacity of materials and/or optimizing the operating voltage range of devices.In this paper,we propose to construct self-supporting independent electrodes with three-dimensional porous structure by using double-layer capacitive graphene and pseudocapacitive materials with excellent charge storage capability(such as PB-Co,NiFe2O4,PPy,MnNb2O6 and CoNb2O6).The specific capacitance of electrodes can be enhanced by the following three points:(1)graphene carbon skeleton with the three-dimensional structure provides sufficient attachment sites for dispersing pseudocapacitive materials,which is conducive to improving the cyclic performance of composites;(2)the introduction of pseudocapacitive materials between graphene sheets can effectively alleviate the secondary accumulation of graphene during charging/discharging poccess,thereby improving rate capability and cyclic performance of composites;(3)the design of electrodes without conductive agents and binders greatly shortens the ion transport path and reduces the contact resistance between electrolyte and electrodes.In addition,the asymmetric ion capacitors are assembled in organic system to further broaden the operating voltage window of the device,so as to improve its energy density.The main research contents are as follows.PB-Co/rGOH was synthesized by one-step hydrothermal method and introducing graphene oxide.The results of scanning electron microscopy and transmission electron microscopy test show that the average particle size of PB-Co is 40 nm,and it is evenly distributed on graphene.The results of electrochemical tests show PB-Co/rGOH with PB-Co loading mass of 0.3 g exhibits the best charge storage capacity(specific capacitance of 220 F g-1 at a current density of 0.5 A g-1).After 10000 charging/discharging cycles,the specific capacitance retention of the electrode is 82.4%.The symmetrical all-solid-state supercapacitor assembled by using PVA/KNO3 as solid electrolyte and PB-Co(0.3)/rGOH as electrodes exhibits an ultra-high operating voltage range of 2.0 V,the maximum energy density of 57 Wh kg-1 and power densityof 10 kW kg-1.After 10000 charging/discharging cycles,the specific capacitance retention can be as high as 92.8%.Through the hydrothermal reaction and the introducing graphene oxide,Ni3[Fe(CN)6]2 was successfully converted into NiFe2O4,and the NiFe2O4@rGOH was obtained.The results of scanning electron microscopy and transmission electron microscopy test show that NiFe2O4 present a cube with a diameter of 35 nm,and are distributed uniformly on the graphene sheet.NiFe2O4@rGOH exhibits high specific capacitance of 488 F g-1 at a current density of 1 A g-1.After 10000 charging/discharging cycles,the specific capacitance retention is 89.8%.The symmetrical all-solid-state supercapacitor assembled by using PVA/KNO3 as solid electrolyte and NiFe2O4@rGOH as electrodes exhibits maximum energy density of 62.5 Wh kg-1 and power density of 13.5 kW kg-1 in the operating voltage range of 0-1.8 V.After 6000 charging/discharging cycles,the specific capacitance retention is up to 93.2%.PPy@rGOH with abundant cross-linked pore structure was obtained by hydrothermal method and electropolymerization method.The content of PPy in PPy@rGOH can be adjusted by controlling the time of electropolymerization.According to the results of electron microscopy tests and electrochemical measurements,the obtained composite gel with the electropolymerization time of 20 s can keep the cross-linking structure of rGOH well without agglomeration,and exhibits the best specific capacitance of 340 F g-1 at a current density of 1 A g-1 and the rate performance(specific capacitance of 285 F g-1 at a current density of 15 A g-1).The asymmetrical all-solid-state supercapacitor assembled by using rGOH as negative electrode,PPy@rGOH-20s as positive electrode and PVA/KNO3 as solid electrolyte shows the maximum energy density of 46.9 Wh kg-1 and power density of 24 kW kg-1 in the operating voltage range of 0-1.6 V.After 10000 charging/discharging cycles,the specific capacitance retention is up to 94.57%.After continuous 2000 charging/discharging cycles under different bending conditions,the specific capacitance retention is 87.1%,indicating excellent cycling stability and flexibility stability.Graphene film was formed on the surface of copper foil based on the corrosion battery composed of copper foil and GO in acidic conditions.Then the PPy layer with a foam-like structure was polymerized on the graphene film by interfacial reaction between organic/water phase.By controlling the polymerization time,the PPy-rGO composite films with different thickness of PPy were obtained.According to the SEM and mapping of N elements,when the polymerization time of PPy is 18 h,the thickness of PPy layer is 5 um,and the PPy-rGO composite films have the best charge storage capacity of 1480 mF cm'2 at a current density of 1 mA cm-2.After 10000 charging/discharging cycles,the specific capacitance retention is 88.2%,showing excellent cyclic stability.The symmetrical all-solid-state supercapacitor assembled by using PVA/KNO3 as solid electrolyte and PPy-rGO-18 h as electrodes displays maximum volumetric energy density of 31.95 Wh L-1 and power density of 9000 W L-1 in the operating voltage range of 0-1.2 V.After 10000 charging/discharging cycles,the capacitance retention can be as high as 89.7%.Calendula-like MnNb2O6 particles with an average particle size of 600 nm and CoNb2O6 particles with an average particle size of 55 nm were prepared by two-step hydrothermal method,respectively.Then the electrochemical properties of the material were further improved by compounding with graphene.The reversible capacity of MnNb2O6@rGO and-CoNb2O6@G is 460 mAh g-1 and 508.5 mAh g-1 at a current density of 50 mA g-1,showing excellent lithium storage performance.The hybrid lithium ion capacitor assembled by using them as anode and activated carbon(AC)as cathode shows maximum energy density of 118Wh kg-1 and 94.1 Wh kg-1,as well as maximum power density of 8000 W kg-1 and 8750.W kg-1 in the operating voltage range of 0-4.0 V and 0-3.5 V,respectively.After 10000 charging/discharging cycles,the capacity retention can remain 88%and 74.5%.