The Research On Flexible Electrochemical Capacitors Based On Polypyrrole Electrodes

As a kind of energy storage devices,flexible electrochemical capacitors have attracted tremendous attention due to their high capacity,fast charging/discharging rates,excellent stability,environmental friendliness,light weight and flexibility.The performances of the flexible electrochemical capacitors depend on many factors,in which the most major are the electrode materials and electrolytes.Among numerous conductive polymers,polypyrrole(PPy)has become an important electrode material due to its larger specific capacitance,good electrical conductivity,flexibility,simple synthesis and good environmental friendliness.However,electrochemical capacitors based on PPy always have poor cycling,which originates from the structural breakdown caused by the repetitive counterion flow during the charge/discharge process.In this paper,some influencing factors of preparing high stability PPy-based electrode materials have been studied,which provided some practical basis for preparing PPy-based ECs with high performance.(1)The PPy-based electrode material(PPy/CFs)doped with chloride ions were deposited on the surface of carbon fiber by chemical oxidation polymerization and electrochemical polymerization.In chemical oxidation polymerization,the content of PPy in PPy/CFs is controlled by changing the concentration of oxidant FeCl3·6H2O.It is found that PPy/CFs prepared under the concentration of oxidizer at 350 mg mL-1 has a high capacitance and good rate capability,and the flexible electrochemical capacitors assembled with the electrode material have a high capacitance and good cycling stability.However,the PPy/CFs prepared by electrochemical polymerization has a considerable specific capacity,but the cycling stability is poor.The PPy/CFs prepared by chemical oxidation polymerization retains85% of the initial capacitance after 5000 cycles,but PPy/CFs prepared by electrochemical polymerization retains only 5% of the initial capacitance after 200 cycles.In addition,the electrochemical capacitor assembled withPPy/CFs prepared by chemical oxidation polymerization shows a very good flexibility with CV curve changing little after bending at different angles.(2)The PPy/CFs composites have been prepared by using CFs as the substrate to deposit PPy via galvanostat method,during which the aqueous solutions containing 0.1 mol L-1 pyrrole monomer and electrolytes such H3PO4,NaH2PO4·2H2O,Na2HPO4·12H2O,Na3PO4 and LiH2PO4 are used as deposition solution.The results show that PPy/CFs prepared with NaH2PO4·2H2O as electrolyte have large specific capacitance and good rate performance.The flexible electrochemical capacitors have assembled by the optimal PPy/CFs as electrode and H3PO4/PVA as electrolyte,the specific capacitance of the active material can reach 287 F g-1 at 2 mV s-1 and the cells can maintain 96.8% of the initial specific capacitance after 13,000 CV cycles.The electrochemical capacitors have connected in series or parallel,three electrochemical capacitors in series can light up a light emitting diode with avoltage threshold of 2.5 V LED,indicating potential of the cells' practical applications.(3)The composite of polypyrrole deposited on titanium mesh(PPy/TiM)has been fabricated via galvanostatic method by using Ti M as substrate to electrochemically synthesize PPy doped with p-toluenesulfonate anion.The influences of deposition time on the characteristics of PPy/TiM samples have been determined.Then the semitransparent flexible electrochemical capacitors have been assembled by using the optimal samples of PPy/TiM as electrodes and H3PO4/PVA as electrolyte.The test results show that PPy in optimal PPy/TiM electrode has a specific capacitance of about 326 F g-1 at0.025 mA cm-2 and the capacitance of the assembled cells exhibits good cycle stability which can still keep unchanged or up even after 20000 cycles.