Stabilities of flexible electrochemical capacitors based on polypyrrole/carbon fibers in different gel electrolytes

Flexible electrochemical capacitors (ECs) are vitally important as the emerging flexible electronics.We have prepared polypyrrole doped with counter ion of Cl-on carbon fibers (PPy-Cl/CFs) via the simple electrochemical deposition method.The flexible ECs based on PPy-Cl/CFs electrodes have been assembled by using H3PO4/poly(vinyl alcohol) (PVA),H2SO4/PVA,LiClO4/PVA,KCl/PVA and LiCl/PVA as gel electrolytes.The capacitive properties of ECs have been systematically evaluated with electrochemical methods.The results show that the cells with H3PO4/PVA electrolyte exhibit good cyclic stability (91.5％ retention after 15000 cycles) although PPy-Cl/CFs has moderate specific capacitance in H3PO4/PVA (41.6 mF·cm-1 or 52.0 F·cm-3).However,the ECs in other electrolytes have poor cyclic stability.Further electrochemical analysis reveals that the doping/dedoping processes of PPy-Cl/CFs are different in the five electrolytes,and X-ray photoelectron spectra demonstrate that the ratios of counter anions in PPy's oxidized states to those in reduced states are obviously different when PPy-Cl/CFs are kept in reduced or oxidized states in the five electrolytes.The results also illustrate why the capacitors using H3PO4/PVA as electrolyte exhibit good cyclic stability.Furthermore,a light emitting diode (LED) with a threshold voltage of 2.5 V can be lighted by three ECs connected in series.