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 center dot cm(-1) or 52.0 F center dot 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.