Flexible, Free-Standing TiO2-Graphene-Polypyrrole Composite Films as Electrodes for Supercapacitors

Flexible, free-standing titania-graphene-polypyrrole (TiO ₂ -G-PPy) composite films as electrodes for supercapacitors were reported in this study, which harness the advantage of both PPy and TiO ₂ . First, titania precursor and graphene oxide (GO) composite films were prepared by the directly mixing and drying strategy. Second, the hydriodic acid (HI) reduction process were employed to reduce GO to obtain graphene/titania precursor composite films. Third, heat treatments at different temperatures were carried out to obtain TiO ₂ -G composite films with different titania crystalline phases. Finally, PPy was deposited on the surface of the as-prepared TiO ₂ -G to form three-phase composite films. The introduction of TiO ₂ remarkably improves the pseudocapacitance and electrochemical stability of G based electrodes. The highest capacitance was obtained with the TiO ₂ content of 14.6%. The PPy coating also improves the capacitance of G-PPy composite electrodes. The role of PPy lies more in the improvement of conductivity, which increases the current density of electrodes under working condition. However, the introduction of PPy decreases the electrochemical stability of composite electrodes. The extremely high capacitance and good electrochemical stability of TiO ₂ -G-PPy composite electrodes reveals that there are synergistic effects of TiO ₂ and PPy on the composites. Different TiO ₂ phases do not have remarkable effects on capacitance of the TiO ₂ -G system. After coated with PPy, different TiO ₂ phases exhibit different behaviors. Anatase-G-PPy has high capacitance whereas rutile-G-PPy has good stability. TiO ₂ -G-PPy heat-treated at 500 °C has the mixed TiO ₂ phases, and therefore has both high capacitance and good stability.