All-solid-state flexible supercapacitor based on nanotube-reinforced polypyrrole hollowed structures

Abstract

Supercapacitors are strong future candidates for energy storage devices owing to their high power density, fast charge-discharge rate, and long cycle stability. Here, a flexible supercapacitor with a large specific capacitance of 443 F g^-1 at a scan rate of 2 mV s^-1 is demonstrated using nanotube-reinforced polypyrrole nanowires with hollowed cavities grown vertically on a nanotube/graphene based film. Using these electrodes, we obtain improved capacitance, rate capability, and cycle stability for over 3000 cycles. The assembled all-solid-state supercapacitor exhibits excellent mechanical flexibility, with the capacity to endure a 180° bending angle along with a maximum specific and volumetric energy density of 7 W h kg^-1 (8.2 mW h cm^-3) at a power density of 75 W kg^-1 (0.087 W cm^-3), and it showed an energy density of 4.13 W h kg^-1 (4.82 mW h cm^-3) even at a high power density of 3.8 kW kg^-1 (4.4 W cm^-3). Also, it demonstrates a high cycling stability of 94.3% after 10 000 charge/discharge cycles at a current density of 10 A g^-1. Finally, a foldable all-solid-state supercapacitor is demonstrated, which confirms the applicability of the reported supercapacitor for use in energy storage devices for future portable, foldable, or wearable electronics.

Fig. 1. Schematic diagram illustrating the fabrication of NTPPy NWs. (a) A CNT solution was filtered through an AAO membrane, resulting in CNT adsorption on the membrane sidewalls. (b) Amine functionalization of AAO membrane. The membrane was treated with O₂ plasma and dipped into APTES solution. (c) Formation of CNT/GO base structure by vacuum filtration. (d) Drying and GO reduction process to give rGO. (e) CNT-wrapped PPy nanowire array formed by electrodeposition. (f) Removal of AAO template using NaOH solution.

Fig. 2. Morphology of the fabricated electrodes. (a) Scanning electron microscopy images of (a) CNT/rGO, (b) FPPy, (c) pristine PPy NWs, and (d) NTPPy NWs fabricated on CNT/rGO hybrid film. (e and f) Transmission electron microscopy images of a NTPPy NW.

Fig. 3. Compositional analysis of CNT/rGO, PPy and NTPPy NWs. (a) Raman and (b) XPS spectra of CNT/rGO, PPy NWs, and NTPPy NWs. High resolution C 1s spectra of (c) NTPPy NWs prepared on the CNT/rGO hybrid base at an optimum deposition charge of 3.6C.

Fig. 4. Electrochemical performance characterization of CNT/rGO, FPPy, PPy NWs, and NTPPy NWs: (a) cyclic voltammetry curves and (b) specific capacitances at different scan rates. (c) Nyquist plots of the four different electrodes. (d) Cycle stability comparison between PPy NWs and NTPPy NWs at a current density of 10 A g⁻¹.

Fig. 5. Electrochemical performance characterization with different PPy loadings (3.0, 3.3, 3.6, and 3.9C). (a) Specific capacitance of NTPPy NW electrodes coated with different PPy loadings. (b) Nyquist plots of NTPPy NW electrodes coated with different PPy loadings (deposition charge: 3.0, 3.3, 3.6, and 3.9C). (c) Capacitive behaviour of a NTPPy NW electrode prepared using a deposition charge of 3.6C at different scan rates. (d) Galvanostatic charge–discharge performance of NTPPy NWs (deposition charge of 3.6C).

Fig. 6. Electrochemical property characterization and flexibility test of all-solid-state symmetric supercapacitor. (a) Pictorial representation of an all-solid-state flexible supercapacitor device based on NTPPy NW electrodes. Electrochemical capacitance performance of the supercapacitor device: (b) CV curves at various scan rates, (c) galvanostatic charging/discharging at different current densities, (d) Ragone plot with specific energy and power densities and comparison to some literature data, (e) CV curves at a scan rate of 50 mV s⁻¹ under different bending conditions, (f) cycling stability measured at a current density of 10 A g⁻¹ over 10 000 cycles (inset shows a green LED lit using three supercapacitors connected in series).