Polyindole Embedded Nickel/Zinc Oxide Nanocomposites for High-Performance Energy Storage Applications

Abstract

Conducting polymers integrated with metal oxides create opportunities for hybrid capacitive electrodes. In this work, we report a one-pot oxidative polymerization for the synthesis of integrated conductive polyindole/nickel oxide (PIn/NiO), polyindole/zinc oxide (PIn/ZnO), and polyindole/nickel oxide/zinc oxide (PNZ). The polymers were analyzed thoroughly for their composition and physical as well as chemical properties by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), ultraviolet-visible spectroscopy (UV-Vis), and thermogravimetric analysis (TGA). The PIn and its composites were processed into electrodes, and their use in symmetrical supercapacitors in two- and three-electrode setups was evaluated by cyclic voltammetry (CV), galvanostatic discharge (GCD), and electrochemical impedance spectroscopy (EIS). The best electrochemical charge storage capability was found for the ternary PNZ composite. The high performance directly correlates with its uniformly shaped nanofibrous structure and high crystallinity. For instance, the symmetrical supercapacitor fabricated with PNZ hybrid electrodes shows a high specific capacitance of 310.9 F g^-1 at 0.5 A g^-1 with an energy density of 42.1 Wh kg^-1, a power density of 13.2 kW kg^-1, and a good cycling stability of 78.5% after 5000 cycles. This report presents new electrode materials for advanced supercapacitor technology based on these results.

Keywords: conductive polymers; one-pot synthesis; oxidative polymerization; supercapacitors; ternary hybrid electrodes.

Scheme 1

Synthesis of PIn-NiO-ZnO (PNZ-1 to -4) via one-pot oxidative polymerization.

Figure 1

SEM-images of (a) NiO, (b) ZnO, (c) PIn, (d) PIn/NiO, (e) PIn/ZnO, (f) PNZ-3.

Figure 2

FTIR spectra of the synthesized PIn and its metal oxide composites.

Figure 3

XRD spectra of the synthesized PIn and its metal oxide composites.

Figure 4

TGA spectra of the synthesized PIn and its metal composites.

Figure 5

(a) CVs of PIn, PIn/NiO, PIn/ZnO, and PNZ-3 in three-electrode setup at 20 mV s⁻¹ in a potential range of 0 V to 0.9 V, (b) CVs of PNZ-3 at different scan rates, (c) calculated specific capacitances at different scan rates (20–100 mV s⁻¹) from the CVs of PNZ-3, (d) logarithmic correlation of the current response of PNZ-3 to the scan rate, (e) comparison of the GCD profiles of PIn, PIn/NiO, PIn/ZnO, and PNZ-3 at a current density of 1 A g⁻¹ in the potential range of 0 V to 0.9 V, (f) GCD curve of PNZ at different current densities (0.35 to 5.0 A g⁻¹ in a potential range of 0 V to 0.9 V), (g) specific capacitances of PIn, PIn/NiO, PIn/ZnO, and PNZ at different current densities (0.35 to 5.0 A g⁻¹), (h) EIS of PNZ at V_DC = 200 mV in the frequency range of 100 kHz to 50 mHz, (i) electrical equivalent circuit (EEC) used for fitting the EIS spectra (the mass of polymer was 0.2 mg for all fabricated electrodes, and all CV, GCD, and EIS plots were recorded in 1.0 M H₂SO₄ aqueous solution).

Figure 6

(a) CVs of PNZ in two-electrode setup at different scan rates (20 to 100 mV s⁻¹) in a potential range between -0.3 V and 0.9 V, (b) GCD of PNZ at different current densities (0.5 to 5.0 A g⁻¹) in the same potential range, (c) calculated specific capacitances of PNZ at the different current densities, (d) coulombic efficiencies of PNZ at the different current densities, (e) Ragoné plot of PNZ, (f) CVs of PNZ at a scan rate of 100 mV s⁻¹ after aging in a potential range between -0.3 V and 0.9 V, (g) cycle stability of PNZ at 100 mV s⁻¹ over 5000 cycles, (h) EIS spectra of PNZ in two-electrode setup at V_DC = 200 mV in the frequency range of 100 kHz to 50 mHz, (i) electrical equivalent circuit used for EIS fitting. (The mass of polymer was 0.2 mg for all fabricated electrodes, and all CV, GCD, and EIS plots were recorded in 1.0 M H₂SO₄ aqueous solution).