3D-graphene hydrogel and tungsten trioxide-MnO2 composite for ultra-high-capacity asymmetric supercapacitors: A comparative study

Researchers worldwide use graphene extensively in its powder form for various applications such as supercapacitors, solar cells, drug delivery, and polymer composites, owing to its considerably valuable properties. However, relatively few studies have been made on the 3D-hydrogel form of graphene for these applications. Here we report a simple and fresh method to prepare 3D graphene hydrogel (3DGH) and its bimetal composite with tungsten trioxide (WO3) and manganese dioxide (MnO2). The structure of 3DGH and its composite is confirmed by advanced characterization techniques viz. Raman Spectroscopy, X-Ray diffraction (XRD), Fourier-transformed infrared spectroscopy (FT-IR), scanning electron microscope (SEM), and tunneling electron microscope (TEM). Further, two symmetric devices using 3DGH, 3DGH-metal composite (3DGHMC), and one asymmetric 3DGHMC-3DGH device are comparatively studied using two electrode systems. The effects of scan rates, current densities, voltage window, and frequency were examined for each device by cyclic voltammetry (CV), galvanic charge-discharge (GCD), and Electron impedance spectroscopy (EIS). The asymmetric device showed a high specific capacitance of 657 F g−1 with an excellent energy density of 48.25 W h kg−1 and a high-power density of 596.06 W kg−1 at the current density of 1 A g−1. Also, the asymmetric device retains 100 % of its initial capacitance after 1000 charging-discharging cycles, also extends its stability after 1000 cycling test. Moreover, the quantitative analysis of the device is also made by Dunn's method to evaluate the percentage contribution from surface and diffusion capacitance.