Abstract:
Due to the increasing demands for energy-storage systems, supercapacitors (SCs) have received significant attention due to their properties/electrochemical performance. Many research papers and technical reports on the development of electrochemical supercapacitors have been published in recent years, inspired by their low specific energy and high production costs. Consequently, research focus in supercapacitors is on improving the specific energy (i.e. ~5˗8 Wh kg-1) without compromising its high specific power (i.e. ~5˗30 kW kg-1).
In this work, the electrochemical properties of both carbonized iron-polyaniline/nickel graphene foam (C-Fe/PANI/Ni-GF) and birnessite-type MnO2/carbon composite electrode materials were investigated for supercapacitor applications. The C-Fe/PANI/Ni-GF electrode material was directly synthesized on a current collector by pyrolysis of the iron-PANI mixture coated on nickel graphene foam in a tube furnace under the N2 atmosphere, and the MnO2-C electrode material with hierarchical nanostructures was successfully synthesized using KMnO4 solution and spent printing carbon grains method. The structural and morphological characterization of the as-synthesized electrode materials was carried out using scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectrometer (EDS), X-ray diffraction (XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS).
The electrochemical characterization of the as-prepared electrode material was evaluated in both three and two-electrode (i.e. symmetric) cell configurations using 1 M NaNO3 electrolyte. The C-Fe/PANI/Ni-GF symmetric device exhibited a maximum specific energy of 68.0 W h kg-1 and specific power of 718.2 W kg-1, at a specific current of 1.0 A g-1 and the maximum potential of 1.7 V. The device further displayed long-term cycling stability with capacity retention of 91% over 10 000 galvanostatic charge-discharge cycles at 5 A g-1. The stability of the device was also tested using the voltage holding and self-discharge approach whereby a slow-discharging process was observed, which suggests the practical application of the device.
To further evaluate the electrochemical performance of C-Fe/PANI/Ni-GF electrode with MnO2-carbon composites electrode, a hybrid electrochemical supercapacitor device was fabricated based on birnessite-type MnO2-C composite electrode and C-Fe/PANI/Ni-GF as the positive and negative electrodes, respectively. At the lowest specific current of 1.0 A g-1 and cell potential of 2.2 V in 2.5 M KNO3 electrolyte, the MnO2-C//C-Fe/PANI/Ni-GF asymmetric device displayed a high specific energy and power of 34.6 W h kg-1 and 1100.0 W kg-1 respectively. The device further displayed long-term cycling stability with capacitance retention of 98% over 10 000 galvanostatic charge-discharge cycles at 10 A g-1. This device is displaying the overall excellent electrochemical performance.
