Asymmetric Supercapacitors based on Graphene-doped Activated Carbon Gels and MnO2, Electrodes with Solid Polymer Electrolyte

In this work, positive electrodes based on manganese oxide (MnO2) and negative electrodes based on graphene-doped activated carbon gels (ACG) were produced. In particular, the negative electrodes were prepared by spreading an ink made by: 80 wt % of carbon gel (ACG); 5 wt % of carbon nanofibers (CNF); 15 wt % of poly(vinylidene fluoride) (PVDF) as binder; and N,N dimethylacetamide (DMA) solvent. Similarly, the positive electrodes were prepared by using a slurry consisting of: 65 wt % of MnO2 , 10 wt % of carbon black (CB), 10 wt % of CNF, 15 wt % of PVDF and DMA [1]. The developed electrodes were electrochemically characterized in the two-electrode system, in which a sodium-exchange Aquivion membrane serves as the polymer electrolyte, so that asymmetric supercapacitors (SCs) can be fabricated. Several electrochemical characterizations, such as cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) with an operating voltage window reaching up to 2 V, were performed to evaluate the characteristics of these energy storage devices. Besides, electrochemical impedance spectroscopy (EIS), long-term cycling stability and self-discharge tests were also performed. The initial specific capacitances of the asymmetric SC calculated by GCD at 0.2 A g-1 were around 100 F g-1 and showed also initial well-rectangular shapes, low resistivities even after long cycle life of 10,000 cycles (with additional > 150 h in floating at 2.0 V) [2]. Furthermore, the supercapacitor achieved a high energy density of 15.3 Wh kg-1 and precisely a stability for more than 50,000 cycles, when it was also tested through regular galvanostatic charge/discharge (GCD) tests at ± 2 Ag-1. In a further development, very low voltage decay (< 30%) was found during self-discharge measurements followed for more than 48 hours.