Graphene-doped Activated Carbon Gels and MnO2 Electrodes with Solid Polymer Electrolyte for Advanced Solid-state Supercapacitors

In this study, positive electrodes of manganese oxide (MnO2) and negative electrodes based on graphene-doped activated carbon gels (aCX) were produced. In specific, the negative electrodes were prepared by spreading an ink comprising by: 80 wt% of carbon gel (aCX); 5 wt % of carbon nanofibers (CNFs); 15 wt% of PVDF binder; and N,N dimethylacetamide (DMAc) solvent. Similarly, the positive electrodes were prepared from a slurry with 65 wt% of MnO2, 10 wt% of carbon black (CB), 10 wt % of CNFs, 15 wt% of PVDF in DMAc [1]. The developed electrodes were electrochemically characterized in a two-electrode system, in which a sodium-exchange Aquivion membrane acts as the polymer electrolyte, so that asymmetric solid-state supercapacitors (SC) were fabricated. Several characterizations, such as cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) in a wide operative voltage window from 0 to 1.8 V, were carried out to evaluate the performance of these energy storage devices. Besides, electrochemical impedance spectroscopy (EIS), long-term cycling stability and self-discharge measurements were also performed and analyzed. The initial specific capacitances of SCs calculated using GCD curves at 0.2 A g-1 reached 100 F g-1 and, showed well-rectangular initial shapes, very low resistivities even after prolonged cycle lifetime consisting of 10,000 GCD cycles and with a further > 150 h floating at 1.8 V [2]. Furthermore, the supercapacitor achieved a high energy density of 11.25 Wh kg-1AM and with a power density of 165 W kg-1AM. In a further development, the best SC device showed a high ability to retain a high voltage (i.e. 1.8 V) from 10 to 70 h without self-discharge (with a voltage decay lower than 30% after 48 h), which corresponds to values in the range of 2.7 to 5.2 mV h-1 (see Figure 1), which are among the lowest values appeared in the literature [3] also in comparison to commercial supercapacitor devices.