Enhancing the capacitance and robustness of graphene supercapacitors by adding a coating of activated-carbon/Bi2Al4O9 on their electrodes

Abstract

Recently, using activated carbon (AC) for supercapacitors (SCs) has attracted attention because such carbon can be easily derived from bio-waste. However, the capacitance of the AC based SCs is in general low. For this reason, we present in this research the synthesis of AC from aquatic water-lily plant, which is considered a pest. First, SCs were fabricated only with graphene electrodes and produced a capacitance (Cs)/Energy-density (E) of 150.3 F g-1/30.1 Wh kg-1. Later, a coating of AC was deposited on the graphene electrodes and now the devices had higher Cs and E of 446.7 F g-1 and 89.4 Wh kg-1, respectively. The Cs/E of the devices was enhanced even more to 641.9 F g-1/128.3 Wh kg-1 after depositing a composite powder of AC/Bi2Al4O9 (BiAl) on the SC electrodes. Thus, the capacitance increased by 197% and 327% after introducing AC and AC/BiAl to the SCs. Other benefits of introducing the AC/BiAl composite to the electrodes were: i) the output voltage was enhanced 40% with respect to the device made with only AC; ii) the capacitance retention for 1000 bending cycles was maintained over 95%, this means that adding the AC/BiAl to the SC electrodes increased the mechanical resistance/stability of the devices; iii) the SCs produced a constant operating voltage of 0.33 V when they immersed in motor oil, which is not commonly reported in the literature; and iv) the SCs maintained a voltage of 0.37 when they are subjected to bending in real time. If no BiAl is added to the SC electrodes, the output voltage is lower (0.15 V). On the other hand, Raman and XPS spectroscopies demonstrated that the devices made with AC/BiAl had on their electrodes oxygen vacancies and chemical species such as: Bi5+/Bi3+/Bi0 and Al3+/Al0, which worked as redox centers for the charge storage. In general, this work demonstrated that it is possible the fabrication of AC-based SCs with capacitances that surpass the values already reported in literature for devices made with AC (50–150 F g-1). Also, we confirmed that adding BiAl to the system increased the robustness (mechanical/electrochemical stability) of the devices, which is of interest for the flexible electronics.