Effect of graphene quantum dot size on the capacitance of rGO-based supercapacitors fabricated with a sustainable electrolyte (seawater)

This work reports high-performance and sustainable supercapacitors (SCs) made with electrodes composed of a SiO 2 /MnO:V (MnVSi) redox powder, graphene quantum dots (GQDs), and reduced graphene oxide (rGO). rGO and GQDs were synthesized from recycled graphite, which was extracted from spent lithium-ion batteries. Further, seawater (an abundant, and eco-friendly electrolyte) was utilized in the SCs. Moreover, the effect of GQD size on the electrochemical performance of SCs was studied. Green emitting GQDs (G-GQDs, size ~6.90 nm) provided a higher active surface area and higher content of defects compared to red emitting GQDs (R-GQDs, size ~15.76 nm). The G-GQDs/MnVSi-SC device (made with electrodes of rGO + G-GQDs + MnVSi) achieved a capacitance of 666.19 F/g, energy density of 66.31 Wh/kg and power density of 140.56 W/kg. Those values of capacitance and energy density surpassed these produced by the R-GQDs/MnVSi-SC device (484.88 F/g and 56.90 Wh/kg), which was made with electrodes of rGO + R-GQDs + MnVSi. In addition, control devices made only with rGO and SiO 2 /MnO:V produced lower capacitances of 56.0 and 88.65 F/g, respectively. Thus, adding the MnVSi redox powder decorated with GQDs on the SC electrodes enhanced their capacitance by 7–11 times. The devices made with G-GQDs had higher capacitance because the G-GQDs contained more oxygen vacancy defects as well C O, C O, and O C O groups on their electrodes, which contributed to the storage of charge. Those defects and functional groups were detected by UV–Vis, Raman and XPS spectroscopies. Additionally, XPS analyses detected changes in the oxidation states for V element (V 5+ → V 4+ ) together with the formation of Si O C and Si C bonds, indicating interfacial coupling between the silicon matrix and the carbon nanostructures.