A sustainable and foldable supercapacitor made with electrodes of recycled soda-label/graphene/ZnO:Ca and its mechanism for the charge storage

This investigation reports the electrochemical performance of foldable supercapacitors (SCs) fabricated with electrodes composed of recycled soda-labels and graphene. To enhance the electrochemical performance of such devices, ZnO:Ca nanoparticles were added on their electrodes, which are quasi-rounded grains with average size of 93 nm. The SC device made without ZnO:Ca showed a maximum capacitance and energy density of 41.2 F/g and 5.7 Wh/kg, respectively. After adding ZnO:Ca in the SC electrodes, such values are increased up to 487.6 F/g and 67.7 Wh/kg, thus, the capacitance and energy density were enhanced by ≈10.8 times. The devices made with ZnO:Ca had a capacitance retention of 72.6% after 6000 cycles of charge/discharge. The analysis by optical absorbance, Raman spectroscopy, photoluminescence and XPS spectroscopy demonstrated that the presence of oxygen vacancy defects on the SC electrodes produced the storage of charge by redox reactions. The Nyquist plots showed that the SCs made with ZnO:Ca had 38–68% lower charge transfer and series resistances than the device made without ZnO:Ca. This confirmed that the ZnO:Ca deposited on the SC electrodes improves the charge storage and ion diffusion. Hence, the results of this research revealed that efficient and foldable SCs could be made from recycled plastics (soda labels from plastic bottles) and could be used in wearable applications.