Synergistic effect between the graphene electrodes and the BiFeO3-BaTiO3 composite to overcome the limit of capacitance in eco-friendly supercapacitors made with a seawater electrolyte

Abstract

To reduce the contamination of the water/soil by wasted polyethylene (generated by daily products), we propose here the fabrication of efficient and flexible supercapacitors (SCs) using recycled polyethylene (PE) from toothpaste tubes. A graphene ink was printed on the PE to make it conductive and the capacitive performance of the SCs was enhanced by adding to their electrodes a BiFeO3/BaTiO3 (BF/BT) ceramic. The devices made without the BF/BT showed a maximum capacitance and energy density of 312.4 F g−1 and 62.4 Wh kg−1, respectively. Moreover, the devices made with BF/BT and PVA/H3PO4 gel-electrolyte, presented a capacitance and energy density of 721.8 F g−1 and 144.6 Wh kg−1, respectively. This last device had 130% more capacitance than that without BF/BT. In order to make an eco-friendly SC, we substituted the PVA/H3PO4 electrolyte with seawater and obtained a capacitance and energy density of 563.7 F g−1 and 112.7 Wh kg−1, respectively. This last value of capacitance is ≈ 22% lower than that for the device made with PVA/H3PO4, but we avoid the toxicity of the acidic electrolyte for the environment. Interestingly, the SC devices reported here maintained a voltage of 0.6–1.05 V after 10 h of steady discharge. Absorbance, XPS and Raman techniques demonstrated the presence of oxygen vacancies, Ti4+/ Ti3+, Bi3+/ Bi2+ and Fe3+/ Fe2+ species (redox centers) on the SC electrodes made with BF/BT and PVA/H3PO4, but only the oxygen vacancies and Bi3+/ Bi2+ species were found in the SC electrodes fabricated with BF/BT and seawater. Thus, the lower performance of the devices made with seawater is associated to the absence of certain redox centers that contributed to the charge storage. Overall, the results of this investigation demonstrated that efficient SCs can be fabricated from recycled plastics and innocuous components (BF/BT and seawater) for wearable technologies, benefiting in this way the environment.