Highly efficient and biodegradable flexible supercapacitors fabricated with electrodes of coconut-fiber/graphene nanoplates

Abstract

We report the fabrication and electrochemical characterization of biodegradable and flexible supercapacitors (SCs). The conductive electrodes of these SCs were pieces of compacted coconut fibers coated by graphene nanoplates. An oral electrolyte solution (OES) and rice paper were used as electrolyte and separator, respectively. The SCs’ encapsulation was made of gelatin/pectin. According to the electrochemical tests, the maximum capacitance and specific energy of the cell made with the biodegradable components were 670.8 F g−1 and 134.2 Wh kg−1, respectively. After adding MgTiO3 nanoparticles (NPs) to the anode, both, the capacitance and specific energy were enhanced by ≈37% and by ≈20%, respectively. For comparison purposes, the OES electrolyte was substituted by a conventional acidic electrolyte (PVA/H3PO4) in the SCs, but the capacitance and specific energy obtained were lower (175.9 F g−1 and 35.2 Wh kg−1). Interestingly, the biodegradable cells exhibited very long discharge times of 550–600 min and a stable output voltage in the range of 0.54–0.71 V, which has not been observed previously for biodegradable SCs. The SCs made with MgTiO3 NPs stored charge by redox reactions and the redox centers were oxygen vacancies (defects), Mg2+/Mg0 and Ti3+/Ti4 (the presence of such centers was confirmed by x-ray photoelectron spectroscopy, Raman and absorbance measurements). In general, we demonstrated that the biodegradable SCs are sustainable/eco-friendly energy sources that are promising to substitute the conventional technologies of Li or alkaline batteries made with toxic/corrosive components that contaminate the environment.