Sustainable flexible quasi solid state supercapacitor Lignin and Graphene based

CO2 emissions from energy production represent one of the main causes of the greenhouse effect. Indeed, about 70% of the world’s energy production comes from the combustion of fossil sources, which are connected to global warming. To mitigate this trend, higher energy production from renewable sources is required. However, since those can’t guarantee continuous production, the energy needs to be stored and then distributed through power grids. Supercapacitors have been developed as a promising energy storage solution due to their high-power density and fast charge/discharge rates. However, they present low energy densities, which can be improved with the right choice of active materials. This study aims to prepare a flexible supercapacitor composed of alkali lignin as anode and ultrasonic exfoliated graphene as a cathode, with a polyvinyl alcohol (PVA) gel electrolyte. Lignin, a solid waste product from paper production and one of the most abundant biopolymers in nature is a promising material for supercapacitor applications. While it is generally graphitized with high-energy processes, it can also be used as an active material, as in this case. Graphene, instead, has a positive effect on the cathode capacitance and can ensure high cycling stability. Finally, gel electrolytes can improve the device's potential window and present lower leakage with respect to liquid flammable electrolytes. The solubility of lignin was first tested in ethanol, methanol, and acetone, with the best results in an 85% methanol and 15% water solution. The lignin was deposited on a carbon paper substrate and the electrode was then electrochemically characterized with cyclic voltammetry, galvanostatic charge-discharge, and electrochemical impedance spectroscopy measurements in different aqueous electrolytes. The lignin electrode was more stable in the H3PO4 1M electrolyte where it reached a 39 mF cm-2 capacitance at 2 mV s-1. The same approach was then repeated for the graphene electrode with the same kind of substrate. For both materials, the best potential window was chosen by monitoring the coulombic efficiency. A first device was then assembled with a liquid H3PO4 electrolyte, and its stability was tested with 10000 charge-discharge cycles. However, a gradual dissolution of the lignin electrode caused problems in the stability of the device, as evidenced by the retention of only 20% of the initial capacitance after 1000 cycles. The electrode morphology was investigated by scanning electron microscopy and they were characterized by energy dispersive X-ray spectroscopy Fourier transformed infrared (FTIR) spectroscopy and Raman spectroscopy before and after the cycling. The preparation of a PVA gel electrolyte was then pursued, studying its crosslinking with glutaraldehyde by FTIR spectroscopy and characterizing its final mechanical properties with compressive tests. A final flexible supercapacitor was then assembled with the gel electrolyte and the lignin and graphene electrodes, and its stability investigated over 100 cycles, monitoring the coulombic efficiency. However, even increasing the concentration of the acid component of the PVA gel, the device could only reach a maximum coulombic efficiency of 18% because of partial dissolution effects.