Utilizing kraft lignin–derived hard carbon as an innovative bio-electrode in electrochemical capacitors

Abstract In recent years, biomaterials are gaining popularity due to high need to make energy storage devices greener and safer. After the carbonization process, lignin has a slightly larger specific surface and a porous structure, which can provide a significant electrochemical double-layer capacity. When combined with kraft lignin as a binder, which helps maintain the structural integrity of the electrode, the efficient use of the active material can be enhanced. The binder facilitates better dispersion of carbonized lignin particles, reducing agglomeration and ensuring greater availability of active sites for electrolyte ions. The aim of this work is to present a novel lignin-based hard carbon as an electrode material for applications in electrochemical capacitors. To this end, a detailed physicochemical and electrochemical analysis was conducted. Kraft lignin was carbonized at temperatures ranging from 600 to 1000 °C. The resulting material is characterized by thermal stability, a low polydispersion index (PDI), and mesoporosity. Thermogravimetric (TG) analysis was used to determine changes in structure, while functional groups were analyzed using Fourier-transform infrared spectroscopy (FTIR). In electrochemical applications, the material exhibits high cyclic stability and no redox reactions, with the primary mechanism of charge accumulation being based on the electrochemical double layer. Additionally, low resistances contribute to improved charge storage. Graphical abstract