Hydrogel Supercapacitor Fabricated by Lignin-Mediated Laser-Induced Graphitization

Supercapacitors have drawn remarkable attention among energy storage devices due to their fast storage capability and enhanced cyclic stability. Hydrogels are one of the promising materials to be used for supercapacitors since they can easily absorb and retain liquid electrolytes. If conductive structures can be directly patterned on hydrogels, the hydrogel can fulfill both roles as an electrolyte retainer and supporting material. Laser-induced graphitization is a compelling technique to directly pattern electrically conductive graphitic carbon on a polymer by simply irradiating a laser beam. To the best of our knowledge, the one-step fabrication of electrically conductive structures on hydrogels by laser-induced graphitization has yet to be demonstrated. In this study, we demonstrated the direct fabrication of electrically conductive graphitic structures on hydrogels by laser-induced graphitization using lignin. Agarose hydrogel containing lignin, both biomass materials, were chosen as a precursor material for realizing the fabrication of a fully-biomass supercapacitor. The irradiation of 522-nm femtosecond laser pulses modified the surface of the hydrogel to a visibly-black colored structure that exhibit electrical conductivity. The Raman spectrum obtained from the structure showed peaks that correspond to D, G, and 2D bands, suggesting the formation of graphitic carbon. The fabricated structures exhibited a higher conductivity for higher lignin concentrations. No visible modification was observed after laser irradiation when no lignin was added to the agarose hydrogel. It should be noted that lignin is an organic material that exhibits high optical absorbance of 522-nm light. Therefore, the addition of lignin into the agarose hydrogel increases the absorbed energy, as well as the carbon content, which may lead to the facilitation of graphitization. To investigate the capability of the fabricated structures for supercapacitors, a pair of structures to be used as electrodes were fabricated on agarose/lignin hydrogels containing NaCl. Capacitance behavior was confirmed from the structures, demonstrating the one-stepped fabrication of hydrogel-based supercapacitors. The prepared hydrogel acts as the electrolyte retainer and supporting material of the fabricated supercapacitor. The proposed method offers a facile fabrication of a hydrogel-based supercapacitor.