Electrochemical impedance of laser-induced graphene: Frequency response of porous structure

Abstract

Laser-induced graphene (LIG) is a promising material for the development of various elements in modern electronics, sensors, and energy storage/conversion devices. Electrochemical impedance spectroscopy (EIS) is a successful and reliable technique for studying electrochemical behavior of different materials in various electrochemical systems and devices, including LIG and LIG-based materials. However, all the published EIS studies on LIG have not comprehensively analyzed the frequency response of its extended, non-homogeneous and hierarchical porous structure. In the present study, we focus on the analysis of EIS data for polyimide-derived LIG films taking into account their porous structure relaxation which depends on both synthesis conditions, in particular, the number of laser beam passages, and duration of their exposure in electrolyte (1 M H2SO4). The frequency response of LIG has been analyzed by an equivalent circuit model taking into account the porous structure hierarchy. Macro-, meso- and micropores which make different contributions to the total capacitance are activated at different characteristic frequencies, and their contributions and frequencies being estimated in the study. It has been demonstrated that long-term electrolyte exposure increases the capacitance due to more efficient impregnation of the porous structure, involving the smallest micropores. Mesopores and macropores have been shown to make the main contribution to the capacitance of LIG. Blocked pores formed under multi passage of the laser beam over LIG surface prevent electrolyte impregnation of the porous structure and significantly reduce the capacitance. A LIG- and PVA/H2SO4-hydrogel-based supercapacitor has been obtained and analyzed. The produced supercapacitor demonstrates high capacitance characteristics (3.2 mF/cm2).