2.4.5 Laser-induced graphene
In 2014, Tour et al.[127] successfully prepared macroporous laser-induced graphene (LIG) by treating polyimide membranes with CO2 lasers. Since then, LIGs prepared by various substrates have been widely used in various fields such as energy storage and sensing. Among them, paper-based materials have proven to be excellent precursor substrates for LIG, but because untreated cellulose paper burns easily when treated at high temperatures in air, it needs to undergo chemical treatment. Lu et al.[128] soaked the paper in 0.1 M flame retardant solution sodium tetraborate for pretreatment. This is to prevent combustion when the laser processes the paper and better produce LIG. Figure 15E shows SEM images of cellulose paper and LIG. When used as a supercapacitor, the paper-based LIG exhibits a specific capacitance of 4.6mF cm-2, and the voltage and energy output of these devices can be controlled by connecting them in series and parallel. Although the LIG obtained by the paper-based material treated with flame retardant has been successfully applied to the field of supercapacitors, the capacitance performance of the device obtained by this method is still not very good. Lin et al. [129] The paper-based device prepared by combining chemical foaming and LIG exhibits a specific capacitance of 23.8mF cm-2. Figure 15G shows the preparation process of this paper-based electrode. The paper is first soaked in the inorganic salt NaHCO3 for processing. Then, the treated paper is patterned and scanned by a CO2 laser to obtain a paper-based electrode. Finally, gel electrolyte is added to obtain a paper-based LIG device. It is worth mentioning that thanks to the foaming effect of NaHCO3(Figure 15F), the CO2 laser produces a large number of porous LIG on the paper when processing it. This structure facilitates the migration of electrons and ions during electrochemical energy storage.