Direct fabrication of high performance moisture-driven power generators using laser induced graphitization of sodium chloride-impregnated cellulose nanofiber films

High performance moisture-driven power generators were developed by laser induced graphitization (LIG) of sodium chloride-impregnated cellulose nanofiber films (CNFs). CNFs impregnated with different amounts of NaCl were obtained by immersing CNFs in NaCl solutions of various concentrations. A CO2 laser engraver was employed to convert the CNFs to porous graphitic carbon films (GCFs) under ambient conditions. By focusing the laser beam on the top surface of the CNF, the laser intensity was the highest on the top surface and gradually decreased toward the bottom surface. Since the focal temperature of the laser beam was higher than the boiling point of NaCl, the NaCl particles near the top surface evaporated more rapidly, creating a NaCl concentration gradient along the thickness direction. When the GCF was exposed to moisture, the dissociated ions migrated between the top and bottom surfaces, producing an electrical current. The maximum voltage and current outputs were 0.65 V and 550 mu A/cm2, respectively, at 90% relative humidity (RH). Due to abundant dissociated ions, the current output was produced continuously rather than pulsed, and was the highest current reported so far. We demonstrated that six vertically stacked GCFs (each 3 mm x 3 mm x 240 mu m in size) at 75% RH were sufficient to turn on green light emitting diodes (LEDs) operating at an onset potential of 2 V for 48 h without any auxiliary devices, such as rectifier circuits and capacitors.