Laser-Induced Carbonization of Sodium Chloride-Impregnated Cellulose Nanofiber Films for Efficient Moisture-Electric Energy Transformation

We developed high-performance moisture-driven power generators (MPGs) using sodium chloride (NaCl) impregnated cellulose nanofiber films (CNFs). CNFs impregnated with different amounts of NaCl were obtained by immersing CNFs in NaCl solutions of various concentrations. An infrared laser was employed to photothermally 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 (~2000 °C) 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. In addition, the top surface became more graphitized than the bottom surface, producing a more porous structure on the top surface. When the GCF was exposed to moisture, the dissociated sodium ions migrated along the preformed concentration gradient, producing electricity. The maximum voltage and current outputs were 0.65 V and 550 μA/cm², respectively, at 90% relative humidity (RH). In particular, the current density output was produced continuously rather than pulsed, and was the highest current reported so far. Such a high current density was obtained by impregnating excess ions and implementing a high NaCl concentration gradient using LIG along the thickness direction of the extremely thin GCF. Furthermore, the porous structure of the GCF was found to produce higher performance than the graphene oxide-cellulose nanofiber composites (GO-CNF) with NaCl due to the presence of more water pathways for ion transport between electrodes. The developed device demonstrated its applicability by turning on four green light-emitting diodes (LEDs) operating at an onset potential of 2 V for 48h using six GCFs (each 3 mm × 3 mm × 240 µm in size) connected in series without a separate rectifier circuit and capacitor. Since moisture is ubiquitous, it has great potential for a wide range of applications such as sweat-powered wearable devices and remote environmental sensors.