Performance Enhancement of Moisture-driven Power Generators by Photofragmentation of Inorganic Salt Particles

We developed a novel method based on the photofragmentation of inorganic salt particles for improving the moisture-electric energy transformation performance of a moisture-driven power generator (MPG). Infrared laser irradiation on cellulose nanofiber films (CNFs) prepared by a TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl radical)-mediated oxidation of bleached pulp induced a photothermal conversion of CNFs to porous graphitic carbon films (GCFs) with the catalyst-derived Na₂O₂ particles. Since the laser beam was focused on the top surface of CNF, the gradients of the photothermal conversion of CNFs and Na₂O₂ concentration were created along the thickness direction. Subsequent irradiation with ultraviolet (UV) light induced the photofragmentation of the micrometer-sized Na₂O₂ particles into smaller ones, which increased the surface area of the salt particles in contact with the GCFs and consequently increased the number of effective dissociable charge carriers. When the GCF was exposed to moisture, the dissociated sodium ions migrated along the preformed concentration gradient, producing continuous outputs of current and voltage. At 90% relative humidity, the maximum voltage and current density outputs of the MPG increased from 0.91 V and 18.7 μA/cm² before UV irradiation to 1.10 V and 56.2 μA/cm² after UV irradiation, respectively. Additionally, we demonstrated that a green light-emitting diode could be turned on without capacitors or rectifiers during normal breathing while wearing a face mask with three GCF arrays attached (each 3 mm × 3 mm × 0.1 mm in size).