Jakyung Eun

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).

We developed a novel hydrovoltaic power generator (HPG) using a Janus bilayer membrane with an asymmetric wettability. The Janus bilayer membrane was fabricated by stacking a hydrophobic graphene oxide (GO)-cellulose nanofiber (CNF) composite layer on a hydrophilic GO-CNF composite layer. Water supplied through the hydrophilic layer stops at the surface of the hydrophobic layer, producing separate wet and dry regions within the thin bilayer. Protons and sodium ions dissociate from oxygen-containing functional groups in the hydrophilic GO-CNF layer and migrate toward the hydrophobic layer, resulting in a maximum output voltage and current of 0.35 V and 20 μA, respectively, in deionized (DI) water. By replacement of DI water with a 0.6 M NaCl solution (i.e., the concentration of seawater), the output voltage and current were further increased to 0.55 V and 60 μA, respectively. This performance was consistent not only under low humidity due to the water supply but also under high humidity, where evaporation was restricted, indicating humidity-independent performance. The asymmetric wettability of the membrane remained stable throughout the experiment (7 days), enabling continuous power generation.