Highly Efficient Moisture-Induced Power Generators through Flashlight-Induced Graphitization of FeCl₃-Impregnated Cellulose Papers

We have developed a method for fabricating and enhancing the performance of moisture-induced power generators (MPGs) through flashlight-induced graphitization. FeCl₃-impregnated cellulose papers (FCPs) were photothermally converted to graphitized cellulose papers (GCPs) with a hierarchically porous structure by flashlight irradiation under ambient conditions. During the photothermal process, a portion of the Fe³⁺ ions was transformed into iron oxide compounds with limited solubility. When a bilayered cellulose paper (BCP), formed by stacking GCP on top of FCP, was exposed to moisture, a potential difference was generated between the collecting electrodes due to the concentration gradient of dissociated Fe³⁺ ions within the BCP, with a higher concentration in FCP and a lower concentration in GCP. The resulting migration of Fe³⁺ ions from FCP to GCP caused electron movement along the external circuit. Fe³⁺ ions were found to serve multiple roles, such as catalysts for graphitization, moisture absorbers, and charge carriers. The BCP-based MPG exhibited a continuous generation of voltage and current outputs, instead of pulsed outputs, with maximum values of 0.39 V for voltage and 28.6 μA/cm² for current density at 45% relative humidity, representing the highest continuous current density value observed under ambient humidity conditions.