Direct Fabrication of a Moisture-Driven Power Generator by Laser-Induced Graphitization with a Gradual Defocusing Method

A CO2 laser was employed to create a rectangle (4 × 2 mm2) of a conductive graphitic carbon layer (GCL) directly on a cellulose substrate. By tilting the substrate while keeping the laser power constant, the laser power density was gradually changed while scanning in the direction of the long side of the rectangle, due to deviation of the laser focus. As the laser beam defocus distance increased, the laser intensity at the substrate decreased, and the oxygen-to-carbon ratio (O/C) of the GCL increased. Upon exposing the GCL substrate to water vapor, the hydrogen-containing groups (carboxyl and hydroxyl groups) in the GCL were hydrolyzed, and a density gradient of hydrogen ions was induced due to the preformed O/C gradient. The resulting voltage and current outputs reached 0.23 V and 0.4 μA/cm2, respectively, at 70% relative humidity. Additionally, it was demonstrated that the electricity obtained during breathing could turn on a green light-emitting diode operating at an onset potential of 2 V when an array of the GCLs was attached to a filter mask.