Facile fabrication of a highly efficient moisture-driven power generator using laser-induced graphitization under ambient conditions

A porous cellulose nanofiber (CNF) substrate was prepared by drying a TEMPO (2,2,6,6-tetramethylpiperidin-1-oxyl radical)-treated bleached pulp solution in a vacuum freeze dryer. A graphitic carbon layer (GCL) was fabricated directly onto the CNF substrate using infrared laser irradiation at ambient conditions. By focusing the laser beam on the top surface of the CNF substrate, higher conversion of the CNF to GCL was achieved on the top surface than the bottom surface. An oxygen-to-carbon ratio (O/C) gradient was established between the top and bottom surfaces during laser induced graphitization (LIG), as the O/C of the GCL decreased with laser intensity. When the GCL with the O/C gradient was exposed to water vapor, hydrolysis of the carboxyl groups in the GCL produced hydrogen ions. The resulting hydrogen ion concentration gradient between the top and bottom surfaces created electricity. At 82% relative humidity, the voltage and current outputs from a 3 x 3 mm(2) GCL were 0.83 V and 5.93 mu A/cm(2), respectively.