Flexible, low-cost and scalable, nanostructured conductive paper-based, efficient hygroelectric generator†
Electric energy harvesting mediated by atmospheric humidity was first demonstrated in 2010, followed by the description of different approaches by many authors. Moisture-based energy-harvesting devices usually contain metals and/or are fabricated using nanomaterials and lithographic processes. Here we present a flexible energy harvesting device that is an asymmetric capacitor, formed by two electrodes made out of Kraft paper coated with exfoliated and reassembled graphite (ERG). The electrodes are doped with different chemicals and finished using standard papermaking processes. The voltage output of a single ERG-based hygroelectric generator (ERG-HEG) cell is as high as 0.5 V and it can deliver 250 nA of electric current through a 2 MΩ resistor, for days. The voltage and current outputs can be scaled up by connecting ERG-HEGs in series or parallel. Energy harvesting is also achieved by short (5 seconds) cyclic capacitor charge/discharge extending for more than two weeks. Examinaton of the electrodes during their operation, using Kelvin probe and microscopy analysis, showed that negative charge carriers are the main actors in the ERG-HEG devices. The low material cost, the simple fabrication processes and the energy output invite further development and scaling-up of this “green” alternative for producing electricity.