Fluorinated graphite paper used for self-powered water speed sensors by immersion-type tribovoltaic effect-dominated triboelectric nanogenerators

The advancement of nanogenerators (NGs) enables self-powered wearable devices　 to respond to environmental parameters, which encourages new application scenarios to be explored. In the work, an immersion-type triboelectric NG (TENG) is fabricated to harvest mechanical energy from running water and acts as a novel self-powered water-speed sensor by fluorinated graphite paper (FGP). The oscillograms of output currents driven by flowing water are comprised of major direct current (DC) and minor alternative current (AC) signals. The DC signals are attributed to tribovoltaic mechanisms between the FGP and water, and the AC signals are attributed to triboelectric mechanisms between the FGP and ions in water, testified by subsequent experiments in NaCl(aq). The FGP-water TENG demonstrates the output current density and voltage of 3.3 μA/cm2and 65mV at a water speed of 4.2cm/sec, showing 84 and 14-fold enhancements, respectively, compared with the GP-water TENG. The sensing of water speed by using the output current of the FGP-water TENG shows a linear calibration curve with R2larger than 0.99. In addition to water speed sensing, bubble sensing in an aquarium, and energy harvesting in dynamic water by ultrasound are also demonstrated by the FGP-based device. The new applications and mechanisms involved in the work provide new insights into the triboelectric and tribovoltaic mechanisms in water and novel ideas to monitor and utilize water resources.