Lotus leaf-inspired droplet-based electricity generator with low-adhesive superhydrophobicity for a wide operational droplet volume range and boosted electricity output

Abstract

As one of the nanogenerators which exploit the potential of the water cycle, a droplet-based electricity generator (DEG) has recently been leading the research field due to its high efficiency. Hence, if the DEG’s effectiveness could be extended to the microscale water cycle, such as raindrop, precipitation, fog, and dew, the application fields of DEG would be countless. While introducing a hydrophobic layer could be a solution to achieving such a wide operational droplet volume range, the hydrophobicity has been so far understood only in terms of increased electricity output of the DEG on the ground that promotes droplet sliding. Herein, we report a lotus leaf-mimicking DEG (simply, LL-DEG) with a low-adhesive superhydrophobic surface of the dielectric layer. The LL-DEG shows not only an increased electricity output with an energy conversion efficiency of 13.7%, but a wide operational droplet volume range to allow normal operation with a droplet volume down to 6 µL. For the first time, we deeply analyze how the lotus leaf-mimicking surface can increase the electricity output of DEG and derive the average rate of droplet contact area change over time by introducing a new parameter, which affects the electricity output. Furthermore, how the lotus leaf-mimicking surface expands the operational droplet volume range is systematically discussed from both the investigations of quasi-static and dynamic states of droplet wetting. The superiority of LL-DEG is confirmed from the demonstration in a rainfall environment including raindrop energy harvesting and self-cleaning property, which is essential for practical utilization in outdoor conditions. Finally, based on the pH-sensitive electricity output, the applicability of the LL-DEG is demonstrated as a raindrop acidity alert. This work, which extends the DEG’s effectiveness to the microscale water cycle, is expected to advance the practical utilization of DEG.