Moisture-driven energy generation by vertically structured polymer aerogel on water-collecting gel

In the realm of energy harvesting, free-standing, film-type, moisture-driven electricity generators (MEGs) based on polymer hydrogels with moisture-retaining capability are promising as a convenient and practical power source, making them potentially suitable for various self-powered, wearable, and patchable electronic devices. Their performance is, however, substantially degraded under low-humidity (<40%) conditions owing to the limited water supply to the device. This study presents a high-performance film-type MEG that functions at a low humidity of 20%. The MEG is based on an ion-selective polymer aerogel as an electricity-generating layer, constructed on a water-collecting gel as a water reservoir. The water-collecting gel in the bi-layered MEG absorbs moisture even under low-humidity conditions and efficiently delivers moisture to the electricity-generating aerogel layer. The energy-harvesting ability of the electricity-generating aerogel layer is further facilitated by its characteristic vertical microstructure, which comprises aerogel pores directionally grown on the frozen water-collecting gel during the freeze-drying process. The low humidity-tolerant MEG generated a stable open-circuit voltage of 1.1 V for more than 500 min even under dry conditions (20% relative humidity). Various self-powered electronic applications are demonstrated with the low-humidity-tolerant, bi-layered MEGs in dry environments. This study provides new insights for designing future MEGs for operation in low-humidity environments, further facilitating the development of green and sustainable power generation.