Nanogenerators with Superwetting Surfaces for Harvesting Water/Liquid Energy

Water covers about 70% of the earth's surface and contains tremendous energy that remains untapped. Despite success in harvesting hydrodynamic energy based on heavy-weight and bulky electromagnetic generators, a great deal of water energies stored in the low-frequency flow of water such as in the form of raindrops, river/ocean waves, and the tide, remain largely untapped. In spite of diversity in development strategies and working mechanisms, engineering efficient water energy harvesting devices, especially nanogenerators, requires the elegant control of interfacial properties of substrates for rapid liquid mass and momentum transfer and effective electron generation/transfer. In particular, inspired by various special wetting phenomena in nature, the design of superwetting surfaces offers a new dimension to fundamentally mediate the way the liquid, as well as the charge, interact with the substrate. Herein, the latest progress in the development of nanogenerators with three distinctive interface types-solid/liquid, solid/solid, and liquid/liquid interfaces-are summarized and their representative applications, challenges, and future perspectives are highlighted.