Hydrovoltaic Effects from Mechanical–Electric Coupling at the Water–Solid Interface

The natural water cycle on the Earth carries an enormous amount of energy as thirty-five percent of solar energy reaching the Earth's surface goes into water. However, only a very marginal part of the contained energy, mostly kinetic energy of large volume bulk water, is harvested by hydroelectric power plants. Natural processes in the water cycle, such as rainfall, water evaporation, and moisture adsorption, are widespread but have remained underexploited in the past due to the lack of appropriate technologies. In the past decade, the emergence of hydrovoltaic technology has provided ever-increasing opportunities to extend the technical capability for energy harvesting from the water cycle. Featuring electricity generation from mechanical-electric coupling at the water-solid interface, hydrovoltaic technology embraces almost all dynamic processes associated with water, including raining, waving, flowing, evaporating, and moisture adsorbing. This versatility in dealing with various forms of water and associated energy renders hydrovoltaic technology a solution for fossil fuel-caused environmental problems. Here, we review the current progress of hydrovoltaic energy harvesting from water motion, evaporation, and ambient moisture. Device configuration, energy conversion mechanism mediated by mechanical-electric coupling at various water-solid interfaces, as well as materials selection and functionalization are discussed. Useful strategies guided by established mechanisms for device optimization are then covered. Finally, we provide an outlook on this emerging field and outline the challenges of improving output performance toward potential practical applications.