Hydrovoltaic\nEffects from Mechanical–Electric\nCoupling at the Water–Solid Interface

The\nnatural water cycle on the Earth carries an enormous amount\nof energy as thirty-five percent of solar energy reaching the Earth’s\nsurface goes into water. However, only a very marginal part of the\ncontained energy, mostly kinetic energy of large volume bulk water,\nis harvested by hydroelectric power plants. Natural processes in the\nwater cycle, such as rainfall, water evaporation, and moisture adsorption,\nare widespread but have remained underexploited in the past due to\nthe lack of appropriate technologies. In the past decade, the emergence\nof hydrovoltaic technology has provided ever-increasing opportunities\nto extend the technical capability for energy harvesting from the\nwater cycle. Featuring electricity generation from mechanical–electric\ncoupling at the water–solid interface, hydrovoltaic technology\nembraces almost all dynamic processes associated with water, including\nraining, waving, flowing, evaporating, and moisture adsorbing. This\nversatility in dealing with various forms of water and associated\nenergy renders hydrovoltaic technology a solution for fossil fuel-caused\nenvironmental problems. Here, we review the current progress of hydrovoltaic\nenergy harvesting from water motion, evaporation, and ambient moisture.\nDevice configuration, energy conversion mechanism mediated by mechanical–electric\ncoupling at various water–solid interfaces, as well as materials\nselection and functionalization are discussed. Useful strategies guided\nby established mechanisms for device optimization are then covered.\nFinally, we provide an outlook on this emerging field and outline\nthe challenges of improving output performance toward potential practical\napplications.