Passive atmospheric water harvesting: Materials, devices, and perspectives

Atmospheric water, being ubiquitous, is extractable in an almost all-scope fashion: from coasts to inland areas, besides being large in amount, thus providing us with a promising path towards global water sustainability. Conventional active atmospheric water harvesting (AWH) technologies can yield a daily water output of over 10 liters, whereas their intrinsic need for electricity supply makes them a non-ideal choice for continuous development. In this context, passive approaches that leverage natural energy sources as their driving force are more favorable, and they can be mainly concluded into fog harvesting, radiative cooling, and sorption-based AWH. Given that current passive AWH technologies are not a mainstream solution to water scarcity despite all their advantages, this review aims to give an in-detailed summary of the recent developments in this field as well as a discussion on some personal perspectives concerning their future advancements, with the hope of providing some technical insights into their further evolvement and bridging the gap between researchers. To sum up, recent years have seen much progress in passive AWH concerning heat and mass transfer, thermal management, overall productivity elevation, and so forth, whereas the scalability, long-term outdoor practicability, and cost-effectiveness of passive AWH technologies need further improvements.