Sprayable porous hydrogel coating for efficient and sustainable evaporative cooling

Liquid spray cooling is extensively used in the thermal management of power electronics. Direct water spraying is simple but unavoidably results in substantial water waste. Enhancing water retention via hydrogel coatings can reduce water consumption, but current nonporous coatings suffer from slow water rehydration. Here, we present a spray-coating process that enables rapid in situ formation of a porous hydrogel coating. Mixed powders of polyvinyl alcohol (PVA) and tannic acid (TA) are sprayed, followed by a glutaraldehyde (GA) aqueous solution. A unique dual-step gelation yields a mechanically robust porous coating originated from the stacking of the powders. When used as the cooling layer, the porosity drastically enables faster water rehydration for prolonged cooling and simultaneously enhances the evaporation rate for more effective thermal management. This simple and scalable approach can be applied to diverse substrates with complex geometries, and the underlying principle can be extended to other cooling liquids.