Materials science: Cracks help membranes to stay hydrated.

Abstract

Membranes have been prepared with a cracked coating that prevents them from drying out in low-humidity conditions — a boon for devices, such as fuel cells, that need hydrated membranes to function. See Letter p.480 Polymeric membranes are important in many processes that rely on ionic or size separation — for example, in water filtration, power generation by reverse electrodialysis, and energy generation and storage in fuel cells and flow batteries. Many applications require the membrane to be hydrated at all times so that ionic transport across it is maintained, and this can be a problem where the membrane is not in a humid environment. Here Young Moo Lee and colleagues report a strategy to keep hydrocarbon membranes from drying out and therefore maintain ionic conductivity across them. Inspired by stomata in cactus plants, which allow water in at night but close up in hotter and drier conditions, they apply a hydrophobic surface coating featuring nanometre-scale cracks 'nanocracks' to hydrocarbon proton-exchange fuel-cell membranes. When the membrane starts to dry out, it shrinks and the cracks in the coating close, acting as nanovalves to delay water desorption and maintain ion conductivity in the membrane upon dehumidification. The authors demonstrate improved performance in an intermediate temperature fuel cell and a reverse-electrodialysis membrane when fitted with surface nanocrack coatings.