Nanocrack-regulated self-humidifying membranes

• Published in: Nature (London) Vol. 532; no. 7600; pp. 480 - 483
• Main Authors: Park, Chi Hoon, Lee, So Young, Hwang, Doo Sung, Shin, Dong Won, Cho, Doo Hee, Lee, Kang Hyuck, Kim, Tae-Woo, Kim, Tae-Wuk, Lee, Mokwon, Kim, Deok-Soo, Doherty, Cara M., Thornton, Aaron W., Hill, Anita J., Guiver, Michael D., Lee, Young Moo
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 28-04-2016; Nature Publishing Group
• Subjects: 639/301/299/893; 639/301/923/1028; 639/638/298; Biomimetic Materials - chemistry; Biomimetics; Cactaceae - metabolism; Cracks; Desiccation; Dialysis; Electrochemistry; Electrodialysis; Environmental regulations; High temperature; Humanities and Social Sciences; Humidity; Hydration; Hydrocarbons; Hydrophobic and Hydrophilic Interactions; Hydrophobic surfaces; letter; Membranes; Membranes (Technology); Membranes, Artificial; Morphology; multidisciplinary; Nanotechnology; Plant Stomata - metabolism; Polymers; Polymers - chemistry; Properties; Protons; Science; Surface Properties; Temperature; Water - analysis; Water content; Water management; Water supply
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature17634

Nanometre-scale cracks in a hydrophobic surface coating applied to hydrocarbon proton-exchange fuel-cell membranes work as tiny valves, delaying water desorption and maintaining ion conductivity in the membrane on dehumidification.
Nanocrack coatings keep critical membranes hydrated
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.
The regulation of water content in polymeric membranes is important in a number of applications, such as reverse electrodialysis and proton-exchange fuel-cell membranes. External thermal and water management systems add both mass and size to systems, and so intrinsic mechanisms of retaining water and maintaining ionic transport
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in such membranes are particularly important for applications where small system size is important. For example, in proton-exchange membrane fuel cells, where water retention in the membrane is crucial for efficient transport of hydrated ions
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, by operating the cells at higher temperatures without external humidification, the membrane is self-humidified with water generated by electrochemical reactions
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. Here we report an alternative solution that does not rely on external regulation of water supply or high temperatures. Water content in hydrocarbon polymer membranes is regulated through nanometre-scale cracks (‘nanocracks’) in a hydrophobic surface coating. These cracks work as nanoscale valves to retard water desorption and to maintain ion conductivity in the membrane on dehumidification. Hydrocarbon fuel-cell membranes with surface nanocrack coatings operated at intermediate temperatures show improved electrochemical performance, and coated reverse-electrodialysis membranes show enhanced ionic selectivity with low bulk resistance.