Water management in polymeric ion-conducting membranes is critical for energy harvesting applications such as in fuel cells and reverse electrodialysis. A group of researchers at Hanyang University in Korea recently reported the regulation of water content in hydrocarbon polymer membranes by coating the membranes with thin hydrophobic layers, patterned with nanometer-sized cracks. These “nanocracks” respond to external environmental stimuli of membrane swelling and humidity and act as ion- and water-selective nanovalves. This function bears striking similarities to the way the cactus Ferocactus schwarzii prevents water loss under dehydration conditions. Cacti plants achieve water control by opening and closing its stomata—little pores in their skin mainly for gas exchange—according to environmental conditions. To decrease water loss, stomata are open at night and closed during the daytime. This self-control mechanism of the cacti pores, which is based on the swelling and deflating of stoma cells, is analogous to the way nanocracks in ion-exchange membranes control water diffusion from the membrane interior, by narrowing or broadening their width.

Hydrophobic barriers, however thin they are, can drastically reduce ion conductivity in ion-exchange membranes, where water is the transport medium. Nanocrack-patterned hydrophobic layers resolve the paradox of water conservation within the bulk membrane, while simultaneously retaining the free movement of ions that co-transport with water molecules through the surface of the membrane.

“To get an ion-conducting membrane to conduct efficiently, the presence of water or another ion carrier is usually needed. We managed to maintain sufficient ion conduction even under adverse conditions, such as at elevated temperatures or lower humidity conditions, where normal membranes might dry out and significantly change their ion-conductive or ion-selective behavior,” says Michael Guiver from Tianjin University in China, co-author and visiting professor at Hanyang University during the time of the research.

Surface nanocracks were at first the surprising and perhaps disappointing outcome of experiments that aimed to improve the interfacial adhesion between membrane and electrodes in fuel cells. “At the onset of our research initiated about seven years ago, we were looking for processes including plasma deposition to thinly coat membranes with lotus leaf-like superhydrophobic surfaces,” remembers Young Moo Lee, a professor in the Department of Energy Engineering at Hanyang University in Korea and leader of the group.

However, the researchers soon recognized this phenomenon as being beneficial for self-humidifying membranes, using a mechanism analogous to water regulation in cacti.

Starting with the fabrication of cation-exchange sulfonated poly(arylene ether sulfone) (BPSH) membranes, thin hydrophobic surface-coating layers were deposited on their surface by atmospheric plasma treatment. The nanocracks did not became visible immediately after the treatment. However, upon hydration in distilled water, membrane swelling triggered the opening of the nanocracks, enabling water absorption. During dehydration of plasma-coated membranes at 30–45% relative humidity, the nanocracks become narrower, thus reducing water loss. To estimate the surface characteristics, atomic force microscopy was used, while dynamic vapor sorption measurements were performed to investigate water vapor sorption and desorption behavior.

One of the most striking findings, according to Lee, was the enhancement of the electrochemical performance of the nanocrack-regulated membranes compared to conventional ion exchange hydrocarbon membranes in fuel cell stack systems (for fuel cell electric vehicles) and stationary fuel cells requiring external thermal and water management systems.

Andrew Livingston, head of the Department of Chemical Engineering at Imperial College, characterizes this new work as ingenious in that “it delivers a solution to the important problem of retaining water in a membrane subjected to a dehydrating environment and yet still allows water (and ion) transport to and from the membrane.” For Livingston “the beauty of this solution lies in the responsiveness of the nanocracks in humid and dehydrating environments.”

According to Benny Freemann, head of the Membrane Research Laboratory of the University of Texas at Austin, “This study represents a clever approach to helping control dehydration of ion transport membranes used in fuel cells, employing readily available and scalable technology to address a pernicious problem plaguing fuel cell membranes.” Freemann adds that the approach has unexpectedly shown surprising improvements in ion permselectivity of such membranes, which is important in the application of membranes for water purification.

“For mass production, optimization and control of the nanocracking depends on the various polymer membrane materials, which have different water swelling properties,” says Lee. “If these membranes can be successfully scaled up, their price can be reduced significantly. Cooling and humidifier systems in fuel cell vehicles or stationary systems could be reduced or eliminated.” Lee’s group is planning to further explore, develop, and scale up nanocrack membranes for fuel cells, four-dimensional printing materials, and reverse electrodialysis.

Read the abstract in Nature