In hydrocarbon proton-exchange membranes, water regulation has been achieved primarily through polymer architecture that induces phase-separated morphology between hydrophilic ion-conducting channels and the hydrophobic matrix, similar to state-of-the-art perfluorosulfonic acid proton-exchange membranes (such as Nafion), allowing high ion conduction with reduced overall membrane hydration. Despite improvements in water retention using this approach, additional challenges have led to the performance being below expectations.

Here, we propose a new concept for regulating membrane hydration in low-humidity or non-humidified environments without modification of the morphology of an ion-exchange membrane, analogous to the water retention mechanisms of the cactus plant. The cactus retains water by opening and closing an array of stomatal openings, which respond to environmental conditions. To decrease water loss, stomata are open at night, during conditions of lower temperature and higher humidity. During the daytime, when hot and arid conditions prevail, the stomata are closed.

—Park et al.

Proton exchange membrane fuel cells (PEMFCs) need to stay constantly hydrated, said CSIRO researcher and co-author Dr. Aaron Thornton. Currently, this is achieved by placing the fuel cells alongside a radiator, water reservoir and a humidifier. However, these components of the balance of plant occupy a large amount of space and consume significant power, he said.

In the new nanocrack membrane, water, generated by an electrochemical reaction, is regulated through the nanocracks within the skin. The cracks widen when exposed to humidifying conditions, and close up when it is drier.

Basic concept of self-humidifying nanocrack membrane. a, A hydrophobic coating layer provides a self-controlled mechanism for water conservation using nanocracks tuned by membrane swelling behavior in response to external humidity conditions. b, AFM images reveal the self-controlled mechanism of plasma-coated membranes. Plasma-coated BPSH membranes have no visible nanocracks immediately after treatment (top panels). However, upon hydration in distilled water, membrane swelling triggers the opening of the nanocracks, enabling water absorption (middle panels). During dehydration of plasma-coated membranes at 30% to 45% relative humidity (RH), the nanocracks become narrower, thus reducing water loss (bottom panels). c, Voronoi diagram analysis and tessellation entropy verified controllable nanocrack surface pattern images of plasma-coated membrane (P-BPSH60R30, where R30 indicates that the plasma treatment was repeated 30 times) in hydration (100% RH) and dehydration (30% to 45% RH). Park et al. Click to enlarge.

This means that fuel cells can remain hydrated without the need for bulky external humidifier equipment. We also found that the skin made the fuel cells up to four times as efficient in hot and dry conditions.

—CSIRO researcher and co-author Dr. Cara Doherty

Professor Young Moo Lee from Hanyang University, who led the research, said that this could have major implications for many industries, including the development of electric vehicles.

At the moment, one of the main barriers to the uptake of fuel cell electric vehicles is water management and heat management in fuel cell systems. This research addresses this hurdle, bringing us a step closer to fuel cell electric vehicles being more widely available.

—Professor Young Moo Lee

The cross-continent team has been working together for more than ten years. For this study, Hanyang University conceived and designed the experiments. Using characterization and modelling expertise, CSIRO researchers were then able to determine how the membranes behaved under changing humidities.

The intermediate operating temperature of over 100 °C can also maximize the advantageous high glass-transition temperature of hydrocarbon proton-exchange membranes for enhanced membrane stability. This unexpected result is noteworthy because the hydrophobic plasma-coating technique is attractive for commercial scale-up. It can be conducted at conditions of atmospheric pressure, which provides a generally applicable process at room temperature and atmospheric humidity. Although hydrocarbon membranes offer some advantages over perfluorosulfonic acid membranes, they also come with drawbacks such as long-term stability at low humidity, so further investigation is needed to determine whether nanocrack coatings will prove useful in commercial fuel-cell applications.

—Park et al.

Resources

• Chi Hoon Park, So Young Lee, Doo Sung Hwang, Dong Won Shin, Doo Hee Cho, Kang Hyuck Lee, Tae-Woo Kim, Tae-Wuk Kim, Mokwon Lee, Deok-Soo Kim, Cara M. Doherty, Aaron W. Thornton, Anita J. Hill, Michael D. Guiver & Young Moo Lee (2016) “Nanocrack-regulated self-humidifying membranes” Nature 532, 480–483 doi: 10.1038/nature17634