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
The synthetic polymer membranes used in fuel cells, water purifiers and systems for harvesting electricity from the sea must be hydrated — desiccation diminishes their performance. This is a problem, because some of these applications (including fuel cells) operate at high temperatures in low-humidity environments. On page 480 of this issue, Park et al.1 describe membranes that limit their own dehydration, substantially improving the membranes' performance in low-humidity environments.
The membranes in question are called ion-exchange membranes (IEMs), and they are made from polymers in which acidic or basic chemical groups are covalently bound to a polymer backbone2. IEMs swell when in contact with water, causing the attached groups to dissociate into fixed and mobile ions, and so making the membranes highly charged. The membranes therefore selectively and efficiently transport counter-ions (ions that have an opposite charge to that of the polymer's charged groups), so that rates of ion transport through the membranes are high. This high ionic conductivity is crucial for many membrane-based technologies because it reduces energy losses and therefore lowers costs.
Water in IEMs facilitates ion transport by keeping ions hydrated and ionized, and by creating or enlarging gaps between polymer chains to reduce the ability of the bulky, slow-moving chains to impede transport of the much smaller, highly mobile ions. In other words, ionic conductivity depends sensitively on water content in IEMs. The membrane's water content is closely linked to membrane swelling — membranes swell as they absorb water and shrink as they lose it. When humidity is low and temperatures are high (both of which are advantageous for applications such as fuel cells), IEMs become desiccated, swelling is reduced relative to that at high humidity and ionic conductivity falls. Controlling IEM dehydration under such conditions has been a formidable technical and scientific barrier that has limited membrane performance.
Park et al. have developed a strategy that helps membranes to retain water in low-humidity environments. They deposited a thin layer (8–260 nanometres thick) of a highly water-repellent material on the surface of IEMs (approximately 50 μm thick) in a relatively low-humidity environment (40% relative humidity), thus endowing the hydrophilic, but partially dehydrated, membranes with a hydrophobic skin.
Such a skin would typically be almost impervious to both ions and water, and would therefore sharply reduce the membrane's ionic conductivity. But when the authors exposed their membranes to high humidity, some water migrated through the thin hydrophobic coating into the highly hydrophilic interior, causing the membrane to swell sufficiently to generate 36–340-nm-wide cracks in the coating. These nanocracks expand in high-humidity environments (100% relative humidity) as the membrane swells, allowing the membrane to absorb more water and rapidly transport ions, but contract when humidity is low (30–45% relative humidity), retaining much of the water that is necessary for high ionic conductivity (Fig. 1).
The nanocracks therefore have a similar role to stomatal pores in a cactus3. These pores open during cool, high-humidity periods at night and in the early morning, so that the plant can take up carbon dioxide with minimal water loss, but close when humidity is low (such as during the day) to limit water loss when it is hot. Similarly, the nanocracks on the membrane's surface close in low-humidity conditions, substantially decreasing water evaporation from the membrane, and so maintaining relatively high ionic conductivity.
The authors tested their surface-modified membranes in proton-exchange membrane fuel cells (PEMFCs) at elevated temperatures (greater than 100 °C) and low humidity. PEMFCs are a widely used class of fuel cell that generates electricity from hydrogen and oxygen. The operation of PEMFCs at moderately high temperatures (100–150 °C) typically requires cumbersome equipment to control the temperature and humidity, but improves the tolerance of electrodes to impurities in the hydrogen-containing gas mixture that stop the fuel cell's catalyst from working at lower temperatures4.
Park et al. observed that their coated membranes displayed up to four times greater power density — a measure of the power (rate of energy conversion) that can be delivered per unit active area of the membrane — than uncoated analogues, with improvements most evident at 120 °C and low humidity. This performance enhancement partly stemmed from reduced water loss, but was also due to enhanced compatibility of the hydrophobic coating with the hydrophobic layers of catalyst in the PEMFC. The increased compatibility improved the cell's stability — in terms of both its performance and its physical stability — over periods of use of up to 220 hours at 120 °C, compared with a state-of-the-art, commercially available membrane.
Previous studies focused on modifying the bulk morphology of IEMs in the attempt to enhance low-humidity performance (see ref. 5, for example). Such approaches often involved introducing into the membrane hydrophobic components that cannot transport ions. In striking contrast to this, Park et al. attacked the problem at its source by modifying the membrane surface through which water loss occurs, thereby leaving the bulk properties of the membranes largely unchanged. Indeed, the authors show that the surface-coating process does not alter either the membrane swelling at equilibrium conditions or the average spacing between the polymer chains through which ion transport occurs. Furthermore, the authors successfully applied their technique to different polymers and molecular architectures, suggesting that it might be applicable to a variety of membranes. Nevertheless, further study is needed to prove the generality of this surface-modification technique, its feasibility for scaling up and the long-term stability of the coated membranes.
The authors also explored the use of surface-modified membranes in reverse electrodialysis, a process that is used to harvest electricity from the energy released when two fluid streams of different salinity (such as river water and seawater) are mixed6. The membranes demonstrated a substantial increase in ion selectivity after surface modification, and ionic conductivity remained high — a combination of features that improves the energy efficiency of reverse electrodialysis. The authors' technique might therefore enhance membrane performance in other processes that require high selectivity and ion transport. Such surface modification has similarly been proposed7 to overcome the ubiquitous trade-off between throughput and selectivity that afflicts gas-separation membranes.
Park and colleagues report an excellent proof of concept for their technique, but many questions remain unanswered. A detailed understanding at the molecular level of the transport of water and ions in IEMs and of how polymer structure affects transport properties is only beginning to emerge8. Systematic studies of the authors' composite membranes will help to clarify this link between structure and transport, and will contribute to achieving the ultimate goal: the rational tailoring of many types of high-performance membrane for various applications.Footnote 1
Park, C. H. et al. Nature 532, 480–483 (2016).
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Improvement in characteristics of a Nafion membrane by proton conductive nanofibers for fuel cell applications
Journal of Membrane Science (2017)