ReviewPhysical and chemical modification routes leading to improved mechanical properties of perfluorosulfonic acid membranes for PEM fuel cells
Highlights
► Approaches to improving membrane stability are critically assessed. ► Consideration given to both mechanical and chemical stabilisation. ► Ionomer cross-linking, inorganic-organic composite and reinforced membranes.
Introduction
Proton Exchange Membrane Fuel Cells (PEMFC) are an attractive, sustainable means for power generation. However, the cost of the cell components and the need to increase fuel cell long-term durability still preclude the immediate and large-scale commercialisation of fuel cell technologies especially in highly demanding applications such as automotives. Perfluorosulfonic acid (PFSA) membranes such as Nafion® from DuPont™ have long been regarded as state-of-the-art membranes for PEMFCs due to their high proton conductivity and chemical stability [1]. In the past, the mechanical properties of fuel cell membranes have not been of the most critical importance [2], principally because initial efforts used thicker (50–200 μm) membranes. However, recent developments have been geared towards the use of thinner membranes (<50 μm) due to the advantages they confer (such as lower membrane resistance and improved water transport) and, considering the various stresses they have to withstand during operation, improvement in membrane mechanical properties becomes an important goal in order to increase resistance of the membrane to premature failure. In an operating fuel cell the membrane has to withstand chemical degradation due to attack by radicals and other reactive species as well as mechanical stresses caused by swelling and dehydration or variation in stack compression. When one considers the importance of membrane durability to the MEA lifetime, improving their mechanical properties becomes an important goal in order to increase the membrane's resistance to failure. Thus, although membrane degradation is a result of both chemical and mechanical effects, the mechanical properties of the membrane remain a metric by which its potential resistance to failure can be gauged [3]. In particular, failure stress and tear resistance can serve as indicators of MEA lifetime as they are sensitive to localised mechanical and chemical degradation of the membrane [4].
Section snippets
Mechanical properties of PFSA
The mechanical properties of commercially available membranes such as Nafion® are generally available in the product literature (Table 1), with properties such as tensile modulus, breaking strength, and elongation at yield typically measured under ambient conditions and 50%RH. Aside from this, much of the literature has been directed more towards identifying thermomechanical transitions that can be assigned to morphological features using techniques such as Dynamic Mechanical Analysis (DMA).
An
Chemical cross-linking
In many types of polymer, chemical cross-linking is an excellent method for increasing mechanical strength [21], and to reduce swelling. As such, cross-linking is a widely used approach in non-fluorinated and also some partially fluorinated polymers when an improvement in mechanical properties is required. However, there is relatively little published work on the cross-linking of PFSA ionomers compared to other polymers, which may be due to the inherent stability of the perfluorinated backbone
Polymer-reinforced PFSA membranes
Currently, the most mature technology in reinforced PFSA membranes is that of PFSA reinforced with a mechanically stable polymer matrix, of which there are many studies and also several commercially available products. A majority of these utilise poly(tetrafluoroethylene) (PTFE) [57], [58], [59], [60], [61], [62], [63], [64], [65] due to its excellent stability and mechanical strength. Such composite membranes possess several advantages in mechanical properties compared to non-modified
PFSA mechanical reinforcement by inorganic fillers
The incorporation (preferably on a nanometric scale) of a solid filler into the polymer matrix can significantly improve the mechanical properties of PFSA membranes [119]. In this type of (nano)composite system, the filler may interact either with the hydrophobic polymer backbone or with the sulfonic functional groups, and the filler – polymer interaction can range from strong (covalent, ionic) bonds to weak physical interaction.
Besides the mechanical properties, there are several membrane
Summary and outlook
Many approaches have been developed for improving the mechanical properties of PFSA membranes, ranging from chemical cross-linking to physical reinforcement, each with its own advantages and disadvantages. Chemical cross-linking should in principle provide durable, chemically stable linkages, however it is often more difficult to accomplish than physical reinforcement. Most studies in this area have focused on long-side chain, Nafion® type PFSAs, with the exception of pioneering approaches at
Acknowledgment
This work was partially supported by funding under the FCH-JU MAESTRO project, contract number 256647.
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