Sulfonated fluoropolymer membranes

PEMFCs have a solid ion exchange membrane made of sulfonated fluoropolymer, or a sulfonated polyetheretherketone (SPEEK) which is the electrolyte, and for the most part uses platinum catalysts. There are other materials in use for combination type membranes. It should be mentioned that currently SPEEK membranes do not hold up as well as the fluoropolymers such as Nafion , but research is ongoing to produce a more reliable and longer lasting membrane. 

Membranes which may be used in the removal of alkali metal ions by electrodialysis are those which are impermeable to anions, but which allow the flow therethrough of cations. Such cation-selective membranes should, of course, possess chemical durability, high resistance to oxidation and low electrical resistance in addition to their ion-exchange properties. Homogeneous-type polymeric membranes are preferred, for example, network polymers such as phenol, phenosulfonic acid, formaldehyde condensation polymers and linear polymers such as sulfonated fluoropolymers and copolymers of styrene, vinyl pyridine and divinylbenzene. Such membranes are well known in the art and their selection for use in the method of the invention is well within the skill of the art. 

In addition to Nafion, the family of sulfonated fluoropolymers includes Dow chemical membranes and Membrane C. Weber and Newman predict that the clusters formed within Dow membranes are smaller than in Nafion due to the higher elastic deformation energy [27]. For sulfonated polyetherketone membranes, which are under investigation due to their potential in lowering costs, separation into hydrophobic and hydrophilic domains is not as well defined as in Nafion [28]. As a result, their structure consists of narrower channels and clusters that are not as well connected as in Nafion [27, 28]. 

Simplicity and promise of low cost - Fuel cells are extremely simple. They are made in layers of repetitive components, and they have no moving parts. Because of this, they have the potential to be mass produced at a cost comparable to that of existing energy conversion technologies or even lower. To date, the fuel cells are still expensive for either automotive or stationary power generation, primarily because of use of expensive materials, such as sulfonated fluoropolymers used as proton exchanged membrane, and noble metals, such as platinum or ruthenium, used as catalysts. 

More recently copolymers have been developed for use in fnel cell membranes. Commercial membranes are often sulfonated fluoropolymers bnt snlfonated PAEK can also have a good level of performance. Victrex has designed polymers which consist of easy-and hard-to-sulfonate blocks which make sulfonation in sulfuric acid easy to control [16]. For example a biphenol or hydroquinone residue is highly reactive to electrophiles and is easily sulfonated in concentrated sulfuric acid. In contrast phenylene groups deactivated... 

One of the first fictional fluoropolymers was poly-1,2,2-trifluorostyrene. On one hand, it has much better oxidation and chemical resistance in comparison with common hydrocarbon polymers and, on the other hand, a wide range of functional groups can be attached to the aromatic ring. A sulfonated polymer was successfully used as a membrane for fuel cells by General Electric Co.3... 

Modem membranes usually consist of at least two layers one from sulfonic type copolymer and another from carboxylic type copolymer. The membranes are usually reinforced by fluoropolymer fabric to provide better mechanical properties and long lifetimes. The most important properties are considered in detail in the reviews mentioned above10,11 and in a basic text by Seko et al.6... 

Grafting of functional monomers onto fluoropolymers produced a wide variety of permselective membranes. Grafting of styrene (with the following sulfonation), (meth)acrylic acids, 4-vinylpyridine, A-vinylpyrrolidone onto PTFE films gave membranes for reverse omosis,32-34 ion-exchange membrane,35-39 membranes for separating water from organic solvents by pervaporation,49-42 as well as other kinds of valuable membranes. 

For instance, the Dow experimental membrane and the recently introduced Hyflon Ion E83 membrane by Solvay-Solexis are "short side chain" (SSC) fluoropolymers, which exhibit increased water uptake, significantly enhanced proton conductivity, and better stability at T > 100°C due to higher glass transition temperatures in comparison to Nafion. The membrane morphology and the basic mechanisms of proton transport are, however, similar for all PFSA ionomers mentioned. The base polymer of Nation, depicted schematically in Figure 6.3, consists of a copolymer of tetrafluoro-ethylene, forming the backbone, and randomly attached pendant side chains of perfluorinated vinyl ethers, terminated by sulfonic acid head groups. °... 

Yi et al. reported a new type of PVDF membrane prepared by blending two very different polymers, a PVDF fluoropolymer such as Kynar with a sulfonated poly-electrolyte. The new membrane is inexpensive and displayed good performance and durability based on 1,000-h test data. 

Since an IPMC functions as a pathway for hydrated cations, its properties will be expected to affect the performance of an IPMC actuator. The membrane materials used in IPMCs have so far been limited to a few commercially available perfluorinated ionic polymers, such as Nafion, and the thickness of the IPMC has also been restricted to the available thickness of the commercial membrane [67]. However, IPMC actuators employing new ionic membranes have now been reported [68]. The membranes are prepared from fluoropolymers grafted with polystyrene sulfonic acid (PSSA). IPMCs assembled with these membranes have been shown to exhibit at least several times larger displacements than the Nafion-based IPMC with similar thickness. 

Ion-permeable membranes are also made by swelling existing films with styrene and DVB, which can then be post-treated to add functional groups, or by grafting of ion-exchange functional groups directly onto the polymer matrix of existing films. For example, free radicals formed by radiation of polyethylene or fluoropolymer films become sites for addition of vinyl sulfonic acid, acrylic acid or vinyl amines [30]. 

To enhance membrane mechanical and hydro-thermal stability, Jiang et al. prepared a blend of side-chain sulfonated PFCB block copolymer and a PVDF fluoropolymer [129]. The chemical structure of the side-chain sulfonated PFCB ionomer is shown in Scheme 6.32. They evaluated the membrane s fiandamental properties, such as proton conductivity, gas permeability, water uptake, and... 

Proton Exchange Membrane (PEM), commonly synonymous with DuPont s Nation membrane and used in both PEM fuel cells and DMFCs, requires aqueous conditions for suitable ionic conductivity and therefore needs insulation against freezing, and requires rigorous humidity control. Although other membrane chemistries are available, the sulfonated tetrafluoroethylene-based fluoropolymer-copolymer chemistry used in Naflon and other brand name membranes is still the material of choice. 

The heart of the PEM fuel cell is the proton exchange membrane, which transports protons from the anode to the cathode. The membrane also serves to separate the fuel and oxidant gas phases and electronically insulates the cathode from the anode. The most typical membrane is a sulfonated per-fluorinated polymer. The Nation family of membranes made by DuPont is representative of this class, and is based on a sulfonated tetrafluoroethylene-based fluoropolymer-copolymer, with the chemical structure represented in Figure 1.2. The sulfonic acid (SO3H) groups on the side chains allow the protons or other cations to "hop" from one acid site to another, in the presence of water. The exact mechanism of the proton movement is an area of significant research. An active area of research is the development of hydrocarbon-based... 

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