Polyetherketone, sulfonated

Figure 11. Stylized view of Kreuer of the nanoscopic hydrated structures of Nafion and sulfonated polyetherketone. (Reprinted with permission from ref 91. Copyright 2003 Elsevier.)...

FIGURE 27.19 Schematic representation of the microstructures of Nation and a sulfonated polyetherketone illustrating the less-pronounced hydrophobic/hydrophilic separation of the latter compared to the former. (Reprinted from Kreuer, K.D., J. Memb. Sci., 185, 29, 2001. With permission from Elsevier.)... 

The ionic conductivities of sulfonated polyetherketone/imidazole (pyrazole) systems were first studied by Kreuer et al. [161]. The intercalation of imidazole (pyrazole) into the polymer with Bronsted add functions was shown to produce a high protonic conductivity (ca. 10 S cm ). However, the volatility of these heterocycles hampers their application for high-temperature PEMFCs. In order to improve the thermal stability, the immobilization of these heterocyde systems was evaluated [162-164]. When Schuster et al. characterized the conductivity of imidazole-terminated ethylene... 

The microstructure of Nation and sulfonated polyetherketones also was a matter of recent investigations [22]. For Nation , the nanoseparation into hydrophobic and hydrophilic domains is well known, with the hydrophilic domain being responsible for the transport of protons and the hydrophobic backbone providing the morphological stability and preventing the membrane from dissolving (Fig. 6). The sulfonated polyetherketones showed a less pronounced nanoseparation due to the less hydrophobic backbone and the less acidic sulfonic groups. 

Solid polymer electrolytes, typically perfluorosulfonic acid (PFSA) membranes, are at the core of Polymer electrolyte membrane fuel cells (PEMFCs). These membranes electrically and mechanically isolate the anode and cathode while, when appropriately humidified, allowing for effective ion migration. Nafion, manufactured by DuPont, is one of the most thoroughly used and studied membranes in the PFSA family. Another family of membranes that holds some promise for use in PEMFCs is the group of sulfonated polyaromatic membranes, typically sulfonated polyetherketones. While research is being performed on other types of membranes, as well as hybrid membranes that might have been better-suited properties, information on these is searce [1-10]. 

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]. 

The physical model can be used to describe trends seen in experimental data. For example, the interconnectivity of the cluster network is predicted to have a profound effect on a membrane s transport properties. The percolation threshold for conductivity should increase when the clusters become smaller, which could be due to a stiflfer and/or more crystalline polymer matrix. These smaller clusters would also mean that the membrane would exhibit lower electro-osmotic coefficients, larger liquid water uptakes, and a greater dependence of the various properties on water content than in Nafion . In fact, these predictions are what is seen in such systems as sulfonated polyetherketones [19, 72] and Dow membranes [73, 74] or when the equivalent weight [22] or drying temperature [4, 6] of Nafion is increased. 

Figure 12.1. The dependence of proton mobility on water content, (a) Proton selfdiffusion coefficients (D

Fig. 4 Schematic representation of the morphology of Nafion and a sulfonated polyetherketone [134]...

Compared to Nafion , a stronger confinement of water in the narrow channels of the sulfonated aromatic polymers leads to a significantly lower dielectric constant of the waters of hydration (20 compared to 64 in fully hydrated Nafion [185,186]). Of particular relevance to macroscopic models are the diffusion coefficients of water. As the amount of water sorbed by the membrane increases and molecular-scale effects are reduced, the properties approach those of bulk water on the molecular scale. Figme 26 shows the trend in proton mobility. Da, and water self-diffusion, Dh20. for Nafion and the sulfonated polyetherketone membrane [134]. 

The investigation of different variants of sulfonated polyetherketones has been widely described in the literature polyetherketone [188], poly(ether ether ketone) [189-191], poly(ether ketone ketone) [192] and poly(ether ether ketone ketone) (sPEEKK) [193, 194], poly(oxa-p-phenylene-3,3-phthalido-p-phenylene-oxa-p-phe-nyleneoxy-phenylene) (PEEK-WC) [195]. An interesting comparison between different structures in this class of material (sulfonated poly (ether ether ketone) (sPEEK) and poly(phenoxy benzoyl phenylene)) (sPPBP) has been published by Rikukawa and Sanui [196]. Both polymers are isomers. In sPEEK the sulfonic groups are in the main chain, while in sPPBP they hang in a side chain. The water uptake at low relative humidity is higher for sPPBP as well as the conductivity at high temperatures. 

The development of polyaryls, in particular polyetherketones (PEEK), as a substitute of perfluorinated polymers was mainly based on cost and stability considerations [1]. Sulfonated polyetherketones has been found to be durable under fuel cell operation conditions over several thousand hours [185]. Victrex Company is the main producer of PEEK polymer and its sulfonation can be performed directly on the polymer backbone or by polymerization of sulfonated monomers [7]. Hoechst-Aventis and Eumatech commercialize sulfonated PEEK (sPEEK) and sulfonated poly(phtalazinone ether ketone) (sPPEK) membranes for fuel cell applications [3]. 

Bauer, B., Jones, D.J., Roziere, J., Tchicaya, L., Alberti, G., Cascioala, M., MassineUi, L., Peraio, A., Besse, S. and Ramunni, E. 2000. Electrochemical characterisation of sulfonated polyetherketone membranes. J. New Mater. Electrochem. Syst. 3 93. 

Bauer B, Jones D J, Rozi re J, Tchicaya L, Alberti G, Casciola M, MassineUi L, Peraio A, Besse S and Rammuni E (2000), Electrochemical characterisation of sulfonated polyetherketone membranes , J New Mater Electrochem Syst, 3, 93-98. 

Fig. 11.15 Left-. A schematic representation of the fully hydrated morphology of a PFSA ionomer (e.g., Nafion) under the assumptions of a cubic lattice model which fitted data from small angle X-ray scattering (SAXS) experiments. Right. SAXS spectra of hydrated Nafion and a hydrated sulfonated polyetherketone. The characteristic hydrophobic/hydrophilic separation lengths are obtained from the position of the ionomer peaks while the internal hydrophobic/hydrophilic interfaces are obtained from the intensities in the Porod regime. First reported in Ref. [66]...

The composite membrane approach can also be used for membranes alternative to Nafion such as sulfonated polyetherketones and polysulfones. Sulfonated polysulfone is one of the most promising polymers for PEMs due to its low cost, commercial availability, and easy processability. Composite polysulfone-based membranes based on silica have been prepared and characterized for DMFC to extend the operating temperature up to 120 °C [16]. As an example, the following describes the preparation and characterization of a... 

D. J. Jones, B. Baur, J. Roziere, L. Tchicaya, G. Alberti, M. Casciola, L. Massinelli, A. Peraio, S. Besse and E. Ramunni, Electrochemical characterization of sulfonated polyetherketone membranes, J. New Mater. Electrochem. Syst. 3, 93-98 (2000). 

Fig. 4.1 Comparison of structures Nafion and sulfonated polyetherketone (SPEEK) [9]. Reprinted from the K.D. Kreuer, On the development of proton conducting polymer membranes for hydrogen and methanol fuel cells. J. Membrane Sci. 185, 32 (2001) with permission from Elsevier...

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