Membranes short side chains

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. °... 

Nafion, a perfluorinated sulfonic acid (PFSA) polymer electrolyte developed and produced by the E. I. Dupont Company, has been extensively studied as a fuel cell membrane. Despite its age, it remains the industry standard membrane because of its relatively high proton conductivity, toughness and quick start capabilities. Attempts to build upon the strengths of Nafion have resulted in a class of PFSA polymer electrolytes, including the short-side-chain (SSC) PFSA polymer electrolyte, originally synthesized by Dow and now produced by Solvay Solexis. Stracturally, PFSA polymer... 

Figure 3. Chemical structures of the monomer used in the simulation (a) Nafion (EW 1144) and (b) Short side chain PFSA membrane (EW 978).

Short Side Chain Perfluorinated Sulfonic Acid Membranes.782... 

E.E. Boakye and H.L. Yeager. Water sorption and ionic-diffusion in short side-chain perfluorosulfonate ionomer membranes. Journal of Membrane Science 69, 155-167 1992. 

Kreuer et al. [25] investigated the membrane properties, including water sorption, transport (proton conductivity, electro-osmotic water drag and water diffusion), microstructure and viscoelasticity of the short-side-chain (SSC) perfluorosulfonic acid ionomers (PFSA, Dow 840 and Dow 1150) with different lEC-values. The data were compared to those for Nafion 117, and the implications for using such ionomers as separator materials in direct methanol and hydrogen fuel cells discussed. Tire major advantages of PFSA membranes were seen to be (i) a high proton conductivity. 

R. Schlogl and F. Helfferich, Theory of exchange membrane potentials, Z. Elektro-chem, 1952, 56, 644-647 N. Ishibashi, T. Seiyama and W Sakai, Electrochemical studies on ion exchangers (Part 10) Mobilities of Ca+ and Cl- in the cation exchange membrane, Denki Kagaku (J. Electrochem. Soc. Jpn.), 1955, 23, 182-186 E.E. Boakye and H.L. Yeager, Water sorption and ionic diffusion in short side chain perfluorosulfonate ionomer membranes, J. Membr. Sci., 1992, 69, 155-167. 

Z.D. Deng and K.A. Mauritz, Dielectric relaxation studies of acid-containing short-side-chain perfluorosulfonate ionomer membranes, Macromolecules, 1992, 25, 2369-2380. 

Y. M. Tsou, M. C. Kimble, and R. E. White, Hydrogen Diffusion, Solubility, and Water-Uptake in Dows Short-Side-Chain Perfluorocarbon Membranes, Journal of the Electrochemical Society, 139,1913 (1992). 

Recently, Yamamoto and Hyodo have employed the DPD method for studying Nafion membranes [20]. The systems considered in this study were built using two distinct molecular species, denoted comb-shaped polymer ip) and water (w). The polymer was presented as a branched sequence of beads. It consisted of a main chain (backbone) of iV = 20 effective monomer units (-CF2CF2CF2CF2 ) linked with rig = 5 short side chains of = 2 units [-0CF2C(CF3)F0 and F2CF2S03H] the total number of interaction sites in the macromolecule was Np= N/, + n xn = 30. A water-like particle was modeled as the same size as the units of the Nafion fragment (<7 = 6.1 A) and represented four water molecules. The x parameters were found using an atomistic calculation. The DPD simulation was performed for water volume... 

The application to fuel cells was reopened by Ballard stacks using a new Dow membrane that is characterized by short side chains. The extremely high power density of the polymer electrolyte fuel cell (PEFC) stacks was actiieved not only by the higher proton conductance of the membrane, but also by the usage of PFSA polymer dispersed solution, serpentine flow separators, the structure of the thin catalyst layer, and the gas diffusion layer. Although PFSA membranes remain the most commonly employed electrolyte up to now, their drawbacks, such as decrease in mechanical strength at elevated temperature and necessity for humidification to keep the proton conductance, caused the development of various types of new electrolytes and technologies [7], as shown in Fig. 2. 

So-called Dow membrane with short side chains once attracted much attention, because it showed excellent performance in fuel cell operation compared with Nation membranes [28-30]. The polymer structure is shown in Fig. 9. 

Fig. 10 Structure of the perfluorosulfonyl monomer of the short side chain membrane...

Originally the monomer in Fig. 9 was prepared by DuPont by the synthetic scheme shown in Fig. 12 [33]. Thermolysis of the acyl fluoride in Fig. 12 did not give a desired monomer but gave a cyclo compound. In order to prevent the cyclization, a new synthetic route was developed as shown in Fig. 13, which was applied to the synthesis of Dow membranes [34]. A chlorine atom was introduced to the acyl fluoride to improve the selectivity of vinyl ether formation. The Dow membrane was also developed for brine electrolysis, but was not commercialized probably because of its high cost. Difficulty in the preparation of the acyl fluoride in Fig. 13 is one of the causes. Recently, new synthetic processes for the short side chain monomer were developed, as represented in Fig. 14. 

Perfluorinated sulfonamide monomers were prepared by DesMarteau. A typical synthetic scheme is given in Fig. 16 [39,40]. The temperature dependency and humidity dependency of proton conductivity of the sulfonamide copolymer with TFE were examined, and the properties were proved to be similar to those of a sulfonic acid type membrane [41-43]. Fuel cell performance was dependent upon membrane thickness and/or lEC, and there do not seem to be large differences depending on the species of the ion-exchange groups. Synthesis of a short side chain type sulfonimide monomer is also reported (see Fig. 17 [44]). 

Bis(trifluoromethyl)-4,5-difluoro-l,3-dioxole (Fig. 18) was copolymerized with TFE and a PSVE monomer shown in Fig. 5. A homopolymer of this third monomer exhibits a glass transition temperatme of 330 °C [56]. The terpolymer exhibits a high softening temperatme like the above short side chain copolymers [57]. This is one of the another approaches to obtain high temperatme membranes. The temperature dependency of the modulus of the terpolymer is compared with that of a conventional copolymer in Fig. 19 [58]. 

The Dow membrane is no longer available. However, Solvay is producing membranes with similar short side chains. 

Wu D, Paddison S J and Elliott J A (2009), Effect of Molecular Weight on Hydrated Morphologies of the Short-Side-Chain Perfluorosulfonic Acid Membrane , Macromolecules, 42,3358-3367. 

Ghielmi, A., Vaccarono, P., Troglia, C., and Arcella, V. (2005) Proton exchange membranes based upon the short-side-chain perfluorinated ionomer. J. Power Sources, 145, 108-115. 

The simplest way to improve the proton conductivity of ionomer membranes is to increase lEC, either by using monomers with short side chains or by increasing copolymer composition of sulfonic acid-containing units. The former approach... 

A modification of the polymer that has been adopted by various groups is to have shorter side chains as compared to Nafion. Short side chains increase the crystallinity of the PFSA, thus reducing the solubility. Solvay Solexis has developed Aquivion, a membrane based on Hyflon, which is a copolymer of Teflon and sulfonyl fluoride vinyl ether with low EW (790-870) and good crystallinity, with proton conductivity values in the order of 30 mS cm at 120°C, 30% RH [22]. A similar approach is followed by 3M, who have shown 580 EW membranes approaching 100 mS cm at 120°C and RH 50% [23]. Gore recently reported values >50 mS cm at 30% RH and > 100 mS cm 50% RH with a new, undisclosed ionomers [24]. DuPont recently presented results on MEAs with new ionomer that showed a much reduced dependence on the RH as compared to Nafion-based membranes [17]. 

Cui and co-workers performed classical molecular-dynamics simulations of two dilferent perfluorosulfonic-acid (PFSA) membranes to investigate the hydrated morphology and the hydronium-ion dilfusion. They put special emphasis on the water content of the membrane (5% to 20%) and compared the properties for two dilferent lengths of the side chains carrying the sulfonic-acid groups. The short side chains lead to a more disperse distribution of water clusters inside the membrane. At low water content this results in a more connected water-channel network, which enhances the proton transport. 

This translated to lower electrical resistance and permited higher current densities than the Nafion membrane, particularly when used in thinner form (9). These short side-chain membranes exhibited good performance and stability, but are no longer supplied by Dow. Furthermore, due toNafion s expense and other engineering issues, new alternative membranes are being developed by a number of different companies. 

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