Short-side-chain ionomer

Asahi Glass has investigated the effect of third compounds and found that cyclic vinyl compounds, for example, 2,2-bis(trifluoromethyl)-4,5-difluoro-l,3-dioxole, functions well to improve the thermal and dimensional stability (Fig. 7.5) [7]. Similar to the short side chain ionomers, the terpolymer PFSAs showed higher elastic modulus and ca. 40°C higher softening temperature than those of the conventional Iraig side chain PFSAs. The water absorbability of the terpolymers was nearly half in a wide range of temperature. 

Fig. 17.2 Structures of some perfluorinated sulfonic acid containing polymers (PFSAs). Polymer 1 is available from DuPont (Nafion ), Asahi Glass (Flemion ), and others Polymer 2 is the short-side-chain ionomer developed at Dow, currently available from Solvacore and Polymer 3 is the ionomer available from 3M Company...

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. °... 

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. 

We will briefly introduce some important ionomers (see Fig. 1), but for a thorough treatment of ionomer chemistry, see, e.g., Refs. . The simplest classification of ionomers is semicrystalline vs. amorphous ionomers. The prototypical semicrystalline ionomer is EMAA (Surlyn, DuPont) neutralized with various cations. Also from DuPont, Nafion is a perfluorinated polyethylene with sulfonic acid or sulfonate groups on short side chains. Other commercial ionomers like Aciplex (Asahi Chemical Company), Flemion (Asahi Glass Company), and Neosepta (Tokuyama) are structurally similar to Nafion. For a recent review on Nafion see Mauritz and Moore. ... 

Until recently, nearly all studies of perfluorlnated ionomers have been carried out with Nafion (110). Materials with shorter side chains were developed by Dow Chemical Co. in the early 198O s (111) and in a later chapter, Tant et. al, contrast the properties of the short side chain perfluorosulfonlc acid lonomers with those of a comparable Nafion material. 

Structure and Properties of Short-Side-Chain Perfluorosulfonate Ionomers... 

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. 

Taut MR, Darst KP, Lee KD, Martin CW (1989) Structures and properties of short-side-chain perfluorosulfonate ionomers. ACS Symp Ser 395 370-400... 

K.D. Kreuer, M. Schuster, B. Obliers, O. Diat, U. Traub, A. Fuchs, U. Klock, S.J. Paddison, J. Maier, Short-side-chain proton conducting perfluorosulfonic acid ionomers why they perform better in PEM fuel cells, J. Power Sources 178 (2008) 499-509. 

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]. 

Recently, scientific literature evidenced that different companies developed per-fluorosulfonic ionomers able to sustain 120°C operating conditions for long time. In some examples the relation with linear short side chain structure is evident, most of all 3M [32] and Solvay [33,34], in other cases the polymer structure is not described, like for Asahi Glass [35] or Asahi Kasey [36], Figure 17.7 summarizes the chemical formula of the most common perfluorinated ionomers. 

Aquivion E87-12S short-side chain perfluorosulfonic acid (SSC-PFSA) membrane with equivalent weight (EW) of 870 g eq and 120 pm thickness produced by Solvay Specialty Polymers was tested in a polymer electrolyte membrane water electrolyser (PEMWE) and compared to a benchmark Nation N115 membrane (EW 1100 g eq ) of similar thickness [27]. Both membranes were tested in conjunction with in-house prepared unsupported Ir02 anode and carbon-supported Pt cathode electrocatalyst. The electrocatalysts consisted of nanosized Ir02 and Pt particles (particle size 2-4 nm). The electrochemical tests showed better water splitting performance for the Aquivion membrane and ionomer-based membrane-electrode assembly (MEA) as compared to Nafion (Fig. 2.21). Lower ohmic drop constraints and smaller polarization resistance were observed for the electrocatalyst-Aquivion ionomer interface indicating a better catalyst-electrolyte interface. A current density of 3.2 A cm for water... 

Arico AS, Baglio V, Di Blasi A et al (2006) Proton exchange membranes based on the short-side-chain perfluorinated ionomer for high temperature direct methanol fuel cells. Desalination 199 271-273... 

Zhang YM, Li L, Tang JK, Bauer B, Zhang W et al. (2009) Development of covalently cross-linked and composite perfluorosulfonic acid membranes . ECS Trans. 25 1469. Gao HR, Jones DJ, Roziere J, Zhang YM (2011) Cross-linking of the short-side-chain perfluorosulfonic acid ionomer . Manuscript in preparation. 

Tant, M.R., K.P. Darst, K.D. Lee et al. 1989. Structure and properties of short-side-chain perfluorosul-fonate ionomers. ACS Symposium Series 395 370-400. 

Kreuer, K. D., Schuster, M., Obhers, B., Diat, O., Traub, U., Fuchs, A., Klock, U., Paddison, S. J., and Maier, J. 2008. Short-side-chain proton conducting perfluorosulfonic acid ionomers Why they perform better in PEM fuel cells. 178(2), 499-509. 

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