Short side chain PFSA

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 PFSA Membranes for DMFCs... 

CF2=CF0CF2CF2CF2CF2S02F (Figure 2.6). The synthesis of the short-chain PFVESF monomer involves numerous steps, and present yields are not satisfying. However, the short-side-chain PFSA (SSC-PFSA) shows significant improvement in performance compared to the long-side-chain PFSA (LSC-PFSA) and the best balance of properties for fuel cell application in terms of proton conductivity, dimensional stability, mechanical strength, and beginning of life fuel cell performance. ... 

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... 

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. 

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. 

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. 

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]. 

Fig. 11.11 Room temperature protrai craiductivity of the short side chain (SSC) PFSA ionraner at 2 different (i.e., Dow 858 and Dow 1084 gionomer per molH+) equivalent weights (EW) and Nafirai as a function of water content expressed as 2 = [H20]/[—SO3H]. The data clearly shows the significant effect the equivalent weight has rat protrai conductivity with the Dow 858 exhibiting conductivity twice that of the higher EW PFS As (Reproduced from K. D. Kieuer et al. [52] by Elsevira Sdraice)...

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...

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. 

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... 

As described earlier, the commercialization of PEMFCs requires lower prices of the membranes and generation systems, compactness of the system, and higher efficiency of power generation. New types of PFSA membranes, new processes of membrane fabrication, and high-temperature membranes have been investigated. This chapter deals with (1) conventional PFSA membranes with long side chains, (2) membranes with short side chains, (3) sulfonimide membranes, and (4) other miscellaneous types of membranes. 

PFSA Polymers with Short Side Chains... 

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