PFSA polymer membranes

With the trend to higher temperature of fuel cell operation, as is needed for both automotive and stationary applications, and the requirement for high performance, recent developments have tended towards the use not only of low-EW PFSA polymer membranes, but also in the employment of membranes of thickness only 25-30 pm (compared with the use of films of ca. 175 pm ten years ago) for their lower area specific resistance and increased water permeation rate, and both of these factors impact the membrane s mechartical strength. The difficulty lies in... 

Perfluorinated membranes are still regarded as the best in the class for PEM fuel cell applications. - These materials are commercially available in various forms from companies such as DuPont, Asahi Glass, Asahi Chemical, 3M, Gore, and Sol-vay. Perfluorosulfonic acid (PFSA) polymers all consist of a perfluorocarbon backbone that has side chains terminated with sulfonated 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... 

The validity of MD simulation is impacted by the choice of system, interaction potential and algorithm implemented. We first discuss the choice of system. In this work we chose to simulate Nafion with an equivalent weight (EW) of 1144 g/mol, which is a practically reasonable EW. In order to have a direct comparison of the effect of side chain length, SSC PFSA polymer electrolyte was simulated with an EW of 978 g/mol. (Commonly used SSC iono-mer has an EW 800 g/mol). The repeat units of these PFSA membranes are shown in Fig. 3. These two materials have the same backbone separating side chains thus the only differentiating feature is side chain length. 

In the development of fuel-cell technology based on this unique polymer electrolyte, special chapters in electrochemical science and engineering have emerged, addressing the fuel-cell ionomeric membrane itself and the optimized fabrication of MEAs. The invention of Nafion, a poly(perfluorosulfonic acid) (poly(PFSA)) at DuPont in the 1960s, was, in fact, a key (if not the key) milestone in the development of PEFC technology. The chemical and mechanical properties of such poly(PFSA) extruded membranes, which are based on a perfluorocar-bon backbone, enabled to achieve stable materials properties and, consequently,... 

Many approaches have been developed for the production of ionic liquid-polymer composite membranes. For example, Doyle et al. [165] prepared RTILs/PFSA composite membranes by swelling the Nafion with ionic liquids. When 1-butyl, 3-methyl imidazolium trifluoromethane sulfonate was used as the ionic liquid, the ionic conductivity ofthe composite membrane exceeded 0.1 S cm at 180 °C. A comparison between the ionic liquid-swollen membrane and the liquid itself indicated substantial proton mobility in these composites. Fuller et al. [166] prepared ionic liquid-polymer gel electrolytes by blending hydrophilic RTILs into a poly(vinylidene fiuoridej-hexafluoropropylene copolymer [PVdF(HFP)] matrix. The gel electrolytes prepared with an ionic liquid PVdF(HFP) mass ratio of 2 1 exhibited ionic conductivities >10 Scm at room temperature, and >10 Scm at 100 °C. When Noda and Watanabe [167] investigated the in situ polymerization of vinyl monomers in the RTILs, they produced suitable vinyl monomers that provided transparent, mechanically strong and highly conductive polymer electrolyte films. As an example, a 2-hydroxyethyl methacrylate network polymer in which BPBF4 was dissolved exhibited an ionic conductivity of 10 S cm at 30 °C. 

Proton diffusion can occur via two mechanisms, structural diffusion and vehicle diffusion [37]. It is the combination of these two diffusion mechanisms that confers protonic defects exceptional conductivity in liquid water. The conductivity of protons in aqueous systems of bulk water can be viewed as the limiting case for conductivity in PFSA membranes. When aqueous systems interact with the environment, such as in an acidic polymer membrane, the interaction reduces the conductivity of protons compared to that in bulk water [37]. In addition to the mechanisms described above, transport properties and conductivity of the aqueous phase of an acidic polymer membrane will also be effected by interactions with the sulfonate heads, and by restriction of the size of the aqueous phase that forms within acidic polymer membranes [32]. The effects of the introduction of the membrane can be considered on the molecular scale and on a longer-range scale, see Refs. [16, 32]. Of particular relevance to macroscopic models are the diffusion coefficients. As the amount of water sorbed by the membrane increases and the molecular scale effects are reduced, the properties approach those of bulk water on the molecular scale [32]. 

Perfluorosulfonic acid (PFSA) membranes as shown in Fig. 1 were first developed for fuel cells by DuPont as Naflon and installed into the Biosatellite spacecraft in 1967 [1,2]. Various types of PFSA polymers, such as Flemion , Aciplex , and Dow membrane, were developed subsequently. They have excellent chemical stability, high proton conductivity, and high water diffusivity in a wide range of temperatures, brought about by the nature of fluorinated compounds and these non-cross-linked structures [3-5]. 

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. 

The most frequently used polymer membranes are proton conducting polymers of the perfluorinated sulfonic acid (PFSA) type, the most well known being Nafion from DuPont. A review on the state of understanding has been given by Mauritz and Moore [10]. A section of Nations chemical formula is given in Fig. 14.4. 

Polymer Electrolyte Fuel Cells, Perfluorinated Membranes, Fig. 3 Decomposition mechanism of PFSA polymer under low humidity conditions. 

---