Ionomer permeability

The term chlor-alkali refers to those products obtained from the commercial electrolysis of aqueous sodium chloride. These are chlorine, sodium hydroxide, and sodium carbonate. The first two are produced simultaneously during the electrolysis while the latter is included because it is also produced in small quantities and shares many of the end uses of sodium hydroxide. Perfluorinated ionomer membranes are permeable to sodium ions but not the chloride ions, and hence they are useful for these electrolytic cells. The arrangement of a typical membrane cell is shown in Figure 10.2. 

But when the contents of Nafion ionomer was increased from 30 to 45 % to find out the better electrode structures, the Pt-Ru/SRaw, which had showed the lowest single cell performance, became the best electro-catalyst. By this result one can conclude that as long as the structure of the electrode can be optimized for the each of new electro-catalysts, the active metal size is a more important design parameter rather than inter-metal distances. Furthermore, when the electro-catalysts are designed, the principal parameters should be determined in the consideration of the electrode structures which affect on the electron conduction, gas permeability, proton conductivity, and so on. 

Mao et al. [174] recently presented research in which Nafion ionomer particles were used as hyperdispersant agents in the MPL of a cathode DL. It was shown that this ionomer helps to decrease the particle size of the PTFE in the MPL. Thus, increasing the Nafion particle content gradually decreased the PTFE size and decreased the hydrophobicity in the layer. In fuel cell testing, an MPL having 1 wt% ionomer showed the best performance it improved the gas permeability and electronic conductivity. 

This condition has been recently used in a vaporization-exchange model for water sorption and flux in phase-separated ionomer membranes. The model allows determining interfacial water exchange rates and water permeabilities from measurements involving membranes in contact with flowing gases. It affords a definition of an effective resistance to water flux through the membrane that is proportional to... 

An improved adsorption of DNA bases has been observed at a chemically modified electrode based on a Nafion/ruthenium oxide pyrochlore (Pb2Ru2-x FhxOj-y modified GC (CME). Nafion is a polyanionic perfiuorosulfonated ionomer with selective permeability due to accumulation of large hydrophobic cations rather than small hydrophilic ones. The Nafion coating was demonstrated to improve the accumulation of DNA bases, while the ruthenium oxide pyrochlore proved to have electrocatalytic effects towards the oxidation of G and A. The inherent catalytic activity of the CME results from the Nafion-bound oxide surface being hydrated. The catalytically active centers are the hydrated surface-boimd oxy-metal groups which act as binding centers for substrates [50]. 

In the area of gas permeability, the loss- crystallinity of a typical ionomer I —. IDT) results in relatively high permeability to oxygen. For packaging of fresh meat this is advantageous, but in other packaging areas, combination with a barrier layer may be required. 

Several organic sealants such as epoxy resins, butyl rubber or silicones prove to be more or less permeable and the tiny amount of solvent in the cell is rapidly lost. Suitable organic sealing materials for this technology turn out to be thermoplastic materials, like polyethylene/carboxylate copolymers. So far, Surlyn 1702 ionomer from Dupont has been the main substance used to optimize cell performance and build module prototypes. However, the softening point of Surlyn is rather low (65° C) and at elevated temperatures (> 70°C), serious solvent loss is observed because the bond between Surlyn and TCO-coated glass is substantially weakened [7]. 

The electrolyte membrane presents critical materials issues such as high protonic conductivity over a wide relative humidity (RH) range, low electrical conductivity, low gas permeability, particularly for H2 and O2, and good mechanical properties under wet-dry and temperature cycles has stable chemical properties under fuel cell oxidation conditions and quick start-up capability even at subfreezing temperatures and is low cost. Polyperfluorosulfonic acid (PFSA) and derivatives are the current first-choice materials. A key challenge is to produce this material in very thin form to reduce ohmic losses and material cost. PFSA ionomer has low dimensional stability and swells in the presence of water. These properties lead to poor mechanical properties and crack growth. 

Good access for the reactants to the electrode-electrolyte interface and sufficient permeability in the ionomer... 

FIGURE 27.11 (See color insert following page S88.) H2 permeability as a function of temperature and RH. Upper limit (solid line) defined by crossover losses (assuming no contribution from O2 crossover), lower Umit (dotted Une) defined by electrode ionomer film-transport requirements, and data are for wet and dry Nafion 1100 EW-based membranes. (Reproduced from Gasteiger, H.A. and Mathias, M. F., in Proceedings of the Symposium on Proton Conducting Membrane Fuel Cells III, 2003. The Electrochemical Society of America. With permission from The Electrochemical Society, Inc.)... 

Asahi Glass is developing an undisclosed new polymeric ionomer with a superior oxygen solubility and permeability especially for electrodes [91]. Table 27.12 gives the values reported by Asahi Glass. A somewhat better performance is achieved with the new polymer in the electrode (Figure 27.45). 

Ionomer with high O2 solubility in High O2 permeability — Soluble fluoropolymer... 

The water distribution within a polymer electrolyte fuel cell (PEFC) has been modeled at various levels of sophistication by several groups. Verbrugge and coworkers [83-85] have carried out extensive modeling of transport properties in immersed perfluorosulfonate ionomers based on dilute-solution theory. Fales et al. [109] reported an isothermal water map based on hydraulic permeability and electro-osmotic drag data. Though the model was relatively simple, some broad conclusions concerning membrane humidification conditions were reached. Fuller and Newman [104] applied concentrated-solution theory and employed limited earlier literature data on transport properties to produce a general description of water transport in fuel cell membranes. The last contribution emphasizes water distribution within the membrane. Boundary values were set rather arbitrarily. 

This is evidenced by the amount of literature on ionomers and by the appearance of two monographs devoted to the subject (J, ). Most of the research effort on the ionomers has focused on only a small number of materials, notably ethylenes (3-9 ), styrenes (10,11), rubbers (12-16) and recently aromatic (17) and fluorocarbon-based ionomers (18). The last material is known for its high water permeability and cation permselectivity. Because of its unique properties, it has been employed as an ion-exchange membrane in chlor-alkali cell operations in electrochemical industries. Perfluorinated ion-exchange membranes are the subject of the present chapter. 

The methanol permeability of the nanocomposite membranes was shown to decrease on addition of the sulfonated titanate. Functionalized montmorillonite (MMT) was also employed to improve PFSA [58, 59] these composite membranes provide a low methanol crossover, without sacrifidng proton conductivity due to the introduction of sulfonic acid groups at the MMT surface, followed by blending with the Nafion ionomer. 

One of these, which has recently become increasingly important in electrochemical applications, and which is one of the materials discussed extensively in this volume, is the Nafion ionomer family. These materials were developed by the duPont company, and consist of hydrophobic fluorocarbon backbone chains, with hydrophilic per-fluorinated ether side chains terminated by sulfonic acid groups or corresponding alkali salts. The Nafions possess many exceptional properties which are not encountered in other ionomer systems, particularly the high water permeability (26,27), permselectivity with regard to ion transport (28-30), durability in strong alkali (26), thermal stability (26,31), and others. 

Copolymerization used in making ionomers has had the effect of depressing crystallinity, although not completely eliminating it, so the materials are also transparent. lonomers also have excellent oil and grease resistance, excellent resistance to stress cracking, and a higher water vapor permeability than does polyethylene. 

---