PFSA ionomers

Healy, J., Hayden, C., Xie, T., Olson, K., Waldo, R., Brundage, A., Gasteiger, H. A. and Abbott, J. 2005. Aspects of the chemical degradation of PFSA ionomers used in PEM fuel cells. Fuel Cells 5 302-308. 

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. °... 

Impregnating these layers with PFSA ionomer for enhanced proton conduction or hydrophobizing agents like Teflon for sufficient gas porosity is optional. However, ionomer impregnation is indispensable in CLs with thicknesses of > 1 ftm. Ultrathin CLs with - 100-200 nm, on the other hand, can operate well without these additional components, based on sufficiently high rates of transport of dissolved reactant molecules and protons in liquid water, which could ensure uniform reaction rate distributions over the entire thickness of the layer. 

Early research of ionomer membrane degradation was conducted in the context of PEM electrolyzers. The detection of fluoride and other chain fragments in the condensed effluent water indicates the decomposition of PFSA ionomer and has long been noticed. Baldwin15 reported the detection of fluoride in the effluent of PEM electrolyzer and believed that it is the result of membrane mechanical failure. Extensive research has been conducted to elucidate the reaction pathways for membrane decomposition. Many controversial results and mechanisms have been reported in the literature, demonstrating the complex nature and the current inadequate understanding of the membrane degradation mechanisms. 

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. 

Figure 10. Cumulative number of molecules in clusters in hydrated SSC PFSA ionomer and Nafion for Rc = 3.5 A as a function of molecular weight - 15mer (gray) and trimer (black).

The membrane and ionomer humidification requirements are of paramount importance for PEMFC operation since the proton conductivity is a fundamental necessity in the membrane as well as in the electrode for the fuel cell to function. The operating conditions of current PEMFCs are dictated by the properties of the membranes/ionomers. Now, the most important membrane type (e.g., Nafion membranes from DuPont) is based on PFSA ionomers that are used in the membrane... 

Fig. 33. Intradiffusion coefficients of water and of protons in concentrated aqueous acid solution, homogeneously sulfonated polyaromatic ionomer, and phase-separated PFSA ionomer [95]. (Reprinted by permission of the Electrochemical Society).

Ionomers with carboxylate anionic groups exhibit both the effect of changing the nature of the anionic site and the effect of microstructural changes. Polyethylenemethacrylic acid (PEMA) ionomers, such as the Surlyn materials made by DuPont, have this anionic group. The positions of the cation motion bands of the alkali metal PEMA ionomers have been found to be even higher than those for the analogous M -PSMA ionomers. They are fisted in Table +1, where they can be compared to the cation-motion bands of the M -PSSA and M PFSA ionomers. 

In order to solve this chemical stability problem, a new proprietary PFSA ionomer synthesis procedure has been developed at DuPont that results in a reduction of the reactive end groups. This approach has been referred as chemical stabilization (CS) technology. Fluoride emission from chemically stabilized polymer in a Fenton s test was found to be eight times lower than a nonchemically stabilized polymer. 

PFSA Ionomer and PTFE Reinforcement at Asahi Kasei (Aciplex Membranes)... 

Although there have been various membranes used, none is more researched or seen as the standard than the Nafion family by E. I. du Pont de Nemours and Company. Like the other membranes used, the general structure of Nafion is a copolymer between polytetrafluoroethylene and polysulfonyl fluoride vinyl ether. These perfluorinated sulfonic acid (PFSA) ionomers exhibit many interesting properties such as a high conductivity, prodigious water uptake, and high anion exclusion to name a few. Nafion is the main membrane studied in this chapter. 

Low cost (final target for electric vehicle apphcations would be cheaper than US 10/m ) need to be taken into account. While a number of PEMs have been developed, no single membranes fulfill aU of these requirements. Currently, the most promising PEMs are perfluoro sulfonic acid (PFSA) ionomers. Another candidate second to the PFSA ionomers is non-fluorinated (or in some cases only slightly fluorinated) hydrocarbon ionomers. The aim of this chapter is to review the most recent progress on these two classes of ionomer membranes for low-temperature fuel cell applications... 

Substitution of currently used perfluorosidfonic acid (PFSA) ionomers and ionomer membranes (e.g., Nafion ) by novel materials with substantially improved proton conductivity at low relative humidity (RH), which would ehminate the need for fuUy humidified reactants and thereby significantly simplify fuel cell system design [2, 4, 5]. 

Fig. 11.15 Left-. A schematic representation of the fully hydrated morphology of a PFSA ionomer (e.g., Nafion) under the assumptions of a cubic lattice model which fitted data from small angle X-ray scattering (SAXS) experiments. Right. SAXS spectra of hydrated Nafion and a hydrated sulfonated polyetherketone. The characteristic hydrophobic/hydrophilic separation lengths are obtained from the position of the ionomer peaks while the internal hydrophobic/hydrophilic interfaces are obtained from the intensities in the Porod regime. First reported in Ref. [66]...

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