PFSA membranes

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. °... 

Transport properties of hydrated PFSA membranes strongly depend on nanophase-segregated morphology, water content, and state of water. In an operational fuel cell, these characteristics are indirectly determined by the humidity level of the reactant streams and Faradaic current densities generated in electrodes, as well as the transport properhes of catalyst layers, gas diffusion layers, and flow... 

The built-in and operation stresses are the consequences of the large swelling and shrinkage of the ionomer membrane when it uptakes and loses water. This is frequently referred to as dimensional instability in the literature. Water in the PFSA membrane is an essential ingredient of its proton conduction behavior. Water affects the morphology13,14 of the ionic clusters (at nanoscale) which... 

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. 

Mittal postulated that radical formation is likely due to the chemical reaction of H2 and 02 on Pt surface, this reaction is chemical in nature and shows strong dependence on the surface properties of Pt particles, and the sulfonic acid groups in the PFSA membrane maybe the key to the radical attack mechanisms.27 Cipollini28 in... 

Figure 14. Strain-to-failure versus number of cycles for two types of MEAs one with a reinforced composite membrane (Gore-5510) and the other with regular PFSA membrane (N- 111). The samples are cycled from 30 to 80% RH and from 80 to 120% RH.37...

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. 

Styrenic polymers, which are easy to synthesize and modify, were studied extensively in the early literature. One example is BAM made by Ballard Advanced Materials (see chemical structure below). This membrane is 75 pm thick and has an ion exchange capacity of about 1.1 to 2.6 meg/g. Its chemical stability is not as good as PFSA even with its perfluorinated backbone. Ballard claimed that this membrane could last for several hundred hours under low RH operating conditions. It is no longer in production due to its high cost and the lack of availability of the monomer. 

Haugen, G.M. et al.. Increased stability of PFSA proton exchange membranes under fuel cell operation by the decomposition of peroxide catalyzed by heteropoly acids, ECS Trans., 3, 551, 2006. 

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. 

The PFSA membranes nsed in the present study were Aciplex-SF-1004 with dimension of 10x10x0.117 tmn. The polymers were irradiated with 1.17 and 1.33 MeV garmna-ray from a cobalt-60 source, installed at the Takasaki Research Establishment of Japan Atomic Energy Agency (IAEA), at room temperatnre and atmospheric pressure. The resultant ionization doses to the polymers by the ganrma-ray irradiation were 530 kGy. 

We have shown that doping 12-sillicotungstic acid (HSiW) into PFSA membranes improves fuel cell performance under hot and dry operating... 

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