Fluorinated ionomers, partially

The use of low-cost basic polymers instead of Nation is an interesting alternative [19,20].The development of new polymers for ionomer membranes including perfluorinated ionomers, partially fluorinated ionomers, nonfluorinated ionomers, high-molecular/low-molecular composite membranes as well as novel polymer modification processes and novel membrane materials is summarised in [21]. 

From the reviewed work, it can be concluded that obviously the use of partially fluorinated aromatic cationomers as ionical cross-linkers leads in most cases to better chemical and thermal stabilities of the blend membranes than if nonfluorinated cationomers would be applied as acidic blend components. Among aU acidic cross-linkers, the sulfonated and partially fluorinated ionomer S9 (Fig. 4.5) leads to the best chemical stability of the referring base-excess PBI blend membranes. 

Partially fluorinated ionomer membranes Among this material class is the BAM3G membrane type composed of sulfonated or phosphonated poly(a,(5,(5-trifluorostyrene) and its copolymers [2-4] and the different types of grafted membranes based on partially fluorinated polymer foils, as developed by Scherer [5], Sundholm [6], and others. 

Dependence of the membrane properties from membrane type (nonfluorinated and partially fluorinated ionomer). We have developed partially fluorinated covalently cross-linked membranes by reaction of disulfmated poly (ethersul-fones) with pentaflnorobenzene sulfochloride and different cross-hnkers [90]. The scheme for the preparation of such partially fluorinated covalent ionomer networks is given in Fig. 8.10. The obtained membranes showed high H -conductivities and moderate SW. In Table 8.5, some of the properties of one... 

Table 8.5 Properties of nonfluorinated and partially fluorinated ionomer membranes...

Research has been actively conducted for the past 20 years in search of alternatives to perfluorinated membranes for PEMFC and DMFC applications. Partially fluorinated ionomers are of great interest among other various candidates in such efforts. Like perfluorinated membranes, partially fluorinated ionomers also have a PTFE-like polymer backbone as a main part to resist chemical attacks. However, ionic groups are attached to styrenic moieties instead of perfluorinated side chains. Due to its availability and easy sulfonation, styrene is a reasonable choice for ionomeric materials. In the late 1990s, Ballard Power Systems introduced a partially fluorinated low-cost membrane for fuel cell applications [24,47,48]. Ballard Advanced Materials (BAM) membrane is a family of sulfonated styrenic co-polymers of a,p,p-trifluorostyrene and substituted a,p,p-trifluorostyrene co-monomers by emulsion polymerization. The chemical structure of BAM membrane is shown in Fig. 13.9. 

Althongh perfluorinated or partially fluorinated ionomers are mostly favored as fuel cell membrane materials due to their unique resistance to chemical attack from their surroundings, fundamental solution to overcome inherent shortcomings of Nafion and other fluorinated membranes would be the development of chemically different types of ionomers. Alternative membranes for PFS A can reduce methanol crossover as low as two orders of magnitude. Such membranes are discussed in the following sections. 

Kerres et al., among others, developed the acid-base blend membranes from sulfonated polymers and aminated or other basic polymers [98] and concluded that the protonation of the basic groups is incomplete if the base is too weak [99]. Very recently, Frutsaert et al. [70] synthesized novel polymers for the development of high temperature PEMFC membranes comprising a blend of s-PEEK and a fluorinated copolymer bearing imidazole functions as pendant groups. The extensive work on intermediate temperature fuel cell membranes are well reviewed in Chap. 4 of this book including polybenzimidazole as the basic component and sulfonated and phosphonated ionomers of either nonfluorinated or partially fluorinated backbones as the acidic component. 

From the abovementioned results, it can be concluded that the combination of the partially lluorinated sulfonated ionomer M with the partially fluorinated PBI B2 is of advantage in terms of chemical stability and compatibility, which can be explained with chemical similarity, since both blend components consist of electron-deficient aromatic ring systems. Good PBI blend membrane stabilities can be achieved by good compatibility between the blend components, electron-deficiency of the aromatic ring systems, or both. To get hints of which is the... 

The search for further chemically highly stable partially fluorinated arylene ionomers as blend partners for polybenzimidazoles was... 

Type of used ionical cross-linker, including sulfonated and phosphonated ionomers which were either composed of nonfluorinated or partially fluorinated backbones... 

Kerres J, Xing D, Schonberger F (2006) Comparative investigation of novel FBI blend ionomer membranes from nonfluorinated and partially fluorinated polyaryiene ethers. J Poiym Sci B Polym Phys 44 2311-2326... 

The mentioned disadvantages of PFI membranes induced many efforts to synthesize PEM based on hydrocarbon-type polymers and brought about the emergence of partially fluorinated and fluorine-free ionomer membranes as alternatives to Nafion membranes. Among them the membranes based on aromatic PEEK were shown to be promising for fuel cell application, as they possess good mechanical... 

Those critical functions of membrane for DMFC are simple but most important. Required functions are ionic conductivity, electrical insulation, gas and liquid (especially methanol) tightness, and chemical and mechanical stability. As indicated in Fig. 13.2, ohmic polarization is mainly due to the ionic resistance of membranes, but the low open circuit potential of cathode is also mainly coming from the voltage drop by mixed potential made of fuel crossover through the membrane. The low cost of material and process is also another factor in terms of commercialization. Especially for mobile applications, membranes have the additional function for mass balance of liquid fuel and water products circulated out of or through the membrane. In this manner, alternative membranes are under development and researchers are focused on four types perfluorinated and partially fluorinated membranes hydrocarbon and composite and other ionomer modifications inorganic materials. The current state of the art and technical approaches to these materials are discussed in detail elsewhere in this volume. 

Before discussing membrane chemical degradation in detail, the factors governing the degradation mechanism must be identified. Among three major types of membrane materials, hydrocarbon, partially fluorinated, and perfluorinated ionomers, perfluorinated sulfonic acid (PFSA) is the most widely used membrane material owing to its high chemical stability (Schiraldi 2006). 

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