Nonfluorinated membranes

A large number of nonfluorinated polymers are now under investigation. Some of these activities have been reviewed in recent publications [165]. Approaches include the sulfonation of commercial polymers and the polymerization of new functionalized polymers. One of the first polymers chosen for sulfonation was polysulfone [166-174]. More recently the synthesis of stable polysulfones from sulfonated monomers has been explored [175]. 

Sulfonated poly(2,6-dimethyl-l,4-phenylene oxide) and copolymers have been considered by some groups [176-178] as proton conductive membranes. The substitution of the methyl groups by phenyl has been tested as one of the initial 

Polyimides have been the topic of investigation for fuel cells for many years [180-184]. By poly condensation of sulfonated amines and phthalic or naphthalen-ic anhydride it is possible to tailor statistic or block copolymers with good proton conductivity. Polyimides have some susceptibility to hydrolysis, better stability being achieved with naphthalenic structures [185]. Sulfonimide membranes [186, 187] have been investigated due their strong superacidity, water uptake and retention above 80 °C. However, although they are thermally quite stable, the hydrolytic stability is not high. 

The investigation of different variants of sulfonated polyetherketones has been widely described in the literature polyetherketone [188], poly(ether ether ketone) [189-191], poly(ether ketone ketone) [192] and poly(ether ether ketone ketone) (sPEEKK) [193, 194], poly(oxa-p-phenylene-3,3-phthalido-p-phenylene-oxa-p-phe-nyleneoxy-phenylene) (PEEK-WC) [195]. An interesting comparison between different structures in this class of material (sulfonated poly (ether ether ketone) (sPEEK) and poly(phenoxy benzoyl phenylene)) (sPPBP) has been published by Rikukawa and Sanui [196]. Both polymers are isomers. In sPEEK the sulfonic groups are in the main chain, while in sPPBP they hang in a side chain. The water uptake at low relative humidity is higher for sPPBP as well as the conductivity at high temperatures. 

Blends of sulfonated polymers with nonsulfonated polymers or with polymers containing basic groups have been well explored for fuel cells [197, 198]. 

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The description given here is a basic outline of the principles of the solid polymer electrolyte fuel cell used in the first Gemini space flights with nonfluorinated membranes (Fig. 13.23). Because the cell is slated for development as part of the electrochemical engine in cars, stages in its modern development are described in another section. 

Over the last few years, membrane development has intensified and numerous new developments have been reported [10]. This increase in the interest in novel proton-conducting membranes for fuel cell applications has resulted in several studies and review publications on the overall subject and also on some related topics (e.g., nonfluorinated membranes). The content of these reviews has been used and is cited in the appropriate sections. 

Nafion membranes, as discussed, have been intensively used for fuel cells because they show high proton conductivity and chemical stability, but their methanol permeability is too high. However, the critical aspect of Nafion is still its high cost. Several nonfluorinated membranes, with potentially lower costs, have been tested for fuel cells. Sulfonated PSF, sulfonated poly(ether ether ketone) (SPEEK), sulfo-nated polyphosphazene, and sulfonated polyamides (PA) with good performance for hydrogen fuel cells are described in several reports (Savadogo 1998 Zaidi et al. 2000 Guo et al. 1999 Vallejo et al. 1999). However, the methanol permeability in many cases is still relatively high. 

F-Release or equivalent for nonfluorine membranes At least every 24 h No target—for monitoring... 

The preparation complexity of perfluorosulfonated membrane and the high cost have restricted PEMFC from commercialization. Many researchers are dedicated to the development of nonflnorinated PEM. The American company Dais has developed styrene/ethylene-bntylene/styrene triblock polymer [51]. This membrane is especially snitable for small power PEMFC working at room temperature. The lifetime of the membrane is up to 4000 h. Baglio did some experiments to test the performance comparison of portable direct methanol fuel cell mini-stacks between a low-cost nonfluorinated polymer electrolyte and Nafion membrane. He found that at room temperature, a single-cell nonfluorinated membrane can achieve maximum power density of about 18 mW/cm. As a comparison, the value was 31 mW/cm for Nafion 117 membrane. Despite the lower performance, the nonfluorinated membrane showed good characteristics for application in portable DMFCs especially regarded to the perspectives of significant cost reduction [52]. 

Roziere, J. and Jones, D. 2003. Nonfluorinated polymer materials for proton exchange membrane fuel cells. Annual Review of Materials Research 33 503-555. 

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]. 

In the late 1990s and early 2000s, nonfluorinated homopolymers were studied as promising alternatives. In simplified terms, however, reduced methanol permeation and reduced conductivity are combined in these materials to achieve a DM FC performance comparable to that of Nafion-based MEAs, and the membranes had to be so thin that it was not possible to reduce substantially the absolute value for fuel loss by permeation. Table 1.3 provides an overview of the most significant membrane modifications. 

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. 

Since PPOCH2Br is a nonfluorinated, electron-rich polymer and FPARCH2Br a partially fluorinated arylene polymer, one could conclude that the partially fluorinated arylene polymer introduces better thermal stabilities into the blend membrane (Fig. 4.15). 

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. 

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... 

Zhang, L. and Mukerjee, S. 2006. Investigation of durability issues of selected nonfluorinated proton exchange membranes for fuel cell application. Journal of the Electrochemical Society 153 ... 

In recent years, many kinds of materials have been developed to synthesize proton-conducting membranes for H2/air PEM fuel cells, and some have exhibited promising performance as potential candidates to replace PFSA membranes. The major membranes are (1) fluorinated membrane, (2) partially fluorinated membrane, (3) nonfluorinated (including hydrocarbon) membrane, and (4) nonfluorinated composite membrane. Among these, the hydrocarbon membrane is considered a promising alternative due to its low cost compared with PFSA membranes [61]. 

Nonfluorinated ionomer membranes Numerous different types of nonfluori-nated ionomer membranes, among them ionomer membranes based on styrene polymers and copolymers containing polystyrene units [7], arylene main-chain polymers of different poly(phenylene) [8], poly(ethersulfone) [9-11], poly(etherketone) [12-15], poly(phenylene oxide) [16,17], poly(phenylene sulfide) [18] types, and such membranes based on an inorganic backbone like poly(phosphazenes) [19,20], poly(siloxane)s [21], have been developed in the past years... 

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