Nonfluorinated materials

Nonfluorinated material 657 exhibits hole mobilities (ph) in vapor-... 

Many studies have appeared in developing alternative nonfluorinated materials for low cost and high performance fuel cells. Among this class, the most widely... 

In Figure 15.2 Pictures 1.1 and 2.1 are SEM images of nonfluorinated (reference) material and fluorinated samples (Batch 15A20PR7), and Pictures 1.2 and 2.2 are the corresponding X-ray spectra. The EDS spectrum shows that Batch 15A20PR is fluorinated, since a fluorine peak can be seen in the left part of the spectrum (Picture 1.2). 

In addition to Nafion-based catalyst layers, additional types have been developed, including CLs with different ion exchange capacities (lECs) [57,58] or with other hydrocarbon-type ionomers such as sulfonated poly(ether ether ketone) [58-60], sulfonated polysulfone [61,62], sulfonated polyether ionomers [63], and borosiloxane electrolytes [64], as well as sulfonated polyimide [65]. These nonfluorinated polymer materials have been targeted to reduce cost and/or increase operating temperature. Unfortunately, such CLs still encounter problems with low Pt utilization, flooding, and inferior performance compared wifh convenfional Nafion-based CLs. 

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

The sulfonic acid moiety has been incorporated into a variety of nonfluorinated polymeric materials (111). Chain-end sulfonated polymers are produced by the reaction of sultones with polymeric organolithiums (112). Polymeric sulfonic acids such as these are incorporated in positive-working photoresist compositions (113). 

However, the low basicity of polyfluorinated materials usually limits this method to the carbocations originating from nonfluorinated compounds (some alcohols, aromatics, etc.) or such relatively basic fluorinated substrates as mono- and dihaloalkanes, and fluoroethylenes,e.g.,CH2=CF2 [21]. 

Materials such as that shown in 17b display a very low productivity in gram of product per gram of catalyst since the amount of immobilized ketone is relatively small (405) under homogeneous conditions, nonfluorinated ketones are mostly used in large, solvent-like amounts in order to achieve appreciable rates. In contrast, better results are achieved with 17a with two equivalents of supported ketone with respect to the olefin, more than a 90% yield is achieved with tram-chalcol (406) ... 

Figure 10.6 Structures of nonfluorinated polymer electrolyte materials.

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]. 

The above results demonstrate that partially fluorinated nonionic surfactants, having a fluorinated hydrophobic chain and a nonfluorinated poly(ethylene oxide) polar chain, can be successfully used to obtain mesoporous materials with controlled morphology and pore size. The specific surface areas can be high, similar to MCM-41 materials obtained using hydrocarbon surfactants. 

Most recently, the Wang group and the Sun group described simultaneously the first NHC-catalyzed oxidative asymmetric a-fluorination of simple aliphatic aldehydes. NFSI serves both as an oxidant and as an F source. Under the optimized conditions, the desired a-fluorinated esters were obtained in up to 92% yield and 98% ee, while no competitive difluorination or nonfluorination occurred. A postulated mechanism of this reaction process is depicted via an NHC-bound enolate intermediate which behaves as a nucleophile to interact with the second NFSI to eventually form the a-flu-oro ester product. Furthermore, acyl fluoride was employed as the starting material under the optimized conditions. Pleasingly, with NFSI (only 1.1 equiv.), the expected a-fluoro ester was obtained in 90% yield and 96% ee, which supported the formation of an NHC-bound acyl azolium intermediate (Scheme 7.94). 

Figure 15.2. SEM Images (LI and 2A) and X-ray spectra (].2 and 2,2) of nonfluorinated reference materials and fluorinated samples,...

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. 

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