Fluorinated Backbone

In PEMFC, the current state-of-the-art fuel cell technology primarily involves the use of perfluorosulfonic acid (PFSA) membrane as electrolyte. PFSA membranes are composed of carbon fluorine backbone chains with perfluoro side chains containing... 

Phosphonated polymers with fluorinated backbones have been obtained by reacting fluorinated poly(atylene ether)s with a H-phosphonate diester in the presence of Pd(PPh3)4 (Scheme 13.3d). Using fluorinated aromatic polymers as the backbone can be expected to improve the dimensional stability of the membranes compared to aromatic polymers with hydrocarbon backbones because of the increased hydrophobicity induced by the presence of the fluorocarbon. The phosphonated polymers with fluorinated aromatic backbones reached a conductivity of 6 mS cm at 95% RH at 80 °C and 2.6 mS cm in water at room temperature, which was below that of Nafion 117. 

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. 

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

Nation. Nation is the trade mark (produced by DuPont) of a perfluorinated material composed of carbon-fluorine backbone chains and perfluoro side chains with sulfonic acid ion-exchange groups ... 

For the purposes of this discussion, PEMs are classified in modified Nafion , membranes based on functionalized non-fluorinated backbones and acid-base polymer systems. The prior approaches adopted by researchers and some characteristics of the resultant membranes under each classification are presented in this section. 

Poly(ether ether ketone) (PEEK) (Fig. 11.5) is a thermostable polymer with an aromatic, non-fluorinated backbone, in which 1,4-disubstituted phenyl groups are separated by ether (-0-) and carbonyl (-CO-) linkages. PEEK can be functionalized by sulfonation and the degree of sulfonation (DS) can be controlled by reaction time and temperature. 

In summary, this section demonstrates that there are many kinds of membranes with functionalized non-fluorinated backbones that have been investigated the recent years in order to substitute the expensive Nafion PEM with properties (especially proton conductivity, methanol permeability, and mechanical stability) superior to those of Nafion. The last category of polymeric PEMs is the acid-base polymer systems, which are presented in the following section. 

Due to the excellent chemical stability of the fluorinated backbone structure of the BAM membrane, its performance is reported to be comparable to perfluorinated membranes, and more than 100,000 h of PEMFC operation is possible [48]. Similarly, by co-polymerizing trifluorostyrene and substituted vinyl monomers. 

Trogadas and Ramani summarized the modification of PEM membranes, including Nafion modified by zirconium phosphates, heteropolyacids, hydrogen sulfates, metal oxides, and silica. Membranes with sulfonated non-fluorinated backbones were also described. The base polymers polysulfone, poly(ether sulfone), poly(ether ether ketone), polybenzimidazole, and polyimide. Another interesting category is acid-base polymer blend membranes. This review also paid special attention to electrode designs based on catalyst particles bound by a hydrophobic poly-tetrafluoroethylene (PTFE) structure or hydrophilic Nafion, vacuum deposition, and electrodeposition method. Issues related to the MEA were presented. In then-study on composite membranes, the effects of particle sizes, cation sizes, number of protons, etc., of HPA were correlated with the fuel cell performance. To promote stability of the PTA within the membrane matrix, the investigators have employed PTA supported on metal oxides such as silicon dioxide as additives to Nafion. 

Fluorinated backbone segments. Fluorinated side chains. 

Polymeric fluorinated surfactants (1) have a fluorinated backbone or (2) contain perfluroalkyl groups, as pendant perfluoroalkyl groups attached directly or indirectly to the polymer backbone, or as one or two fluorinated end groups of the polymer. A perfluoroalkyl tail can be attached to poly(tris(hydroxymethyl)-acrylamidomethane or to a natural product, such as a sugar, polyol, and so forth [180]. A fluorinated group may be attached to a poly(oxyethylene) chain —CH2CH2O—, to an ethylene oxide-propylene oxide block polymer, or to an acrylic comonomer. 

A fluorinated backbone may contain —CF2CF2— or polyether units. The polyether chain may be perfluropoly(oxypropylene)... 

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