Fuel cells phosphonated polymers

Abstract This chapter describes the apphcation of hyperbranched polymer-based electrolyte membranes for high temperature fuel cells. Hyperbranched polymers with a sulfonic acid group or a phosphonic acid one as a proton source were synthesized and the proton conductivity, thermal property and fuel cell performance of their polymers and crosslinked membranes were investigated. The concept of the proton conduction coupled with the polymer chain motion was proposed for high temperature fuel cells. 

Wu et al. [106] prepared hybrid direct methanol fuel cell membranes by embedding organophosphorylated titania submicrospheres (OPTi) into a CS polymer matrix. The pristine monodispersed titania submicrospheres of controllable particle size are synthesized through a modified sol-gel method and then phosphorylated by amino trimethylene phosphonic acid (ATMP) via chemical adsorption. Compared to pure CS membrane, the hybrid membranes exhibit increased proton conductivity to an acceptable level of 0.01 S/cm for DMFC application and a reduced methanol permeability of 5 xlO cm /s at a 2 M methanol feed. 

The conductivity of the polymers at 25 °C in IN HCl was a little smaller than that of Nation . Similar copolymers with an lEC of 2.5-3.5 meq/g were also prepared [60]. Their films exhibited similar proton conductivity to that of a Nation film at 80 °C under saturated water vapor. Preparation of phospho-nic monomers requires many steps [59,61], which makes it difficult to apply phosphonic membranes to fuel cells as well as the estimated high cathodic overpotential. 

Phosphonated polymers have been proposed for fuel cells with the expectation of being thermally more stable and better retaining water than sulfonic groups [210, 211]. Phosphonated poly(phenylene oxide) [212], poly(4-phenoxy-benzoyl-l,4-phenylene) [213] and polysulfones [214, 215] have been reported. Phosphonated fluoromonomers were polymerized [164]. Characterization of phosphonated films in terms of their proton conductivity has been reported for some of the phosphonated polymers polyphosphazene [216], trifluoropolysty-rene [217], poly(4-phenoxybenzoyl-l,4-phenylene) [218]. Relatively low conductivity values were reported for most of the polymers prepared up to now. The values for polyphosphazene [216] and for perfluorocarbon polymers [219] were quite encouraging. Phosphonated poly(phenylene oxide) [211] was evaluated in fuel cell-tests. 

X. Xu, I. Cabasso, Preliminary study of phosphonate ion exchange membranes for PEM fuel cells. Journal of Polymer Material Science 120 (1993) 68. 

Finally, Chapter 13 by Jannasch and Bingol deals with proton conducting phosphonated polymers and membranes for fuel cells. Polymers functionalized with phosphonic acids enable both intrinsic and water-assisted proton conductivity and are thus attractive for use as electrolyte materials in electrochemical devices. 

Proton Conducting Phosphonated Polymers and Membranes for Fuel Cells... 

B. Lafitte and P. Jannasch, On the prospects for phosphonated polymers as proton-exchange fuel cell membranes, in Advances in Fuel Cells, ed. T. S. Zhao, K. D. Kreuer, and T. Van Nguyen, Elsevier, Oxford, 2007, vol. 1, p. 119. 

Lafitte B and Jannasch P (2007), On the prospects for phosphonated polymers as proton-exchange fuel cell membranes , Fuel Cells, 1,119-185. 

Two inorganic acids - phosphonic and phosphoric acid - have been in the focus of several theoretical studies in connection with fuel-cell membranes. Phosphoric acid is used as a proton-conducting electrolyte, whereas phosphonic acid is used as a protogenic group in proton-conducting polymers (e.g. in polyvinyl phosphonic acid). 

Itoh, T., Hirai, K., Tamura, M., Uno, T., Kubo, M., and Aihara, Y. Synthesis and characteristics of hyperbranched polymer with phosphonic acid groups for high-temperature fuel cells. J Solid State Electrochem. 

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