Membranes sulfonic acid degradation

To reach the reductive step of the azo bond cleavage, due to the reaction between reduced electron carriers (flavins or hydroquinones) and azo dyes, either the reduced electron carrier or the azo compound should pass the cell plasma membrane barrier. Highly polar azo dyes, such as sulfonated compounds, cannot pass the plasma membrane barrier, as sulfonic acid substitution of the azo dye structure apparently blocks effective dye permeation [28], The removal of the block to the dye permeation by treatment with toluene of Bacillus cereus cells induced a significant increase of the uptake of sulfonated azo dyes and of their reduction rate [29]. Moreover, cell extracts usually show to be more active in anaerobic reduction of azo dyes than whole cells. Therefore, intracellular reductases activities are not the best way to reach sulfonated azo dyes reduction the biological systems in which the transport of redox mediators or of azo dye through the plasma membrane is not required are preferable to achieve their degradation [13]. 

Yu, J., Yi, B., Xing, D., Liu, R, Shao, Z. and Fu, Y. 2003. Degradation mechanism of polystyrene sulfonic acid membrane and application of its composite membranes in fuel cells. Physical Chemistry Chemical Physics 5 611-615. 

Xie, T., Hayden, C., Olson, K. and Healy, J. 2005. Chemical degradation mechanism of perfluorinated sulfonic acid ionomer. In Advances in materials for proton exchange membrane fuel cell systems, Pacific Grove, CA, Feb. 20-23, abstract 24. 

Figure 11. Degradation rates of perfiuorinated sulfonic acid membranes.

Inaba M (2009) Chemical degradation of perfluorinated sulfonic acid membranes. In Buechi EN, Inaba M, Schmidt TJ (eds) Polymer electrolyte fuel cell durability. Springer, New York... 

Degradation of the D membranes was linked to the reactivity of the alpha C-H bond in the polymer. A series of membranes that did not contain this bond were then synthesized. a, 3, 3-trifluorostyrene sulfonic acid was determined to have chemical and thermal stability, but poor physical properties (Figure 1.11(a) [15]). This was somewhat improved by using a triethyl phosphate plasticizer to combine polyvinylidine fluoride with the trifluorostyrene sulfonic acid polymer, reaching a lifetime of up to 5000 hours at 80 °C. This lifetime was doubled by a fluorocarbon matrix grafted with trifluorostyrene, and then further improved by a membrane composed of trifluorostyrene and substituted trifluorostyrene copolymers [16, 17]. The latter membrane (Figure 1.11(b)), developed by Ballard Power Systems,... 

Figure 1.11. Types of PEMs (a) Monomer units of a,p,P-trifluorostyrene sulfonic acid (b) BAM3G (c) Nafion (d) Dow membrane (e) polysulfone (f) polyetherketone [24]. (Reprinted from International Journal of Hydrogen Energy, 31, Colliera A, Wang H, Yuan XZ, Zhang J, Wilkinson DP. Degradation of polymer electrol3de membranes, 1838-54, 2006 with permission from Elsevier.)...

The sulfonic acid groups in the proton exchange membrane have a high affinity for many cationic species. Replacement of the protons by the contaminant cations will result in reduced conductivity of the membrane and performance loss. In addition, the membrane morphology and structure can be affected. An additional degradation issue associated with contaminant metal ions is the occurrence of Fenton s reactions to produce peroxy and hydroperoxy radicals, which in turn will chemically attack the membrane polymer structure. This mechanism is described in section 1.72.1 of this chapter and in chapter 5. The most common Fenton s metal of concern commonly found in the MEA is iron. 

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