Hydrogen crossover Nafion®-112 membrane

FIGURE 21.27 Hydrogen crossover current density under the long-term RH cycling comparison between Nafion cast membrane and reinforced membrane. (From Choudhury, B., Material challenges in proton exchange membrane fuel cells. International Symposium on Material Issues in a Hydrogen Economy, November 12-15, Richmond, VA, 2007.)... 

Liang ZX, Shi JY, Liao SJ, Zeng JH (2010) Noble metal nanowiies incorporated Nafion membranes for reduction of methanol crossover in direct methanol fuel cells. Int J Hydrogen Energ 35 9182-9185... 

The proton exchange membrane can be a source of fluoride ions as well [143]. Hydroxyl radicals, formed via crossover gases or reactions of hydrogen peroxide with Fenton-active contaminants (e.g., Fe +), could attack the backbone of Nafion, causing the release of fluoride anions these anions in turn promote corrosion of the fuel cell plates and catalyst, and release transition metals into the fuel cell [143]. Transition metal ions, such as Fe, then catalyze the formation of radicals within the Nafion membrane, resulting in a further release of fluoride anions. On the other hand, transition metal ions also can cause decreased membrane and ionomer conductivity in catalyst layers, as discussed in section 2.4 of this chapter. 

FIGURE 6.6 In situ hydrogen crossover current density versus hydrogen partial pressure in Nafion -112 membrane [7],... 

The primary drawbacks of the Nafion membranes are poor conductivity at low relative humidities (and consequently at temperatures >100 C and ambient pressure) and large crossover of methanol in direct methanol fuel ceU (DMFC) applications. As a result, considerable efforts have been made in recent years to overcome these drawbacks. Peihaps the most widely employed approach is the addition of inorganic additives to Nafion m branes to yield organic/inorganic composite membranes. Three major types of inorganic additives that have been studied (zirconium phosphates, heteropolyadds, metal hydrogen sulfates and metal oxides) are reviewed in the following. 

Temperature = 80°C, Pressure = 300 kPa, both gases fully saturated. Calculate cell ionic resistance and hydrogen crossover for two different membranes, namely Nafion 112 and 117. 

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