Page 52 - Introduction to Transfer Phenomena in PEM Fuel Cells

Introduction to Hydrogen Technology 41
with (n) the number of TFE groups per PSEPVE monomer [DOY 03], the
–1
optimum equivalent weight is generally between 900 and 1350 g eq ; a
–1
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value of 1 100 g eq is generally found [SPI 07] and [COL 08]. Nafion
membranes have good stability in an acid environment. They cannot
function in temperatures above 100°C, and they need to be maintained in a
state of sufficient hydration to ensure good ionic conduction. Other products
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are beginning to emerge to replace Nafion membranes, but their
performance must still be assessed.
1.3.1.1.3. The membrane water content
It should be noted that when these membranes are placed in a humid
environment, they adsorb (or desorb) a certain amount of water and swell or
deflate in all directions until reaching a state of equilibrium [ELL 01].
Hydration of the membrane is generally characterized by its water
content (λ m), but it is also defined as being the number of water molecules
contained in the membrane per active site. The amount of water adsorbed is
greater when the environment is wet, since the membrane adsorbs more
water when it is immersed in liquid water than in a saturated humidity
environment; this phenomenon is called the Schröder paradox [VAL 06].

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1.3.1.1.4. The structural model of Nafion
A structural model of hydrated Nafion membrane at room temperature
was envisaged by Gierke et al. [GIE 81] and Hsu and Gierke [HSU 83] in
the late 1970s. For a long time, it was the most widespread and a
representation is given in Figure 1.17.

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Figure 1.17. Structural model of Nafion