Proton Exchange Membranes

With the evolution of renewable energy, such as water, wind, and solar, advanced water electrolysis has attracted numerous concern as one of the most workable and dependable hydrogen production approaches [159]. Unlike conventional alkaline electrolyte water electrolysis, proton electrolyte membrane (PEM) water electrolysis systems using high strength perfluorosulfonic acid membranes as the electrolyte have greater energy efficiency, no pollution, higher hydrogen production rate, and simpler design [160,161]. This section presents the properties of Nation membranes, such as water uptake, swelling ratio, proton conductivity, and electro-osmotic drag coefficient under PEM water electrolysis conditions.

Water Uptake and Membrane Swells

The water uptake by a Nation membrane is exhibited in terms of weight percent of water (w) and water content (A). The water uptake delivered as a is obtained via the weight of the wet sample o>wet and dry sample twdry.

The amount of water molecules per sulfonic acid site can be represented by A. The relationship between A and a is expressed as:

where EW is the equivalent weight, and the value is 1100 g/mol for the commercialized Nation membrane MH,0 is the molar weight of water.

Under proton exchange membrane fuel cell (PEMFC) operation conditions, the Nation membrane is saturated with water vapor, at which point water uptake reduces with the increasing temperature while it is considered to be in equilibrium with liquid water during water electrolysis, which causes a difference in the water uptake and immersion temperature dependency as described by Figure 3.24 between them [162-167]. In the case of membrane equilibration with liquid water, the A is deeply correlated with the pre-treatment of the membrane. As shown in Figure 3.24, Hinatsu et al. obtained that when the film was not subjected to vacuum drying pretreatment (E form), the water absorption rate (A) was considerably high and maintained a constant at around 100°C. They also observed films pre-treated at different drying temperatures (N-form, 80°C and S-form, 105°C). When the immersion

Water uptake (2) of a Nation 117 membrane immersed in liquid water at different temperatures (T)

FIGURE 3.24 Water uptake (2) of a Nation 117 membrane immersed in liquid water at different temperatures (T): ([a] From Zawodzinski, T.A. et al., J. Electrochem. Soc., 140, 1041-1047. 1993; Hinatsu. J.T. et al., J. Electrochem. Soc., 141, 1493-1498. 1994; [b] From Yoshitake, M. et al., Electrochemistry, 64, 727-736, 1996; Parthasarathy, A. et al., J. Electrochem. Soc., 139, 2530-2537, 1992.)

temperature is higher than the glass transition temperature (100°C-110°C), the A is the same under the two treatment modes; below this temperature, the value of A for the S-form is lower than that for the N-form, similar results to Zawodzinski’s [165]. This difference can be explained by the decomposition of ion clusters in the polymer film during drying as Weber and Newman [168] pointed out. In summary, the electrolyte membrane is completely hydrated during the electrolysis process, and the pre-treatment procedure greatly affects water uptake.

After the membrane absorbs water, it expands in three-dimensional space due to the complex interaction between the polymer, the ionic sites, and the membrane structure [169]. Table 3.2 sums up the research data for the expansion ratio of the length (width) [in-plane (5/)] and thickness] through the plane direction (<5f)]. This expansion of the film is anisotropic due to differences in lamination conditions [170].

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