Membrane water uptake

In equilibrium purge, water content of the MEA can be easily calculated from the purge gas RH via the membrane water uptake curve such as19... 

In light of the importance of water storage in partially dry membranes, it is instructive to correlate the cold-start performance quantified by product water (mElo, mg/cm2) with the membrane water uptake potential (Ak) defined as ... 

Figure 3. Relation between product water and membrane water uptake potential for the startup from — 30° C. The dashed line indicates the amount of water stored in the catalyst layer and the extra portion above it denotes the amount of water diffused into the membrane, (reproduced with permission from Tajiri et al.18)...

Figure 3 also shows that the total product water can be separated into the CL and membrane contributions for each water uptake potential in the membrane. While the CL contribution depends mainly on the CL pore volume and weakly on the water uptake potential AA. as indicated in Eq. (7), the membrane contribution varies nonlinearly with AX. This is because longer CL operation before ice filling and shutdown allows more membrane water uptake. 

Water Content and Protonic Conductivity in Ionomeric Membranes Water Uptake by Ionomeric Membranes... 

Interfacial failure during DMFC operation seems to be closely related to water swelling of the ionomer. Typically, the dimensional change of the polymer electrolyte membrane (PEM) under hydration is greater than that of the electrode. As a result, mechanical stress at the membrane-electrode interface is likely to initiate local delamination, which then expands further over the time of DMFC operation. Good correlation between membrane water uptake and the gain in cell resistance was demonstrated (Kim and Pivovar 2005). 

Watanabe et al. used different branching agents in order to synthesize branched/cross-linked SPIs. They introduced either melamine [110, 112] in a fully aromatic SPI (Fig. 24) or tris(aminoethyl)amine in an aro-matic/ahphatic SPI backbone [33,113]. In both cases, the proportion of trifunctional branching agent (leading to a stoichiometric proportion of amine and anhydride groups) was limited to 2 mol %. The authors studied the effect of this new polymer architecture on the membrane water uptake and maximum tensile stress at break. 

Organic-inorganic (sPI-SiOa) interpenetrating networks appear to be very promising materials as sohd electrolytes for fuel cell applications. Lee et al. [144] showed that the presence of sihca reduces the membrane water uptake and methanol permeability, while it increases the membrane selectivity (Fig. 3.10). In addition, the formation of an organic IPN improves the hydrolytic stability of the material. Hence, the mechanical properties (tensile strength and elongation at break) are increased by a factor of 100. 

Figure 6.22 Diffusion coefficient as a function of membrane water uptake from Zowadzinski et al. [14].

Broka and Ekdunge investigated water uptake from the vapor phase by Nafion 117 membrane and recast film. The results show water vapor uptake by both Nafion 117 membrane and recast film decreased with increasing temperature. The lower water uptake at higher temperatures has also been reported by other researchers. Broka and Ekdunge also found PFSA membrane water uptake from the liquid water is higher than those for water vapor. This phenomenon is known as Schroeder s paradox. Zawodzinski et al. explored the isopiestic sorption curve for Nafion 117-H". ... 

They found water uptake from the vapor phase is notably lower with that from hquid water, with 14 waters per sulfonate absorbed from the vapor phase and 22 from the hquid phase. They considered the difficulty in condensing vapor within the pores of the membrane as a possible reason for lower water uptake from the vapor phase. Hinatsu et al. performed an extensive study on the water uptake of PFSl membranes. Water uptake from liquid water increased with decreasing EW of the iono-mer and increased with temperature for absorption. Water content from the vapor phase decreased with temperature down to 10 waters per sulfonate site at 100% RH at 80°C. 

Blended membranes of the sulfonimide polyphosphazene and PVDF in a 15%I25% w/w ratio were cast from Al,Al-dimethylacetamide. No phase separation was observed, with the membrane water uptake reduced to 41 wt% from 119 wt%. It was surprising that the proton conductivity of blended membrane increased more than 20% to 0.060 S cm", even though PVDF is an inert polymer. 

Introduction to polymer electrolyte materials 15 Table 1.3 Polymer electrolyte membranes water uptake... 

Membrane water uptake (WU) may be defined as the amount of water absorbed by it while the specimen is stored in a container at constant temperature and relative humidity (RH) until equilibrium. The type of bonding responsible for the membrane hygroscopicity may be determined by H-MAS-NMR. The weight-based water sorption is expressed as... 

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