Water Uptake of Perfluorosulfonic Acid Membranes from Liquid Water and Water Vapor

Abstract

The water uptake of several perfluorosulfonic acid membranes from liquid water over the temperature range 25 to 130~ and from water vapor at 80~ was determined. For water uptake from liquid water, the water uptake depended on the immersion temperature, the ion exchange capacity of the membrane, and the pretreatment of the membrane. The effects of pretreatment were not significant at immersion temperatures higher than 100 to 110~ For water uptake from water vapor at 80~ the sorption isotherms were similar in shape to those reported by previous investigators for 18.5 to 30~ although the water uptake at 80~ was less than that reported for the lower temperatures. The water uptake from water vapor of some membranes that have been found to give relatively good performance when used in polymer electrolyte fuel cells was higher then that observed with, e.g., Nafion|

Mechanics-based model for non-affine swelling in perfluorosulfonic acid (PFSA) membranes

“…(7)). The water uptake behavior of PFSA membranes is typically character ized by sorption isotherms which define the relationship between the water content, l, and the vapor activity of water in the surrounding air, a w , (or relative humidity, RH) at a given tempera ture [5,24,40,42,44,45,70,71]. In equilibrium, the confining pressure applied by the polymer matrix is balanced by osmotic swelling pressure, or the tendency of external moisture to diffuse into the membrane and increase the swelling.…”

“…In contrast to this, for membranes that are not predried (at elevated temperatures for a certain period of time), but subjected to standard treatment procedure, the water content is [10,38,45,46]. This discrepancy has been attributed to the thermal history of the polymer, which has been shown to play a key role in the water uptake capability of PFSA membranes [8,46].…”

“…However, it has been reported that [8,46,50] the water content in predried 3 membranes is similar, whether the membrane is equilibrated in saturated water vapor or liquid water. Thus, we will here adopt the results from a number of experimental water uptake studies for predried PFSA membranes equilibrated in liquid water and/or water vapor from Zawodzinski et al [8,38], Hinatsu et al [45], Onishi et al [46], and Thompson et al [74]. According to these studies, the water content of the predried membrane saturated with water is in the range of l ¼ 12-20, in close agreement with the predictions of our cylindrical RVE model (Fig.…”

“…For details, interested reader is urged to see the original publications. Water content predictions of the proposed model for a saturated membrane (a ¼ 1) are plotted as a function of temperature using spherical RVE and cylindrical RVE compared with the water uptake data from literature; Zawodzinski et al [8,38], Hinatsu et al [45] and Onishi et al [46] for predried PFSA membranes of 1100 EW in equilibrium with saturated water vapor or liquid water. All the data shown here are for the membranes predried at 105 D C except the S-form membrane of Hinatsu et al [45] which is predried at 80 D C.…”

“…This relationship is referred to as non-affine deformation and has been shown to occur for PFSA membranes by a number of studies in literature [11,[18][19][20][21][22]29,[33][34][35]. Despite the growing body of literature on the sorption behavior [7,8,10,24,34,35,38,40,[44][45][46][47][48][49][50] and nano-structural modeling and characterization [11,13,[17][18][19][20][21][22]28,29,31,[33][34][35][36]39,49,51,52] of PFSA ionomers, the relationship between the water uptake and nanostructure, temperature, mechanical properties and membrane pretreatment is not well established. Understanding of these issues requires a general, yet fundamental model, which is the subject of this study.…”