Nafion water content effects

The effects of equivalent weight (FW = g polymer/mol SO3H) and water content on diffusion coefficient, solubility, and permeability of oxygen for fully hydrated BAM, S-SEBS, ETFE- -PSSA, Nafion 117, and BPSH membranes have been studied. It has been found that the diffusion coefficients of all the studied membranes decrease with increasing EW, while the solubility correspondingly increases. These trends are the same as found in... 

The sensitivity of the impedance to humidification of the reactant gases also depends on the membrane thickness. It has been reported that a thicker membrane is much more sensitive to humidification conditions. For Nafion 117, membrane resistance normally decreases when the reactant gases are better humidified because the conductivity of Nafion increases almost linearly with its water content [60]. If the fuel cell is operated with Nafion 112, the effect of humidifying the reactant gases is much less significant because this thin membrane remains well hydrated by the water produced in the fuel cell reaction [29],... 

A word of cantion is in order. A better connectivity in SSC cannot be instantly eqnated to higher condnctivity. In the simulations of Nafion and SSC, the same amount of water is distributed in the same volume. A change in connectivity is almost certainly accompanied by a change in the characteristic dimension and geometry of the cluster channel. In other words, if one stretches out clusters in SSC in order to better connect them, under the constraint that the volume of the aqueous domain is the same as that in Nafion, then one must accept that the dimension of a channel in a clusters in SSC is smaller. (That the characteristic channel width in SSC is smaller than in Nafion has also been observed experimentally at least for the medium and high water contents.) This change in dimension can affect the environment of the water and hydronium ions. If the channel is smaller and more spread out in SSC PFSA membrane than in Nafion, then it has more surface area with the hydrophobic phase per unit volume. This additional interaction with the hydrophobic phase can be characterized as additional confinement. The effects of confinement on both the diffusion of water and the vehicular and structural components of diffusion of the proton are not fully understood. Thus it is important to corroborate the suggestions of this water cluster distribution analysis with other measures of structure and transport. 

Zawodzinski et al. [64] have reported self-diffusion coefficients of water in Nafion 117 (EW 1100), Membrane C (EW 900), and Dow membranes (EW 800) equilibrated with water vapor at 303 K, and obtained results summarized in Fig. 36. The self-diffusion coefficients were deterinined by pulsed field gradient NMR methods. These studies probe water motion over a distance scale on the order of microns. The general conclusion was the PFSA membranes with similar water contents. A, had similar water self-diffusion coefficients. The measured self-diffusion coefficients in Nafion 117 equilibrated with water vapor decreased by more than an order of magnitude, from roughly 8 x 10 cm /s down to 5 x 10 cm /s as water content in the membrane decreased from A = 14 to A = 2. For a Nafion membrane equilibrated with water vapor at unit activity, the water self-diffusion coefficient drops to a level roughly four times lower than that in bulk liquid water whereas a difference of only a factor of two in local mobility is deduced from NMR relaxation measurements. This is reasonably ascribed to the additional effect of tortuosity of the diffusion path on the value of the macrodiffusion coefficient. For immersed Nafion membranes, NMR diffusion imaging studies showed that water diffusion coefficients similar to those measured in liquid water (2.2 x 10 cm /s) could be attained in a highly hydrated membrane (1.7 x 10 cm /s) [69]. 

To convert the intra(self-)diffusion coefficients (Dseif) to inter(Fickian)diffusion coefficients (Dchem). Zawodzinski and co-workers [64] have corrected the selfdiffusion coefficients they measured for water activity coefficient variations along the membrane thickness dimension and for the effects of swelling of the polymer [87]. The resulting Dchem for water in the Nafion membrane was 2 x lOr cm /s at 30 °C and did not exhibit a strong dependence on water content (however, recent reevalua-... 

Based on the psd, which gives the best approximation to experimental porosity data for Nafion 117 (parameterization (4) in Fig. 5), the effects of other parameters on membrane performance have been studied in Refs. 16, 83. We reproduce here (cf. Fig. 7) water-content profiles at various jp/J-m- The depletion of local water content at the anode side is small for jp < /pc. For jp > 0.8 j-pC it becomes remarkable. At jp = jpC the water content at the anodic membrane boundary drops down to wc, disturbing the performance. Although the depletion is largely localized in the vicinity of the anode, it is sufficient to limit the current density. 

Yeo et al have reported an analysis of the conductivity of Nafion in different alkaline electrolytes, based on the correlation of membrane conductivity with water content. The analysis reveals that larger conductivities arise when the membranes are equilibrated with NaOH solutions than with KOH solutions of equal molarity. Also, it is shown that better conductivity can be realized with thinner and lower-EW membranes. These effects have been proven in an alkaline-water electrolyzer and in relation to the conductivity... 

Influence of concentration. The structural form of the iron, besides depending on chemical treatments, is rather different in slightly and greatly neutralized membranes. We discuss two extremes before considering samples with intermediate iron concentration and the effect of changing the water content of the Nafion membranes (5). ... 

In Table VII are the relative H chemical shifts of water in Nafion at several water contents. The experiments were conducted on specially prepared Nafion spheres in order to eliminate bulk susceptibility effects. These spheres behaved the same as the corresponding Nafion films and powders in limited 23Na NMR experiments. The H linewidths are sufficiently narrow to allow accurate measurement of the chemical shift. With decreasing water content, the resonance shifts upfield, suggesting the breakup of water-hydrogen bonding as for NaCl. The relative shift of pure water and water in saturated Nafion is not known at this time. The increased linewidth indicates decreased water mobility, as seen for the sodium ions. Additional experiments using model electrolytes and a chemical shift standard are warranted. 

The baseline Nafion membrane is deficient in terms of ionic conductivity above 100°C and at low relative humidity required for atmospheric-pressure building applications. Such conditions tend to dry out the membrane, drastically reducing membrane proton conductivity. Furthermore, the loss of water causes membrane embrittlement, resulting in membrane cracking, reactant cross-leakage and poor electrode-membrane contact. Therefore, a cost-effective membrane, with proton conductivity that is less sensitive to change in water content, is needed. 

The physical model can be used to describe trends seen in experimental data. For example, the interconnectivity of the cluster network is predicted to have a profound effect on a membrane s transport properties. The percolation threshold for conductivity should increase when the clusters become smaller, which could be due to a stiflfer and/or more crystalline polymer matrix. These smaller clusters would also mean that the membrane would exhibit lower electro-osmotic coefficients, larger liquid water uptakes, and a greater dependence of the various properties on water content than in Nafion . In fact, these predictions are what is seen in such systems as sulfonated polyetherketones [19, 72] and Dow membranes [73, 74] or when the equivalent weight [22] or drying temperature [4, 6] of Nafion is increased. 

---