Bulk hydrated membrane

Clearly, a fundamental understanding of the key strac-ture/property relationships, particularly membrane morphology and conductivity as a function of polymer electrolyte architecture and water content - both in the bulk hydrated membrane and at the various interfaces within the membrane electrode assembly (MEA), can provide guidance in the synthesis of novel materials or MEA manufacturing techniques that lead to the improvement in the efficiency and/or operating range of PEMFCs. 

Figure 4. Final snapshots at >, = 12.8 (a) hydrated Nafion (b) membrane/vapor interface (c) mem-brane/vapor/catalyst support interface (d) mem-brane/vapor/catalyst interface. In the bulk hydrated membrane we can find the nano segregation of the hydrophilic and hydrophobic domains. Wetting of the catalyst surface is observed while there is none on the catalyst support. Gray, CFx groups orange, sulfur red, oxygen atom of H2O or SO3 green, oxygen atom of HsO" white, hydrogen.

Figure 6. Snapshots of the final configurations of the bulk hydrated SSC PFSA membrane with the ionomers rendered invisible at hydration levels (a) X = 4.4, and (b) X = 9.6. A more connected water network is found at the higher water content.

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]. 

The next question is which of the above-discussed scenarios is most Kkely to be found at the catalyst/hydrated membrane interface. It is generally accepted that the proton conductivity of the membrane depends on the characteristics of ionic clusters formed surrounding the polymer hydrophilic sites, both within the bulk polymeric structure and at the interface with the catalyst [79]. The ionic clusters located at the membrane/catalyst interface are the ones that close the circuit of this electrochemical system. That is, these ionic clusters act as bridges through which protons and other hydrophilic reactants and products may pass from the membrane to the catalyst surface and vice versa during fuel cell operation. To get some insights into the possible formation of ionic clusters, we have analyzed the conformation of a hydrated model nation membrane over Pt nanoparticles deposited on a carbon substrate via classical MD simulations [80] at various degrees of hydration. 

At the level of membrane operation in the fuel cell, considering bulk-hke water as the active medium in well-hydrated membranes implies that hy-drauhc permeation should be regarded as the major mode of water back transport that counterbalances the electro-osmotic drag. This balance between electro-osmotic drag and water backflux determines the degree of membrane dehydration at the interface between membrane and anode as well as the critical fuel cell current density at which voltage losses in the membrane increase dramatically, i.e. in a highly non-ohmic fashion. 

This chapter has given an overview of the structure and dynamics of lipid and water molecules in membrane systems, viewed with atomic resolution by molecular dynamics simulations of fully hydrated phospholipid bilayers. The calculations have permitted a detailed picture of the solvation of the lipid polar groups to be developed, and this picture has been used to elucidate the molecular origins of the dipole potential. The solvation structure has been discussed in terms of a somewhat arbitrary, but useful, definition of bound and bulk water molecules. 

Pulsed field gradient (PFG)-NMR experiments have been employed in the groups of Zawodzinski and Kreuer to measure the self-diffusivity of water in the membrane as a function of the water content. From QENS, the typical time and length scales of the molecular motions can be evaluated. It was observed that water mobility increases with water content up to almost bulk-like values above T 10, where the water content A = nn o/ nsojH is defined as the ratio of the number of moles of water molecules per moles of acid head groups (-SO3H). In Perrin et al., QENS data for hydrated Nation were analyzed with a Gaussian model for localized translational diffusion. Typical sizes of confining domains and diffusion coefficients, as well as characteristic times for the elementary jump processes, were obtained as functions of A the results were discussed with respect to membrane structure and sorption characteristics. ... 

These studies showed that sulfonate groups surrounding the hydronium ion at low X sterically hinder the hydration of fhe hydronium ion. The interfacial structure of sulfonafe pendanfs in fhe membrane was studied by analyzing structural and dynamical parameters such as density of the hydrated polymer radial distribution functions of wafer, ionomers, and protons water coordination numbers of side chains and diffusion coefficients of water and protons. The diffusion coefficienf of wafer agreed well with experimental data for hydronium ions, fhe diffusion coefficienf was found to be 6-10 times smaller than the value for bulk wafer. 

On the other hand, the merits of such insights are obvious. It would become possible to evaluate the relative importance of surface and bulk mechanisms of PT. The transition from high to low proton mobility upon dehydration could be related to molecular parameters that are variable in chemical synthesis. It could become feasible to determine conditions for which high rates of interfacial PT could be attained with a minimal amount of hghtly bound water. As an outcome of great practical value, this understanding could direct the design of membranes that operate well at minimal hydration and T > 100°C. 

Structure diffusion (i.e., the Grotthuss mechanism) of protons in bulk water requires formation and cleavage of hydrogen bonds of water molecules in the second hydration shell of the hydrated proton (see Section 3.1) therefore, any constraint to the dynamics of the water molecules will decrease the mobility of the protons. Thus, knowledge of the state or nature of the water in the membrane is critical to understanding the mechanisms of proton transfer and transport in PEMs. 

Concerning the membrane itself, phospholipid hydration was characterized in terms of the types of water, bound and bulk, and their exchange rates, as well as direct observation of the intermolecular contacts between the phosphate headgroup and bound water via HRMAS HOESY and between lipids in mixed membranes via HRMAS NOESY.112 Significantly, Zhou and co-workers found little dehydration of the membranes even when rotation rates as high as 9 kHz were used, providing some comfort that the centripetal forces of sample rotation are not changing the structure of the membrane. 

When the glass membrane is exposed to water, a hydrated layer, approximately 50-100 nm thick, is formed at its interface. In addition to water, the chemical composition of the glass in this layer is the same as that in dry bulk. The concentration of the anionic binding sites is estimated between 3 and 10 M. The membrane is usually blown into a bulb of a typical thickness of the wall 50-200 jitm. The optimum thickness of the wall is a compromise between mechanical stability and the electrical resistance. The latter is typically on the order of 10MQ. The interior of this bulb is sealed and contains the internal reference electrode. Thus, the glass membrane is bathed on both sides by solution and a similar hydrated layer develops on the inside of the glass bulb as well (Fig. 6.14). 

It is typically on the order of several hundred nanometers. In practice the minimum thickness for polymeric membranes is 50gm or greater, which is far more than one would expect from (6.53). This is apparendy due to the fact that these membranes hydrate in the bulk, thus increasing the dielectric constant. They also form a hydrated layer at the solution/membrane interface (Li et al 1996) which affects their overall electrochemical properties and selectivities. Macroscopic ISEs use relatively thick membranes ( 500jU.m). In contrast, it is desirable to use thin membranes in the construction of asymmetric solid-state potentiometric ion sensors, in order to make their preparation compatible with the thin-layer preparation techniques. 

Balbuena et al. also conducted simulations at various water concentrations for various water contents (Fig. 8). At low water contents (A = 5), small water clusters are almost not connected with each other (Fig. 8a). At a very high water concentration (A. = 45), water forms a continuous phase (Fig. 8c). When A. is about 24, close to the amount in fully hydrated Nation membranes at room temperature, the interface is defined by a semi-continuum water film (Fig. 8b) where some water clusters with diameters of about 1 nm are interconnected by multiple water bridges. The average water density in this phase is estimated to be about 0.682 g cnr3, a much lower value than that of the bulk water phase at 353 K. These observations provide very valuable information for further investigating the OER and really highlight the power of atomistic simulations on the research topics for which currently existing experimental tools are lack of the resolutions in spatial and temporal scales. 

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