While the bulk PEM is at the heart of a working fuel cell, it is also a critical component in the catalyst layer of the membrane electrode assembly (MEA). The polyelectrolyte is typically used as a binder in the electrode(s) where it is in contact with other components including platinum and carbon. These materials can co-exist along with pores (filled with O2, H2O, etc.) in the electrodes to form what is known as the triple-phase interface, or boundary. This term refers to the comingled interfaces of (i) carbon/platinum (C/Pt) particles and pores, (ii) C/Pt particles and polyelectrolyte, and (iii) polyelectrolyte and pores (Fig. 10.4). It has been shown that in these composite electrodes the polyelectrolyte is heterogeneously dispersed and can be confined to films on the order of 2-10 nm thick. It is crucial to the development of such materials for fuel-cell applications to understand how the polyelectrolyte structures at these interfaces impact water transport, proton trans­port, electrochemical reactions, and how certain forms of degradation occur at the

triple-phase boundaries. Furthermore, the structures at interfaces between Nafion® and additive nano-particles may serve to enhance, or improve, properties such as water transport and water retention. Although the structural properties of bulk PEMs have been the focus of many studies using SANS, fewer studies have focused on the thin film and interfacial structural aspects of these materials. Therefore, researchers have employed in situ NR techniques to investigate the structural characteristics of PEMs at interfaces with a variety of materials [38—42].

To date, there have been a limited number of relevant studies that have used NR to investigate the structure of PEM materials. Most of this work has been carried out on Nafion® thin films deposited on various substrates including smooth glassy carbon (GC) [41, 43], sputtered Pt [40, 41, 43], electrochemically oxidized Pt (PtO) [41, 43], SiO2 [38, 44, 45], and gold (Au) [38, 44, 45].

Wood et al. report results from Nafion® thin films spin-coated onto glassy carbon (GC), platinum (Pt) and platinum oxide (PtO) surfaces used to experimentally model the PEMFC electrode interface and by annealing at 140 and 210 °C simulate the decal electrode-preparation method developed by Wilson and Gottesfeld [41, 43, 46, 47]. The films were exposed to 10 % relative humidity H2O and D2O vapour as well as saturated D2O vapour and were found to have different multi-layer structures depending on the substrate. In composite structures of Nafion®/Pt/GC different behaviour was found depending on the relative humidity. At low relative humidity (^10 %) in either H2O or D2O the scattering results were fitted with a single-layer model consisting of hydrated Nafion® with thicknesses on the order of 61-62 nm. The SLD determined for films exposed to 10 % relative humidity H2O and D2O were relatively high (SLDNafion®H2Q = 4.59 x 10-6 A-2; SLDNaflon®D2O = 4.80 x 10-6 A-2) when compared to the value calculated for “dry” Nafion® (SLDNafion®dry = 4.16 x 10-6 A-2) with a known mass density of 1.98 g. cm-3. One would expect that, given the SLD of H2O (-0.56 x 10-6 A-2), the SLD of the hydrated Nafion® film should be lower than that of a dry film. Assuming the water content is unaffected by the isotope the two reported SLD values can be used to calculate that the water volume-fraction at 10 % relative humidity for Wood et al.’s films is approxi­mately 3.2 % by volume and that the SLD of the dry Nafion® would be 4.76 x 106 A-2.

This corresponds to a mass density of 2.27 g. cm-3, which is about 15 % greater than the reported bulk density of 1.98 g. cm-3. One possible explanation for this high density is that the density of the films is higher than that found in bulkNafion® because of the thermal-processing procedure used to prepare the samples, which can increase their crystallinity. However, data for Nafion® on glassy carbon surfaces in ambient air could be fitted as a single layer with SLD =4.12 x 10-6 A-2, which is much lower than for Nafion® in the same conditions on Pt. Another explanation might be the relatively narrow Q-range of the data. When Nafion® on Pt was exposed to saturated D2O vapour the reflectivity curve was best modelled using a two-layer heterogeneous Nafion® film with an approximately 7.5 nm-thick “hydrophobic” layer at the Nafion®/ Pt interface, followed by a thicker (ca. 62 nm) hydrated Nafion® film. This hydro­phobic layer manifests as a “dip” in the SLD profile as shown in Fig. 10.5a. This is in contrast to the work by Murthi and Dura which demonstrated that when Nafion® films that are spin-coated onto Pt or Au are exposed to H2O vapour there is a thin water-rich layer that forms at the polymer/metal interface.

When Nafion® was in direct contact with the GC substrate a more complex scenario evolved (Fig. 10.5b). For the Nafion®/GC systems exposed to a D2O- saturated environment, a three-layer heterogeneous model was used to describe the scattering. In this case, the researchers determined that there was a thin, rough layer (ca. 9 nm thick with 6 nm roughness) sandwiched between two thicker layers. The layer at the Nafion®/vapour interface was the thickest (ca. 57.7 nm) followed by the layer at the Nafion®/GC interface (ca. 26.5 nm). While the water content of each layer was not directly reported, a calculation using the SLD for each layer shows that the layer closest to the GC contained ca. 50 % water by volume. The thick layer at the Nafion®/vapour interface contained about 37 % water, and the middle layer was relatively water depleted at about 24 % water.

Of particular interest are the cyclic-voltammetry results obtained when the Naf — ion®/Pt/GC systems were electrochemically converted to Nafion®/PtO/Pt/GC (Fig. 10.5c). It was reported that although the initial potential-cycle showed no measurable Pt oxidation, subsequent cycles showed clear indications of Pt oxidation and PtO reduction. Once the PtO was formed, the structure was once again probed under saturated D2O conditions. From analysis of the scattering curves Wood and co-workers [41, 43] found that with the PtO layer the interface became more “hydrophilic” compared to the previous Nafion®/Pt interfacial layer. Also of sig­nificance was that, after conversion of the Pt surface to PtO, there was less D2O uptake in the Nafion®/PtO/Pt/GC system. Based on these results a vision of the development of the polymeric structure, specifically for Nafion®, near an interface was formed. Due to strong interactions of the polymer chains with the substrate it was proposed that the typically-isotropic structure reported for bulk Nafion® was modified and becomes anisotropic at the interface. According to Wood et al., the first layer acts as a template and affects the long-range structural properties of the Nafion® thin film. Murthi et al. also showed that the water uptake in Nafion® thin films on metal substrates was measurably lower than that reported for bulk films [40].

Murthi et al. [40] examined Nafion® thin films (59 nm) spin cast onto 6 nm sputtered Pt, using deposition procedures similar to Wood et al., and annealed at

500 Ю00 1500 2000 2500

Thickness (A)

60 °C for one hour or more. From X-ray reflectivity data (to Qmax = 0.7 A 0 it was determined that prior to Nafion® deposition a 0.7 nm PtO surface-oxide layer had formed, presumably by air exposure occurring between deposition steps. Neutron reflectivity data for samples formed in controlled humidity H2O vapour and in liquid water was obtained to measure the water uptake. H2O was chosen over D2O in order to provide a greater contrast between the water domains and the hydro­phobic domains. The data for samples under a controlled humidity-environment at relative humidity values between 0 and 97 % were fitted using a single-layer model and the water content determined from the SLD profiles. In liquid water, a two — layer model was required to describe the data comprised of a thin 16 nm-thick

hydrophilic layer next to the PtO with a water content X (moles H2O per mole SO3-) of 21 and an outer layer of 76 nm with X = 10.2. Similar results were obtained from films prepared on gold substrates.

Dura and co-workers showed a very interesting effect in Nafion® films cast on SiO2 substrates. In contrast to scattering profiles fitted with simple, single-layer, or two-layer models, NR results revealed alternating lamellar layers of water-rich and Nafion®-rich domains that are induced at the interface of hydrated Nafion® films and native silicon-oxide substrates. A cartoon depiction can be seen in Fig. 10.6 which shows the silicon substrate, the native silicon-oxide, and a five-lamella structure. These structures were evidenced by the presence of a peak in the NR curve at approximately Qz = 0.21 A-1 for Nafion® on SiO2 equilibrated at 97 % relative humidity. The lamellar morphology was confirmed by off-specular scat­tering and the location of the lamellar structures at the interface with the SiO2, as opposed to the vapour-polymer interface, was confirmed by comparison of the scattering on thick thermal oxides. The position and intensity of the NR peak were shown to be highly dependent on the hydration level of the film. Detailed mea­surements and analysis including transverse Q-scans led to the conclusion that the

structures were indeed two-dimensional sheets, or lamellae, lying parallel to the substrate surface. The first layer was found to be water-rich at nearly 100 % H2O. The next two water rich layers were found to contain progressively less water, leading to a bulk-like swollen Nafion® layer.

He et al. [39] have used in situ NR to study the structure and kinetic absorption of water in thin films of sulfonated polyphenylene. Films of thickness ranging from 13 to 57 nm cast on oxidized-silicon substrates were exposed to D2O vapour. Typically, NR data collection is too slow to obtain kinetic data, but by limiting the acquisition to the low-Q regime time-averaged data at 10 min intervals could be collected. The NR curves were modelled using a three-layer model. It was deter­mined that there were D2O-rich layers at both the vapour/polymer and polymer/ SiOx interfaces. The kinetic studies revealed that the D2O mass uptake scaled with time1/2 at early times and diverged at later stages. At early stages of water adsorption the effective diffusivity was found to be significantly slower compared to diffusion in the bulk polyelectrolyte.

Our group is investigating the fundamental origins of these lamellar structures in Nafion® and their potential impact on interfacial transport by using a variety of techniques including NR, grazing-incidence SAXS (GISAXS), and quartz-crystal microbalance (QCM). The aim of this work is to investigate the role that specific interactions play on lamellae formation. This is being done by using substrates with tunable chemical characteristics. From our initial studies, it appears that the interfacial structures are generally absent from hydrophobic surfaces, but that in highly hydrophilic substrates there is a strong tendency to form interfacial-lamellar struc­tures [48]. More specifically, we have recently demonstrated that the wettability of the substrate, i. e. hydrophobic or hydrophilic, is a factor governing interfacial structure.

Although a clear and complete picture of how interfacial structure and con­finement in thin polyelectrolyte films influences materials properties, such as water and proton transport, is yet to be gained, it is clear that NR techniques have a significant role to play. In summary, NR has shown that the structure at the interface between a PEM and a substrate depends largely on the surface chemistry, film processing, and even electrochemistry. These factors are certain to have an impact on the transport at these interfaces. Moreover, the confinement of the PEM to a thin film, which certainly has technological relevance, reduces the transport coefficients and can even impact the solubility of water in these materials.