In-plane distribution of water inside Nafion® thin film analyzed by neutron reflectivity at temperature of 80 °C and relative humidity of 30%–80% based on 4-layered structural model

Structures of polymer electrolyte membranes and binders and the distribution of water inside are important for designing new ion-conductive ionomers for polymer electrolyte fuel cells. Neutron reflectometry (NR) was carried out on a Nafion® film with a thickness of 100 nm formed on native SiO2 surface on Si(100) for understanding the in-plane water distribution. The temperature was set at 80 °C and the relative humidity at 30, 50, 65, and 80% for NR measurements, simulating the conditions for the power generation. Clear NR modulation was obtained under each condition. NR data were fit very well with a 4-layer model parallel to the substrate with different densities of Nafion and water. At the interface between the Nafion film and the Si substrate, a 1 nm water-rich layer was observed under all conditions. The water concentration increased with humidity at all 4 layers, but the thickness increased mainly at the bulk layer.


Introduction
Polymer electrolyte fuel cells (PEFCs), anticipated as nextgeneration power sources, have been investigated extensively for use in automotive, residential, and portable devices. 1,2) The membrane electrode assembly (MEA) is comprised of an electrolyte membrane sandwiched by catalyst layers composed of catalysts and polymer binders. Understanding the interfaces in an MEA, or the reaction fields, has become very important, and their structures and properties are intensively studied. [3][4][5] Nafion®, a perfluorosulfonic acid (PFSA) polymer developed by DuPont TM , has a poly-tetrafluoroethylene backbone that provides mechanical properties and chemical stability and perfluorinated side chains with sulfonic acid head groups (-SO 3 H) that confer its protonconduction capabilities. Commonly, to guarantee a low gas permeation and a mechanical strength, Nafion is used in the form of a membrane with a thickness larger than 10 μm. For the transport of protons to the dispersed catalyst, Nafion is used as a solid-electrolyte binder in catalyst layers in the form of thin layers less than 10 nm. In PFSA membranes such as Nafion, water accumulates exclusively in the hydrophilic domains to form proton-conduction channels comprised of sulfonate ions, hydrated protons, and water molecules. [6][7][8] In contrast, the apparent gas diffusion coefficients for the permeation at the gas/ionomer interface decrease with the increase in the relative humidity (RH). [9][10][11] Since a typical ionomer thickness in catalyst layers is several nanometers, the oxygen permeation fluxes through the ionomers in the catalyst layers are dominantly controlled by the gas/ionomer interface permeation. 10,11) Over the past decades, the local water contents inside membranes and binders of the MEAs [12][13][14] have been investigated along with the development of various analytical techniques. In previous works employing a proton nuclear resonance technique, [15][16][17][18][19][20] the in-plane water distribution in a Nafion membrane inside a PEFC was explored under controlled conditions; 15,19) a nominal resolution of 6 μm was obtained in an operating PEFC. 19) Water inside the Nafion membrane was reported to be very mobile and rapidly redistributed. Neutron radiography has been also used to quantify liquid water contents in gas-flow channels and MEAs. [21][22][23] Synchrotron X-ray with relatively low energies (13-30 keV) was used for imaging liquid water inside PEFCs with a spatial resolution of 3-12 μm. [24][25][26][27] In previous works of small-angle neutron scattering [28][29][30][31] with a high sensitivity toward light elements (especially hydrogen, protons, and organic molecules), the quantitative evaluation of humidification, or the averaged water content inside an ionomer film, became possible with a spatial resolution of micrometers. A large body of work exists on bulk Nafion and catalyst layer, but less is known about the structure and behavior of Nafion binder on the nanometer scale especially at the interfaces; thin film studies roughly below 50 nm were fundamentally different, where a concurrent suppression of the proton conductivity was found as compared to bulk, varying on different substrate materials as well as with thermal history. 32,33) Neutron reflectivity (NR) is a powerful method to observe structures of polymers including Nafion on flat substrates with an Angstrom precision in the in-plane direction. [34][35][36][37][38][39][40][41][42][43][44][45][46] In the first NR work on Nafion, Ref. 35 reported the nanometer scale compositions of Nafion and water inside spin-coated Nafion films on native SiO 2 surface on Si(100) at room temperature (RT) at the RH of 0, 35, 60, and 97%. Lamellar region with a thickness less than 10 nm composed of alternating water-rich and Nafion-rich layers was proposed at the interface between SiO 2 surface and the Nafion film. However, this lamellar structure was not found at the Nafion/Pt or Nafion/Au interfaces; instead, a single thin layer with a larger water content than that at the majority of the Nafion film was reported. 34,35) Reference 37 used glassy carbon (GC) and Pt-on-glassy carbon (Pt/GC) substrates. On GC, a water-rich layer was formed, whereas on Pt/GC, a relatively hydrophobic region was reported, 37) contrary to the previous reports. 34,35) Recently, Ref. 46 reported a condensed layer of Nafion molecules approximately 3 nm in thickness on Pt surface. Reference 39 investigated the influences of the hydrophilicity of the supporting substrate and of the thickness of Nafion films on the interfacial nanostructure and the water transport kinetics. A hydrophilic organosilicate substrate induced an interfacial layering of the water transport domains parallel to the substrate, whereas the hydrophobic organosilicate substrate did not trigger this interfacial ordering. 40) Reference 41 reported a new analytical method of NR data for determining the composition depth profiles of multi-layered structures, which is applied to our present study. Reference 42 reported the water uptake and swelling in a very thin (∼15 nm) Nafion film on SiO 2 at nearly a water-saturated conditions at different temperatures from 25 to 60°C. A large swelling strain and water content in a Nafion film were observed at 60°C. 42) Reference 43 reported the effect of film thickness on water uptake using Nafion films ranging from 5 to 153 nm. For the modeling of films with thicknesses of 103 and 120 nm, an additional layer between the gas/Nafion interface was proposed. Beside Nafion films, water penetration into thin hydrocarbon-based membranes, or sulfonated polyphenylene ionomer films, has also been investigated by NR. 36,44) NR investigations of Nafion thin films have been mainly conducted at RT, and the highest temperature reported was 60°C, where the humidity was set only at 95% RH. 42) However, PEFCs used for residences and automobiles are generally operated at temperatures above 70°C at various RHs. To understand the structural changes of and water distribution in Nafion under varying temperature and humidity, we newly designed an environment-controlled chamber for the NR measurements. In this study, Nafion with a thickness of approximately 100 nm on native SiO 2 on Si(100) was used as a model film for the NR measurements carried out at 80°C and at 30, 50, 65, and 80% RH.

Experimental and analytical methods
Prior to the NR measurements, an alcohol-dispersion of 5-wt% Nafion (D-521, Du Pont) was spin-coated at 6000 rpm onto a Si(100) substrate (2 inch f, 1 mm thick) with a native SiO 2 surface (SiO 2 /Si(100)) and annealed at 80°C in air for 1 h. The thickness of the Nafion film was targeted to be 100 nm. The NR measurements were carried out with an environment-controlled chamber (Fig. 1) made of Al for controlling temperature and RH with a continuous flow (200 ml min −1 ) of ultrapure N 2 (99.999%) humidified with ultrapure water (H 2 O, 18.3 MΩ). The temperatures of the specimen and the chamber walls were controlled by rubber heaters. The thickness of the Al windows for neutron beams was 0.5 mm. NR measurements were performed at BL-16 of Materials and Life Science Facility (MLF), Japan Proton Accelerator Research Complex (J-PARC). [47][48][49] The NR experiments were performed at 80°C and at 30, 50, 65, and 80% RH in this order. The Nafion thin film on SiO 2 /Si(100) was kept for 2 h under each condition before each NR measurement. 35,37,[41][42][43] During the 2 h stabilization, NR signals were continuously checked to ensure the steady state of the film. The area of 30 mm × 40 mm was illuminated with pulsed neutrons with the wavelength, λ, of 0.2-0.88 nm at the repetition rate of 25 Hz, where λ was evaluated from the velocity using the time-of-flight method. The neutrons were introduced with changing the incident angle, θ, to be 0.3, 0.6, 1.2, and 2.4°, and the distribution of θ due to the beam divergence was corrected by the reflection angle of neutrons under the assumption of the specular reflection condition. 50) The exposure time of the neutron beam was approximately 100 min to complete each NR measurement. The reflection intensity at each angle was normalized by that of a direct beam, and the reflectivity profile depending on the momentum transfer normal to the substrate, Qz (=4π sin θ∕λ), was obtained with taking the λ and θ of each neutron into consideration.
The analyses of the NR profiles were carried out using the MOTOFIT program. 51) The values of the scattering length density (SLD) for Nafion, silicon, SiO 2 , and H 2 O used in the calculations were 4.16 × 10 −4 , 2.07 × 10 −4 , 3.47 × 10 −4 , and −0.56 × 10 −4 nm −2 , respectively. 41) To calculate the Nafion and water contents at each layer, the "combined" SLD value (SLD) at each Nafion layer containing water was treated as a linear combination of the SLDs of dry Nafion (SLD Nafion ) and water (SLD Water ) as described in Eqs. where V Nafion and V water represent the volume fractions of Nafion and water, respectively. For the Nafion and water contents at each layer, the Nafion density (ρ Nafion _ L ) and water density (ρ Water _ L ) were calculated according to Eqs. (3), (4): Nafion Nafion Nafion Water Nafion Water Water Water Water Nafion water Nafion The densities for solid Nafion (ρ Nafion ) and liquid water (ρ Water ) used were 1.98 and 1.00 g cm −3 , respectively. 52,53)

Results and discussion
Prior to the NR measurements on Nafion, we investigated the structure of an SiO 2 native layer on Si(100) at RT under dry N 2 . This SiO 2 layer is a model for a glass substrate used for filming Nafion membranes. The thickness and roughness of the SiO 2 layer were obtained as 1.06 and 0.75 nm, respectively. Those values were used to analyze all the following data of a Nafion film on SiO 2 /Si(100). Figure 2 shows the NR profiles of Nafion at different RHs. The dots show the NR data, whereas the solid lines show the best-fit curves. We first tried to analyze the Nafion film based on a 3-layered structure model supposedly formed by Nafion/gas interface, Nafion bulk layer, and Nafion/substrate interface. However, the NR curves fit by the 3-layer models showed a poor match to the measured data, especially in the high-Q region around 0.2 Å −1 , where the NR profiles abruptly increased under all humidity conditions (Fig. 2). The characteristic bump could be explained by the existence of a layer with a very different SLD value from that of Nafion. With 4-layer models, the NR data in Fig. 2 were fit very well with calculated values. According to models with more layers, the R factors did not increase very much, showing that the Nafion film can be essentially described with 4 layers. The sublayered structures at the Nafion/substrate interface typically called "lamellar structures" 35,[39][40][41][42][43]45) were not analyzed further in this study. Table I provides the fitting-curve parameters from the NR data at different RHs, including layer thickness, SLD, and roughness. The roughnesses are very small, showing the high flatness of the Nafion specimen, as well as the validity of our 4-layer models. Models of Nafion thin films based on the data in Table I are illustrated in Fig. 3.
By our NR data, the total thickness of the Nafion film at 80°C was 131, 139, 146, and 153 nm at 30, 50, 65, and 80% RH, respectively (Fig. 3). The increase in the film thickness was in good agreement with the water uptakes measured by the change in mass of film at 80°C. 54) At the Nafion plane contacting air or N 2 humidified with water vapor, Ref. 55 proposed Nafion bundles of fused inverted micelles aligning and burying the hydrophilic cores under a thin hydrophobic shell to minimize the surface energy from the results obtained by contact-angle measurements of water, grazing-incidence small-angle X-ray scattering, and atomic force microscopy. The existence of this surface layer was later proposed by NR, 43) and this surface layer was also derived from our NR data as "Topmost Layer" at the Nafion/gas interface as shown in Fig. 3. At topmost layer, the water density increased from 0.08 to 0.20 g cm −3 as the RH increased from 30 to 80%, but the film thickness remained the same, 8-10 nm, regardless of the RH. The thickness of "Bulk Layer" under Topmost Layer monotonically increased from 120 to 125, 132, and 140 nm as the humidity increased from 30 to 50, 65 and 80% RH, respectively, while the water density increased from 0.14 to 0.24 g cm −3 . At thin "Intermediate Layer" under Bulk Layer (Fig. 3), the film thickness increased from 2.3 to 3.0 nm, and the water density slightly increased from 0.38 to 0.42 g cm −3 . Characteristic "Water-rich Layer" existed at the interface between Nafion and SiO 2 as a model glass substrate (Fig. 3), whose thickness was approximately 1.3 nm at all humidities, whereas the water density increased from 0.71 to 0.97 g cm −3 . At 80% RH, the volume fraction of water at Water-rich Layer was almost 1, meaning that Nafion was barely anchored on a Si substrate. The roughness of Waterrich Layer at 80% RH was 0.0 nm (Table I), showing an extremely-uniform thickness. After the NR experiments, the Nafion/SiO 2 /Si(100) sample was carefully examined with a microscope, and no detachment of the Nafion film was observed. At the interface between Nafion and native SiO 2 surface has been reported to exist a water-rich layer, onto which Nafion-rich and water-rich lamellae were alternated with each other. 35,[39][40][41][42][43]45) The lamellar region including the water-rich interfacial layer spanned a total thickness of 6-10 nm at RT depending on the NR analytical models. 35,[39][40][41][42][43]45) Recent data obtained by different experimental methods are showing that so-called lamellar structures at the substrates are not so clearly constructed. 56,57) In our case, the total thickness of Intermediate Layer and Water-rich Layer (Fig. 2) was approximately 4 nm.
Very recently, Ref. 43 reported structural data on a Nafion film with the thickness of 103 nm when dry on SiO 2 /Si(100), a sample very similar to ours, from NR measurements at  29.6°C and 92% RH. Figure S1, available online at stacks.iop. org/JJAP/58/SIID01/mmedia, shows the layered structure of a Nafion film from the SLD profiles reported in Ref. 43 newly calculated based on our analytical method described in Experimental and analytical methods. The total thickness of the Nafion increased from 103 nm under dry Ar to 140 nm at 92% RH (Fig. S1), 43) similar to ours (153 nm at 80% RH). The Nafion and water densities in the layer corresponding to our Topmost Layer were 1.61 and 0.19 g cm −3 , respectively (Fig.  S1), whereas those in Topmost Layer of ours were 1.57 and 0.20 g cm −3 , respectively (Fig. 3). The Nafion and water densities in the layer corresponding to our Bulk Layer were 1.49 and 0.25 g cm −3 , respectively (Fig. S1), whereas those in Bulk Layer of ours were 1.49 and 0.24 g cm −3 , respectively (Fig. 3). The thicknesses of the corresponding layers were similar, too. Therefore, the structures of Topmost Layer and Bulk Layer at high RHs were very similar at 29.6 and 80°C. The lamellar structure shown in Fig. S1 with the thickness of 9.0 nm corresponded to our "Intermediate Layer + Water-rich Layer", 4.2 nm in thickness (Fig. 3). As mentioned, we did not assume a priori the existence of sublayered structures at the Nafion/SiO 2 interface for our structural analysis. Detailed investigations of the Nafion/SiO 2 interface at 80°C are needed.

Conclusions
NR measurements using an environment-controlled chamber was conducted at 80°C and at 30, 50, 65, and 80% RH at BL-16 of MLF, J-PARC. The in-plane water distribution was determined inside a 100 nm thick Nafion film prepared on native SiO 2 on Si(100). The Nafion thin film was analyzed based on a 4-layer model. Topmost Layer was proposed to have a hydrophobic conformation at the interface between Nafion and humidified N 2 , 43,55,58) and its thickness did not change by increasing the humidity. The thickness of Bulk Layer monotonically increased from 120 to 140 nm as the humidity increased from 30 to 80% RH. At Intermediate layer, the film thickness slightly increased from 2.3 to 3.0 nm. The water density in Intermediate Layer was two times higher than that in Bulk Layer. The thickness of Water-rich layer was approximately 1.3 nm regardless of the humidity, while the water density increased up to 0.97 g cm −3 at 80% RH. The structures of Topmost Layer and Bulk Layer were similar at 80°C/80% RH and 29.6°C/92% RH. 43) The lamellar structure at the Nafion/SiO 2 interface reported at 29.6°C corresponded to "Intermediate Layer + Water-rich Layer" at 80°C. Detailed analysis is needed for the Nafion/SiO 2 interface at 80°C. NR measurements on Pt and C substrates at 80°C are now in progress.

Acknowledgments
This work was performed under "Superlative, Stable, and Scalable Performance Fuel Cell" (SPer-FC) project of the New Energy and Industrial Technology Development Organization (NEDO). The NR experiments were performed as projects approved by the Japan Proton Accelerator Research Complex under user programs, Nos. 2016A0246, 2016B0036, and 2017B0316. J.I. thanks Drs. Hideto Imai, Masashi Matsumoto, Naoki Takao, and Yoshiki Iwai of