Water Uptake and Swelling Hysteresis in a Nafion ... - ACS Publications

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Water uptake and swelling hysteresis in a Nafion thin film measured with neutron reflectometry W. Peter Kalisvaart, Helmut Fritzsche, and Walter Merida Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b00764 • Publication Date (Web): 22 Apr 2015 Downloaded from http://pubs.acs.org on April 24, 2015

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Water uptake and swelling hysteresis in a Nafion thin film measured with neutron reflectometry W. Peter Kalisvaarta,*, Helmut Fritzscheb, Walter Méridaa a

Clean Energy Research Center,University of British Columbia, 2360 East Mall, V6T 1Z3, Vancouver BC, Canada

b

Canadian Neutron Beam Centre, Canadian Nuclear Laboratories, Chalk River ON, K0J 1J0, Canada.

Abstract

Water uptake and swelling in a thin (~15 nm) Nafion film on SiO2 native oxide on a Si wafer is studied as a function of relative humidity (8-97%) at room temperature and as a function of temperature (25-60 oC) at 97% relative humidity by neutron reflectometry. This is the first report on the behavior of thin Nafion films at elevated temperatures and high humidity. Large hysteresis is observed during the temperature cycle. The observed swelling strain in the film at 60 oC is 48% as compared to the as-deposited state, which is far above any previously observed trend at room-temperature. A small decrease in the average SLD suggests that part of the additional swelling is due to thermal expansion, but the estimated D2O/SO3 ratio also increases by 70%. Half of the ‘excess’ absorption and 73% of the additional swelling are retained during cooling back down to room temperature. The results provide new insights into the dynamics of Nafion on nanometer scales under fuel cell operating conditions.

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1. Introduction Proton exchange membrane fuel cells (PEMFCs) are becoming a viable alternative to internal combustion engines for automotive drive trains.1 Protons formed from oxidation of hydrogen gas at the anode are transported through the NafionTM membrane towards the cathode to be recombined with oxygen from air to form water. Inside the catalyst layers on either side of the membrane, Nafion exists as nanometer-scale filaments that facilitate proton transport from the membrane to the active sites on Pt nanoparticles.2 Proton transport is enabled by sulphonic acid (SO3H) head groups on the ends of short fluorocarbon sidechains grafted onto a PTFE (CF2)n backbone:

Scheme 1: Model structure of NafionTM The value of ‘x’ in Scheme 1 determines the average spacing between side chains; the lower it is, the more SO3H groups there are per unit length of PTFE backbone. This is usually expressed as the so-called ‘equivalent weight’, EW, which is the mass of dry polymer in grams per mole of SO3H and a lower value means a higher density of SO3H groups in the polymer. The proton conductivity has a very strong correlation with the amount of absorbed water and generally increases with increasing H2O content.3–5 Studying trends in water uptake of nanometer-scale structures of Nafion under temperature and humidity conditions relevant to PEM fuel cell


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operation is therefore of fundamental importance in optimizing catalyst layer composition and, by extension, fuel cell performance. Whereas a large body of work exists on bulk Nafion, see e.g. Mauritz and Moore6 for an extensive review, less is known about the structure and behavior of Nafion on the nanometer scale. Thin film studies have found fundamental differences between thin films and bulk membranes such as suppression of water uptake and swelling for films below, roughly, 50 nm and a concurrent suppression of the proton conductivity as compared to bulk,4,7 although this is found to vary between substrate materials as well as with thermal history.8 So far, all experimental investigations of Nafion thin films have been conducted at room temperature but PEMFCs generally operate at 40-75 oC and it is not known whether increased temperatures induce any additional changes on top of those induced by increasing humidity at roomtemperature in nanometer-scale Nafion structures.9 In this paper, we present a neutron reflectometry (NR) study on a thin, initially ~15 nm, Nafion layer on native SiO2. This particular thickness represents a ‘gap’ in the range of thicknesses studied with reflectometry between ultrathin films of about 8 nm and films larger than 40 nm thickness.7,10,11 Contrary to X-rays, neutrons are highly sensitive to light elements such as lithium,12 hydrogen and deuterium.13 That, combined with its sub-nm depth resolution, makes NR an ideal technique to study water uptake and distribution in a Nafion thin film. The film is subjected to 8% and 97% relative humidity (RH) at room temperature and temperatures between 25-60oC at 95-97% RH. This is the first study that presents reflectometry data on a fully hydrated film at elevated temperatures within the operating range of PEMFCs and studies its evolution as a function of temperature. The results and their implications for the study of PEMFC catalyst layers are discussed.


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2.

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Experimental section

Sample preparation

Four-inch diameter, 0.5 mm thick single crystal Si wafers with (100) orientation were purchased from Silicon Valley Microelectronics (Santa Clara CA, United States) and used as-is. A LIQUIonTM 5wt% Nafion dispersion14 with EW of 1100 was purchased from IonPower Inc. (New Castle DE, United States) and diluted by a factor of 10 by volume with pure isopropanol. To deposit the layer, the Si wafer was first washed with isopropanol twice and spun to dryness for 60 s. Then, the diluted stock solution was dispensed onto the wafer and allowed to spread for 10 s at 300 rpm. Subsequently, rotation was instantly accelerated to 2700 rpm and maintained for 60 s. The film was air-dried at 60oC for 45 minutes and stored under low vacuum at room temperature. During transport from the University of British Columbia to the Canadian Nuclear Laboratories in Chalk River, the wafer was kept in a hermetically sealed bag together with DrieriteTM dessicant.

Neutron Reflectometry measurements

NR measurements were performed on the D3 neutron reflectometer at the NRU reactor in Chalk River, Ontario, Canada. The environmental cell specifically developed for this type of experiment is described elsewhere.15 Saturated KOH in D2O was used to set the low humidity condition (8% RH at 25 oC). For the high humidity (97-95% RH at 25-60 oC) condition, saturated K2SO4 in D2O was used16. Although humidity data are documented for saturated solutions in H2O, RH values over D2O are generally assumed to be identical. When mounting the


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sample in the environmental enclosure and when changing the saturated salt solution to change the humidity, the film was inevitably exposed to ambient conditions for short periods of time. At least 1 hour equilibration time was allowed after each time the cell was opened and/or the temperature was changed, before starting the reflectometry measurement. Based on previous observations on thin (~20 nm) films, the layer was expected to attain its equilibrium hydration within 100 seconds,7 but we allowed additional time for fully exchanging light water from ambient atmosphere with heavy water from the solution. As an additional check, we always first performed a measurement from q = 0.25 to 0.06 nm-1 (which took only 3 minutes) before starting a long scan from q = 2 nm-1 to q = 0.06 nm-1 which typically took 8 hours. This allowed us to check whether the films were changing their structure during these 8 hours by comparing the data between 0.06 < q < 0.25 nm-1. For all our measurements the scans around the critical edge that were about 8 hours apart were always identical. Taking into account the high sensitivity around the critical edge regarding heavy water uptake we can therefore conclude that all our measurements were done in equilibrium.

Fitting the reflectivity data

The GenX program17 was used to fit the reflectivity data. It uses the Parratt formalism18 to simulate the reflectivity curve and a genetic algorithm to find the optimum fit to the data. The program chooses a random starting point for each fit from anywhere between the upper and lower limits defined for each parameter. Each fit was allowed to run for 600 generations and run up to 15 times to find the global minima. The following figure-of-merit (FOM) function was minimized:


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1 FOM = N −P

N

 Yi − Si  Yi i =1 

∑

  

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2

(1)

where Yi and Si are the measured and simulated intensity, N is the number of data points and P the number of independent parameters that are allowed to vary during optimization. This particular FOM function was chosen for the following reasons: the intensity of reflectivity data falls off as q-4 so deviations between measured and simulated data at larger q will be very small19 and contribute little to the FOM, causing the fitting to be biased towards the data closer to the critical edge. Dividing by the measured value removes this bias for the most part and ensures the fitting algorithm strives to fit all data equally well. In principle, increasingly complex models N

should be able to fit the data better and reduce the value of

 Yi − Si  Yi i =1 

∑

  

2

even further. However,

eventually one is simply fitting random variations in the data. In order to reduce the potential for ‘overfitting’, the sum is divided by (N-P) which will become smaller with an increasing number of parameters P and any further improvements in the fit will eventually be negated.

3. Results and discussion


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Figure 1: Measured reflectivity data (open symbols) at 8% RH and fits (solid lines) and scattering length density profiles (inset) corresponding to the optimum fits for increasing model complexity. Fitting was performed using the GenX17 program. For clarity some curves are shifted as indicated in the graphs. Statistical errors fall within the area of the data markers down to a reflectivity of ~10-6 (see Figure S1). Residuals (definitions of Y and S same as in equation (1)) are shown in the bottom panel for three selected models. The number of segments used for the Nafion layer in each model is indicated between brackets on the left side of the SLD profiles in the inset. Color correspondence is maintained throughout all three panels.


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The reflectivity data measured at 8% RH (saturated KOH), are shown in Figure 1. The data around the critical edge (0.06-0.25 nm-1) was always measured twice, ~ 8 hours apart, to verify the film was at its equilibrium hydration. It is clear from Figure 1 that the data cannot be described satisfactorily with only a single-layer model for the Nafion film. The FOM is 0.0492 for this particular model, but it should be noted that the absolute value of the FOM by itself is not sufficient to judge whether the fits are ‘bad’ or ‘good enough’. One should always look whether key features in the reflectivity curve such as oscillation amplitude and position of the oscillation maxima are fitted correctly. The residual plot shows that at 0.05 < q < 0.4 nm-1 the simulated curve is consistently above the measured one, meaning the average SLD in the film is overestimated. Oscillations are also visible in the residuals for the single-layer model, showing that both the measured intensity and the amplitude of the oscillations are not reproduced correctly. Subsequently, the complexity of the model was incrementally increased by dividing the Nafion layer up into an increasing number of segments or ‘lamellae’. The fit markedly improves up to 5 segments after which no further lowering of the FOM was observed. The residuals are now spread more or less evenly around zero and have no remaining distinct features. The total thickness of the Nafion layer at 8% RH is 15.1 nm and the SLD varies along the surface normal from a low of 4.33 to 4.52 and 4.67x10-4 nm-2 at the substrate interface and the surface, respectively, indicating D2O-enrichment and/or denser packing of the polymer chains compared to the central part of the film. The average SLD is 4.45x10-4 nm-2.


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Figure 2 top: Evolution of the reflectivity curve (symbols are data points, solid lines are fits); bottom: corresponding optimized SLD profiles upon increasing the RH from 8 to 97% and subsequently the temperature from 25 to 60 oC. The markers on the left indicate the SLD in the central part of the layer for 8 and 97% RH at 25 oC; 4.3 and 4.8x10-4 nm-2, respectively. The dashed lines represent the optimized SLD profile with higher complexity (7-layer model)

The RH was subsequently increased to about 97% by changing the solution in the reservoir to saturated K2SO4. This salt is particularly suited for this purpose as its equilibrium RH varies by only ~2% between room temperature and 60oC.16 The data are shown in Figure 2 together with 8% RH for comparison. NR is very sensitive to changes in layer thickness d, which can in fact be read directly off the chart as d ≈ 2π/∆q where ∆q is the spacing between oscillation maxima.19 The increased number of oscillations within the measured range up to 2.0 nm-1, first with increasing humidity and subsequent increase in temperature, therefore indicates a large expansion with each step. Throughout the literature, swelling is usually reported as the ‘swelling strain’, defined as ∆d / d0 which represents the increase in thickness compared to the original


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thickness d0. The film first swells by 23% at 97% RH relative to 8% RH, from 15.1 to 18.6 nm. This amount of swelling strain falls within the trends that have been observed before for Nafion films < 20 nm. 4,7 The reflectivity curve was analyzed in the same way as presented in Figure 1, keeping the optimized parameters for the SiO2 layer the same. The surface and interfacial features become more pronounced and the SLD has increased in all parts of the film. The average SLD increases to 4.83x10-4 nm-2 indicating large amounts of heavy water are being absorbed. Even more so than at 8% RH, a single-layer model does not result in a satisfactory fit to the data. The residual plot shows an equal number of, albeit distorted, oscillations as the measurement, meaning the position and amplitude of the oscillation maxima are not fitted correctly (see Figure S2). A satisfactory fit to the data at 25oC and 97% RH can be obtained using a 5-layer model, resulting in a FOM of 0.0109. Increasing the complexity further to a 7layer model leads to only a marginal further improvement down to 0.0107. For the data at 60 oC, a 7-layer model similar to that derived at 25oC does give an additional improvement in the FOM from 0.009 to 0.006 (see Figure S3). The SLD profile that best fits our data in Figure 2 has an SLD of 5.28, 4.51 and 4.80x10-4 nm-2 in the first, second and third segment of the Nafion layer, respectively, counting from the SiO2 interface and has elevated SLD (5.14x10-4 nm-2) at the surface as well. After increasing the temperature, the general shape of the SLD profile is preserved, as can be seen in Figure 2. Such alternating water-rich and water-depleted layers at the Nafion/SiO2 interface have been observed before7,10,11 and are thought to arise from preferential orientation of the Nafion side chains with their hydrophilic SO3 head groups towards the hydrophilic SiO2 surface followed by a narrow zone enriched in PTFE backbone segments. This results in a multi-lamellar micro phasesegregated structure aligned parallel to the substrate where the compositional oscillations quickly


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dampen towards the central part of the film.11 This has been more clearly demonstrated by DeCaluwe et al. by combining data from H2O and D2O-absorbed Nafion films.10 Waterenrichment at the surface has not been observed in reflectometry experiments for films thicker than 10 nm, but this is in line with observations by Paul et al. that films thicker than 55 nm have a hydrophobic surface, whereas films with a thickness of 30 nm and below are hydrophilic.20 It should be noted, however, that the number of lamellae and the contrast between them is too low in our case to generate any clear distinctive features within our q-range such as a high-q peak that is observed for thicker (>40 nm) films using H2O.10,11 As a consequence, there are numerous local minima that the fitting algorithm can get ‘stuck’ in and numerous solutions with comparable FOM. Therefore, each fit was run multiple times for 600 generations from different starting values for each parameter and a summary of the results is included as Figure S4 for the measurement at 97% RH at 25oC for a three-part model of the Nafion film. The FOM is clearly higher and the fit noticeably worse when both lamellae are at the film/substrate interface instead of having increased SLD at the surface as well. We therefore conclude that although the detailed internal structure of the interface and surface regions may be uncertain, there is D2O-enrichment at both the SiO2/Nafion interface and the surface.


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Figure 3: Measured reflectivity curves (open symbols) and fits (solid lines) for spin-cast Nafion film on SiO2 during cooling from 60 to 25 oC are displayed in the top panel. SLD profiles corresponding to optimum fits are shown in the bottom panel. For clarity, some curves are shifted as indicated in the graphs.

Our most striking and important finding, however, is that upon heating the film at high (9597%) RH, there is, besides a slight decrease in the average SLD to 4.75x10-4 nm-2, an additional increase in the layer thickness up to 22.3 nm. Compared to 8% RH, the film has now expanded by 48% which is higher than any previous observations made at room-temperature.4,7 Furthermore, despite the slight decrease in the average SLD, the product of thickness and average SLD increases by another 20%. As will be explained later, it is this product rather than the SLD by itself that is indicative of the water content in the film. Subsequently, the sample was cooled back down to 25 oC in steps of 12 degrees. The reflectivity data and SLD profiles corresponding to the optimum fits are shown in Figure 3. Narrow regions with elevated SLD are still found on both the surface and the interface although, as explained before, fine details are less certain. However, absence of D2O-enrichment at the


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surface still does not produce a satisfactory fit, especially beyond q = 1.0 nm-1 (see Figure S5). The most important observation is that the oscillation spacing remains approximately constant, indicating the layer does not shrink back to its original thickness. A 2.7 nm increase in thickness remains after cooling down, as compared to at 25oC before the film was heated. Compared to 60 o

C, both thickness and SLD decreased, indicating that the amount of absorbed water decreases.

The values are summarized in Table 1 for all temperature-humidity conditions that were measured. Table 1: Thickness, SLD and D2O/SO3 ratio of the Nafion film as a function of temperature and humidity Thickness (nm)

Swelling strain

Average SLD (10-4 nm-2)

CD2O

λ D2O/SO3

∆d/d0

ratio

8% RH, 25 oC

15.1

--

4.45

97% RH, 25 oC

18.6

0.23

4.83

0.31

6.9

95% RH, 60 oC

22.3

0.48

4.75

0.53

11.7

96% RH, 48 oC

21.7

0.44

4.45

0.41

8.9

97% RH, 37 oC

21.5

0.42

4.50

0.41

9.0

97% RH, 25 oC

21.3

0.41

4.57

0.42

9.1

The amount of absorbed water, which is an important predictor of the proton conductivity,3–5 is usually expressed as the ratio between the number of water molecules and SO3H groups and represented by the symbol λ. To calculate the amount of absorbed D2O, the following equation which was originally derived to calculate hydrogen-to-metal atomic ratios in metal hydrides21 but can in principle be adapted for any host-absorbent combination, has been used:


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S d Naf + D2O  bNaf cD2O =  Naf +D2O − 1  S RH =0 d Naf  bD2O

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(2)

SNaf+D2O and SRH=0 are the SLD of the hydrated and dry film, respectively. dNaf+D2O and dNaf are the thickness of the swollen and dry film, respectively. It is useful to note that the ratio between the two is equal to 1+ ∆d/d0 (see Table 1). bNaf and bD2O are the average scattering lengths of Nafion and D2O, respectively, weighed according to their overall atomic composition, which is 5.79 fm for Nafion and 6.38 fm for D2O. It should be kept in mind that CD2O calculated from equation (2) is the atomic ratio between Nafion and D2O. To obtain the D2O-to-SO3 ratio, i.e. λ, CD2O has to be multiplied by the number of atoms in the basic repeating unit containing one SO3 group which is on average 66 for Nafion of 1100 equivalent weight6 and divided by the number of atoms in a D2O molecule. The values for CD2O calculated with equation (2) and λ listed in Table 1 thus differ by a factor of 66/3 = 22. It should be noted that we did not attempt to completely dry the film and the SLD and thickness at 8% RH were used in equation (2). Because the film may not have been entirely water-free at 8%RH and 25 oC the true D2O/SO3 ratio is probably higher. Additional explanations and discussions are included in the Supporting Information. The evolution of layer thickness, SLD, d x SLD and λ with temperature are visualized in Figure 4 for the measurements at 95-97% humidity. Note that while the product d x SLD increases by only 20%, λ nearly doubles from ~7 to ~12 which is not immediately obvious from Equation (2). These values for λ and the swelling strain are nearly twice as high as observed before,7,11 where suppression of water absorption and swelling for films below ~50 nm7 was found. Apparently, this barrier can be overcome at elevated temperature and high humidity. As illustrated in Figure 4, part of this enhanced swelling and absorption is subsequently retained


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upon cooling. The values for d x SLD are 89.69 ± 0.44 and 97.21 ± 0.37 x 10-4 nm-1 before and after heating, respectively.

The evolution of the thickness and average SLD during the

temperature cycle is depicted separately in Figure 4. The average SLD in fact decreases upon heating from 25 to 60 oC, which indicates that part of the observed increase in the thickness is due to thermal expansion and that the polymer chains become less densely packed. This is even more obvious after cooling when the SLD is, on average, ~6% lower compared to before the film was heated. Hysteretic effects have been observed previously in the form of trapped water near the interface with SiO2, but this happened during drying and the bulk of the film still lost all its water.11 Here, we observe two effects that have never been found before; enhanced water uptake at elevated temperatures and a hysteretic effect at high (97%) RH for the bulk of the film. The average SLD at 25 oC is lowered by ~6% and the thickness increased by ~15% after cooling as compared to before the film was heated. Bulk membranes have in fact been shown to absorb less water at 80 oC as compared to 30 oC when in contact with 100% saturated water vapor.22,23 Therefore, it is unlikely that the enhanced water absorption we observed is due to an increase in absolute water concentration with increasing temperature at constant RH as the opposite trend might be expected. Both, the enhanced absorption and hysteresis, probably arise from a change in the structure of the Nafion film. During deposition by spincoating, the solvent evaporates very quickly and the Nafion polymer chains may become ‘frozen’ in a non-equilibrium configuration. Apparently, only a combination of elevated temperature and high water content, which is not achieved during the drying step in ambient humidity during preparation, enables a reorganization of the polymer structure, although the possible role of water as a plasticizer for Nafion is a matter


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of some controversy6. The central part of the layer may form more ‘bulk-like’ structural features such as (spherical) ionic domains and interconnected channels24 that are able to accommodate more water molecules. Even the interfacial structure appears narrower and less distinct during cooling as compared to before the film was heated (compare Figures 2 and 3), which also indicates a reorganization of the polymer structure. In principle, ionic clusters and channels are detectable using off-specular techniques such as grazing-incidence small angle x-ray scattering (GISAXS) which have never shown any ordered structures in films below 20 nm. However, again, only room-temperature experiments have been performed.4,8 Although the fact that more than half the initial increase in λ is lost during cooling may indicate the change is reversible, the time-scale at which this happens is obviously very long, at least several days or perhaps even weeks. After all, the reflectivity curves could always be fitted very well with a single model even though a measurement up to q = 2.0 nm-1 took 8-9 hours. Considering the previously observed confinement effects and the high sensitivity of the measured proton conductivity to the thermal ‘history’, e.g. (vacuum) annealing treatment,25 of the film, it would be very interesting to see whether the film is closer to a truly ‘equilibrated’ state after one temperature cycle or whether multiple cycles at high humidity would affect further changes.


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Figure 4 Top: Thickness x SLD (left y-axis) and D2O/SO3 ratio (right y-axis) as a function of temperature at 95-97% RH. Bottom: SLD (left y-axis) and thickness (right y-axis) separately as a function of temperature. Temperature trajectory is illustrated by the arrows, as well as the large additional water uptake at 60 oC and the hysteresis effect at 25 oC. A comparison of the reflectivity curves at 25oC before and after heating is included as Figure S6

We studied a Nafion film with a thickness comparable to those found in fuel cell catalyst layers, but the substrate material, SiO2, is not routinely included in catalyst layer formulations. However, several studies have found a beneficial effect of including SiO2 nanoparticles either by themselves26–28 or in combination with another hydrophilic oxide29 on overall performance. The biggest improvements were observed for operation under low humidity with SiO2 included on the anode side26. SiO2 on the cathode side has shown mixed results, although surfacefunctionalized silica particles improved performance when included on the cathode catalyst layer as well, both at low and high humidity.28 The enhanced performance at low humidity may be ascribed to the aforementioned trapping of water at the interface where a high conductivity


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pathway for protons would then be maintained. The enhanced ability to absorb water we observed here would indeed be expected to improve fuel cell performance at high humidity and/or high current density as well, as enhanced water uptake by Nafion in contact with SiO2 may delay formation of liquid water at the cathode, mitigating flooding. At room-temperature, the swelling ratio is considerably lower on carbon compared to SiO2 for identically prepared films,4,8 suggesting that this may indeed be true. To study this in more detail, the behavior of thin Nafion films on carbon and Pt, which are known to influence the orientation and morphology of the Nafion and water uptake in different ways,30–32 should be compared with that on SiO2 under identical conditions of high RH combined with elevated temperature. It is worth noting that inclusion of hydrophilic components such as metal oxides, either to induce favorable metal-support interactions29 or protect the conductive carbon phase against corrosion33,34 may have unexpected effects, given how much higher we observed the expansion of Nafion to be. Ex-situ tests of e.g. electrical and ionic conductivity of catalyst layers35 should ideally include at least one temperature excursion at high RH to give a realistic impression of its in-operando behavior. The substantial difference in the SLD throughout the film at 25 oC that is effected by the temperature cycle is indicative of a change in the packing density of the polymer chains and water molecules which may in turn influence other properties such as water and proton mobility.

4. Conclusion

We have studied the evolution of thickness and water content of a 15 nm Nafion film under a combination of high humidity and heat for the first time. Unprecedented swelling strain


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and water content were observed at 60 oC, indicating previously observed suppression of the same are overcome under these conditions. Partial desorption was observed upon cooling back to ambient temperature, but over 45% of the absorption observed in the heating step is retained. The decrease in the average SLD over the temperature cycle, despite the increase in D2O content points to a fundamental change in the structure of the layer, i.e. the polymer chains are now significantly less densely packed. In addition, we discussed how a number of experimental observations on the effects of SiO2 addition to PEM fuel cell catalyst layers can be explained based on thin film model studies such as the one we presented here. To study these phenomena in more detail, NR measurements over multiple temperature cycles at high or intermediate humidity on different substrates would be interesting avenues for further research.

Supporting Information Further details on data fitting and calculation of D2O/SO3 ratio, background correction and statistical errors and alternative models for selected data sets. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author * Peter Kalisvaart, Clean Energy Research Center,University of British Columbia, 2360 East Mall, V6T 1Z3, Vancouver BC, Canada Tel: +1-604-822-6001/+1-778-928-2678, E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


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TOC entry

Large hysteresis in both thickness and water content of a thin Nafion film is observed with neutron reflectometry upon heating/cooling under high relative humidity (95-97%).


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Water Uptake and Swelling Hysteresis in a Nafion ... - ACS Publications

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