NMR and Pulsed Field Gradient NMR Approach of Water Sorption

Apr 22, 2009 - The water uptake and the water self-diffusion coefficient were measured in Nafion membranes at subzero temperatures. NMR spectroscopy ...
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NMR and Pulsed Field Gradient NMR Approach of Water Sorption Properties in Nafion at Low Temperature Armel Guillermo,*,† Ge´rard Gebel,† Hakima Mendil-Jakani,† and Eric Pinton‡ CEA, INAC, SPrAM (UMR5819 CEA/CNRS/UJF), 38054 Grenoble Cedex9, France, and CEA, Liten, LPAC, 38054 Grenoble Cedex9, France ReceiVed: December 15, 2008; ReVised Manuscript ReceiVed: March 4, 2009

The water uptake and the water self-diffusion coefficient were measured in Nafion membranes at subzero temperatures. NMR spectroscopy was used to precisely quantify the actual concentration of water in membranes as a function of the temperature and their hydration rates at room temperature. We find that below 273 K the water concentration decreases with temperature to reach, at around 220 K, a limit value independent of the initial concentration. This regime is observed if the concentration at room temperature is higher than 10%. Below this concentration no membrane deswelling was observed. The water self-diffusion coefficient, measured by pulsed field gradient NMR in function of the temperature, is determined by the actual concentration C(T) whatever the concentration at room temperature. The concentration variation is attributed to a decrease in the relative humidity RH(T) of the water vapor surrounding the membrane induced by the simultaneous presence of supercooled water inside the membrane and ice outside the membrane. 1. Introduction Water management in a polymer electrolyte membrane fuel cell (PEMFC) is a major challenge when the fuel cell is subject to frequent temperature changes. In the specific transport application the effects of negative temperatures will be currently found. It is known that consequences of ice formation inside the cell are strongly unfavorable for its performances; the presence of ice in catalyst layers or inside bipolar plates prevents the start of the cell.1,2 Consequently, the behavior of the membrane at low temperature is one of the main items in the understanding of the impact of freezing on PEM fuel cells. A polymer membrane of interest is the Nafion that is used in most of the current PEMFC applications. Nafion is a perfluorinated ionomer with poly(tetrafluoroethylene) backbone and additional fluorinated side-chains ending with a sulfonic acid group. This chemical structure yields a hydrophobic/hydrophilic duality for the matrix that materializes in a nanoscale phase separation in hydrophobic and hydrophilic domains.3 Regardless of their precise shape, an elongated morphology characterizes these domains, it is recognized that water is confined at a nanometric scale and that the water uptake increases the transverse characteristic size of confinement (typically 1-4 nm).4,5 The network of interpenetrating polymer bundles4 defines a 3D continuum for water diffusion. This structural morphology combined with a nonwetting interfacial behavior between water and polymer aggregates entail the water mobility.6 For instance, the self-diffusion coefficient of water in Nafion is 10-6 cm2/s for five water molecules per sulfonic acid group, whereas it is 5 × 10-8 cm2/s in a sulfonated polyimide at the same water uptake.7 As a consequence, its transport properties and its chemical stability contributed for considering Nafion as a reference membrane in PEMFC applications. However the membrane creep above 80 °C and its swelling rate in the * To whom correspondence should be addressed. E-mail: armel.guillermo@ cea.fr. † CEA, INAC, SPrAM. ‡ CEA, Liten, LPAC.

presence of water limit the improvement of performances of Nafion-based PEMFC. In the context of the PEMFC application that necessitates numerous starting/stopping at subfreezing temperatures, the properties of the Nafion membrane at low temperature have to be characterized. The swelling properties and the determination of the ice location are the main features. Concerning the subfreezing behavior of water in Nafion membranes, the debate is often centered on the location of ice, that is to say: inside8-11 or outside12 the membrane. Arguments against ice crystallization inside the membrane rely on the water confinement, which prevents the crystallites growth; a glassy state of water is expected in this case due to a strong slowing down of the water dynamics.13,14 Recently, a supercooled state of the water was evidenced by recording infrared images of water in membrane electrode assembly operating at subfreezing temperatures15 and a SAXS investigation using a micro X-ray beam demonstrated that ice is formed on the external surfaces of the membrane.16 For this purpose, a hydrated Nafion membrane was first quenched at 203 K, the ice formation being characterized during an annealing at a higher temperature (223 K). The aim of the present study is to measure the water concentration and the water mobility inside hydrated Nafion membranes when the temperature decreases slowly from the room temperature down to 220 K. Such a protocol was chosen to be the most closely related to the standard usage in transport application when the PEMFC have to be turned off and stored at low temperature. The water uptake at room temperature was varied between 9% and 26% in weight. The temperature dependence of the liquid water amount was measured by recording its 1H-magnetization intensity versus temperature. The nuclear magnetic relaxation properties of liquid water and solid ice are different enough to easily discriminate between the NMR signals of these two water phases. Moreover, the fluorinated character of the Nafion yields that the 1H NMR signal is essentially due to the water except the hydrogen of the sulfonic acid group. The in situ measure of the macroscopic swelling of the membranes, owing to a specific NMR experiment, demon-

10.1021/jp8110452 CCC: $40.75  2009 American Chemical Society Published on Web 04/22/2009

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strated that the loss of liquid water stands for an actual decrease in the total water amount in hydrated Nafion, which implies that ice was produced outside the membrane. This result was corroborated by the concentration and temperature dependences of the chemical shift of the water resonance line. Finally, the pulsed field gradient NMR technique (PFGNMR) was applied to measure the water self-diffusion coefficient; according to the design of this method the molecular translational properties are investigated at a micrometer scale. The results of these experiments show that the water diffusivity was fully determined by the temperature and the actual water concentration in the membrane whether its value originate from a desorption process or not. 2. Experimental Methods 2.1. Nafion Samples. The Nafion 112 used in this study was purchased from DuPont Company. The molecular equivalent weight EW is 1100 g/mol (i.e., one sulfonic acid mole per 1100 g of polymer or 15 skeleton C-C bonds per one C5O2F10SO3H side-chain), the thickness of the dry membrane is 50 µm. Acidification of membranes was achieved according to the following treatment: membranes were soaked in a 2 mol/l HCl solution at 80 °C for 2 h then rinsed in deionized water at the same temperature. A second acidic bath was performed in a 1 mol/L nitric acid solution at 80 °C (1 h) then rinsed in water and dried at room temperature with a nitrogen flux. Long strips (8 cm × 1 cm) were cut and rolled up in order to be inserted in a NMR glass tube. The water concentration was obtained by adding liquid water. A Teflon cap was placed just above the sample to avoid water condensation along the NMR tube; a second cap closed the glass tube to ensure a constant water amount in the experimental assembly. The water concentration at room temperature Co was directly obtained by weighing in situ the hydrated sample and by a NMR determination of the water amount (Co ) water mass/hydrated sample mass). In accordance with this sample preparation, the equilibrium swelling of Nafion 112 was found at Ceq (298 K) ) 0.221 ( 0.003 g/g (i.e., λeq ) 17.7 ( 0.4, λ is the molar ratio [water]/[ionic group]). Let us note that the experimental determination of the mass of the dry membrane is no longer necessary for concentration calculation, which avoids uncertainties due to difficulties for achieving an actual dry state of the membrane 2.2. NMR Measurements. a. Determination of the Water Amount in Hydrated Samples. The intensity of the hydrogen nuclear magnetization was used to directly determine the quantity of water in Nafion samples. Indeed, in a defined magnetic field Ho, the nuclear magnetization is directly proportional to the number of nuclear spins in the sample and is inversely proportional to the temperature M(T) ∝ s(T)(N/T), where s(T) denotes the sensitivity of the NMR probe. In a first step, the 1H NMR probe sensitivity was calibrated at room temperature by measuring, for a series of variable quantity of pure water, the 1H NMR signal following a π/2 radio frequency pulse (the so-called NMR-FID): Mx(t,T) ) M(T)Gx(t,T), where Gx(t,T) is the decay of the transverse magnetization. The accuracy of the proportionality between the water 1H magnetization and its mass was (1.5% in the range of water amounts used for hydrated Nafion membranes. The temperature dependence of the nuclear magnetization was calibrated by measuring the 1H signal of viscous silicone oil between 200 and 300 K; this calibration takes also into account the temperature dependence of the probe sensitivity due to its electronic components to obtain Mc(T) ) TM(T)/s(T), which is the NMR measure directly proportional to the number of hydrogen nuclei in samples. The -SO3H contribution in 1H NMR signal of hydrated

Figure 1. Normalized 1H FIDs of hydrated Nafion (EW ) 1100 g/mol). Water concentrations at room temperature are (a) 0.15 g/g and (b) 0.10 g/g, respectively. FIDs are normalized by the temperature dependence of the nuclear magnetization. The arrow marks the magnetization level due to liquid water when ice is formed.

Nafion, that counts for 0.82 × 10-2 equivalent mole of water per polymer mole (EW ) 1100 g/mol), was taken into account to calculate the water concentration. These NMR experiments were conducted using an Apollo Tecmag spectrometer operating at the Larmor frequency νo) 90 MHz with a Bruker electromagnet providing a magnetic field Ho ) 2.11 T, νo ) γHo/2π, where γ is the gyromagnetic ratio of the hydrogen nucleus. The sample temperature was checked with a Pt-100 thermoresistance. The absolute temperature is accurate within (1 K. Typical 1H-FIDs of hydrated membranes are shown in Figure 1. The fast relaxation process (solidlike) observed at 251 K for the sample Co ) 0.15 g/g is the ice part in the NMR signal (Figure 1a); the intensity of magnetization referring to liquid water was measured at the end of this first fast decay. Such a solidlike decay does not exist in the FID of the sample Co ) 0.10 g/g at 240 K, which shows that there is no frozen water in this case (Figure 1b). b. In-Situ Measurement of Sample Length. We measured in situ, in the NMR probe, the length of the Nafion roll during the loss of liquid water. The principle of the experiment is to build a 1D picture of the sample by applying a magnetic field gradient parallel to the axis of the roll during the acquisition of the proton NMR signal. The linear dependence in space of the magnetic field yields a linear spatial dependence of the Larmor frequency (ν ≈ γH). If the sample is smaller than the height of the active volume of the probe radio frequency coil, the width of the NMR spectrum corresponds to the height of the sample. The spectrum was the Fourier Transform of a standard Hahn spin-echo (rf sequence: π/2-τ-π-τ- acquisition) to avoid spectral distortions due to the probe/receiver dead time occurring with FID-records. This experiment is illustrated in Figure 2. The 1H NMR spectrum of a water cylinder (height ≈ 9 mm) inside the 15 mm-height rf coil was recorded in the presence

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Figure 2. Longitudinal profile of a water cylinder in the NMR probe. The height of the sample was roughly 9 mm; the field magnetic gradient was 3.52 Gauss/mm, i.e., 15 kHz/mm.

of a field gradient equal to 3.52 Gauss/mm, which corresponds to a Larmor frequency gradient equal to 15 kHz/mm. c. Water Self-Diffusion Measurements. Water self-diffusion coefficients were measured by PFGNMR with the standard stimulated echo sequence.17 The diffusion time was typically 10 ms, which corresponds to a random displacement at the micrometer scale. The NMR diffusion probe was a Bruker DIFF30 one operating at 200 MHz with a Bruker Avance spectrometer. The maximum value of the magnetic field gradient was 1200 Gauss · cm-1. Measurements were done by increasing the gradient intensity for a given set of time delays of the sequence. With this experimental device the gradient axis was parallel to the axis of Nafion rolled strips. Some additional experiments were performed with the gradient axis perpendicular to the axis of rolls. This gradient orientation was achieved with a 20 MHz Bruker Minispec analyzer. The self-diffusion coefficients measured with both experimental designs agree with a (5% accuracy. The NMR measurements were conducted by decreasing the temperature step by step, typically 7 K steps. The final state of the sample, following a temperature gap, was checked by recording the change of the 1H magnetization intensity in function of the waiting time. Diffusion measurements at 200 MHz were performed when the integral, the chemical shift, and the width of the resonance line have achieved constant values. Depending on the water concentration and the temperature, desorption kinetics after a temperature step ranged between 20 min for the fastest ones and 2 h for the slowest ones. 3. Results and Discussion In this part we present the different NMR characterizations of the samples according to the following scheme. First we determine the amount of liquid water as a function of the temperature and the water concentration at room temperature. The location of produced ice cannot be determined by this experiment alone. To compare with the results obtained by micro-X-ray investigations, that locates ice on membrane surfaces, the macroscopic swelling of the membrane and the change of the chemical shift of the liquid water resonance line were measured. Finally, the diffusivity of water molecules was measured in order to check the effect of the added obstacles that should be ice crystallites inside frozen membranes. All these experiments were done in order to verify if the change of the liquid water amount stands for an actual change of the water concentration in the membrane.

Figure 3. Temperature dependences of the maximum amplitude of liquid water FIDs (a) Co ) 0.19 g/g and (b) Co) 0.08 g/g. The experimental raw data M(T) were corrected from temperature dependences of both nuclear magnetization [TM(T)] and NMR probe sensitivity [s(T)] for obtaining Mc(T) the NMR parameter that actually measures the liquid water amount.

3.1. Variation of the Amount of Liquid Water. Temperature behaviors of M(T), TM(T) and MC(T), the raw data, the temperature corrected data and the probe sensitivity corrected data, respectively, are separately displayed in Figure 3 for two hydrated Nafion samples, Co ) 0.19 g/g and Co ) 0.08 g/g. It comes from these figures that the magnetization temperature dependence is the main correction of the raw data; the probe sensitivity is a second order correction that depends on the quality factor of the probe. The loss of liquid water, measured by the decrease in Mc(T), is 50% at 220 K for the most hydrated sample (Figure 3a). On the other hand, the amount of liquid water keeps constant within the whole temperature range when Co is 0.08 g/g. 3.2. Identification of the Amount of Liquid Water to the Total Amount of Water Inside the Membranes. The above NMR measurements point out a loss of liquid water for highly hydrated membranes. Now, the point is to determine if the frozen part of water is inside or outside the membrane. To determine this point, for the samples of our study, we measured in situ the length of Nafion rolls during the loss of liquid water. These experiments are illustrated in Figure 4; 1H longitudinal profiles of two hydrated Nafion are shown, the water concentrations were 0.19 and 0.08 g/g at room temperature. NMR spectra of liquid water in Nafion rolls (height ≈10 mm) were recorded in the presence of a field gradient equal to 2.34 Gauss/mm, which corresponds to a Larmor frequency gradient of 10 kHz/ mm. A significant decrease in the length (i.e., 5%) is observed for the most hydrated sample (case Co) 0.22 g/g, Figure 4a). On the contrary, the sample length appears constant if the liquid water amount does not depend on the temperature (case Co)

Water Sorption Properties in Nafion

Figure 4. Longitudinal profiles of Nafion samples, (a) Co ) 0.19 g/g and (b) Co ) 0.08 g/g, as a function of the temperature. The magnetic field gradient is 23.4 Gauss per cm, i.e., 10 kHz · mm-1 for NMR spectra.

0.08 g/g, Figures 3b and 4b). The macroscopic swelling properties of Nafion 117 were investigated at room temperature by Morris and Sun;18 they found a quasi-linear dependence of the sample length versus the water amount and the fractional length change, referred to the dry state of the sample, was 6% and 12% for a mass ratio, water to Nafion, equal to 0.12 and 0.24, respectively. The reduction of the sample length that we measured as a function of temperature, 5% for a 50% change of liquid water amount, is quite in agreement with the results obtained at room temperature by decreasing the water concentration. The temperature effect on the sample size was also checked by Thompson et al.11 They conclude that there is no appreciable change in any dimension, a result that might be related to a lack of sensitivity of their measurement, the expected change of length being only 0.5 mm/cm. The deswelling behavior measured for the full hydrated sample indicates that the ice formed at low temperature was not inside the membrane, which is in agreement with scattering X-ray conclusion for a fully hydrated sample.16 On the other hand, NMR results obtained at Co ) 0.08 g/g (Figures 3b and 4b) are supported by a very recent micro-X-ray performed at low temperature on a Nafion film at a water concentration equal to 7.5% g/g.19 The diffraction pattern was recorded after a 1 h annealing at 223 K, following a quenching step at 200 K; no ice signal was observed inside nor onto the membrane surfaces. So, in absence of water desorption (as characterized in Figures 3b and 4b), no ice is formed at low temperature (Figure 1b and ref 19). Temperature dependences of the length of both samples are plotted in Figure 5a; the shape of this dependence differs strongly according to the water content. The slope of the straight line characterizing the nondesorbing sample is equal to 2.4 × 10-4 K-1; it is in the order of a standard thermal expansion factor of polymer matrix. On the other hand the curve shape, for the high hydration rate, looks like the temperature dependence of the liquid water amount (Figure 3a). The length change

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Figure 5. (a) Variation rate of length for Nafion samples Co ) 0.22 g/g (a desorbing sample) and Co ) 0.08 g/g (a nondesorbing sample); the slope of the solid line is 2.4 × 10-4 K-1. (b) The relative length change at Co ) 0.22 g/g induced by the temperature is compared to the concentration dependence of the length of a Nafion 117 membrane measured at room temperature (ref 18). In this figure, L/Lo(T) was corrected with the above thermal expansion factor of the polymer matrix and C(T) was calculated with eq 1a.

of this sample can be plotted versus the water concentration at each temperature, assuming that, inside the membrane, the liquid water is the total amount of water (eq 1b); it is compared in Figure 5b to the variation of length measured by Morris and Sun by varying the water uptake at room temperature.18 Both variations are similar and allow the assumption of an adjustment of water concentration when the temperature decreases. Another confirmation of this conclusion was found at a microscopic scale by recording the variation of the chemical shift of the water resonance line with temperature. Two contributions are expected. The first one is the temperature effect on the lifetime of the hydrogen bond. It is a well-known effect used for calibrating the temperature of NMR probes, the continuous straight line in Figure 6 stands for this water chemical shift variation. The second one is the water chemical shift dependence with the acidity of the solution, which is a well-known property of water in Nafion.20,21 Both the decrease in the temperature and the decrease in the water concentration enhance the magnetic field shielding; it is translated, in terms of NMR understanding, by an increasing chemical shift. The measure of water chemical shift in Nafion is displayed in Figure 6. The temperature dependence of the chemical shift of water in Nafion at Co ) 0.08 and 0.10 g/g are parallel one another, the variation rate is lower than that of bulk water. The same trend was found for the sample at Co ) 0.19 g/g between 298 and 273 K on the one hand and between 245 and 225 K on the other hand. These dependences, parallel to an iso-concentration behavior of the chemical shift variation, border a temperature domain (245-273 K) with a higher degree of chemical shift variation; within this temperature domain the chemical shift

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Figure 6. Chemical shift of the water resonance line. In our study, only the temperature dependence was recorded for each sample; absolute values were calculated by taking into account the published chemical shifts at room temperature (refs 20 and 21). The error bars take into account the discrepancy between the values of literature references. The continuous line is the standard chemical shift for water referred to tetramethysilane (TMS).

variation is also higher than that of bulk water. It suggests for this sample (Co ) 0.19 g/g) a change of its concentration in this intermediate temperature range to reach at 245 K a value slightly larger than to 0.10 g/g. 3.3. Temperature Dependence of the Water Concentration in Nafion Hydrated at Room Temperature. Macroscopic and microscopic properties described above are in full agreement with the picture coming from micro-X-ray experiments.16,19 The loss of liquid water measured at T < 273 K means a decrease in the sample concentration with the ice formed outside the membrane. In this framework, the loss of water was converted into a temperature dependence of the concentration with the eq 1a:

C(T) )

mw(T) R(T)mw(298 K) ) (1a) mw(T) + mdry R(T)mw(298 K) + mdry

where mw and mdry are the mass of water and the dried membrane respectively, R(Τ) is calculated from the decrease in the NMR parameter Mc(T) normalized to 1 at 298 K. Taking into account that mdry/(mw(298K)) ) 1/Co - 1, where Co is the water concentration at room temperature, C(T) was calculated according to

C(T) )

R(T)Co 1 + (R(T) - 1)Co

(1b)

The results are plotted in Figure 7, and two main features have to be commented. First, we note that the concentration change is quite reversible as it is shown with the sample Co ) 0.15 g/g, for which the usual temperature protocol was followed by a step by step temperature rising. Besides, the result does not depend on the way the minimum temperature is reached: it may be step by step or, directly by putting the sample inside the NMR probe that is at 215 K. The second item is the evidence of a common limit value of the concentration reached at low temperature for all the hydrated samples with Co g 10% g/g. This value is slightly higher than those obtained by Pineri et al.12 (C ) 0.08 g/g) on the one hand, and by Thompson et al.11 (λ ) 5, C ) 0.075 g/g) on the other hand, who characterized the amount of nonfrozen water by DSC measurements of hydrated Nafion 117. The continuous line in Figure 7 denotes the equilibrium swelling of Nafion 112 at low temperature Ceq(T); an equilibrium water concentration that is equal to 0.12 g/g is found at -20 °C in full agreement with sorption isotherms

Figure 7. Behavior laws of water concentration in Nafion 112 vs the temperature. Reversibility in the desorption/resorption process is shown for the sample Co) 0.15 g/g: open symbols for decreasing temperatures, full symbols for increasing temperatures. The continuous line (Ceq(T)) denotes maximum values of water concentration vs temperature for Nafion 112 samples hydrated at room temperature.

Figure 8. Self-diffusion coefficients of water in Nafion. Open symbols for temperature domains where the concentration is constant C(T) ) Co, full symbols are for temperature domains where concentrations decrease: (O,b) Co) 0.19 g/g, (],[) Co) 0.135 g/g, (][) Co) 0.10 g/g, and (4) Co) 0.08 g/g. Ds(T) are also shown for water in sulfuric acid (+) ([H2O]/[SO4] ) 15) and for pure water (ref 24).

at low temperature recently published.22 The threshold temperature, below which the loss of liquid water occurs, depends on the concentration at room temperature; above this temperature the concentration is constant. The hydrated samples that verify Ceq(298 K) < 0.10 g/g do not lose water below the water freezing temperature. The merging of chemical shifts previously observed for samples Co ) 0.19 g/g and 0.10 g/g (Figure 6) is a direct consequence of the merging of the actual concentrations of these both samples at low temperature (Figure 7). 3.4. Water Self-Diffusion Coefficient: Temperature and Concentration Dependences. The micro-X-ray diffraction experiments, the measure of the change of the sample length with the temperature, and the properties of the water chemical shift agree in the interpretation of the loss of liquid water in terms of a change of the concentration sample with formation of ice outside the membrane and more specifically on membrane surfaces.16 In this framework, the interpretation of transport properties at low temperature is straightforward considering either the water self-diffusion coefficient Ds(T) or conductivity properties.8,11,23 The behavior of water diffusivity is displayed in Figure 8 for Nafion 112 samples, bulk water, and water in a sulfuric acid solution. This last solution (15 moles of water per mole of sulfuric group) was prepared as an estimate of the acidity effect on the temperature dependence of the water diffusion. For Nafion samples the continuous lines determine the temperature range for which the water concentration keeps

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constant. We first remark that, although absolute values of Ds are very different, their apparent activation energies Eapp(T) are not highly dependent on the water concentration and moreover they are not very much larger than that of pure water; at 300 K, we found 20.6 kJ/mol and 23.8 kJ/mol for Co ) 0.19 g/g and 0.08 g/g, respectively, compared with Ea(300 K) ) 18 kJ/ mol in water. Apparent activation energies Eapp(T) were calculated from

d(log Ds) d(1 ⁄ T) Quite similar results were found previously on both diffusion and conductivity properties at temperatures close to the ambient one.8,11,23 However, the desorption process in subfreezing temperature regime implies that an activation energy at constant concentration cannot be directly determined whatever the temperature domain. So, the high values of the activation energy reported in the literature for hydrated Nafion at subfreezing temperatures8,11,23 are the consequence of the decrease in both temperature and concentration if Co g 0.1 g/g; it is clearly evidenced in Figure 8 from thermal properties of water diffusion for Co ) 0.19 g/g and Co ) 0.14 g/g below 273 and 260 K, respectively. The merging in diffusion coefficient values around 230 K is the direct consequence of the merging of concentrations at this temperature (for 0.10 e Co e 0.19 g/g) as it is depicted in Figure 7. This property of water self-diffusion coefficients of samples undergoing desorption (i.e. Co ) 0.2, 0.19, and 0.135 g/g) where they reach the hydration level of a nondesorbing sample (Co ) 0.10 g/g) is not consistent with the presence of ice crystallites inside the membrane. Indeed, it would assume that the spatial dispersion of crystallites in the highest hydrated samples should alter the water random walk in the same way as the reduction of water channel size related to the water concentration Co, which is not a minor assumption. From our point of view, the desorption process explains in the same simple way the very similar behavior of conductance observed in the same concentration and temperature conditions.11 3.5. The Origin of the Desorption Property. The simultaneous presence of supercooled water inside the membrane and ice outside suggests an analysis involving the difference between saturated vapor pressure of water and ice, respectively. In a recent paper Kowai et al., assuming ice location outside the membrane, attributed the water desorption observed for fully hydrated Nafion with different EW to the fact that, at temperatures below water freezing point, saturated water vapor pressure of ice is smaller than that of supercooled water.25 Sorption isotherms measurements performed at low temperature confirm that the maximum uptake of water is determined by the vapor saturation pressure over ice.22 The vapor pressure of supercooled water was measured up to 235 K.26 Several analytical equations were proposed to calculate the relative humidity RH(T) defined as the saturated vapor pressure of ice referred to the saturated vapor pressure of the supercooled water at the same temperature.27 It comes from the computation of these equations that this ratio decreases quasi-linearly to reach the value 0.74 ( 0.02 at 240 K. It means that at this temperature the RH(T) of the atmosphere surrounding the membrane may be lower than that corresponding to the hydration rate of the sample at room temperature. From water sorption isotherms at room temperature and at -20 °C, a water uptake λ ≈ 6-7 (c ≈ 0.09-0.1 g/g) is found for Nafion EW1100 when RH )

Figure 9. Water uptake for Nafion at room temperature and below 0 °C. (O) At low temperature, for a sample with λo ) 16.2 ( 1 at room temperature: λ(Τ) is calculated from Ceq(T) (the continuous line in Figure 7), and RH(T), the ratio of saturated vapor pressures of ice and supercooled water, is calculated from equations in ref 26 and 27. The continuous line is the sorption curve for Nafion EW1100 at room temperature; the error bars results from the compilation of several data sources (see ref 28).

0.75.28,22 Consequently, when the temperature decreases below the water freezing point and down to 240 K, RH(T) is never lower than 0.75. So, for samples Co ) 0.08 g/g (λo ) 5.4) and Co ) 0.1 g/g (λo ) 6.8), for which RH(298 K) < 0.75, a direct consequence is the absence of any water desorption process as it is shown by our NMR results. On the contrary, samples for which Co corresponds to a relative humidity higher than 0.75 are subject to water desorption to equilibrate with RH(T) when the temperature decreases below 0 °C It can be emphasized that, due to the linear variation of RH(T), the marked dependence Ceq(T) is similar to the wellknown water uptake dependence λ(RH) observed for Nafion between RH 75% and RH 100%.28 Figure 9 shows the decrease in concentration due to temperature variation versus the relative humidity ice/supercooled water calculated according to RH variation equations reported in refs 26 and 27. The RH(T) dependence of the water uptake is compared to the standard sorption curve at room temperature.28 The connection between both results is strong. To sum up, decreasing the temperature below 0 °C appears to be an another way to tune the relative humidity provided that ice is outside the membrane and liquid water inside. These conclusions require a comment about the analysis of DSC records usually observed for hydrated Nafion at low temperature.11,29,30 These records (with scan rates between 5 °C/min and 1 °C/min) clearly show a low-temperature shifted freezing peak and broad endotherms below 273 K for 6 e λ e 15. At a first glance, such thermograms could be in favor of the presence of ice crystallites inside membranes with melting temperatures depending on the confinement size. However, the kinetics of the investigated properties must be taken into account. Kinetics determined from our NMR study reveal quite different characteristic times for desorption and sorption. Water desorption is a very slow process: a typical value of its characteristic time is 1 h at temperatures slightly lower than 0 °C. On the contrary, the resorption process, occurring when temperature is increasing, is faster, typically five times faster in a similar temperature domain. These delay values were obtained for a temperature step of 7 degrees in both cases. So, the abscissa scale of the DSC experiments appears to be in fact a complex temperature/time coordinate: the slow desorption process yields a delay before DSC detects

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ice formation. In the reported DSC experiments the freezing peak occurs at -45 °C for λ ∼11 and at -20 °C for λ ) 22.6 with scan rates equal to 2.5 and 1 °C/min, respectively.11,30 In our NMR study we waited for the a steady state regime of the sample at each temperature step and for this reason, the water desorption and the NMR ice signal are clearly seen at higher temperature than in DSC experiments, for example at -11 °C for λ ) 10.8 and at -2 °C for λ ) 15.7. Lower DSC scan rates than those usually used should be necessary for analyzing the water freezing in real time. The experimental situation is different for endothermic thermograms. Endothermic records were shown to be independent of DSC scan rates between 5 °C/min and 1.25 °C/min;29 this result appears to be coherent with NMR resorption kinetics we measured. However, we relate the endotherms to the sublimation of the thin ice layer on membrane surfaces, which is a reasonable assumption since enthalpies of sublimation and melting have the same order of magnitude. At 220 K, the loss of liquid water yields that the mean thickness of the ice layer formed on each face of the most hydrated membrane is 7 µm. The precise morphology of the interface between the ice layer and the ionomer membrane is not known yet; however, at this step of the study, we may consider two potential mechanisms: a direct rehydration at the interface or/and an heterogeneous layer of ice allowing a water exchange between the membrane and the surrounding vapor. This interpretation of DSC thermograms relies on the central role of the relative humidity (see Figure 9), that necessitates the formation of ice above a supercooled water, and on the reversibility of the desorption/resorption process as it is evidenced in Figure 7 for the sample Co ) 0.15 g/g: no noticeable temperature hysteresis was detected as it should be in the case of a freezing/melting process inside the membrane. 4. Conclusions In this paper the water uptake of Nafion 112 at low temperature was investigated by 1H NMR in order to specify the properties of liquid and frozen water, respectively. The reported results are in full agreement with a previous micro X-ray study showing that there is no ice inside membranes of Nafion swollen at room temperature and cooling down at subfreezing temperature.16 All our NMR experiments, the measure of liquid water amount, the macroscopic swelling rate, the liquid water chemical shift and water self-diffusion properties show that the lowering of the liquid water amount reported in the literature must be understood as a decrease in the water concentration of samples. The nanometric size of the water confinement together with the high acidity prevent water from freezing in membrane pores, the ice being located on the membrane surfaces. So, the interpretation of thermal properties at low temperature must take the concentration change into account if the water concentration at room temperature is higher than 10%. The NMR measurements reported here bring new insights in Nafion behavior at low temperature. The temperature dependence of the water concentration is directly determined by the change in the relative humidity of vapor pressure surrounding the sample, RH(T), resulting from the ratio between saturated vapor pressures of ice and supercooled water at the same temperature. The change in concentration observed within the investigated temperature domain (+25/ -50 °C) reproduces the last step of membrane hydration at room temperature when the relative humidity ranges between

Guillermo et al. 75% and 100%. It concerns water molecules with dynamic properties close to those of bulk water.6,31,32 It appears from this study that the decrease in the relative humidity is a sufficient condition for water desorption in Nafion membranes. However it is highly probable that the matrix elasticity plays an important role in this process, notably in kinetics. The maximum desorption rate is probably dependent on the ionic equivalent capacity and on the water/ matrix interactions. All these points have to be clarified. Finally, a result of practical interest is the determination of the maximum swelling rate for Nafion at subfreezing temperatures between 273 and 220 K in addition to the determination of the water self-diffusion coefficient at low temperature that can be used to model the water management in fuel cells operating with such constraints. Finally, it comes from the Ceq (T) curve that, to avoid water desorption and subsequent freezing, the water concentration must be checked before cooling down the cell to a defined temperature. Acknowledgment. This work was granted by the ANR, the French National Agency for Research, through the program PAN-H: Mephisto. We acknowledge our colleagues O. Diat, F. Volino, and M. Pineri, who initiated this work, for numerous discussions about this study, and we are grateful to the NMR Laboratory of the Department (INAC/SCIB/ LRM) for the access to their Bruker Avance 200 spectrometer. References and Notes (1) Borup, R.; Meyers, J.; Pivovar, B.; Kim, Y. S.; Mukundan, R.; Garland, N.; Myers, D.; Wilson, M.; Garzon, F.; Wood, D.; Zelenay, P.; More, K.; Stroh, K.; Zawodzinski, T.; Boncella, J.; McGrath, J. E.; Inaba, M.; Miyatake, K.; Hori, M.; Ota, K.; Zempachi, Z.; Miyata, S.; Nishikata, A.; Siroma, Z.; Uchimoto, Y.; Yasuda, K.; Kimijima, K.; Iwashita, N. Chem. ReV. 2007, 107 (10), 3904–3951. (2) Pinton, E.; Fourneron, Y.; Rosini, S.; Antoni, L. J. of Power Sources 2009, 186, 80-86. (3) Mauritz, K. A.; Moore, R. B. Chem. ReV. 2004, 104, 4535–4585. (4) Rubatat, L.; Gebel, G.; Diat, O. Macromolecules 2004, 37, 7772– 7783. (5) Schmidt-Rohr, K.; Chen, Q. Nat. Mater. 2008, 7, 75–83. (6) Perrin, J. C.; Lyonnard, S.; Guillermo, A.; Levitz, P. J. Phys. Chem. B 2006, 110, 5439–5444. (7) Perrin J. C. Ph.D. Thesis. Universite´ Joseph Fourier, Grenoble,http:// tel.archives-ouvertes.fr/tel-00115418/en, 2006. (8) Saito, M.; Hayamizu, K.; Okada, T. J. Phys. Chem. B 2005, 109, 3112–3119. (9) Corti, H. R.; Nores-Pondal, F.; Pilar Buera, M. J. Power Sources 2006, 161, 799–805. (10) Iijima, M.; Sasaki, Y.; Osada, T.; Miyamoto, K.; Nagai, M. Int. J. Thermophys. 2006, 27 (6), 1792–1802. (11) Thompson, E. L.; Capehart, T. W.; Fuller, T. J.; Jorne, J. J. Electrochem. Soc. 2006, 153 (12), A2351–A2362. (12) Pineri, M.; Volino, F.; Escoubes, M. J. Polym. Sci., Polym. Phys. Ed. 1985, 23, 2009–2020. (13) Starkweather, H. W., Jr.; Chang, J. Macromolecules 1982, 15, 752– 756. (14) Boyle, N. G.; J; Coey, M. D.; McBrierty, V. J. Chem. Phys. Lett. 1982, 86, 16–19. (15) Ishikawa, Y.; Morita, T.; Nakata, K.; Yoshida, K.; Shiozawa, M. J. Power Sources 2007, 163, 708–712. (16) Pineri, M.; Gebel, G.; Davies, R. D.; Diat, O. J. Power Sources 2007, 172, 587–596. (17) Tanner, J. E J. Chem. Phys. 1970, 52 (2), 2523–2526. (18) Morris, D. R.; Sun, X. J. Appl. Polym. Sci. 1993, 50, 1445–1452. (19) Mendil-Jakani H.; Gebel G. Unpublished work. (20) Tsushima, S.; Teranishi, K.; Hirai, S. Energy 2005, 30, 235–245. (21) Kawamura, J.; Hattori, K.; Hongo, T.; Asayama, R.; Kuwata, N.; Hattori, T.; Mizusaki, J. Solid State Ionics 2005, 176, 2451–2456. (22) Gallagher, K. G.; Pivovar, B. S.; Fuller, T. F. ECS Trans. 2008, 16 (2), 297–307. (23) Cappadonia, M.; Erning, J. W.; Saberi Niaki, S. M.; Stimming, U. Solid State Ionics 1995, 77, 65–69. (24) Holz, M.; Heil, S. R.; Sacco, A. Phys. Chem. Chem. Phys. 2000, 2, 4740–4742.

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