Pore Size Distribution and Water Uptake in ... - Wiley Online Library

ORIGINAL RESEARCH PAPER

DOI: 10.1002/fuce.200800010

Pore Size Distribution and Water Uptake in Hydrocarbon and Perfluorinated Proton-Exchange Membranes as Studied by NMR Cryoporometry~ S. von Kraemer1*, A. I. Sagidullin2, G. Lindbergh1, I. Fur2, E. Persson3, and P. Jannasch3 1

2

3

Applied Electrochemistry, Department of Chemical Engineering and Technology, School of Chemical Science and Technology, KTH-Royal Institute of Technology, SE-10044 Stockholm, Sweden Physical Chemistry and Industrial NMR Centre, Department of Chemistry, School of Chemical Science and Technology, KTH-Royal Institute of Technology, SE-10044 Stockholm, Sweden Polymer and Materials Chemistry, Department of Chemistry, Lund University, P.O. Box 124, SE-221 00 Lund, Sweden

Received January 31, 2008; revised April 03, 2008; accepted April 07, 2008

Abstract Sulfonated polysulfone (sPSU) membranes were analysed by nuclear magnetic resonance (NMR) cryoporometry, conventional gravimetric water uptake measurements as well as by differential scanning calorimetry (DSC). NMR cryoporometry is based on the relation between the pore size and the melting point depression of the pore-filling liquid, i.e. water in fuel cell membranes; thus providing a relation between the amount of molten water and the temperature shift, i.e. the pore size, in hydrated membranes. An sPSU membrane with high ion-exchange capacity (IEC 1.45 mequiv. g–1) possessed a significant amount of large pores after hydrothermal pretreatment at 80 C, which was related to its high hydrophilicity and low resistance towards swelling. An sPSU membrane with low IEC (0.95 mequiv. g–1) showed a significant fraction

1 Introduction Hydrocarbon membranes have been extensively developed and characterised for proton-exchange membrane fuel cell (PEMFC) applications [1]. Compared to the considerable amount of publications concerning the characterisation of novel ionomer membranes in terms of proton conductivity,

– ~

Paper presented at the CARISMA workshop Ionomer Membranes for Medium and High Temperature Fuel Cells’, Stuttgart 2007.

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2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

of small pores (r ~ 1 nm) after hydrothermal pre-treatment at 80 C, implying a controlled water uptake of the membrane. NMR cryoporometry was also carried out on Nafion membranes, the results were found to be in agreement with the water uptake measurements and revealed a pore size distribution peak at r ~ 1 nm. The NMR cryoporometry, gravimetric water uptake and DSC results are also discussed in terms of the state of the water and methodological differences. Keywords: Differential Scanning Calorimetry, Nafion, Nuclear Magnetic Resonance Cryoporometry, ProtonExchange Membrane Fuel Cell, PEMFC, Sulfonated Polysulfone, Water Domain Size, Water Uptake

gravimetric water uptake, thermal stability and fuel cell performance, the structural (i.e. morphological) features of the developed membranes have been studied less frequently, as stated in recent reviews [2, 3]. The size and distribution of membrane water domains are crucial parameters in the context of designing novel ionomer membranes with properties such as high proton conductivity, low reactant gas transport, balanced water transport through the membrane-electrode

– [*] Corresponding author, [email protected]

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von Kraemer et al.: Pore Size Distribution and Water Uptake in Hydrocarbon constituent of the system and is therefore the natural choice as probe liquid. NMR cryoporometry can explore a rather wide range of pore sizes (~nm up to ~lm [18]), which makes it a technique complementary to more traditional analytical tools such as (i) SAXS and small angle neutron scattering (SANS), which are in most studies limited to a narrow scattering-vector space, i.e. only small domains are detectable and (ii) surface characterisation methods such as transmission electron microscopy (TEM) and atomic force microscopy (AFM) [9]. In addition, NMR cryoporometry detects the amount of water freezing and melting in a given temperature range using an equilibrium sweep rate and can therefore be employed as a complement to the conventional method for determination of freezing and non-freezing water, differential scanning calorimetry (DSC), where higher sweep rates are utilised [7, 19, 20]. Compared to other porosimetry methods such as gas and mercury porosimetry, NMR cryoporometry has the advantage of enabling examination of the samples in a hydrated state. In the present work, NMR cryoporometry is utilised for investigating the water domain sizes and water uptake of sPSU membranes, with various ion-exchange capacities (IEC) and hydrothermal pre-treatments. The results are subsequently compared to the cryoporometry results on Nafion (A. Sagidullin, S. von Kraemer, I. Fur, G. Lindbergh, to be submitted). The water uptake, derived from the total amount of molten water in the cryoporometric temperature range, is related to gravimetric water uptake data and the DSC results. In this context, the methodological differences between NMR cryoporometry and DSC are discussed.

2 Experimental 2.1 Membrane Pre-treatment and Gravimetric Water Uptake Measurements The sPSU ionomers, with the molecular structures shown in Scheme 1, were synthesised by direct polycondensation and the membranes were cast from N-methyl-2-pyrrolidinone

O S

O O

O

O

S

x

O

O

Ar

O

y ran

SO 3 H

sPSU A70 x=0. 7 y=0. 3

sPSU P50 Ar =

x=0. 5 y=0. 5

Ar =

Scheme 1 Molecular structures of the sPSUs prepared by direct polycondensation. The molar fraction of sulphonated units and the structure (Ar) of the non-sulphonated units are shown.

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assembly (MEA) and controlled water uptake, to ensure high mechanical stability in dynamic fuel cell conditions [2]. Among other hydrocarbons, sulfonated polysulfones (sPSUs) have been shown to be suitable ionomers for PEMFCs [1, 2, 4]. Recently, sPSU has been incorporated into MEAs, in which the sPSU was utilised in the electrodes as well as in the membrane, and the MEAs were fuel cell evaluated in 60–120 C [5]. The sPSU membranes were characterised by determination of proton conductivity, water uptake and thermal stability, whereas no thorough study concerning the state of the water, i.e. proportions of freezing and non-freezing water, and water domain size was carried out [5]. However, similar hydrocarbon ionomers, sulfonated poly(arylene ether sulfone)s [6, 7], have been studied by measuring the water uptake and by characterising the state of the water. An additional hydrocarbon group, sulfonated polyetherketones, was evaluated in the context of water domain size and water content [8]. In comparison to the structure of novel ionomer membranes, the water domains in the Nafion membrane have been extensively studied, which has resulted in several proposed morphology models, summarised by Mauritz and Moore [9]. Influences of pre-treatment procedure, temperature and humidity on water uptake and transport properties of Nafion have been frequently studied as well [10–12]. 1 H nuclear magnetic resonance (NMR) cryoporometry [13], used for pore size analysis in various micro- and mesoporous materials [14, 15], has recently been presented as a convenient method for studying the pore size distribution in hydrated Nafion where, in combination with small angle X-ray scattering (SAXS), the method was utilised to study the influence of the presence of cyclodextrin in Nafion [16]. NMR cryoporometry is based on the Gibbs–Thomson equation, which describes the relation between melting point depression of a pore-filling liquid and the pore size. Ultimately, the melting point depression arises because the pore wall geometrically constricts the phases of the pore-filling liquid and is dependent on the thermodynamic properties of the probe liquid [17]. In proton-exchange membranes, water is a native


von Kraemer et al.: Pore Size Distribution and Water Uptake in Hydrocarbon solutions in Petri dishes at 65 C under nitrogen flow for 2 days, followed by drying at 65 C under vacuum for 1 day. Experimental details have been described elsewhere [5]. The sPSU membranes were proton-exchanged in 1 M H2SO4 at 25 C for 30 h and rinsed in water three times for 24 h. The Nafion 117 membranes were pre-treated in the conventional way; boiled in 3% H2O2 as well as in 0.5 M H2SO4 followed by rinsing in boiling water. The Nafion membranes were stored in water at room temperature for several months before analysis. To determine the gravimetrical water uptake, the membranes were immersed in water for 1 day at either 25 or 80 C and then weighed in the wet state. The dry weight of the membranes was measured after drying at 100 C for 2 h. 2.2 NMR Cryoporometry For NMR cryoporometry [13], the membranes were dried at 100 C for 2 h, weighed and placed in 5 mm NMR tubes, which were then filled with a known amount of water and sealed, subsequently the samples were equilibrated for 15 h at either 25 or 80 C. The NMR measurements were performed on a Bruker DMX500 spectrometer operating at 500 MHz 1H resonance frequency. A BVT-3000 temperature controller was used to regulate the sample temperature within the range of –90–5 C with a precision of –0.1 C. During the experiments, the temperature was changed at a rate of 0.1 C/5 min, which is slow enough to keep conditions close to thermal equilibrium [17] and 250 times slower than the rate used in the DSC experiments (see below). The 1H NMR signal intensity of molten water, I, in the samples was separated from the ice signal by suppressing the latter via a transverserelaxation filter consisting of the Carr–Parcell–Meiboom–Gill sequence [18] with four 180 pulses and 2.66 ms total spinecho time. By sampling the complete decay of the signal in the Carr–Parcell–Meiboom–Gill sequence, this experimental setting was tested to suppress signal intensity from ice by more than two orders of magnitude and to keep the intensity of water signal largely unaffected. To account for the temperature dependence of the NMR signal arising in part from the Curie law and in part from the temperature-dependent response of spectrometer electronics, all obtained intensities were normalised by the temperature-dependent signal of n-propanol [13] recorded in the same experimental setup and over the temperature range explored. These normalised NMR intensities are presented below. Initial information obtained from NMR cryoporometry [13] is the dependence of signal intensity I on the difference, DT, between the melting temperature of bulk liquid, T0, and the actual temperature T. Since the NMR signal intensity is directly proportional to the molten water volume, m, the melting curves I(DT) can be transformed into the temperature dependence of specific pore volume, ms, of pores which contain molten water at DT as

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ms ðDT Þ ¼

mðDT Þ I ðDT Þ mw ¼ mm I ðT0 Þ qw mm

ð1Þ

where I(T0) denotes water signal intensity at the bulk melting temperature, mm and mw are the masses of membrane and water in the samples, respectively, and qw is the water density. Melting curves scaled as above are presented in Figures 1a and 2a. The relation between the melting point depression DT and pore geometry is provided by the Gibbs–Thomson equation expressed as [17] DT ¼

tcT0 2j DH

ð2Þ

where T0 is the bulk melting temperature of the pore imbibed liquid, t its molar volume, c represents the surface free energy at the liquid–solid (in the present case, the water–ice) interface and DH is the latent heat of melting. The pore geometry is accounted for by the integral mean curvature of the pore wall, j. If one assumes cylindrical pores with radius rx and j = 1/2r, Eq. (2) can be rewritten to DT ¼

tcT0 1 K ¼ r DH r

ð3Þ

where constant K = tcT0/DH characterises the thermodynamic properties of the probe liquid. To re-scale Figures 1a and 2a to pore sizes (see Figures 1b and 2b) and to obtain pore size distributions, Eq. (3) can be used with K = –25 K nm, which was obtained from experiments performed with water confined inside silica-based materials with narrow pore size distribution [18, 21]. The pore size distribution function ps(r) = dms/dr provides information about the specific volume, dms, of pores with radius between r and (r + dr). This function can be derived from the cryoporometry melting curves via numerical differentiation [13, 17] as ps ðrÞ ¼

¶ms K ¶T r2

ð4Þ

For graphical representation, ps(r) is often displayed on a logarithmic scale as dms/d[log10(r)] versus log10(r). Figure 3 presents our pore size distribution data in this form (see also (A. Sagidullin, S. von Kraemer, I. Fur, G. Lindbergh, to be submitted). Proton-exchange membranes are characterised with nanometer-sized pores and the amount of non-frozen water at the pore walls should be included in the analysis. Usually, an addition of approximately two water monolayers (~0.6 nm) to the radius values yielded by Eqs. (3) and (4) is carried out to compensate for the non-frozen water in NMR cryoporometry analysis. This correction has been made for the presented radii in Figures 1–3 and it should be kept in mind that such crude correction is, though customary, not exact and does not take into account any material or temperature dependence of


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von Kraemer et al.: Pore Size Distribution and Water Uptake in Hydrocarbon increased water uptake, and when assuming a 1 g ml–1 density of sPSU, the sPSU ionomers generally had a slightly lower proton concentration than Nafion. 3.2 NMR Cryoporometry

The melting curves in Figures 1a and 2a show the relation between the specific pore volume and the temperature shift, 2.3 Differential Scanning Calorimetry DT, ranging from –81 to –1.1 C. The specific pore volume was derived from the amount of molten water as obtained The state of the water in the sPSU membranes was investifrom the liquid NMR signal retained by the transverse-relaxagated by DSC using a Q1000 differential calorimeter from TA tion filter. Figures 1a and 2a illustrate (i) the amount of nonInstruments. The samples were first equilibrated in liquid frozen water at various temperatures, i.e. various pore radii, water at room temperature or at 80 C for approximately as well as (ii) the cumulative amount of liquid water at values 24 h. Prior to the DSC measurements, the samples equiliclose to 0 C, indicating the total amount of molten water in brated at 80 C were allowed to cool to room temperature in the whole temperature range. The accuracy of the data is limwater. Excess water on the samples was gently wiped off ited by two effects. First, the current NMR cryoporometric before the samples were placed in hermetic aluminium pans, experiments did not detect liquid NMR intensity below which were quickly sealed and weighed. During analysis the –81 C; however water molecules that are most strongly assosamples were first cooled from 25 to –60 C, held isotherciated with the polymer matrix may retain some liquid-like mally for 3 min, and then heated to 25 C. The scan rate was mobility, corresponding to the non-freezing layer of water 5 C min–1. The amount of freezing water in the samples was along the pore walls [24, 25]; this non-frozen water is not calculated by integrating the peak area of the melt endotherm included in the displayed amount of water. Secondly, water and comparing the value with the heat of fusion of pure ice, in pores larger than ~22 nm, corresponding to DT > –1.1 C, 334 J g–1 [22, 23]. By combining these results and the gravimeis excluded due to difficulties in distinguishing between the trically determined amount of total water, the number of melting of water in the largest pores and the melting of bulk freezing water molecules per sulphonic acid unit, kfreezing, water in the NMR tube. It is worthwhile noting that repeated was calculated. freezing–melting cycles carried out on Nafion showed no significant variation, demonstrating that most of the water 3 Results remained in the membrane during the temperature cycling (A. Sagidullin, S. von Kraemer, I. Fur, G. Lindbergh, to be 3.1 Water Uptake by Gravimetric Measurements submitted). The melting curves for the water in the membranes preThe gravimetric water uptake for the sPSU and Nafion treated at 25 C are shown in Figure 1a and the correspondmembranes are presented in Table 1, the estimated error is ing specific pore volume dependence on pore radius, based included as well. As seen, the sPSU A70 membrane showed a on the Gibbs–Thomson equation (see Eq. (3)), is presented in high water uptake, especially after pre-treatment at 80 C. Figure 1b. The sPSU A70 membrane showed a continuous The sPSU P50 membrane was, as expected, less hydrophilic melting of water, i.e. continuous distribution of pore radii, and had a lower water uptake. The water uptake of Nafion throughout the temperature range. The sPSU P50 membrane was in agreement with recent results [12]; however the variaappeared to contain a low amount of water overall and tion in reported values of water uptake in Nafion, due to the Nafion appeared to have a considerable amount of water influence of the membrane history and measurement routine, melting in the lower temperature range, indicating a large is well known. The proton concentration, [H+]est, in the memfraction of smaller pores. Figures 2a and b display water branes was estimated by combining the IEC, the water uptake melting in the membranes when equilibrated in water at and volume fraction of water, as previously proposed by Siu 80 C, showing a significant influence of the pre-treatment et al. [19]. The proton concentration for the ionomers, temperature; this trend was also observed in the gravimetric reported in Table 1, decreased with the temperature due to water uptake measurements listed in Table 1. The water volume located in Table 1 Characteristics of the sPSU membranes as compared to Nafion. pores with radii r £ 22 nm was used (a) (a) (a) –1 + (b) to calculate the amount of water, Ionomer Tg,5% Water uptake (%) [H ]est (M) IEC r (S cm ) (C) (mequiv. g–1) 25 C 80 C 25 C 80 C 25 C 80 C melting in the DT range from –81 to sPSU A70 287 1.45 0.035 0.065 51 – 10 120 – 10 1 0.7 –1.1 C, and the resulting water sPSU P50 298 0.95 0.006 0.015 18 – 1 32 – 1 0.8 0.7 uptake and k (H2O/HSO3–) are listed Nafion 117 336 0.91 0.058 0.15 21 – 2 34 – 2 1.3 1.1 in Table 2. The estimated error of the (a) The Tg,5%, IEC and r were determined as described previously [5]. data, included in Table 2, is related to (b) [H+]est = (IEC densitywater Xwater volume fraction,wet membrane)/water uptake (%) 100 [19]. variation between individual samples

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the amount of non-frozen water [18]. Hence, the position of the peaks in Figure 3 should be taken as a qualitative indicator. This is particularly critical for the smallest pores detected. Yet, the trends observed in the pore characteristics for different materials with different pre-treatments should not be strongly influenced by this compensation.


von Kraemer et al.: Pore Size Distribution and Water Uptake in Hydrocarbon

a) /

a)

/

/

b)

Fig. 1 NMR cryoporometric melting curves presenting the specific pore volume related to (a) melting point depression DT and (b) the corresponding radius r for sPSU A70 (s), sPSU P50 (~) and Nafion 117 (j) membranes, which were pre-treated in water at 25 C.

b)

Fig. 2 NMR cryoporometric melting curves presenting the specific pore volume related to (a) melting point depression DT and (b) the corresponding radius r for sPSU A70 (s), sPSU P50 (~) and Nafion 117 (j) membranes, which were pre-treated in water at 80 C.

Table 2 Water uptake and related k values in pores of the sizes r £ 22 nm and r < 1.5 nm for sPSU A70, sPSU P50 and Nafion membranes, pre-treated in water at 25 and 80 C.

Figures 3a and b present the pore size distributions for the membranes, pre-treated in water at 25 and 80 C, respectively. The sPSU A70 membranes Water uptake(a) (%) Ionomer Pre-treatment k(b) possessed, as the melting curve also implies, pores Twater (C) r £ 22 nm r < 1.5 nm r £ 22 nm r < 1.5 nm over the whole studied size range and the occursPSU A70 25 29 – 3 10 11 – 1 4 rence of large water domains increased when the sPSU A70 80 94 – 9 18 36 – 3 7 sample had been equilibrated at 80 C. The sPSU sPSU P50 25 8–1 3 5–1 2 P50 membrane pre-treated at 25 C showed a low sPSU P50 80 34 – 3 16 20 – 2 9 amount of water throughout the whole pore size Nafion 117 25 21 – 2 10 – 1 13 – 1 6–1 Nafion 117 80 34 – 2 14 – 2 21 – 1 9–1 interval, whereas a considerable increase in occur(a) rence of small pores was observed when the memDetermined from the NMR cryoporometry melting curves at the given pore radius. (b) brane was pre-treated at 80 C. The pore size distriBased on the cumulative water uptake at the given pore radius. bution of Nafion demonstrated a large fraction of small pores, as discussed previously. The shift in the position of the peak in the range of r ~ 1 nm, due to increased pre-treatment temperature, was not detected and to determination of specific pore volume values. The for Nafion and sPSU P50, while sPSU A70 showed a shift to total amount of water located in pores with radii r £ 22 nm higher radii, which was related to the total excessive swelling. was highest for sPSU A70, when pre-treated at 25 C, folHowever, the position of the small radii peaks, relative the lowed by Nafion and then by sPSU P50. For the membranes other samples, should be interpreted with caution due to (i) pre-treated at 80 C a similar cumulative water content was the increasing influence of the pore-wall with decreasing pore observed for Nafion and sPSU P50. The water uptake in pores radius, (ii) the assumption of a constant non-frozen water r < 1.5 nm was also included in Table 2 and these values layer in the three ionomers and (iii) the differences in the were considered as a measure of the fraction of water in the + chemical nature of the ionomers, such as different [H ] consmall pores. centrations, which are estimated in Table 1. In addition, the

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3.3 Differential Scanning Calorimetry DSC has been used extensively in the determination of the fraction of freezing water in fuel cell membranes [7, 19, 20, 26]. Usually, DSC data on freezing water are compared with gravimetrical water uptake data, enabling the determination of non-freezing water; knon-freezing water = ktot water – kfreezing water. Table 3 reports the state of the water present in the two sPSU membranes pre-treated in water at 25 and 80 C. The results indicate a low fraction of freezing water in the sPSU P50 membrane, whereas sPSU A70 possessed a higher proportion of freezing water. The variation of the data, included in Table 3, is related to errors in the integration of the melting peaks, which are seen in Figure 4. It is also worth noting that DSC results generally depend rather strongly on the scan rate and temperature limits. Recently, precaution in the interpretation of freezing and non-freezing water in an ionomer, based exclusively on DSC data, has been recommended [20]. In the present study, DSC is therefore not regarded as a quantitative tool but rather as a complementary method to NMR cryoporometry. In particular, quantitative analysis of the freezing water should be made with precaution due to the DSC scan rate of 5 C min–1, which results in recording melting (and freezing) under non-equilibrium conditions; the fraction of water freezing between 25 and –60 C thus depends on the scan rate. In contrast, NMR cryoporometry employs a far slower scan rate, 0.1 C/5 min [17], and in addition a broader temperature range, 0 to –81 C. Furthermore, the NMR signal is dominantly influenced by the mobility of water molecules while DSC measures the heat transfer related to the phase transition.

Nafion 117 membranes for comparison. The total water uptake in Nafion, when equilibrated in water at 25 and 80 C, was derived from the total amount of water located in pores with r £ 22 nm, and is presented in Table 2. As discussed previously, NMR cryoporometry excludes any non-frozen water at –81 C as well as water in pores larger than the current upper experimental size limit (r > 22 nm). However, for Nafion the close agreement of the total gravimetric water uptake (Table 1) and the amount of water freezing down to –81 C, as detected by NMR cryoporometry (Table 2), indicates only a small amount of non-frozen water below the cryoporometric temperature interval and an insignificant amount of water in large pores (r > 22 nm). As seen in Figure 3, a considerable fraction of the water in Nafion was observed to reside in small pores, i.e. in the range of ~1 nm pore radius with corresponding low melting temperatures, irrespective of whether equilibrated at 25 or 80 C. The finding of a dominance of pores with r ~ 1 nm is in reasonable agreement with previous results based on SANS and SAXS studies [9]. The exact peak position is however influenced by the assumption of the non-frozen water layer thickness as discussed previously. The increased water uptake, due to higher pre-treatment temperature of Nafion, appeared not to influence notably the pore size of pores with r ~ 1 nm, and a slight increase in the amount of water located in the small pores was observed. Additionally, a significant amount of water melted at higher temperatures, corresponding to

a)

4 Discussion The sPSU A70 and sPSU P50 membranes were analysed by NMR cryoporometry (Figures 1–3), by conventional gravimetric water uptake measurements (Table 1) as well as by DSC (Figure 4). NMR cryoporometry was also carried out on

/

b)

Table 3 State of the water in sPSU A70 and sPSU P50 membranes, pretreated in water at 25 and 80 C, as derived from DSC melting curves. Ionomer sPSU A70 sPSU A70 sPSU P50 sPSU P50 (a)

Pre-treatment Twater (C)

ktot(a)

25 80 25 80

19 46 10 19

– – – –

3 4 1 3

kfreezing(b)

knon-freezing(c)

11 – 2 33 – 3 2–1 1–1

8–1 13 – 1 8–1 18 – 2

Based on gravimetrically determined water uptake reported in Table 1.

(b)

Based on integration of DSC melting peak. (c) Calculated as ktotal water – kfreezing water.

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/ Fig. 3 Pore size distributions for sPSU A70, sPSU P50 and Nafion 117 membranes, which were pre-treated in water at (a) 25 C and (b) 80 C.



267


water uptake of the ionomers was not kept constant, as was the case in a previous SAXS study that enabled direct comparison of scattering data, i.e. ionomer peak positions, of sulfonated polyetherketone and Nafion [8].

r > 1–10 nm, which could be related to merging channels in the Nafion membrane. Such water domain sizes are not reported in earlier NMR cryoporometry measurements [16], but are discussed in detail by Sagidullin et al. (A. Sagidullin, S. von Kraemer, I. Fur, G. Lindbergh, to be submitted.). It is of importance to note that a previous desorption porosimetry study of Nafion revealed water channels with radii in the range of r = 0.5–50 nm and that the extraordinary large pores were assigned to the outer surface [27]. The water domains in size of r > 1–10 nm detected in the current work seem too frequent to be considered solely as a surface effect. A fraction of large pores in sPSU A70 was observed in the membrane pre-treated at 25 C (Figure 1b), which was associated with the high hydrophilicity of this ionomer. In the sPSU A70 membrane, equilibrated at 80 C, an increasing fraction of the water melted at higher temperatures and the corresponding large pore radii imply an excessive swelling (Figure 2b), which can be related to the observed low mechanical stability of the sPSU A70 membrane. A comparison of the gravimetric water uptake (Table 1) with the amount of melting water in the cryoporometric temperature range (Table 2) reveals a lower water uptake when derived from the cryoporometry analysis. The discrepancy between the two methods most probably indicates the occurrence of pores exceeding the current instrumental limit of detectable pores (r = 22 nm) and is also supported by the positive slopes at r = 22 nm in Figures 1b and 2b. The DSC melting behaviour of sPSU A70 membranes pre-treated at 25 and 80 C shows a large and rather broad melting peak close to 0 C and suggests a large amount of freezing water; this may be related to the large water domains discussed above, enabling a considerable amount of water to freeze during the high-rate DSC temperature scan. The sPSU P50 membrane pre-treated at 25 C contained an overall low amount of water (Figures 1 and 3). However, an enhanced water uptake, as well as significant increase in smaller water domains, was observed after the pre-treatment at 80 C (Figures 2 and 3). The comparison of the total gravimetric water uptake, Table 1, and the freezing water detected in cryoporometry, Table 2, reveals a discrepancy of the water uptake of the sPSU P50 membrane pre-treated at 25 C, which could be due to a fraction of water melting above –1.1 C, thus not detected by NMR cryoporometry. However, the gravimetric water uptake and cryoporometry results for the sPSU P50 membrane, pre-treated at 80 C, were in agreement, which indicates a small fraction of non-frozen water below the cryoporometric temperature interval. These results are different from the DSC results, where the analysis was carried out with a high scan rate; the DSC results (Table 3 and Figure 4) imply a high fraction of non-freezing water for sPSU P50. This discrepancy illustrates the large method dependency (i.e. choice of scan rate, temperature limits and method of sample preparation) on the DSC determination of non-freezing water in the ionomers, which has been discussed recently [20]. However,

268


sPSU P50 80 ˚C

sPSU P50 25 ˚C 0.5 Wg -1

sPSU A70 25 ˚C

Exo



0.5 Wg

-60

-40

sPSU A70 80 ˚C

-1

-20

Temperature / ˚C

0

20

Fig. 4 DSC melting curves for sPSU A70 and sPSU P50 membranes, which were pre-treated in water at 25 and 80 C.

the DSC results on sPSU P50 could be interpreted also as a confirmation of the large fraction of small pores detected by NMR cryoporometry, in which the water did not freeze at –60 C during the fast temperature scan. In summary, the results derived from the DSC measurements in Table 3 should be interpreted with caution due to the limitations of the method. The fraction of small water domains (r < 1.5 nm) in the various ionomer membranes is reported in Table 2 and shows that Nafion and sPSU A70, pre-treated at 25 C, possessed a large amount of small pores compared to sPSU P50, which also had a low proton conductivity at this temperature. However, the sPSU P50 membrane was most influenced in terms of occurrence of small pores when the pre-treatment temperature was increased. Comparison of the k (r < 1.5 nm) values with k (r £ 22 nm) values reveals that only a part of the water in the membranes, at the most ~50%, is located in the smaller pores. Altogether, this work shows that the pore size distribution of a membrane, i.e. heterogeneous structural features, should be involved in the general analysis of the membrane, and that data, obtained by assuming a homogenous pore size, should be related only with caution to the membrane transport properties.


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References

Hydrated sPSU and Nafion membranes were analysed by NMR cryoporometry and the results were related to gravimetric water uptake and DSC measurements. NMR cryoporometry was shown to be an informative tool, resulting in the following findings: l The total water uptake derived from the NMR cryoporometry data for Nafion was in close agreement with the gravimetric water uptake, indicating a marginal fraction of non-frozen water as well as a minor occurrence of pores with radii r > 22 nm. About half of the water was situated in pores with a small radius, r ~ 1 nm, and the rest, which is a significant amount of water, was located in pores with larger radii r > 1–10 nm. l The two sPSU membranes possessed different water domain size distributions and total water uptake due to varying IEC. The significant amount of larger pores in the sPSU membrane with high IEC when equilibrated in water at 80 C, can be related to the high hydrophilicity and low resistance towards swelling of the membrane. The sPSU membrane with low IEC contained a small amount of water pores when pre-treated at 25 C; however a significant increase in small pores (r ~ 1 nm) was observed in the membrane when pre-treated at 80 C, showing a controlled water uptake. The difference in the water uptake derived from NMR cryoporometry and gravimetric analysis implies occurrence of pores with radii r > 22 nm. l Comparison of NMR cryoporometry and DSC showed that the DSC results should be interpreted with caution in the determination of non-freezing water, since methodological limitations result in the detection of only a fraction of the water located in membrane, i.e. the rapidly frozen water. Additionally, it was shown that the pore size distribution of the ionomer, i.e. structural heterogeneities, should be included in the overall analysis of a membrane. Data based on the assumption of a homogenous pore size should be related only with caution to the membrane transport properties.

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Acknowledgements The financial support from the Swedish Foundation for Strategic Environmental Research (MISTRA), Knut and Alice Wallenberg Foundation and the Swedish Research Council (VR) is gratefully acknowledged.

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5 Conclusions