Effects of contaminant on thermal properties in perfluorinated sulfonic acid membranes

Journal of Membrane Science 363 (2010) 67–71

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Effects of contaminant on thermal properties in perfluorinated sulfonic acid membranes C. Bas ∗ , N.D. Albérola, L. Flandin Laboratoire Matériaux Organiques à Propriétés Spécifiques (LMOPS), UMR CNRS 5041, Université de Savoie, Campus Scientifique, 73376 Le Bourget-du-Lac Cedex, France

a r t i c l e

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Article history: Received 12 February 2010 Received in revised form 28 June 2010 Accepted 3 July 2010 Available online 10 August 2010 Keywords: Perfluorinated sulfonic acid (PFSA) PEMFC Contamination Thermal properties Nafion Counterions

a b s t r a c t The influence of controlled cationic contaminant on the thermal behaviour of proton conducting perfluorosulfonic acid membrane (PFSA) was studied. A series of Ni2+ -exchanged membrane was prepared by varying the ratio between positive charges resulting from the contaminant to the negative charges originating from the sulfonate groups. A large change in properties was evidenced at a ratio close to 0.2. Above this threshold, the PFSA exhibited significantly higher main relaxation and degradation temperatures. The set of experimental data could be described with the help of a phenomenological microstructural model essentially based on ionic interactions. The impurity model could then be employed to understand changes in protonic conductivity from the literature. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Under usual service conditions, the polymer electrolyte membrane of a fuel cell may be exposed to cationic impurities originating from corrosion of fuel cell stack system [1–6], water, and the membrane itself [5,7–9]. The cationic ions can enter the perfluorinated sulfonic acid (PFSA) membrane and interact with the sulphonate groups by displacing protons. They induce detrimental effects on membrane properties with regard to water management [2,10–14]. The literature has shown that the cation contaminant induces a modification of the water content within Nafion [15] as summarized in Table 1. A general trend within the experimental data suggests that larger cationic radii gradually induce a lower countercation hydration energy, and a lower water content sorbed within the membrane [16–20]. This results in a decrease in the water diffusion coefficient within the membrane [21–23] (Table 1). Because of the water content reduction, the presence of foreign cations lower the conductivity of the membrane [24–27] that can reach a tenth of that of the H+ -form [28]. The membrane electrode assembly (MEA) contamination also directly alters the performances of the MEA due to kinetic losses, especially for the oxygen reduction reaction [29,30]. In addition, the cationic contaminants induce a shift towards higher temperatures of the ␣-relaxation related to the glass transition [31–33]. At high temperatures, the thermal sta-

∗ Corresponding author. Tel.: +33 4 79 75 86 24. E-mail address: [email protected] (C. Bas). 0376-7388/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2010.07.001

bility affected by the incorporation of cationic contaminant either alkaline [34–39], alkaline earth metal [18,39], aluminium [39], or alkyl ammonium cations [35,40] seems mainly controlled by the Lewis acid strength (LAS) of the cations [33]. This short literature review shows that the cationic contamination alters the PFSA thermal membrane properties. In addition, the cation effect on the membrane conductivity is largely affected by the concentration of the solution [25–27]. The purpose of this communication is to investigate the influence of the amount of cations on the thermal properties. A critical behaviour will be evidenced as revealed by the presence of a well-defined threshold.

2. Experimental 2.1. Materials and sample preparation The polymer electrolyte membrane used in this study was provided as Nafion® NR 211 membrane (Dupont). Prior to measurement, the membrane in H+-form after concentrated nitric acid treatment was stored in 60% RH atmosphere at room temperature. The unsaturated Ni2+ -exchanged membranes were prepared from immersion of 100 mm × 25 mm × 0.032 mm sample in stirred 100 ppm Ni2+ solution for at least 72 h at room temperature. Solutions were prepared from Ni2 SO4 (Riedel-de-Haën, 99%), with ultra high quality water. The pHs of the solution were not controlled as changed during the exchange. The immersion time of 72 h is long enough to reach the equilibrium.

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Table 1 Water content, ionic conductivity, self-diffusion coefficient of water in Nafion with different counterions. Cation +

H Li+ Na+ K+ Rb+ Cs+ Ag+ Mg2+ Ca2+ Sr2+ Ba2+ Fe2+ Ni2+ Cu2+ Zn2+ Fe3+ a

Cationic radius (nm) +

0.060 (H3 O ) 0.076 0.095 0.133 0.161 0.174 0.115 0.065 0.099 0.126 0.142 0.064 0.069 0.071 0.074 0.064

Lewis acid strength (LAS) [41] +

0.33 (H3 O ) 0.22 0.16 0.13 0.10 0.08 0.12–0.5 0.36 0.29 0.24 0.20 0.40 0.34 0.45 0.36 0.50

Ionic conductivity (mS cm) [28] 90.2 16.1 18.7 13.8 10.1 5.95 24.1 8.9 9.1 8.0 7.3 8.2 9.43 9.1 9.4 3.0

Water content (% wt) a

DH2 O (×10−6 cm2 s−1 )

10.3 [39]–23.4 10.7 [39]–21.1a – 23.3 [16] 8.0 [39]–17.2a –17.8 [16] 7.5 [39]–10.7 [16]–12.3a 9.3a 8.2a 18.8a 16.7 [39] 14.7 [39]–19.4a 17.5a 16a 19.0a

3.9a 3.2a 2.9a –2.7 [21] 2.3a –2.2 [21] 1.9a 1.7a –1.3 [21] 2.9a

26a 21.3a 17a

2.2a 2.9a 2.2a

2.9a 2.3a 2.2a 2.5a

Data from Suresh and coworkers [19,23,42,43].

2.2. Membrane characterizations

3. Results and interpretation

A scanning electron microscope (SEM) Leica Stereoscan 440 microscope coupled to energy-dispersive X-ray (EDX) analyser (Kevex Sigma System) was used to confirm the cation exchange. A simultaneous thermal analyser, Netzsch, TG 209 F1 Iris, coupled to a quadrupole mass spectrometer, Netzsch, MS 403 Aëlos II, was used for the TGA-MS analysis. The experiments were performed under helium atmosphere at a heating rate of 10 ◦ C/min, from 30 ◦ C to 800 ◦ C. A series of fragments related to charge ratios (m/z) was recorded [33]. From the derivate weight loss or 64 MS curves, the onset decomposition temperature can be determined as illustrated in Fig. 1. Dynamic mechanical experiments were carried out with a Polymer Laboratories MKII analyser, on samples of size 10 mm × 8 mm × 0.032 mm. The torque applied to the clamping screws was 20 cN m. Membrane samples were dried for 1 h under nitrogen gas flow and then analysed under nitrogen gas flow in the tensile mode. After a preloaded force of 0.25 N, sinusoidal amplitude of 16 ␮m is applied for 1, 3 and 10 Hz during a heating of 1 ◦ C/min. The storage modulus (E ) and the loss factor tan ı (=E /E ) were recorded as a function of temperature. From this E vs. temperature, Tonset-DMTA can be determined by the straight-line intersection as illustrated in Fig. 2.

Ni2+ -exchanged membrane was prepared with various amount of contaminant by solely changing the volume of the 100 ppm Ni2+ solution from 10 mL to 1000 mL. Energy dispersive X-ray (EDX) analysis confirmed the presence of Ni2+ within the membrane. It can also be assumed that the number of sulfonate groups remained constant because of a strict control of the H+ -form sample surface. The 100 ppm Ni2+ volume variation induces modification of the Ni2+ positive charge to sulfonate anion group ratio, so-designed R± . The R± determined from experimental cation content performed by Kelly et al. [25] is consistent with the calculated one (Table 2). Thus, as cations displaced proton as much as possible, prepared Ni2+ -exchanged membrane ranges from 0.1 to 15 R± (Table 3). PFSA membrane degrades through a three-step process as described in the literature [34,44]. The presence of Ni2+ cationic contaminant induces significant change in both TGA and MS signals as already shown in the literature [33]. To focus on the first step of the perfluorinated sulfonic acid membrane degradation process, the signals of the fragments m/z 64 and 48 related to sulphur dioxide, SO2 , resulting from sulfonic moieties degradation were recorded [34,44] while the 31 m/z fragment is of interest as assigned to perfluorinated moiety degradation [34,44]. Figs. 3 and 4 show MS signals (normalized in intensity) for the 64 and 31 m/z fragments for different contamination levels. The 64 MS signal, related to sulfonic group, was the most sensitive to the presence of cations. According to this indicator, three behaviours could be distinguished depending on the amount of contaminant. The membrane with a R± of 0.3 or less, exhibited a TGA/MS signal very similar to that of the non-contaminated one with two peaks at 345 ◦ C and

Fig. 1. MS signal for fragments 64 and 31 superimposed to derivate weight loss for H+ -form PFSA membrane. Definition of the Tonset-TGA parameter.

Fig. 2. Storage modulus and tan ı as function of temperature at 1 Hz for H+ -form PFSA membrane. Definition of the Tonset-DMTA parameter.

C. Bas et al. / Journal of Membrane Science 363 (2010) 67–71

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Table 2 Analysis of Kelly et al. experiments [25]—Comparison of R± ratio value calculated from experimental cation content determined by EDX analysis and theoretical values estimated from experimental procedure. Cation solution concentration (ppm)

Theoretical R±

Experimental R±

Theoretical R±

Divalent cations: Ni2+ and Cu2+ 7.4 × 10−4 7.4 × 10−3 7.4 × 10−2 7.4 × 10−1

0.1 1 10 100

Experimental R±

Trivalent cations: Fe3+ 7 × 10−4 7 × 10−3 7 × 10−2 7 × 10−1

1.1 × 10−3 1.1 × 10−2 1.1 × 10−1 1.1 × 100

4 × 10−4 6 × 10−3 6 × 10−2 9 × 10−1

Table 3 Relationship between the 100 ppm Ni2+ volume and the contamination level. 100 ppm Ni2+ solution volume (ml)

10

15

20

35

50

75

150

1000

R±

0.15

0.22

0.30

0.52

0.74

1.11

2.22

14.8

Fig. 3. MS signal for both 64 m/z fragments for R± under helium atmosphere.

450 ◦ C. For intermediate ratios ranging from 0.3 to 0.7, both peaks were shifted towards higher temperatures while a third typical behaviour was evidenced for at or above 1: the amplitude of the first peak (at 360 ◦ C) decreased while the second 64 fragment signal reached a large temperature close to 500 ◦ C. Fig. 5 displays the onset decomposition temperature defined as the onset of the 64 MS curve induced by the increase of Ni2+ exchange. The data for 0.01 and 100 R± relate to non-contaminated and saturated membranes respectively [33]. The first step in the degradation process was very sensitive to the cationic contamination, even at low levels. Tonset-TGA reached a plateau at 0.4 R± . In other words, the maximum in decomposition temperature was reached when, in average, five sulfonic anions shared one Ni2+ cation.

Fig. 4. MS signal for both 31 m/z fragments for R± under helium atmosphere.

Fig. 5. Effect of immersion in 100 ppm Ni2+ solutions on characteristic temperature of PFSA membrane. The solid lines result from the best adjustment to Eqs. (1) and (2).

Figs. 6 and 7 show DMTA results for the series of Ni2+ contaminated membrane. As for the TGA results, a gradual temperature increase in both modulus drop and tanı maximum was observed with increasing the amount of contaminant. The beginning of the modulus drop defined as Tonset-DMTA (Fig. 2) was thus carefully determined and plotted versus the contamination level (Fig. 5). This temperature of the membrane was almost constant until Ni2+ absorption reached 0.2 R± . Above this threshold, this transition temperature increases strongly up to a stabilisation temperature at 320 ◦ C when the number of positive Ni2+ charge becomes higher than the negative one. This 320 ◦ C temperature is close to the onset decomposition temperature measured by TGA

Fig. 6. Normalized storage modulus result of Ni2+ -contaminated membrane at different contamination levels.

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Fig. 9. threshold configuration of sulfonate-surrounded cation related to saturated behaviour for TGA and DMTA.

Fig. 7. Temperature dependence of damping factor for Ni2+ -contaminated membrane at different contamination levels.

Fig. 8. Activation energy dependence on the Ni2+ contamination level. The solid line exhibit the best adjustment to Eq. (1).

as shown in Fig. 5. To complete this analysis, the activation energy of the relaxation was determined at the half maximum of tan ı peak. An increase of activation energy and pre-exponential factor with increasing the contaminant level is observed from 160 up to 1200 kJ/mol correlated to an increase of the pre-exponential factor from 1017 to 10100 . This change plotted in Fig. 8 involves a modification of the molecular process at the origin of the tan ı peak. Thus, for contamination level lower than 0.7, the relaxation originates from the glass transition chain motion while for highest cationic impurity ratio, the tan ı peak is related to the degradation process. In this case, as mentioned in previous papers [33], this implies that the glass transition process is not detected as shifted toward temperatures higher than that of degradation. Y = Y0 +

Fig. 10. Nafion conductivity measured at 50 ␮m depth versus the contamination level ratio, R± , for Na+ , Cu2+ , Ni2+ and Fe3+ contaminants extracted from Kelly et al. papers [25,27].

a Boltzmann sigmoid (Eq. (1)). Y = Y0 +

Y 1 + exp[(log Rmean − log R± )/ log dR]

(1)

where Y represent the characteristics under test (temperature or activation energy). Y0 , Y, Rmean and dR are constant, reported in Table 4. On the physical point of view, Y represents the amplitude of the transition while Rmean and dR depict its mean value and width, respectively. For the Tonset-DMTA , two-step process is necessary to fit the experimental data resulting Eq. (2).

Y2 Y1 + 1 + exp[(log Rmean−1 − log R± )/ log dR1 ] 1 + exp[(log Rmean−2 − log R± )/ log dR2 ]

Overall, the cationic contaminant effect on thermal decomposition temperature and activation energy resembled a rather symmetrical S-shaped curve with a long scale on the X-axis. On the quantitative point of view, this behaviour could be well adjusted to

(2)

Based on these curve adjustements, the temperature of the thermal degradation and the glass relaxation displays the same sensitivity to the Ni2+ -impurity as Rmean and dR is the same. Nevertheless, for DMTA, saturated netwok behaviour results for R± of 1 that is two sulfonic anions shared one Ni2+ cation (Fig. 9).

Table 4 Best adjustment parameters to equation 1 for TGA and DMTA parameters. The units of Y0 and Y are the same as the Y parameters. Rmean and dR have no unit. Parameters

Tonset-TGA (◦ C)

Tonset-DMTA – step 1 (◦ C)

Tonset-DMTA – step 2 (◦ C)

Activation energy (kJ/mol)

Rmean dR Y0 Y

0.24 1.13 278 45

0.26 1.18

0.62 1.12 95 150

0.70 1.25 155 1100

64

± ± ± ±

0.09 0.12 20 100

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The contaminant of PFSA membrane also alters its protonic conductivity as demonstrated in the literature. In particular the well-detailed experimental procedure furnished by Kelly and coworkers [25,27], allowed to gain reliable information on the quantitative influence of impurity on the Nafion conductivity. The data extracted from impedance measurements could be plotted as a function of R± for various cation-exchanged membranes (Fig. 10). The previously rather scattered data when plotted against R± nicely fall one a master curve, especially at high contaminant levels. 4. Conclusions This note reported on the influence of the cationic contamination on both mechanical properties and degradation temperature. A large change in properties was evidenced at a positive to negative charge ratio close to 0.2, separating samples close to the H+ form to that with a degraded behaviour. On the application point of view, these results indicate that the performance of the PFSA membrane does not primarily depend on the concentration as usually measured in the literature [1,2,6], but on the actual number of cations that have been in contact with the membrane regardless of the exposure concentration. The detrimental effect of cationic contaminants could thus originate from a cumulative process that could, on the point of view, occur with very low concentrations over long periods of the time. Acknowledgements This research was supported in part by French National Agencies for Research, ANR (PAN-H program). The authors thank their student, Charline Rannou, for technical help. References [1] R.C. Makkus, A.H.H. Janssen, F.A. de Bruijn, R.K.A.M. Mallant, Use of stainless steel for cost competitive bipolar plates in the SPFC, J. Power Sources 86 (2000) 274–282. [2] A. Pozio, R.F. Silva, M. De Francesco, L. Giorgi, Nafion degradation in PEFCs from end plate iron contamination, Electrochim. Acta 48 (2003) 1543–1549. [3] C. Huang, K. Seng Tan, J. Lin, K. Lee Tan, XRD and XPS analysis of the degradation of the polymer electrolyte in H2 –O2 fuel cell, Chem. Phys. Lett. 371 (2003) 80–85. [4] E.A. Cho, U.S. Jeon, S.A. Hong, I.H. Oh, S.G. Kang, Performance of a 1 kWclass PEMFC stack using TiN-coated 316 stainless steel bipolar plates, J. Power Sources 142 (2005) 177–183. [5] X. Cheng, Z. Shi, N. Glass, L. Zhang, J. Zhang, D. Song, Z.-S. Liu, H. Wang, J. Shen, A review of PEM hydrogen fuel cell contamination: impacts, mechanisms, and mitigation, J. Power Sources 165 (2007) 739–756. [6] J. Wind, R. Spah, W. Kaiser, G. Bohm, Metallic bipolar plates for PEM fuel cells, J. Power Sources 105 (2002) 256–260. [7] S. Grot, W. Grot, H. Nguyen, J. Pinto, Fuel Cell Operation Using Components Manufactured With Recycled End-of-life Membrane Electrode Assemblies 2005 Fuel Cell seminar, Palm Springs, CA, 2005. [8] A. Collier, H. Wang, X. Zi Yuan, J. Zhang, D.P. Wilkinson, Degradation of polymer electrolyte membranes, Int. J. Hydrogen Energy 31 (2006) 1838–1854. [9] S.K. Young, S.F. Trevino, N.C. Beck, T.R.L. Paul, Utilization of prompt-gamma neutron activation analysis in the evaluation of Nafion membranes, J. Polym. Sci., Part B: Polym. Phys. 41 (2003) 1485–1492. [10] S. Stucki, G.G. Scherer, S. Schlagowski, E. Fischer, PEM water electrolysers: evidence for membrane failure in 100 kW demonstration plants, J. Appl. Electrochem. 28 (1998) 1041–1049. [11] A. Panchenko, H. Dilger, J. Kerres, M. Hein, A. Ullrich, T. Kaz, E. Roduner, In-situ spin trap electron paramagnetic resonance study of fuel cell processes, Phys. Chem. Chem. Phys. 6 (2004) 2891–2894. [12] T. Xie, C.A. Hayden, J. Healy, K. Olson, Chemical degradation mechanism of perfluorinated sulfonic acid ionomer, in: Advances in Materials for Proton Exchange Membrane Fuel Cell Systems 2005, Pacific Grove, CA, 2005, p. 24. [13] T. Xie, C.A. Hayden, A kinetic model for the chemical degradation of perfluorinated sulfonic acid ionomers: weak end groups versus side chain cleavage, Polymer 48 (2007) 5497–5506. [14] M. Fowler, Materials and system degradation in PEM fuel cells, in: Fuel Cell Technology Day, Kingston, Ontario, 2007. [15] M. Shi, F.C. Anson, Dehydration of protonated Nafion® coatings induced by cation exchange and monitored by quartz crystal microgravimetry, J. Electroanal. Chem. 425 (1997) 117–123.

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