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Aquivion® PerfluoroSulfonic Acid ionomer membranes: A micro-Raman spectroscopic study of ageing
Polymer Degradation and Stability 98 (2013) 1138e1143
Contents lists available at SciVerse ScienceDirect
Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab
AquivionÒ PerfluoroSulfonic Acid ionomer membranes: A micro-Raman spectroscopic study of ageing Stefano Radice a, *, Claudio Oldani a, Luca Merlo a, Massimiliano Rocchia b a b
Solvay Specialty Polymers, R&D Center, Viale Lombardia 20, 20021 Bollate (MI), Italy Thermo Fisher Scientific, Strada Rivoltana, 20090 Rodano (MI), Italy
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 February 2013 Received in revised form 18 March 2013 Accepted 19 March 2013 Available online 28 March 2013
Recent interest in alternative energy technologies promoted the development of perfluorinated polymer membranes for fuel cells; the degradation mechanism occurring during operation and the polymer ageing that may lead to membrane failure are still a matter of investigation with the aim to identify proper testing protocols and analytical methods. This study illustrates some results obtained through micro Raman spectroscopy; this analytical technique showed itself to be a powerful tool in terms of spatial resolution and chemical data gathering. Detailed analytical and structural information along the cross sections of two membrane grades have been presented, shedding light on intrinsic structural modifications related to overall membrane performance. This study clearly shows an ununiformly distributed loss of sulfonic groups in two different AquivionÒ membrane grades, being the spectroscopic technique able to unveil very subtle effects due to equivalent weight and membrane thickness variations. The overall results support the mechanism previously proposed by other authors using different analytical techniques. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Fuel cells PEMFC Electrochemical device Raman spectroscopy Confocal Raman spectroscopy
1. Introduction Thanks to their high efficiency in energy production, low environmental impact and wide range of application, Polymer Electrolyte Membrane Fuel Cells (PEMFCs) are today considered an attractive alternative to fossil fuel in automotive as well as in stationary applications [1]. Generally speaking, PEMFC are electrochemical devices able to convert hydrogen (commonly supplied as a pure gas at the anode side) and oxygen (present in the air feeding the cathode side) in water while producing electric energy. In such a device, polymer membrane is a key element, not only because it is physically placed in the middle of the system but also because it has to play different and fundamental roles where outstanding properties are required. Indeed, polymer membrane has to show i) high impermeability to reactants thus avoiding their contact, ii) high ionic conductivity allowing protons to migrate from anode to cathode, iii) high electron insulating ability in order to force electrons to flow through the external circuit and iv) good mechanical properties to support the catalyst layer (currently Pt on carbon) and to stand dimensional changes triggered by hydration/
* Corresponding author. Tel.: þ39 (0)238356563. E-mail address: [email protected] (S. Radice). 0141-3910/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymdegradstab.2013.03.015
dehydration cycles typical of automotive and discontinuous operations [2]. Amongst the huge number of materials so far proposed to produce membranes able to withstand the high demanding environment typical of fuel cells, PerFluoroSulfonic Acid (PFSA) ionomers are the most promising thanks to their unique combination of ionic conductivity and chemical resistance [3]. AquivionÒ (marketed by Solvay Specialty Polymers SpA) is a PFSA ionomer constituted by a hydrophobic polytetrafluoroethylene (PTFE)-like backbone and hydrophilic side chains ending with eSO3H functional groups able to provide protons; this property is mandatory for fuel cell operation [4]. In spite of the similarity in chemical structure with other ionomers currently available on the market such as NafionÒ (DuPont de Nemours) and 3M ionomer (3M company) (Scheme 1), AquivionÒ shows a shorter side chain length and for this reason is often referred as short side chain (SSC) ionomer whereas its congeners are known as long side chain (LSC) ionomers [5]. The shorter side chain in AquivionÒ allows higher glass transition temperature (i.e. higher softening temperature), higher capability to retain and absorb cathode produced water (i.e. self-humidification ability) and high conductivity and water mobility especially in low humidity conditions [6]. In the last few years in order to shed light on membrane failure events, different mechanisms have been proposed [7]. In spite of the fact that a general agreement in the scientific community is still
S. Radice et al. / Polymer Degradation and Stability 98 (2013) 1138e1143
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Scheme 1. Chemical structure of some ionomers available on the market. From left to right: NafionÒ, 3M ionomer and AquivionÒ.
Table 1 List of AquivionÒ E87-10 and E79-05 membrane characteristics.
Equivalent weight [g/mol] Initial thickness [mm] Final thickness [mm] Chemical stabilization Kind of membrane AST lifetime [h]
E87-10
E79-05
870 70 60 None Extruded 353
790 50 40 None Extruded 218
missing, the most cited mechanisms involve degradation of carboxylic acid end groups (the so-called unzipping reaction) [8] and of the side chain [9] or the loss of SO3H groups [10]. This study has the aim to show some results we obtained using micro Raman spectroscopy to get detailed analytical and structural information along the cross sections of two kind of AquivionÒ membranes (namely E87-10 and E79-05) suitable for fuel cell applications. Chemical and structural changes before and after accelerated stress tests (AST) have been carefully monitored in order to describe a degradation mechanism able to explain membrane failure during operation.
H2O (10% w/w) for 8 h. An additional rinse in deionized water was applied and then wet membranes were dried in vent oven at 80 C for 4 h. Properties of tested membranes are listed in Table 1: 2.2. AST membrane degradation PFSA AquivionÒ membranes were assembled in a 25 cm2 cell with GreenerityÔ H400 gas diffusion electrodes (SolviCore GmbH & Co. KG, Hanau, Germany) and conditioned in a single cell test bench (Fuel Cell Technology, Albuquerque, NM, USA) at 75 C and
2. Material and methods 2.1. Membrane PFSA AquivionÒ membranes have been prepared through melt extrusion of the precursor polymer in eSO2F form. In order to convert eSO2F into SO3H group, membranes were treated in KOH/ H2O (20% w/w) solution at 80 C for 24 h and then, after a careful rinsing in deionized water, treated at room temperature in HNO3/
Fig. 1. Cell voltage versus time during AquivionÒ E87-10 and E79-05 membrane degradation according to AST protocol. End-of-life criteria were fixed at 0.7 V.
Fig. 2. A) Picture of the cross section of the 87-10 membrane before ageing test, (fresh sample). The line indicates the points of analysis (step of 5 micron). Cross section thickness about 70 microns. B)picture of the cross section relative to the 87-10 membrane after ageing test (in the middle is possible to see some deformation and damage). The overall thickness is about 60 microns.
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Fig. 3. A) Picture E79-05 before ageing test (fresh): the line shows points analysed. Thickness around 50 microns. B) Picture of E79-05 after ageing test; the line shows points analysed (thickness around 40 microns).
Fig. 4. Raman spectrum of E87-10 membrane sample before ageing.
100% reactants humidification (air on cathode side and pure hydrogen on anode side) by keeping constant voltage of 0.6 V by an electronic load. No back pressure has been applied to the reactants. After 24 h AST protocol was applied using the following operating conditions: Cell temperature: 90 C. Cathode side reactant: pure oxygen, inlet relative humidity: 30%, no back pressure. Anode side reactant: pure hydrogen, inlet relative humidity: 30%, no back pressure. Cell voltage was maintained in Open Circuit Voltage (OCV), so to drain zero current from the cell, the voltage was continuously monitored during the test and the end-of-life criteria was fixed to 0.7 V which is recognized in the art as corresponding to a very high level of gas crossover, that is to the irreversible membrane damage. Fig. 1 shows the changes of cell voltage over time of the two membranes considered in the study.
Fig. 5. A selection of Raman spectra recorded along the cross section anode-cathod of sample E87-10, aged sample (Fig. 2B). Sampling step: about 5 microns. In the small framework the value of the intensity peak height ratio: (385 cm1)/(731 cm1) these bands are due to normal modes of vibration of the CF2 groups of the polymeric backbone.
S. Radice et al. / Polymer Degradation and Stability 98 (2013) 1138e1143
Fig. 6. Raman data relative to membrane 87-10 before (dashed) and after (continuous) ageing test. Raman peak height ratio: (1062 cm1)/(731 cm1), the parameter KSO3 is proportional to the concentration ratio of SO3 and CF2 groups.
2.3. Raman spectroscopy Raman spectra have been collected with a Thermo Fisher DXR micro Raman instrument: for membrane 87-10 the 532 laser line has been used: power at sample in the range of 10 mW, grating with 900 line/mm provided a spectral resolution of 5 cm1 FWHM. For 79-05 membrane we used the 780 nm laser line, power at sample in the range of 24 mW, grating with 400 line/mm provided the same spectral resolution of 5 cm1. In both cases with a 50 m pinhole aperture and 50 magnification. Samples have been mounted on an automatic stage allowing the control of 1 m step size. 3. Results and discussion Vibrational spectroscopy has been widely used in the characterization of TFE based polymers and copolymers: fundamental
1141
assignments of normal modes are available in literature [11e17]. Perfluorosulphonic membranes have been also extensively studied and characterised by means of different spectroscopies [18e23]. In this study we focused our effort to put in evidence chemical/ structural variations along the cross section of two membranes before and after the ageing test described; the spectroscopic technique used (confocal micro-Raman spectroscopy) allowed us to obtain a spatial resolution around 1 micron. The pictures of the analysed section of the membranes are reported in Figs. 2 and 3 (a,b before and after ageing respectively). In Fig. 4 Raman a spectrum of the copolymer E87-10 before the test is shown (the comparison with aged sample is reported in Fig. 7): The more intense band observed in Raman spectra is located at 732 cm1 and is due to (CF2)n groups of the main backbone; for TFE based polymers and considering the isolated 15/7 helix, at this frequency an A1 (Raman active) normal mode is expected; the same consideration is valid for the 386 cm1 band observed (A1, Raman active, dCF2) [11e13]. In order to get a deeper evaluation of the correct assignment in the Raman spectrum of the copolymer, and especially to be confident that no contribution to their intensity has been given by the presence of the sulphonic comonomer, the intensity ratio of the two bands (732 cm1 and 386 cm1) has been evaluated. This evaluation has been done on all data obtained along the cross section of the membrane. In Fig. 5 Raman data and the intensity ratio values (in the small framework) are reported; values remain constant, apart scattering due to S/N in the spectral data. A characteristic and intense band assigned to the presence of SO3 groups is observed at 1060 cm1 and is due to the antisymmetric stretching of the eSO3 moiety [21e24]. A simple way to evaluate the relative content of SO3 and CF2 groups is to measure the intensity ratio 1060 cm1/732 cm1 along the cross section of the membranes; in the following this parameter will be indicated as KSO3 . In Fig. 6 we report the comparison of the KSO3 parameter along the cross section of the fresh and the aged membrane. It is possible
Fig. 7. Raman spectra relative to the membrane E87-10 before (upper) and after (lower) ageing test. Two points in the inner part or the cross section have been chosen.
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S. Radice et al. / Polymer Degradation and Stability 98 (2013) 1138e1143
Fig. 8. Some Raman spectra relative to the membrane E79-05 after ageing test. Spectral data have been collected along the cross section every 2 micron.
to observe how it shows significative variation along the thickness for the aged membrane; actually it decreases in the center about 40% of its initial value in comparison to virgin membrane and in the nearby of the cathode/anode of the membrane itself. Regarding the thickness of this sample it is worthwhile to observe that starting from a value around 70 microns, after the ageing test the thickness is overall reduced (around 55e60 microns), and in the middle of the membrane itself is possible to observe a more complex and not homogeneous morphological situation (Fig. 2B). In Fig. 8 the spectroscopic data of the membrane E79-05 relative to the aged samples have been reported, while in Fig. 9 is shown the comparison of the KSO3 parameter along the cross section of membranes before and after ageing. The overall KSO3 values for the aged membrane are lower than those of the fresh one. KSO3 has almost a constant value along all the cross section, so that is not possible to observe the previous trend with lower values in the middle of the membrane itself. The overall values are smaller with respect unaged samples in a range of about 20%; we performed also a Raman spectrum, on a single point, in a region of the aged
membrane that had not worked (outside the working area of the mounted fuel cell); the KSO3 value obtained is comparable to that relative to the unaged, fresh membrane. In spite of both the two analysed samples show a clear loss of sulfonic acid groups after accelerated degradation test, the trend of KSO3 parameter versus membrane cross-section shows a different behaviour. E87-10 membrane’s data show a minimum in KSO3 ratio placed in the middle of the membrane suggesting a higher degradation in that position, whereas E79-05’s data show an almost constant KSO3 value indicating a homogeneous loss of sulfonic acid, not depending on localization. This different behaviour can be explained considering the intrinsic differences in membrane structure and polymer properties. It is widely accepted that the ionomer degradation is triggered by the interaction between polymer and peroxide radicals derived from H2 and O2 combination. Indeed, gas diffusion play a key role in material degradation and the differences in KSO3 profile can be explained in terms of different diffusivity of hydrogen and oxygen within the membrane. As a matter of fact the E79-05 membrane is thinner than E87-10 and moreover, having lower equivalent weight than E87-10 membrane, the degree of crystallinity (which hamper the gas diffusion) of the former is lower than that of the latter [4]. 4. Conclusions
Fig. 9. Values of KSO3 parameter relative to the E79-05 membrane before (dashed) and after (continuous) ageing test: I(1060 cm1)/I(731 cm1).
Micro-Raman spectroscopy with confocal capabilities proved to be a very fruitful analytical technique to get chemical, structural and very localised information about the ageing mechanism of PFSA membranes. The experimental set up and the approach proposed resulted to be a powerful, innovative method for such studies and allowed us to measure the degradation profile through the membrane cross section and to demonstrate a degradation mechanism active in AquivionÒ PFSA under AST conditions we applied. Micro-Raman spectroscopy clearly showed in two different AquivionÒ membrane grades a loss of eSO3H groups from the ionomer side chain thus supporting the mechanism previously proposed by other authors using different analytical techniques
S. Radice et al. / Polymer Degradation and Stability 98 (2013) 1138e1143
[10,24]. Further studies are currently ongoing on different AquivionÒ membranes and applying different accelerated testing conditions in order to demonstrate the influence of membrane structure, polymer architecture and ageing conditions on degradation mechanism. Moreover, new grades of thin AquivionÒ reinforced (i.e. having an internal core constituted by an expanded polytetrafluoroethylene support) membrane are under investigation. References [1] Wang Y, Chen KS, Mishler J, Cho SC, Adroher XC. Appl Energy 2011;88: 981e1007; Smitha B, Sridhar S, Khan AA. J Membr Sci 2005;259:10e26. [2] Zhang H, Shen PK. Chem Soc Rev 2012;41:2382e94. [3] Mauritz KA, Moore RB. Chem Rev 2004;104:4535e85. [4] http://www.solvayplastics.com. [5] Grot W. Fluorinated ionomers. Oxford: Elsevier Inc.; 201112. [6] Ghielmi A, Vaccarono P, Troglia C, Arcella V. J Power Sources 2005;145: 108e15. [7] de Bruijn FA, Dam VAT, Janssen GJM. Fuel Cells 2008;1:3e22; Wu J, Yuan XZ, Martin JJ, Wang H, Zhang J, Shen J, et al. J Power Sources 2008;184:104e19;
[8] [9]
[10] [11] [12] [13] [14] [15] [16]
[17] [18] [19] [20] [21] [22] [23] [24]
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Borup R, Meyers J, Pivovar B, Kim YS, Mukundan R, Garland N, et al. Chem Rev 2007;107:3904e51. Curtin DE, Lousenberg RD, Henry TJ, Tangeman PC, Kaz T, Roundner E. Phys Chem Chem Phys 2004;6:2891e4. Zhou C, Guerra MA, Qiu Z-M, Zawodzinski TA, Schiraldi DA. Macromolecules 2007;40:8695e707; Danilczuk M, Perkowski AJ, Schlick S. Macromolecules 2010;43:3352e8. Kadirov MK, Bosnjakovic A, Schlick S. J Phys Chem B 2005;109:7664e70. Zerbi G, Sacchi M. Macromolecules 1974;6:692e9. Masetti G, Cabassi F, Morelli G, Zerbi G. Macromolecules 1973;6:700e7. Rabolt JF. J Polym Sci (Pol Phys Ed) 1983;21:1797e805. Moynihan RE. J Am Chem Soc 1959;81(5):1045e50. Tadokoro H. Structure of crystalline polymers. New York: Wiley; 1979. Milani A, Matteo Tommasini, Chiara Castiglioni, Giuseppe Zerbi, Stefano Radice, Giorgio Canil, et al. Polymer 2008;49(7):1812e22; Quarti C, Milani A, Castiglioni C. J Phys Chem B 2013;117(2):706e18. Abramov SP, Trofimova AA, Barabanov VG. Russ J Appl Chem 2009;82(12): 2187e91. Almeida SH, Kawano Y. Polym Degrad Stabil 1998;62:291e7. Ramaswamy N, Hakim N, Mukerjee S. Electrochim Acta 2008;53:3279e95. Holber M, Johansson P, Jacobsson P. Fuel Cells 2011;11(3):459e64. Kujaski W, Nguyen QT, Neel J. J Appl Polym Sci 1992;44:951e8. Gruger A, Regis A, Schmatko T, Colomban P. Vib Spectrosc 2001;26:215e25. Yamaguchi M, Ohira A. J Phys Chem A 2012;116:10850e63. Fang X, Shen PK, Song S, Stergiopoulos V, Tsiakaras P. Polym Degrad Stab 2009;94:1707e13.
Contents lists available at SciVerse ScienceDirect
Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab
AquivionÒ PerfluoroSulfonic Acid ionomer membranes: A micro-Raman spectroscopic study of ageing Stefano Radice a, *, Claudio Oldani a, Luca Merlo a, Massimiliano Rocchia b a b
Solvay Specialty Polymers, R&D Center, Viale Lombardia 20, 20021 Bollate (MI), Italy Thermo Fisher Scientific, Strada Rivoltana, 20090 Rodano (MI), Italy
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 February 2013 Received in revised form 18 March 2013 Accepted 19 March 2013 Available online 28 March 2013
Recent interest in alternative energy technologies promoted the development of perfluorinated polymer membranes for fuel cells; the degradation mechanism occurring during operation and the polymer ageing that may lead to membrane failure are still a matter of investigation with the aim to identify proper testing protocols and analytical methods. This study illustrates some results obtained through micro Raman spectroscopy; this analytical technique showed itself to be a powerful tool in terms of spatial resolution and chemical data gathering. Detailed analytical and structural information along the cross sections of two membrane grades have been presented, shedding light on intrinsic structural modifications related to overall membrane performance. This study clearly shows an ununiformly distributed loss of sulfonic groups in two different AquivionÒ membrane grades, being the spectroscopic technique able to unveil very subtle effects due to equivalent weight and membrane thickness variations. The overall results support the mechanism previously proposed by other authors using different analytical techniques. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Fuel cells PEMFC Electrochemical device Raman spectroscopy Confocal Raman spectroscopy
1. Introduction Thanks to their high efficiency in energy production, low environmental impact and wide range of application, Polymer Electrolyte Membrane Fuel Cells (PEMFCs) are today considered an attractive alternative to fossil fuel in automotive as well as in stationary applications [1]. Generally speaking, PEMFC are electrochemical devices able to convert hydrogen (commonly supplied as a pure gas at the anode side) and oxygen (present in the air feeding the cathode side) in water while producing electric energy. In such a device, polymer membrane is a key element, not only because it is physically placed in the middle of the system but also because it has to play different and fundamental roles where outstanding properties are required. Indeed, polymer membrane has to show i) high impermeability to reactants thus avoiding their contact, ii) high ionic conductivity allowing protons to migrate from anode to cathode, iii) high electron insulating ability in order to force electrons to flow through the external circuit and iv) good mechanical properties to support the catalyst layer (currently Pt on carbon) and to stand dimensional changes triggered by hydration/
* Corresponding author. Tel.: þ39 (0)238356563. E-mail address: [email protected] (S. Radice). 0141-3910/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymdegradstab.2013.03.015
dehydration cycles typical of automotive and discontinuous operations [2]. Amongst the huge number of materials so far proposed to produce membranes able to withstand the high demanding environment typical of fuel cells, PerFluoroSulfonic Acid (PFSA) ionomers are the most promising thanks to their unique combination of ionic conductivity and chemical resistance [3]. AquivionÒ (marketed by Solvay Specialty Polymers SpA) is a PFSA ionomer constituted by a hydrophobic polytetrafluoroethylene (PTFE)-like backbone and hydrophilic side chains ending with eSO3H functional groups able to provide protons; this property is mandatory for fuel cell operation [4]. In spite of the similarity in chemical structure with other ionomers currently available on the market such as NafionÒ (DuPont de Nemours) and 3M ionomer (3M company) (Scheme 1), AquivionÒ shows a shorter side chain length and for this reason is often referred as short side chain (SSC) ionomer whereas its congeners are known as long side chain (LSC) ionomers [5]. The shorter side chain in AquivionÒ allows higher glass transition temperature (i.e. higher softening temperature), higher capability to retain and absorb cathode produced water (i.e. self-humidification ability) and high conductivity and water mobility especially in low humidity conditions [6]. In the last few years in order to shed light on membrane failure events, different mechanisms have been proposed [7]. In spite of the fact that a general agreement in the scientific community is still
S. Radice et al. / Polymer Degradation and Stability 98 (2013) 1138e1143
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Scheme 1. Chemical structure of some ionomers available on the market. From left to right: NafionÒ, 3M ionomer and AquivionÒ.
Table 1 List of AquivionÒ E87-10 and E79-05 membrane characteristics.
Equivalent weight [g/mol] Initial thickness [mm] Final thickness [mm] Chemical stabilization Kind of membrane AST lifetime [h]
E87-10
E79-05
870 70 60 None Extruded 353
790 50 40 None Extruded 218
missing, the most cited mechanisms involve degradation of carboxylic acid end groups (the so-called unzipping reaction) [8] and of the side chain [9] or the loss of SO3H groups [10]. This study has the aim to show some results we obtained using micro Raman spectroscopy to get detailed analytical and structural information along the cross sections of two kind of AquivionÒ membranes (namely E87-10 and E79-05) suitable for fuel cell applications. Chemical and structural changes before and after accelerated stress tests (AST) have been carefully monitored in order to describe a degradation mechanism able to explain membrane failure during operation.
H2O (10% w/w) for 8 h. An additional rinse in deionized water was applied and then wet membranes were dried in vent oven at 80 C for 4 h. Properties of tested membranes are listed in Table 1: 2.2. AST membrane degradation PFSA AquivionÒ membranes were assembled in a 25 cm2 cell with GreenerityÔ H400 gas diffusion electrodes (SolviCore GmbH & Co. KG, Hanau, Germany) and conditioned in a single cell test bench (Fuel Cell Technology, Albuquerque, NM, USA) at 75 C and
2. Material and methods 2.1. Membrane PFSA AquivionÒ membranes have been prepared through melt extrusion of the precursor polymer in eSO2F form. In order to convert eSO2F into SO3H group, membranes were treated in KOH/ H2O (20% w/w) solution at 80 C for 24 h and then, after a careful rinsing in deionized water, treated at room temperature in HNO3/
Fig. 1. Cell voltage versus time during AquivionÒ E87-10 and E79-05 membrane degradation according to AST protocol. End-of-life criteria were fixed at 0.7 V.
Fig. 2. A) Picture of the cross section of the 87-10 membrane before ageing test, (fresh sample). The line indicates the points of analysis (step of 5 micron). Cross section thickness about 70 microns. B)picture of the cross section relative to the 87-10 membrane after ageing test (in the middle is possible to see some deformation and damage). The overall thickness is about 60 microns.
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Fig. 3. A) Picture E79-05 before ageing test (fresh): the line shows points analysed. Thickness around 50 microns. B) Picture of E79-05 after ageing test; the line shows points analysed (thickness around 40 microns).
Fig. 4. Raman spectrum of E87-10 membrane sample before ageing.
100% reactants humidification (air on cathode side and pure hydrogen on anode side) by keeping constant voltage of 0.6 V by an electronic load. No back pressure has been applied to the reactants. After 24 h AST protocol was applied using the following operating conditions: Cell temperature: 90 C. Cathode side reactant: pure oxygen, inlet relative humidity: 30%, no back pressure. Anode side reactant: pure hydrogen, inlet relative humidity: 30%, no back pressure. Cell voltage was maintained in Open Circuit Voltage (OCV), so to drain zero current from the cell, the voltage was continuously monitored during the test and the end-of-life criteria was fixed to 0.7 V which is recognized in the art as corresponding to a very high level of gas crossover, that is to the irreversible membrane damage. Fig. 1 shows the changes of cell voltage over time of the two membranes considered in the study.
Fig. 5. A selection of Raman spectra recorded along the cross section anode-cathod of sample E87-10, aged sample (Fig. 2B). Sampling step: about 5 microns. In the small framework the value of the intensity peak height ratio: (385 cm1)/(731 cm1) these bands are due to normal modes of vibration of the CF2 groups of the polymeric backbone.
S. Radice et al. / Polymer Degradation and Stability 98 (2013) 1138e1143
Fig. 6. Raman data relative to membrane 87-10 before (dashed) and after (continuous) ageing test. Raman peak height ratio: (1062 cm1)/(731 cm1), the parameter KSO3 is proportional to the concentration ratio of SO3 and CF2 groups.
2.3. Raman spectroscopy Raman spectra have been collected with a Thermo Fisher DXR micro Raman instrument: for membrane 87-10 the 532 laser line has been used: power at sample in the range of 10 mW, grating with 900 line/mm provided a spectral resolution of 5 cm1 FWHM. For 79-05 membrane we used the 780 nm laser line, power at sample in the range of 24 mW, grating with 400 line/mm provided the same spectral resolution of 5 cm1. In both cases with a 50 m pinhole aperture and 50 magnification. Samples have been mounted on an automatic stage allowing the control of 1 m step size. 3. Results and discussion Vibrational spectroscopy has been widely used in the characterization of TFE based polymers and copolymers: fundamental
1141
assignments of normal modes are available in literature [11e17]. Perfluorosulphonic membranes have been also extensively studied and characterised by means of different spectroscopies [18e23]. In this study we focused our effort to put in evidence chemical/ structural variations along the cross section of two membranes before and after the ageing test described; the spectroscopic technique used (confocal micro-Raman spectroscopy) allowed us to obtain a spatial resolution around 1 micron. The pictures of the analysed section of the membranes are reported in Figs. 2 and 3 (a,b before and after ageing respectively). In Fig. 4 Raman a spectrum of the copolymer E87-10 before the test is shown (the comparison with aged sample is reported in Fig. 7): The more intense band observed in Raman spectra is located at 732 cm1 and is due to (CF2)n groups of the main backbone; for TFE based polymers and considering the isolated 15/7 helix, at this frequency an A1 (Raman active) normal mode is expected; the same consideration is valid for the 386 cm1 band observed (A1, Raman active, dCF2) [11e13]. In order to get a deeper evaluation of the correct assignment in the Raman spectrum of the copolymer, and especially to be confident that no contribution to their intensity has been given by the presence of the sulphonic comonomer, the intensity ratio of the two bands (732 cm1 and 386 cm1) has been evaluated. This evaluation has been done on all data obtained along the cross section of the membrane. In Fig. 5 Raman data and the intensity ratio values (in the small framework) are reported; values remain constant, apart scattering due to S/N in the spectral data. A characteristic and intense band assigned to the presence of SO3 groups is observed at 1060 cm1 and is due to the antisymmetric stretching of the eSO3 moiety [21e24]. A simple way to evaluate the relative content of SO3 and CF2 groups is to measure the intensity ratio 1060 cm1/732 cm1 along the cross section of the membranes; in the following this parameter will be indicated as KSO3 . In Fig. 6 we report the comparison of the KSO3 parameter along the cross section of the fresh and the aged membrane. It is possible
Fig. 7. Raman spectra relative to the membrane E87-10 before (upper) and after (lower) ageing test. Two points in the inner part or the cross section have been chosen.
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Fig. 8. Some Raman spectra relative to the membrane E79-05 after ageing test. Spectral data have been collected along the cross section every 2 micron.
to observe how it shows significative variation along the thickness for the aged membrane; actually it decreases in the center about 40% of its initial value in comparison to virgin membrane and in the nearby of the cathode/anode of the membrane itself. Regarding the thickness of this sample it is worthwhile to observe that starting from a value around 70 microns, after the ageing test the thickness is overall reduced (around 55e60 microns), and in the middle of the membrane itself is possible to observe a more complex and not homogeneous morphological situation (Fig. 2B). In Fig. 8 the spectroscopic data of the membrane E79-05 relative to the aged samples have been reported, while in Fig. 9 is shown the comparison of the KSO3 parameter along the cross section of membranes before and after ageing. The overall KSO3 values for the aged membrane are lower than those of the fresh one. KSO3 has almost a constant value along all the cross section, so that is not possible to observe the previous trend with lower values in the middle of the membrane itself. The overall values are smaller with respect unaged samples in a range of about 20%; we performed also a Raman spectrum, on a single point, in a region of the aged
membrane that had not worked (outside the working area of the mounted fuel cell); the KSO3 value obtained is comparable to that relative to the unaged, fresh membrane. In spite of both the two analysed samples show a clear loss of sulfonic acid groups after accelerated degradation test, the trend of KSO3 parameter versus membrane cross-section shows a different behaviour. E87-10 membrane’s data show a minimum in KSO3 ratio placed in the middle of the membrane suggesting a higher degradation in that position, whereas E79-05’s data show an almost constant KSO3 value indicating a homogeneous loss of sulfonic acid, not depending on localization. This different behaviour can be explained considering the intrinsic differences in membrane structure and polymer properties. It is widely accepted that the ionomer degradation is triggered by the interaction between polymer and peroxide radicals derived from H2 and O2 combination. Indeed, gas diffusion play a key role in material degradation and the differences in KSO3 profile can be explained in terms of different diffusivity of hydrogen and oxygen within the membrane. As a matter of fact the E79-05 membrane is thinner than E87-10 and moreover, having lower equivalent weight than E87-10 membrane, the degree of crystallinity (which hamper the gas diffusion) of the former is lower than that of the latter [4]. 4. Conclusions
Fig. 9. Values of KSO3 parameter relative to the E79-05 membrane before (dashed) and after (continuous) ageing test: I(1060 cm1)/I(731 cm1).
Micro-Raman spectroscopy with confocal capabilities proved to be a very fruitful analytical technique to get chemical, structural and very localised information about the ageing mechanism of PFSA membranes. The experimental set up and the approach proposed resulted to be a powerful, innovative method for such studies and allowed us to measure the degradation profile through the membrane cross section and to demonstrate a degradation mechanism active in AquivionÒ PFSA under AST conditions we applied. Micro-Raman spectroscopy clearly showed in two different AquivionÒ membrane grades a loss of eSO3H groups from the ionomer side chain thus supporting the mechanism previously proposed by other authors using different analytical techniques
S. Radice et al. / Polymer Degradation and Stability 98 (2013) 1138e1143
[10,24]. Further studies are currently ongoing on different AquivionÒ membranes and applying different accelerated testing conditions in order to demonstrate the influence of membrane structure, polymer architecture and ageing conditions on degradation mechanism. Moreover, new grades of thin AquivionÒ reinforced (i.e. having an internal core constituted by an expanded polytetrafluoroethylene support) membrane are under investigation. References [1] Wang Y, Chen KS, Mishler J, Cho SC, Adroher XC. Appl Energy 2011;88: 981e1007; Smitha B, Sridhar S, Khan AA. J Membr Sci 2005;259:10e26. [2] Zhang H, Shen PK. Chem Soc Rev 2012;41:2382e94. [3] Mauritz KA, Moore RB. Chem Rev 2004;104:4535e85. [4] http://www.solvayplastics.com. [5] Grot W. Fluorinated ionomers. Oxford: Elsevier Inc.; 201112. [6] Ghielmi A, Vaccarono P, Troglia C, Arcella V. J Power Sources 2005;145: 108e15. [7] de Bruijn FA, Dam VAT, Janssen GJM. Fuel Cells 2008;1:3e22; Wu J, Yuan XZ, Martin JJ, Wang H, Zhang J, Shen J, et al. J Power Sources 2008;184:104e19;
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