PVdF-HFP membranes for fuel cell applications

PVdF-HFP membranes for fuel cell applications: effects of doping agents and coating on the membrane’s properties Tülay Y. Inan, Hacer Doğan & Attila Güngör

Ionics International Journal of Ionics The Science and Technology of Ionic Motion ISSN 0947-7047 Ionics DOI 10.1007/s11581-012-0803-z

1 23

Your article is protected by copyright and all rights are held exclusively by SpringerVerlag. This e-offprint is for personal use only and shall not be self-archived in electronic repositories. If you wish to self-archive your work, please use the accepted author’s version for posting to your own website or your institution’s repository. You may further deposit the accepted author’s version on a funder’s repository at a funder’s request, provided it is not made publicly available until 12 months after publication.

1 23

Author's personal copy Ionics DOI 10.1007/s11581-012-0803-z

ORIGINAL PAPER

PVdF-HFP membranes for fuel cell applications: effects of doping agents and coating on the membrane’s properties Tülay Y. Inan & Hacer Doğan & Attila Güngör

Received: 17 February 2012 / Revised: 24 July 2012 / Accepted: 20 August 2012 # Springer-Verlag 2012

Abstract Poly(vinylidene fluoride-co-hexafluoropropene)– hexafluoropropylene (PVdF-HFP; M n , 130,000)-based membranes were prepared by means of phase inversion technique by coagulating with water and MeOH and then doping with H3PO4 and H2SO4. In order to improve the electrochemical properties of the PVdF-HFP membranes, coagulated membranes were also coated with polystyreneblock-poly(ethylene-ran-butylene)-block-polystyrene (PSEBS) and sulfonated with chlorosulfonic acid in the second stage. The effects of the type of coagulant, coagulation time, doping agents, coating, and sulfonation on the membrane properties were investigated. Membranes were thermally stable up to 400 °C. The conductivity values were measured to be between 1.10E−01 and 6.00E−03 mS/cm for uncoated samples. The proton conductivity value of the PSEBS-coated and sulfonated membrane was increased from 6.00E−03 to 92.1 mS/cm. Water uptake values varied from 0 to 38 % for uncoated samples and from 11.5 to 65.2 % for coated samples. Chemical degradation of PVdF-HFP membranes was investigated via Fenton test. All membranes were found to be chemically stable. Morphology of the membranes was examined by scanning electron microscopy. Different membrane morphologies were observed, depending on different membrane preparation procedures. T. Y. Inan (*) : H. Doğan Chemistry Institute, Marmara Research Center, The Scientific and Technological Research Council of Turkey (TUBITAK), 41470, Gebze, Kocaeli, Turkey e-mail: [email protected] H. Doğan e-mail: [email protected] A. Güngör Department of Chemistry, Faculty of Art and Science, Marmara University, Göztepe, Istanbul, Turkey e-mail: [email protected]

Keywords Poly(vinylidene fluoride-co-hexafluoropropene) . PSEBS . Sulfonation . Polymer electrolyte membrane fuel cell . Phase inversion . Doping . Coagulation

Introduction Due to highly desirable properties, perfluorosulfonated ionomers, such as Nafion™, are widely used for preparation of the proton exchange membrane (PEM). It has high proton conductivity and good chemical and mechanical properties, but it has some critical shortcomings such as lower proton conductivity above 80 °C and poor low resistance to methanol crossover in direct methanol fuel cell (DMFC). There is an increasing demand for low-cost polymeric materials with improved properties such as proton conductivity, mechanical strength, and chemical stability [1]. In recent years, composite membranes (comprising of poly(vinylidene fluoride) (PVDF), poly(vinylidene fluoride-cohexafluoropropene)–hexafluoropropylene (PVdF-HFP ) polymer matrix, and inorganic materials) attracted much interests in fuel cell applications. PVdF-HFP (Fig. 1) has been used as a matrix material for electrolyte films of rechargeable lithium batteries because of its high solubility in organic solvents, lower glass transition temperature, and reduced crystallinity [2]. A PVdF-HFP film is generally weaker than PVDF in mechanical strength because of the amorphous nature of the HFP constituent [3, 4]. In order to obtain highly porous PVDF and PVdF-HFP electrolyte membranes applicable to rechargeable lithium batteries, phase inversion technique using solvent and nonsolvent components simultaneously has been considered as an effective way. The advantages of these materials arise from their low-cost synthesis, good mechanical and chemical resistances, and large liquid uptakes that give a liquidlike conductivity [5, 6]

Author's personal copy Ionics

Fig. 1 Schematic representation of PVdF-HFP

The proton conductivity of such polymers increases as proton-conductive materials are added. It has been shown that doping with strong acids such as phosphoric, sulfuric, and hydrochloric acids also lead to excellent proton conductivity and higher solubility in water; both properties are important in polymer preparation. Phosphoric acid-doped basic polymers are excellent proton conductors under appropriate conditions [7]. Generally, doping with strong acids such as phosphoric, sulfuric, and hydrochloric acid have been used mainly for poly(benzimidazole) (PBI)-based membrane preparations in DMFC and high-temperature fuel cell applications [8–20]. Composite polymer membranes of PVdF-HFP/alumina (Al2O3) were previously prepared by phase inversion technique with poly(ethylene glycol) as an additive. The membranes were soaked in 6 M H3PO4 and immobilized for proton conductivity for fuel cell applications [21]. Soresi et al. [22] have reported on the thermal and morphological behaviors as well as electrical properties of proton-conducting polymers synthesized from PVdF-HFP (5 mol% HFP) copolymer and from composite porous and dense PVDF homopolymer films. High grafting degrees and water uptake (WU) values have been reached, depending on the nature of the polymer matrix used. For these polymers, conductivity exceeding 60 mS/cm has been observed at room temperature and 90 % relative humidity (RH) [23]. Sulfonated polystyrene(ethylene–butylene)polystyrene (PSEBS), a triblock polymer, has been also used as a proton-conductive polymer. The complexation behavior of the sulfonated PSEBS ionomer has been reported by a number of authors [24, 25]. The sulfonated PSEBS polymer prepared by Ehrenberg et al. [26, 27] using SO3 as the sulfonating agent has been claimed to have the proton conductivity in the order of 10−5 S/cm in its fully protonated stage. Sulfonated PSEBS using ClSO3H acid as the sulfonating agent has been claimed to have the proton conductivity in the order of 10−1 S/cm [24]. Our objective is to obtain cheap, time-saving, chemically stable, and highly proton-conductive membranes for fuel cell applications and our study consists of two stages. In the first stage, we prepared porous membranes by using 20 % w/v PVdF-HFP in N,N-dimethylacetamide (DMAc) by means of phase inversion technique using two coagulants of water and methanol and two doping agents with an aqueous highly concentrated acid solution of H3PO4 and H2SO4. With regard to fuel cell applications, the effects of the type of coagulant, coagulant composition, coagulation

time, and doping agents on the membrane properties were examined. In the second stage, although there was minimum amount of leached doped solvents after hydrolytic stability test obtained by inductively coupled plasma (ICP), we coated membranes with PSEBS and then sulfonated by using chlorosulfonic acid to determine the effect of morphology difference on the membrane properties.

Experimental Membrane material and chemicals Phosphoric acid (H 3PO 4 ), sulfuric acid (95–97 wt%), DMAc (for synthesis), and hydrogen peroxide (H2O2) were purchased from Fluka. The chemicals were used as received without further purification. PVdF-HFP (Mn 0130,000) was supplied by Fluka. PSEBS (Mw 0118,000), was supplied by Aldrich. All other reagents were purchased from Aldrich. Membrane preparation Doped membranes Nine membranes were prepared by casting 20 % w/v of PVdF-HFP in DMAc solutions onto a clean glass plate by using a 1,000-μm applicator. One series of the PVdF-HFP films (four membranes) were conditioned at laboratory conditions for 1/4 h (or 15 min) and the other series of the films (four membranes) for 24 h, respectively. Two conditioned membranes were then immersed into coagulation baths of pure methanol–MeOH and the remaining two into the coagulation bath of pure water for 5 h. One of the membranes prepared with solvent casting method was used as a reference sample. All casted membranes on glass surfaces were easily removed during coagulation processes either in water or in methanol bath. It was observed that coagulation processes caused an increase in thicknesses for 1/4-h-conditioned membranes, while 24-h-conditioned membranes’ thicknesses remained constant. Finally, before they were used, the 1/4-h-conditioned and water- or MeOHcoagulated membranes were then doped with 11 M H3PO4 and 11 M H2SO4 for 8 h, respectively. The same procedure was applied for the 24-h-conditioned samples (Scheme 1) as well. The membrane thicknesses were measured at dry state using a digital micrometer. The thicknesses of fabricated membranes are in the range of 80–350 μm. Membranes coded as 24MSO–1/4MSO, 24WSO–1/4WSO, 24MPO–1/ 4MPO, or 24WPO-1/4WPO represent conditioning time of 24 or 1/4 h under room conditions. M and W denote the coagulation medium of methanol and water, respectively. PO and SO denote doping medium of H3PO4 and H2SO4, respectively, for a duration of 8 h (Table 1).

Author's personal copy Ionics Scheme 1 Membrane preparation

Coated membranes

respectively. CSEBS denotes coated membranes sulfonated by chlorosulfonic acid (Table 1).

In the second stage, membranes coagulated under different conditions:

Polymer and membrane characterization

1. coagulated with water for 24 h (24W) and 1/4 h (1/4W) 2. coagulated with methanol for 24 h (24M) and 1/4 h (1/4M)

Instrumentation and methods

were coated with PSEBS solution in chloroform (5 % w/v) by using dip-coating method. The coated membranes were dried at room temperature for about 1.5 h and then sulfonated by using 0.75 M chlorosulfonic acid in dichloromethane for about 1 h at room temperature, following the methodology of Mokrini et al. [28]. Then, membranes were dried at room temperature for about 1.5 h. Obtained membranes coded as 24MCSEBS and 1/4MCSEBS or 24WCSEBS and 1/4WCSEBS stand for samples conditioned for 24 or 1/4 h under room conditions. M and W the denote coagulation medium of methanol and water,

Fourier transform infrared (FTIR) spectra were recorded on a Perkin Elmer Spectrum One FTIR Spectrometer. Differential scanning calorimetry (DSC) analyses were performed with Perkin Elmer Jade DSC at a heating rate of 10 °C/min under nitrogen atmosphere. All data were collected from a second heating cycle and the glass transition temperatures (Tg) were calculated as a midpoint of the thermogram. Thermogravimetric data were obtained using a Thermogravimetric Analyzer Perkin Elmer Pyris 1 operated from 30 to 900 °C under nitrogen with a heating rate of 10 °C/min. Morphologies of the membranes were studied under a

Author's personal copy Ionics Table 1 PVdF-HFP-based membranes’ characterization results Membrane (PVdF-HFP)

Conditioning time (h)

Coagulation solvent

CSEBS 24WCSEBS 1/4WCSEBS 1/4MCSEBS 24MCSEBS 24MSO 24MPO 24WPO 24WSO 24M 1/4WPO 1/4WSO 1/4MSO

24 1/4 1/4 24 24 24 24 24 24 1/4 1/4 1/4

Water Water Methanol Methanol Methanol Methanol Water Water Methanol Water Water Methanol

24 1/4 1/4 1/4

Water Methanol Methanol Water

24W 1/4MPO 1/4M 1/4W Nafion 117

Doping solvent

H2SO4 H3PO4 H3PO4 H2SO4 H3PO4 H2SO4 H2SO4 H3PO4

Water absorption (wt%)

Weight loss after Fenton (wt%)

Conductivity (80 °C) σ (mS/cm)

IECm (meq/g)

11.5 40.3 79.2 21.7 65.2 15.3 4.5 1.2 1.8 2.4 30.6 37.8 1.7

2.18 5.34 3.56 3.2 9.2 0 0 0 0 0 0.7 0.8 0.2

12.2 4.05 0.25 4.3 92.1 1.10E−02 2.60E−02 1.10E−01 3.10E−03 6.00E−03 0.00E+00 4.50E−03 0.00E+00

0.67 0.23 0.12 0.27 1.50 0.01 0.01 0.05 0.01 0.01 0.24 0.01 0.02

1.3 0.3 0 12.1 16.0

0.1 0 0 0 >1 week

4.20E−02 0.00E+00 3.00E−03 3.00E−03 133.3

0.01 0.02 0.01 0.01

IECm membranes ion exchange capacity (in milliequivalents per gram)

scanning electron microscope (SEM; JEOL 6335F). Energydispersive X-ray spectroscopy (EDS) was used for the elemental analysis of the H3PO4- and H2SO4-doped samples. For phosphorous and sulfur analysis, inductively coupled plasma–atomic emission spectroscopy was used.

where L is the distance (in centimeters) between two reference electrodes, W is the width (in centimeters) of the dried sample membrane, and T is the thickness (in centimeters) of the dried sample membrane. At least three measurements were taken for one sample obtained from different parts of the membrane.

Proton conductivity measurements Ion exchange capacity In the literature, conductivity measurements are conducted with two methods, either through-plane conductivity or inplane conductivity. We followed in-plane conductivity measurement for the prepared membranes. The proton conductivity of membranes were determined by four-point probe method using the in-plane membrane conductivity test system (BT-512 Model, BekkTech LLC, Loveland, CO, USA). The sample membrane (approximately 4×30 mm) was connected with four probes: two outside platinum wires to apply the current and two inside platinum wires as reference electrodes. Voltage–current values were set and measured by using the Keithley 2400 Sourcemeter. Sample was tested under different RH between 30 and 100 % at a temperature of 80 °C. Nitrogen gas was passed through the conductivity cell to obtain the desired RH. Fourelectrode conductivity (σ) was determined from measured membrane resistance (R) by using the following equation: L σ¼ RW T

ð1Þ

Ion exchange capacity (IEC) of polymer ionomers was determined by titration method [3]. Dried sulfonated polymer (0.1 to 0.2 g) was dissolved in 50 mL toluene/methanol mixture (90/10 v/v) and titrated with a 0.1-N standardized NaOH solution diluted five times with absolute methanol to a phenolphthalein endpoint. Dried membrane (0.1–0.2 g) was immersed into the 50 mL of saturated NaCl solution at 50 °C for 48 h to replace the sulfonic acid to the sodium salt form. The H+ released from the membrane was titrated with a 0.1-N NaOH solution using bromothymol blue as an indicator. IEC values for both sulfonated polymers (Ep) and membranes (Em) were calculated by Eq. 2 [35]: IEC ¼

VNaOH MNaOH WP

ð2Þ

where VNaOH is consumed NaOH volume (in liters) on titration, MNaOH is the molarity of NaOH (0.1 M), and WP is the dried sulfonated polymer weight (in grams).


Water uptake WU values of the membranes were determined according to ASTM D 570-98 [29]. Membranes were conditioned in an oven first at 105 °C for 1 h, followed by 50 °C for 24 h. The membranes were cooled to room temperature in a desiccator. Upon removal from the desiccator, the membranes were immediately weighed and then were soaked into deionized water at room temperature and kept for 24 h. Then, the membranes were removed from water, wiped free of the water with a tissue paper, and then weighed again immediately (Wwet). The WU (in percent) value was calculated by using Eq. 3 as follows: WUð%Þ100 Wwet Wdry Wdry ð3Þ where Wdry and Wwet are the weights of dry and wet membranes, respectively. Oxidative stability The oxidative stability of the membrane was tested by soaking the film in Fenton’s reagent (3 % H2O2 aqueous solution containing 4 ppm of Fe2+) (e.g., FeSO4) at 68 °C. Membrane specimens were prepared in 2×2 cm dimensions for WU and Fenton tests. All samples were dried in vacuum oven at 60 °C for 12 h before testing. Fenton test was carried out according to the procedure described by Zhang and Mukerjee [30]. The oxidative stability was evaluated by determination of the weight loss of the membrane after 2 h.

For the doping operation, the functionality of the membranes strongly depends on their acid-absorbing ability [21]. The pores formed in the membrane had been entrapped by the acid molecules during the doping process to achieve proton transport. The IR spectra of methanol- or watercoagulated PVdF-HFP membranes and doped in acidic solutions of H2SO4 and H3PO4 are compared in Fig. 2c, d (24MPO, 24MSO, 1/4WPO, and 24WPO). The doped membranes show the vibrational modes of H2PO4−, HPO42−, and PO 4 3− units. The peaks found at 1,073 and 1,186 cm−1 can also be attributed to the P–O and P0O stretching, respectively. The peaks that appeared at 1,073, 881, and 762 cm−1 confirm the presence of PO43−, HPO42−, and H2PO4− anions. The peaks at 570 and 1,073 cm−1 are attributed also as S–O bonds and S0O stretching, respectively. These and the SEM-EDS findings indicate that the acid molecules had been entrapped within the membrane successfully. A broad peak appearing in the region of 3,500–2,850 cm−1 probably indicates O–H stretching. The hydrogen bond seems to play an important role in the proton conductivity system. The peak found at 1,630 cm−1 depicts the absorbed water molecule (Fig. 2c). The 24-h coagulation with either methanol or water gives clearly visible sharper peaks than those of membrane coagulated for 1/4 h. The 24-h coagulation with methanol and subsequent doping with acidic solutions of H2SO4 and H3PO4 give different morphologies and entrapped acid contents that highly depend on the observed morphology. FTIR results clearly depend on the entrapped amounts of the acids. For 24MPO and 24MSO, there is a small difference in the region of 600–800 cm−1, as seen in Fig. 2d.

Results and discussion

Thermal characterizations by TGA and DSC studies

FTIR analysis

Thermogravimetric analysis (TGA) curves of PVdF-HFPbased membranes are given in Fig. 3. Following the preheating stage (100 °C) to remove moisture, the doped polymer samples were heated up to 900 °C at a heating rate of 10 °Cmin−1 under N2. Methanol-coagulated samples (MCS) doped with H3PO4 (24MPO) lead to an early thermal decomposition of the membranes at about 400 °C and also higher residue contents than the 24-h-conditioned and doped with H2SO4 (24MSO), undoped (24M), and original PVdFHFP membranes (Fig. 3a). Water-coagulated samples doped with H3PO4 yielded lower residue upon TGA (Fig. 3b). However, 1/4-h-conditioned water-coagulated and H2SO4doped (WSO) sample showed the highest residue contents than 24WPO, 24WSO, and 24MSO, indicating the highest amount of acid entrapment in the membrane because of morphology differences (Fig. 3b, c). The 24-h-conditioned, methanol- or water-coagulated (24MSO and 24WSO) and H2SO4-doped samples yielded lower residue upon TGA at higher temperatures than PVdF-HFP (Fig. 3c).

The FTIR spectra of the α and β phases of PVDF have been investigated extensively. It is known that the vibrational bands at 531,766, and 976 cm−1 referred to the α phase of PVdF-HFP, while the bands at 489 and 842 cm−1 referred to the β and γ phases [31] (Table 2). FTIR analysis of membranes coagulated with methanol (1/4M, 24M) suggests that the exposure time has effects on the type of crystallites formed in the PVdF-HFP membranes (Fig. 2a). In the spectra of membrane exposed to air and coagulated with methanol for 24 h (24M), all of characteristic peaks (482, 675, 1,007, 1,073, and 1,187 cm−1) are clearly visible and they appear sharper than those of membrane coagulated with methanol for 1/4 h. In the spectra of the 24-h waterconditioned (24W) membrane, all of the characteristic peaks (510, 675, 1,007, 1,035, 1,187, and 1,463 cm−1) are visible and they appear sharper than the 1/4-h water-conditioned (1/ 4W) membrane (Fig. 2b).

Author's personal copy Ionics Table 2 SEM-EDS results of PVdF-HFP-based membranes

Element

Weight %

1/4WPO- SURFACE C O F P

10.4 31.4 43.2 15.0

C O F S

33.1 26.1 19.4 21.4

O F Na S K Ca

25.2 28.8 9.8 19.2 10.7 6.3

1/4MCSEBS

C O F S K Ca

65.0 20.0 8.0 4.1 0.9 2.0

PVDF-HFP/CSEBS

C O F Al S Cl K Ca

59.9 10.1 18.6 2.3 6.6 1.2 0.8 0.5

1/4WSO-SURFACE

1/4WCSEBS

The final melting point of the PVdF-HFP polymer was found to be 146 °C from DSC analysis, as given in Fig. 4a. The thermal behavior of WCS and MCS coated with sulfonated PSEBS after doping with H2SO4 or H3PO4 were analyzed by using DSC. Similar to the findings of Kumar [7], a broad melting peak was obtained between 120 and 160 °C at the end of the second run for all membrane

samples (Fig. 4b, c). Similar values were also obtained for the PSEBS-coated samples, as given in Fig. 4c, and the first and second run results of the methanol-treated, sulfonated PSEBS-coated membrane is given in Fig. 4d. During the first run, a broad peak appearing at 100 °C is interpreted as an indication of absorbed water in the membrane; this peak disappeared in the second run of the thermogram.


Fig. 2 FTIR spectra of a PVdF-HFP membranes prepared at different exposure times and coagulated with methanol (1/4M and 24M), b PVdF-HFP membranes prepared at different exposure times and coagulated with water (1/4W and 24W), c PVdF-HFP membranes prepared

at 24 h exposure time and coagulated with methanol and doped with H3PO4 and H2SO4 (24MPO and 24 MSO), and d PVdF-HFP membranes prepared at different exposure times and coagulated with water and doped with H3PO4 (1/4WPO and 24WPO)

Fig. 3 TGA curves of the PVdF-HFP-based acid-doped membranes. a PVdF-HFP membranes prepared at 24 h exposure time, b PVdF-HFP membranes prepared at 24 h exposure time and coagulated with water,

c PVdF-HFP membranes prepared at 24 h exposure time and coagulated with water and doped with H2SO4, and d 1/4WSO PVdF-HFP membrane treated with water at 80 °C for 24 h


Fig. 4 DSC curves of the PVdF-HFP-based acid-doped membranes. a 24-h air-conditioned samples (24MPO, 24MSO, 24WSO, and 24WPO), b coated samples (1/4WCSEBS, 1/4MCSEBS, 24MCSEBS, and 24WCSEBS), and c 24-h air-conditioned, SEBS-coated PVdF-HFP membrane

SEM micrographs A series of membranes was prepared by casting 20 wt% solution PVdF-HFP in DMAc using different coagulation mediums (water or MeOH). Surface and cross-sectional images for phase inversion membranes are shown in Fig. 5a, b (uncoated samples) and Fig. 5c (coated samples). Depending on the coagulation medium and doping agents, different morphologies were observed. The 24-h-conditioned samples in air show spherical texture (Fig. 5a), while the 1/4-h-conditioned samples in air show sponge-like morphology (Fig. 5b). Membranes coagulated with methanol or water after conditioning in air for 24 h show spherical texture (particles) with pore diameters in the range of 6 and 13 μm; the pore diameters of H3PO4-doped membranes are larger than that of H2SO4doped membranes. Pore diameters are in the range of 5.5– 7 μm for H3PO4-doped membranes and 1.5–2 μm for H2SO4doped membranes (Fig. 5a, b). The thickness of the PSEBS coatings were found to vary between 4.4 and 4.7 μm for PVdF-HFP for 1/4-h-conditioned WCS (1/4WCSEBS) and between 8.7 and 14.8 μm for 1/4-hconditioned MCS (1/4MCSEBS). We could not measure the film thickness reliably for the 24-h air-conditioned samples (24MCSEBS, 24WCSEBS) due probably to the homogeneous

intrusion of the PSEBS solution into the pores of membrane, as depicted in Fig. 5c. Elemental analyses of the membranes doped with H3PO4 or H2SO4 were conducted with EDS in order to verify the presumed presence of mineral acids in pores of the membranes. The analyses for H2SO4-doped and H3PO4-doped samples resulted in the detection of S and P, respectively, as shown in Table 3, verifying entrapment of these acids in the pore structure. In the case of coated membranes, analyses resulted in the detection of S on the film surface, also indicating the sulfonation reaction with chlorosulfonic acid (Table 3). Proton conductivity, ion exchange capacity, and water uptake Proton conductivity (T080 °C, RH0100 %) and WU values of PVdF-HFP-based membranes depending on the coagulation and doping agents are given in Table 1. The IEC values ranged from 0.0 to 1.5 mmol/g; in the highest IEC of 1.5 mmol/g, proton conductivity was 92 mS/cm, which was lower than Nafion 117 (133 mS/cm) at 80 °C and RH of 100 %. Our conductivity measurements ranged from 6.0E − 03 to 1.1E − 01 mS/cm for uncoated samples, obtained with the same conductivity measuring device.


a) 9-PVDFHFP/24M

5-PVDFHFP/24MSO

6-PVDFHFP/24MPO

8-PVDFHFP/24WSO

7-PVDFHFP/24WPO

X1000 CS 14-PVDFHFP/24W

X2000-Surface CS: Cross section

X1000-CS

X1000-CS

b) (CS) 1/4WPO

1/4WPO

1/4WSO

1/4WSO

1/4MSO

1/4MSO

CS: Cross section

Fig. 5 SEM pictures of the PVdF-HFP-based acid-doped membranes. a 24-h air-conditioned samples (24M, 24MSO, 24MPO; 24W, 24WSO, 24WPO), b 1/4-h air-conditioned samples (1/4M, 1/4MSO,

1/4MPO; 1/4W, 1/4WSO, 1/4WPO), and c 24- and1/4-h airconditioned and SEBS-coated PVdF-HFP


c) 24WCSEBS

1/4MCSEBS

1/4WCSEBS

24MCSEBS

PVDF-HFP/CSEBS

PVDF-HFP/CSEBS

CS: CS: Cross section

Fig. 5 (continued)

Methanol (24M)- or water (24W)-coagulated membrane properties, which were undoped with acids, were also given for comparison. The water absorption value of the watercoagulated sample was measured as 1.3 (24W), and the proton conductivity value was measured as 4.20E–02 mS/ cm. For methanol-coagulated and undoped sample (24M), water absorption and proton conductivity values were measured as 2.4 and 6.0E–03 mS/cm, respectively. The higher conductivity value for 24W than 24M could be due to the acidic character of water. Coagulating with methanol and doping with H2SO4 (24MSO) or H3PO4 (24MPO) caused different water absorption and proton conductivity properties. As listed in Table 1, 24MSO sample’s water absorption value (15.3 wt%) is higher than that of 24MPO (4.5 wt%). But contrary to the expectation, proton conductivity values were 1.10E−02 and 2.60E− 02 for 24MSO and 24MPO samples, respectively. A similar trend for water absorption values were obtained for the membranes doped with H3PO4 (24WPO) or H2SO4 (24WSO) for water-coagulated samples (1.2 or 1.8 wt%, respectively). On the other hand, for the H3PO4-doped membrane, 100 times higher conductivity values (1.10E–01 mS/cm) were observed than the H2SO4-doped membrane (3.10E−03 mS/cm) as a result of membrane morphology differences. Membranes produced by coagulating for 1/4 h with methanol and doping with H2SO4 (1/4MSO) have higher water

absorption value (1.7 wt%) than H3PO4-doped (1/4MPO) membrane (0.3 wt%), and both of them did not show any proton-conducting property. Membranes coagulated with water and doped with H2SO4 (1/4WSO) or H3PO4 (1/4WPO) resulted in highest water absorption values (37.8 and 30.6 wt%, respectively), while proton conductivity values were measured as 4.50E − 03 and 0.00E + 00 mS/cm, respectively. The 24MSO membrane has 15.3 wt% WU and 1.10E− 02 mS/cm proton conductivity value, while 1/4MSO has 1.7 wt% WU value and 0.00E+00 mS/cm proton conductivity value. On the other hand, the 24MPO membrane has 4.5 wt% WU and 2.60E−02 mS/cm proton conductivity value, while 1/4MSO has 0.3 wt% WU and 0.00E + 00 mS/cm proton conductivity value. The 24-h airconditioned membranes have higher WU and higher proton conductivity values than the 1/4-h conditioned samples. Proton conductivity values for the 24-h air-conditioned samples were 100 times higher than the 1/4-h conditioned samples. This may be explained by the sponge-like structure of the 1/4-h conditioned samples. Membrane preparation methods were very effective on membrane properties because of the morphology change. Conditioning increased water absorption as well as proton conductivity values. The 24WSO membrane has 1.8 wt% WU and 3.10E− 03 mS/cm proton conductivity value, while 1/4WSO has

Author's personal copy Ionics Table 3 Characteristic FTIR bands Membranes

α (cm−1)

β (cm−1)

γ (cm−1)

Amorphous (cm−1)

Others

PVdF-HFP

531/766/976

489

842

883

24MSO

482

840

881

24MPO

482

840

881

613/C-F wagging 1,085/assym.str CF2vib. 1,190/sym.str CF2vib. 1,403/deformed CH2 vib. 570/S–O bonds 1,073/S0O str vib.3,500–2,850/O–H stretching 1,630/absorbed water 1,073/P–O str vib.

24WPO

675/1,007

482

24M

675/1,007

482

1/4WPO

675/1,007

482

841

882

24W

675/1,007

483

840

882

1/4M

675/1,007

488

1/4W

675/1,007

840

882

37.8 wt% WU value and 4.500E−3 mS/cm proton conductivity value. On the other hand, the 24WPO membrane has 1.2 wt% WU and 1.10E−01 mS/cm proton conductivity value, while 1/4WPO has 30.6 wt% WU and 0.00E+00 mS/ cm proton conductivity value. The H3PO4-doped sample has higher proton conductivity value (24WPO) than the H2SO4doped sample (24WSO) for the 24-h air-conditioned samples. The H3PO4-doped sample has 100 times higher proton conductivity value (1/4WPO) than the H2SO4-doped sample (1/ 4WSO) for the 1/4-h air-conditioned samples. This difference could be explained by the different aspects of the morphology of the membranes, as seen in Fig. 5. The H3PO4-doped sample has a bigger pore diameter than the H2SO4-doped sample (1/4WPO has 10–30 μ H3PO4 and 1/4WSO has15μ H2SO4). Proton conductivity values of the PSEBS-coated and sulfonated membranes were changed between 0.25 and 92.1 mS/cm.

1,186/P0O str vib.3,500–2,850/O–H stretching 1,630/absorbed water 1,073/assym.str CF2vib. 1,187/sym.str CF2vib. 1,406/deformed CH2 vib. 1,073/assym.str CF2vib. 1,187/sym.str CF2vib. 1,633/absorbed water 1,073/assym.str CF2vib. 1,187/sym.str CF2vib. 1,035/assym.str CF2vib. 1,073/assym.str CF2vib. 1,187/sym.str CF2vib. 1,628/absorbed water 1,406/deformed CH2 vib. 1,406/deformed CH2 vib. 1,073/assym.str CF2vib. 1,191/sym.str CF2vib. 1,630/absorbed water 1,035/assym.str CF2vib. 1,180/sym.str CF2vib. 1,633/absorbed water 1,409/deformed CH2 vib.

The highest value was obtained for the 24-h air-conditioned, methanol-coagulated, PSEBS-coated, and sulfonated membrane (24MCSEBS). The proton conductivity value for the unconditioned coated, sulfonated membrane of CSEBS was obtained as 12.2 mS/cm. Coagulation with methanol or water changed membrane morphology and affected the proton conductivity value of the membranes. Chemical stability Chemical stability of the PVdF-HFP-based membranes are given in Table 1. Chemical stability of PEM is one of the most important factors that affect membrane durability. It is known that the formation of HO· and HOO· radicals by the catalytic process during polymer electrolyte membrane fuel cell operation [32, 33] induces membrane degradation.


The obtained weight loss value of the PVdF-HFP membranes after the Fenton test (6 h) was about 0.2 %, and the values changed between 0.8 and 0.0 %, depending on the membrane preparation conditions. Air-conditioned samples’ weight loss values changed between 0.0 and 0.1 wt%, and for unconditioned samples, values changed between 0.0 and 0.8 wt%, indicating the increase of oxidative stability of all membranes. There was no weight loss for the Nafion membrane after testing for a 1-week period. On the other hand, it has been also reported by Zhang and Li that the fuel cell lifetime of membranes based on PBI was over 5,000 h, whereas the average degradation time of these membranes determined by Fenton test was only 30 min [4]. Since the Fenton test exposes the membrane to an unrealistic amount of radicals, it acts only as a fast preview test, and therefore, a long-term durability test in fuel cell must be also done substantially for the determination of the chemical stability of the membrane. Consequently, PVdF-HFP membranes are chemically stable. The 6-h Fenton test results of the PSEBS-coated and sulfonated membranes were changed between 2.2 and 9.3 wt%. On the other hand, coating with PSEBS the decreased chemical stability of all membranes. Hydrolytic stability The hydrolytic stability of the membranes coagulated with water and MeOH, coated with SEBS, and sulfonated with chlorosulfonic acid was investigated at 80 °C for 24 h in water. In order to observe the amount of phosphoric acid and sulfuric acid extracted into the water phase, phosphor (P) and sulfur (S) analysis was performed in the water phase by using ICP, and a maximum of 0.04 % of S and 0.0056 % of P were observed in the water phase of 1/4MPO and 24WSO. respectively. The original phosphorous and sulfur contents were between 15 and 21 %, respectively, as seen from Table 3, proving the chemical interaction of phosphoric acid and sulfuric acid with the polymer. The TGA result of the extracted and unextracted 1/4WSO membrane also supports this result because there is no significant change between the two thermograms for the two membranes, as given in Fig. 3d. On the other hand, the prepared membranes by phase inversion technique were not ruptured after 24 h of treatment with water at 80 °C. This means that all membranes were also hydrolytically stable membranes.

Conclusions PVdF-HFP membranes were prepared by using a solution with 20 % w/v solid concentrations in DMAc by means of phase inversion technique using two different coagulants (water or methanol) and doped with an aqueous highly

concentrated acid solution of H3PO4 and H2SO4. Coagulated PVdF-HFP membranes were also coated with PSEBS and sulfonated with chlorosulfonic acid. The effects of the type of coagulant, coagulation time, doping agents, coating, and sulfonation on the membrane properties were determined. The proton conductivity value of the PSEBScoated and sulfonated membrane was increased from 6.0E −03 to 92.1 mS/cm. Doping with H3PO4 and H2SO4, coating with SEBS, and sulfonation with chlorosulfonic acid apparently increased the conductivity value of the membrane. WU values changed between 0 and 38 % for uncoated samples and between 11.5 and 65.2 wt% for coated samples. With this coating procedure, cheap, time-saving, chemically stable, and highly proton-conductive membranes could be obtained for fuel cell applications. SSEBS has a large aromatic group in the polymer backbone. It makes the SSEBS domain of the SSEBS could have higher WU level than Nafion, as could be seen from Table 1. Sulfonation of SEBS also causes disordered morphology, resulting to the increase in gas permeability, as proposed in the literature, but it could be possible to decrease gas and also methanol permeabilities with the use of montmorillonite [34, 35]. The development of a new PEM with low gas and methanol absorption values is under investigation by our group. Conditioning, type of coagulant, doping agents, coating, and sulfonation are very effective on the membrane properties for fuel cell applications. As a result, it is possible to produce a large number of membranes with different properties depending on the membrane preparation conditions. It is possible to obtain chemically stable, low-cost, and highly proton-conducting membrane using PVdF-HFP polymer by using a suitable membrane preparation method. Acknowledgments This work is financially supported by Türk Demir Döküm Fab. A.Ş. The authors thank Nevin Bekir, Zekayi Korlu, and Mustafa Candemir for their valuable technical assistance in the laboratory. Operation of SEM was possible with help of Orhan İpek and Cem Berk.

References 1. Ren S, Sun G, Li C, Wu Z, Jin W, Chen W, Xin Q, Yang X (2006) Sulphonated poly(ether ether ketone)/polyvinylidene fluoride polymer blends for DMFCs. Mater Lett 60:44 2. Stephan AM, Teeters D (2003) Characterization of PVdF-HFP polymer membranes prepared by phase inversion techniques I. Morphology and charge-discharge studies. Electrochim Acta 48:2143 3. Kim M, Park NG, Ryu KS, Chang SH (2006) Physical and electrochemical characterizations of poly(vinylidene fluoride-cohexafluoropropylene)/SiO2-based polymer electrolytes prepared by the phase inversion technique. J of Applied Polymer Sci 102:140 4. İnan TY, Dogan H, Unveren EE, Eker E (2010) Sulfonated PEEK and fluorinated polymer based blends for fuel cell applications:


5.

6.

7.

8.

9.

10.

11.

12.

13. 14.

15.

16.

17.

18.

investigation of the effect of type and molecular weight of the fluorinated polymers on the membrane’s properties. Int J Hydrog Energy 35:12038 Quartarone E, Carollo A, Tomasi C, Belotti F, Grandi S, Mustarelli P, Magistris A (2007) Relationship between microstructure and transport properties of proton- conducting porous PVDF membranes. J Power Sources 168:126 Weihua P, Xiangming H, Wang L, Jiang C, Wan C (2006) Preparation of PVDF-HFP microporous membrane for Li-ion batteries by phase inversion. J Membr Sci 272(1-2):11 Herrring AM (2006) Inorganic–polymer composite membranes for proton exchange membrane fuel cells. J Macromol Sci C Polym Rev 46:245 Ronghuan H, Quantong C, Baoying S (2008) The acid doping behavior of polybenzimidazole membranes in phosphoric acid for proton exchange membrane fuel cells. Fibers Polym 9(6):679 Oono Y, Sounai A, Hori M (2009) Influence of the phosphoric acid-doping level in a polybenzimidazole membrane on the cell performance of high-temperature proton exchange membrane fuel cells. J Power Sources 189:943 Sezgin A, Akbey Ü, Graf R, Bozkurt A, Baykal A (2009) Proton conductivity survey of the acid doped copolymers based on 4vinylbenzylboronic acid and 4(5)-vinylimidazole. J Polym Sci Part B: Polym Phys 47:1267 Asensio JA, Borros S, Gomez-Romero P (2004) Protonconducting membranes based on poly(2,5-benzimidazole) (ABPBI) and phosphoric acid prepared by direct acid casting. J Membrane Sci 241(1):89–93 Pu HT, Qiao L, Liu QZ (2005) A new anhydrous proton conducting material based on phosphoric acid doped polyimide. Eur Polym J 41(10):2505 Pu HT, Wang D (2006) Studies on proton conductivity of polyimide/H3PO4/imidazole blends. Electrochim Acta 51(26):5612 Fu YZ, Manthiram A (2007) Nafion–imidazole–H3PO4 composite membranes for proton exchange membrane fuel cells. J Electrochem Soc 154(1):B8 Pu HT, Liu QZ, Qiao L (2005) Studies on proton conductivity of acid doped polybenzimidazole/polyimide and polybenzimidazole/ polyvinyl pyrrolidone blends. Polym Eng Sci 45(10):1395 Daletou MK, Gourdoupi N, Kallitsis JK (2005) Proton conducting membranes based on blends of PBI with aromatic polyethers containing pyridine units. J Membrane Sci 252(1–2):115 Zukowska G, Chojnacka N, Wieczorek W (2000) Effect of gel composition on the conductivity of proton-conducting gel polymericelectrolytes doped with H3PO4. Chem Mater 12(12)):3578 Savadogo O, Xing B (2000) Hydrogen/oxygen polymer electrolyte membrane fuel cell (PEMFC) based on acid-doped polybenzimidazole (PBI). J New Mater Electrochem Syst 3(4):343

19. Xiao L, Zhang H, Jan T (2005) Synthesis and characterization of pyridine-based polybenzimidazoles for high temperature polymer electrolyte membrane fuel cell applications. Fuel Cells 5(2):287 20. Schmidt C, Schmidt-Naake G (2007) Proton conducting membranes obtained by doping radiation-grafted basic membrane matrices with phosphoric acid. Macromol Mater Eng 292(10-11):1164 21. Kumar GG, Kim P, Nahm K, Elizabeth RN (2007) Structural characterization of PVdF-HFP/PEG/Al2O3 proton conducting membranes for fuel cells. J Membr Sci 303(1+2):126 22. Soresi B, Quartarone E, Mustarelli P, Magistris A, Chiodelli G (2004) PVDF and P(VDF-HFP)-based proton exchange membranes. Solid State Ionics 166(3–4):383–389 23. Navarra MA, Materazzi S, Panero S, Scrosati B (2003) PVdFbased membranes for DMFC applications. Electrochem Soc Electrochem Soc 150(11):A1528 24. Sangeetha D (2005) Conductivity and solvent uptake of proton exchange membrane based on polystyrene(ethylene–butylene) polystyrene triblock polymer. Eur Polym J 41:2644 25. Li M, Jiang M, Zhu L, Wu C (1997) Novel surfactant-free stable colloidal nanoparticles made of randomly carboxylated polystyrene ionomers. Macromolecules 30:2201 26. Ehrenberg SG, Serpico JM, Wnek GK, Rider JN (1997) US Patent 5,679,482 27. Ehrenberg SG, Serpico JM, Wnek GK, Rider JN (1995) US Pat 5:468,574 28. Mokrini A, Huneault MA (2006) Proton exchange membranes based on PVDF/SEBS blends. J Power Sources 154:51–58 29. Standard test method for water absorption of plastics, ASTM D 570-98 (reapproved 2005) 30. Zhang L, Mukerjee S (2006) Investigation of durability issues of selected nonfluorinated proton exchange membranes for fuel cell application. J Electrochem Soc 153:A1062 31. Kumar GG, Nahm KS, Elizabeth RN (2008) Electro chemical properties of porous PVDF-HFP membranes prepared with different nonsolvents. J Membr Sci 325:117 32. Lu C, Rice C, Masel RI, Babu PK, Waszczuk P, Kim HS, Oldfiled E, Wieckowski AUHV (2002) Electrochemical NMR, and electrochemical studies of platinum/ruthenium fuel cell catalysts. J Phys Chem B 106:9581 33. Panchenko A, Dilger H, Kerres J, Hein M, Ullrich A, Kaz T, Roduner E (2004) In-situ spin trap electron paramagnetic resonance study of fuel cell processes. Phys Chem Chem Phys 6:2891 34. Hwang HY, Koh HC, Rhim JW, Nam SY (2008) Preparation of sulfonated SEBS block copolymer membranes and their permeation properties. Desalination 233:173 35. Dogan H, İnan TY, Koral M, Kaya M (2011) Organo-montmorillonites and sulfonated PEEK nanocomposite membranes for fuel cell applications. Appl Clay Sci 52:285