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Proton conducting hybrid membrane electrolytes of sulfonated poly(ether sulfone)s and poly(ether sulfone)s containing metallophthalocyanine
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 3 ) 1 e1 0
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Proton conducting hybrid membrane electrolytes of sulfonated poly(ether sulfone)s and poly(ether sulfone)s containing metallophthalocyanine Youngdon Lim a, Soonho Lee a, Dongwan Seo a, Hohyoun Jang a, Md Awlad Hossain a, Hyunchul Ju b, Taewhan Hong c, Whangi Kim a,* a
Department of Applied Chemistry, Konkuk Univ., Chungju 380-701, Republic of Korea Schlool of Mechanical Engineering, Inha Univ., 253 Yonghyun-dong, Nam-gu, Incheon 402-751, Republic of Korea c Department of Materials Science and Engineering, Korea National Univ. of Transportation, Chungju 380-702, Republic of Korea b
article info
abstract
Article history:
A role of metallophthalocyanine (MPc) as an anti-oxidizing agent of polymer membrane
Received 30 January 2013
and an accelerating agent of proton conductivity was discussed. The poly(ether sulfone)s
Received in revised form
bearing MPc (Ni, Co and Fe), PMPc were prepared by two-step reaction from phenol-
10 April 2013
phthalein and fumaronitrile and followed reaction with metal (II) chloride (Ni, Co and Fe)
Accepted 12 April 2013
and 1,2-dicyanobenzene in quinoline. The sulfonated polymer was synthesized by
Available online xxx
condensation polymerization using 1,2-bis(4-hydroxyphenyl)-1,2-diphenyl ethylene, bis(4fluorophenyl) sulfone and followed by sulfonation reaction with concentrated sulfuric acid.
Keywords:
A series of hybrid membranes (HeNi, HeCo and HeFe) were prepared from a mixture of the
PEMFC
sulfonated copolymer and PMPcs in dimethylacetamide (DMAc). The structural properties
Fuel cell
of the synthesized polymers were studied by 1H-NMR spectroscopy and FT-IR. The mem-
Membrane
brane properties were investigated by measurements of ion exchange capacity (IEC), water
Metallophthalocyanine
uptake, and proton conductivity, chemical degradation test, and atomic force microscopy
Surface morphology
(AFM) analysis. The cell performance of the membranes was compared with those of normal sulfonated poly(ether sulfone)s and Nafion. Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Polymer electrolyte membrane fuel cells (PEMFCs), which convert chemical energy to electrical energy, are regarded as promising power sources owing to their advantages, such as high efficiency, high energy density, quiet operation, and environmental friendliness [1e3]. For decades, the perfluorinated sulfuric acid membranes (Nafion and Acoplex) have been used because of their excellent chemical, physical,
and electrical properties. However Nafion is not suitable in use above 80 C. Recently, many research groups have been attempting to overcome the drawbacks of Nafion membranes and developing alternative proton-conducting membranes based on sulfonated aromatic hydrocarbon polymers [4e7]. Especially, thermally stable aromatic hydrocarbon polymers are emerging as effective membranes to be used at elevated temperatures of greater than 80 C [8e14]; performances of proton conductivity and power density are closely to
* Corresponding author. Tel.: þ82 438403579. E-mail address: [email protected] (W. Kim). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.04.066
Please cite this article in press as: Lim Y, et al., Proton conducting hybrid membrane electrolytes of sulfonated poly(ether sulfone)s and poly(ether sulfone)s containing metallophthalocyanine, International Journal of Hydrogen Energy (2013), http:// dx.doi.org/10.1016/j.ijhydene.2013.04.066
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perfluorinated membranes, and cost is lower than that. Their durability is still unapproachable to these fluorinated materials. The chemical stability of the membrane is directly related to membrane’s durability under fuel cell operating conditions. It is widely accepted that, in a working PEMFCs, the most likely initiators of membrane chemical decomposition are hydroxy (HO) and hydroperoxy (HOO) radicals generated at the membraneeelectrode interface. Both HO and HOO are very reactive with polymers and causes for their degradation [15,16]. It has been studied that the presence of O2 in the electrolyte system influences both oxygen reduction reaction and the electrochemical and spectral behaviors of the complexes, which indicate electrocatalytic activity of the complexes for the oxygen reduction reaction. Cobalt phthalocyanines (CoPcs) have been known for a long time as electrocatalysts for the oxygen reduction reactions (ORR). The electrocatalytic mechanisms of MPcs for ORR in aqueous solution are well known [17e20]. These processes are accompanied by intramolecular electron transfer from the metal center to the oxygen molecule. Thus, the redox potential of the metal in the phthalocyanine is an important parameter when comparing the electrocatalytic activities of metal macrocyclic complexes for O2 reduction. Randin [21] and Beck [22] were the first to propose the concept of redox catalysis for the reduction of O2 on MPc. According to these authors, when O2 adsorbs on the metal ion in the phthalocyanine, the metal center is oxidized, thereby partially reducing the O2 molecule to the superoxide stage [23]. And also, it has been reported that MPcs act as catalysts for solution phase electrochemical oxidation and reduction of H2O2. Bohrer reported that CoPc responds with current losses to each dose of H2O2, with or without constant humidity, in both the kinetic and saturation regimes [20]. We are interested on MPcs as redox catalyst of oxygen and H2O2 molecules; affecting the durability of polymer membrane. This work is an attempt to synthesize poly(ethersulfone) copolymers having MPcs (Ni, Co and Fe) and normal sulfonated copolymers. We prepared hybrid membranes from a mixture of sulfonated copolymer and PMPc using different wt % ratios. The oxidative stability of the resultant hybrid membranes were investigated with the varying content of PMPc (Ni, Co, Fe). The synthesized polymers and hybrid membranes were characterized by FT-IR, 1H NMR spectroscopy, and thermogravimetric analysis (TGA). Their water uptake, IEC, chemical stability, and proton conductivity were also measured.
2.
Experimental
2.1.
Materials
Diphenylmethane, 2.5 M solution of n-butyl lithium in hexane, 4,40 -dimethoxybenzophenone, p-toluenesulfonic acid, boron tribromide, bis(4-hydroxyphenyl) sulfone, bis(4-fluorophenyl) sulfone, phenolphthalein, zinc powder, fumaronitrile, acetic acid, 4,40 -biphenol, 1,2-dicyanobenzene, nickel(II)chloride, cobalt(II)chloride, iron(II)chloride, quinoline and concentrated sulfuric acid were purchased from TCI and Aldrich Chemical and used as received. Commercial grade sulfolane and
chlorobenzene were dried over calcium hydride and distilled prior to use. Other commercially available solvents - DMSO, tetrahydrofuran, dichloromethane, methanol, acetone, toluene, ethyl acetate, ethanol, and distilled water were used without further purification.
2.2. Synthesis of 4,40 -(2,2-diphenylethenylidene) diphenol, (DHTPE) To a solution of diphenylmethane (10.1 g, 60.0 mmol) in dry tetrahydrofuran (80.0 mL) was added 24.0 mL of 2.5 M solution of n-butyl lithium in hexane (60.0 mmol) at 0 C under nitrogen. The resulting orange-red solution was stirred for 30 min at that temperature. 4,40 -Dimethoxybenzophenone (10.9 g, 45.0 mmol) was then added and the reaction mixture was warmed to room temperature over 6 h while being stirred. The reaction was quenched with aqueous ammonium chloride and the organic layer was extracted with dichloromethane. The combined organic portion was washed with saturated brine and dried over anhydrous magnesium sulfate. The solvent was evaporated and the resulting crude alcohol (containing excess diphenylmethane) was subjected to acidcatalyzed dehydration as follows. The crude alcohol was dissolved in about 300.0 mL toluene in a 500 mL round-bottomed flask fitted with a DeaneStark trap. A catalytic amount of ptoluenesulfonic acid (1.71 g, 9.0 mmol) was added and the mixture was refluxed for 3e4 h and cooled to room temperature. The toluene layer was washed with 10% aqueous sodium bicarbonate, dried over anhydrous magnesium sulfate and evaporated to afford the crude dimethoxytetraphenyl ethylene derivative. The crude product was purified by simple recrystallization from a mixture of dichloromethane and methanol [24]. To dimethoxytetraphenyl ethylene (10.0 g, 25.5 mmol) in dichloromethane (150.0 mL) was added slowly 58.0 mL 1.0 M boron tribromide (60.0 mmol) in dichloromethane at 0 C under nitrogen. The reaction mixture was warmed to room temperature over 24 h while being stirred. The reaction was quenched with water, the organic layer was extracted with ethyl acetate and 1.0 M HCl. The combined organic portion was washed with saturated brine, dried over anhydrous MgSO4 and the solvent was evaporated. The product was purified by simple recrystallization from a mixture of acetonitrile and distilled water.
2.3. Synthesis of sulfonated poly(ether sulfone)s containing 40 mol% DHTPE (SPDHTPE 40) To a 100 mL three neck round-bottomed flask, fitted with DeaneStark trap, condenser, nitrogen inlet/outlet, and magnetic stirrer - monomer DHTPE (0.57 g, 1.57 mmol), bis(4fluorophenyl) sulfone (1.0 g, 3.93 mmol), bis(4-hydroxyphenyl) sulfone (0.59 g, 2.36 mmol), potassium carbonate (0.65 g, 4.72 mmol), DMAc (10 mL), and toluene (10 mL) were charged. The mixture was refluxed for 3 h at 130 C, dehydrated and toluene was slowly removed. It was gradually heated to 160e180 C under continued stirring for about 1e2 h until a highly viscous solution was obtained. The resulting mixture was cooled to room temperature and decanted into a mixture of methanol (100 mL)/water (100 mL)/HCl (10 mL) to give a white fibrous polymer. The polymer was collected by filtration
Please cite this article in press as: Lim Y, et al., Proton conducting hybrid membrane electrolytes of sulfonated poly(ether sulfone)s and poly(ether sulfone)s containing metallophthalocyanine, International Journal of Hydrogen Energy (2013), http:// dx.doi.org/10.1016/j.ijhydene.2013.04.066
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and repeatedly washed with water. And the polymer (PDHTPE) was dried in a vacuum oven at 80 C for 24 h. To a 50 mL roundbottomed flask, 0.5 g of PDHTPE copolymer and 6 mL of concentrated sulfuric acid (95 w 98%) were added, respectively and the mixture was stirred at 45 C for 12 h. The mixture was then decanted into cold water. Then, the sulfonated polymer (SPDHTPE 40) was thoroughly washed 3 times with distilled water and dried in a vacuum oven at 80 C for 24 h.
to 220e240 C for 4e6 h under nitrogen and turned into blue black and then decanted into a mixture of methanol and some drops of hydrochloric acid (36%) with vigorous stirring. The precipitate of deep green to blue particles were washed with acetone, water, dilute hydrochloric acid, and finally by water and ethanol. It was collected by filtration and extracted with chloroform. The chloroform solution was concentrated and precipitated into methanol to give PMPc.
2.4. Synthesis of PHPDCP and poly(ethersulfone) containing metallophthalocyanine (PMPc)
2.6.
The following represents the procedure for the synthesis of 1,4-bis(4-hydroxyphenyl)-2,3-dicyanonaphthalene (HPDCP) monomer. To a dry 25 mL round-bottomed flask equipped with a mechanical stirrer, 2 g (6.24 mmol) of dry & finely powdered reduced phenolphthalein was added. The flask was immersed in an ice-acetone bath for 10 min and then concentrated sulfuric acid (6.5 mL, equilibrated to 0 C) was added. The mixture was vigorously stirred for 2e3 min for dissolving the solid materials. The mixture was then stirred for another 7e8 min at this temperature. The resulting yellowbrown slurry (isobenzofuran) was quickly poured into icewater (120 mL). The solids were separated by filtration and washed with cold water (0 C). The product was dried for 5e15 min before being used directly in the following DielseAlder reaction. To a 50 mL round-bottomed flask, 2.0 g (20.41 mmol) of fumaronitrile in 13 mL acetic acid and isobenzofuran were added. The mixture was quickly heated to the reflux temperature. After 15e20 min, a yellow product e 1,4-bis-(4-hydroxyphenyl)-naphtha-2,3-dicarbonitrilen was precipitated in the reaction mixture. The reaction was continued at this temperature for another 2 h to complete. The product was isolated by filtration and washed twice with a minimum amount of acetic acid.
2.5. Synthesis of poly(ethersulfone)s containing metallophthalocyanine (PMPc) Polymers containing metallophthalocyanine was synthesized as follows: To a 100 mL three neck round bottomed flask, fitted with DeaneStark trap, condenser, nitrogen inlet/outlet, and magnetic stirrer - monomer HPDCP (0.57 g, 1.57 mmol), 4,40 biphenol (0.44 g, 2.36 mmol), bis(4-fluorophenyl) sulfone (1.0 g, 3.93 mmol), potassium carbonate (0.65 g, 4.72 mmol), DMAc (10 mL), and toluene (10 mL) were charged. The mixture was refluxed for 3 h at 130 C, dehydrated and toluene was slowly removed. It was gradually heated to 160e180 C under continued stirring for about 1e2 h until a highly viscous solution was obtained. The reaction mixture was diluted with DMAc, and then the viscous solution was slowly decanted into methanol with a small amount hydrochloric acid and stirred vigorously. The copolymer was precipitated out as a lengthy fiber. The thus-obtained materials were dissolved in chloroform and filtered through a thin layer of celite. The filtrate was slowly poured into methanol with stirring, and then the resulting polymer (PHPDCP) was separated by filtration. To a 100 mL round-bottomed flask, the PHPDCP copolymer (0.5 g), 1,2-dicyanobenzene(0.5 g, 3.9 mmol), metal(II)chloride (0.3 g) and 50 mL of quinoline were added. The mixture was heated
Preparation of hybrid copolymer membranes
Hybrid copolymer membranes were prepared by dissolution of the SPDHTPE 40 with 5, 10 and 15 wt% of PMPc in DMSO, respectively. Hybrid membrane of HeNi 5 is a mixture of 95 wt % of SPDHTPE 40 and 5 wt% of Ni-PMPc; HeNi 10 is 90 wt% of SPDHTPE 40 and 10 wt% of Ni-PMPc. The rest were prepared by changing the wt% of PMPc.
2.7.
Accelerated chemical degradation of membranes
Membrane degradation was assessed by immersion in Fenton reagent (4 ppm Fe2þ, 3% H2O2) at 75 C. Samples were SPDHTPE 40, hybrid membranes, and Nafion 211. The initial weight of the samples were measured and then put into the solution of iron(II)sulfate heptahydrate in Milli-Q water (45 mL) and heated to 75 C and 5 mL of 30% H2O2 was added to this solution. The membrane samples were taken out regularly during the experiment for weighing. Washing the sample in distilled water quenched the degradation reaction and the exposed membranes were dried before the final weight was determined [25].
2.8.
Membrane preparation and characterization
Membranes (25 mm) were prepared by the re-dissolution of the polymer in DMSO to afford 20 wt% transparent solutions and casting at elevating temperatures of 60, 80, 100, and 120 C. The polymer structure was confirmed by Fourier transform infrared (FT-IR) spectroscopy (Nicolet FT-IR spectrometer). 1H NMR spectra were recorded on a Bruker DRX (400 MHz) spectrometer using deuterated dimethyl sulfoxide (DMSO-d6) solvent and tetramethylsilane (TMS) internal standard. Thermogravimetric analysis was performed using a PerkineElmer TGA-7 analyzer. The molecular weight of the polymers was determined relative to polystyrene standards by gel permeation chromatography (GPC) in CHCl3 as the eluent on a Perkin Elmer series 200 high-pressure-liquid chromatographer with a RI detector. For the assessment of water uptake, the membranes were vacuum-dried at 100 C for 24 h, weighed and immersed in deionized water at 30 C and 80 C for 24 h. The wet membranes were wiped to dry and quickly weighed again. The water uptakes of membranes are reported in weight percent as follows: water uptake ¼ {(Wwet Wdry)/Wdry} 100%, where Wwet and Wdry are the weights of the wet and dry membranes, respectively. Membranes’ ion exchange capacities were determined by titration. The membranes in the acid form (Hþ) were converted to the sodium salt form by immersing the membranes in a 1.0 M NaCl solution for 24 h to exchange the Hþ ions with Naþ ions. Then, the exchanged Hþ ions within the solutions
Please cite this article in press as: Lim Y, et al., Proton conducting hybrid membrane electrolytes of sulfonated poly(ether sulfone)s and poly(ether sulfone)s containing metallophthalocyanine, International Journal of Hydrogen Energy (2013), http:// dx.doi.org/10.1016/j.ijhydene.2013.04.066
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BuLi / THF/ 30min
BBr3
4,4-dimethoxybenzophenone 6hrs / rt.
MeO
PTSA / toluene
HO
OMe
OH
DHTPE
OH HO NC H C OH O
H2SO4
Reduced phenolphthalein
HO
OH
O
CN HO
OH NC
Isobenzofuran
CN
HPDCP
Scheme 1 e Synthesis of DHTPE and HPDCP.
were titrated with a 0.02 M NaOH solution. The IEC values were calculated from the degree of sulfonation, obtained from the formula: IEC (meq./g) ¼ Ionic mmol concentration/ Mass of dry membrane at 25 C. Proton conductivity through plane direction of membranes were determined using Scribner membrane test system (MTS-740) with a Newtons 4th Ltd. (N4L) impedance analysis interface (PSM 1735) at 40e80 C under 80% humidity, and at 80 C under 30e90% humidity. EIS was conducted at the open circuit condition by applying a small alternating voltage (10 mV) and varying the frequency from 1 to 1 105 Hz. The film surfaces of the synthesized polymers were observed by atomic force microscope (AFM). PSIA XE100 was applied by non-contact mode with tips P/N 910M-NCHR. Tapping mode atomic force microscopic (AFM) observations were performed with a Digital Instrument - SII-NT SPA400, by using micro fabricated cantilevers with a force constant of w20 N/m.
2.9.
MEA fabrication and testing
Membrane electrode assemblies (MEAs) with an active area of 9 cm2 were prepared using a decal method based on catalyst coated membrane (CCM). A 20 wt% wet-proofed Toray carbon paper (TGPH-060, Toray Inc.) of 190 mm thickness was employed as a gas diffusion layer (GDL) for the anode and cathode sides. Carbon-supported Pt (Hispec 13100, Johnson Matthey Inc.) was used as catalyst for both anode and cathode. The loading of the catalyst layer in this work was 0.2 mg Pt/
cm2 for the anode and cathode. Then, the catalyst layer was transferred on the membrane at 130 C and 10 MPa for 5 min by decal method to produce the CCM. The GDL was placed on both the anode and cathode sides of the CCM to make the MEAs. Cell performance was performed by PEMFC test station (CNL Energy Co.).
3.
Results and discussion
3.1.
Monomer and polymer syntheses
The synthetic method of the monomers, DHTPE and HPDCP, are shown in Scheme 1. DHTPE was synthesized by McMurry carbonecarbon double bond coupling reaction [24] and followed by demethylation using BBr3 with 62% yield. The chemical structure of DHTPE was identified by 1H-NMR shown in Fig. 1(1). The six proton peaks of ortho and para positions (Hc and He) in phenyl rings were observed at 7.02e7.15 ppm and the four proton peaks of meta positions (Hd) in phenyl rings were assigned at 6.94 ppm. The four proton peaks of nearby ethylene (Hb) in phenol group and the four proton peaks of nearby OH (Ha) in phenol group were observed at 6.76 ppm and 6.49 ppm, respectively. And also, OH peaks appeared at 9.35 ppm. And the other monomer, HPDCP was synthesized by DielseAlder reaction with 67% yield [26]. Chemical structure of HPDCP was confirmed by 1HNMR shown in Fig. 1(2). The four proton peaks (Hc and Hd) of
Fig. 1 e 1H-NMR of monomers e DHTPE and HPDCP. Please cite this article in press as: Lim Y, et al., Proton conducting hybrid membrane electrolytes of sulfonated poly(ether sulfone)s and poly(ether sulfone)s containing metallophthalocyanine, International Journal of Hydrogen Energy (2013), http:// dx.doi.org/10.1016/j.ijhydene.2013.04.066
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O S O
HO HO
OH
O S O
F
F
OH DMAC / Toluene K2CO3 160¡É
O
O S O
O
HO3S
O n
O
O S O
O
O S O
m
Sulfonation
SO3H
O
O S O
O S O
O
O n
O S O
m
Scheme 2 e Synthesis of SPDHTPE 40 (n [ 0.4 & m [ 0.6).
naphthalene ring were observed at 7.65e7.93 ppm and the four proton peaks of ortho positions (Ha) in phenol groups were assigned at 6.79 ppm. The four proton peaks of meta positions (Hb) in phenol groups were assigned at 7.11 ppm. And also, OH peaks appeared at 9.71 ppm. The reaction
HO
OH NC
O S O
F
CN
O
O NC
CN
procedures for the preparation of the PDHTPE, SPDHTPE and PMPc are presented in Scheme 2 and Scheme 3. PDHTPE was synthesized by polycondensation polymerization, and the sulfonated polymer (SPDHTPE) was prepared using concentrated sulfuric acid and PDHTPE. PHPDCP was synthesized by
O S O
F
HO
O
OH
O
O S O m
n CN MCl2 CN
O
O N
N
N
O S O
O
O
n
2+ NM N N N N
O S O m
* M = Ni, Co, Fe * n : m = 0.4 : 0.6
Scheme 3 e Synthesis of PHPDCP and PMPcs. Please cite this article in press as: Lim Y, et al., Proton conducting hybrid membrane electrolytes of sulfonated poly(ether sulfone)s and poly(ether sulfone)s containing metallophthalocyanine, International Journal of Hydrogen Energy (2013), http:// dx.doi.org/10.1016/j.ijhydene.2013.04.066
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Fig. 2 e 1H-NMR of the PDHTPE, SPDHTPE, and structure of SPES 40.
poly-condensation polymerization. In addition, a series of PMPcs were synthesized by the reaction of dicyanoarylene containing poly(ether sulfone)s with excess amounts of 1,2dicyanobenzene and the corresponding metal salt in quinoline. The chemical structures of PDHTPE and SPDHTPE 40 were proved by 1H-NMR shown in Fig. 2. The proton peaks of para positions in DHTPE unit of PDHTPE appeared at 7.24 ppm, and disappeared after sulfonation reaction. The new peaks emerged at 7.43 ppm, which are identical to proton peaks of nearby sulfonic acid. The chemical structures of PHPDCP and PMPc were identified by FT-IR and 1H NMR shown in Fig. 3. The spectrum of PHPDCP in Fig. 3(1), stretching peak of eCN observed at 2229 cm-1. From the spectrum of PMPc, it was clear that the peak at 2229 cm-1 is missing indicating the completion reaction at this site. The 1 H NMR spectrum of PMPc is shown in Fig. 3(2). The signals
of protons Ha and Hb appeared at 9.41 and 8.45 ppm, respectively.
3.2.
Properties of polymers and hybrid polymers
Thermooxidative stabilities of SPDHTPE 40 and PMPcs were studied using thermo- gravimetric analysis (TGA) with results shown in Fig. 4. Most of the polymers showed good thermal stability at high temperature under air atmosphere. In SPDHTPE 40, the initial weight loss around 100e250 C was attributed to residual solvent (DMAc) and water release from SPDHTPE 40. The second weight loss over 280 C was assigned to the decomposition of sulfonic acid groups. The third weight loss around 510 C was assigned to the degradation of the polymer main chain. In PMPcs, the initial weight loss around 150e330 C was attributed to residual
Fig. 3 e (1) FTeIR of the PHPDCP and PMPc, (2) 1H-NMR of PMPc. Please cite this article in press as: Lim Y, et al., Proton conducting hybrid membrane electrolytes of sulfonated poly(ether sulfone)s and poly(ether sulfone)s containing metallophthalocyanine, International Journal of Hydrogen Energy (2013), http:// dx.doi.org/10.1016/j.ijhydene.2013.04.066
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 3 ) 1 e1 0
Fig. 4 e Thermogravimetric analysis of membranes.
Fig. 5 e Ion exchange capacities and water uptake of membranes.
solvent (DMAc) and water release from PMPcs, because metal ion becomes hydrated with water. The second weight loss over 520 C was assigned to degradation of the polymer main chain. The molecular weight of the polymers was determined
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by gel permeation chromatography (GPC). The average molecular weight of synthesized copolymers (Mw) and polydispersity (Mw/Mn) were in the range of 68,000 to 83,000 and 2.12e2.43, respectively. The SPDHTPE 40 and PMPcs were found to be soluble in aprotic polar solvents such as DMSO, DMAc, and NMP, etc. A series of hybrid membranes (HeNi 5, 10 and 15) were prepared from mixtures of sulfonated copolymers (95, 90 and 85 wt%) and PMPc-Ni (5, 10 and 15 wt %). The polymer membranes were casted in DMAc solution to form light yellow transparent films. The degree of sulfonation (DS) was expressed in the form of the ion exchange capacity (IEC; meq./g). The introduction of increasing numbers of sulfonic acid groups onto polymers not only makes more hydrophilic nature, but also improves their ionic conductivity. SPDHTPE 40 was selected to obtain a suitable membrane for PEMFCs containing sufficient sulfonic acid groups to provide suitable proton conductivity while maintaining mechanical integrity. As shown in Fig. 5, the IECs of SPDHTPE 40 with HeNi 5, 10 and 15 were 1.40, 1.32, 1.27, and 1.19 meq./g, whereas the IEC of the Nafion 211 was 0.91 meq./g. The IEC of hybrid membranes decreased as PMPc-Ni wt% increased. Fig. 5 shows water uptake of membranes at 80 C. The hybrid membranes shows water uptake around 40% lower than that of 46% for SPDHTPE 40 because of low contents of SPDHTPE 40. Ex-situ hydrogen peroxide tests were carried out for all PMPc membranes, SPDHTPE 40, Nafion 211, and SPES 40 (sulfonated poly(ether sulfone) containing 40 mol% of 4,40 biphenol. The tests were performed in hot Fenton reagent for 5 h as an accelerated chemical degradation assessment of membranes. The remaining weight of the hybrid membranes is presented as a function of time. Losses of weight of HeNi 5, 10 and 15 were shown in Fig. 6(1). HeNi 10 and 15 did not show much differences, whereas HeNi 5 containing low wt% of PMPc showed fast degradation. As a result, we prepared the hybrid membranes with 90 wt% SPDHTPE 40 and 10 wt% PMPc. Fig. 6 showed that afer 5 h, Nafion 211 attained no chemical degradation, and HeNi 10 and HeCo 10 membranes were 6 wt% loss, but HeFe 10 membrane showed 48 wt% loss which is much faster degradation than others. These results explained that PMPc-Co and PMPc-Ni play an important role as anti-oxidizing agents, while PMPc-Fe acts as an accelerator of oxidation. The side chain sulfonated membrane (SPDHTPE
Fig. 6 e Accelerated chemical degradation test in 4 ppm Fe2D Fenton reagent. Please cite this article in press as: Lim Y, et al., Proton conducting hybrid membrane electrolytes of sulfonated poly(ether sulfone)s and poly(ether sulfone)s containing metallophthalocyanine, International Journal of Hydrogen Energy (2013), http:// dx.doi.org/10.1016/j.ijhydene.2013.04.066
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Fig. 7 e Proton conductivities of HeNi 5, 10 and 15, (1): measured at 40e90 C in the 80% RH, (2): measured at 80 C in 30e90% RH.
Fig. 8 e Proton conductivities of membranes, (1): measured at 40e90 C in the 80% RH, (2): measured at 80 C in 30e90% RH.
40) is more chemically stable than main chain sulfonated one (SPES 40). The proton conductivities of hybrid membranes were measured at 40e90 C under 80% RH and at 80 C in the range of 30e90% RH, and shown in Fig. 7. The proton conductivities of HeNi 5, 10 and 15 membranes at different temperatures and relative humidities were similar and the measured values are 72.2, 72.4 and 68.5 mS/cm at 80 C with 90% RH. In these results, HeNi 15 containing high PMPc wt% with low IEC value shows very identical compared to that of HeNi 5. Therefore, PMPc-Ni plays an important role as antioxidant and accelerator of proton conductivity. The proton conductivities of hybrid membranes containing 10 wt% of PMPcs compared with SPDHTPE 40 and Nafion 211 in Fig. 8. The proton conductivities of HeNi 10, HeCo 10 and SPDHTPE 40 were 72.4, 71.6 and 72.3 mS/cm at 80 C with 90% RH when compared with 93.1 mS/cm of Nafion 211. Fig. 9 displays a comparison of measured polarization and power density curves under fully humidified inlet condition (relative humidityanode/relative humiditycathode ¼ 100%/100%). The maximum power densities of HeNi 10, HeCo 10, SPDHTPE 40 and Nafion 211 were roughly 0.475, 0.483, 0.491, and 0.589 W/cm2, respectively. Interestingly, even though the hybrid membranes have low IEC, HeNi 10, HeCo 10 are
comparable power density values to those of SPDHTPE 40 and Nafion. The morphology of polymer surfaces was investigated by using an atomic force microscope (AFM) in Fig. 10. Assessment of the images revealed that the surface pattern
Fig. 9 e Cell performance of membranes.
Please cite this article in press as: Lim Y, et al., Proton conducting hybrid membrane electrolytes of sulfonated poly(ether sulfone)s and poly(ether sulfone)s containing metallophthalocyanine, International Journal of Hydrogen Energy (2013), http:// dx.doi.org/10.1016/j.ijhydene.2013.04.066
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9
Fig. 10 e Atomic force microscopy (AFM) images of SPDHTPE 40 and HeNi 10.
reflecting the hydrophilic-hydrophobic microphase separation were strongly dependent on the chemical structure. From the figure it can be seen that, in SPDHTPE 40 and HeNi 10, numerous large domains with a worm-like shape are clearly discriminable. The hydrophobic domains are easily visible between the dark hydrophilic domains. In SPDHTPE 40, the hydrophilic domains became wider compared to the sample of HeNi 10 which was narrower with uniform distributions. This behavior could be attributed due to the metal ion within phthalocyanine of the polymer. These classes of polymer membranes afford attractive proton conductivities and maximum power densities with high chemical stability.
4.
Conclusions
The DHTPE and HPDCP monomers were synthesized successfully via McMurry coupling reaction and DielseAlder reaction, respectively. The sulfonated poly(ether sulfone) bearing DHTPE was synthesized by polycondensation and sulfonation with concentrated sulfuric acid. The hybrid copolymers containing MPc were synthesized by polycondensation and followed by ring-closing reaction with metal ions (Ni, Co, Fe). The hybrid membranes were prepared by blending with SPDHTPE 40 and 5, 10 and 15 wt% of PMPc-(Fe, Ni & Co). The accelerated chemical degradation tests by hydrogen peroxide combined with determination of weight loss were performed for fast screening of membrane durability. The hybrid membranes (series of HeNi and HeCo) remained over 90% of initial weight after 5 h at 75 C with 4 ppm Fe2þ Fenton reagent. These results show that PMPc-Ni and PMPc-Co play a vital role to improve the durability as opposed to showing in PMPc-Fe. In addition, both PMPc-Ni and PMPc-Co do not show any negative effect on proton conductivity and maximum power density; even though they have low IEC. These PMPcs give positive impact on proton conductivity and maximum power density, and also improve the chemical stability of membranes. These comparisons suggest the feasibility of using hybrid PMPc-Ni
and PMPc-Co as proton exchange membranes for fuel cell applications.
Acknowledgment This work was supported by Konkuk University.
references
[1] Matsumoto K, Higashihara T, Ueda M. Locally and densely sulfonated poly(ether sulfone)s as proton exchange membrane. Macromolecule 2009;42:1161e6. [2] Mauritz KA, Moore RB. State of understanding of nafion. Chem Rev 2004;104:4535e85. [3] Savadogo O. Emerging membranes for electrochemical systems: (I) solid polymer electrolyte membranes for fuel cell systems. J New Mater Electrochem Syst 1998;1:47e66. [4] Alberti G, Narducci R, Di Vona ML, Giancola S. More on nafion conductivity decay at temperatures higher than 80 C: preparation and first characterization of in-plane oriented layered morphologies. Ind Eng Chem Res. http://pubs.acs. org/doi/abs/10.1021%2Fie303628c; 2013. [5] Hickner MA, Ghassemi H, Kim YS, Einsla BR, McGrath JE. Alternative polymer systems for proton exchange membranes (PEMs). Chem Rev 2004;104:4587e611. [6] Hickner MA, Pivovar BS. The chemical and structural nature of proton exchange membrane fuel cell properties. Fuel Cells 2005;5:213e29. [7] Lufrano F, Squadrito O, Patti A, Passalacqua B. Sulfonated polysulfone as promising membranes for polymer electrolyte fuel cells. J Appl Polym Sci 2000;77:1250e7. [8] Bailly C, Williams DJ, Karasz FE, MacKnight WJ. The sodium salts of sulfonated poly(aryl-ether-ether-ketone) (PEEK): preparation and characterization. Polymer 1987;28:1009e16. [9] Zaidi SMJ, Mikhailenko SD, Robertson GP, Guiver MD, Kaliaguine S. Proton conducting composite membranes from polyether ether ketone and heteropolyacids for fuel cell applications. J Membr Sci 2000;173:17e34. [10] Liu B, Robertson GP, Kim DS, Guiver MD, Hu W, Jiang Z. Aromatic poly(ether ketone)s with pendant sulfonic acid phenyl groups prepared by a mild sulfonation method for
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[18] Zhang D, Chi D, Okajima T, Ohsaka T. Catalytic activity of dual catalysts system based on nano-manganese oxide and cobalt octacyanophthalocyanine toward four-electron reduction of oxygen in alkaline media. Electrochim Acta 2007;52:5400e6. [19] Koca A, Kalkan A, Bayir ZA. Electrocatalytic oxygen reduction and hydrogen evolution reactions on phthalocyanine modified electrodes: electrochemical, in situ spectroelectrochemical, and in situ electrocolorimetric monitoring. Electrochim Acta 2011;56:5513e25. [20] Amaral GP, Puntel GO, Corte CLD, Dobrachinski F, Barcelos RP, Bastos LL, et al. The antioxidant properties of different phthalocyanines. Toxicol Vitro 2012;26:125e32. [21] Randin JP. Interpretation of the relative electrochemical activity of various metal phthalocyanines for the oxygen reduction reaction. Electrochim Acta 1974;19:83e5. [22] Beck F. The redox mechanism of the chelate-catalyzed oxygen cathode. J Appl Electrochem 1977;7:239e45. [23] Zagal J, Bindra P, Yeager E. A mechanistic study of oxygen reduction on water soluble phthalocyanines adsorbed on graphite electrodes. J Electrochem Soc 1980;127:1506e17. [24] Banerjee M, Emond SJ, Lindeman SV, Rathore R. Practical synthesis of unsymmetrical tetraarylethylenes and their application for the preparation of [triphenylethylene-spacertriphenylethylene] triads. J Org Chem 2007;72:8054e61. [25] Gode P, Ihonen J, Strandroth A, Ericson H, Lindbergh G, Paronen M, et al. Membrane durability in a PEM fuel cell studied using PVDF based radiation grafted membranes. Fuel Cells 2003;3:21e7. [26] Wana W, Menga YZ, Zhu Q, Tjong SC, Hay AS. Synthesis of a series of metallophthalocyanine end-capped poly(aryl ether sulfone)s from a dicyanoarylene group containing biphenol. Polymer 2003;44:575e82.
Please cite this article in press as: Lim Y, et al., Proton conducting hybrid membrane electrolytes of sulfonated poly(ether sulfone)s and poly(ether sulfone)s containing metallophthalocyanine, International Journal of Hydrogen Energy (2013), http:// dx.doi.org/10.1016/j.ijhydene.2013.04.066
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Proton conducting hybrid membrane electrolytes of sulfonated poly(ether sulfone)s and poly(ether sulfone)s containing metallophthalocyanine Youngdon Lim a, Soonho Lee a, Dongwan Seo a, Hohyoun Jang a, Md Awlad Hossain a, Hyunchul Ju b, Taewhan Hong c, Whangi Kim a,* a
Department of Applied Chemistry, Konkuk Univ., Chungju 380-701, Republic of Korea Schlool of Mechanical Engineering, Inha Univ., 253 Yonghyun-dong, Nam-gu, Incheon 402-751, Republic of Korea c Department of Materials Science and Engineering, Korea National Univ. of Transportation, Chungju 380-702, Republic of Korea b
article info
abstract
Article history:
A role of metallophthalocyanine (MPc) as an anti-oxidizing agent of polymer membrane
Received 30 January 2013
and an accelerating agent of proton conductivity was discussed. The poly(ether sulfone)s
Received in revised form
bearing MPc (Ni, Co and Fe), PMPc were prepared by two-step reaction from phenol-
10 April 2013
phthalein and fumaronitrile and followed reaction with metal (II) chloride (Ni, Co and Fe)
Accepted 12 April 2013
and 1,2-dicyanobenzene in quinoline. The sulfonated polymer was synthesized by
Available online xxx
condensation polymerization using 1,2-bis(4-hydroxyphenyl)-1,2-diphenyl ethylene, bis(4fluorophenyl) sulfone and followed by sulfonation reaction with concentrated sulfuric acid.
Keywords:
A series of hybrid membranes (HeNi, HeCo and HeFe) were prepared from a mixture of the
PEMFC
sulfonated copolymer and PMPcs in dimethylacetamide (DMAc). The structural properties
Fuel cell
of the synthesized polymers were studied by 1H-NMR spectroscopy and FT-IR. The mem-
Membrane
brane properties were investigated by measurements of ion exchange capacity (IEC), water
Metallophthalocyanine
uptake, and proton conductivity, chemical degradation test, and atomic force microscopy
Surface morphology
(AFM) analysis. The cell performance of the membranes was compared with those of normal sulfonated poly(ether sulfone)s and Nafion. Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Polymer electrolyte membrane fuel cells (PEMFCs), which convert chemical energy to electrical energy, are regarded as promising power sources owing to their advantages, such as high efficiency, high energy density, quiet operation, and environmental friendliness [1e3]. For decades, the perfluorinated sulfuric acid membranes (Nafion and Acoplex) have been used because of their excellent chemical, physical,
and electrical properties. However Nafion is not suitable in use above 80 C. Recently, many research groups have been attempting to overcome the drawbacks of Nafion membranes and developing alternative proton-conducting membranes based on sulfonated aromatic hydrocarbon polymers [4e7]. Especially, thermally stable aromatic hydrocarbon polymers are emerging as effective membranes to be used at elevated temperatures of greater than 80 C [8e14]; performances of proton conductivity and power density are closely to
* Corresponding author. Tel.: þ82 438403579. E-mail address: [email protected] (W. Kim). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.04.066
Please cite this article in press as: Lim Y, et al., Proton conducting hybrid membrane electrolytes of sulfonated poly(ether sulfone)s and poly(ether sulfone)s containing metallophthalocyanine, International Journal of Hydrogen Energy (2013), http:// dx.doi.org/10.1016/j.ijhydene.2013.04.066
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 3 ) 1 e1 0
perfluorinated membranes, and cost is lower than that. Their durability is still unapproachable to these fluorinated materials. The chemical stability of the membrane is directly related to membrane’s durability under fuel cell operating conditions. It is widely accepted that, in a working PEMFCs, the most likely initiators of membrane chemical decomposition are hydroxy (HO) and hydroperoxy (HOO) radicals generated at the membraneeelectrode interface. Both HO and HOO are very reactive with polymers and causes for their degradation [15,16]. It has been studied that the presence of O2 in the electrolyte system influences both oxygen reduction reaction and the electrochemical and spectral behaviors of the complexes, which indicate electrocatalytic activity of the complexes for the oxygen reduction reaction. Cobalt phthalocyanines (CoPcs) have been known for a long time as electrocatalysts for the oxygen reduction reactions (ORR). The electrocatalytic mechanisms of MPcs for ORR in aqueous solution are well known [17e20]. These processes are accompanied by intramolecular electron transfer from the metal center to the oxygen molecule. Thus, the redox potential of the metal in the phthalocyanine is an important parameter when comparing the electrocatalytic activities of metal macrocyclic complexes for O2 reduction. Randin [21] and Beck [22] were the first to propose the concept of redox catalysis for the reduction of O2 on MPc. According to these authors, when O2 adsorbs on the metal ion in the phthalocyanine, the metal center is oxidized, thereby partially reducing the O2 molecule to the superoxide stage [23]. And also, it has been reported that MPcs act as catalysts for solution phase electrochemical oxidation and reduction of H2O2. Bohrer reported that CoPc responds with current losses to each dose of H2O2, with or without constant humidity, in both the kinetic and saturation regimes [20]. We are interested on MPcs as redox catalyst of oxygen and H2O2 molecules; affecting the durability of polymer membrane. This work is an attempt to synthesize poly(ethersulfone) copolymers having MPcs (Ni, Co and Fe) and normal sulfonated copolymers. We prepared hybrid membranes from a mixture of sulfonated copolymer and PMPc using different wt % ratios. The oxidative stability of the resultant hybrid membranes were investigated with the varying content of PMPc (Ni, Co, Fe). The synthesized polymers and hybrid membranes were characterized by FT-IR, 1H NMR spectroscopy, and thermogravimetric analysis (TGA). Their water uptake, IEC, chemical stability, and proton conductivity were also measured.
2.
Experimental
2.1.
Materials
Diphenylmethane, 2.5 M solution of n-butyl lithium in hexane, 4,40 -dimethoxybenzophenone, p-toluenesulfonic acid, boron tribromide, bis(4-hydroxyphenyl) sulfone, bis(4-fluorophenyl) sulfone, phenolphthalein, zinc powder, fumaronitrile, acetic acid, 4,40 -biphenol, 1,2-dicyanobenzene, nickel(II)chloride, cobalt(II)chloride, iron(II)chloride, quinoline and concentrated sulfuric acid were purchased from TCI and Aldrich Chemical and used as received. Commercial grade sulfolane and
chlorobenzene were dried over calcium hydride and distilled prior to use. Other commercially available solvents - DMSO, tetrahydrofuran, dichloromethane, methanol, acetone, toluene, ethyl acetate, ethanol, and distilled water were used without further purification.
2.2. Synthesis of 4,40 -(2,2-diphenylethenylidene) diphenol, (DHTPE) To a solution of diphenylmethane (10.1 g, 60.0 mmol) in dry tetrahydrofuran (80.0 mL) was added 24.0 mL of 2.5 M solution of n-butyl lithium in hexane (60.0 mmol) at 0 C under nitrogen. The resulting orange-red solution was stirred for 30 min at that temperature. 4,40 -Dimethoxybenzophenone (10.9 g, 45.0 mmol) was then added and the reaction mixture was warmed to room temperature over 6 h while being stirred. The reaction was quenched with aqueous ammonium chloride and the organic layer was extracted with dichloromethane. The combined organic portion was washed with saturated brine and dried over anhydrous magnesium sulfate. The solvent was evaporated and the resulting crude alcohol (containing excess diphenylmethane) was subjected to acidcatalyzed dehydration as follows. The crude alcohol was dissolved in about 300.0 mL toluene in a 500 mL round-bottomed flask fitted with a DeaneStark trap. A catalytic amount of ptoluenesulfonic acid (1.71 g, 9.0 mmol) was added and the mixture was refluxed for 3e4 h and cooled to room temperature. The toluene layer was washed with 10% aqueous sodium bicarbonate, dried over anhydrous magnesium sulfate and evaporated to afford the crude dimethoxytetraphenyl ethylene derivative. The crude product was purified by simple recrystallization from a mixture of dichloromethane and methanol [24]. To dimethoxytetraphenyl ethylene (10.0 g, 25.5 mmol) in dichloromethane (150.0 mL) was added slowly 58.0 mL 1.0 M boron tribromide (60.0 mmol) in dichloromethane at 0 C under nitrogen. The reaction mixture was warmed to room temperature over 24 h while being stirred. The reaction was quenched with water, the organic layer was extracted with ethyl acetate and 1.0 M HCl. The combined organic portion was washed with saturated brine, dried over anhydrous MgSO4 and the solvent was evaporated. The product was purified by simple recrystallization from a mixture of acetonitrile and distilled water.
2.3. Synthesis of sulfonated poly(ether sulfone)s containing 40 mol% DHTPE (SPDHTPE 40) To a 100 mL three neck round-bottomed flask, fitted with DeaneStark trap, condenser, nitrogen inlet/outlet, and magnetic stirrer - monomer DHTPE (0.57 g, 1.57 mmol), bis(4fluorophenyl) sulfone (1.0 g, 3.93 mmol), bis(4-hydroxyphenyl) sulfone (0.59 g, 2.36 mmol), potassium carbonate (0.65 g, 4.72 mmol), DMAc (10 mL), and toluene (10 mL) were charged. The mixture was refluxed for 3 h at 130 C, dehydrated and toluene was slowly removed. It was gradually heated to 160e180 C under continued stirring for about 1e2 h until a highly viscous solution was obtained. The resulting mixture was cooled to room temperature and decanted into a mixture of methanol (100 mL)/water (100 mL)/HCl (10 mL) to give a white fibrous polymer. The polymer was collected by filtration
Please cite this article in press as: Lim Y, et al., Proton conducting hybrid membrane electrolytes of sulfonated poly(ether sulfone)s and poly(ether sulfone)s containing metallophthalocyanine, International Journal of Hydrogen Energy (2013), http:// dx.doi.org/10.1016/j.ijhydene.2013.04.066
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 3 ) 1 e1 0
3
and repeatedly washed with water. And the polymer (PDHTPE) was dried in a vacuum oven at 80 C for 24 h. To a 50 mL roundbottomed flask, 0.5 g of PDHTPE copolymer and 6 mL of concentrated sulfuric acid (95 w 98%) were added, respectively and the mixture was stirred at 45 C for 12 h. The mixture was then decanted into cold water. Then, the sulfonated polymer (SPDHTPE 40) was thoroughly washed 3 times with distilled water and dried in a vacuum oven at 80 C for 24 h.
to 220e240 C for 4e6 h under nitrogen and turned into blue black and then decanted into a mixture of methanol and some drops of hydrochloric acid (36%) with vigorous stirring. The precipitate of deep green to blue particles were washed with acetone, water, dilute hydrochloric acid, and finally by water and ethanol. It was collected by filtration and extracted with chloroform. The chloroform solution was concentrated and precipitated into methanol to give PMPc.
2.4. Synthesis of PHPDCP and poly(ethersulfone) containing metallophthalocyanine (PMPc)
2.6.
The following represents the procedure for the synthesis of 1,4-bis(4-hydroxyphenyl)-2,3-dicyanonaphthalene (HPDCP) monomer. To a dry 25 mL round-bottomed flask equipped with a mechanical stirrer, 2 g (6.24 mmol) of dry & finely powdered reduced phenolphthalein was added. The flask was immersed in an ice-acetone bath for 10 min and then concentrated sulfuric acid (6.5 mL, equilibrated to 0 C) was added. The mixture was vigorously stirred for 2e3 min for dissolving the solid materials. The mixture was then stirred for another 7e8 min at this temperature. The resulting yellowbrown slurry (isobenzofuran) was quickly poured into icewater (120 mL). The solids were separated by filtration and washed with cold water (0 C). The product was dried for 5e15 min before being used directly in the following DielseAlder reaction. To a 50 mL round-bottomed flask, 2.0 g (20.41 mmol) of fumaronitrile in 13 mL acetic acid and isobenzofuran were added. The mixture was quickly heated to the reflux temperature. After 15e20 min, a yellow product e 1,4-bis-(4-hydroxyphenyl)-naphtha-2,3-dicarbonitrilen was precipitated in the reaction mixture. The reaction was continued at this temperature for another 2 h to complete. The product was isolated by filtration and washed twice with a minimum amount of acetic acid.
2.5. Synthesis of poly(ethersulfone)s containing metallophthalocyanine (PMPc) Polymers containing metallophthalocyanine was synthesized as follows: To a 100 mL three neck round bottomed flask, fitted with DeaneStark trap, condenser, nitrogen inlet/outlet, and magnetic stirrer - monomer HPDCP (0.57 g, 1.57 mmol), 4,40 biphenol (0.44 g, 2.36 mmol), bis(4-fluorophenyl) sulfone (1.0 g, 3.93 mmol), potassium carbonate (0.65 g, 4.72 mmol), DMAc (10 mL), and toluene (10 mL) were charged. The mixture was refluxed for 3 h at 130 C, dehydrated and toluene was slowly removed. It was gradually heated to 160e180 C under continued stirring for about 1e2 h until a highly viscous solution was obtained. The reaction mixture was diluted with DMAc, and then the viscous solution was slowly decanted into methanol with a small amount hydrochloric acid and stirred vigorously. The copolymer was precipitated out as a lengthy fiber. The thus-obtained materials were dissolved in chloroform and filtered through a thin layer of celite. The filtrate was slowly poured into methanol with stirring, and then the resulting polymer (PHPDCP) was separated by filtration. To a 100 mL round-bottomed flask, the PHPDCP copolymer (0.5 g), 1,2-dicyanobenzene(0.5 g, 3.9 mmol), metal(II)chloride (0.3 g) and 50 mL of quinoline were added. The mixture was heated
Preparation of hybrid copolymer membranes
Hybrid copolymer membranes were prepared by dissolution of the SPDHTPE 40 with 5, 10 and 15 wt% of PMPc in DMSO, respectively. Hybrid membrane of HeNi 5 is a mixture of 95 wt % of SPDHTPE 40 and 5 wt% of Ni-PMPc; HeNi 10 is 90 wt% of SPDHTPE 40 and 10 wt% of Ni-PMPc. The rest were prepared by changing the wt% of PMPc.
2.7.
Accelerated chemical degradation of membranes
Membrane degradation was assessed by immersion in Fenton reagent (4 ppm Fe2þ, 3% H2O2) at 75 C. Samples were SPDHTPE 40, hybrid membranes, and Nafion 211. The initial weight of the samples were measured and then put into the solution of iron(II)sulfate heptahydrate in Milli-Q water (45 mL) and heated to 75 C and 5 mL of 30% H2O2 was added to this solution. The membrane samples were taken out regularly during the experiment for weighing. Washing the sample in distilled water quenched the degradation reaction and the exposed membranes were dried before the final weight was determined [25].
2.8.
Membrane preparation and characterization
Membranes (25 mm) were prepared by the re-dissolution of the polymer in DMSO to afford 20 wt% transparent solutions and casting at elevating temperatures of 60, 80, 100, and 120 C. The polymer structure was confirmed by Fourier transform infrared (FT-IR) spectroscopy (Nicolet FT-IR spectrometer). 1H NMR spectra were recorded on a Bruker DRX (400 MHz) spectrometer using deuterated dimethyl sulfoxide (DMSO-d6) solvent and tetramethylsilane (TMS) internal standard. Thermogravimetric analysis was performed using a PerkineElmer TGA-7 analyzer. The molecular weight of the polymers was determined relative to polystyrene standards by gel permeation chromatography (GPC) in CHCl3 as the eluent on a Perkin Elmer series 200 high-pressure-liquid chromatographer with a RI detector. For the assessment of water uptake, the membranes were vacuum-dried at 100 C for 24 h, weighed and immersed in deionized water at 30 C and 80 C for 24 h. The wet membranes were wiped to dry and quickly weighed again. The water uptakes of membranes are reported in weight percent as follows: water uptake ¼ {(Wwet Wdry)/Wdry} 100%, where Wwet and Wdry are the weights of the wet and dry membranes, respectively. Membranes’ ion exchange capacities were determined by titration. The membranes in the acid form (Hþ) were converted to the sodium salt form by immersing the membranes in a 1.0 M NaCl solution for 24 h to exchange the Hþ ions with Naþ ions. Then, the exchanged Hþ ions within the solutions
Please cite this article in press as: Lim Y, et al., Proton conducting hybrid membrane electrolytes of sulfonated poly(ether sulfone)s and poly(ether sulfone)s containing metallophthalocyanine, International Journal of Hydrogen Energy (2013), http:// dx.doi.org/10.1016/j.ijhydene.2013.04.066
4
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BuLi / THF/ 30min
BBr3
4,4-dimethoxybenzophenone 6hrs / rt.
MeO
PTSA / toluene
HO
OMe
OH
DHTPE
OH HO NC H C OH O
H2SO4
Reduced phenolphthalein
HO
OH
O
CN HO
OH NC
Isobenzofuran
CN
HPDCP
Scheme 1 e Synthesis of DHTPE and HPDCP.
were titrated with a 0.02 M NaOH solution. The IEC values were calculated from the degree of sulfonation, obtained from the formula: IEC (meq./g) ¼ Ionic mmol concentration/ Mass of dry membrane at 25 C. Proton conductivity through plane direction of membranes were determined using Scribner membrane test system (MTS-740) with a Newtons 4th Ltd. (N4L) impedance analysis interface (PSM 1735) at 40e80 C under 80% humidity, and at 80 C under 30e90% humidity. EIS was conducted at the open circuit condition by applying a small alternating voltage (10 mV) and varying the frequency from 1 to 1 105 Hz. The film surfaces of the synthesized polymers were observed by atomic force microscope (AFM). PSIA XE100 was applied by non-contact mode with tips P/N 910M-NCHR. Tapping mode atomic force microscopic (AFM) observations were performed with a Digital Instrument - SII-NT SPA400, by using micro fabricated cantilevers with a force constant of w20 N/m.
2.9.
MEA fabrication and testing
Membrane electrode assemblies (MEAs) with an active area of 9 cm2 were prepared using a decal method based on catalyst coated membrane (CCM). A 20 wt% wet-proofed Toray carbon paper (TGPH-060, Toray Inc.) of 190 mm thickness was employed as a gas diffusion layer (GDL) for the anode and cathode sides. Carbon-supported Pt (Hispec 13100, Johnson Matthey Inc.) was used as catalyst for both anode and cathode. The loading of the catalyst layer in this work was 0.2 mg Pt/
cm2 for the anode and cathode. Then, the catalyst layer was transferred on the membrane at 130 C and 10 MPa for 5 min by decal method to produce the CCM. The GDL was placed on both the anode and cathode sides of the CCM to make the MEAs. Cell performance was performed by PEMFC test station (CNL Energy Co.).
3.
Results and discussion
3.1.
Monomer and polymer syntheses
The synthetic method of the monomers, DHTPE and HPDCP, are shown in Scheme 1. DHTPE was synthesized by McMurry carbonecarbon double bond coupling reaction [24] and followed by demethylation using BBr3 with 62% yield. The chemical structure of DHTPE was identified by 1H-NMR shown in Fig. 1(1). The six proton peaks of ortho and para positions (Hc and He) in phenyl rings were observed at 7.02e7.15 ppm and the four proton peaks of meta positions (Hd) in phenyl rings were assigned at 6.94 ppm. The four proton peaks of nearby ethylene (Hb) in phenol group and the four proton peaks of nearby OH (Ha) in phenol group were observed at 6.76 ppm and 6.49 ppm, respectively. And also, OH peaks appeared at 9.35 ppm. And the other monomer, HPDCP was synthesized by DielseAlder reaction with 67% yield [26]. Chemical structure of HPDCP was confirmed by 1HNMR shown in Fig. 1(2). The four proton peaks (Hc and Hd) of
Fig. 1 e 1H-NMR of monomers e DHTPE and HPDCP. Please cite this article in press as: Lim Y, et al., Proton conducting hybrid membrane electrolytes of sulfonated poly(ether sulfone)s and poly(ether sulfone)s containing metallophthalocyanine, International Journal of Hydrogen Energy (2013), http:// dx.doi.org/10.1016/j.ijhydene.2013.04.066
5
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O S O
HO HO
OH
O S O
F
F
OH DMAC / Toluene K2CO3 160¡É
O
O S O
O
HO3S
O n
O
O S O
O
O S O
m
Sulfonation
SO3H
O
O S O
O S O
O
O n
O S O
m
Scheme 2 e Synthesis of SPDHTPE 40 (n [ 0.4 & m [ 0.6).
naphthalene ring were observed at 7.65e7.93 ppm and the four proton peaks of ortho positions (Ha) in phenol groups were assigned at 6.79 ppm. The four proton peaks of meta positions (Hb) in phenol groups were assigned at 7.11 ppm. And also, OH peaks appeared at 9.71 ppm. The reaction
HO
OH NC
O S O
F
CN
O
O NC
CN
procedures for the preparation of the PDHTPE, SPDHTPE and PMPc are presented in Scheme 2 and Scheme 3. PDHTPE was synthesized by polycondensation polymerization, and the sulfonated polymer (SPDHTPE) was prepared using concentrated sulfuric acid and PDHTPE. PHPDCP was synthesized by
O S O
F
HO
O
OH
O
O S O m
n CN MCl2 CN
O
O N
N
N
O S O
O
O
n
2+ NM N N N N
O S O m
* M = Ni, Co, Fe * n : m = 0.4 : 0.6
Scheme 3 e Synthesis of PHPDCP and PMPcs. Please cite this article in press as: Lim Y, et al., Proton conducting hybrid membrane electrolytes of sulfonated poly(ether sulfone)s and poly(ether sulfone)s containing metallophthalocyanine, International Journal of Hydrogen Energy (2013), http:// dx.doi.org/10.1016/j.ijhydene.2013.04.066
6
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 3 ) 1 e1 0
Fig. 2 e 1H-NMR of the PDHTPE, SPDHTPE, and structure of SPES 40.
poly-condensation polymerization. In addition, a series of PMPcs were synthesized by the reaction of dicyanoarylene containing poly(ether sulfone)s with excess amounts of 1,2dicyanobenzene and the corresponding metal salt in quinoline. The chemical structures of PDHTPE and SPDHTPE 40 were proved by 1H-NMR shown in Fig. 2. The proton peaks of para positions in DHTPE unit of PDHTPE appeared at 7.24 ppm, and disappeared after sulfonation reaction. The new peaks emerged at 7.43 ppm, which are identical to proton peaks of nearby sulfonic acid. The chemical structures of PHPDCP and PMPc were identified by FT-IR and 1H NMR shown in Fig. 3. The spectrum of PHPDCP in Fig. 3(1), stretching peak of eCN observed at 2229 cm-1. From the spectrum of PMPc, it was clear that the peak at 2229 cm-1 is missing indicating the completion reaction at this site. The 1 H NMR spectrum of PMPc is shown in Fig. 3(2). The signals
of protons Ha and Hb appeared at 9.41 and 8.45 ppm, respectively.
3.2.
Properties of polymers and hybrid polymers
Thermooxidative stabilities of SPDHTPE 40 and PMPcs were studied using thermo- gravimetric analysis (TGA) with results shown in Fig. 4. Most of the polymers showed good thermal stability at high temperature under air atmosphere. In SPDHTPE 40, the initial weight loss around 100e250 C was attributed to residual solvent (DMAc) and water release from SPDHTPE 40. The second weight loss over 280 C was assigned to the decomposition of sulfonic acid groups. The third weight loss around 510 C was assigned to the degradation of the polymer main chain. In PMPcs, the initial weight loss around 150e330 C was attributed to residual
Fig. 3 e (1) FTeIR of the PHPDCP and PMPc, (2) 1H-NMR of PMPc. Please cite this article in press as: Lim Y, et al., Proton conducting hybrid membrane electrolytes of sulfonated poly(ether sulfone)s and poly(ether sulfone)s containing metallophthalocyanine, International Journal of Hydrogen Energy (2013), http:// dx.doi.org/10.1016/j.ijhydene.2013.04.066
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 3 ) 1 e1 0
Fig. 4 e Thermogravimetric analysis of membranes.
Fig. 5 e Ion exchange capacities and water uptake of membranes.
solvent (DMAc) and water release from PMPcs, because metal ion becomes hydrated with water. The second weight loss over 520 C was assigned to degradation of the polymer main chain. The molecular weight of the polymers was determined
7
by gel permeation chromatography (GPC). The average molecular weight of synthesized copolymers (Mw) and polydispersity (Mw/Mn) were in the range of 68,000 to 83,000 and 2.12e2.43, respectively. The SPDHTPE 40 and PMPcs were found to be soluble in aprotic polar solvents such as DMSO, DMAc, and NMP, etc. A series of hybrid membranes (HeNi 5, 10 and 15) were prepared from mixtures of sulfonated copolymers (95, 90 and 85 wt%) and PMPc-Ni (5, 10 and 15 wt %). The polymer membranes were casted in DMAc solution to form light yellow transparent films. The degree of sulfonation (DS) was expressed in the form of the ion exchange capacity (IEC; meq./g). The introduction of increasing numbers of sulfonic acid groups onto polymers not only makes more hydrophilic nature, but also improves their ionic conductivity. SPDHTPE 40 was selected to obtain a suitable membrane for PEMFCs containing sufficient sulfonic acid groups to provide suitable proton conductivity while maintaining mechanical integrity. As shown in Fig. 5, the IECs of SPDHTPE 40 with HeNi 5, 10 and 15 were 1.40, 1.32, 1.27, and 1.19 meq./g, whereas the IEC of the Nafion 211 was 0.91 meq./g. The IEC of hybrid membranes decreased as PMPc-Ni wt% increased. Fig. 5 shows water uptake of membranes at 80 C. The hybrid membranes shows water uptake around 40% lower than that of 46% for SPDHTPE 40 because of low contents of SPDHTPE 40. Ex-situ hydrogen peroxide tests were carried out for all PMPc membranes, SPDHTPE 40, Nafion 211, and SPES 40 (sulfonated poly(ether sulfone) containing 40 mol% of 4,40 biphenol. The tests were performed in hot Fenton reagent for 5 h as an accelerated chemical degradation assessment of membranes. The remaining weight of the hybrid membranes is presented as a function of time. Losses of weight of HeNi 5, 10 and 15 were shown in Fig. 6(1). HeNi 10 and 15 did not show much differences, whereas HeNi 5 containing low wt% of PMPc showed fast degradation. As a result, we prepared the hybrid membranes with 90 wt% SPDHTPE 40 and 10 wt% PMPc. Fig. 6 showed that afer 5 h, Nafion 211 attained no chemical degradation, and HeNi 10 and HeCo 10 membranes were 6 wt% loss, but HeFe 10 membrane showed 48 wt% loss which is much faster degradation than others. These results explained that PMPc-Co and PMPc-Ni play an important role as anti-oxidizing agents, while PMPc-Fe acts as an accelerator of oxidation. The side chain sulfonated membrane (SPDHTPE
Fig. 6 e Accelerated chemical degradation test in 4 ppm Fe2D Fenton reagent. Please cite this article in press as: Lim Y, et al., Proton conducting hybrid membrane electrolytes of sulfonated poly(ether sulfone)s and poly(ether sulfone)s containing metallophthalocyanine, International Journal of Hydrogen Energy (2013), http:// dx.doi.org/10.1016/j.ijhydene.2013.04.066
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 3 ) 1 e1 0
Fig. 7 e Proton conductivities of HeNi 5, 10 and 15, (1): measured at 40e90 C in the 80% RH, (2): measured at 80 C in 30e90% RH.
Fig. 8 e Proton conductivities of membranes, (1): measured at 40e90 C in the 80% RH, (2): measured at 80 C in 30e90% RH.
40) is more chemically stable than main chain sulfonated one (SPES 40). The proton conductivities of hybrid membranes were measured at 40e90 C under 80% RH and at 80 C in the range of 30e90% RH, and shown in Fig. 7. The proton conductivities of HeNi 5, 10 and 15 membranes at different temperatures and relative humidities were similar and the measured values are 72.2, 72.4 and 68.5 mS/cm at 80 C with 90% RH. In these results, HeNi 15 containing high PMPc wt% with low IEC value shows very identical compared to that of HeNi 5. Therefore, PMPc-Ni plays an important role as antioxidant and accelerator of proton conductivity. The proton conductivities of hybrid membranes containing 10 wt% of PMPcs compared with SPDHTPE 40 and Nafion 211 in Fig. 8. The proton conductivities of HeNi 10, HeCo 10 and SPDHTPE 40 were 72.4, 71.6 and 72.3 mS/cm at 80 C with 90% RH when compared with 93.1 mS/cm of Nafion 211. Fig. 9 displays a comparison of measured polarization and power density curves under fully humidified inlet condition (relative humidityanode/relative humiditycathode ¼ 100%/100%). The maximum power densities of HeNi 10, HeCo 10, SPDHTPE 40 and Nafion 211 were roughly 0.475, 0.483, 0.491, and 0.589 W/cm2, respectively. Interestingly, even though the hybrid membranes have low IEC, HeNi 10, HeCo 10 are
comparable power density values to those of SPDHTPE 40 and Nafion. The morphology of polymer surfaces was investigated by using an atomic force microscope (AFM) in Fig. 10. Assessment of the images revealed that the surface pattern
Fig. 9 e Cell performance of membranes.
Please cite this article in press as: Lim Y, et al., Proton conducting hybrid membrane electrolytes of sulfonated poly(ether sulfone)s and poly(ether sulfone)s containing metallophthalocyanine, International Journal of Hydrogen Energy (2013), http:// dx.doi.org/10.1016/j.ijhydene.2013.04.066
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 3 ) 1 e1 0
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Fig. 10 e Atomic force microscopy (AFM) images of SPDHTPE 40 and HeNi 10.
reflecting the hydrophilic-hydrophobic microphase separation were strongly dependent on the chemical structure. From the figure it can be seen that, in SPDHTPE 40 and HeNi 10, numerous large domains with a worm-like shape are clearly discriminable. The hydrophobic domains are easily visible between the dark hydrophilic domains. In SPDHTPE 40, the hydrophilic domains became wider compared to the sample of HeNi 10 which was narrower with uniform distributions. This behavior could be attributed due to the metal ion within phthalocyanine of the polymer. These classes of polymer membranes afford attractive proton conductivities and maximum power densities with high chemical stability.
4.
Conclusions
The DHTPE and HPDCP monomers were synthesized successfully via McMurry coupling reaction and DielseAlder reaction, respectively. The sulfonated poly(ether sulfone) bearing DHTPE was synthesized by polycondensation and sulfonation with concentrated sulfuric acid. The hybrid copolymers containing MPc were synthesized by polycondensation and followed by ring-closing reaction with metal ions (Ni, Co, Fe). The hybrid membranes were prepared by blending with SPDHTPE 40 and 5, 10 and 15 wt% of PMPc-(Fe, Ni & Co). The accelerated chemical degradation tests by hydrogen peroxide combined with determination of weight loss were performed for fast screening of membrane durability. The hybrid membranes (series of HeNi and HeCo) remained over 90% of initial weight after 5 h at 75 C with 4 ppm Fe2þ Fenton reagent. These results show that PMPc-Ni and PMPc-Co play a vital role to improve the durability as opposed to showing in PMPc-Fe. In addition, both PMPc-Ni and PMPc-Co do not show any negative effect on proton conductivity and maximum power density; even though they have low IEC. These PMPcs give positive impact on proton conductivity and maximum power density, and also improve the chemical stability of membranes. These comparisons suggest the feasibility of using hybrid PMPc-Ni
and PMPc-Co as proton exchange membranes for fuel cell applications.
Acknowledgment This work was supported by Konkuk University.
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Please cite this article in press as: Lim Y, et al., Proton conducting hybrid membrane electrolytes of sulfonated poly(ether sulfone)s and poly(ether sulfone)s containing metallophthalocyanine, International Journal of Hydrogen Energy (2013), http:// dx.doi.org/10.1016/j.ijhydene.2013.04.066