Polym. Bull. (2013) 70:2619–2631 DOI 10.1007/s00289-013-0978-0 ORIGINAL PAPER
Synthesis and characterization of poly(benzimidazolium) membranes for anion exchange membrane fuel cells Hye-Jin Lee • Jieun Choi • Jun Young Han • Hyoung-Juhn Kim • Yung-Eun Sung • Hwayong Kim • Dirk Henkensmeier • Eun Ae Cho Jong Hyun Jang • Sung Jong Yoo
•
Received: 22 January 2013 / Accepted: 16 May 2013 / Published online: 31 May 2013 Ó Springer-Verlag Berlin Heidelberg 2013
Abstract Poly(dimethyl benzimidazolium) iodides were synthesized from polybenzimidazole derivatives by permethylation. They were easily changed to OH-, CO32- and HCO3- ion conducting electrolytes by immersing in 1 M of KOH, K2CO3 and KHCO3. Properties of polymers were changed by the ion exchange process. The anion conducting membranes showed tough and flexible properties. The water uptake, ion exchange capacity and conductivity varied depending on the counter anions. One of the poly(dimethyl benzimidazolium) carbonate membranes, Me-DAB-OBBA-carbonate showed the highest water uptake (59 %) as well as ion conductivity (33.74 mS/cm at 80 °C), and could be a good candidate for an anion exchange membrane for anion exchange membrane fuel cells. Keywords Anion exchange membrane Anion exchange membrane fuel cell Polybenzimidazole Poly(dimethyl benzimidazolium)
Introduction Polymer electrolyte membrane fuel cells (PEMFCs) are devices that directly convert chemical energy to electrical energy and have been receiving attention as a future energy source due to high energy conversion efficiency and low emission rates of H.-J. Lee J. Choi Y.-E. Sung H. Kim (&) School of Chemical and Biological Engineering, Seoul National University (SNU), Seoul 151-744, Republic of Korea e-mail:
[email protected] J. Choi J. Y. Han H.-J. Kim (&) D. Henkensmeier E. Ae Cho J. H. Jang S. J. Yoo Fuel Cell Research Center, Korea Institute of Science and Technology (KIST), Seoul 136-791, Republic of Korea e-mail:
[email protected]
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pollutants such as carbon dioxide [1, 2]. However, expensive PEMFC components such as perfluorosulfonated membrane and platinum catalyst are the drawbacks for the commercialization of PEMFC. Today, anion exchange membrane fuel cells (AEMFCs) start to be an attractive alternative to overcome the disadvantages of PEMFCs. While proton (H?) transfer from anode to cathode through the cation exchange membrane in PEMFC, hydroxyl ion (OH-) diffuses through the anion exchange membrane continuously from cathode to anode in AEMFC. Unique conduction mechanism and electricity generation system of AEMFC have several advantages. In the direct liquid fed AEMFC, the reversed flow of water from the cathode to the anode is expected to significantly reduce the methanol crossover. The main advantages of AEMFCs are the use of non-precious metals such as cobalt, silver or nickel due to the high pH operation condition for alkaline fuel cells. The operation condition facilitates the oxidation reduction reaction at the cathode, reduces electro kinetic over potentials and eases water management since water is generated at the anode side. However, AEMFCs have a few drawbacks such as low stability of the anion exchange functional groups and low conductivity [3]. Many studies have been focused on the quaternary ammonium cation-based polymer electrolyte for anion exchange membranes (AEMs) for AEMFCs. A common method to introduce quaternary ammonium groups is the modification of an existing polymer such as poly(ether sulfone) [4, 5], poly(ether ketone) [6] or poly(phthalazinone ether sulfone ketone) [7] via chloromethylation. The method has disadvantages such as use of carcinogenic chemicals to form the benzyltrimethyl-type quaternary ammonium groups and difficulty in controlling the degree of chloromethylation and side reactions [8]. To overcome these drawbacks, various methods have been proposed to introduce quaternary ammonium cation groups to the polymers. Hickner et al. synthesized quaternary ammonium membranes by bromination of benzylmethyl groups that are attached to poly(sulfone)s, followed by amination and ion exchange processes [9]. Pintauro et al. synthesized ionene block copolymers by introducing hydrophilic quaternary ammonium groups into the main chain [10]. Zhang et al. used 2,20 -dimethylaminomethyl-4,40 -biphenol as a monomer for the synthesis of poly(arylene ether sulfone) followed quaternization with iodomethane [11]. It is well known that quaternary ammonium groups have better stability under alkaline conditions than other functional groups such as methylated phosphonium and sulfonium. Nevertheless, quaternary ammonium groups are known to be unstable due to nucleophilic substitution or Hofmann elimination if they contain beta-hydrogen atoms in the substituents [12]. There have been efforts to overcome the instability of quaternized ammonium groups under high pH conditions. Firstly, the usage of AEMs does not have beta-hydrogen atoms in the quaternized ammonium groups. Recently, Lee and co-workers synthesized phenlytrimethylammonium functionalized AEMs. Their conductivity was improved compared to the benzyl trimethyl ammonium functionalized polymers [13]. The second method is the protection of functionalized groups by steric hindrance or mesomeric stabilization. Xu et al. achieved long-term durable guanidinium groups with the assistance of the p electron-conjugated system [14]. Another method to improve stabilities of AEMs is the use of a different mobile anion. Mustain et al. reported that carbonate ions cause reduced ionic conductivity but the low nucleophilicity
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increases the durability of AEMs [15]. Electrochemical systems using carbonate exchanged membranes are low temperature carbonate fuel cells and electrochemical CO2 pumps [16, 17]. Previously, we reported on polybenzimidazolium based AEMs for AEMFCs [18, 19]. The benzimidazolium systems cannot undergo Hofmann elimination and show mesomeric stabilization. Polybenzimidazolium iodides can be easily prepared by permethylation of polybenzimidazole using metal hydride and iodomethane. This synthetic method is relatively non-toxic compared to chloromethylation. We also found out that the membrane based on hydroxyl exchanged methylated poly[2,20 (p-oxydiphenylene)-5,50 -bibenzimidazole] remained flexible for a longer time than pristine m-PBI-based membranes. In this report, four new polybenzimidazoles were prepared, methylated and ion exchanged in aqueous solutions (1 M KOH, K2CO3 and KHCO3) to form poly(dimethyl benzimidazolium) hydroxide, carbonate and bicarbonate, respectively. The stabilities and properties of poly(dimethyl benzimidazolium) anion conducting electrolytes varied depending on the main chain structure and the kinds of ions after base treatment. The thermal stability, solubility, ion exchange capacity (IEC), water uptake and conductivity of each polymer membrane were characterized for the application in anion exchange membrane fuel cells.
Experimental Materials 3,30 -Diaminobenzidine (DAB), 4,40 -oxybisbenzoic acid (OBBA), isophthalic acid (IPA), 2,20 -bis(4-carboxyphenyl)hexafluoropropane (HFA), 4,40 -sulfonyldibenzoic acid (SDBA), iodomethane, polyphosphoric acid (PPA), dimethyl acetamide (DMAc) and anhydrous N-methyl-2-pyrrolidinone (NMP) were purchased from Aldrich Chemical Co. and used without further purification. Before synthesis, all monomers and polymers were dried under reduced pressure at 60 °C overnight. Synthesis of polybenzimidazoles Synthesis of DAB-OBBA 3,30 -Diaminobenzidine (3.0 g, 14 mmol) and 4,40 -oxybisbenzoic acid (3.6 g, 14 mmol) were mixed in a 4-neck round bottom flask containing polyphosphoric acid (135 g) at 150 °C under argon atmosphere for 5 h. The temperature was elevated to 195 °C and kept overnight while stirring. The polymer mixture was poured into water, and PBI fibers were collected. The polymer fibers were neutralized in 10 vol% of ammonium hydroxide solution at 50 °C for 3 days and washed several times with water and dried under vacuum at 60 °C for 24 h. 1 H NMR (DMSO-d6, ppm): 13.2–12.7 (2H, broad signal, N–H), 8.4–8.1 (4H, broad signal, ArH), 8.1–7.8 (2H, broad signal, ArH), 7.8–7.6 (2H, broad signal, ArH), 7.6–7.5 (2H, broad signal, ArH), 7.5–7.0 (4H, broad signal, ArH). In the same way, DAB-HFA, DAB-IPA and DAB-SDBA were synthesized.
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Synthesis of poly(dimethyl benzimidazolium) iodides Synthesis of Me-DAB-OBBA In a 2-neck round bottom flask, 1 g of DAB-OBBA was dissolved in 30 mL of NMP at 165 °C under argon atmosphere. 60 % sodium hydride (0.34 g) was added at room temperature to make the N-sodium salt of DAB-OBBA. Then, 2 mL of iodomethane was added and stirred at room temperature for overnight. 2 mL of iodomethane was added again and stirred at 80 °C for 24 h. This solution was poured into acetone to precipitate the polymer. It was filtered to remove excess iodomethane and washed with water several times to remove inorganic salt. The product was dried under vacuum at 60 °C for 24 h. 1H NMR (DMSO-d6, ppm): 8.8–8.5 (2H, broad signal, ArH), 8.5–8.2 (4H, broad signal, ArH), 8.2–7.8 (4H, broad signal, ArH), 7.8–7.4 (4H, broad signal, ArH), 4.2–3.8 (12H, broad signal, N-CH3). In the same way, the other poly(dimethyl benzimidazolium) iodides (Me-DABHFA, Me-DAB-IPA, Me-DAB-SDBA) were synthesized. Membrane preparation Poly(dimethyl benzimidazolium) iodide membranes were obtained by pouring filtered 2 wt% polymer solutions in DMSO in a petri dish and drying under vacuum at 80 °C for 48 h. After cooling to room temperature, deionized water was poured in the petri dishes to peel off the membranes. The thickness of the membranes was controlled between 28 and 32 lm. Anion exchange reaction Preparation of Me-DAB-OBBA-hydroxide, carbonate and bicarbonate membrane The membranes (Me-DAB-OBBA) were immersed in 1 M KOH, K2CO3 and KHCO3 solutions, respectively, at room temperature for 24 h to convert the membranes from I- form to the desired anion form such as Me-DAB-OBBAhydroxide, carbonate and bicarbonate. Then, the membranes were washed with deionized water several times and stored in deionized water for 1 day before use. Other anion exchange membranes (Me-DAB-HFA-hydroxide, carbonate, bicarbonate and Me-DAB-IPA-hydroxide, carbonate, bicarbonate and Me-DABSDBA-hydroxide, carbonate, bicarbonate) were prepared from Me-DAB-HFA, Me-DAB-IPA and Me-DAB-SDBA membranes using the same method. Characterization techniques The products were characterized by 1H NMR, FT-IR, and thermogravimetric analysis (TGA). 1H NMR spectra (in DMSO-d6) were recorded on a Varian INOVA 600 MHz NMR Spectrometer. TGA was performed in nitrogen with a SDT Q600
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apparatus (TA Instruments) at a heating rate of 10 °C/min. Before testing, all polymers were dried in a vacuum oven at 60 °C for 24 h. Water uptake (WU) Water uptake of the membranes was determined by measuring the change in weight of the membranes before and after immersing in deionized water. In this study, the membrane samples were soaked in deionized water at 30 °C for 24 h. Then, each sample was weighed immediately after removing the surface water on the membrane. The uptake of water was calculated as follows: Water uptake ð%Þ ¼
Wwet Wdry 100; Wdry
ð1Þ
where Wwet is weight of the wet membrane, and Wdry is weight of the dry membrane. Ion exchange capacity Ion exchange capacity was determined by a back titration method. A dried membrane was immersed into 10 mL of 0.1 M HCl solution for 48 h. 0.02 M NaOH solution was used for back titration to determine the ion exchange capacity. The end point was observed with a pH meter. The IEC values were calculated as follows: IEC ðmeq=gÞ ¼
M0;HCl ME;HCl ; M
ð2Þ
where M0,HCl and ME,HCl are milliequivalents of HCl before and after equilibrium, and M is the weight of the dry membrane. Conductivity In-plane conductivity measurements were made using the four-probe technique. Impedance measurements were carried out in galvanostatic mode with a perturbation amplitude of 5 lA over a frequency range from 1 Hz to 1 MHz, using a ZAHNER IM6 impedance analyzer. The impedance of a membrane was measured in a controlled humidity and temperature chamber via a Nyquist plot. The membrane was immersed in deionized water for at least 24 h prior to the test. The conductivity (r) was calculated as follows: r¼
L ; RA
ð3Þ
where L is the distance between the reference electrode and the sensing electrode and A is the cross-sectional area (thickness 9 width) of a membrane.
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Solubility The solubility of samples was observed at room temperature and 60 °C for 24 h. The concentration of samples was 5 mg/mL.
Results and discussion Polymer synthesis and characterization The four different types of PBIs were obtained through condensation polymerization of DAB with IPA, OBBA, HFA and SBDA in PPA followed by permethylation mentioned in our previous reports (Scheme 1) [18, 19]. The each dicarboxyl compound has different electron withdrawing or electron donating functional group. O
O R
HO
H2N
NH2
H2N
NH2
+ OH
DAB H N
N R
PPA
N H
NaH
N
n
N
N R
CH3I
I-
R
N
N
I-
n
IP
O
OBBA
CF3 CF3
HF
O S O
SDBA
Scheme 1 Synthetic routes for a series of methylated polybenzimidazolium salts
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The introduction of these different functional groups does not affect the polymerization. Figure 1 shows 1H NMR spectra of permethylated PBIs; the methyl peaks are found around 4 ppm. The degree of methylation was calculated by comparing the integrated peak area of the aromatic protons that appear between 7 and 9 ppm with that of the methyl protons. As shown in Table 1, all polymers except Me-DABOBBA (degree of methylation: 94 %) were fully methylated. All series of polymers were flexible and tough enough to fabricate membranes. The ion exchange was simply achieved by immersing the membranes in 1 M of KOH, K2CO3 and KHCO3.
Ha
Ha Hf N
N
N
N
N
N O
n Hb
N
Hb
Hd
Hc
Hc
n N
Hd
He
He
Ha
Ha CF3
N
C N
Hb Hc
CF3 Hd
He
O
N
N
N
N
S
n N
Hb Hc
O Hd
n N
He
Fig. 1 1H NMR spectra of Me-DAB-IPA, Me-DAB-OBBA, Me-DAB-HFA and Me-DAB-SDBA iodides
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Table 1 Methylation degree (%) of Me-DAB-IPA, Me-DAB-OBBA, Me-DAB-HFA, Me-DAB-SDBA
Degree of methylation (%)
Me-DAB-IPA
Me-DAB-OBBA
Me-DAB-HFA
Me-DAB-SDBA
100
94
100
100
Solubility properties and stability towards water Table 2 shows the solubility properties of PBIs and three forms of polybenzimidazolium anions. While pristine PBIs were soluble in the most polar aprotic solvents such as NMP, DMSO and DMF, they were insoluble in methanol. After methylation, poly(dimethyl benzimidazolium) iodides showed similar solubility as the pristine PBIs. The only difference is that DMAc was not a good solvent anymore. When poly(dimethyl benzimidazolium) iodides were exchanged with hydroxide and carbonate, the solubility of poly(dimethyl benzimidazolium) anion adducts was changed. One of the distinct differences is that Me-DAB-IPAhydroxide and carbonate are soluble in methanol. The stability of membranes in water is shown in Table 3. The membranes were soaked in deionized water for 24 h after the treatment with 1 M of KOH, K2CO3 and KHCO3, respectively, for 24 h. Only Me-DAB-OBBA-hydroxide, carbonate and bicarbonate remained stable while other polymers were dissolved or became brittle. Especially, Me-DAB-IPA dissolved completely in water after the ion exchange. Me-DAB-HFA and Me-DAB-SDBA anion adducts were either brittle (a sign for degradation) or partially dissolved in water. Previously, we reported that Me-DAB-OBBA-hydroxide is hydrolytically more stable than Me-DAB-IPA-hydroxide, because the positive charge of the imidazolium system is partially stabilized by the phenyl substituents in position 2. This mesomeric stabilization should be stronger for phenylether linked structures (as in Me-DAB-OBBA, which has 2 phenyl rings and an ether oxygen atom in paraposition to C2) than for benzene linked Me-DAB-IPA [18]. Also in this report, the hydrolytic stability was enhanced when the positive charges of benzimidazolium ions were delocalized over two separate phenyl rings (Me-DAB-OBBA, Me-DABHFA and Me-DAB-SDBA) compared to having one phenyl ring (Me-DAB-IPA). However, the strong electron withdrawing moiety in HFA and SDBA brought less stable properties than electron donating group in OBBA. Based on these results, it would be expected that new polymer design would increase the durability by the proper delocalization of permethylated benzimidazolium ring. Thermal stability of polymers Thermal stability of the films before and after methylation is obtained by TGA analysis as shown in Fig. 2. The first weight losses in PBI samples are related to water evaporation and the second one over 500 °C is due to decomposition of the PBI main chain. After methylation, one more weight loss appeared between about 180 and 250 °C. This is a phenomenon caused by nucleophilic attack of the methyl groups by the anion. Based on the thermal analysis, all four methylated PBI
123
??
??
??
--
DMSO
DMAc
DMF
Methanol
--
??
??
??
??
--
??
??
??
??
--
??
??
??
??
--
??
?-
??
??
DI
--
??
?-
??
??
--
??
??
??
??
DH
--
??
?-
??
??
DS
??
??
--
?-
--
DI
b
a
Ion exchange was carried out in 1 M K2CO3
Ion exchange was carried out in 1 M KOH
--
??
?-
??
?-
DO
--
??
??
??
??
DH
Methylated PBI (OH- forma)
?? soluble (at room temperature), ?- partially soluble (at 60 °C), -- not soluble (at 60 °C)
DO DAB-OBBA, DH DAB-HFA, DI DAB-IPA, DS DAB-SDBA
??
NMP
DS
DO
DH
DI
DO
Methylated PBI (I- form)
Pristine PBI
Table 2 Solubility properties of several PBI derivatives
--
??
??
??
??
DS
??
??
?-
??
??
DI
--
??
??
??
??
DO
--
??
??
??
??
DH
--
??
?-
??
??
DS
Methylated PBI (CO32- formb)
Polym. Bull. (2013) 70:2619–2631 2627
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Table 3 Stability in water of poly(dimethyl benzimidazolium) anions
Stability in water Before alkalization
After alkalization
Me-DAB-IPA
Stable
Brittle
Me-DAB-OBBA
Stable
Stable
Me-DAB-HFA
Stable
Brittle
Me-DAB-SDBA
Stable
Brittle
1 M KOH
1 M K2CO3 Me-DAB-IPA
Stable
Dissolved
Me-DAB-OBBA
Stable
Stable
Me-DAB-HFA
Stable
Partially dissolved
Me-DAB-SDBA
Stable
Brittle
1 M KHCO3 Me-DAB-IPA
Stable
Dissolved
Me-DAB-OBBA
Stable
Stable
Me-DAB-HFA
Stable
Dissolved
Me-DAB-SDBA
Stable
Dissolved
100
Weight / wt.%
80
60 DAB-IPA DAB-OBBA DAB-HFA DAB-SDBA Me-DAB-IPA Me-DAB-OBBA Me-DAB-HFA Me-DAB-SDBA
40
20
0 0
100
200
300
400
500
600
700
800
Temperature / oC Fig. 2 TGA curves of PBIs and poly(dimethyl benzimidazolium) iodides
molecules are shown to be stable up to 180 °C. Figure 3 shows the effect of the counter ion on the thermal decomposition of different Me-DAB-OBBA anion adducts. Decomposition temperature follows the order I- \ CO32- & OH-. According to our previous researches, this is explained that more basic counter ion causes reduction of overall nucleophilicity of benzimidazolium ring [19]. Also, based on the TGA data, three different counter ion forms of Me-DAB-OBBA are thermally stable up to 200 °C after ion exchange.
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100
Weight / %
80
60
-
Me-DAB-OBBA (I form) Me-DAB-OBBA (OH form) 2Me-DAB-OBBA (CO3 form)
40
20 0
100
200
300
400
500
600
700
800
o
Temperature / C Fig. 3 TGA curves of Me-DAB-OBBA with different counter ions 2.0
80
1.2 40 0.8
IEC / mmol g-1
Water Uptake / %
1.6 60
20 0.4
0
OH
-
-2
CO3
-
0.0
HCO3
Fig. 4 Water uptakes and IEC of Me-DAB-OBBA-hydroxide, carbonate and bicarbonate membrane
Ion exchange capacity (IEC), water uptake and conductivity Figure 4 shows IEC and water uptake of Me-DAB-OBBA-hydroxide, carbonate and bicarbonate membrane. IEC values of these electrolytes ranged from 0.9 to 1.52. The water uptake of the polymers was 30–60 %. Generally, the water uptake increases with the IEC. The Me-DAB-OBBA-hydroxide membrane, however, does not show the expected trend, probably because of the low degree of dissociation in polybenzimidazolium hydroxides. The IEC, on the other hand, increases as expected. This shows the reversibility of the hydroxide association under acidic conditions [16]. Carbonate and bicarbonate membrane, however, did not show the same trend. It could be difference of the anion kind and size [20]. Also, the interaction between polymer cation and anion would affect the hydration of anion
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Conductivity / mS cm
-1
100
10
1
OH
2-
CO3
-
HCO3 0.1 40
50
60
70
80
90
100
o
Temperature / C Fig. 5 Temperature dependence of conductivity of Me-DAB-OBBA-hydroxide, carbonate and bicarbonate membrane
exchange membranes. Further study would be very helpful to elucidate the relationship between IEC and water uptake of anion exchange membranes. Me-DAB-OBBA-hydroxide, carbonate and bicarbonate membrane were fully hydrated before conductivity measurement by immersing in water for 24 h after ion exchange and their conductivities were measured in a climate controlled chamber at 95 % RH. The ionic conductivities of membranes with different kinds of anions as a function of temperature are shown in Fig. 5. The conductivity of carbonate form was higher than that of hydroxide or bicarbonate form up to 80 °C and the values were over 10 mS/cm. All forms of membranes conductivity increased with temperature, however, decreased significantly over 80 °C. This behavior is often seen in polymer electrolytes and is known to be related to the dehydration in the membrane [5]. The ion conductivity of the carbonate form was 33.74 mS/cm at 80 °C, which can be useful for alkaline fuel cells.
Conclusions The four types of poly(dimethyl benzimidazolium) iodide (Me-DAB-IPA, Me-DAB-OBBA, Me-DAB-HFA and Me-DAB-SDBA) were synthesized successfully from the corresponding polybenzimidazoles (DAB-IPA, DAB-OBBA, DAB-HFA and DAB-SDBA). Ion exchange was carried out by immersing the poly(dimethyl benzimidazolium) iodide membrane in three different solutions (1 M KOH, K2CO3 or KHCO3). While some polybenzimidazolium anion exchange membranes were dissolved in water or methanol, Me-DAB-OBBA-hydroxide, carbonate and bicarbonate membranes were stable. The conductivity of membrane Me-DABOBBA-carbonate showed 33.74 mS/cm at 80 °C under fully humidified condition ([95 % RH) with IEC of 1.32 mmol/g, which indicates that the membrane could be a good candidate for the AEM of a low temperature carbonate fuel cell.
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Acknowledgments The authors would like to thank the ‘‘COE (Center of Excellence)’’ program of the Korea Institute of Science and Technology. This work was also supported by a Grant (M2009010025) from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy (MKE), Republic of Korea.
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