High Ion Exchange Capacity, Sulfonated Polybenzimidazoles - ACS

Chapter 13

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High Ion Exchange Capacity, Sulfonated Polybenzimidazoles Mahesh P. Kulkarni, Owen D. Thomas, Timothy J. Peckham, and Steven Holdcroft* Department of Chemistry, Simon Fraser University, 8888 University Drive, Burnaby BC V5A 1S6, Canada *Email: [email protected]

A novel, highly sulfonated, polybenzimidazole (sOPBI) was successfully prepared from poly[2,2′-(p-oxydiphenylene)5,5′-bibenzimidazole] (OPBI) through two complementary sulfonation methods: grafting of a pendant sulfonic acid chain via reaction with 1,3-propane sultone, and direct sulfonation of the aromatic rings in the polymer backbone. These membranes possess an IEC of 4.1 mmol·g-1, are soluble in organic solvents, and display high proton conductivity (40.1 mS·cm-1) at elevated temperatures (80 °C, 95 % RH). Fuel cell testing of these membranes provided good performance, comparable to Nafion 112, with a peak power density of 700 mW·cm-2 at 80 °C using humidified H2 and O2.

Introduction Proton exchange membrane fuel cells (PEMFC) have attracted great attention due to their promising applications in power conversion and generation (1). In order to replace current sulfonated perfluorinated polymers, new materials, with high proton conductivity, good mechanical strength and high thermo-chemical stability, are urgently needed. Polybenzimidazoles (PBIs) have currently been proposed as an alternative polymer for high temperature PEMFCs (2–9) because of their outstanding thermal, oxidative, chemical, and hydrolytic stability under fuel cell operating conditions, particularly when exposed to high temperatures. PBI can only conduct protons if it is modified to contain acidic groups, either through doping with acid (10–12) or © 2012 American Chemical Society In Polymers for Energy Storage and Delivery: Polyelectrolytes for Batteries and Fuel Cells; Page, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


by introduction of bound acidic groups. Covalently bound acidic groups have the advantage in that these groups do not leach from the membrane upon contact with water, as can occur with doped acids (13). Three main approaches to prepare sulfonated polybenzimidazoles (sPBI) are being developed: (a) direct sulfonation of the PBI backbone (14–17); (b) chemical grafting of functional monomers such as propanesultone, butanesultone, and sodium (4-bromomethyl) benzenesulfonate via N-linkages (18, 19); (c) co-polycondensation of sulfonated aromatic diacids (e.g., 5-sulfoisophthalic acid and 2-sulfoterephthalic acid) with aromatic tetraamines (20, 21). Direct sulfonation of the PBI backbone and characterization of postsulfonated polybenzimidazoles using sulfuric acid and post-sulfonated thermal treatments were reported by Arija et al. (14) and Staiti et al. (16), but the conductivity of the membranes prepred is not sufficiently high for practical PEMFC applications. Roziere et al. (22) report the direct sulfonation of a poly-[(1-(4, 4′-diphenylether)-5-oxybenzimidazole)-benzimidazole] (PBI-OO) in sulfuric acid under mild reaction conditions. In PBI-OO, the presence of electron donor ether bridges offers four rings per repeat unit for sulfonation, which enables IECs of upto 4.2 mequiv.g-1 to be achieved, providing membranes with proton conductivities of up to 0.05 S/cm. Derivatization of PBI by chemical grafting of sulfonated monomer groups at the N-site of the imidazole ring of the polymer backbone was reported by Reynolds et al. (23, 24) and Glipa et al (18, 25). However, the grafting reaction has to be performed under strict anhydrous conditions and 100% degree of grafting is difficult to achieve. Glipa et al. (18) grafted sulfonated aryl groups onto polybenzimidazole. They achieved a degree of sulfonation up to 75% based on available sites. Sulfonation increases the conductivity from 10-4 S/cm in non-modified PBI to 10-2 S/cm at room temperature for highly sulfonated PBI, doped in 1 mol H3PO4. The direct polycondensation method was invented by Uno et al. (26). In contrast to other methods, direct polycondensation provides advantages in that the the structure of resulting ionomers can be tailored. In recent years many researchers have used this route to synthesize sulfonated polybenzimidazoles using different sulfonated aromatic diacids, such as 5-sulfoisophthalic acid and 4,8-disulfonyl-2,6-naphthalenedicarboxylic acid (27–29), 3,3′-disulfonate-4,4′-dicarboxylbiphenyl (30), 4′-sulfonate-2,5dicarboxyphenyl sulfone (31), 2,2′-disulfonate- 4,4′-oxydibenzoic acid (32), and 4,6-bis(4-carboxyphenoxy)benzene-1,3-disulfonate (33). However, membranes prepared from the majority of these SPBI membranes shows relatively low conductivities since the vast proportion of the sulfonic acid groups are neutralized by the basic imidazole groups (34, 35) and do not contribute to proton conductivity. To achieve higher proton conductivity in SPBI membranes, the degree of sulfonation of PBI must be above 3 to provide a sufficient concentration of “free” protons following neutralization of a proportion of them by basic imidazole groups. In this work, we report the combined use of two approaches, chemical grafting and post-sulfonation of OPBI, to achieve high IEC polymers. The physical properties and proton conductivities of membranes prepared from sOPBI and preliminary testing in H2/O2 PEMFCs are reported. 222 In Polymers for Energy Storage and Delivery: Polyelectrolytes for Batteries and Fuel Cells; Page, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.

Experimental Materials


OPBI was prepared by polycondensation of 3,3′ diaminobenzidine and 4,4′-oxy-bis-benzoic acid, both purchased from Aldrich. Sulfuric acid (98%), dimethylsulfoxides (DMSO) were obtained from Caledon Laboratory Chemicals, Canada. Lithium hydride, calcium hydride, 1, 3-propane sultone and 20% fuming sulphuric acid were purchased from Aldrich.

Synthesis of Sultone-Substituted OPBI In a 500 mL round bottom flask, equipped with a stirrer, gas inlet and a guard tube, OPBI (0.5 g, 1.25 mmol) was dissolved in dry DMSO (180 mL). The solution was heated to 70 °C with stirring under a stream of argon. Lithium hydride (131.1 mg, 16.5 mmol) was added to the solution and the reaction mixture was heated overnight. 1,3-propane sultone (2.015 g, 16.5 mmol) was added slowly and reaction was continued for 48 h. After cooling, the polymer was precipitated in acetone and washed with acetone several times to remove DMSO. The precipitated polymer was isolated by filtration and dried under vacuum overnight at 120 °C. The product, sultone-modified OPBI, was characterized by 1H NMR with no additional purification which indicated that the 1.9-1.95 out of 2.0 N-imidazole units reacted.

Post-Sulfonation of Sultone-Substituted OPBI A 50 mL, single necked round bottom flask, equipped with a magnetic stirrer, argon gas inlet and a guard tube, was charged with 15 mL of H2SO4 and 5mL 20% oleum sultone-modified OPBI (1g) in powder form was added slowly and the mixture was heated to 80 °C with stirring under a stream of argon. The sultone modified OPBI dissolved in 1 h, providing a homogeneous solution. The solution was heated to 120 °C and maintained at this temperature for 48 h. The resulting viscous solution was cooled and poured into 100 mL water to precipitate the polymer as fiber. As the polymer was water soluble, excess NaCl was added to the solution to precipitate it. The polymer precipitate was filtered, and dissolved in 5% KOH solution for further purification by dialysis.

Membrane Preparation and Characterization Membranes were prepared by dissolving the sulfonated OPBI in DMSO and triethylamine and casting on a levelled glass plate maintained at a temperature of 80 °C. Polymer films were dried at 120 °C under vacuum overnight. The membranes (~40 µm thick) were converted to the protonic form by immersing in 2 M HCl overnight. The protonated membranes were washed several times with deionized water and were dried at 120 °C under vacuum overnight, to promote 223 In Polymers for Energy Storage and Delivery: Polyelectrolytes for Batteries and Fuel Cells; Page, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


acid-base self-crosslinking, and placed in water overnight to uptake water. The pre-treated membranes (acidic form) were equilibrated in 2 M NaCl for overnight to release the protons, which were subsequently titrated with 0.001 M NaOH to a phenolphthalein end point. Acid-base control titrations were performed on 2 M NaCl solutions with no membranes present to determine the blank titration volume. After titration, the membranes were immersed in 2 M HCl for a minimum of 4 h to reprotonate the sulfonic sites. After drying at 70 °C under vacuum overnight, the membranes’ “dry” weight was measured. The ion exchange capacity (IEC, mmol/g) of the membrane was calculated by

where VNaOH and MNaOH are the blank-corrected volume (mL) and molar concentration (mol/L) of NaOH solution, respectively. Wdry is the dry weight of the membrane. The membranes were equilibrated in deionized water overnight at room temperature and blotted with a Kimwipe to remove surface water prior to determining the “wet” weight. The water uptake was calculated as the percentage increase in mass over the “dry” weight and given by

where Wwet and Wdry are the wet and dry weight of the membrane, respectively.

Instrumentation 1H NMR spectra were obtained using a Bruker AVANCE III spectrometer operating at 500 MHz. In-plane proton conductivity was measured by ac impedance spectroscopy with a Solartron 1260 frequency response analyzer (FRA) employing a two-electrode configuration, according to a procedure described elsewhere (36). A membrane (5-10 mm) was placed between two Pt electrodes of a conductivity cell, and a 100 mV sinusoidal ac voltage over a frequency range of 10 MHz-100 Hz was applied. The resulting Nyquist plots were fitted to the standard Randles equivalent circuit to determine the membrane resistance. Proton conductivity (σ) was calculated using

where L (cm) is the distance between electrodes, R (Ω) is the membrane resistance, and A (cm2) is the cross-sectional area of the membrane. 224 In Polymers for Energy Storage and Delivery: Polyelectrolytes for Batteries and Fuel Cells; Page, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


Membrane Electrode Assembly (MEA) Fabrication and Fuel Cell Test Conditions A 5 cm2 membrane-electrode assembly (MEA) was composed of commercial gas diffusion layers (SIGRACET® GDL 24 BC, SGL Group) on either side of a catalyst-coated membrane (CCM). The CCM was prepared by using an automatic spray coater (Ultra™ 325TT, EFD) to deposit a catalyst ink to form anode and cathode catalyst layers on the sOPBI membrane. The catalyst ink components included platinum on high surface area carbon (46.4% Pt by weight, TEC10E50E, Tanaka Kikinzoku Kogyo K.K.), 5% by weight commercial Nafion® dispersion (D-520, E. I. du Pont de Nemours and Company) and a dispersant mixture of water (18.2 MΩ) and methanol (>99.8%, Fisher Scientific). All components were used as-received. The ratio of methanol to water was 9:1 by weight. The solid Pt, carbon and Nafion accounted for ~ 1% of the total ink weight; Nafion, itself, accounted for 30% of the total solid mass. After spray coating, the cell was assembled into 5 cm² single cell hardware (Fuel Cell Technologies) with single serpentine flow fields. SIGRACET® GDL 24 BC (SGL Group) was used for both the anode and cathode GDL. After assembly, the cell was installed on a Teledyne Medusa® fuel cell test station (Teledyne Energy Systems) with a Scribner 890CL load. For performance testing, hydrogen and oxygen were supplied to the anode and cathode, respectively, at a constant flow rate of 200 mL/min. The cell was heated to 80 °C and polarization curves were generated with anode and cathode gases supplied at 100%, 70%, 50% and 30% relative humidity. Curves were generated by increasing the current stepwise from 0 A to the maximum achievable current for a given humidity.

Results and Discussion OPBI was synthesized by the solution polymerization in PPA by condensing 3,3′ diaminobenzidine and 4,4′-oxy-bis-benzoic acid. The post-modification of OPBI was further carried out as shown Figure 1. Poly[2,2′-(p-oxydiphenylene)5,5′-bibenzimidazole] (OPBI) was dissolved in dry DMSO by heating at 70 °C with stirring under a stream of argon. After complete dissolution of OPBI in DMSO, the solution was cooled to room temperature and lithium hydride added in a 2.2 molar ratio and the reaction mixture heated overnight. 1,3-Propane sultone was added slowly and the reaction was continued for 48 h. After cooling, the solution was precipitated in acetone and washed with acetone several times to remove DMSO. The precipitated polymer was filtered and dried under vacuum overnight at 120 °C. The product, sOPBI, was characterized by 1H NMR spectroscopy (Figure 2). The disappearance of proton signals at 13.30 δ, assigned to the N-H hydrogen of the imidazole ring, and the emergence of aliphatic protons at 2.3–4.6 δ substantiates the reaction of the sultone with N-H groups in OPBI. 1H-NMR spectroscopy indicated that ~95-97.5% of the total number of the N-H groups reacted with the sultone. From the 1H NMR spectrum, it was calculated that the IEC of the sultone-substituted OPBI is 2.95 mmole/g. The sulfonated OPBI prepared by the chemical grafting method alone was soluble in DMSO, 225 In Polymers for Energy Storage and Delivery: Polyelectrolytes for Batteries and Fuel Cells; Page, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


and provided good film forming ability, and excellent mechanical strength. However, despite the high IEC, the room temperature proton conductivity of these fully-hydrated membranes was only 7 × 10-3 S/cm. The relatively low proton conductivity is due to the self-neutralization of the sulfonic acid groups by the basic imidazole. Thus, no further analysis of this particulalr membrane was undertaken, and the sultone substituted OPBI was further post-sulfonated to increase the IEC. To achieve post-sulfonation of the sultone-substituted OPBI, the polymer was dissolved in a mixture of conc.H2SO4 and 20% oleum at 80 °C with stirring under a stream of argon for 1h and further heated at 120 °C for 48 h. The resulting viscous solution was cooled and poured into 100 mL of water containing excess NaCl. The polymer precipitate was filtered, and dissolved in 5 wt% KOH solution for further purification by dialysis. After dialysis, the water was evaporated. The polymer was dissolved in a DMSO/water mixture and films were cast. The post-sulfonated modified sOPBI also showed solubility in DMSO and good film forming ability.

Figure 1. Synthesis of the sulfonated OPBI, sOPBI.

226 In Polymers for Energy Storage and Delivery: Polyelectrolytes for Batteries and Fuel Cells; Page, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


Figure 2. 1H-NMR spectrum of sultone-substituted OPBI. The IEC of the post-sulfonated OPBI, sOPBI, was calculated, using a combination of 1H-NMR spectrum of the sultone-substituted polymer and the IEC titration of sOPBI to be 4.2mmole/g. However, titration alone, in which only free H+ ions are detected gave a value of only 1.7mmol/g, because acid-base complex formation between benzimidazole and the sulfonic acid groups (34, 35) binds a large fraction of the H+ ions. Water content and water uptake are important parameters that in some instances can provide indirect insights into the connectivity of hydrophobic domains and the ability of the polymer matrix to resist osmotic pressure. Water uptake also has a profound effect on proton conductivity, mechanical properties and long term performance of the membrane in a fuel cell (37). High water uptake leads to higher proton conductivity but excessive water uptake results in unacceptable dimensional instability, which creates weakness between the membrane and electrode in the MEA. The sOPBI membrane exhibit 82 wt% water uptake and 91 vol% swelling. This translates into a water content of 44 wt%. However, because the IEC is high, lambda values, λ, are 11, when considering the total IEC (4.1 mmole/g) and 26, when considering only “free” protons. Similarly, the [H+] is 3.04 M when taking the total IEC into account and 1.3 M when considering the free IEC (1.7 mmole/g) The proton conductivity of the sOPBI was measured at various conditions and compared to Nafion 112. Figure 3 and Table 1 show the conductivity of sOPBI and Nafion 112 under various relative humidity and temperatures. For a given temperature, the proton conductivity increases with increasing relative humidity. sOPBI membrane yield proton conductivities between 2.3 × 10-3 to 4.0 × 10-2 S/cm for increases in RH from 65% to 95% at 80 °C, which is lower than Nafion 112. sOPBI shows a decrease in proton conductivity at 90 °C and 95 % RH which 227 In Polymers for Energy Storage and Delivery: Polyelectrolytes for Batteries and Fuel Cells; Page, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


indicates that its conductivity is highly water dependent. The room temperature, proton conductivity of hydrated membranes is 3 × 10-2 S/cm.

Figure 3. Variation in proton conductivity of sOPBI and Nafion 112 membranes as a function of relative humidity.

Table 1. Proton conductivity values of sOPBI and Nafion 112 at 80 and 90 °C under different RH Relative Humidity (%)

sOPBI σ at 80 °C

Nafion-112 σ at 80 °C

sOPBI σ at 90 °C

Nafion-112 σ at 90 °C

65

2.28 × 10-3

2.62 × 10-2

2.49 × 10-3

2.92 × 10-2

75

3.80 × 10-3

3.79 × 10-2

5.79 × 10-3

4.07 × 10-2

85

1.30 × 10-2

5.57 × 10-2

1.25 × 10-2

6.51 × 10-2

95

4.01 × 10-2

1.43 × 10-1

1.54 × 10-2

1.49 × 10-1

I–V power output characteristics at 80 °C were measured for sOPBI by using CCMs prepared as described in experimental section. Polarization curves are shown in Figure 4. Pure hydrogen and pure oxygen gases were used as the fuel and the oxidant, respectively. The RH of the cathode and anode were maintained at 100% and the gas flow was maintained at 0.2 lpm. A maximum power output of 700mW/cm2 was obtained at 2.1 A/cm2 current density with cell voltage of 0.30 V. The I-V characteristics of Nafion112 are shown as a comparison. Despite the lower proton conductivities determined ex-situ, the I-V characteristics of the sOPBI membranes are comparable to Nafion-112 under these operating conditions. The reasons for this are not yet understood but might be due to compression of the 228 In Polymers for Energy Storage and Delivery: Polyelectrolytes for Batteries and Fuel Cells; Page, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


more hydrated sOPBI membrane, which would effectively squeeze water out of the membrane, increase the acid concentration, and increased conductivity.

Figure 4. I–V characteristics of sOPBI and Nafion 112 with Pt/C electrodes (ETEK, Pt loading 0.4 mg/cm2) at 80 °C by using pure hydrogen and oxygen gases, 0.2 lpm flow. RH, 100%.

Conclusions A novel, sulfonated polybenzimidazole, sulfonated poly[2,2′-(poxydiphenylene)-5,5′-bibenzimidazole] (sOPBI), was successfully prepared by chemical grafting and post-sulfonation reaction of parent polymer, poly[2,2′-(p-oxydiphenylene)-5,5′-bibenzimidazole] (OPBI). sOPBI showed good solubility in DMSO, good film forming ability and proton conductivities of up to 4.1 × 10-2 S/cm at 80 °C. Though these membranes offered no improved performance as compared to Nafion in ex-situ conductivity analyses, they displayed displayed good fuel cell performance in initial tests that were comparable to Nafion 112, despite their showing lower ex-situ proton conductivity. The results are encouraging and provide motivation for further investigation of high IEC PBIs.

Acknowledgments The authors of this paper would like to thank Natural Sciences and Engineering Research Council of Canada for financial support of this research. The authors are grateful to Mark Haldane and Paul Le Marquand of NRC-IFCI, Vancouver, for assistance in fuel cell polarization tests.


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High Ion Exchange Capacity, Sulfonated Polybenzimidazoles - ACS

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