Sulfonated Poly(styrene-co-maleic anhydride)−Poly(ethylene glycol

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Sulfonated Poly(styrene-co-maleic anhydride)-Poly(ethylene glycol)-Silica Nanocomposite Polyelectrolyte Membranes for Fuel Cell Applications Arunima Saxena, Bijay P. Tripathi, and Vinod K. Shahi* Electro-Membrane Processes DiVision, Central Salt and Marine Chemicals Research Institute, BhaVnagar-364002, Gujarat, India ReceiVed: March 21, 2007; In Final Form: August 13, 2007

A method for the preparation of highly conductive and stable organic-inorganic nanocomposite polyelectrolyte membranes with controlled spacing between inorganic segment and covalently bound sulfonic acid functional groups has been established. These polyelectrolyte membranes were prepared by condensation polymerization of the silica precursor (tetraethylorthosilicate) in dimethylacetamide in the presence of poly(ethylene glycol) (PEG) of desired molecular weight, and sulfonated poly(styrene-co-maleic anhydride) was attached to the polymeric backbone by hydrogen bonding. Molecular weight of PEG has been systematically changed to control the nanostructure of the developed polymer matrix for studying the effects of molecular structure on the thermal as well as conductive properties. These polyelectrolyte membranes were extensively characterized by studying their thermo-gravimetric analysis (TGA), ion-exchange capacity (IEC), water content, conductivity, methanol permeability, and current-voltage polarization curves under direct methanol fuel cell (DMFC) operating conditions as a function of silica content and molecular weight of PEG used for membrane preparation. Moreover, from these studies and estimation of selectivity parameter among all synthesized membranes, 30% silica content and 400 Da molecular weight of PEG resulted in the best nanocomposite polyelectrolyte membranes, which exhibited performance comparable to that of the Nafion 117 membrane for DMFC applications.

1. Introduction Recently, there is considerable interest for the development of polyelectrolyte membranes as a key component in the most promising electrochemical devices for convenient and efficient power generation such as polymer electrolyte membrane fuel cells (PEMFCs) and DMFCs.1-4 Among both types of fuel cells, DMFC offers reasonably high fuel energy density, readily stored and available liquid fuel, ease of refueling, and direct and complete electro-oxidation of methanol at moderate temperatures.5,6 Nafion, perfluorosulfonic acid copolymers, are stateof-art membranes for DMFC and hydrogen/air fuel cells due to their high conductivity, good mechanical, and chemical stability.7,8 However, there is much interest in alternative polyelectrolyte membranes because of Nafion’s reduced performance above 80 °C, high methanol crossover and cost.2,9,10 Fluorinefree materials with properties comparable to those of Nafion is one of the directions in the development of cheaper polyelectrolytes. In addition to hydrocarbon-based polymers such as sulfonated styrene-ethylene/butylene-styrene block copolymers,11 which have limited chemical stability, sulfonated aromatic polymers based on poly(arylene ether ketone), poly(imide), poly(ether sulfone), and polybenzimidazole have been also studied.2,12-17 However, to achieve acceptable conductivities, a high degree of sulfonation was often required. This could impart considerable swelling of the membrane upon hydration.2,18,19 Organic-inorganic nanostructured composites constitute an emerging research field, which has opened the possibility of tailoring new materials because they combine in a single solid * Corresponding author. Tel.: +91-278-2569445. Fax: +91-2782567562/2566970. E-mail: [email protected]; [email protected].

both the attractive properties of a mechanically and thermally stable inorganic backbone and the specific chemical reactivity and flexibility of the organo functional groups.19-27 To develop polyelectrolyte membranes, several investigators reported the material, where functional groups were either introduced on the organic part or by doping polyelectrolytes in the host matrix.10,14,28-30 Problems with these types of composite materials were associated to either excessive swelling of the organic part due to its functionalization or leaching out proton carriers on prolonged use at elevated temperature.4,19,23,31 Additionally, less effort has been given to study the effect of spacing between an inorganic segment covalently bound with an organic segment. Nanocomposite polyelectrolyte membranes consisting of SiO2/ polymer (polyethylene oxide, polypropylene oxide, polytetramethylene oxide) with organic bridging groups functionalized with isocyanotopropyltriethoxysilane were reported,32-34 for controlling spacing between inorganic segments. These types of membranes were doped with acidic moieties (proton carriers) such as monododecylphosphate or 12-phosphotungstic acid. Such doping of functional molecules often limits the operational lifetime of a material because of leaching.4 A material with covalently/chemically bound acidic groups would be desirable for preventing this phenomenon. In addition, under strong oxidative fuel-cell conditions, the stability of the carbamate linkages present in the membrane is uncertain, and thus more chemically inert bridge tetraethyl orthosilicate precursors could improve the stability. In the following text, we describe the preparation of an organic-inorganic nanocomposite polyelectrolytes membrane, in which spacing between terminally attached silica was varied by varying molecular weight of

10.1021/jp072244c CCC: $37.00 © 2007 American Chemical Society Published on Web 10/11/2007

PSMA-PEG-Silica Nanocomposite Membranes

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poly(ethylene glycol) and functional groups (sulfonic acid) were covalently bound with the inorganic segment of the composite polymer. 2. Experimental Section 2.1. Materials. Poly(styrene-co-maleic anhydride) (PSMA), poly(ethylene glycol) (PEG of molecular weights 200, 400, 600, and 2000 Da), and tetraethyl orthosilicate (TEOS) were obtained from Sigma Aldrich Chemicals and used as obtained. Chlorosulphonic acid, chloroform, hexane, dimethylacetamide, sodium hydroxide, hydrochloric acid, methanol, etc., were received from S.d. fine Chemicals, Mumbai, India, and also used as obtained. Double distilled water was used in all experiments. 2.2. Membrane Preparation. For the sulfonation, 5 g of PSMA was dissolved in 100 mL of CHCl3 in a 250 mL beaker to obtain a homogeneous solution, and this solution was cooled below 10 °C. Next, a mixture of 3 mL of ClSO3H in 30 mL of CHCl3 was dropwise added to this solution under constant stirring, and the reaction was continued for 2 h. Sulfonated PSMA was precipitated in hexane, washed with water, and dried in the oven at 80 °C. For the membrane preparations, 1 g of sulfonated PSMA was dissolved in 5 mL of dimethylacetamide to obtain a homogeneous solution. Next, a pre-decided amount of TEOS was added, and the mixture was well stirred for 30 min. 0.25 mL of PEG of known molecular weight was also added, and mixture was kept under constant stirring for 12 h. The resulting viscous solution was cast on a glass plate to the desired thickness and dried at 50 °C. The obtained thin film in the form of membranes before being subjected to electrochemical studies was conditioned in 0.10 M HCl and 0.10 M NaOH solutions, alternatively, several times and then equilibrated with experimental solution for further characterization. Prepared membranes were designated as PEG X/Si-Y where X is the molecular weight of PEG used and Y is the silicon content (%) in the membrane phase. 2.3. FTIR and SEM Studies. The Fourier transform infrared (FTIR) spectra were recorded with spectrum GX series 49387 by ATR techniques. For scanning electron microscopy (SEM), gold sputter coatings were carried out on the desired membrane samples at pressure ranging between 1 and 0.1 Pa. Sample was loaded in the machine, which was operated at 10-2-10-3 Pa with EHT 15.00 kV with 300 V collector bias using Leo microscope. SEMs were recorded. 2.4. Thermal and Mechanical Stability Analysis. Thermal degradation processes and stability of the membranes were investigated using thermogravimetric analysis (TGA) (Mettler Toledo TGA/SDTA851c with starc software) under a nitrogen atmosphere using heating rate of 10 °C/min from 50 to 600 °C. The analysis of the mechanical strength of the membranes was carried out using a Mettler Toledo dynamic mechanical analyzer (DMA) 861c instrument with starc software under isothermal conditions. 2.5. Ion-Exchange Capacity and Water Uptake Measurements. Ion-exchange capacity (IEC) was determined by equilibrating the membrane in 1.0 M HCl solution to convert the membrane into H+ form. The membrane was then washed free of excess HCl with distilled water and equilibrated in it to the last traces of acid. Next, it was equilibrated in 0.10 M NaCl for 24 h, and the ion-exchange capacity was determined from increase in acidity, which in turn was determined by acid base titration by the following equation,

IEC (mequiv/g dry membrane) )

CNa+Vsol Wdry

(1)

where CNa+ is the concentration of Na+ in the extraction solution, and Wdry is the dried membrane weight. For the measurement of the volume fraction of water, the membrane was immersed in distilled water for 24 h, and the wet membrane was weighed after mopping the surface with filter paper. This wet membrane was dried at a fixed temperature of 60 °C until constant weight was obtained. φw was estimated by

φw )

∆W/dw ∆W/dw + Wd/dm

(2)

where ∆W is the weight difference between wet and dry membrane, and dw and dm are the densities of water and membrane, respectively. 2.6. Membrane Conductivity Measurements. Membrane conductance measurements were carried out in a cell, composed of two platinum electrodes, which were separated by the membrane with 1 cm2 effective area. There was provision to pass hot solution in each compartment, and the actual temperature at the vicinity of the membrane surfaces was measured with the help of two thermocouples. Equilibrated membrane was sandwiched between both of the electrodes and secured in place by means of a set of screws. Membrane conductance measurements were performed in turn by membrane resistance using potentiostatic two-electrode mode with alternating current (AC), with the help of a digital conductivity meter (century, model CC601, conductance range 0-200 mS, frequency 1-50 kHz, up to (0.001 mS reproducibility). 2.7. Methanol Permeability Measurements. Methanol permeability of the composite membranes was determined using a diaphragm diffusion cell, consisting of two compartments with a capacity of approximately 50 cm3, separated by a vertical membrane with 20 cm2 effective area. The membrane was clamped between the two compartments, which were stirred during the experiments. Before the experiment, membranes were equilibrated in double distilled water for 12 h. Initially, one compartment (A) contained 30% (v/v) methanol-water mixtures, while the other (B) contained double distilled water. Methanol flux arises across the membranes as a result of concentration difference between the two compartments. The increase in the methanol concentration with respect to time in compartment B was monitored by gas chromatography, by using 1 µL samples. Methanol permeability (P) was estimated by the following equation.35

P)

1 CB(t) V L A CA(t - t0) B

(3)

where A is the effective membrane area, L is the thickness of the membrane, CB(t) is the methanol concentration in compartment B at time t, CA(t - t0) is the change in methanol concentration in compartment A between time 0 and t, and VB is the volume of compartment B. All experiments were carried out at room temperature, and the uncertainty of the measured values was less than 2%. 2.8. Electro-osmotic Permeability Measurements. An electro-osmotic permeability-measuring cell contained two chambers with a volume of 25 cm3, made of acrylic glass and separated by the polyelectrolyte membrane of 20.0 cm2 cross-sectional areas. A schematic view of the experimental cell is depicted in Figure 1. Both chambers were kept in a state of constant agitation by means of magnetic and mechanical stirrers. A known potential difference was imposed across the membrane with the help of a potentiostat using Ag/AgCl electrodes fixed

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Figure 1. Experimental cell for measurement of electro-osmotic flux.

in both chambers. Resultant volumetric flux from anodic to cathodic chamber was measured by observing the movement of liquid in a horizontal fixed capillary tube of known radius. The current flowing through the system was also measured using a digital ammeter connected in series. Several measurements were performed to obtain reproducible values. 2.9. Current-Voltage Polarization Curves. The current voltage polarization curves in air operation mode were recorded with the help MTS-150 manual fuel cell test station with controlled gas flow, pressure, and temperature regulation attached with electronic load control ECL-150 (ElectroChem Inc.) using a FC25-01 DM fuel cell. The membranes used for recording current-voltage polarization curves had a planner area of 25 cm2 (5 cm × 5 cm) and were pressed between the electrodes of FC25-01 DMFC. The anode (20 wt % Pt, 10 wt % Ru/C) had a catalyst loading of 1 mg of Pt, 0.5 mg Ru/cm2; the cathode was 20 wt % Pt/C with 1 mg Pt/cm2 loading. The membrane electrode assembly was positioned in a stainless steel cell under controlled conditions. The measurements were performed in the air mode of operation at 10 psi pressure with varied methanol concentration fed at the anode with pressure 7 psi at 70 °C for three membranes. 3. Results and Discussion 3.1. Molecular Design of the Nanocomposite Membrane. Sulfonation of poly(styrene-co-maleic anhydride) was carried out using chlorosulfonic acid in chloroform, and sulfonated PSMA was precipitated in hexane. Membrane forming material was prepared by condensation polymerization of the silica precursor (tetraethylorthosilicate) in dimethylacetamide in the presence of PEG of desired molecular weight. For introducing proton conductivity, sulfonated PSMA was attached to the polymeric backbone by hydrogen bonding. The polyanion clusters were incorporated in the composite polymer matrix, by strong interaction with inorganic silica framework, and never dissociated from the composite membrane. In the present work, the molecular weight of PEG has been systematically changed to control the nanostructure of the developed polymer matrix for studying the effects of molecular structure on the thermal as well as conductive properties. The structure of composite membrane is considered to be an interpenetrating network of nanosized silica skeleton and polymer phase, in which each silica domain seems to have a distance of a few nanometers by bound interior polymer domain.36 A schematic drawing of the organicinorganic nanocomposite is included in Figure 2 along with the reaction scheme. FTIR spectra were obtained for all polyelectrolyte membranes, and for the PEG2000/Si-10 membrane it is presented in Figure 3, as the representative case. All nanocomposite membranes showed a characteristic band at 834 cm-1 (charac-

Figure 2. Reaction scheme for the nanocomposite membrane preparation.

teristic of the symmetric Si-O-Si stretch) and 1100-1200 cm-1 (characteristic of the asymmetric Si-O-Si stretch), indicating that hybridization between the organic and inorganic part was successfully achieved at the molecular level.37 The strong bands at 2924 cm-1 with small bands at 2361 cm-1 aroused due to -CH stretching of the aliphatic chains and aromatic rings. The presence of -SO3H groups was confirmed by the bands at 1126 cm-1 due to S-O stretching. The spectral changes clearly indicate that the hybridization between the organic and inorganic parts was successfully achieved. The broad peak absorption at around 3400 cm-1 in the composite polyelectrolyte membrane indicates a significant number of -OH due to formation of poly(styrene-co-maleic acid) because of sulfonation of PSMA. We suppose that these -OH groups provide the sites of hydrogen bonding between polymer and water, because the silica nanoparticles were observed to retain water, even at high temperatures.38 SEM images for the surfaces of representative nanocomposite polyelectrolyte membranes were also recorded for observing the effect of silica content and molecular weight of PEG on the membrane morphology, and no phase separation on the membrane surface was observed, which suggests that the synthesized polyelectrolyte membranes were homogeneous in nature and dense in nature. Hybridization between organic and inorganic segments was carried out at the molecular level by covalent or hydrogen bonding, in this reported nanocomposite polyelectrolyte membrane. Proton conducting clusters were incorporated by sulfonated PSMA, a hydrophilic part of the composite. The silica surrounded this hydrophilic part. These two parts exhibited their own significance, that is, as hydrophilic proton conducting channels and silica, which reduced water and methanol transmission across the membrane matrix. Also, the distance between these domains was controlled up to the nanometer scale by

PSMA-PEG-Silica Nanocomposite Membranes

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Figure 3. FTIR spectra of PEG2000/Si-10 membranes.

Figure 6. DMA analysis of polyelectrolyte membranes with 15 N force under varying temperature and constant frequency of 10 Hz.

Figure 4. Schematic representation of sulfonated poly(styrene-comaleic anhydride)-poly(ethylene glycol)-silica nanocomposites.

Figure 5. TGA curves for PEG2000/Si-10 and PEG200/Si-10 membranes.

varying molecular weight of PEG, which was used as a spacer in the nanocomposite polyelectrolyte membrane. Thus, it was easy to design a membrane with desired nanostructure and controlled spacing between organic and inorganic segments. A schematic drawing of the nanocomposite is presented in Figure 4. 3.2. Membrane Stability. The thermal stabilities of the membranes were analyzed by recording their TGA curves under flowing nitrogen and are presented in Figure 5 for PEG200/Si10 and PEG2000/Si-10 membranes as representative cases.

These curves were fitted using three main degradation stages, aroused from processes of thermal dissolution, thermal desulfonation, and thermal oxidation of composite polymer matrix. The first weight loss aroused around 100 °C and was attributed to the loss of absorbed water molecules in the membrane matrix. The second weight loss region (250-300 °C) corresponded to the loss of -SO3H. In the third weight loss region (at temperature >350 °C), the polymer residues were further degraded, which corresponds to the decomposition of the main chains of PEG. It was also observed that second stage degradation of membrane with PEG of high molecular weight was delayed in comparison to lower molecular weight. Also, incorporation of silica at the molecular level leads to improvement in membrane thermal and mechanical stability. Figure 6 shows the representative dynamic mechanical analysis for PEG400/Si-10 and PEG600/Si-10 membranes, which was storage modulus (MPa) of the membranes as a function of time and temperature under constant applied force (15 N) and frequency (10 Hz). The membrane exhibited good mechanical stability under these experimental conditions, and no breaking or elongation of the polymeric film was observed. It was also observed that storage modulus values were decreased with increase in the silica content or decrease in the molecular weight of the spacer (PEG). Further, storage moduli of the membranes remained constant up 120 °C and 10 min (12 °C/ min heating rate). Also, before and after the boiling water test, IEC value and membrane conductivity were measured for all prepared membranes, but no appreciable change in the membrane properties was noticed. This indicates that membranes were intact during the boiling water test. The membrane stability was also tested in oxidizing condition, for which the membranes of desired thickness and weight with known IEC values were soaked in Fenton’s reagent (3% H2O2 containing 2 ppm FeSO4) at 60 °C. The stability was evaluated by change in weight, IEC, and appearance of sample membranes. 3.3. IEC and Water Uptake Studies. Ion-exchange capacity (IEC) indicates the density of proton conducting functional

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TABLE 1: IEC, Water Uptake, Proton Conductivity, Methanol Permeability, and Energy of Activation Values for Different Composite Membranesa

d

membrane

IECb (mequiv/g)

water uptakec (%)

proton cond.d ×10-2 (S cm-1)

MeOH permeab.e ×10-7 (cm2 s-1)

energy of activation (K J mol-1)

PEG400/Si-10 PEG400/Si-30 PEG400/Si-50 Nafion 117 PEG200/Si-10 PEG600/Si-10 PEG2000/Si-10

0.549 0.618 0.647 0.900 0.571 0.524 0.518

47.6 52.1 54.7 41.6 49.5 46.9 44.3

8.12 8.74 8.92 9.56 8.25 8.02 7.25

7.54 6.83 6.33 13.10 5.35 7.76 7.96

4.43 5.52 6.51 5.45 4.28 4.59 4.80

a Membrane thickness was 100 µm. b Uncertainty in the measurements was 0.001 mequiv/g. c Uncertainty in the measurements was 0.1%. Uncertainty in the measurements was 0.01 × 10-2 S cm-1. e Uncertainty in the measurements was 0.01 × 10-7 cm2 s-1.

groups in the membrane matrix. IEC values of all synthesized membranes along with Nafion 117 membrane are presented in Table 1. IEC values for synthesized membranes ranged from 0.647 mequiv g-1 for PEG400/Si-50 to 0.518 mequiv g-1 for PEG2000/Si-10, while Nafion 117 showed a slightly higher IEC value (0.900 mequiv g-1). IEC values for synthesized polyelecrolyte membrane with the same molecular weight of PEG increased with the increase in silica content in the membrane forming material. Here, ion-exchangeable -SO3H groups are attached with the inorganic part, and thus their molality in the membrane matrix was proportional to the silica content. Further, with same silica content, that IEC values decreased with the increase in the molecular weight of PEG may be because of an increase in spacing between functional groups in the membrane matrix, which were connected by PEG spacer. Also, with high silica content ( PEG400/Si-30 > PEG400/Si-10. These may be attributed to slightly lower specific conductivities of these membranes in comparison with the Nafion 117 membrane. Membrane conductivity increased with the silica content, while it was reduced by an increase in chain length (molecular weight) of the spacer (PEG) used. In both cases, functional groups were attached with the silica; thus functional group concentration in the membrane matrix played an important role for observed polarization characteristics of the synthesized membranes. Furthermore, current density or power density at given voltage also depends on the methanol concentration fed to the anode (Figure 11A,B). Moreover, comparable voltage and power density at given current density

In this study, sulfonated poly(styrene-co-maleic anhydride)poly(ethylene glycol)-silica nanocomposite polyelectrolyte membranes were prepared with varied silica content using PEG of different molecular weight to have a fine control over spacing between silica domains, up to a few nanometers by chemically bound interior polymer chain. These membranes were extensively characterized for DMFC applications. Physicochemical and electrochemical properties of these membranes were dependent on the silica content in the membrane matrix as well as molecular weight of PEG used for membrane preparation. Although these nanocomposite polyelectrolyte membranes offer no significant advantages over Nafion 117 membrane, so far as IEC and proton conductivity are concerned, the comparable activation energy needed for proton transport, current-voltage polarization characteristics, and relatively lower methanol permeability of these membranes in comparison to Nafion 117 membrane make them applicable to DMFC. Also, the results showed that by controlling distance between inorganic domains, it is possible to tailor the desired architectures of hydrophobic and hydrophilic pathways. However, this new system showed clear improvement over the Nafion membrane as seen by SP values, due to low methanol permeability at temperatures of 30 and 70 °C, while Nafion showed almost the same SP at both temperatures. Relatively high SP values at 70 °C of these membranes indicated a great advantage for PEG-Si composite over Nafion 117 membranes for targeting higher temperature applications. Furthermore, organic-inorganic nanocomposite can be identified as a remarkable family of polyelectrolytes, which potentially provides new technological applications in high-temperature electrochemical devices including ion separations, water electrolysis, and electro-chemical sensors. Acknowledgment. We are grateful to Dr. P. K. Ghosh, Director, CSMCRI, Bhavnagar, for his keen interest and encouragement. Financial assistance received from the Department of Science and Technology, Government of India, for funding project no. SR/S1/PC-15/2003 is also gratefully acknowledged. References and Notes (1) Yamada, M.; Honama, I. Angew. Chem., Int. Ed. 2004, 43, 3688. (2) Hickner, M. A.; Ghassemi, H.; Kim, Y. S.; Einsla, B. R.; McGrath, J. E. Chem. ReV. 2004, 104, 4587. (3) Yamaguch, T.; Miyaza, F.; Nakao, S. C. AdV. Mater. 2003, 15, 1198. (4) Khiterer, M.; Loy, D. A.; Cornelius, C. J.; Fujimoto, C. H.; Small, J. H.; McIntire, T. M.; Shea, K. J. Chem. Mater. 2006, 18, 3665. (5) Arico, A. S.; Srinivasan, S.; Antonucci, V. Fuel Cells 2001, 1, 133. (6) Einsla, B. R.; Kim, Y. S.; Hickner, M. A.; Hong, Y. T.; Hill, M. L.; Pivovar, B. S.; McGrath, J. E. J. Membr. Sci. 2005, 255, 141. (7) Carrette, L.; Friedrich, K. A.; Stimming, U. Fuel Cells 2001, 1, 5. (8) Ding, J.; Chuy, C.; Holdcroft, S. Macromolecules 2002, 35, 1348. (9) Prabhuram, T.; Zhao, T. S.; Liang, Z. X.; Yang, H.; Wong, C. W. J. Electrochem. Soc. 2005, 152, A1390. (10) Kang, M. S.; Kim, J. H.; Won, J.; Moon, S. H.; Kang, Y. S. J. Membr. Sci. 2005, 147, 127. (11) Kim, J.; Kim, B.; Jung, B. J. Membr. Sci. 2002, 207, 129. (12) Steele, B. C. H.; Heinzel, A. Nature 2001, 414, 345. (13) Essafi, W.; Gebel, G.; Mercier, R. Macromolecules 2004, 37, 1431. (14) Kerres, J. J. Membr. Sci. 2001, 185, 3. (15) Yang, Y.; Shi, Z.; Holdcroft, S. Macromolecules 2004, 37, 1678. (16) Xing, P.; Robertson, G. P.; Guiver, M. D.; Mikhailenko, S. D.; Kaliaguine, S. Macromolecules 2004, 37, 7960. (17) Nagarale, R. K.; Gohil, G. S.; Shahi, V. K.; Rangarajan, R. J. Membr. Sci. 2006, 280, 389.

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