J. Alberto Blázquez e-mail: [email protected]

David Mecerreyes Oscar Miguel Javier Rodriguez Ione Cendoya Jon Ajuria CIDETEC, Centro de Tecnologías Electroquímicas, Paseo Miramón 196, 20009, San Sebastián, Spain

Juan J. Iruin Departamento de Ciencia y Tecnología de Polímeros, e Instituto de Materiales Poliméricos (POLYMAT), M. de Lardizábal, 3, 20018 San Sebastián, Spain

J. Ignacio Santos Servicio de Resonancia Magnética Nuclear, UPV/EHU, M. de Lardizábal, 3, 20018 San Sebastián, Spain

Hybrid Proton-Conducting Membranes as Fuel Cells Solid Polyelectrolytes A series of novel hybrid organic-inorganic membranes based on sulfonated naphthalimides and phosphotungstic acid N-methylpyrrolidone were prepared from a NMP solution. These materials, composed of two proton-conducting components, have good mechanical properties, high ionic conductivities (10.5⫻ 10−2 S / cm at 80° C) and good fuel cell performances at 70° C. 关DOI: 10.1115/1.2211636兴 Keywords: sulfonated polyimides, ion-exchange membranes, phosphotungstic acid, fuel cell, PEMFC

Catherine Marestin Régis Mercier Laboratoire des Matériaux Organiques á Propriétés Spécifiques (LMOPS), CNRS-UMR 5041, BP24, 69390 Vernaison, France

1

Introduction

One recent and promising application of the polymeric materials is their use as ion-conductive membranes for batteries 关1兴 or proton exchange membranes for fuel cells 关2–4兴. For instance, perfluorosulfonated ionomer 共Nafion兲 membranes have been used for this purpose due to their efficient proton conduction 共10−1 S cm−1 in the fully hydrated protonic form兲 and long lifetime 关5–8兴. However, the high cost of this ionomer is one of the major drawbacks for the development of this technology. Lower cost polymers with similar properties are therefore strongly desired as alternative materials 关9–13兴. In the last few years, several nonfluorinated membranes have been studied as alternatives to Manuscript received November 22, 2005; final manuscript received November 30, 2005. Review conducted by Roberto Bove. Paper presented at the 1st European Fuel Cell Technology and Applications Conference 共EFC2005兲, December 14, 2005– December 16, 2005, Rome, Italy.

308 / Vol. 3, AUGUST 2006

Nafion such as the sulfonated polynaphthalimides 关14–20兴. The polymers previously reported are, however, only soluble in m-cresol. In this paper, we describe the synthesis of new naphthalenic copolyimides/phosphotungstic acid hybrids. The presence of phosphotungstic acid modifies some important properties of the copolyimides, such as the solubility in water and solvents as well as the mechanical properties 关21–24兴. Then, it has been possible to improve the solubility of such polymers in solvents different from m-cresol, such as N-Methylpyrrolidone 共NMP兲. This improvement allows preparing membranes in better conditions. The objective of this paper is to investigate the properties of these new hybrid membranes such as proton conductivity, density, and water sorption as well as their fuel cell performances.

2

Experimental Part

2.1 Starting Materials. Benzoic acid, triethylamine, N,Ndimethylacetamide, diethyl ether and m-cresol were purchased

Copyright © 2006 by ASME

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Fig. 1 General scheme of the synthesis of a novel sulfonic polyimide BDSA/NTDA/pAPI

from Aldrich and used as received. The 4 , 4⬘-diamino-biphenyl 2 , 2⬘-disulphonic acid 共BDSA兲 was obtained from Tokyo Kasei Co. It was purified before the polycondensation reaction. The 1,4,5,8-naphthalene tetracarboxylic dianhydride 共NTDA兲 and the 4 , 4⬘-共4 , 4⬘-Isopropylidenediphenyl-1 , 1⬘-diyldioxy兲dianiline 共pAPI兲 were purchased from Aldrich and dried at 160° C under vacuum before use. 2.2 Polymer Synthesis. All polyimides were prepared by the same method 关25兴. As a representative example, we describe in detail the synthesis procedure of the BDSA/NTDA/pAPI 共r = nBDSA / nAPMP = 70/ 30兲 copolyimide. In a three-necked flask fitted with mechanical stirrer and nitrogen inlet, 9.0860 g 共0.0262 mol兲 of BDSA containing 1.1% water 共determined by thermogravimetric analysis on a TG-Q500 共TA Instruments兲 under nitrogen at a heating rate of 5 ° C min−1兲 and 6.33 g 共0.0626 mol兲 of triethylamine were introduced with 89 g of m-cresol. This mixture was stirred until solubilization of BDSA. Then 10 g 共0.0372 mol兲 of NTDA, 4.5567 g 共0.0111 mol兲 of pAPI diamine and 6.38 g 共0.0522 mol兲 of benzoic acid were added. This reaction mixture was stirred for a few minutes and then heated at 80° C for 4 h and at 180° C for 20 h. Before cooling, 166 g of m-cresol were added and the viscous polymer solution was poured into ethyl acetate. The precipitated polyimide was collected by filtration, washed with methanol and dried under vacuum at 100° C. 2.3 Film Preparation. Sulfonated polynaphthalimides membranes were obtained by solution casting from a m-cresol polymer solution. Hybrid proton-conducting membranes were obtained by solution casting from a polymer and PTA solution using NMP as solvent. The films were dried on a heating plate for 1 h at room temperature, 2 h at 80° C, 4 h at 120° C, and 2 h at 180° C. The polymer film was separated from the glass plate support by immersion in water. All membranes were washed three times by keeping them in methanol at 50° C for 1 h. A series of tough sulfonated polyimide films were obtained with controlled thickness from 45 to 70 ␮m. Membranes were acidified with a 0.1 M H2SO4 solution at ambient temperature during 14 h and then rinsed with water. 2.4

Polymer Characterization

2.4.1 Density Measurements and FTIR Study. The density values of the different membranes were measured by picnometry Journal of Fuel Cell Science and Technology

using toluene as solvent. Fourier transform infrared 共FTIR兲 spectra were performed on a Nicolet Magna 560 infrared spectrophotometer. 2.4.2 Water Uptake and Water Sorption. The water uptake was determined by soaking the membranes in liquid water at room temperature. Previously, they were dried for one week under vacuum at 100° C and weighted. The dry membranes were then immersed in water at room temperature for different periods of time. Finally, the membranes were wiped with a dry paper and quickly weighted. This procedure was repeated until obtaining a constant weight. The equilibrium water uptake 共WS兲 of a membrane is the amount of water per gram of the original dry membrane 共composed of polymer and inorganic charge兲 and was determined using the following relation: WS =

共WS − Wd兲 Wd

where Wd and Ws are the weight of the dry and the wet membrane, respectively. From this value, we can define the “corrected water uptake” 共WS⬘兲, as the amount of water per gram of polymer using the following relation: WS⬘ =

WS ⫻ 100 %pol

where %pol is the percentage of polymer in the hybrid membrane. With this procedure it is possible to compare the water sorption of the hybrid membranes and that of the pure polymer in order to evidence the real effect of the PTA. The number of water molecules per ionic group was calculated as: ␭=

n共H2O兲 n共SO−3 兲

=

WS⬘ 18IEC

where n共H2O兲 is the H2O mole number, n共SO−3 兲 the SO−3 group mole number, WS⬘ the water uptake previously defined, IEC the theoretical ion-exchange capacity 共in eq H+ / g兲 and 18 corresponds to the water molecular weight 共18 g mol−1兲. Water vapor sorption measurements at different water vapor activities 共aw ⬍ 1兲 were performed at 35° C using the IGA-2 electrobalance supplied by HIDEN, UK. The water uptake was graviAUGUST 2006, Vol. 3 / 309

Fig. 2 IR spectra of hybrid membranes obtained from a sulfonated polyimide „IEC= 1.98 meq H+ / g… and different PTA content

metrically monitorized up to the equilibrium. The dry weight of the used samples was in the range of 35– 75 mg. 2.4.3 Proton Conductivity Measurements and Fuel Cell Test. The conductivity was determined using the complex impedance spectroscopy method in a frequency range between 1 MHz and 1 Hz 共at 20 frequencies per decade兲 and with an amplitude of 10 mV, using an Autolab PGSTAT30 with a FRA module. A membrane 共1.0⫻ 0.5 cm2兲 and two platinum electrodes were set in a Teflon cell. The distance between the two electrodes was 0.5 cm. The cell was placed in a thermostatic chamber in order to control the temperature, as this parameter affects the proton conductivity. All measurements were carried out in de-ionized water 共at 100% relative humidity兲. The resistance value related to the membrane conductance 共R兲 was determined from the highfrequency intercept of the impedance with the real axis. Proton conductivity was calculated from the following equation:

␴=

D L⫻B⫻R

Fig. 3 Water vapor sorption of hybrid membranes composed of a sulfonated polyimide „IEC= 2.5 meq H+ / g… and different PTA content

copolyimides are soluble in NMP when some phosphotungstic acid is previously dissolved in NMP, due to the good interactions between the polymer and the PTA. 3.2 Density Measurements and FTIR Analysis. When the PTA content increases, it increases the density of the membranes. This is related to the higher density value of the inorganic charge with respect to that of the polymer. On the other hand, for a given PTA content, the density decreases as the ionic exchange capacity of the membrane decreases. This is a consequence of the lower proportion of the flexible non-sulfonated diamine 共pAPI兲 incorporated in the polymer chain. Figure 2 shows the IR spectra of membranes having an IEC of 1.98 meq. H+ / g and different PTA contents. It can be noticed from the FTIR spectra that: 共a兲 there is no change in the position of P-O stretching band; 共b兲 the terminal oxygen 共W = Od兲 band appears at the same wave number as that of the secondary structure of the PTA; 共c兲 in the lower contents of polyimide, the wave

where D is the distance between the two electrodes, L and B are the thickness and width of the membrane, respectively, and R is the resistance value measured. In order to perform the fuel cell tests of different membranes the membrane-electrode assemblies 共MEA兲s were prepared by setting a polymer membrane between two electrodes ELAT V2.1 共E-TEK兲 without pressing. The MEAs were then set up in a commercial 16 cm2 single cell 共Fuel Cell Technology兲. Hydrogen and oxygen were humidified by flowing them through bubble humidificators at a temperature of 60° C, and then fed to the anode and cathode at 100 and 80 mL STP/ min, respectively. Operating fuel cell temperature was 70° C at a pressure of 1 atm. Performance of the fuel cell was evaluated by measuring the voltage versus the intensity current using an electronic loader 共Amrel Fel 60-1兲.

3

Results and Discussion

3.1 Synthesis of Sulfonated Polynaphthalimides. All polyimides were prepared by copolymerization of the naphthalic anhydride NTDA with a mixture of the sulfonated diamine BDSA and the aromatic diamine pAPI 共Fig. 1兲. A series of copolymers were prepared by varying the relative ratio between BDSA and pAPI. More precisely, two different values of ion-exchange capacities were fixed: 2.5 and 1.98 meq H + / g. Interestingly, these 310 / Vol. 3, AUGUST 2006

Fig. 4 Temperature dependence of the ionic conductivity values of hybrid membranes obtained from sulfonated polyimides having the same IEC but different PTA content

Transactions of the ASME

brane prepared from NMP solutions with PTA. Indeed, this mild and efficient procedure does not seem to affect the mechanical behavior of the membrane and the performances obtained were better than those obtained with a membrane done from m-cresol solution and without any inorganic charge. One speculative explanation for the better performances should be related to the proton transport mechanism induced by the presence of PTA.

4

Fig. 5 Fuel cell tests of hybrid membranes composed of a sulfonated polyimide I„EC= 1.98 meq H+ / g… and different PTA content. These membranes are compared with the Nafion 112 in the same test conditions.

numbers of the bridging oxygen bands are near to the primary structure. However, a progressive shift of 共W-O-Wc兲 band is observed with increasing polyimide content, whereas there is no change in the case of 共W-Ob-W兲 关21兴. Similar frequency shift was also observed in two cases: 共i兲 PTA-supported silica where it was attributed to the interaction of the corner-shared oxygen of PTA with the silica surface 关26兴; 共ii兲 PTA-poly 共vinyl alcohol兲 关21兴. These specific interactions could explain the solubility of the polymer in NMP in the presence of PTA. 3.3 Water Uptake Analysis. As shown in Fig. 3, for a given IEC value, the water vapor sorption at different relative humidities as well as the liquid water uptake decreases when the PTA content increases. One possible explanation could rely on the lower PTA water sorption capacity compared to the polymer water sorption 关27兴, on the contrary that other systems were reported by Li and Wang 关28兴 where the matrix is less hydrophilic than the PTA. However, the corrected water uptake 共that would have to be constant兲 decreases when the PTA content increases. We have hypothesized that this trend of the corrected value arises from the higher density and lower free volume of our membranes 关21兴. This is an opposite behavior to that of the previous paper 关27兴. Similarly, in partially sulfonated poly共arylene ether sulfone兲 copolymer/ heteropoly acid composite membrane, the water uptake decreases as filler content increases mainly due to strong interaction between sulfonic acid on the polymer backbone and PTA 关29兴. For membranes with the same PTA content and different IEC it is observed that the water uptake decreases when the IEC value decreases because the sulfonic groups are the principal responsible in the absorption of water 关25兴. 3.4 Proton Conductivity Measurements. Not surprisingly, proton conductivity increases with the ion exchange capacity, showing values in the order of 10−1 S / cm. As reported in Fig. 4, conductivity decreases slightly as the PTA content increases. This phenomenon may be due to the lower water sorption of the hybrid membranes. 3.5 Fuel Cells Tests. Figure 5 shows the fuel cell performances of the sulfonated polynaphthalimide membranes and the classic Nafion 112 which is used as a reference. As expected, the membranes having the highest IEC have the best performances. More interesting is the behavior of the mem-

Journal of Fuel Cell Science and Technology

Conclusions

Using a flexible non-sulfonated diamine, it has been possible to synthesize a series of sulfonated polyimides soluble in NMP in the presence of phosphotungstic acid. It has been shown that the nature of the solvent used for casting the membranes induces rather small differences in conductivity. The NMP appeared as a better solvent to this point. The membranes containing PTA and processed in NMP seem to be more efficient than the classical sulfonated polyimide membranes processed in m-cresol given the better fuel cell performances obtained in this case.

Acknowledgment The authors would like to thank the Spanish Ministerio de Ciencia y Tecnología MCYT 共Project No. MAT2001-0055-C0201兲 and the University of the Basque Country 共Programa de Grupos Consolidados 2001兲 for financial support.

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Hybrid Proton-Conducting Membranes as Fuel Cells ...

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