inorganic composite membranes for application in DMFC

Solid State Ionics 162 – 163 (2003) 269 – 275 www.elsevier.com/locate/ssi

Organic/inorganic composite membranes for application in DMFC B. Ruffmann a,*, H. Silva a, B. Schulte b, S.P. Nunes a a

Institute of Chemistry, GKSS Research Centre, Max-Planck-Straße 1, D-21502 Geesthacht, Germany b Institute of Technical Thermodynamik, DLR, Pfaffenwaldring 38, D-70569 Stuttgart, Germany Received 30 August 2002; accepted 31 January 2003

Abstract Zirconium phosphate as inorganic compound was chosen for investigations concerning mainly the swelling behaviour of composite membranes for the direct methanol fuel cell (DMFC). Swelling in liquid systems and in vapour systems at 100% relative humidity conditions was investigated. The fluxes of water and methanol through the membranes were obtained from pervaporation experiments. The conductivity of the developed membranes was determined by impedance spectroscopy. Two different cells for impedance measurements were used. In one cell, the membrane sample is in contact with an electrolyte solution during the measurement. In the second cell, swelling of the membrane sample can be varied by controlling temperature and relative humidity (RH). The in situ generation of inorganic oxides like zirconia by hydrolysis of the alkoxides in the polymer solution leads to a decrease of water and methanol flux through the membranes. The addition of well-dispersed zirconium phosphate to the polymer solution increases the membranes’ conductivity. Both effects can be explained by the swelling behaviour of the composites. The performance of some membranes in a methanol fuel cell test system is discussed with regard to the swelling behaviour and the methanol permeability. D 2003 Elsevier B.V. All rights reserved. Keywords: Conductivity; Composite membrane; DMFC; Permeability; Swelling

1. Introduction Zirconium phosphate as inorganic proton conductor is of particular interest because the layered structure allows intercalation of guest molecules and pillaring [1– 5]. Groups contributing to the proton conductivity can be embedded [5] and delamination increases the acidic surface area [6]. Due to these characteristics, zirconium phosphate is used in organic/inorganic composites for fuel cell membranes [7,8]. It is well established that up to a certain level a higher degree of sulfonation, higher relative humidity * Corresponding author. E-mail address: [email protected] (B. Ruffmann). 0167-2738/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0167-2738(03)00240-6

(RH) or higher temperature improves the conductivity of a polymer membrane [9– 12]. This is also the case for some pure inorganic materials [1,9] and similar behaviour is reported when composites are formed [7– 9,13]. Conductivity measurements at different relative humidities show the dependence of the membranes’ conductivity on the water content or in general on the swelling of the membrane [8]. A swollen membrane usually has lower resistance to both solvent permeation and charge transfer. In hydrogen fuel cells, humidity of electrodes and membrane is of vital importance [14] and a distinct swelling is desired. In direct methanol fuel cells (DMFCs), methanol crossover is a relevant problem since poisoning of the

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catalyst lowers the power output of the fuel cell system. The objective in developing membranes for DMFC application is obtaining a good proton conductor, but with low methanol crossover. Several protonconductive membranes with reduced permeability compared to NafionR are reported [15 – 19]. This work is aimed at presenting a critical comparison of different characterisation methods to determine the swelling, the water/methanol crossover, the proton conductivity and the performance in a DMFC test stand. None of the methods presented here can be regarded as a stand-alone procedure for the assessment of membrane performance. Only the interconnection of available characterisation methods consistently paves the way for reliable characterisation.

tion. In a second step, zirconium phosphate (ZP) was added as dispersion to the polymer solution. The mixture was cast on a glass plate heated to temperatures ranging from 40 to 90 jC for solvent evaporation. The samples were dried in a vacuum oven. The composition of the resulting membranes is depicted as weight percent (wt.%) in Table 1. 2.3. Pervaporation experiments The water and methanol permeabilities across the membranes were measured by pervaporation at 55 jC as described elsewhere [21]. A 20 wt.% methanol solution was circulated on the feed side, the permeate side was evacuated. 2.4. Swelling measurements

2. Materials and methods 2.1. Preparation of zirconium phosphate The phosphate dispersion was prepared using a procedure analogous to that described in US Patent 5,932,361 [20]. The colloidal ZrO2 formed by neutralisation of ZrOCl2 was centrifuged, separated and treated with phosphoric acid in dimethylformamide (DMF) for about 72 h forming zirconium phosphate. This phosphate (ZP) was cleaned by repeated centrifugation and redispersion in DMF before it was added as dispersion in DMF to the polymer solution. Exfoliation experiments where performed by adding 5 g n-propylamine solution (1 M in DMF) to 5.7 g ZP dispersion (5 wt.%). The mixture was stirred for 3 days at 60 jC before 6.2 g poly(benzimidazole) (PBI) solution (2.5 wt.% in DMF) was added and stirred for further 6 days at the same temperature. 2.2. Composite membrane preparation The polymer used for membrane preparation was sulfonated polyetherketone (ion exchange capacity (IEC) = 1.7 mEq/g (SPEK) kindly supplied by Fuma-Tech). It was dissolved in dimethylformamide. Two different types of inorganic modification were performed simultaneously as described in detail in Ref. [21]. Zirconia was formed by in situ hydrolysis and condensation of zirconium n-propoxide (70% solution in isopropanol, Gelest) in the polymer solu-

Swelling of the membranes in water, 0.33 M sulfuric acid and in water/methanol at 25 and 55 jC was determined by batch experiments. Swelling at 70 jC and 100% RH was determined with a magnetic suspension balance (Rubotherm) as described in Ref. [22]. The samples were vacuum-dried in the balance before sorption measurements were performed. 2.5. Conductivity The proton conductivity of each membrane was measured by impedance spectroscopy with two different setups. The first setup consisted of a HP 4284A spectrometer working in the frequency range between 10 and 105 Hz. It was connected to a measuring cell that consists of two compartments filled with 0.33 M Table 1 Water/methanol permeability measured in pervaporation experiments and proton conductivity in setup 1 of SPEK and SPEK composites Membrane composition [wt.%], SPEK/ZP/ZrO2

Thickness [Am]

Total flux [g h 1 m

100/0/0 90/10/0 80/20 ZPexf/0 70/20/10 69/17/14 86/0/14

70 80 75 70 70 73

4114 10 929 5461 2367 1131 234

2

]

Conductivity [mS cm 1] 54.4 99.6 64.4 45.3 34.7 4.0

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H2SO4 solution as liquid electrolyte. The two compartments are separated by the membrane [21]. Measurements were performed at 25 jC, determining the impedance modulus at null phase shift. The second setup consisted of a Zahner IM6 electrochemical workstation connected to a cell as described by Alberti et al. [9]. Measurements were performed in the frequency range of 1 –106 Hz. In these experiments, the membrane was pressed between Etek electrodes and the impedance at temperatures ranging from 50 to 110 jC at 100% RH was determined. 2.6. Polarisation curves Membrane samples were hot pressed with Etek ELAT electrodes (anode: 1 mg/cm2 PtRu on carbon with 0.7 mg/cm2 NafionR/PTFE; cathode: 0.4 mg/ cm2 Pt on carbon with 0.7 mg/cm2 NafionR/PTFE). Membrane electrode assemblies (MEAs) (25 cm2) were investigated with 1.5 mol/l MeOH feed solution (5– 10 ml/min, 2.5 bar) on the anode side and humidified air or O2 (600 ml/min, 3 bar) on the cathode side. The CO2 concentration at the cathode outlet was monitored as a measure of methanol crossover during fuel cell operation [23].

3. Results 3.1. Material characterisation X-ray powder diagrams for SPEK, SPEK/ZP and SPEK/ZrO2 membranes are published in Ref. [21]. Fig. 1 shows the X-ray powder diagrams of zirconium phosphate after different modifications with PBI. The zirconium phosphate was formed by treatment of ZrO2 particles with phosphoric acid (Fig. 1a). The addition of PBI to a phosphate dispersion does not affect the structure of the zirconium phosphate (Fig. 1b). The addition of PBI after treatment with npropylamine (n-PrNH2) leads to an exfoliated zirconium phosphate (Fig. 1c). 3.2. Conductivity in setup 1: the liquid system Up to now, our investigations focused on the water/ methanol crossover of composite membranes deter-

Fig. 1. X-ray powder diagrams of (a) pure zirconium phosphate, (b) zirconium phosphate/PBI and (c) zirconium phosphate/ n-PrNH2/PBI.

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mined by pervaporation experiments and on the membranes’ proton conductivity measured in a setup in which the membrane is in contact with a 0.33 M sulfuric acid solution (setup 1). For both experiments depend on diffusion phenomena, the state of swelling of the membrane is an important aspect, thus making the conditioning of the samples before the measurement a delicate topic: we have to take into account that the long contact with sulfuric acid may change the characteristics of the membrane, leading to higher degree of swelling or leaching out of soluble compounds. In Table 2, membranes characterised without any pretreatment are compared to the same membranes after treatment with sulfuric acid, showing that conditioning leads to an increase in permeability and conductivity. To get information about the initial state of the membranes, characterisation measurements are usually performed with nonconditioned membrane samples. In some cases a leaching procedure was necessary. This was performed in water at room temperature with stamps for conductivity and for pervaporation measurements as well to ensure the equality of the samples. Selective results from Ref. [21] are presented in Table 1. The presence of zirconium phosphate (ZP) in the SPEK membrane increases the water/methanol crossover and the proton conductivity. The in situ generation of zirconia (ZrO2) leads to a reduction of both solvent crossover and conductivity. The utilisation of both modification methods for the preparation of a membrane provides a way to prepare a series of composite membranes with flux and conductivity

Table 2 Permeability measured in pervaporation experiments and conductivity in setup 1 for a SPEEK (DS 60%) membrane and a SPEK composite (SPEK/17 wt.% ZP/14 wt.% ZrO2) Membrane SPEEK SPEK/17 wt.% ZP/14 wt.% ZrO2 a

Thickness [Am]

Total flux [g h 1 m

2

]

depending on the proportion of the inorganic compounds in the membrane (Table 1). A graphical representation is given in Fig. 2. The results for Nafion 112 are presented as well. For comparison with blends reported in Ref. [18], the permeability coefficients for methanol were calculated and are summarised in Table 3. The Antoine constants and Margules constants were taken from Ref. [24]. The water/methanol permeability of a membrane depends on the solubility and diffusivity of the components in the membrane. The correlation of diffusion coefficient and ionic conductivity for membranes in contact with electrolyte solution is described in Ref. [25]. Fig. 2 shows the correlation of water/methanol

Table 3 Permeability coefficients (in Barrer) of methanol in organic – inorganic composites measured at 55 jC (feed = 20 wt.% MeOH) Membrane

Thickness [Am]

Conductivity [mS cm 1]

a

60 70b 103c

a

3754 8195b 553c

50.7a 92.3b 0.8c

135d

8247d

131.5d

Without any pretreatment. After 14 days in 0.3 M sulfuric acid at 25 jC. c After 14 days in water at 60 jC. d After 14 days in 0.3 M sulfuric acid at 60 jC. b

Fig. 2. Total flux determined by pervaporation experiments and conductivity measured in set-up 1.

SPEK/10 wt.% ZP NafionR 112 SPEK/20 wt.% ZPexf SPEK SPEK/20 wt.% ZP/10 wt.% ZrO2 SPEK/17 wt.% ZP/14 wt.% ZrO2 a

1 Barrer = 10

10

Perm. coefficients [Barrer]a

Conductivity at 25 jC [mS cm 1]

80 55 75 70 75

1.8  105 1.4  105 7.8  104 5.5  104 2.05  104

99.6 80 64.4 54.4 45.3

103

9.75  103

0.8

cm3 (STP) cm/(cm2 s cm Hg).

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crossover and proton conductivity obtained by our measurements. Supplementary, swelling measurements under pervaporation conditions (20 wt.% MeOH solution at 55 jC) and under conductivity measurement conditions like in setup 1 (0.33 M sulfuric acid, 25 jC) were performed (Fig. 3). We observe that those membranes, which swell more in water/methanol, also swell more in 0.33 M sulfuric acid. This suggests that both solvent crossover and proton conductivity depend on the same transport phenomena. We have to point out that an increased swelling does not imply higher fluxes and conductivities. In Fig. 4, results of swelling experiments and of pervaporation experiments (both experiments with 20 wt.% MeOH/H2O at 55 jC) for the investigated membranes are shown. For the SPEK composites, the water/ methanol crossover increases linearly with the degree of swelling. Nafion 112 and pure SPEK show a different behaviour. We assume this difference can be traced to the different microstructures of the membranes. NafionR is known to form channels made of side chains terminated by sulfonic acid groups inside the matrix of the hydrophobic backbone. The hydrophilic channels are responsible for a high flux, the hydrophobic matrix hinders an excessive swelling. For a sulfonated polyetherketone, a more homogenous structure with narrow channels and dead-end ‘‘pockets’’ is reported [11]. The sulfonic acid groups in SPEK

Fig. 3. Swelling at pervaporation conditions (20 wt.% Me OH, 55 jC) and at conductivity measurement conditions (0.33 M sulfuric acid, 25 jC).

273

Fig. 4. Swelling in 20 wt.% methanol/water solution at 55 jC and total flux from pervaporation measurements.

are attached to the main chain and are more statistically distributed. This leads to an overall swelling of the membrane, but with a lower flux due to the absence of distinct channels like in Nafion membranes. The behaviour of the SPEK composites lies in between this two extreme values (Fig. 4). The inorganic modification with zirconia reduces the polymer swelling, but we assume regions of lower resistance to water/methanol transport may be generated especially in the interface between polymer and zirconium phosphate. The zirconium phosphate particles are incorporated in the polymer matrix, but no covalent bonding between polymer and this particle exists. Further investigations concerning the microstructure of the composite

Fig. 5. Conductivity and swelling at 70 jC and 100% relative humidity.

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Table 4 Swelling and conductivity of membranes for the gaseous and the liquid system Membrane 100% RH/70 jC composition [wt.%], Water Conductivity SPEK/ZP/ZrO2 uptake [mS cm 1] [wt.%]

Water Conductivity uptake [mS cm 1] [wt.%]

100/0/0 90/10/0 70/20/10 69/17/14 NafionR 112

29.6 35.7 36.2 21.5 17.6

15.12 23.42 13.3 10.0 9.43

10.6 9.1 2.3 0.06 18.7

Water/25 jC

54.4 99.6 45.3 0.8 80

membranes are in progress and will be published soon. 3.3. Conductivity in setup 2: the system at 100% RH Swelling at 70 jC/100% RH of the investigated membranes is much lower than in experiments in which the membranes are immersed in a liquid. This behaviour is well described in literature for NafionR [26]. As conductivity strongly depends on the hydration of the membrane [7,9] as well as on the concentration of the electrolyte in contact with the membrane [25], the conductivity is higher in experiments in which the membrane is immersed in sulfuric acid at 25 jC than in those performed at 70 jC/100% RH (Fig. 5). Again, for the composites, the conductivity increases

Fig. 7. Methanol crossover in a DMFC test stand (vol.% CO2) and in pervaporation experiments. 1 Barrer = 10 10 cm3 (STP) cm/ (cm2 s cm Hg).

with the ratio of zirconium phosphate to zirconia (Table 4). However, the pure SPEK membrane is a better proton conductor in these experiments. 3.4. Polarisation curves The polarisation curves show the current density at an applied potential for the membrane electrode assemblies (Fig. 6). The nonconditioned SPEK composite exhibits good methanol retention (0.4 vol.% CO2),

Fig. 6. Polarisation curves of SPEK and a composite (SPEK/20 wt.% ZP/10 wt.% ZrO2) in the initial and in the preswollen state.

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but the power output is reduced. By conditioning the membrane with sulfuric acid, the electrical performance is improved, but we determine a notable increase of methanol crossover (4.7 vol.% CO2). This value even exceeds the value for the pure SPEK membrane (1.6 vol.% CO2). The methanol crossover determined in these fuel cell experiments is in good agreement with the methanol crossover obtained by pervaporation experiments (Fig. 7).

4. Conclusions Different characterisation methods have been used for the investigation of composite membranes to be used in a direct methanol fuel cell. Both pervaporation experiments and the conductivity measurements in which the membrane is in contact with 0.33 M sulfuric acid are ruled by diffusion processes. Thus, for the samples presented here, a simple correlation between the results of these two methods exists. The proton conductivity obtained by this setup may not reflect the behaviour in a fuel cell test system, because the presence of sulfuric acid dominates the conductivity process across the membrane. The comparison of the water/methanol crossover and the swelling of a membrane can bear relevant information on the membranes microstructure. A membrane like NafionR, with distinct channels, shows a high water/methanol flux while its extent of swelling is low. We observed the opposite behaviour for SPEK and an intermediate behaviour for the composites presented here. Conductivity at higher temperatures/100% RH in setup 2 represents the electrical behaviour in a fuel cell more accurately. Although conductivity values determined with setup 2 and the power output in the DMFC test stand are not improved by the composites presented here, a notable reduction of methanol crossover is achieved. For the membranes presented here, methanol crossover in the fuel cell test system correlates well with the data obtained by pervaporation experiments.

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