Ethanol crossover through alkali-doped polybenzimidazole membrane

Journal of Membrane Science 328 (2009) 86–89

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Ethanol crossover through alkali-doped polybenzimidazole membrane Alexey Y. Leykin a,∗ , Oksana A. Shkrebko a , Michail R. Tarasevich b a Engineering Center for Hydrogen Technologies and Alternative Energy of National Innovation Company “New Energy Projects”, Prechistenka 18, Moscow 119034, Russia b A. N. Frumkin Institute of Physical Chemistry and Electrochemistry of Russian Academy of Science. Leninsky av. 31, Moscow 119991, Russia

a r t i c l e

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Article history: Received 8 September 2008 Received in revised form 7 November 2008 Accepted 28 November 2008 Available online 3 December 2008 Keywords: Alkali-doped polybenzimidazole Ethanol crossover Permeability Direct ethanol fuel cell

a b s t r a c t New reproducible method for investigation of ethanol permeability of polymeric membranes in alkali media has been developed. Using the developed method ethanol crossover through KOH doped poly[2,2 -(4,4 -diphenylether)-5,5 -bibenzimidazole] (DOPBI) has been investigated with respect to alkali concentration. The permeability value, obtained in 3 M KOH was 8.6 × 10−8 cm2 s−1 , which further decreased with the raise of alkali concentration. Preliminary modeled estimation showed that ethanol crossover would drop by a factor of 4 if discharge current of the cell raised by approximately 1 mA cm−2 . The developed method can be expanded to measuring polymeric membrane permeability to alcohols in acidic conditions and various aqueous solutions. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Alkaline direct ethanol fuel cells (ADEFC) recently gained increasing attention due to a number of benefits, such as availability and low toxicity of ethanol, more facile ethanol oxidation in alkaline medium in comparison to acidic medium, and possibility to use relatively cheap non-platinum catalysts [1,2]. The materials for the membranes used in ADEFCs are usually polymers capable of conducting hydroxide ions. Among such polymers are anion exchangers with quaternary ammonium functional groups [3–11]. This group of polymers have shown to perform in alkaline fuel cells, but the performance was quite low due to easy degradation of ammonium base [12,13]. The promising alternative to the above mentioned membranes are polybenzimidazoles, doped with strong bases. It was shown [14], that poly[2,2 -(m-phenylene)-5,5 -bibenzimidazole] (MPBI), doped with strong inorganic bases exhibited high ionic conductivity (up to 9 × 10−2 S cm−1 ), excellent performance in H2 /O2 fuel cell as well as in other electrochemical applications. Potassium hydroxide doped MPBI have been shown to perform well in ADEFC [15]. One of the important issues of all polymer electrolyte fuel cells is fuel crossover. Several methods have been reported in the literature to investigate alcohol permeability through the membranes [16–21]. All methods adhere to the same principle of two cham-

∗ Corresponding author. Tel.: +7 926 764 4987. E-mail address: [email protected] (A.Y. Leykin). 0376-7388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2008.11.047

bers, separated with the membrane. One chamber contains solution of alcohol which diffuses through the membrane and enters the solution in the other chamber. Alcohol concentration in the second chamber is measured either electrochemically [16–18,20], by gas chromatography [19] or even gravimetrically [21]. Different variations of those methods have been used to investigate alcohol crossover through the membranes. All above methods employ rather advanced equipment and in case of electrochemical determination must consider the factor of instability of the system. In addition all permeability experiments have been performed in neutral or acidic media, though it had been shown that permeability of Nafion membranes, for example, depends on the ionic form [21]. We here propose rather simple, reliable and reproducible method for determination of ethanol permeation through polymeric membranes in alkaline medium. We believe that this method can also be adopted for other alcohols and not only limited to alkaline solutions. 2. Experimental 2.1. Chemicals 4,4 -diphenylether dicarboxylic acid was obtained from Aldrich and purified by reprecipitation from disodium salt with activated carbon treatment. 3,3 -diaminobenzidine was obtained from Acros and used after drying in vacuum (10−1 bar) at 55 ◦ C for 2 h. Methanesulfonic acid was obtained from Acros and used as received. Other reagents were obtained from local suppliers and used with no purification.

A.Y. Leykin et al. / Journal of Membrane Science 328 (2009) 86–89

2.2. DOPBI synthesis and membrane casting DOPBI was synthesized from 4,4 -diphenylether dicarboxylic acid and 3,3 -diaminobenzidine in P2 O5 /CH3 SO3 H solution according to previously discovered procedure [22].

Inherent viscosity of the resulting DOPBI was equal to 3.8 dl g−1 (dimethylacetamide, 25 ◦ C). DOPBI was dissolved in dimethylacetamide to obtain 6.5% solution, and the resulting solution was used to cast films on automatic industrial film casting machine (Matis AG). Cast films were dried in vacuum at 160 ◦ C for 2 h. 2.3. Membrane preparation Polymer films were equilibrated in KOH/EtOH/H2 O solution with desired concentration of each of the components at room temperature for 14 days. Right before testing the membranes were extracted from the solution, dried thoroughly with filter paper, wiped twice with wet filter paper to remove excess of KOH from the surface, and dried again. KOH content in the membrane was determined by titration.

mixing. Disc-shaped membrane sample was placed between two rubber o-rings, and the resulting assembly was tightened between two chambers to ensure complete separation of one chamber from the other. Chamber A was filled with KOH solution and chamber B, with the same solution, containing certain concentration of ethanol. The chambers were closed with stoppers with thin openings which served to prevent pressure gradient in the chambers due to osmotic forces. The whole assembly was placed on the shaker to ensure mixing of solutions inside the chambers. After each 30 min 1 ml aliquots of solution were taken from chamber A. The volume of ethanol (v), passed through the membrane up to certain moment of time was calculated by the following equation:

 VA −

Ethanol determination was performed photometrically using Varian Cary 100 Scan UV–vis spectrometer. Determination was based on the change of color intensity of K2 Cr2 O7 solution after reaction with ethanol. Prior to the experiment the solution of K2 Cr2 O7 (0.05 g/100 ml) in HNO3 /H2 O (1:1) was prepared and aged for at least 12 h to ensure time stability. Calibration solutions, containing ethanol at concentration 0.1, 0.3, 0.5, 0.7 and 0.9 mg ml−1 were prepared right before the experiment. 1 ml of the solution being tested was placed into 50 ml measuring flask containing 10 ml of K2 Cr2 O7 solution. The flask was filled with DI water up to the mark and placed into water bath at 80 ◦ C for 20 min. The flask was then removed from the bath, cooled down to room temperature, and the absorbance of the solution was measured against HNO3 /H2 O (1:1) solution at 435 nm. According to the above procedure five calibration solutions and one blank solution were measured to build calibration curve, representing absorbance of the solution as a function of concentration of ethanol in the corresponding calibration solution. Absorbance of the solution was calculated as a difference between the absorbance of the solution from blank experiment and from actual measurement. Ethanol concentration in an unknown solution was calculated using obtained calibration equation. Absorbance of an unknown solution was calculated as a difference between the absorbance of the solution from blank experiment and from actual measurement.

n−1 

 Vn

CnA +

n=1

v=

n−1 

 (Vn CnA ) × 10−3

n=1



here VA is initial volume of solution in chamber A (52 cm3 ); n is experimental point number; Vn is the volume of n-th aliquot (cm3 ); CnA is concentration of ethanol in chamber A (mg cm−3 );  is density of ethanol (0.789 g cm−3 ). Provided that the ethanol concentration in chamber B is nearly constant throughout the experiment, ethanol permeation (cm2 s−1 ) through the membrane was calculated as follows: P=

2.4. Determination of ethanol concentration in solution

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dv(t) L dt SCB

here t is elapsed time (s); L is thickness of the membrane (cm), S is membrane’s active area (cm2 ); CB is concentration of ethanol in chamber B (cm3 of EtOH/1 cm3 of solution). Ethanol crossover rate dv(t)/dt (cm3 s−1 ) was determined as the slope of the straight line dependence of v = F(t).

2.6. Determination of the osmotic drag of water To determine the osmotic drag of water through the membrane we used the developed installation (Fig. 1) except for that chamber stoppers were replaced by two graduated 5 ml pipettes. Chamber A was charged with the solution with certain concentration of KOH, containing 1 mol l−1 EtOH and chamber B, with DI water. Water drag from chamber B to chamber A was defined as an increase of the volume of solution in chamber A, which in turn was determined by raise of the meniscus of solution in the corresponding pipette. During the whole experiment the levels of liquid in both pipettes were kept constant by adding DI water to chamber B.

2.5. Ethanol permeability measurements All experiments were performed at room temperature. For investigation of ethanol permeability through the membrane we used homemade installation whose schematic representation is shown in Fig. 1. The installation consists of two chambers (A and B) of equal volume, made of an inert material, attached to each other with screws. Each chamber was charged with several beads, which served for

Fig. 1. Cross-section of the installation for ethanol crossover measurements: 1, diffusion chamber; 2, beads; 3, membrane sample; 4, rubber o-rings; 5, chamber stopper.

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Fig. 3. EtOH permeability through DOPBI membrane as a function of KOH concentration (EtOH = 1 mol l−1 , membrane thickness = 23 ␮m).

Fig. 2. Content of KOH and water in the membrane as a function of (a) EtOH concentration in 3 M KOH doping solution and (b) KOH concentration in the doping solution with 1 mol l−1 EtOH.

3. Results and discussion During doping of DOPBI with solutions of KOH and ethanol of various concentrations we found that the polymer is stable in the solutions containing up to 2 mol l−1 EtOH at 3–5 mol l−1 KOH. When ethanol concentration reached 1.5 mol l−1 DOPBI turned into soft, mechanically week gel. At 3 mol l−1 EtOH/3 mol l−1 KOH DOPBI drastically softened, and with 5 mol l−1 KOH almost dissolved at 80 ◦ C. Solubility of the polymer in KOH/EtOH mixture makes it possible to cast films from such solutions. This technique could allow to obtain predoped membranes with predefined KOH content. Investigation of production, characteristics and performance of such membranes in ADEFC is undergoing. Due to the fact that addition of EtOH to alkaline solution causes dissolution of the polymer, it is reasonable to suggest that it should increase the amount of KOH introduced into the membrane because of more easy rearrangement of polymer chains and less pronounced diffusion limitations. Fig. 2a shows that KOH concentration in the membrane slightly increased when EtOH concentration raised to 2 mol l−1 . More pronounced increase of doping level was observed when raising concentration of KOH in the doping solution (Fig. 2b). It has been shown earlier that methanol crossover through Nafion membranes depend on the ionic form of the resin [21]. This effect is most likely due to different content of water in the membrane and thus different solubility of alcohol in polymer gel. As it was shown in Fig. 2 different doping levels of DOPBI membrane correspond to different water content and thus can affect ethanol permeation. Fig. 3 shows that ethanol crossover reached its maximum at approximately 3.5 mol l−1 KOH. Further raising of alkali concentration led to decrease of EtOH permeability.

We attribute this fact to the increasing association of ethanol molecules in solutions with high ionic strength. Such behavior of amphiphilic substances is well known and is used in salting out process [23]. In our case ethanol molecules may, for example, self-assemble into micelles with polar hydroxide groups facing the solution phase [24]. Formation of bulky associates suppresses crossover due to diffusion limitation. Thus, the maximum ethanol permeability for DOPBI membrane was 8.6 × 10−8 cm2 s−1 . To ensure reproducibility of the developed method measurements were performed five times with various ethanol concentrations and membrane thicknesses. It was found that the obtained results fell within 10% relative. It is known, that in direct alcohol alkaline fuel cells ion transport through the membrane occurs in the direction opposite to fuel crossover which leads to its significant reduction [7]. However, this effect have not been yet characterized quantitatively. This effect can be evaluated if ethanol crossover is measured in the presence of counter flow through the membrane. Such flow can be established by employing osmotic drag of water. If pure water is placed on one side of the membrane and KOH solution on the other, water would be forced to diffuse through the membrane and dilute alkali solution.

Fig. 4. EtOH crossover reduction factor as a function of the flow of water and current density expressed in terms of in terms of the flow of molecular species through the membrane.

A.Y. Leykin et al. / Journal of Membrane Science 328 (2009) 86–89

We experimentaly determined that with different alkali concentration (C, mol l−1 ) in the range 1.5–3 mol l−1 against 1 mol l−1 EtOH on one side of the membrane and DI water on the other osmotic drag of water (OD, mol cm−1 s−1 ) is described by the simple equation: OD × 109 = 1.1 × C − 1.3 Reduction factor for ethanol crossover in the presence of osmotic drag of water was plotted against the latter (Fig. 4). We also expressed the osmotic drag as current density in terms of the flow of molecular species through the membrane, assuming that one H2 O molecule represents one OH− species to which corresponds one electron in the external circuit. If rough assumption is made that the flow of water molecules imposes comparable resistance to counter current of EtOH as OH− species, it can be concluded that increase of current density by 1 mA cm−2 leads to decrease of EtOH permeability by a factor of 4. It should be noted, that another flow in the membrane is KOH diffusion. When pure water was placed on one side of the membrane experimentally determined alkali flux was equal to 3.3 × 10−8 cm2 s−1 . 4. Conclusions • The developed rather simple method for investigation of ethanol permeability through polybenzimidazole membrane showed excellent reproducibility which falls within 10% relative. • Crossover measurements in alkaline solution provide more reliable data, than those, obtained in pure water, because in the first case hydration of the membrane due to doping is encountered, which can affect alcohol permeability. • Preliminary modeled estimation shows that increase of discharge current of the cell by 1 mA cm−2 leads to the reduction of the ethanol crossover through the membrane by a factor of 4. Acknowledgements This paper represents a part of the research project implemented by National Innovation Company “New Energy Projects”. References [1] A. Verma, A.K. Jha, S. Basu, Manganese dioxide as a cathode catalyst for a direct alcohol or sodium borohydride fuel cell with a flowing alkaline electrolyte, J. Power Sources 141 (2005) 30.

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