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Anion exchange composite membranes composed of poly(phenylene oxide) containing quaternary ammonium and polyethylene support for alkaline anion exchange membrane fuel cell applications
Solid State Ionics 344 (2020) 115153
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Anion exchange composite membranes composed of poly(phenylene oxide) containing quaternary ammonium and polyethylene support for alkaline anion exchange membrane fuel cell applications Tae Yang Sona, Tae Ho Koa, Vijayalekshmi Vijayakumara, Kihyun Kimb, Sang Yong Nama,b, a b
T
⁎
Department of Material Engineering and Convergence Technology, Engineering Research Institute, Gyeongsang National University, Jinju 52828, Republic of Korea Department of Polymer Science and Engineering, Engineering Research Institute, Gyeongsang National University, Jinju 52828, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Alkaline anion exchange membrane fuel cell Anion exchange composite membrane Poly(phenylene oxide) Pore-filling method quaternary ammonium
Novel anion exchange composite membranes with controlled thicknesses have been developed by a simple porefilling method using poly(phenylene oxide) containing a quaternary ammonium group as an anion conducting polymer electrolyte and a polyethylene porous substrate support for alkaline anion exchange membrane fuel cell applications. The thickness optimization could be done by controlling the viscosity of the polymer electrolyte solution. The resulting composite membrane prepared using 5 centipoise (cP) viscous polymer electrolyte solution composed of the co-solvent system such as dimethylacetamide (DMAc) and ethanol showed a uniformly thin structure of about 40 μm thickness. Among the composite membranes developed here, the membrane with the optimized thickness exhibited high tensile strength (45.6 MPa), and a low contact angle (13.19°) indicative of a hydrophilic surface, as well as high hydroxide conductivity (38.9 mS/cm) due to the combined effect of the reinforcement of the robust PE substrate and the sound impregnation of aminated poly(phenylene oxide). Membrane electrode assembly using A-PPO30E200 revealed excellent cell performance (peak power density:153 mW/cm2 at 0.43 V) than those of commercial FAA-3-50 Fumatech anion exchange membrane (peak power density:114 mW/cm2 at 0.43 V) under the operating condition of 60 °C and 100% RH.
1. Introduction At present, environmental problems caused by industrialization and urbanization are becoming more serious. The pollutants produced not only harm public health but also harm the environment, bringing about climate change. In addition, carbon dioxide emissions and methane emissions from agriculture and livestock industries are affecting climate change to a level that cannot be ignored. There are many such impacts on climate change, but most of them are due to electric energy production and greenhouse gas emissions from vehicles. Specifically, carbon dioxide emissions from automobiles have been increasing steadily over the years, and emissions of carbon dioxide have increased, which is accelerating the rate of climate change. There are many types of eco-friendly cars, such as electric cars, hybrid cars, and fuel cell cars [1–4]. An electric vehicle is an automobile that does not use fossil fuel at all, generating propulsion through electric energy. However, electric vehicles are expensive, twice the price of domestic automobiles of the same class, have a charging time of up to 6 h, and have short charge
travel ranges [5]. Fuel cell vehicles use electricity produced from hydrogen and oxygen fuels and are known to be the most eco-friendly type, as they emit only water and water vapor as byproducts. Fuel cell vehicles have greater ranges and better energy efficiency than electric vehicles, but they have shortcomings with regard to the storage and transportation of hydrogen and the cost. However, numerous benefits of fuel cell vehicles relative to other types have directed progressive research in this field [6]. Fuel cells are systems that convert chemical energy into electrical energy, offsetting various disadvantages such as the emission of environmental pollutants and the difficulties related to energy production and transportation [7]. Among the various types of fuel cells, alkaline anion-exchange membrane fuel cells (AAEMFCs), which use anion-exchange membranes, have attracted significant interest and are the current focus in the fuel cell research community, as they use Pt-free catalysts, resulting in a lower cost. Unlike alkaline fuel cells (AFC), these fuel cells have the advantage of a solid anion-exchange electrolyte membrane rather than a liquid electrolyte [8–16]. The alkaline anion-exchange membrane fuel cell (AAEMFC) has higher
⁎ Corresponding author at: Department of Material Engineering and Convergence Technology, Engineering Research Institute, Gyeongsang National University, Jinju 52828, Republic of Korea. E-mail address: [email protected] (S.Y. Nam).
https://doi.org/10.1016/j.ssi.2019.115153 Received 20 July 2019; Received in revised form 10 October 2019; Accepted 14 November 2019 0167-2738/ © 2019 Published by Elsevier B.V.
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synthesized by varying the NBS quantity of 0.014 mol (1.83 g), 0.0125 mol (2.22 g), 0.0166 mol (2.96 g) and 0.0208 mol (3.70 g), respectively following the same above mentioned procedures.
efficiency than the polymer electrolyte membrane fuel cell (PEMFC) because the activation energy of the oxygen reduction reaction (ORR) is lower than that of the polymer electrolyte membrane fuel cell (PEMFC) [17]. Many hydrocarbon polymers such as poly(arylene ether sulfone), poly(ether ether ketone), poly(styrene-ethylene-butylene-styrene), poly (phenylene), poly(phenylene oxide), poly(imide) and AEMs based on poly(benzimidazole) are being developed for use in anion-exchange membrane fuel cells [18–31]. However, the ion conductivity and stability of anion-exchange membranes always exist in a trade-off relationship. Therefore, many researchers have focused on improving the ion conductivity and stability simultaneously. To avoid the dilemma between ionic conductivity and stability, various techniques are often utilized, including chemical cross-linking and physical reinforcement through a pore-filling process [32–34]. The pore-filling technique is one of the most effective methods for the preparation of composite membranes, showing high ion conductivity and stability upon the filling of an ionic polymer into the pores of a porous support. However, thicknesses are often different depending on the manufacturer. Therefore, in this study, we undertook thickness optimization of composite membranes using a PE support and a poly(phenylene oxide) polymer containing quaternary ammonium for anion-exchange membrane fuel cells. Detailed membrane properties, including the morphology, ion-exchange capacity, water uptake capability, mechanical properties and ion conductivity are also discussed here.
2.4. Preparation of a low viscous aminated poly(phenylene oxide) solution The use of a low viscous aminated poly(phenylene oxide) (A-PPO) solution can assist in the fabrication of a composite membrane with an optimized thickness. In order to obtain low viscosity, the A-PPO polymer solution was prepared using a co-solvent composed of a mixture of solvent that dissolves the A-PPO polymer and a low viscous solvent. The low viscous co-solvent used in this study was prepared using a mixture of DMAc and ethanol, and the viscosity could be controlled by adjusting the percentage of ethanol. Here, 1.5 g of 10 w/w % solution of A-PPO in DMAc with varying ethanol contents (i.e., ethanol/ A-PPO solution weight ratio of 0, 66, 100, 133, 160 and 200%) was used to make the polymer electrolyte solutions. The A-PPO solution prepared by co-solvent system was designated as A-PPOXEY solution, where X represent the molar ratio of quaternary ammonium group and Y represent the weight ratio of ethanol. For example, A-PPO30E200 solution indicates the solution composed of A-PPO with DS of 30.30% is dissolved in DMAc as 10 wt%, and the added amount of ethanol to the polymer solution is 200%. 2.5. Preparation of the anion exchange composite membrane
2. Experimental The membranes were fabricated by using a pore – filling method, in which the pores of the supporting substrate were filled with polymer electrolyte. An ultra-thin substrate (20 μm thick) was selected to minimize the membrane resistance. A-PPO was used as the anion exchange polymer owing to its excellent physicochemical properties. PE served as the porous substrate in this work due to their chemical stability and sufficient mechanical strength to suppress the swelling of the polymer electrolyte filled into the pores. The porous PE support was extended over a petri dish, after which the low viscous polymer solution was poured into the PE support. The petri dish was then dried in a vacuum oven at 60 °C for 12 h. During this process, the low viscous aminated poly(phenylene oxide) (A-PPO) penetrated into the pores of the PE support, and upon the evaporation of the co-solvent, the A-PPO polymer could completely fill the micropores. To prepare pristine APPO membranes without PE substrate, 0.25 g of A-PPO with different molar contents of quaternary ammonium group was dissolved in 4.75 g of DMAc. Then the A-PPO solution was poured into a glass petri-dish and removed the solvent at 60 °C in a vacuum oven for 12 h. The pristine and composite membranes after complete removal of the solvent were subjected to alkalization in 1 M KOH solution for 24 h at room temperature. The OH– exchanged membranes were washed several times with deionized water to remove any residual KOH and stored in DI water until the characterization process. Pristine membranes made by casting from A-PPO with different molar contents of quaternary ammonium group are referred to as the A-PPOX membrane. The PE supported composite membranes are named as A-PPOXEY membrane, where, X represents the molar ratio of quaternary ammonium group and Y indicates the weight ratio of ethanol. For example, APPO30E200 membrane is prepared by the A-PPO30E200 solution and porous PE substrate through pore-filling method.
2.1. Materials Poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) was purchased from Asahi Kasei Corp. N-bromosuccinimide (NBS), 2,2′-azobis(2-methylpropionitrile) (AIBN) and the trimethylamine solution (TMA solution) used here were obtained from Sigma Aldrich Chemical Corp. Chlorobenzene, ethyl acetate (EA), methyl alcohol (MeOH), ethyl alcohol (EtOH), dimethylacetamide (DMAc), potassium hydroxide (KOH), sodium hydroxide solution and hydrochloric acid solution were purchased from Daejung Chemical. The polyethylene support was sourced from W-SCOPE Korea. 2.2. Synthesis of brominated poly(phenylene oxide) (Br-PPO) The brominated poly(phenylene oxide) (Br-PPO) was synthesized in a two-neck round bottom flask fitted with a reflux condenser. In addition, the reaction was maintained in a nitrogen atmosphere. The PPO solution was prepared by dissolving poly(phenylene oxide) (PPO, 5 g, 41.61 mmol) in chlorobenzene as a solvent. In the next step, measured amount of 2,2′-azobis(2-methylpropionitrile) (AIBN, 0.05 g, 0.21 mmol) and n-bromosuccinimide (NBS, 3.7 g, 20.81 mmol) were added to the solution in the flask. The temperature was then raised to 135 °C and the reaction was continued for 3 h. After the reaction, the temperature was lowered to room temperature and the solution was precipitated using methyl alcohol, the Br-PPO obtained was washed several times using the same non-solvent. Finally, the product was dried in a vacuum oven at 60 °C for 24 h. 2.3. Synthesis of aminated poly(phenylene oxide) (A-PPO) 5 g Br-PPO was dissolved in 95 g DMAc solvent at 60 °C, after which 2.54 g of a 45% trimethylamine solution (TMA solution) was added to the Br-PPO solution. The reaction was sufficiently carried out so that all of the bromine ions could be replaced with ammonium ions. Afterwards, the resulting mixture was precipitated in ethyl acetate and the obtained solid aminated poly(phenylene oxide) (A-PPO) was washed with deionized water until the unreacted TMA was removed. The purified polymer was dried in a vacuum oven at 60 °C for 24 h. A-PPO with different molar contents of quaternary ammonium group were also
2.6. Characterization The 1H NMR spectra of Br-PPO and A-PPO were analyzed with a 1H nuclear magnetic resonance spectrometer (DRX300, Bruker). Chloroform-d was used as the solvent. Furthermore, the degree of substitution was calculated from the area integral of the peak. The theoretical ion exchange capacity was calculated according to the area ratio of the hydrogen in the benzene ring, which is not affected by the substitution reaction. The viscosity of the polymer solution was 2
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Fig. 1. Scheme of the aminated poly(phenylene oxide) (A-PPO).
60, and 80 °C and at a relative humidity of 100% by the impedance method in a range from 100 MHz to 1 MHz using electrochemical spectroscopy (SP-300, Bio Logic Science Instrument, UK). Finally, the ion conductivity was calculated using the following equation:
measured using a Brookfield viscometer (Dial Reading Viscometer, Brookfield, USA). The Brookfield viscometer is a rotary viscometer, and the viscosity can be determined by converting the torque value into the centipoise. Because the temperature condition during the preparation of the composite membrane was room temperature, the viscosity was also measured at room temperature. The contact angle between the polymer solution and the support surface was measured using contact angle analyzer (Phoenix-300 Touch, Surface Electro Optics Co., Ltd., Republic of Korea) according to the sessile droplet method. Briefly, a drop of polymer solution was deposited onto the support surface and the contact angle of the droplet with the surface was measured. Each contact angle was measured five times at different locations on each substrate sample and the average value was reported. The morphology of the composite membranes was observed by means of a FE-SEM (Field-Emission Scanning Electron Microscope, Philips XL30S FEG). The observations of the composite membranes consisted of surface and cross-section assessments. The FE-SEM images were used to confirm the impregnation of the polymer into the supports. The thickness of the composite membranes was reported by measuring the average thickness in nine different locations. For the cation exchange membranes, the acid – base titration method was used to determine the ion exchange capacity (IEC). However, for the anion exchange membrane, the back-titration method was used, in contrast to the cation exchange membranes. The composite membrane was immerged in a 0.01 M HCl solution for 24 h, and the IEC value was measured by titration using a 0.01 M NaOH solution. After finishing the titration process, the composite membrane was removed and dried in a vacuum oven at 60 °C. The weight of the dried composite membrane was then measured. Finally, the ion exchange capacity was calculated using the following equation,
IEC(meq/ g ) =
σ = L/ RA σ is the ion conductivity, R is the electrical resistance, and L and A are respectively the thickness and area of the membrane. Single cell performance using the membranes were analyzed at 60 °C under 100% RH and H2 and O2 feed flow rates of 200 mL/min and 400 mL/min respectively. The catalyst slurry was prepared using a mixture of Pt/C catalyst (46.2 wt%, Tanaka, Japan), 2-propanol and ionomer solution (Fumion FAA-3, 10 wt% in NMP, Fumactech, Germany) by ultra-sonicating method. The CCMs (Catalyst Coated Membranes) were prepared by spraying the slurry on the membrane (0.4 mg Pt/cm2) with an air spray gun. The MEA (Membrane Electrode Assembly) was sandwiched between gas diffusion layer (GDL, Sigracet 39BC) and gasket (Teflon, CNL energy) and the effective area was 9 cm2. 3. Results and discussions 3.1. Structural analysis Fig. 1 shows the preparation route of the aminated PPO (A-PPO). The A-PPO was synthesized by amination via a bromination reaction. In the first step, PPO was subjected to a bromination reaction with Nbromosuccinimide (NBS) and AIBN to yield Br-PPO. The Br-PPO was further reacted with trimethyl amine, yielding aminated PPO. The structure of the polymer at each step was confirmed by 1H NMR spectroscopy. Fig. 2 shows the 1H NMR spectra of the Br-PPO and A-PPO polymers. In the Fig. 2(a), the peak at 4.48 ppm was assigned to the hydrogen peak of the Br substituted portion in the methyl group of the PPO polymer (2H, -CH2Br). The chemical shifts at 6.60 ppm and 6.40 ppm were attributed to the proton peaks of the substituted benzene ring (1H, AreH) and the unsubstituted benzene ring (1H, AreH) respectively [35]. Finally, the introduction of the quaternary ammonium group was confirmed by the appearance after the reaction of trimethylamine of 9H at 3.0 ppm, corresponding to the methyl groups of quaternary ammonium (9H, -N(CH3)3). Moreover, the bromomethyl proton (2H, -CH2-Br) at 4.48 ppm completely shifted to 4.5 ppm after the reaction, confirming the complete conversion to the quaternary ammonium group (Fig. 2(b)) [35]. In addition, as shown in Fig. 2(c), the sizes of the peaks at 6.60 and 4.48 ppm were relative to the amounts and the degree of substitution was determined from their integral area values (Table 1).
(VHCl × MHCl ) − (VNaOH × MNaOH ) Wdry
where MHCl (M) and VHCl (mL) are correspondingly the concentration and volume of the initial HCl solution. MNaOH (M) and VNaOH (mL) are the concentration and volume of standard NaOH solution used for titration, respectively. Wdry (g) is the weight of the dry composite membrane. The water uptake was calculated according to the weight change of the composite membrane. To determine the water uptake value, dried (60 °C, 12 h) composite membrane was cut into rectangles of an appropriate size and then weighed (Wdry). These were then immersed in DI water for 24 h at room temperature. Excess water on the sample surface was wiped away with tissue paper and weighed (Wwet). The water uptake was calculated using the following equation:
Water uptake (%) =
Wwet − Wdry Wdry
× 100
The mechanical properties of the composite membrane were measured by using a universal testing machine (UTM, LR10K at LLOYD). Specimens for this test were prepared and measured according to ASTM D6385. The test was carried out at a tension speed of 5 mm/min. The ion conductivity of the composite membrane was determined using the measured value of the membrane resistance. The ion conductivity tests of the composite membranes were carried out at 25, 40,
3.2. Viscosity of the polymer solution For the successful fabrication of pore filling anion exchange membranes, the polymer electrolyte must be completely filled into the pores of the support substrate. Hence, control of the viscosity of the polymer solution is essential. In the present study, the low viscous polymer 3
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Fig. 2. 1H NMR spectra of the synthesized polymers: (a) brominated poly(phenylene oxide) (Br-PPO), (b) aminated poly(phenylene oxide) (A-PPO), and (c) Br-PPO polymers with different degrees of substitution.
Table 1 Synthesis and properties of aminated poly(phenylene oxide) (A-PPO). Sample
Feed molar ratio of NBS to PPO (%)
Degree of substitution (%)
IECNMR(meq/g)
IECEXP (meq/g)
A-PPO11 A-PPO15 A-PPO24 A-PPO30
25 30 40 50
10.73 15.38 23.89 30.30
0.85 1.19 1.78 2.18
0.96 1.32 1.49 2.31
solution was prepared using a co-solvent consisting of DMAc and ethanol. Ethanol has lower viscosity than DMAc such that the viscosity of the polymer solution is lowered. Simultaneously, ethanol is well volatilized and plays an important role in the fabrication of composite membranes with uniform thicknesses. Fig. 3 shows the viscosity of the polymer solution for the composite membranes according to the different weight ratio of ethanol. As the amount of ethanol in the polymer solution was increased, a gradual decrease in the viscosity was noted.
Fig. 3. Viscosity change of A-PPO30EY solution according to the different weight ratio of ethanol to A-PPO solution (Y = 66, 100, 133, 166 and 200).
support demonstrated a steady tendency to decrease upon an increase in the weight percentage of ethanol, which indicates the hydrophilicity and wettability of the polymer electrolyte solution onto the substrate. The polymer solution with the lowest contact angle would easily be impregnated inside the porous substrate and can be used in the fabrication of the composite membranes.
3.3. Effect of the co-solvent concentration on the contact angle of the polymer electrolyte solution The contact angles of the PE support against the polymer solution concentration are presented in the Fig. 4. The contact angles of the PE 4
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Fig. 4. Contact angles of the polymer electrolyte (A-PPO30EY) solutions containing different weight ratio of ethanol to A-PPO solution on the PE support: Y (%) = (A) 0, (B) 66, (C) 100, (D) 133, (E) 166, and (F) 200.
Fig. 6. FE-SEM images of the surface and cross–section of the porous PE support and A-PPO30E200 membrane prepared by pore-filling process.
3.4. Morphology of the composite membranes with the optimum thickness
membranes were about 40 ± 0.14 μm. The composite membrane with nearly uniform thicknesses was observed. As the amount of ethanol increased, thin composite membranes with a more uniform thickness were obtained. A-PPO30E200 showed the optimum performance in terms of viscosity and contact angle. The low contact angle observed indicates the best electrolyte filling inside the pores. Hence, the APPOXE200 was selected for further studies. The FE-SEM surface and cross-section of the PE porous support before and after pore filling are shown in Fig. 6. The surface and crosssection of the composite membrane showed a pore-filled structure without visible pores, indicating that the pores of the porous PE support are uniformly filled with the A-PPO polymer electrolyte. These
Fig. 5 shows the photographs of the composite membranes containing polymer electrolyte with different co-solvent concentrations. Before the characterization step, all the composite membranes were immerged in a 1 M KOH solution for conversion into the OH– form. As indicated in the figure, all the composite membranes are transparent and flexible. Filling the polymer electrolyte into the porous substrate is expected to allow the construction of a high density structure while suppressing excessive swelling. Thinner membranes are expected to show improved water transport and reduced IR resistance during the operation of the fuel cells in which they are applied. The thicknesses of these composite
Fig. 5. Photographs of porous substrate and composite membranes: (a) porous polyethylene substrate, (b) A-PPO30E66, (c) A-PPO30E100, (d) A-PPO30E133, (e) APPO30E166 and (f) A-PPO30E200. 5
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Fig. 7. Ion exchange capacity of A-PPOX and A-PPOXE200 membranes with different degree of substitution (X = 11, 15, 24 and 30).
Fig. 9. Water uptake values of A-PPOX and A-PPOXE200 membranes with different degree of substitution (X = 11, 15, 24 and 30).
observations confirm that good penetration of the A-PPO30E200 solution into the substrate occurs, leading to the successful formation of composite membranes.
of hydrophilic sites capable of absorbing water, thereby increasing the water uptake. This can be seen in the Fig. 9, in which the water uptake is proportional to the degree of substitution of quaternary ammonium and follows a trend similar to that of the IEC. The ion conductivity and mechanical properties of the anion exchange membranes were also affected by the water uptake. The water uptake in the composite membrane is lower than that of the free-standing membrane due to the hydrophobility of the PE support [36,37].
3.5. Ion exchange capacity (IEC) and water uptake The IEC was measured to evaluate the density of the active cation groups in the membrane, as this factor can significantly affect the water uptake and ion conductivity of the AEMs. Fig. 7 shows the ion exchange capacities of the prepared composite membranes. The IEC values of all the A-PPOXE200 membranes showed relatively lower than those of the corresponding A-PPOX membranes. The lower IEC values observed for the composite membranes were ascribed to the hydrophobic porous PE substrate, which accounted for a large portion in the AEM as compared to the APPO polymer electrolyte. Moreover, the ion exchange capacity of the composite membranes is proportional to the equivalent. A minimum IEC value of 0.30 meq/g was observed for A-PPO11E200, and it reached a maximum value of 1.32 meq/g for the A-PPO30E200. In addition, the effects on the ion exchange capacity according to the ethanol content in the polymer electrolyte are shown in Fig. 8. No significant change in the ion exchange capacity was noted when varying the ethanol content, implies ethanol only serves to optimize the thickness of the composite membrane and that the ion exchange capacity was dependent only on the amount of polymer filled in the porous support. In general, membranes with higher IEC values have a great number
3.6. Mechanical properties of thickness optimized composite membranes Fig. 10 shows the difference in the mechanical strength of the nonsupported cast membranes and pore filled composite membranes. The introduction of the PE support increased the overall mechanical strength. For the pristine membranes, when the molar ratio was high, due to the moisture absorption and the resulting hydration, the mechanical strength was weakened. However, when A-PPO was impregnated into the porous PE support, the overall mechanical strength was improved, which indicates that the introduction of the porous support can prevent the membrane from being damaged during the cell assembly and operation. The most dramatic improvement in the mechanical properties was observed in the A-PPO30E200, whose value increased from 28.2 to 45.6 MPa confirm the reinforcement effect of the mechanically robust PE support. 3.7. Ion conductivity The ion conductivity is a pivotal factor on which the fuel cell
Fig. 8. Ion exchange capacity of composite membranes prepared by APPO30EY solutions with different ethanol weight ratio (Y = 0, 66, 100, 133, 166 and 200).
Fig. 10. Mechanical properties of the A-PPOX and A-PPOXE200 membranes with different degree of substitution (X = 11, 15, 24 and 30). 6
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Fig. 11. Ion conductivity of (a) A-PPOXE200 membranes with different quaternary ammonium (X = 11, 15, 24 and 30) and (b) A-PPO30EY membranes with different ethanol weight ratio (Y = 66, 100, 133, 166 and 200).
performance was checked under the operating condition of H2/O2 feed flow rate of 0.2/0.4 cc min−1 at 60 °C and 100% relate humidity. As seen in Fig. 12, A-PPO30E200 based MEA showed an open circuit voltage (OCV) of 0.93 V and a maximum power density of 153 mW/cm2 at a current density of 454 mA/cm2. These values are higher than the Fumatech sample (FAA-3-50) (maximum power density: 114 mW/cm2 at current density: 266 mA/cm2, OCV: 0.91 V) in the fuel cells. Therefore, A-PPO30E200 membrane is expected to be an efficient anion exchange membrane candidate for the AAEMFC application. 4. Conclusions Anion exchange composite membrane with an optimized and uniform thickness was developed by a simple pore-filling method using poly(phenylene oxide) containing a quaternary ammonium group as the polymer electrolyte and polyethylene support as the reinforcing porous substrate. All of the composite membranes exhibited good mechanical strength and ion conductivity. Specifically, the composite membrane prepared with the A-PPO30E200 solution and the PE support showed a uniformly thin structure and ion conductivity of 38.9 mS/cm at 80 °C. MEA fabricated using A-PPO30E200 exhibited a maximum power density of about 153 mW/cm2 at 60 °C, which is comparatively better than the commercial Fumatech (FAA-3-50) membrane. These results indicate that the porefilled modified PPO membrane is a promising candidate for AAEMFC applications.
Fig. 12. Single cell test result of the MEA fabricated using A-PPO30E200 under H2/O2 conditions and 100% RH at 60 °C.
efficiency depends. The ion conductivities of the prepared composite membranes with different quaternary ammonium molar ratio of the APPO polymer electrolyte (A-PPOXE200) were measured at 25, 40, 60 and 80 °C. As shown in Fig. 11(a), the ion conductivity values increased with increase in the temperature and the molar ratio of the quaternary ammonium group. The A-PPO30E200 showed the highest overall conductivity. This was attributed to the high density of ion exchange sites, the proper connection of ion conduction channels, and appropriate level of water uptake, making the composite less resistant to hydroxide ion mobility. The A-PPO30E200 exhibited a maximum conductivity of 38.9 mS/cm at 80 °C due to the active ion mobility at high temperature [38]. To investigate whether the concentration of ethanol (co-solvent) has any effect on the conductivity of the composite membranes, the membranes with A-PPO30EY with varying ethanol quantities were subjected to conductivity tests at different temperatures (Fig. 11(b)). Similar to the ion exchange capacity outcomes, no difference in the ion conductivity of the composite membranes with different ethanol weight ratio was observed because the ion conductivity of the composite membranes is mainly affected by the ion conducting groups of the impregnated polymer electrolyte (Fig. 11(b)) [38].
Acknowledgement This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Ministry of Science, ICT & Future Planning (NRF-2015M1A2A2058028). References [1] J.M. Rust, T. Rust, Climate change and livestock production: a review with emphasis on Africa, S. Afr. J. Anim. Sci. 43 (2012) 256–257, https://doi.org/10.4314/ sajas.v43i3.3. [2] L.H. Baumgard, R.P. Rhoads, M.L. Rhoads, N.K. Gabler, J.W. Ross, A.F. Keating, R.L. Boddicker, S. Lenka, V. Sejian, Impact of climate on livestock production, in: V. Sejian, M. Naqvi, T. Ezeji, L.R. Lakritz (Eds.), Environmental Stress and Amelioration in Livestock Production, Springer Inc, Berlin, 2012, pp. 413–468. [3] F. Grelier, CO2 Missions from Cars: The Facts, Transport & Environment, Brussles (2018). [4] Green vehicle, Wikipedia, https://en.wikipedia.org/wiki/Green_vehicle, (2018) (accessed 21 October 2018). [5] What is an electric car, conserve energy future, https://www.conserve-energyfuture.com/advantages-and-disadvantages-of-electric-cars.php, (2018). [6] Advantages and disadvantages of fuel cell vehicle, car treatments, https:// cartreatments.com/fuel-cell-vehicle/, (2017) (accessed 14 August 2017). [7] A Basic Overview of Fuel Cell Technology, Smithsonian Institution, 2017, http:// americanhistory.si.edu/fuelcells/basics.htm (accessed 21 October 2018).
3.8. Single cell performance A-PPO30E200 was selected as the best performance composite membrane based on the IEC, water uptake and conductivity performances. Single cell test was carried out using A-PPO30E200 to check its applicability in AAEMFCs and the performance was compared with commercially available Fumatech FAA-3-50 membrane. The cell 7
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Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
Anion exchange composite membranes composed of poly(phenylene oxide) containing quaternary ammonium and polyethylene support for alkaline anion exchange membrane fuel cell applications Tae Yang Sona, Tae Ho Koa, Vijayalekshmi Vijayakumara, Kihyun Kimb, Sang Yong Nama,b, a b
T
⁎
Department of Material Engineering and Convergence Technology, Engineering Research Institute, Gyeongsang National University, Jinju 52828, Republic of Korea Department of Polymer Science and Engineering, Engineering Research Institute, Gyeongsang National University, Jinju 52828, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Alkaline anion exchange membrane fuel cell Anion exchange composite membrane Poly(phenylene oxide) Pore-filling method quaternary ammonium
Novel anion exchange composite membranes with controlled thicknesses have been developed by a simple porefilling method using poly(phenylene oxide) containing a quaternary ammonium group as an anion conducting polymer electrolyte and a polyethylene porous substrate support for alkaline anion exchange membrane fuel cell applications. The thickness optimization could be done by controlling the viscosity of the polymer electrolyte solution. The resulting composite membrane prepared using 5 centipoise (cP) viscous polymer electrolyte solution composed of the co-solvent system such as dimethylacetamide (DMAc) and ethanol showed a uniformly thin structure of about 40 μm thickness. Among the composite membranes developed here, the membrane with the optimized thickness exhibited high tensile strength (45.6 MPa), and a low contact angle (13.19°) indicative of a hydrophilic surface, as well as high hydroxide conductivity (38.9 mS/cm) due to the combined effect of the reinforcement of the robust PE substrate and the sound impregnation of aminated poly(phenylene oxide). Membrane electrode assembly using A-PPO30E200 revealed excellent cell performance (peak power density:153 mW/cm2 at 0.43 V) than those of commercial FAA-3-50 Fumatech anion exchange membrane (peak power density:114 mW/cm2 at 0.43 V) under the operating condition of 60 °C and 100% RH.
1. Introduction At present, environmental problems caused by industrialization and urbanization are becoming more serious. The pollutants produced not only harm public health but also harm the environment, bringing about climate change. In addition, carbon dioxide emissions and methane emissions from agriculture and livestock industries are affecting climate change to a level that cannot be ignored. There are many such impacts on climate change, but most of them are due to electric energy production and greenhouse gas emissions from vehicles. Specifically, carbon dioxide emissions from automobiles have been increasing steadily over the years, and emissions of carbon dioxide have increased, which is accelerating the rate of climate change. There are many types of eco-friendly cars, such as electric cars, hybrid cars, and fuel cell cars [1–4]. An electric vehicle is an automobile that does not use fossil fuel at all, generating propulsion through electric energy. However, electric vehicles are expensive, twice the price of domestic automobiles of the same class, have a charging time of up to 6 h, and have short charge
travel ranges [5]. Fuel cell vehicles use electricity produced from hydrogen and oxygen fuels and are known to be the most eco-friendly type, as they emit only water and water vapor as byproducts. Fuel cell vehicles have greater ranges and better energy efficiency than electric vehicles, but they have shortcomings with regard to the storage and transportation of hydrogen and the cost. However, numerous benefits of fuel cell vehicles relative to other types have directed progressive research in this field [6]. Fuel cells are systems that convert chemical energy into electrical energy, offsetting various disadvantages such as the emission of environmental pollutants and the difficulties related to energy production and transportation [7]. Among the various types of fuel cells, alkaline anion-exchange membrane fuel cells (AAEMFCs), which use anion-exchange membranes, have attracted significant interest and are the current focus in the fuel cell research community, as they use Pt-free catalysts, resulting in a lower cost. Unlike alkaline fuel cells (AFC), these fuel cells have the advantage of a solid anion-exchange electrolyte membrane rather than a liquid electrolyte [8–16]. The alkaline anion-exchange membrane fuel cell (AAEMFC) has higher
⁎ Corresponding author at: Department of Material Engineering and Convergence Technology, Engineering Research Institute, Gyeongsang National University, Jinju 52828, Republic of Korea. E-mail address: [email protected] (S.Y. Nam).
https://doi.org/10.1016/j.ssi.2019.115153 Received 20 July 2019; Received in revised form 10 October 2019; Accepted 14 November 2019 0167-2738/ © 2019 Published by Elsevier B.V.
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synthesized by varying the NBS quantity of 0.014 mol (1.83 g), 0.0125 mol (2.22 g), 0.0166 mol (2.96 g) and 0.0208 mol (3.70 g), respectively following the same above mentioned procedures.
efficiency than the polymer electrolyte membrane fuel cell (PEMFC) because the activation energy of the oxygen reduction reaction (ORR) is lower than that of the polymer electrolyte membrane fuel cell (PEMFC) [17]. Many hydrocarbon polymers such as poly(arylene ether sulfone), poly(ether ether ketone), poly(styrene-ethylene-butylene-styrene), poly (phenylene), poly(phenylene oxide), poly(imide) and AEMs based on poly(benzimidazole) are being developed for use in anion-exchange membrane fuel cells [18–31]. However, the ion conductivity and stability of anion-exchange membranes always exist in a trade-off relationship. Therefore, many researchers have focused on improving the ion conductivity and stability simultaneously. To avoid the dilemma between ionic conductivity and stability, various techniques are often utilized, including chemical cross-linking and physical reinforcement through a pore-filling process [32–34]. The pore-filling technique is one of the most effective methods for the preparation of composite membranes, showing high ion conductivity and stability upon the filling of an ionic polymer into the pores of a porous support. However, thicknesses are often different depending on the manufacturer. Therefore, in this study, we undertook thickness optimization of composite membranes using a PE support and a poly(phenylene oxide) polymer containing quaternary ammonium for anion-exchange membrane fuel cells. Detailed membrane properties, including the morphology, ion-exchange capacity, water uptake capability, mechanical properties and ion conductivity are also discussed here.
2.4. Preparation of a low viscous aminated poly(phenylene oxide) solution The use of a low viscous aminated poly(phenylene oxide) (A-PPO) solution can assist in the fabrication of a composite membrane with an optimized thickness. In order to obtain low viscosity, the A-PPO polymer solution was prepared using a co-solvent composed of a mixture of solvent that dissolves the A-PPO polymer and a low viscous solvent. The low viscous co-solvent used in this study was prepared using a mixture of DMAc and ethanol, and the viscosity could be controlled by adjusting the percentage of ethanol. Here, 1.5 g of 10 w/w % solution of A-PPO in DMAc with varying ethanol contents (i.e., ethanol/ A-PPO solution weight ratio of 0, 66, 100, 133, 160 and 200%) was used to make the polymer electrolyte solutions. The A-PPO solution prepared by co-solvent system was designated as A-PPOXEY solution, where X represent the molar ratio of quaternary ammonium group and Y represent the weight ratio of ethanol. For example, A-PPO30E200 solution indicates the solution composed of A-PPO with DS of 30.30% is dissolved in DMAc as 10 wt%, and the added amount of ethanol to the polymer solution is 200%. 2.5. Preparation of the anion exchange composite membrane
2. Experimental The membranes were fabricated by using a pore – filling method, in which the pores of the supporting substrate were filled with polymer electrolyte. An ultra-thin substrate (20 μm thick) was selected to minimize the membrane resistance. A-PPO was used as the anion exchange polymer owing to its excellent physicochemical properties. PE served as the porous substrate in this work due to their chemical stability and sufficient mechanical strength to suppress the swelling of the polymer electrolyte filled into the pores. The porous PE support was extended over a petri dish, after which the low viscous polymer solution was poured into the PE support. The petri dish was then dried in a vacuum oven at 60 °C for 12 h. During this process, the low viscous aminated poly(phenylene oxide) (A-PPO) penetrated into the pores of the PE support, and upon the evaporation of the co-solvent, the A-PPO polymer could completely fill the micropores. To prepare pristine APPO membranes without PE substrate, 0.25 g of A-PPO with different molar contents of quaternary ammonium group was dissolved in 4.75 g of DMAc. Then the A-PPO solution was poured into a glass petri-dish and removed the solvent at 60 °C in a vacuum oven for 12 h. The pristine and composite membranes after complete removal of the solvent were subjected to alkalization in 1 M KOH solution for 24 h at room temperature. The OH– exchanged membranes were washed several times with deionized water to remove any residual KOH and stored in DI water until the characterization process. Pristine membranes made by casting from A-PPO with different molar contents of quaternary ammonium group are referred to as the A-PPOX membrane. The PE supported composite membranes are named as A-PPOXEY membrane, where, X represents the molar ratio of quaternary ammonium group and Y indicates the weight ratio of ethanol. For example, APPO30E200 membrane is prepared by the A-PPO30E200 solution and porous PE substrate through pore-filling method.
2.1. Materials Poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) was purchased from Asahi Kasei Corp. N-bromosuccinimide (NBS), 2,2′-azobis(2-methylpropionitrile) (AIBN) and the trimethylamine solution (TMA solution) used here were obtained from Sigma Aldrich Chemical Corp. Chlorobenzene, ethyl acetate (EA), methyl alcohol (MeOH), ethyl alcohol (EtOH), dimethylacetamide (DMAc), potassium hydroxide (KOH), sodium hydroxide solution and hydrochloric acid solution were purchased from Daejung Chemical. The polyethylene support was sourced from W-SCOPE Korea. 2.2. Synthesis of brominated poly(phenylene oxide) (Br-PPO) The brominated poly(phenylene oxide) (Br-PPO) was synthesized in a two-neck round bottom flask fitted with a reflux condenser. In addition, the reaction was maintained in a nitrogen atmosphere. The PPO solution was prepared by dissolving poly(phenylene oxide) (PPO, 5 g, 41.61 mmol) in chlorobenzene as a solvent. In the next step, measured amount of 2,2′-azobis(2-methylpropionitrile) (AIBN, 0.05 g, 0.21 mmol) and n-bromosuccinimide (NBS, 3.7 g, 20.81 mmol) were added to the solution in the flask. The temperature was then raised to 135 °C and the reaction was continued for 3 h. After the reaction, the temperature was lowered to room temperature and the solution was precipitated using methyl alcohol, the Br-PPO obtained was washed several times using the same non-solvent. Finally, the product was dried in a vacuum oven at 60 °C for 24 h. 2.3. Synthesis of aminated poly(phenylene oxide) (A-PPO) 5 g Br-PPO was dissolved in 95 g DMAc solvent at 60 °C, after which 2.54 g of a 45% trimethylamine solution (TMA solution) was added to the Br-PPO solution. The reaction was sufficiently carried out so that all of the bromine ions could be replaced with ammonium ions. Afterwards, the resulting mixture was precipitated in ethyl acetate and the obtained solid aminated poly(phenylene oxide) (A-PPO) was washed with deionized water until the unreacted TMA was removed. The purified polymer was dried in a vacuum oven at 60 °C for 24 h. A-PPO with different molar contents of quaternary ammonium group were also
2.6. Characterization The 1H NMR spectra of Br-PPO and A-PPO were analyzed with a 1H nuclear magnetic resonance spectrometer (DRX300, Bruker). Chloroform-d was used as the solvent. Furthermore, the degree of substitution was calculated from the area integral of the peak. The theoretical ion exchange capacity was calculated according to the area ratio of the hydrogen in the benzene ring, which is not affected by the substitution reaction. The viscosity of the polymer solution was 2
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Fig. 1. Scheme of the aminated poly(phenylene oxide) (A-PPO).
60, and 80 °C and at a relative humidity of 100% by the impedance method in a range from 100 MHz to 1 MHz using electrochemical spectroscopy (SP-300, Bio Logic Science Instrument, UK). Finally, the ion conductivity was calculated using the following equation:
measured using a Brookfield viscometer (Dial Reading Viscometer, Brookfield, USA). The Brookfield viscometer is a rotary viscometer, and the viscosity can be determined by converting the torque value into the centipoise. Because the temperature condition during the preparation of the composite membrane was room temperature, the viscosity was also measured at room temperature. The contact angle between the polymer solution and the support surface was measured using contact angle analyzer (Phoenix-300 Touch, Surface Electro Optics Co., Ltd., Republic of Korea) according to the sessile droplet method. Briefly, a drop of polymer solution was deposited onto the support surface and the contact angle of the droplet with the surface was measured. Each contact angle was measured five times at different locations on each substrate sample and the average value was reported. The morphology of the composite membranes was observed by means of a FE-SEM (Field-Emission Scanning Electron Microscope, Philips XL30S FEG). The observations of the composite membranes consisted of surface and cross-section assessments. The FE-SEM images were used to confirm the impregnation of the polymer into the supports. The thickness of the composite membranes was reported by measuring the average thickness in nine different locations. For the cation exchange membranes, the acid – base titration method was used to determine the ion exchange capacity (IEC). However, for the anion exchange membrane, the back-titration method was used, in contrast to the cation exchange membranes. The composite membrane was immerged in a 0.01 M HCl solution for 24 h, and the IEC value was measured by titration using a 0.01 M NaOH solution. After finishing the titration process, the composite membrane was removed and dried in a vacuum oven at 60 °C. The weight of the dried composite membrane was then measured. Finally, the ion exchange capacity was calculated using the following equation,
IEC(meq/ g ) =
σ = L/ RA σ is the ion conductivity, R is the electrical resistance, and L and A are respectively the thickness and area of the membrane. Single cell performance using the membranes were analyzed at 60 °C under 100% RH and H2 and O2 feed flow rates of 200 mL/min and 400 mL/min respectively. The catalyst slurry was prepared using a mixture of Pt/C catalyst (46.2 wt%, Tanaka, Japan), 2-propanol and ionomer solution (Fumion FAA-3, 10 wt% in NMP, Fumactech, Germany) by ultra-sonicating method. The CCMs (Catalyst Coated Membranes) were prepared by spraying the slurry on the membrane (0.4 mg Pt/cm2) with an air spray gun. The MEA (Membrane Electrode Assembly) was sandwiched between gas diffusion layer (GDL, Sigracet 39BC) and gasket (Teflon, CNL energy) and the effective area was 9 cm2. 3. Results and discussions 3.1. Structural analysis Fig. 1 shows the preparation route of the aminated PPO (A-PPO). The A-PPO was synthesized by amination via a bromination reaction. In the first step, PPO was subjected to a bromination reaction with Nbromosuccinimide (NBS) and AIBN to yield Br-PPO. The Br-PPO was further reacted with trimethyl amine, yielding aminated PPO. The structure of the polymer at each step was confirmed by 1H NMR spectroscopy. Fig. 2 shows the 1H NMR spectra of the Br-PPO and A-PPO polymers. In the Fig. 2(a), the peak at 4.48 ppm was assigned to the hydrogen peak of the Br substituted portion in the methyl group of the PPO polymer (2H, -CH2Br). The chemical shifts at 6.60 ppm and 6.40 ppm were attributed to the proton peaks of the substituted benzene ring (1H, AreH) and the unsubstituted benzene ring (1H, AreH) respectively [35]. Finally, the introduction of the quaternary ammonium group was confirmed by the appearance after the reaction of trimethylamine of 9H at 3.0 ppm, corresponding to the methyl groups of quaternary ammonium (9H, -N(CH3)3). Moreover, the bromomethyl proton (2H, -CH2-Br) at 4.48 ppm completely shifted to 4.5 ppm after the reaction, confirming the complete conversion to the quaternary ammonium group (Fig. 2(b)) [35]. In addition, as shown in Fig. 2(c), the sizes of the peaks at 6.60 and 4.48 ppm were relative to the amounts and the degree of substitution was determined from their integral area values (Table 1).
(VHCl × MHCl ) − (VNaOH × MNaOH ) Wdry
where MHCl (M) and VHCl (mL) are correspondingly the concentration and volume of the initial HCl solution. MNaOH (M) and VNaOH (mL) are the concentration and volume of standard NaOH solution used for titration, respectively. Wdry (g) is the weight of the dry composite membrane. The water uptake was calculated according to the weight change of the composite membrane. To determine the water uptake value, dried (60 °C, 12 h) composite membrane was cut into rectangles of an appropriate size and then weighed (Wdry). These were then immersed in DI water for 24 h at room temperature. Excess water on the sample surface was wiped away with tissue paper and weighed (Wwet). The water uptake was calculated using the following equation:
Water uptake (%) =
Wwet − Wdry Wdry
× 100
The mechanical properties of the composite membrane were measured by using a universal testing machine (UTM, LR10K at LLOYD). Specimens for this test were prepared and measured according to ASTM D6385. The test was carried out at a tension speed of 5 mm/min. The ion conductivity of the composite membrane was determined using the measured value of the membrane resistance. The ion conductivity tests of the composite membranes were carried out at 25, 40,
3.2. Viscosity of the polymer solution For the successful fabrication of pore filling anion exchange membranes, the polymer electrolyte must be completely filled into the pores of the support substrate. Hence, control of the viscosity of the polymer solution is essential. In the present study, the low viscous polymer 3
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Fig. 2. 1H NMR spectra of the synthesized polymers: (a) brominated poly(phenylene oxide) (Br-PPO), (b) aminated poly(phenylene oxide) (A-PPO), and (c) Br-PPO polymers with different degrees of substitution.
Table 1 Synthesis and properties of aminated poly(phenylene oxide) (A-PPO). Sample
Feed molar ratio of NBS to PPO (%)
Degree of substitution (%)
IECNMR(meq/g)
IECEXP (meq/g)
A-PPO11 A-PPO15 A-PPO24 A-PPO30
25 30 40 50
10.73 15.38 23.89 30.30
0.85 1.19 1.78 2.18
0.96 1.32 1.49 2.31
solution was prepared using a co-solvent consisting of DMAc and ethanol. Ethanol has lower viscosity than DMAc such that the viscosity of the polymer solution is lowered. Simultaneously, ethanol is well volatilized and plays an important role in the fabrication of composite membranes with uniform thicknesses. Fig. 3 shows the viscosity of the polymer solution for the composite membranes according to the different weight ratio of ethanol. As the amount of ethanol in the polymer solution was increased, a gradual decrease in the viscosity was noted.
Fig. 3. Viscosity change of A-PPO30EY solution according to the different weight ratio of ethanol to A-PPO solution (Y = 66, 100, 133, 166 and 200).
support demonstrated a steady tendency to decrease upon an increase in the weight percentage of ethanol, which indicates the hydrophilicity and wettability of the polymer electrolyte solution onto the substrate. The polymer solution with the lowest contact angle would easily be impregnated inside the porous substrate and can be used in the fabrication of the composite membranes.
3.3. Effect of the co-solvent concentration on the contact angle of the polymer electrolyte solution The contact angles of the PE support against the polymer solution concentration are presented in the Fig. 4. The contact angles of the PE 4
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Fig. 4. Contact angles of the polymer electrolyte (A-PPO30EY) solutions containing different weight ratio of ethanol to A-PPO solution on the PE support: Y (%) = (A) 0, (B) 66, (C) 100, (D) 133, (E) 166, and (F) 200.
Fig. 6. FE-SEM images of the surface and cross–section of the porous PE support and A-PPO30E200 membrane prepared by pore-filling process.
3.4. Morphology of the composite membranes with the optimum thickness
membranes were about 40 ± 0.14 μm. The composite membrane with nearly uniform thicknesses was observed. As the amount of ethanol increased, thin composite membranes with a more uniform thickness were obtained. A-PPO30E200 showed the optimum performance in terms of viscosity and contact angle. The low contact angle observed indicates the best electrolyte filling inside the pores. Hence, the APPOXE200 was selected for further studies. The FE-SEM surface and cross-section of the PE porous support before and after pore filling are shown in Fig. 6. The surface and crosssection of the composite membrane showed a pore-filled structure without visible pores, indicating that the pores of the porous PE support are uniformly filled with the A-PPO polymer electrolyte. These
Fig. 5 shows the photographs of the composite membranes containing polymer electrolyte with different co-solvent concentrations. Before the characterization step, all the composite membranes were immerged in a 1 M KOH solution for conversion into the OH– form. As indicated in the figure, all the composite membranes are transparent and flexible. Filling the polymer electrolyte into the porous substrate is expected to allow the construction of a high density structure while suppressing excessive swelling. Thinner membranes are expected to show improved water transport and reduced IR resistance during the operation of the fuel cells in which they are applied. The thicknesses of these composite
Fig. 5. Photographs of porous substrate and composite membranes: (a) porous polyethylene substrate, (b) A-PPO30E66, (c) A-PPO30E100, (d) A-PPO30E133, (e) APPO30E166 and (f) A-PPO30E200. 5
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Fig. 7. Ion exchange capacity of A-PPOX and A-PPOXE200 membranes with different degree of substitution (X = 11, 15, 24 and 30).
Fig. 9. Water uptake values of A-PPOX and A-PPOXE200 membranes with different degree of substitution (X = 11, 15, 24 and 30).
observations confirm that good penetration of the A-PPO30E200 solution into the substrate occurs, leading to the successful formation of composite membranes.
of hydrophilic sites capable of absorbing water, thereby increasing the water uptake. This can be seen in the Fig. 9, in which the water uptake is proportional to the degree of substitution of quaternary ammonium and follows a trend similar to that of the IEC. The ion conductivity and mechanical properties of the anion exchange membranes were also affected by the water uptake. The water uptake in the composite membrane is lower than that of the free-standing membrane due to the hydrophobility of the PE support [36,37].
3.5. Ion exchange capacity (IEC) and water uptake The IEC was measured to evaluate the density of the active cation groups in the membrane, as this factor can significantly affect the water uptake and ion conductivity of the AEMs. Fig. 7 shows the ion exchange capacities of the prepared composite membranes. The IEC values of all the A-PPOXE200 membranes showed relatively lower than those of the corresponding A-PPOX membranes. The lower IEC values observed for the composite membranes were ascribed to the hydrophobic porous PE substrate, which accounted for a large portion in the AEM as compared to the APPO polymer electrolyte. Moreover, the ion exchange capacity of the composite membranes is proportional to the equivalent. A minimum IEC value of 0.30 meq/g was observed for A-PPO11E200, and it reached a maximum value of 1.32 meq/g for the A-PPO30E200. In addition, the effects on the ion exchange capacity according to the ethanol content in the polymer electrolyte are shown in Fig. 8. No significant change in the ion exchange capacity was noted when varying the ethanol content, implies ethanol only serves to optimize the thickness of the composite membrane and that the ion exchange capacity was dependent only on the amount of polymer filled in the porous support. In general, membranes with higher IEC values have a great number
3.6. Mechanical properties of thickness optimized composite membranes Fig. 10 shows the difference in the mechanical strength of the nonsupported cast membranes and pore filled composite membranes. The introduction of the PE support increased the overall mechanical strength. For the pristine membranes, when the molar ratio was high, due to the moisture absorption and the resulting hydration, the mechanical strength was weakened. However, when A-PPO was impregnated into the porous PE support, the overall mechanical strength was improved, which indicates that the introduction of the porous support can prevent the membrane from being damaged during the cell assembly and operation. The most dramatic improvement in the mechanical properties was observed in the A-PPO30E200, whose value increased from 28.2 to 45.6 MPa confirm the reinforcement effect of the mechanically robust PE support. 3.7. Ion conductivity The ion conductivity is a pivotal factor on which the fuel cell
Fig. 8. Ion exchange capacity of composite membranes prepared by APPO30EY solutions with different ethanol weight ratio (Y = 0, 66, 100, 133, 166 and 200).
Fig. 10. Mechanical properties of the A-PPOX and A-PPOXE200 membranes with different degree of substitution (X = 11, 15, 24 and 30). 6
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Fig. 11. Ion conductivity of (a) A-PPOXE200 membranes with different quaternary ammonium (X = 11, 15, 24 and 30) and (b) A-PPO30EY membranes with different ethanol weight ratio (Y = 66, 100, 133, 166 and 200).
performance was checked under the operating condition of H2/O2 feed flow rate of 0.2/0.4 cc min−1 at 60 °C and 100% relate humidity. As seen in Fig. 12, A-PPO30E200 based MEA showed an open circuit voltage (OCV) of 0.93 V and a maximum power density of 153 mW/cm2 at a current density of 454 mA/cm2. These values are higher than the Fumatech sample (FAA-3-50) (maximum power density: 114 mW/cm2 at current density: 266 mA/cm2, OCV: 0.91 V) in the fuel cells. Therefore, A-PPO30E200 membrane is expected to be an efficient anion exchange membrane candidate for the AAEMFC application. 4. Conclusions Anion exchange composite membrane with an optimized and uniform thickness was developed by a simple pore-filling method using poly(phenylene oxide) containing a quaternary ammonium group as the polymer electrolyte and polyethylene support as the reinforcing porous substrate. All of the composite membranes exhibited good mechanical strength and ion conductivity. Specifically, the composite membrane prepared with the A-PPO30E200 solution and the PE support showed a uniformly thin structure and ion conductivity of 38.9 mS/cm at 80 °C. MEA fabricated using A-PPO30E200 exhibited a maximum power density of about 153 mW/cm2 at 60 °C, which is comparatively better than the commercial Fumatech (FAA-3-50) membrane. These results indicate that the porefilled modified PPO membrane is a promising candidate for AAEMFC applications.
Fig. 12. Single cell test result of the MEA fabricated using A-PPO30E200 under H2/O2 conditions and 100% RH at 60 °C.
efficiency depends. The ion conductivities of the prepared composite membranes with different quaternary ammonium molar ratio of the APPO polymer electrolyte (A-PPOXE200) were measured at 25, 40, 60 and 80 °C. As shown in Fig. 11(a), the ion conductivity values increased with increase in the temperature and the molar ratio of the quaternary ammonium group. The A-PPO30E200 showed the highest overall conductivity. This was attributed to the high density of ion exchange sites, the proper connection of ion conduction channels, and appropriate level of water uptake, making the composite less resistant to hydroxide ion mobility. The A-PPO30E200 exhibited a maximum conductivity of 38.9 mS/cm at 80 °C due to the active ion mobility at high temperature [38]. To investigate whether the concentration of ethanol (co-solvent) has any effect on the conductivity of the composite membranes, the membranes with A-PPO30EY with varying ethanol quantities were subjected to conductivity tests at different temperatures (Fig. 11(b)). Similar to the ion exchange capacity outcomes, no difference in the ion conductivity of the composite membranes with different ethanol weight ratio was observed because the ion conductivity of the composite membranes is mainly affected by the ion conducting groups of the impregnated polymer electrolyte (Fig. 11(b)) [38].
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3.8. Single cell performance A-PPO30E200 was selected as the best performance composite membrane based on the IEC, water uptake and conductivity performances. Single cell test was carried out using A-PPO30E200 to check its applicability in AAEMFCs and the performance was compared with commercially available Fumatech FAA-3-50 membrane. The cell 7
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