Article pubs.acs.org/IECR
Effect of Methylimidazole Groups on the Performance of Poly(phenylene oxide) Based Membrane for High-Temperature Proton Exchange Membrane Fuel Cells Wei Wu,†,‡ Gongwen Zou,†,‡ Xuying Fang,§ Chuanbo Cong,†,‡ and Qiong Zhou*,†,‡ †
Beijing Key Laboratory of Failure, Corrosion and Protection of Oil/Gas Facilities Materials and ‡Department of Materials Science and Engineering, China University of Petroleum Beijing, Beijing 102249, China § Marsh (China) Insurance Brokers Co., Ltd., Beijing 100001, China ABSTRACT: A series of phosphoric acid (PA) doped imidazolium poly(phenylene oxide) (PPO) membranes with different methylimidazole (MeIM) contents were prepared to tailor the performance of solidstate membranes. First, brominated poly(phenylene oxide) (BPPO) was synthesized by methyl bromination. Then BPPO was reacted with MeIM and doped with PA. The process was confirmed by Fourier transform infrared spectroscopy and 13C NMR. The PA absorption ability of the imidazolium poly(phenylene oxide) (PPO-MeIM) membranes changed with increasing MeIM content. PPO-MeIM membrane showed the best proton conductivity of 6.79 × 10−2 S cm−1 at 0% relative humidity, the highest mechanical strength of 4.8 MPa at the molar ratio of 4:10 (M-3#) at 30 °C, and high power density of 260 mW cm−2 without additional humidification at 160 °C. Results indicated that incorporation of an appropriate amount of MeIM groups can achieve the best proton conduction performance and mechanical properties.
1. INTRODUCTION The proton exchange membrane fuel cell (PEMFC) is a kind of electrochemical energy conversion device, known for its good security, high energy conversion efficiency, environmental safety, and low operation noise.1−4 Proton exchange membranes (PEMs) are one of the key materials of PEMFCs, and they play important roles in blocking fuel gas, conducting protons, and attaching electric catalyst. To date, commercial PEMs, such as Nafion,5 which is perfluorosulfonic acid ionomer membrane, are widely used in industrial PEMFC systems. However, these types of PEMs normally work below 100 °C due to the limitation of water-assisted proton conductivity, and high cost.6,7 Recently, PEMs operating at temperatures of 120− 200 °C have a lot of advantages: their high tolerance to CO, reduced use of Pt, fast electrode kinetics, and simplified management systems of fuel, water, and heat.8−11 Development of nonfluorinated PEM which can tolerate high temperature and low humidity in PEMFCs is a key issue to be solved. In the process of developing high-temperature PEMs, alkaline polymers are used as proton acceptors in the normal acid−base reaction, constructing macromolecular ion pairs, which is known as the complexation of polymeric bases and inorganic acids. Since phosphoric acid (PA) can form dynamic hydrogen bonds by developing proton donor and proton acceptor groups without the prerequisite of H2O, it becomes the most commonly used proton acid. As Kreuer et al. indicate, the protons can break and form hydrogen bonds to achieve transportation, as shown in the Grotthuss mechanism.12−15 © XXXX American Chemical Society
Moreover, phosphoric acid also has a great performance in the field of thermal stability and vapor pressure while in hightemperature circumstances. Considering the features of excellent thermal stability and superior mechanical strength, PA-doped polybenzimidazole (PBI) membranes have been generally examined as a solution for high-temperature proton exchange membranes (HTPEMs).16,17 The imidazole groups and PA molecules form acid−base pairs in the backbone of PBI to construct hydrogen bond networks.18 Proton conduction depends on hydrogen bond networks. PBI membranes must possess high PA doping levels to achieve high proton conductivity. However, abundant PA results in migration of PA and poor durability of PEMFC after long time operations. To solve this problem, a large number of PBI nanocomposites, including those with added inorganic fillers, have been developed and studied.19−21 Interfacial interaction of inorganic fillers with PBI chains improves mechanical properties of nanocomposite membranes.22 Meanwhile, PBI presents poor solubility and processability of costing.23 The toxicity of 3,3′,4,4′-tetraaminobiphenyl (TAB), the monomer of PBI, also poses a concern.24 Researchers improved the solubility of PBI by N-alkyl grafting25 and replaced TAB with tetraamine 2,6-bis(3′,4′-diaminophenyl)-4-phenylpyridine (Py-TAB).26 Received: Revised: Accepted: Published: A
May 22, 2017 August 23, 2017 August 28, 2017 August 28, 2017 DOI: 10.1021/acs.iecr.7b02094 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
2. MATERIALS AND METHODS 2.1. Materials. PPO was purchased from Gaodong Plastic Co. MeIM (99%), chlorobenzene (99%), N-methyl pyrrolidone (NMP, 99%), and PA (85 wt %) were obtained from Tianjing Guangfu Fine Chemical Research Institute. Liquid bromine (Br2, 99%) was supplied by Sinopharm Chemical Reagent Co., Ltd. All the materials and chemicals were used without humidification. 2.2. Synthesis of Imidazolium PPO (PPO-MeIM) and Fabrication of Membranes. As shown in Scheme 1, PPOMeIM membranes were fabricated by two steps: benzyl bromination and reaction with MeIM. First, PPO was completely dissolved in chlorobenzene, and homogeneous solutions were added to a three-necked flask. The polymer concentration was 10 wt %. Br2 was dissolved in chlorobenzene to form 10 wt % solution by dropwise addition for 5 h. The mixture was maintained at 130 °C for 24 h under nitrogen atmosphere.28 After bromination, the mixture was rapidly poured into methanol to get a light-yellow precipitate. The product was washed thoroughly with methanol and deionized water and finally dried in a vacuum oven at 60 °C. Fabrication of PPO-MeIM membrane was accomplished through gentle and facile heating and curing method.28 BPPO polymer was dissolved in NMP at 80 °C for 3 h in an oven and used to prepare 10 wt % homogeneous solutions. Then, different mole ratios of MeIM were added to BPPO/NMP solution under ultrasonication for 30 min. Molar ratios of MeIM to BPPO were 1:10 (M-1#), 2:10 (M-2#), 4:10 (M-3#), and 8:10 (M-4#). The obtained mixtures reacted at 70 °C for 24 h. Membranes were fabricated by solution casting of the above reaction product onto dishes, followed by drying at 80 °C for 12 h and 100 °C for 24 h to graft the MeIM groups. Obtained membranes were peeled off, washed thoroughly with demineralized water to remove excess Br2, and dried at 40 °C for 12 h. Prepared membranes were immersed in 85 wt % H3PO4 for 24 h at room temperature, dried at 80 °C, and kept in the oven as standby application. 2.3. Characterizations. 2.3.1. Structural Analysis. 13C nuclear magnetic resonance (NMR) spectra of membranes were tested on a 400 MHz WB solid-state NMR spectrometer. The degree of bromination of BPPO was calculated from 13C NMR data. Fourier transform infrared spectroscopy (FT-IR) was performed on a Bruker (TensorII, Germany) spectrometer equipped with a DTGS detector and a ZnSe crystal as
Other researchers had paid attention to PA-doped HTPEMs based on sulfonated polyether ether ketone (SPEEK) and polysulfone (PSU) by introducing the ionic liquid cation of 1methylimidazolium (MeIM).27,28 Yang et al.2 studied the influence of the structure of imidazole groups on properties of PSU-based HTPEMs. Membrane with a long-tailed decyl side chain in imidazole groups achieved the highest conductivity of 0.038 S cm−1 at 160 °C without humidifying and a tensile strength of 5 MPa at room temperature. Poly(phenylene oxide) (PPO) is an engineering polymer exhibiting good thermal stability, high strength, and low cost. PPO has been used as a PEM operating by sulfonation29 and bromination.30 Acid−base pairs composed of imidazole and phosphoric acid can realize proton conductivity. Previous studies prepared proton exchange membranes with saturated imidazole content to dope more PA, leading to migration of PA and poor durability of PEMFC after long operation. Researchers observed that unsaturated imidazole in sulfonated PPO improved conductivities.29 Unfortunately, no systematic study has been carried out to analyze the effect of imidazole content on the performance of high-temperature membranes. Table 1 shows the mechanical properties of membranes prepared by doping phosphoric acid with different bromination Table 1. Properties of Different Levels of PPO-MeIM Membranes at 0% RH, 30 °C, and Thickness of Each Membrane within 250−350 μm tensile strength (MPa)
B8-MeIM B12-MeIM B16-MeIM
undoped sample
doped PA
conductivity (160 °C) (S cm−1)
50.60 42.90 35.30
12.10 5.20 1.50
0.01 0.05 0.03
degrees of brominated PPO (BPPO) pick-MeIM in our previous work. The PEM obtained based on B12 which is 12 g of PPO with 12 g of Br2, exhibited the best comprehensive properties. Surprisingly, membranes obtained by reacting B12 with different amounts of MeIM-doped PA can exhibit different properties. This paper will report such processes and explain the important role played by hydrogen bonding which is influenced by the amounts of MeIM in the conductivity, thermal stability, and mechanical strength of PEM. Scheme 1. Synthetic Procedure of PPO-MeIM
B
DOI: 10.1021/acs.iecr.7b02094 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
was sprayed using a sprayer with a loading of 0.5 mg cm−2, whereas that of the anode was 0.3 mg cm−2. The spraying temperature was 50 °C, and the spraying time was 3 h. Carbon paper was placed on both sides of the proton exchange membrane and then hot-pressed at 60 psi for 60 s at 60 °C. Proton exchange membranes were tested from 120 to 160 °C on a fuel cell system (Greenlight G20, Canada) with unhumidified H2 and O2 at a rate of 150 mL min−1. The initial dimensions of cut membrane samples were 30 mm × 30 mm. The thickness of each membrane was within 250−350 μm measured by a spiral micrometer.
attenuated total reflection accessory between 4000 and 600 cm−1. bromination degree = [−CH3(Cghop) − −CH3(Cgho)]/−CH3(Cghop)
(1)
where −CH3(Cgho) and −CH3(Cghop) are integral areas from the 13C NMR spectra. 2.3.2. Thermal Properties. Thermogravimetric analyses (TGA; NETZSCH TG209F3) were performed in a nitrogen atmosphere at a flow rate of 100 mL min−1and heating rate of 10 °C min−1 from 30 to 600 °C.30 2.3.3. Acid Doping and Swelling. Acid-doped membranes were fabricated by immersing the membranes in 85 wt % PA solution at 30 °C for 24 h. The membranes were then placed in a vacuum oven at 100 °C for 24 h to reduce absorbed water. The acid-doping level (ADL) of a membrane is explained as the molar number of PA per mole of MeIM group in the membrane and calculated as below: ADL (mol) = 82(W2 − W1)/(98W )
3. RESULTS AND DISCUSSION 3.1. Structural Analyses. Chemical structures of synthesized PPO-MeIM were characterized by 13C NMR spectroscopy. Figure 1 shows 13C NMR spectra of PPO, BPPO, and membranes M-1# to M-4#. Contrasting curves of BPPO and PPO, with the resonance at 26.9 ppm attributed to the bromomethyl group (−CH2Br), showed good agreement with the successful preparation of BPPO. Meanwhile, the bromination degree of BPPO can be calculated according to the integral area ratio of −CH3(Cgho) and −CH3(Cghop). The substitution degree of BPPO reached 29%, indicating that 29% of methyl groups in PPO were substituted by bromine in the present condition.28 As shown in Figure 1, the grafted imidazole groups were present on NMR spectra between M-x# (x = 1, 2, 3, 4) membranes and BPPO. Taking M-1# membrane as an example, characteristic peaks at 36.9 and 48.5 ppm in the spectrum were assigned to MeIM. This finding indicates successful incorporation of MeIM in the PPO system. Furthermore, the peak at 26.9 ppm, which originated from the bromomethyl group in BPPO, disappeared and was replaced by a new one at 29.8 ppm. The peak shifted toward low-field regions probably because of the deshielding of electrophilic methylimidazole groups.2 Similar results were observed for the other three imidazolium membranes of M-2#, M-3#, and M-4#. NMR results indicated successful synthesis of PPO-MeIM membranes. Synthesized PPO-MeIM polymers were first characterized by FT-IR. As shown in Figure 2, two strong absorption peaks at 2850 and 2925 cm−1 in the spectra of M-1# to M-4# membranes corresponded to the C−H stretching vibration of saturated alkyl in MeIM,31 respectively, whereas strong absorption peaks at 1571 cm−1 corresponded to the stretching vibration of imidazolium cations;32−34 these values proved the existence of MeIM groups in prepared membranes. We also could see the characteristic wide bands at 3400 cm−1 were probably related to the stretching vibration of O−H groups of water.35 As previously reported, BPPO membranes are hydrophobic in nature and can hardly absorb water from ambient atmosphere. After doping with MeIM groups, PPOMeIM membranes become hydrophilic and can easily absorb water from ambient apmosphere. Although the membranes had been predried, stretching vibration peaks of O−H groups from water were observed in the FT-IR spectra. A similar phenomenon was also observed for PBI membranes.36 FT-IR results confirmed nucleophilic substitution reaction or electrostatic interactions between BPPO and MeIM. 3.2. Thermal Stability. Another important property of polymer based membranes for high-temperature fuel cells is thermal stability. Figure 3 shows TGA curves for BPPO and M1# to M-4# samples. The BPPO sample showed three
(2)
where W2 and W1 represent the weights of membrane after and before PA doping treatment, respectively, and W is the weight of MeIM doping bromomethylated membranes.29 The swelling ratios of the area and the volume of a membrane sample by the acid doping were calculated as follows: area swelling ratio (%) = (S2 − S1)/S1 × 100%
(3)
volume swelling ratio (%) = (V2 − V1)/V1 × 100%
(4)
where S2 and S1 correspond to area dimensions after and before PA doping, and V2 and V1 are the volume dimensions after and before PA doping, respectively.2 2.3.4. Mechanical Properties. Tensile stress−strain curves of membranes were tested by using a mechanical property testing instrument (CMT6502, SANS Company, China). Dumbbellshaped membrane samples were cut at dimensions of 40 mm length, 10 mm width, and membrane thickness within 250−350 μm, as measured by a spiral micrometer. Measurements were performed with a constant rising speed of 5 mm min−1 at 0% relative humidity (RH) and 30 °C. 2.3.5. Conductivity Tests. Conductivity was measured by using a two-electrode conductivity cell with an ac impedance of 1−1000 kHz and an oscillating voltage of 10 mV. The membrane thickness was recorded. Whole cells were heated by an oil bath to control the test temperature. All conductivity measurements were performed under the atmosphere which was without any humidification. To remove water from the membranes, membrane samples were preheated at 120 °C for 2 h until constant weight was reached. The thickness of each membrane was within 250−350 μm, as determined by a spiral micrometer. Conductivity σ was calculated as σ (S cm−1) = L /(RA)
(5)
where L refers to the membrane thickness, R is the impedance value of a membrane, and A represents the area of the electrodes. 2.3.6. Single Cell Performance Test. The membrane electrode assembly was accomplished by sandwiching the blended membrane between a Pt/C catalyst anode and a Pt catalyst cathode to evaluate the single cell performance of the high-temperature fuel cells (HT-FCs). Proton exchange membranes (30 mm × 30 mm) were prepared. Pt/C catalyst C
DOI: 10.1021/acs.iecr.7b02094 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
Figure 2. FT-IR spectra of BPPO and PPO-MeIM membranes.
Figure 3. TGA curves of BPPO and PPO-MeIM membranes.
was attributed to degradation, which was mainly cleavage of the ether bond of the PPO backbone. With doping of additional quantities of MeIM, the temperature for initial degradation reached 200 °C. The first loss was due to volatilization of MeIM (the MeIM boiling point is 198 °C). As shown in Figure 3, a small mass loss (
