Bencai Lin*ab,
Gang Qiaoa,
Fuqiang Chua,
Shuai Zhanga,
Ningyi Yuanab and
Jianning Ding*abc
aSchool of Materials Science and Engineering, Jiangsu Collaborative Innovation Center for Photovolatic Science and Engineering, Changzhou University, Changzhou, 213164, Jiangsu, China. E-mail: linbencai@cczu.edu.cn; dingjn@cczu.edu.cn
bJiangsu Province Cultivation Base for State Key Laboratory of Photovoltaic Science and Technology, Changzhou University, Changzhou, 213164, Jiangsu, China
cMicro/Nano Science and Technology Center, Jiangsu University, Zhenjiang, 212013, China
First published on 4th January 2017
In this study, phosphoric acid doped hydrophobic ionic liquid-based composite membranes are successfully synthesized and characterized. 1-Vinyl-3-butylimidazolium bis(trifluoromethylsulfonyl)-imide ([VBIm][NTf2]) was synthesized and used as hydrophobic phase in the composite membranes. The H3PO4 uptake of the composite membranes increases with the increasing content of [VBIm][NTf2] and then decreases. The resultant composite membranes showed good thermal stability, mechanical properties and high proton conductivity (up to the order of 10−2 S cm−1 at 180 °C) at high temperatures under anhydrous conditions. The results of this study suggest that this type of PEMs have good perspectives for high temperature proton exchange membrane fuel cell applications.
Recently, several studies have been conducted for the preparation of novel or functionalized commercially proton exchange membranes with good fuel cell performance at elevated temperatures.8–14 Development of sulfonated polymers with the sulfonic acid groups is an important approach to preserve water and obtain high temperature PEMs.15 Though high degree of sulfonation results in high proton conductivity of PEMs, it also reduces the mechanical properties of membranes because of high swelling ratio.16 Both proton conductivity and mechanical properties of PEMs could be enhanced at elevated temperatures by adding hydrophilic inorganic additives into sulfonated polymers.17,18 However, this type of composite PEMs still cannot maintain high proton conductivity above 130 °C due to the evaporation of water.19
Phosphoric acid could be used as high temperature protonic conductors; it is an effective way to prepare high temperature PEMs by replacement of water with phosphoric acid.20–24 The dynamic hydrogen bond networks that formed from phosphoric acid, favor the transfer of protons.25,26 On the other hand, the incorporation of phosphoric acid could improve the water uptake of PEMs at high temperatures.27 Therefore, phosphoric acid doped PEMs have attracted much attention due to the high proton conductivity at elevated temperatures. Generally, more phosphoric acid uptake of the polymer systems leads to a higher conductivity of the membranes. However, too high phosphoric acid uptake of the PEMs often reduces the mechanical properties and chemical stabilities of the membranes significantly,28 making them unsuitable for practical use.
Recently, syntheses of new types of ionic polymers29,30 have been pursued to obtain PEMs with high proton conductivity and excellent mechanical properties. Si et al.29 prepared phosphoric acid doped cross-linked proton-conducting membranes, and the resultant composite membranes showed high thermal stability and high proton conductivity (at 160 °C). However, the membranes showed a high phosphoric acid uptake, and the mechanical properties of the membranes were not satisfactory. Therefore, synthesis of novel PEMs with excellent chemical and mechanical stability as well as high proton conductivity at high temperature is in great demand.
Yan et al. reported novel composite PEMs by doping protic ionic liquid into the membranes, and the composite membranes showed good mechanical properties and high proton conductivity due to the microscopic phase separation formed in the membranes.19 It is usually considered that membranes with a well-defined phase separation morphology show higher conductivity and mechanical properties than those without microscopic phase separation structure.31 In the present study, a hydrophobic ionic liquid, 1-vinyl-3-butylimidazolium bis(trifluoromethylsulfonyl)-imide, was synthesized and used for the preparation of high-temperature PEMs. A microscopic phase separation morphology is expected to be formed in the membranes due to the presence of a hydrophobic ionic liquid and phosphoric acid. The effect of the hydrophobic ionic liquid content on the membrane properties, such as thermal stability, mechanical properties, phosphoric acid uptake, swelling ratio, microstructure and proton conductivity, were systematically investigated.
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Scheme 1 Reaction scheme for the preparation of the phosphoric acid doped hydrophobic ionic liquid-based PEMs. |
A mixture of styrene/acrylonitrile (weight ratio, 1:
3, 30 wt%), [VBIm][Bi] (15–55 wt%), N-vinylimidazole (55–15 wt%), divinylbenzene (4 wt% for the formulation based on the weight of monomer), and 2 wt% of photoinitiator benzoin isobutyl ether was stirred and ultrasonicated to obtain a homogeneous solution, which was then cast onto a glass mold and photo-cross-linked by irradiation with UV light of 250 nm wavelength at room temperature. The resultant membrane samples were washed with ethanol and distilled deionized water and vacuum dried at 120 °C for 24 h. The prepared membranes were soaked into 85 wt% of H3PO4 at 80 °C for 24 h. The acid-doped membranes were wiped with filter paper and vacuum dried at 70 °C for 36 h to remove the absorbed water before characterization.
The swelling ratio (SR) of the composite membranes was determined from the dimensional differences between the wet and dry samples. The calculation was based on the following equation:
The chemical structures of [VBIm][NTf2]x-VImy membranes without phosphoric acid were characterized by FT-IR, and the results are shown in Fig. 1. All the samples showed the absorption bands of the cyano groups (CN) at 2241 cm−1, and the absorption bands at about 2934 cm−1 arose from the stretching vibration of methylene and methyl. The absorption bands at 1400–1600 cm−1 indicated the presence of polystyrene. Absorption bands at around 1553 and 704 cm−1 were attributed to the breathing vibration of imidazole ring. The absorption bands present at 1054 and 1134 cm−1 correspond to the symmetric and asymmetric stretching vibration of sulfone groups. The absorption peaks at 1348 and 1185 cm−1 indicate the presence of C–F groups in the membranes. The content of [VBIm][NTf2] increases from [VBIm][NTf2]15-VIm55 to [VBIm][NTf2]55-VIm15, and this trend was confirmed by a gradual increase in the intensity of the characteristic peaks of C–F groups.
Thermal properties of [VBIm][NTf2]x-VImy were characterized by thermogravimetric analysis (TGA) as shown in Fig. 2. A small weight loss was observed at about 100 °C due to the dehydration of [VBIm][NTf2]x-VImy. The main weight loss stage took place at above 300 °C that can be ascribed to the main-chain degradation of [VBIm][NTf2]x-VImy. No noticeable decomposition was observed for [VBIm][NTf2]x-VImy till 300 °C, confirming that this type of hydrophobic ionic liquid-based PEMs indeed confers a high thermal stability for their applications in high temperature PEMFCs.
The H3PO4 uptake and swelling ratio of [VBIm][NTf2]x-VImy are listed in Table 1. An imidazolium salt could be obtained from the reaction of N-vinylimidazole and H3PO4 (Scheme 1B). Generally, the H3PO4 uptake and swelling ratio in H3PO4 of the membranes decrease upon decreasing the content of N-vinylimidazole and increasing the content of [VBIm][NTf2] in the membranes. However, in the present study, the H3PO4 uptake of the membranes increases with an increase in the content of [VBIm][NTf2], and then decreases. This is probably because of loose structure formed by the increase of the [VBIm][NTf2] content due to its abundant pendent imidazolium cations and bistrifluoromethanesulfonimide anion, which results in a more free volume for adopting H3PO4 molecules,34 and this can be confirmed by the height images shown in Fig. 3A and Fig. 3C. Microscopic phase separation structure formed in the membranes can be confirmed by the AFM phase images. The darker areas represent the copolymer composed of styrene, acrylonitrile and N-vinylimidazole, and the brighter areas are associated with hard hydrophobic domain composed of [VBIm][NTf2] (Fig. 3B and D). However, [VBIm][NTf2] showed poor compatibility with H3PO4 due to its hydrophobic nature. Addition of excess [VBIm][NTf2] resulted in a decrease of H3PO4 uptake of the membranes, and [VBIm][NTf2]35-VIm35 showed the highest H3PO4 uptake (134.74 wt%) among the produced membranes. Just as expected, the change in the trends of the swelling ratio of [VBIm][NTf2]x-VImy is similar to that of H3PO4 uptake. Therefore, the H3PO4 uptake and swelling ratio of the membranes could be adjusted by varying the content of [VBIm][NTf2] in the membranes.
Membrane | H3PO4 uptake (%) | Swelling ratio in H3PO4 (%) |
---|---|---|
[VBIm][NTf2]15-VIm55 | 74.18 | 37 |
[VBIm][NTf2]25-VIm45 | 98.10 | 49 |
[VBIm][NTf2]35-VIm35 | 134.74 | 63 |
[VBIm][NTf2]45-VIm25 | 107.85 | 54 |
[VBIm][NTf2]55-VIm15 | 84.62 | 41 |
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Fig. 3 AFM tapping height and phase images of [VBIm][NTf2]15-VIm55 (A, B) and [VBIm][NTf2]35-VIm35 (C, D). |
The mechanical properties of [VBIm][NTf2]x-VImy were measured using a universal testing machine at room temperature with a crosshead speed of 5 mm min−1. Table 2 lists the tensile strength, tensile modulus and elongation at the break of [VBIm][NTf2]x-VImy membranes. The tensile strength of [VBIm][NTf2]x-VImy is in the range of 11.75–18.01 MPa, with the tensile modulus of 400–947 MPa, and the values of elongation at break of 8–24%. In the present study, [VBIm][NTf2] acts as a hydrophobic phase and endows the membranes good mechanical properties just like the backbone of Nafion membranes. Therefore, the tensile strength and tensile modulus of membranes increase with increasing [VBIm][NTf2] content in the membranes due to its hydrophobic nature. The results show that [VBIm][NTf2]x-VImy membranes possess sufficient mechanical strength for fuel cell applications, and the mechanical properties of [VBIm][NTf2]x-VImy could be adjusted easily by changing the content of [VBIm][NTf2] in the membranes.
Membranes | Tensile strength (MPa) | Tensile modulus (MPa) | Elongation at break (%) |
---|---|---|---|
[VBIm][NTf2]15-VIm55 | 11.75 ± 0.3 | 400.94 ± 20.0 | 8.56 ± 1.0 |
[VBIm][NTf2]25-VIm45 | 12.82 ± 1.1 | 553.70 ± 33.1 | 13.64 ± 0.6 |
[VBIm][NTf2]35-VIm35 | 14.44 ± 0.5 | 631.97 ± 34.3 | 24.36 ± 0.9 |
[VBIm][NTf2]45-VIm25 | 15.30 ± 0.8 | 841.52 ± 26.5 | 17.52 ± 0.5 |
[VBIm][NTf2]55-VIm15 | 18.01 ± 0.4 | 947.23 ± 17.8 | 10.77 ± 1.2 |
Proton conductivity is a key property for PEM materials. Herein, anhydrous proton conductivities of the membranes containing a variety amount of H3PO4 under 80–180 °C are depicted in Fig. 4, and enhancement in proton conductivity upon rising temperature has been observed for all the samples. For example, [VBIm][NTf2]15-VIm55 shows the conductivity of 5.6 × 10−3 S cm−1 at 80 °C and the value increases to 1.9 × 10−2 S cm−1 at 180 °C. Similar to the H3PO4 uptake and swelling ratio, the conductivity of the membranes at a given temperature increased with increasing content of [VBIm][NTf2] at first, and then decreased. No decay in proton conductivity was observed even at 180 °C for [VBIm][NTf2]x-VImy, which further confirms the excellent thermal stability of the membranes. Generally, higher H3PO4 uptake, leads to higher conductivity of the membranes. Among the [VBIm][NTf2]x-VImy membranes, [VBIm][NTf2]35-VIm35 showed the highest H3PO4 uptake. Furthermore, a well-defined phase separation morphology was formed in [VBIm][NTf2]35-VIm35, which favors the transmission of proton.35–38 It is not surprising that [VBIm][NTf2]35-VIm35 showed the highest conductivity (4.14 × 10−2 S cm−1 at 180 °C).
The conductivity activation energy (Ea) of [VBIm][NTf2]x-VImy can be obtained from the proton conductivity of the membranes.33 Fig. 5 displays the plots of ln(σ) vs. 1000/T (T is the absolute temperature) for [VBIm][NTf2]x-VImy. The Ea obtained from the slope of the line ranged from 14.67 to 17.27 kJ mol−1. The changing trend of Ea values is strongly related to the H3PO4 uptake of membranes, and [VBIm][NTf2]35-VIm35 showed the lowest Ea (14.67 kJ mol−1), indicating that proton transfer needs the least energy among the produced membranes. It could be concluded that the proton transport mechanism of [VBIm][NTf2]x-VImy is not changed under 80–180 °C with no change observed in the slope of the Arrhenius plot for the membranes (Fig. 5).29
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