Sulfonated Poly(arylene ether nitrile)-Based Composite Membranes Enhanced with Ca2+ Bridged Carbon Nanotube-Graphene Oxide Networks

As the core component of proton exchange membrane fuel cells, proton exchange membranes (PEM) have attracted much attention of researchers. To trade-off the proton conductivity, dimensional stability and anti-oxidation ability of PEM, graphene oxide (GO) and acidized multi-walled carbon nanotubes (MWCNT) using calcium ion as coordination bridge (GO-Ca2+-MWCNT) was synthesized, and then incorporated into sulfonated poly(arylene ether nitrile) (SPEN) to fabricate SPEN/GO-Ca2+-MWCNT organic–inorganic composite membranes by solution-casting method and explore the influence of varying loading on performances as PEM. It was found that the proton conductivity of the composite membranes was higher than that of SPEN, while maintaining better dimensional stability, excellent anti-oxidation ability and good mechanical properties. All of these were attributed to the formation of three-dimensional structure between GO and MWCNT bridged by Ca2+ and the interaction between the sulfonic acid group and calcium ions in SPEN/GO-Ca2+-MWCNT composites. Particularly, the SPEN/GO-Ca2+-MWCNT-1 composite membrane exhibited excellent tensile strength of 71.45 MPa, better thermal stability as well as high proton conductivity (0.054 S/cm at 30 °C, and 0.193 S/cm at 90 °C), above 10–2 S/cm, satisfying the requirement of PEM. All in all, the results indicate that the filler with three-dimensional network structure can effectively improve the performances of SPEN, and the prepared composite membranes show potential applications in many fields.


Introduction
Recently, with the increasing serious problem of energy shortages, exploring new pollution-free energies are urgently needed. Proton exchange membrane fuel cells have become the focus of many researchers due to its advantages, such as high energy density, environment-friendly and low pollution from combustion products [1][2][3]. Proton exchange membranes (PEM) play a pivotal role in proton conduction and simultaneously act as separator between fuel and oxidant, which can directly affect the performances of fuel cells [4]. Therefore, it is necessary to improve the quality of PEMs, such as high proton conductivity, better dimensional stability and outstanding anti-oxidation ability.
As a kind of high-performance engineering plastics, poly(arylene ether nitrile)s (PEN) possess excellent comprehensive properties of high-temperature resistance, radiation resistance, mechanical strength, molding and processing, which have broad application prospect in many fields 1 3 [5][6][7][8]. To explore the use value of PEN as PEM, sulfonic acid groups were introduced into the main chain of PEN to synthetize a series of sulfonated poly(arylene ether nitrile) s (SPEN) with different structures. In previous works, the researchers found that the SPEN with large quantity of sulfonic acid groups possesses high proton conductivity, but may cause excessive swelling and poor dimensional stability [9]. That is to say, it is difficult for single SPEN to balance the relationship of various performances. The fabrication of organic-inorganic hybrid composite membranes is an efficient approach to achieve excellent performances by combining the advantages of both organic and inorganic components [10].
Graphene oxide (GO) and multi-walled carbon nanotubes (MWCNT) have attracted extensive attention due to the superior properties such as low-weight, large specific surface area, high electrical conductivity, extraordinary mechanical, optical, and thermal properties [11][12][13]. In the previous works, the doping of single GO (or MWCNT) and SPEN not only can greatly improve the proton transfer ability and impede methanol permeability, but also can significantly enhance the mechanical and thermal properties of PEMs [14][15][16]. As we know, hybrid fillers consisting of two or more different components may play synergistic enhancement effect on polymer matrix and endow better performances than single filler/polymer composites [17][18][19]. Zhang et al. reported the enhanced performance of mixed fillers composed of functionalized graphene and functionalized MWCNT on the electrical conductivity and tensile modulus of poly(ether sulfone) (PES) composites [20]. However, the complicated synthesis procedures limit the broad applications, while a simple, eco-friendly and inexpensive strategy is urgently needed for industrial applications. Divalent metal ions possess strong coordination ability for carboxyl, hydroxyl and other oxygen-containing functional groups. Among these, calcium ion (Ca 2+ ) attracts our attention for the low price, environmental friendly and extensive sources. The metal ions coordination not only can effectively bridge between GO and acidified MWCNT, forming threedimensional structure, but also can be fitted with a variety of polymer matrices by π-π interaction [21,22], significantly improving the performances.
In this work, the GO and MWCNT bridged by calcium ion (GO-Ca 2+ -MWCNT) was prepared and incorporated into SPEN to explore the effects of GO and MWCNT on dimensional stability, anti-oxidation ability, proton conductivity, tensile and thermal properties. The possible proton conducting mechanism of the SPEN/GO-Ca 2+ -MWCNT composite membranes was also proposed. All the results prove that the three-dimensional structured GO-Ca 2+ -MWCNT exhibits a favorable synergistic enhancement effect, which could provide an alternative approach in fields of pollution-free energies.

Preparation of MWCNT
Acidified MWCNT was prepared by the traditional method of strong acid oxidation [26]. The detailed steps were as follows: 5 g MWCNT, 300 mL H 2 SO 4 and 100 mL HNO 3 (volume ratio of 3:1) were mixed and slowly put into threeneck flask, vibrating by ultrasonic sound and stirring for 3 h at 60 °C. Then, the mixed solution was filtered and washed with deionized water until the PH = 7. Finally, acidified MWCNT can be obtained after grinded and dried at 80 °C for 48 h.

Preparation of the GO-Ca 2+ -MWCNT
GO (25 mg) and acidulated MWCNT (75 mg) (the mass ratio of GO and MWCNT is 1:3) were added into NMP solution (20 mL), vibrating by ultrasonic sound for 4 h to obtain homogeneous dispersion. Then, different amounts of CaCl 2 was added into the mixed solution, which the ratio of Ca 2+ and C was controlled to be 0.001, 0.002, 0.004 and 0.006 mmol mg −1 , respectively, to explore the optimal Ca 2+ concentration. The mixture was stirred simultaneously with sonication for 48 h at 45 °C to ensure the complete coordination of Ca 2+ with GO and MWCNT. Finally, the GO-Ca 2+ -MWCNT suspension was put into glass bottles in liquid nitrogen for preliminary frozen about 15 min, and transferred into vacuum freezing dryer at − 40 °C for 48 h. The GO-Ca 2+ -MWCNT with three-dimensional structure can be obtained. Besides, the blended particle of GO and MWCNT (1:3) without Ca 2+ coordination was also prepared for comparison based on above same method.

Characterizations
Fourier transform infrared (FTIR) spectra were recorded on Nicolet IS10 (Thermo Fisher, USA) in KBr pellets between 4000 and 400 cm −1 . UV-Vis spectra were tested by UV-Visible spectrophotometer (TU1800, China) in the wavelength range of 200-600 nm. Scanning electron microscope (SEM, FEI INSPECT F50, USA) was used to observe the morphologies of GO-Ca 2+ -MWCNT and SPEN-based composite membranes. The membranes were fractured in liquid nitrogen and sprayed gold for 20 min. The thermogravimetric analysis was performed on TA Instruments Q50 (TA Instruments Ltd., USA) under N 2 atmosphere and the program was as follows: heating from room temperature to 150 °C at the rate of 10 °C min −1 to remove the thermal history, and then cooling to room temperature naturally, heating to 600 °C at the rate of 20 °C min −1 . Mechanical properties of the membranes were conducted by the SANS microcomputer-controlled electronic universal testing machine (CMT6104) at fully dry state and room temperature. The sizes of sample were 1 cm × 10 cm and the stretching rate was 5 mm min −1 .
The oxidation stability of the membranes was measured the residual weight by soaking in Fenton's reagent (3 wt% H 2 O 2 containing 4 ppm Fe 2+ ) for 8 h at 80 °C. The water uptake (WU), swelling ratio (SR) and proton conductivity (σ) of the membranes were determined as reported literatures [27][28][29].

Optimal Concentration of Ca 2+
To explore the optimal concentration of Ca 2+ in GO-Ca 2+ -MWCNT filler, Fig. 2   , which is ascribed to the coordination of bivalent metal ions and carboxylic acids [31]. Figure 4 shows the ultraviolet absorption spectra of MWCNT, GO, MWCNT/GO and GO-Ca 2+ -MWCNT. Both GO and MWCNT exhibit one absorption peak between 200 and 300 nm due to the strong absorption of NMP solution, suggesting good dispersion of GO and MWCNT in NMP [32]. Besides, the peak at ~ 250 nm of GO-Ca 2+ -MWCNT has obvious decrease, presumably due to the consumption of the C=O band, indicating the successful coordination of divalent metal ions and carboxylic acids. The similar conclusion has been obtained in previous literatures on Zn 2+ and Cu 2+ [33,34].

Characterization of GO-Ca 2+ -MWCNT
To intuitively observe the three-dimensional structure of GO-Ca 2+ -MWCNT, the SEM morphologies of MWCNT/  Fig. 5a, the MWCNT/GO particle possesses no structural cross-linking between GO and MWCNT due to the simple blend of GO and MWCNT. Besides, the SEM image of MWCNT/GO has little differences with that of pure GO, which is ascribed to the fact that the nanoscale of acidified MWCNT is very small compared with GO. Figure 5b exhibits that GO and MWCNT were cross-linked via bridge action to form three-dimensional structure, which can be seen clearly from the amplification area, as shown in Fig. 5c. SEM images demonstrate the successful coordination of the Ca 2+ between GO and MWCNT. This structure provides more efficient transport pathways for protons to transfer in SPEN membranes.
TGA curves were used to characterize the weight loss of GO, MWCNT and GO-Ca 2+ -MWCNT from room temperature to 600 °C under N 2 atmosphere, as shown in Fig. 6. From Fig. 6, all of the GO, MWCNT and GO-Ca 2+ -MWCNT display two significant weight loss states at about 100-230 °C and 230-600 °C, respectively. One is the decomposition of oxygen-containing groups such as carboxyl, hydroxyl and epoxy groups, and the other is the splitting of carbon skeleton structure. In the decomposition temperatures of 100-230 °C, the decomposition degree of oxygen-containing groups of GO is about 34.9%, while the MWCNT is about 2.43%. In addition, the decomposition degree of oxygen-containing groups of GO-Ca 2+ -MWCNT is about 4.79%, far lower than that of GO, which is speculated to be the uncoordinated calcium ions crosslinking with oxygen-containing functional groups on GO and MWCNT. Therefore, the above results confirm that the GO and MWCNT were coordinated by calcium ions. Figure 7 shows the SEM images of pure SPEN and PEN/ GO-Ca 2+ -MWCNT-1 composite membrane. As shown in Fig. 7a, the morphology of SPEN is relative smooth, while the addition of GO-Ca 2+ -MWCNT filler makes the crosssectional of composite membranes rougher than pure SPEN, as shown in Fig. 7b. The GO-Ca 2+ -MWCNT particles cannot cause obvious cracks or pinholes in composite membranes, indicating the excellent interfacial adhesion to SPEN matrix, which can be seen clearly from the marked rectangular boxed areas in Fig. 7b. Besides, Fig. 7b also well demonstrates that the particles possess better dispersion in SPEN matrix, which is the prerequisite for better performances of composite membranes. Therefore, the GO-Ca 2+ -MWCNT particles with three-dimensional structure play an important role in improving the performances of SPEN-based composite membranes.

Thermal and Mechanical Stability of the Membranes
Thermal stability and tensile properties are important factors to determine the performances of proton exchange   Table 1 exhibits the thermogravimetric decomposition data of SPEN and SPEN-based composite membranes with different GO-Ca 2+ -MWCNT contents. The thermal stability of composites increases with the increasing content of GO-Ca 2+ -MWCNT, which also validates the above results. In addition, the 5% and 10% decomposition temperatures of composite membranes are all above 320 °C and 365 °C, respectively, far higher than that of pure SPEN, indicating that GO-Ca 2+ -MWCNT as the filler has a positive impact on the thermal stability of SPEN-based composite membranes. Table 2 presents the tensile strength and Young's modulus of pure SPEN and SPEN/GO-Ca 2+ -MWCNT composite   37 MPa, respectively, as the GO-Ca 2+ -MWCNT content increases to 2 wt%, which is due to the agglomeration phenomenon of excessive GO-Ca 2+ -MWCNT particles. Even so, the tensile strength and Young's modulus of SPEN/ GO-Ca 2+ -MWCNT composites are still higher than that of SPEN. The combination effect of sulfonic acid groups and calcium ions also can synergistically enhance the tensile properties of the membranes [36]. All the above results affirm the good mechanical strength of SPEN/GO-Ca 2+ -MWCNT composites, which is enough for the applications as proton exchange membrane.

Oxidative Stability of the Membranes
Oxidation stability is vulnerable to judge the ability to resist free radicals of the membranes. All the membranes were soaked into Fenton's reagent (3 wt% H 2 O 2 containing 4 ppm Fe 2+ ) for 8 h at 80 °C to simulate the environment of oxidative radicals attacking membranes and measure the residual weight percentages of membranes. As shown in Table 3, the residual weight percentages of pure SPEN and SPEN/GO-Ca 2+ -MWCNT composite membranes are 94.3%, 96.2%, 98.7% and 99.9%, respectively. The residual weight percentages of SPEN/GO-Ca 2+ -MWCNT are far higher than that of SPEN, and it increases with the increasing filler content, indicating that the GO-Ca 2+ -MWCNT filler possesses excellent ability to resist free radicals. This phenomenon can be explained by the following aspects. Firstly, GO possesses strong stability and oxidation resistance due to the large number of oxygen-containing functional groups, which can effectively resist the attack of oxidative free radicals [37,38]. Secondly, the oxygen-containing functional groups of GO-Ca 2+ -MWCNT and SPEN can form hydrogen bonds interaction, enhancing the interfacial adhesion and resisting the oxidation of the membranes [39]. Finally, the interaction between the calcium ions of GO-Ca 2+ -MWCNT and the sulfonic acid groups of SPEN also can function positively on the resistance to free radicals [38]. Therefore, the SPEN/ GO-Ca 2+ -MWCNT composite membranes possess excellent oxidation stability against Fenton's reagent. Figure 9 and Table 3 present the water uptake and swelling ratio of SPEN and SPEN/GO-Ca 2+ -MWCNT composite membranes at 30 °C and 80 °C, respectively. The water uptake and swelling ratio of SPEN/GO-Ca 2+ -MWCNT composite membranes have a slight decrease compare with SPEN, which is due to the reduced free volume occupied by the inorganic filler [40]. The interaction between the sulfonic acid group and calcium ions also can suppress the penetration of water molecules. Besides, temperature has significant effect on the water uptake and swelling ratio, as shown in Fig. 9. The water uptake and swelling ratio of all membranes at 80 °C are higher than that of 30 °C, which is caused by the increased transmission rate of water molecular and the formation of more hydronium ions (H 3 O + ). In addition, the influence on the water uptake and swelling ratio at 80 °C is greater than 30 °C, which is mainly due to the fact that the water absorbing capacity of sulfonic acid groups is weaker under low temperature condition. It's worth noting that the all the swelling ratio of SPEN/GO-Ca 2+ -MWCNT composite membranes is less than 40% at 80 °C, suggesting outstanding dimensional stability, speculating that the GO-Ca 2+ -MWCNT particles limit the infinite expansion of the membranes.

Proton Conductivity of the Membranes
The influences of the GO-Ca 2+ -MWCNT filler on the proton conduction properties of SPEN were also examined. Figure 10 provides a comparison among the proton conductivity of pure SPEN and the three different kinds of SPEN composite membranes (filler contents = 0.5, 1.0 and 2.0 wt%) at different temperatures and 100% humidity. Obviously, the proton conductivity of SPEN-based composite membranes rises with the increase content of GO-Ca 2+ -MWCNT and reaches the maximum value at 1 wt%. The three-dimensional structured GO-Ca 2+ -MWCNT particles provide more efficient channels for proton transmission and increase proton transport efficiency [41]. However, the proton conductivity of SPEN/GO-Ca 2+ -MWCNT-2 membrane has a downward trend, even lower than the SPEN. This is caused by the aggregation of excessive filler content, blocking the channels of proton transport in the composite membrane and greatly reducing the efficiency of proton transport [42,43]. In addition, Fig. 10 also exhibits that the proton conductivity of SPEN and composite membranes rises dramatically in the range of 30-90 °C. The substantial increasing of water absorption can lead to the sharp rise of proton conductivity due to the carrier effect of water molecules on proton conduction. Especially, the proton conductivity of SPEN/ GO-Ca 2+ -MWCNT-2 reaches the maximum of 0.193 S/cm at 90 °C, 19.2% higher than that of SPEN, showing excellent proton transport capability. Therefore, the GO-Ca 2+ -MWCNT filler with three-dimensional structure can significantly promote the proton conductivity of SPEN. The enhancement effect of GO-Ca 2+ -MWCNT on proton conduction of composites can be explained by the Grotthuss mechanism. The surface of GO-Ca 2+ -MWCNT filler still contains a large number of oxygen-containing functional groups, such as hydroxyl and uncoordinated carboxyl groups, which can form hydrogen bonds networks with the sulfonic acid groups of SPEN chain. According to the Grotthuss mechanism, the protons can be transferred from one carrier (sulfonic acid groups) to another (oxygen-containing functional groups) by the hydrogen bond networks. By this way, the continuous proton transport channels can be formed to accelerate proton transmission and trigger the synergistic enhancement effect between GO-Ca 2+ -MWCNT and SPEN, thus improving proton conductivity of SPEN-based composites. The three-dimensional structured GO-Ca 2+ -MWCNT can facilitate the construction of long-term and continuous proton transmission paths and accelerate the proton transfer rate in the composite membranes [44]. Besides, the interaction between the sulfonic acid group of SPEN and calcium ions of GO-Ca 2+ -MWCNT can endow excellent interface adhesion to SPEN composites, which provides a prerequisite for the effective proton transport.

Conclusions
Herein, the GO-Ca 2+ -MWCNT filler was synthetized by the coordination of calcium ions between GO and MWCNT to form three-dimensional structure, and then incorporated into SPEN to fabricate SPEN/GO-Ca 2+ -MWCNT composite membranes. The dispersion and interfacial adhesion of GO-Ca 2+ -MWCNT and SPEN matrix was improved due to the interaction between the sulfonic acid group of SPEN and calcium ions of GO-Ca 2+ -MWCNT. The three-dimensional structured GO-Ca 2+ -MWCNT particles provided more efficient channels for proton transmission, resulting in the improvement of proton conduction, and synchronously Fig. 10 Proton conductivity of pure SPEN and SPEN/GO-Ca 2+ -MWCNT composite membranes at different temperatures endowed the excellent oxidation stability against Fenton's reagent to the composite membranes. As a result, the SPEN/ GO-Ca 2+ -MWCNT composite membranes exhibited good resistance to high-temperature, high capability of efficient transferring protons, excellent dimensional stability, good mechanical strength and outstanding anti-oxidation ability. Especially, the SPEN/GO-Ca 2+ -MWCNT-1 composite membrane presented the optimal performances, such as the strongest tensile strength of 71.45 MPa, and the highest proton conductivity (0.054 S/cm at 30 °C, and 0.193 S/ cm at 90 °C), while maintaining better anti-oxidation and dimensional stability. Therefore, a convenient and effective strategy was provided and confirmed to design high-performances SPEN-based composite membranes.