3.1 Optimal concentration of Ca2+
To explore the optimal concentration of Ca2+ in GO-Ca2+-MWCNT filler, Fig. 2 displays the dispersion states of GO-Ca2+-MWCNT with different Ca2+/C ratio (i.e., 0.001, 0.002, 0.004 and 0.006 mmol mg−1) in NMP solvent (20 mL), standing for 48 h. It is obvious that the dispersity becomes worse as the concentration increases. At 0.006 mmol mg−1 Ca2+/C ratio, the particles precipitate completely, while remains disperse and stable at 0.001 mmol mg−1 Ca2+/C ratio. Although high concentration ratio of Ca2+/C provides larger probability to coordinate MWCNTs and GO by the oxygen-containing functional groups, the bad dispersion of GO-Ca2+-MWCNT in NMP can also result in the bad dispersion in SPEN matrix. Therefore, 0.001 mmol mg−1 Ca2+/C ratio was selected in this work.
3.2 Characterization of GO-Ca2+-MWCNT
Figure 3 shows the FTIR spectra of MWCNT/GO and GO-Ca2+-MWCNT. For GO/MWCNT, the spectra possess five distinct characteristic peaks, which are assigned to C=O (carboxyl/carbonyl) at 1628 cm−1, C=C (aromatic ring) at 1579cm−1, C-O (carboxyl) at 1400 cm−1, C-O (epoxide/ether) at 1172 cm−1and C-O (alkoxy) at 1079 cm−1, respectively [24]. However, for GO-Ca2+-MWCNT, the characteristic peak of C=O (carboxyl/carbonyl) at 1628 cm−1 disappears, and the peak of C-O (carboxyl) at 1400 cm−1 shifts to lower wavenumber, which is ascribed to the coordination of bivalent metal ions and carboxylic acids [25]. Besides, Figure 3 also shows that the intensity of C-O (epoxide/ether) band at 1172 cm−1 decreases and the intensity of C-O (alkoxy) band at 1079 cm−1 increases as compared with MWCNT/GO. This is due to the fact that the coordination of Ca2+ breaks the C-O band of epoxy groups and forms more alkoxy bonds [26]. All of these peaks provide evidences to the coordination of GO and MWCNT by Ca2+.
Figure 4 shows the ultraviolet absorption spectra of MWCNT, GO, GO/MWCNT and GO-Ca2+-MWCNT. Both GO and MWCNT exhibit one absorption peak between 200-300 nm due to the strong absorption of NMP solution, suggesting good dispersion of GO and MWCNT in NMP [24]. Besides, the peak at ~250 nm of GO-Ca2+-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 Zn2+ and Cu2+ [27–28].
To intuitively observe the three-dimensional structure of GO-Ca2+-MWCNT, the SEM morphologies of GO/MWCNT and GO-Ca2+-MWCNT were characterized and showed in Fig. 5. As shown in Fig. 5a, the GO/MWCNT particle possesses no structural cross-linking between GO and MWCNT due to the simple blend of GO and MWCNT. Besides, the SEM image of GO/MWCNT 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. Fig. 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 Ca2+ 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-Ca2+-MWCNT from room temperature to 600 oC under N2 atmosphere, as shown in Fig. 6. From Fig. 6, the GO displays two significant weight loss states at about 100-200 ℃ and 200-600 ℃, 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. The weight loss of MWCNT is about 14.58% from 200-600 ℃, proving that the surface of MWCNT also contains a certain amount of oxygen-containing groups such as hydroxyl and carboxyl. Besides, the curve of GO-Ca2+-MWCNT also shows two significant decomposition states at about 100-230 ℃ and 230-600 ℃, respectively. The decomposition degree at about 100-230 ℃ has a slight decrease trend than that of GO, which is speculated to be the uncoordinated calcium ions crosslinking with oxygen-containing functional groups on GO and MWCNT. The second decomposition state at 230-600 ℃ is also the splitting of carbon skeleton structure. Therefore, the above results confirm that the GO and MWCNT were coordinated by calcium ions.
3.3 SEM Characterization of SPEN and composite membranes
Figure 7 shows the SEM images of pure SPEN and PEN/GO-Ca2+-MWCNT-1 composite membrane. As shown in Fig. 7a, the morphology of SPEN is relative smooth, while the addition of GO-Ca2+-MWCNT filler makes the cross-sectional of composite membranes rougher than pure SPEN, as shown in Fig. 7b. The GO-Ca2+-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 the better performances of composite membranes. Therefore, the GO-Ca2+-MWCNT particles with three-dimensional structure play an important role in improving the performances of SPEN-based composite membranes.
3.4 Thermal and mechanical stability of the membranes
Thermal stability and tensile properties are important factors to determine the performances of proton exchange membranes. Fig. 8 displays the TGA results of the SPEN and SPEN/GO-Ca2+-MWCNT composite membranes. All the composites membranes exhibit two weight loss stages under N2 atmosphere. The first stage at around 250 ℃ is related to the decomposition of sulfonic acid groups of SPEN and the organic groups of GO-Ca2+-MWCNT [29]. The second stage at about 400 ℃ is attributed to the degradation of the main chain of SPEN and the carbon skeleton of GO-Ca2+-MWCNT. As shown in Fig. 8, the weight losses of SPEN/GO-Ca2+-MWCNT membranes at 250 ℃ and 400 ℃ are all lower than that of pure SPEN, suggesting that the sulfonic acid groups of SPEN can combine with the calcium ions of GO-Ca2+-MWCNT, which can lead to higher thermal stability. Table 1 exhibits the thermogravimetric decomposition data of SPEN and SPEN-based composite membranes with different GO-Ca2+-MWCNT contents. The 5% and 10% decomposition temperatures of composite membranes are higher than that of pure SPEN, and it increases with the increasing content, showing that the GO-Ca2+-MWCNT can improve thermal stability of composite membranes, which validates the above results. In addition, the SPEN/GO-Ca2+-MWCNT composites present same decomposition trends with SPEN, which can be considered that GO-Ca2+-MWCNT as the filler has a positive impact on the thermal stability of SPEN-based composite membranes.
Table 1
The thermal stability of pure SPEN and SPEN/GO-Ca2+-MWCNT composite membranes
Membranes
|
T5% (℃)
|
T10% (℃)
|
Tmax (℃)
|
SPEN
|
311.50
|
354.45
|
369.51
|
SPEN/GO-Ca2+-MWCNT-0.5
|
320.58
|
367.92
|
380.46
|
SPEN/GO-Ca2+-MWCNT-1
|
326.74
|
371.18
|
371.22
|
SPEN/GO-Ca2+-MWCNT-2
|
343.61
|
380.29
|
380.86
|
Table 2 presents the tensile strength and Young's modulus of pure SPEN and SPEN/GO-Ca2+-MWCNT composite membranes at fully dry state and room temperature. The tensile strength and Young's modulus of the membranes increase from 62.41 to 71.45 MPa and from 1636.50 to 1715.09 MPa, respectively, as the GO-Ca2+-MWCNT content increases to 1 wt%, which is attributed to the impediment of particles on the movement of polymer chain. The synergistic effect between calcium ions and SPEN also can enhance the tensile properties of the membranes [30]. However, the tensile strength and Young's modulus reduce to 63.88 MPa and 1643.37 MPa, respectively, as the GO-Ca2+-MWCNT content increases to 2 wt%, which is due to the agglomeration phenomenon of excessive GO-Ca2+-MWCNT particles. As shown in Table 2, the tensile strength and Young's modulus of SPEN/GO-Ca2+-MWCNT composites are higher than that of SPEN membrane, even in the composite membranes with high filler content, suggesting the outstanding enhancement effect of GO-Ca2+-MWCNT to tensile properties of SPEN membrane.
Table 2
Mechanical stability of pure SPEN and SPEN/GO-Ca2+-MWCNT composite membranes at room temperature
Membranes
|
Tensile strength (MPa)
|
Tensile modulus (MPa)
|
SPEN
|
62.41
|
1636.50
|
SPEN/GO-Ca2+-MWCNT-0.5
|
64.12
|
1661.35
|
SPEN/GO-Ca2+-MWCNT-1
|
71..45
|
1715.09
|
SPEN/GO-Ca2+-MWCNT-2
|
63.88
|
1643.37
|
3.5 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% H2O2 containing 4 ppm Fe2+) for 8 h at 80 ℃ to simulate the environment of oxidative radicals attacking membranes, and then measure the residual weight percentages of the membranes. As shown in Table 3, the residual weight percentages of pure SPEN and SPEN/GO-Ca2+-MWCNT composite membranes are 94.3%, 96.2%, 98.7% and 99.9%, respectively. The residual weight percentages of SPEN/GO-Ca2+-MWCNT composite membranes are far higher than that of SPEN, and it increases with the increasing filler content, indicating that the GO-Ca2+-MWCNT filler possesses excellent ability to resist free radicals, especially for the SPEN/GO-Ca2+-MWCNT-2 membrane. This phenomenon can be explained by the following reasons. 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 [31–32]. Secondly, the oxygen-containing functional groups of GO-Ca2+-MWCNT and SPEN can form hydrogen bonds interaction, enhancing the interfacial adhesion and resisting the oxidation of the membranes [33]. Finally, the interaction between the uncoordinated calcium ions of GO-Ca2+-MWCNT and the SPEN also can function positively on the resistance to free radicals [32]. Therefore, the SPEN/GO-Ca2+-MWCNT composite membranes possess excellent oxidation stability against Fenton's reagent.
Table 3
The oxidative stability, water uptake and swelling ratio of pure SPEN and SPEN/GO-Ca2+-MWCNT composite membranes
Membranes
|
aOxidative stability
bw (%)
|
WU (%)
|
SR (%)
|
30 ℃
|
80 ℃
|
30 ℃
|
80 ℃
|
SPEN
|
94.3
|
34.71
|
81.80
|
17.99
|
44.40
|
SPEN/GO-Ca2+-MWCNT-0.5
|
96.2
|
34.88
|
50.17
|
17.74
|
38.42
|
SPEN/GO-Ca2+-MWCNT-1
|
98.7
|
33.31
|
49.38
|
17.56
|
36.80
|
SPEN/GO-Ca2+-MWCNT-2
|
99.9
|
32.27
|
45.77
|
16.11
|
36.40
|
a Measured at 80°C in Fenton's reagent (4 ppm FeSO4 and 3% H2O2) |
b Refers to the residual weight percentages of the membranes after soaked in Fenton's reagent (4 ppm FeSO4 and 3% H2O2) for 8 h |
3.6 Water uptake and swelling ratio of the membranes
Figure 9 and Table 3 present the water uptake and swelling ratio of SPEN and SPEN/GO-Ca2+-MWCNT composite membranes at 30 oC and 80 oC, respectively. The water uptake and swelling ratio of SPEN/GO-Ca2+-MWCNT composite membranes have a slight decrease compare with SPEN, which is due to the reduced free volume occupied by the inorganic filler [34]. The interaction between the sulfonic acid group of SPEN and uncoordinated calcium ions also can suppress the penetration of water molecules. Besides, temperature has a significant effect on the water uptake and swelling ratio, as shown in Figure 9. The water uptake and swelling ratio of all membranes at 80 oC are higher than that of 30 oC, which is caused by the increased transmission rate of water molecular and the formation of more hydronium ion (H3O+). Meanwhile, the influence on the water uptake and swelling ratio at 80 oC is greater than 30 ℃, which is mainly due to the fact that the water absorbing capacity of sulfonic acid groups of SPEN is weaker under low temperature conditions. It's worth noting that the swelling ratio of SPEN/GO-Ca2+-MWCNT composite membranes is all less than 40% at 80 ℃, suggesting outstanding dimensional stability, speculating that the GO-Ca2+-MWCNT particles limit the infinite expansion of the membranes.
3.7 Proton conductivity of the membranes
The influences of the GO-Ca2+-MWCNT filler on the proton conduction properties of SPEN were also examined. Fig. 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 composite membranes rises with the increase content of GO-Ca2+-MWCNT and reaches the maximum value at 1 wt%. The three-dimensional structured GO-Ca2+-MWCNT provides more efficient channels for proton transmission and increases proton transport efficiency [35]. However, the proton conductivity of SPEN/GO-Ca2+-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 membrane and then reducing greatly the efficiency of proton transport [36–37]. In addition, Fig. 10 exhibits that the proton conductivity of SPEN and composite membranes rises dramatically in the range of 30-90 ℃. The substantial increasing of water absorption can led 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-Ca2+-MWCNT-2 reaches the maximum of 0.193 S/cm at 90 ℃, 19.2% higher than that of SPEN, showing excellent proton transport capability. Therefore, the GO-Ca2+-MWCNT filler with three-dimensional structure can significantly promote the proton conductivity of SPEN.
The enhancement effect on proton conduction of membranes can be explained by the Grotthuss mechanism. The surface of GO-Ca2+-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 transports channels can be formed to accelerate proton transmission and trigger the synergistic enhancement effect between GO-Ca2+-MWCNT and SPEN, improving the proton conductivity. The three-dimensional structured GO-Ca2+-MWCNT can facilitate the construction of long-term and continuous proton transmission paths and accelerate the proton transfer rate in the composite membranes [38]. Beside, excellent interface adhesion provides a prerequisite for the proton transport.