3.1. Thermal Stability Study of Void-Free SP/e-spunCL Membranes
The fuel cells that exhibit a better performance when it is operated at a high temperature is said to possess a very high thermal stability. Hence, thermal characteristic is an important subject to be studied to obtain a high performance fuel cell. Thermogravimetric analysis for the void-free SP/e-spunCL membranes were performed with the aims of finding the thermal stability behavior and to observe the vulnerable functional group that evolve when heat applied.
Figure 3 shows the degradation stages for all samples. Three steps of degradation occurred as a result of thermal solvation process, desulfonation and oxidation of SPEEK as polymer matrix [
20].
Table 2 tabulates the degradation temperature (
Td) and weight loss of SP/e-spunCL membranes. The first weight loss (
Td1) occurred at temperature ranging from 163.1 to 209.1 °C caused by the loss of absorbed water molecules and the range also represents the decomposition of Cloisite 15A
®[
21]. The weight loss in the T
d1 ranged from 9.444 to 20.558 wt %. The absorption of water can be explained as a polymer with sulfonic acid group such as SPEEK is naturally hydrophilic and will absorb moisture from surrounding [
22]. The absorbed water molecules mostly exist in a bound state rather than being in free molecule state [
23].
The second weight loss (
Td2) that occurred at 388.1–410 °C with the mass loss of 12.703–15.306 wt % can be related to the crosslinked bonds breakage which is the –CO–O– bonds that resulted from the desulfonation of sulfonic acid [
24]. In addition, sulfonic groups tend to decompose at the earliest at 288 °C due to thermal instability [
24].
In the third weight loss region (
Td3) at temperature ranging from 564.1 to 617.1 °C with mass loss from 11.126 to 17.914% occurred due to the breakdown of SPEEK backbone [
25]. As shown in the SPEEK degradation curve in
Figure 2, SPEEK started to degrade from 163.1 to 564.1 °C, whereas the other membranes only begin to decompose at around 192.1–617.1 °C. This result showed that all membranes have good thermal stability for the DMFC application, which all was recorded to be thermally stable within the range of 60 to 120 °C of the DMFC operating temperature.
In addition, it was also realized that the degradation temperature increased as the addition of Cloisite increased. Referring to the results, the SP/e-spunCL membranes showed higher degradation temperature in contrast to pure SPEEK membrane. The SP/e-spunCL10 showed degradation temperature of 602.1 °C and, for e-spunCL30, it was 617.1 °C, which is, respectively, 38 and 53 °C increased with 10 and 30 wt % of the Cloisite. The decomposition temperatures for others also increased: SP/e-spunCL10 (606.1 °C), SP/e-spunCL15 (610.1 °C) and SP/e-spunCL25 (613.1 °C).
The above results indicate that the membranes with addition of inorganic fillers/additives, i.e. Cloisite, were thermally more stable than their pure polymer. This is in agreement with previous research that was carried out for polymer/clay nanocomposites [
26,
27,
28]. It can be discussed that the impact of Cloisite had clearly noted as mass transport barrier and superior insulation against the volatile compound resulted during the degradation of polymer when heat was applied [
29].
In addition, Cloisite is also reported to be an element that causes the formation of layered carbonaceous char during degradation [
29]. This can be reaffirmed by the observation of the Cloisite content of SP/e-spunCL membranes remaining as residue after heating [
30]. The residuals can be explained by the inorganic materials that are more thermally stable in the ranges of temperature where the organic compounds (SPEEK) were already degraded into volatile compounds [
30]. As for the TGA, by means of the degradation and thermal stability of the membranes study, SP/e-spunCL membranes have been thermally improved as compared to pure SPEEK membrane.
3.2. Wettability Analysis of the Void-Free SP/e-spunCL Membranes
Water uptake played an important role as it affects membrane transport properties, for example water diffusion coefficient and proton conductivity. Low proton conductivity will increase cell resistance and low water diffusion coefficient will hinder methanol to diffuse to catalyst sites in taking reaction. The drawbacks lead to the fuel low efficiency. Hence, water uptake is an important element in DMFC operation.
Figure 4,
Figure 5 and
Figure 6, shows the physico-chemical properties (water uptake, Proton conductivity and methanol permeability) of SP/e-spunCL membranes and Nafion
®.
The dependence of water uptake on membrane dehydration conditions may have important implications in the use of these membranes in DMFC. Less water is taken up by the membrane during cell operation, a decrease in the maximum obtainable membrane conductivity occurs, since the conductivity depends roughly linearly on membrane water uptake as proton carrier.
Figure 5 shows the proton conductivity of all SP/e-spunCL membranes.
In
Figure 5, SP/e-spunCL15 had the highest conductivity values with 12.12 mS·cm
−1. The lowest conductivity was SP/e-spunCL10, at 10.30 mS·cm
−1. As the amount of Cloisite added in electrospun SP/Cloisite increased, the proton conductivity increased up to 12.12 mS·cm
−1. The increasing value of proton conductivity did not happen for the following formulations: SP/e-spunCL25 and SP/e-spunCL30. The different results are not only related to water uptake which played role as proton carrier but also the fineness of fibers which embedded into the membrane. Finer fibers with less beaded structures show higher proton conductivity of the membrane, promoted by the interactions of electrospun fibers with SPEEK polymer matrix [
31].
S direct methanol fuel cell also requires that the membranes have low methanol permeability.
Figure 6 shows methanol permeability for all SP/e-spunCL membranes.
It is observed that the overall membrane characteristics was increased from SP/e-spunCL10 to SP/e-spunCL15 and the selectivity was dramatically decreased from the SP/e-spunCL10 to SP/e-spunCL25 membranes. The selectivity was increased after adding electrospun fibers with 0.15 wt % of Cloisite loading and decreased when Cloisite content was further increased to 0.25 wt %. The reduction of the value was due to the decrease in proton conductivity that is more significant compared to the methanol permeability. As a result, the maximum overall membrane characteristics was recorded at 0.15 wt % of Cloisite (SP/e-spunCL15), which seems to be the best membranes compared to the other SP/e-spunCL samples. Methanol permeability decreased as the diameter of fibers decreased and became more packed, as performed by SP/e-spunCL15. The fiber diameter of SP/e-spunCL15 is shown in
Figure 7a, while the beaded electrospun fibers of SP/e-spunCL10 is shown in
Figure 7b, and the summary for average fiber diameters of all samples is tabulated in
Table 3.
Fibers with diameters of 386.17–67,680.00 nm were obtained from the electrospinning processed with 20 cm of distance from needle tip to screen collector and constant voltage applied of 22.5 kV. The concentration varied as 0.05, 0.10, 0.17, 0.25 and 0.30 wt %. The variation of fibers structure due to different concentrations can also be explained by the relation between viscosity and surface tension. The viscosity increased when the concentration increased. As the surface tension caused by high voltage supplied tried to reduce the surface area unit per mass, the formation of beads occurred [
17]. Viscoelastic forces prevented the formation of beads and allowed the smooth fibers to form.
The lesser is the continuity of fiber, the higher methanol permeation through the membrane, as shown in SP/e-spunCL10. The fibrous structure with good dispersion of Cloisite and exfoliated the membrane surface which not only acted as constraint for methanol to pass through but also created smaller and windier paths in preventing methanol crossover problem. This statement is proven by the value of selectivity, of which SP/e-spunCL15 had the highest of 99.34 × 104.
Further investigation were carried out to reiterate the characteristics of water uptake thermally. Since the TGA results showed the degradation temperature of SPEEK is above 200 °C, the DSC experiments were all handled in the temperature range 50 to 275 °C, which corresponds to the plotted curves in
Figure 8. All samples showed the endothermic peak during heating range 85–98 °C due to absorbed moisture.
Tg can be observed as a peak that is often misread as melting peak or the substantial endorthermic peak of
Tg can also appear in base line [
32]. The endothermic peak that appeared (near 160 °C) after the absorbtion of water was caused by the glass transition of the amorphous SPEEK.
It can be explained that
Tg of SPEEK and the SP/e-spunCL samples lies in the range of 150–164 °C. From the comparison of
Figure 8 and
Figure 9, it can be interpreted that the associated moisture is removed during reheating at the range of temperature around 85–98 °C.
Figure 9 shows the
Tg values of all samplesobtained after reheating process, while
Table 4 shows the
Tg according to reheat curves.
The Tg value of SPEEK (150.20 °C) increases with the incorporation from different concentration of SP/e-spunCL (150.20 to 164.00 °C). The introduction of cloisite fibers into the SPEEK polymer matrix increases Tg by as much as 13.8 °C for e-spunCL30, 10.13 °C for SP/e-spunCL25, 6.47 °C for SP/e-spunCL15, 2.8 °C for SP/e-spunCL20 and 0.8 °C for e-spunCL10.
From the results, it should be noted that, when the SP/e-spunCL fibers content increases, it gives only small effect to thermal stability (
Td and
Tg), whereas it improves tensile strength significantly. The tensile strength and Young’s modulus of all membrane samples are shown in
Table 5. It can be discussed that the notable decrease in tensile strength of SP/e-spunCL25 and SP/e-spunCL30 membranes might be due to the significant exfoliation of SP/e-spunCL fibers or it can be said that the concentration of the SP/e-spunCL higher than that of SP/e-spunCL15 can cause the weak interaction of organic (SPEEK)–inorganic (Cloisite) thus made the contents of Cloisite no longer work as helpful filler but as a defect factor instead. To support the statement, the result from the previous study on composite membranes of Nafion/MMT by Jung reported a similar trend [
33].
Many definitions have been made to describe the correlation between hydrophilic polymer and water. In general, the states of water are known as bound water, non-freezing water and free water [
34]. Bound water can be recognized when there are small amounts of water entrapped and connected with polar and ionic group that exist in polymer chain. It occurs when absorption of water increased at certain volume, and then, the polar and ionic groups become saturated. The level of bound water relying on the content of ionic groups and the polarity exist in polymer itself [
35].
Free water is known as water that has similar characteristic with bulk water and also has the same phase transition, that is 0 °C [
35]. Free water can also be found in free space in membrane and crystallizes at higher temperature comparing to bound water. The free water also happens because of the weak interaction of chains within the polymer. Non-freezing bound water is water that has undefined phase transition and different from free water, non-freezing bound water happens because of the strong chains interaction in polymer [
36].
Table 6 summarizes the states of water according to heat of melting value (Hfree). The differential scanning calorimetric (DSC) curves correspond to determine the type of water content of SP/e-spunCL10, SP/e-spunCL15, SP/e spunCL20, SP/e-spunCL25 and SP/e-spunCL30 and SPEEK membranes. The test is crucial to study the ability of the membranes in upholding water to provide a high proton conduction.
Since the endothermic peaks in DSC ice-melting diagrams are attributed to the freezable water, for example, freezable bound water and free water, the amount of freezable water in the nanocomposite membranes can be estimated from DSC profile [
37]. The endothermic peaks correspond to the freezing free and freezing bound water. The melting enthalpy is obtained through integration and normalization in unit of J·g
−1 of the swollen membrane. The latent heat of water, 333 J·g
−1, was taken for the calculation. Based on the calculation, the free water and non-freezing bound water in water uptake are summarized in
Table 6.
Only one peak at temperature around 0 °C is observed for all the samples. Hence, the peak is considered to happen from free water. Bound water is suggested to happen due to the hydrogen bond of –SO3H in SPEEK polymer chain.
The methanol permeability and proton conductivity of SP/e-spunCL membranes notably e-spunCL15 were improved as compared to Nafion112
® commercial membrane. It can be discussed from involving several factors that give rise to a better SP/e-spunCL membrane characteristics: (1) good dispersion of Cloisite with exfoliated surface membrane; and (2) sufficient water uptake.
Table 6 shows the composition of water for all membrane samples.
It is natural for a membrane with low water uptake to perform low methanol permeability. It is generally agreed that methanol permeability increases with the amount of soaked water, but it dominantly occurs via free water (freezing water) inside the interconnected membrane structure channels and insignificantly via non-freezing bound water associated with the ionic sites. As shown in
Table 6, SP/e-spunCL15, SP/e-spunCL25 and SP/e-spunCL30 nanocomposite membranes exhibited low amount of free water content and hence low methanol permeability is not surprising. The clearly different values of the three SP/e-spunCL membranes with Nafion
® has resulted a significant drop of methanol permeability value from Nafion
®. The low methanol permeability in SP/e-spunCL membranes can be related to the statement that a membrane with low water uptake has low methanol permeability. Methanol only permeates through free water inside the membrane structure channel (central space) [
38].
On the other hand, an increase in the proton conductivity of SP/e-spunCL15 nanocomposite membrane can be interpreted in the following way. The absorbed water molecules are present mostly in the ionic cluster domains and ionic cluster channels. In particular, in the ionic cluster channels, the water molecules exist in two different forms. One is the protonated water (mostly non-freezing bound water) that is bound strongly to the ionic site. The other is free water that occupies the central space free from the influence of the ionic sites. The proton transfer through the ionic cluster channel occurs by two different mechanisms: (1) near the channel wall via the bound water, in which proton is transported by the Grotthuss mechanism, hopping from one ionic site to the other; and (2) via free water by vehicle mechanism, in which proton is facilitated by the water molecules moving through the interconnected central channel space. However, the contribution from the Grotthuss mechanism is more essential. Results presented in
Table 6 for SP/e-spunCL15 show the highest value for non-freezing water. Thus, water retention capacity of the nanocomposite membrane was enhanced due to the presence of electrospun fibers with good dispersion of Cloisite, which was expected to reduce the dehydration of membrane. Therefore, it can be said that the enhanced proton conductivity of the SP/e-spunCL15 nanocomposite membrane is attributed to the water retention capability. From these results, it is clear that the incorporation of appropriate amounts of electrospun SPEEK/Cloisite15A
® fibers has synergistic effect to lower the methanol permeability and increasing proton conductivity.
This condition has given a significant benefit to SP/e-spunCL membranes.
Figure 10a,b shows the model of methanol and proton mobility in parent SPEEK and SP/e-spunCL membranes, respectively.
Figure 10a shows the typical SPEEK membrane without addition of electrospun fibers. It can be seen that, without having any constraint, methanol can easily pass through the channel in SPEEK membrane together with the high amount of free water. Hence, it will result in a membrane with high methanol permeability and possibly cannot curb methanol crossover problem. As for the membrane in
Figure 10b, there is the impact of the addition of electrospun fibers which make the methanol pathway become smaller and winding. This phenomenon can also explain the result shown in
Table 6, in which electrospun fibers had lower methanol permeability compared to SPEEK membrane and Nafion
®.
The new configuration of PEM with addition of electrospun fibers is also responsible for the increase in proton conductivity. The Grotthus mechanism (proton hopping) played an important role in bringing the proton from one site to another [
39]. The proton was delivered through ionic cluster channel (free water and SO
3 that linked with non- freezing bound water). It is desirable for a membrane to have high value of bound water (so called non-freezing bound water) since it is a crucial element in delivering protons. To prove the statement, it can be observed that SP/e-spunCL15 showed the highest proton conductivity and also the highest non-freezing bound water. The existence of Cloisite is another factor that help to reduce the hydration in membrane by retaining water [
40]. Water retention capability of Cloisite has successfully enhanced the proton conductivity of the SP/e-spunCL membranes.