## Results
### Effect of the Binder Concentration on the Thermal Evolution and Structure of Membranes
Figure 2 shows typical SEM images of the surface and bulk of the 90 PAA membrane obtained with the optimized parameters. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & & \multicolumn{2}{c}{**Temperature, \({}^{\circ}\)C**} & & \(\Delta\)**m, \%** \\ \hline
**Sample** & **Endo1-** & **Endo2-** & **Additional Endo Effects** & **Exo-** & **\textless{300 \({}^{\circ}\)C**} \\ \hline
100 PAA & 57.1 & 126.3 & - & 369 & 14.8 \\ \hline
90 PAA & 100.5 & 123.8 & 190 & 303 & 14.7 \\ \hline
80 PAA & - & 127 & 166 & 302 & 14.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The types and temperatures of the effects according to the DTA curves and mass losses calculated from the TG curves of 100 PAA, 90 PAA and 80 PAA samples. Overall, macroscopic defects such as large pores, cracks or voids are absent from the surface of the 90 PAA membrane. As seen from Figure 2a, the membrane is composed of brighter and darker areas. According to EDX spectra (see Figure S1 in the Supplementary Material), brighter areas correspond to the PAA phase, whereas the darker areas correspond to fluoroplastic binder. Close packing of the spherical PAA particles, both on the surface and on the membrane bulk, can be observed in Figure 2b,d. The small cracks at the cross-section are the result of brittle fracture of the membrane during the sample preparation (Figure 2c). The binder filled up the voids between PAA particles, but its content was not sufficient to isolate individual particles or particle agglomerates, which allowed direct contact between them and enabled faster ion transport through the bulk membrane. Similar results were obtained for the 80 PAA membrane. In Figure 3, the logarithmic differential curves versus pore size distribution, for both 90 PAA and 80 PAA samples, are plotted. Regarding the overall pore volume, both binder content samples seemed to present quite similar pore volume, once the traces largely overlapped, only showing significant differences in the mesoporous region. Concerning pore size distribution, and consequent surface contact area, the 10% binder content sample exhibits a higher pore volume in the mesoporous region (pores which average size is lower than 0.1 \(\upmu\)m), and consequently a slightly higher overall contact surface area. What is clear is that both samples have a quite significant number of pores in the mesoporous region, and this type of porosity is known to favor the appearance of diffusion mechanisms, which are typical ionic conductivity contributions to the overall electric conduction of the materials. Figure 2: SEM images of (**a**,**b**) surface and (**c**,**d**) the bulk of the 90 PAA membrane. A set of in situ X-ray patterns obtained for 100 PAA sample without the binder upon heating in the range of 30-330 \({}^{\circ}\)C is shown in Figure 4. All reflexes in the XRD pattern obtained at 30 \({}^{\circ}\)C correspond to a crystalline phase of hydrated polyantimonic acid, Sb\({}_{2}\)O\({}_{5}\cdot\)nH\({}_{2}\)O, with n = 3-4. It had a pyrochlore-type structure with space group \(Fd\overline{3}m\). No admixtures of other phases are observed. Heat treatment of the pure PAA sample induced a shift in the reflexes towards higher 2\(\Theta\) angles. For all temperatures in the range 30 to 330 \({}^{\circ}\)C, no phase transformation took place. The intensity of several peaks in the XRD pattern, as were the cases of those found at 2\(\Theta\) = 40.6, 45, 58.7, 63, 76.3\({}^{\circ}\), decreased significantly up to 100 \({}^{\circ}\)C. In the range 100-250 \({}^{\circ}\)C, almost no changes in the position and intensity of the reflexes were observed. For temperatures higher than 250 \({}^{\circ}\)C, further gradual decreases in the reflex intensity, and even in the background intensity, are seen (Figure 5). Reflex positions also shifted significantly towards higher 2\(\Theta\) angles. The intensities ratio of main reflexes at 33 and 34.5\({}^{\circ}\) changed from \(\sim\)1 to \(\sim\)0.7.