## 3 Results
### Membrane Characterization
Figures 7 and 8 show the high-resolution XPS of C 1s and S 2p of the considered membranes, respectively. The spectra of O 1s can be found in the Supporting Information in Figure S2. The main constituents in the sPEEK sample were carbon and sulfur in the ionomer and oxygen in the ketone and ether linkages. The high-resolution C 1s spectra of the non-crosslinked sPEEK were fitted to four peaks with binding energies of about 284.6, 284.9, 286.3, and 288.2 eV (Figure 7a,b). The peak at 284.6 eV was assigned to the aryl C=C network, whereas the peaks at 284.9, 286.9, and 288.2 eV corresponded to alkyl (C-H), C-O-C/C-S, and carboxyl (C=O) groups, respectively [44; 45; 46]. A peak corresponding to C-C bonds appeared in the spectra of all membranes crosslinked with DCX (Figure 7c-f),  
Figure 5: SEM images of the membrane surface of (**a**) sPEEK, (**b**) sPEEK/DCX, (**c**) sPEEK/DCX-s90\({}^{\circ}\)C, (**d**) sPEEK/DCX-CX24h, and (**e**) sPEEK/DCX-CX190\({}^{\circ}\)C.  
Figure 6: SEM images of the membrane cross-section of (**a**) sPEEK, (**b**) sPEEK/DCX, (**c**) sPEEK/DCX-s90\({}^{\circ}\)C, (**d**) sPEEK/DCX-CX24h, and (**e**) sPEEK/DCX-CX190\({}^{\circ}\)C.  
whereas the intensity of the C-O and C=O peaks decreased. As can be seen in Figure 3, DCX is the only source of sp3 carbons (C-C) in the membrane polymer. The appearing C-C peak in Figure 7c-f, therefore, is only possible if crosslinking has occurred. Probably, the most interesting result from the XPS is that the sp2/sp3 carbon ratio gives information on the crosslinking degree of the polymer. More specifically, the lower the ratio, the higher the crosslinking degree. The ratios are given in Table 2. The sp2/sp3 ratio is the lowest for sPEEK/DCX-CX24h and sPEEK/DCX-CX190\({}^{\circ}\)C, which shows that a higher crosslinking time or temperature significantly increases the crosslinking degree. The sp2/sp3 ratio of sPEEK/DCX-CXs90 is higher than sPEEK/DCX due to its lack of crosslinking sites, which is caused by the high degree of sulfonation. As these membranes were kept in water, a broad peak at 288.5-289.0 eV, corresponding to surface carbonates, was found mainly in the C 1s spectra of sPEEK/DCX-CX24h membranes, which could be attributed to physisorption of dissolved CO\({}_{2}\)[47]. Figure 8 shows the S 2p spectra with two peaks at 168.2 and 169.5 eV, corresponding to 2p3/2 and 2p1/2 sulfur. No discernible difference could be found in the S 2p spectra between the non-crosslinked and crosslinked sPEEK membranes, demonstrating that crosslinking had no influence on the sulfonation of the polymer. As shown in Figure S2 in the Supporting Information, the O 1s signal was deconvoluted to the main peaks of C-O-C at 533.3 eV, C=O at 531.5 eV, and -SO3H around 532 eV. The strong shoulder at a higher binding energy was seen primarily in the O 1s spectra of sPEEK/DCX-CX24h and \begin{table}
\begin{tabular}{c c c c} \hline \hline  & \multicolumn{3}{c}{**C 1s, Binding Energies (eV)**} \\ \cline{2-4}
**Membrane** & **284.5** & **284.9** & **Ratio** \\  & **C=C** & **C-C** & **C=C/C-C** \\ \hline sPEEK & 67.76 & 6.81 & **9.95** \\ sPEEK/DCX & 48.11 & 12.39 & **3.88** \\ sPEEK/DCX-s90\({}^{\circ}\)C & 58.60 & 9.88 & **5.93** \\ sPEEK/DCX-CX24h & 55.82 & 20.94 & **2.66** \\ sPEEK/DCX-CX190\({}^{\circ}\)C & 50.57 & 27.20 & **1.86** \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Quantification of C-C and C=C surface carbons. Figure 7: High-resolution C 1s XPS spectra of the synthetized membranes; (**a**) comparison of all membranes before deconvolution, (**b**) sPEEK, (**c**) sPEEK/DCX, (**d**) sPEEK/DCX-s90\({}^{\circ}\)C, (**e**) sPEEK/DCX-CX24, and (**f**) sPEEK/DCX-CX190\({}^{\circ}\)C.  
sPEEK/DCX-CX190\({}^{\circ}\)C membranes, which could be attributed to the physical adsorption of dissolved H2O/CO2. This is in agreement with the carbon O 1s spectra [47].