## 5 Hybrid Membranes
Despite the fact that the additive surface in this case is also hydrophilic, actually, its basic functional groups interact with the acid groups of the pore walls, reducing their solvation and turns out to be an additional factor that reduces the water uptake and leads to an increase in selectivity of these membranes [228]. In fact, in this case, the role of doping turns out to be similar to that of cross-linking--the membrane conductivity decreases while selectivity increases. The influence of the dopant surface acidity on the properties of cation-exchange membranes based on polymethylpentene with grafted sulfonated polystyrene has been studied in [229]. ZrO\({}_{2}\), TiO\({}_{2}\), and SiO\({}_{2}\) with increased acidic properties were used as dopants. Their pK\({}_{4}\) values vary somewhat depending on the preparation method and are 11-12, 8-9, and 5-6, respectively [230]. If the first of them mainly exhibits basic properties, those of the last one are acidic. The ion-exchange capacity and conductivity of hybrid membranes naturally increase in this series of dopants (Table 2). If in the case of zirconia and titania the ion exchange capacity decreases due to the formation of salt bridges (hydrogen ionic bonds of the type -M-O\({}_{3}\)S-), silica does not change it. Based on Poisson-Boltzmann equations, the fraction of negatively charged groups on the surface of a silica nanoparticle at pH = 6 estimated to be in the range of 5-10% [231]. This result is in good agreement with the electrokinetic experimental data reported by [PERSON] et al. [232]. Dissociation of OH-groups of silica showing weak acid properties is suppressed in the presence of a strong sulfonic acid (sulfonated polystyrene). At the same time, the ionic conductivity of this membrane significantly increases as result of increased pore size and water uptake. At the same time, the selectivity of ion-exchange membranes significantly decreases in the ZrO\({}_{2}\)-TiO\({}_{2}\)-SiO\({}_{2}\) series. When zirconia is used as a dopant, the apparent cation transport numbers increase by 7% compared with the initial membrane, for silica it decreases by the same 7%. After the ZrO\({}_{2}\) introduction, the proton conductivity sharply decreases due to the formation of salt bridges. At the same time, after membrane treatment with 1 M alkali solution, its conductivity increases and \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Property** & **Initial Membrane** & \multicolumn{3}{c}{**Dopant**} \\  & **ZrO\({}_{2}\)** & **TiO\({}_{2}\)** & **SiO\({}_{2}\)** \\ \hline IEC mg eq/g & 2.1 & 1.1 & 1.9 & 2.1 \\ \(\omega\) (H\({}_{2}\)O), wt\% & 50 & 29 & 45 & 55 \\ \(\sigma_{\text{Na+}}\), mS cm\({}^{-1}\) & 25.3 & 10.1 & 22.3 & 31.6 \\ (in 0.5 M NaCl) & & & & \\ t\({}_{\text{+app}}\)1 & 88 & 95 & 88 & 81 \\ \hline \hline \end{tabular}  
* apparent transport number measured between 0.5 M and 0.1 M NaCl solutions.  
\end{table} 
Table 2: Ionic conductivities and apparent transport numbers of some grafted hybrid CEM in Na\({}^{+}\) form with the dopant content of 8–10 wt% [229]. reaches almost the same values as for the initial membrane. This change is due to the destruction of salt bridges in the alkaline medium. It should be noted that this effect is reversible--during cyclic treatment with acid and alkali solutions, the conductivity almost reversibly returns to the values of the hydrogen and sodium forms of hybrid membranes, respectively [229]. An increase in the selectivity of transport processes was also noted upon membrane doping with such a weak base as polyaniline [233; 234; 235; 236]. Recently, much attention has been paid to carbon nanomaterials, primarily carbon nanotubes (CNT). They have gained attention due to unique structure and properties such as extraordinary mechanical properties, high surface area, electronic conductivity and chemical stability. Carbon nanotubes are mostly used in fuel cells as a catalytic support for the oxygen reduction reaction [237].