## 5 Hybrid Membranes
They are also displaced by polar water molecules and cations from the double electric layer and are localized mainly in the electroneutral solution. The increase in selectivity of hybrid membranes can be explained by the model of limited elasticity of their pore walls (Figure 6). When additive particles are introduced, they displace a part of the electroneutral solution localized in the pore center, thereby decreasing the solubility of gases or alcohols as well as crossover in fuel cells. However, considering the conductivity and selectivity of hybrid membranes, it is necessary to take into account the nature of additive particles or their surface. It has already been noted above that the particle surface should, whenever possible, be hydrophilic for conductivity enhancement. The acid properties of its surface are no less important. If the additive surface contains strong acid groups, as result of their dissociation, an additional number of counterions (protons, charge carriers) are formed. In this case, the nanoparticle surface acquires a charge of the same sign as the pore walls of cation-exchange membranes. These filler particles should repel from them due to the electrostatic effects, be located in the pore center, and displace only the electrically neutral solution. Moreover, their surface charged negatively will create an additional \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Material** & **Proton Conductivity** & **Methanol Permeability** & **Reference** \\  & **(S/cm)** & **(10\({}^{-7}\) cm\({}^{2}\)/s)** & \\ \hline SPAEEKK & 0.012 & 7.83 & \\ SPAEEKK/sulfonated silica & 0.043 & 4.86 & [215] \\  & (20 \({}^{\circ}\)C, 100 RH \({}^{1}\)) & & \\  & 0.043 & 7.53 & \\  & (20 \({}^{\circ}\)C, 100 RH) & & \\  & 0.038 & 85.9 & \\  & 0.0021 & 20.7 & [216] \\  & 0.0025 & 4.31 & \\  & (60 \({}^{\circ}\)C, 60 RH) & & \\  & 0.0991 & 1.34 & \\  & 0.0667 & 0.25 & [184] \\  & 0.075 & & \\  & 0.1404 & 14 & [187] \\  & (20 \({}^{\circ}\)C, 100 RH) & & \\  & 0.0179 & 6.51 & \\  & 0.0624 & 5.82 & \\  & (20 \({}^{\circ}\)C, 100 RH) & & \\  & 0.051 & 6.4 \(\pm\) 0.1 & \\  & 0.084 & 4.8 \(\pm\) 0.1 & \\  & 0.137 & 1.4 \(\pm\) 0.1 & \\  & 0.100 & 2.5 \(\pm\) 0.1 & \\  & 0.000 & 2.5 \(\pm\) 0.1 & \\  & 0.000 & 2.5 \(\pm\) 0.1 & \\  & & & \\ \hline \hline \end{tabular} \({}^{1}\) RH—relative humidity.  
\end{table} 
Table 1: Comparison of the proton conductivity and methanol permeability of various PEMs for DMFCs application. double electric layer, contributing to the displacement of coions and non-polar molecules as a result of competition with polar water molecules, which interact much more strongly with the charged pore walls. Moreover, such nanoparticles should not decrease the counterion transport along the pore walls, on the contrary, they will contribute to its increase due to an increase in the charge carrier concentration [67,205]. Heteropoly acids, their acid salts with alkali metal cations, or silica particles with heteropoly acids were shown to improve conductivity in such way [218, 219, 220, 221, 222, 223, 224, 225]. On the other hand, the surface of such particles is hydrophilic, which contributes to an increase in water uptake. When the composite membranes Nafion/SiO\({}_{2}\)/PWA and Nafion/SiO\({}_{2}\) were employed as an electrolyte in H\({}_{2}\)/O\({}_{2}\) PEMFC, the higher current densities (540 and 320 mA/cm\({}^{2}\) at 0.4 V, respectively) were obtained than that of the Nafion-115 membrane (95 mA/cm\({}^{2}\)), under the operating condition of 110 \({}^{\circ}\)C and at the humidified temperature of 100 \({}^{\circ}\)C [219]. The opposite situation can be realized if the membrane is doped by nanoparticles with the basic surface [226,227]. In this case, \"salt bridges\" are formed between the functional groups of the membrane and the basic surface groups, e.g., of the -SO\({}_{3}\)-HNR\({}_{3}\) type. Their formation decreases the ion-exchange capacity of the membrane and constricts the pore walls, decreasing pore size and water uptake.