*3.5.2 The electrolyte (membrane)*

The electrolytes are thin plastic-like or solid polymer membranes. Their main function is to pass the hydrogen positive ions from the anode to the cathode while completely block the free electrons from moving in this direction [18]. If the free electrons could pass through the electrolyte, they would hold up the chemical reaction. The filtration process is based on the semi-permeability of the membrane. There are several mechanisms of ions permeability through the membrane including absorption and adsorption, distillation, extraction, physical filtration, and stripping.

The compositions of the electrolytes are perfluorosulfonic acids, which are Teflon-like fluorocarbon polymers that have side chains ending in sulfonic acid groups (–SO32–). Such acidic polymer electrolytes require water to conduct hydrogen ions. Therefore, the reaction gas (H2) in contact with the electrolyte must be saturated with water. The commercial name of Teflon based polymer electrolyte is Nafion [20]. It has a high ionic conductivity at 80°C, good thermal and chemical stability [24–27], and high chemical resistance. Its chemical chain is shown in **Figure 5** [28].

Fuel cells performances improve largely with increasing temperature to 90o C due to the decline in ohmic resistance of their electrolytes [29]. But at higher temperatures, the fuel cell will dry. Since Nafion ionic conductivity is strongly dependent on water content, the Nafion membranes will have an osmotic swelling problem and maybe potentially dissolve in methanol solution when increasing methanol concentration and temperature.

$$\begin{array}{c} \cdots \left[ \left< \mathbf{CF\_2\cdots CF\_2} \right> \left< \mathbf{CF\_2\cdots CF} \right> \right]\_\mathbf{x} \\ \mathbf{O} \stackrel{\textstyle \mathbf{---CF\_2\cdots CF\cdots CF\_3}}{\longrightarrow} \\ \mathbf{O} \stackrel{\textstyle \mathbf{---CF\_2\cdots CF\_2\cdots SO\_3H}}{\longrightarrow} \end{array}$$

**Figure 5.** *Nafion chemical chain [28].*

### *3.5.3 The catalysts*

The catalysts are special chemical materials used to increase the rate of reactions without being consumed. The main condition of the material to be used as a catalyst is to stay unchanged after reactions. Therefore, they could be recovered from the reaction mixture chemically. The existing of catalysts reduces the reaction energy barrier and thus speeds up the reaction at low temperatures [30]. Most types of fuel cells used platinum (Pt) and its group as a catalyst at the two electrodes. At the anode, the Pt catalyst helps in splitting the hydrogen molecules into free electrons and positive cations. While at the cathode, it enables oxygen reduction [31]. A perfect catalyst for a fuel cell is the one with high electric conductivity, stable during contact with the reactant gases and the electrolyte.
