## 9 Application of Derivatives of Styrene in Membrane of Fuel Cells
### Styrene-Ethylene-Butylene-Styrene (SEBS)  
SEBS is a polymer produced by the hydrogenation of SBS. During functionalization, this method provides protection from a side reaction in the non-aromatic double bonds. SEBS exhibits good flexibility and chemical inertness. However, its use as a membrane in fuel cells is limited due to its tendency to swell and low tensile strength. Various attempts have been made to use SEBS as an AEM by introducing ion-conducting head groups. These efforts have resulted in AEMs with high ionic conductivity due to excellent phase separation, despite relatively low IEC values. SEBS can also be easily copolymerized with other styrene-based polymers, enhancing the physical and electrochemical properties of membranes. In one of the examples, [PERSON] and colleagues [122] synthesized modified SEBS by live anion copolymerization with polystyrene and 1,4-butadiene units. 1,4-butadiene become part of the membrane materials, while polystyrene high contents provide high mechanical strength to the membrane. With 70% PS, the modified membrane shows IEC and proton conductivity values of 3.37 mmol g\({}^{-1}\) and 190 mS cm\({}^{-1}\) when tested at a temperature of 80 \({}^{\circ}\)C [Figure 6a-d]. Figure 6e,f demonstrates the alkaline stability and fuel cell performance of the synthesized poly(styrene-b-(ethylene-co-butadiene)-b-styrene) anion exchange membrane (AEM), showcasing its practical viability. The figure includes polarization and power density curves, where voltage and power output are plotted against current density. The polarization curve reveals three key regions: an initial voltage drop due to activation losses at low current densities, a linear decline from ohmic resistance (dominated by the membrane), and a sharp drop at high current densities caused by mass transport limitations. Moreover, the modified membrane is able to attain a power density value of nearly 1000 mW cm\({}^{-2}\) with a current density value of 2.09 A cm\({}^{-2}\) when tested in H\({}_{2}\)/O\({}_{2}\) single cell assembly at a temperature of 80 \({}^{\circ}\)C. The results show that the modified SEBS anion exchange membrane is able to apply commercial fuel cells [Figure 6e,f]. The presence of styrene allows for the sulfonation of SEBS, making the membrane more hydrophilic and enhancing its ability to attract water molecules. As an example, [PERSON] and his fellow colleagues [123] synthesized sulfonated SEBS and crosslinked it with glutaraldehyde of different amounts from 2 to 10%. Glutaraldehyde acts as a crosslinking agent which favors the dimensional stability of the prepared membrane. The modified membrane shows a high proton conductivity value of 10 mS cm\({}^{-1}\) at a temperature of 25 \({}^{\circ}\)C. The modified sulfonated membrane shows power density values of 68 mW cm\({}^{-2}\) and 58 mW cm\({}^{-2}\) with a current density value of 200 mA cm\({}^{-2}\) for a crosslinked glutaraldehyde amount of 2 and 10%, respectively, when assembled with 2 M concentration of methanol solution at 25 \({}^{\circ}\)C. Moreover, the sulfonated modified membrane resists high methanol crossover due to its blocking effects. Ultimately, the methanol crossover value from 16.22 cm\({}^{2}\) s\({}^{-1}\) to 11. \(89\times 10^{-7}\) cm\({}^{2}\) s\({}^{-1}\) could pass through modified membrane which is much lower than the commercial Nafion 117 membrane whose value is 35.2 \(\times\) 10\({}^{-7}\) cm\({}^{2}\) s\({}^{-1}\). [PERSON] and his fellow colleagues [124] used one derivative of styrene to make another derivative of styrene. In the article, two types of SBS named SEBS 1301 and SEBS1401 were chosen, containing 87% 1,4 butadiene. These types were converted into SEBS-based AEMs through methods such as hydrogenation, quaternization, alkalization, and chloromethylation. The results show that the membrane shows a high mechanical strength which is a result of physical self-crosslinking provided by poly(ethylene-co-butylene).