Furthermore, we choose Nafion as a solid BAY 80-6946 electrolyte (SPE) since Nafion has good proton conductivity, high gas permeability, and chemical stability, and it has been widely used in chlor-alkali, fuel-cell, and sensor applications. SPE became important during the mid-1970s because of the inefficiencies and maintenance requirements of liquid electrolytes . And it is often used in gas sensors , ,  and , especially for trace gas analysis . In the Nafion-based AGS design, Nafion film is used both as the electrolyte and as a support for the electrode structure. The ionic conductivity results from the mobility of the hydrated hydrogen ions that move through the polymer sheet by passing from one sulfuric acid group to another. Being a cation-exchange membrane, Nafion permitted the passage of cations and blocked the passage of anions. Thus, the hydrogen ions produced at the working electrode through oxidation of CO were transported through the ion-exchange membrane to the counter electrode, where they reduced oxygen with the addition of electrons to produce molecular water. In addition to its excellent ion-exchange capacity, the Nafion electrolyte exhibited other positive features, such as high structural stability, resistance to acids and strong oxidants, and good thermal stability. As a result of its useful characteristics, Nafion has been used to detect gases not only CO  and , but H2S  and , and so on.