Irreversibility of proton conductivity of Nafion and Nafion–Titania composites at high relative humidity

The temperature dependence of the proton conductivity of Nafion and Nafion–Titania composite in the hydrated state is investigated. The proton conductivity of Nafion–Titania measured in two successive heatings displays irreversibility, a behavior also observed in bare Nafion. The relationship between the proton transport and the dynamics of α-relaxation as proved by dielectric spectroscopy is discussed for Nafion and the composite electrolytes. The results indicate that in the first heating, the proton conductivities of Nafion and Nafion–Titania are modulated only by the glass transition of the ionomer, whereas in the second heating both the α-relaxation and the polymer glass transition are coupled to the charge transport. Such features are important for the development of high-performance electrolytes for high-temperature proton exchange membrane fuel cells.