Nafion–Zirconium Phosphate Nanocomposite Membranes with High Filler Loadings: Conductivity and Mechanical Properties†

Zirconium phosphate (ZrP) was incorporated in preformed Nafion 117 membranes by exchange of the ionomer protons with zirconium cationic species and subsequent treatment with phosphoric acid. Reiteration of these treatments under the same reaction conditions allowed to obtain membranes with filler loading in the range of 20–40 wt.-%. 31P MAS NMR investigations showed that the filler is mainly α-ZrP and that the fraction of monohydrogen phosphate groups in the α-type environment increases with the number of treatments. All membranes were characterised by stress–strain mechanical tests at room temperature and turned out to be stiffer than Nafion 117. Through-plane conductivity determinations were carried out as a function of temperature and relative humidity (RH). At 100 °C and RH in the range of 30–90%, the composite membranes were less conductive than Nafion 117, and the higher the filler loading the larger the conductivity decrease occurring with decreasing RH. The conductivity of the composite membranes showed however better long-term stability at high temperature and RH values. Moreover, the partial replacement of the phosphate groups of ZrP with sulphophenylphosphonate groups leads to a significant improvement of conductivity and stiffness with respect to both Nafion 117 and the parent composite membrane.