Zirconium phosphate Nafion® composites — a microstructure-based explanation of mechanical and conductivity properties

It is known that by addition of zirconium phosphate to perfluorinated sulfonic acid ionomers the proton conductivity of the resulting PFSA-ZP-composite decreases. Nevertheless, at high temperatures the stability and efficiency of the composite is greatly increased. In order to clarify these, at first sight, contradicting observations mechanical properties and conductivity of a PFSA ionomer (Nafion® 117) and PFSA-ZP-composites were examined under conditions similar to fuel cell application. Water sorption-isotherms were used to directly relate mechanical and electrical properties to the water content at 75 °C. The results point at a strong coulombic interaction between organic and inorganic phase which at the same time increases the stiffness and TG by ionic crosslinking and decreases conductivity. The interaction is the more pronounced when the water activity is lower. A threshold of about three water molecules per sulfonic acid group was identified, above which a significant proton conductivity is achieved and, concurrently, most of the mechanical stiffening effect has vanished.