Preparation, characterization and proton conductivity of titanium phosphate sulfophenylphosphonate

Titanium phosphate sulfophenylphosphonate, Ti(HPO4)1.00(O3PC6H4SO3H)0.85(OH)0.30·nH2O was synthesised and characterized by X-ray diffraction, solid state NMR, impedance measurements, TG and DSC analysis. The MAS-NMR spectrum of 31P nucleus presents two resonances characteristic of monohydrogen phosphate and sulfophenylphosphonate groups coordinated to three Ti(IV) atoms through three non-protonated oxygen atoms. Water content, protonic conductivity (σ) and activation energy for conduction (Ea) were determined in the range 20 to −20°C for materials equilibrated at relative humidity (r.h.) in the range 5–90%. By increasing the number of water molecules per sulfonic group from 2 to 14, σ rises by 3 orders of magnitude up to about 0.1 S cm−1, while Ea decreases from 12.6 to 4.3 kcal/mol. Measurements were also carried out at 100°C as a function of relative humidity in the range 30–100%; σ values higher than 0.1 S cm−1 were found at r.h. ≥65%. The titanium phosphonate was used as a filler of Nafion 1100 hybrid membranes. At 100°C and 80% r.h., the conductivity of the hybrid membranes containing from 5% to 20% phosphonate loading is very similar to that of a Nafion 1100 membrane prepared in-house by casting technique, but it drops by an order of magnitude when the loading is increased to 30%.