Preparation and proton conductivity of composite ionomeric membranes obtained from gels of amorphous zirconium phosphate sulfophenylenphosphonates in organic solvents

Abstract

Amorphous zirconium phosphate sulfophenylenphosphonates (Zr(HPO₄)_2−x(SPP)_x, where SPP = O₃PC₆H₄SO₃H and 0.5 ≤ x ≤ 1.3) were precipitated from solutions of zirconyl chloride, H₃PO₄ and H₂SPP and characterised by thermogravimetric analysis, solid state NMR and impedance measurements. Zr(HPO₄)_0.7(SPP)_1.3 showed the highest proton conductivity (0.07 S cm⁻¹ at 20 °C, 90% relative humidity) and turned out to be suitable to form a gel in dimethylformamide (DMF) containing about 7 wt% solid phase. The gel was used to prepare composite membranes of sulfonated polyetherketone (I. E. C. = 0.95 meq g⁻¹) filled with 10 and 20 wt% Zr(HPO₄)_0.7(SPP)_1.3. SEM pictures showed that the composite membranes are dense and homogeneous with filler particles not larger than 200 nm. The proton conductivity of both composite membranes and that of a pure s-PEK membrane was determined as a function of temperature up to 150 °C, at 90% relative humidity, by means of the impedance technique. While the conductivity of the s-PEK membrane was nearly constant within the overall temperature range (7–9 × 10⁻⁴ S cm⁻¹), the conductivity of the composite membranes was independent of temperature up to 110 °C but at higher temperatures it decreased to a value approaching the conductivity of s-PEK. In the range 80–110 °C, the conductivity of the composite membrane with 20 wt% Zr(HPO₄)_0.7(SPP)_1.3 was around 8 × 10⁻³ S cm⁻¹. The possibility that the increase of conductivity arises from the presence of filler rich and/or filler continuous conduction pathways is discussed.