Preparation and characterization of composite membranes composed of zirconium tricarboxybutylphosphonate and polybenzimidazole for intermediate temperature operation

Abstract

Zirconium tricarboxybutylphosphonate, Zr(O 3PC(CH 2) 3(COOH) 3) 2, was synthesized by slowly decomposing zirconium fluoro-complexes in the presence of 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTC) to prepare inorganic/organic composite membranes based on polybenzimidazole (PBI). Composite membranes composed of zirconium tricarboxybutylphosphonate (Zr(PBTC)) particles embedded in PBI were formed by evaporating the solvent from a suspension of Zr(PBTC) particles in a PBI solution. Composite membranes with 50 wt.% Zr(PBTC) exhibited uniform distribution of the particles throughout the thickness, as ascertained by scanning electron microscopy. The conductivities measured for compacted Zr(PBTC) powder and PBI-based membranes with 50 wt.% Zr(PBTC) were 6.74 × 10 −2 and 3.82 × 10 −3 S cm −1, respectively, at 200 °C for P H 2O / P s=1. Functionalization of the PBI with phosphoric acid groups obtained by treatment with phosphoric acid and with sulfonic acid groups obtained by post-sulfonation thermal treatment with sulfuric acid improved the proton conductivity of 50 wt.% Zr(PBTC) membranes to 5.24 × 10 −3 and 8.14 × 10 −3 S cm −1 under the same measurement conditions. Zr(PBTC)/PBI membranes thus have great potential as proton exchange membrane fuel cells (PEMFCs).