Water-free, Proton-Conducting Hybrid Materials for Elevated-Temperature Electrochemical Systems

By reacting TiCl4 with nitrilo tris(methylenphosphonic acid) (NTMPA) in water solution, a titanium complex gel (Ti-NTMPA) has been prepared. After drying, this gel was powdered and mixed with benzimidazole. The resulting hybrid material, which is insoluble in water and thermally stable up to at least 210 °C, possesses proton conductivity on the order of 10-2 Ω-1 cm-1 in fully anhydrous conditions. Due to these characteristics, such Ti-NTMPA/benzimidazole hybrid may be considered for application as a proton conductor in fuel cells or water electrolyzers operating under elevated-temperature and low-humidity conditions. X-ray diffraction, thermogravimetric analysis, and FT-IR spectroscopy show the presence of hydrogen-bonding interaction between Ti-NTMPA and benzimidazole in the hybrid. It appears that the Ti-NTMPA/benzimidazole system forms a solid protic ionic with the "free" (not coordinated to Ti) phosphonic acid moieties and benzimidazole acting, respectively, as Brønsted acid and base. In this way the imidazolic rings can vehiculate the protons provided by the phosphonic groups, thereby conferring hopping-site proton conduction upon this hybrid material even in the absence of water. This work provides proof of the above concepts, thereby demonstrating a general paradigm to design a whole class of hybrid materials with effective water-free proton conductivity. (Figure Presented)