Proton conductive inorganic materials for temperatures up to 120° C and relative humidity down to 5%

The proton conductivities of inorganic conductors in a temperature range of 25-120°C and relative humidity (RH) from 1 to 100% were studied. A conductivity cell was designed and built. A conductivity measurement method was developed including inorganic sample preparation, a reliable condition-controlled experimental system, conductivity measurement protocol, and an electrochemical impedance spectroscopy equivalent circuits. Alpha zirconium phosphate was used as a reference material to test the designed experimental approach based on literature study and laboratory measurements. Solid acid materials, such as sulfated zirconia (S-ZrO₂), phosphosilicate gels (P-SiO₂), and sulfonic-functionalized silica (SBA-15) were synthesized, characterized, and their conductivities were measured. The impedance characteristics and transport mechanisms were discussed. Under a fully hydrated condition at 120°C, S-ZrO₂ presented a proton conductivity of 10 mS cm-1. P-SiO₂ was found to be a pure proton conductor. The materials with a P:Si ratio of 1.5 reached a high proton conductivity of 100 mS cm-1, which was higher than that of Nafion at 120°C and 70% RH. In the study, SBA-15 appeared to be a partially electron-conductive material.