Proton conductivity in mixed cation phosphate, KMg1-xH2x(PO3)•yH2O, with a layered structure at low-intermediate temperatures

Proton solid electrolytes, which exhibit high proton conductivity at a wide range of low-intermediate temperatures (150-300 degrees C), are key materials for the development of fuel cells for automobiles and cogeneration systems. In this study, a benitoite-type polyphosphate, KMg1-xH2x(PO3)center dot yH(2)O, which has a non-combustible and layered structure, was investigated as a new proton conductor. The benitoite-type KMg1-xH2x(PO3)center dot yH(2)O was synthesised by a coprecipitation method. The solid solution formed in the range of x = 0-0.100 in KMg1-xH2x(PO3)(3)center dot yH(2)O. Multi-step weight loss due to dehydration was observed for TG/DTA measurement at 30 degrees C and 150 degrees C. We observed enhanced peaks of the vibration bands at around 1117 cm(-1) and 1229 cm(-1), which were attributed to the symmetric and asymmetric PO2 vibration modes, and at 743 cm(-1) and 970 cm(-1) due to the n(s)(P-O-P) and n(as)(P-O-P) modes as well as broad absorbance peaks at 2300 cm(-1) and 2700 cm(-1) corresponding to the vibration modes of n(s)(P-O-H) with increasing x for FTIR spectra, which suggest the introduction of protons to the crystal structure. Proton conductivity increased from x = 0 to 0.10 and then decreased at x = 0.125, where the impurity phase was observed. The sample with x = 0.10 in benitoite-type KMg1-xH2x(PO3)(3)center dot yH(2)O exhibited high proton conductivity of 1.4 x 10(-3) S cm(-1) at 150 degrees C and 6.5 x 10(-3) S cm(-1) at 250 degrees C under a non-humidified N-2 gas flow.