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

Abstract

Proton solid electrolytes, which exhibit high proton conductivity at a wide range of low-intermediate temperatures (150–300 °C), are key materials for the development of fuel cells for automobiles and cogeneration systems. In this study, a benitoite-type polyphosphate, KMg_1−xH_2x(PO₃)·yH₂O, which has a non-combustible and layered structure, was investigated as a new proton conductor. The benitoite-type KMg_1−xH_2x(PO₃)·yH₂O was synthesised by a coprecipitation method. The solid solution formed in the range of x = 0–0.100 in KMg_1−xH_2x(PO₃)₃·yH₂O. Multi-step weight loss due to dehydration was observed for TG/DTA measurement at 30 °C and 150 °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 KMg_1−xH_2x(PO₃)₃·yH₂O exhibited high proton conductivity of 1.4 × 10⁻³ S cm⁻¹ at 150 °C and 6.5 × 10⁻³ S cm⁻¹ at 250 °C under a non-humidified N₂ gas flow.