Humidity-sensitive irreversible phase transformation of open-framework zinc phosphate and its water-assisted high proton conduction properties

Abstract

Open-framework zinc phosphate (NMe₄)(ZnP₂O₈H₃) undergoes irreversible phase transformation. Structural transformation with α (NMe₄·Zn[HPO₄][H₂PO₄] the low-temperature phase) and β (NMe₄·ZnH₃[PO₄]₂ the high-temperature phase) (T_c = 149 °C) and conduction properties were investigated by single-crystal X-ray diffraction, differential scanning calorimetry, and alternating current (ac) impedance. The open-framework material was sensitive to humidity and β proton conductivity was higher than 10⁻² S cm⁻¹ at room temperature and 98% relative humidity (RH). Given that the high proton conductivity of the open-framework material can compete with that of many advanced proton conductors based on metal–organic frameworks (MOFs), it has broad application prospects in various electrochemical devices.