High-temperature thermal behaviors of XH2PO4 (X = Cs, Rb, K, Na) and LiH2PO3

XH2PO4 ionic compounds have emerged as a viable electrolyte for intermediate temperature fuel cells, and here have been subjected to thermal analysis to clarify their high-temperature properties. Thermoanalytical peaks were identified at 231.5, 239 and 349 °C for CsH2PO4; 127, 250 and 354 °C for RbH2PO4; 232, 270 and 319 °C for KH2PO4; 223, 330 and 352 °C for NaH2PO4; also, 195 and 220 °C for LiH2PO3 (peak temperature values as measured at the same heating rate of 10 K/min). The thermal events at 231.5 °C in CsH2PO4 and 127 °C in RbH2PO4 were previously interpreted as thermal decomposition by numerous researchers, but we confirm their origin in structural phase transition. The high-temperature variations in KH2PO4 and NaH2PO4 are entirely due to thermal dehydration rather than phase transition. We have also examined LiH2PO3, and found, for the first time, an endothermic peak at 195 °C, and attributed it to structural phase transition.