Bismuth phosphates as intermediate temperature proton conductors: From polycrystalline powders to amorphous glasses

•Polycrystalline and amorphous glasses of bismuth phosphates prepared.•Major crystalline phases of monoclinic Bi2P4O13 and monoclinic BiPO4.•Intrinsic anhydrous proton conductivity of up to 10−5 S/cm.•High conductivity of 10−2 S/cm level under humidified atmosphere.•Good conductivity stability during humidity cycling for over 100 h.

Proton conducting electrolyte materials operational in the intermediate temperature range of 200-400 °C are of special interest for applications in fuel cells and water electrolysers. Bismuth phosphates in forms of polycrystalline powders and amorphous glasses are synthesized and investigated by scanning electron microscopy, X-ray diffraction, FT-IR, thermogravimetric analysis and AC impedance. Under dry atmosphere the pure crystalline and amorphous phosphates exhibit an intrinsic conductivity of up to 10−5 S cm−1 at 250 °C. In the presence of atmospheric humidity the conductivity of both types of phosphates is significantly enhanced, reaching about 10−2 S cm−1 at a water vapor partial pressure above 0.5 atm. During a period of more than 100 h with four humidity cycles from zero to 0.58 atm of the water vapor partial pressure, the phosphates show good stability, suggesting the potential as an intermediate temperature electrolyte.