A mixed cation phosphate, KCo 1−x H 2x (PO 3 ) 3 • yH 2 O, with a tunnel structure was synthesized by a coprecipitation method and investigated as a proton conductor. KCo 1−x H 2x (PO 3 ) 3 • yH 2 O formed a solid solution in the range of x = 0-0.18, and the enhancement of the absorption peaks corresponding to the vibration and stretching modes of OH and POH were observed for increasing x. The sample with x = 0.18 exhibited a high proton conductivity in the temperature range of 25-250°C, and the value of 2.6 x 10 −3 Scm −1 was obtained at 250°C. This sample maintained a high proton conductivity of over 10 −3 Scm −1 at 150°C for 9 h, and no electron conductivity was confirmed in a hydrogen or oxygen gas atmosphere.

Proton solid electrolytes, which exhibit high proton conductivity at a wide range of low-intermediate temperatures (150–300ºC), are key materials for the development of fuel cells for automobiles and cogeneration systems....

Proton solid electrolytes, which exhibit high proton conductivity and thermal stability at a wide range of intermediate temperatures, are desirable for operating fuel cells at a temperature suitable for applications...