Increasing the electrical conductivity of La2Ce2O7 thin films by hydration

Proton conductors are essential for a wide range of applications including solid oxide fuel cells, sensors and resistivity switches. However, commonly used proton-conducting materials such as Nafion, doped barium zirconate, and cesium dihydrogen phosphate suffer from significant limitations - including poor long-term stability and microfabrication compatibility. Recent work has shown that bulk La 2 Ce 2 O 7 (LCO50) ceramics hydrate in moist environments and exhibit increased conductivity at temperatures below 473 K. In this work, we investigate the hydration behavior of magnetron-sputtered LCO50 thin films. We demonstrate that water can be incorporated into the crystal lattice, leading to an approximately fivefold increase at 363 K in the total conductivity of the as-deposited film. Extrapolating the exponential temperature dependence to room temperature, yields an approximately 10 3 fold conductivity enhancement, in agreement with values derived from XPS/CREM. To confirm that the total conductivity of the hydrated film is at least partially protonic, and to demonstrate that LCO50 can function as a mixed ion conductor, we demonstrate its use as the electrolyte in a layered tungsten oxide electrochromic device.