Use of interplay between A-site non-stoichiometry and hydroxide doping to deliver novel proton-conducting perovskite oxides

The magnitude of ionic conductivity is known to depend upon both mobility and number of available carriers. For proton conductors, hydration is a key factor in determining the charge–carrier concentration in ABO₃ perovskite oxides. Despite the high reported proton mobility of calcium titanate (CaTiO₃), this titanate perovskite has thus far been regarded as a poor proton conductor due to the low hydration capability. Here, the enhanced proton conductivity of the defective calcium titanate Ca_0.92TiO_2.84(OH)_0.16 prepared by replacing lattice oxygens with hydroxyl groups via a solvothermal route is shown. Conductivity measurements in a humidified Ar atmosphere reveal that, remarkably, this material exhibits one order of magnitude higher bulk conductivity (10⁻⁴ Scm⁻¹ at 200 °C) than hydrated stoichiometric CaTiO₃ prepared by traditional solid-state synthesis due to the higher concentration of protonic defects and variation in the crystal structure. The replacement of Ca²⁺ by Ni²⁺ in the Ca_1−xTi1O_3−2x(OH)_2x, which mostly exsolve metallic Ni nanoparticles along orthorhombic (100) planes upon reduction, is also demonstrated. These results suggest a new strategy by tailoring the defect chemistry via hydration or cation doping followed by exsolution for targeted energy applications.