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 chargecarrier
concentration in ABO3 perovskite oxides. Despite 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 we show the enhanced proton conductivity
of the defective calcium titanate Ca0.92Ti1O2.84(OH)0.16 prepared by replacing lattice oxygens
with hydroxides via a solvothermal route. Conductivity measurements in humidified Ar
atmosphere revealed that, remarkably, this material exhibits more 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. We also demonstrate the replacement of Ca2+ by Ni2+ in the
Ca1-xTi1O3-2x(OH)2x, which mostly exsolve metallic Ni nanoparticles along orthorhombic (100) planes upon reduction. These results suggest a new strategy by tailoring the defect chemistry
via hydration or cation doping followed by exsolution for targeted energy applications.