Protons in lattice confinement: Static pressure on the Y-substituted, hydrated BaZrO3 ceramic proton conductor decreases proton mobility

Abstract

Yttrium substituted BaZrO3, with nominal composition BaZr0.9Y0.1O3, a ceramic
proton conductor, was subject to impedance spectroscopy for temperatures 300 K
< T < 715 K at mechanical pressures 1 GPa < p < 2 GPa. The activation energies
Ea of bulk and grain boundary conductivity from two perovskites synthesized by
solid-state reaction and sol-gel method were determined under high pressures.
At high temperature, the bulk activation energy increases with pressure by 5%
for sol-gel derived sample and by 40% for solid-state derived sample. For the
sample prepared by solid-state reaction, there is a large gap of 0.17 eV
between the activation energy at 1.0 GPa and > 1.2 GPa. The grain boundary
activation energy is around a factor two times as that of the bulk, and it
reaches a maximum at 1.25 - 1.5 GPa, and then decrease as the pressure
increases, indicating higher proton mobility in the grain boundaries at higher
pressure. Since this effect is not reversible, it is suggested that the grain
boundary resistance decreases as a result of pressure induced sintering. The
steady increase of the bulk resistivity upon pressurizing suggests that the
proton mobility depends on the space available in the lattice. In return, an
expanded lattice with a/a0 > 1 should thus have a lower activation energy,
suggesting that thin films expansive tensile strain could have a larger proton
conductivity with desirable properties for applications.