Chemical stability and proton conductivity of doped BaCeO3–BaZrO3 solid solutions

Barium cerate has a high proton conductivity but rather poor chemical stability in CO2-containing atmospheres. Barium zirconate, in contrast, is a rather stable material but one that exhibits low proton conductivity. In the present work, the conductivity and chemical stability of solid solutions between these compounds have been investigated, in an attempt to find a composition exhibiting both high conductivity and good stability. Compounds of general formula BaCe0.9−xZrxM0.1O3−δ, where M was Gd or Nd and x ranged from 0 to 0.4, were prepared by solid state reaction and characterized by X-ray powder diffraction, thermal gravimetric analysis (TGA) in flowing CO2, differential thermal analysis (DTA) in flowing CO2, and AC impedance spectroscopy in dry and H2O-saturated argon. Introduction of Zr into doped barium cerate greatly enhanced the chemical stability: for the Nd-doped system, compositions with x=0.2 or higher did not react with CO2 (under the experimental, non-equilibrium conditions), whereas for the Gd-doped system, the composition with x=0.4 did not react. Not unexpectedly, introduction of Zr also led to a decrease in conductivity and an increase in the activation energy for proton transport. Overall, Nd-doped samples exhibited higher chemical stability and lower conductivity than those doped with Gd. The composition BaCe0.7Zr0.2Nd0.1O3−δ appears to give a good compromise between conductivity and stability for fuel cell applications.