Thermal evolution of structures and conductivity of Pr-substituted BaZr0.7Ce0.2Y0.1O3−δ: potential cathode components for protonic ceramic fuel cells

Abstract

A complete solid solution forms between the perovskite proton conductor BaZr_0.7Ce_0.2Y_0.1O_3−δ (BZCY72) and BaPr_0.9Y_0.1O_3−δ (BPY) on synthesis by the Pechini method and high-temperature annealing. Phase fields of selected members of the Ba(Zr_0.7Ce_0.2)_1−(x/0.9)Pr_xY_0.1O_3−δ series were studied as a function of composition and temperature by high-resolution neutron powder diffraction revealing symmetry changes in the sequence Pnma → Imma → → Pmm. Higher symmetry is favoured for low Pr contents and high temperatures, as consideration of tolerance factor suggests. A volume contraction, ascribed to dehydration, is observed by synchrotron X-ray diffraction on heating in air for lower x. Magnetic measurements and structural data support the presence of Pr in the IV valence state on the perovskite B site. Thermogravimetric analysis in CO₂ to ∼1253 K indicates better chemical stability for x ≤ 0.445, whereas decomposition occurred for higher x. Electrical conductivity increases by over two orders of magnitude in dry air at lower temperature from x = 0.225 to 0.675; total conductivity reaches a value of 0.4 S cm⁻¹ at 1173 K for x = 0.675. The series exhibits electron–hole transport with a positive pO₂ dependence which increases with temperature, consistent with participation of oxygen vacancies in charge compensation of the Y³⁺ acceptor dopant. The activation energy for thermally activated hole hopping in air in the range 523–773 K decreases from ∼1 eV for BZCY72 to ∼0.4 eV for x = 0.675. Conductivity is generally lower in humidified N₂ and air (pH₂O ≈ 0.023 atm) than the corresponding dry atmospheres, consistent with consumption of holes by less mobile protonic species; however for x ≤ 0.225 the lower concentration of electron holes concomitant with higher oxygen-vacancy content in N₂ results in slightly higher conductivity in wet conditions due to hydration of vacancies.