Effect of nickel substitution on defect chemistry, electrical properties, and dimensional stability of calcium-doped yttrium chromite

Abstract

The effect of nickel substitution on defect chemistry, electrical properties, and dimensional stability of calcium-doped yttrium chromite was studied for use as an interconnect material in high temperature solid oxide fuel cells (SOFCs). The compositions of Y 0.8Ca 0.2Cr 1 − x Ni xO 3 ± δ (x = 0–0.15), prepared using the glycine nitrate process, showed single phase orthorhombic perovskite structures over a wide range of oxygen partial pressures (4.6 × 10 − 20 atm ≤ pO 2 ≤ 0.21 atm at 900 °C). X-ray diffraction (XRD) analysis indicated that most of the nickel ions replacing chromium ions are divalent and act as acceptor dopants, leading to a substantial increase in conductivity. In particular, the conductivity at 900 °C in air increased from 10 S/cm to 34 S/cm with 15% nickel substitution, and an increase in charge carrier density was confirmed by Seebeck measurements, which validated the predominant Ni 2+ oxidation state. A point defect model was derived, and the relationship between electrical conductivity and oxygen partial pressure was successfully fitted into the proposed model. The defect modeling results indicated that nickel substitution improves the stability of calcium-doped yttrium chromite toward reduction and suppresses the oxygen vacancy formation, which results in significantly increased electrical conductivity in reducing environment. The electrical conductivity of Y 0.8Ca 0.2Cr 0.85Ni 0.15O 3 ± δ at 900 °C in reducing atmosphere ( pO 2 = 10 − 17 atm) was 5.8 S/cm, which was more than an order of magnitude higher than that of Y 0.8Ca 0.2CrO 3 ± δ (0.2 S/cm). Improved stability in reducing atmosphere was further confirmed by dilatometry measurements showing reduced isothermal “chemical” expansion, and the isothermal expansion in reducing atmosphere ( pO 2 = 10 − 17 atm) at 900 °C decreased from 0.07% for Y 0.8Ca 0.2CrO 3 ± δ to 0.03% for Y 0.8Ca 0.2Cr 0.85Ni 0.15O 3 ± δ . Based on these results, enhanced electrical performance and mechanical integrity is expected with nickel substitution on calcium-doped yttrium chromite in SOFC operating conditions.