Structural, electrical and electrochemical properties of calcium-doped lanthanum nickelate

The present work focuses on the structural, thermo-mechanical and electrical properties of La 2–x Ca x NiO 4 + δ with different calcium content (x = 0–0.4) and the electrochemical performance of the electrodes on their base in contact with a Ce 0.8 Sm 0.2 O 1.9 electrolyte. It was shown that the introduction of even a small amount of calcium (x = 0.1) stabilized the crystal structure in the tetragonal syngony across the whole temperature range 25–800 °C. Dilatometric study of the compact samples in the entire concentration range of Ca showed no phase transitions up to 800 °C. The total electrical conductivity, measured by a dc four-probe technique, increased with an increase in calcium content. However, electrochemical activity of the La 2–x Ca x NiO 4 + δ – based electrodes, contrarily, drastically decreased. The polarization resistance R η , derived from the impedance spectroscopy data, increased from 0.18 Ω·cm 2 (x = 0) to 1.24 Ω·cm 2 (x = 0.3) at 800 °C. The coefficients of oxygen surface exchange ( k ⁎ ) and oxygen tracer diffusion ( D ⁎ ) calculated from the oxygen isotope exchange data for La 2 NiO 4 + δ were considerably higher than those for La 1.7 Ca 0.3 NiO 4 + δ (4.54 ∙ 10 − 7 cm ∙ s − 1 and 3.02·10 − 8 cm 2 ∙ s − 1 (x = 0); 2.91 ∙ 10 − 8 cm ∙ s − 1 and 5.82·10 − 10 cm 2 ∙ s − 1 (x = 0.3) at 800 °C, respectively). It was found that for La 1.7 Ca 0.3 NiO 4 + δ the rate-determining stage of oxygen surface exchange is incorporation of oxygen, while for La 2 NiO 4 + δ it is oxygen dissociative adsorption.