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

Abstract

The present work focuses on the structural, thermo-mechanical and electrical properties of La2–xCaxNiO4 + δ with different calcium content (x=0–0.4) and the electrochemical performance of the electrodes on their base in contact with a Ce0.8Sm0.2O1.9 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 La2–xCaxNiO4 + δ – based electrodes, contrarily, drastically decreased. The polarization resistance Rη, derived from the impedance spectroscopy data, increased from 0.18Ω·cm2 (x=0) to 1.24Ω·cm2 (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 La2NiO4+δ were considerably higher than those for La1.7Ca0.3NiO4+δ (4.54∙10−7cm∙s−1 and 3.02·10−8cm2∙s−1 (x=0); 2.91∙10−8cm∙s−1 and 5.82·10−10cm2∙s−1 (x=0.3) at 800°C, respectively). It was found that for La1.7Ca0.3NiO4+δ the rate-determining stage of oxygen surface exchange is incorporation of oxygen, while for La2NiO4+δ it is oxygen dissociative adsorption.