Electrochemical behavior of perovskite-type Ce1-xCaxVO3 under reducing conditions

The incorporation of calcium into the zircon-type lattice of Ce1-xCaxVO4 (x = 0.2-0.4), stable at atmospheric oxygen pressure, leads to a modest decrease in the unit cell volume and electrical conductivity, predominantly electronic. The average linear thermal expansion coefficients (TECs) of vanadate ceramics in air vary in the range (6.6-9.9) x 10(-6) K-1 at 393-1123 K, increasing with temperature and x. On reduction in hydrogen, zircon-type Ce1-xCaxVO4 undergoes phase transformation into perovskite-like Ce1-xCaxVO3. This transition is accompanied by a 32% crystal lattice contraction and a drastic increase in the conductivity, up to 120 S/cm at 1123 K for Ce0.6Ca0.4VO3. The average TECs of reduced metavanadates, (9.7-12.7) x 10(-6) K-1 at 393-1123 K, are compatible with those of solid oxide electrolytes. As for the variations of electronic resistivity, the polarization resistance of porous Ce1-xCaxVO3 electrodes in reducing atmospheres decreases with calcium additions. The electrochemical activity of Ce1-xCaxVO3 in contact with zirconia-based electrolyte, and protective ceria interlayer is higher for H2O electrolysis with respect to H-2 oxidation.