Electrical properties and thermal expansion of cobalt doped apatite-type lanthanum silicates based electrolytes for IT-SOFC

The thermal expansion and conductivities have been investigated for Co3+ doped lanthanum silicates. The apatite-type lanthanum silicates with formula La10Si6−xCoxO27−x/2 (x = 0.2, 0.4, 0.6, 0.8, 1.0, 1.5) were synthesized by sol–gel process. The thermal expansion coefficient (TEC) of La10Si6−xCoxO27−x/2 was improved with increasing cobalt content because of the lower valence and larger radius of Co3+ ion compared to Si4+. Analysis of AC impedance spectroscopy showed that conductivity increased first and then decreased with increasing cobalt content. There is an optimum doping amount of cobalt and La10Si5.2Co0.8O26.6 exhibits the highest conductivity of 3.33 × 10−2 S/cm at 800 °C. When x ≤ 0.8, the local distortion caused by doping with Co3+ can significantly affect the oxygen channels and assist the migration of the interstitial oxide ions, resulting in the improvement of ionic conductivity. However, excess Co3+ dopant (0.8 < x ≤ 1.5) reduced the number of interstitial oxide ions and decreased the conductivity.

The figure shows the dependence of conductivity on the Co content. It can be seen that La10Si5.2Co0.8O26.6 exhibits the highest ionic conductivity of 3.33 × 10−2 S/cm at 800 °C. When x ≤ 0.8, as doping Co weakens the binding energy and aids the migration of the interstitial oxide ions, the ionic conductivity improves. On the other hand, excess dopant of Co (0.8 < x ≤ 1.5) can decrease the number of interstitial oxide ions and reduce the ionic conductivity.Figure optionsDownload as PowerPoint slideHighlights
► The unit volumes of La10Si6−xCoxO27−x/2 increase with increasing cobalt content.
► Doping Co can increase the thermal expansion because of the larger radius of Co3+ ion.
► Conductivities of La10Si6−xCoxO27−x/2 first increase and then decrease with cobalt content.
► Above 550 °C, La10Si6−xCoxO27−x/2 shows low activation energies of around 0.7 eV.