Ionic conductivity of apatite-type solid electrolyte material, La10−XBaXSi6O27−X/2 (X = 0-1), and its fuel cell performance

We prepared Ba substituted lanthanum silicate (La_10−XBa_XSi₆O_27−X/2) and examined the effect of Ba substitution on the crystal structure and conductivity. The X-ray diffraction (XRD) results for a series of compositions showed that Ba ion can occupy the La site of an apatite structure with the composition La_10−XBa_XSi₆O_27−X/2 (X = 0-1). Rietveld analysis of the synchrotron XRD profiles revealed Ba occupation in the La 4f site rather than the La 6 h site and a decrease in the occupation factors of the oxide ions of the SiO₄ tetrahedra.The conductivity of La_10−XBa_XSi₆O_27−X/2 exhibited a maximum at X = 0.6 and the value was the same as that of YSZ (8 mol% Y₂O₃ doped ZrO₂) at 750 °C. On the other hand, the activation energy of about 50 kJ mol⁻¹ for La_9.4Ba_0.6Si₆O_26.7 was smaller than that of YSZ. Thus the conductivity of La_9.4Ba_0.6Si₆O_26.7 was a higher than those of YSZ below 750 °C. The conductivity parallel to the c-axis which is attributed to the 2a site oxide ions migration is known to be dominant in La₁₀Si₆O₂₇. However, Ba substitution seems to produce oxygen vacancies and create another pathway for oxide ions perpendicular to the c-axis. The increase in the pathways leads to an increase in the conductivity. We also reported the solid oxide fuel cell (SOFC) performance of La_10−XBa_XSi₆O_27−X/2 with a maximum power density of 65 mW cm⁻² using La_0.9Sr_0.1CoO_3−δ as a cathode and NiO-SDC (Sm doped CeO₂) as an anode.