Y2T2O7-Based Pyrochlores As Alternative Materials for Diffusion Barrier Layer at the Fuel Electrode-Electrolyte Interface in SOFC/SOEC
Authors: Yaremchenko Aleksey , Bamburov Aleksandr 
Published: December 2024

The introduction of a ceria-based diffusion barrier layer between electrolyte and Sr-containing oxygen electrode is a common approach to avoid the formation of undesirable low-conducting SrZrO3 phase at the electrolyte/electrode interface with negative effects on the performance of solid oxide cell (SOFC/SOEC). However, partial reduction of cerium cation to 3+ oxidation state under reducing conditions induces noticeable chemical expansion of doped ceria ceramics and may cause delamination of the ceria-based buffer layer applied at the fuel electrode side of solid oxide cell. The present work explores pyrochlore-type Y2Ti2O7-based titanates as alternative interlayer materials operating at the interface between the electrolyte and Sr-containing fuel electrodes.
A- and/or B-site Y2T2O7-based compositions including (Y0.9Ca0.1)2Ti2O7, (Y0.85Ca0.15)2Ti2O7, (Y0.9Ca0.1)2(Ti0.6Zr0.4)2O7, (Y0.9Ca0.1)2(Ti0.9Mn0.1)2O7, (Y0.9Mg0.1)2Ti2O7, and Y2(Ti0.9Mg0.1)2O7 were selected as candidate materials. The materials were prepared by solid reaction method and sintered at 1600-1700°C in air. The characterization included structural and microstructural studies, controlled-atmosphere dilatometry, and measurements of electrical transport properties (electrical conductivity as a function of temperature and oxygen partial pressure by AC impedance spectroscopy, oxygen-ion transference numbers by modified EMF method).
The results showed that the solubility of calcium cations in the yttrium sublattice is limited to 10 at.%. Apart from (Y0.9Ca0.1)2Ti2O7, only (Y0.9Ca0.1)2(Ti0.6Zr0.4)2O7 and (Y0.9Mg0.1)2Ti2O7 were phase-pure pyrochlores, while other materials contained different types of secondary phases. Total electrical conductivity of single-phase ceramics in air increased in the row (Y0.9Ca0.1)2(Ti0.6Zr0.4)2O7 < (Y0.9Mg0.1)2Ti2O7 < (Y0.9Ca0.1)2Ti2O7 reaching 0.02 S/cm at 800°C (Fig.1). The conductivity is predominantly oxygen-ionic at intermediate oxygen pressures, while minor contributions of p-type and n-type electronic transport increase under oxidizing and reducing conditions, respectively. Average oxygen-ion transference numbers under air/(10%H2-N2) gradient at 800°C were 0.995 for (Y0.9Ca0.1)2Ti2O7 and 0.983 for (Y0.9Mg0.1)2Ti2O7. Pyrochlore-type titanate ceramics showed moderate thermal expansion coefficients, 10.2-10.4 ppm/K, under both oxidizing and reducing conditions at 25-1100°C and negligible chemical expansion. On the contrary, gadolinia-doped ceria Ce0.9Gd0.1O2-δ ceramics underwent a substantial chemical expansion and cracking on heating above 700-800°C in 10%H2-N2 atmosphere.
(Y0.9Ca0.1)2Ti2O7 was selected as the preferred interlayer candidate material with the highest ionic conductivity. A procedure for the fabrication of a thin interlayer in a model electrolyte-supported button cell was developed. Chemical compatibility was verified in contact with solid electrolyte, yttria-stabilized zirconia, and model fuel electrode materials, Sr0.85Pr0.15TiO3-δ and SrFe0.75Mo0.25O3-δ.
Figure 1. Electrical conductivity of pyrochlore-type Y2T2O7-based ceramics in air.
Acknowledgments: This work was done within the scope of project CLEARNESS (funded by the ERA-MIN3 program), project SFRH/BD/150704/2020 (funded by the FCT), and project CICECO-Aveiro Institute of Materials (UIDB/50011/2020, UIDP/50011/2020 & LA/P/0006/2020) financed by national funds through the FCT/MCTES (PIDDAC), Portugal.

Figure 1