SrVO3-Based Fuel Electrode Material for Solid-Electrolyte Cells: Stability, Electrical Properties and Thermochemical Expansion

Abstract

Perovskite-like SrVO3-δ exhibits high electrical conductivity under reducing conditions and good resistance to sulfur poisoning and carbon deposition, and can be considered, therefore, as a promising fuel electrode material for application in solid oxide fuel cells (SOFCs) and reversible solid electrolyte cells [1-3]. The drawback of SrVO3-δ as potential electrode material is a narrow stability domain matching the anodic conditions. Under oxidizing conditions typical for SOFC fabrication, V4+-based SrVO3 perovskite is easily oxidized to insulating V5+-based strontium pyrovanadate Sr2V2O7; this transformation is accompanied with significant dimensional and microstructural changes and is not completely reversible at T < 1000°C [4]. The present work aimed at the development of SrVO3-δ - based fuel electrode materials with improved stability using two approaches. First approach relates to partial donor-type substitutions into strontium and vanadium sublattices, which may be expected to stabilize V3+/V4+ oxidation state and to oppose the oxidation to V5+ state, and also may improve the structural stability by adjusting tolerance factor. Another approach is to find a compromise between high metallic conductivity of SrVO3 and remarkable stability of SrTiO3 perovskite lattice. Sr1-x Ln x V1-y Nb y O3-δ (Ln = La and Y, x = 0-0.2, y = 0-0.3) and SrV1-y Ti y O3-δ (y= 0-1.0) compositions were assessed as potential electrode materials with emphasis on phase relationships, phase stability, redox behavior, electrical conductivity and thermomechanical behavior. Ceramic materials were prepared by solid-state synthesis assisted with high-energy milling, and sintered at 1500°C in 10%H2-N2atmosphere. XRD analysis confirmed the formation of single-phase materials with cubic perovskite structure for all compositions, while microstructural analysis (SEM/EDS) indicated that the solid solubility of Nb cations in vanadium sublattice under applied conditions corresponds to ~25 at.%. Donor-type substitutions by La, Y and Nb were found to decrease electronic conductivity, which still remains sufficiently high for electrode application (> 100 S/cm at temperatures ≤ 950°C), and to suppress thermochemical expansion thus improving thermomechanical compatibility with solid electrolytes. The upper-p(O2) stability boundary at 900°C was found to shift from ~10-15 atm for the parent strontium vanadate to ~6×10-13 atm for Sr0.8Y0.2VO3-δ and ~ 10-12 atm for SrV0.8Nb0.2O3-δ, whereas oxidative decomposition of Sr0.8La0.2VO3-δ occurs in the p(O2) range between 10-10 and 10-5atm (Fig.1). Increasing titanium concentration in SrV1-y Ti y O3-δ was demonstrated to have qualitatively similar effects suppressing oxygen nonstoichiometry and dimensional changes with temperature, and leading to a transition from metallic to semiconducting behavior at y > 0.5. Upper p(O2) stability limit also gradually shifts to higher oxygen partial pressure with Ti substitution and corresponds to p(O2) > 10-10 at 900°C for y> 0.3. In air, the substituted materials still undergo oxidative decomposition accompanied with substantial dimensional and irreversible microstructural changes. On the other hand, sluggish oxidation kinetics in inert gas environment, demonstrated by electrical, thermogravimetric, dilatometric and structural studies, results in a nearly reversible behavior of substituted vanadates after exposure to inert atmosphere, thus enabling the fabrication of solid-electrolyte cells with SrVO3-δ-based anodes under these conditions. Electrochemical behavior of Sr0.8La0.2VO3-δ-, SrV0.8Nb0.2O3-δ- and SrV0.5Ti0.5O3-δ-based porous electrode layers in contact with yttria-stabilized zirconia electrolyte was evaluated by electrochemical impedance spectroscopy employing symmetrical cells.