Structural and electrochemical characterization of two proton conducting oxide thin films for a microfabricated solid oxide fuel cell

Abstract

The use of proton conducting oxide materials as an electrolyte offers the potential to reduce the operating temperature of a solid oxide fuel cell (SOFC), leading to improved thermal management and material compatibility. However, many proton conducting materials have not yet been investigated for residual stress and electrochemical properties in thin film form. This research characterizes the thermomechanical and electrochemical properties of two promising materials: Yttria doped Barium Cerate (BaCeo.9Yo0.O) and Terbium doped Strontium Cerate (SrCeo0.95TbYo.050O), for use in a microfabricated SOFC (gSOFC). Uniform, crack-free thin films of both compositions were produced by sputter deposition. Films with thickness ranging from 150 nm to 600 nm were deposited on a fused silica substrate. The desired composition was achieved for both films at a deposition temperature of 5750C, though minor secondary phases were also present. Residual stress for different film thicknesses was measured as a function of temperature using the wafer curvature technique.

(cont.) All films exhibited an initial compressive residual stress (from -200 to -600 MPa) and a significant tensile stress hysteresis upon thermal cycling, leaving a final residual stress at room temperature that ranged from -200 to +200 MPa. The average modulus-CTE product for films of each material was found to be - -0.65 MPa/ OC, which is 3-4 times smaller than that of the bulk materials. Electrochemical performance was assessed using impedance spectroscopy. Measurements were taken in three gas atmospheres (ambient air, dry air, and 5% H2 + 95% Ar) from 200 to 5000C. Bulk conductivity in air was found to be 6.57x10-4 S/cm at 473°C for BaCeYO and 2.64x10-4 S/cm at 4240C for SrCeTbO. Though somewhat lower, these values compare well with the conductivity for bulk yttria stabilized zirconia (YSZ), the most common SOFC electrolyte: [approx.] 1 x 10-3 S/cm at 5000C. Prior research indicates that conductivity and activation energy are a strong function of the quantity of water vapor in the test atmosphere. The lack of water vapor in this work most likely explains the lower conductivity and higher activation energy.

(cont.) The activation energy in ambient air was 0.7 eV for BaCeYO and 1.0eV for SrCeTbO, suggesting that these materials should have comparable or better conductivity at lower temperatures than bulk YSZ which has a value of 0.8 eV. This research shows that these proton conducting oxides are viable and promising materials for lowering the temperature of a pSOFC. Future work on these films should include electrochemical tests in a controlled, humidified atmosphere and performance tests of these materials in a pSOFC.