MTL Annual Report

Micro-solid oxide fuel cells (SOFCs) are currently under intense investigation for portable power applications, such as notebook computers and mobile phones ^[1] [2].  In this work, thin-film SrTi_1-xFe_xO₃ (STF) cathodes were fabricated on top of single-crystal yttria-doped zirconia (YSZ) solid electrolyte substrate by a pulsed laser deposition (PLD), (see Figure 1), and their material and morphological properties were systematically investigated in relation to the cathodic performance of the STF.

Impedance spectra were collected on symmetrical cells of the type STF/YSZ/STF. Measurements were performed over the temperature range of 570 – 650°C and the pO₂ range of 2×10^-5 – 1 atm. The STF cathode was observed to exhibit typical mixed ionic-electronic behavior, demonstrating fast surface oxygen exchange kinetics comparable to those exhibited by other popular mixed ionic-electronic conductors such as (La,Sr)(Co,Fe)O₃, thereby confirming the suitability of STF as a model mixed-conducting cathode material.

The magnitude of the surface exchange coefficient, k, was found to be only weakly dependent on the magnitudes of the electronic and ionic conductivities as well as carrier densities (over nearly five orders of magnitude change in s_el) over the x values (0.05-1.0) examined in this study. On the other hand, an appreciable degree of Sr-excess near the surface of the STF thin film was found by means of XPS, indicating a possible correlation between the cathode performance and the surface chemistry.

Figure 1: A photograph (left) and a schematic illustration (right) of STF thin films deposited by pulsed layer deposition (PLD).

1. C.D. Baertsch, K.F. Jensen, J.L. Hertz, H.L. Tuller, S.T. Vengallatore, S.M. Spearing, and M.A. Schmidt, Journal of Materials Research 19 (9), 2604 (2004). [↩]
2. D. Nikbin, The Fuel Cell Review 3, 21 (2006). [↩]