PNNL

Abstract

In this study, the polarization resistance of the La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) solid oxide fuel cell (SOFC) cathode was investigated by I-V sweep and electrochemcial impedance spectroscopy under a series of dc bias voltages and cathode environments (i.e. stagnant air, flowing air, and flowing oxygen) at temperatures from 550 to 750 °C. In flowing oxygen, the polarization resistance of the fuel cell decreased considerably with the applied current density. A linear relationship was observed between the ohmic-free over-potential and the logarithm of the current density of the fuel cell at all the measuring temperatures. In stagnant or flowing air, a new arc related to the molecular oxygen diffusion in the pores of the cathode was identified at high temperatures and high current densities. The magnitude of this arc increased with the applied current density due to the decreased oxygen partial pressure at the interface of the cathode and the electrolyte. It is found that the performance of the fuel cell in air is mainly determined by the oxygen diffusion process. Elimination of this process by flowing oxygen to the cathode improved the cell performance significantly. At 750 °C, for a fuel cell with a laser-deposited Sm0.2Ce0.8O1.9 (SDC) interlayer, an extraordinarily high power density of 2.6 W cm-2 at 0.7 V was achieved in flowing oxygen, as a result of reduced ohmic and polarization resistance of the fuel cell, which were 0.06 ? cm2 and 0.03 ? cm2, respectively. The results indicate that optimization of the microstructure of the LSCF cathode or adoption of a new cell design which can mitigate the diffusion problem in the cathode might enhance cell performance significantly.