PNNL

Abstract

The oxygen reduction reaction at the cathode of solid oxide fuel cell (SOFC) is a complicated process which involves the interaction of oxygen molecules, electrons, and oxygen ions. Therefore, it can only proceed at locations where gas, electronic conductor, and an oxygen ion conductor meet. Although the extension of the reaction zone beyond the traditional so-called triple-phase-boundary (TPB) is widely accepted for a mixed ionically and electronically conductive cathode, work in this area has yet to reach a consensus on how far the reaction zone can be extended. In this study, anode-supported fuel cells with a variety of LSCF cathode thicknesses were fabricated and tested in two cathode environments, flowing oxygen and flowing air. In flowing oxygen, the cell performance increased with LSCF cathode thickness over the entire range investigated (from 5 to 33 ?m). In flowing air, the cell performance also increased with the LSCF cathode thickness from 5 to 13 ?m, but then remained almost constant with further increase in cathode thickness. In flowing oxygen, since there was no concentration polarization related to oxygen diffusion, the polarization resistance of oxygen reduction decreased with the LSCF cathode thickness because of the increased number of reaction sites. A linear relationship was established between the reciprocal of the polarization resistance and the cathode thickness, which was explained by a model developed for aqueous gas diffusion electrodes.