Active Cathodes for Super-High Power Density Solid Oxide Fuel Cells Through Space Charge Effects

Abstract

This report briefly summarizes the work done over the duration of the project, beginning October 1, 2003 and ending September 30, 2005. This project was on understanding cathode mechanisms in intermediate temperature solid oxide fuel cells (SOFC) and developing superior cathodes. A particular emphasis was on the role of defect chemistry and the role of space charge effects. It was hypothesized that the space charge should have a major effect on cathode activation polarization. If the space charge increases oxygen vacancy concentration, it should be beneficial. On the other hand, if space charge suppresses oxygen vacancy concentration, it may adversely affect the cathodic reaction. It was determined that a combined effect of porous body microstructure/defect chemistry can be significant on transport and polarization. Also explored extensively was the role of interfaces, where electrode reactions occur, on transport through oxygen ion conducting materials. In particular, fundamental aspects of transport were theoretically examined. The analysis has significant implications on the very measurement of overpotential using the so-called three electrode system. In the area of cathode polarization, an important property concerns the chemical surface exchange coefficient, chem k . An experimental technique was developed which allows for the measurement chem k using porous bodies. Some of the work conducted under this project has been published in the open literature. Copies of the papers are included here in lieu of a detailed report. Much of it has been reported in text form in the quarterly reports submitted. Also included in this report are the manuscripts yet to be submitted for publication. 4 LIST OF PAPERS PUBLISHED 1) "Theoretical Analysis of the Role of Interfaces in Transport through 5 BRIEF SUMMARY OF THE WORK PERFORMEDIn what follows, brief summary of the work done is given. This is given as summaries of each of the papers (and the preprint) described here. Copies of the papers are attached as part of this final report.Paper 1: "Theoretical Analysis of the Role of Interfaces in Transport through Oxygen Ion and Electron Conducting Membranes", A. V. Virkar, J. Power Sources, 147 8-31 (2005). SummaryThis manuscript examines transport through oxygen ion and electronic conducting membranes including electrode/membrane interfaces by explicitly incorporating both ionic and electronic transport through the membranes and across interfaces. Spatial variation of electrochemical potential of oxygen ions, Thus, in the case of oxygen separation under an applied voltage, the transport properties of the material and the interfaces determine membrane stability. Implications of the analysis concerning the applicability of the so-called the three-electrode system under an applied voltage to investigate electrode polarization are presented. It is shown that the use of the three-electrode system for the estimation of electrode kinetics can lead to significant errors at high applied voltages, and may result in overestimati...