ArticlePublisher preview available

Materials for Methane-Fueled SOFC Systems

Trans Tech Publications Ltd
Key Engineering Materials
Authors:
To read the full-text of this research, you can request a copy directly from the authors.

Abstract

Abstract: A short overview of recent work on electroceramic materials relevant to methane-fueled SOFC systems is given. Various fuel feed options are considered, such as pure methane, biogas, and the addition of reforming agents. The principle and implementation into SOFC systems of mixed conducting ceramics for oxygen separation is described, as well as their characterisation by electrochemical methods. Ceramic anodes capable of operating with methane-rich fuel injection are presented. The document concludes with electrochemical results on planar anode supported ceramic fuel cells operating at reduced temperature.
Key Engineering Materials Online: 2001-12-15
ISSN: 1662-9795, Vols. 206-213, pp 1213-1220
doi:10.4028/www.scientific.net/KEM.206-213.1213
© 2002 Trans Tech Publications Ltd, All Rights Reserved
All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans
Tech Publications Ltd, www.scientific.net. (Research Gate for subscription journals-14/04/25,17:37:40)
... careful selection of raw materials [102], adjustment of particle sizes [103] and grading of nickel content in the structure [104] in the last few years. Some of these materials optimizations are reported in [105][106][107][108] and some in a more general context [36] [68][84] [109][110][111]. Möbius recently reviewed the history of solid electrolyte fuel cells and especially the anodes herein [112]. ...
Article
Full-text available
A solid oxide fuel cell (SOFC) is a solid-state energy conversion system that converts chemical energy into electrical energy and heat at elevated temperatures. Its bipolar cells are electrochemical devices with an anode, electrolyte, and cathode that can be arranged in a planar or tubular design with separated gas chambers for fuel and oxidant. Single chamber setups have bipolar cells with reaction selective electrodes and no separation between anode and cathode compartments. A nickel/yttria-stabilized-zirconia (YSZ) cermet is the most investigated and currently most widespread anode material for the use with hydrogen as fuel. In recent years, however, doped ceria cermet anodes with nickel or copper and ceria as the ceramic phase have been introduced together with ceria as electrolyte material for the use with hydrocarbon fuels. The state-of-the-art electrolyte material is YSZ of high ionic and nearly no electronic conductivity at temperatures between 800–1000°C. In order to reduce SOFC system costs, a reduction of operation temperatures to 600–800°C is desirable and electrolytes with higher ionic conductivities than YSZ are aimed for such as bismuth oxide, lanthanum gallate or mixed conducting ceria and the use of thin electrolytes. Proton conducting perovskites are researched as alternatives to conventional oxygen conducting electrolyte materials. At the cathode, the reduction of molecular oxygen takes place predominantly on the surface. Today's state-of-the-art cathodes are LaxSr1–xMnO3–? for SOFC operating at high temperature i.e. 800–1000°C, or mixed conducting LaxSr1–xCoyFe1–yO3–? for intermediate temperature operation, i.e. 600-800°C. Among the variety of alternative materials, SmxSr1–xCoO3–? and BaxSr1–xCoxFe1–xO3–? are perovskites that show very good oxygen reduction properties. This paper reviews the materials that are used in solid oxide fuel cells and their properties as well as novel materials that are potentially applied in the near future. The possible designs of single bipolar cells are also reviewed.
Article
Full-text available
Chemical diffusion (D̃) and surface exchange (k) coefficients for SrFeO3-δ were measured using an electrochemical cell combined with electrochemical impedance spectroscopy (EIS) and potential step technique (PS) in the temperature range of 850-915°C. A D̃ value of ∼ 4.10-5 cm2/s and a k value of ∼ 8.10-5 cm/s were obtained at 900°C. Slow scan (0.5-3 μV/s) cyclic voltametry (CV) was performed in the same temperature range and using the same electrochemical cell to obtain oxygen nonstoichiometry data. The oxygen nonstoichiometry (δ) at 900°C in air was determined as 0.4. A plateau corresponding to δ = 0.5 was observed below an oxygen partial pressure (P(O2)) of 10-6 atm. These results were shown to be consistent with the literature data. Nonstoichiometry data were further analysed using the existing defect models, and the limits of the independent point defect approximation and the necessity of considering interactions between point defects and clusters were established.
Article
Thyssengas GmbH has been involved since 1998 in a 3 yr worldwide field test with Hexis Fuel Cells, Winterthur, Switzerland. The system has a maximum output of 1 kw electrical and 19 kw heat, including the integrated gas boiler. The goal is to assess the suitability of the Hexis Fuel Cell to generate electricity and heat for domestic use and its market opportunities. There were many breakdowns at the start of the field test mainly caused by peripheral components. The system was continually improved as the study continued, leading to an increase in system reliability. By the end of July 2000, almost 10,000 operating hours had been reached. The discussion includes history; features that make the fuel cell attractive; how the fuel cell functions; field test with the Hexis system (background, construction, test data, prior experiences); and outlook. The Hexis Fuel Cell will be introduced to the market in small quantities in 2001 and its properties observed with respect to, e.g., natural gas use. Mass production is due to begin in 2004 in Switzerland.
Article
Oxygen diffusion is treated in a dense electronically conducting cobaltate pellet blocked ionically on one surface, electronically on the other, and sealed on its cylindrical periphery. A procedure is developed for extracting the chemical diffusion and surface exchange coefficients for oxygen by use of the asymptotic equations derived for the current response to a potential step at short and long times. It is shown that, while the formation of interfacial phases by reaction between the sample and the electrolyte may affect the surface exchange coefficient, the chemical diffusion coefficient data determined by the present approach are independent of such interfacial phenomena. The consistency of data obtained from several specimens with varying thickness and manner of interfacing with the electrolyte validates the diffusion model and the method used for data analysis. An oxygen permeation cell is also developed in this work as a modification of the diffusion cell. The new cell allows monitoring of the permeation rate by electrochemical means. The steady-state permeation data obtained by the permeation cell are consistent with the chemical-diffusion and surface- exchange coefficients measured by the blocked diffusion cell as long as the assumptions of the related theoretical models are satisfied. This is a further validation of the diffusion model and the related methodology developed here for obtaining the necessary data for characterizing oxygen exchange and transport in such materials.