Materials Engineering for Solid Oxide Fuel Cell Technology

Materials engineering plays vital role in Solid Oxide Fuel Cell (SOFC) technology. For example, engineered porous materials are needed to support delicate electrolyte membranes, where mechanical integrity and effective diffusivity to fuel gases is critical; and to construct fuel cell electrodes, where an optimum combination of ionic conductivity, electronic conductivity, porosity and catalyst distribution is critical. Material engineering also underpins selection of cell designs and material systems to minimise failure, particularly during transient operations such as thermal cycling. The paper will address these issues, making reference to high temperature (>900C) SOFCs for integration with gas turbines, and metal supported SOFCs designed to operate at temperatures of 500-600C. Introduction to Fuel Cell Technology A fuel cell is a device for directly converting the chemical energy of a fuel into electrical energy in a constant temperature process. Fuel Cells operate on a wide range of fuels, including hydrogen, and are seen as a clean, high efficiency power source, and an enabling technology for the hydrogen economy. Potential applications for fuel cells range from battery replacement in consumer goods and portable computers, through residential scale combined heat and power (CHP), to distributed energy generation. Information on fuel cell types and technology is provided by a variety of books [e.g. 1,2,3,4]. Useful historical surveys have been provided by Kordesch [5] and Appleby [6]. Summaries of the current technological and commercial status of fuel cells are provided in the Fuel Cell Handbook issued by the US Department of Energy [7], Proceedings of the Grove Fuel Cell meetings [8], and Fuel Cells Bulletin [9]. This paper will focus on SOFCs, the leading fuel cell technology for stationary power, or combined heat and power applications (operating on natural gas, biofuels or LPG). Solid oxide fuel cells operate at elevated temperatures, generally around 900°C for the all ceramic high temperature variant, down to 500-800°C for metal-ceramic Intermediate Temperature Solid Oxide Fuel Cells (IT-SOFCs). The electrolyte is a dense ceramic, usually yttria stabilised zirconia (YSZ), which is an oxide ion conductor at elevated temperatures. This means that in an SOFC the reaction product, steam (and also carbon dioxide if the fuel cell is fed directly on hydrocarbon fuels), is formed on the anode side of the fuel cell. The cathode is typically a perovskite material such as strontium doped lanthanum manganite, often mixed with YSZ in the form of a composite. The anode is generally a cermet of nickel and YSZ. The main difference between the high temperature SOFC and the IT-SOFC lies in (i) the thickness of the electrolyte, which tends towards 20 μm thick films for IT-SOFCs, to reduce ionic resistance, and (ii) the interconnect material, with stainless steel being used at the lower temperatures of the IT-SOFC, whereas more expensive high chrome alloys, or oxides such as lanthanum chromite, are needed at higher temperatures. SOFCs lend themselves to applications in which their high-temperature heat can be used. This heat can be used in two basic ways – for heating processes such as those in industry or in homes, or for Materials Science Forum Vols. 539-543 (2007) pp 20-27 Online available since 2007/Mar/15 at www.scientific.net © (2007) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/MSF.539-543.20 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 the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 130.203.133.123-09/08/10,17:36:39) integration with turbines in hybrid cycles for very high efficiency electricity production. Examples in which SOFCs may be used include decentralised electricity generation of 250 kWe to 30 MWe; off-grid applications of 1-25 kWe, and domestic CHP applications of 1-5 kWe. Intermediate Temperature SOFCs are also of interest for vehicle auxiliary power unit (APU) applications, operating on diesel or gasoline. Carbon monoxide is not a poison for SOFCs, meaning that a wide range of fuels can be used, together with a simpler, and therefore cheaper, fuel processor. It is also possible to recuperate heat from the fuel cell within the fuel reformer, improving system efficiency when compared to low temperature fuel cells when operating on hydrocarbon fuels. The Role of Engineered Porous Materials in SOFCs Porous materials play two key roles in SOFC technology. The first is that of transporting gases to/from the fuel cell electrodes. Porous ceramics are commonly used to provide the mechanical support for thin and delicate ceramic oxide electrolytes, in many cases these porous materials also play an important role in current collection on the anode or cathode side. The second vital role of porous materials is within the fuel cell electrodes, be they anode or cathode. The electrodes play a vital role in minimising losses attributable to electrode kinetics, and in some cases mass transport. This is achieved by maximising the length of the so-called triple phase or three-phase boundary (TPB), a term describing the conjunction of a pore space, an ionically conducting phase, and an electronically conducting phase. In practice this is achieved by the use of porous composite electrode structures containing both ionically and electronically conducting materials. Porous Support Materials for SOFCs: Depending upon the fuel cell design, porous support materials for SOFCs can be fabricated either from the anode material – i.e. a cermet of nickel and yttria stabilised zirconia such as that used by Versa Power [10], the cathode material – generally strontium doped lanthanum manganite as used by Siemens Westinghouse [11], an inert ceramic as used by Rolls Royce Fuel Cell Systems [12], or a metal such as stainless steel as used by Ceres Power [13-15]. These materials are generally all characterised by a relatively coarse microstructure (particle size generally in the range of 1 to 20 μm) with porosity in the range of 30-40%. A key characteristic of all the support materials is that they provide the mechanical support for the delicate fuel cell electrolyte, meaning that the mechanical properties of the support are important. Fig.1 illustrates the support material of an anode supported design, showing the coarse support microstructure, with a finer electro-catalytically active anode region in contact with a thick film electrolyte.