ICMCTF2008 Session TS2: Coatings for Fuel Cells

A new solid oxide fuel cell (SOFC) generation is promising to reduce the working temperature in an intermediate range (600 - 700°C), to minimize ceramic matter quantity and so to decrease the global cost of SOFC systems¹. It is composed of a stacking of electrodes and electrolyte coatings of a few ten microns thick, constituents of the unit cell, deposited on a metallic interconnect used as cell support.@In this work, the metallic supports could be either dense or porous. Porous ones, which are particularly interesting to control the gas inlets, will be prepared by powder metallurgy². Powder metallurgy, which is a low cost process using soft chemistry, leads to compact metallic substrates with controlled porosity after compaction of the powders.

In order to keep a good interconnect material mechanical behavior and to reduce corrosion of the supports, sol-gel process is used to prepare anodic coatings at moderate thermal treatment temperature. Indeed, we take advantage of the potentialities of the sol-gel route to synthesize first Ni-YSZ homogeneous cermets (common anodic material³) and then to process both thin and thick films on metallic interconnects by dip-coating. By this route, the mixture of precursors occurs at a molecular scale and the control of compounds stoichiometry is possible. The simultaneous growth of yttria stabilised zirconia and nickel oxide within the same synthesis should allow obtaining a nanocomposite material with an homogeneous distribution of each component.

In this way, duplex microstructured anodes are prepared from both a thin and a thick layer, directly deposited on metallic interconnects. The interfacial anodic layer, of about 100 nm thick, is going to improve adhesion and also to accommodate stresses between the metallic interconnect and the active thick anode. Moreover, by dipping the substrate into an optimized slurry containing sol-gel composite powders, thick films with a thickness of few microns are prepared and constitute the active anodic part. A heat treatment at only 800°C leads to coherent anodic duplex stacking which is continuous, homogeneous and adherent.

Working characteristics of this new generation of SOFC material are evaluated in this paper and correlations between microstructure and properties are discussed.

¹ S. Visco, www.lbl.gov

² V. Baco-Carles, P. Combes, P. Tailhades, A. Rousset New method to prepare iron particles with different morphologies : a way to get high green strength metal compacts Powder Metall. 45 (1), 33-38 (2002)

³W.Z. Zhu, S.C. Deevi A review on the status of anode materials for solid oxide fuel cells Mater. Sci. Eng. A362, 228-239 (2003) .

Major challenges facing the development of solid oxide fuel cells (SOFC) are related to the need of materials with long term stability and of time- and cost-effective preparation processes.

In this work, we report the sol-gel potentialities to prepare cathode and electrolyte thin and thick layers on an anodic NiO-YSZ support also made from powders prepared by sol-gel route. It is necessary to optimize and to control both the electrode and electrolyte composition and microstructure in order to obtain good electrocatalytic properties. In our center, we have synthesized, by several sol-gel or slurries routes, conventional and new materials as electrolyte and cathode for SOFC applications.

Electrolyte yttria stabilized zirconia (YSZ) thick films were coated onto porous NiO-YSZ composites substrates by dip-coating process using a new formulation suspension. These suspensions are obtained from the addition of a polymeric matrix to a stable suspension of YSZ powders in an azeotropic MEK-EtOH solution¹. YSZ powders are previously prepared by a sol-gel route derived from the Pechini process. To ensure continuity, homogeneity, adherence and gas-tightness of the electrolyte thick layer, the suspension composition is modified by addition of a part of YSZ precursor colloidal sol². During heat treatment, the in-situ growth of these colloids significantly increases the layers density. After thermal treatment, different microstructures are obtained depending on the synthesis parameters and the layers are adherent, homogeous and continuous with thicknesses ranging from 10 to 20 micrometers. In addition, permeation measurements on the electrolyte thick layers before and after reduction treatment of the anode support confirm the good gas-tightness of the YSZ coatings.

Mixed oxides, particularly those corresponding to Ruddlesden-Popper structure are very good candidates for cathode materialsparagraph2³. To improve the cathode conductivity, it is necessary to optimise its microstructure. First, we propose to use a dip-coating process to prepare a duplex microstructure cathode constituted of both Lasub1.98niosub4+dthin (few nanometers) and thick (few micrometers) films on YSZ substrates. The choice to prepare such architectured cathode has been considered by numerical models pointing out the influence of the microstructure on the electrical properties, gas transport and catalytic activities. By this method, films in the range of few nanometers to few micrometers are obtained according to the nature of the dip-coated solution (polymeric sol or adequate suspension). Then microstructure of these coatings will be correlated to electrochemical performances.

Finally, these archictectured cathodes are deposited on the half anodic cell in order to constitute an elementary cell composed of both cathode and electrolyte thick layers on anodic supports.

¹P. Lenormand, S. Castillo, J.R. Gonzalez, C. Laberty, F. Ansart. Solid State Science, 7, 2005, 159-163.paragraph2² A. Lecomte, P. Lenormand, A. Dauger. J. Appl. Cryst., 33 (3-1), 2000, 496-499.

³ M.L. Fontaine, C. Laberty-Robert, M. Verelst, J. Pielaszeck, P. Lenormand, F. Ansart, P. Tailhades. Mater. Res. Bull., 41, 2006, 1747-1753.

Nickel oxide is soluble in zirconia up to a few percent, making it unavoidable that some dissolved NiO will be present in zirconia coatings whenever these two oxides are in direct contact during elevated thermal processing. This is certainly the case for zirconia electrolyte coatings on anode supports containing NiO in solid oxide fuel cells. Upon subsequent long-term exposure to reducing atmosphere, NiO diffuses out of the electrolyte forming nano-sized metallic nickel accretions. These accretions reduce the ionic conductivity and modify the mechanical properties of the resulting composite material. This effect is difficult to observe directly in thin electrolyte coatings, but relatively easy to see in bulk specimens.

This paper describes experiments on bulk specimens of yttria-stabilized zirconia (8YSZ) with 1 wt% NiO. Ionic conductivity was measured at elevated temperatures for extended durations in reducing atmosphere, during which an exponential conductivity relaxation behavior was observed. The reduced specimen in cross-section resembled an "Oreo cookie", with a sharp interface between the white, NiO-depleted zirconia region in the center, and the black exterior, where the nickel accretions were present. Magnetometer measurements confirmed that most of the nickel was present in the metallic state. TEM analysis was used to study the microstructure of the phases on either side of the interface. These experiments underscore the challenges facing long-term SOFC operation and the difficult trade-off between the temperatures required to obtain dense electrolytes while minimizing the amount of dissolved NiO.