ICMCTF2009 Session TS2: Coatings for Fuel Cells
Wednesday, April 29, 2009 1:30 PM in Room Sunset
TS2-1 Electroplated Coatings on Ferritic Steels for SOFC Interconnect Application
J.H. Zhu (Tennessee Technological University)
In the planar design of a solid oxide fuel cell (SOFC) stack, the interconnect acts not only as electrical connection between the various cells but also as the mechanical support for the thin electroactive ceramic parts and as gas-proof separation of air and fuel gas. With the reduction of the SOFC operating temperatures to 600-800°C, chromia-forming ferritic steels are widely used as interconnect materials in the planar-type SOFC stacks currently under development. These ferritic steels such as Crofer 22 APU and SS 430 possess an overall combination of properties desirable as SOFC interconnect materials such as low cost, excellent manufacturability, adequate match in coefficient of thermal expansion (CTE) with other cell components, high electronic conductivity and thermal conductivity. Two major concerns with these ferritic interconnect alloys are (1) their long-term oxidation resistance and oxide scale electrical conductivity, and (2) Cr evaporation and associ ated “poisoning” of the cathode under the operating environments of SOFC. To address these issues, protective coatings need to be developed, which should be economically viable, electrically conductive, and chemically compatible with the substrate alloy and other cell components. The present talk provides an overview of the recent research efforts in the development of electroplating as a processing route to fabricate protective coatings on ferritic steels. The progress and the issues with electroplating for synthesis of the protective coatings are summarized. Several promising approaches are highlighted for mitigating the current problems with the electroplated coatings. Some examples are given where electroplated alloy coatings are utilized for protecting the ferritic steels.
TS2-3 Improved Properties of SOFCs using Pre-Coated Sandvik Sanergy HT 22% Cr Ferritic Interconnect Steel
U. Bexell, M. Schuisky (AB Sandvik Materials Technology, Sweden)
Ferritic stainless steel has attracted a great deal of attention for its use as an interconnector in solid oxide fuel cells (SOFCs). The ferritic Sandvik Sanergy HT chromium steel is specially developed for interconnectors in SOFC with a unique chemical composition, which gives the alloy a good high temperature corrosion resistance as well as good surface conductivity in the formed chromium oxide scale. However, chromium evaporation from metallic interconnectors in SOFC fuel cells tends to poison the cathode of the fuel cell. Furthermore, the evaporation of chromium species from the oxide surface tends to increase the oxidation rate resulting in increased contact resistance. To reduce chromium evaporation from the interconnectors, rather thick coatings have been deposited using various spraying techniques such as plasma spraying. In this study, a 22% Cr ferritic steel, Sandvik Sanergy HT has been coated with thin metallic films. These coated samples are compared to unco ated material. The idea is to promote the formation of a dense cap layer thus reducing chromium evaporation and increasing scale conductivity. @paragraph@ Oxidation studies have been carried out on pre-coated and uncoated samples of Sandvik Sanergy HT in air at different temperatures. The samples were analyzed by XRD, SEM/EDX on polished and/or FIB cut cross sections. Also, the surface morphology and chemistry were studied by SEM/EDS.
TS2-5 Development of Spinel Protection Layers for Steel-Based SOFC Interconnects
J.W. Stevenson, Z.G. Yang, G.B. Xia, X.S. Li, Z.M. Nie, C.M. Wang (Pacific Northwest National Laboratory)
Due to their low cost, high temperature oxidation resistance, and appropriate thermal expansion match to anode-supported cells, ferritic stainless steels are among the most promising candidate materials for interconnect applications in intermediate-temperature planar SOFC stacks. For long term operation, however, several issues remain, including long-term surface stability, electrical resistance due to scale growth, and chromia scale volatility that can lead to cell poisoning and performance degradation. To improve their performance, ferritic stainless steels can be surface-modified via application of a conductive oxide coating. In particular, (Mn,Co)@sub3@O@sub4@ based protection layers are being developed and optimized at PNNL for several candidate ferritic stainless steels. Recent progress will be summarized in this paper.
TS2-7 Structure and Conductivity of Apatite-Like Lanthanum Silicate Films for SOFcs Electrolytes
J.C. Oliveira (University of Coimbra, Portugal); M. Vieira (Polytechnic Institute of Leiria, Portugal); A.L. Shaula, A. Cavaleiro (University of Coimbra, Portugal)
The development of Intermediate Temperature Solid Oxide Fuel Cells (IT-SOFCs) will require electrolyte materials with ionic conductivity higher than the conventional yttria-stabilised zirconia (YSZ) at moderate temperatures. Recently, lanthanum silicates materials (La@sub 9.33@Si@sub 6@O@sub 26@) with an apatite-like structure have attracted considerable interest as potential low cost electrolyte materials. Some of these materials show conductivities comparable to, or better than, YSZ at 875 K, and are thus potential electrolytes for economic feasible fuel cells. Their high level of oxide ion mobility is related to the presence of oxygen channels along the c axis which facilitate the diffusion of anionic species. Magnetron sputtering has already been used to synthesize thin film electrolytes for SOFCs owing to its versatility as well as the ability to control composition and morphology. Most of the reported work focuses on the deposition of thin dense yttria-stabilised zirconia (YSZ) gadolinium doped ceria (GDC) and lanthanum gallate electrolyte layers. The main objective of this work is the production of apatite-like lanthanum silicates thin films by magnetron sputtering. Thin films with the appropriate La/Si atomic ratios were deposited by reactive magnetron sputtering from LaSi and Si targets and subsequently annealed in controlled atmosphere to obtain the targeted lanthanum silicate oxide. The chemical composition of the coatings was determined by electron probe microanalysis (EPMA). The structure of the coatings was studied by X-ray diffraction (XRD) using a Phillips diffractometer operated in Bragg-Brentano configuration with Co(Kα) radiation. The cross section and surface topography of the La-Si films were examined on a JEOL scanning electron microscope (SEM) equipped with an EDAX energy dispersive spectrometer (EDS). The electrical properties of the films were measured by AC impedance spectroscopy (HP4284A precision LCR meter, 20 H z – 1 MHz).
TS2-8 The Corrosion Properties and Interfacial Contact Resistance of TiN, TiAlN and CrN PVD Coatings in Simulated PEM Fuel Cell Environments
L. Wang, D.O. Northwood (University of Windsor, Canada); J. Housden, E. Spain (Tecvac Ltd.); X. Nie (University of Windsor, Canada)
Metallic bipolar plates, especially stainless steels are widely accepted as promising candidates to replace graphite in PEM fuel cell as electrodes. The major concerns on their corrosion susceptibility and contact resistance increments after formation of surface passivation films have induced the increasing interest in finding promising protective coatings on metallic bipolar plates to prevent the degradation of plates. In this study, the interfacial contact resistance (ICR) and electrochemical properties of TiN, CrN and TiAlN PVD coatings and their stainless steel 316 substrate were investigated in a simulated PEM fuel cell environment. The potentiodynamic polarization corrosion tests were conducted with purged O@sub2@ or H@sub2@, and the potentiostatic corrosion tests were performed under both simulated cathodic (+0.6V vs. SCE purged with O@sub2@) and anodic conditions (-0.1V vs. SCE purged with H@sub2@) for a long period (4 hrs). SEM was used to observe the surface m orphologies of the samples after corrosion tests. The test results showed that the TiAlN and CrN coatings had a low ICR but the TiN coating exhibited a slightly high ICR, compared with the uncoated SS 316. TiN and CrN-coated metallic plates could potentially be used as anode plates in PEM fuel cell environment based on their anti-corrosion performance. However, since the main corrosion initiated at the pinholes on the PVD coatings, more efforts are needed to be explored to eliminate the pinholes resulted from the PVD deposition process.