ICMCTF2007 Session TS2: Coatings for Fuel Cells
Friday, April 27, 2007 8:00 AM in Room Sunset
TS2-1 Application of Vacuum Deposition Methods to Solid Oxide Fuel Cells
L. Pederson, X.-D. Zhou (Pacific Northwest National Laboratory)
The application of vacuum deposition techniques to the fabrication of solid oxide fuel cell materials and structures are reviewed, focusing on magnetron sputtering, vacuum plasma methods, laser ablation, and electrochemical vapor deposition. A description of each method and examples of use to produce electrolyte, electrode, and/or electrical interconnects are given. Generally high equipment costs and relatively low deposition rates have limited the use of vacuum deposition methods in solid oxide fuel cell manufacture, with a few notable exceptions. Vacuum methods are particularly promising in the fabrication of micro fuel cells, where thin films of high quality and unusual configuration are desired.
TS2-3 ASR Evaluation of Different Kinds of Coatings on Ferritic Stainless Steel as SOFC Interconnects
P. Piccardo (DCCI, Italy); P.E. Gannon (Montana State University); S. Chevalier (LRRS, UMR 5613 CNRS, France); M. Viviani (National Research Council, Italy); A. Barbucci (DICHeP - Universitat di Genova, Italy); G. Caboche (LRRS, UMR 5613 CNRS, France); R. Amendola (DCCI, Italy); S. Fontana (LRRS, UMR 5613 CNRS, France)
The demanding technical requirements for solid oxide fuel cell (SOFC) interconnect materials pose development challenges for commercially-viable systems. SOFC interconnects physically separate and manifold fuel (reducing) and oxidant (oxidizing) gases, while electrically connecting adjacent cell electrodes (anode to cathode) into series, and also permitting SOFC stacking and sealing. Due to the challenging multifunctional requirements, SOFC interconnects material investigations are intensifying to enable deployment of highly-efficient and inexpensive SOFC systems. Commercial ferritic stainless steels are used as interconnects in SOFC operating around 800°C. Their compositions contain elements such as Mn, Rare Earths, Al or Ti as microalloying elements, designed to produce a thin surface oxide layer with specific characteristics: high mechanical strength; excellent adhesion to the metallic substrate; low growth rate (interconnect durability must be around 40,000 hours); high chemical resistance (to avoid Cr depletion and electrode poisoning); and, high electronic conductivity (ASR <10@super -1@ @ohm@-cm@super 2@). The performance of the best commercial ferritic stainless steels are close to the desired behaviour, but need to be improved by surface treatments. These treatments have several important tasks, with the most important being to stabilize Cr(III) compounds in order to limit formation of volatile Cr(VI) compounds, which are associated with SOFC cathode poisoning. This communication will discuss the efficacy of two different kind of promising coatings: reactive element nanolayers, produced by MOCVD; and, modified CrAlYO diffusion barriers, obtained by filtered arc deposition. The coatings were applied on commercial ferritic stainless steels and tested under conditions simulating the highly aggressive service environment of an SOFC interconnect.
TS2-4 Ion Beam Analysis of Thermal Stability and Oxidation Resistance of (Al,Cr,Ti,Co,Mn,Y) Oxide Coatings on Ferritic Steels for SOFC Interconnect Applications@super 1@
R.J. Smith (Montana StateUniversity); A. Kayani, H. Chen, W. Priyantha, T.L. Buchanan, M. Kopczyk, R. Hutchison (Montana State University); M. Finsterbusch (TU Ilmenau, Germany); M. Binney (Winona State University); J. Lucas, C. Collins (Montana State University); V. Gorokhovsky (Arcomac Surface Engineering, LLC); P.E. Gannon, M.C. Deibert (Montana State University)
The requirements of low cost and high-temperature corrosion resistance for interconnects in SOFC stacks have directed attention to the use of steel plates with electron conducting, oxidation resistant coatings. We have studied coatings from the AlCrTiCoMnYO system, deposited on 430 steel coupons using filtered arc deposition technology at Arcomac Surface Engineering. Ion beam analysis (IBA) was used to characterize the thermal stability of the coatings, including the use of RBS with He@super +@ ions for interdiffusion of substrate and coating constituents, non-Rutherford scattering with H@super +@ ions for enhanced sensitivity to oxygen and other light elements, and nuclear reaction analysis using the @super 18@O(p,@alpha@)@super 15@N reaction to study oxygen transport through the coatings. The volatility of Cr-derived species was measured for the coated steel coupons using IBA. Analysis was performed as a function of annealing time in air for up to 100 hours at 800°C. Significant reductions in oxidation rates as well as reduced Cr volatility were seen for these coatings. @paragraph@ @super 1@HiTEC is funded by DOE as a subcontract from Battelle Memorial Institute and Pacific Northwest National Laboratory under Award No.DE-AC06-76RL01830.
TS2-5 Structure and Properties of Spinel Protection Layers on Ferritic Stainless Steels for SOFC Interconnect Applications
Z.G. Yang, G.-G. Xia, C.-M. Wang, X.-L. Li, G.D. Maupin, S.P. Simner, Z.-M. Nie, J.W. Stevenson (Pacific Northwest National Laboratory)
Due to their low cost, good 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 SOFC stacks. For long term operation, however, several issues remain, including long-term surface stability, electrical resistance and thus power loss arising from the surface 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 conductive oxides. In particular, (Mn,Co)@sub 3@O@sub 4@ based protection layers have been successfully applied onto varied ferritic stainless steels at PNNL. A systematic investigation has been carried out on performance and structure/properties of the spinel protection layers. This paper will present details of this work.
TS2-6 Influence of a Ce Surface Treatment on the Behavior of Ferritic Stainless Steel for SOFC Interconnect Applications
D. Alman, G. Holcomb, R. Wilson, T. Adler, P. Jablonski (National Energy Technology Laboratory)
Pack cementation-like Cerium based surface treatments have been found to be effective in enhancing the oxidation resistance of ferritic steels (Crofer 22APU) for solid oxide fuel cell (SOFC) applications. The application of either a CeN- or CeO@sub 2@ based surface treatment results in a decrease in weight gain by a factor of three after 4000 hours exposure to moist air at 800C. Similar oxide scales formed on treated and untreated surfaces, with a continuous Cr-Mn outer oxide layer and a continuous inner Chromia layer formed on the surface. However, the thickness of the scales, and the amount of internal oxidation were significantly reduced with the treatment, leading to the decrease in oxidation rate. This presentation will detail the influence of the treatment on the electrical properties of the interconnect. Half-cell experiments (LSM cathode sandwiched between two steel interconnects) and full SOFC button cell experiments were run with treated and untreated interconnects. Preliminary results indicate the Ce treatment can improve SOFC performance.
TS2-7 Volatilization of Cr Vapor Species from Coated and Uncoated SOFC Interconnect Alloys
J.W. Stevenson, G.D. Maupin, P Singh, Z.G. Yang, G.G. Xia (Pacific Northwest National Laboratory)
Volatilization of Cr species from SOFC interconnects and other balance-of-plant components can have adverse effects on cathode performance. Accordingly, there is a need to understand and possibly mitigate this volatilization during SOFC system operation. In the present study, Cr volatility from alloys and oxide coatings was measured quantitatively using an apparatus which heated the alloy samples in a sealed, flowing air stream and then collected the transported Cr species via downstream condensation. The condensed Cr species were then analyzed by inductively coupled plasma-mass spectroscopic (ICP-MS) analysis to quantitatively determine the amount of Cr volatilized during the test. The experimentally obtained Cr transport rates for coated and uncoated alloys will be reported and compared with previously published results and thermodynamic calculations. Mitigation approaches will also be briefly discussed.
TS2-8 Large Area Filtered Arc (Al,Cr,Ti,Co,Mn,Y) Nanocomposite Oxide Coatings on T430 Ferritic Steel: Solid Oxide Fuel Cell Interconnect Performance and Substrate Surface Finish Effects
P.E. Gannon (Montana State University); V. Gorokhovsky (Arcomac Surface Engineering, LLC); M.C. Deibert (Montana State University); R.J. Smith (Montana StateUniversity); H. Chen, W. Priyantha, P. White, E. Musz (Montana State University)
This investigation is aimed at further evaluation of large area filtered arc deposition (LAFAD) oxide coatings on ferritic steels and their behavior in solid oxide fuel cell interconnect SOFC(IC) relevant environments. LAFAD coatings from the AlCrTiCoMnYO system were deposited on T430 ferritic steel with 3 different surface finishes: bead blasted (rough); ground (less rough); and, bright annealed (smooth). Coating adhesion, chemical and phase composition were assessed and compared with uncoated steels as a function of SOFC(IC) relevant exposures. Significantly enhanced SOFC(IC) performance was observed with many coated steels. Influences of substrate surface finish, coating elemental composition and thickness on high temperature behavior in moisturized air will be discussed in the context of improved LAFAD SOFC(IC) coatings.
TS2-9 Preparation and Characterization of Model Oxides used for Optimization of the Properties for Interconnect Materials used in High Temperature Fuel Cells
C.C. Mardare, H. Asteman, M. Spiegel (Max Planck Institute for Iron Research, Germany)
With the decrease of operating temperature of SOFC from 1000@super o@C to 600-800@super o@C it is possible to replace ceramic interconnects with the metallic ones. Stainless steels can in this case be used and offer a cheaper materials costs and a more feasible solution in terms of fabrication. They form thin Cr-rich (Cr,Fe)@sub 2@O@sub 3@ which is prone to form volatile CrO@sub 2@(OH)@sub 2@ (g) in the presence of water vapor at temperatures above 600@super o@C. At the cathode side both O@sub 2@ and H@sub 2@O are present if air is used as oxidant and it has been observed that Cr-species evaporate and reduces the lifetime of the SOFC due to poisoning. In order to prevent this it has been suggested to coat the surface with a conductive material, such as spinel type oxides based on the Mn-Co-Fe system. It is more or less impossible to prevent the formation of Cr-rich (Cr,Fe)@sub 2@O@sub 3@ beneath the spinel layer, therefore both of these oxides in combination will determine the overall conduction. It is therefore of interest to investigate each of the two oxide systems, (Cr,Fe)@sub 2@O@sub 3@, (Fe,Cr,Mn,Co)@sub 3@O@sub 4@), first independently and then together as a layered film. In this work candidate oxide films have been prepared on top of Pt, SiO@sub 2@/Si substrates and alloys used as interconnects. The first step has been to deposit Fe, Cr, Mn, and Co, and alloys of these metals by PVD and then to establish a reproducible and well controlled way to convert them into the desired oxide by thermally oxidizing them in O@sub 2@ and H@sub 2@/H@sub 2@O atmospheres. The SiO@sub 2@/Si substrates have been used for this. The next step was then to repeat the procedure using Pt and alloys substrates in order to be able later on to measure the conduction properties. The metal precursor films and the prepared oxides have been characterized by SEM, XRD, XPS and ToF-SIMS. The change in semi-conducting properties from p- to n-type was followed by EIS using Mott-Schottky plots.
TS2-12 Deposit of Dense YSZ Electrolyte and Porous NiO-YSZ Cermet Anode for SOFC Device by a Low Pressure Plasma Process
F. Rousseau, S. Awamat (Universite Pierre et Marie Curie, France); M. Nikravech (Universite Paris 13, France); D. Morvan, J. Amouroux (Universite Pierre et Marie Curie, France)
A low pressure plasma process was used in the laboratory to synthesize very pure Sr-doped LaMnO@sub 3@ porous layers that could be used as SOFC cathode. In recent works, the process was adapted to synthesize both the YSZ dense electrolyte and NiO-YSZ cermet porous anode in one step. The raw material was a mixture of Y and ZrO nitrates dissolved in water. Nitrates were introduced in the argon / oxygen plasma discharge in order to obtain a very pure YSZ layer at low temperature on quartz substrate (T@<=@360 K). After the deposit of YSZ, Ni nitrates were progressively added to the mixture of Y and ZrO nitrates in order to obtain a NiO-YSZ cermet layer presenting concentration gradients. SEM-EDX analyses were performed on the fractured cross section of the YSZ / NiO-YSZ stack. Micrographs showed that the YSZ layer was dense, contrary to the porous NiO-YSZ cermet. EDX analyses confirmed the purity and the composition of the YSZ layer. The presence of concentration gradients of Ni and YSZ was demonstrated along the cross section. The Ni concentration increased from the YSZ layer up to the surface, contrary to the YSZ concentration. Y and Zr elements were not detected at the surface of the YSZ / NiO-YSZ stack.
TS2-13 Hydrophobic Coatings on Carbon Electrodes for PEMFC
K.-F. Chiu, K.W. Wang (Feng Chia University, Taiwan)
The carbon electrodes for proton exchange membrane fuel cell have been modified by sputter deposited PTFE in pure Ar (99.9%) atmosphere. The PTFE target was placed on a stainless steel electrode powered by a radio frequency (13.56MHz) power supply. This technique produced fluorine contented functional groups on the carbon electrodes, which enhanced the hydrophobicity of the carbon electrodes. Furthermore, this is a clean process without heat treatments and FEP solution as compared to conventional process. The properties of the plasma treated and untreated carbon electrodes were investigated by contact angle measurement, electron spectroscopy for chemical analysis (ESCA) and scanning electron microscopy (SEM). The results showed that sputtered deposition of PTFE effectively modified the hydrophobic properties of the electrode surface. The modified hydrophobic properties can be interpreted by the additional functional groups found on the treated carbon electrodes. Sputter deposition of PTFE was only carried out on the top-side of the electrode; therefore the gas channels within the carbon electrode can form a hydrophobicity gradient, which induced an osmosis effect. As a result, the treated carbon electrodes were more efficient in water repelling and greatly enhance the cell performance.