ICMCTF2006 Session TS3: Coatings for Fuel Cells

The requirements of low cost and high-temperature corrosion resistance for bipolar interconnect plates in solid oxide fuel cell stacks has directed attention to the use of metal plates with oxidation resistant coatings. We have investigated the performance of steel plates with homogenous coatings of CrAlN (nitrides). The coatings were deposited using RF magnetron sputtering, with Ar as a sputtering gas and N gas added during the growth process. The Cr/Al composition ratio in the coatings was varied in a combinatorial approach. The coatings were subsequently annealed in air for up to 100 hours at 800°C. The composition of the coated plates and the rate of oxidation were characterized using Rutherford backscattering (RBS) and nuclear reaction analysis (NRA). The results are compared with similar studies for uncoated steels and for CrAlN coatings deposited by filtered arc deposition.

*HiTEC is funded by DOE as a subcontract from Battelle Memorial Institute and Pacific Northwest National Laboratory under Award No.DE-AC06-76RL01830.

The requirements of low cost and high-temperature corrosion resistance for bipolar interconnect plates in solid oxide fuel cell stacks has directed attention to the use of steel plates with oxidation resistant coatings. However, volatile Cr species from the Cr₂O₃-based oxide scales on these steels find their way to the triple-phase boundary, leading to rapid degradation of fuel cell performance. Coatings can serve not only to slow oxidation rates, but also as diffusion barriers for the Cr-derived species from the steel, slowing the degradation process. We have measured the vaporization rates of Cr from several steels, and from steel coupons with CrAlN coatings, deposited using a filtered arc process and rf magnetron sputtering. Chromium containing vapors from the steel coupons in a tube furnace at 800°C were transported with various flow rates of humid air to a Si wafer at ~100°C near the end of the quartz tube in the furnace, where the vapors adsorbed on the Si surface. The wafers were subsequently analyzed for Cr surface concentration using Rutherford backscattering. Separate experiments with Cr₂O₃ powder as the vapor source established the quantitative reliability of this approach. Results are presented for several steel surfaces with and without CrAlN coatings.

*@HiTEC is funded by the DOE as a subcontract from Battelle Memorial Institute and Pacific Northwest National Laboratory under Award No.DE-AC06-76RL01830.

Interconnects are important elements of SOFC and in most cases represent the failure point of the device. Optimal interconnects must be chemically stable at the working temperature (i.e. ca. 800°C for actual SOFC) and in aggressive atmospheres. Good corrosion resistance, long term performances and negligible chemical interaction with electrodes are also required. Coatings applied on metallic interconnects represent a promising methodology allowing to improve the chemical stability, through the slow formation of thin oxides layers tightly bonded to the metallic substrate, combined with high electrical conductivity. Previous papers (1, 2) discussed about the potential application of MOCVD for the realisation of nanoscaled coating of RE oxides on the surface of ferritic stainless steels and their behaviour under oxidising atmospheres. In this work the ferritic stainless steels and Ni/Cr alloys are tested with and without MOCVD coatings of R.E. oxides under reducing atmospheres (H₂/H₂O) at 800°C in order to simulate the aggressive environment at the anode side. Characterisation methods are: TGA/DT on samples electrochemically polished and eventually MOCVD coated, surface (LOM, SEM-EDXS, XRD, Raman Microspectroscopy) and cross section (SEM, TEM) characterisation. On the couple â?ospecimen-coating's showing the best behaviour long term tests (500h) on chemical stability and ASR (Areas Specific Resistance) are performed in order to investigate the compatibility of this couple as a SOFC interconnect.

(1) G. Cabouro et alii, Opportunity of Metallic Interconnectors for ITSOFC: Reactivity and Electrical Property, J. in Power Sources, Elsevier, in press.

(2)P. Piccardo et alii, Metallic interconnects for SOFC: characterisation and conductivity evaluation at operating temperature of differently coated ferritic stainless steels, Paper No. EFC2005-86124, ASME 1st European Fuel Cell Technology & Applications Conference, Dec.2005, Rome, Italy.

Nowadays, the technological development of solid oxide fuel cells (SOFC) is limited because of their high operating temperature of about 1200 K. In this temperature range, low-cost interconnect materials cannot be used and a long-term stability of the cell cannot be improved as chemical reactions occur between electrodes and an electrolyte. A reducing of operating temperature without a significant decrease of power density is the challenge of this technology. However, two main problems have been identified: the low ionic electrolytes conductivity and the high polarisation resistance of cathode. Various solutions have been proposed to overcome these drawbacks. One of them consists in a reducing of electrolyte thickness in order to decrease its ohmic drop at low temperature. Concerning cathode materials, using a mixed ionic and electronic conducting material appears to be a promising way to transform the triple-phase boundary into a double contact.

Up to now, most of the studies of SOFC cathode materials are devoted to perovskite type oxides such as La_{1-x√sub x}MnO₃. Recently, a new family of oxides of general formulation A₂MO_4+y was investigated: their structure of K₂NiF₄-type may exhibit some oxygen overstoichiometry.

In this paper, we investigate the feasibility of both (La_{1-x√sub x})MnO₃ and La₂NiO₄ cathode materials by co-sputtering of metallic La(Sr) and Mn or La and Ni targets in the presence of argon-oxygen reactive gas mixture. After a short description of the used sputtering device, we present the main relations between deposition conditions and chemical, structural and morphological properties of coatings. A special care is taken to the crystallisation of coatings in the convenient stoichiometry. A comparison of their electrical characteristics, measured by four-probe technics and by complex impedance spectroscopy, is finally presented.