Protective Coating from Manganese Cobalt Oxide Powders Obtained By High Energy Ball Milling: Materials Characterization and Cell Environment Testing

Abstract

Solid Oxide Fuel Cells (SOFC) competitiveness in the energy market requires improvement of the longevity of the stacks and production affordability. Lowering operating temperatures is one of the evaluated routes, since significantly reduces components degradation and materials requirements. At operating temperature below 800°C Stainless Steels (SS) are suitable as base materials of several stack components like cell housings and interconnects (IC). Ferritic alloys, in particular, are characterized by Coefficients of Thermal Expansion (CTE) similar to the other cell component materials in addition to low production cost and high mechanical stability, high thermal and electrical conductivities and ease of fabrication [1]. Poor corrosion resistance in SOFC operating environment represents the main drawback for the use of ferritic SS. The formation of chromium oxide layers on the surfaces affects significantly cell performances, due to an overall stack electrical resistance increase related to the insulating layer growth and especially due to the cathode poisoning originated by volatile Cr species evaporation [2]. To limit these issues the application of protective coatings is required. Mixed Manganese Cobalt Oxides (MCO) represent one of the most studied and promising materials due to their high electrical conductivity and good CTE match with ferritic SS. Wet powder methods represent cost effective ways to apply such coatings, and are based on the formulation of powders as appropriate inks. MCO powders production is usually carried out with sol-gel or high temperature solid state methods, each method having its drawback, such as the use of large amount of solvents or the need for prolonged heat treatments. High Energy Ball Milling (HEBM) is a low cost, environmental friendly powder processing mechanochemical technique. This method consists in repeated energy transfers to the powder using hitting balls as milling media: kinetic energy of the balls is released to the compound promoting several physico-chemical transformation, including chemical reactions. The process usually produces nanostructured materials, characterized by high surface areas and defectivity. The high energy content of milled particles results in high reactivity of the materials [3]. In this work a HEBM route was evaluated for the production of MCO coatings precursor powders. By milling Mn and Co oxides, nanostructured multi-phase MCO powder is produced. The obtained mixture easily reacts to form the equilibrium spinel compound in a very short time at temperatures below 800°C. The milled mixture was formulated into a slurry ink, and deposited on Crofer 22H ferritic stainless steel substrate. The coated material was subjected to thermal treatment to promote the equilibrium spinel formation and particles sintering. The so obtained coated sample was finally subjected to long term exposure at 800°C in SOFC environment, and Area Specific Resistance (ASR) was measured during the test. X-Ray Diffraction (XRD) analysis and Scanning Electron Microscopy were used to evaluate materials evolution during the production process and during the high temperature oxidizing atmosphere exposure. Bibliography [1] Wu J, Liu X. Recent Development of SOFC Metallic Interconnect. J. Mater. Sci. Technol. 2010;26:293–305. [2] Tucker MC. Progress in metal-supported solid oxide fuel cells: A review. J. Power. Sources 2010;195:4570–82. [3] BalហP, Achimovičová M, BalហM, Billik P, Cherkezova-Zheleva Z, Criado JM, et al. Hallmarks of mechanochemistry: from nanoparticles to technology. Chem. Soc. Rev. 2013;42:7571–637.