Vacuum-sintered stainless steel porous supports for inkjet printing of functional SOFC coatings

Mater Renew Sustain Energy (2015) 4:14 DOI 10.1007/s40243-015-0056-7 ORIGINAL PAPER Vacuum-sintered stainless steel porous supports for inkjet printing of functional SOFC coatings R. I. Tomov1 • M. Krauz2 • A. Tluczek2 • R. Kluczowski2 • Venkatesan V. Krishnan3 • K. Balasubramanian3 • R. V. Kumar1 • B. A. Glowacki1,4,5 Received: 7 May 2015 / Accepted: 14 July 2015 / Published online: 31 July 2015 Ó The Author(s) 2015. This article is published with open access at Springerlink.com Abstract Porous metal supports for SOFC applications were produced via conventional powder metallurgy routes—tape casting and high-pressure injection moulding. The supports were sintered in vacuum at different vacuum levels and temperatures. Commercially accessible low-cost stainless steel 430L powder was chosen as source material. The relations between the vacuum sintering temperature and the supports properties were studied. The density and the open porosity distribution of sintered supports were determined by Archimedes’ method, Optical Image Analysis and Hg-porosimetry. The microstructure and the stainless steel grain surface composition evolution were investigated by scanning electron microscope and energy dispersive X-ray spectrometry. direct ceramic inkjet printing (DCIJP) was employed as coating technology for depositing anode (NiO/GDC) and electrolyte GDC coatings. Suspension anode and electrolyte inks were developed in-house and the printing procedure was optimized to produce uniform coatings with thicknesses below 15 lm. The analyses confirmed that the as-produced substrates & R. I. Tomov 1 Department of Materials Science and Metallurgy, University of Cambridge, 27 Charles Babbage Road, Cambridge CB3 0FS, UK 2 Ceramic Department CEREL, Institute of Power Engineering, Boguchwała, Poland 3 Non-Ferrous Materials Technology Development Center, Hyderabad, India 4 Department of Physics and Energy, University of Limerick, Plassey, Ireland 5 Institute of Power Engineering, Warsaw, Poland were suitable to support DCIJP deposited SOFC functional coatings. Keywords SOFC
Metal supports
Cell fabrication
Inkjet printing Introduction Solid oxide fuel cells (SOFCs) have been object of continuous research efforts for decades due to their highly efficient direct conversion of chemical energy into electricity, fuel flexibility and environmental benefits. The SOFC cover wide range of applications including stationary (MW) and auxiliary (kW) power units, combined heat and power installations and decentralised off-grid applications. Depending on the design SOFCs can operate at temperatures within the region of 600–1000 °C [1, 2]. The state-of-the-art commercial SOFCs are based on a combination of cermet anodes (e.g. Ni–YSZ) and ion-conducting ceramic electrolyte materials, most often yttria-stabilized zirconia (8YSZ) or doped ceria (Gd:CeO2). Both materials offer the required chemical and thermal stability in oxidizing and reducing atmospheres and good oxygen ionic conductivity over a wide range of conditions [3, 4]. The commonly used anode-supported SOFC design is based on porous cermets providing mechanical support and permeation path for the fuel and the reaction products. Ni–YSZ anodes are preferred due to their sufficient electrical conductivity and mechanical strength, as well as minimal chemical interaction with the electrolyte [5]. However, such supports having thicknesses of *0.5–1.0 mm thickness can contribute significant cost due to the high-volume fraction of rare earth containing materials. In addition, the cermet electrodes are mechanically fragile and do not 123 14 Page 2 of 11 sustain thermal shock stresses. The operating temperatures of SOFCs at levels of 800–1000 °C introduce further limitations in SOFCs fabrication and operation. Such temperatures require utilization of expensive corrosive resistant interconnects and are detrimental to the durability of the cell causing functional materials degradation. As a consequence, the main barriers for SOFCs commercialization have been recognized as the high cost of production and the operational durability. Currently a shift towards intermediate temperatures (\800 °C) is considered essential for the commercialization of SOFCs technology. The advantages of reduced-temperature operation also include systems design compactness and wider choice of materials [6]. The strategies for compensating the incurred drop in ionic conductivity include lowering the electrolyte resistance either by implementing thinner electrolyte or using higher ionic conductive materials as well as reduction of the potential interfacial polarization losses and enhancement of the electrochemical activity of the electrodes [7]. Lowering the operational temperature permits utilization of less expensive stainless steel as support and interconnector materials. Porous metal substrates with thicknesses of few hundreds of micrometers are advantageous for SOFCs since they provide good electrical conduction, high mechanical strength, favourable thermal distribution due to the high thermal conduction and as a consequence rapid start-up times [8–10]. Metal supported SOFCs can also enable conventional metal joining techniques in the stack assembly. Ferritic stainless steels offer well-matched thermal expansion coefficient (TEC) with commonly used ceramics (TEC8YSZ * 10.4–11.0 ppmK-1, TECGDC * 12.7 -1 -1 ppmK , TECNiO/YSZ * 12.3 ppmK for an NiO/YSZ composite with 53 vol %NiO and TEC430L * 11.4 ppmK-1) which is beneficial for withstanding repeated thermal stresses caused by rapid thermal cycling [8–12]. One of the main obstacles for the use of metal supports at elevated temperatures is their corrosion. Partial oxygen pressures during SOFC operation can vary from values *1 atm on the cathode side to values *10-20 atm on the anode side. Although the latter is a very low value it is thermodynamically in the metal oxide formation region. The problem can be partially resolved by the formation of Cr2O3 protective coating onto the metal scaffolding which effectively stops further degradation and retains good electrical conductivity. Variety of different ferritic steels and alloys have been studied for SOFCs applications including standard commercial 316 and 430 series and as well as more expensive specialized (higher Cr content) ones-Crofer22 APU (ThyssenKrupp VDM), E-Brite (Allegheny Ludlum), ZMG232 (Hitachi Metals Ltd.), FeCr (70:30) (AMETEK) and Ti-Nb stabilized 17 % Cr ferritic stainless steel used by CERES Power [8–10, 13, 14]. Some austenitic nickel-based alloys (Haynes 230 and Haynes 123 Mater Renew Sustain Energy (2015) 4:14 242-Haynes International) have also been experimented with. The higher percentage of chromium content promotes the formation of a protective scale but chromia is volatile under certain conditions and can degrade the performance of the cell. Thus, a number of protective coatings preventing the effects of Cr poisoning and depletion are cur (...truncated)