ICMCTF2013 Session F6-1: Coatings for Fuel Cells & Batteries
Friday, May 3, 2013 8:00 AM in Room Sunrise
F6-1-1 Prototyping Solid-Oxide Fuel Cells with Pulsed Laser Deposition
Samuel Mao (Lawrence Berkeley National Laboratory, US)
Among various approaches to depositing thin films, pulsed laser deposition is a mature technique that has been utilized for prototyping and optimizing energy conversion devices, such as solid-oxide fuel cells, particularly their electrolytes and electrodes. With pulsed laser deposition, thin films in good contact with the substrate can be grown at lower temperature, which also suppresses chemical reactions between the electrode and the electrolyte for fuel cells. This presentation will start with an overview of the state-of-the-art pulsed laser deposition technology, in particular an advanced high throughput thin film material screening and discovery platform; followed by a discussion of recent progress in the development of prototype high performance solid-oxide fuel cells where pulsed laser deposition has played an important role.
F6-1-3 High Performance Nano-Coatings for Ferritic Stainless Steel Strips used as Solid Oxide Fuel Cell Interconnects
JanGustav Grolig, Jan Froitzheim, Lars-Gunnar Johansson, Jan-Erik Svensson (Chalmers University of Technology, Sweden)
Solid oxide fuel cells (SOFCs) are seen as promising concerning decentralized electricity and heat production. To reach a marketable product - SOFCs have to be improved concerning their long term stability, efficiency, and most of all less cost intensive materials need to be developed. The use of metallic interconnect materials, mainly ferritic stainless steels has reduced costs substantially compared to ceramic interconnect materials. To achieve sufficiently low corrosion rates special alloys such as Crofer 22 APU, Crofer 22 H or Sanergy HT have been developed. These materials are characterized by a chromium content of approximately 22 wt % which leads to a chromia protection layer and a manganese content of about 0.5 wt % to form a chromium manganese spinel layer on top which lowers chromium evaporation. Even though the corrosion performance of these special steels is improved, coatings against Cr evaporation on top these steels are needed to reach acceptable performance.
The above mentioned high performance alloys are relatively expensive compared to mass manufactured 441 stainless steel. This work investigates 441 in combination with a multifunctional coating, that inhibits Cr evaporation and also leads to lower corrosion rates and higher electrical conductivity. Therefore 10 nm thin reactive element coatings of cerium or lanthanum have been applied to increase corrosion resistance and were later combined with a 640 nm thick cobalt coating to minimize chromium evaporation. Samples were exposed to a cathode side environment (air, 850 °C, 3 % water) and a recently developed “Denuder Technique” has been used to measure chromium evaporation in a time resolved manner. Scanning electron microscopy is used to investigate the evolved microstructure and analyze the corrosion performance. Simple ASR characterization has been performed to monitor influences of the coatings.
It has been observed that the uncoated stainless steel 441 suffers relatively high corrosion in a cathode side environment and even spallation has been observed. The reactive element coatings decreased the corrosion significantly. Cobalt coated samples showed slightly higher corrosion, which can be linked to the oxidation of cobalt and successfully reduced chromium evaporation about 90 %. Finally, coating combinations of reactive elements with cobalt showed a superior corrosion and chromium evaporation performance.
F6-1-4 Strontium Diffusion in Magnetron Sputtered Gadolinia-doped Ceria Thin Film Barrier Coatings for Solid Oxide Fuel Cells
Steffen Sonderby, Petru Lunca Popa, Jun Lu (Linköping University, Sweden); Bjarke Christensen, Klaus Almtoft, Lars Pleth Nielsen (Danish Technological Institute, Denmark); Per Eklund (Linköping University, Sweden)
Strontium diffusion through sputtered Gd2O3-doped CeO2 (CGO) thin films is investigated by in-situ and ex-situ X-ray diffraction (XRD) and electron microscopy. A model system consisting of a screen printed (La,Sr)(Co,Fe)O3-δ (LSCF) layer and magnetron sputtered thin films of CGO and Y2O3-ZrO2 (YSZ) were prepared with the CGO sandwiched between the LSCF and YSZ. This system simulates a solid oxide fuel cell setup and allows Sr diffusion to be probed by XRD by tracing the formation of SrZrO3 upon annealing. CGO films were prepared with different thicknesses and at different substrate bias voltage. For CGO barriers with thicknesses up to 600 nm, SrZrO3 formation was observed at temperatures above 900 °C. However, by use of substrate bias the temperature could be increased to 950°C. Observation of SrZrO3 precipitates by transmission electron microscopy (TEM) confirmed the observation done by XRD and proved the use of XRD as a suitable method for assessing the quality of CGO barrier coatings. Furthermore, the combined XRD and TEM study yielded understanding of the Sr diffusion mechanism. Sr was found to diffuse along grain/column boundaries in the CGO film. By modifying film thickness and microstructure, the Sr diffusion could be decreased.
F6-1-5 High Performance Duplex Coatings for PEMFC Metallic Bipolar Plates by CFUBMSIP and HIPIMS Technology
Hailin Sun, Kevin Cooke, Philip Hamilton (Teer Coatings Limited, Miba Coating Group, UK); Papken Hovsepian, ArutiunP. Ehiasarian, A.A. Sugumaran (Sheffield Hallam University, UK)
Coatings are essential to maximise performance and longevity of metallic Bipolar Plates (BPPs) in Polymer Electrolyte Membrane Fuel Cells (PEMFC). Thin stainless steel foils, around 0.1mm thick, are attractive for automotive PEMFC stacks, providing mechanical and structural integrity, and minimisation of size and weight. However, the intrinsic chromium-oxide passivation on the stainless steel surface raises interfacial contact resistance (ICR) and will not withstand long term exposure in the aggressive electrochemical environment of the cells, leading to corrosion with further degradation of the electrical conductivity and the release of metal ions, which can poison the cell’s membrane. In order to meet such challenging performance, economic and serial manufacturing requirements, thin, sub-micron duplex coatings consisting of a transition metal nitride and a highly conductive carbon top coat have recently been demonstrated to dramatically improve both the ICR and the corrosion resistance of the plates. TiN+C & CrN+C have been deposited by industrially compatible, magnetron-based PVD techniques.In this research, closed field unbalanced magnetron sputter ion plating (CFUBMSIP) and High Power Impulse Magnetron Sputtering (HIPIMS) are used to produce dense, well adhered coatings, of transition metal nitrides and non-hydrogenated amorphous carbon. CFUBMSIP coatings include a thin metallic adhesion layer, followed by a graded interface with increasing nitrogen content and modulus, supporting the main stoichiometric nitride and the topmost amorphous carbon contact surface. HIPIMS utilises a high energy bombarded interface region resulting in intimate, extremely dense, and fine structured coating without the need for a separate gradient layer. HIPIMS coatings have also been shown to provide exceptional corrosion resistance. The critical characteristics of the coatings produced by both processes have been determined and their relative merits are discussed.Duplex coatings improve the ICR of the metallic plates, compared to a single transition nitride coating, achieving ICR close to that of thin (e.g. >20nm) Au, which is widely regarded as an industry benchmark. Their corrosion resistance has been assessed ex-situ by potentiostatic and potentiodynamic tests under simulated anode and cathode conditions and exceeds the requirements of the relevant industry specifications at both low and high potentials. Surface roughness, coating structure and composition have been assessed by profilometry, SEM and TEM.Finally, the potential for the future industrial exploitation of the duplex coatings is discussed.
F6-1-6 Industrial, Low Cost Ceramic MaxPhase™ Protective Coatings for Stainless Steel Bipolar Plates
Henrik Ljungcrantz, Kristian Nygren, Mattias Samuelsson (Impact Coatings, Sweden)
Bipolar plates (BPP) provides cell separation, gas distribution, current collection, and structural integrity of the fuel cell (FC) stack. While typically made from graphite, this makes the BPP heavy, bulky, fragile, and costly. Producing plates from stainless steel circumvents the aforementioned shortcomings. However, uncoated steel cannot withstand the corrosive environment in the FC, which will result in deterioration of the membrane from corrosion products and subsequent cell failure. Noble metal coatings, such as Au, will not deteriorate during operation, but they fail to meet the US Department of Energy (DOE) cost target. The lack of available coatings that meet the cost target and that are chemically stable under FC operation conditions impedes the transition from graphite to stainless steel BPP.
Impact Coatings offers Ceramic MaxPhase™ (CMP) coatings as a low cost material suitable for stainless steel BPP, in conjunction with the InlineCoater™ deposition system which provides high throughput, short cycle times, and simultaneous two-sided coating. In the present work, CMP deposited by physical vapor deposition is compared to Au coatings. Ex-situ evaluations of chemical stability and electrical properties of CMP coated stainless steel show high oxidation resistance and low contact resistance (<10 mΩcm2). Moreover, the electrical properties were not affected by the corrosion treatment. Life-time tests in commercial PEMFC systems show equal stack performance of CMP coated plates as compared to Au coatings for at least 2,000 hours of operation. In addition, cost calculations for high volume production of the coatings were performed, showing that the cost target of the DOE is within reach. Thus, the CMP coatings and the production concept enable the transition from graphite to stainless steel BPP for use in PEMFC and DMFC.
F6-1-7 Pre-coated Steel Stripes for PEMFC and SOFC Interconnects
Gayatri Dadheech (General Motors Research and Development Center, US); Hakan Holmberg (Sandvik Coromant R&D Materials and Processes, Sweden); Mikael Schuisky (Sandvik Machining Solutions, Sweden)
Metal interconnects are getting widely popular due to their fast and easy high volume manufacturing. One of the metals of choice is stainless steel materials which have good corrosion stability due to the naturally occurring native oxide on its surface. However, stainless steel offers a high contact resistance when connected in series, mainly due to the passive oxide surface layer, which is the sole reason behind the corrosion resistance of stainless steels. Typical interfacial contact resistance (ICR) values for stainless steels at the compaction pressures of interest for PEMFC bipolar plates, are at least an order of magnitude higher than the DOE target of 10 mΩ cm2. The use of uncoated stainless steel leads to undesirable high electrical contact resistance and significant fuel cell stack performance loss. In this talk we would review reducing the ICR and improving corrosion resistance using graphite like carbon (GLC) coatings precoated on stainless steel strips for easy high volume production.
F6-1-8 R.F. Magnetron Sputtered Li-Mn-O Thin Films
Julian Fischer, Thomas Bergfeldt (Karlsruhe Institute of Technology, Germany); Keke Chang (RWTH Aachen University, Germany); Harald Leiste, Torsten Scherer, Sven Ulrich, Hans-Michael Bruns (Karlsruhe Institute of Technology, Germany); Carlos Ziebert (Karlsruhe Institute of Technology, Germay); HansJürgen Seifert (Karlsruhe Institute of Technology, Germany)
The increasing demand on lithium ion batteries (LIB) and more efficient energy storage solutions for portable consumer electronic devices like Smartphones, Camcorders and Tablet-PCs makes thin film technology more and more interesting as an serious alternative to conventional produced micro battery applications. With thin film and nano-technology it is possible to develop electrochemical storage systems that are both, small and powerful. While the most conventional liquid-electrolyte-based lithium ion secondary batteries hold the risk of leakage, burning and undesirable side-reactions, these problems can be completely eliminated with all solid state technology. Like in the liquid based systems the diffusion of the lithium ions through the cathode material is one of the most limiting factors and makes additional studies on the understanding and improvement of intercalation compounds desirable and necessary.
This work is about the investigation and development of environmental friendly manganese oxide based thin film cathodes. R.f. magnetron sputtering was used in combination with furnace annealing to produce amorphous and crystalline Li-Mn-O thin films. All samples were investigated on their elemental ratios Li:Mn:O by inductively coupled plasma optical emissions spectroscopy (ICP-OES) and carrier gas heat extraction (CGHE). Atomic force microscopy (AFM) and scanning electron microscopy (SEM) techniques were used to investigate thin film surface roughness and morphology of the surface and cross-section. Further a structural investigation was carried out by X-ray diffraction (XRD), Raman-spectroscopy and transmission electron microscopy (TEM) in combination with focused ion beam (FIB) produced TEM-lamella. The distribution of the elemental compositions in the as deposited and annealed films was investigated with time of flight secondary ion mass spectroscopy (TOF-SIMS) depth profiles. To complete these measurements finally electrochemical battery tests in Swagelok half-cells were carried out to investigate both the electrochemical reactions and the galvanostatic cycling behavior.
F6-1-9 The Effect of Reactive Element Coatings on the Oxidation Properties of Ferritic Steels for Solid Oxide Fuel Cell Interconnect Applications
Rakshith Sachitanand, Jan Froitzheim, Jan-Erik Svensson, Lars-Gunnar Johansson (Chalmers University of Technology, Sweden)
Ferritic stainless steels are used as interconnector materials in solid oxide fuel cells owing to their combination of low cost, compatible mechanical properties and electronically conductive oxide scales. The applicability of these materials is however limited by their unsatisfactory high temperature oxidation resistance and chromium volatilization of the oxide scale leading to catastrophic cell degradation. Solid oxide fuel cell interconnectors operate in dual atmospheres wherein the oxygen partial pressure differs significantly on either side of the bipolar interconnector plate. This entails oxidizing conditions on one side and reducing on the other, which throws up varying material challenges on the same piece of steel under operating conditions.
This study investigates the effect of metallic thin films of Ce and La on the oxidation properties of the commercial interconnector material Sanergy HT (Sandvik Materials Technology) in a typical cathode side SOFC environment.Exposures are carried out in tubular furnaces at 850°C, with 6l/min airflow and 3% H2O to simulate the cathode side atmosphere in a SOFC. Test durations range from 1 minute to 1000 hours .In addition to the oxidation tests, in-situ chromium evaporation measurements are carried out using a novel denuder technique to investigate the effect of these coatings on chromium volatilization. The surface morphology and microstructure of the oxide scales are characterized using scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX). Secondary ion mass spectroscopy (SIMS) and X-ray photoelectron spectroscopy (XPS) are applied to better understand the effect that these coatings have on near surface chemistry.