ICMCTF2013 Session F6-1: Coatings for Fuel Cells & Batteries

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.

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.

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.

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.