Robert Mücke

Development of High Power Density Solid Oxide Fuel Cells (SOFCs) for Long-Term Operation

Solid oxide fuel cells (SOFCs) enable environmentally friendly energy to be produced with high efficiency. The market entry of SOFC systems depends on the functionality of the components and on the costs. The SOFC has not yet reached market maturity. This presentation focuses on the possibilities for manufacturing SOFCs with high power outputs and long-term durability by using manufacturing technologies feasible in industry. For the past 15 years, FZ Jülich has been developing large-size so-called anode-supported SOFCs (up to 200 x 200 mm²) with reproducibly high power output (> 2 A/cm² at 800°C). Novel technologies for high-capacity manufacturing such as tape casting and roller coating have been introduced. Additionally, newly developed thin-film techniques have led to power outputs of more than 3 A/cm² at 800°C and more than 1.5 A/cm² below 700°C. These high power densities open up new possibilities for the operation of SOFCs at low temperatures to ensure low degradation and therefore long lifetimes.

Investigations on the Initial Stress Evolution During Atmospheric Plasma Spraying of YSZ by In Situ Curvature Measurement

The residual stresses within plasma-sprayed coatings are an important factor that can influence the lifetime as well as the performance in operation. The investigation of stresses evolving during deposition and post-deposition cooling for atmospheric plasma spraying of yttria-stabilized zirconia coatings using in situ measurement of the samples curvature is a powerful tool for identifying the factors that contribute to stress generation. Under various spray conditions, the first deposition pass leads to a significantly larger increase in samples curvature than the subsequent passes. It is shown in this work that the amount of curvature change at the onset of spraying is significantly influenced by the spray conditions, as well as by the substrate material. More information on the origin of this steep curvature increase at the onset of spraying was obtained by single splat experiments, which yielded information on the splat bonding behavior under various conditions. A comparison of the compressive yield strength for different substrate materials indicated the influence of substrate residual stress relaxation. Residual stress measurements using the incremental hole-drilling method and x-ray diffraction confirmed that the coating deposition affects the substrate residual stress level. The yield strength data were combined with the substrate near-surface temperature during deposition, obtained by finite element simulations, and with the measured residual stress-profile. This revealed that residual stress relaxation is the key factor for the initial curvature increase.

Preparation of Thin Functional Layers for Anode Supported SOFC by Roll Coating Process

A new method for the application of functional layers for solid oxide fuel cells was tested. So-called “roll coating” is a promising process for an industrial production of SOFCs, because it allows a high degree of automation. Nickel/yttria-stabilized zirconia (Ni/YSZ), YSZ, YSZ-LSM, and La0.65Sr0.3MnO3-δ (LSM) were used as materials for the anode, anode functional layer, electrolyte, cathode and current collector. The anode substrates were produced by tape casting. The anode and electrolyte were subsequently applied by roll coating on unsintered green tapes as well as presintered substrates. After co-sintering at 1400°C for 5h, half-cells with an electrolyte thickness of 10 µm were obtained. Finally, an LSM/YSZ cathode functional layer and an LSM current collector were deposited by screen printing and co-sintered at 1100°C for 3 h. The cells had current densities of 1.0 A/cm 2 at 800°C and 0.7 V, which is lower than the performance of screen-printed Julich standard cells with the same type of LSM/YSZ cathode. It is expected that the performance of roll-coated cells can be improved by further reducing the thickness of the functional layers. The results show that the anode-supported planar half-cells can be fabricated in a cost-effective way by combining roll coating with subsequent co-firing.

Residual Stress Depth Distributions for Atmospheric Plasma Sprayed MnCo1.9Fe0.1O4 Spinel Layers on Crofer Steel Substrate

In solid oxide fuel cells (SOFC) for operating temperatures of 800 °C or below, the use of ferritic stainless steel can lead to degradation in cell performance due to chromium migration into the cells at the cathode side [1]. Application of a coating on the ferritic stainless steel interconnect is one option to prevent Cr outward migration through the coating. MnCo1.9Fe0.1O4 (in the following designated as MCF) spinels act as a diffusion barrier and retain high conductivity during operation [2]. Knowledge about the residual stress depth distribution throughout the complete APS coating system is important and can help to optimize the coating process. This implicitly requires reliable residual stress analysis in the coating, the interface region and in the substrate.For residual stress analysis on these specific layered systems diffraction based analysis methods (XRD) using laboratory X-ray sources can only by applied at the very surface. For larger depths sublayer removal is necessary to gain reliable residual stress data. The established method for sublayer removal is electrochemical etching, which fails, since the spinel layer is inert. However, a mechanical layer removal will affect the local residual stress distribution.As an alternative, mechanical residual stress analyses techniques can be applied. Recently, we established an approach to analyse residual stress depth distributions in thick film systems by means of the incremental hole drilling method [5, 6]. In this project, we refined our approach for the application on MCF coatings with a layer thickness between 60 – 125 μm.

Properties of Screen Printed Ceramic Green Films Determined by Optical Laser Profilometry

The green density and roughness of green ceramic layers determine their mechanical and micro structural properties after final sintering. These properties can be measured precisely by laser profilometry. The green density of thin layers (20-50 µm) could quickly be determined as accurately as 0.5% theoretical density. The influence of paste parameters (powder conditioning, solid content, binder, and dispersing agent) on the green density was studied systematically for electrolyte pastes (8 mol.-% yttria stabilised zirconia = 8YSZ) typically used in solid oxide fuel cell applications. It could be shown that a minimal binder content is required to achieve acceptable green densities. Pre-calcination of the powder yielded also significantly higher film densities. Dispersant agents contributed to a smoother surface in any case, however a significant effect on the packing density was only observed for the fine, non-calcined powder.

Systematic Investigation on the Influence of Spray Parameters on the Mechanical Properties of Atmospheric Plasma-Sprayed YSZ Coatings

In the atmospheric plasma spray (APS) process, micro-sized ceramic powder is injected into a thermal plasma where it is rapidly heated and propelled toward the substrate. The coating formation is characterized by the subsequent impingement of a large number of more or less molten particles forming the so-called splats and eventually the coating. In this study, a systematic investigation on the influence of selected spray parameters on the coating microstructure and the coating properties was conducted. The investigation thereby comprised the coating porosity, the elastic modulus, and the residual stress evolution within the coating. The melting status of the particles at the impingement on the substrate in combination with the substrate surface condition is crucial for the coating formation. Single splats were collected on mirror-polished substrates for selected spray conditions and evaluated by identifying different types of splats (ideal, distorted, weakly bonded, and partially molten) and their relative fractions. In a previous study, these splat types were evaluated in terms of their effect on the above-mentioned coating properties. The particle melting status, which serves as a measure for the particle spreading behavior, was determined by in-flight particle temperature measurements and correlated to the coating properties. It was found that the gun power and the spray distance have a strong effect on the investigated coating properties, whereas the feed rate and the cooling show minor influence.

Comparison of the Simplification of the Pressure Profiles Solving the Binary Friction Model for Asymmetric Membranes

The gas flow through porous media including that of multiple species is frequently described by the binary friction model (BFM) considering the binary diffusion, Knudsen diffusion, and viscous flow. Therefore, a numerical simulation was performed on a microporous support of an asymmetric oxygen transport membrane. As its exact numerical solution is complicated and not always possible, the results of two different levels of simplification of the pressure profiles within the porous support are compared to the exact numerical solution. The simplification using a constant pressure equal to the gas pressure outside the support leads to a deviation by up to 0.45 mL·min−1·cm−2 from the exact solution under certain operating condition. A different simplification using a constant pressure averaged between the outside of the support and the support/membrane interface reduces this deviation to zero. Therefore, this is a useful measure to reduce computational efforts when implementing the Binary Friction Model in computational fluid dynamics simulations.

Monitoring and Improving the Reliability of Plasma Spray Processes

Monitoring and improving of process reliability are prevalent issues in thermal spray technology. They are intended to accomplish specific quality characteristics by controlling the process. For this, implicit approaches are in demand to rapidly conclude on relevant coating properties, i.e., they are not directly measured, but it is assumed that the monitored variables are in fact suggestive for them. Such monitoring can be performed in situ (during the running process) instead of measuring coating characteristics explicitly (directly) and ex situ (after the process). Implicit approaches can be based on extrinsic variables (set from outside) as well as on intrinsic parameters (internal, not directly adjustable) having specific advantages and disadvantages, each. In this work, the effects of atmospheric plasma spray process variables are systemized in process schemes. On this basis, different approaches to contribute to improved process reliability are described and assessed paying particular attention to in-flight particle diagnostics. Finally, a new test applying spray bead analysis is introduced and first results are presented.