Application of composite coatings as protection/contacting layers for metallic highchromium- content SOFC interconnect material

Oxidation of the surface of metallic chromium oxide forming metallic interconnect (MIC) can cause up to one third of the total SOFC stack degradation during the long-time operation at elevated (750 - 850 °C) temperatures. The application of protective coatings is the most effective method not only for reduction of the growth of oxide scales but also for prevention of evaporation of Cr-containing species from MIC and of the poisoning of the air electrode. Two approaches to form the protective layers on the surface of CFY interconnect material with high chromium content (~ 94 %) have been tested. The CuNiMn-spinel (CNM) coatings were deposited using the wet powder spraying (WPS) of the slurries. As an alternative approach physical vapour deposition (PVD) method was used to apply thin metallic films on the surface of MIC and to form the protection layer by in-situ oxidation under the stack relevant conditions. The experiments were carried out at first using the model samples of different geometries to evaluate the properies and efficiency of the coatings. Composite pastes with addition of perovskite powders were also tested, because the CNM layers densify at SOFC operating conditions and shrinkage during long-term operation can cause the decrease of the contact area between the components and accelerate the degradation of the stack performance. The experiments have shown that the perovskite additive can efficiently reduce the shrinkage compared to the pure CNM material and match it well to the shrikage of other stack components. Moreover, the perovskite additive do not deteriorate the electrical properties of the composite since the perovskites have electrical conductivity comparable to CNM. The PVD coatings were tested in combination with CNM containing contacting layers applied by screen printing to reduce the chromium release rate. The experiments have shown a good compatibility and mechanical stability between the contacting layer and PVD protective coating during operation and thermal cycling. The materials and composites have been characterized by scanning electron microscopy (SEM/EDX), optical dilatometry and electrical conductivity measurements. Finally, the most promising material combinations obtained for model samples were transferred to SOFC stacks MK35x and evaluated under real operation condition.Federal Ministry for Economic Affairs and Energy (BMWi) for funding of these researches (support code 03ET6120A)

Lateral speckle size in Phase Retrieval systems

Phase Retrieval (PR) techniques provide a complimentary approach to Digital Holography (DH) for estimating the complex amplitude of a wavefield. If one uses a Gerchberg-Saxon type iterative approach, generally two intensity distributions at different optical planes are captured. Fast Fourier Transform (FFT) based algorithms are then employed to iteratively recover the phase information. Once we have estimated the complex amplitude of the wavefield we can use numerical propagation algorithms to calculate the distribution at any optical plane. We expect however that the finite size of the camera aperture and the sampling effect introduced by the camera pixels will act to reduce the performance of the optical system. In this manuscript we attempt to examine these performance limiting factors in more detail using a special field as the input into our system; namely a speckle field. This particular type of field contains high spatial frequencies which in turn are related to the lateral speckle size. A series of PR experiments at different capture planes are conducted and we report on how the system performance is effected

Green on-site power generation : environmental considerations on small-scale biomass gasifier fuel-cell CHP systems for the residential sector

Contemporary combined heat and power (CHP) systems are often based on fossil fuels, such as natural gas or heating oil. Thereby, small-scale cogeneration systems are intended to replace or complement traditional heating equipment in residential buildings. In addition to space heating or domestic hot water supply, electricity is generated for the own consumption of the building or to be sold to the electric power grid. The adaptation of CHP-systems to renewable energy sources, such as solid biomass applications is challenging, because of feedstock composition and heat integration. Nevertheless, in particular smallscale CHP technologies based on biomass gasification and solid oxide fuel cells (SOFCs) offer significant potentials, also regarding important co-benefits, such as security of energy supply as well as emission reductions in terms of greenhouse gases or air pollutants. Besides emission or air quality regulations, the development of CHP technologies for clean on-site small-scale power generation is also strongly incentivised by energy efficiency policies for residential appliances, such as e.g. Ecodesign and Energy Labelling in the European Union (EU). Furthermore, solid residual biomass as renewable local energy source is best suited for decentralised operations such as micro-grids, also to reduce long-haul fuel transports. By this means such distributed energy resource technology can become an essential part of a forward-looking strategy for net zero energy or even smart plus energy buildings. In this context, this paper presents preliminary impact assessment results and most recent environmental considerations from the EU Horizon 2020 project "FlexiFuel-SOFC" (Grant Agreement no. 641229), which aims at the development of a novel CHP system, consisting of a fuel flexible smallscale fixed-bed updraft gasifier technology, a compact gas cleaning concept and an SOFC for electricity generation. Besides sole system efficiencies, in particular resource and emission aspects of solid fuel combustion and net electricity effects need to be considered. The latter means that vastly less emission intensive gasifier-fuel cell CHP technologies cause significant less fuel related emissions than traditional heating systems, an effect which is further strengthened by avoided emissions from more emission intensive traditional grid electricity generation. As promising result, operation "net" emissions of such on-site generation installations may be virtually zero or even negative. Additionally, this paper scopes central regulatory instruments for small-scale CHP systems in the EU to discuss ways to improve the framework for system deployment

Interconnect Corrosion in Steam Containing Fuel Gas

Iron or chromium based alloys are currently the most preferred materials for SOFC interconnects. The poisoning of the cathode of the membrane electrode assembly (MEA) due to evaporation of chromium species from the metallic interconnect (MIC) and the oxidation of the interconnect surface during stack operation at elevated (800-900 °C) temperatures are regarded to be the major mechanisms affecting the degradation behaviour of the SOFC stack. The oxidation at the cathode side of the interconnect can cause up to one third of the total stack degradation. Despite intensive investigations on corrosion stability of MIC for SOFC application over the past decades, the main attention was focused mainly on the material properties in the cathode side gas conditions (ambient air). The growth behavior and the composition of oxides grown in the SOFC anode gas environment as well as the influence of the oxides on the contact resistance between MIC and anode contacting are still not investigated comprehensively. The MIC materials have to be stable over more than 40,000 hours to satisfy a demand for a continuous increase of operation times of the SOFC stacks. Real time tests over such long time scales are cost intensive. A modification of standard material test procedures, with the aim to accelerate material degradation, is therefore necessary. In order to realize this, three ferritic steels (Crofer 22 APU, Crofer 22 H, AISI 441) and a chromium-based alloy were tested in SOFC anode gas environment in a temperature range between 725 – 875°C. The experiments were carried out under variation of the water vapor content in the gas mixture for different exposure times in order to create the accelerated degradation testing conditions for MIC and to investigate the behavior of these materials caused by the formation of growing chromium oxide based scales. Both gravimetric measurements and FESEM/EDX data, as well as polished microimage sections were analyzed to characterize the oxidation kinetics and the microstructure of the oxide scales and interconnect materials. A clear correlation between increasing temperatures and increasing oxide growth rate constants kp,w can be demonstrated in all materials. This interrelation results in thicker surface oxide scales. A comparison of surface oxide thicknesses after 1000 h oxidation in reducing atmosphere show an increase between 725°C and 875°C with a maximum factor of about 3.8 in ferritic steels and 2.4 for CFY. The comparison of kp,w values at temperatures between 725 and 875°C show acceleration factors of 8,2-11.9 in ferritic steels and 2.3 for CFY. No significant dependence of the oxide growth rate in ferritic steels and CFY by variation of the water content in fuel gas for concentrations within H2/H2O=87/13 vol.% and H2/H2O= 50/50 vol.% was found. The structures of the oxide layers are specific for each material and consist mainly of Cr2O3 (CFY) and Cr2O3/(Cr,Mn)3O4 in ferritic steels with different element distributions and thickness ratios. Beside this, the zone of an inner oxidation of MICs with the Al-, Si- and Ti- rich oxide inclusions can be seen in ferritic samples, whose microstructures differ depending on analyzed materials and temperatures. In Nb containing materials (Crofer H, AISI 441), Silicon precipitations as Laves phases as well as continuous SiO2 scales were observed beneath the formed Cr2O3 top layer. Furthermore, an increase of the internal oxidation zone was identified with increasing temperatures in ferritic steels. In CFY, the internal oxidation zone with Cr2O3 inclusions in the material bulk remains almost constant over 1000 h oxidation in the tested temperature range. Electrical measurements in the dual gas atmosphere reveal also an increase of resistance within 1000 h material exposure under anode gas conditions. Comparison of these results suggests that materials, forming such isolating SiO2 layers, show a higher resistance increase. The results of the oxide scale formation under the anode gas conditions in tested Crofer 22 APU and CFY samples are compared with data derived from the real SOFC stacks. Only minor differences in the oxide thicknesses were observed between the gas inlet and gas outlet (region with higher water content) at the anode side of the stacks. This behavior is similar to the results of our artificially oxidized MIC samples in which we also have not found a clear evidence for an increase in oxide scale growth rate with higher water content in the humidification range tested. Such effects can indicate, that the reactions at phase boundary are dominated by solid-state diffusion and the rate supply of adsorbed oxygen species (from H2O) is not the limiting factor (in the tested H2O concentrations range)

Glass ceramics sealants for SOFC interconnects based on a high chromium sinter alloy

The operation of solid oxide fuel cells requires gas tight and stable sealing materials. Glass ceramic materials have been established to seal different stack designs and can be optimized to match the requirements of components and joining conditions. It is known that glass based sealants are sensitive to the formation of undesired chromia species in contact with ferritic steels used as interconnects. Their formation depends on the earth alkaline type and content in the glass as well as the chromium content of the interconnect. A robust interconnect material is the chromium based sinter alloy CFY with > 93 wt.% chromium what in turn enforces the formation of chromia phases. SOFC stacks with chromium based interconnects and electrolyte supported cells (ESC) with 10SCSZ electrolyte are a candidate for SOFC systems with high efficiency in a wide power range. This study presents results on the development of sealing glasses adapted for the CFY by optimizing the glass components BaO, CaO, SiO2 and Al2O3. Depending properties like crystallization behavior, thermal expansion, adhesion and reactivity in contact with CFY have been regarded. The glasses have been tested under operating conditions of SOFC (dual atmosphere, electric potentials) in samples as well as in real stacks. It has been shown that optimized glass compositions especially regarding a limited BaO-content combine a good joining behavior and minimized BaCrO4 formation in contact with the CFY alloy

Towards a wastewater energy recovery system: The utilization of humidified ammonia by a solid oxide fuel cell stack

This study presents the results of investigations on performance and durability of an ammonia-supplied MK352 solid oxide fuel cell stack with electrolyte supported cells and chromium based interconnects. The performance evaluation revealed no significant differences between ammonia and equivalent hydrogen/nitrogen gases as fuel, which was a result of the excellent ammonia conversion rates up to 99.99%. When using high ammonia flow rates, temperature measurements inside the stack revealed a temperature drop due to the endothermic ammonia decomposition of up to 18.8 K, which proceeded preferentially at the fuel inlet region. An 1000 h durability test with humidified ammonia in 80% fuel utilization condition was performed, which resulted in a stack performance degradation rate of about 1.1%/1000 h. Tests with hydrogen/nitrogen fueled reference stacks revealed similar degradation rates during the initial 1000 h. Post-mortem analyses by scanning electron microscopy and energy-dispersive X-ray spectroscopy revealed no significant micro-structural deterioration of the functional layers of the anode, but nitriding effects on the nickel contact meshes and chromium nitrides were found in the material structure of the interconnects. Also, an oxide layer was found between interconnect and contact meshes at the anode, which appears to be the main cause of the performance degradation

Portable 100 W power generator based on efficient planar SOFC technology: Presentation held at ICACC 2012 - 9th International Symposium on Solid Oxide Fuel Cells (SOFC): Materials, Science and Technology, Jan. 27, 2012, Daytona Beach, FL

Portable power generators for camping and industrial applications require start-up times around 30 minutes and need to achieve a life time of 3,000 h including 300 cycles. Preferably they operate on available fuels and have a compact and lightweight system design. Eneramic®, a portable solid oxide fuel cell (SOFC) system in the 100 Watt class has been developed. A planar SOFC stack based on electrolyte supported cells and ferritic interconnects is used for the eneramic® system. The long-term stability of SOFC stacks was tested over more than 3,000 hours with power degradation below 1.0 %/1,000 h. However, hotbox testing of 40-cell stacks and stack operation in the system environment revealed slightly higher degradation rates between 2.1 and 2.3 %/1,000 hours. SOFC stacks exposed to thermal cycles showed no power losses. The results show that the compact planar SOFC stack is capable to survive the expected system life time

Green on-site power generation : environmental considerations on small-scale biomass gasifier fuel-cell CHP systems for the residential sector

Contemporary combined heat and power (CHP) systems are often based on fossil fuels, such as natural gas or heating oil. Thereby, small-scale cogeneration systems are intended to replace or complement traditional heating equipment in residential buildings. In addition to space heating or domestic hot water supply, electricity is generated for the own consumption of the building or to be sold to the electric power grid. The adaptation of CHP-systems to renewable energy sources, such as solid biomass applications is challenging, because of feedstock composition and heat integration. Nevertheless, in particular smallscale CHP technologies based on biomass gasification and solid oxide fuel cells (SOFCs) offer significant potentials, also regarding important co-benefits, such as security of energy supply as well as emission reductions in terms of greenhouse gases or air pollutants. Besides emission or air quality regulations, the development of CHP technologies for clean on-site small-scale power generation is also strongly incentivised by energy efficiency policies for residential appliances, such as e.g. Ecodesign and Energy Labelling in the European Union (EU). Furthermore, solid residual biomass as renewable local energy source is best suited for decentralised operations such as micro-grids, also to reduce long-haul fuel transports. By this means such distributed energy resource technology can become an essential part of a forward-looking strategy for net zero energy or even smart plus energy buildings. In this context, this paper presents preliminary impact assessment results and most recent environmental considerations from the EU Horizon 2020 project "FlexiFuel-SOFC" (Grant Agreement no. 641229), which aims at the development of a novel CHP system, consisting of a fuel flexible smallscale fixed-bed updraft gasifier technology, a compact gas cleaning concept and an SOFC for electricity generation. Besides sole system efficiencies, in particular resource and emission aspects of solid fuel combustion and net electricity effects need to be considered. The latter means that vastly less emission intensive gasifier-fuel cell CHP technologies cause significant less fuel related emissions than traditional heating systems, an effect which is further strengthened by avoided emissions from more emission intensive traditional grid electricity generation. As promising result, operation "net" emissions of such on-site generation installations may be virtually zero or even negative. Additionally, this paper scopes central regulatory instruments for small-scale CHP systems in the EU to discuss ways to improve the framework for system deployment

Operation of a SOFC CHP system with wood gas from a fixed-bed updraft gasifier

This paper will show the results of operating an SOFC system with the product gas from a biomass gasifier for more than 200 hours under constant and dynamic operation. The SOFC system was operated at different load cases, 50 % and 100 % load, and achieved a gross electrical efficiency of 42 %. It shows a high dynamic behavior, so that electric load changes could be performed within a few seconds from full load to part load. Furthermore, AVL developed a simulation software by which a SOFC stack model can be simulated dynamically. A comparison of the measured and simulated values of the stack module will be presented. By using the results of the test runs the software could be validated successfully