No abstract

SOFC modules with a power of 5 to 50 kW are a serious competitor to commercially available combined heat and power plants for decentralized electricity and heat supply with high electrical efficiency. Electrical efficiency and electrical power of a SOFC stack are essential for its profitable operation. The highest power density and electrical efficiency can only be achieved with planar SOFC stacks. Electrolyte supported cells with 10ScSZ electrolytes allow power densities of more than 500 mW/cm². On account of the thermomechanical properties of electrolyte, this potential can only be used in stacks with chromium based interconnects (CFY from Plansee SE). High power, robustness and long-term stability are essential for stationary SOFC systems. In order to fulfill these requirements, Fraunhofer IKTS in close collaboration with Plansee SE develops a new stack design with larger active area, simple cathode contacting, external cathode manifold and optimized interconnect layer. This paper will give an overview of the current SOFC stack development focused on power output, long-term stability, temperature cycling and reliability of SOFC stack assembling. Material Combination for SOFC Stacks with High EfficiencyPlanar Solid Oxide Fuel Cells (SOFC) are considered to be highly efficient power generators with low emission levels even for small power units (1-200 kW el ). The recently developed and up-scaled net-shape technology for powder metallurgical production of bipolar plates allows new options for application of electrolyte supported cells with area specific resistances (ASR) <0.3 Ωcm² and are a match to stacks with anode supported cells (1-4). The electrolyte supported cells have benefits in long-term stability and thermal cyclability which were previously tested and are important parameter for a robust SOFC-system.The development of SOFC stacks with interconnects made from powder metallurgical processed chromium-iron-yttria based alloy (CFY) and scandia stabilized zirconia electrolytes (10ScSZ) have a long history and have surpassed the line to mass production in recent years.The bipolar plates of the new stack design (MK35x) have dimensions of 130x150x3.2 mm³. Currently, it is the biggest CFY interconnect and has a good ratio of active area to ECS Transactions, 35 (1) 269-277 (2011) 10.1149/1.3570002 © The Electrochemical Society 269 ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 160.39.73.191 Downloaded on 2015-03-24 to IP

Fraunhofer IKTS and Plansee SE continuously develop SOFC stacks with Chromium based interconnects (CFY) and electrolyte supported cells based on Scandia doped Zirconia. This technology has a great potential for cost-effective manufacturing and rapid up-scaling due to availability of major stack components and essential improvements in manufacturing made last year. The stacks are already available on the market and are in test at several potential customers. The paper gives an overview of the current status of CFY-stack development focused on performance map and system cycling. The results of operation of CFY-stacks in modules with stacks working on different fuels with and without internal steam reforming in partial and full load are presented.

Chromium-based interconnects (CFY) in combination with electrolyte supported cells are perfectly matching the requirements of different SOFC applications. Furthermore, this type of stacks and the related manufacturing of cells and interconnects is already proven in very high volumes today. Plansee SE and Fraunhofer IKTS, together with partners in the SOFC20 project and lead customers, have developed the advanced MK351 stack-platform, which is now available for interested new customers. A suitable, well-adjusted combination of materials, especially regarding the cathode protection and contacting, has been developed. Sample test results have been verified in long term stack testing for more than 8,000 h. The stack has proven very low power degradation of less than 0.7 %/ 1,000 h.

The stack concept of electrolyte supported cells is applicable for both, SOFC as well as SOEC. High power densities and efficiencies comparable to stacks based on anode supported cells can be achieved, however, in unison with higher robustness. Incorporating an electrolyte made of fully scandia-stabilized zirconia paired with chromium-base CFY interconnects of matched CTEs, a feasible stack concept was created with the focus on long-term stability in regard to high temperature corrosion and thermal cycling. Onward development of the stack design as well as stack data of more than 20,000 hours of a single stack are presented. Using the same material combinations and the same footprint as for the MK351-design, the newly developed MK352-design allows for easier stack assembling and system integration. Slight modifications in interconnect design enable lower pressure drops over the stack, which, paired with enhanced tolerance chains lead to enhanced stack performance, especially in regard to system cycling.