The presentation will cover highlights from the SOFC/SOEC activities at DTU Energy, which span over a broad range in the value chain, from materials to cells, stacks and analyses at energy system level. These highlights also include progress of method development that supports SOFC/SOEC research such as ceramic processing methods, micro structural analysis, electro chemical characterization, and modelling.
Recent achievements involve electrode development on Ni-free fuel electrodes with high carbon tolerance, redox stability, and tolerance towards impurities in the gas. Building on the concept of nanostructured fuel-electrodes, symmetric cells with zirconia based backbones were successfully prepared by freeze-casting and showed a small area specific resistance of ca. 0.012 Ohm cm2 at 650 oC after infiltration of nano sized ceria.
DTU Energy has been among leading groups in the area of metal supported SOFCs. Our approach is based on a “co-firing” route utilizing ceramic manufacturing methods such as tape casting for the supporting metal layer. Recently the focus has been on developing a new cell concept that enables long term operation under conditions of high fuel utilization, where previous cell types have failed. Progress was achieved through micro structural optimization and use of an active and stable electrode material, LSFNT.
Apart from development and improvement of fuel and electrolysis cells, comprehensive testing with the aim to understand the processes determining performance and durability has been carried out. Specific emphasis has been on matching real application conditions also in the lab. Such realistic conditions comprise both, the operating regimes – such as dynamic profiles (considering SOFC/SOEC applications in an energy system dominated by wind or solar power) – and the fuels – such as biogas.
Biogas - a mixture of mainly methane and CO2 – is formed through anaerobic digestion of for example domestic or farming waste and is already produced at large scale throughout the world. SOFC allow for the direct use of biogas as fuel and offer the unique potential to produce electricity with high efficiency. Recently, a successful SOFC test was performed under dry reforming conditions, i.e. a mixture of a real biogas from a landfill unit and added CO2.
With the SOFC technology approaching longer life times of thousands and tens of thousands of hours, durability testing for degradation analysis and life time prediction becomes more time consuming. Accelerated aging tests are therefore highly desired. DTU Energy has several activities in this area and has for example applied a new strategy for degradation analysis, recently. It was shown how the steam content determined by the inlet gas composition and the operating conditions current load and fuel utilization is one of the possible accelerating parameters for SOFC degradation.
Also in the area of SOEC an extensive test program has been running over the past 10 years, including durability tests of several thousands of hours, both for steam and co-electrolysis of steam and CO2. This has involved test of various generations of cells, all based on a Ni/YSZ-structural support, but with different oxygen electrodes, as well as long term tests at stack level. Recent learnings on the impact of running cells in glavano static or potentio static regime and of the conversion degree will be presented.
In electrolysis operation, the oxygen activity on the oxygen electrode will be higher than in fuel cell mode. This has an adverse effect on interconnect corrosion. The impact of oxygen activity on corrosion and performance of recently developed protective coatings will be presented. Finally, the importance of mechanical robustness for reliable operation of stacks shall be discussed based on newly acquired data on the mechanical properties of both cells and interfaces in the stack.