and useful electrical power from a fuel gas (e.g., natural gas), at much higher conversion efficiencies than conventional combustion methods. [1,2] This method of heat and electricity co-generation makes SOFC technology ideal for use in micro-combined heat and power (µ-CHP) units, particularly in the 1-5 kW (electrical power) output class, which may be used to satisfy the total heat and electricity demand of family homes and small businesses. An example of such a unit is the Galileo 1000 N, produced by HEXIS AG between 2013 and 2018. [3] This µ-CHP unit was successfully commercialized with more than 100 units being installed in a variety of locations across Western Europe, each providing 1 kW of electrical power and 20 kW of heat (from an auxiliary burner). [4,5] Currently, the Swiss SOFC manufacturer is focusing on the development of the next-generation SOFC-based µ-CHP unit, designed to provide an increased electrical power output of 1.5 kW at a higher electrical efficiency. [3,6] Typically, high-temperature SOFC (operating at 700-850 °C) are produced using traditional, well-studied, and effective material sets, which have been tailored to suit the requirements of each component. For example, air electrodes (cathodes) are often made from composites of yttria-stabilized zirconia (YSZ) and strontium-substituted lanthanum manganite (LSM) [7][8][9] or composites of cerium gadolinium oxide (CGO) and strontium-substituted lanthanum cobaltite (LSC), [9,10] ferrite (LSF), [11] or cobaltite ferrite (LSCF). [8,[12][13][14][15][16] Electrolytes used in SOFC operating within this temperature regime are zirconia stabilized, commonly, with scandia or yttria (ScSZ or YSZ), [2] while fuel electrodes (anodes) traditionally comprise composites of Ni and either YSZ or CGO. [2,17] However, despite the excellent performance that can be obtained using these materials, there are several challenges posed, specifically, by the anode materials, that must be addressed in order to provide greater resistance to harsh operating conditions in next-generation SOFC systems. These ceramic-metal composites (cermets) of YSZ/CGO and Ni exhibit redox instability, due to the propensity of Ni to agglomerate, in addition to sulfur poisoning and carbon deposition during exposure to unprocessed natural gas feeds. [1,18] Several different materials design approaches have been identified and explored in an attempt to mitigate the limitations of the Solid oxide fuel cell (SOFC) stack technology offers a reliable, efficient, and clean method of sustainable heat and electricity co-generation that can be integrated into micro-combined heat and power (µ-CHP) units for use in residential and small commercial environments. Recent years have seen the successful market introduction of several SOFC-based systems, however, manufacturers still face some challenges in improving the durability and tolerance of traditional Ni-based ceramic-metal (cermet) composite anodes to harsh operating conditions, such as redox and thermal cycling, overload exposure, sulfur poisonin...
Regardless of the material used for interconnects in SOFC stacks, whether it is based on ferritic stainless steels or on chromium based material systems, chromium oxide scales will be formed on the interconnects’ surface during operation. In connection to the oxide scale formation, chromium evaporation is a commonly observed and researched issue. It was shown that evaporated chromium species did not harm the cathode’s performance significantly for more than 40.000 h of continuous operation in both short stack and full stack level with the Hexis repeat unit concept. Short stacks with varying operation times were subjected to post-mortem analysis and characterized for oxide scale thicknesses. Additionally, an effective chromium filter system based on a perovskite material system was designed and integrated into the exhaust gas stream before the heat exchanger. The filter concept was operated for more than 10.000 h in Hexis CHP-systems, some on full and, some on part load. During the experiments, condensate samples were taken in regular intervals and none of them contained Cr species.
Recent research into Rh and Ce0.80Gd0.20O1.90-impregnated La0.20Sr0.25Ca0.45TiO3 fuel electrodes for solid oxide fuel cells has demonstrated the high-stability of these material sets to a variety of harsh operating conditions at small scales (button cells with 1 cm2 active area), as well as full commercial scales (100 cm2 cells) in short stacks (5 cells) and full micro-combined heat and power systems (60 cells). In this work, the authors present a comprehensive evaluation of the ability of these novel titanate-based materials to function as fuel electrodes in solid oxide electrolysis cells (SOECs). Short-term and durability testing of button cell scale SOECs, under CO2 and steam electrolysis conditions, highlighted the limited stability of lanthanum strontium manganite-based air electrodes with lanthanum strontium cobaltite ferrite-based air electrodes offering improved degradation. Upscaling of this optimized cell chemistry to a 16 cm2 active area SOEC and testing under CO2, CO2/steam and steam electrolysis conditions demonstrated encouraging performance over a period of ~600 hours.
Currently, solid oxide fuel cell systems rely on upstream desulfurization units to prevent sulfur poisoning of the state-of-the-art anodes based on Ni-cermets. Next-generation anode materials should be sulfur tolerant, without compromising the performance, to reduce system complexity and cost. In this study, all-ceramic La0.4Sr0.4Fe0.03Ni0.03Ti0.94O3 (LSFNT) anodes, infiltrated with Ni:Ce0.8Gd0.2O1.9 (CGO) or FeNi:CGO electrocatalysts, were integrated into large-area electrolyte supported cells. The cells were tested together with a SoA cell with Ni/CGO cermet anode in a short stack configuration using reformed grid natural gas and an upstream, bypassable desulfurization unit. The cell performances before exposure to sulfur were similar for all cell types at 850 °C. Exposure to sulfur revealed different degradation mechanisms. TheSoA cell shows a fast, initial degradation followed by limited further degradation. In contrast, the FeNi:CGO infiltrated LSFNT anode appears sulfur tolerant initially, followed by accelerated degradation of >50% kh-1 over long-term sulfur exposures. The Ni:CGO infiltrated LSFNT anode is remarkably sulfur tolerant.
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