Assessing the Performance and Degradation of Metal Supported SOFCs Operating with Natural Gas

Abstract

High operating temperatures endow SOFCs with many advantages including fuel flexibility and high conversion efficiencies, distinguishing them from many other types of fuel cells. In order to leverage this role SOFCs must be able to operate well with a variety of fuels including H2, CH4, natural gas, syngas, and higher molecular weight hydrocarbons. This requirement necessitates keeping the SOFC anodes clear from excessive carbon deposition or “coking” when exposed to hydrocarbon fuels. Standard SOFC anodes composed of a Ni-YSZ cermet are highly susceptible to coking which can be detrimental to overall cell performance as it can block catalytic sites and impede gas transport. One way to mitigate this effect is to introduce steam into the fuel feed to remove carbon via oxidation. However, steam also risks oxidizing the catalytically active Ni metal resulting in volume changes that may damage the fragile electrode microstructure, requiring material or mechanical solutions to improve overall cell resilience. LBNL has developed metal supported SOFCs (MS-SOFCs) with unique symmetrical architecture and infiltrated catalysts with numerous advantages over conventional cells including low cost, mechanical ruggedness, quick start-up capability, and excellent tolerance to redox cycling and thermal cycling under standard H2 conditions. The high mechanical stability of LBNL’s MS-SOFCs suggest that they may be well suited for long-term hydrocarbon operation, even when high levels of steam are present in the fuel stream.In this work, the use of LBNL’s MS-SOFCs are extended to natural gas. As the least carbon-intensive fossil fuel, natural gas has the potential to serve as a bridging fuel, facilitating the transition into a low carbon energy economy. The durability of the MS-SOFCs under natural gas and reformate fuels have been evaluated at 700 °C as well as the resilience toward thermal cycling under equivalent fuel conditions to demonstrate the feasibility of using the cells in applications that require rapid start/stop cycles (with ramp rates <15 minutes). These experiments suggest that cell exposed to natural gas reformates operate and degrade in a similar manner to cells operating with pure H2 and have high tolerance to repeated thermal cycling. Cells operating with natural gas under internal reformate conditions experience faster degradation rates and the ability to improve the performance of these cells with a high entropy alloy catalyst will be discussed. Ongoing experiments will examine durability of the MS-SOFCs to reformate and natural gas fuels with sulfur contamination in order to determine the effect of carbon species present in the fuel stream on sulfur reactions at the electrode. This talk will give an overview of the current status of LBNL’s MS-SOFC technology operating with natural gas fuel and reformates, the effect of sulfur poisoning, resilience to thermal cycling, as well as determine future improvements required for improved cell performance and commercialization.