Long-Term Sulfur Tolerance of Solid Oxide Fuel Cells with Alternative Titanate-Based Fuel Electrodes

Abstract

The future renewable energy system relies on devices capable of energy conversion and storage. Solid oxide fuel cells (SOFCs) convert the chemical energy of a fuel into electricity and heat with high electrical efficiency. The fuel flexibility of SOFCs allows for the use of not only hydrogen, but also C-containing fuels, including biogas and green fuels from power-to-X. Such fuels contain trace amounts of sulfur, either being present from the original source or added for safety reasons as odorant, which causes loss of performance in state-of-the-art (SoA) SOCFs with Ni-based cermet fuel electrodes, due to sulfur adsorption on the Ni. An upstream desulfurization unit mitigates this problem in commercial SOFC-systems, increasing CAPEX and system complexity.Hence, alternative S-tolerant fuel electrode materials are highly desirable. All-ceramic doped strontium titanates perovskite compounds (ABO3) are interesting candidates due to their high electronic conductivity and chemical stability. Among these, La0.4Sr0.4Fe0.03Ni0.03Ti0.94O3 (LSFNT), is particularly interesting, due to its B-site exsolution of electrocatalytically active Fe and Ni nanoparticles under reducing conditions. However, the electrochemical activity from the exsoluted catalyst is not sufficient and integration of additional electrocatalyst by infiltration ensures electrochemical performance comparable to SoA.In this study, the electrochemical performance and sulfur tolerance of three different, electrolyte supported cells are examined: Cell 1 contains a standard Ni/CGO cermet fuel electrode, Cell 2 contains a LSFNT electrode backbone with Ni:CGO electrocatalyst and Cell 3 contains LSFNT electrode backbone with Fe-Ni:CGO electrocatalyst.. Fe is added to the Ni:CGO to investigate if alloy formation on the nanoscale can improve the sulfur tolerance. All the cells contain LSM-YSZ oxygen electrodes. The cells were integrated in a short, 5-cell stack and tested at HEXIS AG. The fuel was a natural gas directly supplied from the grid, pre-reformed in a catalytic partial oxidation (CPOx) unit, where a desulfurization unit was connected or by-passed for determining the sulfur tolerance. The electrochemical performance was investigated for each cell in the stack individually by iV-curves and electrochemical impedance spectroscopy (EIS) with and without the use of an upstream desulfurization unit. This was followed by a durability test for ca. 400 hours with sulfur exposure. While the initial performances with short term exposure to sulfur indicate reversible degradation on both types of infiltrated LSFNT fuel electrodes, Cell 1 with the Ni/CGO cermet fuel electrode experiences loss of performance related to an increase in the Ohmic resistance. Cell 1 together with Cell 2 (LSFNT infiltrated with Ni:CGO) are both stable over several hundred hours of operation in sulfur, whereas accelerated degradation of cell voltage is observed on Cell 3 where Fe is present in the infiltrate. Details on electrochemical behavior during the durability test and origins of degradation will be presented together with a microstructural analysis. Figure 1