Performance of Metal Supported SOFCs Operated in Hydrocarbon Fuels and at Low (<650˚C) Temperatures

Abstract

Solid oxide cells (SOCs) are a key enabling technology for the required future renewable energy systems by providing an efficient link between the power, gas, and heat sectors. Specifically, solid oxide fuel cells (SOFCs) convert the chemical energy of a fuel into electricity and heat with high electrical efficiencies. Hydrogen is the obvious choice of fuel due to the only emissions being steam, however hydrocarbon fuels are an interesting and safe alternative to hydrogen, especially for mobile applications, facilitating easy integration into existing infrastructure. The state-of-the-art (SoA) Ni:YSZ (yttria-stabilized zirconia) composite fuel electrode is capable of direct oxidation of hydrocarbons or of reforming of hydrocarbon gas mixtures. Unfortunately, the material that facilitates the above reactions, nickel, is also an excellent carbon deposition catalyst. This is problematic, due to the associated volumetric changes and reduction in electrochemically active sites. Ultimately, the fuel electrode can delaminate from the electrolyte, causing a catastrophic failure of the cell. Alternative fuel electrode materials are the key to avoid this. A-site deficient lanthanum doped strontium titanate (La0.4Sr0.4Fe0.03Ni0.03Ti0.94O3, LSFNT) based anodes, coated with Ni:CGO (gadolinium-doped ceria) electrocatalyst are a promising alternative - while avoiding Ni as a major part of the backbone, the infiltrated Ni ensures electrocatalytic activity. The co-infiltrated CGO serves to improve the ionic conductivity. Integrating these anodes into a metal supported (MS) design ensures lower cost and mechanical robustness. Full cells are manufactured through scalable tape casting and screen printing technology, including the metal support. Possible pitfalls for the MS-cell include vulnerability to corrosion of the metal support and decreased reforming activity due to less electrocatalyst (Ni) compared to the SoA-cell. In this study, the reforming activity of methane in 3 different SOFCs is investigated: Cell 1 is with a SoA Ni:YSZ anode, cell 2 is with a LSFNT anode and cell 3 is with a LSFNT-FeCr anode. The investigation involves electrochemical characterization by impedance spectroscopy (EIS) and performance under applied current (I/V-curves) at temperatures 750˚C, 700˚C, 650˚C and 620˚C, so-called fingerprints, with special emphasis on the anode activity for reforming and water gas shift reaction. Furthermore, galvanostatic durability tests are conducted for the MS-cells (cell 2 and cell 3) with focus on the durability at low temperature (650˚C) and corrosion resistance at medium to high fuel utilization (ca. 50%). The degradation of the tested cells is examined by comparing the electrochemical performance before and after durability testing.