Understanding and Preventing Ni Oxidation in Ni-YSZ Anode-Supported SOFCs

Date
2014-04-17
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Abstract
Ni-yttria-stabilized zirconia (YSZ) anode-supported cell architectures are the current state-of-the-art in solid oxide fuel cells (SOFCs), but their lifetime is too short due to sulphur (S) poisoning, coking and deleterious Ni oxidation. The aim of this work was to understand the mechanisms by which Ni oxidation (‘redox cycling’) damages the cell and develop solutions to this problem. It was shown that degradation due to air exposure increases as the temperature was increased from 600 to 900˚C and as pO2 was decreased. The severity of degradation was correlated to the steepness of the NiO gradient in the anode layer. It was also shown that when oxidation is homogeneous in the anode, some NiO particles were pushed out of the air-anode interface. A pre-oxidation cycle, aimed at intentionally forcing NiO out onto the surface, increased the oxidation tolerance at 600˚C from 74% to 80%, but the tolerance at 800˚C was not changed by the process, although the severity of the electrolyte degradation was lowered. Raman spectroscopy was used to quantitatively map the NiO gradient across partially oxidized anodes. It is shown that this technique can quickly and accurately map the NiO distribution through the thickness of a Ni-YSZ anode layer and even showed 20% NiO content in regions of the anode that were not expected to contain any NiO. A reverse bias (a negative bias applied to the Ni-YSZ anode) technique was also used during air exposure to attempt to prevent Ni oxidation. Ex situ microscopy showed that Ni adjacent to the electrolyte is protected preferentially. A new technique, in operando TGA, that allows electrochemical evaluation while concurrently monitoring cell mass was developed. It was shown that the application of > -1.5 V or > -13 mA∙cm-2 to a Ni-YSZ electrode during air exposure (800˚C) can prevent a large fraction of Ni oxidation. It was also demonstrated with in operando TGA that S-define could be removed by an air exposure with a reverse bias applied, although it may be necessary to oxidize the Ni surface in the region of the electrolyte to fully remove the S.
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Chemistry--Physical
Citation
Young, J. L. (2014). Understanding and Preventing Ni Oxidation in Ni-YSZ Anode-Supported SOFCs (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca. doi:10.11575/PRISM/28557