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Title: Chromium poisoning of cathodes in the SOFC
Author: Lee, Soo-na
ISNI:       0000 0004 5348 5427
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2014
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Mixed Ionic Electronic Conductors (MIECs) such as La0.6Sr0.4Co0.2Fe0.8O3δ (LSCF) and La2NiO4+δ (LNO) are gaining much attention as candidate cathode materials at reduced working temperatures (600-800°C), because they exhibit better oxygen transport properties, in comparison to earlier cathode materials such as LaxSr1xMnyO3δ(LSM). The main objective of the study in this thesis is establishing the relationship between the amount of chromium and performance degradation of LSCF and Sr-free LNO cathodes, and improving understanding of the mechanism. LSCF were screen printed onto Ce0.9Gd0.1O1.95 (CGO10) electrolyte pellets and infiltrated with chromium nitrate solutions to different Cr levels up to 2wt%. Electrochemical impedance spectroscopy at 500 ~ 800°C showed that even very low levels of Cr give a significant increase in cathode polarisation resistance, which increases with Cr concentration. The impedance response was analysed using the model of Adler, Lane and Steele (ALS model) to extract oxygen self-diffusion (Do) and surface exchange (ko) parameters for the LSCF. The results show that Cr reduces both Do and ko, the latter being the more affected. However, the activation energies for polarisation resistance, Do and ko are not significantly affected by Cr content. This indicates that the Cr poisoning mechanism involves the de-activation of sites for oxygen exchange on the LSCF surface and that the cathode's residual activity is by means of remaining active sites. The surface reaction and diffusion of oxygen in dense LSCF was studied by oxygen isotope exchange using bulk specimens and depth profile analysis. The specimen surfaces were coated with different thicknesses of Cr2O3 by sputtering prior to exchange. The results showed that the Cr2O3 surface layer had a large inhibiting effect on ko, in agreement with the electrochemical measurements, but a negligible effect on Do because diffusion of Cr into the bulk was very slow. The effect of Cr on Sr-free LNO electrodes deposited symmetrically onto Ce0.9Gd0.1O1.95 (CGO10) electrolytes by screen printing was studied in a similar way. XRD of LNO/Cr2O3 powder mixtures annealed at 1000°C for 24h showed that LNO reacts readily with Cr2O3 to form LaCrO3 and LaNiO3. Despite this reactivity, the electrochemical experiments show that the polarisation resistance of LNO cathodes is insensitive to Cr introduced by solution infiltration for Cr concentrations up to 1%. For concentrations above this level the Cr increases the polarisation resistance, but to a much lesser degree in comparison to LSCF. Cr reduces the rate of the oxygen reduction reaction on the LNO surfaces, but does not change the activation energy significantly. The relatively good tolerance of LNO cathodes to Cr ingress is probably due to the reaction products themselves having some useful catalytic activity for oxygen reduction.
Supervisor: Atkinson, Alan; Kilner, John Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral