New cathodes for intermediate temperature solid oxide fuel cells (IT-SOFCs)

Hodgeman, Darren
(2014) New cathodes for intermediate temperature solid oxide fuel cells (IT-SOFCs). Doctor of Philosophy thesis, University of Liverpool.

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The work presented in this thesis focuses on the synthesis of long axis A-site ordered perovskites, which have ordered oxygen vacancies. The materials discussed have also been assessed as potential cathodes for solid oxide fuel cells (SOFCs), targeting an intermediate temperature (IT) SOFC operating range of 500 800 °C. In chapter 3 of this thesis, a 16ap phase (ap = 1 perovskite unit, ABO3-δ), Y2.24Ba2.28Ca3.48Fe7.44Cu0.56O21-δ, was first observed in a powder X-ray diffraction (PXRD) pattern that resulted from the attempted Cu doping of a 10ap phase (Y0.9Ba1.7Ca2.4Fe5 xCuxO13 δ). This phase was indexed as ap √2 × 16ap × ap √2 by selective area electron diffraction (SAED). Mössbauer spectroscopy identified that Fe3+ existed in three different coordination environments and a Rietveld refinement was carried out using combined powder synchrotron (S) XRD and powder neutron diffraction (ND) data. High Angle Annular Scanning Transmission Electron Diffraction (HAADF-STEM) confirmed the A-site ordering from the refinement. The 16ap phase exhibited good thermal stability, CO2 stability and chemical compatibility with state of the art electrolytes (GDC, SDC and LSGM), as well as a close matching thermal expansion coefficient (TEC) with the same electrolytes. Although possessing low electronic conductivity, 3.5 at 750 °C, a good area specific resistance (ASR) of 0.12 Ω.cm2 at 750 °C was achieved, within the IT-SOFC operating range. In chapter 4, a 10ap phase, Y0.9Ca2.4Sr1.7Fe5O13-δ (YCSFO), was discussed. The space group Imma was identified from SXRD data, while the structure was closely related to brownmillerite. A Rietveld refinement carried out with the addition of SXRD data collected at the K absorption edge for Sr determined A-site ordering. The ASR of YCSFO was three times higher than that of the 16ap phase, highlighting the ordering of oxygen vacancies. The final phases investigated in chapter 5 of this thesis belong to a family of Co doped 10ap phases (Y0.9Ba1.7Ca2.4Fe5 xCoxO13 δ). A range of compositions were synthesised by varying the cation ratios, in order to obtain high Co content phase pure samples. The highest Co content was x = 1.85 for compositions Y1.24Ba1.85Ca1.91Fe3.15Co1.85O13-δ (annealed in O2) and Y1.6Ba1.8Ca1.6Fe3.15Co1.85O13-δ. AC impedance measurements carried out showed that increased Co content reduced the ASR, with the values at 700 °C of 0.19 Ω.cm2 and 0.23 Ω.cm2 respectively.

Item Type: Thesis (Doctor of Philosophy)
Additional Information: alt_title: Isolation of A-Site ordered perovskites for potential application as solid oxide fuel cell cathodes Date: 2014-04 (completed)
Subjects: ?? QD ??
Divisions: Faculty of Science and Engineering > School of Physical Sciences
Depositing User: Symplectic Admin
Date Deposited: 26 Aug 2014 11:06
Last Modified: 12 Mar 2021 12:10
DOI: 10.17638/00018675