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Title: Electrochemical performance and transport properties of La2NiO4+σ
Author: Sayers, Ruth
ISNI:       0000 0004 2688 3331
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2010
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Oxygen excess lanthanum nickelate, La2NiO4+δ (LNO), is a candidate cathode material for intermediate temperature solid oxide fuel cells (IT-SOFCs). The aim of this work is to investigate the properties of LNO in the intermediate temperature regime (500 – 700°C). The structure and stability of LNO has been studied by in-situ high resolution synchrotron x-ray diffraction and thermal analysis. A bi-phasic orthorhombic room temperature structure was identified, which undergoes a transition to a tetragonal phase. The phase change occurs over the temperature range 250°C to 450°C and is associated with loss of oxygen on heating. LNO undergoes an oxidation reaction, catalysed by platinum, above 800°C where it begins to form the higher order Ruddlesden-Popper phases, La3Ni2O7-δ and La4Ni3O10-δ. The oxygen ion transport properties of LNO have been studied by determining the oxygen tracer diffusion and surface exchange coefficients (D* and k*, respectively). LNO displays high D* and reasonable k* values and exhibits low activation energies for these processes (0.54eV and 0.63eV, respectively). The low activation energy for diffusion is associated with a high oxygen interstitial concentration between 350°C – 550°C. The compatibility of LNO with IT-SOFC electrolytes was investigated using high resolution x-ray synchrotron diffraction techniques. The stability of composites of LNO with Ce0.9Gd0.1O2-δ was found to be highly dependent on oxygen partial pressure and temperature and no reaction phase was observed in composites exposed to atmospheric oxygen. Studying composites in-situ revealed a series of reaction processes that have not previously been identified from ex-situ diffraction techniques. The performance of LNO as a cathode was studied by AC impedance of symmetrical cells with Ce0.9Gd0.1O2-δ and La0.8Sr0.2Ga0.8Mg0.2O3-δ electrolytes. Significant enhancement of the cathode performance was achieved by the addition of a thin compact layer of LNO at the electrode/electrolyte boundary; an area specific resistance (ASR) of 0.5 Ω.cm2 was measured at 800°C in a symmetrical cell with this layered structure. The decrease in ASR is believed to be a result of improved contact at the electrolyte/cathode boundary enhancing the oxygen ion transfer to the electrolyte, and an increase in the cathode surface area for the oxygen reduction reaction to occur.
Supervisor: Kilner, John Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral