Electrochemical performance and transport properties of La2NiO4+σ
Author(s)
Sayers, Ruth
Type
Thesis
Abstract
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.
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.
Date Issued
2010
Date Awarded
2010-06
Copyright Statement
Attribution NoDerivatives 4.0 International Licence (CC BY-ND)
Advisor
Kilner, John
Creator
Sayers, Ruth
Publisher Department
Materials
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)