Characterization and optimization of new fluorite-related oxide ion conductors

Lead Research Organisation: Imperial College London
Department Name: Materials


It is commonly assumed that solids which conduct electricity do so via the movement of charged electrons under an applied electric field, and electrically conducting liquids require the movement of charged ions. However, this is not always the case. Liquid mercury provides, perhaps, the best example of a liquid showing metallic (electronic) conduction, and there are a variety of solids whose conductivity is primarily due to ionic migration. Such solids may therefore show similar properties, and applications, as liquid electrolytes. An important application for solids which conduct via the migration of O2- ions is as electrolytes for high temperatures solid oxide fuel cells, SOFCs, where the electrolyte separates the active electrode materials, which may simply be O2 and H2. Other applications include O2- conducting membranes which can, for example, be utilised for the production of pure oxygen from impure sources, typically air, by passing a current through the membrane. Such devices could be used for large scale oxygen production or in portable devices, e.g. for medical purposes. For efficiency and technological stability, low temperature operation is a requirement, and the need for new materials which show high O2- conductivity at low temperatures provides the stimulus for this proposal. We have made an important observation that certain cations, when partially substituting Bi in bismuth oxide, produce what appear to be the best low temperature isotropic oxide ion conductors (i.e. the conductivity is independent of direction). It is now vital that we fully characterise these materials in order to:i optimise their properties;ii fully check (and possibly improve) their stability to long term usage at low temperatures;iii explore their potential for real applications;iv explore the possible extension of our observation to other systems.In particular, we need to explore the detailed structure of the materials we have already synthesised, especially the local structure around the ions substituted into the bismuth oxide framework, and the surface properties which are important for applications in real devices. We also need to have a better understanding of the mechanism applicable to the O2- migration in these materials, in order to rationalise the improved conductivity observed. This objective cannot be achieved experimentally, but requires the use of theoretical modelling. The proposal therefore will bring together four high quality research groups from different research centres, each of which will provide unique, internationally leading expertise in the areas necessary to achieve the objectives:Birmingham / synthesis, structure evaluation;Imperial College / surface properties and O2- ion diffusion parameters;Sheffield / conductivity measurements under variable atmospheric conditions;Bath / theoretical simulations to explore the mechanism of O2- migration and provide information on other possible methods to achieve similar, or better, properties.


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Kotsantonis S (2011) Oxygen ion diffusion measurements in Bi0.775La0.225O1.5 in Solid State Ionics

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N/a Kotsantonis (2010) Secondary ion instabilities in the SIMS analysis of Bi2O3 based materials in Surface and Interface analysis

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Perkins J (2010) Anomalous Oxidation States in Multilayers for Fuel Cell Applications in Advanced Functional Materials

Description The high temperature face centered cubic (fcc) phase of bismuth oxide (delta-Bi2O3) has the highest oxide ion conductivity of all oxide ion conductors known so far, one to two orders of magnitude higher than that of stabilized zirconia at the corresponding temperatures. Despite the fact that the stabilised bismuth oxide exhibits very high ionic conductivity, its use as an electrolyte in SOFCs is prevented by its instability against reduction at low oxygen partial pressures. At high temperatures, solid electrolytes based on bismuth oxide tend to reduce towards metallic Bi under low oxygen pressure, decreasing the ionic conductivity and increasing the electronic. Despite this fact, Bi2O3 still serves as a model material for understanding fluorite structures and highly defective materials. The main focus of this research was to measure transport data for pure Bi2O3, to characterise Bi-based doped materials, and to investigate the possibility of stabilising other Bi2O3 phases.

For the first time in the literature we managed to obtain information for the diffusion of oxygen in undoped Bi2O3. Specifically we:

i) Determined the oxygen ion diffusivity in both the low and high temperature polymorphs (alpha and delta phase) of Bi2O3

ii) Obtained new results about the oxygen surface exchange coefficient of Bi2O3. The material appears to have a catalytic behaviour towards the dissociative adsorption of oxygen, suggesting the possibility of electrolyte materials without the need for a cathode as a catalyst.

iii) Confirmed that the conductivity in the delta-phase is purely ionic by comparing SIMS data with total conductivity data from AC Impedance

For the fluorite related doped bismuth based materials,e.g. the Rhenium substituted bismuth oxides, their high conductivity and its pure ionic character was confirmed by means of Secondary Ion Mass Spectrometry. Unfortunately, investigation of their long term stability, showed that these materials tend to transform to more stable phases with prolonged annealing, a phenomenon followed by a decrease in their conductivity, making them unsuitable for practical applications.

Bi2O3 can also be stabilized in structures other than the fcc phase, depending on the dopant and its concentration, that are also interesting becasue of their high conductivity. One of these phases is the rhombohedral phase in the Bi-La-O system. We investigated this family of materials and derived oxygen diffusion data. Their oxygen ion diffusion appears to be high, almost 2 orders of magnitude higher than YSZ at 500C. The other important finding for these materials is that they appear to be chemically stable, with no transformations or decays of conductivity observed after prolonged ageing and, as such, are suitable candidates for practical applications.
Exploitation Route Development of new electrolytes using Bi2O3 based materials by others working on thin film materials for low temperature fuel cells
Sectors Energy