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Modulated Composite Structures: A 3D Bulk Approach to Incoherent Hetero-Structure Interface Engineering in Oxides for Future Low Temperature Energy Applications

Periodic Reporting for period 2 - MODCOMS (Modulated Composite Structures: A 3D Bulk Approach to Incoherent Hetero-Structure Interface Engineering in Oxides for Future Low Temperature Energy Applications)

Reporting period: 2019-02-05 to 2020-02-04

Current technology that requires fast oxygen ionic transport at lower temperatures such as solid oxide fuel cells, exhaust gas cleaning heterogeneous catalysts, gas sensors, and gas separation are limited by the current approaches used to achieve improved performance. Some of these approaches involve expensive and time consuming thin film methods, nano-sized particles and combining separate phases. Despite success in reaching lower temperatures < 200C, most of the methods are not stable long term and will degrade with time.

The impact on society is largely a delay in new technology which depends on fast oxygen ionic conduction at lower temperatures, such as solid oxide fuel cells, where temperatures > 600C-700C is commonly required to start the oxygen ionic conductivity and the surface reactions at the electrode where oxygen is reduced.

The high temperature brings about problems such as degradation of the material, and reaction with components, but also the need to use expensive technical solutions that can withstand high temperatures instead of e.g. steel.

For both catalysts, sensors and gas separation membranes the material has to withstand large variations in oxygen partial pressures, and be able to absorb and release oxygen in response to variations in the oxygen partial pressure.

The heterogeneous catalysts in vehicles are important for the removal of fine particulate matter and e.g. NOx gases to decrease the number of deaths related to bad air quality in cities with heavy traffic. The oxygen gas sensors are used in e.g. combustion engines or the manufacturing of steel to precisely control the oxygen content required.

In gas separation membranes one of the important factors to maximize is the total amount of oxygen that can be absorbed and releases, but also the rate. Shifting between two separate temperatures where the material absorbs and desorbs the oxygen can be an efficient way of separating oxygen without expensive cryogenic distillation methods that will, in addition, leave contamination of other gases due to difficulty of separating gases with similar boiling points. Oxygen is the second-largest volume industrial gas and is used in many industries such as the steel and metals manufacturing industry, petrochemical industry for production of different hydrocarbons etc.

These are just a couple of important societal challenges where such materials will be beneficial for novel energy technologies at lower temperatures, air quality and more energy efficient production of oxygen gas.
In the YMnO3 type oxides, a novel reversible phase transition takes place under high oxygen partial pressures at 150-400C where oxygen is absorbed and reversibly desorbed in reducing atmospheres or high temperature.
Since the material is taking up an excess amount of oxygen, the oxygen ions are assumed to be located at interstitial sites in the structure. From theoretical calculations their mobility is expected to be high in the structure and will therefore be of interest for e.g. low temperature SOFCs. To find additional efficient systems for low temperatures it is therefore necessary to first understand the YMnO3 system in detail. One step is to understand the driving forces behind the phase transition.

The overall objectives are therefore to:

1) Synthesize compositions that work under atmospheric conditions and study how the average crystal structure and oxidation states change vs. temperature and cation composition.
2) Acquire detailed average and local structural, chemical, kinetic and transport properties for the selected compositions at different stages in the phase transition.
3) Model bulk systems and interfaces to link the basic theoretical models with experimental data to design and improve OSC, kinetics and temperature for such materials.
+Materials based on the YMnO3 type oxides have been synthesized with different additional cations by standard solid state synthesis
+Materials analyzed by both in-house and synchrotron x-ray powder diffraction, and thermogravimetry clearly shows that one of the cations significantly improves the rate at which oxygen is absorbed, thus changing the structure
+Synchrotron x-ray spectroscopy by XANES clearly shows the increase in oxidation state of the Mn cation after treatment in pure oxygen.
+Synchrotron x-ray spectroscopy by EXAFS clearly shows a difference between the cations in the materials before and after oxygen treatment
+Thermodynamic properties of the cation doped samples differ significantly from the undoped samples, and are supporting the theoretical results for the pure compound published recently
+Electronic conductivity was also measured in different atmospheres

-Unexpected problems of high porosity and too high melting point to achieve fully dense samples to complete electronic conductivity relaxation during the period, moreover, oxygen gas sensors were only functional above ~500C

+Samples were sent to a collaborator in Spain for TEM studies of the samples, in order to confirm the presence of a modulated composite structure in the samples with excess oxygen.
+The ER has achieved training in the use of Linux operative system and Ab-initio calculations through the VASP software using the Swedish national supercomputing resources.
+Initial scripts to calculate ground state energy of simple YMnO3 type oxide was made.

-Calculating transport energies for the MODCOMS were not made due to lack of time and the necessary information that is required from the TEM studies, which will hopefully be finished after the project time.
+Strong indications of two phases existing on a local scale
+Strong indications of a thermodynamic behaviour where interstitial oxygen ions are likely favoured compared to oxygen vacancies as shown for the pure YMnO3 only by computations previously

The results are expected to be connected together to give a clearer picture of the chemistry governing the phase transition. The theoretical models that will be tested, will if being successful, provide details about the effects on energetics of vacancy formation and migration energies govern the phase transition, and how it can be expanded to other systems with more abundant cations than lanthanides to facilitate an easier implementation of the material industrially.