Oxygen absorption in hexagonal manganites

Abstract

Oxygen is one of the worlds most used chemicals, with over 100 million tons of pure O2 being used for industrial and medical purposes each year. The demand for O2 is high, and expected to rise even further due to its possible application within green technologies such as oxyfuel combustion for CO2 capture. Today, most of the industrial oxygen gas is produced through cryogenic distillation, which is an energy intensive and expensive air separation method. Alternative methods for large-scale O2 separation from air are therefore highly desired, e.g. by oxygen permeable ceramic membranes or chemical looping air separation (CLAS). These methods can possibly reduce the energy cost of O2 production by up to 35 %, and can in addition be easily integrated into existing coal fired power plants for oxyfuel combustion. Both ceramic membranes and CLAS require a material that can transport O2 at elevated temperatures. Perovskites have been in the spotlight for these applications in the last decades, but these materials have problems with low thermal and chemical stability, and often require operating temperatures as high as 800-1000 °C.

Hexagonal manganites RMnO3 (R = Sc, Y, Ho-Lu) have emerged as possible oxygen storage materials (OSMs) due to their ability to store and release large amounts of oxygen at temperatures as low as 300 °C. Due to the hexagonal P63cm crystal structure being less dense than the perovskite structure, hexagonal manganites can incorporate oxygen as interstitials, making them fundamentally different from other OSMs which typically use oxygen vacancies for transport. As hexagonal manganites are quite new in the OSM research field, there are still many unknowns when it comes to how the properties of the materials can be improved. In this work, we wanted to understand more of how the oxygen storage properties of hexagonal manganites can be tailored by changes in composition, both on the R3+ site and with aliovalent doping on the Mn4+ site.

In the first part of the work, the oxygen absorption properties of RMn1-xTixO3 (R = Y, Ho, Dy; x = 0, 0.15) were investigated. The materials were studied using thermogravimetric analysis (TGA) and high-temperature X-ray diffraction (HT-XRD), showing larger oxygen storage capacities for samples with smaller crystallites, larger R cations, and with added Ti. For the last two, the increased oxygen absorption was believed to be the result of the expanded ab-plane found in these materials. Ti-doping also increased the thermal stability of interstitial oxygen. Density functional theory (DFT) calculations were used to compare the trends observed experimentally. In addition, a thermodynamic model for enthalpy and entropy of oxidation was fitted to the experimental data.

In the second part of the thesis, the kinetics of oxidation of RMn1-xTixO3 (R = Ho, Dy; x = 0, 0.15) was investigated, as the Ti-doped compositions from Manuscript I showed indications of increased oxidation rates. The materials were studied using TGA with different heating and cooling rates, and time resolved X-ray absorption near edge structure (XANES) and HT-XRD with in situ switching of atmosphere from N2 to O2. It was showed that the oxidation kinetics increase with larger R, and with the addition of Ti, as the expanded ab-plane lessens the electrostatic repulsion between interstitial oxygen and the planar oxygen surrounding it.

In the third part of the thesis, the oxidation properties of high entropy hexagonal manganites with different compositions and particle sizes were examined, using the same experimental methods as in Manuscript I and II. The high compositional entropy stabilized formation of a single hexagonal phase, even for large R cations and high tolerance factors. All high entropy compositions displayed high oxygen storage capacities, with bulk materials showing increased oxygen absorption compared to single R cation bulk materials measured using fast cooling rates. The oxidation kinetics were also improved compared to low entropy hexagonal manganites, due to the high configurational entropy improving ionic conductivity, and due to the large R cations expanding the ab-plane promoting oxygen transport.

In addition to the work presented in the attached manuscripts, this thesis includes work on preparation of an asymmetric oxygen permeable membrane using [Ho0.98In0.02]0.97Mn0.85Ti0.15O3 as the membrane material. The membrane film was deposited on top of porous Y1.05Mn0.85Ti0.15O3 substrates using spray coating, giving films with thicknesses ranging from 5 to 30 µm. All the prepared membranes showed problems with surface cracking, originating from stresses formed in the film during constrained sintering, and from the anisotropic thermal expansion of the material, which results in crack formation during cooling. The aim of the project was therefore shifted towards material development of hexagonal manganites for oxygen production methods that use OSMs in powder form, such as CLAS.