AN EXPERIMENTAL STUDY OF PEROVSKITE-STRUCTURED MIXED IONIC- ELECTRONIC CONDUCTING OXIDES AND MEMBRANES

Date of Award

May 2014

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical and Aerospace Engineering

Advisor(s)

Jeongmin Ahn

Second Advisor

Dacheng Ren

Keywords

Air separation, Ceramics, Composite oxides, Methane oxidation, Oxygen permeation membrane, Perovskite oxides

Subject Categories

Engineering

Abstract

In recent decades, ceramic membranes based on mixed ionic and electronic conducting (MIEC) perovskite-structured oxides have received many attentions for their applications for air separation, or as a membrane reactor for methane oxidation. While numerous perovskite oxide materials have been explored over the past two decades; there are hardly any materials with sufficient practical economic value and performance for large scale applications, which justifies continuing the search for new materials. The main purposes of this thesis study are: (1) develop several novel SrCoO3-δ based MIEC oxides, SrCo1-xMxO3-, based on which membranes exhibit excellent oxygen permeability; (2) investigate the significant effects of the species and concentration of the dopants M (metal ions with fixed valences) on the various properties of these membranes; (3) investigate the significant effects of sintering temperature on the microstructures and performance of oxygen permeation membranes; and (4) study the performance of oxygen permeation membranes as a membrane reactor for methane combustion.

To stabilize the cubic phase structure of the SrCoO3-δ oxide, various amounts of scandium was doped into the B-site of SrCoO3-δ to form a series of new perovskite oxides, SrScxCo1-xO3- (SSCx, x = 0-0.7). The significant effects of scandium-doping concentration on the phase structure, electrical conductivity, sintering performance, thermal and structural stability, cathode performance, and oxygen permeation performance of the SSCx membranes, were systematically studied. Also for a more in-depth understanding, the rate determination steps for the oxygen transport process through the membranes were clarified by theoretical and experimental investigation. It was found that only a minor amount of scandium (5 mol%) doping into the B-site of SrCoO3-δ can effectively stabilize the cubic phase structure, and thus significantly improve the electrical conductivity and oxygen permeability of the SrCoO3-δ membrane. Among all the disk-shaped SSCx (x = 0-0.7) membranes with a thickness of 0.91 mm, both SSC0.05 and SSC0.1 exhibit the highest oxygen permeation rate of about 3.2 mL.cm-2.min-1 (STP) at 900 oC, SSC0.1 also shows excellent cathode performance for a solid oxide fuel cell. Therefore SSC0.1 is of special interest, and thus investigated regarding the performance as a membrane reactor for methane combustion. The performance was evaluated based on the results of methane conversion rates and CO2 selectivity.

Inspired by the above findings, a series of mixed-conducting perovskite oxides SrCo0.95M0.05O3- (SCM, M = Bi5+, Zr4+, Ce4+, Sc3+, La3+, Y3+, Al3+ , Zn2+) were prepared to study the effects of different dopants M on the performance of SrCo0.95M0.05O3-. It was found that the M cations significantly affect the crystal phase structure, grain growth, membrane porosity, electrical conductivity, and the oxygen permeability of the SCM membranes. Specifically, it is postulated in this study that the formation of the cubic perovskite structure is dependent on the electron configuration in the outer orbits of M cations, which may provide theoretical guidance for future development of high oxygen permeation ceramic membranes based on the perovskite materials.

To study the significant effects of grain sizes on the oxygen permeation behaviors of La0.6Sr0.4Co0.2Fe0.8O3- (LSCF) and SrSc0.1Co0.9O3-δ (SSC0.1) membranes, the LSCF and SSC0.1 membranes were sintered at various temperatures to form different microstructures. Properties of these membranes with varied grain sizes were compared. Results showed that the oxygen permeation rate of the LSCF membrane increases with increasing the grain size, however, it is interesting that the oxygen permeation rate of the SSC0.1 membrane decreases with increasing the grain size. This implies that oxygen transport occurs more, however, less rapidly along grain boundaries than through the bulks in the LSCF and SSC0.1 membranes, respectively.

A LSCF hollow fiber membrane and a SSC0.1 planar membrane were applied as membrane reactors for methane combustion. To improve their performances, LSCF powder and SSC0.1 powder were dip-coated and spray-coated on the permeation sides of LSCF hollow fiber membranes and SSC0.1 planar membranes, respectively. The exhaust gas components were analyzed by Gas Chromatography (GC). The performance was evaluated based on the results of methane conversion rates and CO2 selectivity. The highest CO2 selectivity of the LSCF hollow fiber membrane and the SSC0.1 planar membrane is about 88 and 85 %, respectively. This indicates that the application of an oxygen permeation membrane as methane combustion reactor is feasible.

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