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Mixed oxygen ionic and electron conducting perovskite oxides [Elektronische Ressource] : issues and possible solutions / Konstantin Efimov

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119 pages
Mixed oxygen ionic and electron conducting perovskite oxides: issues and possible solutions Von der Naturwissenschaftlichen Fakultät der Gottfried Wilhelm Leibniz Universität Hannover zur Erlangung des Grades Doktor der Naturwissenschaften Dr. rer. nat. genehmigte Dissertation von Dipl.-Chem. Konstantin Efimov geboren am 29. März 1980 in Iwanowo (Russland) 2011 Referent: Priv.-Doz. Dr. Armin Feldhoff Korreferent: Prof. Dr. Klaus-Dieter Becker Tag der Promotion: 29.09.2011 Abstract Alkaline-earth and cobalt-based perovskite oxides stand out in the family of mixed oxygen ionic and electron conducting (MIEC) materials as a result of their extraordinary transport properties. These cubic perovskites are renowned for their potential applications toward oxygen separating membranes used in membrane reactors, as well as for cathodes in solid-oxide fuel cells. However, their reliable use can be hindered by phase decomposition of the cubic perovskite structure at intermediate temperatures (773-1073 K), as well as poor chemical stability in the presence of CO . Within the 2scope of the presented thesis, both of these major issues were extensively studied. Seven original research articles were produced during the course of this work, including a discussion of how these problems can be solved. In chapter 2, the decomposition process of (Ba Sr )(Co Fe )O and 0.5 0.5 0.8 0.
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Mixed oxygen ionic and electron conducting
perovskite oxides:
issues and possible solutions


Von der Naturwissenschaftlichen Fakultät
der Gottfried Wilhelm Leibniz Universität Hannover
zur Erlangung des Grades


Doktor der Naturwissenschaften
Dr. rer. nat.


genehmigte Dissertation
von
Dipl.-Chem. Konstantin Efimov

geboren am 29. März 1980 in Iwanowo (Russland)

2011






















Referent: Priv.-Doz. Dr. Armin Feldhoff
Korreferent: Prof. Dr. Klaus-Dieter Becker
Tag der Promotion: 29.09.2011
Abstract
Alkaline-earth and cobalt-based perovskite oxides stand out in the family of mixed oxygen ionic and
electron conducting (MIEC) materials as a result of their extraordinary transport properties. These
cubic perovskites are renowned for their potential applications toward oxygen separating membranes
used in membrane reactors, as well as for cathodes in solid-oxide fuel cells. However, their reliable
use can be hindered by phase decomposition of the cubic perovskite structure at intermediate
temperatures (773-1073 K), as well as poor chemical stability in the presence of CO . Within the 2
scope of the presented thesis, both of these major issues were extensively studied. Seven original
research articles were produced during the course of this work, including a discussion of how these
problems can be solved.
In chapter 2, the decomposition process of (Ba Sr )(Co Fe )O and 0.5 0.5 0.8 0.2 3-δ
(Ba Sr )(Co Fe )O at temperatures below 1173 K was elucidated using powder X-ray 0.8 0.2 0.8 0.2 3-δ
diffraction (XRD) and various transmission electron microscopy (TEM) techniques. Transformation of
the cubic perovskite structure into hexagonal or hexagon-related perovskite phases was observed in
both systems. This process was driven by a combined valence and spin-state transition of cobalt
cations, leading to considerable diminution of their effective ionic radii. Large effective ionic radii are
not tolerated in cubic perovskite structures that contain large barium and strontium cations.
Hence, the cubic perovskite structure of alkaline earth-based perovskite at intermediate
temperatures can be stabilized by the substitution of cobalt with iron in the crystal lattice. This aspect
is discussed in chapter 3 via the introduction of a novel cobalt-free perovskite material
(Ba Sr )(Fe Cu )O . Apart from its excellent phase stability at relevant temperatures, the 0.5 0.5 0.8 0.2 3-δ
(Ba Sr )(Fe Cu )O membrane exhibits the highest oxygen permeation flux of known cobalt-free 0.5 0.5 0.8 0.2 3-δ
materials so far.
Chapter 4 summarizes the effect of CO on (Ba Sr )(Fe Zn )O and Ba(Co Fe Zr )O 2 0.5 0.5 0.8 0.2 3-δ x y z 3-δ
perovskites, which were developed as an alternative to cobaltites because of their enhanced phase
stability at intermediate temperatures and because of their desirable thermo-mechanical properties. It
was found that the oxygen permeation performance of both membrane materials broke down
completely in the presence of CO . Furthermore, the perovskites were partially decomposed into 2
carbonates and distorted perovskite phases after contact with CO , which was accompanied by the 2
segregation of zinc or cobalt, respectively, as confirmed through (in-situ) XRD and TEM investigation.
It was also discovered that the oxygen permeation and membrane microstructures were fully
recovered under CO -free conditions. 2
The assumption that calcium-containing, perovskite-like (La Ca )(Co Fe )O and 0.6 0.4 0.8 0.2 3-δ
(La Ca )FeO may provide CO -stable membrane materials, owing to the lower thermodynamic 0.6 0.4 3-δ 2
stability of calcium carbonate compared with barium- and strontium-carbonate, is discussed and finally
confirmed in chapter 5. An alternative approach to overcome CO -intolerance lies in the concept of 2
alkaline-earth free dual-phase membranes. This concept is also presented in chapter 5. Membranes
containing 40 wt. % NiFe O , as the electronic conductor, and 60 wt. % Ce Gd O , as the 2 4 0.9 0.1 2-δ
electrolyte, can be considered promising materials for applications that require the presence of CO . 2

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Keywords: mixed ionic and electron conductor, oxygen transporting membrane, perovskite

ii


Zusammenfassung
Oxide mit Perowskitstruktur, die Erdalkalimetalle sowie Cobalt enthalten, ragen aufgrund ihrer
außergewöhnlichen Transport-Eigenschaften in der Klasse der gemischt Sauerstoff-Ionen und
elektronenleitenden Materialien heraus. Allerdings wird ihre großtechnische Anwendung, z.B. in
Membranreaktoren oder als Kathode in Fest-Oxid Brennstoffzellen, erschwert, da sich die kubischen
Perowskit Strukturen im mittleren Temperaturbereich (773-1073 K) zersetzen. Desweiteren zeigen
Erdalkalimetall-haltige Oxide eine schlechte chemische Stabilität gegenüber CO . Im Rahmen der 2
vorgelegten Dissertation werden die genannten Problemstellungen sowie mögliche Lösungsansätze
intensiv diskutiert, woraus sieben Forschungsarbeiten resultierten.
In Kapitel 2 wird der Zersetzungsprozess, der unterhalb von 1173 K eintritt, von
(Ba Sr )(Co Fe )O und (Ba Sr )(Co Fe )O mit Hilfe der Röntgenpulverdiffraktometrie 0.5 0.5 0.8 0.2 3-δ 0.8 0.2 0.8 0.2 3-δ
(XRD) und der Transmissionselektronenmikroskopie (TEM) untersucht. In beiden Systemen findet
eine Umwandlung in hexagonale oder hexagonal verzerrte Phasen statt. Triebkraft dieser
Umwandlung ist ein gekoppelter Valenz- und Spinübergang der Cobaltkationen, wodurch der
Ionenradius verringert wird, was durch die kubische Perowskitstruktur nicht toleriert wird.
Die kubische Perowskitstruktur kann im mittleren Temperaturbereich stabilisiert werden, indem
Cobalt durch Eisen ersetzt wird. Dies wird in Kapitel 3 anhand des neuen Cobalt-freien
Perowskitmaterials (Ba Sr )(Fe Cu )O gezeigt. Neben exzellenter Stabilität im relevanten 0.5 0.5 0.8 0.2 3-δ
Temperaturbereich, weist die (Ba Sr )(Fe Cu )O Membran den höchsten Sauerstofffluss aller 0.5 0.5 0.8 0.2 3-δ
bekannten Cobalt-freien Materialien auf.
Kapitel 4 behandelt den Einfluss von CO auf die Perowskite (Ba Sr )(Fe Zn )O und 2 0.5 0.5 0.8 0.2 3-δ
Ba(Co Fe Zr )O . Diese wurden als Alternative zu Cobaltiten entwickelt, da sie erhöhte x y z 3-δ
Phasenstabilität im mittleren Temperaturbereich sowie verbesserte thermomechanische
Eigenschaften besitzen. In Anwesenheit von CO jedoch brach die Sauerstoffionenleitfähigkeit beider 2
Membranmaterialien komplett ein. (In-situ) XRD sowie TEM Untersuchungen zeigten, dass sich die
Perowskite unter CO -Einwirkung teilweise in Carbonate und verzerrte Perowskitphasen zersetzten. 2
Unter CO -freien Bedingungen wurden die Mikrostruktur und die Sauerstoffionenleitfähigkeit beider 2
Membranen vollständig regeneriert.
In Kapitel 5 wird die Annahme bestätigt, dass die Calcium-haltigen Perowskite
(La Ca )(Co Fe )O und (La Ca )FeO CO -stabil sind, was auf die schlechtere 0.6 0.4 0.8 0.2 3-δ 0.6 0.4 3-δ 2
thermodynamische Stabilität von Calciumcarbonat gegenüber Barium- und Strontiumcarbonat
zurückgeführt werden kann. In diesem Kapitel wird zusätzlich das Konzept Erdalkali-freier Dual-
Phasen Membranen vorgestellt, um dem Problem der CO -Intoleranz zu begegnen. Das Material mit 2
der neuen Zusammensetzung aus 40 Gew.-% NiFe O (Elektronenleiter) und 60 Gew.-% 2 4
Ce Gd O (Elektrolyt) kann in Anwendungen unter CO -haltiger Atmosphäre als besonderes 0.9 0.1 2-δ 2
vielversprechend angesehen werden.

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Schlagwörter: gemischt Ionen- und Elektronenleiter, Sauerstofftransportierende Membran, Perowskit

iv


Preface

The presented thesis summarizes the results, which were achieved in the past three years during
my work as scientific co-worker at the Institut für Physikalische Chemie und Elektrochemie of the
Gottfried Wilhelm Leibniz Universität Hannover. Financial support for this work was granted by the
department chair Prof. Dr. Jürgen Caro, by the Deutsche Forschungsgemeinschaft (DFG, Grant FE
928/ 1-2), as well as from the State of Lower Saxony (Germany, NTH bottom-up project, No. 21-
71023-25-7/09) under the guidance of Priv. Doz. Dr. Armin Feldhoff.

Seven selected research articles are presented within this thesis; I am the first author in four of
these papers. The following statements assign my contributions to the articles included in this thesis.
For all articles, I greatly acknowledge the beneficial encouragement of my co-authors, in particular
from Priv. Doz. Dr. Armin Feldhoff and Prof. Dr. Jürgen Caro.
The two articles presented in chapter 2 deal with the inherent phase instability of
(Ba Sr )(Co Fe )O (BSCF) and (Ba Sr )(Co Fe )O perovskites at temperatures below 0.5 0.5 0.8 0.2 3-δ 0.8 0.2 0.8 0.2 3-δ
1173 K. In the first article, entitled TEM study of BSCF perovskite decomposition at intermediate
temperatures, experimental investigations and data interpretation were carried out by me. I appreciate
the collaboration of Dr. Qiang Xu from TU Delft (Netherlands), who provided me with the opportunity to
work with an FEI Titan transmission electron microscope (TEM), according to the 026019 ESTEEM
integrated infrastructure initiative from the European Union. The second article in this assembly,
Oxygen-vacancy related structural phase transition of Ba Sr Co Fe O , was written by Dr. Zhèn 0.8 0.2 0.8 0.2 3- δ
Yáng from ETH Zurich (Switzerland). I am very glad to have contributed to this publication, which
includes refinements of the X-ray diffraction (XRD) patterns for this material and the interpretation of
data achieved from the XRD and TEM investigations.
The experimental work for the article A long-term stable cobalt-free oxygen-permeable perovskite-
type membrane in chapter 3 was conducted by the competent student apprentice Torben Halfer and
me in equal shares. I carried out the data interpretation and manuscript preparation for this study. I
would like to thank Dipl.-Chem. Alexander Kuhn for electrical conductivity measurements and Prof. Dr.
Paul Heitjans for fruitful discussions.
Chapter 4 provides an understanding of studies concerning the poisoning effect of CO on 2
Ba(Co Fe Zr )O (BCFZ) and (Ba Sr )(Fe Zn )O . The article In-situ X-ray diffraction study of x y z 3-δ 0.5 0.5 0.8 0.2 3-δ
carbonate formation and decomposition in perovskite-type BCFZ arose from the successful and very
pleasant cooperation of my colleague Dr. Oliver Czuprat, who carried out oxygen permeation
experiments on BCFZ hollow fiber. I performed in-situ XRD and TEM investigations, as well as the
data analysis and manuscript development for this work. Within the second article, Performance of
zinc-doped perovskite-type membranes at intermediate temperatures for long term oxygen permeation
and under carbon dioxide atmosphere, I carried out scanning electron microscopy (SEM) and TEM
examinations. Furthermore, Dr. Lars Robben from the Institut für Mineralogie of the Gottfried Wilhelm
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Leibniz Universität Hannover and I conducted the XRD measurements and data interpretation in
similar shares. I am grateful to Dr. Julia Martynczuk for writing the manuscript and for her cooperation
during this research.
The last two articles in chapter 5 introduce CO -tolerant, mixed oxygen-ionic and electron-2
conducting materials (MIEC). I am the first author for the publication entitled Ca-containing CO -2
tolerant perovskite materials for oxygen separation. I express gratitude to my colleagues Dipl.-Chem.
Tobias Klande and MSc. Nadine Juditzki, who supported me by carrying out membrane fabrication
and oxygen permeation flux measurements.
The article CO -stable and cobalt-free dual-phase membrane for oxygen separation was written by 2
my esteemed colleague MSc. Huixia Luo. My contribution to this publication was in conducting the
XRD investigations for this work, as well as performing in-situ XRD experiments in different gaseous
atmospheres. Additionally, I provided data interpretation and manuscript preparation support.

First of all, I would like to deeply thank Priv. Doz. Dr. Armin Feldhoff, who was not only a supervisor
of my work but also a tutor. I learned a lot from him concerning both the theory and application of
electron microscopy, as well as how to perform conscientious scientific work. He always provided me
with help and advice, whether it was in regard to complex TEM questions or the preparation of
manuscripts.
I am especially grateful to Prof. Dr. Jürgen Caro for the allocation of MIEC materials as a research
topic. He supported me in all respects of the research, and I greatly appreciate his ambitions to
establish the best possible working conditions for Ph.D. students.
I want to express my gratitude to Prof. Dr. Klaus-Dieter Becker from the Institut für Physikalische
und Theoretische Chemie of the Technische Universität Braunschweig for his interest in this work and
for his willingness to provide his expertise. Furthermore, I would like to thank Prof. Dr. Peter Behrens
from the Institut für Anorganische Chemie of the Gottfried Wilhelm Leibniz Universität Hannover for his
willingness to host my thesis defense.
Additional acknowledgements are dedicated to all of my co-authors who were involved in the
elaboration of articles not included in this thesis.
I kindly thank Dr. Mirko Arnold and Dr. Julia Martynczuk for their initial introduction of MIEC-
relevant topics. I would also like to extend my gratitude to all members of the Institut für Physikalische
Chemie, in particular to Frank Steinbach, Tobias Klande, Oliver Merka, Monir Sharifi, Oliver Czuprat,
Helge Bux, and Kerstin Janze for accomplishing a nice and uncomplicated atmosphere in the group.
Last, but not least, I kindly acknowledge my friends and family, who have always encouraged me.
My special thanks are dedicated to my wife Olga, who gave me two beautiful children and made it
clear to me what is truly important in life.






vi

Contents

Abstract………………………………………………………………………………...…………………i
Zusammenfassung……………………………………………………………………………………..iii
Preface………………………………………………………………………………………...…………v

1 Introduction…………………………………………………………………….…....1
1.1 Motivation……………………………………….………………………………………….….1
1.2 Structure of perovskite oxides…………………………………………….………4
1.2.1 Ideal cubic perovskite structure………………………………………..…………..4
1.2.2 Distorted perovskite structure……………………………………...………………5
1.3 Preparation of perovskite oxide membranes……………………………….………...……9
1.3.1 Sol-gel process………………………………………………………………………9
1.3.2 Solid state reactions……………………………………………………..10
1.3.3 Preparation of dense perovskite membranes……………………...……………11
1.3.4 Preparation of asymmetric membranes……………………...…………………..11
1.4 Mixed oxygen ion and electron conductors…………………………….…..…………..…12
1.4.1 Defect chemistry of perovskite oxides……………………………………………12
1.4.2 Electrical conductivity of perovskite oxides……………………...……13
1.4.3 Oxygen permeation through perovskite membranes ………………………..…14
1.4.4 Measurement of oxygen permeation……………………………………………..16
1.5 MIEC perovskite oxides: issues and possible solutions……………………………..…..17
1.5.1 Inherent phase instability of BSCF perovskite at intermediate temperatures..17
1.5.3 Intolerance of alkaline-earth containing perovskite against CO ……………...18 2
1.6 Bibliography………………………………………………………..…………………………20
2 Inherent phase instability of BSCF perovskite at intermediate
temperatures......................................................................................................................27
2.1 Summary………………………………………..…………………………………………….27
2.2 TEM study of BSCF perovskite decomposition at intermediate temperatures………..28
2.3 Oxygen-vacancy related structural phase transition of Ba Sr Co Fe O ………40 0.8 0.2 0.8 0.2 3-δ
3 Novel cobalt-free perovskite for intermediate temperatures
applications……………………………………………………………………………………….51
3.1 Summary……………………………………………………………………………..……....51
3.2 A novel cobalt-free oxygen-permeable perovskite-type membrane……………...…….52
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4 Intolerance of alkaline-earth containing perovskite against
CO ……………………………………………………………………………………………………59 2
4.1 Summary…………………………………………………………………………..………….59
4.2 In-situ X-ray diffraction study of carbonate formation and decomposition
in perovskite-type BCFZ…………………………………………………………………….60
4.3 Performance of zinc-doped perovskite-type membranes at intermediate temperature
for long-term oxygen permeation and under carbon dioxide atmosphere………….....66
5 CO -stable MIEC materials…………………………………………………………....….…77 2
5.1 Summary……………………………………………………………………………………...77
5.2 Ca-containing CO -tolerant perovskite materials for oxygen separation………………78 2
5.3 CO -stable and cobalt-free dual-phase membrane for oxygen separation………..…..96 2

Publications and conferences……………………………………………………….………………....I
Curriculum Vitale……………………………………………………………………………………….V
Erklärung zur Dissertation......……………………………………………………………………….VII




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