Functional Materials for Solid Oxide Fuel Cells: Processing, Microstructure and Performance E-Book
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- Herausgeber: Bentham Science Publishers
- Kategorie: Wissenschaft und neue Technologien
- Serie: Frontiers in Ceramic Science
- Sprache: Englisch
Solid Oxide Fuel Cells (SOFCs) have received great attention among researchers in the past few decades due to their high electrochemical energy conversion efficiency, environmental friendliness, fuel flexibility and wide range of applications. This volume is a contribution from renowned researchers in the scientific community interested in functional materials for SOFCs. Chapters in this volume emphasize the processing, microstructure and performance of electrolyte and electrode materials. Contributors review the main chemical and physical routes used to prepare ceramic/composite materials, and explain a variety of manufacturing techniques for electrode and electrolyte production and characterization. Readers will also find information about both symmetrical and single fuel cells. The book is a useful reference for students and professionals involved in SOFC research and development.
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Seitenzahl: 292
Veröffentlichungsjahr: 2017
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Frontiers in Ceramic Science
(Volume 1)
Edited by
Moisés Rómolos Cesário
&
Daniel Araújo de Macedo
BENTHAM SCIENCE PUBLISHERS LTD.
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FOREWORD
This fascinating e-book clusters contributions from researchers who have dedicated the last years of their carrier to study materials, manufacturing processes and characterization techniques applied to the development of Solid Oxide Fuel Cells (SOFCs). These electrochemical devices that convert chemical energy into electricity are promising alternatives to traditional mobile and stationary power sources. Among their many advantages deserve special attention the high energy conversion efficiency and the excellent fuel flexibility. The development of high-performance functional SOFC is an important step towards reducing the operating temperature to 500 – 750 °C or lower. By doing this, the cell components can be easily and cost-efficiently produced. With this in mind, recent research around the world has focused on novel synthesis methods and processing routes to develop high performance components and single cells operating at reduced temperatures.
I am sure that this e-book reviews how processing conditions affect both microstructure and performance of functional SOFC materials.
Dr. Daniel Araújo de Macedo Department of Materials Engineering Federal University of Paraíba BrazilPREFACE
Solid Oxide Fuel Cells (SOFCs) are identified as a major technological promise for clean energy production. The development of functional materials for SOFC operating at intermediate temperatures (550 – 750 °C) requests not only a strict control of synthesis and processing conditions of ceramic/composite powders, but also a good understanding about the correlation between microstructure and electrochemical properties.
This e-Book aims to cluster contributions from the most productive and well-recognized researchers studying SOFC functional materials. Emphasis is on novel chemical/physical/mechanical processing routes towards the attainment of electrolyte and electrodes powdered/layered materials. Furthermore, the potential of the resulting microstructures toward SOFC applications has been checked using a combination of electron microscopy and electrical/electrochemical characterization techniques using symmetrical and/or single fuel cell configurations.
The book begins with an introductory chapter addressing the working principle of a SOFC and basic characteristics of SOFC electrodes. The second chapter is dedicated to cathode materials applied to intermediate and low-temperature SOFCs. The author proposes a comprehensive discussion on the cathode development, emphasizing its reaction mechanism, microstructural, characterization, and electrical performance. Studies of long-term chemical and mechanical stability have also been discussed.
The third chapter describes a review on anode materials, with focus on materials composition, synthesis methods, and electrical properties.
The forth chapter reports on the study of lanthanum silicate apatite based materials, drawing attention to their properties as electrolytes for SOFC. The authors propose a discussion on different synthetic methods to obtain apatite type electrolytes.
The fifth chapter presents a brief review on chemical/physical routes to prepare electrolyte and electrode materials for SOFC.
The sixth chapter reports on a recently phase inversion technique that is used to fabricate micro tubular solid oxide fuel cells (MT-SOFC). The authors propose a discussion on the development of this important manufacturing technique and their effects on the fuel cell performance.
The seventh chapter also discusses the use of the phase inversion based extrusion technique to fabricate MT-SOFC. Emphasis is given on the fabrication of electrolyte and how the fabrication parameters could affect the structure of the obtained electrolyte layer.
The eighth chapter reports on the study of proton conducting ceramic oxides with perovskite structure. The authors propose the development of electrolyte and electrode materials with combined properties of proton conductivity, high sinterability (in case of electrolytes), and chemical stability which make quite innovative research.
We would like to express our gratitude to all the eminent contributors for their excellent contributions and we believe that this e-book will be a reference to academic/industrial scientists from chemistry, physics, and materials science interested in the processing-microstructure-performance of SOFC materials.
Dr. Moisés Rómolos Cesário Unit of Environmental Chemistry and Interactions on Living - EA 4492 University of the Littoral Opal Coast (ULCO) France&Dr. Daniel Araújo de Macedo Department of Materials Engineering Federal University of Paraíba BrazilList of Contributors
INTRODUCTION
The Solid Oxide Fuel Cell (SOFC) technology has attracted significant attention due to the fuel flexibility and environmental advantages of this high efficient electrochemical device. However, typical SOFC operating temperatures near 1000 °C introduce a series of drawbacks related to electrode sintering and chemical reactivity between cell components. Aiming to solve these problems, researchers around the world have attempted to reduce the SOFC operating temperature to 500 – 750 °C or lower. It would result in the use of inexpensive interconnect materials, minimization of reactions between cell components, and, as a result, longer operational lifetime. Furthermore, decrease the operation temperature increases the system reliability and the possibility of using SOFCs for a wide variety of applications such as in residential and automotive devices. On the other hand, reduced operating temperatures contributes to increase ohmic losses and electrode polarization losses, decreasing the overall electrochemical performance of SOFC components. Thus, to attain acceptable performance, reducing the resistance of the electrolyte component and polarization losses of electrodes are two key points. Losses attributed to the electrolyte can be minimized by decreasing its thickness or using high conductivity materials such as doped ceria and apatite-like ceramics. Regarding electrode losses, the higher activation energy and lower reaction kinetics of the cathode compared with those of the anode, limits the overall cell performance. Therefore, the development of new functional SOFC materials with improved electrical/electrochemical properties combined with controlled microstructures become critical issues for the development of solid oxide fuel cells. These topics are systematic discussed along this e-book.
Introduction to Solid Oxide Fuel Cells
João Paulo de Freitas Grilo1,Caroline Gomes Moura2,Daniel Araújo de Macedo3,*Abstract
Fuel cells are electrochemical devices that convert chemical energy into electrical energy with high potential for commercial power generation applications. Among various types of existing fuel cells, solid oxide fuel cell (SOFC) is one of the most promising types of fuel cells, due mainly to its ability to utilize several types of fuels such as hydrogen, CO, hydrocarbon fuels, and ethanol. This chapter introduces the reader into the fundamentals of SOFCs, including its working principle and the main components used as electrodes.
INTRODUCTION
Fuel cells are electrochemical devices that convert directly and efficiently chemical energy of a fuel gas into electrical energy. Furthermore, fuel cells are environmentally friendly devices whose efficiency is not limited to the Carnot-cycle and compared to others power generation systems with internal combustion, they do not produce significant amount of NOx, SOx, COx, and pollutants. The main fuel of fuel cells is hydrogen or hydrogen-rich fuels, this requirement makes fuel cell development a great challenge to researchers worldwide due to a number of problems involving hydrogen generation and storage. Fuel cells technology usage can be done in large stationary industrial plants as well as in vehicles and portable devices [1-5].
Solid Oxide Fuel Cell (SOFC) is one of the most widely studied fuel cells, mainly because of their larger stability compared to other cell types, since it has a solid electrolyte, high efficiency and fuel flexibility. The main SOFC components are: porous cathode, porous anode, dense electrolyte, and sealants. The cathode is typically a solid state oxide which catalyzes oxygen reduction reaction, while anode is an oxide or cermet which catalyzes oxidation of a fuel, which can be either hydrogen or reformed hydrocarbons. The SOFC electrolyte must be an electronic insulating but ion-conducting material that allows only oxygen ions to pass through. Furthermore, this SOFC component must be dense to separate air and fuel, chemically and structurally stable over a wide range of partial pressures of oxygen and temperatures. Sealant materials, often used during the manufacture of single SOFCs, should provide a viscous behavior for coupling the compensating tolerances and other materials avoiding failures, which guarantees a hermetic seal.
Usually a SOFC operates at high temperatures in a range of 600 - 1000 °C allowing internal reforming of fuel. The characteristics of high operating temperature of SOFC present great challenges related to the cell lifetime and materials degradation. Therefore, there is a great interest in reducing the SOFC operating temperature to a range of 500 – 800 °C or lower, which reduces their production costs as well as stability and degradation issues. The operating principle of SOFC is schematically illustrated in Fig. (1). The fuel, hydrogen or a hydrocarbon gas, permeates into the anode compartment and the oxygen, from the air, into the cathode. At the anode (fuel electrode side), fuel is oxidized according to the reaction (Eq. 1):
(1)The electrons are transported to the cathode through an external circuit. At the cathode the oxygen is reduced with the incoming electrons from external load according to the reaction (Eq. 2):
(2)Generated oxygen ions migrate to anode across the electrolyte, hence, the fuel is oxidized by incoming oxygen ions. Therefore, this electrical connection allows a continuous supply of oxygen ions from the cathode to the anode, whilst maintaining an overall balance of electrical charge, thus producing electrical energy. The products of these reactions (Eq. 1 and 2) are only water and heat (Fig. 1). Most of the electrochemical reactions in a cell occur in the so-called triple phase boundary (TPB), which is the contact region between gas phase, electrode and electrolyte [2, 6-8].
Fig. (1)) The working principle of a SOFC.Besides showing the working principle of a typical SOFC, this chapter is also focused on a brief review on the main SOFC electrodes (cathode and anode). Materials, processing and obtaining methods of electrodes and electrolyte materials will be discussed in the following chapters.
SOFC ELECTRODES
Cathode
The cathodes for SOFC must have several properties, including: (a) high electrical conductivity; (b) high catalytic activity for the oxygen reduction; (c) good compatibility with others cell components; (d) suitable porosity (approximately 30-40%); (f) thermal expansion coefficient matching those of other components; (g) chemical stability during fabrication and operation; (h) low manufacturing cost and (i) extensive TPB (triple phase boundary). In the early development of SOFC, platinum was used as cathode, nevertheless it seemed very costly for commercial use. The cathode has the function to reduce the oxygen molecule, transport ion to the electrolyte and provides electrical current resulting from reduction reaction of oxygen. Thus, the choice of electrode material is important for high performance of the cell and, thereby, avoids undesirable chemical reactions [9-12]. In the cathode, the reaction is shown in Eq. 3:
(3)The TPB area is schematically illustrated in Fig. (2). Any disruption in connectivity among these phases decreases the number of points to occur the electrochemical reaction [13].
Fig. (2)) Schematic representation of triple phase boundaries.Anode
The anode is the electrode where the fuel oxidation occurs. As the cathode, this component must also exhibit high electronic conductivity, good catalytic activity for the fuel oxidation reactions and sufficient porosity to allow the transport of fuel to the anode/electrolyte interface and the removal of reaction products. In addition, the anode should be chemically stable and thermally compatible with the other SOFC components [1, 14].
The electrochemical performance of the anode depends on the charge transport resistance (electrons and ions), inside the anode and the anode/electrolyte interface, and the resistance of gas transport. The increase of the triple phase boundaries (TPB) length, by microstructural optimization and phase composition, are the most efficient ways to improve the electrochemical performance of anodes [15, 16].
Internal reform and tolerance to sulfur-containing compounds are also essential to the anodes, especially when a hydrocarbon fuel is used, e.g. methane. The porosity of the anodes is a very important factor, not only because it is related to high densities of triple phase boundaries, but also because it avoids mass transport limitation. In this regard, many studies have reported the use of pore formers (graphite, starch, citric acid, etc.) in order to obtain suitable porosity in anodes [17-19]. However, due to the tendency to agglomerate of pore-forming agents, it is sometimes difficult to ensure good structural performance and permeation of gases in these electrodes [20].
CONFLICT OF INTEREST
The authors confirm that they have no conflict of interest to declare for this publication.
ACKNOWLEDGEMENTS
Declared none.
REFERENCES
[1]Atkinson, A.; Barnett, S.; Gorte, R.J.; Irvine, J.T.; McEvoy, A.J.; Mogensen, M.; Singhal, S.C.; Vohs, J. Advanced anodes for high-temperature fuel cells. Nat. Mater.,2004, 3(1), 17-27. [http://dx.doi.org/10.1038/nmat1040] [PMID: 14704781][2]Brett, D.J.; Atkinson, A.; Brandon, N.P.; Skinner, S.J. Intermediate temperature solid oxide fuel cells. Chem. Soc. Rev.,2008, 37(8), 1568-1578. [http://dx.doi.org/10.1039/b612060c] [PMID: 18648682][3]Sun, C.; Stimming, U. Recent anode advances in solid oxide fuel cells. J. Power Sources,2007, 171, 247-260. [http://dx.doi.org/10.1016/j.jpowsour.2007.06.086][4]Murray, E.P.; Tsai, T.; Barnett, S.A. A direct-methane fuel cell with a ceria-based anode. Nature,1999, 400, 649-651. [http://dx.doi.org/10.1038/21781][5]McIntosh, S.; Gorte, R.J. Direct hydrocarbon solid oxide fuel cells. Chem. Rev.,2004, 104(10), 4845-4865. [http://dx.doi.org/10.1021/cr020725g] [PMID: 15669170][6]Carrette, L; Friedrich, A; Stimming, U. Fuel Cells - Fundamentals and Applications. Wiley online Libr.,2001, 1, 5-39.[7]Stambouli, A.B.; Traversa, E. Solid oxide fuel cells (SOFCs): A review of an environmentally clean and efficient source of energy. Renew. Sustain. Energy Rev.,2002, 6, 433-455. [http://dx.doi.org/10.1016/S1364-0321(02)00014-X][8]Taroco, H.A.; Santos, J.A.; Domingues, R.Z.; Matencio, T. Ceramic Materials for Solid Oxide Fuel Cells. Adv Ceram Synth Charact Process Specif Appl,2011, 5, 423-446.[9]Yokokawa, H.; Horita, T. Singhal, C.S.; Kendall, K. Cathodes. High temperature solid oxide fuel cells: fundamentals, design and applications.; Elsevier Science, 2003. pp. 119-147. [http://dx.doi.org/10.1016/B978-185617387-2/50022-2][10]Jiang, S.P.; Jian, L. Fergus, J.W.; Hui, R.; Li, X. Cathodes. Solid oxide fuel cells: materials properties and performance.; CRC Press, 2009. pp. 131-171.[11]Sun, C.; Hui, R.; Roller, J. Cathode materials for solid oxide fuel cells: a review. J. Solid State Electrochem.,2010, 14, 1125-1144. [http://dx.doi.org/10.1007/s10008-009-0932-0][12]Steele, B.C.; Bae, J.M. Properties of La0.6Sr0.4Co0.2Fe0.8O3-x (LSCF) double layer cathodes on gadolinium-doped cerium oxide (CGO) electrolytes. Solid State Ion.,1998, 106, 255-261. [http://dx.doi.org/10.1016/S0167-2738(97)00430-X][13]Adler, S.B. Factors governing oxygen reduction in solid oxide fuel cell cathodes. Chem. Rev.,2004, 104(10), 4791-4843. [http://dx.doi.org/10.1021/cr020724o] [PMID: 15669169][14]Liu, J.; Barnett, S.A. Operation of anode-supported solid oxide fuel cells on methane and natural gas. Solid State Ion.,2003, 158, 11-16. [http://dx.doi.org/10.1016/S0167-2738(02)00769-5][15]Holzer, L.; Münch, B.; Iwanschitz, B.; Cantoni, M.; Hocker, T.; Graule, T. Quantitative relationships between composition, particle size, triple phase boundary length and surface area in nickel-cermet anodes for Solid Oxide Fuel Cells. J. Power Sources,2011, 196(17), 7076-7089. [http://dx.doi.org/10.1016/j.jpowsour.2010.08.006][16]Mizusaki, J.; Tagawa, H.; Saito, T.; Yamamura, T.; Kamitani, K.; Hirano, K. Kinetic studies of the reaction at the nickel pattern electrode on YSZ in H2-H2O atmospheres. Solid State Ion.,1994, 70-71, 52-58. [http://dx.doi.org/10.1016/0167-2738(94)90286-0][17]Zhu, W.Z.; Deevi, S.C. A review on the status of anode materials for solid oxide fuel Cells. Mater. Sci. Eng.,2003, 362, 228-239. [http://dx.doi.org/10.1016/S0921-5093(03)00620-8][18]Haslam, J.J.; Pham, A.Q.; Chung, B.; Dicarlo, J.F.; Glass, R.T. Effects of the Use of Pore Formers on Performance of an Anode Supported Solid Oxide Fuel Cell. J. Am. Ceram. Soc.,2005, 88, 513-518. [http://dx.doi.org/10.1111/j.1551-2916.2005.00097.x][19]Clemmer, R.M.; Corbin, S.F. Influence of porous composite microstructure on the processing and properties of solid oxide fuel cell anodes. Solid State Ion.,2004, 166, 251-259. [http://dx.doi.org/10.1016/j.ssi.2003.12.009][20]Hu, J.; Lü, Z.; Chen, K.; Huang, X.; Ai, N.; Du, X.; Fu, C.; Wang, J.; Su, W. Effect of composite pore-former on the fabrication and performance of anode-supported membranes for SOFCs. J. Membr. Sci.,2008, 318, 445-451. [http://dx.doi.org/10.1016/j.memsci.2008.03.008]Cathode Materials for High-Performing Solid Oxide Fuel Cells
Hanping Ding1,2,*Abstract
It is well recognized that the development of low-temperature solid oxide fuel cells (LT-SOFCs) replies on the exploration of new functional materials and optimized microstructures with facilitated oxygen reduction reaction (ORR) that involves complicated electrochemical processes occurring at triple-phase boundaries (TPB). This urgent and critical demand promotes great research efforts on pursuing superior catalysts as electrodes owing comprehensive electrochemical and physicochemical properties, and relevant catalyst optimization on materials and microstructures. The material development is mostly based on perovskite with extensive doping strategies to maximize the catalytic activity while other properties such as stability, thermal and chemical compatibility, etc. are well compromised. Other types of materials such as K2NiF4, double perovskite were also studied as potential candidates, owing to the excellence of catalytic activity resulting from the special features of crystal structures. In this chapter, the fundamental knowledge of cathode is briefly introduced, such as reaction processes of ORR, catalysis mechanism and defect transport. Several typical perovskites are reviewed to better understand the required specific material properties for an excellent ORR catalyst as cathode material that can be operated at practical low temperatures (350~500 °C). Particularly, recent development of the layered perovskites is specifically introduced because they show very promising performance at low temperatures due to the fast oxygen exchange and oxygen diffusion yielded by the ordered cation distribution in crystal.
INTRODUCTION
In this chapter, the recent development of cathode materials, which are operated at low operating temperature (350 ~ 600 oC) is discussed. With emphasis on how the candidate materials are selected as potential low-temperature operating cathodes, mechanism of oxygen reduction reaction, criteria of promising cathode and role of mixed ionic-electronic conductors are also discussed.
Technical Parameters for SOFC Cathodes
As an air electrode, the oxygen reduction reaction (ORR) occurs at the three phase boundaries where the oxygen gas, electrolyte and cathode surfaces meet. The produced negatively charged oxygen ions transfer through the electrolyte conducting membrane and then react with hydrogen molecules to form water while the electrons released from fuel of hydrogen have to pass the external circuit to form the current. Therefore, the ORR is a very critical step to determine the initial kinetics of total reaction. The oxygen reduction on the cathode surface is believed to include several sub-steps which separately determine the limiting step, such as oxygen absorption, charged, dissociation and desorption, etc. (Fig. 1) shows the reaction steps at TPB area (pure electronic conductor is used to illustrate for simplicity). The oxygen molecules are absorbed on the surface or the TPB sites first and then move towards TPB area to be dissociated, where oxygen ions are formed through electrochemically charged by the electrons. Consequently, the oxygen ions should have to leave the sites and move towards electrolyte and incorporate into it. If a mixed ionic and electronic conductor is used, the places for oxygen dissociation can be extended to the whole cathode surface. Therefore, the oxygen ions can reach the electrolyte membrane by another pathway of bulk cathode. The reaction kinetics can be significantly increased by this extension of reaction sites and diffusion paths.
In order to facilitate ORRs to proceed fast, several technical requirements have to be satisfied. (a) Electrical conductivity: since ORR is an electrochemical reaction, a certain conductivity is needed to allow electron conduction. The mixed conductor of electrons and oxygen ions is preferred because the more active sites are created; (b) Catalytic activity: the property of surface chemistry need to allow the absorption and desorption of various oxygen-related species on the cathode surface; (c) porosity: the gas diffusion from the layer surface to the cathode/electrolyte interface should be fast to minimize the concentration over-potential; (d) thermal and chemical compatibility with electrolyte: the thermal expansion of cathode bulk should be close to that of electrolyte to avoid the potential delamination between two layers and to increase the resistance against thermal shocks. The cathode should not react with electrolyte to form any insulating phases that slower down or block the further proceeding of oxygen reduction; (e) chemical stability: the cathode must be chemically stable in some case of low oxygen partial pressures when large amount of oxygen is consumed by reactions, causing the oxygen absent at electrolyte/cathode interface area. Overall, the cathode performance is determined by all of these physical, chemical, or electrochemical parameters; one bad-done aspect can deteriorate the cathode behavior in operation.
Fig. (1)) The schematic of catalysis mechanism for ORR on the cathode surface (pure electronic conductor).Typical Materials for Cathode and their Conducting Nature
Perovskite
Searching for a conductive oxide which can sustain good relevant properties after the processes of material synthesis and fuel-cell fabrication conditions. Many oxides with perovskite structure are ideal material candidates, which are still in great interest in current activities. Perovskite is a class of compounds which have the same type of crystal structure as CaTiO3 (A2+B4+X2-3). Due to the high tolerance factor of the crystal structure, perovskite offers wide flexibility for improving the properties of materials, such as catalytic activity, electronic or ionic conductivity, chemical stability and thermal behavior, etc [1]. Many useful properties of the perovskite oxides are primarily determined by the B-site cations while they can be also tuned by A-site cations. In this perovskite structure as shown in Fig. (2), oxides typically adopt a cubic structure (sometimes it also distorts to other crystal structure depending on the atoms and preparation conditions).
Fig. (2)) The unit-cell structure of ABO3-type perovskite. The A-site cation occupies the larger spaces of 12-fold oxygen coordinated interstitials; B-site cation occupies the smaller octahedral holes (6-fold coordination).A large alkaline earth or rare earth cation occupies A-site locating at the corners of the cube, while smaller transition metal element sits at the body center, and oxygen atoms at the centers of six cubic faces. The A-site cations (such as La, Ca, Sr, Ba, Pr and Sm, etc.) with lower valence at the interstitial sites are surrounded by four octahedron and coordinated to twelve oxygen anions while the B cations (such as Cr, Ti, Fe, Cu, Ni, Ce, Y, Yb and Zr, etc.) at the center of octahedron are coordinated to six oxygen atoms at the corners.
Perovskite oxides generally involve the structural distortion such as the tilting of the octahedron and displacement of cations at either A or B site, resulting from equilibrium states of charged cations and anions with different ionic sizes and valences. While this distortion may achieve some required electrical properties, it also leads to the structural instability. In order to quantify the degree of this distortion, a term called Goldschmidt tolerance factor (t) is used to describe it, according to the equation (1) [2]:
(1)
