Electrochemical Energy

Advanced Materials and Technologies

Pei Kang Shen, Chao-Yang Wang, San Ping Jiang, Xueliang Sun, Jiujun Zhang, Pei Kang Shen, Chao-Yang Wang, San Ping Jiang, Xueliang Sun, Jiujun Zhang

• Publisher: CRC Press
• Year: 2018
• ISBN:
  9781351231206
• Edition: 1
• Topic: Tecnología e ingeniería
• Subtopic: Combustibles fósiles

Electrochemical Energy: Advanced Materials and Technologies covers the development of advanced materials and technologies for electrochemical energy conversion and storage. The book was created by participants of the International Conference on Electrochemical Materials and Technologies for Clean Sustainable Energy (ICES-2013) held in Guangzhou, China, and incorporates select papers presented at the conference.

More than 300 attendees from across the globe participated in ICES-2013 and gave presentations in six major themes:

• 
  
  

• Fuel cells and hydrogen energy
• Lithium batteries and advanced secondary batteries
• Green energy for a clean environment
• Photo-Electrocatalysis
• Supercapacitors
• Electrochemical clean energy applications and markets

Comprised of eight sections, this book includes 25 chapters featuring highlights from the conference and covering every facet of synthesis, characterization, and performance evaluation of the advanced materials for electrochemical energy. It thoroughly describes electrochemical energy conversion and storage technologies such as batteries, fuel cells, supercapacitors, hydrogen generation, and their associated materials. The book contains a number of topics that include electrochemical processes, materials, components, assembly and manufacturing, and degradation mechanisms. It also addresses challenges related to cost and performance, provides varying perspectives, and emphasizes existing and emerging solutions.

The result of a conference encouraging enhanced research collaboration among members of the electrochemical energy community, Electrochemical Energy: Advanced Materials and Technologies is dedicated to the development of advanced materials and technologies for electrochemical energy conversion and storage and details the technologies, current achievements, and future directions in the field.

Section V

Advanced Materials and Technologies for Fuel Cells

12	Advanced Materials for High-Temperature Solid Oxide Fuel Cells (SOFCs) Kuan-Zong Fung

CONTENTS

12.1	Introduction
12.2	Solid Electrolytes
	12.2.1	Oxygen-Ion Conductors with Fluorite-Type Structure
		12.2.1.1	Doped Zirconia or Zirconium Oxide
		12.2.1.2	Ceria or Cerium Oxide
		12.2.1.3	Bismuth Oxide
	12.2.2	Perovskite Structure
	12.2.3	Proton Conductors
		12.2.3.1	Proton Conductors with Perovskite Structure
12.3	Anode of SOFCs
	12.3.1	Requirements of Anode
	12.3.2	Development of Anode Materials
		12.3.2.1	Conventional Ni–YSZ Cermet Anode Materials
		12.3.2.2	Other Cermet Anode Materials
	12.3.3	Conducting Oxides
		12.3.3.1	Perovskite Anode Materials
		12.3.3.2	Pyrochlore Anode Materials
		12.3.3.3	Tungsten Bronze Anode Materials
	12.3.4	Sulfur-Tolerant Anode Materials
12.4	Cathode
	12.4.1	Category of the Cathode Material
		12.4.1.1	Perovskite-Type Structure Cathode Materials
		12.4.1.2	Ln1−xAxM1−yMnyO3-δ, Where Ln = La, Nd, Pr, etc., A = Ca, Sr, and M = Transition Metal
		12.4.1.3	Other Ln1−xAxM1−yMnyO3-δ
		12.4.1.4	La1−xSrxCoO3-δ
		12.4.1.5	La1−xSrxFe O3-δ
		12.4.1.6	La1−xSrxFe1−yCoyO3-δ
	12.4.2	K2NiF4-Type Structure Cathode Materials
	12.4.3	Ordered Double Perovskites
	12.4.4	Surface Modification
		12.4.4.1	Mixed Ionic–Electronic Conductor (MIEC)
		12.4.4.2	Noble Particle Deposition on MIEC Cathode
		12.4.4.3	Thin Film Coating on MIEC Cathode
	12.4.5	Summary
12.5	Interconnect
	12.5.1	Ceramic-Based Interconnect
	12.5.2	Metallic Interconnect
		12.5.2.1	Problems for Metallic Materials as Interconnect
		12.5.2.2	Excessive Growth and Spallation of Oxide Scale
		12.5.2.3	Chromium Poisoning
	12.5.3	Contact Resistance of Metallic Interconnect
	12.5.4	Materials for Metallic Interconnects
		12.5.4.1	Chromium-Based Alloys
		12.5.4.2	Fe–Cr-Based Alloys
		12.5.4.3	Ni–Cr-Based Alloys
	12.5.5	Modification Coating
		12.5.5.1	Nitride Coatings
		12.5.5.2	Perovskite Coatings
		12.5.5.3	Spinel Coatings
	12.5.6	Summary
References

12.1 INTRODUCTION

Among several types of fuel cells, the solid oxide fuel cell (SOFC) has shown advantages, such as high conversion efficiency, no need for noble metal catalysts, use of hydrocarbon fuel, no liquid in the fuel cell, etc. In recent years, a total of 10 MW Bloom Energy servers (100 kW per server) based on SOFC technology have been installed in California and North Carolina in the United States. The power generation of a 700-W unit of “Ene-Farm Type S,” which realized a power generation efficiency of 46.5% (net AC, LHV) as a commercially available residential-use fuel cell system, was developed jointly by Osaka Gas, Aisin Seiki, Kyocera, and Toyota Motor. Although the cost of an SOFC unit is still high, the future looks promising for SOFCs.

The major components of an SOFC are the solid electrolyte, the anode, and the cathode (see Figures 12.1 and 12.2). In addition, an interconnect is also needed when cells are arranged in series as a stack. Since each component is operated under a different environment, it must provide a different function and meet special requirements. For example, a solid electrolyte and an interconnect need to provide ionic conduction and electronic conduction, respectively. Both of them are separated by the fuel and the oxidant. Thus, they need to have adequate stability against oxidizing and reducing atmospheres. On the other hand, cathode and anode need to have ionic–electronic conduction in addition to adequate stability in either an oxidizing or a reducing environment.

FIGURE 12.1 Schematic diagram of the operating concept of an SOFC.

FIGURE 12.2 SOFC consists of two porous electrode layers and a dense electrolyte layer.

Fundamentally, the conductivity and stability of functional materials is strongly affected by their crystal structure and/or atomic arrangements. Therefore, in this chapter, some of the materials used in SOFCs are categorized on the basis of their unique crystal structures.

12.2 SOLID ELECTROLYTES

Solid electrolytes, namely solid-state ionic conductor, are materials that exhibit high ionic conduction (as high as 1 S cm⁻¹) with negligible electronic conduction. The mobile ions involved in the electrochemical reaction of SOFCs are either oxygen ions (O²⁻) and/or protons (H⁺). Since ionic conduction in an ionic conductor is highly dependent on its defect concentration and crystals structure, oxygen-ion conductors used for SOFC electrolytes commonly have similar crystal structures, mainly fluorite- and perovskite-type structures.

12.2.1 OXYGEN-ION CONDUCTORS WITH FLUORITE-TYPE STRUCTURE

The cubic fluorite structure consists of relatively large oxygen ions that form 8-fold coordination. Every other cube is formed by the simple cubic packing of the oxygen ions having a cation at its center. On the basis of Pauling’s rules, the radius ratio R_cation/R_anion needs to be greater than 0.73. Oxides with CaF₂ structure such as ZrO₂, CeO₂, and Bi₂O₃ have been extensively studied for the application.

12.2.1.1 Doped Zirconia or Zirconium Oxide

ZrO₂ with proper dopant or stabilizer is the most common electrolyte used for SOFC applications. However, ZrO₂ in its pure form cannot be used as a proper electrolyte because of its structural instability and low ionic conductivity. At temperatures from a melting point of 2680°C to 2370°C, undoped ZrO₂ shows a cubic (c) structure. With further cooling down to 2370°C, the cubic phase will change to a tetragonal (t) form with slight distortion. As the temperature reaches 1170°C and below, the tetragonal ZrO₂ exhibits a martensitic transformation into a monoclinic (m) form. The tetragonal–monoclinic transformation is accompanied with a large volume expansion (~5%) that may result in a catastrophic failure. Such a transformation may be rationalized by the undesired radius ratio of R_Zr+4/R_O of well below 0.732. Thus, destabilization of the fluorite-structure ZrO₂ is expected when the temperature is below 1170°C.

With proper addition of larger cations with lower valence, such as Y³⁺ and Ca²⁺, not only the radius ratio, R_cation/R_anion, is greater than 0.73, but also the positive oxygen vacancies are also created due to compensation of YZr or CaZr. Consequently, the cubic phase of doped ZrO₂ may be stabilized to room temperature. In addition, the doped ZrO₂ shows enhanced conduction of oxygen ions. Thus, 8 mol% Y₂O₃–stabilized ZrO₂ (YSZ) is the mo...