Solid Oxide Fuel Cell Technology

Principles, Performance and Operations

K Huang, J B Goodenough

• Editore: Woodhead Publishing
• Anno: 2009
• ISBN:
  9781845696511
• Argomento: Technologie et ingénierie
• Categoria: Ingénierie de la chimie et de la biochimie

High temperature solid oxide fuel cell (SOFC) technology is a promising power generation option that features high electrical efficiency and low emissions of environmentally polluting gases such as CO2, NOox and SOx. It is ideal for distributed stationary power generation applications where both high-efficiency electricity and high-quality heat are in strong demand. For the past few decades, SOFC technology has attracted intense worldwide R&D effort and, along with polymer electrolyte membrane fuel cell (PEMFC) technology, has undergone extensive commercialization development.This book presents a systematic and in-depth narrative of the technology from the perspective of fundamentals, providing comprehensive theoretical analysis and innovative characterization techniques for SOFC technology. The book initially deals with the basics and development of SOFC technology from cell materials to fundamental thermodynamics, electronic properties of solids and charged particle transport. This coverage is extended with a thorough analysis of such operational features as current flow and energy balance, and on to voltage losses and electrical efficiency. Furthermore, the book also covers the important issues of fuel cell stability and durability with chapters on performance characterization, fuel processing, and electrode poisoning. Finally, the book provides a comprehensive review for SOFC materials and fabrication techniques. A series of useful scientific appendices rounds off the book.Solid oxide fuel cell technology is a standard reference for all those researching this important field as well as those working in the power industry.

• Provides a comprehensive review of solid oxide fuel cells from history and design to chemistry and materials development
• Presents analysis of operational features including current flow, energy balance, voltage losses and electrical efficiency
• Explores fuel cell stability and durability with specific chapters examining performance characterization, fuel processing and electrode poisoning

1

Introduction to solid oxide fuel cells (SOFCs)

Abstract

Technology advancement to address the world’s growing demand for clean and affordable energy will require simultaneous advances in materials science and technology in order to meet the performance demands of new power-generating systems. Fuel cells emerge as highly efficient, fuel flexible, and environmentally friendly electricity producing devices. These unique characteristics advantageously differentiate fuel cells from conventional heat engines for power generation and therefore have attracted worldwide attention – from research and development activities in institutes to commercialization efforts in industries – for the last few decades. In this chapter, the history, advantages, applications, and designs of solid oxide fuel cells (SOFCs) are briefly reviewed.

Key words

solid oxide fuel cell

advantages

applications

functionality

stack design

1.1 A brief history of the solid oxide fuel cell (SOFC)^1,2

Fuel cells have been known to science for more than 150 years. As early as 1839, the Swiss scientist Christian Friedrich Schoenbein first asserted the possibility of a fuel cell that combined hydrogen with oxygen.³ One month later, the English scientist William Robert Grove published the experimental observation of voltage in a concentration cell (called a ‘gas cell’ at the time) when combining hydrogen with oxygen in the presence of platinum.⁴ A few years later, in 1845, he published the paper ‘On the gas voltaic battery – voltaic action of phosphoros, sulphur and hydrocarbons’,⁵ which formally confirmed the technical feasibility of a fuel cell as a power-generating device. However, it was not until the end of the nineteenth century with the discovery by the German scientist Walther Nernst of the so-called ‘Nernst mass’⁶ of a ceramic material consisting of 85 mol% ZrO₂ and 15 mol% Y₂O₃ that the key solid electrolyte material for modern SOFCs was identified. Since then, many mixtures of ZrO₂ with the rare-earth and alkaline-earth oxides have been systematically studied, revealing a range of compositions with high oxide-ion conductivity. After electrochemistry was connected with thermodynamics, the basic principle that establishes the relationship between the chemical energy of a fuel and the voltage of a fuel cell was explained by H. von Helmholtz in 1882.⁷ In 1894, W. Ostwald correctly pointed out that a fuel cell could produce electricity in a more efficient way than a conventional steam engine.⁸ Such a realization undoubtedly became a stimulant for pursuing fuel cells as potential highly efficient power-generating devices in the twentieth century. If the nineteenth century was considered as an era of curiosity in fuel cells, the twentieth century was certainly the epoch for fuel cells to become the subject of intense research and development (R&D) and commercialization efforts.

The conceptual SOFC was probably first demonstrated in 1937 by the Swiss scientists Emil Bauer and Hans Preis using zirconia ceramics as the electrolyte, Fe₃O₄ as the cathode, and C as the anode.⁹ Clearly, the problems of stability of the electrode materials and gas-phase diffusion were not recognized at the time. However, more concentrated and systematic studies on SOFCs started after the pioneering 1943 work by the German scientist Carl Wagner, who first recognized the existence of oxygen vacancies in mixed oxides such as doped ZrO₂, and attributed the observed electrical conductivity at high temperatures to the movement of these oxygen vacancies under a gradient of oxygen partial pressure.¹⁰ In 1957, Kiukkola and Wagner published another landmark work describing thermodynamic investigations with concentration cells based on the solid electrolyte Zr_0.85Ca_0.15O_1.85.¹¹ It was this work that laid the theoretical foundation for the modern solid-state electrochemistry of the SOFC. A few years later, two scientists, Joseph Weissbart and Roswell Ruka, from the Westinghouse Electric Corporation, reported in 1961 the first solid-electrolyte-based device for measuring the oxygen concentration of a gas phase with a concentration cell,¹² which later led to their patent ‘A solid electrolyte fuel cell’ issued in 1962.¹³ Based on these initial efforts, a group of Westinghouse engineers developed and successfully tested the first tubular ‘bell-and-spigot’ SOFC stack from 1962 to 1963. This development eventually became the foundation of today’s cathode-supported, tubular seal-less SOFCs developed by Westinghouse/Siemens.

During the same period, advances in electrode materials for SOFCs have also taken place. The most noticeable progress was in the evolution of the cathode material. It started with the noble metals such as platinum and transitioned to doped In₂O₃¹⁴ and finally settled on today’s doped LaMnO₃. The evolution of cathode materials was clearly driven by the performance requirement, viz. the capability to activate effectively the oxygen-reduction process. The unique electrical and catalytic properties possessed by rare-earth, transition-metal perovskite oxides best satisfy the cathode requirement. However, the requirement for a thermal-expansion match between cathode and electrolyte has narrowed the practical cathode material to the doped LaMnO₃ for ZrO₂-electrolyte-based SOFCs. Another important material, developed by Meadowcroft in 1969, was the doped LaCrO₃ perovskite that is stable in both oxidizing and reducing atmospheres;¹⁵ it immediately found use as an interconnect in SOFCs. A patent filed by Spacil in 1964 described a composite anode consisting of Ni metal with a ZrO₂-based electrolyte that has remained the standard choice of anode for SOFCs.¹⁶

Historically speaking, the period from the 1970s to the 1990s marks an important era in the technical development of SOFCs. In the 1970s, the electrochemical vapor deposition (EVD) process was invented in Westinghouse by an engineer of genius, Arnold Isenberg, who demonstrated the making of a perfectly dense ZrO₂ electrolyte thin film on the substrate of a porous, tubular substrate at relatively low temperatures. Based on this important invention, Westinghouse successfully manufactured and tested a series of SOFC generator systems in the range of 5–250 kWe from the 1970s to 1990s and clearly positioned itself as the world leader in modern SOFC technology. It was also during this period that various SOFC stack designs flourished, from tubular to planar in geometry, and alternative materials for the cathode, the anode, and the interconnect were also explored as the substrate.

A real advancement of anode-supported planar SOFCs took place after the pioneering work of de Souza et al. of Berkeley National Laboratory, published in 1997;¹⁷ they essentially demonstrated that an electrolyte on a porous anode substrate can be co-fired at high temperatures into a dense thin film without invoking chemical reactions. The cathode was applied afterwards and sintered at much lower temperatures to minimize chemical reactions. As a result, the single-cell performance has been significantly improved, which in turn has allowed an anode-supported SOFC to operate at lower temperatures where commercially available oxidation-resistant alloys such as thermal- expansion-compatible ferritic steels can be utilized as interconnect materials for SOFC stacks. A majority of today’s SOFC designs adopt the anode-supported planar geometry based on considerations of cost and performance. However, the reliability and stability appear to be the leading issues for commercialization at the present time.

Looking through the history of SOFCs, it is not difficult to find that the ZrO₂-based materials have remained the mainstream electrolytes since the discovery by Nernst over 100 years ago. As early as 1990, Goodenough et al.¹⁸ had pointed out that high oxide-ion conduction can exist in the perovskites and hence in other structures than the classical fluorite structure, giving hopes for finding a new family of oxide-ion conductors in other crystal structures. This prediction was favorably vindicated by the noteworthy discovery of the high oxide-ion conductivity perovskite Sr- and Mg-doped LaGaO₃ (LSGM) by Ishihara et al¹⁹ in 1994, immediately confirmed by Feng and Goodenough²⁰ in the same year, followed by a systematic characterization of the system by Huang et al.^{21, 22} The high oxide-ion conductivity and the crystallographic compatibility with cathode materials make LSGM even more attractive for low-temperature SOFCs. Mitsubishi Materials has recently demonstrated an excellent stack performance of an SOFC based on LSGM electrolyte operating at 800 °C.²³

In summary, the major driver for sustaining the development of SOFC technology is the intrinsically high electrical efficiency compared with a conventional heat engine. After a century of scientific research and commercial engineering, development in the areas of materials, designs, and system integration has advanced dramatically. A thorough review of SOFC materials and fabrication techniques is given in Chapter 12. The feeling is that commercialization of the technology is on the horizon.

1.2 Advantages of the solid oxide fuel cell

The fuel cell is a device that directly converts the chemical energy in fossil fuels into electrical power in an electrochemical manner. Unlimited by the Carnot cycle, a fuel cell has an inherently higher electrical efficiency than conventional heat engines, particularly for the less than 1 MW class. Higher electrical efficiency infers a reduced CO₂ emission per unit electricity produced if hydrocarbons are used as fuels; this influence has become increasingly important as we endeavor to minimize the emission of greenhouse gases in future power generation. Another conceivable benefit is the minimal environmental impact from a fuel-cell generator compared with a conventional heat engine. Owing to its relatively low operating temperature, the formation and therefore the emission of nitrogen oxides (collectively known as NO_x) are negligible. Use of a desulfurizer subsystem in a fuel-cell generator ensures almost zero emission of sulfur oxides (collectively known as SO_x). In addition, fuel-cell power generators are much quieter and exhibit less vibration than a conventional engine during operation; they therefore represent a competitive alternative in distributed, stationary power generation.

For SOFCs operating at higher temperatures, there are added advantages. High-temperature operation, typically in the range of 600–1000 °C, not only provides high-quality wast...