Solid Oxide Fuel Cells (SOFC) - Developments, Designs, Materials and Current Status

Topics Covered

Background

Introduction

SOFC Performance

Operating Principle and Thermodynamics of SOFC

Factors Affecting the Efficiency of SOFC

SOFC Technology

Concepts

Basic Stack Designs

Materials for SOFC

Fabrication of Cells

Status of SOFC Development

Potential Markets For Fuel Cells

Challenges

Background

Solid Oxide Fuel Cells (SOFC) are an environmentally friendly (high efficiency and low pollution) power generation technology that is currently under development in a number of countries. An overview of the technology, techniques used for fabricating fuel cell components and materials currently in use is given.

Introduction

Fuel cells and in particular Solid Oxide fuel cells promise to revolutionise electric power generation in the 21st century. Fuel cells convert gaseous fuels such as hydrogen, natural gas or gasified coal to electricity in an electrochemical process. They are therefore not limited in efficiency by the Carrot cycle of heat engines, and they are also an environmentally clean technology producing substantially lower pollutant emissions compared with conventional power generation technologies. The search for “cleaner technologies” resulted in a revival of interest in fuel cells over the past decade, and resources for their development as power sources have increased substantially. Several different types of fuel cells are currently under development, but the Solid Oxide Fuel Cell (SOFC) for stationary power generation and the Solid Polymer Electrolyte Fuel Cell (PEMFC) for transport applications are considered to be the most promising fuel cell systems. Current estimates are that about 1 billion US$ will be spent on development of SOFC technology alone over the next five years.

SOFC Performance

SOFC’s can operate on multiple fuels, including carbon based fuels because of their high operating temperature. This results in high fuel to electric efficiency of around 60% for single cycles and up to 85% total system efficiency. SOFC’s deliver power densities of about 1 MW/m³, the load following capability is excellent and they have the ability to co-generate heat and electric power with the balance in favour of electric power.

Operating Principle and Thermodynamics of SOFC

An SOFC cell, the basic building block, consists of an Y₂O₃-doped ZrO₂ electrolyte (oxygen ion conductor), onto which a cathode (La-manganite) and an anode (Ni/ZrO₂-cermet) are deposited. At the cathode oxygen from air is converted to oxygen ions, which move through the electrolyte membrane and react with the fuel at the anode/electrolyte interface. The process results in an open circuit (E_OCV) or reversible voltage (E_r) according to E_OCV(E_r) = -DG/nF (DG = free energy of the fuel oxidation reaction, n = number of electrons transferred, F = Faraday constant). This voltage is typically 1.1 - 1.2 V for a single cell. Under load conditions a single cell produces about 0.6 to 0.9 V and current densities between 500 to 800 mA.cm^-2 are possible.

Factors Affecting the Efficiency of SOFC

The efficiency is determined by the electric and fuel efficiencies. Fuel efficiencies depend on the type of fuel and temperature of operation, and electric efficiencies are determined by internal losses (resistive (IR) within the electrolyte and overpotential (h) at both electrode/electrolyte interfaces) in the cell.

SOFC Technology

Concepts

Single cells are combined to form multi-cell units, the fuel cell stack. A number of different cell stacking configurations have been reported, which include:

•         Self-supporting concepts, where the electrolyte (80-250gm in thickness) forms a structural element of the design

•         Supported concepts where the electrolyte is deposited as a thin layer (<50gm) onto porous support/electrode structures.

Cells are connected with interconnects (highly conducting ceramic or metal) which may also carry fuel and air distribution channels. Fuel and air are supplied to stacks through manifolds. The total system consists of an air/fuel supply/conditioning unit, the stack, a power conditioning system and a heat recovery system.

Basic Stack Designs

Three basic stack designs are being developed:

•         The tubular design (Westinghouse design) is by far the most advanced SOFC concept with 40 kW prototypes operating. Tubes up to 1 m in length represent single cells and stacks are formed by stacking the tubes together. The concept is a supported cell design where thin adherent layers are deposited on tubular supports by costly fabrication methods such as electrochemical vapor deposition (EVD) or low pressure plasma deposition.

•         The planar or flat plate design is the most common concept under development as its fabrication is potentially least costly. Single cells can be produced by conventional ceramic mass production routes such as tape casting and screen printing. The single cells are stacked together and sealed with a high temperature sealing material. Numerous variations of the concept, including external and internal co-flow, counter-flow and cross-flow manifolded stacks are under development.

•         In the monolithic design green laminates of air electrode/electrolyte/fuel electrode and interconnect are formed and co-sintered. This eliminates the need for high temperature seals, but requires forming the stack by co-sintering, a rather difficult task considering the different materials involved and microstructure requirements for each layer.

Hybrid designs between monolithic and planar are also under development.

Materials for SOFC

Development of SOFC’s at this stage is predominantly a materials challenge, dictated by the high temperature of operation and the multi-component nature of the fuel cell stack (each component cannot be viewed in isolation). There are a number of common requirements of cell components such as: stability (chemical, phase and microstructural in SOFC operating environment), matched thermal expansion, reasonable mechanical properties (strength, toughness, thermal shock resistance), low vapour pressure and cost competitive fabrication. The requirements for fuel cell components are summarised in the following table.

Table 1. Suitable materials for the components of a SOFC.

Fabrication of Cells

Fabrication methods (listed in the Table below) are determined by (i) the type of cell design, (ii) the performance required and (iii) economical manufacture (costs and mass production).

Table 2. Fabrication methods for the various types of fuel cell concepts and their components.

The fabrication methods (CVD/EVD, low pressure plasma spraying) for the sealless tubular Westinghouse concept are very costly, whereas monolithic and flat plate cells are produced by well established green ceramic processing methods (tape casting, calender rolling, screen printing) followed by high temperature sintering, techniques with the potential to be cost effective and suitable for mass production.

Status of SOFC Development

The most recent effort in SOFC development started in 1985 in the USA and in Japan, in 1989 in Europe, and in 1991 in Australia. A large number of companies and government research organisations worlwide are involved in R&D programs on SOFC development, supported largely by public funding agencies (DOE, GRI, MITI, CEC). Westinghouse is generally regarded as the world leader in SOFC development. The company has developed the tubular concept to an advanced state, having built a 40 kW prototype unit. Other players (Siemens, Ceramatec, Fuji Electric, Sanyo, Sulzer) are developing planar stacks with either metallic or ceramic interconnects and units up to 1 kW have been constructed. Mitsubishi Heavy Industries is developing tubular, planar and a monolithic/planar hybrid design and has constructed 1 kW units for each design. In Australia, a SOFC development project started in CSIRO-DMST in 1991. In 1992 a consortium consisting of CSIRO, major utilities, BHP and government R&D funding bodies established Ceramic Fuel Cells Ltd. The company focusses on development of materials and cost effective fabrication technology for planar SOFC stacks.

Potential Markets For Fuel Cells

Several different types of markets have been identified for fuel cells and range from remote area power supply to on-site combined heat and electricity generation, and central station power generation. Local market studies conservatively estimate market size of several hundred million dollars a year in Australia alone by the turn of the century with much higher markets in developing countries in the Asian region. Similar overseas studies indicate market potential of several billion dollars per year for fuel cells.

Challenges

Solid oxide fuel cells have enormous potential provided solutions to the problems of interconnect materials and seals, cost competitive fabrication and long term stability can be found. Efforts are currently under way in reducing the operating temperature (largely determined by the ionic conductivity of the electrolyte) to 800-850°C. This would assist in improving life times and reducing production costs. However, substantial R&D effort to develop electrolytes with higher conductivity or high conductance (thin film technology), more effective electrodes and seals for this temperature range are necessary.

Note – A complete set of references can be obtained by referring to the original text.

Primary author: S.P.S. Badwal and K. Foger

Source: International Ceramic Monographs Vol. 1 no. 2, pp. 761-65, 1994.

For more information on this source please visit The Australasian Ceramic Society.