Fatih Dogan

Department of Materials Science and Engineering University of Missouri-Rolla

CONTENTS

Introduction………………………………………………………………………………………………….. 204

Fuels for Solid-Oxide Fuel Cells…………………………………………………………………. 205

Single-Chamber Solid-Oxide Fuel Cells and Hydrocarbon Fuels…………………… 209

Summary………………………………………………………………………………………………………. 211

References…………………………………………………………………………………………………….. 212

Abstract This chapter addresses utilization of alcohol and other hydrocarbon — based fuels to generate electricity in solid-oxide fuel cells (SOFCs). One of the key advantages of SOFC is that both external as well internal fuel reforming is possible to operate the fuel cell under stable conditions. While alcohol fuels can be obtained sulfur-free and in high purity, hydrocarbon fuels have higher energy density and existing infrastructure of production and distribution. Development of more energy-efficient and chemically stable electrode materials is necessary for SOFC operating at high (800-1000°C) and intermediate (500-800°C) tem­peratures. Significant progress has been made in recent years in the development of carbon monoxide-tolerant fuel electrodes (anodes) to prevent carbon deposition on the catalyst that results in a reduced performance of the fuel cell. Development of fuel electrodes compatible with alcohol and hydrocarbon fuels will lead to more efficient and widespread applications of SOFCs in double-chamber and single-chamber modes.

INTRODUCTION

Fuel cells are viewed as environmentally compatible and efficient energy conver­sion systems. A fuel cell works much like a battery with external fuel supplies. Chemical fuels are electrochemically converted into electricity at high efficiencies without producing significant amount of pollutants such as nitrogen oxides as compared to combustion engines. Hydrogen is the ideal fuel since it reacts with oxygen in the air to produce water and an electric current, but hydrogen is expensive and difficult to store. Until the hydrogen economy is well established, other fuels can be used indirectly with an external reformer or directly to operate fuel cells. Hydrogen is stored naturally in alcohols (e. g., ethanol and methanol) or hydrocarbons such as propane and methane, which are available to produce cleaner power if the electrochemical processes of hydrocarbon oxidation reactions are well understood.

Among various fuel cells, solid oxide fuel cells (SOFCs) and molten car­bonate fuel cells can be operated using hydrogen as well as carbon monoxide. Particularly, SOFC is viewed as the most flexible fuel cell system that can operate using various fuel gases directly supplied to the fuel electrodes [1-3]. Removal of CO from H2 fuel is essential for polymer electrolyte membrane fuel cells, which are generally considered to be the most viable approach for mobile applications.

The application of high and intermediate temperature SOFCs range from small-scale domestic heat and power to large-scale distributed power generation. SOFCs offer high efficiencies up to 60-70% in individual systems and up to 80% in hybrid systems by extracting the energy present in the high-temperature exhaust gases, e. g., by using gas or steam turbines [4]. High-temperature SOFC applica­tions include multimegawatt-scale centralized power generation, distributed power generation up to 1 MW and combined heat/power (CHP) plants in the 100-kW to 1-MW range. Potential areas of application for intermediate SOFCs are in the transport sector (up to 50 kW), military and aerospace (5 to 50 kW), domestic CHP (up to 10 kW) and miniaturized fuel cells “palm-power” in the 10-W range.

In SOFC, the electrolyte is typically a dense yttria-stabilized zirconia (YSZ), which is an ionic conductor blocking electron transport as shown in Figure 11.1. The electrolyte allows the transport of oxygen ions via the oxygen vacancies from the interface at the air electrode (cathode) to the interface with the fuel electrode (anode). The cathode is typically composed of a porous lanthanum strontium manganese oxide with YSZ and facilitates the reaction for the reduction of oxygen gas to oxygen ions at the electrode/electrolyte interface. The anode material is typically a porous Ni-YSZ composite allowing the oxi­dation of the fuel and transport of the electrons from the electrolyte/electrode interface to the interconnect of the fuel cell stack. The interconnect material is typically lanthanum strontium chromite for high-temperature operation while corrosion-resistant metallic alloys are employed in the development of SOFCs operating at intermediate temperatures. The role of the interconnect is to transfer

the electrons between the individual cells in the stack and to prevent mixing of fuel and oxidant gases [5].

A diverse range of fuels can be used in SOFCs since the internal temperature is high enough to initiate fuel conversion reactions. Hence, SOFCs have an efficiency advantage over polymer electrolyte membrane fuel cells when alcohol or hydrocarbon fuels are to be used, even though direct-methanol fuel cells with polymer electrolyte membranes are widely studied. The use of these fuels in SOFCs without preprocessing, however, requires further advances in development of appropriate electrode materials toward preventing unwanted reactions such as carbon formation on the anode, which significantly affects the performance of the fuel cell.