Science and Technology of Solid- Oxide Fuel Cells
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Science and
Designs, Materials, and Fabrication Processes
Technology of SolidOxide Fuel Cells S.C. Singhal
Introduction The high oxygen-ion conductivity over wide ranges of temperature and oxygen pressure in stabilized cubic zirconia has led to its use as a solid-oxide electrolyte in a variety of electrochemical applications. Zirconia-based oxygen sensors are widely used for combustion control, especially in automobiles, for atmosphere control in furnaces, and as monitors of oxygen concentration in molten metals. Other applications include electrochemical pumps for control of oxygen potential, steam electrolyzers, and high-temperature solidoxide fuel cells (SOFCs). High-temperature SOFCs offer a clean, pollution-free technology to electrochemically generate electricity at high efficiencies. These fuel cells provide many advantages over traditional energy-conversion systems, including high efficiency, reliability, modularity, fuel adaptability, and very low levels of NOx and SOx emissions. The quiet, vibrationfree operation of SOFCs also eliminates the noise usually associated with conventional power-generation systems. Furthermore, because of the high temperature of operation (1000C) of SOFCs, naturalgas fuel can be reformed within the cell stack, eliminating the need for an expensive external reformer system. Also, pressurized SOFCs can be successfully used as replacements for combustors in gas turbines; such hybrid SOFC/gas-turbine power systems are expected to reach efficiencies approaching 70%.
conducting electrolyte. At the anode, oxygen ions combine with H2 (and/or CO) in the fuel to form H2O (and/or CO2), liberating electrons. Electrons flow from the anode through the external circuit to the cathode. The overall cell reaction is simply the oxidation of fuel (H2 and/or CO), and the open circuit voltage, E0, of the fuel cell is given by the Nernst equation: E0
RT PO oxidant ln , 4F PO fuel 2
2
(1)
where R is the gas constant, T is the cell temperature, F is the Faraday constant, and PO is the oxygen partial pressure. Under cell operating conditions—that is, when a current passes through it—the cell voltage V is given by 2
V E0 IR A F ,
(2)
where I is the current passing through the cell, R is the electrical resistance of the cell, and A and F are the polarization voltage losses associated with the air electrode and the fuel electrode, respectively. To
SOFCs of several different designs have been investigated; these include planar, monolithic, and tubular geometries.1–7 The materials for cell components in these different designs are the same or very similar in nature. In the planar design, illustrated in Figure 2, the cell components are configured as thin, flat plates. The interconnection, which is ribbed on both sides, forms gas-flow channels and serves as a bipolar gas separator contacting the anode and the cathode of adjoining cells. The dense electrolyte and interconnection are fabricated by tape casting, powder sintering, or chemical vapor deposition (CVD),8,9 wh
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