The SOFC has the most desirable properties for generating electricity from hydrocarbon fuels. The SOFC uses a solid electrolyte and is very effi­cient. It can internally reform hydrocarbon fuels and is tolerant to impu­rities. The SOFC operates at a very high temperature (700-1000°C) and so does not require any cooling system for maintaining a fuel cell oper­ating temperature. For small systems, insulation has to be provided to maintain the cell temperature. In large SOFC systems, the operating temperature is maintained internally by the reforming action of the fuel and by the cool outside air (oxidant) that is drawn into the fuel cell. At high operating temperatures, chemical reaction rates in the SOFC are high and air compression is not required. This results in a simpler
system, quiet operation, and high efficiencies. Westinghouse has worked at developing a tubular style of the SOFC that operates at 1000°C (see Fig. 9.10) for many years [1-3, 26, 27]. These long tubes have high elec­trical resistance but are simple to seal. Many other manufacturers are now working on a planar SOFC composed of thin ceramic sheets which operate at 800°C or even less. Thin sheets offer low electrical resistance, and cheaper materials such as stainless steel can be used at these lower temperatures [3, 6, 26]. One big advantage of the SOFC over the MCFC is that the electrolyte is a solid. Therefore, no pumps are required to cir­culate a hot electrolyte, and very compact, small planar SOFC systems of a few kW range could be constructed using very thin sheets.

A major advantage of the SOFC is that both hydrogen and carbon monoxide are used in the cell. Therefore, in the SOFC, many common hydrocarbon fuels such as natural gas, diesel, gasoline, alcohol, and coal gas can be safely used. The SOFC can reform these fuels into hydrogen

and carbon monoxide inside the cell, and the high-temperature waste thermal energy can be recycled back for fuel reforming. During oper­ation, the SOFC is at the same time a generator and a user of heat. Heat is generated through exothermic chemical reactions and ohmic losses, while it is absorbed by the reforming reaction. It is possible to design the SOFC to be thermally balanced, thereby eliminating the requirement for external insulation and heating. Small SOFC systems are not thermally self-sustaining and may require an external heat source to start and maintain operation. In large systems, the heat gen­erated is not fully absorbed by fuel reforming, and the excess heat can be used in gas turbines for generating electricity or for cogeneration. Another advantage of the SOFC is that expensive catalysts are not required. However, a few minutes of fuel burning is required to reach the operating temperature of the SOFC at the start. This time delay is a disadvantage for an automotive application, but for stationary electric power plants, this is not a problem as they run continuously for long periods of time.

Electrochemistry of SOFCs. Hydrogen or carbon monoxide in the fuel stream reacts with oxide ions (O2 ) from the electrolyte to produce water or CO2 and to deposit electrons into the anode. The electrons pass out­side the fuel cell, through the load, and back to the cathode, where oxygen from the air receives the electrons and is converted into oxide ions, which are injected into the electrolyte. In the SOFC, oxygen ions are formed at the cathode. The reaction at the cathode is

O2 + 4e~ ^ 2O2~

At the operating temperature, the electrolyte offers high ionic con­ductivity and low electrical conductivity; therefore, oxygen ions migrate through the electrolyte to the anode. The overall reaction occurring at the anode is as follows:

The hydrogen in the fuel reacts with the oxygen ions to produce water and releases two electrons.

H2 + O2 ^ H2O + 2e

Carbon monoxide present in the fuel causes a shift reaction to produce additional fuel (H2).

CO + H2O ^ H2+ CO2

The following internal reforming reaction for the hydrocarbon fuel takes place on the anode side:

CxHy + xH2O ^ xCO + (x + t^)H2

For methane-rich fuels, this reforming reaction is CH4 + H2O — CO + 3H2

This reaction is generally not in chemical equilibrium, and the CO shift reaction takes place to provide more hydrogen. The overall cell reaction is

H2+ |o2 — H2O

Electrolyte. The use of a solid electrolyte in the SOFC eliminates the electrolyte management problems associated with the liquid electrolyte fuel cells and also reduces corrosion considerations to a great extent. In the SOFC, it is the migration of oxygen ions (O2) through the elec­trolyte that establishes the voltage difference between the anode and cathode. Therefore, the electrolyte must be a good conductor of O2~ ions and a bad conductor of electrons; it must also be stable at the high oper­ating temperature. Some ceramics possess these properties and there­fore are good candidates for this application. With the help of modern ceramic technology and solid-state science, many ceramics can be tai­lored for electrical properties unattainable in metallic or polymer mate­rials. These tailored ceramic materials are termed electroceramics, and one group is known as fast ion conductors or superionic conductors. These superionic conductors when used as a solid electrolyte allow easy passage of ions from the cathode to the anode in an SOFC. The material generally used as an electrolyte in the SOFC is dense yttria-stabilized zirconia. It is an excellent conductor of negatively charged oxygen ions at high temperatures (1000°C), but its conductivity reduces drastically with the drop in temperature. Other materials such as scandia-stabilized zirconia (ScSZ), which shows good ionic conductivity at a lower temper­ature (800°C), are also being investigated, but the electrolyte developed with ScSZ-based materials is very expensive and they degrade very fast.

Electrode. The anode is made of metallic Ni and Y2O3-stabilized ZrO2 (YSZ). Ytrria-stabilized zirconia is added in Ni to inhibit sintering of the metal particles and to provide a thermal expansion coefficient close to those of the other cell materials [26]. Nickel structure is normally obtained from NiO powders; therefore, before starting the operation for the first time, the cell is run with hydrogen in an open-circuit condition to reduce the NiO to nickel. The anode structure is fabricated with a porosity of 20-40% to facilitate mass transport of the reactant and prod­uct gases. The Sr-doped lanthanum manganite (La1_x Srx MnO3, x = 0.10-0.15; known as LSM) is most commonly used for the cathode mate­rial. LSM is a p-type semiconductor. Similar to the anode, the cathode is also a porous structure that permits rapid mass transport of the reac­tant and product gases.

Hardware. In the SOFC, both CO and hydrogen are used as direct fuel. Therefore, it is important that the fuel and air streams are kept sepa­rate, and a thermal balance should be maintained to ensure that oper­ating temperatures remain within an acceptable range. Several designs of the SOFC (tubular and planer) have been developed to accommodate these requirements. The SOFC is a solid-state device and shares certain properties and fabrication techniques with semiconductor devices.

Individual cells in the stack are connected by interconnects, which carry an electrical current between cells and can also act as a separa­tor between the fuel and oxidant supplies. In high-temperature SOFCs, the interconnects that are used are ceramic such as lanthanum chromite, or if the temperature is limited to less than 1000°C, a refractory alloy based on Y/Cr may be used. The interconnects constitute a major pro­portion of the stack cost. Stack and other plant construction materials that are used also need to be refractory to withstand the high-temperature gas streams. Volatility of chromium-containing ceramics and alloys can result in contamination of the stack components, and the presence of a toxic material such as Cr6+ requires special disposal procedures.

The high operating temperature (1000°C) of the SOFC requires a sig­nificant start-up time. The cell performance is very sensitive to operat­ing temperatures. A 10% drop in temperature results in an ~12% drop in cell performance due to the increase in internal resistance to the flow of oxygen ions. The high temperature also demands that the system include significant thermal shielding to protect personnel and to retain heat. Also, the materials required for such high-temperature operation, particularly for interconnect and construction materials, are very expen­sive. Operating the SOFC at temperatures lower than 700°C would be very beneficial as low-cost metallic materials, such as ferritic stainless steels, that can be used as interconnect and construction materials. This will make both the stack and balance of a plant cheaper and more robust. Using ferritic materials also significantly reduces the problems associated with chromium. The other advantages of low/intermediate — temperature operation are rapid start-up and shutdown and signifi­cantly reduced corrosion rates.

However, to operate at reduced temperatures, several changes are required in stack design, cell materials, reformer design and operation, and operating conditions. With the reduction in operating temperature, the ionic conductivity of the electrolyte decreases and the parasitic losses due to the conductivity of the electrodes and interconnects increase. This results in a rapid deterioration of the performance of the SOFC. This can be overcome to some extent by reducing the thickness of the electrolyte to compensate for its reduced ionic conductivity. The thick­ness reduction that is required to accommodate, say a 200°C reduction in the operating temperature, leads to impracticably thin membranes.

Some designs in which a thin, dense layer of the electrolyte is physically supported on one of the electrodes (electrode-supported design) are sug­gested. This structure of a very porous support is difficult to manufac­ture, and an expensive thin-film deposition technique such as chemical vapor deposition (CVD) is needed to manufacture these systems. Even then, the mechanical strength of the structure (defined by the porous electrode) is often poor, and the handling of the structure through sub­sequent processing and assembly is difficult. Another approach to improve SOFC performance at low operating temperatures is to use different materials for the electrolyte and the electrode. Several mate­rials options are being investigated [2, 6, 26, 27].