• Introduction
• Principles of Operation
• Cell, Stack, and Module Description
• SOFC Power Systems
• Summary & Conclusions

Introduction
A fuel cell converts the chemical energy stored in gaseous fuel (e.g., H₂, CO, CH₄, etc.) to DC electricity and thermal energy. The cell consists of a positive electrode (cathode or air electrode), a negative electrode (anode or fuel electrode), and an electrolyte, sandwiched between the two electrodes. The electrodes are electronic (e^-) conductors, and the electrolyte is an ionic conductor, but not a conductor of electrons. (As discussed in a later section, an interconnect is required when two or more cells are to be connected in electrical series.) Fuel cells are named after their electrolyte material, the selection of which will dictate the cell operating temperature. Two common, low temperature fuel cells are the Proton Exchange Membrane (PEM), which operates at ~100°C, and the Phosphoric Acid Fuel Cell (PAFC), which operates at ~200°C. The PEM and PAFC electrolytes conduct the hydrogen ion (H⁺).

The high temperature fuel cells are the Molten Carbonate Fuel Cell (MCFC), which operates at ~600°C, and the Solid Oxide Fuel Cell (SOFC), which operates in the 650 to 1000°C range, depending on material/geometry selections. The MCFC electrolyte conducts the carbonate ion (CO₃⁼) and the SOFC electrolyte conducts the oxygen ion (O⁼). The SOFC has a number of broad advantages compared to other fuel cell types, such as:

1. Rating scalability, from a few kWe to >100 MWe net system output,
2. Fuel flexibility and the ability for direct-internal-reformation of hydrocarbon fuels,
3. Very high electrical efficiencies, particularly in combined cycle applications due to its high
   operating temperature, and
4. The potential for very low voltage degradation rates and long life.

Additionally, the technology handles well the common gaseous contaminants found in coal syngas, such as HCl, ammonia, and H₂S. Relative to these contaminants, test results show no voltage degradation with syngas at the 1 ppm HCl level, a NETL warm syngas cleanup system target, and no voltage degradation at 1000 ppm ammonia levels, also a cleanup target. Sulfur-bearing compounds, which can cause reversible voltage declines, are readily reduced to SOFC-acceptable levels by conventional pre-SOFC desulfurization techniques. (Additional information on SOFC and other fuel cell types can be found in the Fuel Cell Handbook.

The planar SOFC has been selected for development under the Department of Energy's Solid State Energy Conversion Alliance (SECA) program, largely because of its potential for high power density and low manufacturing cost, and its description follows.