Operation of SOFCs
The electrochemical reactions in most fuel cells require hydrogen fuel and are rendered inoperable if any compounds other than hydrogen are present in the fuel stream. SOFCs are more durable and can be fueled with a mix of hydrogen and carbon monoxide (CO). Hydrogen fuel must be produced internally from the fuel cell by a separate process such as steam reforming. Because of their high operating temperatures and tolerance of CO, SOFCs can sustain the endothermic reaction necessary to reform pipeline natural gas internally to hydrogen and CO. This gives SOFCs a significant advantage in cost, flexibility, and simplicity over other processes that require external reforming.
When oxygen and fuel are supplied to an SOFC, electricity is produced by movement of ions across an electrolyte. A tubular SOFC operates at a temperature of around 1000 degrees C (1800 degrees F) and requires an external heat source only during start up. During operation oxygen in the air is ionized at the cathode (Figure 2). Oxygen ions flow toward the cathode through the solid electrolyte, a crystal lattice typically made up of zirconium oxide and calcium oxide. Fuel is introduced outside the fuel electrode. Oxygen ions combine with the fuel at the anode and oxidize hydrogen to H2O and CO to CO2 releasing electrons. If the anode and cathode are connected in a circuit, current will flow. After startup it is self sustaining and will continue to operate long as fuel and air are supplied.