Molten Carbonate Fuel Cells (MCFC)

Molten Carbonate Fuel Cells (MCFC) are in the class of high-temperature fuel cells. The higher operating temperature allows them to use natural gas directly without the need for a fuel processor and have also been used with low-Btu fuel gas from industrial processes and other sources and fuels. Developed in the mid 1960s, improvements have been made in fabrication methods, performance and endurance.

MCFCs work quite differently from other fuel cells. These cells use an electrolyte composed of a molten mixture of carbonate salts. Two mixtures are currently used: lithium carbonate and potassium carbonate, or lithium carbonate and sodium carbonate. To melt the carbonate salts and achieve high ion mobility through the electrolyte, MCFCs operate at high temperatures (650ºC).

When heated to a temperature of around 650ºC, these salts melt and become conductive to carbonate ions (CO₃^2-). These ions flow from the cathode to the anode where they combine with hydrogen to give water, carbon dioxide and electrons. These electrons are routed through an external circuit back to the cathode, generating electricity and by-product heat.

Anode Reaction:	CO32- + H2 => H2O + CO2 + 2e-
Cathode Reaction:	CO2+ 1/2O2 + 2e- => CO32-
Overall Cell Reaction:	H2(g) + ½O2(g) + CO2 (cathode) => H2O(g) + CO2 (anode)

The higher operating temperature of MCFCs has both advantages and disadvantages compared to the lower temperature PAFC and PEFC. At the higher operating temperature, fuel reforming of natural gas can occur internally, eliminating the need for an external fuel processor. Additional advantages include the ability to use standard materials for construction, such as stainless steel sheet, and allow use of nickel-based catalysts on the electrodes. The by-product heat from an MCFC can be used to generate high-pressure steam that can be used in many industrial and commercial applications.

The high temperatures and the electrolyte chemistry also has disadvantages. The high temperature requires significant time to reach operating conditions and responds slowly to changing power demands. These characteristics make MCFCs more suitable for constant power applications. The carbonate electrolyte can also cause electrode corrosion problems. Furthermore, since CO₂ is consumed at the anode and transferred to the cathode, introduction of CO₂ and its control in air stream becomes an issue for achieving optimum performance that is not present in any other fuel cell.