The molten carbonate electrolyte is usually a Lithium/Potassium Carbonate  (Li₂CO₃/K₂CO₃) or Lithium/Sodium Carbonate (Li₂CO₃/Na₂CO₃) in an aluminium-based ceramic matrix (LiAlO₂). At the high operating temperatures, the alkali carbonates form a highly conductive molten salt, with carbonate ions providing the ionic conductitvity. The matrix can be supported with Al₂O₃ fibres for mechanical strength. Sodium instead of potassium stabilises the electrolyte and is expected to increase its lifetime.  Since carbonate ions (CO₃^2-) are transported from the cathode through the electrolyte to the anode, CO₂ in addition to oxygen is required at the cathode. Usually it is recycled from the anode exhaust.

Due to the very high operating temperatures (600-700°C), the cathode kinetics are drastically improved compared to PEMFC and PAFC, and there is no need for a precious metal as the catalyst. On the cathode there is usually a nickel oxide, but lithium oxide materials have also been investigated. For the anode, nickel-aluminium or nickel-chrome alloys are used. The electrode and overall reactions are shown below:

Anode oxidation of hydrogen:

H₂ + (CO₃)^2- H₂O + CO₂ + 2e^-

Cathode reduction of oxygen:

CO₂ + ½O₂ + 2e^- (CO₃)^2- 

Total MCFC reaction:

H₂ + ½O₂ H₂O

Provided a reforming catalyst is added, internal reforming of fossil fuels can take place in the cell. The rejected (waste) heat is also of sufficiently high temperature to run a gas turbine and/or produce high-pressure steam for use in a steam turbine or for co-generation of heat and power. Because of the exploitation of the waste heat, system efficiencies up to 80% can be reached, with a fuel cell efficiency exceeding 50%.

MTU Friedrichshafen MCFC stacks.

MTU Friedrichshafen Hot Modules.

In contrast to the low-temperature fuel cells, the MCFC has no difficulties with CO₂ and CO in the fuel. However, the low sulphur tolerance of the reforming catalyst presents a problem. All fossil fuels contain some sulphur, and due to the relatively low reforming temperature, this poisons the catalyst. Other disadvantages are the degradation of materials due to the high temperature combined with the very corrosive electrolyte. Important issues in the material selection are therefore degradation, sealing and thermal expansion properties. Special stainless steel types based on Ni, Co, Fe or Cr/Al have proven to be suitable under these operating conditions.

The MCFC is being developed for power plants based on natural gas and coal, from some hundred kW to several MW. Companies like MTU Friedrichshafen, Fuel Cell Energy (former Energy Research Corporation) and some Japanese companies (Hitachi, IHI and Mitsubishi Electric) have already developed prototype systems.

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