Molten carbonate fuel cells (MCFCs) use a molten carbonate salt suspended in a porous ceramic matrix as the electrolyte. Salts commonly used include lithium carbonate, potassium carbonate and sodium carbonate.

They operate at high temperature, around 650ºC and there are several advantages associated with this. Firstly, the high operating temperature dramatically improves reaction kinetics and thus it is not necessary to boost these with a noble metal catalyst. The higher temperature also makes the cell less prone to carbon monoxide poisoning than lower temperature systems. As a result, MCFC systems can operate on a variety of different fuels, including coal-derived fuel gas, methane or natural gas, eliminating the need for external reformers.

Disadvantages associated with MCFC units arise from using a liquid electrolyte rather than a solid and the requirement to inject carbon dioxide at the cathode as carbonate ions are consumed in reactions occurring at the anode. There have also been some issues with high temperature corrosion and the corrosive nature of the electrolyte but these can now be controlled to achieve a practical lifetime.

MCFCs are used in large stationary power generation. Most fuel cell power plants of megawatt capacity use MCFCs, as do large combined heat and power (CHP) and combined cooling and power (CCP) plants. These fuel cells can work at up to 60% efficiency for fuel to electricity conversion, and overall efficiencies can be over 80% in CHP or CCP applications where the process heat is also utilised.