Molten Carbonate Fuel Cell
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 (600-700ºC). Peak power output is between 100-200 .
When heated to a temperature of around 650ºC, these salts melt and become conductive to carbonate ions (CO32-). 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.
CO32- + H2 => H2O + CO2 + 2e-
CO2+ 1/2O2 + 2e- => CO32-
Overall Cell Reaction:
H2(g) + ½O2(g) + CO2 (cathode) => H2O(g) + CO2 (anode)
MCFCs exhibit up to 60 percent efficiency, and this can rise to 75 percent if the waste heat is utilized for cogeneration.
High-temperature MCFCs can extract hydrogen from a variety of fuels using either an internal or external reformer. They are also less prone to carbon monoxide "poisoning" than lower temperature fuel cells, which makes coal-based fuels more attractive for this type of fuel cell. MCFCs work well with catalysts made of nickel, which is much less expensive than platinum. Currently, demonstration units have produced up to 2 megawatts (MW), but designs exist for units of 50 to 100 MW capacity.
Two major difficulties with molten carbonate technology put it at a disadvantage compared to solid oxide cells. One is the complexity of working with a liquid electrolyte rather than a solid. The other stems from the chemical reaction inside a molten carbonate cell. Carbonate ions from the electrolyte are used up in the reactions at the anode, making it necessary to compensate by injecting carbon dioxide at the cathode.
Molten carbonate fuel cells demand such high operating temperatures that most applications for this kind of cell are limited to large, stationary power plants. Yet consumers might benefit from this type of cell, even if they never see it in their homes. The high operating temperature opens the opportunity of using waste heat to make steam for space heating, industrial processing, or in a steam turbine to generate more electricity. Many modern gas-fired power plants exploit this type of system, called cogeneration.