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 (CO32-) 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.
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:
oxidation of hydrogen:
H2O + CO2 + 2e-
reduction of oxygen:
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%.
Friedrichshafen MCFC stacks.
Friedrichshafen Hot Modules.
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
250 kW MCFC Fuel Cell Energy.
MW MCFC Fuel Cell Energy.
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.