Molten carbonate fuel cell
Molten carbonate fuel cells (MCFCs) were developed for natural gas, biogas (produced as a result of anaerobic digestion or biomass gasification), and coal-based power plants for electrical utility, industrial, and military applications.
MCFCs are high-temperature fuel cells that use an electrolyte composed of a molten carbonate salt mixture suspended in a porous, chemically inert ceramic matrix of beta-alumina solid electrolyte (BASE).
Since they operate at extremely high temperatures of 650 °C (roughly 1,200 °F) and above, non-precious metals can be used as catalysts at the anode and cathode, reducing costs.
[1] Improved efficiency is another reason MCFCs offer significant cost reductions over phosphoric acid fuel cells (PAFCs).
[1] Unlike alkaline, phosphoric acid, and polymer electrolyte membrane fuel cells, MCFCs don't require an external reformer to convert more energy-dense fuels to hydrogen.
Because they are more resistant to impurities than other fuel cell types, scientists believe that they could even be capable of internal reforming of coal, assuming they can be made resistant to impurities such as sulfur and particulates that result from converting coal, a dirtier fossil fuel source than many others, into hydrogen.
Alternatively, because MCFCs require CO2 be delivered to the cathode along with the oxidizer, they can be used to electrochemically separate carbon dioxide from the flue gas of other fossil fuel power plants for sequestration.
[1] Molten carbonate FCs are a recently developed type of fuel cell that targets small and large energy distribution/generation systems since their power production is in the 0.3-3 MW range.
[3] Due to the production of CO2 during reforming of the fossil fuel (methane, natural gas), MCFCs are not a completely green technology, but are promising due to their reliability and efficiency (sufficient heat for co-generation with electricity).
Due to the high operating temperatures of MCFCs, the materials need to be very carefully selected to survive the conditions present within the cell.
The following sections cover the various materials present in the fuel cell and recent developments in research.
These alloying elements allow for formation of LiCrO2/LiAlO2 at the grain boundaries, which increases the materials' creep resistance and prevents sintering of the anode at the high operating temperatures of the fuel cell.
[7] A reduction in operating temperature would extend the lifetime of the fuel cell (i.e. decrease corrosion rate) and allow for use of cheaper component materials.
This dissolution leads to precipitation of Ni metal in the electrolyte and since it is electrically conductive, the fuel cell can get short circuited.
Therefore, current studies have looked into the addition of MgO to the NiO cathode to limit this dissolution.
[10] Magnesium oxide serves to reduce the solubility of Ni2+ in the cathode and decreases precipitation in the electrolyte.
The high temperatures of the fuel cell is required to produce sufficient ionic conductivity of carbonate through this electrolyte.
In addition, scientists have also looked into modifying the matrix of the electrolyte to prevent issues such as phase changes (γ-LiAlO2 to α-LiAlO2) in the material during cell operation.
The phase change accompanies a volume decrease in the electrolyte which leads to lower ionic conductivity.
Through various studies, it has been found that an alumina doped α-LiAlO2 matrix would improve the phase stability while maintaining the fuel cell's performance.
[12] The German company MTU Friedrichshafen presented an MCFC at the Hannover Fair in 2006.
The unit weighs 2 tonnes and can produce 240 kW of electric power from various gaseous fuels, including biogas.
