Background edit

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.^[2] The operating pressure is between 1-8 atm while the temperatures are between 600 and 700 °C.^[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). Current MCFC efficiencies range from 60 to 70%.^[4]

Reactions^[5] edit

Internal Reformer (methane example):

${\displaystyle CH_{4}+H_{2}O=3H_{2}+CO}$ 

Anode (hydrogen example):

${\displaystyle H_{2}+CO_{3}^{2-}=H_{2}O+CO_{2}+2e^{-}}$ 

Cathode:

${\displaystyle {\frac {1}{2}}O_{2}+CO_{2}+2e^{-}=CO_{3}^{2-}}$ 

Cell:

${\displaystyle H_{2}+{\frac {1}{2}}O_{2}=H_{2}O}$ 

Nernst Equation:

${\displaystyle E=E^{o}+{\frac {RT}{2F}}log{\frac {P_{H_{2}}P_{O_{2}}^{\frac {1}{2}}}{P_{H_{2}O}}}+{\frac {RT}{2F}}log{\frac {P_{CO_{2},cathode}}{P_{CO_{2},anode}}}}$ 

Materials edit

Due to the high operating temperatures of MCFC's, 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.

Anode edit

The anode material typically consists of a porous (3-6 μm, 45-70% material porosity) Ni based alloy. Ni is alloyed with either Chromium or Aluminum in the 2-10% range. 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.^[6] Recent research has looked at using nano Ni and other Ni alloys to increase the performance and decrease the operating temperature 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. At the same time, a decrease in temperature would decrease ionic conductivity of the electrolyte and thus, the anode materials need to compensate for this performance decline (e.g. by increasing power density). Other researchers have looked into enhancing creep resistance by using a Ni₃Al alloy anode to reduce mass transport of Ni in the anode when in operation.^[8]

Cathode edit

On the other side of the cell, the cathode material is composed of either Lithium metatitanate or of a porous Ni that is converted to a lithiated nickel oxide (lithium is intercalated within the NiO crystal structure). The pore size within the cathode is in the range of 7-15 μm with 60-70% of the material being porous.^[9] The primary issue with the cathode material is dissolution of NiO since it reacts with CO₂ when the cathode is in contact with the carbonate electrolyte. 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. Alternatively, replacement of the conventional cathode material with a LiFeO2-LiCoO2-NiO alloy has shown promising performance results and almost completely avoids the problem of Ni dissolution of the cathode.^[10]

Electrolyte edit

MCFC's use a liquid electrolyte (molten carbonate) which consists of a sodium(Na) and potassium(K) carbonate. This electrolyte is supported by a ceramic (LiAlO₂) matrix to contain the liquid between the electrodes. The high temperatures of the fuel cell is required to produce sufficient ionic conductivity of carbonate through this electrolyte.^[3] Common MCFC electrolytes contain 62% Li₂CO₃ and 38% K₂CO₃.^[11] A greater fraction of Li carbonate is used due to its higher ionic conductivity but is limited to 62% due to its lower gas solubility and ionic diffusivity of oxygen. In addition, Li₂CO₃ is a very corrosive electrolyte and this ratio of carbonates provides the lowest corrosion rate. Due to these issues, recent studies have delved into replacing the potassium carbonate with a sodium carbonate.^[12] A Li/Na electrolyte has shown to have better performance (higher conductivity) and improves the stability of the cathode when compared to a Li/K electrolyte (Li/K is more basic). 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. If fueled by fuels that contain carbon such as natural gas, the exhaust will contain CO₂ but will be reduced by up to 50% compared to diesel engines running on marine bunker fuel.^[13] The exhaust temperature is 400 °C, hot enough to be used for many industrial processes. Another possibility is to make more electric power via a steam turbine. Depending on feed gas type, the electric efficiency is between 12% and 19%. A steam turbine can increase the efficiency by up to 24%. The unit can be used for cogeneration.

• Glossary of fuel cell terms
• Hydrogen technologies

• https://web.archive.org/web/20060927032111/http://www.eere.energy.gov/hydrogenandfuelcells/fuelcells/fc_types.html#molten

• LLNL: The Carbon/Air Fuel Cell Conversion of Coal-Derived Carbons
• DoD
• MTU 240kW fuel cell presented on the Hannover Fair 2006
• Logan Energy Limited integrate, install and operate all fuel cell technologies
• [1] molten carbonate fuel cells distributed generation challenge
• [2] presentation to Fourth Annual Conference on Carbon Capture and Sequestration