The MCFC has evolved from work in the 1960s, aimed at producing a fuel cell that would operate directly on coal [23, 24]. Although direct oper­ation on coal is no longer a goal, a remarkable feature of the MCFC is that it can directly operate on coal-derived fuel gases or natural gas and is therefore also called a direct fuel cell (DFC). MCFCs operate at high temperatures (600-650°C) compared to phosphoric acid (180-220°C) or PEM fuel cells (60-85°C). Operation at high temperatures eliminates the need for external fuel processors that the lower temperature fuel cells require to extract hydrogen from naturally available fuel. When natu­ral gas is used as fuel, methane (the main ingredient of natural gas) and water (steam) are converted into a hydrogen-rich gas inside the MCFC stack (“internal reforming”) (see Fig. 9.9). High operating temperatures also result in high-temperature exhaust gas, which can be utilized for heat recovery for secondary power generation or cogeneration. MCFCs can therefore achieve a higher fuel-to-electricity and an overall energy use efficiency (>75%) than the low-temperature fuel cells. The MCFC

Fuel + steam

CO,

Internal

reforming

Anode

is a well-developed fuel cell and is a commercially viable technology for a stationary power plant, compared to other fuel cell types. A number of MCFC prototype units in the power range of 200 kW to 1 MW and higher are operating around the world. The cost and useful life issues are the major challenges to overcome before the MCFC can compete with the existing (thermal or other) electric power generation systems for widespread use.

Electrochemistry of MCFC. The electrochemical reactions occurring in the cell are:

Anode half reaction. At the anode, hydrogen reacts with carbonate ions to produce water, carbon dioxide, and electrons. The electrons travel through an external circuit—creating electricity—and return to the cathode.

H2 + CO32~ ^ H2O + CO2 + 2e~

Cathode half reaction. At the cathode, oxygen from the air and carbon dioxide recycled from the anode react with the electrons to form car­bonate ions that replenish the electrolyte and transfer the current through the fuel cell, completing the circuit.

2 O2 + CO2 + 2e S CO3

The overall cell reaction is

H2 + 2 O2 + CO2 (cathode) s H2O + CO2 (anode)

If a fuel such as natural gas is used, it has to be reformed either exter­nally or within the cell (internally) in the presence of a suitable cata­lyst to form H2 and CO by the reaction:

CH4 + H2O ^ 3H2 + CO

Although, CO is not directly used by the electrochemical oxidation, but produces additional H2 by the water gas shift reaction:

CO + H2O ^ H2 + CO2

Typically, the CO2 generated at the anode is recycled to the cathode, where it is consumed. This requires additional equipment to either trans­fer CO2 from the anode exit gas to the cathode inlet gas or produce CO2 by combustion of anode exhaust gas and mix with the cathode inlet gas.

Electrolyte. The MCFC uses a molten carbonate salt mixture as its electrolyte. At operating temperatures of about 650°C, the salt mixture is in a molten (liquid) state and is a good ionic conductor. The composi­tion of salts in the electrolyte may vary but usually consist of lithium/potassium carbonate (Li2CO3/K2CO3, 62-38 mol%) for operation at atmospheric pressure. For operation under pressurized conditions, lithium/sodium carbonate (LiCO3/NaCO3, 52-48 or 60-40 mol%) is used as it provides improved cathode stability and performance. This allows for the use of thicker Li/Na electrolyte for the same performance, resulting in a longer lifetime before a shorting caused by internal precipitation. The composition of the electrolyte has an effect on electrochemical activ­ity, corrosion, and electrolyte loss rate. Li/Na offers better corrosion resistance but has greater temperature sensitivity. Additives are being developed to minimize the temperature sensitivity of the Li/Na elec­trolyte. The electrolyte has a low vapor pressure at the operating tem­perature and may evaporate very slowly; however, this does not have any serious effect on the cell life. The electrolyte is suspended in a porous, insulating, and chemically inert ceramic (LiAlO2) matrix. The ceramic matrix has a significant effect on the ohmic resistance of the electrolyte. It accounts for almost 70% of the ohmic polarization. The electrolyte management in an MCFC ensures that the electrolyte matrix remains completely filled with the molten carbonate, while the porous electrodes are partially filled, depending on their pore size distributions.

Electrode. The anode is made of a porous chromium-doped sintered Ni-Cr/Ni-Al alloy. Because of the high temperatures resulting in a fast anode action, a large surface area is not required on the anode as com­pared to the cathode. Partial flooding of the anode with molten carbon­ate is desirable as it acts as a reservoir that replenishes carbonate in the stack during prolonged use. The cathode is made up of porous lithi — ated nickel oxide. Because of the high operating temperatures, no noble catalysts are needed in the fuel cell. Nickel is used on the anode and nickel oxide on the cathode as catalysts. Bipolar plates or interconnects are made from thin stainless steel sheets with corrugated gas diffusion channels. The anode side of the plate is coated with pure nickel to pro­tect against corrosion.

Performance. At the high operating temperatures of an MCFC, CO is not a poison but acts as a fuel. In the MCFC, CO2 has to be added to oxygen (air) stream at the cathode for generation of carbonate ions. The anode reaction converts these ions back to CO2, resulting in a net transfer of two ions with every molecule of CO2. The need for CO2 in the oxidant stream requires that CO2 from the spent anode gas be separated and mixed with the incoming air stream. Before this can be done, any resid­ual hydrogen in the spent fuel stream must be burned. Systems devel­oped in the future may incorporate membrane separators to remove the hydrogen for recirculation back to the fuel stream to increase efficiency.

Internal reforming of natural gas and partially cracked hydrocarbons is possible in the inlet chamber of the MCFC, eliminating the separate fuel processing of natural gas or other hydrogen-rich fuels. The require­ment for CO2 makes the digester gas (sewage, animal waste, food pro­cessing waste, etc.) an ideal fuel for the MCFC; other fuels such as natural gas, landfill gas, propane, coal gas, and liquid fuels (diesel, methanol, ethanol, LPG, etc.) can also be used in the MCFC system. The elimination of the external fuel reformer also contributes to lower costs, and high-temperature waste heat can be utilized to make additional elec­tricity and cogeneration. MCFCs can reach overall thermal efficiencies as high as 85%.

With the increase in operating temperature, the theoretical operat­ing voltage for a fuel cell decreases, but increases the rate of the elec­trochemical reaction and therefore the current that can be obtained at a given voltage. This results in the MCFC having a higher operating volt­age for the same current density and higher fuel efficiency than a PAFC of the same electrode area. As size and cost scale roughly with the elec­trode area, the MCFC is smaller and less expensive than a PAFC of com­parable output. Another advantage of the MCFC is that the electrodes can be made with cheaper nickel catalysts rather than the more expen­sive platinum used in other low-temperature fuel cells. Endurance of the cell stack is a critical issue in commercialization, and MCFC manufac­turers report an average potential degradation of — 2 mV/1000 h over a cell stack lifetime of 40,000 h. The high temperature limits the use of materials in the MCFC, and safety issues prevent their application for home use. MCFC units require a few minutes of fuel burning at the start up to heat up the cell to its operating temperature and therefore are not very suitable for use in automobiles. However, they are very good for sta­tionary power applications and units with up to 2 MW have been con­structed, and designs for units with up to 100 MW exist [3, 23-25].