In the 1930s, Emil Baur and H. Preis experimented with high-temperature, solid oxide electrolytes in Switzerland. They encountered problems with electrical conductivity and unwanted chemical reactions between the electrolytes and various gases (including carbon monoxide). The following decade, O. K. Davtyan of Russia explored this area further, but met with little success. By the late 1950s, Dutch scientists G. H. J. Broers and J. A. A. Ketelaar began building on this previous work.
They determined that limitations on solid oxides at that time made short-term progress unlikely. Instead, they focused on electrolytes of fused (molten) carbonate salts. By 1960, they reported making a fuel cell that ran for six months using an electrolyte
"mixture of lithium-, sodium- and/or potassium carbonate, impregnated in a porous sintered disk of magnesium oxide." However, they found that the molten electrolyte was slowly lost, partly through reactions with gasket materials. At approximately the same time, Francis T. Bacon was developing a molten cell using two-layer electrodes
on either side of a "free molten" electrolyte. At least two groups were working with semisolid or "paste" electrolytes and most MCFC research groups were investigating
"diffusion" electrodes rather than solid electrodes. In the mid-1960s, the U.S. Army's Mobility Equipment Research and Development Center (MERDC) at Fort Belvoir tested several molten carbonate cells made by Texas Instruments. These cells ranged in size from 100 watts to 1,000 watts output and were designed to run on "combat gasoline" using an external reformer to extract hydrogen. In particular, the Army wanted to use fuels already available rather than a special fuel that might be difficult to supply to field units.
Molten Carbonate fuel cells (MCFCs) contain a liquid solution of lithium, sodium and/or potassium carbonates, soaked in a matrix for an electrolyte. They promise high fuel-to-electricity efficiencies, about 60% normally or 85% with cogeneration, and operate at approximately 1,200 ℉ (650 ℃). This high operating temperature is necessary to achieve sufficient electrolyte conductivity. Because of this high temperature, noble metal catalysts are not required for the fuel cell's electrochemical oxidation and reduction processes. To date, MCFCs have been operated with hydrogen, carbon monoxide, natural gas, propane, landfill gas, marine diesel, and simulated coal gasification products. MCFCs from 10 kW to 2 MW have been tested on a variety of fuels, and are primarily targeted toward electric utility applications. Carbonate fuel cells for stationary applications have been successfully
demonstrated in Japan and Italy. Their high operating temperatures create a big advantage because this allows higher efficiency and the flexibility to use more types of fuels and inexpensive catalysts. This is because reactions involving the breaking of carbon bonds in larger hydrocarbon fuels occur much faster at higher temperatures. A disadvantage to these phenomena, however, is that high temperatures enhance the corrosion and breakdown of cell components. The higher working temperature of these fuel cells, between 650~1000℃, and the heat transfer caused by conduction and convection, generates radiation heat transfer. The mechanism in this case is electromagnetic radiation propagated because of temperature difference, called thermal radiation.[1]
Davtyan was the first to realize the necessity of “support” for the electrolyte, i.e.
a matrix which holds the electrolyte in place and prevents direct combination of reacting gases. In 1964, Broers reported on LiAlO , which was chemically stable and 2 gave much performance. Broers was also the first to introduce porous nickel as the anode material. Clauss and Genin showed that porous nickel oxide, oxidized in situ, provides stable performance for the cathode. An MCFC uses a salt mixture of alkali carbonates as the electrolyte. This mixture provides mass and charge transfer from the cathode to the anode via carbonate ions. The electrolyte in modern applications is a mixture of lithium carbonate and potassium carbonate. Mixtures of lithium carbonate and sodium carbonate and carbonates of alkaline-earth metals are also in use. The typical operating temperature of a MCFC is about 650°C. At that high operating
temperature, the carbonate mixture is in a molten state and becomes a good ionic conductor. The molten electrolyte is contained in a porous electrolyte matrix of
LiAlO , which is an electrically insulating and chemically inert ceramic. Thus, royal 2
metals are not required to act as catalysts, reducing the material cost of the MCFC. A molten carbonate fuel cell has many features as follows.
1. The electrolyte material is a eutectic mixture of lithium carbonate and potassium carbonate. It is in liquid phase at temperatures higher than 500°
C.
2. An MCFC exhibits an internal reforming ability because of its high operating temperature. Therefore, it does not require pure hydrogen as fuel, but can use hydrocarbons such as natural gas and coal gas etc., Moreover, a MCFC produces 40% lower carbon dioxide emission than a thermal power plant.
3. The waste heat of reacting gases emitted by the MCFC can be utilized to generate electric power through gas turbines.
4. An MCFC can be used as a device for separating and concentrating carbon dioxide because the anode gas has the ability to concentrate carbon dioxide[2]
There are several important issues for molten carbonate fuel cell are:[3]
(1). Cell Sealing
In addition to the cathode, anode, and electrolyte, each cell structure also contains an electrolyte matrix that holds the liquid electrolyte in place. This matrix structure is composed of a mixture of ceramic powder (usually lithium aluminates, LiAlO ) and carbonate electrolyte. The mixture is 2 semisolid (paste-like) and the molten carbonate electrolyte is immobilized by the capillary force. The resulting matrix structure is stiff and impermeable to the reactant gases, but also deformable. The plasticity of the matrix provides a gastight seal around the periphery of the cell. Gas sealing is a major challenge in high-temperature fuel cells. This edge sealing technique is often called a wet seal. The wet seal concept is very similar to the sealing technique used in PEM fuel cells in that both techniques use the electrolyte itself as the sealing material to provide gas-tight sealing. This works because the electrolyte itself is gas impermeable, and is compatible with the rest of the cell components. In the molten carbonate fuel cell, however, wet sealing the cell is the only feasible sealing technique when the cell housing is made of metals. This is because the carbonate electrolyte is very corrosive and very few materials can remain stable under MCFC operating conditions. Although high-density alumina and other dense ceramics are suitable sealing materials, they cannot withstand thermal cycling.
(2). Current Collectors
Current collectors enhance the rate of electric current collection and reduce ohmic losses. They are usually made of stainless steel or nickel metal screens and are located between the electrodes and the cell housing for good electrical contact between both components. The cell housing is made of metal shells with flow distribution channels built on its inside surface for proper distribution of the gas supply to the respective electrode.
(3). Electrolyte Management
Another unique feature of the molten carbonate fuel cell structure is its unique method of electrolyte management. PAFC and PEMFC electrolyte management uses hydrophobic materials such as PTFE. The dispersed PTFE in the porous electrodes acts as a binder for the integrity of the electrode structure and as a wet-proofing agent for the establishment of a stable gas-liquid interface. However, this method cannot be used for MCFCs because similar de-wetting materials do not exist in molten carbonate under oxidizing conditions. Hence, capillary equilibrium is used to control electrolyte distribution in the porous electrodes, and stable electrolyte/gas interfaces in MCFC porous electrodes (the so-called three-phase zone).
An MCFC power plant is one of the most attractive new types of power plants
available, and has the potential to replace conventional thermal power plants. The principal reason for this is that MCFC power plants have a higher energy conversion efficiency and are able to use both LNG and coal gas as fuel. Furthermore, an MCFC power plant can be applied in the electric power industry as a dispersed power source or a central power source fueled by LNG or coal. Many manufacturers and organizations have developed conceptual designs of MCFC power plants. The efficiency in most of these designs is 45%~70% (LHV, low heat value). MCFC power plants therefore carry great promise as primary power plants in the future, especially for a decentralized power supply.
Research and development on MCFCs is conducted primarily in the USA, Japan, and Europe. The USA led MCFC technology initially, but Japan and several European countries, which started their own R&D programs in the 1980s, have greatly increased their activities. The goal of the development programs in all of these countries is to develop and commercialize simple, low-cost power plants that can compete favorably with conventional thermal power plants. Many R&D programs have now reached the commercial stage, where prototype stacks and plants are being constructed and tested.
The principle of an MCFC is that, at high operating temperatures, carbonate ions migrates in a molten electrolyte. This carbonate ion produced from carbon dioxide and oxygen in the cathode passes through the electrolyte, and reacts with the hydrogen in anode. At the same time, this reaction in the anode produces carbon
dioxide, vapor, and electrons. The electrons are conducted to the external load circuit through the anode electrode, and back to the cathode through the cathode electrode.
Figure 1.1 shows the principle of electric power generation in an MCFC, and Fig. 1.2 shows the basic components of a fuel cell. The key materials in an MCFC are anode electrode, cathode electrode, electrolyte, and bipolar plate. The chemical reaction equations in the anode and cathode of an MCFC are as follows.
In the cathode,
2
2 2 3
1 2
2O +CO + e− →CO− (1.3)
In the anode,
2
2 3 2 2 2
H +CO− →CO +H O+ e− (1.4)
The total reaction is
2 2 2 2 2
1
2O +H +CO →CO +H O (1.5)
In equation (1.1) and (1.2), the carbon dioxide is the product and reactant in both the anode and the cathode. The overall reaction in MCFCs is similar to other fuel cells, but CO2 is produced at the anode and consumed at the cathode. This implies that a CO2 recycling system is needed to supply CO2 from the anode chamber to the cathode chamber in a power plant. When carbon dioxide produced in the anode is transferred to the cathode as the reactant, it creates a closed cycle and reduces overall carbon dioxide emissions.
Because of their many advantages such as low pollution, low noise, high efficiency, wide application, etc., MCFCs can assist or even replace thermal electric generators in the future. Therefore, this study investigates the thermal and electrical performance of a MCFC based on its potential development in electric power generation [4, 5].