Fuel cell, high-temperature molten salt carbonate

Molten carbonate fuel cells use a molten salt electrolyte of lithium and potassium carbonates and operate at about 650 °C. MCFCs promise high fuel-to-electricity efficiencies and the ability to consume coal-based fuels. A further advantage of the MCFC is the possibility of internal reforming due to the high operating temperatures (600-700 °C) and of using the waste heat in combined cycle power plants. 

Molten Carbonate Fuel Cell. The electrolyte ia the MCFC is usually a combiaation of alkah (Li, Na, K) carbonates retaiaed ia a ceramic matrix of LiA102 particles. The fuel cell operates at 600 to 700°C where the alkah carbonates form a highly conductive molten salt and carbonate ions provide ionic conduction. At the operating temperatures ia MCFCs, Ni-based materials containing chromium (anode) and nickel oxide (cathode) can function as electrode materials, and noble metals are not required. 

The electrolyte in this fuel cell is generally a combination of alkali carbonates, which are retained in a ceramic matrix of LiA102 [8], This fuel cell type works at 600°C-700°C, where the alkali carbonates form a highly conductive molten salt with carbonate ions providing ionic conduction. At the high operating temperatures in the molten carbonate fuel cell, a metallic nickel anode and a nickel oxide cathode are adequate to promote the reaction [9], Noble metals are not required. 

Molten carbonate fuel cells (MCFC) have the electrolyte composed of a combination of alkali (Li, Na, K) carbonates. Operating temperatures are between 600 and 700°C where the carbonates form a highly conductive molten salt, with carbonate ions providing ionic conduction. These fuel cells are in the precommercial / demonstration stage for stationary power generation [1]. 

When natural gas is considered as a possible fuel, an increase in the rate of the electrode reactions is needed. Since catalysts are either very expensive or unknown, temperature is raised to lower the overpotential. Since the products of oxidation of natural gas are carbon dioxide and water, these will always be present in the gas mixture over the cell. An equilibrium between the gases and the molten salt electrolyte will be established, and part of the electrolyte will be converted to carbonate, regardless of the nature of the original anion. Therefore, it seems reasonable to use molten carbonates, not other salts, as electrolytes. On the other hand, since carbonates dissociate at high temperatures to give carbon dioxide, it is necessary to keep the partial pressure of CO2 above the cell at such a value as to retard any change in the composition of the electrolyte. Both the fuel gas and the air are premixed with carbon dioxide before being fed to the fuel cell. 

The poor efficiencies of coal-fired power plants in 1896 (2.6 percent on average compared with over forty percent one hundred years later) prompted W. W. Jacques to invent the high temperature (500°C to 600°C [900°F to 1100°F]) fuel cell, and then build a lOO-cell battery to produce electricity from coal combustion. The battery operated intermittently for six months, but with diminishing performance, the carbon dioxide generated and present in the air reacted with and consumed its molten potassium hydroxide electrolyte. In 1910, E. Bauer substituted molten salts (e.g., carbonates, silicates, and borates) and used molten silver as the oxygen electrode. Numerous molten salt batteiy systems have since evolved to handle peak loads in electric power plants, and for electric vehicle propulsion. Of particular note is the sodium and nickel chloride couple in a molten chloroalumi-nate salt electrolyte for electric vehicle propulsion. One special feature is the use of a semi-permeable aluminum oxide ceramic separator to prevent lithium ions from diffusing to the sodium electrode, but still allow the opposing flow of sodium ions. 

Molten carbonate fuel cells (MCFCs) are currently being developed for natural gas 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 lithium aluminium oxide (LiAI02) matrix. Since they operate at extremely high temperatures of 650°C and above, non-precious metals can be used as catalysts at the anode and cathode, reducing costs. 

Molten carbonate fuel cell technology was developed based on the work of Bauers and Ehrenberg, Davy tan, and Broers and Ketelaar in the 1940s [8], The electrolyte is a molten salt such as sodium carbonate, borax, or cryolite. This type of fuel cell requires a high temperature to keep the electrolyte in a molten state. The following 30-40 years saw great successes, with the development of MCFCs and MCFC stacks that could be operated for over 5000 hours. 

Hot corrosion is designated as the accelerated attack of metals and ceramics in oxidizing environments by the presence of a thin molten salt film, for example, a fused sulfate, carbonate, chloride, or nitrate. In many high-temperature processes, molten salts are present either in partially molten ashes, as deposits on boiler tubes from conventionally fired plants such as waste fired boilers (chlorides, sulfates), as a single salt deposits on gas turbines (Na2S04), or as the electrolytes in molten carbonate fuel cells [(Li,K)2C03]. 

Recently, combining ceramics with molten salts has been of growing interest for innovative high-temperature fuel cell applications. In the last ten years, doped-ceria oxides mixed with molten salts, such as chlorides, fluorides, carbonates and sulphates (Zhu and Mat, 2006 Di et al, 2010 Lapa et al, 2010 ... 

The two types of high temperature fuel cell are quite different from each other (Table 6). The molten carbonate fuel cell, which operates at 650°C, has a metal anode (nickel), a conducting oxide cathode (e.g. lithiated NiO) and a mixed Li2C03/K2C03 fused salt electrolyte. Sulphur attack of the anode, to form liquid nickel sulphide, is a severe problem and it is necessary to remove H2S from the fuel gas to <1 ppm or better. However, CO is not a poison. Other materials science problems include anode sintering and degradation, corrosion of cell components and evaporation of the electrolyte. Work continues on this fuel cell in U.S.A. and there is some optimism that the problem will be solved within 10 years. 

A dense and electronically insulating layer of L1A102 is not suitable for providing corrosion resistance to the cell current collectors because these components must remain electrically conductive. The typical materials used for this application are 316 stainless steel and Ni plated stainless steels. However, materials with better corrosion resistance are required for long-term operation of MCFCs. Research is continuing to understand the corrosion processes of high-temperature alloys in molten carbonate salts under both fuel gas and oxidizing gas environments (29, 28) and to identify improved alloys (30) for MCFCs. Stainless steels such as Type 310 and 446 have demonstrated better corrosion resistance than Type 316 in corrosion tests (30). 

Molten carbonate fuel cells (MCFCs) are high-temperature systems that use an immobilised liquid molten carbonate salt as the electrolyte. Salts commonly used include lithium carbonate, potassium carbonate and sodium carbonate. Typically MCFC units have an operating temperature of around 650°C and an efficiency of around 60%. (This can rise to as much as 80% if the waste heat is used for cogeneration.)... 

Molten carbonate fuel cell - The typical running temperature of a MCFC is around 650°C. Molten carbonate salt is used as the electrolyte. Due to the high working temperature, a MCFC can be fuelled with various kinds of fuel such as derived gas and natural gas without an external reformer. The corrosion effect of sulfur should be taken into consideration when choosing the type of fuel. High efficiency and non-noble catalyst are the major advantages of MCFC. To date, MCFC has retained more than 40% of the fuel cell market share. 

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