Fuel Cells Using Molten Electrolyte

Wire gauze Metal powder Central electrode 

Current Density (mA/cnf) Polarization Voltage (V) with Fuel Used  

Upper percentage values are associated with the fuels used, and the lower percentage values indicate the carbon-dioxide gas produced from the electrochemical reaction. For example, in the case of hydrogen fuel, 65% is the hydrogen, whereas 35% is the carbon dioxide. In another example of methane fuel, the percentages of fuel to carbon dioxide are identical, but the polarization voltage is extremely low. 

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Two distinct electrolytes, namely molten electrolyte and solid electrolyte, have been investigated for high-power sources. A fuel cell using molten electrolyte, hydrogen fuel, and air as oxidant that produced a power density of 45 W/ft. at 0.7 V has demonstrated a remarkable reliability. A two-cell device demonstrated a power density of 58 W/ft. at 1.3 V and a continuous operation exceeding 4,000 h with no deterioration in cell performance. 

Figure 3.2 Critical elements of a high-capacity fuel cell using an electrolyte paste consisting of fine-grain solid MgO and molten electrolyte for optimum performance.

Molten carbonate fuel cells use a mixture of lithium and potassium carbonate as an electrolyte and have an OT of 630 to 650°C. 

For natural-gas-fuelled CHP plants, the same line of argumentation holds as for the stationary use of hydrogen from biomass. It is more reasonable to use natural gas directly than to convert it to hydrogen first and then to heat and electricity. High electrical efficiencies can be reached in the stationary sector by feeding natural gas to molten-carbonate fuel cells (MCFC) and solid-oxide fuel cells (SOFC). Molten-carbonate fuel cells have the added advantage of using C02 for the electrolyte (see also Chapter 13). 

Molten carbonate fuel cell (MCFC)—Carbonate electrolyte with conventional metal catalyst. It can use coal gas and natural gas fuel, and is suited for 10 kW to 2 mW power plants. 

Molten carbonate fuel cells use a mixture of carbonates that are liquid at operating temperature—600°C to 650°C. MCFC, like SOFC, operates at a higher temperature than the PEMFC does it does not require a fuel reformer and it can be operated with a hydrogen-rich fuel. The MCFC s liquid electrolyte means more handling issues. It does not have the ability to be pressurized. The MCFC could serve a niche market of data centers and hospitals. FuelCell Energy has recently made a commercial offering of MCFCs. These fuel cells will probably not have the same market penetration potential as SOFCs and thus would likely have little or no impact as a transition strategy for H2 use. 

Chapters I to III introduce the reader to the general problems of fuel cells. The nature and role of the electrode material which acts as a solid electrocatalyst for a specific reaction is considered in chapters IV to VI. Mechanisms of the anodic oxidation of different fuels and of the reduction of molecular oxygen are discussed in chapters VII to XII for the low-temperature fuel cells and the strong influence of chemisorhed species or oxide layers on the electrode reaction is outlined. Processes in molten carbonate fuel cells and solid electrolyte fuel cells are covered in chapters XIII and XIV. The important properties of porous electrodes and structures and models used in the mathematical analysis of the operation of these electrodes are discussed in chapters XV and XVI. 

Lithium aluminates are also important in the development of molten carbonate fuel cells (MCFC) [82, 83], In these fuel cells, a molten carbonate salt mixture is used as an electrolyte. These fuel cells operate through an anode reaction, which is a reaction between carbonate ions and hydrogen. A cathode reaction combines oxygen, C02, and electrons from the cathode to produce carbonate ions, which enter the electrolyte. These cells operate at temperatures of 650°C and the electrolyte, which is usually lithium and potassium carbonate, is suspended in an inert matrix, which is usually a lithium aluminate. 

Finally we come to the fuel cell itself. We have already mentioned the original Grove fuel cell, and the alkaline and phosphoric acid fuel cells used in space technology. Three other types of cell are the molten carbonate fuel cell (with a molten Li2C03/Na2C03 electrolyte), the solid oxide fuel cell (containing a solid metal oxide electrolyte) and the... 

CO2 was used for the initial heating of the fuel cell with molten carbonate electrolyte. The rate of heating was 5°C/min for the uniform fuel cell heating. After heating of the fuel cell to the necessary temperatures, synthesis-gas and air were added into the streams of carbon dioxide and then CO2 consumption was reduced. 

Samples of fuel cells with molten carbonate electrolyte were made in CETl. Molten carbonate electrolyte on the basis of Na2C03, K2CO3, and Li2C03 was used. For the decrease in melting temperature of electrolyte, the part of Li2C03 was 50 mass %. Operating temperature of electrolyte was from 600 up to 650°C. The fixing of electrolyte was executed by the matrix. The matrix made of ceramics (MgO) with a 40% porosity. The anodes were made of nickel. The cathodes were made of NiO and NiO with Li addition. Ni and NiO were put on the matrix by plasma deposition. 

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. 

Current research is centred on making compact cells of high efficiency. They are described in terms of the electrolyte that is used. The principle types are alkali fuel cells, described above, with aqueous KOH as electrolyte, MCFCs (molten carbonate fuel cells), with a molten alkali metal or alkaline earth carbonate electrolyte, PAFCs (phosphoric acid fuel cells), PEMs (proton exchange membranes), using a solid polymer electrolyte that conducts ions, and SOFCs, (solid oxide fuel cells), with solid electrolytes that allow oxide ion, 0 , transport The... 

The working temperature of molten carbonate fuel cells is around 600-650°C. Mixed carbonate melts containing 62-70 mol% of lithium carbonate and 30-38 mol% of potassium carbonate, with compositions close to the eutectic point, are used in molten carbonate fuel cells as an electrolyte. Sometimes, sodium carbonate and other salts are added to the melts. This liquid melt is immobilized in the pores of a ceramic fine-pore matrix, made of sintered magnesium oxide or lithium aluminate powders. 

There are several types of fuel cells, which are classified primarily by the kind of electrolyte they employ. The materials used for electrolytes have their best conductance only within certain temperature ranges (Hirschenhofer 1994). A few of the most promising types include phosphoric acid fuel cell (PAFC), molten carbonate fuel cell (MCFC), solid oxide fuel cell (SOFC), alkaline fuel cell (AFC), proton exchange membrane fuel cell (PEMFC), and direct methanol fuel cell (DMFC). 

Morita H, Komoda M, Mugikura Y, Izaki Y, Watanabe T, Masuda Y and Matsuyama T (2002), Performance analysis of molten carbonate fuel cell using a Li/Na electrolyte ,/ Power Sources, 112,509-518. 

There exist a variety of fuel cells. For practical reasons, fuel cells are classified by the type of electrolyte employed. The following names and abbreviations are frequently used in publications alkaline fuel cells (AFC), molten carbonate fuel cells (MCFC), phosphoric acid fuel cells (PAFC), solid oxide fuel cells (SOFC), and proton exchange membrane fuel cells (PEMFC). Among different types of fuel cells under development today, the PEMFC, also called polymer electrolyte membrane fuel cells (PEFC), is considered as a potential future power source due to its unique characteristics [1-3]. The PEMFC consists of an anode where hydrogen oxidation takes place, a cathode where oxygen reduction occurs, and an electrolyte membrane that permits the transfer of protons from anode to cathode. PEMFC operates at low temperature that allows rapid start-up. Furthermore, with the absence of corrosive cell constituents, the use of the exotic materials required in other fuel cell types is not required [4]. 

Fuel cells do not use a solid material to store their charge. Instead, low-temperature proton exchange membrane fuel cells use gases such as hydrogen and liquid ethanol (the same form of alcohol found in vodka) or methanol as fuels. These materials are pumped over the surface of the fuel cells, and in the presence of noble-metal catalysts, the protons in these fuels are broken away from the fuel molecule and transported through the electrolyte membrane to form water and heat in the presence of air. The liberated electrons can, just as in the case of batteries, be used to drive an electric motor. Other types of fuel cells, such as molten carbonate fuel cells and solid oxide fuel cells, can use fuels such as carbon in the form of coal, soot, or old rubber tires and operate at 800 degrees Celsius with a very high efficiency. 

Many different types of fuel-cell membranes are currently in use in, e.g., solid-oxide fuel cells (SOFCs), molten-carbonate fuel cells (MCFCs), alkaline fuel eells (AFCs), phosphoric-acid fuel cells (PAFCs), and polymer-electrolyte membrane fuel cells (PEMFCs). One of the most widely used polymers in PEMFCs is Nalion, which is basically a fluorinated teflon-like hydrophobic polymer backbone with sulfonated hydrophilic side chains." Nafion and related sulfonic-add based polymers have the disadvantage that the polymer-conductivity is based on the presence of water and, thus, the operating temperature is limited to a temperature range of 80-100 °C. This constraint makes the water (and temperature) management of the fuel cell critical for its performance. Many computational studies and reviews have recently been pubhshed," and new types of polymers are proposed at any time, e.g. sulfonated aromatic polyarylenes," to meet these drawbacks. 

The electrolyte in the molten carbonate fuel cell uses a liquid solution of lithium, sodium, and/or potassium carbonates, soaked in a matrix. MCFCs have high fuel-to-electricity efficiencies ranging from 60 to 85 % with cogeneration, and operate... 

Since the type of electrolyte material dictates operating principles and characteristics of a fuel cell, a fuel cell is generally named after the type of electrolyte used. For example, an alkaline fuel cell (AFC) uses an alkaline solution such as potassium hydroxide (KOH) in water, an acid fuel cell such as phosphoric acid fuel cell (PAFC) uses phosphoric acid as electrolyte, a solid polymer electrolyte membrane fuel cell (PEMFC) or proton exchange membrane fuel cell uses proton-conducting solid polymer electrolyte membrane, a molten carbonate fuel cell (MCFC) uses molten lithium or potassium carbonate as electrolyte, and a solid oxide ion-conducting fuel cell (SOFC) uses ceramic electrolyte membrane. 

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