Fuel cells ceramic electrolyte

Fast-ion conductors general comments The essential element of ceramics-based fuel cells and batteries is the electrolyte, a solid, fast-ion conductor. 

Badwal, S.P.S. and Foger, K., Solid oxide electrolyte fuel cell review. Ceramics International, 1996, 22, 257-265. 

A high temperature solid electrolyte fuel cell (SOFC) will be considered now. Modern SOFC technology employs calcia-stabilized zirconia as the support tube and yttria-stabilized zirconia as the solid electrolyte. In addition, special oxide ceramic materials are employed as electrodes and interconnection materials. These materials and the solid electrolyte are deposited as thin layers on the support tube by electrochemical vapor deposition. 

The three components of the fuel cell, anode, cathode, and electrolyte form a membrane-electrolyte assembly, as, by analogy with polymer electrolyte fuel cells, one may regard the thin layer of solid electrolyte as a membrane. Any one of the three membrane-electrode assembly components can be selected as the entire fuel cell s support and made relatively thick (up to 2 mm) in order to provide mechanical stability. The other two components are then applied to this support in a different way as thin layers (tenths of a millimeter). Accordingly, one has anode-supported, electrolyte-supported, and cathode-supported fuel cells. Sometimes though an independent metal or ceramic substrate is used to which, then, the three functional layers are applied. 

Fuel cells were invented more than 150 years ago, but their commercialization has been very slow. To date, they have been used primarily in space vehicles, but these systems are quite costly and not suitable for commercial applications. In the last decade, however, there has been a dramatic increase in research, and it is now clear that fuel cells will enter the commercial marketplace in the not too distant future. Currently, most attention is focused on two types of fuel cells, polymer-electrolyte membrane (PEM) fuel cells and solid-oxide electrolyte fuel cells (SOFCs) (Carrette et al., 2000 Minh 1993). PEM systems use a proton-conducting polymer as the electrolyte and operate at low temperatures SOFCs use an oxygen ion-conducting ceramic membrane as the electrolyte and operate at temperatures of 700 to 1,000°C. 

G. Meng, G. Ma, Q. Ma, R. Peng, X. Liu, Ceramic membrane fuel cells based on solid proton electrolytes. Solid State Ionics 178, 697-703 (2007)... 

Indium oxide with different additives was proposed as a cathode material in 1956 [109] and frequently used (e.g. [110,107,108]). How ever, electronically conducting perovskites soon began to dominate the developments for both cathode and interconnect. The use of Lai- -Sr CoOs for the air electrode of solid oxide fuel cells marked the beginning [111], followed in 1967 by recommendations of PrCoOs [112] and of mixtures of the oxides of Pr. Cr, Ni and Co [113]. Strontium-doped lanthanum chromite, even now the most important ceramic interconnection material, was proposed by Meadowcroft in 1969 [114]. For cathodes, the situation in 1969 was summarised [115] as It Is apparent that a fully satisfactory air electrode for high temperature zirconia electrolyte fuel cells is still lacking. ... 

D. D. Button and D. H. Archer, Development of Lai xSrxCo03 air electrodes for solid electrolyte fuel cells. Amer. Ceram. Soc. Washington, Meeting, May 1966. 

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. 

Solid Oxide Fuel Cell In SOF(7s the electrolyte is a ceramic oxide ion conductor, such as vttriurn-doped zirconium oxide. The conduetKity of this material is 0.1 S/ern at 1273 K (1832°F) it decreases to 0.01 S/ern at 1073 K (1472°F), and by another order of magnitude at 773 K (932°F). Because the resistive losses need to be kept below about 50 rn, the operating temperature of the... 

In the ceramics field many of the new advanced ceramic oxides have a specially prepared mixture of cations which determines the crystal structure, through the relative sizes of the cations and oxygen ions, and the physical properties through the choice of cations and tlreh oxidation states. These include, for example, solid electrolytes and electrodes for sensors and fuel cells, fenites and garnets for magnetic systems, zirconates and titanates for piezoelectric materials, as well as ceramic superconductors and a number of other substances... 

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. 

There are five classes of fuel cells. Like batteries, they differ in the electrolyte, which can be either liquid (alkaline or acidic), polymer film, molten salt, or ceramic. As Table 1 shows, each type has specific advantages and disadvantages that make it suitable for different applications. Ultimately, however, the fuel cells that win the commercialization race will be those that are the most economical. 

In a simple version of a fuel cell, a fuel such as hydrogen gas is passed over a platinum electrode, oxygen is passed over the other, similar electrode, and the electrolyte is aqueous potassium hydroxide. A porous membrane separates the two electrode compartments. Many varieties of fuel cells are possible, and in some the electrolyte is a solid polymer membrane or a ceramic (see Section 14.22). Three of the most promising fuel cells are the alkali fuel cell, the phosphoric acid fuel cell, and the methanol fuel cell. 

The stability of ceramic materials at high temperatures has made them useful as furnace liners and has led to interest in ceramic automobile engines, which could endure overheating. Currently, a typical automobile contains about 35 kg of ceramic materials such as spark plugs, pressure and vibration sensors, brake linings, catalytic converters, and thermal and electrical insulation. Some fuel cells make use of a porous solid electrolyte such as zirconia, Zr02, that contains a small amount of calcium oxide. It is an electronic insulator, and so electrons do not flow through it, but oxide ions do. 

Sol-gel techniques have been widely used to prepare ceramic or glass materials with controlled microstructures. Applications of the sol-gel method in fabrication of high-temperature fuel cells are steadily reported. Modification of electrodes, electrolytes or electrolyte/electrode interface of the fuel cell has been also performed to produce components with improved microstructures. Recently, the sol-gel method has expanded into inorganic-organic hybrid membranes for low-temperature fuel cells. This paper presents an overview concerning current applications of sol-gel techniques in fabrication of fuel cell components. 

Ionic conductors, used in electrochemical cells and batteries (Chapter 6), have high point defect populations. Slabs of solid ceramic electrolytes in fuel cells, for instance, often operate under conditions in which one side of the electrolyte is held in oxidizing conditions and the other side in reducing conditions. A signihcant change in the point defect population over the ceramic can be anticipated in these conditions, which may cause the electrolyte to bow or fracture. 

Liu J and Barnett SA. Thin yttrium-stabilized zirconia electrolyte solid oxide fuel cells by centrifugal casting. J Am Ceram Soc 2002 85 3096-3098. 

Du Y and Sammes NM. Fabrication of tubular electrolytes for solid oxide fuel cells using strontium- and magnesium-doped LaGa03 materials. J. Eur. Ceram. Soc. 2001 21 727-735. 

Ishihara T, Sato K, and Takita Y. Electrophoretic deposition of Y203-stabilized Zr02 electrolyte films in solid oxide fuel cells. J. Am. Ceram. Soc. 1996 79 913-919. 

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