The history of solid oxide electrolyte started at the end of the 19th century. The discovery was that the very high electrical resistance of pure solid oxide could be largely minimized by the addition of some oxides was an important step in the development. (4)(1)(7) It was mostly zirconia (ZrO2) and small amount of Yttrium. Baur and Pries obtained open circuit voltage between 1.1 and 1.2 volt, although factors such as use of relatively unreactive carbon as fuel and excessive thickness of the electrolyte with its accompanying high internal resistance made the cell impracticable. (4)(2)(8) They varied the composition of the ceramic crucibles, paying particular attention to the resistance of the solid electrolyte as a function of temperature and current flow. Pervoskite is the name of the mineral; it was discovered by Gustav Rose and named after L.A Perovski who was a Russian mineralogist. (4)(1)(2)
Solid oxide fuel cells (SOFCs) have an electrolyte which is solid, non-porous metal oxide; the electrolytes are called Yttria-stabilized zirconia (Y2O3-stablilized ZrO2). They operate at 600-1000oC, this point ionic conduction by oxygen ions take place. Typically, the anode is a Nickel-Zirconia (Ni-ZrO2) cermets and the cathode is Sr-doped LaMnO3 (LSM). (8)(6) There is no liquid electrolyte with its attendant material corrosion or electrolyte management problems. Rigorous requirements are placed on the materials due to high temperature of the solid oxide fuels cell. The development of good affordable materials of ceramic structures is presently the key technical challenges facing Solid Oxide Fuel Cells. (4)(1)(3)(8)
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The ideal properties of the solid oxide electrolyte are
- It has to be chemically stable under oxidizing and reducing condition
- It must be physically stable
- It should be impermeable to gases at elevated temperature
- It must enable quick movement of oxide ions through it.
- No electrical conductivity and High ionic conductivity.
- It must be fully dense to avoid short circuiting (4)
HOW IT WORKS.
The molecular oxygen is changed to oxygen ion and moves to the cell fuel side through the electrolyte. For the migration to happen, the electrolyte must have no electrical conductivity and should have huge ionic conductivity. Just like other materials, it should be structurally stable; it should have good thermal and chemical stability within a wide temperature range. To prevent short circuiting of reacting gases through it, it must be very dense and also to reduce loss of resistivity in the cell, it should be very thin. (1) (2)
Solid fuels cell electrolyte is used mostly in solid oxide fuels cell applications. The electrolyte is used in modern day innovation due to the high prospect of the fuels cell being used to generate power in the technology world. The following examples are some of the current application and future application of the fuel cell. (4)(3)
- Car companies like BMW are making cars that are hydrogen and oxide powered. Hybrid engines are manufactured from the leading car companies giving the fuels cell great future prospects. Example is BMW H2R and Mazda RX-8 Hydrogen RE (8)
- “Stand-alone Power Supply (SPS) systems are small-scale (<50 kW) self-contained units, providing electricity independent of the main electricity grid or mini grid network. These systems are sometimes also known as Remote Area Power Supply (RAPS) systems. Solid oxide fuel cell is used in this situation to create alternative power supply”. (2)(4)
- Portable household equipment that uses electricity can be made using the solid oxide fuel cell. Mobile equipment is made to use the electrolyte as their generators.
- Electrolytes are also used in various aircraft and space vehicles, they are considered to replace batteries as a non-time limited emergency power supply.(8)
There are a lot of ceramic materials used for solid oxide fuel cell electrolyte like YSZ, doped cerium oxide (Gd doped CeO2), Lanthanum gallate, scandium stabilized zirconia (Sc-ZrO2) and doped bismuth oxide (Bi2O3). The doped bismuth oxide has operating temperature lower than 800oC and high oxygen ion conductivity; it should also be without enough crystalline stability at increased temperature. The most practical solid oxide electrolytes investigated so far has been based on zirconia. (4)
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Pure zirconia is chemically stable even under severe oxidizing and reducing condition but the weakness is its tendency to fracture on thermal cycling. (6) Pure zirconia is a typical insulator with specific electrical resistance of the order of 1014 ohm-cm at room temperature, decreasing to about 107 ohm-cm at 1000oC (ohm cm is the measure of resistivity of a material). In addition to the stabilizing effect, little amount of oxide materials such as CaO, MgO, Y2O3 are always added to enhance the conductivity and electrical stability. (7)
Solid solutions are formed which exhibit remarkably high electrical conductivities at temperature above about 800oC or 900oC. Other example of materials under consideration is Gadolinium Doped Ceria (10% Gd or 20% Gd), which is used for sintering aid, support catalyst and used for enhancing catalytic activity. It is softly agglomerated. It is suitable for ink manufacture, tape casting and pellet pressing. (5)(7)
Gadolinium doped ceria is made by stabilization of cerium oxide by different doping levels of gadolinium oxide, it is a material that when fired forms a very ionically conductive thin film electrolyte layer that is used in solid oxide fuel cell electrochemistry structures. Gadolinium Oxide doped Ceria is in a class of compounds (doped ceria) whose ionic conductivity exceeds that of Yttria Stabilized Zirconia (YSZ) electrolytes.
YSZ has a number of applications:
- It is also used as a refractory in space ships and jet engine.
- YSZ is also used in gas turbines as a thermal barrier.
- Due to its hardness and optical properties in mono-crystal form, it is used as jewellery.
- YSZ is used for D.I.Y ceramics and cements in homes.
- YSZ doped with rare-earth materials are used for thermo graphic phosphor and a luminescent material. (4)(6)(2)(7)
(1)Bottom of Form
- Fuel cells compendium by Nigel p Brandon and Dave Thompsett. 2005 ELSEVIER
- Fuel cells, Engines and Hydrogen, an energy approach. Frederick J. Barclay. John Wiley and son
- [Seabaugh] Matthew M. Seabaugh, Scott L. Swartz, William J. Dawson, “Developing Colloidal Fabrication Processes for YSZ Solid Electrolyte Membranes,” Processing and Characterization of Electrochemical Materials and Devices. Proc. Symp. Indianapolis, 25-28 April 1999.
- Singhal] S. C. Singhal, “Science and Technology of Solid-Oxide Fuel Cells,” MRS Bull. Vol.25, No.3, 2000, p.16-21.