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Solid Oxide Fuel Cells are a high-temperature device with a working temperature of about 600-10000C where the liquid electrolyte is replaced with a ceramic membrane which is a good oxygen ion conductor. It is therefore one of the simplest fuel cells in concept with only two phases required. An electrolyte in this type of fuel cell is usually a ceramic layer made of zirconium dioxide stabilised by di-yttrium trioxide (with a molar concentration of 8-10 per cent). There is number of different electrolytes used in SOFCs with a structure of fluorite, perovskite (crystal ceramic, based on CaTiO3, with a cubical structure; discovered in 1830 by Perowski), brownmillerite and apatite.
The idea of solid oxide fuel cells developed probably on 1899, when Nernst discover and describe conduction of an oxygen ion through the zirconia about 800-11000C (which is the highest working temperature for all fuel cells). The construction of the SOFC allows a transport of oxygen ions through the electrolyte; oxygen ions undergo ionisation on a cathode. Ions interact with gas fuel when reaching the anode (similar to the MCFC). In this reaction the products are water (in a case of hydrogen fuel) and electrons which may flow in external circuit giving electric energy (Figure 4.0).
A very important feature is the possibility of using different kind of fuels such as hydrogen, carbon monoxide or more complex fuels (methane, propane and ethane)
Figure 4.0 Diagram of operation of SOFC (showing oxygen as a green spots, hydrogen as red, and electrons as small pink; arrows indicate the direction of process)
A complex of ceramic and metal, called zirconia cermet is usually used for anode. A stabile, high electronic conductive nickel is used as the metal element, which brings the advantage of being internal reforming catalyst. Cathodes are made of different conductive oxides or ceramics with a strontium-doped lanthanum manganite as the most popular. The simplicity of SOFC system, no carbon dioxide recycling makes this kind of fuel cell widely and commonly used. As SOFCs do not require a liquid electrolyte there is no problem with corrosion and electrolyte leakage.
4.1 MECHANISM OF WORKING OF SOFC
The single fuel cell has three main components: dense, gas-tight electrolyte, porous cathode and anode. Reactions in solid oxide fuel cells are:
Cathode reaction: O2 + 4e- -> 2O2-
Anode reaction: H2 + O2- -> H2O +2e-
Summary reaction: O2 + 2H2 -> 2H2O
The fuel, usually hydrogen, is delivered to the anode, and the oxygen is delivered to the cathode. Electrons on a cathode react with oxygen to form negatively charged oxygen ions which flow through the electrolyte to the anode. Hydrogen (in anode) reacts with oxygen ions creating water and free electrons. Those electrons, as they cannot be transferred to the positively charged cathode, flow through the external circuit producing electric current. On a cathode electrons reacts with oxygen making new oxygen ions and the whole process is being repeated (Figure 4.0).
The perfect fuel cell could provide infinite fastness of all these processes going through the whole cycle but there are kinetic limitations that limit the fuel cell in reality such as; particle diffusion to the electrode, particle adsorption and desorption in electrode et cetera.
4.2. SOFC COMPONENTS
There are three elementary parts of single fuel cell: electrolyte, anode and cathode. In addition there are also interconnectors which join single fuel cells into stacks. Nowadays the most popular electrolyte material is a zircon oxide stabilised by yttrium oxide called YSZ; nickel oxide mixed with YSZ is used for an anode; lanthanum manganese with strontium, with a perovskite structure used for cathode; and ceramic (LaCrO3 with strontium, magnesium or calcium) or metallic (bromine alloys) materials are used as interconnectors.
Conducting oxygen ions is not only requirement for material to be a good electrolyte. These materials have to also be a good isolator for electrons, moreover require to have a great density to be a good barrier for gases and be resistant to high pressure and temperature. The great majority of solid oxide fuel cells are using ceramic materials such as zircon dioxide (ZrO2) which stays stabile in reduction and oxidation environment. The zircon dioxide has different crystal structure under different temperatures therefore stabilizers may have to be introduced that stabilise only the needed kind of that oxide (tetragonal) under wide range of temperatures. Stabilizers also support the conductivity. Most often used stabilisers are Y2O3, Sc2O3, CaO, MgO. The YSZ is the most commonly used electrolyte over better ion conductors such as zircon oxide stabilised by the scandium oxide or the cerium oxide that is support through subsidies of gadolinium oxide which are more expensive, less resistant and therefore not so often used.
Oxidising hydrogen is a main function of anode therefore the anode must conduct electrons and ions. The anode has to demonstrate chemical stability under high pressures, must be chemically and physically consistent with an electrolyte. The anode transport oxygen ions and fuel through the fuel cell and is responsible of disposal of water produced. The metal in the anode is the main material conducting electrons, and is blended with electrolyte material such as YSZ to be effective at transporting oxygen ions. It has been show (Figure 4.2) that one of the best materials for anode is the Ni-YSZ, with about 60% volume of nickel.
Figure 4.2. Nickel cermet and its conductivity over different volume of nickel
Moreover the anode has a high porosity for greater surface for a transport of different particles. Between an anode and electrolyte loss of polarisation may occur therefore other techniques of producing anode are developing; new materials, higher porosity or bi-layered anodes, control of size of the YSZ particle may be crucial for evolution of anode in SOFCs.
Cathode has a similar porous structure to the anode, but it has a different chemical structure. Popular, perovskite structured, intricate oxides, such as strontium doped lanthanum manganite (LSM) has high electronic and ionic conductivity. Cathode has to be produced of materials that are catalytically active under oxygen reduction (O2 + 4e- -> 2O2-), be chemically and physically stabile under certain conditions. These materials have to harmonise with electrolyte and interconnectors to stay same and not to create intermediate phases.
Solid oxide fuel cells are one with the longest continuous development. From the early 50sââ‚¬â„¢ in twenties century materials for the cell were developed to increase efficiency and durability. Nowadays cathode and electrolyte materials are well developed but the anode material is still not perfect. Many studies are undertaken to decrease the temperature of working to 600-8000C, which would also increase the durability and expand the environment of use. More expensive materials introduced to fuel cell are great answer for the specific conditions of utilisation; long lasting thermal and chemical stability; high temperature resistance as well as oxidation and reduction resistance. On the other hand other forms of cell may be introduced as the solid type of fuel cell enables to change the geometry. Tubular design was introduced in the late 1970s which originally used porous calcia-stabilised zirconia tube with thin cylindrical anodes.
Figure 4.3.1 Diagram showing example of tubular fuel cell (top part of figure) and cross-section of solid oxide fuel cell (bottom diagram)
Unfortunately the high cost and low power density, results from the length of a path for electron to be endured, make this original design not very efficient and finally not accepted as the best type of a cell. To increase a power of a tubular cell the zirconia support tube were replaced by air-electrode support (AES) materials. The tubular design allows removing high temperature seals which prevent a temperature loss. This three-layered tube has an internal part that has a contact with a fuel and acts as an anode, the middle part is an electrolyte and the outer part is a cathode with an oxidant flows outside the tube. The tubular design has great tightness and therefore minimise leaks (Figure 4.3.1). Whole fuel cell is usually made of a more than one single tube in a form of stacks. Recently more studies are undertaken to improve that great type of solid oxide fuel cell.
Cheaper and much higher density (results from low ohmic losses) type of the cell may be that with a planar design (Figure 4.3.2) which does not need a long current path. In this case the gas-tight, high-temperature sealing is needed.
Figure 4.3.2 Schematic diagram of a planar design for a solid oxide fuel cell (exploded view)
Glass and ceramic sealing were developed to improve the planar cell. These cells are also formed into stacks but in this case they need to be built into a window frame to prevent thermal stresses that lead to mechanical degradation and also to obviate decay of very thin components.
Other designs, such as integrated planar and flat-tube, were developed based on planar and tubular types of cells, which induce high-power density. Introducing new types of sealing based on connection through the interconnectors rather than bipolar plates was the minor difference. Also shortening of a cell length brings many advantages to non-perfect previous designs and also reduces a cost of use.
Solid oxide fuel cells are simple in their construction, cheaper (due to absence of expensive metals such as platinum) than the other types of fuel cells and resistant to abrasion caused by maintain.
Fuel cells are widely used in number of different environments. SOFCs find practical application as a main source of power in small airliners and transport aircraft as well as in bigger aeroplane as an auxiliary source of power. SOFCs are also being commonly used in stationary power plants or power systems and installations as well as small (20W) batteries. Offering a power with making no fuss and pollutions, causing no increase in costs is also used in underground mining industry to power train engine and as a research device in research institutes; is often used as a backup power or as a solution for generators and in telecommunication. Solid oxide fuel cell may also be used in close future as a power source in cars and in maritime ships.
4.5 FUTURE ââ‚¬" IT-SOFCs AND SOFC COMBINED SYSTEMS
As the fuel cells are under continuous studies new better technologies are introduced to increase their productivity combined with decreased cost. Nowadays aspirations to streamlining of SOFC become a motivation for development of IT-SOFC (which stands for: Intermediate Temperature SOFC). High costs were triggered by a high degradation rate under high (over 9000C) operation temperatures in normal type of SOFCs. Smaller system sizes of operating fuel calls influence on the cost as the temperature of each fuel cell may be reduced in this type of environment. Operating SOFCs under lower temperature (600-8000C) had a great impact on the costs as well as reduced the degradation of materials. New materials for cathode, anode and electrolyte are researched currently. Also catalytic activity of the cathode is researched as the oxygen reaction may be responsible for a loss in performance of a cathode. Metallic bipolar plates (such as ferritic stainless steel) may be used to reduce the operating temperature up to 6000C. Operating the cell at about 5000C may give a new ways of designing fuel cells, such as stacking them with more flexible gaskets than the glass. This is now highly important to search for new techniques and materials that allow producing highly efficient and economical fuel cells that may operate under lower temperatures.
Solid oxide fuel cell combined with a gas turbine (SOFC/GT) is an idea being studied for many years to become an efficient and practical system. This concept pioneered by Siemens Westinghouse is now under research to produce a SOFC which may be used under pressure and therefore have a higher operating efficiencies in which the hot, high pressure exhaust of the fuel cell can be used to spin the turbine, generating a second source of electricity.. The idea is to use a pressure of atmospheric circulation that is given from anode and cathode. Using heat and pressure from a one fuel cell may have a great impact on overall efficiency.