27-04-2011, 04:24 PM
Submitted by:
Pathan Mohsinkhan
Sachin Kanungo
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Abstract
Solid oxide fuel cells (SOFCs), which will be operated at reduced temperature, are becoming a frontier of R and D. These compact size SOFCs will fit well with intermittent loads, of which share in energy system is increasing today, whereas the “conventional SOFCs” will be effectively operated with stationary mode. For such compact size SOFCs, throttle down operation following intermittent loads will be profitable because low current density gives higher efficiency. SOFCs are not suitable for quick start up. It was estimated that the hot standby mode would be more acceptable than cold start mode from the viewpoint of heat loss. The merit of internal reforming will also be lost for the reduced operation temperature. Solid Oxide Fuel Cells are becoming the most revolutionary platforms in making a greener earth with low cost electricity production. Within its category SOFC is the cheapest, high temperature working and output efficient. The project contains the explanation of primary elements for SOFC like anode, cathode and electrolyte with 3D diagram and actual working of SOFC. Applications and merits/demerits are concisely discussed. Blooming new technologies within SOFC and improvements after its foundation and the new process stacks are defined. Bloom Energy Server, which is the power plant box containing Solid Oxide Fuel Cells is the revolutionary product in large scale applications is discussed.
Introduction:
Solid oxide fuel cells are a class of fuel cell characterized by the use of a solid oxide material as the electrolyte. In contrast to proton exchange membrane fuel cells (PEMFCs), which conduct positive hydrogen ions (protons) through a polymer electrolyte from the anode to the cathode, the SOFCs use a solid oxide electrolyte to conduct negative oxygen ions from the cathode to the anode. The electrochemical oxidation of the oxygen ions with hydrogen or carbon monoxide thus occurs on the anode side.
They operate at very high temperatures, typically between 500 and 1,000°C. At these temperatures, SOFCs do not require expensive platinum catalyst material, as is currently necessary for lower temperature fuel cells such as PEMFCs, and are not vulnerable to carbon monoxide catalyst poisoning. However, vulnerability to sulfur poisoning has been widely observed and the sulfur must be removed before entering the cell through the use of adsorbent beds or other means.
Solid oxide fuel cells have a wide variety of applications from use as auxiliary power units in vehicles to stationary power generation with outputs from 100 W to 2 MW. Theoretical efficiency of a SOFC device can exceed 60 percent. The higher operating temperature make SOFCs suitable candidates for application with heat engine energy recovery devices or combined heat and power, which further increases overall fuel efficiency.
Because of these high temperatures, light hydrocarbon fuels, such as methane, propane and butane can be internally reformed within the anode. SOFCs can also be fueled by externally reforming heavier hydrocarbons, such as gasoline, diesel, jet fuel (JP-8) or biofuels. Such reformates are mixtures of hydrogen, carbon monoxide, carbon dioxide, steam and methane, formed by reacting the hydrocarbon fuels with air or steam in a device upstream of the SOFC anode. SOFC power systems can increase efficiency by using the heat given off by the exothermic electrochemical oxidation within the fuel cell for endothermic steam reforming process.
Thermal expansion demands a uniform and well-regulated heating process at startup. SOFC stacks with planar geometry require on the order of an hour to be heated to light-off temperature. Micro-tubular fuel cell design geometries promise much faster start up times, typically on the order of minutes.
Unlike most other types of fuel cells, SOFCs can have multiple geometries. The planar fuel cell design geometry is the typical sandwich type geometry employed by most types of fuel cells, where the electrolyte is sandwiched in between the electrodes. SOFCs can also be made in tubular geometries where either air or fuel is passed through the inside of the tube and the other gas is passed along the outside of the tube. The tubular design is advantageous because it is much easier to seal air from the fuel. The performance of the planar design is currently better than the performance of the tubular design however, because the planar design has a lower resistance comparatively. Other geometries of SOFCs include modified planar fuel cell designs (MPC or MPSOFC), where a wave-like structure replaces the traditional flat configuration of the planar cell. Such designs are highly promising, because they share the advantages of both planar cells (low resistance) and tubular cells.
SOFC technology dominates competing fuel cell technologies because of the ability of SOFCs to use currently available fossil fuels, thus reducing operating costs. Other fuel cell technologies (e.g. molten carbonate, polymer electrolyte, phosphoric acid and alkali) require hydrogen as their fuel. Widespread use of such fuel cells would require a network of hydrogen suppliers, similar to our familiar gas stations.
High efficiency and fuel adaptability are not the only advantages of solid oxide fuel cells. SOFCs are attractive as energy sources because they are clean, reliable, and almost entirely nonpolluting. Because there are no moving parts and the cells are therefore vibration-free, the noise pollution associated with power generation is also eliminated.
WHAT IS SOLID OXIDE FUEL CELL?
A solid oxide fuel cell (SOFC) is an electrochemical conversion device that produces electricity directly from oxidizing a fuel. Fuel cells are characterized by their electrolyte material and, as the name implies, the SOFC has a solid oxide, or ceramic, electrolyte. Advantages of this class of fuel cells include high efficiencies, long term stability, fuel flexibility, low emissions, and cost. The largest disadvantage is the high operating temperature which results in longer start up times and mechanical/chemical compatibility issues.
Solid oxide fuel cells are a class of fuel cell characterized by the use of a solid oxide material as the electrolyte. In contrast to proton exchange membrane fuel cells (PEMFCs), which conduct positive hydrogen ions (protons) through a polymer electrolyte from the anode to the cathode, the SOFCs use a solid oxide electrolyte to conduct negative oxygen ions from the cathode to the anode. The electrochemical oxidation of the oxygen ions with hydrogen or carbon monoxide thus occurs on the anode side. They operate at very high temperatures, typically between 500 and 1,000°C. At these temperatures, SOFCs do not require expensive platinum catalyst material, as is currently necessary for lower temperature fuel cells such as PEMFCs, and are not vulnerable to carbon monoxide catalyst poisoning. However, vulnerability to sulfur poisoning has been widely observed and the sulfur must be removed before entering the cell through the use of adsorbent beds or other means.
HOW DOES IT WORK?
A solid oxide fuel cell is made up of four layers, three of which are ceramics (hence the name). A single cell consisting of these four layers stacked together is typically only a few millimeters thick. Hundreds of these cells are then connected in series to form what most people refer to as an "SOFC stack". The ceramics used in SOFCs do not become electrically and ionically active until they reach very high temperature and as a consequence the stacks have to run at temperatures ranging from 600 to 1,000°C. Reduction of oxygen into oxygen ions occurs at the cathode. These ions can then diffuse through the solid oxide electrolyte to the anode where they can electrochemically oxidize the fuel. In this reaction, a water byproduct is given off as well as two electrons. These electrons then flow through an external circuit where they can do work. The cycle then repeats as those electrons enter the cathode material again
Anode
The ceramic [anode] layer must be very porous to allow the fuel to flow towards the electrolyte. Like the cathode, it must conduct electrons, with ionic conductivity a definite asset. The most common material used is a cermet made up of nickel mixed with the ceramic material that is used for the electrolyte in that particular cell, typically YSZ (yttria stabilized zirconia). The anode is commonly the thickest and strongest layer in each individual cell, because it has the smallest polarization losses, and is often the layer that provides the mechanical support. Electrochemically speaking, the anode’s job is to use the oxygen ions that diffuse through the electrolyte to oxidize the hydrogen fuel. The oxidation reaction between the oxygen ions and the hydrogen produces heat as well as water and electricity. If the fuel is a hydrocarbon, for example methane, another function of the anode is to act as a catalyst for steam reforming the fuel into hydrogen. This provides another operational benefit to the fuel cell stack because the reforming reaction is endothermic, which cools the stack internally
Electrolyte
The electrolyte is a dense layer of ceramic that conducts oxygen ions. Its electronic conductivity must be kept as low as possible to prevent losses from leakage currents. The high operating temperatures of SOFCs allow the kinetics of oxygen ion transport to be sufficient for good performance. However, as the operating temperature approaches the lower limit for SOFCs at around 873 K, the electrolyte begins to have large ionic transport resistances and affect the performance. Popular electrolyte materials include yttria stabilized zirconia (YSZ) (often the 8% form Y8SZ) and gadolinium doped ceria (GDC) The electrolyte material has crucial influence on the cell performances. Detrimental reactions between YSZ electrolytes and modern cathodes such as LSCF have been found, and can be prevented by thin (<100 nm) ceria diffusion barriers.
If the conductivity for oxygen ions in SOFC can remain high even at lower temperature (current target in research ~773 K), material choice for SOFC will broaden and many existing problems can potentially be solved. Certain processing technique such as thin film deposition can help solve this problem with existing material by
- reducing the traveling distance of oxygen ions and electrolyte resistance as resistance is inversely proportional to conductor length;
- producing grain structures that are less resistive such as columnar grain structure;
- controlling the micro-structural nano-crystalline fine grains to achieve "fine-tuning" of electrical properties;
- Building composite with large interfacial areas as interfaces have shown to have extraordinary electrical properties.
Interconnect
Just as an internal combustion engine relies on several cylinders to provide enough power to be useful, so too must fuel cells be used in combination in order to generate enough voltage and current. This means that the cells need to be connected together and a mechanism for collection of electrical current needs to be provided, hence the need for interconnects. The interconnect functions as the electrical contact to the cathode while protecting it from the reducing atmosphere of the anode.
The high operating temperature of the cells combined with the severe environments means that interconnects must meet the most stringent requirements of all the cell components: 100% electrical conductivity, no porosity (to avoid mixing of fuel and oxygen), thermal expansion compatibility, and inertness with respect to the other fuel cell components. It will be exposed simultaneously to the reducing environment of the anode and the oxidizing atmosphere of the cathode.
For an YSZ SOFC operating at about 1000 C, the material of choice is LaCrO3 doped with a rare earth element (Ca, Mg, Sr, etc.) to improve its conductivity. Ca-doped yttrium chromate is also being considered because it has better thermal expansion compatibility, especially in reducing atmospheres [Chou]. Interconnects are applied to the anode by plasma spraying and then the entire cell is co-fired.
Cathode
The cathode, or air electrode, is a thin porous layer on the electrolyte where oxygen reduction takes place. The overall reaction is written in Kröger-Vink Notation as follows:
Cathode materials must be, at minimum, electronically conductive. Currently, lanthanum strontium manganite (LSM) is the cathode material of choice for commercial use because of its compatibility with doped zirconia electrolytes. Mechanically, it has similar coefficient of thermal expansion to YSZ and thus limits stresses built up because of CTE mismatch. Unfortunately, LSM is a poor ionic conductor, and so the electrochemically active reaction is limited to the triple phase boundary (TPB) where the electrolyte, air and electrode meet. LSM works well as a cathode at high temperatures, but its performance quickly falls as the operating temperature is lowered below 800°C. In order to increase the reaction zone beyond the TPB, a potential cathode material must be able to conduct both electrons and oxygen ions. Composite cathodes consisting of LSM YSZ have been used to increase this triple phase boundary length. Mixed ionic/electronic conducting (MIEC) ceramics, such as the perovskite LSCF, are also being researched for use in intermediate temperature SOFCs as they are more active and can makeup for the increase in the activation energy of reaction.
The charge carrier in the SOFC is the oxygen ion (O2-). At the cathode, the oxygen molecules from the air are split into oxygen ions with the addition of four electrons. The oxygen ions are conducted through the electrolyte and combine with hydrogen at the anode, releasing four electrons. The electrons travel an external circuit providing electric power and producing by-product heat.
