A solid oxide fuel cell (or SOFC) is an electrochemical conversion device that produces electricity directly from oxidizing a fuel. Fuel cells are characterized by their electrolyte material; the SOFC has a solid oxide or ceramic electrolyte. Advantages of this class of fuel cells include high combined heat and power efficiency, long-term stability, fuel flexibility, low emissions, and relatively low cost. The largest disadvantage is the high operating temperature, which results in longer start-up times and mechanical and chemical compatibility issues.
Introduction Solid oxide fuel cells are a class of fuel cells characterized by using a solid oxide material as the electrolyte. SOFCs use a solid oxide electrolyte to conduct negative oxygen ions from the cathode to the anode. The electrochemical oxidation of the hydrogen, carbon monoxide or other organic intermediates by oxygen ions thus occurs on the anode side. More recently, proton-conducting SOFCs (PC-SOFC) have been developed to transport protons instead of oxygen ions through the electrolyte with the advantage of running at lower temperatures than traditional SOFCs. They operate at very high temperatures, typically between 600 and 1,000 °C. At these temperatures, SOFCs do not require expensive platinum group metals catalysts, 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. For lower-quality fuels, such as gasified biomass, coal, or biogas, fuel processing becomes increasingly complex and, consequently, more expensive. The gasification process, which transforms the raw material into a gaseous state suitable for fuel cells, can generate significant quantities of compounds like methane and toluene, as well as larger polyaromatic and short-chain hydrocarbon compounds. These substances can lead to carbon buildup in SOFCs. The expenses associated with reforming and desulfurization are comparable in magnitude to the cost of the fuel cell itself. These factors become especially critical for systems with lower power output or greater portability requirements. Solid oxide fuel cells have a wide variety of applications, from using them as auxiliary power units in vehicles to stationary power generation with outputs from 100 W to 2 MW. In 2009, the Australian company, Ceramic Fuel Cells, successfully achieved an efficiency of an SOFC device up to the previously theoretical mark of 60%. The higher operating temperature makes SOFCs suitable for application with heat engine energy recovery devices or combined heat and power, further increasing 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 from the exothermic electrochemical oxidation within the fuel cell for an endothermic steam reforming process. Solid fuels, such as coal and biomass, may also be gasified to form syngas suitable for fueling SOFCs in integrated gasification fuel cell power cycles. Thermal expansion demands a uniform and well-regulated heating process at startup. SOFC stacks with planar geometry require an hour to heat to operating temperature. Micro-tubular fuel cell design geometries promise much faster start-up times, typically in 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 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.
Operation
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 temperatures. As a consequence, the stacks have to run at temperatures ranging from 500 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, water byproduct and two electrons are given off. 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.
Balance of plant Most of the downtime of an SOFC is caused by the mechanical balance of the plant (from components like the air preheater, prereformer, afterburner, water heat exchanger, and anode tail gas oxidizer) and the electrical balance of the plant (including the power electronics, hydrogen sulfide sensor and fans). By using internal reforming (converting methane into hydrogen internally), lower cooling requirements are needed, helping decrease the complexity and costs of the balance of plant.
… excerpt ends here. Continue reading the full article.
