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Solid Oxide Fuel Cells Solid oxide fuel cells (SOFCs) have an electrolyte that is a solid, non-porous metal oxide, usually Y2O3-stablilized ZrO2. The cell operates at 600-1000 oC where ionic conduction by oxygen ions takes place. Typically, the anode is a Ni-ZrO2 cermet and the cathode is Sr-doped LaMnO3. There is no liquid electrolyte with its attendant material corrosion or electrolyte management problems. The high temperature of the SOFC, however, places stringent requirements on its materials. The development of suitable low cost materials and the low-cost fabrication of ceramic structures are presently the key technical challenges facing SOFCs. The cell is constructed with two porous electrodes that sandwich an electrolyte. Air flows along the cathode. When an oxygen molecule contacts the cathode/electrolyte interface, it acquires electrons from the cathode. The oxygen ions diffuse into the electrolyte material and migrate to the other side of the cell where they contact the anode. The oxygen ions encounter the fuel at the anode/electrolyte interface and react catalytically, giving off water, carbon dioxide, heat, and electrons. The electrons transport through the external circuit, providing electrical energy. Solid oxide fuel cells (SOFC) allow conversion of a wide range of fuels, including various hydrocarbon fuels. The relatively high operating temperature allows for highly efficient conversion to power, internal reforming, and high quality by-product heat for cogeneration or for use in a bottoming cycle. Indeed, both simple-cycle and hybrid SOFC systems have demonstrated among the highest efficiencies of any power generation system, combined with
minimal air pollutant emissions and low greenhouse gas emissions. These capabilities have made SOFC an attractive emerging technology for stationary power generation in the 2 kW to 100s MW capacity range. More recently, (planar) SOFC systems with high power densities operating at lower temperatures (700 to 850 °C instead of 900 to 1000 °C as was previously the norm) have been developed. Combined with the ability of SOFC to use conventional fossil fuels, this could help reduce the cost of the fuel cell because less-expensive materials of construction could be used at lower temperatures. This would improve the economy of applications ranging from small-scale stationary power (down to ~2 kW) to auxiliary power units for vehicles and mobile generators for civilian as well as military applications. There is even the possibility that SOFC could eventually be used for part of the prime power in vehicles. The present challenge for developers is to produce robust, high-performance stack technologies based on suitable low-cost materials and fabrication methods. Derivatives from SOFC technology, such as oxygen sensors used in automobiles, are already in widespread commercial use.
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