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| Fig. 1: The three ceramic layers of an SOFC. (Source: Wikimedia Commons.) |
A Solid Oxide Fuel Cell (SOFC) is a device that converts chemical energy from a fuel, such as hydrogen or methane, into electricity via a series of electrochemical reactions. The electrical work in these reactions is directly converted due to the large fraction of the enthalpy associated with the electrochemical oxidation of the fuel into water and carbon dioxide. Therefore, in comparison to traditional technologies, such as coal power plants, fuel cells are able to deliver higher electrical conversion efficiencies. Furthermore, due to their high temperature operation, SOFCs also allow the use of a variety of different fuels (hydrogen, carbon monoxide, hydrocarbons, etc), and they are capable of producing a significant amount of exhaust heat. [1]
Solid Oxide Fuel cells were originally made for use as a commercial light source, in an effort to replace carbon filament lamps. The device used the Nernst mass, which is made of yttria-stabilized zirconia, a conductor of oxide ions in the air, at temperatures from 600 to 1000 degrees Celsius. In 1937, Baur and Preis developed the first solid oxide fuel cells using coke as the fuel and magnetite as the oxidant. In the 1960s, a period of much research began on solid fuel cells, particularly with a focus on space, submarine, and military applications. Currently, the Department of Energy and electrical power companies are actively working on solid oxide fuel cells, focusing on the research, development, and commercialization of the technology. [2]
How do SOFCs work? Five components make up this cell: (1) an electrolyte, (2) an anode, (3) a cathode, and (4 &5) two interconnect wires. A SOFC contains a solid oxide electrolyte made from yttria-stabilised zirconia. This ceramic material permits "oxygen atoms to be reduced on its porous cathode surface by electrons, thus being converted into oxide ions, which are then transported through the ceramic body to a fuel-rich porous anode zone." A critical component of SOFC design is the heat that is produced by the electrochemical reaction. The heat, which is generated in the SOFC by ohmic losses and electrode over-potentials, cannot be eliminated, but in a good design, must be integrated into a heat management system. This would allow it to maintain the operating temperature of the cells; the higher temperature of the exhaust heat also makes the utilization of such technology simple and mass-producing. [3,4]
High temperature fuel cells allow for hybrid systems to be engineered, allowing the generation of electricity with higher efficiency. SOFCs work at temperatures of 800 to 1,000 degrees Celsius. This elevated temperature allows for the possibility of using internal conversion of hydrocarbon fuels into hydrogen. Methane, methanol, petroleum, and other hydrocarbons can be converted to hydrogen directly within the fuel cell. Taking all of this into account, the fuel cell has delivered power generation efficiency of up to 50%, while connecting fuel cells could achieve an even higher efficiency, of up to 70%. [5]
Solid oxide fuel cells have a number of advantages over traditional generators and other types of fuel cells. For example, the high quality of exhaust heat (over 800 degrees Celsius) makes it a useful application in industry for cogeneration. Because of combined cycles, high efficiency for electricity production can be achieved. Also, due to the modular nature of solid fuel cells, they offer flexibility in the planning of power generation capacity. Finally, carbon dioxide emission is considerably reduced. [2]
Wachsman et al have identified that the key technical issue "that has limited the development and deployment of [SFOCs] to be its high operating temperature, resulting in higher systems costs and performance degradation rates, as well as slow start-up and shutdown cycles." Low-temperature SOFCs (less than 650 degrees Celsius) can reduce system cost due to wider material choices for interconnects, as well as reduced balance of plant costs. Furthermore, with temperatures less than 600 degrees Celsius, both radiative heat transfer and sintering rates exponentially drop off, reducing insulation costs. [6]
According to the paper "A high-performance cathode for the next generation of solid-oxide fuel cells," published in Nature, a new cathode material for reduced-temperature SOFC operation is presented. The primary function of the cathode in a fuel cell is to facilitate an electrochemical reduction of oxygen. A major hindrance to reduced-temperature operation of SOFCs is the poor activity of traditional cathode materials for electrochemical reduction of oxygen. In this paper, the new cathode material, known as BSCF, incorporates a "thin-film doped ceria fuel cell," and exhibits high power densities when operated with "humidified hydrogen as the fuel and air as the cathode gas." [7]
© Danielle Rasooly. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] J. T. S. Irvine and P. Connor, Solid Oxide Fuels Cells: Facts and Figures (Springer, 2013).
[2] P. J. Gellings and H. J. M. Bouwmeester, The CRC Handbook of Solid State Electrochemistry (CRC Press, 1997).
[3] S. C. Singhal and K. Kendall, High Temperature Solid Oxide Fuel Cells: Fundamentals, Design and Applications (Elsevier, 2003).
[4] N. Chowdhury, "Solid Oxide Fuel Cells and The Bloom Box," PH240, Stanford University, Fall 2011.
[5] J. Milewski et al., Advanced Methods of Solid Oxide Fuel Cell Modeling (Springer, 2011).
[6] E. D. Wachsman and K. T. Lee, "Lowering the Temperature of Solid Oxide Fuel Cells," Science 334, 935 (2011).
[7] Z. Shao and S. M. Haile, "A High-Performance Cathode For the Next generation of Solid-Oxide Fuel Cells," Nature 431, 170 (2004).
