The Solid Oxide Fuel Cell

One leading prototype of a high-temperature fuel cell is the solid oxide fuel cell, or SOFC. The basic principle of the SOFC, like the PEM, is to use an electrolyte layer with high ionic conductivity but very small electronic conductivity. Figure B shows a schematic illustration of a SOFC fuel cell using carbon monoxide as fuel. 

The principle of the fuel cell was first demonstrated by Grove in 1839 [W. R. Grove, Phil. Mag. 14 (1839) 137]. Today, different schemes exist for utilizing hydrogen in electrochemical cells. We explain the two most important, namely the Polymer Electrolyte Membrane Fuel Cell (PEMFC) and the Solid Oxide Fuel Cell (SOFC). 

The principle of the A-probe is shown in Fig. 10.2. It is a simple oxygen sensor made in a similar manner to the solid oxide fuel cell discussed in Chapter 8. An oxide that allows oxygen ions to be transported is resistively heated to ensure sufficiently high mobility and a short response time ( 1 s.). 

There are six different types of fuel cells (Table 1.6) (1) alkaline fuel cell (AFC), (2) direct methanol fuel cell (DMFC), (3) molten carbonate fuel cell (MCFC), (4) phosphoric acid fuel cell (PAFC), (5) proton exchange membrane fuel cell (PEMFC), and (6) the solid oxide fuel cell (SOFC). They all differ in applications, operating temperatures, cost, and efficiency. 

A fuel cell is a form of battery. An ordinary battery consists of internal reactants that are converted into electrical energy, whereas in a fuel cell the chemical reactants are supplied from an external source. There are several designs of fuel cell, one of which is the solid oxide fuel cell (SOFC). These employ calcia- or yttria-stabilized zirco-nia. The cells operate at temperatures of about 900°C, this high temperature being needed to maintain a high enough oxygen transport for useful cell output. 

Jiang SP and Badwal SPS. An electrode kinetics study of H2 oxidation on Ni/Y203-Zr02 cermet electrode of the solid oxide fuel cell. Solid State Ionic 1999 123 209-224. 

Huang Y-H, Dass RI, Denyszyn JC, and Goodenough JB. Synthesis and characterization of Sr2MgMo06 s an anode material for the solid oxide fuel cell. J Electrochem Soc 2006 153 A1266-A1272. 

We discuss both the Proton Exchange Membrane as well as the Solid Oxide Fuel Cells in this chapter (PEMFC and SOFC). Both types are in full development, the PEMFC for mobile and stationary applications, and the SOFC for stationary applications as well as for auxiliary power generation for transport. 

Bove R. and Sammes N., 2005. The effect of current collectors configuration on the performance of a tubular SOFC. Proceedings of the Solid Oxide Fuel Cells IX, Quebec City, Canada, S.C. Singhal and J. Mizusaki (Eds.), Electrochemical Society, Vol. 1, pp. 780-789. 

One of the more important fuel cells is the solid oxide fuel cell Solid oxide fuel cell (SOFC), since they offer a realistic opportunity for use in electric utility operations. A schematic of a fuel cell is shown in Figure 38. These can be developed in stacks having 5-10 kW capacity that would be suitable for both stationary and mobile power units. 

Fuel cells are classified primarily according to the nature of the electrolyte. Moreover, the nature of the electrolyte governs the choices of the electrodes and the operation temperatures. Shown in table 10.1 are the fuel cell technologies currently under development. "" Technologies attracting attention toward development and commercialization include direct methanol (DMFC), polymer electrolyte membrane (PEMFC), solid-acid (SAFC), phosphoric acid (PAFC), alkaline (AFC), molten carbonate (MCFC), and solid-oxide (SOFC) fuel cells. This chapter is aimed at the solid-oxide fuel cells (SOFCs) and related electrolytes used for the fabrication of cells. 

At 1-10 W (watts), fuel cells could be used as battery replacements at 100 W to 1 kW, fuel cells could find military applications which require lightweight portable power sources for communications and weapon power at 1 - 10 kW, fuel cells could supply power to residential buildings and serve as auxiliary power units in vehicles and trucks. At higher power levels, the solid oxide fuel cell (SOFC) could be an effective approach for the distributed power generation and the cogeneration (i.e., combined heat and power). Above 1 MW, the SOFC could be integrated with a turbine power plant to improve the overall efficiency of power generation and reduce emissions. ... 

For the solid oxide fuel cells (SOFCs), a number of environmentally critical items have been identified (Zapp, 1996). The carrier sheet electrolyte may be produced from yttrium-stabilised zirconium oxide with added electrodes made of, e.g., LaSrMn-perovskite and NiO-cermet. Nitrates of these substances are used in manufacturing, and metal contamination of wastewater is a concern. The high temperature of operation makes the assembly very difficult to disassemble for decommissioning, and no process for recovering yttrium from the YSZ electrolyte material is currently known. 

The most important fuel cells that are in use nowadays are the polymer electrolyte membrane fuel ceU (PEMFC), the molten carbonate fuel cell (MCFC), and the solid oxide fuel cell (SOFC). In a PEMFC, the electrolyte is a polymer membrane that conducts protons, in an MCFC the electrolyte is a carbonate melt in which oxygen is conducted in the form of carbonate ions, CO , and in an SOFC the electrolyte is a solid oxide that conducts oxygen ions, While a PEMFC can be operated at low temperatures of about 80 °C, an MCFC works at intermediate temperatures of about 650 °C, and an SOFC needs relatively high temperatures of 800-1000 °C (see next sections). 

R. S. Gordon, W. Fischer, A, V, Virkar, in Ceramic Transactions Vol. 65, Role of Ceramics in Advanced Electrochemical Systems, P. N. Kumpta, G. S. Roher, U. Balachadran, eds., American Ceramic Society, Westerville, OH, 1996, pp. 203-237. Current review on the application of ceramics in the sodium sulfur battery and the solid oxide fuel cell. 

N. Q. Minh, J. Am. Ceram. Soc., 76, 563 (1993). Excellent review of the solid oxide fuel cell including a thorough treatment of the ceramic components (electrolyte, anode, cathode, and interconnect). 

The ion conductivity of bismuth oxide is decreased with increasing concentration of Y2O3 dopant. Dopant concentrations of at least 25 mol% Y2O3 are necessary to stabilize the cubic structure at temperatures below 730°C. The higher conductivity of stabilized bismuth oxide compared to yttria-stabilized zirconia offers the possibility of its use as a solid electrolyte in the solid oxide fuel cell at reduced temperatures. However, the... 

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