Solid-oxide fuel cells temperature stability

This reaction is of great technological interest in the area of solid oxide fuel cells (SOFC) since it is catalyzed by the Ni surface of the Ni-stabilized Zr02 cermet used as the anode material in power-producing SOFC units.60,61 The ability of SOFC units to reform methane "internally", i.e. in the anode compartment, permits the direct use of methane or natural gas as the fuel, without a separate external reformer, and thus constitutes a significant advantage of SOFC in relation to low temperature fuel cells. 

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. 

Primdahl S, Sprensen BF, and Mogensen M. Effect of nickel oxide/yttria-stabilized zirconia anode precursos sintering temperature on the properties of solid oxide fuel cells. J Am Ceram Soc 2000 83 489 -94. 

Figure 29. Conductivity of some intermediate-temperature proton conductors, compared to the conductivity of Nafion and the oxide ion conductivity of YSZ (yttria-stabilized zirconia), the standard electrolyte materials for low- and high-temperature fuel cells, proton exchange membrane fuel cells (PEMFCs), and solid oxide fuel cells (SOFCs).

H. Hammou outlines the thermodynamic concepts and rate processes relevant to solid oxide fuel cells. Recent advances in materials research concerning electrical properties, and stability at high temperatures, are thoroughly reviewed. The most promising hardware developments are described, along with problems to be resolved. 

The difficulties in the development of HTSO fuel cells are in the area of stability of materials rather than in catalysis. Different materials, some of them ionic conductors with no electronic conductivity and others electronic conductors with no ionic conductivity, must be compatible with each other chemically at a high temperature and mechanically during temperature cycling. Improvements in materials are steadily made, but the more sophisticated materials developed for this purpose tend to increase the cost. Once the materials problems have been overcome, the inherent simplicity of the design and operation of high temperature solid oxide fuel cells may make them the most useful... 

Finally, Steve Chuang (University of Akron) reviews catalysts for use in solid oxide fuel cells. These fuel cells hold the promise of significant improvements in energy efficiency with minimum emissions, but challenges remain - particularly for the catalysts. A particular challenge is the effect of the high temperature operation on the oxide electrolyte. Also important is the stability of the anode in... 

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... 

Solid oxide fuel cell (SOFC) uses solid ceramic material, such as Y2O3 stabilized Zr02 (YSZ), as an electrolyte. As SOFC operates at high temperature (600-1000° C), a variety of fuels, e.g., hydrogen, methane, and carbon monoxide, can directly be utilized. The high temperature places severe constraints on material selection and results in difficult fabrication process. Co-ZrO (or Ni-ZrO) and SrO doped LaMn03 have often been used for anode and cathode materials, respectively. 

The electrolyte in a solid oxide fuel cell (SOFC) is a solid oxide. The state-of-the-art material for the electrolyte is 8 mol% Y203 stabilized Zr02. The operating temperature is about 1000°C. 

Since these first reports, Iwahara and other investigators have studied the conductivities (both ionic and electronic), conduction mechanism, deuterium isotope effect, and thermodynamic stability of these materials. The motivation for most of this work derives from the desire to utilize these materials for high temperature, hydrogen-fiieled solid oxide fuel cells. In a reverse operation mode, if metal or metal oxide electrodes are deposited onto a dense pellet of this material and heated to temperature T, the application of an electric potential to the electrodes will cause a hydrogen partial pressure difference across the pellet according to the Nemst equation ... 

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