General Electric, solid oxide fuel cell

In some ceramics the only species that can move in an applied electric field are the ions in the structure. Generally, the movement of ions is slow, but in a class of ceramics called fast ion conductors, they can move very rapidly. In cubic zirconia the diffusion of oxygen ions at high temperature is particularly fast, and this ceramic is used as the electrolyte in solid oxide fuel cells. Fuel cells are becoming a key part of a diverse energy plan for the twenty-first century. 

In general, the performance of the anode is defined by its electrical and electrochemical profjerties and therefore has a strong dependence on its microstructure. Thus, the control pjarameters, such as composition, size and distribution of perticles and pores, are very important for optimizing the performance of the anode material of a solid oxide fuel cell. 

Minh, N. (2(X)4) SECA solid oxide fuel cell program-General Electric SECA industry team. Proceedings of the SECA Annual Workshop and Core Technology Program peer reviewed abstracts. May 11-13, 2004, Boston, MA, p. 2. 

Minh, N., Andrews, R. and Campbell, T. (2(X)4) General Electric Solid State Energy Conversion Alliance (SECA) solid oxide fuel cell program. Fuel Cell program Annual Report, Office of Fossil Energy, U.S. Department of Energy, pp. 27-32. 

In the discussion so far, the diffusional and electrical fluxes of the ionic and electronic carriers were treated separately. However, as will become amply clear in this section and was briefly touched upon in Sec. 5.6, in the absence of an external circuit such as the one shown in Fig. 7.7, the diffusion of a charged species by itself is very rapidly halted by the electric field it creates and thus cannot lead to steady-state conditions. For steady state, the fluxes of the diffusing species have to be coupled such that electroneutrality is maintained. Hence, in most situations of great practical importance such as creep, sintering, oxidation of metals, efficiency of fuel cells, and solid-state sensors, to name a few, it is the coupled diffusion, or ambipolar diffusion, of two fluxes that is critical. To illustrate, four phenomena that are reasonably well understood and that are related to this coupled diffusion are discussed in some detail in the next subsections. The first deals with the oxidation of metals, the second with ambipolar diffusion in general in a binary oxide, the third with the interdiffusion of two ionic compounds to form a solid solution. The last subsection explores the conditions for which a solid can be used as a potentiometric sensor. 

The properties of the interface at which the formation of oxide ions occurs have been of special interest [6, 7, 28—35]. While solid electrocatalysts, Pt [28, 29, 31, 32] and C [30], were studied mainly, a molten silver cathode was employed in another type of zirconia-electrolyte fuel cell developed [34,35] at the General Electric Research and Development Center in Schenectady. Since the hindrance of the electrochemical steps of the O2 reduction at the cathode surface is small [28, 32] on platinum around 1000 °C, it is hard to elucidate the reaction mechanism beyond the net reaction 1. Analysis [33] of the potential distribution curves inside Zro 9Yo 2 02.i in contact with two platinum electrodes showed at 1380°C that the electronic hole contribution to the conductivity in the bulk of the specimen depended upon as would be expected from the equilibrium of reaction 15. The partial oxygen pressure had values between 10 and 10 atm. However, if the production of oxide ions is assumed to occur at the cathode solely by reaction 15, the rate of production is much lower than the rate of loss at the anode. A cathodic reaction of the type... 

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