Ceria electrolytes conditions

In the real non-equilibrium conditions of a present-day MCFC with very successful electrode reform, the cell electrode reaction, voracious for fuel, consumes the reformer product and favourably influences the reform process. The latter turns out to operate well at 600 °C, compared with about 800 °C in a fired reformer coupled, say, to much less voracious hydrogen separation and storage. In the practical SOFC, 1000 °C at the anode promotes excessively vigorous electrode reform, which leads to a local electrode cold spot. There are also stability considerations (Gardiner, 1996). Hence the contemporary movement towards lower SOFC temperatures, via new ceria electrolytes, and interconnect change from ceramic to steel. A PEFC near Tq, must have a combustion-operated 800 °C reformer, since a Tq electrochemical reform process does not exist in practice. 

Whereas the ionic conductivity is always much lower than the electronic conductivity in pure reduced ceria, the situation is quite different in ceria doped with oxides of two- or three-valent metals due to the introduction of oxide ion vacancies, cf eqs. 15,2 and 15.3. A high vacancy concentration will shift eq. 15.1 to the left. This means that the ionic domain extended down to 10 atm or even lower in the temperature range of 600 - 1000 0. The electronic conductivity in air may be very low, and the doped cerias are under these conditions excellent electrolytes. The conductivity mechanism is the hopping of oxide ions to vacant sites, and the ionic conductivity, a may be expressed as... 

Ceria doped with heterovalent cations, such as alkaline earth and rare earth ions, has been considered one of the most promising electrolyte materials for intermediate-temperature solid oxide fuel cells. It was found that doped ceria materials exhibit relatively high ionic conductivity under nonreducing conditions and relatively lower temperatures in comparison to that of YSZ electrolyte. Among the various dopants studied, Gd " and singly doped Sm ceria have been reported to have a high conductivity [8] and to be relatively stable in a reducing environment [49]. 

The microscopic mechanisms responsible for the electrochemical behavior of metallic and oxide electrodes were exhaustively analyzed in the literature [17-22, 24, 60, 62-69] and are outside of the main scope of this chapter. One should only mention that the solid-electrolyte additions into the electrode composition make it possible to increase reversibility and to enlarge the domain of temperatures and chemical potentials where the electrode can be safely used, a result of the TPB expansion and microstructural stabilization. Although the mixed-conducting and catalytically active additives such as doped ceria might also be useful from the cell impedance point of view, their use for the reference electrodes is limited if oxygen nonstoichiometry changes may occur under the cell operation conditions. 

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