Oxide ion mixed conductors

In general, oxides that show both ionic and electronic conductivity are called mixed conductors. These have applications in electrochemical cells, particularly SOFCs (Section 5.7) where both ioiuc and electronic components are essential. The ionic conductivity is usually several orders of magnitude less than that of the electronic component and is often more difficult to quantify. 

At high oxygen pressures the incorporation of neutral oxygen to form oxygen interstitials requires the abstraction of two electtons to form oxide ions, thus generating an equal number of holes  

The hole population results in p-type conductivity, the magnitude of which is proportional to the hole concentration  

The form of the curve drops more or less symmetrically to a minimum which depends upon the temperature of the sample. 

Many other perovskites containing mixed valence cations behave in a similar way, and these can be modified with A- or B-site substitutions in order to improve either aspects of stabihty or conductivity. For example, the perovskite SrFej Sc Oj shows the same conductivity dependence as SrjFe Og  

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The oxide ion diffusivity of Ea2Ni04+d over the temperature range 640 -840 °C is shown in Fig. 2 and is compared with the data for the most common perovskite oxide ion mixed conductors. 

Iwahara, H., Yajima, T., Hibino, T., and Ushida, H. (1993). Performance of solid oxide fuel cell using proton and oxide ion mixed conductors based on BaCei-jcSmjtOs-a Electrochem. Soc. 140 1687-1691. 

In response to other needs in the energy and transportation sector, membranes are evolving that transport molecular species, ions, electrons, and combinations of these species. For example, mixed oxide ion-electronic conductors that become the wall of tubular reactors will soon move out of the laboratory and be used to oxidize methane by transport of oxygen from the air side to the fuel side, where the methane is converted to carbon monoxide and hydrogen. This technology eliminates the need for huge and expensive air separation plants to supply oxygen. 

An example of a layer structure mixed conductor is provided by the cathode material L CoC used in lithium batteries. In this solid the ionic conductivity component is due to the migration of Li+ ions between sheets of electronically conducting C0O2. The production of a successful mixed conductor by doping can be illustrated by the oxide Cei-jPxx02- Reduction of this solid produces oxygen vacancies and Pr3+ ions. The electronic conductivity mechanism in these oxides is believed to be by way of electron hopping between Pr4+ and Pr3+, and the ionic conductivity is essentially vacancy diffusion of O2- ions. 

Fig. 3. Oxygen transport in solids. 02 is dissociated and ionized at the reduction interface to give O2 ions, which are transferred across the solid to the oxidation interface, at which they lose the electrons to return back to 02 molecules that are released to the stream, (a) In the solid electrolyte cell based on a classical solid electrolyte, the ionic oxygen transport requires electrodes and external circuitry to transfer the electrons from the oxidation interface to the reduction interface (b) in the mixed conducting oxide membrane, the ionic oxygen transport does not require electrodes and external circuitry to transfer the electrons to the reduction interface from the oxidation interface, because the mixed conductor oxide provides high conductivities for both oxygen ions and electrons.

Perovskite-related oxides of the K2MF4 (A2BO4+J B-transition metal ion) structure (Figure 16) have been investigated by several groups as alternative mixed conductors for SOFC. 

The electrolyte in an SOFC must consist of a good ion conductor, which has essentially no electronic conductivity. Otherwise the cell will be internally short-circuited. An often-used electrolyte material is yttria-stabilised zirconia (YSZ). The electrodes must pos.scss good electron conductivity in order to facilitate the electrochemical reaction and to collect the current from the cell. The fuel electrode usually contains metallic nickel for this purpose. The anodic oxidation of the fuel (H or CO) can only take place in the vicinity of the so-called three-phase boundary (TPB), where all reactants (oxide ions, gas molecules and electrons) are present. Thus, it is advantageous to extend the length and width of the TPB zone as much as possible. One way to do this is by making a composite of Ni and YSZ called a Ni-YSZ-cermet. Another way is to use a mixed ionic and electronic conductor, which in principle can support the electrochemical reaction all over the surface as illustrated in Fig. 15.1. Partially reduced ceria is a mixed ionic and electronic... 

CeO is both an n-type semi-conductor and an oxide ion conductor, i. e. a mixed conducior even though the ionic conductivity of pure reduced ceria at 1000°C is... 

Ceramic electrochemical reactors are currently undergoing intense investigation, the aim being not only to generate electricity but also to produce chemicals. Typically, ceramic dense membranes are either pure ionic (solid electrolyte SE) conductors or mixed ionic-electronic conductors (MIECs). In this chapter we review the developments of cells that involve a dense solid electrolyte (oxide-ion or proton conductor), where the electrical transfer of matter requires an external circuitry. When a dense ceramic membrane exhibits a mixed ionic-electronic conduction, the driving force for mass transport is a differential partial pressure applied across the membrane (this point is not considered in this chapter, although relevant information is available in specific reviews). 

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