Ionic-electronic conductors

Observing NEMCA, and actually very pronounced one, with Ti0224 and Ce0271 supports was at first surprising since Ti02 (rutile) and Ce02 are n-type semiconductors and their ionic (O2 ) conductivity is rather low so at best they can be considered as mixed electronic-ionic conductors.77... 

A special class of conductors are ionically and electronically conducting polymers (Sections 2.6.4 and 5.5.5). 

Fig. 4. Configuration of a ceramic membrane reactor for partial oxidation of methane. The membrane tube, with an outside diameter of about 6.5 mm and a length of up to about 30 cm and a wall thickness of 0.25-1.20 mm, was prepared from an electronic/ionic conductor powder (Sr-Fe-Co-O) by a plastic extrusion technique. The quartz reactor supports the ceramic membrane tube through hot Pyrex seals. A Rh-containing reforming catalyst was located adjacent to the tube (57).

Ionic conductors are ionic compounds. Therefore it is appropriate to start with ions rather than atoms to construct the electronic energy level diagrams. Fig. 3.2 illustrates such a construction for the electronic and ionic insulator MgO. The energy levels and 0 correspond... 

Electrical conductivity can also be an important consideration in p3rrotechruc theory [7]. This phenomenon results from the presence of mobile electrons in the solid that migrate when an electrical potential is applied across the material. Metals are the best electrical conductors, while ionic and molecular solids are generally much poorer, serving well as insulators. 

Hitherto we have dealt with model FICs that are mostly useful as solid electrolytes. The other class of compounds of importance as electrode materials in solid state batteries is mixed electronic-ionic conductors (with high ionic conductivity). The conduction arises from reversible electrochemical insertion of the conducting species. In order for such a material to be useful in high-energy batteries, the extent of insertion must be large and the material must sustain repeated insertion-extraction cycles. A number of transition-metal oxide and sulphide systems have been investigated as solid electrodes (Murphy Christian, 1979). 

It will be shown further on that the phases on either side of the boundary become charged to an equal and opposite extent and this gives rise to a potential difference across the boundary. There are several ways in which this potential difference can arise. If one of the phases is an electronic conductor and the other is an ionic conductor, electron-transfer reactions can occur at the boundary and lead to the development of a potential difference. A discussion of this type of mechanism will be reserved for Section 7.5. Or, the electronic conductor can be deliberately charged by a flow of electrons from an external source of electricity. The electrolyte side of the boundary then responds with an equal and opposite charge, and a potential difference develops across the boundary. However, even without an external connection or the occurrence... 

Figure 9. Four modes of spontaneous oxygen permeation (a) in a short-circuited electrochemical cell (b) through a mixed conducting single phase, (c) through a composite phase mixture comprising an ionic and an electronic conductor, and (d) through an ionic (electronic) conductor the grain boundaries of which are predominandy electronically (ionically) conducting.

In the technology of ceramics, electronic conductors (semiconductors), ionic conductors (solid electrolytes) and mixed electronic-ionic conductors are encountered. In all cases the conductivity is likely to vary with temperature according to... 

FIGURE 8.12 Oxygen transport in a mixed ion electron ionic conductor. 

Q.25.3 Explain how electroneutrality is maintained in an electrical circuit that has an electrolytic conductor as a circuit element. Be sure to address the three general aspects of the circuit s behavior, that is, electronic, ionic, and electrodic. 

As intensive studies on the ECPs have been carried out for almost 30 years, a vast knowledge of the methods of preparation and the physico-chemical properties of these materials has accumulated [5-17]. The electrochemistry ofthe ECPs has been systematically and repeatedly reviewed, covering many different and important topics such as electrosynthesis, the elucidation of mechanisms and kinetics of the doping processes in ECPs, the establishment and utilization of structure-property relationships, as well as a great variety of their applications as novel electrochemical systems, and so forth [18-23]. In this chapter, a classification is proposed for electroactive polymers and ion-insertion inorganic hosts, emphasizing the unique feature of ECPs as mixed electronic-ionic conductors. The analysis of thermodynamic and kinetic properties of ECP electrodes presented here is based on a combined consideration of the potential-dependent differential capacitance of the electrode, chemical diffusion coefficients, and the partial conductivities of related electronic and ionic charge carriers. 

Scheme 11.1 Classification of conductive/ electroactive solids and films, (a) Molecular charge-transfer (CT) complexes and jt-conjugated linear oligomers (b) Mixed electronic-ionic conductors inorganic jt-conjugated polymers and polymer-like...

These are presented by two subclasses of electroactive polymer (i) -conjugated polymers of both organic and inorganic nature [5-15] and (ii) conventional redox polymers [26], and by inorganic ion-insertion (intercalation) compounds [27, 28[ (see the top of Scheme 11.1b). Despite the different nature of their chemical bonds, all of these compounds are mixed, electronic-ionic conductors [29], and hence, their electronic and/or ionic conductivity is expected to change with the applied potential in a predictable, characteristic manner (see Section cl 1.4). 

Figure 11.16 Potential dependences of the differential capacitance for three different electronic-ionic conductors, (a) V2O5 film (1600 A thick) in 1 M LiCIO4/PC solution. Scan rates of 0.2 mVs (b) Freshly prepared 0.1 4.m-thick PPy cycled in 0.25 M TBACIO4/PC solution up to 4 V (versus Li), v = 5 mV s (c) The same PPy film as in panel b, after 100 consecutive cycles.

Electronics of circuitry to control. potential across I interfaces 1 Electronic e conductor 1 Ionic 1 0 conductor Physical chemistry of solutions Statistical mechanics of particle distribution near interface in field... 

O conductor, Na-p"-Al203, a Na conductor, CaZro.9lno.i03-a, a H" conductor [22], or Ti02, a mixed electronic-ionic conductor [23]). Catalyst, counter and reference electrode preparation and characterization details have been presented in detail elsewhere [9,14] together with the gas analysis system for on-line monitoring of the rates of catalytic reactions via gas chromatography, mass spectrometry and infrared spectroscopy. 

Another physical property of ionic compounds is their tendency to dissolve in water. When they dissolve in water, the solution conducts electricity, as you saw in Figure 4.2. Ionic compounds also conduct electricity in the liquid (melted) state. Any compormd that conducts electricity when melted or dissolved in water is an electrolyte. Therefore, ionic compounds are electrolytes. In order to conduct electricity, ions must be free to move because they must take on or give up electrons. Ionic compormds in the solid state do not conduct electricity because the ions are locked into position. Ionic compormds become good conductors when they melt. This is evidence that the ions are less bormd and free to move in the liquid state. 

Electrocatalysis can modify the composition of the electrode surfaces and the nature of the electrolytic products. The perchlorate decomposition (cathodic production of chloride) on platinum catalysts is one of the examples [57] and the IrCT decomposition during the sodium chlorate production [58]. The electropolymerization of the organic substances is critically dependent on the type of the electronic/ionic conductors, electrolyte characteristics, and the electrolysis resident time of the monomer [59]. 

There are several advantages to using a dual-phase membrane over a singlephase mixed electronic ionic conductor. These include the fact that the ionic conductors such as YSZ are much more chemically and thermally stable compared to most perovskites. Thus, dual-phase membranes are Ukely to be able to tolerate the harsh conditions of an oxygen separation device. They also show good tolerance to both CO2 and steam. The difficulty comes in the selection of an electronically conducting material. The cost of noble metals makes their incorporation into commercial devices unlikely. Therefore, the electron-conducting phase is Umited to a... 

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