Mixed Electronic-Ionic Conductivity

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... 

Having discussed the functional equivalence of classical promotion, electrochemical promotion and metal-support interactions on 02 -conducting and mixed electronic-ionic conducting supports, it is useful to also address and systematize their operational differences. This is attempted in Figure 11.15 The main operational difference is the promoter lifetime, Tpr, on the catalyst surface (Fig. 11.15). 

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). 

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... 

Dense membranes are made from solid layers of metals (e.g. Pd alloys) for hydrogen separation, or of mixed (electronic, ionic) conducting oxides for oxygen separation. A special form are the LIMs (liquid immobilised membranes) which consist of a porous support filled with a liquid or molten salt which is semipermeable. 

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...

Electroactive Solids and Polymeric Films with Mixed Electronic-Ionic Conductivity... 

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). 

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... 

Of gallium, chromium and titanium B-site cations, only galhum perovskites have substantial oxygen-ion conductivity. Therefore, if chromium or titanium are used as part of a single-phase mixed electronic-ionic conducting membrane, they need to be used in conjunction with iron or small amounts of cobalt [26]. GaUium perovskites, while possessing potential for excellent ionic conductivity, do not have sufficient electronic conductivity and need the addition of iron, cobalt or chromium. 

Sutija, D. P., Norby, T., Bjombom, P. (1995). Transport number determination by the concentration-ceU/open-circuit voltage method for oxides with mixed electronic, ionic and protonic conductivity. Solid State Ionics, 77, 167—174. 

Gas sensors based on the MOS and FET structures is a new trend in ISSP of University of Latvia It is supported by cooperation with the Laboratory of Applied Physics of Linkoping University Our attention is given to the use of non-stoichiometric oxides of transition metals with mixed electronic-ionic conductivity (WOx, MoOx, IrOx, NiOx) instead of layers of metal and/or oxide in MOS structures... 

Ftikos, C., Carter, S., and Steele, B.C.H. (1993) Mixed electronic/ionic conductivity... 

Impedance spectroscopy (IS) is a versatile and powerfiil characteization technique for the investigation of frequency dependent electrical properties of materials and interfaces. It can be used to investigate the dynamics of boimd or mobile charge, both in the bulk and in interfacial regions of any kind of soUd or Uquid material with electronic, ionic, semiconducting, mixed electronic-ionic conductivity or even dielectric properties (Macdonald, 1987a). 

It appears that Cu primarily provides electronic conductivity to the anode and is otherwise catalytically inert. This is confirmed by data showing that Au-ceria-SDC (samaria-doped ceria) composites exhibit a similar performance to that of Cu-ceria-SDC anodes, as Au would not be expected to add catalytic activity [86,87]. It is proposed that the function of ceria is primarily that of an oxidation catalyst, although mixed electronic-ionic conductivity (MEIC) could also enhance anode performance. In general, finely dispersed ceria seems more active than doped ceria ceramics, thus the redox oxygen exchange ability of ceria at fuel conditions might be considered as a key factor. 

Particular attention has been focused on how this TMO could improve mixed electronic-ionic conduction as well as how it could develop fast ion conductivity (FlC) in glasses [36]. 

As previously reported, the peculiarity of alkali vanado-phosphate glasses is ascribed to the mixed electronic-ionic conductivity. The electronic conductivity is due to the small polaron hopping between the TM ions with different valence state and it could be related to the presence of V +-0-y + species in the glass network. For NaVP systems, TableS.lO reports the various contributions that are included in the V-O-V linkages. It could be highlighted that the increase of V content results in a major number of V -O-V " linkages, which might be correlated to an increase of the electronic conductivity. 

P. Jozwiak, J. Garbarczyk, Mixed electronic-ionic conductivity in the glasses of the LiO-VO-PO system. Sohd State Ionics 176(25-28), 2163-2166 (2(X)5). Aug H. Takahashi, T. Karasawa, T. Sakuma, J.E. Garbarczyk, Electrical conduction in the vitreous and crystalhzed Li20-V205-P205 system. Solid State Ionics 181(1-2), 27-32 (2010)... 

Thus, Avenir Akimovich Velikanov was the first who brought a wide variety of objects with mixed electronic-ionic conductivity into electrochemistry. This contribution to the modem electrochemical science cannot be overestimated. 

The separation layer may be dense (non-porous), such as Pd or Pd-alloy membranes for hydrogen separation and mixed (electronic, ionic) conducting oxide membranes for oxygen separation, or porous, such as metal oxides, silicalite, or zeolite membranes. Inorganic membranes are generally named for this separation layer, since it determines the properties and application of the membrane. The flux and selectivity of inorganic membranes are mainly determined by the quality of the separation layer, which is required to be defect-free and as thin as possible. 

Diffusion in Mixed Electronic-Ionic Conducting Oxides (MEICs)... 

The challenge is to develop a single-phase oxide material that catalyzes fuel oxidation, reduces significantly the problems described above, and possesses a level of mixed electronic ionic conductivity comparable with the nickel YSZ cermet. 

During electrochemical reduction (charge) of the carbon host, lithium cations from the electrolyte penetrate into the carbon and form a lithiated carbon Li rCn. The corresponding negative charges are accepted by the carbon host lattice. As for any other electrochemical insertion process, the prerequisite for the formation of lithiated carbons is a host material that exhibits mixed (electronic and ionic) conductance. 

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