Solid mixed conduction

There are a number of ways in which this desirable state of affairs can be achieved. In one, a material that is a good ionic conductor by virtue of structural features (the layer structure (3-alumina, for example) can have the rest of the structure modified to become electronically conducting. In another approach, impurities can be introduced into a matrix to balance populations of both electronic and structural defects to generate a mixed conducting solid. Both approaches have been exploited in practice. 

Lin, Y. S., Wang, W., and Han, J. (1994). Oxygen permeation through thin mixed-conducting solid oxide membranes, AIChE J. 40(5), 786. 

In summary, all these local measurements demonstrate the power of microelectrodes i) to determine local bulk conductivities in ionic solids and ii) to study an important phenomenon in solid state ionics, namely the occurrence of nonstoichiometry profiles in mixed conducting solids. 

A. Haffehn, J. Joos, M. Ender, et al., Time-Dependent 3D Impedance Model of Mixed-Conducting Solid Oxide Fuel Cell Cathodes, Journal of the Electrochemical Society, vol. 160, no. 8, F867-F876, 2013. 

The situation is often encountered where, upon the passage of current through an electrochemical cell, only one of the mobile species is discharged at the electrodes. Examples are (a) the use of a liquid or polymeric electrolyte, where both ions are mobile, and yet where only one is able to participate in the electrode reaction and (b) a mixed conducting solid in which current is passed by electrons, but in which cations also have a significant transport number. 

Fleig, J. (2003). On the width of the electrochemicalfy active region in mixed conducting solid oxide fuel cell cathodes. J. Power Sources. 

For the measurement of the chemical diffusion coefficient D, an auxiliary electrolyte is used both to measirre the flux of ions into and out of the sample and to determine the concentration of the mobile species at the interface with the electrolyte. The rate-determining factor is assumed to be the diffusion of species within the mixed conducting solid and not the transport of ions across the interface. The change in the double layer charge at the interface must also be small compared to the charge transported into or out of the bulk of the sample. In order to verify this assnmption, typical valnes for the double layer capacities may be assumed and, better, samples with different lengths shonld be employed. 

Oxygen separation through thick mixed-conducting solid oxide membrane... 

There is a wide variety of solid electrolytes and, depending on their composition, these anionic, cationic or mixed conducting materials exhibit substantial ionic conductivity at temperatures between 25 and 1000°C. Within this very broad temperature range, which covers practically all heterogeneous catalytic reactions, solid electrolytes can be used to induce the NEMCA effect and thus activate heterogeneous catalytic reactions. As will become apparent throughout this book they behave, under the influence of the applied potential, as active catalyst supports by becoming reversible in situ promoter donors or poison acceptors for the catalytically active metal surface. 

Today, the term solid electrolyte or fast ionic conductor or, sometimes, superionic conductor is used to describe solid materials whose conductivity is wholly due to ionic displacement. Mixed conductors exhibit both ionic and electronic conductivity. Solid electrolytes range from hard, refractory materials, such as 8 mol% Y2C>3-stabilized Zr02(YSZ) or sodium fT-AbCb (NaAluOn), to soft proton-exchange polymeric membranes such as Du Pont s Nafion and include compounds that are stoichiometric (Agl), non-stoichiometric (sodium J3"-A12C>3) or doped (YSZ). The preparation, properties, and some applications of solid electrolytes have been discussed in a number of books2 5 and reviews.6,7 The main commercial application of solid electrolytes is in gas sensors.8,9 Another emerging application is in solid oxide fuel cells.4,5,1, n... 

In solid electrolyte fuel cells, the challenge is to engineer a large number of catalyst sites into the interface that are electrically and ionically connected to the electrode and the electrolyte, respectively, and that is efficiently exposed to the reactant gases. In most successful solid electrolyte fuel cells, a high-performance interface requires the use of an electrode which, in the zone near the catalyst, has mixed conductivity (i.e. it conducts both electrons and ions). Otherwise, some part of the electrolyte has to be contained in the pores of electrode [1]. 

Figure 8.16 Mixed conductivity in SrThFe O -j, (a) schematic variation of conductivity and (h) experimental conductivity for SrTio.5Feo.5C>3 8. [Data adapted from S. Steinsvik, R. Bugge, J. Gjpnnes, J. Taftp, and T. Norby, J. Phys. Chem. Solids, 58, 969-979 (1997).]...

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.

Bebelis S, Kotsionopoulos N, Mai A, Rutenbeck D, and Tietz F. Electrochemical characterization of mixed conducting and composite SOFC cathodes. Solid State Ionics 2006 177 1843-1848. 

Mixed conducting (i.e., electronic and ionic) materials for anodes may be advantageous if H2 oxidation can occur over the entire surface of the electrode to enhance current production, instead of only in the region of the three-phase interface (gas/solid electrolyte/electrode). Similarly, mixed conductors also may be advantageous for cathodes. 

Figure 18. Possible pathways by which oxygen is reduced in a porous mixed conducting electrode. Following dissociative adsorption (which forms a surface-polarizing species 0, where n represents the unknown partial charge state of adsorbed oxygen), 0 either travels by surface diffusion to the TPB (where it is fully reduced) or is incorporated directly into the mixed conductor as 0 , where it then diffuses to the solid/solid interface. (Adapted with permission from ref 203. Copyright 1987 The Electrochemical Society, Inc.)...

One limit of behavior considered in the models cited above is an entirely bulk path consisting of steps a—c—e in Figure 4. This asymptote corresponds to a situation where bulk oxygen absorption and solid-state diffusion is so facile that the bulk path dominates the overall electrode performance even when the surface path (b—d—f) is available due to existence of a TPB. Most of these models focus on steady-state behavior at moderate to high driving forces however, one exception is a model by Adler et al. which examines the consequences of the bulk-path assumption for the impedance and chemical capacitance of mixed-conducting electrodes. Because capacitance is such a strong measure of bulk involvement (see above), the results of this model are of particular interest to the present discussion. 

Effective temperature control of large fixed beds can be difficult because such systems are characterized by a low heat conductivity. Thus in highly exothermic reactions hot spots or moving hot fronts are likely to develop which may ruin the catalyst. In contrast with this, the rapid mixing of solids in fluidized beds allows easily and reliably controlled, practically isothermal, operations. So if operations are to be restricted within a narrow temperature range, either because of the explosive nature of the reaction or because of product distribution considerations, then the fluidized bed is favored. 

The macrocyclic phthalocyanine ligand will form a complex Pt(phthalocyanine).1106 The crystal structure shows two polymorphs present because of molecular packing.1107 The platinum is in a square planar coordination geometry with a mean Pt—N distance of 1.98 A. The complex can be partially oxidized with iodine to give conducting mixed valence solids.1108 Eighteen fundamental and overtone combination bands are observed in the resonance Raman spectrum of platinum phthalocyanine, and from this data the symmetry of the excited singlets are found to be Dy.. Qlv or D2.1109... 

Another way to decrease the anodic overpotential is to intercalate a mixed conductor between the yttria stabilized zirconia electrolyte and the metallic anode. Such a combination enlarges the reaction area which theoretically lowers the anodic overpotential. Tedmon et al. [93] pointed out a significant decrease of polarization when ceria-based solid solutions like (Ce02)o.6 (LaO, 5)04 are used as anode materials for SOFCs. This effect is generally attributed to the mixed conductivity resulting from the partial reduction of Ce4+ to Ce3+ in the reducing fuel atmosphere. A similar behaviour was observed in water vapor electrolysis at high temperature when the surface zirconia electrolyte is doped with ceria [94, 95]. 

Adler S.B., Wilson J.R., Schwartz D.T. (2003) Nonlinear harmonic response of mixed-conducting SOFC cathodes. In Solid Oxide Fuel Cells VIII (SOFC-VIII), Electrochemical Society Proceedings, 2003-07, S.C. Singhal and M. Dokiya (Eds.), The Electrochemical Society, Pennington, NJ, pp. 516-524. 

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