Conduction, mixed

P. Birke, S. Doring, S. Scharner, W. Weppner, Paper presented on the 192th Meeting of the Electrochemical Society, Paris, France, 1997, to be published in Proc. Ionic and Mixed Conducting Ceramics III. 

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. 

Chapter 11 analyzes the recently discovered mechanistic equivalence of electrochemical promotion and metal-support interactions on ionic and mixed conducting supports containing Zr02, Ce02 or Ti02. The analysis focuses on the functional identity and operational differences of promotion, electrochemical promotion and metal support interactions. 

J. Xue, and R. Dieckmann. Oxygen partial pressure dependence of the oxygen content of zirconia-based electrolytes in Ionic and Mixed Conducting Ceramics Second International Symposium 94-12, 191-208 (1994) ES Meeting San Francisco, California. 

Ceria is another type of mixed conducting oxide which has been shown already to induce electrochemical promotion.71 Ceria is a catalyst support of increasing technological importance.73 Due to its nonstoichiometry and significant oxygen storage capacity it is also often used as a promoting additive on other supports (e.g. y-A Cb) in automobile exhaust catalysts.79 It is a fluorite type oxide with predominant n-type semiconductivity. The contribution of its ionic conductivity has been estimated to be 1-3% at 350°C.71... 

Promotion, electrochemical promotion and metal-support interactions are three, at a first glance, independent phenomena which can affect catalyst activity and selectivity in a dramatic manner. In Chapter 5 we established the (functional) similarities and (operational) differences of promotion and electrochemical promotion. In this chapter we established again the functional similarities and only operational differences of electrochemical promotion and metal-support interactions on ionic and mixed conducting supports. It is therefore clear that promotion, electrochemical promotion and metal-support interactions on ion-conducting and mixed-conducting supports are three different facets of the same phenomenon. They are all three linked via the phenomenon of spillover-backspillover. And they are all three due to the same underlying cause The interaction of adsorbed reactants and intermediates with an effective double layer formed by promoting species at the metal/gas interface (Fig. 11.2). 

During the last year a bipolar particular cell has been described (Fleischmann et al., 1971d). The cell shown in Fig. 18 consists of a packed bed of mixed conducting and non-conducting particles a d.c. potential is applied between two feeder electrodes situated at the ends... 

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]. 

Lasch K, Hayn G, Jdrissen L, Garche J, Besenhardt O (2002) Mixed conducting catalyst support materials for the direct methanol fuel cell. J Power Sources 105 305-310... 

The first sample has the greatest deviation from stoichiometry. Samples JV°1 - JV°4 have been found to be nonstoichiometric oxide -hydroxide type. They have a mixed conductivity - ionic (o,) and electronic (ae). The ionic one is due to the presence of OH" - groups. Namely, they stabilize the defects of chemical nature in such compounds. These defects are determined by the presence of Mn4+ and Mn3+ in the same crystallographic position. 

Elangovan, S., B. Nair, J. Hartvigsen, and T. Small, Mixed Conducting Membranes for Pressure Driven Hydrogen Separation from Syngas, 225th American Chemical Society National Meeting, Fuels Division, New Orleans, LA, March 2003. 

The material would be expected to be a hole (p-type) semiconductor. However, in this compound the interstitial oxygen ions can diffuse fairly quickly, and the oxygen diffusion coefficient is higher than normal, so that the compound shows both high oxygen diffusivity and electronic conductivity, a situation referred to as mixed conductivity (Section 8.7). 

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. 

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).]...

Acceptor doping in perovskite oxides gives materials with a vacancy population that can act as proton conductors in moist atmospheres (Section 6.9). In addition, the doped materials are generally p-type semiconductors. This means that in moist atmospheres there is the possibility of mixed conductivity involving three charge carriers (H+, O2-, and h ) or four if electrons, e, are included. 

The main defects responsible for mixed conductivity in Ceo.8Pr0.202-s are ... 

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. 

Defining the Role of the Bulk—Dense Thin-Film Mixed-Conducting Electrodes... 

Summary Importance of the Bulk for Mixed-Conducting SOFC Cathodes... 

Figure 4. Some mechanisms thought to govern oxygen reduction in SOFC cathodes. Phases a, and y refer to the eiectronic phase, gas phase, and ionic phase, respectiveiy (a) Incorporation of oxygen into the buik of the electronic phase (if mixed conducting) (b) adsorption and/or partial reduction of oxygen on the surface of the electronic phase (c) bulk or (d) surface transport of or respectively, to the oJy interface, (e) Electrochemical charge transfer of or (f) combinations of and e , respectively, across the aJy interface, and (g) rates of one or more of these mechanisms wherein the electrolyte itself is active for generation and transport of electroactive oxygen species.

One of the first such kinetic studies of a perovskite mixed conducting electrode was reported by Ohno and co-workers in 1981, who found Lai /la/IoOs-a to have better kinetic properties than Pt as an SOFC cathode at 1000—1100 °C. °° A number of other JiepomOKizeo of general formula Lai jSr rMOs a (M = Cr, Mn, Fe, Co) were later studied by Takeda et al. ° To avoid reaction of the perovskites with the YSZ... 

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