The Use of Mixed 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 

Nevertheless both transient rate analysis24,71 and XPS24 have shown that in both cases the electrochemical promotion mechanism is identical with that obtained with YSZ, i.e. electrochemically controlled migration (back-spillover) ofO2 onto the gas-exposed catalyst-electrode surface.24,71 

Ethylene oxidation on Pt/Ti02 was investigated at temperatures 450°C to 600°C and was found to exhibit strong electrochemical promotional behaviour at temperatures near 500°C.24 

This is a truly exciting electrochemical promotion system which can serve as an excellent example for illustrating the two local and three of the four global promotional rules described in Chapter 6. The reason is that under open-circuit conditions the reaction is positive order in both reactants, as can be seen in subsequent figures. 

Although the p values are comparable (p=26 for Pt/YSZ, p=21 here) the A value is now a factor of 37 smaller (A=74-103 for Pt/YSZ, A 2-103 here) and now x is a factor of 3 larger than 2FNG/I (Fig. 8.68) while x is a factor of 3 smaller than 2FNG/I in the case of Pt/YSZ (e.g. Fig. 4.13), i.e. now x is a factor of 9 longer than in the case of Pt/YSZ. Both observations, i.e. the smaller A and the longer x values are consistent with the fact that only a fraction (3-15%) of the current, I, is ionic (O2 ) while the rest is electronic,77 an idea also corroborated by electrical conductivity data and transient work function measurements and XPS spectra.24 

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The Use of Mixed Conductors Instead of Solid Electrolytes in Fuel Cells... 

The use of mixed ionic-electronic conductors (MIECs) as ORR electrocatalysts is quite common in solid-state electrochemistry [125], because the reaction zone is extended over the entire electrode/gas interface, contrary to the case of metal electrodes where the reaction is, to a large extent, restricted to the tpb zone [23]. 

It will also be shown that the absolute electrode potential is not a property of the electrode but is a property of the electrolyte, aqueous or solid, and of the gaseous composition. It expresses the energy of solvation of an electron at the Fermi level of the electrolyte. As such it is a very important property of the electrolyte or mixed conductor. Since several solid electrolytes or mixed conductors based on ZrC>2, CeC>2 or TiC>2 are used as conventional catalyst supports in commercial dispersed catalysts, it follows that the concept of absolute potential is a very important one not only for further enhancing and quantifying our understanding of electrochemical promotion (NEMCA) but also for understanding the effect of metal-support interaction on commercial supported catalysts. 

Solid mixed ionic-electronic conductors (MIECs) exhibit both ionic and electronic (electron-hole) conductivity. Naturally, in any material there are in principle nonzero electronic and ionic conductivities (a i, a,). It is customary to limit the use of the term MIEC to those materials in which a, and 0, 1 do not differ by more than two orders of magnitude. It is also customary to use the term MIEC if a, and Ogi are not too low (o, a i 10 S/cm). Obviously, there are no strict rules. There are processes where the minority carriers play an important role despite the fact that 0,70 1 exceeds those limits and a, aj,i< 10 S/cm. In MIECs, ion transport normally occurs via interstitial sites or by hopping into a vacant site or a more complex combination based on interstitial and vacant sites, and electronic (electron/hole) conductivity occurs via delocalized states in the conduction/valence band or via localized states by a thermally assisted hopping mechanism. With respect to their properties, MIECs have found wide applications in solid oxide fuel cells, batteries, smart windows, selective membranes, sensors, catalysis, and so on. 

This expression is independent of molecular chain length and so is suitable for use with polymers of mixed molecular weight. The turn molecular rotation contribution can be obtained from either of the models for optical rotation we have presented 12-14), either as a sum of contributions from four-atom units or by use of helical conductor equation (Eq. 1) ... 

The use of a mixed oxygen ion-electronic conductor membrane for oxygen separation with direct reforming of methane, followed by the use of a mixed protonic-electronic membrane conductor for hydrogen extraction has also been proposed in the literature [34]. The products are thus pure hydrogen and synthesis gas with reduced hydrogen content, the latter suitable, for example, in the Fish-er-Tropsch synthesis of methanol [34]. 

One approach to chiral conductors using the counterions as the source of asymmetry is that employed in the preparation of a conductor based on bis(ethylenedithio)-tetrathiafulvalene (BEDT, Fig. 3) [39]. When this organic donor is electrocrystallised in the presence of the L-tartrate salt K2[Sb2 (L-tart)2] the compound that is formed is BEDT3Sb2(L-tart)2 MeCN. Thus, the BEDT is in a mixed valence state, with two third charge per molecule on average. The salt, which pertains to the P2i2i2i space group, has layers of donor molecules and ions derived from BEDT which alternate with layers of the chiral counter-ions. 

The basic problem with activated carbon is that, intrinsically, it is a poor electrical conductor. Moreover, the use of small particles instead of a bulk crystal adds a contribution to the contact resistance. A binder must be mixed with the powder to stick the carbon particles together. The choice of binder material type and amount is influenced by the carbon surface properties. 

The class of ionic conductors is not unambiguously defined in literature. Depending on context, this term maybe used either for solid electrolytes where the ion transference number is higher than 0.99, or for any solid material where ions are mobile, including mixed ionic-electronic conductors where the partial ionic and electronic diffusivities are comparable. The latter term is used for materials where the ion transference numbers are lower than 0.95-0.99, and also in conditions when a minor contribution to the total conductivity (ionic or... 

The DCNQI-Cu salt indicates another way to the higher dimensional system with the use of the coordination bond [30]. The crystal structure of the anion radical salt (DCNQI)2Cu is shown in Fig. 8. Planar DCNQI molecules stack to form one-dimensional columns. These DCNQI columns are interconnected to each other through tetrahedrally coordinated Cu ions to form the three-dimensional DCNQI-Cu network. If there is no interaction between Cu and DCNQI, this structure gives only one-dimensional tt (LUMO) band, as is the case of ordinary molecular conductors. But, in this case, the Cu is in the mixed valence state [31] and provides three-dimensional band structure. 

The limits of integration are the oxygen partial pressures maintained at the gas phase boundaries. Equation (10.10) has general validity for mixed conductors. To carry the derivation further, one needs to consider the defect chemistry of a specific material system. When electronic conductivity prevails, Eqs. (10.9) and (10.10) can be recast through the use of the Nemst-Einstein equation in a form that includes the oxygen self-diffusion coefficient Dg, which is accessible from ionic conductivity measurements. This is further exemplified for perovskite-type oxides in Section 10.6.4, assuming a vacancy diffusion mechcinism to hold in these materials. 

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