Mixed-conducting solid oxide

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

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. 

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

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

Kilner, JA., De Souza, R.A., and Fullarton, I.C. (1996) Surfece exchange of oxygen in mixed conducting perovskite oxides. Solid State Ionics, 86-88, 703-709. 

J.A. Kilner, R.A. De Souza, I.C. Fullarton, Surface Exchange of Oxygen in Mixed Conducting Perovskite Oxides . Solid State Ionics 86-88, 703-709 (1996)... 

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... 

Double Substitution In such processes, two substitutions take place simultaneously. For example, in perovskite oxides, La may be replaced by Sr at the same time as Co is replaced by Fe to give solid solutions Lai Sr Coi yFey03 5. These materials exhibit mixed ionic and electronic conduction at high temperatures and have been used in a number of applications, including solid oxide fuel cells and oxygen separation. 

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. 

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.

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. 

A systematic study to identify solid oxide catalysts for the oxidation of methane to methanol resulted in the development of a Ga203—M0O3 mixed metal oxide catalyst showing an increased methanol yield compared with the homogeneous gas-phase reaction.1080,1081 Fe-ZSM-5 after proper activation (pretreatment under vacuum at 800-900°C and activation with N20 at 250°C) shows high activity in the formation of methanol at 20°C.1082 Density functional theory studies were conducted for the reaction pathway of the methane to methanol conversion by first-row transition-metal monoxide cations (MO+).1083 These are key to the mechanistic aspects in methane hydroxylation, and CuO+ was found to be a likely excellent mediator for the reaction. A mixture of vanadate ions and pyrazine-2-carboxylic acid efficiently catalyzes the oxidation of methane with 02 and H202 to give methyl hydroperoxide and, as consecutive products, methanol and formaldehyde.1084 1085... 

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... 

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. 

Strontium ferrite perovskite. The nonstoiehiometric strontium ion oxide SrFeOx (2.5solid state electrolyte itself, doped with rare earths or transition metals, it forms materials, whieh exhibit even greater ionie conductivity. These oxides are used as materials for self-supported oxygen conduetivity membranes. 

Fig. 10.1. Different membrane concepts incorporating an oxygen ion conductor (a) mixed conducting oxide, (b) solid electrolyte cell (oxygen pump), and (c) dual-phase membrane. Also shown is the schematics of an asymmetric porous membrane (d), consisting of a support, an intermediate and a barrier layer having a graded porosity across the membrane.

---