Metallic mixed conductors

Although several metals, such as Pt and Ag, can also act as electrocatalysts for reaction (3.7) the most efficient electrocatalysts known so far are perovskites such as Lai-xSrxMn03. These materials are mixed conductors, i.e., they exhibit both anionic (O2 ) and electronic conductivity. This, in principle, can extend the electrocatalytically active zone to include not only the three-phase-boundaries but also the entire gas-exposed electrode surface. 

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. 

In many transition-metal oxides and sulfides, ionic conductivity is augmented by electronic conductivity, and transport numbers need to include contributions from electrons and holes. These mixed conductors are described in Section 8.8. 

Following the introduction of basic kinetic concepts, some common kinetic situations will be discussed. These will be referred to repeatedly in later chapters and include 1) diffusion, particularly chemical diffusion in different solids (metals, semiconductors, mixed conductors, ionic crystals), 2) electrical conduction in solids (giving special attention to inhomogeneous systems), 3) matter transport across phase boundaries, in particular in electrochemical systems (solid electrode/solicl electrolyte), and 4) relaxation of structure elements. 

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]. 

Figure 1. Sketch of a general electrochemical cell, as referred to in the text, with the mixed conductor MX as the central phase. The currentcollecting metal is denoted by m .3 Reprinted from J. Maier, Z. Phys. Chem. NF (1984) 191-215. Copyright 1984 with permission from Oldenbourg Verlagsgruppe.

As well known from semiconductor physics, in non-metals electrons are, at finite temperatures, excited from the highest occupied band to the lowest unoccupied band to form excess electrons in the conduction band and electron holes in the valence band. Owing to long-range order each crystal possesses a certain amount of free electronic carriers. The mixed conductor which exhibits both ionic and electronic conductivity, will play an important role in this text, since it represents the general case, and pure ionic and electronic (semiconductors) conductors follow as special cases. 

Perovskite-related oxides of the K2MF4 (A2BO4+J B-transition metal ion) structure (Figure 16) have been investigated by several groups as alternative mixed conductors for SOFC. 

The distinction between electronic and electrolytic conductors is not sharp, for many substances behave as mixed conductors that is, they conduct partly electronically and partly electrolytically. Solutions of the alkali and alkaline earth metals in liquid ammonia are apparently mixed conductors, and so also is the jS-form of silver sulfide. Fused cuprous sulfide conducts electronically, but a mixture with sodium or ferrous sulfide also exhibits electrolytic conduction a mixture with nickel... 

Y. Shen, M. Liu, D. Taylor, S. Balagopal and A. Joshi, Mixed ionic electronic conductors based on Bi-Y-O-Ag metal-ceramic system, in T.A. Ramanarayan et al. (Eds.), Proceedings of the 2nd International Symposium on Ionic and Mixed Conductors. Proceedings Vol. 94.12, The Electrochemical Society Inc., pp. 574r-583. 

The complex phase diagrams and rich crystal chemistry of the transition metal-containing oxide systems, and great diversity in the defect chemistry and transport properties of mixed-conducting materials known in these systems, make it impossible to systematize all promising compositions in a brief survey. The primary attention here is therefore centered on the comparison of major families of the oxide mixed conductors used for dense ceramic membranes and porous electrodes of SOFCs and other high-temperature electrochemical devices. 

The term electron transfer may be ambiguous since it is also used for the shift of electrons from one medium to another in electrochemical systems where metals are in contact with mixed conductors. In such cases mobile electrons exist in both phases (e.g., solvated electrons in a molten salt) and the current flow corresponds partly to a simple electron transfer from one phase to the other, without any redox reaction occurring. 

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