Mixed conducting materials

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. 

As for the permeability measurements, most techniques based on the analysis of transient behavior of a mixed conducting material [iii, iv, vii, viii] make it possible to determine the ambipolar diffusion coefficients (- ambipolar conductivity). The transient methods analyze the kinetics of weight relaxation (gravimetry), composition (e.g. coulometric -> titration), or electrical response (e.g. conductivity -> relaxation or potential step techniques) after a definite change in the - chemical potential of a component or/and an -> electrical potential difference between electrodes. In selected cases, the use of blocking electrodes is possible, with the limitations similar to steady-state methods. See also - relaxation techniques. 

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. 

Figure9.11 Total conductivity of various mixed-conducting materials in air [68,101, 133, 139—147].

The grain size of the mixed-conducting material of dense membrane can have a great influence on ... 

Although a variety of protonceramic materials has been found in the past decade, only very limited studies have dealt with experimental measurements of H2 flux, and little effort has been made to model the experimentally measured data in these studies. H2 permeation flux data on more perovskite-based membranes are needed so that the mathematical models can be verified with experimental data. Such models, when rigorous and accurate, can provide essential guidance in the design and understanding of the mixed conducting materials. Fundamental study should be conducted to understand the stractural,... 

High-Temperature Reactions, Anode, Fig. 3 Enhancement of electrode reactive surface area for fuel oxidation in an SOFC anode by mixed conducting materials as compared to Ni-YSZ anode considering both carbon... 

The calculated results of tolerance factors for some mixed conducting materials are described in Fig. 2.1. Shannon s ionic radii referring to the coordination numbers 12 (A-site) and 6 (B-site) has been used, although it is known that oxygen deficiency influences the coordination number and therefore the ionic radii. From the results, we can find that the tolerance factors of several popular materials such as LSM, LSCF are close to 1. Therefore the tolerance factor should be considered first in designing a new material. 

Fig.2.1 Calculated tolerance factors of the mixed conducting materials...

Since the membrane conducts oxygen ions, in the great majority of the cases through a mechanism involving anion vacancy diffusion, an equivalent counterflux of electrons should take place for charge neutrality membrane materials should be mked conductors. Perovskite-type mixed-conducting materials are considered suitable candidates for use in dense membrane reactor chemical looping processes, since they fulfill most of the required characteristics. 

In addition to single-phase samples, electrochemical methods may also be employed to investigate reactions of mixed conducting materials with other phases. If the reaction rates are determined by bulk diffusion, the parabolic tarnishing rate constant k, results from a comparison of the parabolic tarnishing rate law and Tick s first law. By integration of this... 

Electric resistance, R, of either electron or ion (or mixed) conductive material is an important characteristic in electrochemical science and engineering and is defined... 

---