MIEC systems

The calculation also indicates that the oxygen recovery should be limited within 20 — 40% so as to reduce the overall oxygen production cost of the MIEC membrane systems. 

As follows from the previous chapters, a complex interface Metal/MIEC/Electrolyte (MIEC = mixed ion-electron conductor) appears in many processes related to the electrochemistry of polyvalent metals. The model of MIEC in terms of the concept of polyfunctional conductor (PFC) can be a useful approach to deal with the mechanisms of the processes in such systems. The qualitative classification of EPS has been given based on this approach. Further on, we are going to demonstrate that this concept is useful for quantitative (or at least, semi-quantitative) modelling of macrokinetics (dynamics) of the processes in highly non-equilibrium systems. Before doing this, it is worthwhile to outline some basic ideas related to the MIEC. These considerations will also show some restrictions and approximations that are commonly applied in electrochemical practice and which are no longer valid in such kind of systems. 

As far as other MIEC membrane materials are concerned. Tan et al. developed a pilot-scale membrane system (shown in Figure 4.17) comprising 889 LSCF6428... 

In the present section of this chapter, we will review our work on the MIEC block copolymer systems and also point out come of the advantages of these systems compared with purely electronic conductive polymers. 

The work carried out in our laboratory has led to the development of synthetic methods required for the preparation of MIEC block copolymers. It will now be necessary to obtain a clear understanding of the structure-microstructure-function interrelationship of the MIEC block copolymers. MIEC block copolymers of controlled structures may be ideal for testing the different hypotheses of coupling between the electronic and the ionic species in mixed conductors [56]. Methods are available to determine the diffusion constants of the ionic species in mixed conducting systems [65]. The diffusion constants of ionic species in mixed con-... 

The influence of carbon dioxide on the oxygen permeation of MIEC membranes has been extensively studied compared with the effect of other gases, such as SO2, which can also be present in the gas streams. Of course, CO2 will be the major component of a hypothetical gas mixture where the MIEC membrane should work (e.g., oxy-fuel process). Nevertheless, the influence of SO2 must be mandatorily evaluated and represents one of the biggest concerns in this area. For example, the SO2 concentration in the flux gas can be around 400 ppm or even higher than 1000 ppm, as reported before the scrubber system in some power plants in China [82]. Up to 2011, few reports can be found dealing with membrane operation in SO2 environments. However, on related topics, for example, catalysis, several studies revealed the formation of sulfetes and sulfldes... 

Seidel- Morgenstern, A. Max Planck Institute for Dynamics of Complex Technical Systems, Magdeburg, Germany 28 Mainly MIEC Dehydrogenation reactions... 

The acceptor-doped perovskites (general formula of ABOs-a) have shown much promise as MIECs. Much attention has been focused on these versatile metal oxides as they exhibit catalytic activity [24] and therefore their use in some applications can avoid the need for catalytic modification of the membrane surface. This activity has meant that MIECs can be used as electrode materials in solid-oxide fuel cell systems for the reduction of oxygen simultaneously, this inherent catalytic activity means that problems with chemical stability can arise. 

When used as a dense membrane in a reactor system the MIEC acts as a barrier between two isolated chambers allowing only the ionically conducted species to pass through the membrane under a chemical potential gradient. Air can be used as a freely available oxidant that is supplied to one chamber with no mixing of nitrogen with the product stream from an oxidation reaction as depicted in Figure 3.7,... 

The Sr-Co-Fe perovskite systems have the highest oxygen permeabilities of all the perovskite MIECs but can suffer from degradation as a result of the formation of a brownmillerite phase. Experimental and modeling work concerning the transport... 

One can, however, consider cpiasi-coupling terms, between ionic and electronic currents in MIECs which conduct ionic defects having different z values. Let us consider a MIEC that conducts A/, M ", and e. The coupling terms arise if one attempts to describe the three particle systems in terms of two only." The coupling terms then depend on which of the particles is used to describe the system. Thus, if these are M "" and e, then M is presented as Mi""+ e. Then, eliminating jiM ) the current ecpiations causes the appearance of coupling terms between the remaining current densities J(MH and y e ... 

Pt - Reference system / oxide electrolyte / MIEC - Pt. The measurement of the emf of the electrochemical chain allows the determination of the oxygen activity in the MIEC. The following sections are devoted to the description of devices allowing the determination of the oxide activity on the surface of a MIEC, and of the deviation from equilibrium of the oxide surface. 

Mini-probes (Figure 6) have been developed for the determination of thermodynamic properties of MIEC as a function of composition Zirconia- or thoria-based tubes, a few mm in diameter, have been used. A metal-metal oxide system serves as a reference. Such cells have been used for the measurement of oxygen potential in urania-based solid solutions or for continuous control of oxygen redistribution in UO2+X under a thermal gradient [Ducroux et al, 1980 Une Oguma, 1982]. 

The ionic conductivity in a MIEC can be determined using the "short-circuiting" method [Riess, 1991]. Both electrodes are reversible systems with different compositions. The electronic current is brought to zero by short-circuiting the MIEC on a low impedance amperometer (V = 0). Consequently, it can be demonstrated that the ionic resistance Ri obeys the following equation [Riess, 1991] ... 

Electrode reactions involve charge transfer as a fundamental step, wherein a neutral species is converted into an ion, or an ion is converted into a neutral species. Both reactions thus involve electron transfer. At the cathode, the charge transfer reaction involves the conversion of an oxygen molecule into oxide ions. The electrodes in solid state electrochemical devices may either be purely electronic conductors, or may exhibit both ionic and electronic conductivity (the so-called mixed ionic electronic conduction, MIEC). In addition, the electrodes may be either single phase or composite, two-phase. For the purposes of illustration, in what follows we will examine the overall cathode reaction in a system with a single phase, purely electronically conducting electrode. 

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