Mixed ionic electronic conduction electrodes

Generally, the activation layer is a functionally graded porous structure made of the same composition as the membrane layer. A second case is when the two porous interfaces, acting as mixed ionic/electronic-conducting electrodes, are made of materials different from the membrane (a purely ion-conducting electrolyte), as shown in Figure 9.10c. In this case, the oxygen flux can be precisely controlled by the... 

The electrochemical reduction of a solid compound characterized by mixed ionic/ electronic conductivity, immobilized on an electrode surface and in contact with an electrolyte solution, has been further studied on a theoretical basis [26]. Here, the coupled uptake or expulsion of electrons from the electrode and of cations from the electrolyte solution according to... 

In Riga we are planning to use materials with mixed ionic-electronic conductivity as gate electrodes, thereby making the available region of selectivity and sensitivity of different elements in an array broader... 

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. 

The diffusivity is independent of the motion of any other species (e.g. electrons or holes) and is not influenced by internal electrical fields as in the case of chemical diffusion processes which require the simultaneous motion of electronic or other ionic species. The partial ionic conductivity of the mixed ionic and (predominantly) electronic conducting electrode is given by the product of the concentration and the diffusivity and may be related to the variations of the steady state and transient voltage ... 

The fluoride electrode is a typical example of an ion selective electrode. Its sensitive element is a crystal of lanthanum trifluoride that allows fluorine atoms to migrate into the network formed by lanthanum atoms (Fig. 18.3). Other electrodes use a mineral membrane obtained as agglomerates of crystalline powders (for measurement of Cl-, Br-, I , Pb++, Ag+ and CN ). Generally, the internal electrolyte can be eliminated (by dry contact). However, it is preferable to insert a polymer layer with a mixed-type conductivity to ensure the passage of electrons from the ionic conductivity membrane to the electronic conductivity electrode (Fig. 18.3). 

Similar approaches are used for most steady-state measurement techniques developed for mixed ionic-electronic conductors (see -> conductors and -> conducting solids). These include the measurements of concentration-cell - electromotive force, experiments with ion- or electron-blocking electrodes, determination of - electrolytic permeability, and various combined techniques [ii-vii]. In all cases, the results may be affected by electrode polarization this influence should be avoided optimizing experimental procedures and/or taken into account via appropriate modeling. See also -> Wagner equation, -> Hebb-Wagner method, and -> ambipolar conductivity. 

NEMCA effect — The term NEMCA is the acronym of Non-faradaic Electrochemical Modification of Catalytic Activity. The NEMCA effect is also known as electrochemical promotion (EP) or electropromotion. It is the effect observed on the rates and selectivities of catalytic reactions taking place on electronically conductive catalysts deposited on ionic (or mixed ionic-electronic) supports upon application of electric current or potential (typically 2 V) between the catalyst and a second (counter or auxiliary) electrode also deposited on the same support. The catalytic reactants are usually in the gas phase. 

Continuous air separation by an oxygen-conducting membrane which constitutes the wall of a CPO reactor is another approach which has received much interest, also from industry. Two types of membrane materials have been studied zirkonia-based membranes, which are efficient oxygen ion conductors but require electrodes to transfer electrons to the reduction interface, and perovskites (of general formula ABO3, with dopants in the A and/or B site), which are mixed ionic/electronic conductors (MIEC). ... 

Chebotin s scientific interests were characterized by a variety of topics and covered nearly all aspects of solid electrolytes electrochemistry. He made a significant contribution to the theory of electron conductivity of ionic crystals in equilibrium with a gas phase and solved a number of important problems related to the statistical-thermodynamic description of defect formation in solid electrolytes and mixed ionic-electronic conductors. Vital results were obtained in the theory of ion transport in solid electrolytes (chemical diffusion and interdiffusion, correlation effects, thermo-EMF of ionic crystals, and others). Chebotin paid great attention to the solution of actual electrochemical problem—first of all to the theory of the double layer and issues related to the nature of the polarization at the interface of the solid electrol34e and gas electrode. 

An important point to note in solid state electrochemistiy is that electrolyte and electrode behavior may coincide in compounds showing both ionic and electronic conduction, the so-called mixed ionic-electronic conductors (often abbreviated to mixed conductors). 

Solid mixed ionic-electronic conductors (MIECs) exhibit both ionic and electronic (elec-tron/hole) conductivity. Naturally, in aity material there are in principle nonzero electronie and ionic conductivities (Oei, Oj). It is customary to limit the use of the name MIEC to those materials in which Oj, and Oei do not differ by more than 2 orders of magnitude. It is also customary to use the term MIEC if Oj and 0 1 are not too low (Oj, > 10 Q- cm ). Obviously, these are not strict rales. There are processes where the minority carriers play an important role despite the fact that exceeds those limits and Oj, < 10 Q cm For example, the small electronic conductivity in a pmely ionic conductor (Oj 0 1), i.e., in a solid electrolyte (SE), is a necessary condition for ion permeation through the SE and therefore, e g., shortens the lifetime of a battery based on this SE. On the other hand, a small ionic conductivity in an electronic conductor is a necessary condition for permeation of ions through the electronic conductor, which may be of advantage in some applications (e.g., as electrode material). 

---