MIEC electrodes

The best performance, i.e., the lowest electrode impedance, for the cathodes is found for electrodes made of MIECs. It is expected that the same will be true for anodes. However, no suitable MIEC was found that can be used as anode, having a low resistance to electronic as well as ionic current, being stable under the reducing conditions prevailing there, not reacting with the SE, and having a thermal expansion coefficient similar to that of the SE. A quasi-MIEC electrode can, however, be prepared by mixing fine powders of the SE material and the metal Ni (for YSZ- or CeOz-based SE). 

Insertion electrodes are MIECs that provide a source or sink for material as well as a conductive path for electric charge. For example, TiS2 serves as a cathode in Li batteries. It allows the intercalation of Li ions that arrive through a lithium-conducting SE. One expects that the charge transfer process across the SE/MIEC interface will exhibit a Butler-Volmer-type current-overpotential relation and that the diffusion in the MIEC electrode will yield a diflusion-limited current at high cnrrent densities. A detailed analysis confirms this mechanism. ... 

The shape of the ac impedance plots may deviate from that expected for the simple RC and Warburg elements. There are different reasons for deviations. Typical reasons are rough siufaces, constriction resistance, and distribution of elements with different characteristic parameters, mainly in the bulk. The constriction resistance is due to a smaller contact area of the electrode than the nominal electrode area. At low frequencies the capacitance reflects the actual contact area, while at high frequencies the capacitance reflects the area of the electrode material which may be larger. Thus the contact caimot be described by a single capacitance. It has also been shown that for a MIEC electrode the impedance of transfer of oxygen from the gas phase into the MIEC and the impedance of diffusion inside the MIEC, though coupled in series, do not yield separated parts in the Cole-Cole plot. 

Figure 12.21 (left) The triple-phase boundaries (circled areas) in an SOFC with composite electrodes, (right) If using mixed ionic and electronic conducting (MIEC) electrodes the reactions can in principle be spread out over a larger surface as indicated by the outlined particles. 

Figure 9.3(a) shows a schematic of such a charge transfer reaction. In a singlephase MIEC electrode, on the other hand, the charge transfer reaction is not restricted to the linear feature (e.g. TPB), but can occur over the entire electrode/ gas phase interface. In such a case, the exchange current density is given by... 

Cathodic and anodic activation polarisations, in light of MIEC electrodes, are discussed below. 

Figure 9.4 shows a schematic of a porous MIEC electrode used in solid state electrochemistry, in which the pathways for the various species are shown. The MIEC characteristics can be realised in two ways (1) Use of a single phase, porous MIEC material, such as Sr-doped LaCoOs (LSC), or (2) Use of a composite, two-phase porous mixture of an electronic conductor (e.g. LSM) and an ionic conductor (e.g. YSZ). In the case of a composite, two-phase mixture, the MIEC properties are realised globally (at the microstructural level, not at the atomistic... 

Theoretical aspects of porous MIEC electrodes, both using single-phase and two-phase materials, have been analysed by many authors [18,27,30-34]. While the particulars of the models vary from model to model, general features of the porous MIEC electrodes can be summarised as follows (1) Gaseous species... 

The basic concepts of composite or single-phase MIEC electrodes are equally applicable to anodes. Traditionally, however, the typical anode used to date has been a composite mixture of Ni and YSZ. The presence of YSZ not only suppresses the thermally induced coarsening of Ni, but it also introduces MIEC characteristics. Other anodes currently under investigation are based on cermets of copper, which are being explored for direct oxidation of hydrocarbon fuels [39]. These types of anodes are in an early stage of development and thus their polarisation behavior is not discussed here. In so far as single-phase anodes are concerned, some work has been reported in the literature, most notably on La-SrTi03 [40, 41]. Work on this as well as other perovskite-based anodes is in its infancy, and is not elaborated upon further. The discussion in this chapter is confined to Ni + YSZ cermet anodes. 

Models of Mixed Ionic and Electronic Conducting (MIEC) Electrodes These specialised electrode models usually consider the MIEC electrode in combination with the electrolyte and focus on correlating performance with the semiconductor characteristics of the electrode (and sometimes electrolyte) [70-72]. Recent modelling of oxygen reduction and oxygen permeation at perovskite electrodes includes both MIEC effects and classical diffusion-type analysis [73-75]. 

The addition of an ionic conductive phase, such as GDC, also promotes the elec-trocatalytic activity of an MIEC cathode. Hwang et al. [108] studied the electrochemical activity of LSCF6428/GDC composites for the 02 reduction and found that the activation energy decreased from 142 kJmol-1 for the pure LSCF electrode to 122 kJmol1 for the LSCF/GDC composite electrodes. Thus, the promotion effect of the GDC is most effective at low-operation temperatures (Figure 3.12). This is due to the high ionic conductivity of the GDC phase at reduced temperatures. 

The electrocatalytic activity of MIEC cathodes also depends strongly on the properties of the electrolyte, as shown by Liu and Wu [109], The electrode polarization resistances, RE, or area specific resistance (ASR) measured by the electrochemical... 

Figure 15.1. Illusuation of the difference in location of the electrode reaction on two different SOFC electrode types. Upper In an electrode where the electrode material is exclusively an electronic conductor, the reaction zone is restrained to the vicinity of the triple phase boundary (TPB). Lower In a mixed ionic-electronic conductor (MIEC) the electrode reaction can take place on the entire electrode surface...

The use of mixed ionic-electronic conductors (MIECs) as ORR electrocatalysts is quite common in solid-state electrochemistry [125], because the reaction zone is extended over the entire electrode/gas interface, contrary to the case of metal electrodes where the reaction is, to a large extent, restricted to the tpb zone [23]. 

Sr-doped LaMnOj (LSM) cathode have been extensively investigated and developed as electrode materials. For oxygen reduction in SOFCs, mixed ionic and electronic conducting (MIEC) materials such as (La, Sr)(Co, FejOj (LSCF) show much higher electrochemical activity than that of LSM. However, MIEC materials based on cobaltites react readily with YSZ electrolytes to form resistive La2Zr207 and SrZrOj phases at the... 

A modification of the cell shown in Fig. 2(b) is presented in Fig. 2(d) in which nonporous MIECs are used as electrodes. In a fuel cell, oxygen has then to diffuse from the gas phase through the MI EC cathode to reach the SE and, correspondingly, the fuel and the exhaust gas have to diffuse through the anode. (In reality, the MI EC electrodes are porous, which increases the electrode active area). The MI EC can be either a semiconductor or a metal. An example of a continuous cathode is YSZ Ag air, with the Ag layer as the MI EC electrode. This is possible because silver is permeable to oxygen at elevated temperature [17,18]. 

A simple way to make the driving force for electrons vanish (V/ie = 0) is to short circuit the electrodes [47]. This sets the difference Aftg = 0 and for most common MIECs, it also sets V/ie = 0. 

A different electrode is one that is not porous but a continuous MIEC and the source or sink for material is not the MIEC but the gas phase, as shown schematically in Eig. 2(d), Then diffusion of material in the form of ions through the MIEC, as well as charge transfer of ions at the MIEC/SE interface has also to be considered. 

A simplified situation exists when one can neglect the core and the corresponding space charge that exists in the free surface of a solid (shown in Fig. 3a). Then a space charge is formed only due to the contact between the SE/MIEC and the metal electrode. This situation is shown in Fig. 3(c) for an inert metallic electrode. This last case is similar to the situation in the metal-LE double layer in LSE mentioned before, when no ions... 

Fig. 5 Potential energies [pP, p, zq

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