MIEC performance

One technique to characterize the MIEC performances as electrode for SOFC or SOEC is the electrochemical impedance spectroscopy (EIS) applied to symmetrical cells made of a dense electrolyte, which must be a pure oxide ion conductor and porous electrode, once the lack of reactivity between the electrolyte and the electrode has been verified. An example of such a cell is given in Figure 8.6. 

The suitability of lanthanum nickelate as an SOFC cathode has been examined by Virkar s group [138], They showed that LN performed poorly as a single-phase cathode in an anode-supported YSZ cell. However, with an SDC/LN composite interlayer the performance of the LN cathode increased substantially and the maximum power density of the cell with a YSZ thin electrolyte (-8 pm) was -2.2 Wear2 at 800°C, considerably higher than 0.3 to 0.4 Wcm-2 of similar cells with only LN or SDC interlayer. The results are significant as it shows that the composite MIEC cathodes perform much better than single-phase MIEC in the case of LN despite its mixed ionic and electronic conductivity. 

The phase inversion/sintering technique has to be further improved to produce in a large scale MIEC hollow fiber membranes with constant macrostructure and performances so as to reduce the membrane costs. 

Oxy Fuel Combustion Power Production Using High Temperature O2 Membranes 99 Table 4.2 Comparison of oxygen permeation performance of C02 tolerant MIEC materials... 

Despite the apparent simplicity of this reaction, the process by which the oxygen reduction occurs followed by incorporation of the ionic species into the electrolyte is the subject of some debate and is dependent on the mode of operation of the cathode material. Two typical cathode types are currently utilized in SOFCs -electronic conductors and mixed ionic-electronic conductors (MIECs). The cathode reactions, while nominally the same in both types of materials, occur at different locations, and hence, the active region varies, leading to differences in the operating regime and ultimately performance. In the case of a single phase electronic conductor. 

MIEC Electrochemical Performances as SOFC or SOEC Electrodes... 

The most important consideration for a MIEC membrane is the delivery of a stable continuous transmembrane flux. This flux is central to membrane performance and therefore oxygen permeation studies in MIEC membrane characterization are fundamental. From an economic standpoint, an oxygen flux of 1 to 10 ml cm min (STP) has been cited as the requirement for future needs [25,26]. In order to provide a broad overview of this research area we will briefly outline synthesis and characterization methods applied to the MIEC perovskites before moving on to look at selected MIEC membranes currently used in laboratory-scale tests. 

The I-V relations of solid electrochemical cells based on MIECs are of primary interest in this review. They describe the performance of these cells and are also the theoretical basis that allows one to extract the partial ionic and electronic conductivities from I-V measurements. The geometry of the cell considered here is mainly one dimensional (taken to be in the X direction) as shown in Figure 7.2. Reference will also be made to the Van der Pauw configuration, where the MIECs have a flat, singly coimected shape and the electrodes are applied on the periphery of the MIEC. We refer in particular to circular flat samples with electrodes equally spaced as shown in Figure 7.3. 

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). 

Figure 9.9 Effect of temperature on the MIEC membrane reactor performance. Reproduced with permission from [23]. John Wiley Sons.

MIECs used in combination with the electrolyte material (usually CSO or CGO) to form a composite also present superior performance in comparison with the pure MIEC, as shown, for example, in the Smo.5 Sro.5Co03-Smo.2Ceo.80i.9 (75/25 wt%), SrCoo.8Feo.203-S"Lao.45 Ceo.5502-s (50/50 LaNio.6Feo.403-CSO (50/50 wt%)[ l... 

Activation polarisation is the voltage drop due to the sluggishness of reactions occurring at the electrode-electrolyte interfaces. Several processes are necessary for electron transfer to take place, especially at the cathode. Because LSM has little ionic conductivity, these processes are localised at the TPBs. Recently, it has become common to use MIEC (composite or single-phase) cathodes to spread the TPB and extend the reaction zones this has had a beneficial effect on reducing the activation polarisation and allowed better SOFC performance at lower temperatures. 

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 isotope-kinetic equations (30) - (31) allow the elucidation of the mechanism of surface reactions (adsorption-desorption and other processes) and the estimation of the bond strength of the smface oxygen [48-56] considered to be important parameters in analysis of porous MIEC cathodes performance [9]. 

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