Dense electrolyte/porous electrodes

Investigations were conducted to determine whether jet vapor deposition (JVD) could be substituted for EVD, which is capital intensive. JVD is a thin film technique in which sonic gas jets in a low vacuum fast flow serve as deposition sources. Results showed that the YSZ films can be made dense and pinhole free they seal highly porous electrode surfaces and are gas tight. Conductivity needs to be improved, which should be obtainable. The ultimate goal will be to fabricate thin film SOFCs, both electrolyte and the electrodes, in an unbroken sequence of JVD steps. This would also allow the use of alternate metal cathode, such as Ag thin films (19). 

The bipolar plate design is illustrated in Fig. 47. It consists of a cross-flow arrangement where the gas-tight separation is achieved by dense ceramic or metallic plates with grooves for air and fuel supply to the appropriate electrodes. A porous cathode, a dense and thin electrolyte and a porous anode form a composite flat layer placed at the top of the interconnected grooves. The deposition of the porous electrodes can be achieved by mass production methods. Moreover, the bipolar plate configuration technology makes it possible to check for defaults, independently and prior to assembly of the interconnection plate and the anode-electrolyte-cathode structure. 

SOFC can be manufactured in different geometrical configurations, i.e. planar, tubular or monolithic. Regardless of the geometrical configuration, a solid oxide fuel cell is always composed of two porous electrodes (anode and cathode), a dense electrolyte, an anodic and a cathodic gas channel and two current collectors. For the sake of simplicity the planar configuration is taken as reference, as shown in Figure 3.1. 

The single cells consist of a dense solid electrolyte membrane and two porous electrodes. In most cases, at least one of the electrodes is exposed to an oxygen-containing gas (often, ambient air), while the other electrode is exposed to an inert gas, a liquid metal, a partial vacuum, or a reacting mixture (hydrogen, water vapor, hydrocarbons, CO, CO2, etc.). The single-chamber reactor (SCR) has been also proposed either as a membrane reactor or as a fuel cell. In this case, the solid-electrolyte disk, with two different electrodes that are coated either on opposite sides or on the same side of the pellet, is suspended in a flow of the reacting mixture (see Section 12.6.3). 

The electrochemical vapor deposition (EVD) process has been successfully developed as fabricating a dense electrolyte film on porous cathodes and fixing anode nickel particles on the electrolyte plate with YSZ. This provides a nearly ideal microstmcture for electrolyte/electrode interfaces. 

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. 

Figure 8.6 A symmetrical cell made of a dense electrolyte and porous electrodes. Thus,...

Figure 9.4 Nyquist (a) and Bode (b) plots for ( ) electrode/ electrolyte solution (c)/dense layer—porous sublayer/electrolyte solution (c)/electrode and (x) electrode/electrolyte solution (c)/ electrode. e = electrolyte p.sl. = porous sublayer d.l. = dense layer.

Figure 1 illustrates a typical SOFC, which essentially consists of two porous electrodes separated by a dense, oxide ion conducting electrolyte. Oxygen gas... 

Fig. 7.5 Equivalent circuit for (a) a local mixed conductor, (b) dense electrode/electrolyte system, and (c) porous electrode lectrolyte system...

The other approach is an attempt for a thinner electrolyte supported on a porous electrode. Recently, many researchers have been working on a thin-film-type SOFC to realize an intermediate-temperature fuel cell [6-9]. In this approach, the eleetrolyte must be supported on either the anode or cathode. The main diffieulty regarding this approach hes in making a solid thin film on a porous electrode. To get a dense electrolyte layer, the electrolyte thickness should be several times larger than the maximum pore size of the support electrode. Thus, the preparation method of the support electrode with good pore-size distribution is a key technology of this approach not only it is difficult, but... 

Adlerhas investigated the behaviour of LSC porous electrodes by AC impedance spectroscopy and has found that the oxygen reduction reaction is limited by surface chemical exchange and solid state diffusion, contrary to the commonly accepted view that the electrode reactions are charge transfer limited. Furthermore he has also found that the reduction reaction extends over several micrometres and under certain conditions may approach the thickness of the electrode. Isotope exchange measurements on dense LSC deposited on a Ceo.9Cao.iOi,9 electrolyte were performed by Kawada et These experiments showed that the... 

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