**4. Microstructure and properties of functional layers of multi-layer electrode**

The electrochemical reactors for selective NOx decomposition can be represented by the following reactor arrangements:

$$\text{(Covering layer \mid Electric-catalitytic electrode \mid Cathode) \mid YSZ \mid \text{(Anode)}}\tag{5a}$$

$$\text{(Covering layer \mid Cathode \mid Electric-catalitytic electrode) \mid YSZ \mid \text{(Anode)}}\tag{5b}$$

Let us consider in detail the arrangement of the electrochemical reactor with a functional multi-layer electrode and the properties of each functional layer. The cross-section view of the functional multi-layer electrode is shown in Fig.6.

Fig. 6. The cross-section view of the multi-layer electro-catalytic electrode.

Electrochemical Cells with Multilayer Functional Electrodes for NO Decomposition 187

(TPB) on the surface of the pores inside the electro-catalytic electrode. Oxygen ionic transport takes place perpendicular to the YSZ-Pt cathode plane from the electro-catalytic

YSZ-Pt Composite Cathode

0 10 20 30 40 50 60 70 80 90 100

YSZ, vol%

Fig. 7. The experimental dependence of the current on the volume fraction of YSZ in the YSZ-Pt cathode for applied voltages of 1.4 Volts and 1.6 Volts, and comparison with

YSZ in the YSZ-Pt composite cathode (Fig. 7) (Bredikhin et al., 2004).

Figure 8 shows the cross-sectional view of the electrochemical cells for different YSZ-Pt cathode compositions. It is seen that the addition of YSZ particles to the cathode leads to the formation of oxygen-conducting YSZ bridges through the electronically conducting Pt cathode and that the number of such bridges increases with an increasing amount of YSZ in the cathode. From Fig. 8 it is seen that the average size of the electronically insulating (YSZ) particles and electronically conducting (Pt) particles are of the order of 2-3 mm and are the same as the thickness of the cathode layers. These observations give us the possibility to conclude that the YSZ-Pt cathode is a quasi two-dimensional system and that the twodimensional percolation model can be used to describe the electronic conductivity along the plane of the YSZ-Pt cathode. In accordance with this model a sharp transition in electronic conductivity along the cathode should be observed at 50 vol % of the electronically conducting Pt phase (Bredikhin et al., 2004). This means that the electronically conducting Pt phase is continuous when the volume fraction of the electronically insulating YSZ phase is less than 50 vol % and the Pt phase becomes disconnected when the volume fraction of insulating YSZ phase is greater that 50 vol %. This two-dimensional percolation model prediction is in good agreement with the experimentally observed sharp threshold of the value of the current through the electrochemical cell as a function of the volume content of

 1.6V Model 1.4V Model

electrode through the network of YSZ particles to the YSZ disk.

0

20

40

60

80

Current, mA

calculated dependencies.

100

120

140

The cathode is a dense Pt(55vol%)-YSZ(45vol%) composite with a thickness of about 2-3 m. The nano- porous NiO-YSZ electro-catalytic electrode with a thickness of about 6-8 m was deposited over the cathode. The porous YSZ layer with a thickness of about 2-3 m was deposited over the cathode. It is seen that the multi-layer electrode consists from three main functional layers: 1. Cathode; 2. Electro-catalytic electrode; 3. Covering layer.
