**3. Electrochemical reactors with multi–layer functional electrode**

To solve the problem of effective electrochemical reduction of nitric oxide in the presence of the excess oxygen *S.Bredikhin et al.* (Awano et al., 2004; Bredikhin et al., 2001a, 2001b) proposed the concept of artificially designed multilayer structure which should operate as an electrode with high selectivity. At present time a new type of electrochemical reactor with a functional multi-layer electrode has been successfully designed in *National Institute of Advanced Industrial Science and Technology* (*AIST), Nagoya, Japan* (Awano et al., 2004; Bredikhin et al., 2004). The typical values of current efficiency in such electrochemical reactors are of the order of 10% - 20% at gas composition: 1000 ppm NO and 2% O2 balanced in He and at gas flow rate 50 ml/min. The value of current efficiency depends on the functional multi-layer electrode composition, structure and operating temperature. Such electrochemical reactors show the value of NO selectivity (sel) with respect to oxygen gas molecules sel > 5. This means that the probability for NO gas molecules to be adsorbed and decomposed is at least 5 times higher than for oxygen gas molecules.

The arrangement of the electrochemical reactor with a functional multi-layer electrode is illustrated schematically in Fig.4. An YSZ disc with a thickness of 500m and a diameter of

Electrochemical Cells with Multilayer Functional Electrodes for NO Decomposition 185

The electrochemical reactor was set in a quartz house and connected to a potensio galvanostat (SI1267 and 1255B, SOLARTRON). The applied voltage and current dependence of NO decomposition behavior was investigated. The range of the applied voltage to the electrochemical cell was from 0 to 3V. The electrochemical decomposition of NO was carried out at 573 - 873 K by passing a mixed gas of 500-1000 ppm of NO and 2-10% of O2 in He (balance) at a flow rate = 50 ml/min. The concentrations of NO and of N2 in the outlet gas ([NO]out) were monitored using an on–line NOx (NO, NO2 and N2O) gas analyser (Best Instruments BCL-100uH, BCU-100uH) and a gas chromatograph (CHROMPACK Micro-GC

*S. Bredikhin et al.* (Awano et al., 2004b; Bredikhin et al., 2004; Hiramatsu et al., 2004) and *K. Hamamoto et al.* (Hamamoto et al., 2006, 2007) have shown that electrochemical reactors with multi-layer electro-catalytic electrode effectively operate even at low concentration of NOx (300-500ppm) and at the high concentration of oxygen (10%). From Fig.5 it is seen that the efficiency of NO decomposition by electrochemical reactors with the functional multi-layer

**4. Microstructure and properties of functional layers of multi-layer electrode**  The electrochemical reactors for selective NOx decomposition can be represented by the

(Covering layer |Electro-catalytic electrode |Cathode) **|**YSZ**|** (Anode) (5a)

 (Covering layer |Cathode |Electro-catalytic electrode) **|**YSZ**|** (Anode) (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

electrode far exceeds the efficiency of the traditional type of electrochemical cells.

CP 2002), respectively.

following reactor arrangements:

the functional multi-layer electrode is shown in Fig.6.

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

20mm was used as the solid electrolyte. The composite Pt(55vol%)-YSZ(45vol%) paste was screenprinted with an area of 1.77 cm2 on one surface of the YSZ disk as the cathode, and then calcined at 1673 K for 1 hour, to produce a dense Pt(55vol%)-YSZ(45vol%) composite with a thickness of about 3 m and diameter of 15 mm (Bredikhin et al., 2004). A dense Pt collector was connected with a cathode. The NiO-YSZ paste was screen-printed with an area of 2 cm2 over the cathode and sintered at 1773 K for 4 hours to produce a nano-porous NiO-YSZ electro-catalytic electrode with a diameter of 16mm and a thickness of about 5-6 m (Aronin et al., 2005; Awano et al., 2004a; Bredikhin et al., 2006; Hiramatsu, 2004). The nanoporous YSZ layer with a thickness of about 2 m was deposited over the electro-catalytic electrode as a covering layer (Awano et al., 2004b). The commercial TR-7070 (Pt-YSZ) paste was screen-printed with an area of 1.77 cm2 on to the other surface of the YSZ disk as the anode, and then calcined at 1473 K for 1 hour. Platinum mesh and wire were attached to the cathode and the anode, for connection with the power supply unit.

Fig. 4. Conceptual representation of the electrochemical cell with multilayer electro-catalytic electrode.

Fig. 5. The dependence of NO conversion on the value of the current for electrochemical reactors with functional multi-layer electrodes (-●- 2% and -○- 10% of oxygen) and for a reactor with a monolayer electro-catalytic electrode (-■- 2% of oxygen) and a traditional type of electrochemical cell with Pt-YSZ types of cathode (-▲- 2% of oxygen).

20mm was used as the solid electrolyte. The composite Pt(55vol%)-YSZ(45vol%) paste was screenprinted with an area of 1.77 cm2 on one surface of the YSZ disk as the cathode, and then calcined at 1673 K for 1 hour, to produce a dense Pt(55vol%)-YSZ(45vol%) composite with a thickness of about 3 m and diameter of 15 mm (Bredikhin et al., 2004). A dense Pt collector was connected with a cathode. The NiO-YSZ paste was screen-printed with an area of 2 cm2 over the cathode and sintered at 1773 K for 4 hours to produce a nano-porous NiO-YSZ electro-catalytic electrode with a diameter of 16mm and a thickness of about 5-6 m (Aronin et al., 2005; Awano et al., 2004a; Bredikhin et al., 2006; Hiramatsu, 2004). The nanoporous YSZ layer with a thickness of about 2 m was deposited over the electro-catalytic electrode as a covering layer (Awano et al., 2004b). The commercial TR-7070 (Pt-YSZ) paste was screen-printed with an area of 1.77 cm2 on to the other surface of the YSZ disk as the anode, and then calcined at 1473 K for 1 hour. Platinum mesh and wire were attached to the

Fig. 4. Conceptual representation of the electrochemical cell with multilayer electro-catalytic

0 50 100 150 200 250 300 350 400

Current (mA)

Fig. 5. The dependence of NO conversion on the value of the current for electrochemical reactors with functional multi-layer electrodes (-●- 2% and -○- 10% of oxygen) and for a reactor with a monolayer electro-catalytic electrode (-■- 2% of oxygen) and a traditional type

cathode and the anode, for connection with the power supply unit.

of electrochemical cell with Pt-YSZ types of cathode (-▲- 2% of oxygen).

NOx Conversion (%)

electrode.

The electrochemical reactor was set in a quartz house and connected to a potensio galvanostat (SI1267 and 1255B, SOLARTRON). The applied voltage and current dependence of NO decomposition behavior was investigated. The range of the applied voltage to the electrochemical cell was from 0 to 3V. The electrochemical decomposition of NO was carried out at 573 - 873 K by passing a mixed gas of 500-1000 ppm of NO and 2-10% of O2 in He (balance) at a flow rate = 50 ml/min. The concentrations of NO and of N2 in the outlet gas ([NO]out) were monitored using an on–line NOx (NO, NO2 and N2O) gas analyser (Best Instruments BCL-100uH, BCU-100uH) and a gas chromatograph (CHROMPACK Micro-GC CP 2002), respectively.

*S. Bredikhin et al.* (Awano et al., 2004b; Bredikhin et al., 2004; Hiramatsu et al., 2004) and *K. Hamamoto et al.* (Hamamoto et al., 2006, 2007) have shown that electrochemical reactors with multi-layer electro-catalytic electrode effectively operate even at low concentration of NOx (300-500ppm) and at the high concentration of oxygen (10%). From Fig.5 it is seen that the efficiency of NO decomposition by electrochemical reactors with the functional multi-layer electrode far exceeds the efficiency of the traditional type of electrochemical cells.
