**4.1 Microstructure and the properties of composite cathode**

The external voltage in the electrochemical reactor with a functional multi-layer electrode is applied between the cathode and anode. This voltage leads to the polarization of the YSZdisk and to the generation of a high concentration of oxygen vacancies in the near cathode region (Fig. 4). Due to the gradient in the concentration of the oxygen ions between the nearcathode region of the YSZ disk and the electro-catalytic electrode, diffusion of oxygen ions from the electro-catalytic electrode to the YSZ disk takes place. Since the oxygen ions are a charged species, their diffusion from the electro-catalytic electrode to the YSZ disk leads to an equal flux of electrons from the cathode to the electro-catalytic electrode (Aronin et al., 2005; Awano et al., 2004a; Bredikhin et al., 2006; Hiramatsu, 2004). As a result of electroneutrality, a decrease in the flux of the electrons should lead to the same decrease in the flux of the oxygen ions and the diffusion of oxygen ions should be stopped when the transport of electrons is blocked. In accordance with this consideration we can conclude that for effective reactor operation the cathode should be an electronic and oxygen ionic current conductor with high electronic conductivity along the cathode plane and high oxygen ionic conductivity from the electro-catalytic electrode through the cathode to the YSZ solid electrolyte (Awano et al., 2004b).

In 2004 *S.Bredikhin et al.* (Bredikhin et al., 2004) studied the correlation between the efficiency of NO decomposition by electrochemical cells with electro-catalytic electrode and the YSZ-Pt cathode compositions. In this study, *S.Bredikhin et al.* (Bredikhin et al., 2004) examined YSZ(*X*)-Pt(1-*X*) composite as a cathode for electrochemical cells with a functional multilayer electrode. Electrochemical cells with electro-catalytic electrode and YSZ(*X*)-Pt(1-*X*) composite cathode with 0, 15.0, 24.8, 35.2, 45.4, 49.9, 55.0, and 64 vol. % of YSZ were obtained. Investigation of the current-voltage (*I*-*V*) characteristics of the electrochemical cells with multilayer electrode has shown a strong dependence on the composition of the YSZ-Pt cathode (Bredikhin et al., 2004). The best performance was observed for electrochemical cells with YSZ volume contents slightly lower than 50 vol %. To investigate this behavior in detail the value of the current has been plotted as a function of the volume content of YSZ in the YSZ-Pt composite cathode for different values of the electrochemical cell operating voltage. These experimental data are shown in Fig. 7. From this figure, it is seen that an increase of the YSZ content from 0 to 49.9% leads to a four to five times increase in the value of the current through the cell at the same value of the cell operating voltage. At the same time a small change in the composition of the YSZ-Pt cathode by increasing the YSZ content to more than 50 vol % leads to an abrupt decrease in the value of the current passed through the cells (Fig. 7) and at 55 vol % of YSZ current higher than 1 mA cannot be passed even at applied voltages to the cells of more than 3 V. To describe these phenomena let us consider the processes of electronic and ionic transport through the composite YSZ-Pt cathode. The geometry of the employed electrochemical cell means that electronic transport take place along the cathode through the network of Pt particles and from the cathode through the three-dimensional network of pathways from NiO or Ni particles to three-phase boundary

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

The external voltage in the electrochemical reactor with a functional multi-layer electrode is applied between the cathode and anode. This voltage leads to the polarization of the YSZdisk and to the generation of a high concentration of oxygen vacancies in the near cathode region (Fig. 4). Due to the gradient in the concentration of the oxygen ions between the nearcathode region of the YSZ disk and the electro-catalytic electrode, diffusion of oxygen ions from the electro-catalytic electrode to the YSZ disk takes place. Since the oxygen ions are a charged species, their diffusion from the electro-catalytic electrode to the YSZ disk leads to an equal flux of electrons from the cathode to the electro-catalytic electrode (Aronin et al., 2005; Awano et al., 2004a; Bredikhin et al., 2006; Hiramatsu, 2004). As a result of electroneutrality, a decrease in the flux of the electrons should lead to the same decrease in the flux of the oxygen ions and the diffusion of oxygen ions should be stopped when the transport of electrons is blocked. In accordance with this consideration we can conclude that for effective reactor operation the cathode should be an electronic and oxygen ionic current conductor with high electronic conductivity along the cathode plane and high oxygen ionic conductivity from the electro-catalytic electrode through the cathode to the YSZ solid

In 2004 *S.Bredikhin et al.* (Bredikhin et al., 2004) studied the correlation between the efficiency of NO decomposition by electrochemical cells with electro-catalytic electrode and the YSZ-Pt cathode compositions. In this study, *S.Bredikhin et al.* (Bredikhin et al., 2004) examined YSZ(*X*)-Pt(1-*X*) composite as a cathode for electrochemical cells with a functional multilayer electrode. Electrochemical cells with electro-catalytic electrode and YSZ(*X*)-Pt(1-*X*) composite cathode with 0, 15.0, 24.8, 35.2, 45.4, 49.9, 55.0, and 64 vol. % of YSZ were obtained. Investigation of the current-voltage (*I*-*V*) characteristics of the electrochemical cells with multilayer electrode has shown a strong dependence on the composition of the YSZ-Pt cathode (Bredikhin et al., 2004). The best performance was observed for electrochemical cells with YSZ volume contents slightly lower than 50 vol %. To investigate this behavior in detail the value of the current has been plotted as a function of the volume content of YSZ in the YSZ-Pt composite cathode for different values of the electrochemical cell operating voltage. These experimental data are shown in Fig. 7. From this figure, it is seen that an increase of the YSZ content from 0 to 49.9% leads to a four to five times increase in the value of the current through the cell at the same value of the cell operating voltage. At the same time a small change in the composition of the YSZ-Pt cathode by increasing the YSZ content to more than 50 vol % leads to an abrupt decrease in the value of the current passed through the cells (Fig. 7) and at 55 vol % of YSZ current higher than 1 mA cannot be passed even at applied voltages to the cells of more than 3 V. To describe these phenomena let us consider the processes of electronic and ionic transport through the composite YSZ-Pt cathode. The geometry of the employed electrochemical cell means that electronic transport take place along the cathode through the network of Pt particles and from the cathode through the three-dimensional network of pathways from NiO or Ni particles to three-phase boundary

functional layers: 1. Cathode; 2. Electro-catalytic electrode; 3. Covering layer.

**4.1 Microstructure and the properties of composite cathode** 

electrolyte (Awano et al., 2004b).

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

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 calculated dependencies.

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

Electrochemical Cells with Multilayer Functional Electrodes for NO Decomposition 189

The external voltage (V0) in the electrochemical reactor is applied between the cathode and the anode (Fig. 4), and the voltage drop is equal to the sum of the polarizing voltage (Vpol)

 V0= Vpol + VOhm= Vpol + RYSZIox, (6) where RYSZ is the resistance of the YSZ disk and Iox is the value of oxygen ionic current through the cell. The formation of the gradient in the concentration of the oxygen ions in the YSZ disc under the DC polarization voltage Vpol can be described in accordance with the well-known equation (Hamamoto et al., 2007; Kobayashi et al., 2000; Schoonman, as cited in

where o and Po are standard values and where the chemical potential of the oxygen at the surface of the electro-catalytic electrode (CE) and in the anode region of YSZ disc (anode) is fixed by the oxygen gas pressure (PO2) (CE = anode = o + ½ kT ln(PO2/Po)). Then the difference in the chemical potential between the near cathode region and the surface of the electro-catalytic electrode should result in the Nernst potential formation () through the

The value of the oxygen ionic current through the electro-catalytic electrode depends on the value of the Nernst potential and on the value of the electro-catalytic electrode ambipolar

 Iox = amb (9) As follows from equations (8) and (9) the value of ionic current through the electrochemical cell depends on the value of ambipolar conductivity of electro-catalytic electrode (amb) and

 Iox = (V0amb)/(1RYSZ amb). (10) It is seen that when RYSZamb 1, there is a linear dependence between ionic current

 Iox (V0amb). (11) In the year 2001 *S.Bredikhin et al*. (Bredikhin et al., 2001a, 2001b, 2001c) have shown that for electrochemical cells with nano-porous electro-catalytic electrode there is a linear dependence between the value of NO conversion (NO) and value of oxygen ionic current

on the value of oxygen ionic conductivity (RYSZ) of YSZ disc as

through the cell and the value of ambipolar conductivity

cathode = anode 2eVpol = o + ½ kT ln(PO2/Po) 2eVpol, (7)

Vpol= (V0 RYSZIox) (8)

NO = (1/(Fn)) Iox , (12)

a. Peculiarity of ambipolar conductivity of the electro-catalytic electrode

Chowdari & Radakrishna, 1998; Wagner, as cited in Delahay & Tobias, 1966)

and to the Ohms voltage (VOhm = RYSZIox) drop through the YSZ disk

**4.2 Electro-catalytic electrode** 

electro-catalytic electrode

conductivity (amb) as

passed through the cell

Fig. 8. SEM images of the cross-section of the multi-layer cells with different YSZ-Pt cathode compositions (a - 15vol% of YSZ, b - 25vol% of YSZ, c - 35vol% of YSZ, d - 45vol% of YSZ).

As follows from Figs. 7 and 8 an increase of the YSZ content leads to an increase in the oxygen ionic current through the cell at a given value of the applied external voltage. At the same time when the YSZ content exceeds the percolation threshold (50 vol %) the Pt phase becomes disconnected, and the flux of electrons from the cathode to the electro-catalytic electrode is blocked. As a result the oxygen ionic diffusion from the electro-catalytic electrode to the YSZ-disk is stopped. In accordance with the above results it is seen that the most critical place for oxygen ion diffusion from the electro-catalytic electrode to the YSZdisk is the oxygen transport through the composite YSZ-Pt cathode, and that compositions with a volume fraction of YSZ slightly lower than the two-dimensional percolation threshold (1/2) should have the highest efficiency for charge transport through the electrochemical cell (Bredikhin et al., 2004).

#### **4.2 Electro-catalytic electrode**

188 Electrochemical Cells – New Advances in Fundamental Researches and Applications

Fig. 8. SEM images of the cross-section of the multi-layer cells with different YSZ-Pt cathode compositions (a - 15vol% of YSZ, b - 25vol% of YSZ, c - 35vol% of YSZ, d - 45vol% of YSZ).

As follows from Figs. 7 and 8 an increase of the YSZ content leads to an increase in the oxygen ionic current through the cell at a given value of the applied external voltage. At the same time when the YSZ content exceeds the percolation threshold (50 vol %) the Pt phase becomes disconnected, and the flux of electrons from the cathode to the electro-catalytic electrode is blocked. As a result the oxygen ionic diffusion from the electro-catalytic electrode to the YSZ-disk is stopped. In accordance with the above results it is seen that the most critical place for oxygen ion diffusion from the electro-catalytic electrode to the YSZdisk is the oxygen transport through the composite YSZ-Pt cathode, and that compositions with a volume fraction of YSZ slightly lower than the two-dimensional percolation threshold (1/2) should have the highest efficiency for charge transport through the

electrochemical cell (Bredikhin et al., 2004).

#### a. Peculiarity of ambipolar conductivity of the electro-catalytic electrode

The external voltage (V0) in the electrochemical reactor is applied between the cathode and the anode (Fig. 4), and the voltage drop is equal to the sum of the polarizing voltage (Vpol) and to the Ohms voltage (VOhm = RYSZIox) drop through the YSZ disk

$$\mathbf{V}\_0 = \mathbf{V}\_{\rm pol} + \mathbf{V}\_{\rm Chm} = \mathbf{V}\_{\rm prod} + \mathbf{R}\_{\rm YSZ} \times \mathbf{I}\_{\rm cov} \tag{6}$$

where RYSZ is the resistance of the YSZ disk and Iox is the value of oxygen ionic current through the cell. The formation of the gradient in the concentration of the oxygen ions in the YSZ disc under the DC polarization voltage Vpol can be described in accordance with the well-known equation (Hamamoto et al., 2007; Kobayashi et al., 2000; Schoonman, as cited in Chowdari & Radakrishna, 1998; Wagner, as cited in Delahay & Tobias, 1966)

$$
\mu\_{\text{cathode}} = \mu\_{\text{anode}} + 2\mathbf{e} \times \mathbf{V}\_{\text{pol}} = \mu\_{\text{o}} + \mathbb{V} \,\mathrm{kT} \, \ln(\mathbf{P}\_{\text{O2}}/\mathbf{P}\_{\text{o}}) + 2\mathbf{e} \times \mathbf{V}\_{\text{pol}} \tag{7}
$$

where o and Po are standard values and where the chemical potential of the oxygen at the surface of the electro-catalytic electrode (CE) and in the anode region of YSZ disc (anode) is fixed by the oxygen gas pressure (PO2) (CE = anode = o + ½ kT ln(PO2/Po)). Then the difference in the chemical potential between the near cathode region and the surface of the electro-catalytic electrode should result in the Nernst potential formation () through the electro-catalytic electrode

$$
\Delta\phi = \mathbf{V}\_{\text{pol}} \equiv \left(\mathbf{V}\_0 - \mathbf{R}\_{\text{YSZ}} \times \mathbf{I}\_{\text{ox}}\right) \tag{8}
$$

The value of the oxygen ionic current through the electro-catalytic electrode depends on the value of the Nernst potential and on the value of the electro-catalytic electrode ambipolar conductivity (amb) as

$$\mathbf{I}\_{\rm ox} = \Delta\Phi \times \mathbf{\sigma}\_{\rm amb} \tag{9}$$

As follows from equations (8) and (9) the value of ionic current through the electrochemical cell depends on the value of ambipolar conductivity of electro-catalytic electrode (amb) and on the value of oxygen ionic conductivity (RYSZ) of YSZ disc as

$$\mathbf{I}\_{\rm ox} = (\mathbf{V}\_0 \times \boldsymbol{\sigma}\_{\rm amb}) / \{\mathbf{1} + \mathbf{R}\_{\rm YSZ} \times \boldsymbol{\sigma}\_{\rm amb}\}. \tag{10}$$

It is seen that when RYSZamb 1, there is a linear dependence between ionic current through the cell and the value of ambipolar conductivity

$$\mathbf{I}\_{\rm ox} \approx (\mathbf{V}\_0 \times \boldsymbol{\sigma}\_{\rm amb}).\tag{11}$$

In the year 2001 *S.Bredikhin et al*. (Bredikhin et al., 2001a, 2001b, 2001c) have shown that for electrochemical cells with nano-porous electro-catalytic electrode there is a linear dependence between the value of NO conversion (NO) and value of oxygen ionic current passed through the cell

NO = (1/(Fn)) Iox , (12)

where n=2 is the charge of the oxygen ions, F- is a Faraday constant, is a total gas flow rate and is current efficiency. From equations (11) and (12) it is follows that the rate of NO decomposition depends on the external voltage applied to the cell and on the value of ambipolar conductivity as

$$
\Delta \text{NO} = \left( 1/\left( \text{F} \times \text{v} \times \text{n} \right) \right) \times \eta \times V\_0 \times \sigma\_{\text{amb}} \tag{13}
$$

Electrochemical Cells with Multilayer Functional Electrodes for NO Decomposition 191

 NiO(82.5vol%) NiO(35vol%)

0.0 0.5 1.0 1.5 2.0 2.5

External voltage, V

Fig. 10. The dependence of the value of the ambipolar conductivity on the cell operating voltage for two NiO-YSZ composite electrodes with NiO volume content 82.5% and 35%.

b. Reduction-oxidation processes and the structure of the electro-catalytic electrode

From Figs.9 and 10 it is seen that for the composition range from 1/3 to 2/3 the application of external voltage leads to 6-10 times increase of the value of ambipolar conductivity. Such unusual dependence of the value of conductivity on the applied voltage shows that change of the chemical composition of NiO-YSZ electro-catalytic electrode takes place under the cell

The distinguishing feature of such reactors is the artificial nano-structure formed in the NiO/YSZ interface of the electro-catalytic electrode under operation. The essential changes occur in the interfacial boundary region between NiO and YSZ grains. After the electrochemical cell operation for 22 hours at a cell voltage lower than 2.2 Volts they are as

1. New grains nucleate and grow in the pore zone of NiO near-boundary regions. The zone of new grains spreads into the depth of the NiO grain (Fig. 11). In some cases this zone of new grains devours "old" NiO grain completely (Figs. 12a and 12b). The electron diffraction patterns from these regions contain ring reflections of Ni in addition to NiO reflections (Aronin et al., 2005). The size of Ni grains is 5-20 nm and they are seen in the dark field electron microscopy image (Fig. 12b). This image was obtained in the reflection marked by the arrow in the electron diffraction pattern (Fig. 12a, insert). The new grains are both Ni

0.00

0.02

0.04

0.06

0.08

amb

operation.

follows (Aronin et al., 2005).

and NiO phases.

0.10

0.12

0.14

0.16

It is obvious that for optimization of the electrochemical cell for NO decomposition it is necessary to design the cell with the highest value of the ambipolar conductivity. To analyze the specific features of the ambipolar conductivity of the NiO-YSZ composite electrode the calculated value of amb (see Eq.11) has been plotted as a function of the volume content of NiO in the NiO-YSZ composite electrode. These data are shown in Fig.9.

Fig. 9. The dependence of the value of the ambipolar conductivity on the volume fraction of NiO in Ni-YSZ electro-catalytic electrode

It is seen that when the external voltage is higher than 1.5 volt, the two percolation transitions of electronic and ionic conductivities lead to a composition range from 1/3 to 2/3, in which the ambipolar conductivity is much higher than those in the other regions. This suggests that in order to achieve high value of ambipolar conductivity, the volume fractions of each phase have to be within 1/3 ~ 2/3 and external voltage should be higher then 1.5 volt. It should be noted that in the composition range from 1/3 to 2/3 the value of ambipolar conductivity increases with increasing of the voltage applied to the cell. As an illustration Fig.10 shows the dependence of the value of the ambipolar conductivity on the voltage for two NiO-YSZ composite electrodes with NiO volume content 82.5% and 35%.

where n=2 is the charge of the oxygen ions, F- is a Faraday constant, is a total gas flow rate and is current efficiency. From equations (11) and (12) it is follows that the rate of NO decomposition depends on the external voltage applied to the cell and on the value of

It is obvious that for optimization of the electrochemical cell for NO decomposition it is necessary to design the cell with the highest value of the ambipolar conductivity. To analyze the specific features of the ambipolar conductivity of the NiO-YSZ composite electrode the calculated value of amb (see Eq.11) has been plotted as a function of the volume content of

1st percolation limit 2nd percolation limit

0 10 20 30 40 50 60 70 80 90 100

YSZ NiO

vol.%

Fig. 9. The dependence of the value of the ambipolar conductivity on the volume fraction of

It is seen that when the external voltage is higher than 1.5 volt, the two percolation transitions of electronic and ionic conductivities lead to a composition range from 1/3 to 2/3, in which the ambipolar conductivity is much higher than those in the other regions. This suggests that in order to achieve high value of ambipolar conductivity, the volume fractions of each phase have to be within 1/3 ~ 2/3 and external voltage should be higher then 1.5 volt. It should be noted that in the composition range from 1/3 to 2/3 the value of ambipolar conductivity increases with increasing of the voltage applied to the cell. As an illustration Fig.10 shows the dependence of the value of the ambipolar conductivity on the voltage for two NiO-YSZ composite electrodes with NiO volume content 82.5% and 35%.

NiO in the NiO-YSZ composite electrode. These data are shown in Fig.9.

 2.25V 2V 1.75V 1.5V 1.25V 1V

0.00

NiO in Ni-YSZ electro-catalytic electrode

0.02

0.04

0.06

0.08

0.10

, (Ohm)-1

0.12

0.14

0.16

0.18

0.20

NO = (1/(Fn)) V0amb (13)

ambipolar conductivity as

Fig. 10. The dependence of the value of the ambipolar conductivity on the cell operating voltage for two NiO-YSZ composite electrodes with NiO volume content 82.5% and 35%.

From Figs.9 and 10 it is seen that for the composition range from 1/3 to 2/3 the application of external voltage leads to 6-10 times increase of the value of ambipolar conductivity. Such unusual dependence of the value of conductivity on the applied voltage shows that change of the chemical composition of NiO-YSZ electro-catalytic electrode takes place under the cell operation.

b. Reduction-oxidation processes and the structure of the electro-catalytic electrode

The distinguishing feature of such reactors is the artificial nano-structure formed in the NiO/YSZ interface of the electro-catalytic electrode under operation. The essential changes occur in the interfacial boundary region between NiO and YSZ grains. After the electrochemical cell operation for 22 hours at a cell voltage lower than 2.2 Volts they are as follows (Aronin et al., 2005).

1. New grains nucleate and grow in the pore zone of NiO near-boundary regions. The zone of new grains spreads into the depth of the NiO grain (Fig. 11). In some cases this zone of new grains devours "old" NiO grain completely (Figs. 12a and 12b). The electron diffraction patterns from these regions contain ring reflections of Ni in addition to NiO reflections (Aronin et al., 2005). The size of Ni grains is 5-20 nm and they are seen in the dark field electron microscopy image (Fig. 12b). This image was obtained in the reflection marked by the arrow in the electron diffraction pattern (Fig. 12a, insert). The new grains are both Ni and NiO phases.

Electrochemical Cells with Multilayer Functional Electrodes for NO Decomposition 193

Fig. 13. HREM images of a near-boundary region of an YSZ grain and small NiO grains

3. An orientation relationship has been found to exist between the lattices of YSZ and new

The presence of the orientation relationship between the lattices of YSZ and NiO grains indicates that the new NiO grains may nucleate on the YSZ grains as on the substrate and in this case their surface energy will decrease. This result is a direct evidence of the process of oxygen spillover from YSZ to Ni grains even in the presence of oxygen in the surrounding

The distinguishing feature of the microstructure of the YSZ/(Ni-NiO) interface is the 10-50 nm Ni grains reversibly produced during the reactor operation. Schematically the microstructure of the YSZ/NiO interface reversibly produced during the cell operation is represented in Fig.14. It is well known that adsorption and decomposition of NOx gas molecules occurs in preference to oxygen gas molecules on Ni grain surfaces (Lindsay et al., 1998; Miura et al., 2001; Rickardsson et al., 1998). In addition, we should mention that rough surfaces and nano-size Ni grains are much more active for breaking of NO chemical bonds than smooth, flat surfaces (Garin, 2001; Lindsay et al., 1998). Based on the above results, the following reaction mechanism was proposed for NO decomposition on the nano-size Ni

 2NiNO 2NiO N2 (15) NO gas molecules are first chemisorbed on Ni. As a second step the chemisorbed NO

Oxygen ionic current passed though the network of YSZ particles surrounding the Ni grains. This process removed oxygen species from the electrode and permitted the reactions (14) and (15) to reoccur. The regeneration reaction of the reduction of NiO to Ni takes place

NO + Ni NiNO (14)

formed during the cell operation (a and b different magnification).

(310)YSZ (110)NiO, [001]YSZ [] NiO

grains produced during the reactor operation.

decomposes to form N2, oxidizing Ni to NiO.

at the NiO/YSZ interface under the reactor operation

NiO grains:

gas.

Fig. 11. Interfacial boundary between YSZ and NiO grains of the sample after cell operation.

Fig. 12. Bright field TEM image (a) and dark field TEM images (b) of new NiO and Ni grains zone devoured "old" NiO grain.

2. In the region with new NiO grains the pores are located both in the interfacial vicinity and before the front of new growing grains. Sometimes small NiO grains are surrounded by the pores on all sides. Separate pores are also observed in the region of new grains, located between the new grains (Fig. 11). The high resolution electron microscopy images of the near-boundary region of YSZ grain and small NiO grain formed during the cell operation at different magnifications are shown in Fig. 13. The size of the new NiO grains varies from 10 to 100 nm depending on the location.

Fig. 11. Interfacial boundary between YSZ and NiO grains of the sample after cell operation.

Fig. 12. Bright field TEM image (a) and dark field TEM images (b) of new NiO and Ni grains

2. In the region with new NiO grains the pores are located both in the interfacial vicinity and before the front of new growing grains. Sometimes small NiO grains are surrounded by the pores on all sides. Separate pores are also observed in the region of new grains, located between the new grains (Fig. 11). The high resolution electron microscopy images of the near-boundary region of YSZ grain and small NiO grain formed during the cell operation at different magnifications are shown in Fig. 13. The size of the new NiO grains varies from 10

zone devoured "old" NiO grain.

to 100 nm depending on the location.

Fig. 13. HREM images of a near-boundary region of an YSZ grain and small NiO grains formed during the cell operation (a and b different magnification).

3. An orientation relationship has been found to exist between the lattices of YSZ and new NiO grains:

#### (310)YSZ (110)NiO, [001]YSZ [] NiO

The presence of the orientation relationship between the lattices of YSZ and NiO grains indicates that the new NiO grains may nucleate on the YSZ grains as on the substrate and in this case their surface energy will decrease. This result is a direct evidence of the process of oxygen spillover from YSZ to Ni grains even in the presence of oxygen in the surrounding gas.

The distinguishing feature of the microstructure of the YSZ/(Ni-NiO) interface is the 10-50 nm Ni grains reversibly produced during the reactor operation. Schematically the microstructure of the YSZ/NiO interface reversibly produced during the cell operation is represented in Fig.14. It is well known that adsorption and decomposition of NOx gas molecules occurs in preference to oxygen gas molecules on Ni grain surfaces (Lindsay et al., 1998; Miura et al., 2001; Rickardsson et al., 1998). In addition, we should mention that rough surfaces and nano-size Ni grains are much more active for breaking of NO chemical bonds than smooth, flat surfaces (Garin, 2001; Lindsay et al., 1998). Based on the above results, the following reaction mechanism was proposed for NO decomposition on the nano-size Ni grains produced during the reactor operation.

$$\text{NiO} + \text{Ni} \rightarrow \text{Ni} - \text{NO} \tag{14}$$

$$\text{2Ni-NO} \rightarrow \text{2NiO} + \text{N}\_2 \tag{15}$$

NO gas molecules are first chemisorbed on Ni. As a second step the chemisorbed NO decomposes to form N2, oxidizing Ni to NiO.

Oxygen ionic current passed though the network of YSZ particles surrounding the Ni grains. This process removed oxygen species from the electrode and permitted the reactions (14) and (15) to reoccur. The regeneration reaction of the reduction of NiO to Ni takes place at the NiO/YSZ interface under the reactor operation

$$\text{NiO} + \text{V}\_{\text{O}}(\text{ZrO}\_{2}) + 2\text{e} \rightarrow \text{Ni} + \text{O}^{\text{2}}(\text{ZrO}\_{2}) \tag{16}$$

Electrochemical Cells with Multilayer Functional Electrodes for NO Decomposition 195

To optimize the characteristics of the electrochemical cells with multi-layer electrode, we have carried out the investigations of rate of NOx decomposition depending of the upper layer microstructure. Our investigations has shown that the best characteristics of the cells for selective NO decomposition can be reached for the electrochemical cells with the thin

Fig. 15. TEM images of the structure of the covering YSZ layer sintered at 14500C.

shape with a size 50-100 nm.

The transmission electron microscopy investigations of the upper layer and of the electrocatalytic electrode were carried out on a JEOL 4000 FX microscope. Foils for the electron microscopy investigations were prepared by mechanical polishing followed by ion milling. The structure of the upper YSZ layer sintered at temperature 14500C is shown in Fig.15. This figure shows some typical microstructures of the upper layer. It is seen that upper layer is porous and that the pores form channels with a size of 200-500 nm, or they have ellipsoidal

The structure of the electro-catalytic electrode in the electrochemical cell with multi-layer electrode is shown in Fig.16. This figure shows a typical structure of electro-catalytic electrode after the cell operation. Data of chemical composition of this structure obtained by

upper YSZ layer (2-3 m) sintered at temperature 14500C.

Therefore, the reduction of NiO grains into Ni grains and the oxidation of Ni grains into NiO take place continuously during reactor operation. As a result the catalytic activity for NO decomposition is independent of the operation time.

At the same time oxygen gas molecules have a preference for adsorption by F-type centers on the surface of YSZ.

$$\text{O}\_2 + 4\text{e}^- + 2\text{ V}\_\text{O} \text{(ZrOO}\_2) \rightarrow 2\text{ OF}^- \text{(ZrOO}\_2) \tag{17}$$

From this consideration it follows that under the reactor operation adsorption and decomposition of NO and O2 gas molecules occur on the surface of Ni grains and by F – centers on the surface of YSZ grains, respectively. The design of an electrochemicallyassembled electrode with two kinds of active sites provides a way to suppress the unwanted reaction of oxygen gas adsorption (Eq.(17)) and to increase the desirable reaction of NO gas decomposition Eqs.(14,15).

Fig. 14. Schematic representation of the microstructure of the YSZ-NiO interface and of the sites for NO and Oxygen gases adsorption in the self-assembled electro-catalytic electrode.

#### **4.3 Covering layer**

The deposition of a thin (2–3 nm) covering YSZ layer leads to a suppression of the oxygen adsorption and decomposition. Additionally, the deposition of the covering layer leads to an increase of the amount of nanosize Ni grains located in the near interface boundary porous region between the grains of NiO and YSZ. As a result, electrochemical reactors with the functional multilayer electrode show much better selectivity for NO gas decomposition even with respect to the electrochemical cells with electro-catalytic electrode but without a covering layer.

 NiO VO(ZrO2) 2e Ni O2-(ZrO2) (16) Therefore, the reduction of NiO grains into Ni grains and the oxidation of Ni grains into NiO take place continuously during reactor operation. As a result the catalytic activity for

At the same time oxygen gas molecules have a preference for adsorption by F-type centers

+2 VO(ZrO2) 2 O2

From this consideration it follows that under the reactor operation adsorption and decomposition of NO and O2 gas molecules occur on the surface of Ni grains and by F – centers on the surface of YSZ grains, respectively. The design of an electrochemicallyassembled electrode with two kinds of active sites provides a way to suppress the unwanted reaction of oxygen gas adsorption (Eq.(17)) and to increase the desirable reaction of NO gas

Fig. 14. Schematic representation of the microstructure of the YSZ-NiO interface and of the sites for NO and Oxygen gases adsorption in the self-assembled electro-catalytic electrode.

The deposition of a thin (2–3 nm) covering YSZ layer leads to a suppression of the oxygen adsorption and decomposition. Additionally, the deposition of the covering layer leads to an increase of the amount of nanosize Ni grains located in the near interface boundary porous region between the grains of NiO and YSZ. As a result, electrochemical reactors with the functional multilayer electrode show much better selectivity for NO gas decomposition even with respect to the electrochemical cells with electro-catalytic electrode but without a

(ZrO2) (17)

NO decomposition is independent of the operation time.

on the surface of YSZ.

decomposition Eqs.(14,15).

**4.3 Covering layer** 

covering layer.

O2 + 4e

To optimize the characteristics of the electrochemical cells with multi-layer electrode, we have carried out the investigations of rate of NOx decomposition depending of the upper layer microstructure. Our investigations has shown that the best characteristics of the cells for selective NO decomposition can be reached for the electrochemical cells with the thin upper YSZ layer (2-3 m) sintered at temperature 14500C.

Fig. 15. TEM images of the structure of the covering YSZ layer sintered at 14500C.

The transmission electron microscopy investigations of the upper layer and of the electrocatalytic electrode were carried out on a JEOL 4000 FX microscope. Foils for the electron microscopy investigations were prepared by mechanical polishing followed by ion milling. The structure of the upper YSZ layer sintered at temperature 14500C is shown in Fig.15. This figure shows some typical microstructures of the upper layer. It is seen that upper layer is porous and that the pores form channels with a size of 200-500 nm, or they have ellipsoidal shape with a size 50-100 nm.

The structure of the electro-catalytic electrode in the electrochemical cell with multi-layer electrode is shown in Fig.16. This figure shows a typical structure of electro-catalytic electrode after the cell operation. Data of chemical composition of this structure obtained by

Electrochemical Cells with Multilayer Functional Electrodes for NO Decomposition 197

0 20 40 60 80 100 120 140 160

Current (mA)

Fig. 17. The dependence of NO conversion on the value of the current for electrochemical cells with multi-layer electro-catalytic electrodes in the presence of 2% of Oxygen (-- 500ppm and -- 1000 ppm of NO gas) and of 10% of Oxygen (-- 500 ppm and --

The design of the self-assembled electrode with two kinds of active sites provides a way to suppress the unwanted reaction of oxygen gas adsorption and to increase many times the desirable reaction of NO gas decomposition. For the first time, an electrochemical cell with multi-layer electro-catalytic electrode for selective NO decomposition in the presence of excess oxygen (10%) operating at a low value of electrical power was designed. These results indicate that electrochemical reactors with multi-layer electro-catalytic electrode can

**5. Intermediate and low temperature electrochemical reactors with multilayer** 

In 2006 *K.Hamamoto et.al.* (Hamamoto et.al., 2006, 2007) proposed to use an electrochemical rector with multilayer functional electrode for intermediate temperature operation. The cell

8YSZ (8 mol % Y2O3-doped ZrO2), 10YSZ (10 mol % Y2O3-doped ZrO2) and ScCeSZ (10 mol % Sc2O3 and 1 mol % CeO2-doped ZrO2) were selected as solid electrolytes. The electrocatalytic electrodes with compositions NiO–8YSZ, NiO–10YSZ and NiO–ScCeSZ (with 55 mol % of NiO) were deposited on the surface of 8YSZ, 10YSZ and ScCeSZ solid electrolyte disks, respectively. Figure 18 shows the values of current efficiency () plotted as a function of applied voltage for such electrochemical reactors operated at 475C. The value of selectivity (sel) of such reactors for NO gas molecules decomposition is also displayed in Fig. 18 (Hamamoto et.al., 2006). From Fig.18 it is seen that in electrochemical reactors with electro-catalytic electrodes the probability for NO gas molecules to be adsorbed and

performance represented on (5b) was used for intermediate temperature operation.

decomposed is at least 5 times higher than for oxygen gas molecules.

 Ox 2% NO- 500ppm Ox 2% NO-1000ppm Ox10% NO- 500ppm

Ox10% NO-1000ppm

0

10

20

30

NOx Conversion (%)

1000ppm of NO gas).

be used for practical applications.

**functional electrode** 

40

50

60

70

EDS method are also displayed in the same figure. From Fig.16 it is seen that the structure consists of NiO, Ni and YSZ grains. The Ni grins are located in the near interface boundary porous region between the grains of NiO and YSZ. From above consideration it follows that this structure is the same as a structure of the field-quenched electro-catalytic electrode in the cells without upper layer (Aronin et al., 2005; Bredikhin et al, 2006). It is obvious that deposition of the upper layer leads to suppressing of the oxygen gas adsorption and to an increase of the concentration of the oxygen vacancies in the YSZ grains. As the result both the rate of the reduction of the NiO to Ni and of the amount of new Ni grains increase in the YSZ/(Ni-NiO) interface region. Therefore the electrochemical cells with multi-layer electrode should show a much higher selectivity for NOx gas decomposition in the presence of excess oxygen than all known cells.

Fig. 16. TEM image of the structure of the electro-catalytic electrode (A) and the chemical composition of this structure obtained by EDS method (B).

One more important advantage of the electrochemical reactor with multi-layer electrode should be discussed. Our investigations have shown that electrochemical cells with multilayer electro-catalytic electrode effectively operate even at low concentration of NOx (300- 500 ppm) and at the high concentration of oxygen (10%) in the exhaust gas. In Fig.17 the NO conversion is plotted as a function of the current for one compartment electrochemical cell with multi-layer electrode at different concentration of NO (500 ppm or 1000 ppm) and oxygen (2% or 10%) at a gas flow rate 50 ml/min and at temperature 5600C. From this figure it is seen that the decrease of the NO concentration from 1000ppm to 500ppm leads to the two times decrease of the value of the current required for 30% NO decomposition for both 2% and 10% of oxygen in the gas mixture. Direct proportion between NO concentration in the exhaust gas and the value of the current required for NO decomposition confirms our proposal that the process of NO gas adsorption and decomposition is practically independent of the oxygen gas adsorption and decomposition. Additional we should mention that increase of the oxygen content in the investigated gas from 2% to 10% at fixed NO concentration leads to the 1.5 times increase only of the value of the current required for NO decomposition. This result shows that the oxygen adsorption and decomposition in the electrochemical cells with multi-layer electrode is suppressed. In accordance with above we can conclude that new type of electrochemical reactor with multi-layer electro-catalytic electrode can be used for effective NO decomposition even in the presence of high oxygen concentration.

EDS method are also displayed in the same figure. From Fig.16 it is seen that the structure consists of NiO, Ni and YSZ grains. The Ni grins are located in the near interface boundary porous region between the grains of NiO and YSZ. From above consideration it follows that this structure is the same as a structure of the field-quenched electro-catalytic electrode in the cells without upper layer (Aronin et al., 2005; Bredikhin et al, 2006). It is obvious that deposition of the upper layer leads to suppressing of the oxygen gas adsorption and to an increase of the concentration of the oxygen vacancies in the YSZ grains. As the result both the rate of the reduction of the NiO to Ni and of the amount of new Ni grains increase in the YSZ/(Ni-NiO) interface region. Therefore the electrochemical cells with multi-layer electrode should show a much higher selectivity for NOx gas decomposition in the presence

Fig. 16. TEM image of the structure of the electro-catalytic electrode (A) and the chemical

One more important advantage of the electrochemical reactor with multi-layer electrode should be discussed. Our investigations have shown that electrochemical cells with multilayer electro-catalytic electrode effectively operate even at low concentration of NOx (300- 500 ppm) and at the high concentration of oxygen (10%) in the exhaust gas. In Fig.17 the NO conversion is plotted as a function of the current for one compartment electrochemical cell with multi-layer electrode at different concentration of NO (500 ppm or 1000 ppm) and oxygen (2% or 10%) at a gas flow rate 50 ml/min and at temperature 5600C. From this figure it is seen that the decrease of the NO concentration from 1000ppm to 500ppm leads to the two times decrease of the value of the current required for 30% NO decomposition for both 2% and 10% of oxygen in the gas mixture. Direct proportion between NO concentration in the exhaust gas and the value of the current required for NO decomposition confirms our proposal that the process of NO gas adsorption and decomposition is practically independent of the oxygen gas adsorption and decomposition. Additional we should mention that increase of the oxygen content in the investigated gas from 2% to 10% at fixed NO concentration leads to the 1.5 times increase only of the value of the current required for NO decomposition. This result shows that the oxygen adsorption and decomposition in the electrochemical cells with multi-layer electrode is suppressed. In accordance with above we can conclude that new type of electrochemical reactor with multi-layer electro-catalytic electrode can be used for effective NO decomposition even in the presence of high oxygen

composition of this structure obtained by EDS method (B).

of excess oxygen than all known cells.

concentration.

Fig. 17. The dependence of NO conversion on the value of the current for electrochemical cells with multi-layer electro-catalytic electrodes in the presence of 2% of Oxygen (-- 500ppm and -- 1000 ppm of NO gas) and of 10% of Oxygen (-- 500 ppm and -- 1000ppm of NO gas).

The design of the self-assembled electrode with two kinds of active sites provides a way to suppress the unwanted reaction of oxygen gas adsorption and to increase many times the desirable reaction of NO gas decomposition. For the first time, an electrochemical cell with multi-layer electro-catalytic electrode for selective NO decomposition in the presence of excess oxygen (10%) operating at a low value of electrical power was designed. These results indicate that electrochemical reactors with multi-layer electro-catalytic electrode can be used for practical applications.
