**2. Traditional type of electrochemical cells for NO decomposition**

Electrochemical cells have become an important technology, which contributes to many aspects of human life, industry and environment. Now it is understandable that the reduction of NOx emission can be achieved not only by catalytic NOx decomposition but also by electrochemical decomposition, where the removal of oxygen by a gaseous reducing reagent is replaced by the more effective electrochemical removal. Additional reducing reagents such as hydrocarbons, CO, H2 or ammonia can lead to the production of secondary pollutants like oxygenated hydrocarbons, CO, CO2, N2O or ammonia or, even, as was often reported in the past, cyanate and isocyanate compounds.

Without coexisting oxygen the successful decomposition of NO gas into oxygen and nitrogen in a primitive electrochemical cell (Fig.1) was first demonstrated over 25 years ago (Gur & Huggins, 1979; Pancharatnam et al., 1975). In 1975 *Pancharatnam et al.* (Pancharatnam et al., 1975) proposed to use for NO gas decomposition an electrochemical cell represented by the following cell arrangement

$$\text{Pt(Cathode)} \mid \text{YSZ} \mid \text{Pt(Anode)} \tag{1}$$

On applying a voltage to such cells NO gas is directly reduced at the triple-phase boundary *(tpb)* (cathode - yttrium-stabilized zirconia (YSZ) - gas) forming gaseous N2 and solid –phase oxygen ions:

$$2\text{NO} + 4\text{e}^- + 2\text{ V}\_\text{O}(\text{ZrO}\_2) \rightarrow \text{N}\_2 + 2\text{O}^{\text{-}}(\text{YSZ})\tag{2}$$

Under the external voltage the oxygen ions are transported through the solid electrolyte from cathode to anode and gaseous O2 is evolved at the anode.

Unfortunately, excess O2 in the combustion exhaust gas is adsorbed and decomposed at the *tpb* in preference to the NO gas (Fig.1):

$$\text{O}\_2 + 4\text{e}^- + 2\text{ V}\_\text{O}(\text{ZrO}\_2) \rightarrow 2\text{OF}^-(\text{YSZ}) \tag{3}$$

NO+CO CO2+1/2 N2

**Third,** the selective catalytic reduction of NO in the presence of hydrocarbons and more particularly methane, a method which has not yet reached industrial use but can be applied both for automotive pollution control and in various industrial plants (Armor, 1995;

**Fourth,** the direct decomposition of NO. The decomposition of NO would represent the most attractive solution in emission control, because the reaction does not require that any reactant be added to NO exhaust gas and could potentially lead to the formation of only N2

The goal of this paper is to represent a **fifth** direction of an intense research effort focused on electrochemical cells for the reduction of NOx gases due to the need to design an effective

Electrochemical cells have become an important technology, which contributes to many aspects of human life, industry and environment. Now it is understandable that the reduction of NOx emission can be achieved not only by catalytic NOx decomposition but also by electrochemical decomposition, where the removal of oxygen by a gaseous reducing reagent is replaced by the more effective electrochemical removal. Additional reducing reagents such as hydrocarbons, CO, H2 or ammonia can lead to the production of secondary pollutants like oxygenated hydrocarbons, CO, CO2, N2O or ammonia or, even, as was often

Without coexisting oxygen the successful decomposition of NO gas into oxygen and nitrogen in a primitive electrochemical cell (Fig.1) was first demonstrated over 25 years ago (Gur & Huggins, 1979; Pancharatnam et al., 1975). In 1975 *Pancharatnam et al.* (Pancharatnam et al., 1975) proposed to use for NO gas decomposition an electrochemical cell represented

 Pt(Cathode)YSZPt(Anode) (1) On applying a voltage to such cells NO gas is directly reduced at the triple-phase boundary *(tpb)* (cathode - yttrium-stabilized zirconia (YSZ) - gas) forming gaseous N2 and solid –phase

+ 2 VO(ZrO2) N2 + 2O2

Under the external voltage the oxygen ions are transported through the solid electrolyte

Unfortunately, excess O2 in the combustion exhaust gas is adsorbed and decomposed at the

+2 VO(ZrO2) 2O2

(YSZ) (2)

(YSZ) (3)

Hamada et al., 1991; Iwamoto, 1990; Libby, 1971; Miura et al., 2001; Sato et al., 1992).

and O2 (Garin, 2001; Lindsay et al., 1998; Miura et al., 2001; Rickardsson et al., 1998).

method for the purification of the exhaust gases from lean burn and diesel engines.

**2. Traditional type of electrochemical cells for NO decomposition** 

reported in the past, cyanate and isocyanate compounds.

2NO + 4e

from cathode to anode and gaseous O2 is evolved at the anode.

by the following cell arrangement

*tpb* in preference to the NO gas (Fig.1):

O2 + 4e

oxygen ions:

NO+H2 1/2 N2 +H2O

As a result, the additional ionic current though the cell associated with the oxygen ions produced due to this unwanted reaction (Eq.(3)) far exceeds the current associated with the desired reaction (Eq.(2)). In 1997 *Hibino et al.* (Hibino et al., 1997) has shown that at first stage the electrochemical oxygen pumping is carried out without NO decomposition, and that NO decomposition began at corresponding currents after the electrochemical oxygen pump is complete. As illustration Fig.2 shows the dependence of NO conversion on the value of the current passing through the two chambers cell at 1000ppm of NO without oxygen (Curve 1) and at 2% of Oxygen (Curve 2) in He (the balance) at gas flow rate 50ml/min. It is seen that in the presence of oxygen the decomposition of NO take place only when all oxygen should be pumped away from the near electrode area.

Recently, many attempts to improve the properties of electrochemical cells operating in the presence of excess oxygen have been carried out by using different catalysts as the cathode material (Hibino, 2000a, 2000b; Marwood & Vayenas, 1997; Nakatani et al., 1996; Walsh & Fedkiw, 1997). *Walsh* (Walsh & Fedkiw, 1997) proposed substitute dense Pt electrodes to the porous platinum and to use a mixture of ionic (CeO) and electronic (Pt) conductors as a porous cathode. It is well known that substitution of dense electrode to the porous should increase gas penetration to the *tpb* on the surface of the YSZ-disc solid electrolyte and using of the mixture of ionic (CeO) and electronic (Pt) conductors should lead to the increase of the *tpb* surface area inside the cathode. As the result both oxygen and nitrogen oxide decomposition take place in such cells and for effective NO adsorption and decomposition the *tpb* should be free from the adsorbed oxygen. This conclusion agrees well with a fact that the NO decomposes after the oxygen pumping is completed.

Fig. 1. Conceptual representation of the electrochemical cell for NO decomposition.

To improve the selectivity for NO gas adsorption and decomposition in the presence of the oxygen excess *K. Iwayama* (Iwayama & Wang, 1998; Washman et al., 2000) proposed to coat Pt cathode by different metals or metal oxide. Decomposition activity was measured on metal oxide/Pd(cathode)/YSZ/Pd(anode) at 773–973 K and 3.0V of applied voltage in a flow of 50 ml/min containing 1000 ppm of NO and 6% of O2 in helium. Coating of various metal oxides onto the cathode electrode greatly changed the decomposition activity; the order was RuO2>>Pt>Rh2O3>Ni>none>Ag>WO3. The activity of the system modified by RuO2 has been investigated as a function of the kind of electrode, the applied voltage, and

Electrochemical Cells with Multilayer Functional Electrodes for NO Decomposition 183

Fig. 3. Current efficiency vs. current curves of LSC u YSZ u Pt and LSPC u YSZ u Pt

presence of the excess oxygen and can't be used for practical application.

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

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

cathode materials are needed.

electrochemical cells between 600 and 800°C. (-●-,-○-, 800°C), (-▲-,-- 700°C), (-■-,-□- 600°C).

several types of all solid-state electrochemical cells is possible, and proposed, that in order to reduce nitric oxide in an atmosphere containing excess oxygen further development of

In accordance with above we can conclude that all known cathode materials used up now for electrochemical reduction of nitric oxide show a low selectivity for NO reduction in the

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

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

the reaction temperature. The cell of RuO2/Ag(cathode)/YSZ/Pd(anode) was found to show the most excellent activity among the cells examined.

Fig. 2. Dependence of NO conversion on the value of the current passing through the two chambers cell at 1000 ppm of NO without oxygen (-●- Curve 1) and at 2% of Oxygen (-□- Curve 2) in He (the balance) at gas flow rate 50 ml/min.

Later (Hwang et al., 2001; Matsuda et al., 2001; Washman et al., 2000) a series La1-*x*A*x*BO3 perovskite were prepared and systematically evaluated for substitution of the Pt or Pd electrodes. A major target of all these researches was the promotion of NO reduction by F – center type defects in the YSZ surface or inside perovskite type cathodes (Hwang et al., 2001; Matsuda et al., 2001; Washman et al., 2000).

An important characteristic of the efficiency of electrochemical cell is the value of the current efficiency coefficient (). Current efficiency () can be defined from the value of the oxygen ionic current (INO) due to the oxygen from decomposed NO gas (see eq.2) and a total ionic current flux through the cell (I = INO + IO2) as:

$$\mathbf{I}\eta = \mathbf{I}\_{\text{NO}} / \text{ (I}\_{\text{NO}} + \text{I}\_{\text{O2}}) \tag{4}$$

As illustration current efficiency for NO decomposition against current is plotted in Fig.3 for LSC|YSZ|Pt and LSCP|YSZ|Pt cells (Hwang et al., 2001). It is seen that relatively high value of current efficiency, *ca.* 1.5% can be obtained between 300 and 350 mA. This result shows that the unwanted reaction (Eq. (3)) of oxygen gas adsorption and decomposition fare exceed the desirable reaction of NO gas adsorption and decomposition.

In 2007 *Simonsen et al.* (Simonsen et al., 2007) studied spinels with composition CoFe2O4, NiFe2O4, CuFe2O4, and Co3O4 as electro-catalyst for the electrochemical reduction of nitric oxide in the presence of oxygen. It was shown that spinels are active for the reduction of both nitric oxide and oxygen. The composition CuFe2O4 shows the highest activity for the reduction of nitric oxide relative to the reduction of oxygen. However now information was given on the characteristics of the electrochemical cells based on these cathode materials.

*K. Kammer (*Kammer, 2005) reviewed the investigations in the field of electrochemical reduction of nitric oxide. He has shown that the electrochemical reduction of nitric oxide in

the reaction temperature. The cell of RuO2/Ag(cathode)/YSZ/Pd(anode) was found to

0 50 100 150 200 250 300

Current (mA)

Fig. 2. Dependence of NO conversion on the value of the current passing through the two chambers cell at 1000 ppm of NO without oxygen (-●- Curve 1) and at 2% of Oxygen (-□-

Later (Hwang et al., 2001; Matsuda et al., 2001; Washman et al., 2000) a series La1-*x*A*x*BO3 perovskite were prepared and systematically evaluated for substitution of the Pt or Pd electrodes. A major target of all these researches was the promotion of NO reduction by F – center type defects in the YSZ surface or inside perovskite type cathodes (Hwang et al., 2001;

An important characteristic of the efficiency of electrochemical cell is the value of the current efficiency coefficient (). Current efficiency () can be defined from the value of the oxygen ionic current (INO) due to the oxygen from decomposed NO gas (see eq.2) and a total ionic

As illustration current efficiency for NO decomposition against current is plotted in Fig.3 for LSC|YSZ|Pt and LSCP|YSZ|Pt cells (Hwang et al., 2001). It is seen that relatively high value of current efficiency, *ca.* 1.5% can be obtained between 300 and 350 mA. This result shows that the unwanted reaction (Eq. (3)) of oxygen gas adsorption and decomposition fare

In 2007 *Simonsen et al.* (Simonsen et al., 2007) studied spinels with composition CoFe2O4, NiFe2O4, CuFe2O4, and Co3O4 as electro-catalyst for the electrochemical reduction of nitric oxide in the presence of oxygen. It was shown that spinels are active for the reduction of both nitric oxide and oxygen. The composition CuFe2O4 shows the highest activity for the reduction of nitric oxide relative to the reduction of oxygen. However now information was given on the characteristics of the electrochemical cells based on these cathode materials.

*K. Kammer (*Kammer, 2005) reviewed the investigations in the field of electrochemical reduction of nitric oxide. He has shown that the electrochemical reduction of nitric oxide in

exceed the desirable reaction of NO gas adsorption and decomposition.

= INO/ (INO + IO2) (4)

show the most excellent activity among the cells examined.

NOx Conversion (%)

Curve 2) in He (the balance) at gas flow rate 50 ml/min.

Matsuda et al., 2001; Washman et al., 2000).

current flux through the cell (I = INO + IO2) as:

Fig. 3. Current efficiency vs. current curves of LSC u YSZ u Pt and LSPC u YSZ u Pt electrochemical cells between 600 and 800°C. (-●-,-○-, 800°C), (-▲-,-- 700°C), (-■-,-□- 600°C).

several types of all solid-state electrochemical cells is possible, and proposed, that in order to reduce nitric oxide in an atmosphere containing excess oxygen further development of cathode materials are needed.

In accordance with above we can conclude that all known cathode materials used up now for electrochemical reduction of nitric oxide show a low selectivity for NO reduction in the presence of the excess oxygen and can't be used for practical application.
