J-P Viricelle

P Vernoux

J Gao

I Romanytsia

P Breuil

C Pijolat

NO 2 -selective electrochemical sensors for Diesel exhausts

Keywords: NOx sensor, exhaust, YSZ, mixed potential, polarization, overpotential

Electrochemical NOx sensors based on yttria-stabilized zirconia (YSZ) are commercialized to detect overall NOx concentrations in Diesel exhausts without distinction between NO and NO 2 . To overcome this inconvenient, a high-sensitive sensor based on selective electrochemical reduction of NO 2 is proposed. An electrochemical cell consisting of three metallic electrodes on solid-state electrolyte (YSZ) operating under electric polarization between a gold working electrode and a platinum counter electrode allows selective NO 2 detection at 400-550°C with high sensitivity, and without any significant interferences to other gases like hydrocarbons, carbon monoxide and also ammonia and nitrogen monoxide. A mechanism based on cathodic overpotential is proposed.

Introduction

The interest of reliable electrochemical gas sensors for automotive exhausts application to on-board control nitrogen oxides emissions is growing with the strengthening of international legislations. Although commercial sensors still exist and despites numerous studies on mixed potential YSZ based devices, there are few solutions to distinguish NO and NO 2 [START_REF] Fischer | Method for detection of NOx in exhaust gases by pulsed discharge measurements using standard zirconia-based lambda sensors[END_REF][START_REF] Wang | Sensing Exhaust NO2 Emissions Using the Mixed Potential Principle[END_REF]. However, NO 2 is much more harmful than NO because at the origin of acidic rains and acting as a precursor of ozone formation. In addition, studies on air quality in European urban zones have evidenced that the ratio NO 2 /NOx slightly increases year after year mainly due to Diesel Oxidation Catalysts which equipped Diesel cars. Hence, the objective is to develop a NO 2 -selective electrochemical sensor for Diesel exhausts.

Experimental

Elaboration of 3 electrodes YSZ based sensor and gas response measurement

An electrochemical planar sensor composed of an YSZ porous layer and three metallic electrodes was prepared by screen-printing on an alumina substrate equipped with a platinum heater on reverse side (Fig. 1). Low currents (10/100 nA) were applied between the working gold electrode (WE) negatively polarized, and the platinum counter electrode (CE) to induce electrochemical reductions at the Au/YSZ interface. The sensor response (ΔV) was measured as the potential difference between the reference platinum electrode (RE) and WE. NOx sensitivity and selectivity of this sensor were investigated in a test bench in the temperature range 400-550°C for atmospheres containing O 2 (1-20 vol.%), H 2 O (2 vol.%) and various NO and NO 2 concentrations (25-300 ppm).

Electrochemical measurement and overpotential calculation

In galvanostatic mode, the measured signal, ∆V, which is the sensor response, is defined by equation [START_REF] Fischer | Method for detection of NOx in exhaust gases by pulsed discharge measurements using standard zirconia-based lambda sensors[END_REF].

WE RE V V V    (1)
where V RE and V WE are respectively the potentials of the Pt reference and Au working electrode, under the applied current (i pol ). Value of ∆V can also be expressed by equation [START_REF] Wang | Sensing Exhaust NO2 Emissions Using the Mixed Potential Principle[END_REF].

WE pol o i R V V       ' (2)
where R' is the electrical resistance of solid electrolyte between RE and WE; i pol is the polarization current between WE and CE, ΔV° is the open-circuit voltage (when i pol =0), η WE is the electrochemical overpotential of the working electrode.

The overpotential of the reference electrode η RE is negligible as no current is issued from the reference electrode.

V and V o can be measured respectively at fixed i pol and i pol =0. Electrolyte resistance R' can also be determined experimentally from impedance spectroscopy measurement. Therefore, the overpotential of the working electrode can be calculated, for a fixed temperature and gas conditions, directly from equation [START_REF] Miura | Selective detection of NO by using an amperometric sensor based on stabilized zirconia and oxide electrode[END_REF].

pol o WE i R V V '       (3)
In a same way, it is possible to consider platinum counter electrode CE versus reference electrode RE. The polarisation voltageV pol applied to fix the desired current can be expressed by equation [START_REF] Sekhar | Application of commercial automotive sensor manufacturing methods for NOx/NH3 mixed potential sensors for on-board emissions control[END_REF].

CE WE pol o WE CE pol Ri V V V V           (4)
where R is the electrical resistance of solid electrolyte between CE and WE; i pol is the polarization current between WE and CE, ΔV° is the open-circuit voltage (when i pol =0), η WE and η CE are respectively the electrochemical overpotential of the working and counter electrodes.

The experimental measurement of ΔV, ΔV° and R allows to calculate the sum η WE + abs(η CE ) from equation ( 4), and finally combining with equation 3 to determine both η WE and η CE . Their variations were recorded upon various applied currents in a wide range of oxygen partial pressure, in addition with injections of NO 2 (10 to 100 ppm).

Results and discussions

Gas sensor responses

At temperatures lower than 500°C, the sensor was found to be highly sensitive and selective to NO 2 . For instance, at 450°C, for an applied current of 100 nA, ΔV value drastically decreases from 2.1 V in O 2 /H 2 O atmosphere down to 1.7 V when 100 ppm of NO 2 is introduced (Fig. 2). For low NO 2 concentrations (< 100 ppm), the sensitivity of the sensor is around 5 mV/ ppm of NO 2 . Furthermore, when reducing gases are introduced, such as NO (200 ppm), CO (300 ppm), H 2 (300 ppm), hydrocarbons (300 ppm of a mixture 25 vol.% methane, 25 vol.% butane, 50 vol.% propene) or NH 3 (50 ppm), the sensor signal is practically not modified (Fig. 3). Hence the sensor is selective to NO 2 .

Detection mechanism

It is known that the modification of sensitivity and selectivity can be enhanced by the polarization of the sensing electrode [START_REF] Miura | Selective detection of NO by using an amperometric sensor based on stabilized zirconia and oxide electrode[END_REF][START_REF] Sekhar | Application of commercial automotive sensor manufacturing methods for NOx/NH3 mixed potential sensors for on-board emissions control[END_REF]. If the electrode is polarized negatively as a cathode like WE gold electrode here, only reducible gas can modify its potential and then can be detected. The obtained experimental result proves that in the used temperature and polarization conditions, only O 2 and NO 2 can be electrochemically reduced on the cathodic polarized gold working electrode (equations 5, 6).

(5) (6) Fig. 2. Sensor response (V /mV) at 450°C to 100ppm NO2/NO (O2 12vol.%, H2O 2vol.%) at different polarizations (i /nA). Fig. 3. Relative sensor response at 450°C to 300 ppm CO, 300 ppm H2, 200 ppm NO, 300 ppm HC (25 % methane, 25 % butane, 50 % propene), 50 ppm NH3 at ipol=100 nA (O2 12vol.%, H2O 2vol.%).

x o O O NO e V NO       2 2 x O O O e V O       2 2 1 2
where V O °° is an oxygen vacancy and O O x is a lattice position of O 2-ion in YSZ. Calculations of Au-WE and Pt-CE overpotential during NO 2 (Fig. 4) and NO, CO (not shown here) injections confirms that the sensor response is mainly linked to the cathodic overpotential at the Au (WE)/ YSZ interface which is drastically reduced by the presence of NO 2 . Pt-CE overpotential (absolute value) is quite lower than Au-WE one and do not vary significantly under any gases, neither NO 2 nor reducing gases.

The influence of oxygen concentration on Au-WE overpotential was also investigated (Fig. 5). The overpotential linearly depends on the logarithm of the NO 2 partial pressure. The slopes are respectively 151, 129 and 129 mV/decade for 5.7%, 9.7% and 13.7% O 2 . These values are more than two times higher than those measured for the similar sensor in potentiometric sensor (no polarization): 52 mV/decade at 450°C [START_REF] Romanytsia | Application of advanced morphology Au-X (X = YSZ, ZrO2) composites as sensing electrode for solid state mixed-potential exhaust NOx sensor[END_REF], or 35 mV/decade at 500 °C [START_REF] Gao | Improvement of the NOx selectivity for a planar YSZ sensor[END_REF]. They are also higher than the ones reported (112 mV/decade) for a polarized device based on lambda probe [START_REF] Fischer | Method for detection of NOx in exhaust gases by pulsed discharge measurements using standard zirconia-based lambda sensors[END_REF]. Such high sensitivity to NO 2 is related to the effect of polarization. In potentiometric mode, the mixed potential of the electrodes [START_REF] Miura | Selective detection of NO by using an amperometric sensor based on stabilized zirconia and oxide electrode[END_REF][START_REF] Sekhar | Application of commercial automotive sensor manufacturing methods for NOx/NH3 mixed potential sensors for on-board emissions control[END_REF][START_REF] Romanytsia | Application of advanced morphology Au-X (X = YSZ, ZrO2) composites as sensing electrode for solid state mixed-potential exhaust NOx sensor[END_REF][START_REF] Gao | Improvement of the NOx selectivity for a planar YSZ sensor[END_REF] is mainly linked to the difference in electrocatalytic properties of the two electrodes. In galvanostatic mode, the sensitivity is only driven by the overpotential of the sensing electrode. In case of polarized sensing electrodes, the applied current fixes the rate of the electrode reactions, both the electrochemical reduction of oxygen and NO 2 . As demonstrated by study of the exchange current (not presented here), the rate of NO 2 electrochemical reduction is higher than the one of oxygen; NO 2 can be more easily electrochemically reduced than O 2 . The strong decrease of the overpotential is a consequence of the fast electrochemical reduction of NO 2 and is proportional to the logarithm of its concentration. Moreover, the impact of O 2 on NO 2 sensitivity is quite weak in the studied range. As a conclusion, a high sensitive and selective NO 2 sensor running in a polarized mode was developed, based on the overpotential drop of the Au-WE electrode.

Fig. 1 .

 1 Fig. 1. Schematic representation of sensing (A) and heating (B) sides of YSZ planar sensor

  published by Elsevier Ltd. Volume 168, Pages 7-10, doi: 10.1016/j.proeng.2016.11.112

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Fig. 4 .

 4 Fig. 4. Overpotential of Au-WE and Pt-CE electrodes ( /mV) at 450°C with various polarization (20, 50nA) and NO2 concentrations (10, 100ppm) (O2 12vol.%, H2O 2vol.%). at different polarizations (i /nA).

Fig. 5 .

 5 Fig. 5. Overpotential of Au-WE electrode ( /mV) as a function of NO2 concentration, at 450°C, 25nA, in H2O (2 vol.%), O2 (≈6, 10, 14 vol.%), balanced with N2.