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Solid State Ionics 107 (1998) 213216

NOx detection using the electrolysis of water vapour in a YSZ cell

Part I. NOx detection

a , a b b b*T. Hibino , Y. Kuwahara , T. Otsuka , N. Ishida , T. Oshimaa

National Industrial Research Institute of Nagoya, 1-1, Hirate-cho, Kita-ku, Nagoya 462, Japanb

NGK Spark Plug Co., Ltd., 2808 Iwasaki, Komaki-shi, Aichi 485, Japan

Received 3 October 1997; accepted 4 November 1997

Abstract

The reaction of NO with the hydrogen formed by electrolyzing water vapour in a YSZ cell has been applied to detect NO

with output signal of magnitude in millivolt or milliampere. The experimental apparatus consisted of two YSZ cells,

PtuYSZuPt, which served as an electrolysis cell and an oxygen sensor, respectively. A mixture of 03000 ppm NO and 3%H O in argon was successively fed to the two cells at 800 8C. In the upstream cell, the hydrogen formed by electrolyzing

2

water vapour in the sample gas reacted with NO in the sample gas. In the downstream cell, the electromotive force (EMF)

value was measured using air as a reference gas. The EMF value of the oxygen sensor was used as a sensor signal, when a

current of 6.7 mA was applied to the electrolysis cell. The EMF signal decreased with increasing NO concentration in the

sample gas. Furthermore, the current applied to the electrolysis cell was used as another sensor signal, when the EMF valueof the oxygen sensor was held at 700 mV. The current signal increased with increasing NO concentration in the sample gas.

1998 Elsevier Science B.V. All rights reserved.

Keywords: YSZ cell; Water vapour electrolysis; NOx detection

1. Introduction potentiometric [12] and amperometric ones [13,14].

In the former, the YSZ cells using metal nitrates as

NOx sensors are becoming increasingly important sensing electrodes show the EMF value following

for the improvement of the environmental pollution Nernsts equation. However, the operation tempera-

such as acid rain and photochemical smog and for ture of the sensor is limited up to the melting point

the further introduction of lean-burn and diesel of the nitrate electrodes, many of which are lower

vehicles into the market. So far many kinds of NOx than the highest temperature of the exhaust gases

sensors have been investigated using oxide semi- from lean-burn or diesel engines. In the latter, when

conductors [14], metal phytalocyamine [5,6], solid a constant voltage is applied to the YSZ cells, the

electrolytes [714] and so on. Among these sensors, current based on the electrolysis of NOx is propor-

those using YSZ can be expected to exhibit the best tional to its concentration. However, the current

durability. As for such NOx sensors, there has been signal observed is only several mA per 1000 ppm

NOx.*Corresponding author. Fax: 81 52 911 2422 We have proposed a new NOx sensor in order to

0167-2738/98/$19.00 1998 Elsevier Science B.V. All rights reserved.P I I S 0 1 6 7 - 2 7 3 8 ( 9 7 ) 0 0 5 3 8 - 9

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solve the above problems. When water vapour is the oxygen sensor, where the operation temperature

present in the sample gas, it can be easily elec- was 8008C and air was used as a reference gas. The

trolyzed to hydrogen in the YSZ cell. Since hydrogen EMF value of the oxygen sensor was measured by an

is generally reactive to oxidizing gases, it will react electrometer (Hokuto Denko HE-104). As an addi-

with NOx in the sample gas. This paper deals with tional experiment, the concentrations of NO andthe application of the electrolysis of water vapour to hydrogen in the outlet gas from the electrolysis cell

the detection of NOx. It is shown that this method were analyzed by an NOx analyzer (Shimazu NOA-

makes it possible to detect NOx with output signals 7000) and GC (Shimazu GC 8A), respectively.

in a practical use. Furthermore, it is demonstrated

that this method can be applied to both potentiomet-

ric and amperometric sensors. 3. Results and discussion

Fig. 2 shows the changes in EMF of the oxygen2. Experimental sensor with the current applied to the electrolysis

cell. The EMF value drastically increased at more

The experimental apparatus consisted of an elec- than 6 mA, indicating that water vapour is elec-trolysis cell and an oxygen sensor, as shown in Fig. trolyzed to hydrogen. The EMF value, however, was1. YSZ disks were used as solid electrolytes of the strongly dependent on the concentration of NO in thetwo elements. The YSZ powders were purchased sample gas. At the same current, the EMF valuefrom Tosoh. These powders were pressed at 250 became smaller as the concentration of NO in-

22kg cm and then sintered at 14508C for 10 h in air. creased. In order to elucidate this reason, we mea-The sintered compact was sliced into the form of a sured the gas composition in the outlet gas from thedisk (12 mm diameter and 1.0 mm thickness). Two electrolysis cell. The results are summarized in Fig.platinum electrodes, in the form of a porous film, 3. The formation of hydrogen was depressed in thewere attached to the surfaces of the disk by heating presence of NO. Also, the concentration of NOthe assembly to 9008C for 1 h. decreased at more than 6 mA. These suggest that the

The electrolysis cell was heated to 8008C. A formed hydrogen reacts with NO in the sample gasmixture of 03000 ppm NO and 3% H O in argon2 as follows:

was introduced into the upper gas compartment at a21 2 22

flow-rate of 10 ml min . Direct currents were H O1 2e H 1O , (1)2 2

applied from a galvanostat (Hokuto Denko HA-501)

to the cell via two platinum wires. The outlet gas

from the electrolysis cell was then introduced into

Fig. 2. Plots of EMF of oxygen sensor against current applied to

electrolysis cell: solid electrolyte YSZ; operation temperature

8008C; 03000 ppm NO and 3% H O in argon at a flow-rate of 102

21Fig. 1. Experimental apparatus consisting of two YSZ cells. ml min .

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T. Hibino et al. / Solid State Ionics 107 (1998) 213216 215

Fig. 4. Change in EMF of oxygen sensor with NO concentration

in sample gas: operation temperature 8008C; 0500 ppm NO and21

3% H O in argon at a flow-rate of 10 ml min ; current applied to2

electrolysis cell was kept at 6.7 mA.

sensor signal, when a constant current was applied toFig. 3. Concetrations of NO (a) and H (b) in outlet gas from2

the electrolysis cell. Fig. 4 shows a typical change inelectrolysis cell: experimental conditions are as in Fig. 2.EMF of the oxygen sensor with the concentration of

NO in the sample gas. In this case, the current

H 1NO

H O1 1 /2N . (2) applied to the electrolysis cell was kept at 6.7 mA.2 2 2 The EMF values observed at less than 200 ppm NO

Probably, this reaction will occur on the platinum drastically decreased from 700 to 200 mV, while

electrode of the electrolysis cell. The consumption of those observed at more than 200 ppm NO were held

hydrogen for Eq. (2) will decrease the EMF value of constant at 200 mV. This is due to the fact that the

the oxygen sensor in the presence of NO. As another hydrogen formed by electrolyzing water vapour at

reason, it can be considered that the electrolysis of 6.7 mA is entirely consumed by the reaction with

NO directly occurs at the platinum electrode of the 200 ppm NO in the sample gas. As described in the

electrolysis cell. The electrolysis of water vapour is following part of this paper, when currents of more

inhibited during this process, so that the formation of than 6.7 mA are applied to the electrolysis cell, the

hydrogen is suppressed in the presence of NO. As drastic decreases in EMF are observed at more than

shown in Fig. 3, however, the decrease in NO 200 ppm NO.

concentration began at 6 mA, where the voltage In the secondary manner, the current applied to the

between the two electrodes of the electrolysis cell electrolysis cell was used as another sensor signal,

was 1.2 V. The electrolysis of NO is thermody- when the EMF value of the oxygen sensor was kept

namically possible from 0 V. Thus, this consideration at a constant value. Fig. 5 shows the result keeping

seems to be not suitable for the above reason. the EMF at 700 mV. The applied current almost

We applied the reaction represented by Eq. (2) to linearly increased with NO concentration. This is due

detect NO. Two potentiometric and amperometric to additional application of current to the electrolysis

manners were adopted to observe the sensor signal cell so as to compensate for the amount of hydrogen

for the NO concentration. In the first manner, the consumed by reacting with NO. The present manner

EMF value of the oxygen sensor was used as a can be characterized by the fact that the relationship

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216 T. Hibino et al. / Solid State Ionics 107 (1998) 213216

present method disappear when excess oxygen is

present in the sample gas. In the following part of

this paper, we will propose an approach for such a

disadvantage.

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