new auxiliary sensing materials for solid electrolyte no2 sensors

Solid State Ionics 70/71 (1994) 572-577 North-Holland

SOLID STATE lowlcs

New auxiliary sensing materials for solid electrolyte NO2 sensors

Norio Miura, Sheng Yao, Youichi Shimizu ’ and Noboru Yamazoe Department of Materials Science and Technology, Graduate School OfEngineering Sciences, Kyushu University, Kasuga-shr, Fukuoka 816, Japan

Solid-state electrochemical devices were fabricated by combining an Na+ conductor (NASICON) with auxiliary phases of

NaN02-M&O3 (M=Na or Li). These devices showed good sensing characteristics to NO2 in synthetic air. The use of NaNO,-

L&CO, (9: 1 in molar ratio) gave quite excellent sensing properties especially to extremely dilute NOZ. The electromotive force

of the device followed the Nemst equation in the wide NO, concentration range of 0.005 ppm (5 ppb)-200 ppm at 15O”C, its

slope suggesting the one-electron reduction of NOz. The 90% response times of the sensor to 10 ppm and 5 ppb were ca. 8 s and

ca. 3 min, respectively. In addition, the device was found to be utterly insensitive to CO2 up to 40 vol%.

1. Introduction

Nitrogen oxides (NO,: NO and NOz) are known to cause air pollution problems such as acid rain and photochemical smog as well as to give damages to human nerves and respiratory organs. NO, is mainly emitted from automobiles, stationary combustion facilities and homes. Measurements of NO, concentration have so far been done by means of spectro- scopic instruments based on chemical luminescence or infrared absorption, but such instruments cannot lit well to on-site feedback control systems because of time-consuming analytical procedures, bulky sizes and high costs. Therefore there has been a great de- mand for compact, low-priced solid-state sensors which can monitor in real time NO, concentrations in combustion exhausts or environment in urban area.

Recently many works have been reported on solid- state NO, sensors using various materials, e.g., sem- iconductive oxides [ l-3 1, organic semiconductors [ 4,5 1, and solid electrolytes [6-lo]. However, very few have been able to detect environmental NO2 which is as dilute as several tens ppb or to detect NO which dominates in the NO, of combustion exhausts. Among various types of NO, sensors, solid

’ Present address: Kyushu Institute of Technology, Tobata-ku,

Kitakyushu-shi, Fukuoka 804, Japan

electrolyte NO2 sensors, especially those attached with an auxiliary phase (gas-sensing material ), have appeared to be very promising with regard to sensitivity, selectivity and simple device structure. An NO2 sensor of this type was first reported in 1986 [ 71, as a combination of an Ag+-P-alumina with an auxiliary phase of AgNOJ. Thereafter there have been several reports on NO, sensors using Na+ conductors or Ag+ conductors [ 8-101.

We have been investigating NASICON (Na+ super ionic conductor)-based sensors for NO and NO2 [ 11-l 71. It has been found that the use of NaN02 auxiliary phase brings about far better sensing characteristics to NO2 than that of the conventional NaN03, in addition to making the sensor device to be sensitive to NO [ 13-l 51. It seems that selection and design of the auxiliary materials are decisively important for the sensors of this type. In this study, we tried to improve further the NO2 sensing properties of the NASICON-based device by using a binary system of NaN02-M2C03 (M=Na or Li) in- stead of pure NaNO, or NaNO,. It turned out that the optimized device could detect as dilute as 5 ppb N02, without being disturbed by CO, as rich as 40 vol%, as described below.

0167-2738/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved.

N. Miura et al./New auxiliary sensing materials 573

2.Experimental 3. Results and discussion

The structure and fabrication method of sensor devices were the same as those already reported else- where [ 15,16 1. NASICON ( Na3Zr2Si2P0i2) was synthesized by calcining a stoichiometric mixture of ZrSi04 and Na3P04 at 1200°C. A sintered NASI- CON disc, 8 mm in diameter and 0.5 mm thick, was attached on one end of a quartz glass tube with an inorganic adhesive, as shown in fig. 1. The outside surface of the disc was covered with a layer (ca. 0.1 mm thick) of either pure NaNO, or a nitrite-carbonate mixture of NaN02-M2C03 (M = Li or Na) by a melting-quenching method, on top of which an Au mesh ( 100 mesh) was placed as the sensing electrode. The counter electrode (Pt black and Pt mesh) was attached onto the inside surface of the NASI- CON disc and connected to an Au lead wire.

NO* sensing experiments were carried out in a conventional flow apparatus equipped with a heat- ing facility under atmospheric pressure. Sample gases containing various concentrations of NOz at a fixed oxygen concentration (21 ~01%) or in N2 balance were allowed to flow over the sensing electrode at a flow rate of 100 cm3/min, while the counter electrode was always open to static atmospheric air. In some cases, the synthetic air containing various concentration of CO2 or water vapor was used as a sample gas. In the experiments related to sensing mechanism, oxygen concentration was also varied with N2 balance in the presence of N02. The electromotive force (EMF) of the device was measured with a dig- ital electrometer (Advantest, TR 8652) mainly at 150°C.

D

Fig. 1. Structure of NO* sensor device based on NASICON. A:

NASICON disc, B: NaNO*-M2COI(M=Na, Li) or NaN02, C: Au-mesh, D: Au-wire, E: Inorganic adhesive, F: Pt-black, G: Pt-

mesh, H: Quartz glass tube.

3.1. NaN02-Na2C03 system

As we reported previously, the electrochemical cell fitted with NaNO, as an auxiliary phase was more sensitive to dilute NOz in air than one fitted with NaN03 [ 13 1. In the present study it was found that the sensing characteristics to dilute NO2 could be improved further with NaN02-Na2C03. Fig. 2 com- pares the typical response transients of the devices fitted with pure NaNO* or NaN02-Na2C03 (3 : 1 in molar ratio). The device using NaNO* could re- spond rather sharply to NO1 diluted with air at 150’ C when NO* concentration was 0.2 ppm and above.

On the other hand, the device using NaN02-Na2C03 showed a sharp response even to 0.05 ppm NOz, with the times for 90% response and 90% recovery being

ca. 40 s and ca. 5 min, respectively. With increasing NO2 concentration, the response became sharper and the 90% response times to 2 ppm NO1 and above were as short as 8 s.

Fig. 3 shows the dependence of the EMF of the same devices on NOz concentration at 150°C. In a wide range of NOz concentrations, the EMF was linear to the logarithm of NOz concentration for both devices, with the Nernst’s slopes close to that cor- responding to n= 1, where n is the number of elec- trons involved in the electrode reaction per NO* molecule. The linearity was kept down to the lowest NO, concentration examined (0.05 ppm) for the device using NaN02-Na2C03, while deviation from the linearity became significant at 0.1 ppm NOz for the device using NaN02. The response and recovery rates of the modified device increased with increasing op-

Fig. 2. Response transients to NOZ at 150°C of the sensor de-

vices. Auxiliary phase: (a) NaNO,, (b) NaNO,-Na,CO, (3 : 1 in

molar ratio).

N. Mura et al./New auxiliary sensing materials

100 -

> o- E

i z s

-100

-200 I .c.....: 0.0 I 0. I I IO 100 IO00

NO2 concentration i ppm

Fig. 3. Dependence of EMF on NO2 concentration at 150°C.

Auxiliary phase: (a) NaNO*, (b) NaNOZ-Na&O, (3 : 1). NOz

in synthetic air, (b’ ) NaNO*-Na*CO, (3: 1 ), NO1 in nitrogen.

-200

s -30c)

ix. E w

-400

-400

(IXO’C) /

0.01 0.1 I IO I 00 IO00

NO2 concentration / ppm

1

Fig. 4. NO2 sensing characteristics of the device fitted with

NaN02-Na2C03 (3 : 1) at 180°C. (a) Response transients to very

dilute NO*; (b) Dependence of EMF on NO2 concentration.

eration temperature. At 180” C, for example, the 90% response and 90% recovery times to 0.05-20 ppm NO* were ca. 8 s and ca. 2 min, respectively, as shown by response transients in fig. 4. Again the EMF of the device was perfectly linear to the logarithm of NOz concentration over the whole range examined (0.0% 200 ppm) at this temperature, with the Nernst’s slope agreeing with n = 1 .O.

3.2. NOz sensing mechanism

All the above experiments were carried out at a constant oxygen concentration of 21 ~01%. The de-

pendence of the EMF on oxygen concentration of the devices was examined under a fixed NOz concentration. As shown in fig. 5, the EMF values of the device fitted with pure NaNO,( a) or NaN02- Na,C03( b) to 20 and 4 ppm NO*, respectively, were totally independent of oxygen concentration over the whole tested range. It was confirmed by another ex- periment that the EMF versus NOz concentration correlation of both devices remained unchanged no matter how oxygen was present or not, as shown by b and b’ in fig. 3. The same conclusion was also de- rived for the device using pure NaNO, or pure NaNO,.

It has been assumed so far that the device fitted with NaNO, auxiliary phase undergoes the following reaction at the sensing electrode.

Na++NOz+ ( f)Oz +e-=NaNO,. (1)

If this is correct, the EMF sensor should be suscep- tible to a change in oxygen concentration under a fixed concentration of NOz. However, the oxygen- independent property mentioned above indicates that the NO, sensing mechanism for a device fitted with NaNOz is totally different from what has been pro- posed even for the device fitted with NaNO,. We tentatively assume that the following sensing electrode reaction should take place on the device fitted with NaNO,-based auxiliary phases.

Nat+NOz +e-=NaNOz (2)

This can explain the observed one-electron reaction

Fig. 5. Influence of oxygen concentration on the EMF to a fixed

concentration of NO,. Auxiliary phase (a) NaNOz (20 ppm NO,.

230°C); (b) NaNO*-Na&O, (3: 1,4ppmNO,, 150°C).

N. Mura et al./New auxiliary sensing materials 575

of NO1 as well as the oxygen-independent characteristics. As for the counter electrode reaction, the following one can be considered as usual.

Na++ (a)O*+e-

= (f )NazO(in NASICON) . (3)

Then, the total reaction for the electrochemical cell will be given by

NOz+(i)Na,0=NaN0,+($)02. (4)

However, such a sensing mechanism is still open for further investigation.

3.3. NaN02-L&CO3 system

We found that NaN02-Li2C03 system could also improve the NOz sensing properties similarly to the case of NaN02-Na2C03 just mentioned. For example, the EMF of the device fitted with NaN02- L&CO3 (3 : 1 molar ratio) varied logarithmically with NO* concentration in the tested range of 0.05-200 ppm at 150°C with the Nernst’s slope correspond- ing to n= 1.0, as shown in fig. 6. These results indicate that mixing of the nitrite with a carbonate provides an advantageous effect for the NO2 sensing characteristics. By the way, the cross sensitivity to COz or water vapor is important for a practical sensor. This is especially so for the present sensors because introduction of a carbonate may induce the

Hz0 vapor pressure / kPa

0 I 2 3 4

r r ’ I

0.01 0.1 I 10 loo loo0 10000

Gas concentration / ppm

Fig. 6. Dependence of EMF on gas concentration for the device fitted with NaN02-Li2C09 (3 : 1) at 150°C.

sensitivity to COz. In fact, the device using NaN02- Li2C03 (3 : 1) responded to 10 ppm CO* and above at 150 ’ C, giving rise to a Nemst’s correlation ( n = 2 ) , as also shown in fig. 6. The EMF response to C02, however, was far smaller than that to NO1 of the same

concentration, e.g., the EMF to 350 ppm CO2 (typical concentration in environment) was roughly comparable to that to 0.05 ppm NOz. This means that this device will be able to detect higher than 0.05 ppm NO2 in environment. As also shown in fig. 6, EMF was hardly affected by water vapor pressure up to 3.5 kPa.

3.4. Further approach to lower the detection limit

In the above devices, the composition of NaN02-

Li2C03 (or Na&O,) was set to 3: 1 in molar ratio. It has turned out that the cross sensitivity to COz de- pends on the kind and composition of the auxiliary phases. Fig. 7 shows the sensitivity to 8000 ppm CO* in synthetic air for the devices using NaNO,- Li2C03 (a ) and NaN02-Na2C03 (b ) as a function of carbonate content. The sensitivity here was ex- pressed in terms of AE, an increment of the EMF on

0 20 40 60 80 100

NaNOz Li,C@ mol%

LizCO3

200 (b)

$ . loo

4

~ 0 __________(_~_b_as_e)_________

1, 8 8, 1, I 0 20 40 60 80 100

NaNOz NazC@ mol%

Na>CO,

Fig. 7. Dependence of CO2 sensitivity (AI?) on carbonate content in binary auxiliary systems (8000 ppm C02, 150°C). Aux-

iliary phase: (a) NaN02-Li2COJ, (b) NaN02-Na2C03.

516 N. Miura et a/./New auxiliary sensing materials

switching from synthetic air to CO,-containing air. In the case of NaNO,-Li,CO, (a), A_!? decreased with decreasing Li,C03 content, being diminished to zero at 10 mol% Li2C03 and below. It is suspected that all the Li2C03 added dissolved into NaN02 phase to form a solid solution or an amorphous phase in the region insensitive to CO*. Similar behavior was also observed in the NaNO,-Na,CO, system (b), in which the CO1 sensitivity was completely lost at 1 mol% Na2C03. These results indicate that even if a carbonate is introduced in the auxiliary phase, the cross-sensitivity to CO* can be eliminated completely by choosing a proper composition. It is em- phasized that, even when AE became zero at a low carbonate content, EMF to air (air level) or to NO* in air (signal output ) could be different significantly from those of the device fitted with pure NaNO,, as

described shortly. Fig. 8 shows the EMF-NO2 concentration corre-

lation (a) of the device using NaNO,-Li2C03 (9 : 1 in molar ratio) at 150°C. A linear correlation (n= 1.0) can hold in the wide NOz concentration range of 0.005-200 ppm. It is surprising that the device could detect as low as 5 ppb NO*, with a rea- sonable 90% response time of ca. 3 min. It was confirmed that the EMF response was stable after sensor operation for three weeks. Furthermore, the device was insensitive to CO2 up to 40 vol%, as shown by the correlation (c) in fig. 8. It was confirmed as well that the EMF response to NOz was totally unaffected by coexistence of COZ. For comparison, the EMF re-

(IS0 “Cl

Fig. 8. Dependence of EMF on concentrations of NO2 (a) and

(b) or CO* (c) as well as the air levels (a’) and (b’) at 150°C.

(a), (a’) and (c) for the device fitted with NaNO>-Li,C03 (9: 1);

(b) and (b’) for the device fitted with NaNO*.

sponse (b) of the device using NaN02, are again shown in fig. 8. The Nernst-type linearity hold only at NO1 concentrations above 0.3 ppm in this case, as already mentioned. It is noted that the Nernst-type correlation shifted upward from (b) to (a) as NaNO, was replaced by NaNO*-Li2C03, while air levels shifted downward from (b’) to (a’). It is thus evi- dent that the increment of the EMF upon switching from air to NOz-containing air can be made far larger with the replacement of the auxiliary phase. This sit- uation appears to be mainly responsible for lowering the NO* detection limit of the present device using NaN02-Li2C03 to be less than 5 ppb.

In conclusion, the mixing of NaNOz with a proper amount of Na or Li carbonate is quite effective for improving the sensitivity to very dilute NO2 without

inducing the cross sensitivity to CO,. The solid electrolyte device developed appears to be applicable to the direct monitoring of environmental NOz in urban area.

Acknowledgement

This work was partly supported by a Grant-in-Aid for Scientific Research from Ministry of Education, Science and Culture of Japan, and a grant from Iwa- tani Naoji Foundation.

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new auxiliary sensing materials for solid electrolyte no2 sensors

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