Solid electrolyte sensors for gaseous oxides for pollution monitoring

Materials Science and Engineering, A 146 ( 1991 ) 81-89

81

Solid electrolyte sensors for gaseous oxides for pollution monitoring Toshio Maruyama Department of Metallurgical Engineering, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152 (Japan) (Received March 28, 1991 ; in revised form April 19, 1991 )

Abstract Solid electrolyte sensors for CO2, SO x and NO x have been developed on the basis of electrochemical concentration cells. These sensors give the e.m.f, expressed by the Nernst relation. The performance of the sensor depends on the choice of materials for the electrolyte and electrodes. Application of alkali metal salts of carbonate, sulphate and nitrate to the electrolyte is used in primary designs. However, it is difficult to sinter these salts to an appreciable density to prevent permeation of gases. The cationconducting oxide solid electrolytes such as NASICON and fl-Al203 provide superior sensors with auxiliary electrodes of carbonate, sulphate or nitrate salts. The miniaturization of the sensor depends on development of the solid reference electrode.

1. Introduction

The e.m.f. E of the cell is given as

Modern human life has been supported by science and technology and by consuming a huge amount of energy. Fossil fuels are one of the most important sources of energy. The combustion of fossil fuels gives off carbon oxides, sulphur oxides and nitrogen oxides, which pollute the atmosphere. Carbon dioxide is expected to be the reason for global warming. Sulphur oxides give acid rain, and nitrogen oxides which are emitted from the exhaust of automobiles cause air pollution in large cities. The continuous and accurate monitoring of these oxides is the first step in pollution control. Solid electrolyte sensors are suitable not only for pollution monitoring but also for industrial process control because of their capability for high temperature use. Gauthier and Chamberland [1] were the first to point out that alkali salts are applicable as solid electrolytes in sensors for various gaseous oxides such as sulphur oxides, carbon oxides and nitrogen oxides. The sensors were constructed on the basis of the electrochemical cell. The sulphur oxide sensor, for example, was expressed as follows:

E=R~Tln(Pso2111)Po2m)]

et, SO2(I), O2(1), SO3(I)1K2SO41SO2(II), O2(II),

so3(II), pt 0921-5093/91/$3.50

(1)

2F

\ Pso2il)P%i~)/

_RT In [Ps°'(n)P°2(ml- 2/ 2F k Pso,(,,Po~<1/2]

(2)

In the same manner, they constructed a carbon oxide sensor and a nitrogen oxide sensor using K 2 C O 3 and Ba(NO 3)2 respectively. These sensors require a reference gas with a well-defined composition. Following this study, many investigations have been performed not only to improve sensor characteristics but also to develop new sensors with different designs. This paper summarizes sensors which consist of an alkali ion conductor as a solid electrolyte and alkali salts as electrode materials.

2. Carbon dioxide (CO2) sensor Gauthier and Chamberland [1] have shown that the detection of CO 2 is possible using K 2 C O 3 as a solid electrolyte. This sensor requires a reference gas. Maruyama [2] have developed a small tipshaped CO2 sensor by combination of a sodiumion-conducting oxide and N a 2 C O 3 ; this sensor

et al.

© Elsevier Sequoia/Printed in The Netherlands

82

does not require any reference gas. Figure 1 shows a schematic illustration of the sensor. The solid electrolyte is Na3Zr2Si2PO~2, which is a three-dimensional sodium ion conductor called NASICON [3]. Sodium fl-Al203 is a possible alternative, which is a two-dimensional sodium ion conductor. The electrodes are gold and sodium carbonate is superficially applied on one electrode. 2.1. The principle of the tip-shaped C02 sensor The sensor shown in Fig. 1 is expressed as the following cell:

mu, CO2, O21Na2CO3UNASICONIO2,mu

(3)

The possible anode reaction is Na2CO3 ~

2Na + + CO2 + ½02 + 2e-

(4)

and the cathode reaction is 2Na ÷ +½02 + 2e- ~

Na20 (in NASICON)

(5)

The thermodynamic relation at the anode is /~Ya~CO3= 2kiNa+/~CO, + ½ktO=+ 2fie-

(6)

and that at the cathode is 2fiN,. + ½,Uo2+ 2kie- =

(7)

fl-4Na20

In the above equations,/~ and/2 are the chemical potential and the electrochemical potential respectively. At equilibrium, the electrochemical potentials of sodium ions are identical at the two electrodes. The chemical potentials of oxygen at both electrodes are the same because both electrodes are exposed to the same atmosphere. Consequently, the e.m.f, is expressed as follows: E = (/~ °r%c% -/~°N.zo -/~ °c%) 2F x tn(aN.~oPco~P* - l)

Pco2 the partial pressure of CO 2 and P* the atmospheric pressure equal to 1.01 x 105 Pa. In the case when the activity of Na20 is constant at a fixed temperature, the e.m.f, depends only on the partial pressure of CO2. 2.Z Materials and procedure 2.2.1. NASICON The NASICON was fabricated according to Pober's [4] procedure. Sodium orthophosphate dodecahydrate (Na3POa.12H20) and zircon (ZrSiO4) were mixed in the molar ratio of 1 to 2. The mixture was calcined at 1420 K for 173 ks and was ground in an alumina mortar for 3 ks. The resulting powder was pressed at 110 MPa into a pellet with a diameter of 20 mm and was sintered at 1520 K for 43 ks. The density of the sintered body was approximately 98% of the theoretical density. 2.2.2. Sodium "t~"-A120~ Figure 2 presents the phase diagram of the Na20-AI203 system [5]. There are three intermediate compounds of NaAIO2 (7), fl"-m1203 (3Bfl) and fl-Al203 (2Bfl). The phase diagram indicates that two-phase electrolytes of NaAIO2-fl"-A1203, f l ' - A l e O 3 - f l - A l 2 0 3 o r flAI203-AI203 exhibit the fixed activities of Na20. Sodium carbonate and aluminum hydroxide were mixed in various ratios and calcined at 1070 K for 43 ks. This process provides powders of sodium aluminium oxides. The ratio is defined as the molar ratio of Na20 to A1203. The resulting powders were sintered at 1570 K for 43 ks. Figure 3 shows the X-ray diffraction patterns of

RT 2F

I

(8)

where F is the Faraday constant, R the gas constant, T the absolute temperature, aNa2Othe activity of Na20 in the solid electrolyte (NASICON),

2 200 2000

~ A u W~ re 5mm I

l

Fig. 1. Schematic illustration of the tip-shaped CO2 sensor of the cell in eqn. (3)[2].

I

I

I "3' I Liq. * ~ Ai203 ~

I

Liquid

-~'2.B~-

Liq..81:l

/

Liq..2BJ3

aAI203_

1853K

y-

1800

Na2CO3 NASICON

I

~/

81:1"2B13

r

"2BI} -

1 3BI3

5~ 16a3K 61:1.3BI3 1 600 Y

1:1 • 3Bl}

L

2BP

Jl

I z.00

3B!B~ J J

I

50 60 Na20 .AI203

I

I

I

I

70 80 MoI.%

[

90

100 AI203

Fig. 2. Phase diagram of the Na20-AI203 system [5].

83 ct(o), !8(o)13 (v) NaAIO2(*) 0

: A

•

o t

•

I

I • ~ A

I

I

~--[ . . . .Ar . . . ]+ O~ , 1/10 ~ tluz I.,...u~j

o

1• •oho°oo

•

07

Na20/AI203

I

,

1/6

OE

ino

I

CO~+O~ 800K

|•

•

NASICON •e

6

O4

i

>

1/

i

•

" " •

•

Oo

T o o.o

°-o °

'~o.

t~.L/I.,.~_Ao

JLLA__

1/5

11 1/3

L

I

I

I

I

I

I0

20

30

40

50

2010

(Cu Ka)

]

[

__

1

Fig. 3. X-raydiffraction patterns of"fl"-Al203 [2].

I

I

I

2

4

5

Io g ( PcoJPa )

Fig. 4. E.m.f.sof the sensors of the cell in eqn. (3)[2]. the products. The sample with an Na20-to-Al203 ratio of 1 to 10 is composed of a- and fl-A1203. In the samples with Na20-to-Al203 ratios of 1 to 6 and 1 to 5, fl-Al203 is a major phase but a small amount of a-A1203 is observed. Three phases of fl-A}203, fl"-Al203 and NaA102 exist in the sample with an Na20-to-AlzO 3 ratio of 1 to 3. According to the phase diagram shown in Fig. 2, these products are not in equilibrium and are referred to as "fl"-Al203.

2.3. E.m.f. characteristic Figure 4 shows the e.m.f.s of the sensors shown in Fig. 1 at 800 K as a function of the logarithm of the partial pressure of CO2. The e.m.f.s lie on straight lines the slopes of which correspond to 2.3RT/2F, which is predicted by eqn. (8). In the sensors using "fl"-A1203, the e.m.f, decreases with increasing Na20-to-A1203 ratio. On the basis of eqn. (8), this result indicates that the activities of Na20 in the electrolytes remain constant, and that the activity increases with increasing NazOto-A1203 ratio. However, the e.m.f.s deviate from the straight lines at high partial pressures of CO 2 and reach zero in sensors using "fl"-Al203 with the Na20-to-Al203 ratios of 1 to 3 and 1 to 2. From these data, the activity of Na20 in the solid electrolytes can be evaluated with the help of eqn. (8) using the standard free energies of formation for Na2CO3, Na20 and C O 2 [6]. The activities of Na20 are calculated for NASICON and "fl"-A1203 with Na20-to-Al203 ratios of 1 to

i

i

i

! ~-~ -10~-

i

i

""

~"~e,> 0 v.

/

~

CO~

/2~--.~--~~ e C

o

I

800 K

0

1/ 3 "---~'--~'--"~ -- "" ~ " - - v - ~ . ~

o

Na20/AI203

-15 NASICON

I

_20 L 0

I 1

I 2

I 3

I 4

L 5

I o g ( PcoJPa ) Fig. 5. Activities of N a 2 0 in the solid electrolytes [2].

2, 1 to 3 and 1 to 4. Figure 5 presents the activities of Na20 at 800 K as a function of the logarithm of the partial pressure of CO:. In this figure, the boundary indicating the stability region of Na2CO 3 is also shown by the thin line. In the region with the higher Na20 activity than shown by the line, Na2CO3 will be formed on NASICON and "fl"-Al203. The activity of Na20 is constant well below the thin line and an increase in the Na20-to-A1203 ratio increases the activity of Na20 in "fl"-A1203. However, the activity of Na20 lies on the thin line at high partial pressures of CO 2 in " f l " - A l 2 0 3 with higher Na20-to-A1203 ratios. This fact suggests that Na2CO3 is formed at the cathode of the cell in eqn. (3) and that

84

identical electrode reactions occur at the anode and at the cathode, giving the e.m.f, as zero, These results indicate that the use of NASICON is recommended because the activity of Na20 is so low that Na2CO3 is not formed even at a CO2 partial pressure of 105 Pa. From this point of v i e w , " f l " - A 1 2 0 3 with a lower Na20 activity is also suitable. The response of the cell in eqn. (3) at 800 K is shown in Fig. 6 only for two sensors of "fl"-Al203 with Na20-to-Al203 ratios of 1 to 10 and 1 to 5 as an example. When the partial pressure of CO 2 is changed, the e.m.f, varies and reaches a constant value within 300 s. The response is quite rapid because about 300 s is required to change the gas composition in the vicinity of the sensor. The response of cells using NASICON and the other "fl"-Al203 is the same as that of the two examples in Fig. 6. 2.4. Simultaneous monitoring of carbon dioxide and oxygen The industrial processes in which fossil fuels undergo combustion require simultaneous monitoring of CO2 and 02. Maruyama et al. [2] designed a hybrid sensor made of NASICON and yttria-stabilized zirconia (YSZ) as shown in Fig. 7(a). The oxygen sensor made of YSZ is joined to the CO2 sensor made of NASICON. The mixture of Cu2 O and CuO is packed into the YSZ tube as a reference for the oxygen sensor. 0.7

i

i

1

2

3

4

Au, CO2, O2 INa2CO3 IINASICON JlYSZ JO2, Au

(9) In this cell, the anode reaction (4) occurs at interface 1. Sodium ions migrate and reach interface 3 through interface 2. At interface 3, Na + ions react with 0 2 - ions supplied by YSZ and form Na20 in NASICON, i.e. 2Na ÷ + 0 2 - ---'- Na20

(10)

The cathode reaction of ½02 + 2e- ~

O 2-

(11)

takes place at interface 4. The overall reaction is the same as in the cell in eqn. (3) so that the e.m.f. is expressed by eqn. (8). Figure 7(b) shows a cell which is identical with the CO2 sensor. Four sensors were prepared by the same procedure. The e.m.f, characteristics at 800 and 1000 K are shown in Fig. 8. All the e.m.f.s are well expressed by eqn. (8). The activity of Na20 differs slightly in different cells. The part of the system which is the 02 sensor is Pt-Au, O2(CuzO/CuO)IYSZIO2, Pt

(12)

The anode reaction is Cu20 + 0 2 - --'- 2CuO + 2e-

(13)

and the cathode reaction is

i

I_..~_~,

The part of the system which is the CO 2 sensor is expressed as

½02 + 2e- ~

800 K

[ 110 1

02-

(14)

NASICON YSZ GlassSeal

I[

05L'--'~,__ 756x10 ~

I

ZlI66X102 t

04

" , ~ 1.11xlOa ~

•

$

-

Na20/AI203

I

(a)

~, 8.22xi03 ~ * 1/10 0.3

NASICON

~,~ 4.76×104Pa"~"L

1/5

0.2 I

0

5

I

10 t ~ ks

Fig. 6. Response of the cell in eqn. (3) [2].

I

i5

20

(b) Fig. 7. Schematic illustration of (a) the hybrid sensor for CO2 and 0 2 and (b) the cell in eqn. (9) [2].

85 0.8

i

i

j

I

I

tx

I

I

1000 K

o.o : obsd. --: calcd.

[

0.5

0.7

0.6 LU 0.4

>

.....

0.5

0.4 QI

I -12

I -8

0.3

I -4 l o g (Po~ / Pa)

I 0

4

Fig. 9. E.m.f.s of the hybrid s e n s o r as a function of oxygen

partial pressure [2]. 0.2 I

I

I

[

I

1

2

3

4

5

that of C 0 2 , reaction (17) has been found to be predominant at the anode. The hybrid sensor is potentially applicable to monitoring the gases evolved in industrial processes.

[ o g (Pco2/Pa)

Fig. 8. E.m.f.s of the cell in eqn. (9) [2].

The overall reaction is expressed as Cu20

4- ½ 0 2 ~

2CuO

(15) 3. Sensors for sulphur oxides

The e.m.f, of this sensor is given by E - / x °Cu2O- 2~ °CuO t- --R T In ( Po, P* 1) 2F 4F -

(16)

Employing the hybrid sensor, the working limit of the CO2 sensor was examined at low oxygen partial pressures. The sensor was placed in CO2 at 1.01 x 105 Pa, and then some of CO 2 was electrochemically reduced by an oxygen pump of stabilized zirconia. Figure 9 shows the e.m.f.s of the CO 2 sensor at 1000 K as a function of the logarithm of the oxygen partial pressure which was obtained from the hybridized oxygen sensor. The broken line indicates the calculated e.m.f, of the CO 2 sensor. The observed e.m.f, is consistent with the calculated value above the oxygen partial pressure of 10 -4 Pa. However, the e.m.f, deviates markedly from the calculated value below the pressure. The deviation of e.m.f, from the calculated value at low oxygen pressures has been found and this suggests that the mixed potential arises from the following competitive reactions at the anode

[7, 8]: NazCO 3 ~

2Na + + CO 2 + ½02 + 2e

NazCO3 + C O --,- 2Na+ + 2CO2+ 2e -

(4) (17)

Under the very low oxygen pressures at which the partial pressure of CO is comparable with

In the last decade, damage to the forests by acid rain has been spreading rapidly. Air pollution caused by sulphur oxides is a major reason for acid rain. Many investigations have been performed to develop a sensor for monitoring gaseous sulphur oxides.

3. I. Sensors using solid electrolytes of alkali metal sulphates Gauthier and Chamberland [1] demonstrated the applicability of K z S O 4 as a solid electrolyte for a potentiometric sensor for sulphur oxides. Following this study, N a z S O 4 and Li2SO 4 were examined by Jacob and Rao [9] and Liu and Worrell [ 10] respectively. The SO2-O2-SO 3 concentration cell using alkali metal sulphates as the solid electrolytes is expressed as Pt, 802(I), O2(1), 803(1)1M28041802(I1), O2(II), SO3(II), Pt

(18)

The anode reaction is M2SO4 ~

2M + + SO3 + ½02 + 2e-

(19)

and the reverse reaction occurs at the cathode. The overall reaction is given as SO3(II ) + ½02(II) ~

SO3(I ) + ½02(I)

(20)

86

The resulting e.m.f, is

RT In [Ps°~(ll)P°z(u)'/21 I eso,(,)Po2,,) 1/2 ]

E = 2-F

(2)

For 5 0 2 , 0 2 and SO3, the equilibrium reaction is as follows: SO2 +½02 ~

SO3

(21)

The equilibrium constant of the above reaction is K=

Ps%

(22)

Pso2P%1/2

From this relation, one obtains two kinds of expression for the e.m.f, in eqn. (2). Figure 10 is a schematic illustration of the SO2O2-SO3 concentration cell, which is frequently used in e.m.f, measurements. A solid electrolyte is interposed between alumina tubes to separate the anode from the cathode. Gold rings are inserted between the electrolyte and alumina tubes to ensure the gas-tightness of the electrodes. The SO2-O 2 gas mixtures, in which the partial pressure of 502 is Ps%~,, are introduced into the electrodes by the inner alumina tube. Pso~i, is related to the equilibrium pressures of Pso2 and Pso~ at elevated temperatures by Pso2in= Pso~ + Pso~

(23)

When the oxygen partial pressures are equal at the two electrodes, eqn. (2) is simplified:

E = R~T ln ( Ps°2i"m)I 2F

Pt l e a d ~

(24)

~ Ps%in(l)/

I~

Figure 11 shows the e.m.f.s obtained by the cell using a K2504 solid electrolyte [1]. The observed e.m.f.s deviate from the calculated values for a large difference in PS%n. This is probably caused by permeation of gases through the sulphate electrolyte because of the low sinterability of the sulphates [11]. The alkali sulphates consist of a small alkali cation and a large sulphate anion in which sulphur is tetrahedrally coordinated by oxygen. The bonding between sulphur and oxygen is covalent in nature. Therefore the sulphate anion is almost immobile and the sintering rate is determined by the diffusion of the sulphate anions.

3.2. Application of oxide sodium ion conductors

as the solid electrolyte Maruyama et al. [12] applied NASICON as the solid electrolyte. NASICON is easily sintered to an appreciable density to prevent permeation of gases. The e.m.f, is identical with those obtained by the cell using the Na2SO 4 electrolyte as shown in Fig. 12. The X-ray diffraction on the NASICON surface after the e.m.f, measurement proved that Na2SO 4 was formed at the electrodes. The cell is expressed as Pt, 502(I), O2(I), 503(I)[ Na250411NASICON II Na25041502(II), O2(II), SO3(II), Pt

Figure 13 shows the e.m.f, of the cell in eqn. (25) at 1049 K. Pso2i,m/is varied between 6.6 and 660 Pa, and Pso2i,(~)is fixed at 3.2 or 0.17 Pa. The observed e.m.f, agrees with that calculated from eqn. (24). Even for a large difference between PSO2~n at the anode (0.17 Pa) and that at the cathode (660 Pa), no lowering of the e.m.f, is

i

S02 ÷ 02 200 T= 1093 K

tube I00 Pt mesh electrode ~_~

f

tv

°j'°

~Au ring k~j

electrolyte

Exp. ~ . -iO0

Alumina tube

-2 ~k--s02÷O 2

Fig. 10. Schematic illustration of the SO2-O2-SO 3 concentration ceil.

oJ

o/

" ~ / /

/of z~" Calcd. -200

Pt lead. - ~ '

(25)

o//,~~" J _

I 0 log(Pso2in/Pa)

Fig. 11. E.m.f.s obtained by the cell using the solid electrolyte K2SO4 [1].

87 180

I

Calcd,--

]

400

I

,~ :660Pa I I "°',~',~J20

E R/ in81

:TF

160

,6o:

E=7z,oI~) L

300

/

oo0s0 oo8o'o.

in i

2so

T.

oj ° /

1so

2o

-o

616

I

iI'

'

l

/7~I021n=017Pa

I

0

!

,

2

&

Calcd

I ~

~. . . . . . .

I

I

i

I

6

8

10

I t

i

12

1&

t/ks

7-?

950

I

~

[ O NASICON

Obsd, ~ A

900

i

i i

200

i

,4o

i

o

/

Fig. 14. Response of the cell in eqn. (25) [ 12].

1000 1050 1100 1150 TIK

Fig. ] 2. E.m.f.s of the cell in eqn. (l 8) using the electrolytes NASICON and Na2SO4 [ 12].

400 I

R 7"

'PS%2i n

CalcdI E=~-ffln P',so2in

,I

/

I049K

~/

/

300

200 l.u

Selectivity is one of the most important factors in a sensor. As clarified in Fig. 12, the formation of Na2SO4 on the NASICON surface makes it feasible to use NASICON as the electrolyte in the sulphur oxide sensor. If Na2CO 3 or NaNO3 is formed on NASICON when CO2 or NO 2 coexists in the measuring gas mixture, the e.m.f, should be affected by the coexisting CO2 or NO2. Figure 15 shows the e.m.f, at 1049 K when CO2 or NO2 is added to the SO2-O2 mixture. No significant change in the e.m.f, is observed in either case. The sulphur oxide sensor using the NASICON electrolyte shows excellent selectivity. These results are supported by the thermodynamic stability of the sodium salts involved.

o

100

! o// L/I

o / I 10

3.3. Solid reference electrodes f o r S O x sensors

t

[

20

3.0

Iog (Pf%2inI Pa) Fig. 13. E.m.f.s of the cell in eqn. (25)[12].

The SO x sensors mentioned above require reference gases with a fixed composition. This requirement limits the miniaturization of the sensors. The use of solid reference electrodes offers the possibility of miniaturizing the sensor. In a good reference electrode equilibrium is established rapidly, the long-term stability is exhibited and no reaction with the solid electrolyte Occurs.

observed. This result confirms that the NASICON electrolyte is dense enough to prevent permeation of gases, fl-A1203 can be an alternative to NASICON [13]. Figure 14 presents the response of the e.m.f, to the change in Ps%i, for the cell in eqn. (25) at 1049 K. Pso2int~)was fixed at 0.17 Pa and Pso2in(ll ) was varied between 6.6 and 660 Pa. Immediately after the change in Pso~m(m, the e.m.f, alters stepwise and attains the new value within 180 s. The major part of the response time is probably due to the time taken for the gas with the new composition to be swept into the electrode compartment.

Gauthier et al. [14] examined the AgJAg + electrode using the following cell: AglK2SO4(1 mol.°/o Ag2SO4)ISO 2, O2(air), SO3, Pt

(26)

The e.m.f, of the cell (26) is expressed as RT E = C27-

2~

RT

lnaAg2S°'+2F

lnPso2i,

(27)

where C27 is the constant involving the standard free energies of formation for Ag2SO4 and SO3, the oxygen partial pressure of the measuring elec-

88 20C ~--- 502- 0 2 - - - - @ . Ps~ozin: 730 Pa ] Lu 100 . . . . . . .

o' . . . . . . .

- ~ ; ' v - s Z ° ~ - ~. . . . . . . .

1049K 0

Ps]ozin=81 Pa

i

o

separate it from the ambient air. The seal requires a complicated structure and makes it difficult to miniaturize the sensor. A two-phase mixture of metal sulphate and oxide can be utilized as the reference electrode. The reaction between sulphate and oxide is

S02-02-C0z. PsUozin= 730 Pa

--

;

1

~

lb

,2

MSO 4 ~

t/ks

20C --o

o--o

502-02 :~Lu 1013 F~-o2in=3.1Pa

....

o . . . . . . . .

~

-

/

o ....

o

40

o-

P~Ozin=3"lPa PNIO2in = 4.4 Pa

1058 K 0

o . . . . . . . . . . .

502-O2-NO 2

P~o2in=81Pa i ~oo

3;0

,oo

s;0

i ~oo

7;0 8;0 9;0

t lks

Fig. 15. Selectivity of the cell in eqn. (25) against NO: [12].

C O

2

and

trode and the equilibrium constant shown by eqn. (22). The measured e.m.f, was gradually changed, possibly because of the dissolution of silver into K2SO 4 as Ag2SO4 or the formation of Ag20, altering the activity of silver in the electrolyte. Liu and Worrell [10] used a two-phase electrolyte in the Li2SO4-Ag2SO 4 system, in which the activity of Ag2SO4 is basically constant. The e.m.f, was stable over a long duration. Ito et al. [15] proposed the two-phase reference electrode of gold and Au2Na, in which the activity of sodium is fixed. Argon was flowed into the reference electrode to prevent the oxidation of sodium in the electrode. They also examined the feasibility of a two-phase mixture of fl- and fl"-A1203 in air. In this electrode, the activity of sodium remains constant because both the activity of NazO and the oxygen partial pressure are fixed. An alternative cell connected to stabilized zirconia has been proposed in order to use the oxygen in the air as the reference electrode [14]. The cell is Pt, O2(air)l ZrO2( + CaO)ll K2SO41SO2, O2(air), SO 3, Pt (28) and the e.m.f, is RT E = C2q -t- ~ In Pso2i. 2F

(29)

This cell has proved to work well only when the Z r O 2 - K e S O 4 interface is sealed perfectly to

MO + SO 3

(30)

In equilibrium, the partial pressure of SO3 is fixed. The e.m.f, of the SOx sensor also depends on the oxygen partial pressure so that the oxygen pressure must be fixed. Gauthier et al. [14] examined the feasibility of MgSO4-MgO and MnSOa-Mn203 mixtures as the reference electrode; the mixtures are exposed to air through a small hole to fix the oxygen pressure. These electrodes worked well except with respect to the long-term stability. The lack of stability is due to the high equilibrium pressure of SO3, especially for MnSO4, which causes the sulphate to be consumed and shortens the life. Saito et al. [16] have pointed out the use of the three-phase mixture in metal-S-O systems sealed in the electrode compartment; this mixture fixes both the SO3 and the 02 partial pressures simultaneously. They examined the phase relation in the Ni-S-O, M g - S - O and C u - S - O systems and recommended a C u 2 0 - C u O - C u O ' C u S O 4 mixture as the solid reference electrode. 4. Sensors for nitrogen oxides

Solid electrolyte sensors for nitrogen oxide can be designed by the same concept as those for carbon dioxide. Hotzel and Weppner [17] have shown that the sensor for nitrogen dioxide is constructed using the following cell: Ag IAg-fl-Al203 I]AgNO3 ]NO 2, 02, Pt

(31)

The e.m.f, of the cell in eqn. (31) is shown in Fig. 16. In this experiment, the measuring gas was prepared by mixing NO2 and air. The e.m.f, is expressed well by the Nernst relation. 5. Selectivity of sensors

Solid electrolyte sensors for gaseous oxides function on the basis of the electrochemical reaction between the metal salt of the electrode and the gaseous oxide to be detected. The selectivity is determined by the thermodynamic stability of the metal salt in the measuring electrode. In the SO x sensor using NazSO4, for example, the effect

89

various gaseous species can be designed by employing appropriate materials as the electrolyte and the electrodes. The selectivity of the sensor is largely dependent on the stability of the electrode materials in the working conditions.

0.9 !

0.8 -

~

t.~

0.7

References 0.6 0

~

L_._

1

2

~ 3

r 4

log (PNo~/Pa)

Fig. 16. E.m.f.s of the cell in eqn. (31 ) [17].

of C O 2 o n the performance of the sensor is evaluated by considering the following reaction: Na2SO 4 + C O 2 ~

N a 2 C O 3 + SO 3

The Gibbs energy change of this reaction is a measure of the stability of N a 2 S O 4. When the gas composition makes the Gibbs energy change negative, N a 2 S O 4 is converted into N a 2 C O 3 and the sensor acts as a CO 2 sensor. For a CO~ sensor, the reverse situation should be considered. Generally speaking, sulphates are more stable than carbonates, and nitrates are less stable than carbonates. In moderate conditions therefore the e.m.f, of an SO x sensor is not affected by CO2 and NO2 but in operating the CO2 sensor, one must pay attention to the concentration of coexisting SOx in the measuring gas. In the case of an NO~ sensor, both SOy and CO2 should be taken into account. 6. Conclusion Gaseous oxide sensors based on electrochemical concentration cells using solid electrolytes provide the e.m.f, expressed by the Nernst equation and a rapid response. The sensors for

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