Electrical conductivity of Li2SO4Ag2SO4 solid electrolytes

Solid State lonics 18 & I9 (1986) 524-528 North-Holland, Amsterdam

524

ELECTRICAL CONDUCTIVITY OF Li2SO4-Ag2SO 4 SOLID ELECTROLYTES

Q.G. LIU

and

W.L. WORRELL

Department of Materials Science and Engineering, University of Pennsylvania, 3231 Walnut Sreet, Philadelphia, PA 19104

New solid-state electrochemical sensors using LigSOA-Ag2SO 4 electrolytes have been recently developed in our laboratory for measuring SO 2 an~/o~ SO~ in gas mixtures. The electrical conductivities of these solid electrolytes-are reported in this paper. An A. C. impedance technique has been used to measure the electrical conductivities of several Li2SO4-Ag2SO d solid electrolytes. Results indicate that the electrical conductivities of the LIgSO4-A~gSO 4 solid electrolytes are much higher than those for other sulfate electrolytes. The ~arlati~n in conductivity with gas composition has also been investigated. The independence of the electrical conductivity with the SO 2 and SO 3 concentrations indicates that the conduction is ionic. electrolytes using an A. C. impedance technique.

i. INTRODUCTION New solid-state electrochemical sensors using Li2SO4-Ag2SO 4 solid electrolytes have been recently developed (1-3) in our laboratory to measure SO 2 and/or SO 3 in gas mixtures.

The

2. EXPERIMENT Three different compositions in the Li2SO 4Ag2SO 4 system

were investigated:

sensors which consist of a two-phase electrolyte

a) (Li2SO4)0.77-(Ag2SO4)0.23

and a solid Ag-(Ag2SO 4) reference electrode

b) (Li2SO4)0.45- (Ag2S04)0.55

exhibit excellent behavior.

For example, they

have long-term electrochemical stability and

c) pure Ag2SO 4 As shown in the Li2SO4-Ag2SO 4 phase diagr~n

accurate potentiometric responses are obtained

(Figure i), sample (a) is in the two-phase

over a six month period.

region when the temperature is between 515-

The presence of signi-

ficant concentrations of CO 2 and H20 in the gas

560°C, and sample (b) is in a single phase

mixture has no effect on the sensor response.

region when the temperature is above 420°C. Pure

Thus these sensors have potential applications

Ag2SO 4 and Li2SO4.H20 were used to prepare the

as reliable detectors and monitors of SO 2 and/or

electrolyte samples. The Li2SO4.H20 was dried at

SO 3 concentrations in

170°C for 15 hours and dehydration was verified

various gaseous atmos-

using thermogravimetric analysis. The Ag2SO 4 and

phere. The electrical conductivity of solid electro-

Li2SO 4 powders were carefully mixed,

ground in

lytes is an important parameter which influences

a corundum mortar and isostatically pressed in a

the range and types of their applications.

rubber mold at 80,000 psi. The pellets were sin-

Although, the electrical conductivity of some

tered for 50 hours at 540°C except for the pure

Li^SO.-based electrolytes have been z ~ (4-7) measured , there are no reported values of

Ag2SO 4 which was sintered at 620°C.

the electrical conductivity of the Li2SO4-Ag2SO 4

and polished. Gold was then sputtered on the two

system and influence of the SO 2 and SO 3 concen-

base surfaces to insure good electrical contact.

trations in the gas atmosphere.

This paper sum-

From the

sintered pellet a square column was cut, sanded

The conductivity was measured in the experi-

marizes the results of electrical conductivity

mental apparatus shown in Figure 2.

measurements of several Li2SO4-Ag2SO 4 solid

was sandwiched between two gold meshes, and a

0 167-2738/86/$ 03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

The sample

Q. G. Liu, W.L. Worrell / Electrical conductivity o f Li2SO 4 - Ag2SO 4 solid electrolytes

800

L,quid

S

525

A

OLD

MESH

700

L

\

\

aA,2SO,"*c~/J

~

Li)2SO,' .

.

.

.

.

~'.., I

500

/

/

QUARTZ

WOOL

QUARTZ

TUBE

~ AI2OSRODS

400

3o0 LizSO4

GAS IN 20

40

60

80

Ag2S04

MO %

Figure 2

Figure i

Experimental apparatus Of conductivity

Phase diagram of Li2SO4-Ag2SO4 system

measurements spring loading was used to maintain a constant pressure on the gold mashes to ensure the inti-

ties for the three sample compositions are shown

mate electrical contact.

in Figures 3, 4 and 5.

Both the sintering and

conductivity measurements were carried out in a gas mixture of SO 2 (ii0 ppm) and air.

The gas

mixture was passed through a vanadium pentoxide

The variation of Log(6T)

with I/T is linear, and the equations obtained using a least squares analysis are listed in Table i.

catalyst to insure an equilibrium mixture of Table i. Variation of Conductivity with

SO 2, SO 3 and 02 according to reaction (i),

Temperature ................................................

SO 2 (g) + 1/2 02 (g) = SO3(g) log K = 5106 / T - 4.845

(i) Composition

Equations

a

log(6T)=-4247/T+8.263(515-560°C)

The A. C. impedance was measured between i00

a

log(6T)=-4031/T+7.723(420-515°C)

and 999,000 Hz using a Solartron 1174 Frequence

a

Iog(6T)=-3579/T+4o297(<390°C)

b

Iog(6T)=-2165/T+5.567(420-560°C)

Response Analyser interfaced to a Hewlard Parkard 9845 Computer 3. RESULTS The variation of the electrical conductivi-

b

log(6T)=-2573/T+2.818(<390°C)

Alpha-Ag2SO 4

log(6T)=-5060/T+7.492(420-660°C)

Beta-Ag2SO 4

Iog(6T)=-3670/T+3.829(<420°C)

...........................

_ ....................

Q.G. Liu, W.L. Worrell / Electrical conductivity o f Li2SO 4 - Ag2SO 4 solid electrolytes

526

T(°C) 450 400

600 550 500 I

I

I

I

I

I

I

1

:550

t

I

Li ~ SO4 (77mole

I

i

%)-A(:J2 S04(23

IlOppm

LOG (~T)

I

LOG (o-T)

500

SO 2 IN AIR

2--

0

0--

I

150

Li2SO4(45%)_Ag2S04(55%) IlOppm SO 2 IN AIR

2 I

I I I I

, 200 I

I

5

%)

3--

I--

T (°C) 500 I I

600 500 400 I I I I I

;. :1: II +

I

i

i

-I -2 I

-:5 2

-4

5

-5 I

I.I

1.2

I

I

I

1.3 1.4 1.5 I/TxlO 3 (°K-I)

I

I

1.6

1.7

18

I

I

I

I

I

I

1.2

1.4

1.6

1.8

2.0

2.2

I/T Figure 3

2.4

x 1031°K -T}

Figure 4

Variation of conductivity of composition (a)

Variation of conductivity of composition (b)

with tempararure

with temperature

The change in conductivity with the gas com-

three orders of magnitude higher than the value

position has also been investigated at 490°C. As

of Na2SO 4 at 700°C and that of K2SO 4 at 800°C.

shown in Figure 6, the conductivity is constant

The higher the conductivity of the electrolyte,

when the SO 2 concentration of inlet gas varies

the lower is the static interference from stray

from 20 to i0,000 ppm.

electrical fields.

A high ionic diffusion coef-

ficient is also associated with a high ionic 4. DISCUSSION

conductivity.

The conductivities for some alkali metal sulfates are composed in Table 2.

The typical

The independence of electrical conductivity with SO 2 or SO 3 concentrations is a necessory

working temperature is 530°C for the two-phase

but not sufficient evidence of a pure ionic con-

(composition (a)) electrolyte

ductor.

sensor (1'3) ,

700°C for Na^SO. electrolyte sensor (8) , and

However, the emf of the galvanic cells

made by these Li2SO4-Ag2SO 4 electrolytes is

800°C for K2SO4~sensor j (I0) . The conductivity of

identical (3) to that calculated from known ther-

the two-phase electrolyte (1.17 (ohm cm) -I) is

modynamic data.

This indicates that the conduc-

Q. G. Liu, W.L. Worrell / Electrical conductivity o f Li2SO 4 - Ag2SO 4 solid electrolytes I

T (°C)

600 2 |

500

1

I

400

I

I

~

I

527

I

I

I

500 [

I

q

500

Ag2S04

(SO 2) -971

I ]]OPPM

530=C

pprn

(SO2)-971 ppm 530°C

SO2 ;N ~IF~

0

/°

/ -j

° ~ ° °11~ 9.

I

I

-. Pso2=lO00 791°C

I

400

LOG (GT) -2

o7

o ___

o

I

( S O Z) • 19,8 ppm 5 3 0 ° C

> E

-5

L

4

----

500

I

EXPERIMENTAL DATA OF THIS WORK FROM MGAUTHIER (10}

L~ I 0

I r

I 2

I 3

14

I/Tx

I5

I000

1.6

17

1.8

20

19

(°K)-I

/o-~"o._.. °

20(

--i I

Pso2 = IOppm 79]°C ~ o ~

~ o

0

Figure 5

o

o

Variation of conductivity of pure Ag2SO 4 with temperature I0( 0.8

I

0.7 --

I

I I I I0 20 TIME (doys)

I

I

50

I

Figure 7

Li2SO4 (45mole%)- Ag 2 SO 4 (55mole %)

Results of two-phase electrolyte sensor for

06 T= 490%

detection of SO 2 and/or SO 3 concentration

Q5 04 (~cm) 03

Table 2. Comparison of Conductivity __

LizS04 {77mo~e%)-Ag 2S04 (23mole % )

for Some Sulfates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

(2 Li2SO4

02

Composition OI

Conductivity (ohm cm) -I

530°C h I0

I O0

Ag2SO*. ~ = 0 . 0 0 ] (~,cm!-' 1000 SO 2 ( p p m ) IN AIR

J I0000

Figure 6 Variation of conductivity with SO 2 concentration in inlet gas

Alpha-Ag2SO 4

700°C

800 °C

1.78

2.56

0.0193

b

0.88

a

1.17

L L S O . (4)z 4(8 ~

0.5

Na2SO • "

3.1X10 - 4

2 . 1 x 1 0 - 3 5x10 - 3

K2SO4 -9)

0.Tx10 -5

4x10 -4

1.6X10 -3

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

tivity mechanism is essentially ionic and that the electronic conduction is negligible in these electrolytes. A distinct advantage of a two-phase electrolyte sensor is its long-term chemical stability. Figure 1 indicates that a variation of Ag2SO 4

Ag2SO 4 concentration or activity of each phase in the two phase region. Thus, a solid-reference electrode prepared by emdedding silver powder into part of this two phase electrolyte should ensure a stable reference potential for long

concentration from 21 to 35 mol% at 530°C will only change the amount of each phase but not the

time period for cell (A).

Q. G. Liu, W.L. Worrell / Electrical conductivity o f Li2SO 4 - Ag2SO 4 solid electrolytes

528

Ag/(Li2SO4)0.77-(Ag2SO4)0.23/SO2,SO3,O 2

(A)

Results for one of our long-term tests, in which the SO 2 concentration in the gas was changed at the days of 7th and 27th, are shown in Figure 7. The measured potentials are within +3 mv of the calculated values.

For c~nparison, earlier

results of a single phase K2SO 4 electrolyte sensor at 791°C (dotted lines) are also shown in Figure 7. It is evident that the potentials measured by the two-phase sulfate sensor are more stable and reliable.

Even after 6 months, the

2. Q. G. Liu and W. L. Worrell, U. S. Patent Appl. Serial No. 303,320. 3. Q. G. Liu and W. L. Worrell, Electrochemical Sensors Using Li2sO4-Ag?SO a Electrolytes for the Detection of SO? ana/o~ SOq, in: Physical Chemistry of ExtracTive Metall0rgy, edited by V. Kudryk and Y. K. Rao, (Conference Proceedings, Metallurgical Society of AIME 1985) pp. 387-396. 4. A. Kvist and A. Lunden, Z. Naturforschg, 20a (1965) 235. 5. R. T. Johnson, Jr, R. M. Biefeld, Fast Ion Transport in Solid, Proceeding of International Conference, Wiscosin, May, 21-25 (1979), 457.

two-phase sensor potentiometric response are still accurate.

6. A Kvist, Z. Naturforschg, 21a (1966) 1221. 7. A Kvist, Z. Naturforschg, 21a (1966) 1966.

ACKNOWLEDGEMENT Financial support from the NSF Materials Research Laboratory Program (DMR-792367) at the University of Pennsylvania is gratefully acknowledged

REFERENCES i. W. L. Worrell and Q. G. Liu, J. Electroanal. Chem. 168 (1984) 355.

8. K. T. Jacob, D. B. Rao, J. Electrochem. Soc. 126 (1979) 1842. 9. M. Natarajan, E. A. Secco, Can. J. Chem. 53 (1975) 1572. i0. M. Gauthier, A. Chamberland, A. Belanger, M.Poirier, J. Electrochem Soc., 128 (1981) 371.