Electromotive force of the COCO2O2 concentration cell using Na2CO3 as a solid electrolyte at low oxygen partial pressures

SolidStateIonics23(1987)113-117 North-Holland, Amsterdam

ELECTROMOTIVE FORCE OF THE CO-C02-O2 CONCENTRATION CELL USING Na2C03 AS A SOLID ELECTROLYTE AT LOW OXYGEN PARTIAL PRESSURES Toshio MARUYAMA,

Xu-Yun YE * and Yasutoshi

SAITO

Research Laboratory ofEngineering Materials, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama 22 7, Japan

Received 27 June 1986; accepted for publication 3 October 1986

The electromotive force (EMF) of the CO-CO,-0, concentration cell is examined using Na2C03 as a solid electrolyte at temperatures between 1004 and 1160 K. The electrical conductivity measurements reveal that the solid electrolyte of NazCOJ is stable even in the CO-CO2 atmospheres with low oxygen partial pressures. The electrical conductivity is independent of atmospheres and the activation energy for conduction is 156 kJ mol-‘. The polarization measurement at a fixed current suggests that the anode reaction is NaZC03+C0+2Na+ +2C02+ 2e- in CO-CO2 atmospheres, whereas the reaction is NazC0,+2Na+ + CO1 + 1/202 + 2e- in CO*-O2 atmospheres. The EMF depends on log Pso, P& in CO-CO2 atmospheres, and the response is rapid.

1. Introduction The present authors have reported that a small tipshaped CO1 sensor is feasible by the combination of Na2C03 and NASICON ( NasZrzSizPOlz) [ 11. The sensor is expressed as the following cell: Au, COz, O2 INa, CO3 (INASICON The anode reaction

(02, Au

[Al

is

Na,C0,+2Na++C0,+fOz+2eand the cathode reaction

(1) is

2Na+ + $0, +2e-

-+NazO .

The electromotive as follows:

force (EMFE)

E=E,

.

-~n(0Na20PC02P*-1)

(2) of the cell is given

,

(3)

where &, is the constant involving the standard free energies of formation for Na2C03, NazO and COz, F the Faraday constant, R the gas constant, Tthe absolute temperature, &.&o the activity of NazO in NASICON, Pcozthe partial pressure of CO, and P *

* On leave from the 3rd Design and Research

1nstitute;Ministi-y

of Machine Building, China. 0 167-2738/87/$

(North-Holland

03 SO 0 Elsevier Science Publishers B.V. Physics Publishing Devision)

the atmospheric pressure of 1.O1 x 1O5Pa. The activity of NazO is found to remain constant at a fixed temperature so that EMF depends only on the partial pressure of COz. In the preceeding paper [ 21, an EMF characteristic has been examined in detail for sensors using NASICON and “beta”-alumina with various activities of NazO. The EMF is well expressed by eq. (3) in the wide range of partial pressure of COz, and the response is quite rapid. The higher activity of Na,O gives the lower EMF in accordance with eq. ( 3). Such a high activity of NazO results in the formation of Na2C03 on the solid electrolyte, which gives the EMF to be zero. A hybrid sensor is designed by joining the CO2 sensor and a stabilized zirconia oxygen sensor. The hybrid sensor has revealed that the CO2 sensor functions well at oxygen partial pressures above 1Op4 Pa, and that the EMF deviates from the calculated value below the pressure. The aim of the present study is to clarify the reason of the deviation of EMF at low oxygen partial pressures. The electrode reactions both at the anode and at the cathode should be considered as the possible reasons. The reaction (2) occurs reversibly on p-alumina at oxygen partial pressures over Fe-Fe0 and Ni-NiO coexistences [ 31. Therefore, there may

T. Maruyama et al./EMF of the CO-CO*-O2 concentration

114

be little problem in the cathode reaction (2) on NASICON because NASICON and p-alumina exhibit similar properties except for the conduction path (three-dimensional in NASICON and two-dimensional in P-alumina). For the anode, two possibilities are considered. One is the phase change of Na2C03 and the other is the change in the electrochemical reaction between Na2C03 and the gaseous species. In the present study, the EMF characteristic of the CO-CO*-O2 concentration cell using Na,CO, as the solid electrolyte was examined in the wide range of the gas composition. Furthermore, the electrical conductivity of Na2C03 and the polarization for the electrochemical reaction at Na$O, were measured in various CO-C02-O2 atmospheres.

2. Experimental Fig. 1 shows the schematic illustration of the cell. The sintered NASICON is sandwiched with sinterd Na2C03. The fabrication of NASICON was described elsewhere [4], and NazC09 was sintered at 1050 K for 14.4 ks. The electrodes are attached to Na2COS using gold wires and paste. A mixture of CO* and O2 (1: 1) is flowed into the cathode (lower compartment) as a reference. Mixtures of CO, COZ and 0, with various compositions are introduced into the

anode as:

(upper

cell

compartment).

The cell is expressed

Au, CO’, CO:, 0:

CO”, CO:‘, 0:‘) Au .

PI

The anode reaction is expected to be the reaction (1)) and the reverse reaction occurs at the cathode. The EMF of the cell [B] is E= (RT12F)ln(P~02P~,L’21P,&2P~~‘2)

.

(4)

The oxygen partial pressure at the anode is monitored using an oxygen sensor of yttria-stabilized zirconia with the Fe-Fe0 coexistence as a reference. The electrical conductivity of Na2C03 was measured in various CO-CO*-O2 mixtures by the ac fourterminal technique with 1 kHz. Sodium carbonate was sintered and cut into a parallelepiped of 7 x 6 x 20 mm. Four gold wires were wrapped on the specimen as electrodes. The two outer electrodes were painted with gold paste and sintered in order to assure the electrical contact. In the measurement of the polarization the voltage between a pair of outer and inner electrodes was monitored passing the dc of 10 PA through the parallelepiped of Na2C0,. 3. Results and discussion

)

Fig. 1. Schematic cell.

illustration

co-coz-02

of the CO-CO*-O2

concentration

Fig. 2 presents the typical EMF of the cell [B] at 1004 K. The thin line shows the calculated EMF. The EMF is consitent with the calculated value in the region I. In the region I, the gases introduced into the anode are mixutres of CO, and 02, and thus the oxygen partial pressure is relatively high. However, the EMF deviates markedly from the calculated value in the region II. In this region, the gas in the anode is CO2 which is partly reduced electrochemically by an oxygen pump of stabilized zirconia. The oxygen partial pressure in the region is low, and this region corresponds to the region where the EMF of the tipshaped CO2 sensor deviates from the calculated value [2]. In the region III where the gases in the anode are mixtures of CO and COZ, the EMF lies on a straight line but is not consistent with calculated value. The oxygen partial pressure is extremely low

T. Maruyama et al./EMF of the CO-COZ-0,

concentration cell

0.8

> ;

0.6

0.4

0.2

-4

-2

0

2

4

6

0

log ( Pco,Po~* I Pa3’*1

0

-4

0

-2

2

4

6

0

Fig. 4. Electrical conductivity of Na&O,

log(&,g2/Pa3’*)

Fig. 2. Typical EMF of the cell [B] at 1004 K.

in this region. The EMF’s in the region III are shown in fig. 3 as a function of log( PLO, P&k’*). The shaded line denotes the boundary where carbon deposits from the CO-CO2 mixture. The EMF’s lie on the straight lines of which slopes are steeper than the line expected from eq. (4). /

I

I

t

,

1

,

I

Calcd.

~

0 Cell I

0.8

-4

1

1

-3

’

1

I

I

Iog(/&z~~‘2/Pa

I

I

-1

-2 3/2

1

Fig. 3. EMF in the region III as a function of log(P&, P!$).

Two possible reasons should be taken into account for the deviation of EMF from the calculated value. One possibility depends on the phase stability of Na2C03 at low oxygen partial pressures, that is, at high partial pressures of CO. If Na2C03 changes into the another phase, the anode reaction may be altered, and eq. (4) is no longer valid. Even if Na2C03 is stable, the other possibility may appear, whether the anode reaction is rapid enough to provide the equilibrium EMF. The X-ray diffraction after EMF measurements in the regions II and III has failed to detect any other phase than Na2C03. The electrical conductivity is more sensitive to the phase change. Cersier and Roux [ 51 have reported that the electrical conductivity is sensitively influenced by atmospheres and that the transport number of Na+ ions is reported to be unity in Na2C03. Therefore, the electrical conductivity obtained in the present study corresponds to the ionic conductivity of Na+ ions. Fig. 4 shows the electrical conductivity of Na2C03 as a function of log( PC-* Pg:). The electrical conductivity is independent of the compositions of atmospheres in contrast with that reported by Cersier and Roux [ 5 1. This result indicates that Na2C03 is stable in the entire regions of I, II, and III. Fig. 5 presents the Arrhenius plot of the electrical conductivity. The slope of the line gives the apparent activation energy of 156 kJ mol-‘. Cersier and Roux [ 51 have obtained the

T. Maruyama et al./EMF of the CO-CO,-O2 concentration cell

116

0.9

0.9 5

1.0

T-‘I lO=jK-’

NazC03 +CO’+2Na+

Fig. 5. Arrhenius plot of the electrical conductivity.

activation energy of 92 kJ mol-’ in the corresponding temperature range. Overall polarization consists of resistance, activation and concentration polarizations. The resistance polarization is constant at a fixed current. Therefore, the variation in polarization is due to the change in activation and concentration polarizations, and is a measure of a rate of the electrode reaction. Fig. 6 shows the polarization at 1093 K under various atmospheres. The numbers on abscissa show the atmospheres where the polarization is measured, and correspond to those shown in fig. 4. The polarization is small in the region I. The smaller polarization is due to the higher rate of the electrode reaction. As

100

shown in fig. 2, the EMF in this region agrees well with the calculated value assuming that the reaction (1) occurs in the anode. In the region II where the large deviation is marked between observed and calculated EMF’s, the polarization becomes large. However, the polarization decreases in the region III, and it is clear that the higher partial pressure of CO gives the smaller polarization. This result recommends that one assumes the following anode reaction: +2CO: +2e-

(5)

In the cathode where the mixture of CO* and O2 is introduced, the reverse reaction of (1) 2Na++CO$‘+$0:*+2e-+Na,CO,

(1’)

takes place. In this case, the EMF is expressed as:

where AGP is the standard free energy. of formation for species i. Eq.( 6) indicates that the plot of E against log(P& P!-3) gives a straight line. Fig. 7 shows the EMF in the region III against logU% P&d). At 1160 K, Na2COS is in a molten state. The EMF’s agree fairly well with the calculated value

-

1093K

~00.

a0

60

.

Calcd. Obsd.

1.1

40 20 : b%

0 -20 -40 0.8

-60 -80 -100 0.8 b

Number

Fig. 6. Polarization at 1093 K under various atmospheres.

Fig. 7. EMF in the region III 8s a function of log(P!& P&s).

T. Maruyama et al./EMF of the CO-COz-O, concentrationcell r-

117

4. Conclusion

Sodium carbonate is stable even in CO-CO2 atmospheres. The anode reaction is expressed as: Na2 CO3 + CO+2Na+ + 2C02 + 2ein CO-CO2 atmospheres, whereas the reaction is NazC03+2Na++COz+~02+2ein C02-O2 atmospheres. The EMF depends on log(P&,P&) and the response is rapid. 1093

K

Acknowledgement 0.8 L

0

I 5

I 10

I 15

I

20

2

t 1 ks Fig. 8. Response of the EMF in the region III.

based on eq.( 6) using the standard free energies of formation for CO and CO1 [ 61. Fig. 8 presents the response of the EMF in the region III. Taking into account the time required to change gas composition near the sensor, the response is quite rapid. The sensor using NazC03 gives the EMF which is related to the value of Pi&P& in the CO-CO* atmosphere. On the other hand, the oxygen sensor of stabilized zirconia provides the value of P& PC0 - ‘. The simultaneous use of these sensors makes it possible to determine partial pressures of CO and CO, separately.

X. -Y. Ye is indebted to Kanto Yakin Kogyo Co. Ltd. for providing the opportunity to join the present research.

References [ 1] Y. Saito, T. Maruyama and S. Sasaki, Report of Research Laboratory of Engineering Materials, Tokyo Institute of Technology 9 (1984) 17. [ 21 T. Maruyama, S. Sasaki and Y. Saito, Solid State Ionics 23 (1987) 107, the preceding paper. [3] N.S. Choudhury, J. Electrochem. Sot. 120 (1973) 1663. [4] T. Maruyama, Y. Saito, Y. Matsumoto and Y. Yano, Solid StateIonics 17 (1985) 281. [ 51 P. Cerisier and F. Roux, J. Solid State Chem. 22 (1977) 245. [6] JANAF Thermochemical Tables, NSRDS-NBS 37 (U.S. Dept. Commerce, Washington, D.C., 197 1).