Beta″-alumina solid electrolytes for solid state electrochemical CO2 gas sensors

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Solid State Communications, Vol. 76, No. 3, pp. 311-313, 1990. Printed in Great Britain.

0038-i098/9053.00+.00 Pergamon Press plc

BETA"-ALUMINA SOLID ELECTROLYTES FOR SOLID STATE E L E C T R O C H E M I C A L CO2 GAS SENSORS Ion Liu and Wemer Weppner Max-Planck-lnstitut for Festk{kperforschung Heisenbergsm I, 7000 Stuttgart 80, Fed. Rep. Germany (Received 9 August 1990 by M. Cardona) The application of surface modified Na-~"-alumlna solid ionic conductors for the potentiometric detection of CO2 partial pressures at ambient and moderately increased temperatures has been investigated. The galvanic cell voltage is decreased compared to the thermodynamically expected value for the formation of Na2COs. This indicates kinetically more favorable cell reactions that produce metasmble compounds. The redox process involves 4 electrons for each CO2 and 2 electrons for each 02 molecule at the applied temperature (150 °C). The investigations show that Na-~"-alnmina allows to build up potentials which depend on the CO2 partial pressure.

Introduction Gas sensors based on solid state galvanic cells have become increasingly important in practical applications for process control automation and environmental pollution protection. These gas sensors were classified into three types depending on the interaction with the gas [1]: I) the measured gaseous species agree with the mobile component of the electrolyte, II) the measured gaseous species are the immobile component of a binary (or quasibinary) electrolyte, III) other gaseous species interact with the solid ionic conductor by employing auxiliary solid phases at the surface (surface modifications) of the electrolyte. The application of type Ill solid-state galvanic cells has considerably increased the number of gases that may be detected by fast solid ionic conductors and especially allows the detection of gases that consist of several different atomic species. The monitoring of C12, NO2 and 02 partial pressures has been demonstrated in recent years [2,3]. The thin film of the auxiliary solid phase controls the virtual cell reaction in such a way that the activity of the measured gaseous component is related to that of the mobile species in the electrolyte. Thermodynamically, the gas equilibrates with the kinetically active components of the solid electlolyte and the auxiliary phases.The activities are related to each other by Gibbs-Duhem's equation [4]. Kinetic deviations occur frequently which are reproducible and make that system also applicable for gas sensors. The application of Ha I Na-[3"-alumina I Na2CO3/Pt, CO2(g),O2(g) galvanic cells for monitoring CO2 partial pressures has been considered previously [3]. But it turned out to be difficult to observe stable voltages and to fabricate a suitable Ha reference electrode. Other work has used Na2CO3 in combination with a sodium ion conductor (NASICON or Ha-p-alumina) at 52"/ "C by Saito and Maruyama [5]. In an earlier work also K2CO3 and K2CO3Ag2SO4 mixtures have been used as electrolyte in CO2 solid state detectors at 753 "C [6] and 726 "C [7]. in the present work, a stable sodium reference eleclnxie has been prepared by an elecm>cbemical method. Since Na[~";alum.ina is a very good ionic conductor even at or near ambient temperature, the interaction of this electrolyte and Na2COs as a surface modification has been investigated in

view of the possibility to come up with a low temperature CO2 sensor. Such devices are in need for a large variety applications such as air conditioners, green houses, brewieries, wine cellars etc. [8] and also in many cases for chemical process control. CO2 sensors are also very much of interest commercially, especially since competing sensing devices based on other principles are presently not available.

I.

2.

Theoretical Considerations In general, the electrochemical cell reaction includes a charge transfer process at the interface of the electrode and the elecmolyte. This results in a potential E which is related to the Gibbs energy ~ r of the actual electrochemical reaction: AGr = -nqE

(1)

where n and q are the number of electrons involved in the reaction and the elementary charge, respectively. A sensor is obtained for those gaseous species that are included in the galvanic cell reaction. Seiectivity requires that only one type of gaseous species interacts. In reality, several types of gases are often involved in the galvanic cell reaction and each reaction contributes correspondingly to the overall Gibbs energy of reaction. A mixed potential appears and depends on a variety of experimental conditions such as temperature, flow rates and the types and concentrations of the gases. E.g., in the case of the 0 2 sensor, a cross-sensitivity to H2S has been described and discussed in terms of the metastable formation of Ag2S [9]. The following galvanic cell Na I Na+ -~"-alumina I Na2C-"O,~ CO2(g),O2(g) was employed. Thermodynamically, the CO2 in the gas phase (g) at the right hand side is assumed to undergo the energetically most favorable over~l cell reaction I

2Na+CO2 + ~ 02 -+ Na2C'O3

(2)

AGr = -617.5 kJ/mol (150 °C) in the presence of oxygen and the emf E is given according 311

312

Vol. 76, NO. 3

BETA"- ALUMINA SOLID ELECTROLYTES

to eqn. (1) by

3.3,

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(3)

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T = 423K

E = ---~ [G(Na2CO3) - 2G(Na) - G(CO2) - ~G(O2)]

kT kT + ~ In pcoz+ ~ In POz

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E3.2 m

z

or

+~p~

3.1 tiJ

kT + ~ In PO2

(4)

where AG ° is the Gibbs energy of formation from the elements as'listed in standard thermodynamic tables [4]. Na is transferred to the right hand side of the galvanic cell and reacts with the CO2 and 02 gas to form Na2CO3 in-

situ. The thermodynamically most stable compound is Na2CO3 but also Na20, Na202 are possibly formed because of higher kinetic Incferences. IfNa20, Na202 and Na2C02 are formed simultaneously by the cell reaction (required for the voltage measurement), the galvanic cell will show the characteristicsof mixed potentialsas given by the relativecontributions of the Gibbs energies of formation of the sodium carbonate and the oxides. 3. Experimental The experimental arrangement of the galvanic cell is shovm in Fig. 1. The sodium reference electrode was formed electrochemically by coulometric titration. A voltage was appried to the Na-13"-AI:~93pellet which has been brought in contact with liquid sodium at the face which later became exposed to the gas and with a metal foil sealed from the gas environment by epoxy resin (type 700, Kager GmbH, Frankfurt) at the other side. Sodium was coulometrically tiu'ated into the space between the Na-13"-aluminapellet and the metal foil. The unity sodium activity was conf'wmed by measuring the open circuit voltage relative to the liquid sodium which then became removed. Pt paint or evaporated porous Pt films were applied to form the measuring electrode. Two processes were used in fabricating the surface modification of the C02 solid electrochemical sensor. In one type, the thin fllrn auxiliary gas sensitive layer was formed by evaporation. Pt was evaporated simultaneously or subsequenfly as a thin porous layer. In an alternative way the auxiliary phase was formed by applying a voltage between the measuring (gas) electrode and the sodium reference electrode. A small quantity of Na is transferred electrochemically from the reference to the measuring electrode and reacts Porous Pt with Na 2 COa Film Electrode

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Fig. 2. Cell voltage E as a function of the CO2 partial pressure for an oxygen partial pressure of 0.2 atm. Experimental results at 3 times of exposure to the gas are shown. The voltage increases with time and shows a 4electron-process for the reaction with each C02 molecule. For comparison, the thermodynamically expected relation for the formation of Na2CO3 (2-electron-process) is shown. with the COz and 02 gas to form in-situ a thin film gassensitive layer of Na2C02. Tylan type FC-280 and FC-260 mass flow controllers were used to obtain a defined gas mixture of CO2, 02 and At. A Keithley 617 Electrometer with an input impedance of 10t4 ~ was employed for the emf measurements. The temperature was maintained within + 1 *C by a Eurotherm proportional temperature conu'oller using Chromel-Alumel thermecouples. 4. Results and Discussion

The emf of the galvanic cell as a function of the CO2 partial pressure at a given 02 pressure of 0.2 arm at 423K is shown in Fig. 2. The slope of the curve corresponds to a four-elec~on mechanism (n--4), i.e., 4 electrons and 4 sodium ions are required for each C02 molecule to react electrochemically. This is not expected from reaction (2). Also, the experimental emf data are about 0.2 V lower at high CO2 pressures than the calculated values assuming reaction (2). The end" dependence from the 02 partial pressure is presented in Fig. 3. Again, the slope does not correspond to the value expected from eqn. (3), a two-electron mechanism (n=2) is observed which can be explained by the formation of sodium peroxide: 2Na + 02 ~ Na202 AGr = -422.6 kJ/mol (150 *C)

(5)

PI Foil

Fig. 1. Schematic representation of the galvanic cell arrangement. The Na-y-AI203 electrolyte is contacted at one side by a hemetically sealed sodium reference electrode and is exposed to the gas at the opposite side through a gas sensitive Na2CO3 layer.

Considering the stability of Na202 in the presence of 02, it appears possible that both Na2CO3 and Na202 were formed at the surface of tbe measuring electrode side. A drift toward the thermodynamicaily expected higher voltages is observed over long periods of time, see Fig. 4. This phenomon indicates that the reaction at the electrode

BETA"- ALUMINA SOLID ELECTROLYTES

Vol. 76, No. 3

3.06

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(b)

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(c)

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Fig. 4. Equilibration of the cell voltage for 3 different CO2 partial pressures (0.01, 0.1, 0.6 arm).

Fig. 3. Oxygen partial pressure dependence of the cell voltage for 3 different CO2 pressures. The slope indicates a 2-electron process for the electrochemical reaction with 02 molecules.

Na20 + CO2 = Na2CO3 AGr = -255.6 kJ/mol (150 °C)

(7)

Na202 + CO2 = Na2CO3 + ~O2 1 surface is not in equilibrium. After several days the emf of the cell gets close, but not exactly, to the theoretical value. At higher CO2 pressure and lower 02 pressures, Na20 may be formed which will cause a change of the slope of the ¢mf-log PCO2curve according to 2 Na + 1 02 ffiNa~) AGr = -362.4k J/tool (150 °C)

(6)

The experimentally observed voltage may be interpreted in terms of a mixed potential set by the three reactions (2), (5) and (6). The slow drift of the voltage may be explained by the reaction of Na20 or Na202 with CO2 to form predomihandy N a ~ :

AGr = -194.9 kJ/mol (150 °C)

(8)

The potential increases toward the equilibrium value for the formation of Na2CO3. From the curves of Fig. 2, the relative contribution of formation of Na2CO3 and Na202 to the cell voltage is calculated according to eqn. (1). The result shows 77% Na2CO3 and about 23% Na=f)2 formation. The relative changes of the galvanic cell voltage to variations of the CO2 partial pressure were observed to the largest extent within about ten minutes at 150 °C, but a stable value needed about 3 to 4 hours. Acknowledgment

The authors would like to thank Bettina Schoch for her help to prepare the soclium reference electrode.

References:

(1) W. Weppner, Sensors and Actuators 12 (1987)107 (2) G. H6tzel and W. Weppner, Solid State Ionics 18&19 (1986) 1223 (3) G. H6tzel and W. Weppner, Sensors and Actuators 12 (1987) 449 (4) I. Barin and O. Knacke, Thermodynamic P r o j ~ ' e s of Inorganic Substances, Springer Veflag, Berlin, 1973. (5) Y. Saito and T. Maruyama, in: Proceedings of the ~ternafional Seminar on Solid State Ionic Devices, 1823 July 1988, World Publisher, Singapore, p.225.

(6) M. Gauflfier, A. Belanger, D. Fauteux, C. Bale and R. Cote, Chemical Sensors, Pro(:. International Meeting on Chemical Sensors, Japan, Sept.19-22, 1983, p.353. (7) M. Ganthier and A. Chamberland, J. Electrocbem. Soc. 124 (1977) 1579 (8) B.C.Tofield in: Sofid State Gas Sensors (P.T.Moseley and B.C.Tofield, ed.) Adam Hilger, p.198. (9) A. Menne and W. Weppner, Solid State Ionics, in press