Atmosphere-dependent potentials at oxide interfaces

Sensors and Ar&&m

55

3, f5-16 (1993) 55-62

Atmosphere-dependent potentials at oxide interfaces A. A. Arias de Velasco, P. T. Moseley*, R. Peat and J. G. PelLez AEA Mustriul Technology,Harwelf Laboratory, Didcot, Oxon OX11 QRA(UK)

Abstract Symmetrical potentiometric gas sensors in which a potential difference is generated between identical metal electrodes by a single reaction but involving different activities of the same chemical species at each of the electrodes are well known. Cells of this sort with stabilized zirconia electrolytes are widely used to monitor the oxygen partial pressure in vehicle exhaust systems. When the oxygen partial pressures over the two electrodes of such a cell are equal then there is no potential difference between them. However,at temperatureslower than is normal for zirconia oxygen cells the potentials of electrodes of two different materials on a zirconia electrolyte in a common atmosphere are, in general, not equal. The potential difference arising in such a system is extremely sensitive to the presence of reducing gases (carbon monoxide, hydrogen, methane etc.) in a backwood atm~ph~e of air. Such a system can be used to monitor the presence of reducing gases in air. The measured signal arises principally from a difference in mixed potential between the electrodes but additional contributions to the change in potential resulting from a change in atmospheric ~ompo~tion could arise from surface states and from ~~ovolta~es. The nse of a solid electrolyte cell in which one electrode was held in a reference atmosphere inside a tube with one closed end has enabled contributions to the gas response of the external electrode to be studied in isolation. In particular the apparatus allows us to show that the con~butions from the two electrodes are additive in the potential response of a single gas space device. The atmosphere-dependent modulation of potential in such devices is robust and reproducible, though temperature dependent. Since the mixed potential depends on the electrocatalytic activity of the electrode material there are some prospects for selective response between more and less reactive gases. Thus the variation of response with temperature for methane is quite different from those for carbon monoxide and hydrogen.

Introduction

The interaction between gas molecules and a solid surface has been exploited in a variety of ways in the development of solid state gas sensors; release of heat of combustion leading to a change in resistance, reaction leading to a change in charge carrier density, adsorption leading to a change in frequency of propagation of surface acoustic waves etc. Potentiometric gas sensors, which were the earliest type of solid state gas sensor to become established, function by developing an eJectromotive force which is related in ma~tude to the partial pressure of the gas to be measured. Three broad categories of potentiometric gas sensor have been described [ 11. Type A is defined as a device in which the mobile ion in the solid electrolyte participates directly in an equilibrium with a chemical species that fixes the potential. There are two subdivisions of this category. In the first the electrodes are ‘of the first kind’ in which only one chemical potential of a neutral component is involved. An example is the classical potentiametric oxygen sen-

*Author to whom ~~~nden~ shouM be addressed. Present address: Capteur Sensors and Analysers Ltd., 66 Milton Park, Abingdon, OX14 4RY, UK.

sor in which a solid electrolyte separates two regions of oxygen partial pressures, po2’ and poz2, and the potential across the membrane is given by the Nemst expression

The second sub-category of type A involves electrodes of ‘the second kind’ at which secondary ~u~ib~a relate the electrochemical potential of the species to be measured to that of the mobile ions. An example of this sub-category would be the detection of CO> via an equilibrium with Na,C03 on a sodium ion conductor [2] in the galvanic cell NafNa+ beta alu~na~Na*CO~~ Pt, COz(g), O,(g). In general it is necessary to operate both sub-categories of type A potentiometric sensors at elevated temperature. Type B ~tentiomet~c sensors are chamcte~ed by the use of an electrolytic material in which the mobile ion is not in equilibrium with the species to be sensed. Rather, an ion related to the analyte is accommodated (dissolved) in the structure of the solid eiectrolyte to allow ~u~ibration with the atmosphere. This mechanism has been invoked to account for the successful use of LaF, as an electrolyte in the sensing of oxygen at room temperature [ 3,4].

@ 1993 -

ElsevierSequoia.All rights reserved

56

The third category of potentiometric sensors, type C [I], are those in which more than one reaction is available to take place at the electrodes so that a ‘mixed potential’ is generated. It is this type of sensor that is the subject of the present paper.

C02+Ze--C0. O*-

Mixed potentials Mixed potentials arise at electrodes where at least two reactions are available simultaneously. At open circuit no net current is allowed to flow so that at each electrode the sum of the cathodic currents must be equal to the sum of the anodic currents [5]. This is illustrated for the simple case of one reaction contributing to each component in Fig. 1 where it will be seen that the potential of an electrode sustaining the two reactions shown is uniquely determined by the point at which the magnitudes of the two local currents are equal. An expression for the mixed potential of a single electrode can be derived by an analysis of the reaction kinetics of the processes involved [6]. An example would be the case of an electrode on an oxygen ion conductor exposed to an atmosphere containing oxygen, carbon monoxide and carbon dioxide. If Bo, 8,,, 0co2 and Bv are the fractional surface coverage of atomic oxygen, CO, CO, and vacancies, respectively, then two potential-determining reactions may be considered as follows. (i) The reduction of oxygen Q,+h2++2e-

&O”$& k-1

(21

(ii) The equilibrium of oxygen ions and carbon monoxide with carbon dioxide Oc-, + h2+ -+2e - & 0” + e,, k-2 Reaction

/

A

current f

Fig. 1. Schematic representation of the*formation of a mixed potential at a single electrode. At Emlxedthe net current is zero.

E,(Au)

E .fPt)

ElecTrode Potential

Fig. 2. Schematic representation of the development of an output voltage deriving from the mixed potentials at platinum and gold electrodes,respectively,on an electrolyteexposed to an atmosphere containing oxygen and carbon monoxide. AE = sensor output voltage.

where h*+ and 0” are surface concentrations of doubly charged oxygen ion vacancies and of oxygen ions in the electrolyte, respectively. Reaction rate theory then gives (61 the following expression for, 4, the e.m.f. difference between the electrode and the electrolyte

00+ (W~ho2 t’v+(k_z/k_,)8,

>- 1 h2+ 0”

(4)

Now electrodes exposed to the same two contributing reactions (e.g. by sharing a common atmosphere) will stand at identical potentials, given by eqn. (4), if the electrodes are identical. However, since the mixed potential at each electrode depends on surface coverage of several species and reaction rate constants, the mixed potential on electrodes of different metals will, in general, be different. The difference in electrocatalytic properties will cause the null current position to occur at separate points on the potential axis (Fig. 2). It is clear that, under such circumstances, a change in the concentration of one of the gases contributing to the current balance will alter the potential of null current flow to a different degree on different electrocatalytic surfaces. Mixed potential gas responses have been demonstrated at elevated temperature for devices fabricated on stabilized zirconia [7]. The potential difference between gold and platinum electrodes was found to move rapidly when the composition of the common atmosphere was altered. These results differ from those reported elsewhere [8] for Ptlstabilized zirconia/SnO, devices in respect of their behaviour in the temperature range up to around 400 “C. E.m.f. measurements with a high impedance meter [8] show non-zero potential for such devices for most of the temperature range from ambient to 400 “C and an opposite sign for gas response below 300 “C as compared with that above 300 “C. It was pointed out

that the use of a high impedance measurement via a platinum electrode on stabilized zirconia appears to provide a means for monitoring the density of oxygen ions on the tin dioxide and it is these which are thought to play a crucial role in the function of gas sensitive resistors. However such measurements, in a single gas space, do not eliminate contributions from reactions at the metal electrode nor contributions from thermovoltages arising out of differential heating from the catalysed combustion of reducing gases on some parts of the semiconductor rather than others [9]. The series of experiments described in the present paper was intended to allow the origins of the atmosphere-dependent potentials generated in such systems to be assigned to particular interfaces.

A tube of yttria-stabilized-zirconia was placed coaxially inside a horizontal glass chamber which was, in turn, placed axially in a horizontal tube furnace (not shown). An external electrode was formed by a pressure contact of the closed end of the zirconia tube onto an oxide pellet or a metal foil which was located in the hot zone of the furnace and was contacted via an external lead wire to the potential measurement system. An electrode on the internal surface of the zirconia tube (also near the closed end) was formed by throwing a silver mirror. External and internal gas atmospheres could be supplied separately and gas concentrations controlled, in early experiments using a signal gas blender and later using a pair of Tylan mass flow controllers. Gas switching, temperature control and data recording were as described for the preliminary experiments.

Experimental

Results

In a preliminary experiment a section of yttria stabilized zirconia tube with both ends open was prepared with a silver electrode painted on the inner surface and a platinum electrode painted on the outer surface. Both electrodes were exposed to the same atmospheric composition. The device was placed inside a horizontal silica tube placed axially in a tube furnace. The composition of the atmosphere passing down the silica tube was switched via a system of solenoid valves. The sequence of atmospheric changes was controlled by a BBC ‘B’ microcomputer which also recorded potential readings made via a Keithley electrometer (impedance 1Oi4ohms). The temperature of the system could be varied continuously, at a slow rate, with the aid of a ramp generator. The major series of experiments was performed with an apparatus designed to isolate the two different electrodes in two separate atmospheres, as in a conventional Nernst probe. The apparatus is shown in Fig. 3.

The preliminary experiments, with a zirconia-based device having both electrodes exposed to a single atmosphere, produced results typified by the example shown in Fig. 4, in response to the introduction of 1% of hydrogen or carbon monoxide. The potential in air (shown by a broken line) is near to zero at temperatures above around 500 “C and passes through a minimum value between 200 and 100 “C. At temperatures below 380 “C the introduction of the reducing gas produces a positive-going departure from the air-only line. Above 380 “C the sign of the gas response is reversed and a negative-going shift in potential results. Responses to the introduction of 1% methane were a negative-going shift occurring at temperatures above 550 “C. The temperature-dependent sign of response is comparable with the result obtained with a Pt/YSZ/ SnO, cell in a single gas space [8]. The need to study the behaviour of the two electrodes separately was quite clear.

Internal gas out

electrdyte

tube

Silver metal electrode

Fig. 3. Schematic of apparatus designed to allow assessment of the effect of changing the atmosphere around one electrode (semiconducting oxide) on the potential across an oxygen ion conducting solid electrolyte while maintaining a constant atmosphere at a second (reference) electrode. The apparatus is run in a horizontal tube furnace (not shown) placed so that the electrodes are located in the hot zone.

58

1% hydro/gen

1% hbdrogen lair

-0-81

I

I

I

I

I

I

I

I

I

500

LOO

300

200

100

200

300

LOO

500

Temperature

I air

I°C

Fig. 4. Voltage responses of ( +) F’t/YSZ/Agsensor to pulses of 1% hydrogen in air (15min) alternated with pulses of air (10 min) as the temperature was reduced at a constant rate from near 600 “C to around 200 “C and then back up to 600 “C. The responses to the introduction of the hydrogen were a negative move in potential (away from zero) at temperatures above 380 “C and a positive move (towards zero) at lower temperatures.

potential responses of each of the electrodes, on expo-

shift and those at the platinum appear as a negativegoing shift because a self-consistent system of connec-

sure to a reducing gas, independently. Figure 5 shows that, in isolation, the silver and platinum electrodes only provide responses of a single sign. The responses at the silver electrode always appear as a positive-going

tions to the measurement system was used. However, since the temperature dependences of the responses at the two individual electrodes are different it is clear that the temperature dependence of the sign of the response

shown in Fig. 3 was used to study the

The apparah~s

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l0.2 -

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Air

-

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-0

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&I

(a)

Fig. 5. (a)

I

500

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Temperature

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59 1

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1% hydrogen I air

I

500

I LOO

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300

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200

200

300

Temperature

(b)

I LOO

I 500

I OC

Fig. 5. (a) Voltage responses of an isolated (-) Ag/YSZ electrode (as in Fig. 3) to pulses of 1% hydrogen in air (15 min) alternated with pulses of air (10 mm) as the temperature was reduced from near 600 “C to around 200 “C and back. (b) The equivalent voltage responses of an isolated ( t) F’t/YSZ electrode.

Theoretica

I

slope

60

> E

20 -

k% o0 z s -20 -

-LO -

-60 -

Theoretical

630* C

-80

slope

of the device with both electrodes exposed to the test gas (Fig. 4) can arise from the summation of the contributions from the two electrodes. Although the major interest of the present study relates to measurements made in a background of air a supplementary investigation was made of the oxygen response of the system. At any fixed ratio of oxygen partial pressure between the two gas spaces, within the range used, the potential measured was very stable but, for each electrode, the po, dependence departed from the theoretical value for a four electron reaction when the electrode was exposed to low oxygen partial pressures (Fig. 6). The remainder of the experimental programme was directed to a study of the potential responses at the semiconducting oxide electrode, with the silver electrode held constantly in a reference atmosphere. Figure 7 shows the variation in potential when a tin dioxide electrode was exposed to a series of pulses of 1% hydrogen during a temperature cycle from 600 “C down to 100 “C and back. The locus of the potential for the air-only condition is shown by a broken line. It is clear that, as for the yttria-stabilized zirconia/platinum electrode the gas response is a negative-going move in

-100 i

I

I

2

I 3

log I PO2wmI

I

I

L

5

Fig 6. Variation of the potential generated in the apparatus shown in Fig. 3 as a function of the logarithm of the oxygen partial pressure (a) at the internal, silver electrode (-) and (b) at the external tin dioxide electrode ( +). In each case the counter electrode was held in a reference atmosphere of air.

60

+o 1 0

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P

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zo -0.1

I oil

1 % hydrogen

-05 t - 0.6 t -0

71

I

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I 300

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200

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300

Temperature

I

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OC

Fig 7. Voltage responses of the system shown in Fig. 3 with a constant atmosphere of air around the internal (silver) electrode (-) and an atmosphere of 1% hydrogen in air (15 min) alternating with air (10 min) around the external (tin oxide) electrode (+). The temperature was ramped down from 600 to 100 “C and back in 17 h.

potential over the whole temperature range with a maximum response occurring at around 300 “C. Similar data were obtained for exposures to 1% of carbon monoxide and there was no significant difference between data that was collected with a platinum outer connection to the tin oxide pellet and data that was collected with a silver outer connection. In all cases the response characteristic in the temperature-decreasing

part of the cycle was mirrored well by the temperatureincreasing part and the data were entirely reproducible. When measurements were made with 1% of methane in place of hydrogen on carbon monoxide the temperature of maximum response shifted up to around 400 “C. The response characteristic was evidently dependent on the type of oxide. The temperature of maximum response to hydrogen for an electrode comprising a

to.1

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-0.t

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30

I 500

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I 300

I 200

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Temperature

I Oc

I 300

I LOO

I 500

I 600

Fig. 8. Voltage responses in an experiment with the tin dioxide replaced by a pellet of titanium dioxide. Other conditions are as in the experiment shown in Fig. 7.

61

0

! 10

.

11...,

.

.

.

.

.

. .

.

.

.

..‘..I

1000 10000 100 Hydrogen Coneentratiod~m

.

.

-1 100000

Fig. 9. Dependence on the hydrogen concentration of the potential generated at the tin dioxide electrode ( t) at 300 “C.

pellet of TiO, was 300 “C (Fig. 8). This response was generally larger and sustained to higher temperatures than in the case of the cell with a tin dioxide electrode (Fig. 7). The size of the potential response to the introduction of hydrogen to the tin oxide electrode at 300 “C showed a logarithmic concentration dependence (Fig. 9).

Discussion

The potential measured across a composite including an ionic conductor with electronically conducting or semiconducting oxide comprises the sum of the interface potentials, the electromotive force arising from any available chemical reactions and thermovoltages where appropriate. Single gas space potentiometric devices can clearly show substantial responses to low concentrations of reducing gases and may have useful merits in terms of long term stability but they can only be fully understood when consideration is given to all possible contributions to the measured potential individually. Use of the present apparatus isolating the two electrodes in different gas spaces and a systematic substitution of each of the materials used at the outer electrode has enabled the effects of the potential-contributing reactions to be isolated. Changes of sign of potential response to the introduction of reducing gas as a function of temperature are only seen when the test gas impinges on the two electrodes simultaneously (Fig. 4 and ref. 8). The response of isolated electrodes, in the temperature range lOOW500“C is, generally, of uniform sign but reaches a maximum value at different temperatures on different materials (metals/semiconductors) and in gases of

different reactivity. The temperature-dependent change of sign of response is thus attributable to a summation of the effects at the two separate electrodes. The function of potentiometric sensors for reducing gases up to around 600 “C depends on their failure to achieve equilibrium between the surface concentration of oxygen ions and the oxygen partial pressure surrounding the two electrodes. Measurements of the oxygen partial pressure dependence at silver and tin oxide electrodes show that thermodynamic equilibrium is not sustained even at the highest temperatures used in the study. On the semiconducting oxide electrodes the potential is non-zero for all temperatures below around 350 “C. This is the temperature range within which the surface of tin oxide participates in the chemisorption of oxygen [lo]. The potential responses to the gases tested normally appeared over the same range as responses are found on gas sensitive resistors incorporating the same oxides. A mechanism involving reaction between molecules of the reducing gas and chemisorbed or lattice oxygen would be consistent with the observed temperature range and with the concentration dependence of the potential modulation. Gas-prompted shifts in potential have been measured in excess of 400 mV. Since temperature differences across the electrodes are not expected to be more than a few “C (e.g. on tin oxide contacted with silver) and Seebeck coefficients of the materials involved are not expected to exceed a few mV/“C, the contribution of thermovoltage to the measured potentials is expected to be very small. Differences in temperature dependence of the responses at different electrode materials lead to possible reversal of sign in a single gas volume sensor. Since gas reactions on which the responses depend are controlled by electrocatalytic processes there are good prospects for achieving a degree of selectivity between more reactive gases (hydrogen, carbon monoxide) and less reactive gases (e.g. methane) by choice of materials and temperature. The responses seen here have been stable and reproducible.

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62 4 J. P. Lukaszewicz, N. Miura and N. Yamazoe, A LaF,-based oxygen sensor with perovskite-type oxide electrode operative at room temperature, Sensors and Acrwrors, BI (1990) 195198. 5 W. C. Maskell, in P. T. Moseley, J. W. Norris and D. E. Williams (eds.), Techniques and Mechanisms in Gas Sensing, Adam Hilger, Bristol, 1991, Ch. 1, pp. l-45. 6 J. E. Anderson and Y. B. Graves, Steady-state characteristics of oxygen concentration cell sensors subjected to non-equilibrium gas mixtures, J. Electrochem. Sot., 128 (1981) 294300. 7 D. E. Williams, P. McGeehin and B. C. Tofield, in R. Metselaar,

H. J. M. Heijligers and J. Schoonman (eds.), Sfudies in Inorganic Chemistry, Vol. 3, Elsevier, Amsterdam, 1983. 8 J. F. McAleer, A. Maignan, P. T. Moseley and D. E. Williams, Tin dioxide gas sensors Part 3-Solid state electrochemical investigation of reactions taking place at the oxide surface, J. Chem. SOL, Faraday Trans. 1, 85 (1989) 183-799. 9 J. F. McAleer, P. T. Moseley, P. Bourke, J. 0. W. Norris and R. Stephan, Tin dioxide gas sensors: use of the Seebeck effect, Sensors and Actuators, 8 (1985) 251-258. 10 P. T. Moseley and D. E. Williams, in P. T. Moseley, J. 0. W. Norris and D. E. Williams (eds.), Techniquesand Mechanisms in Gas Sensing, Adam Hilger, Bristol, Ch. 2, pp. 46-60.