a new so2 gas sensor based on an mg2+ conducting solid electrolyte
TRANSCRIPT
A new SO2 gas sensor based on an Mg2� conducting solid electrolyte
Ling Wang, R.V. Kumar *
Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB2 3QZ, UK
Received 23 August 2002; received in revised form 6 November 2002; accepted 27 November 2002
Abstract
A new SO2 gas sensor based upon a magnesium ion conducting solid electrolyte MgZr4(PO4)6 (MZP) and a Na2SO4 auxiliary
electrode has been fabricated and tested. Electromotive force (emf) results with variation in the SO2 and the O2 partial pressures at
various temperatures from 873 to 1023 K are presented. The results have shown that the response to SO2 is rapid and the measured
emf of the sensor is found to vary linearly as a function of the logarithm of the partial pressure of SO2. As MZP does not react with
SO2, both electrodes of the sensor can be exposed to the same atmosphere containing SO2 and thus there is neither any need for
sealing the electrode compartment nor using any semi-permeable membranes.
# 2002 Elsevier Science B.V. All rights reserved.
Keywords: SO2; Sensor; Solid electrolyte; Mg2� conductor
1. Introduction
SO2 is a major air pollutant from power plants,
metallurgical processes and acid plants. The conven-
tional analysis methods of SO2 include solution-absorb-
ing chemical analysis, chromatography and
electrochemical analysis and spectroscopy, but these
analyses are carried out at room temperature, take a
long time and are non-continuous, so it is difficult to
apply these methods to production control and in situ
environmental monitoring. There is an increasing need
for reliable, selective and continuously detecting SO2
sensors. It is anticipated that sensors based on solid
electrolytes will fulfil these requirements.
Because no solid electrolyte for transporting the
sulphate ions is available, in the early stages, SO2
sensors were based upon alkali-metal sulphates. The
first SO2 sensor was based on a SO2 concentration cell,
which was proposed by Gauthier et al. [1] in which
K2SO4 was used as the electrolyte. Furthermore, Gau-
thier et al. [2] investigated various sensors by selecting
different reference materials, such as SO2-containing
gas, Ag�/Ag2SO4 and MgO�/MgSO4, but stable signals
were obtained only when a gas reference was used.
Because K2SO4 easily forms low melting point poly-
sulphate at high temperature, Jacob and Rao [3] used
Na2SO4�/I as the electrolyte.
Worrell and Liu [4,5] used Ag2SO4�/Li2SO4 as the
solid electrolyte and a metal�/metal sulphate mixture,
namely Ag�/Ag2SO4 as the reference. In fact the
thermodynamic properties of the Ag/O/S system showthat at temperatures higher than 463 K, only the Ag and
Ag2SO4 phases coexist. The sensor can be described as:
Pt; Ag ½ Ag2SO4 (25%)�Li2SO4 (75%) ½ SO2; O2; Pt
The electromotive force (emf) values of these cells
were essentially consistent with those theoretically
calculated using the Nernst equation within a certain
SO2 partial pressure range.However, sulphates suffer from poor sinterability,
phase transitions with large volume change, which cause
cracks and pores in the electrolyte during preparation or
operation. In order to avoid these problems, more
practical devices were prepared by using Na� ion
conductors such as Na3Zr2Si2PO12 (Nasicon) and Na-
b-Al2O3. They exhibit much higher ionic conductivity
than sulphates and can be easily sintered and show nophase transformation with volume change in the wide
temperature range. Saito and co-workers [6] first
prepared a SO2 sensor based on a Nasicon electrolyte.
* Corresponding author. Tel.: �/44-1223334327; fax: �/44-
1223334567
E-mail address: [email protected] (R.V. Kumar).
Journal of Electroanalytical Chemistry 543 (2003) 109�/114
www.elsevier.com/locate/jelechem
0022-0728/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0022-0728(02)01438-9
The cell can be expressed as
Pt; SO2 (I); O2 (I); SO3 (I) j NASICON
j SO3 (II); O2 (II); SO2 (II); Pt
The cell gave exactly the same emf as the cell using a
Na2SO4 based electrolyte, because a thin film of Na2SO4
is formed on the electrodes during operation. In order to
improve the sensor performance further, sandwicharrangement SO2 sensors were proposed by using
Nasicon or Na-b-Al2O3 as the electrolyte with a
Na2SO4 auxiliary electrode [7�/9]. Itoh et al. [10]
proposed sensor systems using solid reference materials
such as Au2Na�/Au or Na-bƒ�/b-Al2O3 in order to fix
the Na activity at the reference electrode.
CaF2 has also been used as an electrolyte for a SO2
sensor with a CaSO4 auxiliary electrode [11]. However,the sensor demonstrated slow responses to SO2 (re-
sponse time 2�/8 h) and CaF2 is found not to be stable in
a humid atmosphere due to the following reaction
CaF2�H2O�SO3�2HF�CaSO4 (1)
Despite progress, because these sensors are used upon
reference gases or solid references, which required
protection, gas tight electrolytes and good sealing
between the sensing and the reference side are critical.Normally, the sensors operate at high temperatures, so
sealing problems due to differential expansion coeffi-
cients of the materials used are commonly experienced.
It is difficult to construct gas tight, long lasting cells,
especially for cyclic temperature use. Application of a
reference gas makes sensor construction and operation
complicated and is not practical. In addition, the emf
values of sensors with sealed references not only dependon SO2 partial pressure but also on O2 partial pressure.
In order to overcome these problems, Rao et al.
proposed a temperature gradient SO2 sensor for which
sealing is not necessary [12]. The emf obtained due to the
thermoelectric effect is not sufficiently sensitive for
practical application. Yan et al. suggested the use of
stabilized zirconia interfaced with a Li� ion conductor
in the form of a mixed sulphate [13,14]. Both theelectrolytes were not exposed to the same test gas.
According to their suggested mechanism, while the
Li2SO4-based electrolyte was sensitive to SO3, and
MgO-stabilised zirconia (MSZ) was responsive to O2,
they postulated that, at the interface between MSZ and
the Li2SO4-based electrolyte, a small amount of Li2ZrO3
was formed. According to their suggested electrode
reactions, the virtual cell reaction can be deduced to be:
1=2O2 (test)�SO3�Li2ZrO3
�Li2SO4�ZrO2�1=2O2 (reference) (2)
In another approach, yttria-stabilised zirconia (YSZ)
has been interfaced with either Nasicon or Na b-alumina
and the same test gas is exposed to both electrodes such
that the two compartments are not separated, which
greatly simplifies the design and the operation of the
sensors in practice [15,16]. The arrangement of the cell
can be described as
Test gas; Pt ½ YSZ ½ Nasicon ½ Na2SO4 ½ Pt; Test gas
These sensors are essentially SO3 sensitive and the
electrode reactions for SO3 sensing can be expressed as
Na2SO4�2Na��SO3 (g)�1=2O2 (g)�2e� (3)
1=2O2 (g)�2e��O2� (4)
2Na��O2��Na2O (Nasicon) (5)
Reaction (5) is deemed to occur at the YSZ j Nasicon
interface. Oxygen, involved in both the anodic andcathodic reactions, cancels out of the final relationship
between the emf and gas composition, due to the partial
pressure of O2 on both sides being same. YSZ also
serves to protect the Nasicon from reacting with SO2/
SO3 to form Na2SO4 on the reference side.
The sensor has been further modified to measure SO2
by providing a Pt/alumina catalyst for in situ catalysis of
SO2 oxidation with O2 in the atmosphere to anequilibrium concentration of SO3 at high temperature.
Thus for measuring SO2, a knowledge of PO2in the test
gas becomes necessary [16].
Recently a thin film SO2 sensor was developed by
using a mixed sulphate of Na2SO4�/BaSO4�/Ag2SO4
film as the electrolyte with metal Ag as the reference
[17]. Both the electrodes of the sensor were exposed to
the same atmosphere during operation. The responsewas fairly fast, but can be used only at low temperatures
due to the low melting point of the sulphate mixture.
Another SO2 sensor [18] which also does not need any
sealing has been constructed using the oxygen ion
conductor YSZ and a silver sulphate based salt as the
auxiliary phase along with a silver sensing electrode and
a Pt/O2 reference electrode. The sensor has been
reported to work in a wide temperature range withfast responses. However, in sensors using silver as a
reference material, depletion of silver in an SO2 atmo-
sphere can result in short lifetime.
MgZr4(PO4)6 is a magnesium ion conductor which
was introduced by Ikeda et al. [19] and later successfully
used to develop CO2 sensors [20,21].
In this work, a new SO2 sensor was fabricated and
tested by using a divalent cationic Mg2� conductorMgZr4(PO4)6 with Na2SO4 as the auxiliary electrode.
The response properties and the working mechanism of
this sensor are reported below.
2. Suggested mechanism
The electrochemical cell can be described as:
SO3; O2; Pt ½ Na2SO4 ½ MgZr4(PO4)6 ½ Pt; O2; SO3
L. Wang, R.V. Kumar / Journal of Electroanalytical Chemistry 543 (2003) 109�/114110
The operation of such a cell presupposes an exchange
reaction at the surface of the MZP electrolyte with the
auxiliary interface:
Na2SO4�(MgO)in MZP�MgSO4�(Na2O)in MZP (6)
Assuming both Na2SO4 and MgSO4 activity are at
unity, the ratio of aNa2O=aMgO is equal to the equilibrium
constant K for the above reaction:
K�aNa2O=aMgO (7)
Currently no information is available concerning the
values of either aMgO or aNa2O in MZP. Assuming that
aMgO in MZP is relatively low given the low value of K ,
it is anticipated that aNa2O in MZP is very low indeed,
thus promoting the decomposition of Na2SO4. Accord-
ingly the following cell reactions are proposed:
Anode: Na2SO4�(MgO)in MZP
�Mg2��(Na2O)in MZP�2e�1=2O2�SO3 (8)
Cathode: 1=2O2�2e�Mg2��(MgO)in MZP (9)
Overall: Na2SO4�(Na2O)in MZP�SO3 (10)
Thus the Nernst equation can be derived as:
emf��2:303RT
2Flog PSO3
�2:303RT
2Flog aNa2O�
DG�
2F
�a�b log PSO3(11)
where a��(2:303 RT=2F )log aNa2O�DG�=2F ; b�2:303 RT=2F and DG8 is the standard Gibbs energy of
the reaction (10).
At the Pt catalyst, the following equilibrium isexpected to be established:
SO2�1=2O2�SO3 (12)
and thus for the equilibrium constant K for the above
reaction (12), it has been shown Eq. (13) that:
PSO3�PSO2
KPO2
1=2
1 � KPO2
1=2(13)
where PO2is the partial pressure of oxygen in the gas
phase.
Thus the emf is a function of the SO2 partial pressure
as follows:
emf��2:303 RT
2Flog aNa2O�
DG�
2F
�2:303 RT
2Flog
KPO2
1=2
1 � KPO2
1=2
�2:303 RT
2Flog PSO2
�A�B log PSO2(14)
where A��(2:303 RT=2F )log aNa2O�DG�=2F�
(2:303 RT=2F )log(KPO2
1=2=1�KPO2
1=2) and B��2:303 RT=2F : Thus the emf is independent of PO2
for a SO3 sensor. It is however dependent upon PO2for a
SO2 sensor through the SO2/SO3 equilibrium.
3. Experimental
3.1. Materials
MgZr4(PO4)6 powder was prepared using conven-
tional solid reaction method [17] precursors of magne-sium oxide, zirconia and ammonium dihydrogen
phosphate. These powders were mixed according to
the required ratio and ball-milled in acetone for a total
of 24 h. The powder slurry was dried and then placed in
an alumina crucible and calcined for 4 h at 483 K and
for 6 h at 1173 K in air. After milling and drying, pellets
15 mm in diameter and 2 mm thick were prepared, and
then the pellets surrounded with powder of the samecomposition were sintered in an alumina crucible at
1623 K for 24 h.
3.2. Preparation of SO2 sensor
The construction of the sensor device is shown in Fig.
1. Pt ink electrodes were coated on both sides of the
sintered MgZr4(PO4)6 pellet and then were fired at 1173K for 10 min. A Pt wire lead was fixed on the Pt
electrodes by firing. As the sensing material, a porous
film of Na2SO4 was attached to one of surfaces of
MgZr4(PO4)6 by coating with Na2SO4 paste and sinter-
ing at 1173 K for 5 h. Since the sensor is expected to
respond to SO3, for SO2 measurement the SO2 must be
converted into SO3. In order to obtain an in situ
conversion of SO2 to SO3, alumina pellets containing1% Pt are used as a catalyst, adjacent to the auxiliary
Na2SO4 electrode in the gas path.
3.3. Emf measurement
The SO2 sensors were tested under controlled condi-
tions at different SO2 concentrations, and subjected to
variation in operating temperature and partial pressureof O2. Measurements were carried out in a conventional
gas-flow apparatus equipped with a heating facility. One
Fig. 1. Schematic diagram of the SO2 sensor device using
MgZr4(PO4)6 as a solid electrolyte with the Na2SO4 auxiliary
electrode.
L. Wang, R.V. Kumar / Journal of Electroanalytical Chemistry 543 (2003) 109�/114 111
percent Pt loaded alumina pellets, used as the catalyst,
which was used to convert SO2 to SO3, were inserted
and fixed in front of the sensor to ensure that the test gas
flow passed through the catalyst. The sensors wereexposed to a flow (100 cm3 min�1) of the required
sample gases, which were prepared by diluting the
parent gas (5000 ppm in air�/N2) with synthetic air
using a gas blender. The gas mixtures of SO2/air had an
SO2 concentration varying from several ppm to 1000
ppm. Temperatures from 873 to 1073 K were investi-
gated. The emf of the sensor was measured with a digital
electrometer and transferred to a computer-based datacollecting system.
4. Results and discussion
Fig. 2(a and b) show typical emf responses to a
stepwise increase in the SO2 content from 0 to about
1000 ppm for planar SO2 sensors at 972 and 1021 K and
PO2�21%: The emf is decreased, as PSO2
(and therefore
PSO3) is increased. The response was relatively rapid and
continuous. The response time to achieve 90% of the
emf value was less than 3 min and the response became
faster with increasing SO2 content. The recovery time on
switching off SO2 was somewhat slower at 5 min.
The influence of the temperature on the emf response
to different SO2 contents was also evaluated. Fig. 3
shows the dependence of the emf response of the sensor
on the logarithm of SO2 partial pressure (log PSO2);
measured at different temperatures between 873 and
1023 K for a fixed value of PO2�21 atm: The emf
responses of the sensor were linear, with negative slopes,
with respect to the logarithm of SO2 partial pressure
(log PSO2) at all the temperatures tested. The numbers of
electrons z , as derived from the pseudo-Nernstian
equation applied to the best linear fit, are calculated as
1.56 at 1021 K, 1.54 at 972 K, 1.16 at 923 K and 0.95 at
673 K. As z approaches the theoretical value of 2 at
higher temperature, the Nernstian values are not
achieved. At the lower end of the experimental tem-
peratures, the discrepancy is larger. These problems may
arise from the difficulty in achieving equilibrium at low
temperatures and low PSO2values. On disregarding the
scattered values, the value of z approaches two,
especially at temperatures higher than 973 K.
According to the mechanism proposed above in
Section 2, oxygen in the gas mixture, which is involved
in electrode reactions should not interfere with the
sensor response to SO3, but it affects SO2 conversion
to SO3, so it affects the emf indirectly. Fig. 4 shows the
emf output change with the logarithm of the SO2
content when the oxygen partial pressure in the gas
mixture varies from several tens of ppm to 21%.
Fig. 2. Emf response with time to various SO2 contents (ppm in air)
with a Pt-loaded alumina catalyst, measured at 972 (a) and 1021 K (b)
with PO2�0:21 atm:/
Fig. 3. Emf response as a function of the logarithm of SO2 partial
pressure, measured at four different temperatures and a fixed PO2of
0.21 atm.
L. Wang, R.V. Kumar / Journal of Electroanalytical Chemistry 543 (2003) 109�/114112
Experimentally it was found that the emf is dependent
on the pressure of oxygen in the gas mixture when the
oxygen content is low, but when the oxygen content is
high enough, it hardly interferes with the emf values.
This is a great advantage when the sensor is applied to
an environment where the oxygen pressure is high and
variable.
Since at a given temperature, for a fixed PO2; the emf
decreases with increasing SO2 content, as suggested by
the mechanism in Section 2, the virtual cell reaction
leads to a decomposition reaction forming SO3. This
implies that the solid electrolyte MZP is stable with
respect to the formation of MgSO4 on the reference Pt/
O2 electrode. Therefore, MZP can be utilised without
any need for protection from a semi-permeable mem-
brane or YSZ. On the other hand, Na2SO4 is unstable incontact with MZP, and can result in the dissolution of
the Na2SO4 forming Na2O in the electrolyte accompa-
nied by precipitation of MgSO4 at the interface (see Eq.
(6)) which in turn can impede any further dissolution of
the Na2SO4. It is suggested that the dissolution�/
precipitation mechanism is the reason why a stable
auxiliary phase can coexist in a system, which is
inherently unstable. Using Eq. (14) as the model, sinceall other values are known except aNa2O; the values of
aNa2O are estimated at different temperatures and
reported in Table 1.
Since the auxiliary phase is a porous system, it is also
possible that the overall cell reaction, due to the
formation of MgSO4 at the MZP j Na2SO4 interface,
is given by:
MgSO4�MgO�SO3 (15)
and accordingly
emf ��2:303 RT
2Flog aMgO�
DG�
2F
�2:303 RT
2Flog
KPO2
1=2
1 � KPO2
1=2
�2:303 RT
2Flog PSO2
(16)
where DG� is the Gibbs energy of decomposition of
MgSO4 and aMgO is the activity of MgO in MZP. Using
the experimental results and considering this alternative
scenario, a MgO in MZP is also estimated and reported in
Table 1.
Only by measuring the aMgO in MZP, by an alter-
native method, will it be possible to comment on
whether the emf dependence upon log PSO2is deter-
mined by either Eq. (14) or Eq. (16). The possibility of
inter-dissolution between Na2SO4 and MgSO4 should
also be considered. It is suggested that in order to
stabilise MgSO4 in contact with MZP, the use of
Na2SO4 as an auxiliary phase is essential.
Since MZP (unlike Nasicon) does not react with SO2
(or SO3) to form a sulphate, the reference Pt/O2
electrode is simplified, without the need to use anadditional electrolyte such as YSZ [15] or a semi-
permeable membrane such as Au�/Pd [16].
5. Conclusion
A planar type of SO2 gas sensor based on the
magnesium ion conducting electrolyte MgZr4(PO4)6
with an Na2SO4 auxiliary electrode has been prepared
and tested. The main feature of this kind of SO2 sensor
is that both electrodes of the sensor are exposed to the
same atmosphere, without the requirement of any
sealing, and the auxiliary phase Na2SO4 does not share
the Mg2� ion. The experimental results have shown that
this kind of sensor clearly responds to SO2 and the
variation of emf with the logarithm of the partialpressure of SO2 is found to be linear. Since both
electrodes are in a common gas mixture, the oxygen
partial pressures at both electrodes are equal and the
Fig. 4. Emf response as a function of the logarithm of SO2 partial
pressure for different oxygen partial pressures in the gas mixture,
measured at 993 K.
Table 1
Estimation of the activity values of Na2O and MgO in MZP at
different temperatures based on Eq. (14) for Na2O and Eq. (16) for
MgO
T /K /log aNa2O (in MZP) /log aMgO (in MZP)
873 �/65.68 �/17.55
923 �/61.83 �/16.03
973 �/63.30 �/19.58
1023 �/61.72 �/19.89
L. Wang, R.V. Kumar / Journal of Electroanalytical Chemistry 543 (2003) 109�/114 113
sensor is less sensitive to oxygen when the O2 partial
pressure is high. It is suggested that MgSO4, which is
unstable in contact with MZP, is precipitated in-situ and
stabilized by the presence of Na2SO4. Further work isrequired to test the long-term stability and the cross-
sensitivity to H2O and CO2 of such a sensor.
Acknowledgements
The financial support of EPSRC for this project is
gratefully acknowledged.
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