Tunable NO2 -sensing characteristics of YSZ-based mixed-potential-type sensor using Ni1-x Cox O -sensing electrode

Article abstract of DOI:10.1149/1.3174390

Tunable NO₂ -sensing characteristics using Ni_1-x Co_x O -sensing electrodes (SEs) at high temperature are reported. Various compositions of cobalt (Co)-doped NiO solid solutions (Ni_1-x Co_x O, 0 ≤ x ≤ 0

Full text of DOI:10.1149/1.3174390

J288
Journal of The Electrochemical Society, 156 ͑9͒ J288-J293 ͑2009͒
0013-4651/2009/156͑9͒/J288/6/$25.00 © The Electrochemical Society
Tunable NO2-Sensing Characteristics of YSZ-Based
Mixed-Potential-Type Sensor Using Ni1−xCoxO-Sensing
Electrode
Perumal Elumalai,a,b,z Vladimir V. Plashnitsa,a,c Yuki Fujio,d, and
Norio Miura
*
a,**
aArt, Science and Technology Center for Cooperative Research , cResearch and Education Centre of
Carbon Resources and , dInterdisciplinary Graduate School of Engineering Sciences, Kyushu University,
Fukuoka 816-8580, Japan
bJapan Society for the Promotion of Science, Tokyo 102-8471, Japan
Tunable NO2-sensing characteristics using Ni1−xCoxO-sensing electrodes ͑SEs͒ at high temperature are reported. Various compo-
sitions of cobalt ͑Co͒-doped NiO solid solutions ͑Ni1−xCoxO, 0 Յ x Յ 0.3͒ were synthesized by the citrate-gel method, and each
of the Ni1−xCoxO samples obtained was utilized as SE for yttria-stabilized zirconia ͑YSZ͒-based planar sensor for detection of NO2
at 800°C under wet condition. As a result, the NO2 sensitivity increased with increasing cobalt content ͑x͒ in Ni1−xCoxO, and the
maximum sensitivity was observed for Ni0.9Co0.1O. The sensitivity of the sensors was almost invariant to the change in water
vapor concentration in the sample gas. Moreover, the sensor exhibited excellent long-term stability for about 6 months. Interest-
ingly, the sensor attached with Ni0.8Co0.2O SE showed excellent selectivity to NO2. The effect of Co substitution in NiO on the
rates of electrochemical reactions at the Ni1−xCoxO/YSZ interface and the catalytic activity for the gas-phase conversion of NO2
to NO was evaluated, and the sensing mechanism of the present sensor was discussed.
© 2009 The Electrochemical Society. ͓DOI: 10.1149/1.3174390͔ All rights reserved.
Manuscript submitted May 11, 2009; revised manuscript received June 11, 2009. Published July 22, 2009.
NOx ͑NO and NO2͒ emissions from automobiles and various
Various compositions of Co-doped NiO solid solutions ͑Ni1−xCoxO,
0 Յ x Յ 0.3͒ were synthesized by the citrate-gel method, and the
obtained samples were used as SE for the YSZ-based planar sensor
for detection of NO2 at 800°C. The citrate-gel method has been used
here because it is a simple and an efficient method for producing
high quality oxides with better homogeneity and rather fine grain
size.26,27 As a result, NO2 sensitivity was found to increase with
increasing Co content in Ni1−xCoxO, and the maximum sensitivity
was observed at x = 0.1. Interestingly, the sensor attached with
Ni0.8Co0.2O SE exhibited excellent selectivity to NO2. Thus, we
report here the detailed sensing characteristics of YSZ-based sensor
using Ni1−xCoxO SE.
industrial furnaces are harmful to the environment and are a health
concern to human and animals. For example, NOx emission leads to
several adverse effects, such as respiratory problems, acid rain, etc.,
so stringent emission regulations are set for automobile exhausts
across the world. For example, the Tier 2 ͑in USA͒ set the lower
limit value of NOx emissions from a passenger car to be 0.05 g/km,
whereas the value set by Euro V ͑in Europe or in Japan͒ is 0.06
g/km.1-3 Due to the large expansion of automotive industries includ-
ing the developing countries, it is certain that soon these limit values
can be further reduced. Therefore, to meet such stringent regula-
tions, the automotive industries are enforced to adopt DeNOx pro-
cesses, such as NOx-storage catalyst and urea-based selective-
catalytic reduction system, to remove NOx.4-10 As long as these
catalysts can work well, toxic NOx can be decomposed to environ-
mentally safe N2 and H2O, and released through the tailpipe. In this
regard, to monitor the efficiency of these catalysts and to detect the
catalyst decay, the development of high performance sensors for in
situ monitoring of NOx in automobile exhausts is strongly required.
So far, several kinds of NOx sensors were examined and
reported.11-18 Among them, mixed-potential-type sensor based on
yttria-stabilized zirconia ͑YSZ͒ has been one of the potentially in-
teresting candidates due to several attractive features, such as fast
response, portability, low cost, simplicity of fabrication, and opera-
tion at elevated temperatures in a harsh environment.18-24 Although
the sensors based on YSZ and catalytically active metal oxide sens-
ing electrodes ͑SEs͒ have been examined and reported to date, sev-
eral key issues, including poor sensitivity to lower concentration of
pollutant gases, low sensitivity at higher operating temperatures, and
unsatisfactory selectivity, are challenging to be solved. Moreover,
because the temperature of a car engine occasionally reaches even
higher than 800°C while accelerating the vehicle, the proposed sen-
sor should work well for the long term even at such an elevated
temperature.
Experimental
Synthesis and characterization.— The Co-doped NiO solid solu-
tions ͑Ni1−xCoxO, 0 Յ x Յ 0.3͒ were synthesized by the citrate-gel
method. Aqueous solutions of Ni͑NO3͒2·6H2O and Co͑NO3͒2·6H2O
were prepared to obtain the required compositions and were kept to
be stirred at 80°C on a hot plate. An aqueous solution of citric acid
͑2 M͒ was added dropwise into the obtained nitrate solution. On the
process of heating the solution mixture at 80°C for 12 h, a gel was
formed. The obtained gel was heat-treated at 150°C ͑overnight͒ and
650°C ͑6 h͒ and then utilized as SE. The crystal structures of the
as-prepared samples and the 1000°C calcined samples were deter-
mined by X-ray diffraction ͑XRD, Rigaku, 2100VLR/PC͒ analysis.
The surface morphology of the samples was observed by a field-
emission scanning-electron microscope ͑JEOL JSM-340F͒ operating
at 15 kV. The Ni and Co compositions in Ni1−xCoxO samples were
analyzed by using an energy-dispersive X-ray analyzer ͑EDX,
Horiba, EX-220SE͒.
Fabrication and evaluation of sensor.— A YSZ ͑8 mol
%
Y2O3-doped stabilized zirconia͒ plate ͑10 ϫ 10 mm, 0.2t mm͒ was
used for the fabrication of a sensor device. A Pt paste was screen
printed on both sides of the YSZ plate and then calcined at 1000°C
for 2 h in air. Each of the prepared Ni1−xCoxO samples was thor-
oughly mixed with ␣-terpinol organic solvent and was applied on
the YSZ plate by a screen printing technique to obtain Ni1−xCoxO
thick films. The obtained oxide-printed YSZ plates attached with a
Pt lead and Pt reference electrode ͑RE͒ were sintered at 1000°C for
2 h. Figure 1 shows schematic views of ͑a͒ front and ͑b͒ back sides
We have recently found that NiO-based SE can give high NO2
sensitivity even at high temperature.20,25 However, sensitivity and
selectivity to NO2 were not satisfactory. In this study, we found that
NO2-sensing characteristics can be tuned by using Ni1−xCoxO SEs.
*
Electrochemical Society Student Member.
Electrochemical Society Active Member.
**
z E-mail: elumalai_75@yahoo.com
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Journal of The Electrochemical Society, 156 ͑9͒ J288-J293 ͑2009͒
J289
(∆ YSZ; ● Ni1-xCoxO)
Pt leads
∆
∆
∆
YSZ only
∆
∆
∆
∆
Ni1-xCoxO-SE
∆
●
●
●
x (Co) = 0
∆
●
∆
∆
●
●
●
x = 0.01
∆
∆
●
●
∆
∆
∆
∆
∆
x = 0.03
(a)
(b)
Pt-RE
∆
∆
∆
∆
●
●
x = 0.05
YSZ
∆
∆
●
●
∆
∆
∆
∆
●
Figure 1. ͑Color online͒ Schematic views of ͑a͒ front and ͑b͒ back sides of
x = 0.10
the fabricated planar sensor.
∆
∆
●
∆
●
x = 0.15
∆
∆
of the planar sensor fabricated in the present study. Though two Pt
strips were printed on the back side of the YSZ plate, only one was
used as RE during all measurements.
●
∆
∆
∆
∆
●
∆
∆
●
●
x = 0.20
The measurements of sensing characteristics were carried out by
using a conventional gas-flow apparatus equipped with a furnace
operating at 800°C. The base gas was composed of 5 vol % O2
+ N2 balance. The concentration of NO2 in the sample gas was
changed from 10 to 400 ppm, whereas the concentration of other
gases ͑CO, CH4, C3H8, C3H6, NO, and NH3͒ was kept constant at
400 ͑or 100͒ ppm. The base gas and the sample gas were humidified
with 5 vol % water vapor by using a water-vapor generator coupled
with a small homemade evaporator and a microflow pump ͑Hitachi,
L-2100; Japan͒. The flow rate of the base gas or the sample gas was
kept constant at 100 cm3/min. The potential difference between SE
and Pt RE was measured with a digital electrometer ͑Advantest,
R8240͒ as a sensing signal. The positive and negative terminals of
the electrometer were connected to oxide SE and Pt RE, respec-
tively. The sensitivity ͑⌬emf, where emf is the electromotive force͒
to each target gas was defined as a difference between the emf value
of the sensor in the sample gas and that in the base gas.
The current–voltage ͑polarization͒ curves were measured by a
potentiostat ͑Hakuto Denko, HZ-3000͒ based on a potential-sweep
method at a scan rate of 2 mV/min for a two-electrode configuration
in the base gas ͑5 vol % O2 + N2 balance͒ and in 200 ppm NO2 ͑+
base gas͒. The current axis of the anodic polarization curve was
subtracted from that of the cathodic polarization curve at each po-
tential so as to obtain the modified polarization curve in which the
current axis was expressed in an absolute scale.
●
●
∆
∆
●
x = 0.30
∆
∆
∆
∆
●
30
35
40
45
50
55
60
65
2θ / degree
Figure 2. XRD patterns of 1000°C calcined Ni1−xCoxO thick films formed
on the YSZ plate.
solid solution should vary linearly with composition, according to
Vegard’s law.28 By considering the radius of Co2+ ͑0.84 Å͒ and Ni2+
͑0.83 Å͒, a solid solution should be formed for any composition.29
Thus, in the present case shown in Fig. 3, a clear increase in the
lattice parameter ͑a͒ is seen with increasing x in N1−xCoxO, obeying
Vegard’s law. Such an increase in the a value is apparent because a
larger Co2+ is substituted in place of a smaller Ni2+
.
The morphology was observed for each Ni1−xCoxO-SE layer.
Figure 4 shows the representative scanning electron microscopy
͑SEM͒ images of Ni0.9Co0.1O SE sintered at 1000°C. It is seen that
the average size of the oxide grains is less than 1 ␮m and the
Evaluation of gas-phase catalytic activity.— The catalytic activ-
ity against the gas-phase decomposition of NO2 to NO was evalu-
ated in the temperature range of 200–800°C for each of the
Ni1−xCoxO samples ͑0.1 g each͒ by using a conventional gas-flow
apparatus as well as a chemiluminescence NOx analyzer ͑Yanaco,
ECL-88A͒.
4.18
4.17
Results and Discussion
4.16
●
Crystal structure and morphology of SE.— Figure 2 shows XRD
patterns of 1000°C calcined Ni1−xCoxO thick films formed on the
YSZ plate. In all cases, each set of Bragg peaks for YSZ and
Ni1−xCoxO was distinctly seen while no other peaks were observed.
The set of Bragg peaks corresponding to Ni1−xCoxO was indexed to
cubic phase, according to JCPDS no. 47-1049. It can be seen that as
the Co content in Ni1−xCoxO is increased, the position of the Bragg
peaks shifts to a lower diffraction angle ͑2␪͒ ͑or higher d spacing͒,
indicating the formation of solid a solution. A solid solution between
two metals or metal ions can be formed if the difference in radius of
two metals or metal ions is within 30% and the crystallographic
phase of end members is similar. The lattice parameter of formed
4.15
4.14
0.0
0.1
x in Ni1-xCo
0.2
0.3
x
O
Figure 3. Variation in lattice parameter ͑a͒ with Co content ͑x͒ in
Ni1−xCoxO.
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J290
Journal of The Electrochemical Society, 156 ͑9͒ J288-J293 ͑2009͒
(a)
100
●
80
●
60
●
●
●
Ni1-xCoxO-SEs
800°C
40
5 vol.% O
2
5 vol.% H2O
20
400 ppm NO
2
●
●
0
0
0.05
0.10
0.15
0.20
2 µm
x in Ni1-xCoxO
(b)
Figure 5. Dependence of NO2 sensitivity on x in Ni1−xCoxO SEs at 800°C
for the YSZ-based planar sensors under wet condition.
gas and that in the base gas͒ on Co content ͑x͒ in Ni1−xCoxO. It is
seen that the NO2 sensitivity increases with increasing x and the
maximum sensitivity is observed at x = 0.1 ͑Ni0.9Co0.1O͒. This im-
plies that the NO2 sensitivity can be tuned by varying Co content in
Ni1−xCoxO and high sensitivity can be obtained if 10 mol % Co-
doped NiO is used. Because the sensor attached with Ni0.9Co0.1O SE
showed the highest NO2 sensitivity, its further sensing characteris-
tics were examined in detail.
Figure 6 shows the response transients to various concentrations
of NO2 at 800°C under the wet condition for the sensor attached
with Ni0.9Co0.1O SE. The emf of the sensor in the base gas was
almost zero and changed quickly from the base value upon switch-
ing to the sample gas. The sensor reached the steady-state emf value
in each concentration in due time and returned to the original base
value upon turning on the base gas. A good repeatability of ⌬emf to
NO2 was observed with nearly the same response. The 80%
response/recovery times were about 80/180 s. Figure 7 shows the
dependence of ⌬emf on NO2 concentration at 800°C under the wet
condition for the present sensor using Ni0.9Co0.1O SE. ⌬emf varies
logarithmically with the NO2 concentration in the range of 50–400
ppm ͑Fig. 7a͒, as has been reported elsewhere.20,23-25,30 On the other
hand, in the lower concentration range of 10–100 ppm NO2, ⌬emf
varies linearly with the NO2 concentration ͑Fig. 7b͒, as has also
been reported similarly.21,31 The present sensor exhibits rather high
oxide
YSZ
5 µm
∗
(c)
Ni
Co
∗
Ni
Co
0
2
4
6
8
10
Binding enegry / eV
Figure 4. SEM images of ͑a͒ surface and ͑b͒ cross section of the Ni0.9Co0.1
sample. ͑c͒ EDX profile observed for the Ni0.9Co0.1O sample ͑ء Au͒.
O
Ni0.9Co0.1O-SE
800°C
120
100
80
60
40
20
0
5 vol.% O2
5 vol.% H2O
400 ppm NO2
300
200
average thickness of the SE layer is about 8 ␮m. Almost similar
morphology and thickness were observed for other samples. A rep-
resentative EDX profile for Ni0.9Co0.1O SE is shown in Fig. 4c. The
EDX analysis showed that Ni and Co contents were about 88.6 and
11.3 atom %, respectively. Based on these contents, the observed
Ni͓=Ni/͑Ni + Co͔͒ and Co͓=Co/͑Ni + Co͔͒ composition ratios
were estimated as 0.89 and 0.11, respectively, almost matching with
the nominal composition. An almost similar coincidence of nominal
and observed composition ratios of Ni and Co was seen for other
Co-substituted samples.
100
80
50
20
10
Performances of planar sensor using Ni1ϪxCoxO SEs.— The sen-
sitivity to 400 ppm NO2 was measured at 800°C under a wet con-
dition ͑5 vol % H2O͒ for the YSZ-based planar sensors attached
with Ni1−xCoxO SEs. Figure 5 shows the dependence of NO2 sensi-
tivity ͑⌬emf is the difference between the emf value in the sample
0
50
100
150
Time / m in
200
250
300
Figure 6. Response transients to NO2 at 800°C for the YSZ-based sensor
attached with Ni0.9Co0.1O SE under wet condition.
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Journal of The Electrochemical Society, 156 ͑9͒ J288-J293 ͑2009͒
J291
120
100
80
120
100
80
60
40
20
0
(a)
(a)
Ni0.9Co0.1O-SE
800°C
5 vol.% O2
400 ppm NO2
60
Ni0.9Co0.1O-SE
40
800°C
5 vol.% O2
5 vol.% H2O
20
2
4
H
6
8
10
12
2
O concentration / vol.%
0
100
400
200 300
50
140
120
100
80
120
(b)
(b)
100
80
60
40
20
0
Ni0.9Co0.1O-SE
800°C
60
5 vol.% O2
40
5 vol.% H2O
400 ppm NO2
Ni0.9Co0.1O-SE
800°C
20
5 vol.% O2
5 vol.% H2O
0
0
50
100
150
200
Time / day
20
40
60
80
100
Figure 8. ͑a͒ Dependence of ⌬emf to NO2 under various water-vapor con-
centrations and ͑b͒ time course of NO2 sensitivity at 800°C for the YSZ-
based sensor attached with Ni0.9Co0.1O SE.
NO
2
concentration / ppm
Figure 7. Dependence of ⌬emf to NO2 in ͑a͒ high concentration and ͑b͒ low
concentration ranges at 800°C for the YSZ-based sensor attached with
Ni0.9Co0.1O SE under wet condition.
dized to CO2 and H2O before arriving at the NOx-sensing chamber.
Then, NO can be electrochemically converted to NO2 and the total
concentration of NOx can be detected by the NO2 sensor.
sensitivity even to 10 ppm NO2. Such a high sensitivity to low NO2
concentration has not been reported so far for this kind of sensor at
this elevated temperature. Thus, there is a good possibility that the
present sensor can give significant sensitivity to even below 10 ppm
NO2.
Explanation of sensing characteristics.— According to our pre-
vious reports, the sensing mechanism in the present case seems to be
based on a mixed-potential model.20,23-25,30 When an SE is exposed
to NO2 in O2 containing atmosphere, a non-Nernstian potential is
developed on the SE due to the following two competitive electro-
chemical reactions
Because the concentration of water vapor in a real car exhausts
usually changes extensively, the effect of water vapor on the
NO2-sensing characteristics of the sensor attached with Ni0.9Co0.1
O
O2−
→
21 O2 + 2e− ͑anodic͒
͓1͔
SE was examined. Figure 8a shows the sensitivity to 400 ppm NO2
for the present sensor at 800°C in the concentration range of 2–11
vol % H2O. The sensitivity is not altered much by a change in
water-vapor concentration. The other important requirement of a
practical sensor is to have a good stability. To examine the stability
of the present sensor, the emf response to 400 ppm NO2 was moni-
tored for about 6 months ͑Fig. 8b͒. The sensor retains almost the
same sensitivity and the similar response/recovery rates even on the
180th day of the sensor operation. The above-mentioned perfor-
mances are highly valuable in the viewpoint of an actual application.
Although the sensor attached with Ni0.9Co0.1O SE showed a high
sensitivity to NO2 and a stable response for about 6 months, its
selectivity to NO2 was conformed to be unsatisfactory. This kind of
NO2 sensor can be used for the laminated-type total NOx-sensing
system, which can be constructed with an oxidation catalyst and a
NO2 + 2e− → NO + O2− ͑cathodic͒
͓2͔
On Pt RE, the following electrochemical reaction is equilibrating
O2 + 4e− = 2O2−
͓3͔
When the rates of Reactions 1 and 2 are equal, a nonequilibrium
potential ͑mixed potential͒ appears at the interface of SE/YSZ.
To rationalize the aforementioned NO2-sensing characteristics of
the sensors using Ni1−xCoxO SEs and to examine the effect of Co
doping in NiO on the rates of the electrochemical Reactions 1 and 2,
the current–voltage ͑polarization͒ curves were measured at 800°C in
the base gas ͑5 vol % O2 + N2 balance͒ as well as in the sample gas
containing 200 ppm NO2 ͑+5 vol % O2 + N2 balance͒. Figure 9a
shows the obtained anodic polarization curves in the base gas and
the modified cathodic polarization curves in NO2 containing sample
gas for the sensor using each of NiO and Ni0.9Co0.1O SEs at 800°C.
NOx conversion electrode, as has been reported elsewhere.32 On the
oxidation catalyst, the reducible gases ͑CO and HCs͒ can be oxi-
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J292
Journal of The Electrochemical Society, 156 ͑9͒ J288-J293 ͑2009͒
100
800°C
150
100
Solid lines: 5 vol.% O2
100 ppm NO2
5 vol.% O2
Dotted lines: 200 ppm NO2
(a)
80
60
40
20
0
Est.: 91 mV
Obs.: 94 mV
Est.: 3 mV
50 Obs.: 1 mV
0
200
300
400
500
600
700
800
0
20
40
60
80
100
Temperature / °C
Potential / mV
Figure 10. Temperature dependence of the degree of NO2 decomposition to
NO for 1000°C sintered Ni1−xCoxO samples.
(b)
Ni1-xCoxO-SEs
150
100
50
800°C
5 vol.% O2
Effect of Co doping on NO2 selectivity.— To check the effect of
Co doping in NiO on the NO2 selectivity, the cross sensitivities to
C3H6, CH4, and NO2 were measured at 800°C for the sensors at-
tached with Ni1−xCoxO-SEs under the wet condition. Figure 11a
shows the dependence of cross sensitivities to different gases on x in
Ni1−xCoxO. Each cross sensitivity increased initially with increasing
x and gave the maximum values at 0.05 Յ x Յ 0.1. When x is 0.2,
0
0
20
40
60
80
100
100
(a)
Potential / mV
NO
2
75
50
Figure 9. ͑a͒ Anodic polarization curves recorded in O2 and modified ca-
thodic polarization curves recorded in NO2 for NiO and Ni0.9Co0.1O SEs and
͑b͒ the anodic polarization curves recorded in O2 for the sensors using
Ni1−xCoxO SEs at 800°C under wet condition.
25
0
-25
-50
-75
-100
It can be seen that the intersection between the anodic and the ca-
thodic polarization curves increased largely from 3 mV to about 91
mV when the SE of the sensors was changed from NiO to
Ni0.9Cr0.1O. Evidently, these intersection values agree with the ac-
tual ⌬emf values of the sensor observed under potentiometric mode
͑Fig. 7͒. Such a close coincidence of the estimated and the observed
values confirms that the sensing mechanism of the present sensor is
based on mixed-potential model.
Ni1-xCoxO-SEs
CH
4
800°C
5 vol.% O
2
2O
5 vol.% H
400 ppm each
C
3H
6
0
0.05
0.1
x in Ni1-xCoxO
0.15
0.2
Figure 9b shows the obtained polarization curves for anodic re-
action of O2 for the sensors using each of various Ni1−xCoxO SEs. It
is clear that as the Co content in Ni1−xCoxO is increased, the anodic
polarization curve largely shifts downward. This implies that the
catalytic activity to the electrochemical reaction of O2 at the SE/
YSZ interface is lowered with increasing Co content in Ni1−xCoxO.
Such a lower catalytic activity can bring about a positive influence
on NO2 sensitivity.20
To quantify the effect of Co substitution in NiO on the catalytic
activity for the NO2 gas-phase decomposition of NO2 to NO, which
also plays an important role in deciding NO2 sensitivity,20 the degree
of NO2 conversion to NO was evaluated for the various Ni1−xCoxO
samples. The obtained results are shown in Fig. 10 for the represen-
tative compositions of Ni1−xCoxO. The degree of NO2 conversion to
NO increases with an increase in Co content in Ni1−xCoxO. This
means that the concentration of NO2 at the interface of Co-rich NiO
SE/YSZ is less than that of the interface of Co-less NiO SE/YSZ.
Thus, such a high degree for the gas-phase conversion of NO2 to NO
can contribute to lower the NO2 sensitivity in the sensor using
higher Co-content NiO SE.
NH
CH
3 (100 ppm)
(b)
4
C
3H
6
Ni0.8Co0.2O-SE
C
3
H
8
800°C
5 vol.% O
2
2O
CO
NO
5 vol.% H
400 ppm each
NO
2
-10
0
10
20
30
40
50
60
Sensitivity (∆emf)/mV
Figure 11. ͑a͒ Dependence of cross sensitivities to CH4, C3H6, and NO2 on
x in Ni1−xCoxO SEs and ͑b͒ comparison of cross sensitivities to various gases
for the sensor using Ni0.8Co0.2O SE at 800°C for the YSZ-based sensor under
wet condition.
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Journal of The Electrochemical Society, 156 ͑9͒ J288-J293 ͑2009͒
J293
NO2
O2
NO2
O2
NO2
O2
varying the Co content in Ni1−xCoxO. While the sensor attached
with Ni0.9Co0.1O SE exhibited the maximum NO2 sensitivity, the
sensor attached with Ni0.8Co0.2O SE showed excellent selectivity to
NO2 with acceptable sensitivity. It was confirmed that Co doping in
NiO resulted in a large decrease in the rate of anodic reaction of O2
and led to a higher degree for gas-phase decomposition of NO2 to
NO. While the lower rate of anodic reaction of O2 gave higher NO2
sensitivity in Ni0.9Co0.1O SE, the higher degree of NO2 conversion
to NO resulted in a moderate NO2 sensitivity in Ni0.8Co0.2O SE. The
sensitivity of the examined sensors varied linearly on the logarithm
of NO2 concentration and was almost invariant to the change in
water-vapor concentration in the sample gas. It was confirmed that
the NO2 sensitivity of the sensor using Ni0.9Co0.1O SE was highly
stable for about 180 days when tested.
NiO
Ni0.9Co0.1O
Ni0.8Co0.2O
Interface
Interface
Interface
YSZ
YSZ
YSZ
YSZ
YSZ
YSZ
Rate of anodic
reaction of O2
Rate of cathodic
reaction of NO2
Gas-phase
catalytic activity
NO2 conc.
(at interface)
+++
+
+
+
+
+
++
++
+++
++
+++
+
NO2 sensitivity
NO2 selectivity
poor
insignificant
high
poor
moderate
excellent
Acknowledgments
This work was supported in part through “The Grant-in-Aid for
Scientific Research on Priority Area, Nanoionics ͑439͒,” “Grants-in-
Aid for JSPS Fellowship ͑P07071͒” by MEXT, and “Kyushu Uni-
versity Global-COE Program” “Novel Carbon Resources Sciences.”
(+ means degree of process)
Figure 12. ͑Color online͒ Various processes occurring at SE/YSZ layers.
Kyushu University assisted in meeting the publication costs of this ar-
ticle.
the cross sensitivities to CH4 and C3H6 become nearly negligible.
Interestingly, the sensitivity to NO2 for Ni0.8Co0.2O SE is still high.
This implies that the sensor using Ni0.8Co0.2O SE can detect NO2
selectively. To verify this, the cross sensitivities to seven kinds of
gases were measured for the sensor attached with Ni0.8Co0.2O SE at
800°C under the wet condition. The obtained results are shown in
Fig. 11b. Undoubtedly, the sensor exhibits excellent selectivity to
NO2. It is seen that while the cross sensitivities to other gases are
less than Ϯ10 mV, the sensitivity to NO2 is about 50 mV.
Unlike the sensor using Ni0.9Co0.1O SE, which shows large cross
sensitivities to various gases, the sensor using Ni0.8Co0.2O SE can
detect NO2 selectively. Thus, if this type of selective NO2 sensor
would be installed in the laminated-type total NOx-sensing system, a
much simpler device can be constructed without using an oxidation
catalyst. In this case, the production cost of the laminated-type total
NOx-sensing device is expected to be lower.
Figure 12 shows the summary of various processes occurring at
each of the NiO-, Ni0.9Co0.1O-, and Ni0.8Co0.2O-SE/YSZ interfaces.
In NiO SE, a high rate for anodic reaction of O2 can be seen. When
Co content is increased in NiO, this anodic rate tends to become
lower. However, the increase in Co content results in a higher degree
for gas-phase NO2 decomposition to NO. Thus, while the high rate
for anodic reaction of O2 leads to very low NO2 sensitivity for the
sensor using NiO SE, the lower catalytic activity for anodic reaction
of O2 as well as the moderate degree of gas-phase NO2 decomposi-
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