Stability of NASICON-based CO2 sensor under humid conditions at low temperature.pdf

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Sensors and Actuators B 75 (2001) 179±187
Stability of NASICON-based CO
2
sensor under
humid conditions at low temperature
Tetsuya Kida
a
, Kengo Shimanoe
a
, Norio Miura
b
, Noboru Yamazoe
a,*
a
Department of Molecular and Material Sciences, Interdisplinary Graduate School of Engineering Sciences,
Kyushu University, Kasuga-shi, Fukuoka 816-8580, Japan
b
Advanced Science and Technology Center for Cooperative Research, Kyushu University,
Kasuga-shi, Fukuoka 816-8580, Japan
Received 30 October 2000; received in revised form 10 January 2001; accepted 12 January 2001
Abstract
The NASICON-based CO
2
sensor using Li
2
CO
3
-BaCO
3
auxiliary phase was tested for stability under exposure to the humid air
containing 3.6 kPa H
2
O and 1000 ppm CO
2
at 508C. For this purpose, the sensor devices were attached with a reference electrode (RE)
which was always kept from contacting the humid air, in addition to the sensing electrode (SE) and counter electrode (CE). With this
structure, the SE and CE potentials to CO
2
containing atmospheres at 4508C were measured relative to RE before and after the humid air
treatment for 3 days. It was found that the humid air treatment caused the CE potential to shift largely from the original value, while the SE
potential remained intact. The shift of CE potential decreased gradually with increasing time of operation at 4508C, disappearing almost
completely in several days. The SEM observation revealed that a number of tiny deposits showed up on the surface of NASICON after the
3-day treatment and the deposits grew into discrete crystalline particles after the 14-day treatment. It was found in separate experiments
that, when the NASICON disk was soaked in hot water, a signi®cant amount of Na
3
PO
4
was eluted from the disk, and that the resulting disk
could have the CE potential more stabilized to the humid air treatment. Based on these results, it is concluded that the elution of Na
3
PO
4
from the bulk of the NASICON disks to the surface is responsible for the degradation of CO
2
sensing properties in this type sensor after
being kept in a humid atmosphere at low temperature.
#
2001 Elsevier Science B.V. All rights reserved.
Keywords:
NASICON; CO
2
sensor; Electrode potential; Interfacial structure; Stability
1. Introduction
NASICON (Na
3
Zr
2
Si
2
PO
12
), high Na
-ion conducting
material discovered by Hong and coworkers [1,2], has
widely been applied for electrochemical sensors [3±9].
Maruyama et al. [3,4] ®rst reported a new type solid
electrolyte gas sensor for SO
2
or CO
2
, for which NASICON
was combined with an auxiliary phase of Na
2
SO
4
or
Na
2
CO
3
, respectively. The potentiometric sensor of this
type, classi®ed later as Type III sensor by Weppner [10],
opened a way to exploring new sensors for oxidic gases.
Among them, CO
2
sensors are particularly important for air
quality monitoring in of®ces and homes as well as for
control of bio-related processes. It turned out soon, however,
that the CO
2
sensors of this type are not always stable
Corresponding author. Tel.:
81-92-583-7539;
fax:
81-92-583-7539.
E-mail address:
yamazoe@mm.kyushu-u.ac.jp (N. Yamazoe).
*
enough under practical conditions and that the selection
of the auxiliary phase holds key to the stability. As we found,
for example, the auxiliary phase should be changed from
Na
2
CO
3
to Li
2
CO
3
, or more preferably, Li
2
CO
3
-based
mixed carbonates such as Li
2
CO
3
-CaCO
3
or Li
2
CO
3
-
BaCO
3
, in order to obtain a stable CO
2
sensor [11±13].
The NASICON device on which the binary carbonate aux-
iliary phase was attached as a thin layer through melting and
quenching proved high stability against disturbance by
humidity, beside excellent CO
2
sensing performances.
These sensors have been shown to work satisfactorily for
a long time under actual ®elds as long as they are operated
continuously at elevated temperature (e.g. 4008C).
However, even these CO
2
sensors have turned out to have
still a problem in practice, i.e. the sensing properties are
often deteriorated after the sensors have been kept under
humid conditions at low (room) temperature for a prolong
time with the heater switched off, as reported by Futata
and Ogino [14]. They suggested that the deterioration might
be caused by the adsorption of water vapor. The same
0925-4005/01/$ ± see front matter
#
2001 Elsevier Science B.V. All rights reserved.
PII: S 0 9 2 5 - 4 0 0 5 ( 0 1 ) 0 0 5 4 9 - 4
180
T. Kida et al. / Sensors and Actuators B 75 (2001) 179±187
phenomenon was also reported by Kaneyasu et al. [15], but
the degradation mechanism has been left unsolved. Under
these circumstances, we tried to elucidate what happens on
the NASICON-based CO
2
sensor during storage under
humid air at low temperature. At the start of this study,
we suspected from literature that the degradation problem
could be related with the stability of NASICON itself.
Kuriakose and coworkers [16,17] observed a change in
pH when NASICON powder was suspended in water. Yagi
and Saiki [6] reported that the humidity sensors based on
NASICON could be more stable in highly humid atmo-
sphere when phosphorus-free NASICON was used. These
results would suggest that the stability of NASICON could
be associated with the phosphorous component in NASI-
CON. Recently, Mauvy et al. [18] reported that, when
NASICON was immersed in water, a limited amount of
Na
3
PO
4
dissolved out into the water phase, and that this
should be attributed to the dissolution of Na
3
PO
4
as an
impurity phase in NASICON.
In this paper, the NASICON-based CO
2
sensor devices
were kept under a humid air containing CO
2
at 508C for a
prolonged time, to investigate its in¯uences on the CO
2
sensing properties at 4508C. The morphology of NASICON
surface as well as the structure of interfaces, NASICON/
auxiliary phase and NASICON/platinum electrode, were
observed or analyzed by means of SEM-EPMA. In separate
experiments, NASICON disks were soaked in hot water
(908C) to con®rm the dissolution of Na
3
PO
4
from the disks.
The results of these investigations indicated that the degra-
dation in problem can be attributed to the segregation of
Na
3
PO
4
phase at the counter electrode.
2. Experiments
2.1. Sensor fabrication and electrochemical measurement
The powder of NASICON (Na
3
Zr
2
Si
2
PO
12
) was prepared
conventionally through a solid-state reaction of Na
3
PO
4
and
ZrSiO
4
[19]. It was sintered and polished into a disk (8 mm
in diameter and 0.8 mm thick) as reported elsewhere [9].
Fig. 1 shows a schematic drawing of the CO
2
sensor devices
fabricated in this study. Unlike a conventional device having
a sensing electrode (SE) and a counter electrode (CE)
attached to the both side of the NASICON disk, the present
devices had the both electrodes attached on the same plane
of the disk, and in addition, a third electrode was attached to
the back side as a reference electrode (RE), to which the
potentials of SE and CE could be referred separately. For CE
and RE, Pt paste was applied on both sides of the disk as
indicated and ®red at 9008C. For SE, gold mesh and
auxiliary phase were attached as follows. A small piece
of gold mesh mounting a small quantity of auxiliary phase,
Li
2
CO
3
-BaCO
3
(1:2 in molar ratio) composite in the present
study, was placed in a designated area on the NASICON
disk. The whole assembly was inserted into a furnace
(preheated to 7508C) for 3 min, and taken out. This proce-
dure allowed the auxiliary phase to spread over by melting
and recrystallization, resulting in its good adhesion to the
gold mesh as well as to the NASICON surface. The disk was
®xed on the end of a quartz glass tube (6 mm in diameter)
with an inorganic adhesive.
The CO
2
sensing measurements were carried out in a
conventional gas ¯ow apparatus. The sensor device was set
Fig. 1. Three-electrode attached CO
2
sensing device fabricated.
T. Kida et al. / Sensors and Actuators B 75 (2001) 179±187
181
Fig. 2. Experimental set-up for CO
2
sensing measurement and humid air treatment.
in a quartz glass chamber which was heated externally, as
shown in Fig. 2. With this setting, RE (inside) of the device
was always exposed to synthetic air (O
2
±N
2
), while SE and
CE were exposed to the ¯ow (100 cm
3
/min) of sample gases.
The sample gases, consisting of CO
2
, air and water vapor,
were prepared by mixing at prescribed proportions a parent
CO
2
gas (2000 ppm diluted in air), synthetic air, and the
humidi®ed synthetic air after bubbling through warm water
(408C). The potential of SE and CE under exposure to CO
2
containing atmospheres were measured relative to RE on an
electrometer (ADVANTEST TR8652) at 4508C to detect
changes in the respective electrode potentials after humid air
treatments at low temperature, which were carried out also
in the same apparatus by exposing SE and CE to the ¯ow of
the synthetic air containing humidity (3.6 kPa H
2
O) and CO
2
(1000 ppm) at 508C. CO
2
was added to the treatment gas
because the sensor is always exposed to CO
2
in practice.
Eventually, however, the addition of CO
2
does not seem to
have exerted any particular effects in the present study.
2.2. SEM observation and EPMA analysis
After the completion of all the electrochemical measure-
ments, the sensor devices were fractured in two pieces to
observe or analyze the structure of interfaces, NASICON/Pt
and NASICON/carbonate, by means of scanning electron
microscope (SEM) instrument (JEOL JSM-6340F). In sepa-
rate experiments, NASICON disks were subjected to the
same humid air treatments as above to examine the stability
of NASICON under humid conditions. The surfaces of the
treated NASICON disks were observed or analyzed on a
SEM equipped with an electron-probe micro-analyzer
(EPMA; NORAN VOYAGER).
2.3. Hot water treatment of NASICON disk
The NASICON disks were soaked in 100 ml of hot water
(908C) for a designated time (1±5 h). To avoid contamina-
tion by atmospheric CO
2
, nitrogen was passed through the
hot water. This hot water treatment was carried out for two
purposes. One was to con®rm the release of phosphate ions
from the NASICON disks, and another to check the proper-
ties of the resulting NASICON disks as a base solid electro-
lyte of CO
2
sensor. The amount of phosphate ion released
was determined by a standard molybdenum blue method.
The method was composed of three steps, i.e. formation of
molybdophosphoric acid, reduction of the acid with tin(II)
chloride to yield a blue-colored complex (molybdenum
blue), and spectrophotometric determination of the complex
on a spectrometer (HITACHI U-2001).
3. Results and discussion
3.1. Behavior of electrode potentials of CO
2
sensor as
prepared
The device fabricated in this study was tested for the CO
2
sensing capability under usual operation conditions. Various
concentrations of CO
2
in the presence of water vapor
(1.6 kPa) were fed over the device at 4508C. Fig. 3 shows
the potentials of SE and CE of the device relative to RE as a
function of CO
2
concentration. The SE potential responded
Fig. 3. Response of the sensing- and counter-electrodes potential (relative
to the reference electrode) to variations in CO
2
concentration at 4508C
(fresh device).
182
T. Kida et al. / Sensors and Actuators B 75 (2001) 179±187
quickly to the variations in CO
2
concentration and the
dependence of the steady values on the CO
2
concentrations
obeyed Nernst equation for
n
2 where
n
is the number of
reaction electrons, as indicated. The counter electrode
potential, on the other hand, remained constant to the
variations in CO
2
concentration, as also indicated. These
characteristics of the SE and CE potentials are typical to the
CO
2
sensor of this type, providing a base on which the SE
potential can be measured relative to the CE potential in
practical device.
It should be pointed out that, theoretically speaking, the
potential of CE must be practically the same as that of RE
under the present conditions because the electrodes are both
made of Pt. Actually, however, a signi®cant difference in
potential existed between the two electrodes, as observed.
Such differences between the CE and RE potentials were
observed for all the devices fabricated. The differences
scattered device to device, but remained within a range of
about 50 mV through the devices. The reason for this
anomaly is not clear at present. It is suggested, however,
that the surface of NASICON disk used was not uniform,
showing ¯uctuations in composition or impurity phases,
depending on the site selected. If so, the potential of Pt
electrode attached on NASICON can also ¯uctuate depend-
ing on the location of Pt electrode. This may be a reason for
the discrepancy of the CE and RE potentials. It is empha-
sized that, for a given device, the CE potential relative to RE
was stable as long as the device was operated under usual
conditions, as already mentioned.
3.2. Behavior of electrode potentials after the humid air
treatments
The devices were exposed to the humid air containing
3.6 kPa H
2
O and 1000 ppm CO
2
at 508C for 3 days, and
tested for the CO
2
sensing properties at 4508C again. It was
found that the SE potential relative to RE reproduced almost
exactly the same CO
2
sensing properties as it had before the
humid air treatment. For the particular device (Device 1)
used in Fig. 3, for example, the SE potential to 200 ppm CO
2
reproduced the original value (À462 mV) immediately after
the device temperature was set to 4508C, as shown in Fig. 4.
The CE potential, on the other hand, exhibited utterly
different behavior. The potential after the treatment deviated
largely from the original one before the treatment. For
Device 1, for example, the original potential was at
À51
mV, while the new potential was at 7 mV, having
shifted upward by 58 mV. Quite notably, the CE potential
after the treatment was not stable but moved very gradually
toward the original potential. For Device 1, the potential
almost recovered the original value in 3 days. Such transient
behavior of CE potential was common to all of three devices
tested, as shown in Fig. 5, where the deviation of CE
potential from the original value are plotted as a function
of time. The deviation tended to decrease with increasing
time of sensor operation at 4508C, although the details of the
Fig. 4. Behavior of the potentials of sensing- and counter-electrodes to
continuous exposure to 200 ppm CO
2
at 4508C after the humid air
treatment at 508C for 3 days.
behavior depended on the individual devices. These results
indicate that something had happened at CE during the
humid air treatment at low temperature.
3.3. Formation and stability of deposit on NASICON
surface
In order to specify the cause of the CE potential shift
mentioned above, the NASICON surface after the humid air
treatment at 508C was inspected by means of SEM and
EPMA. Fig. 6 shows SEM images (top-view) of the NASI-
CON after the humid air treatment for varying periods. On
the fresh surface (a), one could see the straight scratches
formed by polishing and hollows resulting from pores. After
the treatment for 3 days (b), the surface was covered by thin
clouds consisting of a number of tiny deposits formed at
numerous sites on the NASICON surface. The clouds
thickened after the 7 days treatment (c), and condensed
or grew into deposits having clear peripheries after 14 days
(d). These observations show that something is segregated
on the NASICON surface when NASICON is exposed to the
humid air at low temperature.
Fig. 5. Behavior of counter electrode potential to continuous exposure to
200 ppm CO
2
at 4508C after the humid air treatment, as compared among
three individual devices.
T. Kida et al. / Sensors and Actuators B 75 (2001) 179±187
183
Fig. 6. SEM images of NASICON surfaces before (a) and after the humid air treatment for 3 days (b), 7 days (c), 14 days (d).
Fig. 7. SEM images of NASICON surfaces after heating at 4508C for 3 days following the humid air treatment at 508C for 3 days (a), 7 days (b) and 14 days (c).

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