Application of Srh-alumina solid electrolyte to a CO2 gas sensor

Takashi Goto a,*, Gang He b, Takayuki Narushima b, Yasutaka Iguchi b

aInstitute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba, Sendai 980-8577, JapanbDepartment of Metallurgy, Tohoku University, Sendai 980-8579, Japan

Received 21 April 2002; received in revised form 25 June 2002; accepted 3 July 2002

Abstract

Srh-alumina was synthesized in a SrO–MgO–Al2O3 system by solid state sintering, and the electrical conductivity was

measured at 873–1673 K by impedance spectroscopy. The oxygen concentration cell method was applied to measure the ionic

transport number. The ionic transport numbers were close to 1 in the temperature range between 1200 and 1600 K. Open- and

closed-type CO2 gas sensors were constructed using the Srh-alumina solid electrolyte and a Pt–SrCO3 auxiliary electrode. The

CO2 sensors responded well to the changes of CO2 partial pressures at temperatures of 1073–1373 K.

D 2003 Elsevier Science B.V. All rights reserved.

PACS: 66.10.Ed

Keywords: Strontium h-alumina; CO2 gas sensor; Open-type cell; A.c. impedance spectroscopy; Transport number

1. Introduction

Since CO2 gas emission is a recent serious environ-

mental issue, highly selective and sensitive CO2 gas

sensors should be developed. Several kinds of CO2

gas sensors have been fabricated using alkali metal

solid electrolytes [1–5]. However, alkali metals in the

solid electrolytes would easily vaporize at high tem-

peratures. Therefore, they can be hardly used in the

industrial fields such as steel making processes and

many chemical plants. On the other hand, alkaline-

earth metal h-aluminas would be a good candidate

material as the solid electrolyte at high temperatures

because of their high chemical stability and mechan-

ical strength. Either ion exchanging of NahW-alumina

or solid state sintering has been used to prepare the

alkaline-earth metal h-aluminas. The alkaline-earth

metal hW-aluminas synthesized by the ion exchanging

would easily transform their structure into a magneto-

plumbite phase at high temperatures [6,7], and the

phase transformation often causes the formation of

micro-cracks leading to a leakage of gas through the

material. There have been several reports on the

synthesis of alkaline-earth metal h-aluminas by solid

state sintering [8–11]; however, they have not been

well applied to CO2 gas sensors.

In the present work, Srh-alumina in a SrO–MgO–

Al2O3 system was synthesized by solid state sintering.

The electrical property and the transport number were

measured in the temperature range between 873 and

1673 K. The performance of CO2 gas sensors using

the Sr-alumina as a solid electrolyte was evaluated.

0167-2738/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved.

PII: S0167 -2738 (02 )00681 -1

* Corresponding author. Tel.: +81-22-215-2105; fax: +81-22-

215-2107.

E-mail address: goto@imr.edu (T. Goto).

www.elsevier.com/locate/ssi

Solid State Ionics 156 (2003) 329–336

2. Experimental

SrCO3, MgCO3 and g-Al2O3 (Kanto Chemical,

99.9%) powders were used as starting materials. The

powders were dehydrated at 773 K for 24 h. The dried

powders were weighed and mixed in an agate mortar

by adding a small amount of ethanol. The mixtures of

powders were then pressed into disks, and were

calcined twice in an alumina crucible for 24 h at

1573 K. The calcined specimens were pressed again

into disks. The specimens were sintered in an alumina

crucible at 1573–2073 K for 24 h, and then slowly

cooled at 5.8 K/s in a furnace. The calcining and

sintering were both conducted in a dried O2 atmos-

phere. The structure and lattice parameters were

analyzed by powder X-ray diffraction (Ni filtered,

CuKa).

Pt paste (Degussa, 308A) was printed on both

surfaces of the specimens. The electrical conductivity

was measured by an a.c. impedance analyzer (Solar-

tron 1260) in the frequency range between 0.05 Hz

and 5 MHz at 873–1673 K in air and in Ar–O2

atmospheres. The oxygen partial pressure (PO2)

dependence of the electrical conductivity was inves-

tigated. The specimens were kept at a constant tem-

perature for 30 min before the measurements.

The ionic transport number was measured with the

oxygen concentration cell represented as:

Pt;O2V; =Srh� alumina=O2W; Pt ðPO2V > PO2

W Þ

Thin Pt paste was printed on both sides of the speci-

mens, and heated at 1473 K in air for 10 min. The

specimens were attached to an Al2O3 tube by using

Fig. 1. Cell structure of CO2 gas sensors using Srh-alumina. (a) Closed-type; (b) open-type.

T. Goto et al. / Solid State Ionics 156 (2003) 329–336330

alumina cement. Dried 100% O2 was passed in the

cathode side of the cell, and mixtures of O2–Ar gases

ranging from 5% to 45% O2 were used to control the

oxygen partial pressure in the anode side. The gas

flow rates in the cathode and anode sides were kept at

400 ml/min. The measurements were conducted at the

temperature range between 873 and 1473 K.

Srh-alumina, SrCO3 powder and Pt paste were used

to construct the cell (CO2 gas sensor) represented as:

Pt;CO2V;O2V; SrCO3=Srh� alumina=O2W; Pt

Porous Pt electrodes were printed on both surfaces

of the Srh-alumina pellet and heated at 1473 K for 10

min in air. Then, mixtures of Pt paste and SrCO3

powder were printed on one side of the Pt electrode

several times to make an auxiliary electrode. The

amounts of SrCO3 powder in the mixture were

increased gradually in the electrode. The specimens

with the auxiliary electrode were heat-treated at 1273

K in CO2 atmosphere for 10 min. The cell was

constructed in two types, closed- and open-types, as

shown in Fig. 1(a) and (b), respectively. In the closed-

type, the reference electrode and the working electrode

were separated into two gas chambers; in the open-

type, the whole cell was kept in the same flowing gas.

For the closed-type cell, reference gases of 10–50%

O2–Ar mixtures were passed in the cathode, and

working gases of 10–50% O2–CO2 mixtures in the

anode. For the open-type cell, 10–90% O2–CO2

mixture gases were passed around the specimens.

The electrode reactions for the cell may be given as

Eqs. (1) and (2):

Anode : SrCO3ðsÞ ¼ Sr2þ þ CO2VðgÞ

þ 1=2 O2VðgÞ þ 2e� ð1Þ

Cathode : Sr2þ þ 1=2 O2WðgÞ þ 2e�

¼ SrO ðin Srh� aluminaÞ ð2ÞThe overall cell reaction may be expressed as Eq. (3):

SrCO3ðsÞ þ 1=2 O2WðgÞ

¼ SrO ðin h� aluminaÞ þ CO2VðgÞ þ 1=2 O2VðgÞð3Þ

Since SrCO3 is a pure material, the activity of SrCO3

would be unity. The EMF value of the cell may be

given as Eq. (4):

EMF ¼ �DG0ð3Þ=2F

� ðRT=2FÞlnðaSrOPCO2V PO2

V1=2=PO2W1=2Þ ð4Þ

where DG(3)0 is the standard Gibbs free energy change

of the Eq. (3) [12], aSrO is the activity of SrO in the

Fig. 2. XRD pattern of bulk Srh-alumina.

T. Goto et al. / Solid State Ionics 156 (2003) 329–336 331

Srh-alumina, and PO2V , PCO2

V and PO2W are the partial

pressures of O2 and CO2 in the anode and O2 in the

cathode, respectively. RT/2F has the usual meaning.

In the open-type cell (Fig. 1(b)), Eq. (4) will be sim-

plified as Eq. (5), because PO2V and PO2

W have the same

value.

EMF ¼ �DG0ð3Þ=2F � ðRT=2FÞlnðaSrOPCO2

Þ ð5Þ

Hence, the EMF changes only with PCO2in the open-

type cell at every temperature.

3. Results and discussion

Srh-alumina in a single phase was obtained in the

present work. Fig. 2 shows the XRD pattern of bulk

Srh-alumina with the composition of SrO/MgO/

Al2O3 = 1:1:5 (SrMgAl10O17). The bulk specimen

had greater intensity of (002) and (004) peaks than

those of powdered specimens. This indicates a slight

c-orientation of the specimens. Iyi and Gobbels [13]

synthesized single crystals of Srh-aluminas by a

floating zone (FZ) method, and studied detailed crys-

tal structures. Table 1 summarizes the synthesis of

Srh-aluminas reported in literatures and the present

work. The sintering temperatures in the present work

were almost the highest among the past reports. This

may enable us to obtain dense specimens. Fig. 3

shows the relationships between density and sintering

temperature for the Srh-aluminas in the present work.

The specimens sintered at more than 2023 K were

almost translucent and had a high relative density

(98% of the theoretical value). The specimens sintered

at temperatures lower than 1873 K could not be used

for a gas sensor, because the specimens were porous

resulting the leakage of gases.

Fig. 4(a) and (b) shows a.c. impedance spectra of

the Srh-alumina measured at 1673 and 873 K, res-

Table 1

Comparison of present and reported results on the synthesis of Srh-alumina

Sintering temperatre (K) Lattice constant (a, c/nm) Reference

1473–1873 0.56255, 2.23902 [14]

1923–2038 0.5561, 2.2334 [10]

1923 – [8]

(FZ method) 0.5620, 2.2400 [13]

1873–2073 0.5621, 2.2238 present work

Fig. 3. Relationship between density and temperature for Srh-alumina.

Fig. 4. A.c. impedance spectra of Srh-alumina. (a) At 1673 K; (b) at

873 K.

T. Goto et al. / Solid State Ionics 156 (2003) 329–336332

pectively. At high temperatures, a slightly depressed

semicircle was obtained in the high frequency region

with a straight line in the low frequency region. At

low temperatures, the depressed semicircle was sepa-

rated into two semicircles. Since the Srh-alumina is in

a single phase, these two semicircles may be associ-

ated with bulk and grain boundary impedance.

Fig. 5 shows Arrhenius plots of the total conduc-

tivity for the Srh-aluminas obtained in the present

work compared with other literature values. The

activation energy for the conduction was 103 kJ

mol� 1 for Srh-alumina in the present work. Yama-

guchi et al. [11] synthesized Srh-alumina by solid

state sintering, and measured the conductivity by a.c.

impedance spectroscopy in the temperature range

between 700 and 1373 K. The total conductivity and

activation energy of Srh-aluminas in the present work

had almost the same values as those of Yamaguchi et

al. [11]. Schafer et al. [9] synthesized Srh-alumina by

solid state sintering. The total conductivity of Srh-alumina in the present work was 10–100 times greater

than that of Schafer et al. [8,9]. SrhW-aluminas were

also prepared by an ion exchange method [6,7]. The

total conductivity of the specimens prepared from

NahW-alumina by ion exchanging showed greater

values than those by solid state sintering.

Fig. 6 shows the relationship between the total

conductivity and oxygen partial pressure (PO2) for

Srh-alumina. The total conductivity was independent

of PO2. This suggests that the Srh-alumina may be

predominantly an ionic conductor.

The ionic transport number may be determined

from the EMF value of the oxygen concentration cell

[15]. According to Nernst equation, the theoretical

EMF value of the cell can be expressed as Eq. (6):

EMF ¼ RT=4F ln ðPO2W =PO2

V Þ ð6Þ

The ionic transport number (tion) may be calculated

from Eq. (7),

tion ¼ EMFth=EMFexp ð7Þ

where EMFth and EMFexp are the theoretical and

experimental EMF values of the cell, respectively.

George and Virkar [16] used the oxygen concentration

cell to measure the ionic transport number of La3 + in

LaNb3O9. The oxygen concentration cell measurement

cannot identify the mobile species; however, this

Fig. 5. Temperature dependence of total conductivity of Srh-aluminas.

Fig. 6. Relationships between total conductivity and oxygen partial

pressure for Srh-alumina.

T. Goto et al. / Solid State Ionics 156 (2003) 329–336 333

method enables one to determine the ionic transport

number irrespective of cation and anion conductors.

Fig. 7 shows the effect of temperature on the ionic

transport number obtained from the oxygen concen-

tration cell for Srh-alumina and NahW-alumina as a

standard specimen. The ionic transport number of

Srh-alumina calculated from Eq. (7) was 0.89–0.94,

close to 1, at 1073–1473 K. These values decreased

significantly with decreasing temperature at low tem-

peratures. It is known that the electrode reactions may

be not in equilibrium at low temperatures because of

the slow charge transfer at the electrode/electrolyte/

gas interface. This might have resulted to the decrease

of the EMF values.

Fig. 8 shows the temperature dependence of EMF

values for the closed- and open-type cells at 1023–

1373 K. The EMF values of both types increased with

increasing temperature, and had the same values. The

EMF values were not stabilized below 1073 K. Since

the electrical conductivity is too small at low temper-

atures, the electrode reactions could hardly reach the

equilibrium at temperatures less than 1073 K.

Fig. 9 shows the CO2 partial pressure dependence

of EMF values for the open-type cell at 1173–1373

K. The EMF values increased with decreasing PCO2.

Fig. 7. Relationships between ionic transport number and temper-

ature for Srh-alumina and NahW-alumina.

Fig. 8. Temperature dependence of EMF for the closed- and open-

type cells.

Fig. 9. CO2 partial pressure dependence of EMF for the open-type

cell.

T. Goto et al. / Solid State Ionics 156 (2003) 329–336334

This trend corresponds well with Eq. (5). Fig. 10

shows the time response of EMF values of the open-

type cell measured at 1373 K. The EMF values were

stabilized within a few minutes after the change of

CO2 partial pressures. However, the electron transfer

number calculated from Fig. 9 was about 15, which

was far greater than the theoretical value of 2.

Although the open-type sensor responded to the

change of ambient CO2 partial pressures, the true

equilibrium might not have been established in the

sensor.

4. Summary

Sr aluminate with the h-alumina structure was

synthesized in a SrO–MgO–Al2O3 system by solid

state sintering. Srh-alumina in a single phase was

obtained at a composition of SrO/MgO/Al2O3 = 1:1:5.

The activation energy of the conduction was 103 kJ

mol� 1. The ionic transport numbers measured by the

oxygen concentration cell were close to 1 in the

temperature range between 1200 and 1600 K. The

CO2 gas sensor using the Srh-alumina as a solid

electrolyte showed quick response to the change of

CO2 partial pressure at temperatures from 1073 to

1373 K. The open-type CO2 sensor is particularly

advantageous because the EMF values are independ-

ent of the oxygen partial pressure.

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