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carbonate composite solid electrolytes for potentiometric CO 2 sensors
SOLID STATE
loMcs
Solid State Ionics 96 (1997) 201-208
EISE3’IER
Yttriakarbonate
composite
solid electrolytes sensors
for potentiometric
CO,
A. Dubbe, M. Wake, Y. Sadaoka” Dept.of Materials
Science and Engineering,
Received
8 February
Ehime
University,
1996; accepted
Bunkyo-cho,
20 December
790- 77 Matsuyama,
Japan
1996
Abstract The effect of yttria addition to Na,CO,/BaCO, composites for potentiometric CO, sensors is studied in respect to ionic conductivity, microstructure, and sensing properties. The observed dependence of the sensing properties on both electrolyte and electrode materials is ascribed to varying contributions of yttria, Na2C0,, and BaCO, phases to a mixed potential and to surface carbonate formation on yttria grains. Kf~word.~: CO? sensor; Yttria; Potentiometric
gas sensor
1. Introduction
a thin layer between
Potentiometric solid-electrolyte CO, gas sensors are important for application in domestic, industrial, medical, agricultural, and other areas. Their advantages are simple construction, high selectivity, and the definite Nernstian relation between partial pressure and cell voltage. Their function principle is based on a solid carbonate/electrode interface, which generates the cell voltage and thus is responsible for the sensing properties. In early investigations, K,CO, pellets were used as sensitive carbonate [ 1,2]. More recently, other alkali carbonates, including Li,CO, [3], which lowers the water cross-sensitivity [4], Na,CO, [3,5-81, and Cs,CO, [2], as well as mixtures of alkali carbonates [9] were employed either as pellets or as *Corresponding author. Tel.: + 89-927-9891; 9856: e-mail: [email protected] 0167-2738/97/$17.00 0 1997 Elsevier Pff SOl67-2738(97)00010-6
fax:
+ 89.927.
Science B.V. All rights reserved
the electrode and another alkali ion conductor. By addition of high concentrations of alkali earth carbonates, CaCO,, SrCO,, and BaCO,, as a second component to Li,CO, or Na,CO,, the water cross-sensitivity was suppressed completely at 770-820 K [lo-131. In the present study, Na,CO,/BaCO, composites containing yttria (Yz03) as a third component are employed as sensitive material in potentiometric CO, sensors [14]. As a similar approach, properties of CO, sensors using an AI,O,/(Li, Na, K&CO, composite were reported recently [ 151. A resistive CO, sensor using the similar material Na,CO,/ BaCO,/CaO/Y,O, operating at 773 K was described in [ 161. A&O, /K,SO, (Na,SO,) composites were suggested for potentiometric SO, sensors [ 171. The addition of fine particles of an insulating material (heterogeneous doping) is known to increase the ionic conductivity of many solid electrolytes. This effect is explained by increased defect
202
A. Duhhr
et al. I Solid State Ionics
96 (1997)
l-208 P
concentration and thus increased ionic conductivity near the particle/electrolyte interface [ l&19]. However, addition of Al,O, to Li,CO, decreases the conductivity due to formation of-aluminates [ 181 and addition of MgO decreases the conductivity of Na,CO, [20]. Yttria addition was found to increase ionic conductivities of Li,SO, [ 181, K,SO, [ 171, and Na,CO,
test
gas
It
working efectrode
+ BaCO, + Y203 Na$03 + BaCO,
Na$03
Ag adhesive
WI. reference gas I
-
reference electrode
Fig. I. Cross-sectional view of the CO?-sensor
2. Experimental The sensors consisted of Na,CO,-based electrolyte discs with electrodes made from Au, Pd, or Pt paste (Demetron M8000, M8070, and Pt-black, respectively, fired for 2 h at 873 K), resulting in cells of the type P co,, PO?, M/carbonate M’, P;o,,
P;,
composite/
(M, M’ = Au, Pd, Pt).
(1)
The solid-electrolyte discs were prepared by melting mixtures of Na,CO, and BaCO, (analytical grade, dried for 12 h at 673 K) in an alumina crucible and quenching the melt by pouring onto a brass plate. The product was ground in a stabilized-zirconia ballmill, compressed into pellets and sintered for 1.5 h at lo-20 K below the melting point in a closed crucible filled with Na2C0, powder. Yttria powder (99.9%, dried for 12 h at 673 K) was added to some of the samples prior to ball-milling. The Na,CO, /BaCO, pellets were sealed on to a-alumina plates by heating for 0.5 h up to the onset of plasticity while being lightly pressed against each other by a weight. This resulted in a strongly bonded connection, presumably due to formation of a sodium aluminate interface layer. Yttria-containing discs were sealed by the same method on top of the Na,CO,/BaCO, pellets. Working electrodes were formed on the free carbonated surfaces and reference electrodes at the opposite carbonate surfaces in small holes (1 mm diameter) in the alumina plates. The plates were mounted at the ends of alumina tubes, resulting in two separated gas spaces at both electrodes (Fig. 1). Standard gases, CO, in artificial air (21 vol.% 0, in N,), were introduced to the working electrode and 100 Pa CO, in artificial
air was taken
as reference
gas. The cell
voltage was measured with a digital electrometer (input impedance > lOI a). Response time t,,, i.e., the time at which AE(t)/ E(a)=0.9 after a step change of partial pressure at t=O, were determined by switching from 100 to 10 Pa CO,/air. In order to subtract the influence of the finite gas-exchange time in the sample chamber, measurements were taken at several flow rates and extrapolated to infinite flow rates in plots t,, versus reciprocal flow rate. Cross-sensitivity to water vapor was measured by passing the test gas through water at 295 K, which resulted in 67% r.h., i.e., 1.64 kPa H,O. The elemental distribution of Y, Na, and Ba at the surface of the composites was measured with a scanning-Auger electron microprobe (SAM, Perkin Elmer Phi 650) using the Y(MNN), Na(KLL), and Ba(MNN) lines and a low beam current, 0.2 nA at 2 kV, which avoided any electron-microscopically visible electron beam damage. For X-ray photoelectron spectroscopy (XPS, Perkin Elmer Phi 1600), Al Ka radiation was used at 0.1 pPa base pressure and the binding energies were referenced to the adventitious carbon contamination C*ls at 285.0 eV. Both the samples for SAM and XPS were prepared in the same way, by grinding the surface of the compressed pellets with #lOOO emery paper and annealing for 10 h at 773 K in 100 Pa CO,/artificial air in a closed quarts tube. The samples were transferred under pure nitrogen into the XPS and SAM instruments and were not sputtered. Powder X-ray diffraction (PXD) measurements were carried out on crushed and ground pellets with Ni-filtered Cu Ka radiation. For measuring the electrolyte conductivity, Au electrolytes (4X4 mm’) were prepared on the oppo-
A. Dubbe et ul. i Solid Stute lonks
203
96 (1997) 201-208
site sides of ground electrolyte discs (10 mm diameter) by vacuum evaporation and annealed for 0.5 h at 960 K. This resulted in essentially non-porous thick films. Impedance spectra from 100 Hz to 100 kHz were measured with 50 mV amplitude in a 200 Pa CO, /artificial air atmosphere. After equilibration for 2 h at the highest measurement temperature of 623 K, measurements were carried out in 50 K steps down to 293 K.
...... . .......... ... ..... ......_.. r.‘i’.‘t Au pd
IdPr
3. Results and discussion Na,CO,/BaCO, composites with Ba : Na atomic concentration ratios of 0.005 and 0.30 were investigated. The small Ba content of 0.5 at.% limits the growth of Na,CO, crystallites, so that the ceramics are less porous as compared with pure Na,CO, and are plastically deformable at lo-20 K below the melting point. The Ba : Na atomic ratio of 0.30 is the eutectic composition [21], which was chosen in order that the composition of the solidified melt is not changed by rests in the crucible. From each of these composites, samples containing 0 and 30 wt.% yttria were prepared and applied as compact discs on whose smooth surfaces porous Au, Pd, or Pt electrodes were attached, so that the influence of an additional ionic conductor/electrode interface as found in set-ups employing thin carbonate films [22] was avoided. At four of the resulting 12 sensors, hermetic connection between the carbonate and the alumina plate could be achieved, or the Pt electrodes were non-conducting due to reaction with Na,CO,, so that they are not considered further here. The cell voltage E was measured between 573 and 923 K for 1, 10, 100, and 1000 Pa CO, in artificial air. The effective electron numbers rr for the reduction of CO, are calculated from the slopes of E versus log Pco2 p lots (Fig. 2) between 1 and 100 Pa CO, partial pressure after the Nemst equation,
Fig. 2. CO,
ln(P,,,~P~~~P’,~?P’U~
‘).
partial pressure dependence of cell voltage at 773 K
(meaning of symbols is the same as in Fig. 3). compared with theory
from
Eq.
(2)
for
n =2
(A).
Electrode
materials
are
indicated on the graphs. For clarity, the curves are shifted along the abscissa and normalized to 0 mV for IO Pa.
voltage for 1 Pa CO, increased by 5-20 mV when the test-gas flow was lowered from 24 to 3 l/h, while for 10 Pa CO,, similar increases of up to 2 mV were found in only some cases. This was probably caused by diffusion of reference gas through the pores of the carbonate disc to the working electrode and was corrected by extrapolation to infinite flow rates. We have found that for practical sensor applications this problem can be avoided by bonding the carbonate composite on other highly densified ionic conductors by hot pressing. However, such set-ups were not used in the present study since any possible influence of an additional carbonate/solid electrolyte interface on the sensing properties should be avoided. The yttria-containing Ba-poor composite showed the highest sensitivity. RTfnF, about 95% of the theoretical. The response times follow an Arrhenius-type law in the measured range of t,, larger than 1 s (Fig. 3), t
E = (RTInF)
H 1 decade
1% Pwq
90
=
A exp(E,, lkT)
(3)
(2)
The theoretical electron number for the reaction of CO, and O2 forming carbonate ions is n =2. the experimental n-value did not depend strongly on the temperature, so that the mean value over the temperature is listed in Table 1. At all sensors the cell
as frequently found for various solid-electrolyte potentiometric gas sensors 120.23-251. However. earlier investigations of yttria-free electrolytes with Ba : Na=0.005 had shown that for some electrode materials Eq. (3) is not valid for response times well below 1 s 181. The activation energies E;, (Table 1)
A. Dubbe
204
Table I Effective electron number n in Eq. (2) between composition and electrode material Ba
: Na
I Solid
State
96 (1997)
I and 100 Pa CO> and activation energy
AU
Pd
0 0 30 30
2.80 2.15 2.12 2.27
2.44
1.4
1.5
2.11 2.33
1.6
1.7
(K-l)
Fig. 3. Temperature dependence of response times (meaning of symbols is the same as in Fig. 4). The dotted line was extrapolated from measurements at higher temperatures [8].
as well as t,, values drastically depend on both electrode and electrolyte materials. In all cases, yttria addition increased the response times and BaCO, addition decreased them. For the yttria-containing sensors with Au and Pd electrodes, long response times correlate with high sensitivity. Admixture of water vapor to the test gas resulted in reversible increases of the cell voltage for the Ba-poor composite (Fig. 4). By yttria addition this
0
-s 5
-10
8
-20
n
Ba/Na=O.30,
30 wt 8 Y
&
-30
0 Ba/Na=O.OOS
0 wt % Y
-40
550 600 650 700 750 800 850 T
201-208
E, of the response
time depending
on electrolyte
E, (ev)
Y,O,
1000/T
^x
Ionics
n
Wt.%
0.005 0.30 0.005 0.30
et al.
(K)
Fig. 4. Cross-sensitivity to water vapor for 100 ppm CO, (CION) and 1000 ppm CO, (0) in artificial air for Au electrodes.
Pt
AU
Pd
Pt
I .59 I .65
I .44
1.64 2.13
1.13 I .86
effect was decreased to about 50%, at 773 and 823 K. By raising the Ba content to Ba : Na=0.30, the water cross-sensitivity nearly vanished at high temperatures, and a reversible decrease of the cell voltage was only observed below 673 K. Yttria addition to the Ba-rich composite decreased this effect to about 50%. In Ref. [ 131 however, the water cross-sensitivity was reported to vanish only for Ba : Na atomic ratios higher than 0.5 for Pt electrodes (873 K, 39 Pa CO,). The reason for this discrepancy may be the influence of the different electrode materials on the water cross-sensitivity. The surface of the yttria-free and Ba-rich (Ba : Na=0.30) composite consists of grains with diameters in the order of 10 pm, which consist either of Na,CO, or BaCO, and are separated by clearly visible grain boundaries (Fig. 5). This was confirmed by Auger spectra acquired on small rectangles on a Na,CO, grain, where no Ba-peak was detected, and on a BaCO, grain, where no Na-peak was detected (Fig. 6), within the experimental error, given by the standard deviation of the background noise divided by the peak height, of 3.6% and 5.7% for Ba and Na, respectively. The surface of the composite with 30 wt.% yttria and a Ba : Na atomic ratio of 0.30 consists of Na,CO,, BaCO,, and yttria grains with diameters of about l-5 pm, obtained from the elemental distribution maps, and no visible grain boundaries. In the XPS spectra, a second component in the Y3d doublet (Y3d,,, at 156.1 eV) was found (Fig. 7a), which is attributed to carbonatic yttrium. A proof is that the XPS of a compressed pellet of pure yttria, pretreated for 3 h in 10 Pa CO, in artificial air at 473 K (Fig. 7b), revealed a similar shoulder in the at 157.2 eV) and also carY3d doublet (Y3d,,, bonatic carbon (Cls at 289.3 eV). Both vanished after sputtering (2 kV Ar+, Fig. 7~). Vasquez [26]
A. Dubbe
et al. I Solid State Ionics
96 (1997)
201-208
Fig. 5. Secondary electron micrographs and SAM maps of yttria-free (left) and 30 wt.% yttria-containing (right) composites (Ba after annealing for IO h at 773 K in IO0 Pa CO,
.
‘
I
560 580 600 kin. energy (eV)
. 960 km.
980 loo( energy (eV)
Fig. 6. Auger electron spectra of Na and Ba regions acquired ( I ) on a BaCO,
grain and (2) on a Na,CO,
composite in Fig. 5.
: Na=0.30)
in artificial air
I I
205
grain of the yttria-free
reported Y,(C0,),*3H20 binding energies (relative to C* 1s at 285.0 eV) of 288.6 and 157.1 eV for C 1s and Y3d,,*, respectively, the latter being in good agreement with the present results. By PXD (Fig. 8) of Ba : Na=0.30 samples with O-40 wt.% yttria, only Na,CO,, BaCO,, and yttria phases were detected and no indication of the known mixed phases NaYO?, Y2Ba0,, Y2Ba407, Y,Ba,O,, and Y,Ba,O,CO, was found, which are only stable at higher temperatures and lower CO, partial pressures [27,28]. Furthermore, no hint of glass formation was found; i.e., the diffraction patterns of the three detected phases were the same as in their pure state. The ionic conductivities of composites with a Ba : Na atomic ratio of 0.30 and yttria content from 0 to 40 wt.% were determined from curve fits of their impedance spectra. In the high-frequency parts of the Cole-Cole plots, semicircles resulting from the
206
A. Dubbe et al. I Solid State tonics 96 (1997) 201-208
_.
‘-;..:.*- .....~.,,,,;-i-j-i.“..“~? 0
92
0
0
288
I
1
284 binding
162
158
154
energy (eV)
composite
20
mixture
30
40 50 2 8 (deg.)
Fig. 9. Conductivity of composite electrolyte pellets (Ba: Na= 0.30) in 200 Pa CO, in artificial air. Solid lines: curve fits after Eq. (4).
Fig. 7. XPS spectra of (a) Na,CO,-BaCO,-Y,O, composite (Ba: Na=0.30,40 wt.% Y,O,) annealed for 10 h at 773 K in 100 Pa CO, in artificial air, (b) pure Y,O, after 3 h at 473 K in 10 Pa CO2 in artificial air, and (c) the same sample after sputtering. Cls regions are displayed with the same scale, while Y3d spectra are normalized to unity.
1
1.2 1.4 I.6 1000/T (K-‘)
;; C
J
: ,a_
60
Fig. 8. X-ray diffraction diagrams of the Na2COJ-BaC03-Y201 composite (Ba : Na=0.30, 40 wt.% Y,O,) and of a mixture of Na,CO,, BaCO,, and Y,O, powders with the same composition. displayed with negative intensities.
The parameters A *, A CB, E,, and EGB were determined from least-squares fits of the log(aT) versus 1000/T plots. The term with the higher activation energy, E,, of 2.0-2.4 eV is assigned to intrinsic conduction of the Na,CO, crystallites, since it decreases with increasing yttria concentration and vanishes at about 20 wt.% yttria (Fig. 10). The reason for this is seen in the successive isolation of the Na,CO, grains from each other by yttria grains until, above 20 wt.% yttria, any closed conduction path through a chain of interconnected Na,CO, grains has vanished. Over the whole observed temperature range, the conductivity is dominated by the term with the low activation energy, E,,, of 0.8- 1.1 eV, which is interpreted as grain-boundary conduction. a;,, is increased with increasing yttria content,
geometrical capacity and the electrolyte resistance were found, followed by incomplete depressed semicircles of the electrode impedance [29] in the low-frequency parts. The log(&) vs. 1000/T curves have a concave shape (Fig. 9), which is explained by a parallel resistance circuit of two conduction processes with different activation energies, expressed as u = a, + a,,
=A,T-’
yttria
exp(-E,lkT)
+ A,,T-’
exp(-E,,lkT).
(4)
cont.
(wt
96)
yttria
cont.
(wt
%)
Fig. 10. Conductivity parameters of composite electrolyte pellets (Ba : Na = 0.30) in 200 Pa CO? in artificial air from curve-fits after Eq. (4).
A. Dubbe et al. I Solid State Ionics 96 (1997)
up to 20 wt.%, and decreases with higher yttria content. This is explained by improved grain boundary conduction due to decreased grain sizes for low yttria content and a blocking effect of insulating yttria grains at higher yttria content. At 873 K in O,, the conductivity of yttria, mainly due to electronic hole conduction with a minor contribution of 02ionic conduction [30,31], is 3.2X lo-” S/cm and is lower than the conductivity of the carbonate composites by a factor of lo4 to 105, so that yttria can be regarded as an insulator. Also for Na,CO,/CaO composites, concave log(cTT) vs. 1000/T curves were reported and the transition points at 843-893 K were tentatively assigned to a phase transition involving the rotation or vibration of the carbonate anion [32]. Well-known phase transitions of Na,CO, occur at 758 and 699 K, which result in changes of the activation energy for ionic conduction only for pure or nearly pure Na,CO,, but have smaller or no effects on the conductivity of Na,CO,/CaO or Na,CO,/BaCO, composites [20,32].
4. Conclusions Deviations from the theoretical sensitivity, i.e., n>2 in Eq. (2) were also reported for sensors employing Na,CO,-impregnated electrolytes in contact with other sodium ionic conductors below 673 K [5,22] and were ascribed to the presence of coexisting CO,-sensitive electrode/carbonate interfaces and CO,-insensitive electrode/ion-conductor interfaces. Their cell voltage is thus a mixed potential with contributions from these two interfaces [22]. This explanation, however, does not apply to the deviations from the Nernstian sensitivity of the sensors in the present investigation. A similar effect for sensors with electrodes on compact carbonate discs was also reported in [20] for Na2C0,/BaC0, composites (Ba : Na = 0.005) for an operation temperature below 773 K. In this case, we see the reason in a slow equilibration of the volume of the Na,CO, crystallites with the CO, and 0, activities in the surrounding gas phase. This effect is less disturbing in thin carbonate films, but electrodes formed on smooth surfaces of compact carbonate discs have the further advantage of a more favorable electrode morphology for short response times [8]. Since yttrium carbonate is formed on the surface
201
201-208
of the yttria grains, it is expected that, to a certain degree, Y 3f ions diffuse into the Na,CO, phase, i.e., a solid solution of yttrium carbonate forms in the vicinity of the yttria grains. Yttria as a pure phase has an activity of 1 and thus the activity of the dissolved Y”+ ions is a thermodynamic function of the CO, partial pressure. The expected diffusion into the Na,CO, phase thus enhances CO, equilibration kinetics. This explains the observed prolongation of both response time and sensitivity upon yttria addition; only the yttria-free, Ba-rich sensor with Au electrodes is an exception. A similar acceleration effect of solid carbonate equilibration with the surrounding gas phase was found in thermal decomposition studies of Y/Gd-doped Ag,CO, by Wydeven and coworkers [33,34]. Also Ag’ ion diffusion from Ag electrodes into the carbonate phase has been found to improve sensitivity and elongated response times of CO2 sensors [35]. However, Ag electrodes have the disadvantages of blocking the electrode kinetics at both high CO, and 0, partial pressures [8,35] and of expected high affinity to sulfur-containing contaminating gases. A further indication that the yttria particles play an active electrochemical role in CO, sensing is the decrease of water cross-sensitivity upon yttria addition. This means that the interfaces between the surface carbonate on the yttria particles and the electrode particles are contributing to the cell voltage in the same way as the NazCO,/electrode interfaces, but are not less sensitive to water vapor. The water cross-sensitivity of Na,CO, is explained by formation of NaOH defects [36]. A similar effect is believed to be responsible for the drastic reduction of water cross-sensitivity upon BaCO, addition to Na,CO,: Both water vapor sensitive Na,CO,/electrode interfaces and water vapor insensitive BaCO,/ electrode interfaces contribute to a mixed potential. Sublimation or decomposition of Na2C0, in contact with Au or Pt as reported in 1371 may further reduce the water cross-sensitivity.
References [I] M. Gauthier, A. Btlanger, D. Fauteux, C. Ball and R. Cetht, Anal. Chem. Symp. Ser. 17 (1983) 353. [21 R. Cbht, C.W. Bale and M. Gauthier, 131 (1984) 63.
.I. Electrochem.
Sot.
208 [3] T. Ogata,
A. Dubbe et al. I Solid State lonics 96 (1997)
J. Mater. Sci. Lett. 5 (1986) 285. [4] N. Imanaka, T. Kawasato and G. Adachi, Chem. Lett. (1991) 1743. [5] J. Liu and W. Weppner, Eur. J. Solid State Inorg. Chem. 28 (1991) 1151. [6] T. Maruyama, Solid State lonics 24 (1987) 281. [7] Y. Sadaoka, Y. Sakai, M. Matsuguchi and T. Manabe, J. Mater. Sci. 28 (1993) 5783. [S] A. Dubbe, H.D. Wiemhofer, Y. Sadaoka and W. Giipel, Sens. Actuat. B25 (1995) 600. [9] N. Imanaka, T. Murata, T. Kawasato and G. Adachi, Chem. Lett. (1992) 103. [IO] S. Yao, Y. Shimizu, N. Miura and N. Yamazoe, Jpn. J. Appl. Phys. 31 (1992) L197. [ 1 I] S. Yao, Y. Shim& N. Miura and N. Yamazoe, Chem. Lett. (1990) 2033. [12] S. Yao, S. Hosohara, Y. Shimizu and N. Miura, Chem. Lett. (1991) 2069. [13] N. Miura, S. Yao, Y. Shimizu and Y. Yamazoe, J. Electrothem. Sot. 139 (1992) 1384. [ 141 A. Dubbe, Y. Sadaoka and Y. Sakai, in: Chemical Sensors 10 A, Proc. 18th Chemical Sensor Symp., Sendai, Japan (1994) p. 81. [15] Y. Niizeki, T. Nishina and I. Uchida, in: Chemical Sensors 10 A, Proc. 18th Chemical Sensor Symp. Sendai, Japan (1994) p. 77. [ 161 T. Sugai and T. Matsuzawa, Sens. Actuat. Bl3-14 (1993) 480. [ 171 U. Guth, S. Brosda, B. Loscher, A. Simmich, P. Schmidt and H.H. Mijbius, Mat. Sci. Forum 76 (1991) 137. [18] N.F. Uvarov, V.F. Isupov, V. Sharma and A.K. Shukla, Solid State Ionics 51 (1992) 41.
201-208
[ 191 J. Maier, J. Electrochem. Sot. 134 (1987) 1524. [20] S. Brosda, Dr. Thesis, University of Greifswald, Germany (1992). (211 IN. Belyaev and M.L. Sholokhovich, Sbmik Statei Obshchei Khim. Akad. Nauk SSSR 1 (1953) 141. [22] N. Yamazoe and N. Miura, in: Proc. Transducers ‘95/ Eurosensors IX, Stockholm, Sweden (1995) p, 841. [23] A.J.A. Winnubst, A.H.A. Scharenborg and A.J. Burggraaf, J. Appl. Electrochem. 15 (1985) 139. [24] S.P.S. Badwal, M.J. Bannister and F.T. Ciacchi. J. Appl. Electrochem. 18 ( 1988) 608. [25] R. Akila and K.T. Jacob, Sens. Actuat. 16 (1989) 31 I. [26] R.P. Vasquez, J. Electron Spectrosc. Relat. Phenom. 50 (1990) 167. [27] P. Karen and A. Kjekshus, J. Solid State Chem. 94 (1991) 298. [28] H. Brunn and R. Hoppe, Z. Anorg. Allg. Chem. 430 (1977) 144. [29] A. Dubbe, H.D. Wiemhofer and W. Giipel, J. Electrochem. Sot. 142 (1995) 2757. [30] T. Norby and P. Kofstad, J. Am. Ceram. Sot. 69 (1986) 784. [31] Z.S. Volchenkova and D.S. Zubankova, Izv. Akad. Nauk SSSR, Neorg. Mater. 12 (1976) 1695. [32] P. Cerisier and F. Roux, J. Solid State Chem. 22 ( 1977) 245. [33] T. Wydeven and E. Rand, J. Catal. 12 (1968) 271. [34] T. Wydeven, J. Catal. 16 (1970) 82. [35] A. Dubbe, H.D. Wiemhofer and Y. Sadaoka, J. Electrochem. sot. 144 (1997) 943. [36] Y. Sadaoka, Y. Sakai and T. Manabe, J. Mater. Chem. 2 (1992) 945. [37] H.P. Bonzel, G. Broden and H.J. Krebs, Appl. Surf. Sci. 16 ( 1983) 373.
loMcs
Solid State Ionics 96 (1997) 201-208
EISE3’IER
Yttriakarbonate
composite
solid electrolytes sensors
for potentiometric
CO,
A. Dubbe, M. Wake, Y. Sadaoka” Dept.of Materials
Science and Engineering,
Received
8 February
Ehime
University,
1996; accepted
Bunkyo-cho,
20 December
790- 77 Matsuyama,
Japan
1996
Abstract The effect of yttria addition to Na,CO,/BaCO, composites for potentiometric CO, sensors is studied in respect to ionic conductivity, microstructure, and sensing properties. The observed dependence of the sensing properties on both electrolyte and electrode materials is ascribed to varying contributions of yttria, Na2C0,, and BaCO, phases to a mixed potential and to surface carbonate formation on yttria grains. Kf~word.~: CO? sensor; Yttria; Potentiometric
gas sensor
1. Introduction
a thin layer between
Potentiometric solid-electrolyte CO, gas sensors are important for application in domestic, industrial, medical, agricultural, and other areas. Their advantages are simple construction, high selectivity, and the definite Nernstian relation between partial pressure and cell voltage. Their function principle is based on a solid carbonate/electrode interface, which generates the cell voltage and thus is responsible for the sensing properties. In early investigations, K,CO, pellets were used as sensitive carbonate [ 1,2]. More recently, other alkali carbonates, including Li,CO, [3], which lowers the water cross-sensitivity [4], Na,CO, [3,5-81, and Cs,CO, [2], as well as mixtures of alkali carbonates [9] were employed either as pellets or as *Corresponding author. Tel.: + 89-927-9891; 9856: e-mail: [email protected] 0167-2738/97/$17.00 0 1997 Elsevier Pff SOl67-2738(97)00010-6
fax:
+ 89.927.
Science B.V. All rights reserved
the electrode and another alkali ion conductor. By addition of high concentrations of alkali earth carbonates, CaCO,, SrCO,, and BaCO,, as a second component to Li,CO, or Na,CO,, the water cross-sensitivity was suppressed completely at 770-820 K [lo-131. In the present study, Na,CO,/BaCO, composites containing yttria (Yz03) as a third component are employed as sensitive material in potentiometric CO, sensors [14]. As a similar approach, properties of CO, sensors using an AI,O,/(Li, Na, K&CO, composite were reported recently [ 151. A resistive CO, sensor using the similar material Na,CO,/ BaCO,/CaO/Y,O, operating at 773 K was described in [ 161. A&O, /K,SO, (Na,SO,) composites were suggested for potentiometric SO, sensors [ 171. The addition of fine particles of an insulating material (heterogeneous doping) is known to increase the ionic conductivity of many solid electrolytes. This effect is explained by increased defect
202
A. Duhhr
et al. I Solid State Ionics
96 (1997)
l-208 P
concentration and thus increased ionic conductivity near the particle/electrolyte interface [ l&19]. However, addition of Al,O, to Li,CO, decreases the conductivity due to formation of-aluminates [ 181 and addition of MgO decreases the conductivity of Na,CO, [20]. Yttria addition was found to increase ionic conductivities of Li,SO, [ 181, K,SO, [ 171, and Na,CO,
test
gas
It
working efectrode
+ BaCO, + Y203 Na$03 + BaCO,
Na$03
Ag adhesive
WI. reference gas I
-
reference electrode
Fig. I. Cross-sectional view of the CO?-sensor
2. Experimental The sensors consisted of Na,CO,-based electrolyte discs with electrodes made from Au, Pd, or Pt paste (Demetron M8000, M8070, and Pt-black, respectively, fired for 2 h at 873 K), resulting in cells of the type P co,, PO?, M/carbonate M’, P;o,,
P;,
composite/
(M, M’ = Au, Pd, Pt).
(1)
The solid-electrolyte discs were prepared by melting mixtures of Na,CO, and BaCO, (analytical grade, dried for 12 h at 673 K) in an alumina crucible and quenching the melt by pouring onto a brass plate. The product was ground in a stabilized-zirconia ballmill, compressed into pellets and sintered for 1.5 h at lo-20 K below the melting point in a closed crucible filled with Na2C0, powder. Yttria powder (99.9%, dried for 12 h at 673 K) was added to some of the samples prior to ball-milling. The Na,CO, /BaCO, pellets were sealed on to a-alumina plates by heating for 0.5 h up to the onset of plasticity while being lightly pressed against each other by a weight. This resulted in a strongly bonded connection, presumably due to formation of a sodium aluminate interface layer. Yttria-containing discs were sealed by the same method on top of the Na,CO,/BaCO, pellets. Working electrodes were formed on the free carbonated surfaces and reference electrodes at the opposite carbonate surfaces in small holes (1 mm diameter) in the alumina plates. The plates were mounted at the ends of alumina tubes, resulting in two separated gas spaces at both electrodes (Fig. 1). Standard gases, CO, in artificial air (21 vol.% 0, in N,), were introduced to the working electrode and 100 Pa CO, in artificial
air was taken
as reference
gas. The cell
voltage was measured with a digital electrometer (input impedance > lOI a). Response time t,,, i.e., the time at which AE(t)/ E(a)=0.9 after a step change of partial pressure at t=O, were determined by switching from 100 to 10 Pa CO,/air. In order to subtract the influence of the finite gas-exchange time in the sample chamber, measurements were taken at several flow rates and extrapolated to infinite flow rates in plots t,, versus reciprocal flow rate. Cross-sensitivity to water vapor was measured by passing the test gas through water at 295 K, which resulted in 67% r.h., i.e., 1.64 kPa H,O. The elemental distribution of Y, Na, and Ba at the surface of the composites was measured with a scanning-Auger electron microprobe (SAM, Perkin Elmer Phi 650) using the Y(MNN), Na(KLL), and Ba(MNN) lines and a low beam current, 0.2 nA at 2 kV, which avoided any electron-microscopically visible electron beam damage. For X-ray photoelectron spectroscopy (XPS, Perkin Elmer Phi 1600), Al Ka radiation was used at 0.1 pPa base pressure and the binding energies were referenced to the adventitious carbon contamination C*ls at 285.0 eV. Both the samples for SAM and XPS were prepared in the same way, by grinding the surface of the compressed pellets with #lOOO emery paper and annealing for 10 h at 773 K in 100 Pa CO,/artificial air in a closed quarts tube. The samples were transferred under pure nitrogen into the XPS and SAM instruments and were not sputtered. Powder X-ray diffraction (PXD) measurements were carried out on crushed and ground pellets with Ni-filtered Cu Ka radiation. For measuring the electrolyte conductivity, Au electrolytes (4X4 mm’) were prepared on the oppo-
A. Dubbe et ul. i Solid Stute lonks
203
96 (1997) 201-208
site sides of ground electrolyte discs (10 mm diameter) by vacuum evaporation and annealed for 0.5 h at 960 K. This resulted in essentially non-porous thick films. Impedance spectra from 100 Hz to 100 kHz were measured with 50 mV amplitude in a 200 Pa CO, /artificial air atmosphere. After equilibration for 2 h at the highest measurement temperature of 623 K, measurements were carried out in 50 K steps down to 293 K.
...... . .......... ... ..... ......_.. r.‘i’.‘t Au pd
IdPr
3. Results and discussion Na,CO,/BaCO, composites with Ba : Na atomic concentration ratios of 0.005 and 0.30 were investigated. The small Ba content of 0.5 at.% limits the growth of Na,CO, crystallites, so that the ceramics are less porous as compared with pure Na,CO, and are plastically deformable at lo-20 K below the melting point. The Ba : Na atomic ratio of 0.30 is the eutectic composition [21], which was chosen in order that the composition of the solidified melt is not changed by rests in the crucible. From each of these composites, samples containing 0 and 30 wt.% yttria were prepared and applied as compact discs on whose smooth surfaces porous Au, Pd, or Pt electrodes were attached, so that the influence of an additional ionic conductor/electrode interface as found in set-ups employing thin carbonate films [22] was avoided. At four of the resulting 12 sensors, hermetic connection between the carbonate and the alumina plate could be achieved, or the Pt electrodes were non-conducting due to reaction with Na,CO,, so that they are not considered further here. The cell voltage E was measured between 573 and 923 K for 1, 10, 100, and 1000 Pa CO, in artificial air. The effective electron numbers rr for the reduction of CO, are calculated from the slopes of E versus log Pco2 p lots (Fig. 2) between 1 and 100 Pa CO, partial pressure after the Nemst equation,
Fig. 2. CO,
ln(P,,,~P~~~P’,~?P’U~
‘).
partial pressure dependence of cell voltage at 773 K
(meaning of symbols is the same as in Fig. 3). compared with theory
from
Eq.
(2)
for
n =2
(A).
Electrode
materials
are
indicated on the graphs. For clarity, the curves are shifted along the abscissa and normalized to 0 mV for IO Pa.
voltage for 1 Pa CO, increased by 5-20 mV when the test-gas flow was lowered from 24 to 3 l/h, while for 10 Pa CO,, similar increases of up to 2 mV were found in only some cases. This was probably caused by diffusion of reference gas through the pores of the carbonate disc to the working electrode and was corrected by extrapolation to infinite flow rates. We have found that for practical sensor applications this problem can be avoided by bonding the carbonate composite on other highly densified ionic conductors by hot pressing. However, such set-ups were not used in the present study since any possible influence of an additional carbonate/solid electrolyte interface on the sensing properties should be avoided. The yttria-containing Ba-poor composite showed the highest sensitivity. RTfnF, about 95% of the theoretical. The response times follow an Arrhenius-type law in the measured range of t,, larger than 1 s (Fig. 3), t
E = (RTInF)
H 1 decade
1% Pwq
90
=
A exp(E,, lkT)
(3)
(2)
The theoretical electron number for the reaction of CO, and O2 forming carbonate ions is n =2. the experimental n-value did not depend strongly on the temperature, so that the mean value over the temperature is listed in Table 1. At all sensors the cell
as frequently found for various solid-electrolyte potentiometric gas sensors 120.23-251. However. earlier investigations of yttria-free electrolytes with Ba : Na=0.005 had shown that for some electrode materials Eq. (3) is not valid for response times well below 1 s 181. The activation energies E;, (Table 1)
A. Dubbe
204
Table I Effective electron number n in Eq. (2) between composition and electrode material Ba
: Na
I Solid
State
96 (1997)
I and 100 Pa CO> and activation energy
AU
Pd
0 0 30 30
2.80 2.15 2.12 2.27
2.44
1.4
1.5
2.11 2.33
1.6
1.7
(K-l)
Fig. 3. Temperature dependence of response times (meaning of symbols is the same as in Fig. 4). The dotted line was extrapolated from measurements at higher temperatures [8].
as well as t,, values drastically depend on both electrode and electrolyte materials. In all cases, yttria addition increased the response times and BaCO, addition decreased them. For the yttria-containing sensors with Au and Pd electrodes, long response times correlate with high sensitivity. Admixture of water vapor to the test gas resulted in reversible increases of the cell voltage for the Ba-poor composite (Fig. 4). By yttria addition this
0
-s 5
-10
8
-20
n
Ba/Na=O.30,
30 wt 8 Y
&
-30
0 Ba/Na=O.OOS
0 wt % Y
-40
550 600 650 700 750 800 850 T
201-208
E, of the response
time depending
on electrolyte
E, (ev)
Y,O,
1000/T
^x
Ionics
n
Wt.%
0.005 0.30 0.005 0.30
et al.
(K)
Fig. 4. Cross-sensitivity to water vapor for 100 ppm CO, (CION) and 1000 ppm CO, (0) in artificial air for Au electrodes.
Pt
AU
Pd
Pt
I .59 I .65
I .44
1.64 2.13
1.13 I .86
effect was decreased to about 50%, at 773 and 823 K. By raising the Ba content to Ba : Na=0.30, the water cross-sensitivity nearly vanished at high temperatures, and a reversible decrease of the cell voltage was only observed below 673 K. Yttria addition to the Ba-rich composite decreased this effect to about 50%. In Ref. [ 131 however, the water cross-sensitivity was reported to vanish only for Ba : Na atomic ratios higher than 0.5 for Pt electrodes (873 K, 39 Pa CO,). The reason for this discrepancy may be the influence of the different electrode materials on the water cross-sensitivity. The surface of the yttria-free and Ba-rich (Ba : Na=0.30) composite consists of grains with diameters in the order of 10 pm, which consist either of Na,CO, or BaCO, and are separated by clearly visible grain boundaries (Fig. 5). This was confirmed by Auger spectra acquired on small rectangles on a Na,CO, grain, where no Ba-peak was detected, and on a BaCO, grain, where no Na-peak was detected (Fig. 6), within the experimental error, given by the standard deviation of the background noise divided by the peak height, of 3.6% and 5.7% for Ba and Na, respectively. The surface of the composite with 30 wt.% yttria and a Ba : Na atomic ratio of 0.30 consists of Na,CO,, BaCO,, and yttria grains with diameters of about l-5 pm, obtained from the elemental distribution maps, and no visible grain boundaries. In the XPS spectra, a second component in the Y3d doublet (Y3d,,, at 156.1 eV) was found (Fig. 7a), which is attributed to carbonatic yttrium. A proof is that the XPS of a compressed pellet of pure yttria, pretreated for 3 h in 10 Pa CO, in artificial air at 473 K (Fig. 7b), revealed a similar shoulder in the at 157.2 eV) and also carY3d doublet (Y3d,,, bonatic carbon (Cls at 289.3 eV). Both vanished after sputtering (2 kV Ar+, Fig. 7~). Vasquez [26]
A. Dubbe
et al. I Solid State Ionics
96 (1997)
201-208
Fig. 5. Secondary electron micrographs and SAM maps of yttria-free (left) and 30 wt.% yttria-containing (right) composites (Ba after annealing for IO h at 773 K in IO0 Pa CO,
.
‘
I
560 580 600 kin. energy (eV)
. 960 km.
980 loo( energy (eV)
Fig. 6. Auger electron spectra of Na and Ba regions acquired ( I ) on a BaCO,
grain and (2) on a Na,CO,
composite in Fig. 5.
: Na=0.30)
in artificial air
I I
205
grain of the yttria-free
reported Y,(C0,),*3H20 binding energies (relative to C* 1s at 285.0 eV) of 288.6 and 157.1 eV for C 1s and Y3d,,*, respectively, the latter being in good agreement with the present results. By PXD (Fig. 8) of Ba : Na=0.30 samples with O-40 wt.% yttria, only Na,CO,, BaCO,, and yttria phases were detected and no indication of the known mixed phases NaYO?, Y2Ba0,, Y2Ba407, Y,Ba,O,, and Y,Ba,O,CO, was found, which are only stable at higher temperatures and lower CO, partial pressures [27,28]. Furthermore, no hint of glass formation was found; i.e., the diffraction patterns of the three detected phases were the same as in their pure state. The ionic conductivities of composites with a Ba : Na atomic ratio of 0.30 and yttria content from 0 to 40 wt.% were determined from curve fits of their impedance spectra. In the high-frequency parts of the Cole-Cole plots, semicircles resulting from the
206
A. Dubbe et al. I Solid State tonics 96 (1997) 201-208
_.
‘-;..:.*- .....~.,,,,;-i-j-i.“..“~? 0
92
0
0
288
I
1
284 binding
162
158
154
energy (eV)
composite
20
mixture
30
40 50 2 8 (deg.)
Fig. 9. Conductivity of composite electrolyte pellets (Ba: Na= 0.30) in 200 Pa CO, in artificial air. Solid lines: curve fits after Eq. (4).
Fig. 7. XPS spectra of (a) Na,CO,-BaCO,-Y,O, composite (Ba: Na=0.30,40 wt.% Y,O,) annealed for 10 h at 773 K in 100 Pa CO, in artificial air, (b) pure Y,O, after 3 h at 473 K in 10 Pa CO2 in artificial air, and (c) the same sample after sputtering. Cls regions are displayed with the same scale, while Y3d spectra are normalized to unity.
1
1.2 1.4 I.6 1000/T (K-‘)
;; C
J
: ,a_
60
Fig. 8. X-ray diffraction diagrams of the Na2COJ-BaC03-Y201 composite (Ba : Na=0.30, 40 wt.% Y,O,) and of a mixture of Na,CO,, BaCO,, and Y,O, powders with the same composition. displayed with negative intensities.
The parameters A *, A CB, E,, and EGB were determined from least-squares fits of the log(aT) versus 1000/T plots. The term with the higher activation energy, E,, of 2.0-2.4 eV is assigned to intrinsic conduction of the Na,CO, crystallites, since it decreases with increasing yttria concentration and vanishes at about 20 wt.% yttria (Fig. 10). The reason for this is seen in the successive isolation of the Na,CO, grains from each other by yttria grains until, above 20 wt.% yttria, any closed conduction path through a chain of interconnected Na,CO, grains has vanished. Over the whole observed temperature range, the conductivity is dominated by the term with the low activation energy, E,,, of 0.8- 1.1 eV, which is interpreted as grain-boundary conduction. a;,, is increased with increasing yttria content,
geometrical capacity and the electrolyte resistance were found, followed by incomplete depressed semicircles of the electrode impedance [29] in the low-frequency parts. The log(&) vs. 1000/T curves have a concave shape (Fig. 9), which is explained by a parallel resistance circuit of two conduction processes with different activation energies, expressed as u = a, + a,,
=A,T-’
yttria
exp(-E,lkT)
+ A,,T-’
exp(-E,,lkT).
(4)
cont.
(wt
96)
yttria
cont.
(wt
%)
Fig. 10. Conductivity parameters of composite electrolyte pellets (Ba : Na = 0.30) in 200 Pa CO? in artificial air from curve-fits after Eq. (4).
A. Dubbe et al. I Solid State Ionics 96 (1997)
up to 20 wt.%, and decreases with higher yttria content. This is explained by improved grain boundary conduction due to decreased grain sizes for low yttria content and a blocking effect of insulating yttria grains at higher yttria content. At 873 K in O,, the conductivity of yttria, mainly due to electronic hole conduction with a minor contribution of 02ionic conduction [30,31], is 3.2X lo-” S/cm and is lower than the conductivity of the carbonate composites by a factor of lo4 to 105, so that yttria can be regarded as an insulator. Also for Na,CO,/CaO composites, concave log(cTT) vs. 1000/T curves were reported and the transition points at 843-893 K were tentatively assigned to a phase transition involving the rotation or vibration of the carbonate anion [32]. Well-known phase transitions of Na,CO, occur at 758 and 699 K, which result in changes of the activation energy for ionic conduction only for pure or nearly pure Na,CO,, but have smaller or no effects on the conductivity of Na,CO,/CaO or Na,CO,/BaCO, composites [20,32].
4. Conclusions Deviations from the theoretical sensitivity, i.e., n>2 in Eq. (2) were also reported for sensors employing Na,CO,-impregnated electrolytes in contact with other sodium ionic conductors below 673 K [5,22] and were ascribed to the presence of coexisting CO,-sensitive electrode/carbonate interfaces and CO,-insensitive electrode/ion-conductor interfaces. Their cell voltage is thus a mixed potential with contributions from these two interfaces [22]. This explanation, however, does not apply to the deviations from the Nernstian sensitivity of the sensors in the present investigation. A similar effect for sensors with electrodes on compact carbonate discs was also reported in [20] for Na2C0,/BaC0, composites (Ba : Na = 0.005) for an operation temperature below 773 K. In this case, we see the reason in a slow equilibration of the volume of the Na,CO, crystallites with the CO, and 0, activities in the surrounding gas phase. This effect is less disturbing in thin carbonate films, but electrodes formed on smooth surfaces of compact carbonate discs have the further advantage of a more favorable electrode morphology for short response times [8]. Since yttrium carbonate is formed on the surface
201
201-208
of the yttria grains, it is expected that, to a certain degree, Y 3f ions diffuse into the Na,CO, phase, i.e., a solid solution of yttrium carbonate forms in the vicinity of the yttria grains. Yttria as a pure phase has an activity of 1 and thus the activity of the dissolved Y”+ ions is a thermodynamic function of the CO, partial pressure. The expected diffusion into the Na,CO, phase thus enhances CO, equilibration kinetics. This explains the observed prolongation of both response time and sensitivity upon yttria addition; only the yttria-free, Ba-rich sensor with Au electrodes is an exception. A similar acceleration effect of solid carbonate equilibration with the surrounding gas phase was found in thermal decomposition studies of Y/Gd-doped Ag,CO, by Wydeven and coworkers [33,34]. Also Ag’ ion diffusion from Ag electrodes into the carbonate phase has been found to improve sensitivity and elongated response times of CO2 sensors [35]. However, Ag electrodes have the disadvantages of blocking the electrode kinetics at both high CO, and 0, partial pressures [8,35] and of expected high affinity to sulfur-containing contaminating gases. A further indication that the yttria particles play an active electrochemical role in CO, sensing is the decrease of water cross-sensitivity upon yttria addition. This means that the interfaces between the surface carbonate on the yttria particles and the electrode particles are contributing to the cell voltage in the same way as the NazCO,/electrode interfaces, but are not less sensitive to water vapor. The water cross-sensitivity of Na,CO, is explained by formation of NaOH defects [36]. A similar effect is believed to be responsible for the drastic reduction of water cross-sensitivity upon BaCO, addition to Na,CO,: Both water vapor sensitive Na,CO,/electrode interfaces and water vapor insensitive BaCO,/ electrode interfaces contribute to a mixed potential. Sublimation or decomposition of Na2C0, in contact with Au or Pt as reported in 1371 may further reduce the water cross-sensitivity.
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