Preparation of K2OSm2O3nSiO2-based solid-state electrolyte and its application to electrochemical CO2 gas sensor

ELSEVIER

Sensors and Actuators

CHEMICAL

B 24-25 (1995) 282-286

Preparation of K,O-Sm,O,-nSiO,-based solid-state electrolyte and its application to electrochemical CO, gas sensor Yoshihiko Sadaoka a, Susumu Nakayama b, Yoshiro Sakai a, Makoto Wake a aDepartment of Applied Chemishy, Faculty of Engineering Ehime

Cbdversi&Matmyama 790, Japan ‘New MaterialsResearch Center, Shimgawa RefractoriesCo. Ltd., Bizen 705, Japan

Abstract

Ceramic-glass composites (&O-SmzOs-GO?) may be produced directly by the sintering of starting mixtures. The ionic conductivity is lowered with an increase in SiO, content for the sample with hexagonal structure. The CO,-sensing characteristics of an electrochemical gas sensor with a structure expressed as Pt~~K~O-Sm203~Si0,)Na,C0,(IPt, CO, have been investigated in terms of a two-electron electrochemical reaction. The e.m.f. is sensitive to the CO, gas concentration and increases with n in KIOSm,O,-nSiO,. The introduction of water vapour induces a lowering of the e.m.f. in which the decrement increased with a decrease in CO, concentration, and a prolongation of the response time. The sensing characteristics recover completely after the water vapour is cut off. Kqvords: Ceramic-glass composites; Carbon dioxide sensor; Solid-state electrolyte

1. Introduction Recently, there has been considerable interest in dense ionic conductors because they are suitable materials for solid-state batteries and/or chemical sensors [l-7]. As part of a screening program for well-compacted and conductive alkali-ion solid electrolytes, the electrical properties of some lanthanoid-silicates have been studied, in which the anion is a three-dimensional network of interconnected LnO, octahedra and SiO, tetrahedra. For M,O-Ln,O,-2Si0, (M = Li, Na, K, Rb, Cs; Ln =I.a, Nd, Sm, Gd, Dy, Y, Er, Yb), the highest ionic conductivity was observed for K,O-Nd/Sm203-2SiOZ with a hexagonal phase [8]. The material acts as a good potassium ionic conductor. Here we present the results electrical properties of of studies of the K,O-Sm,O,-nSiO, and the CO, gas-sensing characteristics of an electrochemical solid-state cell with Pt~~K,0-Sm,0,+Si0,jNa~C03~~Pt, CO, structure.

2. Experimental As starting materials, reagent-grade K,CO,, Sm,O, and SiO, were used. The raw materials were ball-milled with ethanol, dried and calcined in air. The obtained powders were ball-milled again with ethanol into fine powders, to which 15% of polyvinylalcohol solution was OY25-4005/95/$09.506 1995 Elsevier Science S.A. All rights reserved SSDI 0925-4005(94)01359-P

added. After drying, discs were pressed at 1000 kg cm-’ and sintered in air. The discs were 8 mm in diameter and about 2 mm thick. Some discs were ballmilled again and then immersed in water. The treated powders were pressed again and then sintered (watertreated ceramics). The crystal parameters and sintering tcmperatnre used to obtain the discs are summarized in Table 1. Platinum paint was applied to opposite faces of the disc and heated to 950 “C. The electrical properties were measured using a multi-frequency LCR meter (Yokogawa Hewlett-Packard, 4292A) in the frequency range 100 Hz-10 MHz. The sample was heated

Table 1 Crystal parameters and sintering temperature Materials

K20-Sm,03-2Si02 K&Sm,O,-3Si02 K&Sm,O&iO, K@-Sm203-5Si02 K,O-Sm,0,-6SiOz K@-Smz09-7Si02 KzO-Sm,0,-8SiOz K,O-Sm,O,-lOSi0, K,O-Sm,0r12Si01 Kz~Sm,0r14SiO~

u

c

(4

W-4

Sintering temp. rC)/time (h)

0.9478

0.6992 0.7012 0.7Wfi 0.6991 0.6998 0.6996 0.6998 0.6Y90 0.6983 0.6985

1350/10 1200/3 1050/3 9.50/3 95013 95013 95013 95013 1000/3 1000/3

0.9511 0.9512 0.9507 0.9511 0.9511 0.9509 0.9509 0.9509 0.9505

y. Sadaoka et al. I Sensors and Actuators B 24-25 (1995) 282-286

283

up to 300 “C to eliminate adsorbed water before mcasurements were made. The crystalline phases were identified at room temperature by an X-ray diffraction technique. The microstructures were examined using scanning electron microscopy (SEM). Elemental analysis was carried out by the X-ray fluorescence method.

3. Results and discussion 3.1. Scanning electron microscopy For K,O-Sm,O,-nSiO,, dense ceramics were obtained, except for n = 1 and 2, as shown in Fig. 1. For the samples with higher contents of SiO, (n > 4), welldefined fine particles were not observed and densification had progressed well. It is clear that crystalline phases are doped within glass-like phases and that dense glass-ceramic composites are formed. The morphology of K,OSm,O,-SiO, was distinctly different from that of the others. For K,O-Sm,O,+SiO, (ri > 2), the surface of the water-treated samples is smoother than that of untreated ones, and the grain boundary is unclear. 3.2. Elechical properties To determine the conductivity component, complexplane impedance analysis was performed. In the lower temperature region, the result was represented by a slightly depressed semicircle that passed through the origin in the higher-frequency region, and by a spur in the lower-frequency region. The intercept with the Z’-axis indicates the resistance of the electrolyte, and the spur is attributable to the electrode-electrolyte junction. When the temperature was increased, the semicircle diminished, and only a quasi-straight line due to the electrolyte-electrode behaviour was observed, in which the intercept with the Z’-axis indicates the total resistance of the electrolyte. A depressed semicircle appearing in the high-frequency region in the compleximpedance plot can be adequately expressed by z(w)=z,/[l+o’o/w,)~~-“~]

(I)

where Z(w) is the complex impedance at angular frequency o, Z, the low-frequency real-axis intercept, o, the relaxation angular frequency at the maximum height of the semicircle, (Ythe depression parameter (0= ?ra? 2) and j = (- 1)‘“. The equivalent circuit corresponding to the impedance spectrum consists of a frequencydependent capacitor C,(o) and a frequency-independent resistor R,. These parameters are described by

Fig. I. Scanning electron micrographs (bar=2 jnn): (a) KzO-SmzO,-2SiOz; (b) K2GSmz034Si02; (c) K2@Sm2034Si0~.

&=R,,

c,(0)=c,cj&$“,

R&c4)= 1

The limiting case, B=O, represents an equivalent circuit consisting of lumped RC components with zero depression angle. It is confirmed for the K,O-Ln,O,-nSiO, series that the depression angle decreases with an increase in the

Y Sadaokaetal.I Sensors and Acluators B 24-25 (199s)282-286

2.34

resistance (the resistance increases with SiO, content, except for n = 1) and that the o, value increases with the temperature. Graphically estimated resistances from the complex-plane impedance results were parameterized by the Arrhenius equation, GT=G,

exp( -E/kT)

(3

where G is the conductivity (=1/R), G, is the preexponential factor, E is the activation energy, k is Bohzmann’s constant and T is the absolute temperature. Arrhenius plots are shown in Fig. 2. Electrical parameters are summarized in Table 2. The highest conductivity and lowest activation energy are observed for K&I-Sm,O,-2SiO,, and the conductivity decreases with increasing SiO, content. K,O-Sm,O,-SiO, shows poor conductive behaviour. For K,O-Sm,O,-nSiO, (n = 24, the conductivity decreased and the activation energy increased with thewater treatment, while for the samples with a higher SiO, content, the electrical characteristics were only slightly influenced by the water treatment.

-5 0.5

A decreased elution of potassium with water is confirmed. For samples with n <4, the activation energy is considerably lower than Hakim and Uhlmann’s results [9] for potassium silicate. For samples with a higher SiO, content, the crystal layers are covered by the glass-like phase and well-defined crystal phases can be detected only for K,O-Sm,03-2SiO,, as shown in Fig. 1. A proposed crystal structure for K,O-Sm,O,-2Si0, is shown in Fig. 3. Two possible sites for potassium and/ or Ln are considered. One of these is situated in a layer formed by the SiO, network and the other one is situated in the interlayer. The possibility of the two types of site for potassium is also interpreted from the results as follows: the water treatment results in a shorter c-lattice constant, whereas the a-lattice is hardly influenced. The potassium content decreases with water treatment, especially for samples with a lower SiO, content. It seems that the decrease in the c-lattice constant due to water treatment is mainly caused by the decrease in the potassium ions situated in the A site. The water treatment results in an increase in the activation energy and a decrease in the pre-exponential factor of K,O-Sm,O,-2SiO,,

I 1

1.5

Fig. 2. Temperature dependence in K+Smz0,-nSi02.

2

2.5

3

3.5

1oooKfr of conductivity. Figures indicate n

Table 2 Electrical parameters Materials

Conductivity

(S cm-‘)

E (kJ

K~@Sm~0,-Si02 K,OSm,O,-ZSiO, KzO-Sm,O,-3SiOz K~O-Sm20,-4Si0~ K#-Sm20,-6Si02 K@-Sm20,-7SiOz K20-SmZO,-8Si02 KzO-Sm,O~-l@SiOP KzO-Sm20r12Si0, K+Sm20,-14Si0,

500 ‘C

600 “C



mol-‘)

161 32.8 (18.0) 39.9 (20.9) 50.6 (42.9) 19.7 86.1 87.4 89.5 104.9 94.9

c

-0

b

Ln,K

Fig. 3. View of hexagonal K,O-Ln,O,ZSiO,.

OSi

00

Y. Sodaok

et al. I Sensors and Actuators B 24-25 (1995) 282-286

285

3.3. C02-sensing characteristics For the cell expressed as 0,, Ptl/K ionic electrolytellPt, Na2C03, COz, 0, it is assumed that the chemical potential at the anode is controlled by the reaction Na,CO, -

2Na + CO2 + (l/2)0,

(3)

whereas that at the cathode is controlled by the equation 2K+ (l/2)0,

----) K,O

(4)

The overall reaction is predicted Na,CO, + 2K -

to be

K,O t 2Na + CO,

(5) a.~

When P, in the cathode side is the same as that in the anode side, the e.m.f. of the cell is then expected to be

20”

eet I

0

100

lcaxl

I

200

300

I 4w

so0

Timeimin

e.m.f. = - [AG,o + AGcti - AGNazC&F - (RTI2F) ln[u KzoaNa2Pc&K21

(6)

where AG,, is the standard free energy of formation and PCi,the concentration of species i. If the activities of K,O, K and Na remain constant, the e.m.f. gives the concentration of CO,. Fig. 4 shows. the sensing characteristic at 470 “C for the sensor with the structure 0,, PtllK,O-Sm,O,-4SiO,/Na,CO,IIPt, COz, 0,. On switching from 1000 ppm CO,/air flow to 100 ppm COJair flow, the e.m.f. increased quickly and a steadystate value was observed within 2 min. The rise and recovery times were very fast; the 90% response time was less than 1 min (test gas flow rate=40 ml min-’ and measurement chamber volume = 60 ml). When the CO, concentration changed from 10 000 to 100 ppm, from 100 to 10 ppm and from 10 ppm to air, the response time became longer. It seems that the con-

Fig. 5. Sensing characteristic of K#-Smz0,-lSi02-based element under wet conditions at 470 “C. Figures indicate the CO1 concentration (PPm).

400 I

600

Fig. 6. Sensing characteristic of K#-SmzO&Si02-based element when the COP concentration was repeatedly changed from 100 to 1000 ppm and reversed in wet and then in dly conditions.

500

>

E

a

400

300

Timelmin. Fig. 4. Sensing characteristic of K,O-Sm,O,-NO,-based element under dry conditions at 470°C. Figures indicate the COZccmcentration (PPG

centration in the measuring cell could not be obtained in an early period after changing from a higher to a lower concentration since the response time becomes faster with an increasing flow rate. The response time is shortened by an increase in the flow rate of the test gases and with an increase in the number of measuring cycles. With increasing SiO, content, the observed e.m.f. was increased due to the lowering of K,O activity in the electrolyte. In order to detect the CO, concentration in ambient air, the influence of the humidity in the test gas on the sensing characteristics was examined. The result is shown in Fig. 5. When humid test air (30 “C dew point) was passed, the e.m.f. value was lower than that con-

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and Actuators B 24-25 (1995) 282-286

firmed for dry test air. The e.m.f. in concentrations of CO, less than 100 ppm was lower than the value expected from the relation confirmed in a higher range (> 100 ppm CO& The decrement of the e.m.f. had a tendency to increase with a decrease in CO, concentration, while the sensing characteristics recovered completely after the water vapour was cut off, as shown in Fig. 6. These decrements are interpretable by the formation of sodium oxides such as Na,O and Na,O, in the Na&O, layer and/or Pt electrode-body interface, as reported previously [7].

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Roth and H. BGhm, Ionic conductivity of sodium-nepheline single crystals, Solid Slale lonics, 18-19 (1986) 553-556. 131 H. Aono, E. Sugimoto, Y. Sadaoka, N. Imanaka and G. Adachi, Ionic conductivity of solid electrolytes based on lithium titanium phosphate, I. E[ecfrochem. Sot., 137 (1990) 1023-1027. [41 K. Jakowska and A.R. West, Ionic conductivity of Li&jiO, solid solutions in the system L&O-Al,O,-SiO,, J. Mater. Sci, 1X (1983) 2380-2384. I51 J. Liu and W. Weppner, Potentiometric CO1 gas sensor based on Na-p//3”-alumina solid electrolytes at 450 “C, Eur. I. Solid State Inorg. Chem., 2X (1991) 1151-1160. I61 N. Miura, S. Yao, Y. Shim& and N. Yamazoe, High-performance solid-electrolyte carbon dioxide sensor with a binary carbonate electmde, Senmrs and Acrunrm B, 9 (1992) 165-170. [71 Y. Sadaoka, Y. Sakai and T. Manabe, CO1 sensing characteristics of a solid-state electrochemical sensor based on a sodium ionic conductor, I Mater. Chem., 2 (1992) 945-947. PI S. Nakayama and Y. Sadaoka, Ionic conductivity of ceramics prepared by sintering of M20-Ln,O,-2Si02 mixtures (M=Li, Na, K, Rb, Cs; Ln=La, Nd, Sm, Gd, Dy, Y, Ho, Er, Yb), J. Muter. Chem., 3 (1993) 125-1257. 191 R.M. Hakim and D.R. Uhlmann, Electrical conductivity of alkali silicate glasses, Phys. Chem. Glasses, 12 (1971) 132-138.