Fabrication and characteristics of CO2-gas sensor using Li2CO3–Li3PO4–Al2O3 electrolyte and LiMn2O4 reference electrode

Sensors and Actuators B 76 (2001) 594±599

Fabrication and characteristics of CO2-gas sensor using Li2CO3±Li3PO4±Al2O3 electrolyte and LiMn2O4 reference electrode Dong-Hyun Kim, Ji-Young Yoon, Hee-Chan Park, Kwang-Ho Kim* Department of Inorganic Materials Engineering, Pusan National University, Pusan 609-735, South Korea

Abstract Fabrication of solid-state potentiometric CO2 sensor was simpli®ed by using Li2CO3±Li3PO4±Al2O3 mixture for both a solid electrolyte and sensing electrode. LiMn2O4 was used as the reference electrode to improve an electrochemical stability of the sensor. The emf of the sensor showed a good accordance with theoretical Nernst slope (n ˆ 2) for CO2 gas concentration in the range of 100±10,000 ppm above 3508C. The emf of the sensor was constant regardless of oxygen partial pressure at the high temperature above 5 kPa. It however, a little depended on oxygen partial pressure as the pressure decreased below 5 kPa. The oxygen-dependency of the sensor gradually disappeared as the operating temperature increased. The sensing behavior of CO2 sensor was affected by the presence of water vapor, but its effect was small comparing with other sensors. The CO2 sensor also showed superior long-term stability. # 2001 Published by Elsevier Science B.V. Keywords: CO2; Gas-sensor; Li2CO3; LiMn2O4; Solid electrolyte

1. Introduction The concentration of carbon dioxide gas in air has been gradually increased as carbon-contained fuels were continuously used. It is a well-known factor that this increase directly causes a global warming of the earth. Recently, various CO2 gas sensors were investigated to monitor and control an emission of carbon dioxide [1±8]. Among these sensors, a solid-state potentiometric CO2 sensor which consists of a sensing electrode, a solid electrolyte and a reference electrode has the advantage of superior sensitivity, selectivity and long-term stability to CO2 gas. Carbonates, such as Li2CO3, Na2CO3, CaCO3, BaCO2, were generally used as the sensing electrode of CO2 sensor [9,10,20]. Solid electrolyte must have essentially a structure which ions can migrate into. Thus b-alumina [11], Nasicon [12,13], lithiumion conductor [14,17,18], and stabilized-zirconia [15,16] with mobile ions of alkali or oxygen were used as the solid-electrolyte of CO2 sensor. Also, LiCoO2 [17], LiMn2O4 [18], and Na2SnO3 [19] were selected as materials of the reference electrode to produce metal ions stably. In this work, solid-state potentiometric CO2 sensor was fabricated using Li2CO3±Li3PO4±Al2O3 mixture as a solidelectrolyte and sensing electrode. And to improve the elec-

trochemical stability of the sensor, LiMn2O4 which was mainly selected as reversible cathode in lithium ion battery, was used as the reference electrode. Li2CO3 phase was less affected by the presence of water vapor for sensing CO2 gas [20], and had a lower ®ring temperature than other solid electrolytes. However, pure Li2CO3 phase has a low ionic conductivity in general working temperature range of electrochemical CO2 sensor [18,21,22]. Thus, Li3PO4 phase was doped in Li2CO3 phase to get a suf®cient ionic conductivity of solid electrolyte for CO2 sensor in this work. And alumina was added on Li2CO3±Li3PO4 to remove a decline phenomenon of mechanical strength due to crystallographically heteropic orientation of them [21,22]. 2. Theoretical background The CO2 sensor used in this work has a structure of the following electrochemical cell which consists of a reference electrode, a solid electrolyte, and a sensing electrode. Au; LiMn2 O4 jLi2 CO3 Li3 PO4 Al2 O3 jAu; CO2 ; O2 …Solid electrolyte† …Reference electrode† …Sensing electrode† (1) The cell reactions at both the sensing and the reference electrode can be described by the following expressions:

* Corresponding author. Tel.: ‡82-51-510-2391; fax: ‡82-51-512-0528. E-mail address: [email protected] (K.-H. Kim).

0925-4005/01/$ ± see front matter # 2001 Published by Elsevier Science B.V. PII: S 0 9 2 5 - 4 0 0 5 ( 0 1 ) 0 0 6 4 2 - 6

2 LiMn2 O4 ˆ 2 Li1 x Mn2 O4 ‡ 2x Li‡ ‡ 2x e …ref:†

(2)

D.-H. Kim et al. / Sensors and Actuators B 76 (2001) 594±599

2x Li‡ ‡ 2x e x CO2 ‡ 12 x O2 ˆ x Li2 CO3 …sens:†

595

(3)

During sensing CO2 gas, LiMn2O4 in the sensor is partially transformed into Li1 xMn2O4, as shown in the Eq. (2), and lithium ions were released from the reference electrode to the solid electrolyte. According to the Eq. (3), lithium ions react with carbon dioxides into lithium carbonates on the surface of the sensing electrode. Using Eqs. (2) and (3), the total cell reaction is shown by Eq. (4). 2 LiMn2 O4 ‡ x CO2 ‡ 12 x O2 ! x Li2 CO3 ‡ 2 Li1 x Mn2 O4

(4)

Fig. 1. XRD pattern of the powder synthesized for reference electrode.

The emf of the electrochemical cell is derived from Eqs. (2) and (3).     RT RT E ˆ E0 ‡ ln pCO2 …sens:† ‡ ln pO2 …sens:† n1 F n2 F (5) where E0 (standard electrochemical force) is constant, F the Faraday constant, R the gas constant, T the absolute temperature, n1 and n2 are the numbers of electrons which react with one CO2 and O2 molecule for electrochemical reaction, and p is the partial pressure of the gas in the sensing electrode.

Fig. 2. Schematic diagram of electrochemical CO2 sensor.

3. Experimental

sealed with a Pb-contained glass of low melting point to increase a mechanical strength of the junction between the solid-electrolyte and the reference electrode. The schematic structure of our CO2 sensor was depicted in Fig. 2.

3.1. Sensor fabrication

3.2. Characterization

To improve the ionic conductivity of Li2CO3 (Aldrich Chemical Co., 99.997), Li3PO4 (Aldrich Chemical Co., 99.9) was doped on it in the concentration range from 0 to 6 mol%. The Li2CO3±Li3PO4 powder was mixed with Al2O3 (Aldrich Chemical Co., 99.8) in the weight ratio of 75:25 to increase a mechanical strength of the electrolyte. The Li2CO3±Li3PO4±Al2O3 powder was melt in alumina crucible by heating to 7508C in air, and cooled down in metal mold. The ionic conductivity of the solid electrolyte was measured using an impedance analyzer (Hewlett-Packard, 4192A) from room temperature to 5008C with a ramping rate of 5 K min 1 in air. To synthesize LiMn2O4 powder as a reference electrode of sensor, Li2CO3 and MnCO3 (Aldrich Chemical Co., 99.9) were mixed in the molar ratio of 1:4. The mixed powder was heated at 4808C for 12 h, and then at 8008C for 12 h. Fig. 1 shows the XRD pattern of the synthesized powder. The diffraction peaks in Fig. 1 were exactly corresponding to those of LiMn2O4. The LiMn2O4 powder was pressed into a pellet, which was then sintered at 9508C for 1 h. The prepared solid electrolyte and LiMn2O4 were joined by melting and cooling the solid-electrolyte on the surface of LiMn2O4. Layers of metal electrodes were formed with Au-paste on the top and bottom of the joined body, and two Pt-wires were attached on the Au-painted surfaces, respectively. A side surface of the sample were

The emf-responses of sensor to CO2 gas were investigated in a glove box of 23.8 l. Before testing, the chamber was ®lled with arti®cial air (79% N2 ‡ 21% O2), and then CO2 gas was introduced into the chamber with a ¯ow rate of 40 cm3 min 1. The working temperatures of sensor were varied between 300 and 4508C, and the CO2 concentrations in the chamber were varied from 100 to 10,000 ppm using volume ratio of VCO2 =…Vair ‡ VCO2 †  105 ppm. To investigate an oxygen partial pressure dependence of the sensor, the testing box was ®lled with gases of N2 and CO2, and then the oxygen partial pressure was varied from 0 to 0.4 atm using volume ratio of VO2 =…VN2 ‡CO2 ‡ VO2 †  1 atm. The relative humidity (RH) in the chamber was controlled by injecting air gas through a water bubbler into the testing box. The CO2 sensitivity of the sensor was de®ned as a difference of emf between the sensing electrode and the reference electrode with a variation of CO2 gas concentration. The emf of the electrochemical sensor was measured using a voltmeter (Keithly, 182 Sensitive Digital Voltameter). 4. Results and discussion Fig. 3 shows temperature-dependence of electrical conductivity for a few kinds of lithium-based compounds. The

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Fig. 4. Dependence of ~emf/decade on temperature in air gas. Fig. 3. Temperature-dependence of electrical conductivity: (&), Li3PO4 only; (5), Li2CO3 only; (~), Li2 CO3 ‡ Li3 PO4 (2 mol%); (*), Li2 CO3 ‡ Li3 PO4 (4 mol%).

ing temperatures of the electrochemical CO2 sensors using Li2CO3 with different solid electrolytes were shown in Table 1. The conductivity of Nasicon [9,10] was 102 times higher than that of Li2CO3±Li3PO4 (4 mol%)±Al2O3. Nevertheless, low limits of sensor working temperatures were 3508C regardless of electrolytes. This re¯ects an evidence that the emf of sensor was chie¯y controlled by chemical reactions with CO2 gas on the surface of the sensing electrode. Therefore, the reason why the sensor showed low response to CO2 gas below 3508C in this work is believed to be controlled more by the decline of chemical reaction rate on the surface of the sensing electrode than by that of conductivity of the electrolyte, and it was found that the proper working temperature of our sensor is over 3508C. Fig. 5 shows the emf-variation of the sensor with the electrolyte of Li2CO3±Li3PO4 (4 mol%)±Al2O3 as a function of CO2 concentration in air. The CO2 concentration was varied from 100 to 10,000 ppm at two working temperatures of 370 and 4208C. The emf of the sensor increased with increasing temperature. Also, the emf decreased linearly with increase of CO2 concentration. This decrease of the emf was almost consistent with the theoretical Nernst slope. Transient responses of the sensor to CO2 gas with the electrolyte of Li2CO3±Li3PO4 (4 mol%)±Al2O3 were shown in Fig. 6. The working temperature of the sensor was 4208C, and the CO2 concentration was varied from 100 to 2000 ppm with various steps. It was found that the sensor exhibited the good characteristic of the transient response to CO2 gas, and

log s linearly decreased with increase of 1000/T for all lithium compounds. The conductivity of Li2CO3 phase showed the change of slop at around 4008C. This change of conductivity was believed to be because a phase-transformation of Li2CO3 occurred [18,22,23]. The small addition of Li3PO4 into Li2CO3 matrix removed this phenomenon, and improved the conductivity of lithiumbased compound. It was found that the conductivity became irregular over the doping amount of 4 mol% Li3PO4. Hereafter, the adding amount of Li3PO4 into Li2CO3 was ®xed at 4 mol%. Fig. 4 shows Demf/decade, that is, emf-variation of the cell with 10 times increase of CO2 concentration, of the sensor with the electrolyte of Li2CO3±Li3PO4 (4 mol%)± Al2O3 at various temperatures in order to investigate the working temperature of the CO2 sensor. The experimental values of Demf/decade have a good accordance with the theoretical ones of Demf/decade from the Nernst equation (n ˆ 2) in the temperature range over 3508C. However, the deviation of experimental and theoretical values of Demf/ decade existed below 3508C. It may be considered as the reason of the deviation below 3508C whether the rate of chemical reactions with CO2 on the surface of the sensing electrode was a very small or the conductivity of the solid electrolyte was insuf®cient to operate the sensor. The work-

Table 1 Temperatures of electrochemical CO2 sensors using Li2CO3 sensing electrode with different electrolytes Solid electrolyte

Conductivity, s (O 2008C

Nasicon Li2CO3 ‡ Li3PO4 ‡ Al2O3 Li2CO3 ‡ MgO

8.5  10 2  10 5 1.3  10

1

cm 1) 3008C

2 6

1.5  10 2.5  10 3.2  10

4008C 1 4 5

4  10 1 1  10 3 3.1  10

4

Operating temperature (8C)

References

350±500 350±600 350±405

[9,10] [17,21,22] [18]

D.-H. Kim et al. / Sensors and Actuators B 76 (2001) 594±599

Fig. 5. Dependence of emf on CO2 concentration at different temperatures in air gas.

that 90% response time of the sensor was between 90 and 110 s. Sadaoka et al. [24] reported that the 90% response time of electrochemical CO2 sensors increased with increasing the ¯ow rate of CO2 gas. Considering that CO2 gas was injected into the chamber with the low ¯ow rate of 40 cm3 min 1 in this work, our electrochemical CO2 sensor was thought to have a fast response time to CO2 gas. According to the equation (5), the emf of the sensor was depended on only CO2 concentration at ®xed O2-partial pressure. However, with variation of the O2-partial pressure, both CO2 and O2 partial pressure have an effect on the emf of the sensor. To investigate the dependence of the emf on O2 partial pressure, the CO2 sensor was exposed in atmosphere of the O2-partial pressure from 0.001 to 0.4 atm at ®xed CO2 concentration. Fig. 7 shows the emf-variation of the sensor with the electrolyte of Li2CO3±Li3PO4 (4 mol%)±Al2O3 as a function of O2-partial pressure in N2 ‡ CO2 atmosphere. The CO2 gas concentration was ®xed at 200 ppm and the working temperature of the sensor was maintained at 370,

Fig. 7. Dependence of emf on oxygen partial pressure concentration (CO2: 200 ppm) at various temperatures in N2 gas.

400 and 4308C. In atmosphere of high O2-partial pressure above 0.05 atm, the emf of the sensor was almost constant regardless of variation of O2-partial pressure. On the other hand, it increased with increase of O2-partial pressure in atmosphere of low O2-partial pressure below 0.05 atm. Considering that the O2-partial pressure in air is 0.21 atm, our sensor has no oxygen partial pressure dependency in air. From these experimental results, the numbers of electrons (n2) which react with one O2 molecule for electrochemical reaction were 9.8, 10.6 and 25.5 at the temperature of 370, 400 and 4208C, respectively. According to the equation [5], the value of n2 corresponding to the theoretical Nernst slope was four. The increase of n2 means to the decrease of Nernst slope, and it indicates that the emf-response of the sensor to the variation of O2-partial pressure decrease. The value of n2 increased with increasing the sensor working temperature in Fig. 7. Therefore, it was concluded that the oxygen-dependency of our sensor gradually disappeared as the working temperature increased. As shown in Fig. 7, the oxygen pressure dependence of our sensor was different from that of other CO2 sensor having a value of constant n2. The reason of this oxygen dependence can be thought that Li2O and LiO phase were transformation from Li2CO3 in process of melting and cooling solid electrolyte, and that the following reaction mainly occurred on the surface of sensing reaction [25,26]. As shown in Eq. (6), the oxygen partial pressure is not related to the sensing reaction. 2 Li‡ ‡ CO2 ‡ Li2 O2 ‡ 2 e ˆ Li2 CO2 ‡ Li2 O

Fig. 6. Response transients of sensor to variations of CO2 concentration.

597

(6)

Fig. 8 shows the humidity-dependency of the sensor with the electrolyte of Li2CO3±Li3PO4 (4 mol%)±Al2O3 during CO2 sensing reaction. The CO2 concentration was varied from 100 to 10,000 ppm at sensor working temperature of 4208C. The CO2 sensor exhibited the emf-deviation of 2±3 mV in dry and wet (50% RH) air. This emf-deviation by the presence of water vapor was smaller than that of CO2 sensors

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sensor was constant regardless of oxygen partial pressure at the high temperature above 0.05 atm. It however, a little depended on oxygen partial pressure as the pressure decreased below 0.05 atm. The oxygen-dependency of sensor gradually disappeared as the operating temperature increased. The sensing behavior of CO2 sensor was affected by the presence of water vapor, but its effect was small comparing with other sensors. The CO2 sensor also showed superior long-term stability. Acknowledgements

Fig. 8. Sensor responses to CO2 gas at 4208C in dry and wet (50% RH) air.

The authors wish to acknowledge the ®nancial support of the Korea Research Foundation made in the program year of 1997. References

Fig. 9. Long-term stability of sensor in 1000 ppm CO2 at 4208C.

using other sensing electrode, such as Na2CO3 [24,27,28] and Li2CO3±Li2O [4,14]. Also, the response and recovery times of the CO2 sensor were increased in 50% RH air. This change of CO2 sensing characteristics with the presence of water vapor was believed to be because of formation of LiOH by decomposing of H2O on the surface of sensing electrode. Fig. 9 shows the variation of emf for sensor operating time of 20 day. The operating temperature of sensor was 4208C, and CO2 concentration was 1000 ppm. It was found that the CO2 sensor showed superior long-term stability. 5. Conclusions The emf of the sensor showed a good accordance with theoretical Nernst slope (n ˆ 2) for CO2 gas concentration in the range of 100±10,000 ppm above 3508C. The emf of

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Department of Inorganic Materials Engineering, Pusan National University in 2000.

Biographies

Kwang-Ho Kim received his BS degree from Department of Metallurgical Engineering Seoul National University in 1980, and MS and PhD degrees from Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST) in 1982 and 1986, respectively. Since 1985, he has been with Department of Inorganic Materials Engineering in Pusan National University, first as a full-time lecturer and presently as a professor. He is head of Division of Materials Science and Engineering. His main research areas are thin film processing and electronic ceramics.

Dong-Hyun Kim received his BS and MS degree from Department of Inorganic Materials Engineering, Pusan National University in 1998 and 2000, respectively. Now, he is a PhD candidate of Department of Inorganic Materials Engineering in Pusan National University. Ji-Young Yoon received her BS degree from Department of Materials Engineering, Miryang National University in 1998, and MS degree from

Hee-Chan Park received his MS degree in Metallurgical Engineering from the University of Alabama, Tuscaloosa, in 1975, and his PhD degree in Materials Engineering from Pusan National University, Pusan, in 1979. He is a professor in the Department of Materials Engineering at Pusan National University. Professor Park joined Pusan National University as an assistant professor in 1977 and was named to his current position in 1984. His main research interests are concerned with sol±gel depositions of oxides and electrical properties of glass-ceramics.