JP4849634B2 - CO2 sensor manufacturing method - Google Patents

Description

  The present invention relates to a potentiometric type CO2 sensor using a Na ion conductor, Li2CO3, a rare earth compound, and a working electrode containing an electronic conductive material and a method for manufacturing the same, and more particularly to improvement of the stability thereof.

  A CO2 sensor is used in which a working electrode made of a mixture of Li2CO3 and Pt, Au, etc. is provided on one side of NASICON, and a reference electrode of Au or Pt is provided on the back side of NASICON by shielding it from the atmosphere to be detected. The problem with this CO2 sensor is that the electromotive force decreases when left unheated, especially when left unheated in a humid atmosphere. In order to compensate for such a decrease in electromotive force, practically, learning of the electromotive force in the atmosphere is performed. That is, when it is detected that the electromotive force is high and stable, this is regarded as an electromotive force in the atmosphere, and the CO2 concentration is obtained from the difference from this electromotive force. As described above, stabilizing the electromotive force of the CO2 sensor is a basic problem with the CO2 sensor.

  An object of the present invention is to improve the stability of a CO2 sensor.

The present invention is provided with a reference electrode that is cut off from the atmosphere to be detected so as to be in contact with the Na ion conductor, and a working electrode that includes Li2CO3, an Nd compound, and an electron conductive material and is in contact with the atmosphere to be detected. In the manufacturing method of the CO2 sensor,
By firing a mixture of Li compound and Nd compound in CO2 at 850-900 ° C., a mixture of Nd oxycarbonate and Li2CO3 containing Li ions between layers,
Using the mixture and the electron conductive material, a working electrode is formed on the surface of the Na ion conductor, and then refired at 700 to 850 ° C. in CO 2.

  The inventor has found that the reason why the electromotive force of the CO2 sensor changes with time is that the activity of Li ions at the working electrode changes. That is, Li ions react with a Na ion conductor such as NASICON and form an intermediate layer at the interface between the working electrode and the Na ion conductor. When Li ions react with NASICON, etc., the Li ion activity at the working electrode decreases and the electromotive force changes.

  The inventor has found that light rare earth elements having atomic numbers of 57 to 64 form oxycarbonates and include Li ions between oxycarbonate layers. The space between the layers is a space formed by CO3 ions in the oxycarbonate, and since the CO3 ions have a large size, Li ions can freely enter and leave the layers.

  If the CO2 sensor experiences unheated exposure and the working electrode touches water vapor, a part of Li2CO3 decomposes into Li hydroxide. Li hydroxide produces more active and free Li ions than Li2CO3, but the generated Li ions are absorbed between layers of light rare earth oxycarbonates and can prevent reaction with Na ion conductors. With the above mechanism, changes in the electromotive force of the CO2 sensor can be prevented.

  Nd is particularly preferable as the light rare earth element, and La, Pr, Sm, Eu, etc. may be used in addition to this. At the working electrode, a part of the Li element is contained in the rare earth element oxycarbonate, and the remainder exists in the form of Li2CO3. Especially when firing at 700 ° C or higher, the Li2CO3 melts and the rare earth oxycarbonate It is distributed in layers on the surface of the particles. The ratio of Li2CO3 to rare earth oxycarbonate is, for example, about 1: 1 to 1: 5, and preferably about 3: 4 to 1: 5 in molar ratio.

  As shown in FIG. 8, FIG. 9, etc., the stability of the working electrode is increased by firing a mixture of Li compound and Nd compound in CO2 at 850 to 900.degree. In other words, it is difficult for CO2 to desorb from the working electrode. Here, the mixture of Li2CO3 and Nd oxycarbonate calcined at 850 to 900 ° C further increases in weight at 700 to 850 ° C in CO2. This is considered that CO2 is desorbed by the initial firing and absorbs CO2 again at 700 to 850 ° C. When the initial firing temperature is 800 ° C or 700 ° C, CO2 reabsorption near 800 ° C is not observed. In this specification, CO2 means a CO2 partial pressure of 200 KPa (0.2 atm) or more, preferably a CO2 partial pressure of 1 atm or more.

  For these reasons, a mixture of Li compound and Nd compound is baked in CO2 at 850 to 900 ° C., and then an electroconductive substance such as Pt or Au is mixed with this mixture to form a working electrode. When re-firing in CO2 at ~ 850 ° C, the Li compound not incorporated into the Nd oxycarbonate is almost entirely Li2CO3.

  The inventor has found that the characteristics of the CO2 sensor can be stabilized even when a heavy rare earth element oxide or Y2O3 is mixed with Li2CO3 instead of the light rare earth element (FIGS. 11 to 13). In the following, Y is added to the heavy rare earth element having the atomic number of 65 to 71 and simply referred to as heavy rare earth element. Since heavy rare earth elements generally do not form oxycarbonates, the mechanism that stabilizes the properties of the CO2 sensor is different from the incorporation of Li ions between oxycarbonate layers. The inventor has found that when Li2CO3 and an oxide of heavy rare earth element are baked in CO2 or in air, infrared absorption derived from CO3 ions occurs on the surface of the oxide of heavy rare earth element. This suggests that the surface of the heavy rare earth oxide is covered with CO3 ions, and the negatively charged field composed of CO3 ions captures free Li ions. In fact, the working electrode containing heavy rare earth oxide and Li2CO3 is more stable than the working electrode made only of Li2CO3 in addition to the electron conductive material (FIG. 13). These facts suggest that the field of CO3 ions on the surface of heavy rare earth oxides can adsorb free Li ions, and the electromotive force can be stabilized by making the Li ion activity at the working electrode close to constant. .

  In the following, an optimum embodiment for carrying out the present invention will be shown.

Structure of CO2 Sensor FIGS. 1 to 13 show the CO2 sensor 2 of the embodiment and its characteristics. FIG. 1 schematically shows the structure of the CO2 sensor 2, and 4 is a NASICON in the form of pellets, which is expressed by the composition of Na3Zr2Si2PO12. Reference numeral 6 denotes a working electrode, which includes a porous Pt layer 8 inside. The porous Pt layer 8 is formed on the surface of the NASICON 4, and the working electrode 6 is formed so as to cover the surface. Note that Pt may be dispersed throughout the working electrode 6, and Au or the like may be used instead of Pt. Hereinafter, the working electrode 6 will be described excluding Pt. The working electrode 6 is a mixture of Li2CO3 and an oxycarbonate of a light rare earth element such as Nd, such as Nd2O2CO3, and the oxycarbonate contains Li ions. Li2CO3 is heat-treated at a temperature above its melting point, so it is distributed in layers on the surface of oxycarbonate.

  Instead of using oxycarbonates of light rare earth elements, heavy rare earth oxides may be used.In this case, CO3 ions are adsorbed on the surface of the heavy rare earth oxide, and Li2CO3 is oxidized as in the case of light rare earths. Distribute in layers on the surface. A reference electrode 10 is made of porous Pt, Au, or the like, and is shielded from the atmosphere to be detected by the glass tube 12. In the embodiment, pellet-shaped NASICON 4 is used. For example, a NASICON film is provided on the surface of an alumina substrate, a working electrode 6 is provided on a part of the surface, a reference electrode 10 is provided on the other part, and the reference electrode 10 is made of glass. It may be shielded from the atmosphere by, for example.

Material preparation Material preparation is described below. NASICON was prepared from Zr oxynitrate and Na nitrate and an acidic solution of tetraethyl silicate in nitric acid and ammonium hydrogen phosphate ((NH4) 2HPO4). The starting material excluding ammonium hydrogen phosphate was mixed in a proportion corresponding to the composition of Na3Zr2Si2PO12, precipitated by adding ammonium hydrogen phosphate, and calcined at 600 ° C. in air. The obtained product was pulverized by a ball mill, pressure-formed into a pellet, and fired at 1200 ° C. to obtain a NASICON 4 in a pellet form. A Pt paste was used for the reference electrode 10 and the porous Pt layer 8, applied to both surfaces of NASICON 4, and baked at 600 ° C. in air, for example.

  When light rare earth elements are used for the working electrode 6, light rare earth oxides, light rare earth element oxycarbonates such as Nd2O2CO3, or light rare earth element carbonates and Li2CO3 are mixed, for example, at a maximum temperature of 850 to 900 ° C. Heat in CO2 and hold for about 1 hour, crush after cooling. Glycerine or the like is added to the pulverized product to form a paste, and the porous Pt layer 8 and the reference electrode 10 are applied to the formed NASICON 4 so as to cover the working electrode 6, and CO2 is applied at a maximum temperature of 700, for example. Re-fire to keep at ~ 850 ° C for about 1 hour.

  When heavy rare earth oxide is used as the rare earth compound, the heavy rare earth oxide and Li2CO3 are mixed, heated in CO2 to a maximum temperature of, for example, about 800 ° C., held for about 1 hour, and pulverized after cooling to obtain a working electrode. Material. The obtained working electrode material is made into a paste, applied to the surface of NASICON 4 so as to cover the porous Pt layer 8, and fired again at about 700 ° C., for example. The refiring atmosphere is preferably in CO2.

Reaction of Li2CO3 and NASICON Fig. 2 shows the product when a mixture of Li2CO3 and NASICON is heat treated. Li2CO3 and NASICON were mixed and heated to the temperature shown in the figure, then cooled to room temperature and subjected to X-ray diffraction. A peak corresponding to Na3.4Zr2Si2.4P0.6O12 is seen around 600 ° C, and a peak corresponding to Na4Zr2Si3O12 is seen around 800 ° C. At about 1000 ° C., a ZrO2 peak can be detected. FIG. 2 shows that when a mixture of NASICON and Li2CO3 is heated at 600 ° C. or higher, a peak of a compound rich in Na than NASICON is generated and a peak of Li2CO3 disappears. Therefore, it can be presumed that Li2CO3 reacts with NASICON to produce a compound containing Li ions.

  The operating temperature of the CO2 sensor 2 is about 400 to 500 ° C., in the example, 460 ° C. When used at this temperature for a long time, a part of Li2CO3 reacts with NASICON to form an intermediate layer containing Li ions. Conceivable. In addition, the reaction between Li2CO3 and NASICON is considered to go through free Li ions generated by decomposition of Li2CO3 by leaving it unheated. Therefore, the inventor presumed that the long-term stability of the CO2 sensor could be improved by suppressing the reaction between LiICON and NASICON at the working electrode-NASICON interface.

Light Rare Earth Oxycarbonate Figure 3 shows X-ray diffraction results when a 1: 1 mixture of neodymium oxide (Nd2O3) and Li2CO3 in a molar ratio was heated in CO2 (1 atm). The numbers 300 to 1000 in the figure indicate the heating temperature, held at this temperature for 1 hour, and then cooled to room temperature to perform X-ray diffraction. From the figure, Nd oxycarbonate (Nd2O2CO3) is formed around 600 ° C, the Li2CO3 peak disappears around 700 ° C, and the lattice constant of Nd oxycarbonate slightly changes around 800 ° C. The change in lattice constant (FIG. 4) shows that Nd oxycarbonate (Nd2O2 + 2x (CO3) 1-xLi2x) including Li ions is formed. Oxycarbonate is decomposed by heating at about 1000 ° C and converted to Nd oxide. Since Li2CO3 melts at over 600 ° C, it is considered that Li2CO3 is distributed in layers on the surface of the oxycarbonate after cooling.

  Fig. 4 shows the change in lattice constant when Nd and La oxides and Li2CO3 are mixed at a molar ratio of 1: 1 and calcined in CO2. For Nd oxycarbonate, the c-axis parameter is In La, ab plane parameters are shown. Numbers such as 700 to 900 in the figure indicate the maximum heating temperature, and x on the vertical axis is a parameter indicating the degree of Li conversion used to indicate the composition of Nd oxycarbonate in FIG. When Li is incorporated into Nd oxycarbonate, the composition is known to be Nd2O2 + 2x (CO3) 1-xLi2x, and the overall composition is Nd2O2 + 2x (CO3) 1-xLi2x and (1-x ) Li2CO3. From this and the thermal sample analysis described later, the Li degree was estimated. As the firing temperature is increased, Li ions are taken into the oxycarbonate, and in the case of Nd, a large amount of Li ions are taken into the oxycarbonate upon firing at 800 to 900 ° C. When a mixture of Nd oxide and Na2CO3 was calcined in CO2, Na ions were not taken into the oxycarbonate.

FIG. 5 shows the FTIR spectrum of the product when a 1: 1 mixture of Li2CO3 and Nd2O3 is calcined in CO2 (solid line) and in air (dashed line). By baking at 600 ° C. or higher in CO2, the absorption of hydroxyl groups near 3600 cm ⁻¹ disappears. On the other hand, in the baking in air, the absorption of hydroxyl groups has not disappeared even at around 800 ° C. Therefore, it can be seen that the formation of Nd oxycarbonate by calcination in CO2 effectively prevents the formation of Li hydroxide.

FTIR Spectra of Heavy Rare Earth Oxycarbonates FIG. 6 and FIG. 7 show FTIR spectra after a 1: 1 molar mixture of Li2CO3 and Y2O3 was heated to 400-800 ° C. and then cooled to room temperature. When a mixture of Y2O3 and Li2CO3 is calcined, the absorption of hydroxyl groups in the vicinity of 3600 cm ⁻¹ disappears even when the calcining temperature is as low as 400 ° C. and even when the calcining atmosphere is in the air. Also, absorption of CO3 ions adsorbed on Y2O3 is observed near 900 cm ⁻¹ , and absorption of CO adsorbed on Y2O3 is also observed near 550 cm ⁻¹ . From these facts, it can be seen that when Y2O3 and Li2CO3 are mixed and fired, the surface of Y2O3 is covered with CO3 ions regardless of whether the firing atmosphere is CO2 or air. In other words, absorption corresponding to CO3 ions and CO on Y2O3 can be seen both in the air and in CO2. In addition, it is thought that the CO3 ion in the case of baking in air was supplied from Li2CO3. Even when a mixture of heavy rare earth oxide and Li2CO3 is fired, it is considered that Li2CO3 melts and is distributed in layers on the surface of the heavy rare earth oxide at 600 ° C. or higher.

Thermomass analysis Fig. 8 shows the results of thermal mass spectrometry when Nd2O3 and Li2CO3 at a molar ratio of 1: 1 were pre-fired in CO2 at temperatures of 700 to 900 ° C and heated again in CO2. . The black mark in the figure indicates the temperature rising side, the white mark indicates the temperature decreasing side, the numbers in the figure indicate the pre-baking temperature, and the system state is a mixture of Li2O and Nd2O3 above 900 ° C . If the pre-baking temperature is 800 ° C or 700 ° C, if you experience a temperature of 900 ° C or higher, it will not return to its original mass even after cooling. On the other hand, in the case of a sample fired at 900 ° C. in advance, the mass once increases at 700 to 850 ° C., and returns to the original mass when cooled.

  FIG. 9 shows the characteristics when the same sample is reheated in air. In either case, the mass does not return to the original mass after cooling, but when the pre-calcination temperature is 900 ° C., the decomposition of Li 2 CO 3 is gradual compared to 800 ° C. firing and 700 ° C. firing. When the results of FIGS. 8 and 9 are combined, the stability of Li2CO3 is increased by firing at 900 ° C. in advance. However, in this sample, CO2 is desorbed and deficient from Li2CO3 or the like, and CO2 reabsorption occurs at 700 to 850 ° C. Therefore, light rare earth oxides such as Nd2O3 and Li2CO3 are baked at 850 to 900 ° C. so that oxycarbonates of light rare earth elements contain Li ions, and Li2CO3 is attached to the surface. Next, the obtained product is pulverized and a working electrode is formed, and then calcined again at 700 to 850 ° C. in CO 2 to obtain a working electrode having a high content of CO 3 ions and being thermally stable.

  FIG. 10 shows mass changes when four kinds of heavy rare earth element oxides of Y, Er, Yb, and Lu and Li2CO3 are mixed at a molar ratio of 1: 1 and fired in air. The black symbols in the figure indicate the results when the mixture was previously baked in 900 ° C. CO 2, and the white symbols indicate the results when the preliminary calcination was not performed. The firing atmosphere in FIG. 10 is in the air. All the samples were converted to double oxides of Li and heavy rare earth elements at a temperature higher than about 900 ° C., and the formation of oxycarbonate was not detected. The resulting double oxide was unstable and when heated in CO2, it decomposed above 300 ° C and changed to a heavy rare earth oxide and Li2CO3. Pre-firing in CO2 shifts the decomposition temperature of Li2CO3 to a higher temperature, indicating that there is a strong interaction between heavy rare earth oxides and Li2CO3. From the FTIR results, it can be inferred that the CO3 ion layer on the surface of the heavy rare earth oxide delays the decomposition of Li2CO3.

Characteristics of the working electrode using Nd oxy carbonate Figure 11, Figure 12 shows the characteristics of the working electrode starting from a mixture of Li2CO3 and Nd2O3. The working electrode is obtained by baking a mixture of Nd2O3 and Li2CO3 at 800 ° C. in CO 2 and re-baking at 700 ° C. in CO 2 after film formation. A ratio such as 0.25: 1 in FIG. 12 indicates a molar ratio between Li2CO3 and Nd2O3, and a ratio 1: 0 at the bottom of FIG. 12 indicates a working electrode prepared using only Li2CO3 without using Nd2O3. The middle part of FIG. 11 shows a sample obtained by calcining a mixture of Li2CO3 and Nd2O3 at a molar ratio of 1: 1 without pre-firing in CO2 at 600 ° C. in air after film formation.

  The upper and lower widths of the plots in FIGS. 11 and 12 indicate the CO2 sensitivity, and in the working electrode using only Li2CO3, the electromotive force rapidly decreases for the first 100 hours and then increases. In the case of a mixture of Li2CO3 and Nd2O3, or a mixture of Nd oxycarbonate (including Li ions) and Li2CO3, the electromotive force changes in the initial stage, but the characteristics stabilize in about several tens of hours. Then, in the mixture of Nd oxycarbonate and Li2CO3, the drift of electromotive force thereafter becomes particularly small after about 60 hours.

  As shown in FIG. 12, the stability of the electromotive force increases when Nd2O3 is excessive with respect to Li2CO3. The stability of the electromotive force is most excellent with 4 mol of Nd2O3 starting material with respect to 1 mol of Li2CO3, and the ratio of Li and Nd at the working electrode is preferably 1: 5 to 3: 4 in terms of atomic ratio from FIG. Clearly, particularly preferably about 1: 4.

Characteristics of Working Electrode Using Heavy Rare Earth Oxide FIG. 13 shows characteristics at a working electrode using a 1: 1 mixture of heavy rare earth oxide and Li2CO3. Li2CO3 and heavy rare earth oxide were pre-fired in CO2 at 900 ° C, pulverized into a paste, formed into a working electrode, and refired in CO2 at 700 ° C. In contrast to Li2CO3 alone, the electromotive force is stable both in the case of a mixture with Y2O3 and with a mixture of Yb2O3. It was also confirmed by Tb, Dy, and Tm that the electromotive force was stabilized by mixing heavy rare earth oxides.

CO2 sensor stabilization mechanism The mechanism of sensor characteristic stabilization is discussed. In the detection of CO2, the reaction at the working electrode is represented by equation (1), and the reaction at the interface between NASICON and the reference electrode is represented by equation (2).
^{Li2CO3 → 2Li + + CO2 + 1} /2 O2 + 2e - (1)
^{2Na + +1/2 O2 + 2e - →} Na2O (2)
Therefore, the electrochemical potential of each component involved in the reaction is represented by μ, the working electrode is represented by the subscript a, the reference electrode is represented by the subscript r, and assuming the equilibrium at the working electrode and the equilibrium at the reference electrode and the NASICON interface, Equation (3) is established at the pole and Equation (4) is established at the reference pole.
^{μaLi2CO3 = 2μaLi + + μaCO2 + 1} / 2μaO2 + 2μae - (3)
^{2μrNa + + 1 / 2μrO2 + 2μre} - = μrNa2O (4)

When the activity is represented by the symbol a, the change in Gibbs free energy is represented by ΔG ⁰ and the Farady constant is represented by F, the electromotive force of the CO2 sensor is represented by the equation (5) from the equations (3) and (4). . If the term relating to the CO2 sensitivity is separated from the other term from this, the electromotive force is expressed by Equation (6).
EMF = −ΔG ⁰ / 2F− (RT / 2F) ln [(aaLi + ² · PaCO2 · aaO2 ^0.5 · arNa2O) /
(aaLi2CO3 ・ arNa + ²・ arO2 ^0.5 )] (5)
EMF = − (RT / 2F) ln [PaCO2] − (RT / 2F) ln [(aaLi + ² · arNa2O) / (aaLi2CO3 · arNa + ² )]
+ Constant (6)
The Li2CO3 activity on the working electrode side is considered to be almost constant, and the ratio of the Na2O activity to the Na ion activity on the reference electrode side is also the formula if the oxygen activity at the reference electrode is constant. It is considered almost constant from (4). Then, what brings about the change of electromotive force becomes the change of the activity of Li ion. This corresponds to the change in Li ion activity caused by the reaction between NASICON and Li2CO3, which was confirmed in Fig. 2, and light rare earth oxycarbonates and heavy rare earth oxides act as Li ion buffers. This agrees with the result of preventing power drift.

In the examples, the light rare earth element oxycarbonate absorbs free Li ions, prevents reaction with NASICON, and stabilizes the activity of Li ions. As a result, the electromotive force is stabilized. In the case of heavy rare earth oxides, a layer of CO3 ions is formed on the oxide surface. When free Li ions are generated from Li2CO3 at the working electrode, they are adsorbed by CO3 ions on the surface of the heavy rare earth oxide. For this reason, free Li ions are trapped, and similarly, the change of Li ion activity can be prevented and the reaction with NASICON can be prevented.

The figure which shows the CO2 sensor of the example typically X-ray diffraction diagram showing the reaction between NASICON and Li ions X-ray diffraction diagram showing the formation of Li ion-containing Nd oxycarbonate when Nd2O3 and Li2CO3 are calcined in CO2. The figure which shows the relationship between the c-axis direction parameter and Li ion content in FIG. FTIR spectrum when calcining a 1: 1 mixture of Li2CO3 and Nd2O3 in CO2 (solid line) and in air (dashed line) FTIR spectrum when calcining a 1: 1 mixture of Li2CO3 and Y2O3 in CO2 (solid line) and in air (dashed line) In the FTIR spectrum of the same sample as Figure 6, an enlarged view of a wave number range of 400cm ^-1 ~2000cm ^-1 The figure which shows the mass change pattern when the temperature of the mixture of Nd2O3 and Li2CO3 previously reacted at 700-900 ℃ in CO2 is changed in CO2. The figure which shows the mass change pattern at the time of changing the temperature in the air the same sample as FIG. Diagram showing the mass change pattern of a mixture of heavy rare earth oxide and Li2CO3 calcined in CO2 when the temperature is changed in air This figure shows the CO2 detection characteristics of a sensor using a working electrode obtained by firing a 1: 1 mixture of Li2CO3 and Nd2O3 in carbon dioxide at 800 ° C. The atmosphere was switched between 370 ppm and 10000 ppm for CO2 every 30 minutes. The blank area in the figure indicates unheated neglect. FIG. 11 is a characteristic diagram similar to FIG. 11 and shows the result when the molar ratio of Li2CO3 and Nd2O3 is changed in the range of 0.25: 1 to 1: 1. In the same characteristic diagram as in FIG. 11, the results when using a working electrode starting from a 1: 1 mixture of Li2CO3 and Yb2O3 and a working electrode starting from a 1: 1 mixture of Li2CO3 and Y2O3 are shown. Show

Explanation of symbols

2 CO2 sensor 4 NASICON
6 Working electrode 8 Porous Pt layer 10 Reference electrode 12 Glass tube

Claims (1)

Production of a CO2 sensor provided with a reference electrode cut off from the detected atmosphere so as to be in contact with the Na ion conductor, and a working electrode containing Li2CO3, an Nd compound and an electron conductive substance and in contact with the detected atmosphere In the method
By firing a mixture of Li compound and Nd compound in CO2 at 850-900 ° C., a mixture of Nd oxycarbonate and Li2CO3 containing Li ions between layers,
Using the mixture and the electron conductive material, a working electrode is formed on the surface of the Na ion conductor, and then re-fired at 700 to 850 ° C. in CO 2. .

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