A CO2 sensor operating under high humidity

A CO2 sensor operating under high humidity

Journal of Electroanalytical Chemistry 522 (2002) 173– 178 www.elsevier.com/locate/jelechem A CO2 sensor operating under high humidity T. Oho, T. Ton...

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Journal of Electroanalytical Chemistry 522 (2002) 173– 178 www.elsevier.com/locate/jelechem

A CO2 sensor operating under high humidity T. Oho, T. Tonosaki, K. Isomura, K. Ogura * Department of Applied Chemistry, Yamaguchi Uni6ersity, Tokiwadai, Ube 755 -8611, Japan Received 6 December 2001; received in revised form 22 January 2002; accepted 24 January 2002

Abstract The development of a CO2 sensor capable of operating in a highly humid atmosphere is desirable in order to preserve a clean environment in an airtight room, to apply to medical equipment such as a metabolic breathing system, etc. The CO2 sensor proposed here can monitor the CO2 concentration, which equilibrates with water vapor, permitting the detection of CO2 under high humidity. The sensing material was a composite film consisting of base-type poly(anthranilic acid) (PANA) and poly(vinyl alcohol) (PVA). The ac impedance was enhanced with increasing CO2 concentration in a frequency region above 100 Hz. A log–log plot of impedance versus CO2 concentration obtained at the frequency of 100 kHz showed a good linear relationship in the concentration range between 3 ×102 and 1.5 ×105 ppm under as high a relative humidity as 80%. The response curve of the composite film to CO2 concentration was not affected by the presence of NH3 and HCl in the concentration regions below 1000 and 10 ppm, respectively. Furthermore, no effect of coexisting gases such as O2 and N2O was observed at all for the linear relationship of the log–log plot of dc resistance versus CO2 concentration. © 2002 Elsevier Science B.V. All rights reserved. Keywords: CO2 sensor; Coexisting gas; Conducting polymer; Impedance

1. Introduction Consecutive detection of CO2 concentration has been recognized to be very important for preserving a clean environment in an airtight room and controlling agricultural and biological processes. For this purpose, infrared spectroscopic and gas chromatographic techniques are unsuitable because the available apparatus is expensive and large in scale. Hence, a number of simplified CO2 sensors have been proposed which permit the routine measurement of CO2 concentration. The principle of CO2 sensing rests chiefly on the basis of the electrical response of sensing materials upon the change in concentration of CO2; e.g. the electromotive force and capacitance are measured in electrolyte- [1,2] and capacitance- [3,4] type CO2 sensors, respectively. The practical operation of these sensors requires a high temperature (\ 400 °C) for activation. This entails high running costs and some risks. Furthermore, in the electrolyte-type CO2 sensor for which alkali carbon-

* Corresponding author. Tel.: + 81-836-35-9417; fax: +81-836-322886. E-mail address: [email protected] (K. Ogura).

ates are generally used, the deterioration of the sensing property is not slight under high humidity, because the solubility of alkali carbonate is quite high in water. In general, CO2 sensors are applied in an environment with rather high humidity as in an airtight room or a greenhouse. Hence, the development of a CO2 sensor workable under high humidity is strongly desired. In the present study, a sensor, which monitored the CO2 concentration in equilibrium with water vapor, has been developed, permitting the detection of CO2 under high humidity. We have previously reported that a composite film consisting of base-type conducting and insulating polymers responds to CO2 with a high sensitivity in the manner such that the dc resistance of the composite film changes depending on the CO2 concentration at a constant humidity [5,6]. In this system, however, the change in resistance upon the variation of CO2 concentration became quite small under relative humidities higher than 60%, and such a CO2 sensor is difficult to apply in a highly humid atmosphere. Nevertheless, it was found here that the measurement of ac impedance instead of dc resistance permitted us to determine the CO2 concentration with accuracy under a high humidity.

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2. Experimental Poly(anthranilic acid) (PANA) was used as a conducting polymer, and prepared by the standard procedure [5]. The starting monomer and oxidizing agent were anthranilic acid and ammonium peroxosulfate, respectively. The polymerization was carried out at room temperature (r.t.), and the precipitate was finally dried under vacuum. The resultant PANA is in a salt-type form (Scheme 1) at this stage, and it is conducting due to the self-doping of ionized carboxyl groups. A base-type PANA (Scheme 2) was obtained by further heating the salt-type PANA at 280 °C for 8 h and eliminating the carboxyl groups. The transformation of the salt-type PANA to the base-type was confirmed by Fourier transform infrared spectroscopy (Shimadzu FTIR, 8100M). In the heattreatment of the salt-type PANA, the bands at 1690 and 1450 cm − 1 disappeared owing to the decarboxylation, while bands around 1600 and 1510 cm − 1 with comparable optical intensities were observed which are attributable to the quinoid and benzenoid ring stretching vibration of base-type PANA, respectively [5]. A given amount of the base-type PANA was dissolved in dimethyl sulfoxide (DMSO). This solution was then mixed with a DMSO solution containing a constant concentration of poly(vinyl alcohol) (PVA) used as an insulating polymer. Various composite solutions were made by changing the mass ratios of the two solutions. The composite solution was cast on a comb-shaped microelectrode, and the solvent was removed under vacuum. The thickness of the applied composite was always 0.1 mm. The comb-shaped microelectrode (BAS Co., No.51-2042) was made by depositing a thin platinum film in a comb shape onto a glass substrate in vacuum. The base-type PANA/PVA composite-cast microelectrode was used as a CO2 sensor, and put in a measuring cell to monitor the dc resistance and ac impedance. The cell was attached to a vacuum line to enable us to admit a controlled pressure of CO2 and water at 25 °C. The dc resistance and ac impedance were measured after N2 was introduced to the cell and the pressure was restored to 101 kPa. CO2 and N2 gases were passed before use through a trap cooled with liquid nitrogen for refinement. The dc resistance was

Scheme 1.

Scheme 2.

measured with an ultra high resistance meter (Advantest, model R8340), and the applied voltage was 1 V. The measurement of the ac impedance was performed with an EG&G model 263A potentiostat/galvanostat interlinked with an EG&G model 5210 lock-in amplifier controlled by a computer system. The amplitude of the sine ac potential was 10 mV, and the frequencies applied were in the range from 10 mHz to 100 kHz. The relative humidity in the cell was monitored with a hygrometer (Shinyei Co., TRH-3A). The effects of coexisting gases on the sensing properties of the CO2 sensor were examined by monitoring the dc resistance under a relative humidity in the presence of O2, N2O, O2 + N2O, NH3 or HCl gas.

3. Results and discussion

3.1. Relationship between dc resistance and CO2 concentration A base-type PANA/PVA composite with the mass ratio of 1:2 was placed in the cell, and the dc resistance was measured at 25 °C under the various conditions of CO2 concentration and humidity. As is shown in Table 1, the dc resistance obtained in the presence of either CO2 (run 1) or water vapor (run 9) were very large: 5.8× 1011 or 1.9×1010 V, respectively. In the presence of both CO2 and water vapor (runs 2–8), however, the dc resistance became smaller as the humidity and CO2 concentration were increased. These results can be rationalized by the following. There are no ionic species in the composite consisting of base-type PANA and PVA, and besides no generation of ion occurs when the composite is in contact with either 100% CO2 or a relative humidity of 100%. Hence, a base-type PANA/ PVA composite should be non-conducting under such conditions. On the other hand, in the presence of both CO2 and water vapor, the hydrolysis of CO2 should lead to the formation of hydrogencarbonate ions and Table 1 Dc resistance of a base-type PANA/PVA composite with the mass ratio of 1:2 at 25 °C under various conditions of CO2 concentration and humidity Run

CO2/%

Humidity/%RH

dc resistance/V

1 2 3 4 5 6 7 8 9

100 10 10 10 10 0.01 0.01 0.01 0

0 20 30 50 70 30 50 70 100

5.8×1011 4.5×109 1.7×108 3.6×107 3.5×107 2.9×109 8.1×108 1.3×108 1.9×1010

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protons, and these ions may come in contact with the non-conducting base-type polymer, resulting in the partial conversion of this polymer to the salt-type. Thus, the electrical resistance of the PANA/PVA composite becomes low enough to be measured. The change in resistance of the composite should be dependent on the concentration of hydrogencarbonate ions, which equilibrate with surrounding CO2 and water vapor. The electrical resistance of a base-type PANA/PVA composite with the mass ratio of 1:2 was measured by the dc method in a humid CO2 atmosphere, and the results are shown as a function of CO2 concentration in Fig. 1. At the relative humidities of 30 and 50%, the logarithmic resistance is linearly related to the log of CO2 concentration, indicating an excellent property as a CO2 sensor. At the humidities of 20 and 70%, however, the linear relationship is valid only in a limited region of CO2 concentration. The dc resistance of the composite film was about 1.5× 1010 V, independent of the CO2 concentration in a dry atmosphere (B20%) with a lower concentration of CO2 ( B104 ppm). This is

Fig. 1. Relationship between dc resistance and CO2 concentration for a base-type PANA(1)/PVA(2) composite under relative humidities of 20 ( ), 30 (), 50 ( ), 70 ()%. Temperature, 25 °C.

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attributed to small changes in concentration of hydrogencarbonate ion upon the variation of CO2 concentration under such dry conditions. On the other hand, the resistance showed a constant value of about 3×107 V in a wet atmosphere (\ 70% RH) with higher concentration of CO2 (\ 5×103 ppm). This value approximated to that of 2.7×107 V obtained for a salt-type PANA entirely doped with hydrogencarbonate ions. This polymer was prepared by soaking a base-type PANA in a 0.1 M KHCO3 solution for 6 h [7]. Hence, it is suggested that the base-type PANA is completely converted to the salt-type in such a wet atmosphere with a higher concentration of CO2 and hence the dc resistance is independent of the CO2 concentration. The resistance versus CO2 concentration curve for a base-type PANA/PVA composite was found to depend also on the mass ratio of two components. As shown in Fig. 2, the resistance of PANA(1)/PVA(1) and PANA(1)/PVA(2) composites is proportional to the CO2 concentration on a log–log scale. On the other hand, for PANA(1)/PVA(3) and PANA(1)/PVA(6) composites, the resistance reached a constant value in the concentration region beyond 104 and 3× 103 ppm, respectively. This result is consistent with the above view. The composite film with a smaller ratio of PANA to PVA should have a smaller amount of PANA, and a complete conversion of the base-type PANA to the salt-type may be achieved by the reaction with a smaller amount of carbonate ions; i.e. with a lower concentration of CO2. Thus, in the application of the base-type PANA/PVA composite as a CO2 sensor, a log–log plot of dc resistance versus CO2 concentration gives a straight line only under a moderate humidity, and it is difficult to use such a sensor under highly humid conditions. As described below, however, a log–log plot of impedance versus CO2 concentration obtained by the ac method showed very good linearity in a highly humid atmosphere with CO2; hence the system would be suitable for use as a CO2 sensor.

3.2. Relationship between ac impedance or capacitance 6ersus CO2 concentration

Fig. 2. Relationship between dc resistance and CO2 concentration for composites with various mass ratios of base-type PANA to PVA: 1/1 ( ), 1/2 (), 1/3 ( ), 1/6 () under a relative humidity of 50%. Temperature, 25 °C.

In Fig. 3a, the ac impedance for a base-type PANA(1)/PVA(3) composite at the relative humidity of 80% is plotted as a function of frequency. As seen from this figure, the impedance decreases with increasing frequency and is independent of the concentration of CO2 until about 100 Hz, but beyond this value the impedance is considerably affected by the CO2 concentration. In Nyquist plots (Fig. 3b), a semicircle was observed in a high-frequency range, followed by the start of a second semicircle in a region of lower frequency. Hence, the electrochemical system is considered to be represented by a parallel circuit of the film resistance (Rfilm) and capacitance (Cfilm).

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obtained in the higher frequency region in the presence of a larger concentration of CO2 (Fig. 3a) is attributed to the decrease in capacitance of the composite film. In the following, the impedance property of a CO2 sensor was measured by applying a constant frequency of 100 kHz under various conditions. A log–log plot of ac impedance versus CO2 concentration is shown in Fig. 5. The slope of this curve was steeper at higher humidity, indicating that the sensitivity for the CO2 detection with this sensor was more favorable at high rather than low humidity. This is completely different from the properties for usual CO2 sensors, because these sensors can hardly respond to CO2 at such a high relative humidity as 80% owing to the deterioration of the sensing material. Hence, the base-type PANA/PVA composite film is promising as a CO2 sensor capable of working in a highly humid atmosphere. The ac impedance of the composite films with various mass ratios of base-type PANA to PVA is plotted versus the CO2 concentration at the relative humidity of

Fig. 3. Relationship between ac impedance and applied frequency (a) and Nyquist plots (b) for a base-type PANA(1)/PVA(3) composite under a humidity of 80% in the presence of 0.03% () and 15% () CO2. Temperature, 25 °C.

Fig. 5. Relationship between ac impedance and CO2 concentration for a base-type PANA(1)/PVA(3) composite under relative humidities of 60 (), 70 ( ), and 80 ()%. Applied frequency, 100 kHz; temperature, 25 °C.

Fig. 4. Relationship between charge capacitance and CO2 concentration for a PANA(1)/PVA(3) composite under relative humidities of 70 ( ) and 80 ()%. Applied frequency, 100 kHz; temperature, 25 °C.

Zfilm =

1 1 +j…Cfilm Rfilm

As is shown in Fig. 4, the film capacitance decreased with an increase in concentration of CO2, but the film resistance was almost independent of the CO2 concentration (\5×103 ppm) under highly humid conditions (Fig. 1). Hence, the larger value of the impedance

Fig. 6. Relationship between ac impedance and CO2 concentration for base-type PANA/PVA composites with various mass ratios: 1/2 (), 1/3 ( ), 1/4 (), 1/6 ( ). The measurement was performed at 25oC by applying a frequency of 100 kHz under a relative humidity of 70%.

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PANA to PVA (Fig. 2). Hence, in the ac method, the preparation of a CO2 sensor with the base-type PANA/ PVA composite may be much simplified. This would be another favorable feature, which does not occur for the usual sophisticated sensors.

3.3. Effects of coexisting gases on the CO2 -sensiti6e properties

Fig. 7. Response of a base-type PANA(1)/PVA(3) composite to 1% CO2 +1.6% H2O gases in the presence of various concentrations of NH3 (a): 10 ( ), 1000 ( ), 3000 () ppm and HCl (b): 0 ( ), 10 ( ), 20 () ppm, and to dynamic pumping at 25 °C. The sample gas was admitted to the measuring cell at an arrow ¡ and pumped out at an arrow  .

The response of a base-type PANA/PVA composite to 1% CO2 + 1.6% H2O (corresponding to the relative humidity of 50%) gases were examined in the presence of various concentrations of NH3 and HCl (Fig. 7). The measuring cell with a CO2 sensor was first pumped out, the sample gases were then introduced into the CO2 sensor, and again pumped out. As shown in Fig. 7a, the response curves are affected only slightly by the addition of NH3 until 1000 ppm, and beyond this concentration the resistance becomes much smaller. This indicates that the cation–anion interaction in the conducting polymer becomes smaller since the dielectric constant of the composite film is enhanced by the adsorption of higher concentration of NH3. On the other hand, the effect of HCl on the response curve is more noticeable as is seen from Fig. 7b; that is, a considerable decrease in resistance is observed even in the addition of 20 ppm HCl. This is due to the preferential interaction of HCl with the base-type polymer, which is transformed partially to the salt-type polymer. In Fig. 8, various concentrations of CO2 were prepared by diluting pure CO2 gas with N2, O2, N2O and 67% N2O+ 33% O2 gases. As seen from this figure, the log–log plot of resistance versus CO2 concentration is linear irrespectively of the kind of coexisting gases. Hence, it is found that the CO2-sensitive property of the composite film is not affected at all by these inorganic gases.

4. Conclusions

Fig. 8. Dependence of dc resistance of a base-type PANA(1)/PVA(3) composite on the CO2 concentration at a relative humidity of 50%. Sample gases were prepared by diluting with N2 ( ), O2 ( ), N2O () and 67% N2O +33% O2 ().

70% in Fig. 6. A linear relationship is effective for the composites with any ratios, although the sensitivity for the CO2 measurement seems to be better for the composite with smaller mass ratio of PANA/PVA. This also differs from the result obtained from the dc measurement in which the linear relationship between the logarithmic resistance and CO2 concentration was observed only for the composite film with a specific ratio of

The logarithmic ac impedance of the base-type PANA/PVA composite was linearly related with the log of CO2 concentration under high humidity. This relationship was valid in the range of CO2 concentration from 3× 102 to 1.5× 105 ppm under high relative humidities of up to 80%. The impedance was found to respond to the CO2 concentration, which equilibrates with water vapor. At a high humidity, an increase in concentration of CO2 caused a decrease in film capacitance, while the film resistance was almost independent of CO2 concentration (\ 5× 103 ppm), resulting in a linear relationship between the impedance and CO2 concentration in a log–log plot. Coexisting gases such as O2 and N2O did not have any effect on the linear relationship of log–log plot of dc resistance versus CO2

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concentration, but the existence of HCl in concentration higher than 10 ppm resulted in a considerable decrease in dc resistance of the composite film, owing to the preferential interaction between HCl and the basetype polymer. References [1] M. Gauthier, A. Chamberland, J. Electrochem. Soc. 124 (1977) 1579.

[2] S. Yao, Y. Shimizu, N. Miura, N. Yamazoe, Chem. Lett. (1990) 2033. [3] T. Ishihara, K. Kometani, Y. Mizuhara, Y. Takita, Sens. Actuat. B 5 (1991) 97. [4] S. Matsubara, S. Kaneko, S. Morimomto, S. Shimizu, T. Ishihara, Y. Takita, Sens. Actuat. B 65 (2000) 128. [5] K. Ogura, H. Shiigi, Electrochem. Solid-State Lett. 2 (1999) 478. [6] K. Ogura, H. Shiigi, T. Oho, T. Tonosaki, J. Electrochem. Soc. 147 (2000) 4351. [7] H. Shiigi, T. Oho, T. Tonosaki, K. Ogura, Electrochemistry 69 (2001) 216.