Anal. Chem. 1993, 65, 2736-2739
2736
Amperometric Carbon Dioxide Gas Sensor Based on Electrode Reduction of Platinum Oxide Toru Ishiji'J Riken Keiki Co. Ltd., 2-7-6 Azusawa, Itabashi-ku, Tokyo 174, Japan
Katsuo Takahashi and Akira Kira The Institute of Physical and Chemical Research (Riken), Wako-shi, Saitama 351-01, Japan
An amperometric sensor based on the pH-dependent electrode reduction of platinum oxide was developed for monitoring carbon dioxide in the atmosphere. The sensor, using a gas-permeable platinum oxide electrode and aqueous electrolytic solution, could measure the C02 concentration without interference from oxygen reduction. The platinum oxide electrode was formed on a gaspermeable PTFE membrane by reactive sputter coating. It was found that platinum oxide is much better for C02 sensors than platinum. At the optimumelectrode potential, the reduction current was found to be proportional to the square root of the C02 gas concentration, indicating that the electrode reduction is controlled by the hydrogen ion concentration supplied from COZ. The sensor showed good response and stability. INTRODUCTION Carbon dioxide in the atmosphere has been noted to be a major cause of global warming. Therefore, the monitoring of COZin the atmosphere and controlling COZin bioresearch fields are growing in importance. Infrared sensors, conductivity sensors,1-3 and Severinghaus-type sensorsu are commercially available for the detection of COz; however, these sensors have some disadvantages. Although the infrared spectroscopic system offers better precision, it is large and relatively expensive. The thermal conductivity sensor is convenient to use; however, other gases interfere. The Severinghaus-type sensor, which consists of a glass pH electrode covered by a gas-permeable membrane, is commonly available for industrial and scientific usages because of its simplicity and low cost. However, it is affected by electromagnetic disturbances because of its high impedance, and the signal fluctuates because of the hydrogen ions generated by COz in sample gases. Recently, sensors using solid electrolytes combined with potentiometry' or conductometry have been developed. Such sensors are simple and stable, but interfered with combustible gases. Amperometric gas sensors are widely used for the detection of toxic gases such as carbon monoxide and nitrogen dioxide t Saitama University, Shimo-ohkubo 255, Urawa-shi, Saitama 338, Japan. (1)Amoudse, P. B.; Pardue, H. L.; Bourland, J. D.; Miller, R.; Geddes, L. A. Anal. Chem. 1992,64,200-204. (2) Brukenstein, S.; Symanski, J. S. Anal. Chem. 1986,58,1766-1770. (3)Shimizu, Y.;Komori, K.; Egashira, M. J.Electrochem. SOC.1989, 136,2256-2260. (4)Severinghaus, J. W.Ann. N.Y. Acad. Sci. 1958,148, 115-137. (5)Jensen, M.A,; Rechnitz, G. A. Anal. Chem. 1979,51,1972-1977. (6)Kobos, R.K.; Parks, S. J.; Meyerhoff, M. E. Anal. Chem. 1982,54, 1976-1980. - - . - - - - -. (7)Miura, N.;Yao, S.; Shimizu, Y.; Yamazoe, N. J.Electrochem. SOC. 1992,139,1384-1388.
0003-2700/93/0365-2736$04.00/0
in ambient air.8~9An amperometric sensor for COZ,however, has not been developed for practical use because the electrochemical reduction of COZ is difficult because of the interference of the hydrogen ion reduction in aqueous solutions. Amperometric COz sensors consisting of nonaqueous electrolytic solutions10 and using a pH-dependent reaction of a copper diamine complexll have been studied, but they have not been successful in practical use. We have reported an acidic gas sensor based on reduction of iodine liberated by the sensing acidic gases, but it has not been successful in practical COZ sensing.lZJ3 In this paper, we describe a study to develop an amperometric sensor for monitoring COZin the range 0.04-0.1 5% , corresponding to the COZ concentration in an office. The platinum and platinum oxide films were deposited on the gas-permeable membrane by reactive sputter coating. The electrode reduction of the platinum oxide was applied to COz detection because the reduction current depends on the acidity of the electrolytic solution. The effect of oxygen reduction on the detection current, the reaction mechanism of the electrode, the stability, and the reproducibility were investigated to evaluate the characteristics of the sensor for the monitoring of COz in ambient air. The COz concentration in an office measured by our COZsensor was compared with that obtained by a conventional nondispersive infrared gas analyzer.
EXPERIMENTAL SECTION A schematicdiagram of the sensor and sensing system is shown in Figure 1. The sensor was constructed as a three-electrodecell systemcontaining17 cm30f0.1 M (M = mol dm-9)KCl supporting aqueoussolution. The reference electrodewas a Ag wire electrode whose surface was oxidized to AgCl in a KC1 solution. The potential of the reference electrode was 0.105 V vs Ag/AgCl/ saturated KCl. The counter electrode was Ag wire. All the potentials, E, in the text will be presented against the reference electrode in the sensor. The working electrode (WE) consisted of a platinum or a platinum oxide film formed on aporous poly(tetrafluoroethy1ene) membrane (PTFE). The membrane, Model FX-30, was purchased from Sumitomo Electric Co. Ltd. Typical porosity of the membrane was 34%. The diameter of the working electrode was 27 mm. The working electrodewas prepared by reactive sputter coating using a Model BMS-6021 (Shinku-Kikai Kogyo Co. Ltd.) magnetron sputtering system. The sputtering unit with a platinum plate target was used to deposit the electrode films on the membrane substrates. The discharge voltage and current were 580 V and 450 mA, respectively. The total pressure,of the (8)Sedlak, J. M.; Blurton, K. F. J.Electrochem. SOC.1976,123,14761478. (9) Chang, S. C.; Stetter, J. R. Electroanalysis 1990,2,359-365. (IO)Albery, W.J.; Barron, P. J.Electroanal. Chem. 1982,138,79-87. (11)Evans, J.; Pletcher, D.; Warburton, P. R. G.; Gibbs, T. K. Anal. Chem. 1989,61,577-580. (12) Ishiji, T.; Takahashi, K. Denki Kagaku 1990,58,1178-1183. (13)Ishiji, T.; Takahashi, K. Sem. Actuators, 1993,B13-14,583-584. 0 1993 Amerlcan Chemlcal Society
ANALYTICAL CHEMISTRY, VOL. 65, NO. 20, OCTOBER 15, 1993
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Flgure 1. Schematic diagram of the sensor and gas-sensing system: (A)air or nitrogen gas reservoir; (B) standard(0.25 % COP)gas reservoir; (SV) stop valve; (P) suction pump; (WE) PtO electrode on gas-permeable membrane; (RE) reference electrode; (CE) counter electrode; (ES) electrolytic solution; (PS) potentiostat; (REC) pen recorder.
oxygen-argon mixture reactive gas was maintained at 6.7 X 10-1 Pa, and the partial pressure of oxygen, P(02), was varied from 0 to 3.3 X 10-3Pa. The substrate membrane was maintained at 50 “C and sputter coated for 60 min. The thickness of the electrode film was typically ca. 0.5 pm. The properties of the sputter-coated films were analyzed by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD).14J5 The film formed with the oxygen-free reactive gas showed that the film mainly consisted of metallic platinum, assigned from the binding energy of the XPS peak; this film is abbreviated as “Pt-film”in the text. The film prepared with the reactive gas containingAr and 02 (P(02)= 3.3 X 10-3Pa) was assigned as PtO from the binding energy of the XPS peak, and is abbreviated as “PtO-film”in the text. The structure of PtO was microcrystalline platinum oxide which was estimated from the broadening of the peaks on the XRD pattern.ls Both Pt- and PtO-films had low resistivity (less than 2.5 X le2fl cm) which is suitable for an electrode for an amperometric sensor. Standard gases containing 0.25% CO2 in nitrogen and in the air (the air mentioned below contains nitrogen and oxygen gases with volume fractions of 79% and 21%, respectively) were purchased from Taiyo Sans0 Co., Ltd. Sample gases containing various concentrations of C02 were prepared by mixing the standard gas and nitrogen or air in a gas samplingbag and sucked to the sensor by a pump as shown in Figure 1. The gas flow rate into the sensor was 0.5 dm3 min-l. The current-potential characteristics of the sensors were measured by a potentiostatic method at each electrode potential. The potential was held constant for 30 min to achieve a stable background current for both air and nitrogen samples. The current response for the sample gases was recorded on a pen recorder chart. The change in the C02 concentrationin the office was measured by our sensor and a nondispersive infrared gas analyzer, Model RI-411A, produced by Riken Keiki Co. Ltd. The gas flow rate into the analyzer was 1.0 dm3min-l, and the inlet was at the same point as that of the PtO-film sensor. All measurements were carried out at 25 f 2 “ C .
RESULTS AND DISCUSSION Potential Dependence of Current Response for ( 2 0 2 . Typical current response curves for C02 measured by the PtO-film sensor at E = 0.2 and 0.1 V are shown in Figure 2. The residual current under flowing air and N2 became stable at ca. 30 min after application of the electrode potential, whose residual current is indicated as I a in Figure 2. When sample gas containing COZ was allowed to flow through the cell, the reduction current increased rapidly. The current showed a maximum on curves A and B; however, this type of maximum does not appear with good reproducibility. The reduction current observed at 3 min after the introduction of a gas (14) McBride, J. R.; Graham, G. W.; Peters, C. R.; Weber, W. H. J. Appl. Phys. 1991,69,1596-1604. (15) Nakano, N.; Ishiji,T.;Okamoto, A.; Suzuki, Y.; Ogawa, S.Hyomen Gijutsu 1992,43,785-788.
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Figure 2. Response curves of the PtO-film sensor for air, 0.25 % Con in air (CA), N2, and 0.25% C02 in N2 (CN). Electrode potentials are 0.2 V (A, B) and 0.1 V (C) vs Ag/AgCl (0.1 M KCI). Gas flow rate is 0.5 dm3 min-I. 5 0 d
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0.1 0.2 0.3 0.4 Electrode potential / V
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0.6
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Flgure 3. Current-electrode potential relationships obtained by the PtO-film sensor for N2and air with and without COP: (0)N2; (0)air; (+) 0.25% C02 in N2; (0)0.25% C02 in air; (m) AI in air; (A)AI in N2-
containing CO2, I b , is shown in Figure 2. Curves A and B show the CO2 response curves in air and N2, respectively. The response curve obtained at E = 0.1 V is also shown in Figure 2 as curve C. The potential dependence of I a and I b for the sample and balance gases (the air or N2) is shown in Figure 3a. The residual current in the case of air increases a t potentials below 0.2 V, which is attributed to the reduction of oxygen. Although the reduction current for CO2-containing gas also increased a t potentials below 0.2 V, the high residual current is undesirable for reliability and stability in COS sensing. The current due to the added CO2, AI (=pa- I&,is also plotted in Figure 3b. There is a difference between AI in air and A I in N2 at potentials below 0.2 V, which may be caused by the oxygen reduction current dependence on the pH change induced by the dissolution of CO2 into the solution. At a potential of 0.2 V or above, the effect of the oxygen reduction on AI becomes negligibly small. We chose the electrode potential 0.2 V as the optimum condition of this sensor for C02 detection. Pt- and PtO-Film Sensors. To characterize the PtOfilm sensor, the current response and its stability were compared with those of the Pt-film sensor by using air containing 0.25 % C02 or 6 ppm hydrogen fluoride (HF) gas. The CO2 response (sensitivity) for Pt- and PtO-film sensors obtained by the above-mentioned method (E = 0.2 V) is listed in Table I, line A. AI(0.25% C02) for the Pt-film sensor is
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 20, OCTOBER 15, 1993 2.5
Table I. Current Responses for COa and HF Obtained by Pt- and PtO-Film Sensors
,
sensor Pt-film line A
1 AI(0.2575 C02)/pA AI(6ppmHF)/pA
B
1.7
PtO-film
2
3
4
5
6
1.8
1.6
3.1 -
2.8 -
2.8 20.8
-
11.8 1.3
-
8.2 After Flowing HF (65 h) AI(6ppmHF)IpA 0.2 Al(0.2575 C02)IpA 0
C D 0
10
-
-
-
No data. 0 j (a) PtO-film sensor
0
-10 -
%
