Anal. Chem. 1995, 67,2409-2414
Electrochemical Composite Carbon-Ceramic Gas Sensors: Introduction and Oxygen Sensing Michael Tsionsky and Ovadia Lev*
Fredy and Nadine Herrmann School of Applied Science, Division of Environmental Sciences, The Hebrew University of Jerusalem, Jerusalem 91904, Israel
A new class of sol-gel-derived electrochemical gas sensors, comprised of a homogeneous dispersion of catalystmodified carbon powder in porous, organically modified silica, is introduced. The porous structure of the electrode material permits high gas permeability. Their hydrophobic surface rejects water, leaving only a very thin layer at the outermost surface in contact with the electrolyte and thus mixhizing effects of liquid side mass transfer. The carbon powder provides electric conductivity, and the catalystguarantees selectivity and sensitivity. Heat-treated Co tetramethoxymesoporphyrin modified composite ceramic-carbon electrodes exemplifjrthis new class of gas sensors. Detection of several gases by anodic (sulfur dioxide) or cathodic (carbon dioxide and oxygen) reactions is demonstrated. Metrological characteristics of a prototype oxygen sensor are presented. This paper demonstrates catalyst-modifiedcomposite carbonceramic electrodes (CCEs) and shows that CCEs can be used for gas analyses as exempliiied by a Co tetramethoxymesoporphyrin (Co-TMMP)-modifed fuel cell type renewable oxygen sensor. Despite the rapid development of solid electrolyte gas sensors,1j2 fuel cell type sensors are often preferred due to their convenient handling, low-temperature operation, and (often) higher selectivity. CCEs are comprised of a dispersion of carbon black or graphite powder in organically modifed or unmodified ~ i l i c a . ~CCEs , ~ are prepared by mixing carbon powders with sol-gel precursors, such as acidic solutions of water and tetraalkoxysilane, organotrialkoxysilane, or another organofunctional alkoxysilane. When organofunctional akoxysilane precursors are used, the Si-C bond does not hydrolyze but remains intact during the polymerization, and the corresponding organic group remains exposed on the xerogel surface. Addition of carbon powder to the sol-gel starting solution yields black, rigid xerogels in which the carbon powder percolates through the silica or modified silica backbone and provides high conductivity. Hydrophobically modified CCEs exhibit low background current, and their response can approach that of ensembles of microelectrodes,4 because water is rejected by the hydrophobic (1) Janata, J.; Bezegh, A. Anal. Chem. 1988,60.62R-74R (2) Seiyama, T.; Yamazoe, N. Recent advances in gas sensors in Japan. In Fundamentah and Applications of Chemical sensors;Schuetzle, D., Hammerle, R , Eds.; ACS Symposium Series 309 American Chemical Society: Washington, DC. 1986; p 39. (3) Tsionsky, M.; Gun, G.; Glezer, V.; Lev, 0.; Anal. Chem. 1994,66, 1747. (4) Gun, G.; Tsionsky, M.; Lev, 0. In Better Ceramics through Chemistry VI; Sanchez, C., Mecartney, M. L,Brinker C. J., Cheetham, A, Eds., Proceedings of the Materials Research Society Symposium 346; Materials Research Society: Pittsburgh, PA, 1994; p 1011.
0003-2700/95/0367-2409$9.00/0 0 1995 American Chemical Society
material and does not penetrate the porous structure, leaving only the outermost surface in contact with the electrolyte. The modified silica matrix provides a brittle, rigid backbone, and owing to its high resistance to plastic deformation, it can be reproducibly renewed by mechanical polishing. Gas sensing by heat-treated, carbon-supported Cc-TMMP was chosen as a model system that demonstrates a useful application (for oxygen sensing), illuminates the advantages as well as the disadvantages of the new class of gas sensors, and also demonstrates the possibility of electrocatalysis by bulk-modified CCEs. The choice of Co-TMMP-modifed CCEs was in part motivated by the high sensitivity and high stability of oxygen reduction by heat-treated, Cc-TMMP-supported which are imperative conditions for chemical sensing. EXPERIMENTAL DETAILS Materials. Unless otherwise specified, analytical grade reagents and triply distilled water (resistivity greater than 20 MQ/ cm) were used. Methyltrimethoxysilane (MTMOS) was purchased from ABCR, Inc. (Karlsruhe, Germany). High-purity carbon powder (-20-40 pm) was purchased from Bay Carbon, Inc. may City, MI). Gas mixtures of different partial oxygen pressures were made from pure oxygen (99.9%) and pur8ed air or pure nitrogen (99.999%);99.6%carbon dioxide and 99.98%sulfur dioxide (E. Merck, Darmstadt, Germany) were also used. The carbon dioxide gas contained -0.4% oxygen impurities, and these traces of oxygen were removed by passing the carbon dioxide feed through freshly reduced (in hydrogen stream) copper catalyst at 200 "C. Apparatus. An EG&G PARC Model 273 potentiostat-galvanostat, in conjunction with a Watanabe WX 4421 X-Y recorder, was used for voltammetric studies. Steady state currents were measured with a Keithley 485 picoamperometer. Sensor Configuration. Figure 1 presents a scheme of the three-electrode cell used in this research and details of the gas CCE. A 20 mL cell equipped with a Pt-flag counter electrode and a saturated Ag/&Cl or reference hydrogen electrode was used. The reaction cell was placed in a thermostated bath (Fried Electric, Israel) held at 30 "C f 0.5 "C unless otherwise specified. All experiments were conducted in 0.5 M sulfuric acid solution (Frutarom, Israel) unless otherwise specified. The working electrode comprised a T-shaped glass tube (1.0 mm id.) with one side sealed by a few millimeters of composite (5) van Veen, J. A R; Visser, C. Electrochim. Acta 1979,24, 921. (6)Scherson, D. A; Yao, S. B.; Yeager, E. B.; Eldrich, J.; Kordesch, M. E.; Hoffman, R W. J Phys, Chem. 1983,87,932. (7) Levina, 0. A; Radyushkina, K A Otktytia, Izobret., Prom. Obraztsy, Touamye Znaki 1977,54, 205.
Analytical Chemistry, Vol. 67, No. 14, July 15, 1995 2409
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7
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Figure 1. Scheme of a CCE gas sensor.
material made of methykmodified silica and heat-treated, graphite supported Co-TMMP. The modified electrode was prepared according to the followingpreparation protocol 0.1 g of graphite powder was thoroughly mixed with 4 mL of 0.1% (w/w) C e TMMP-toluene solution for 10 min. The solvent was then evaporated under vacuum at 80 "C. The graphitesupported catalyst was further heated at 800 "C for 2 h under constant flow of helium according to the procedure described by Yeager." To a freshly preprepared solution of 1.0 mL of MTMOS (ABCR), 1.5 mL of methanol (Frutarom). and 0.05 mLof 11M HCI was added 380 mg of pyrolyzed, Co-TMMP-modified graphite powder, and the mixture was vigorously shaken for 1-2 min. The leg of the T-shaped glass tube was dipped in the sol-gel precursors, removed from the solution, and allowed to dry under ambient conditions for at least 2 days. The electrode was then polished with 600 grit polishing paper, and its back side was connected to an external electric lead. Control blank electrodes were made of unmodified graphite powder using an identical preparation protocol. RESULTS AND DISCUSSION Operation Principle. The principle of operation of the gas sensor is illustrated by cyclic voltammetic studies of three different gases (COz, SOZ, 02) that are known to exhibit elec-
boreactivity on metal macrocycles-modified eledrodes.8-" These studies are depicted in F i i r e 2. Oxygen. Some of the fundamental features of the electrodes can already be seen from the (100 mV/s) CV response of blank (8)Meshiuuka. S.: Ichikawa. M.: Tamaru. K J. Chem. 5 3 ,~Chm. Conmum. 1974,II.158. (9)Raduyshkina. K A Tarasevieh. M. R Ahundov. E. A E/ekfmkhimin 1979, 12,1884. (10) Hiratsub. K, Takahashi. K Saraki. H.: Toshima. S. C h m . IO. 1137. (11) JaJinsky. RJ.Elrrhochm. Soc. 1965. 112, 526
Len. 1977.
2410 Analytical Chemistry. Vol. 67, No. 14, July 15, 1995
and modified CCEs (Figure 2A). When the electrode is fed by a nitrogen stream and held in deoxygenated 0.5 M HzSOl solution (by nitrogen bubbling), a flat current response between the oxygen evolution and hydrogen reduction is observed. Featureless voltammograms with small background current were o b served (under these conditions) for the blank (curve 1, 2) and the Co-TMMP-modfied (curve 3, 4) CCEs. The observed electrode capacitance, calculated as the average of the anodic and cathodic current densities divided by the scan rate, was 110 f 10 pF/mmz for the two types of CCEs. The observed capacitance was approximately twice that of a polished glassy carbon electrode." When air was introduced through the back of the blank electrode, a steep current response was initiated at --0.1 V/rhe. but the other characteristics of the electrode were not affected (curve 2). A cyclic voltammogram of air-fed Co-TMMP CCEs showed an additionalpronounced cataiytic activity (curve 3). Curve 4 demonsbates the response of a modified electrode in aerated solution when nitrogen is fed from the gas side. The current contribution of the dissolved oxygen (curve 4) is negligible compared to the contribution of the gas side stream (curve 3). Indeed, stirring of the solution had no effect on the electrode response when air was supplied from the gas side in aerated as well as in deoxygenated solutions. The electrode response was also indifferent to the gas flow rate (at least in the range 10-400 mL/min). These and other observations, which will be outlined in the next sections, prove pure kinetic control operation. The low observed capacitance of the electrodes and the indifference to agitation indicate that only a thin layer at their external surface is in contact with the electrolyte. This is especially favorable for dynamic operation since it implies that the reactants are contined to a very narrow region near the g a liquid interface and liquid side diffusion does not affect the (12)
Gun. J.: Tsionsky. M.: Lev. 0. Awl. Chim. A m 1994.294. 261.
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Figure 3. Steady state polarization curves of Co-TMMP-modified and 1.Oatm oxygen partial pressure (0.5 M sulfuric CCE in 0.0,0.03, acid, T = 30 "C).
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E, V (rhe) Figure 2. Cyclic voltammograms (0.1 V/s) of several gas feeds. (A) Oxygen reduction: (1) nitrogen-fed unmodified CCE in deoxygenated solution; (2) air-fed unmodified CCE in deoxygenated solution; (3) air-fed heat-treated, Co-TMMP-modified CCE in deoxygenated solution; and (4) nitrogen-fed Co-TMMP modified-CCE in air-saturated solution (0.5 M sulfuric acid, T = 30 "C). (B)Carbon dioxide reduction: (1) nitrogen-fed unmodified CCE; (2) carbon dioxide-fed unmodified CCE; (3) carbon dioxide-fed Co-TMMPmodified CCE; and (4) Co-TMMP-modified CCE fed with 99.6% carbon dioxide and 0.4% oxygen (pH 10; boric chloride buffer, T = 30 "C). (C) Sulfur dioxide oxidation: (1) nitrogen-fed unmodified CCE; (2) sulfur dioxide-fed unmodified CCE; and (3) sulfur dioxide-fed CoTMMP-modified CCE (0.5 M sulfuric acid, T = 30 "C).
response rate. Indeed, cyclic voltammeby of several dissolved redox couples in blank and bulk modified CCEs demonstrated that internal diffusion is negligible in hydrophobic CCES.'~ carbon Dioxide. Figure 2B compares the response of a blank CCE with that of a Co-TMMP-modified electrode for the electrochemical reduction of carbon dioxide. The cell solution contained
0.087 M NaOH, 0.013 M boric acid, and 0.013M KCl adjusted at pH 10. The responses of the blank CCE were identical under carbon dioxide and nitrogen feed streams (curve 2) up to -1.1 Vhhe, while the modified CCE exhibited a pronounced wave that was initiated at -0.8 Vhhe when pure carbon dioxide stream was fed (curve 3). The response of 0.4% oxygen in 99.6% carbon dioxide gas under the same operating conditions is presented in curve 4 and illustrates the relative sensitivity of the sensor to the two gases and the possibility of detection of the two analytes by the same sensor. Sulfur Dioxide. Cyclic voltammograms of sulfur dioxide-fed blank (curves 1and 2) and Co-TMMP-modified (curve 3) CCEs are depicted in Figure 2C. The oxidation waves of the modified and unmodified CCEs start at 0.4 Vhhe, but the catalytic action of the Co-TMMP is clearly demonstrated by the steeper slope of the anodic wave of curve 3 as compared to curve 2. In all these cases, the electrode was operated under a kinetic control mechanism, and its response was not influenced by stining of the solution or changes in gas flow rate. The fact that the potentials of the oxygen and carbon dioxide reduction and sulfur dioxide oxidation are far apart makes it possible to determine all three gases with the same sensor by employing ordinary polarographic techniques. Oxygen Sensing. (a) Polarization Curves. The steady state polarization curves of Co-TMMP-modified CCE subjected to several partial pressures of oxygen are presented in Figure 3. Exponential Tafel-type response is observed up to -0.45 Vhhe, and then the polarization curve becomes less steep. The mechanism of oxygen reduction on heat-treated, Co-TMMP-modified CCE in sulfuric acid solution will be presented in a subsequent paper.I3 For E > 0.45 V/rhe, oxygen reduction proceeds via firstorder electroreduction of adsorbed dioxygen, which is in equilibrium with the partial pressure of oxygen gas and obeys a Langmuir adsorption isotherm. For higher overvoltage (E < 0.2 Vhhe), the reaction rate is controlled by hydrogen peroxide reduction and shows half-order reaction rate with respect to the oxygen partial pressure, and at still higher overpotentials (E < -0.2 V/rhe), hydrogen evolution commences. The oxygen reduction was virtually independent of the pH (in the range 0-2). The fact that the response is independent of pH is of practical (13) Tsionsky, M.; Lev, 0.1.Electrochem. Soc., in press.
Analytical Chemistry, Vol. 67, No. 74, July 15, 7995
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Figure 4. Linear calibration curves of Co-TMMP-modified CCE polarized at (1) 0.570, (2) 0.520, and (3) 0.470 V/rhe. Curve 4 represents the linearized (Langmuir-type) l/current-llpressure at 0.520 V (0.5 M sulfuric acid, T = 30 "C).
importance since water loss by evaporation does not affect the electrode response. (b) Calibration Curves and Detection Iimits. Figure 4 presents typical calibration curves (curves 1-3) depicting the relationship between the partial pressure of oxygen and the reduction current for several electrode potentials. All the calibration curves exhibited zero intercept and linear response in the range 0-0.21 atm with correlation coefficients R > 0.9995. This range of linear response in combination with the zero intercept is important from a practical point of view because it makes onepoint air calibration of the sensor possible. In the 0.21-1 atm range, the current density overpotential dependence is best described by an electrochemical charge transfer step preceded by a reversible Langmuir-type oxygen adsorption, i.e.,
The linearized form (l/current vs Upresure) of the Langmuir isotherm (Figure 4,curve 4) exhibits good linearity (R= 0.9998; n = 13) for the 0.2-1 atm range. Best-fit parameters were a = 0.38, K = 0.803 f 0.061 Watm, and k = 25.6 It 1.9 mA/mm2. At still higher overpotentials, hydrogen evolution reaction starts, so mass transport limited conditions were never observed for this reaction unless a special gas-limiting barrier was added. The limit of reliable measurement, PLM, Le., the lowest oxygen partial pressure that can be distinguished from zero with a risk of 5%for false negative and false positive (type I and type I1 errors, respectively), was calculated by the AOAC recommended procedure.14 The upper (95%) confidence limit for a false positive reading (Le., positive reading when oxygen concentration was 0), Pub, was calculated from
where 6 is the correlation slope, to.05 is the one-tailed Student's t distribution at 0.95 confidence, S, is the standard deviation about (14) Wemimont, G. T. Use ofstatistics to develofi and evaluate analytical methods; Association of Official Analytical Chemists: Arlington, VA, 1985. 2412
Analytical Chemistry, Vol. 67, No. 14, July 15, 7995
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Figure 0. Response of air-fed Co-TMMP-modifiedCCE: (1) smooth Pt counter electrode; (2) platinized Pt counter electrode (E= 0.520 Vhhe; 0.5 M sulfuric acid, T = 30 OC).
the regression line, n is the number data points, and Pa, is the average oxygen partial pressure. PLM is given by
Pub = 0.002, 0.003, and 0.005 atm and PLM = 0.003, 0.0045, and 0.007 atm were found for curves 3, 2, and 1 (Figure 4), respectively. Better detection limits are observed for the high overpotential curves due to their improved sensitivity (higher b). However, at high overpotentials (below -0.35 V/rhe) and high partial pressures, hydrogen peroxide reduction influences the reduction rate (half-order oxygen dependence is observed) and limits the acceptable working range. The low detection limit can also be improved by using larger electrodes or better noise shielding. (c) Long-Term Stability and Reproducibility. CCE gas sensors benefit from the excellent mechanical properties of the ceramics and the high stability of pyrolyzed Co porphyrin catalysts. Thus, stable and reproducible signals can be obtained. Figure 5 depicts several random step changes of oxygen partial pressure which illustrate the reproducibility of the signal. The in-use stability, Le., stability of the sensor response in continuous operation, was found to be largely affected by the accumulation of hydrogen peroxide in the cell. Figure 6 (curve 1) demonstrates a continuous drift of the electrode upon 7 days of oxygen sensing in compressed air (operated continuously at 0.52 V/rhe; 30 "C). The fact that the accumulation of hydrogen peroxide is responsible for the current drift was verified by
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Figure 7.
Response time of an oxygen sensor. 4.2
voltammetric determination of hydrogen peroxide using a different set of electrodes. Introduction of platinized platinum counter electrode, which is known for its efficient electrocatalyticoxidation of hydrogen peroxide, improved the long-term response characteristics. Less than 1%drift per day was observed during a period of 7 days of continuous operation (at 0.520 V/rhe and air feed) when a platinized platinum counter electrode was in use. Alternatively, the signal can be restored by periodic replacement of the cell solution or by insertion of an additional cathode to decompose and prevent accumulation of hydrogen peroxide (the contribution of the oxygen byproduct to the signal is negligible). The shelf life, i.e., storage stability, of the electrode was not determined intentionally in this research, but electrodesthat were prepared more than a year ago are routinely used (after calibration) in our laboratory. (d) Response Time. Typical dynamic response of the electrode is demonstrated in Figure 7. The time required to achieve 90%of the ultimate response, T0.9,is 0.8 s for a 0.8 mm long electrode. Similar response time was found in upward and downward stepping of the oxygen partial pressure. The dependence of the square root of T0.9 on the thickness of the composite material (L) shows good linearity in the range L = 0.8-6 mm (R = 0.96 n = 6). This indicates that the response time can be attributed to oxygen diffusion into and out of the porous composite material and that the response time can be further improved by using thinner carbon-ceramic membranes. (e) Surface Renewal. An inherent feature of hydrophobic CCEs is their renewal capability. Mechanical polishing of the electrode reveals a new, uncontaminated, yet chemically modfied surface. The fact that the electrodes are brittle makes it possible to renew the electrode surface by mechanical polishing without blocking the porous structure by plastic deformations. Some of the carbon and its surface modifier is shaved off by the polishing step, but still the remaining catalytic activity is very pronounced (compare curve 2 vs 3 in Figure 2A for E > 0.1). A set of five cycles of repeated polishing of the same electrode (starting with rough polishing paper followed by a 600grit paper) revealed
