10648
J. Phys. Chem. B 2001, 105, 10648-10652
Mixed Potentials at Metal-Electrode and Proton-Conducting Electrolyte Interfaces in Hydrocarbon-Oxygen Mixtures Takashi Hibino,*,† Atsuko Hashimoto,† Ken-taro Mori,‡ and Mitsuru Sano‡ National Institute of AdVanced Industrial Science and Technology, Hirate-cho, Kita-ku, Nagoya 462-8510, Japan, and Nagoya UniVersity, Furo-cho, Chikusa-ku, Nagoya 466-0804, Japan ReceiVed: June 1, 2001; In Final Form: August 25, 2001
Potentiometric sensors using different solid electrolytes, including proton, oxide-ion, and mixed proton and oxide-ion conductors, were fabricated to investigate a relationship between the mixed potential for C1-C4 hydrocarbons and ion conduction in the electrolyte in a mixture of hydrocarbons and oxygen at 600 °C. There was an increase in the mixed potential for the hydrocarbons with increasing proton conductivity or decreasing oxide-ion conductivity. The sensor using a SrCe0.95Yb0.05O3-R (SCY) electrolyte with a Pt electrode showed an enhancement of the mixed potential for the hydrocarbons by increasing the carbon number, branching the chain structure, and unsaturating the C-C linkage. The polarization curves for hydrocarbon oxidation and for oxygen reduction at the Pt/SCY interface were also measured. These results revealed that the mixed potential at the Pt/SCY interface was strongly dependent on the rate of abstraction of hydrogen atom from the hydrocarbon molecules.
Introduction Some perovskite-type oxides such as SrCe0.95Yb0.05O3-R (SCY), SrZr0.9Y0.1O3-R (SZY), and CaZr0.9In0.1O3-R (CZI) show proton conduction in hydrogen-containing atmospheres at elevated temperatures.1 These materials are used as the solid electrolytes of a hydrogen or water-vapor concentration cell, which can determine their gas concentration in such atmospheres from the electromotive force (EMF) of the cell. More recently, Le et al. have reported that a single-chamber sensor using the CZI electrolyte with a Ni-CZI cermet anode and a Pt cathode can respond to methane in natural gas, where hydrogen is formed through the reforming of methane by carbon dioxide over the anode surface, while such a formation proceeds at a very slow rate over the cathode surface.2 Similarly, Pham et al. has fabricated a hydrocarbon sensor that is capable of operating in exhaust gases on the basis of a difference in the catalytic activity for the decomposition of hydrocarbons between a Pt anode coated with iron oxide and a Pt cathode.3 However, since these sensors are regarded as hydrogen concentration cells, it would be difficult to use them as hydrocarbon sensors in the presence of excess oxygen at high temperatures, because the hydrogen formed over the anode surface is subject to combustion under such conditions. On the other hand, Chiang et al.4 and our group5 have proposed that methane could be dimerized to ethane and ethene through the formation of methyl radical, CH3•, by the anodic polarization of a metal electrode attached to the SCY electrolyte, indicating that a hydrogen atom in the methane molecule is directly dissolved as proton into the electrolyte bulk. Based on these results, one could extrapolate that if a mixture of hydrocarbon and oxygen is fed into such a working electrode, * Corresponding author. Tel: +81 52 911 3614. Fax: +81 52 911 2422. E-mail:
[email protected]. † National Institute of Advanced Industrial Science and Technology. ‡ Nagoya University.
the following electrode reactions would proceed under opencircuit conditions:
Cathodic reaction 1/4O2 + H+ + e- f 1/2H2O
(1)
Anodic reaction RCH3 f [RCH2•] + H+ + e-
(2)
As a result, the working electrode would show a mixed potential based on eqs 1 and 2. The mixed potential becomes more negative as the working electrode favors eq 2 rather than eq 1, thus making it possible to utilize this signal for determining the hydrocarbon concentration under oxidizing conditions. To our knowledge, no mixed-potential hydrocarbon sensor has been reported using a proton-conducting electrolyte, although there are some sensors using zirconia- or ceria-based electrolytes.6-9 This paper deals with the mixed potential of the sensors using not only proton conducting but also oxide-ion conducting electrolytes with different metal electrodes for C1-C4 saturated and unsaturated hydrocarbons. We show a relationship between the mixed potential for the hydrocarbons and ion conduction in the electrolyte. In addition, the mechanism of the mixed potential is also discussed by studying kinetics of the hydrocarbons at the metal-electrode and proton-conducting electrolyte interface. Experimental Section Fabrication of Sensor. An electrochemical cell was fabricated as shown in Figure 1. Various types of ion conductors were used as the solid electrolytes, which were purchased in a sintered disk form (13-15 mm diameter, 1-2 mm thickness) from different commercial sources: TYK (SCY, SZY, CZI, and BaCe0.8Y0.2O3-R (BCY)); Nikkato (8 mol %-yttria-stabilized zirconia (YSZ)); Anan Kasei (Ce0.8Sm0.2O1.9 (SDC)); NGK Spark Plug (La0.9Sr0.1Ga0.8Mg0.2O3 (LSGM)). These electrolyte surfaces were polished to a thickness of 0.8 mm with an abrasive paper and then washed with distilled water in an ultrasonic cleaner. Commercial Pt, Pd, and Au pastes (Tokuriki) were
10.1021/jp012094k CCC: $20.00 © 2001 American Chemical Society Published on Web 10/10/2001
Interfacial Mixed Potentials
J. Phys. Chem. B, Vol. 105, No. 43, 2001 10649 potential of the working electrode vs the reference electrode, which was exposed to atmospheric air, was controlled by a potentiostat (Hokuto Denko HA-501). The polarization curve was obtained after IR correction, where the ohmic and electrodereaction resistances were measured using an impedance analyzer (Solartron SI-1260 and -1287). Results and Discussion Mixed Potential for Hydrocarbons. Table 1 summarizes EMFs generated from the sensors using different ion conductors as the solid electrolytes by feeding a mixture of 0 or 1000 ppm propene and 10 vol % oxygen in argon into the Pt working chamber at 600 °C. In the absence of propene, the sensors using the oxide-ion conductors such as YSZ, SDC, and LSGM showed EMF values between -10 and -14 mV, which are roughly in agreement with the value calculated from Nernst’s equation:
E ) (RT/4F) ln[PO2(anode)/PO2(cathode)] Figure 1. Schematic illustration of the electrochemical cell.
smeared as the working electrodes on the bottom surface (superficial electrode area: 0.5 cm2) of the electrolyte as thinly as possible with a brush. The Pt counter and reference electrodes were similarly smeared on the top surface (superficial electrode area: 0.5 cm2) and the side surface (superficial electrode area: ca. 0.3 cm2), respectively, of the electrolyte. After the assembly was dried at 130 °C in air, it was calcined at 900 °C in air for 1 h. For the working and counter electrodes, a Au wire and a Au mesh served as the output terminal and the electrical collector, respectively. For the reference electrode, a Au wire served as both output terminal and electrical collector. Two gas chambers were set up by interposing the obtained cell between two alumina tubes (9 mm internal diam, 13 mm outer diam). Each chamber was sealed by melting a glass ring gasket at 900 °C. Measurement of Mixed Potential. The working chamber was supplied with a mixture of 0-1000 ppm hydrocarbon (methane, ethane, ethene, ethyne, propane, propene, butane, 1-butene, or 2-methylpropene), 0-1000 ppm hydrogen, or 0-1000 ppm carbon monoxide, or 0-1000 ppm nitrogen monoxide, and 0-10 vol % oxygen in argon, which was saturated with water vapor at room temperature, at a flow rate of 100 mL min-1 and at an operating temperature of 600 °C. The counter chamber was statically exposed to atmospheric air. The EMF value of the cell was measured by an electrometer (Hokuto Denko HE-104) connected with a potable recorder (Yokogawa 3057). Characterizations of Electrode-Electrolyte Interface. The catalytic activities of the working electrode and its polarization curves at 600 °C were also measured using the above cell. The measurement of the catalytic activities was carried out by feeding a mixture of 1000 ppm propene and 10% oxygen in argon into the working electrode at a flow rate of 100 mL min-1 and then analyzing the outlet gas by on-line gas chromatography (Shimazu GC-8A). The separation of the hydrocarbons, carbon dioxide, and water vapor were performed using a Porapak Q column at 120 °C, and the other gases were analyzed using a molecular sieve 5A column at 60 °C. The anodic polarization in propene diluted to a given concentration with argon was carried out in the absence of oxygen, and the cathodic polarization in oxygen diluted to a given concentration with argon was carried out in the absence of propene. In both cases, the flow rate of the sample gases was 100 mL min-1. The
(3)
The EMF values of the sensors using the proton conductors such as SCY, SZY, and CZI were somewhat more negative than the values observed above. This is because these sensors are not an oxygen concentration cell, but a water-vapor concentration cell:
E ) -(RT/2F) ln[PH2O(anode)/PH2O(cathode)] × [PO2(cathode)/PO2(anode)]1/2 (4) In the presence of propene, the EMF values of the sensor using the oxide-ion conductors were near the values at 0 ppm propene, indicating that the Pt working electrode barely produced a mixed potential for propene on the surface of the oxide-ion conductors. Largely negative EMFs were generated from the sensors using the proton conductors. The order of the negative EMF value was SCY > SZY > CZI, which is consistent with that of their proton conductivity. This suggests that the rate of eq 2 is speeded up by the increase in proton conductivity in the electrolyte. However, the sensor using the BCY electrolyte showed a moderately negative EMF value, regardless of its high proton conductivity. We presume that this is due to mixed proton and oxide-ion conduction in the BCY electrolyte.10 Since oxideion conduction can be assumed to have a negative effect on the production of a mixed potential at the Pt working electrode as described above, such an effect would compensate for a positive effect of proton conduction. Table 2 shows the dependencies of the EMF value of the two sensors using the SCY and YSZ electrolytes for propene at 600 °C on the electrode material. The negative EMF value of the sensor using the YSZ electrolyte was reduced in the order of Au > Pd > Pt working electrodes. According to the mechanism of a mixed potential phenomenon of YSZ-based sensors proposed,7-9 the mixed potential is established by the competition between the following two electrode reactions:
Cathodic reaction 3/2O2 + 6e- f 3O2Anodic reaction HC + 3O2- f CO2 + H2O + 6e-
(5) (6)
Mukundan et al. demonstrated that the large mixed potential at the Au working electrode is due to an extremely slow reaction rate of eq 5 and not to a significantly fast reaction rate of eq 6. The sensor using the SCY electrolyte showed an opposite order of the electrode material (Table 2). Since the Pt working electrode can be expected to be a good electrocatalyst for eq 1,
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TABLE 1: EMF Generated from Sensors Using Different Ionic Conductors as the Electrolyte for Propene at 600 °C)a EMF/mV electrolyte
sensing electrode
conducting ion
ohmic resistance of sensorb/Ω
0 ppm propene
1000 ppm propene
∆E
YSZ Ce0.8Sm0.2O1.9 La0.9Sr0.1Ga0.8Mg0.2O3
Pt Pt Pt
O2O2-
26 21 40
-14 -13 -10
-16 -16 -11
-2 -3 -2
BaCe0.8Y0.2O3-R
Pt
H+ and O2-
17
-11
-21
-10
SrCe0.95Yb0.05O3-R SrZr0.9Y0.1O3-R CaZr0.9In0.1O3-R
Pt Pt Pt
H+ H+ H+
59 70 517
-33 -20 -27
-78 -55 -28
-45 -35 -1
O2-
a The experiment was carried out by feeding a flowing mixture of 1000 ppm propene and 10 vol % oxygen in argon, which was saturated with water vapor at room temperature, into the workig chamber. b These values are ac impedances at 10 kHz under open-circuit conditions.
TABLE 2: EMF of Oxygen Sensors Using SrCe0.95Yb0.05O3-r and YSZ Solid Electrolytes with Different Working Electrodes for Propene at 600 °Ca EMF/mV solid electrolyte
sensing electrode
ohmic resistance of sensorb/Ω
0 ppm propene
1000 ppm propene
∆E
SrCe0.95Yb0.05O3-R SrCe0.95Yb0.05O3-R SrCe0.95Yb0.05O3-R
Pt Pd Au
59 103 61
-33 -34 -33
-78 -64 -40
45 30 7
YSZ YSZ YSZ
Au Pd Pt
23 27 26
-15 -19 -14
-103 -96 -16
88 77 2
a The experiment was carried out by feeding a flowing mixture of 1000 ppm propene and 10 vol % oxygen in argon, which was saturated with water vapor at room temperature, into the working chamber. b These values are ac impedances at 10 kHz under open-circuit conditions.
Figure 2. EMF response of sensor using SrCe0.95Yb0.05O3-R electrolyte with Pt working electrode to propene at 600 °C. The oxygen concentration in the gas mixture was kept a constant value of 10%.
the same logic as above does not apply to the present case. It is more likely that the Pt working electrode essentially favors eq 2 rather than eq 1. Further details of the mixed potential were investigated using the sensor constructed from the SCY electrolyte with the Pt working electrode in the subsequent experiments. Figure 2 shows the EMF response of the sensor to propene at 600 °C, where the oxygen concentration was kept at a constant value of 10 vol %. The negative EMF value was enhanced with an increase in the propene concentration and reduced with a decrease in the propene concentration. In addition, the response transient was sharp and stable with 90% response and recovery times of less than 30 s. It is thus evident that the electrode reactions based on eqs 1 and 2 reversibly and rapidly proceed at the Pt working electrode. Similar tendencies were also observed for the other hydrocarbons as shown in Figure 3. The negative EMF values for all the tested hydrocarbons were more or less enhanced by the increase in gas concentration. More important are three dependencies of the EMF value for the hydrocarbons on the
hydrocarbon species: (1) The EMF values for the hydrocarbons became more negative as the carbon number increased; (2) The EMF value for 1-butene was enhanced when it isomerized to 2-methylpropene; (3) The EMF values for the alkenes, especially propene and 1-butene, and 2-methylpropene, were extremely negative. From the kinetics of eq 2, it can be predicted that the mixed potential for the hydrocarbons is closely related to the abstraction of hydrogen atoms from the hydrocarbon molecules. It is generally known that the fission of C-H binding in hydrocarbons is promoted by increasing the carbon number and branching the chain structure. These correspond to the first and second results. Furthermore, the C-H binding energy in hydrocarbons is in order of ethynylic C-H (CH3 ≡ C-H) > vinylic C-H (CH2dHC-H) > primary C-H (R-H2C-H) > secondary C-H ((R)2-HC-H) > tertiary C-H ((R)3-C-H) > allylic C-H (CH2dCH-H2C-H), which can best explain the third result, because propene, 1-butene, and 2-methylpropene have the allylic C-H in the molecule. In other words, ethyne and ethene, having the ethynylic C-H and the vinylic C-H, respectively, barely contributed to the production of the mixed potential (Figure 3). This is largely distinguishable from the result observed at the Au/YSZ interface, where an EMF value of -182 mV for ethyne was more negative than values of -103 mV for propene and -126 mV for 2-methylpropene. Since the rate of eq 6 would be determined by the reactivity of the hydrocarbons, the EMF value of the sensor using the YSZ electrolyte was maximum for ethyne, which is the most reactive among C1-C4 hydrocarbons. Figure 4 shows the EMF values of the sensor for the other reducing gas at 600 °C. The sensor had no response to hydrogen, carbon monoxide, and nitrogen monoxide, thus indicating that the sensor has extremely high selectivity to the hydrocarbons over the other reducing gases. From the analysis of the outlet gas from the working chamber, it was found that both hydrogen and carbon monoxide were almost completely subject to combustion in the gas phase and/or over the Pt surface, thus
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J. Phys. Chem. B, Vol. 105, No. 43, 2001 10651
Figure 3. Dependencies of EMF values of sensor using SrCe0.95Yb0.05O3-R electrolyte with Pt working electrode upon hydrocarbon gas concentration at 600 °C. The oxygen concentration in the gas mixture was kept a constant value of 10 vol %. The EMF values after equilibrium were recorded as data.
Figure 4. Dependencies of EMF values of sensor using SrCe0.95Yb0.05O3-R electrolyte with Pt working electrode upon concentration of different reducing gases at 600 °C. The experimental conditions are the same as in Figure 3.
inhibiting the subsequent electrochemical reaction. Although we could not analyze the concentration of nitrogen monoxide by our gas chromatography, this gas can be presumed to contribute the following cathodic reaction rather than the anodic reaction analogous to eq 2:
Cathodic reaction 1/2NO + H+ + e- f 1/2N2 + 1/2H2O (7) This is supported by the fact that the EMF was slightly shifted to a positive value with an increase in the gas concentration (Figure 4). Anyhow, no response to those reducing gases is an advantageous feature over the sensor using the YSZ with the Au working electrode, which showed the EMF values for hydrogen and carbon monoxide comparable to the value for propene.
Mechanism of Mixed Potential. As pointed out above, the mixed potential for reducing gases is reduced when their nonelectrochemical oxidation occurs in the gas phase and/or the electrode surface as a parallel reaction. We thus analyzed the outlet gas from the Pt working chamber at 600 °C. These results are summarized together with those of the sensor using the YSZ electrolyte with the Au working electrode in Table 3. About one-half of propene reacted with oxygen in both cases, suggesting that the residual propene can contribute to the anodic reaction. Here, it should be noted that the amount of the carbon dioxide formed at the Pt/SCY interface was less than that at the Au/YSZ interface, thus resulting in a conversion of propene into carbon dioxide at the Pt/SCY interface lower than that at the Au/YSZ interface. In fact, ethene, methane, and carbon monoxide were observed as the products other than carbon dioxide in the Pt working chamber. This might reflect that the
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Figure 5. Anodic polarization curves of Pt and Au working electrodes for propene in argon and their cathodic polarization curves for oxygen in argon at 600 °C.
TABLE 3: Inlet- and Outlet-Gas Compositions from Pt and Au Sensing Electrodes on SrCe0.95Yb0.05O3-r and YSZ Solid Electrolytes, Respectively, at 600 °C solid electrolyte
sensing electrode
SrCe0.95Yb0.05O3-R YSZ none
Pt Au none
inlet gas propene/ppm oxygen/vol % 1000 1000 1000
propene/ppm
10 10 10
anodic reactions of propene at the Pt/SCY interface proceeds according to eq 2. Based on the above results, we measured the anodic polarization curve of 493 ppm propene in the absence of oxygen and the cathodic polarization curve of 9.7 vol % oxygen in the absence of propene at the Pt/SCY interface. These results are summarized together with those at the Au/YSZ interface in Figure 5, where the direction of the cathodic current is reversed. The intersection of the two curves at the Pt/SCY interface, which corresponds to the mixed potential, was -84 mV, which was near the EMF value for propene of -78 mV shown above. A similar agreement was observed at the Au/YSZ interface: the mixed potential of -101 mV was near the EMF value of -103 mV. Accordingly, it is seen that the model based on eqs 1 and 2 is valid for the Pt/SCY interface, as well as the model based on eqs 5 and 6 for the Au/YSZ interface. The sensing properties of the present sensor for the hydrocarbons meet the need for hydrocarbon monitoring in exhaust gases from lean-burn or diesel engines, which are similar to the model gases used in this study. However, Scholten et al. reported that SCY easily reacted with carbon dioxide,11 which is usually included to the extent of ca. 10 vol % in the exhaust gases. Accordingly, SZY, where such a reaction is kinetically inhibited,12 would be appropriate for automotive applications. Conclusion The sensor using a proton-conducting SCY as the solid electrolyte exhibited largely negative EMF values, which deviated significantly from the values calculated from Nernst’s equation, in mixtures of C1-C4 hydrocarbons and oxygen at
493 460 997
outlet gas oxygen/vol % carbon dioxide/ppm 9.7 9.1 10
924 1366 0
conversion of propene into carbon dioxide/% 31 46 0
600 °C. This deviation was due to the mixed potential at the working electrode, where a cathodic reaction of oxygen and an anodic reaction of the hydrocarbons were in competition with each other. The mixed potential was strongly affected by the electrolyte and electrode materials and the hydrocarbon species. These effects could be explained by using a model based on the abstraction of hydrogen atoms from the hydrocarbon molecules at the Pt/SCY interface. References and Notes (1) Iwahara, H.; Yajima, T.; Hibino, T.; Ozaki, K.; Suzuki, H. Solid State Ionics 1993, 61, 65. (2) Le, J.; Van Rij, L. N.; Schoonman, J. Solid State Ionics 2000, 147, 4343. (3) Pham, A. Q.; Visser, J. H.; Ejakov, S.; Glass, R. S.; Abstract 875, The Electrochemical Society Meeting Abstracts, Phoenix, October 22-27, 2000. (4) Chiang, P. H.; Eng, D.; Stoukides, H. J. Electrochem. Soc. 1991, 138, L11. (5) Hamakawa, S.; Hibino, T.; Iwahara, H. J. Electrochem. Soc. 1993, 140, 459. (6) Thiemann, S.; Hartung, R.; Wulff, H.; Klimke, J.; Mobius, H. H.; Guth, U.; Schonauer, U. Solid State Ionics 1996, 86-88, 873. (7) Hibino, T.; Kuwahara, Y.; Wang, S.; Kakimoto, S.; Sano, M. Electrochem. Solid-State Lett. 1998, 1, 197. (8) Mukundan, R.; Brosha, E. L.; Brown, D. R.; Garzon, F. H. Electrochem. Solid-State Lett. 1999, 2, 412. (9) Miura, N.; Shiraishi, T.; Shimanoe, K.; Yamazoe, N. Electrochem. Commun. 2000, 2, 77. (10) Knight, K. S.; Soar, M.; Bonanos, N. J. Mater. Chem. 1992, 2, 709. (11) Scholten, M. J.; Schoonman, J.; van Miltenburg, J. C.; Oonk, H. A. J. Solid State Ionics, 1993, 61, 83. (12) Yajima, T.; Suzuki, H.; Yogo, T.; Iwahara, H. Solid State Ionics, 1992, 51, 101.
