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A novel application of gas-diffusion electrodes as electrochemical gas sensors
69
J. Electroanal. Chem., 248 (1988) 69-15 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
A NOVEL APPLICATION OF GAS-DIFFUSION ELECTROCHEMICAL GAS SENSORS
ELECTRODES
AS
CHUAN-SIN CHA, PEI-FANG LIU, ZHONG-BE1 CHOU, ZHI-GAN WU and WEN-YUEN LU Laboratory of Electrochemistry, Department of Chemistry, Wuhan University, Wuhan 430072 (People’s Republic of China) (Received 25th January 1988)
ABSTRACT Gas-diffusion electrodes of the hydrophobic type were employed as thin-layer gas electrode (TLGE) sensors for monitoring the concentrations of gas components either in the gas phase or dissolved in solution. The sampling and flushing manipulations of such thin-layer devices were designed to be exceedingly simple, and the temperature coefficient of the calibration curves is significantly lower than that of gas sensors based on the limiting current concept. Multi-component determination could also be achieved comparatively easily with the TLGE sensor, as illustrated by the case of analysis of Or +N,O mixtures.
INTRODUCTION
Both potentiometric sensors, such as various types of ion-selective electrodes, and amperometric sensors, such as the Clark type dissolved oxygen probe, are in wide use. Potentiometric sensors are more suitable in those cases where the concentration of the monitored species may vary over several orders of magnitude, while amperometric sensors give more accurate readings within a limited range of concentration. Most amperometric sensors work on the principle of a steady limiting rate of diffusion. However, a steady amperometric signal can be obtained only with special types of electrodes (DME, RDE, microelectrodes, etc.), or if some type of steady hindrance (membrane, capillary, etc.) is imposed on the process of transfer of electro-active species to the surface of the electrode. Besides, the temperature coefficient of the response of amperometric sensors is usually relatively high as the result of a significant temperature dependence of the rate of diffusion. If the thin-layer cell concept is used in the design of amperometric sensors, then the response of the sensor would be a transient current signal, and the total amount of charge (Q = j1 dt) consumed in the exhaustive electrolysis of a solution confined in the thin layer should be proportional to the concentration of the species
70
monitored. As Q depends only on the geometric parameters of the thin layer, the temperature dependence of Q is definitely smaller than that of the rate of diffusion. Besides, a response in the form of a peak or peaks is desirable if detection of more than one component is attempted. Nevertheless, the construction of good thin-layer cells requires a high degree of skill, and filling and flushing them is usually not easy. In principle, the thin-layer concept should also be applicable to the construction of gas sensors by combining a gas electrode with a thin gap as gas reservoir. Since the basic prerequisite of a thin-layer device may be expressed as 1 e (2Dt)l/* (where I is the width of the thin layer and r is the duration of the measurement), and D values are usually several orders of magnitude higher in the gas phase than in the liquid phase, it should be easier to construct thin-layer gas electrodes. We have found that the hydrophobic type of gas-diffusing electrodes widely used in fuel cells and metal-air batteries could be employed as a very simple and effective thin-layer electrode. With such a thin-layer gas electrode (TLGE) we have been successful in developing a new series of electrochemical gas sensors for monitoring the concentration of a gas component either in the gas phase or dissolved in solution.
EXPERIMENTAL
The construction of a hydrophobic gas-diffusing electrode is shown schematically in Fig. 1. It consists of a hydrophobic porous layer made from a mixture of teflon and acetylene black or pure teflon, a thin hydrophilic layer of catalyst, and a current collector between both layers. The principle of operation of such an electrode, e.g. a TLGE, is also illustrated in the same figure. The electrode is first exposed briefly to the gas phase (5-10 s) so that the composition of the gas within the pores of the
Fig. 1. Structure and operation of the thin-layer gas electrode sensor. (1) Hydrophobic layer; (2) catalyst layer. (a) Position of electrode for sampling; (b) position of electrode for measurement.
71
_
Fig. 2. Apparatus for TLGE measurements. (1) TLGE sensor; (2) counter electrode; (3) reference electrode; (4) potentiostat with ramp generator; (5) x-y recorder; (6) levelling bulb; (7) gas mixer; (8) flowmeters; (9) inlets for gases.
electrode becomes in equilibrium with the environment. Then the electrolyte level is raised and the immersed electrode works as a TLGE. The actual experimental setup is shown in Fig. 2, and the electrolysis cell for measuring the concentration of dissolved gases is shown in Fig. 3. Voltammograms of the TLGE were recorded with the common LPSV technique. The amount of charge consumed in exhaustive electrolysis (Q) was obtained by integration of the peak area in the I-t curves.
0.9
0.7 f ;a5
03
0.1
--___----_____.
7
-_.-._
-j’B
-0:2 -6.4 E/ Vvs HgO
Fig. 3. Electrolytic cell for measurement of dissolved gas. (1) Reference electrode; (2) counter electrode; (3) TLGE sensor; (4) 30% KOH; (5) water sample; (6) stirrer. Fig. 4. Voltammograms of the TLGE (oxygen in air). Scan rate: (1) 3; (2) 6; (3) 12; (4) 18; (5) 24; (6) 30; (7) 60 mV/s.
72 RESULTS AND DISCUSSION
Cyclic voltammograms of the TLGE after exposure to air are shown in Fig. 4. A peak appears on the voltammogram as the result of exhaustive electrolysis. The leading edges of all peaks obtained with different scan rates coincide. This is a clear indication that this part of the curve is controlled by the rate of the surface reaction, which increases exponentially with potential. The effect of concentration polarization becomes more and more pronounced as electrolysis goes on, so the curve bends downward and passes through a maximum before it declines rapidly to zero. The width of the peaks depends on scan rate (u) and the activity of the electrocatalyst. If polarization within the catalyst layer of the TLGE is homogeneous, then in the case of totally irreversible electrode reactions, such as oxygen reduction, the Ii,+ and E,-log( u) relationships should be linear [l]. However, in Fig. 5, departure from ideal linear behaviour is significant in the region of high scan rates. This seems to indicate that the polarization is not homogeneous at higher current densities. Localization of the current density in the outer layer of the porous electrode adjacent to the electrode/electrolyte interface becomes more and more serious as the current increases. Nevertheless, non-homogeneous polarization of the electrode apparently has no effect on the charge consumed in exhausive electrolysis, as shown in Fig. 6 (region u z 12 mV/s). There is another factor that must be considered. In the case of an ideal thin-layer gas electrode, all electro-active species should come only from cavities within the porous matrix (equivalent to the thin layer of electrolyte in conventional thin-layer cells). However, in the case of the TLGE, electro-active species may come from
Lg(
v/mV-s-1
)
v /mV.
5-l
Fig. 5. Dependence of peak current (I,) and peak potential (E,) on scan rate. Fig. 6. Dependence of Q on scan rate. (1) In 0, saturated solution; (2) in N, saturated solution,
36 Q < H
24
i.2
E / Vvs HgO
0,
/ %
Fig. 7. Voltammograms of the TLGE with or without catalyst. (1) Pure carbon electrode; (2) with noble-metal catalyst. Scan rate: 12 mV/s. Fig. 8. Dependence of Q on relative amount of 0, in the gas sample.
solution outside the TLGE, or diffuse away from the TLGE. In Fig. 6, Q values were found to be too high when the solution was saturated with 0, and the scan rate was too low, while too low Q values were observed in nitrogen saturated solution in the region of low scan rates. It is welI known that oxygen may reduce either to peroxide or to OH- (or to a mixture of both) at different electrode surfaces [2]. Therefore, special attention must be paid to the choice of electrocatalyst for oxygen probes. Without application of a catalyst (pure carbon electrode) the voltammogram of oxygen reduction consists of two distinct peaks of equal area (Fig. 7, curve l), while with an appropriate electrocatalyst a single peak of twice the area appears on the voltammogram (curve 2), indicating a clear-cut four-electron reduction. The linear relation between Q and oxygen concentration in the gas samples is shown in Fig. 8. In 30% KOH, Q decreases at a rate of ca. 0.3% per day, apparently due to gradual wetting of the hydrophobic matrix in strong alkali. Q remains constant over a long period if the TLGE sensor is washed after use and stored dry. The small change in the volume of the gas pores could easily be accounted for by calibration with oxygen in air before use. The temperature coefficient of Q lies in the range -0.1 to -0.3%/O C, which is much lower than that of many other types of oxygen probes. The magnitude of the temperature coefficient can be explained by the ideal-gas law. The general characteristics of voltammograms obtained with the TLGE sensor immersed in aqueous solutions are the same as those shown in Fig. 4. However, the following two factors make the behaviour of TLGE sensors less satisfactory in water samples or dilute solutions. Firstly, the sampling process, by which the composition of the gas in the pores of the TLGE comes into equilibrium with the partial pressure of dissolved gas in the liquid phase, involves such slow processes as diffusion and evaporation of dissolved
oI 0
,
I -0.2
1 I -04
E/‘Vvs
I -0.6
,
I -0.8
HgO
Fig. 9. Q values measured in solution saturated with gas of different oxygen content,
Fig. 10. Volt~o~~s N,O; (c) pure N20.
of the TLC33 sensor (4 +N20 mixtures). (a) Pure oxygen; (b) 50% 0, + 50%
molecules. Several minutes are required to attain equilibrium, even in stirred solutions. Secondly, since the solubility of oxygen in water or dilute solutions is significantly higher than in 30% KOH, the scan rate must be higher in order to avoid excess transport of dissolved gas from the solution during the reduction cycle. Nevertheless, with the appropriate choice of electrocatalyst and scan rate, the linear relation between Q and the partial pressure of dissolved oxygen could still be quite satisfactory, as shown in Fig. 9. An important merit of the TLGE sensor for dissolved oxygen me~~ement is that the sensor can be calibrated very easily with the oxygen content in the atmosphere. Another possible application of TLGE sensors is the analysis of multi-component gas mixtures. Some preliminary results of the analysis of anaesthetic 0, + N,O mixtures are presented in Fig. 10. The reduction peaks of both components are well-defined and well-separated. This seems to indicate that current peaks with a
15
100
80
60
40
20
0
20
40
60
80
LOO
40 0 f
30
a yl 20 0 -
10
0
Fig. 11. Dependence of Q(0,) and Q(N20) on composition of
-nitrous oxide mixture.
difference of their potentials >, 0.5 V can be well-separated even if the electrode processes are totally irreversible, as in the reduction of dioxygen and nitrous oxide. Thus, it might be possible, at least in favourable cases, to achieve well-separated consecutive reduction of 2 to 3 components within a potential range of 1.0 to 1.5 V. The linearity of the reponses of both components was found to be satisfactory (Fig. ll), and the ratio of the slopes of the calibration curves is in good accordance with the ratio of number of electrons involved in the reduction processes (4 : 2). Details will be published in ref. 3. ACKNOWLEDGEMENT
This work was supported by the China National Science Foundation. REFERENCES 1 A.J. Bard and L.R. Faulkner, Electrochemical Methods, Wiley, New York, 1980, pp. 409-411. 2 M.R. Tarasevich, A. Sadkowski and E. Yeager in B.E. Conway, J.O’M. Bock&, E. Yeager, S.U.M. Khan and R.E. White (Eds.), Comprehensive Treatise of Electrochemistry, Vol. 7, Plenum, New York, 1983, Ch. 6. 3 Z.B. Chou, C.S. Cha and Z.G. Wu, Chin. J. Appl. Chem., in press.
J. Electroanal. Chem., 248 (1988) 69-15 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
A NOVEL APPLICATION OF GAS-DIFFUSION ELECTROCHEMICAL GAS SENSORS
ELECTRODES
AS
CHUAN-SIN CHA, PEI-FANG LIU, ZHONG-BE1 CHOU, ZHI-GAN WU and WEN-YUEN LU Laboratory of Electrochemistry, Department of Chemistry, Wuhan University, Wuhan 430072 (People’s Republic of China) (Received 25th January 1988)
ABSTRACT Gas-diffusion electrodes of the hydrophobic type were employed as thin-layer gas electrode (TLGE) sensors for monitoring the concentrations of gas components either in the gas phase or dissolved in solution. The sampling and flushing manipulations of such thin-layer devices were designed to be exceedingly simple, and the temperature coefficient of the calibration curves is significantly lower than that of gas sensors based on the limiting current concept. Multi-component determination could also be achieved comparatively easily with the TLGE sensor, as illustrated by the case of analysis of Or +N,O mixtures.
INTRODUCTION
Both potentiometric sensors, such as various types of ion-selective electrodes, and amperometric sensors, such as the Clark type dissolved oxygen probe, are in wide use. Potentiometric sensors are more suitable in those cases where the concentration of the monitored species may vary over several orders of magnitude, while amperometric sensors give more accurate readings within a limited range of concentration. Most amperometric sensors work on the principle of a steady limiting rate of diffusion. However, a steady amperometric signal can be obtained only with special types of electrodes (DME, RDE, microelectrodes, etc.), or if some type of steady hindrance (membrane, capillary, etc.) is imposed on the process of transfer of electro-active species to the surface of the electrode. Besides, the temperature coefficient of the response of amperometric sensors is usually relatively high as the result of a significant temperature dependence of the rate of diffusion. If the thin-layer cell concept is used in the design of amperometric sensors, then the response of the sensor would be a transient current signal, and the total amount of charge (Q = j1 dt) consumed in the exhaustive electrolysis of a solution confined in the thin layer should be proportional to the concentration of the species
70
monitored. As Q depends only on the geometric parameters of the thin layer, the temperature dependence of Q is definitely smaller than that of the rate of diffusion. Besides, a response in the form of a peak or peaks is desirable if detection of more than one component is attempted. Nevertheless, the construction of good thin-layer cells requires a high degree of skill, and filling and flushing them is usually not easy. In principle, the thin-layer concept should also be applicable to the construction of gas sensors by combining a gas electrode with a thin gap as gas reservoir. Since the basic prerequisite of a thin-layer device may be expressed as 1 e (2Dt)l/* (where I is the width of the thin layer and r is the duration of the measurement), and D values are usually several orders of magnitude higher in the gas phase than in the liquid phase, it should be easier to construct thin-layer gas electrodes. We have found that the hydrophobic type of gas-diffusing electrodes widely used in fuel cells and metal-air batteries could be employed as a very simple and effective thin-layer electrode. With such a thin-layer gas electrode (TLGE) we have been successful in developing a new series of electrochemical gas sensors for monitoring the concentration of a gas component either in the gas phase or dissolved in solution.
EXPERIMENTAL
The construction of a hydrophobic gas-diffusing electrode is shown schematically in Fig. 1. It consists of a hydrophobic porous layer made from a mixture of teflon and acetylene black or pure teflon, a thin hydrophilic layer of catalyst, and a current collector between both layers. The principle of operation of such an electrode, e.g. a TLGE, is also illustrated in the same figure. The electrode is first exposed briefly to the gas phase (5-10 s) so that the composition of the gas within the pores of the
Fig. 1. Structure and operation of the thin-layer gas electrode sensor. (1) Hydrophobic layer; (2) catalyst layer. (a) Position of electrode for sampling; (b) position of electrode for measurement.
71
_
Fig. 2. Apparatus for TLGE measurements. (1) TLGE sensor; (2) counter electrode; (3) reference electrode; (4) potentiostat with ramp generator; (5) x-y recorder; (6) levelling bulb; (7) gas mixer; (8) flowmeters; (9) inlets for gases.
electrode becomes in equilibrium with the environment. Then the electrolyte level is raised and the immersed electrode works as a TLGE. The actual experimental setup is shown in Fig. 2, and the electrolysis cell for measuring the concentration of dissolved gases is shown in Fig. 3. Voltammograms of the TLGE were recorded with the common LPSV technique. The amount of charge consumed in exhaustive electrolysis (Q) was obtained by integration of the peak area in the I-t curves.
0.9
0.7 f ;a5
03
0.1
--___----_____.
7
-_.-._
-j’B
-0:2 -6.4 E/ Vvs HgO
Fig. 3. Electrolytic cell for measurement of dissolved gas. (1) Reference electrode; (2) counter electrode; (3) TLGE sensor; (4) 30% KOH; (5) water sample; (6) stirrer. Fig. 4. Voltammograms of the TLGE (oxygen in air). Scan rate: (1) 3; (2) 6; (3) 12; (4) 18; (5) 24; (6) 30; (7) 60 mV/s.
72 RESULTS AND DISCUSSION
Cyclic voltammograms of the TLGE after exposure to air are shown in Fig. 4. A peak appears on the voltammogram as the result of exhaustive electrolysis. The leading edges of all peaks obtained with different scan rates coincide. This is a clear indication that this part of the curve is controlled by the rate of the surface reaction, which increases exponentially with potential. The effect of concentration polarization becomes more and more pronounced as electrolysis goes on, so the curve bends downward and passes through a maximum before it declines rapidly to zero. The width of the peaks depends on scan rate (u) and the activity of the electrocatalyst. If polarization within the catalyst layer of the TLGE is homogeneous, then in the case of totally irreversible electrode reactions, such as oxygen reduction, the Ii,+ and E,-log( u) relationships should be linear [l]. However, in Fig. 5, departure from ideal linear behaviour is significant in the region of high scan rates. This seems to indicate that the polarization is not homogeneous at higher current densities. Localization of the current density in the outer layer of the porous electrode adjacent to the electrode/electrolyte interface becomes more and more serious as the current increases. Nevertheless, non-homogeneous polarization of the electrode apparently has no effect on the charge consumed in exhausive electrolysis, as shown in Fig. 6 (region u z 12 mV/s). There is another factor that must be considered. In the case of an ideal thin-layer gas electrode, all electro-active species should come only from cavities within the porous matrix (equivalent to the thin layer of electrolyte in conventional thin-layer cells). However, in the case of the TLGE, electro-active species may come from
Lg(
v/mV-s-1
)
v /mV.
5-l
Fig. 5. Dependence of peak current (I,) and peak potential (E,) on scan rate. Fig. 6. Dependence of Q on scan rate. (1) In 0, saturated solution; (2) in N, saturated solution,
36 Q < H
24
i.2
E / Vvs HgO
0,
/ %
Fig. 7. Voltammograms of the TLGE with or without catalyst. (1) Pure carbon electrode; (2) with noble-metal catalyst. Scan rate: 12 mV/s. Fig. 8. Dependence of Q on relative amount of 0, in the gas sample.
solution outside the TLGE, or diffuse away from the TLGE. In Fig. 6, Q values were found to be too high when the solution was saturated with 0, and the scan rate was too low, while too low Q values were observed in nitrogen saturated solution in the region of low scan rates. It is welI known that oxygen may reduce either to peroxide or to OH- (or to a mixture of both) at different electrode surfaces [2]. Therefore, special attention must be paid to the choice of electrocatalyst for oxygen probes. Without application of a catalyst (pure carbon electrode) the voltammogram of oxygen reduction consists of two distinct peaks of equal area (Fig. 7, curve l), while with an appropriate electrocatalyst a single peak of twice the area appears on the voltammogram (curve 2), indicating a clear-cut four-electron reduction. The linear relation between Q and oxygen concentration in the gas samples is shown in Fig. 8. In 30% KOH, Q decreases at a rate of ca. 0.3% per day, apparently due to gradual wetting of the hydrophobic matrix in strong alkali. Q remains constant over a long period if the TLGE sensor is washed after use and stored dry. The small change in the volume of the gas pores could easily be accounted for by calibration with oxygen in air before use. The temperature coefficient of Q lies in the range -0.1 to -0.3%/O C, which is much lower than that of many other types of oxygen probes. The magnitude of the temperature coefficient can be explained by the ideal-gas law. The general characteristics of voltammograms obtained with the TLGE sensor immersed in aqueous solutions are the same as those shown in Fig. 4. However, the following two factors make the behaviour of TLGE sensors less satisfactory in water samples or dilute solutions. Firstly, the sampling process, by which the composition of the gas in the pores of the TLGE comes into equilibrium with the partial pressure of dissolved gas in the liquid phase, involves such slow processes as diffusion and evaporation of dissolved
oI 0
,
I -0.2
1 I -04
E/‘Vvs
I -0.6
,
I -0.8
HgO
Fig. 9. Q values measured in solution saturated with gas of different oxygen content,
Fig. 10. Volt~o~~s N,O; (c) pure N20.
of the TLC33 sensor (4 +N20 mixtures). (a) Pure oxygen; (b) 50% 0, + 50%
molecules. Several minutes are required to attain equilibrium, even in stirred solutions. Secondly, since the solubility of oxygen in water or dilute solutions is significantly higher than in 30% KOH, the scan rate must be higher in order to avoid excess transport of dissolved gas from the solution during the reduction cycle. Nevertheless, with the appropriate choice of electrocatalyst and scan rate, the linear relation between Q and the partial pressure of dissolved oxygen could still be quite satisfactory, as shown in Fig. 9. An important merit of the TLGE sensor for dissolved oxygen me~~ement is that the sensor can be calibrated very easily with the oxygen content in the atmosphere. Another possible application of TLGE sensors is the analysis of multi-component gas mixtures. Some preliminary results of the analysis of anaesthetic 0, + N,O mixtures are presented in Fig. 10. The reduction peaks of both components are well-defined and well-separated. This seems to indicate that current peaks with a
15
100
80
60
40
20
0
20
40
60
80
LOO
40 0 f
30
a yl 20 0 -
10
0
Fig. 11. Dependence of Q(0,) and Q(N20) on composition of
-nitrous oxide mixture.
difference of their potentials >, 0.5 V can be well-separated even if the electrode processes are totally irreversible, as in the reduction of dioxygen and nitrous oxide. Thus, it might be possible, at least in favourable cases, to achieve well-separated consecutive reduction of 2 to 3 components within a potential range of 1.0 to 1.5 V. The linearity of the reponses of both components was found to be satisfactory (Fig. ll), and the ratio of the slopes of the calibration curves is in good accordance with the ratio of number of electrons involved in the reduction processes (4 : 2). Details will be published in ref. 3. ACKNOWLEDGEMENT
This work was supported by the China National Science Foundation. REFERENCES 1 A.J. Bard and L.R. Faulkner, Electrochemical Methods, Wiley, New York, 1980, pp. 409-411. 2 M.R. Tarasevich, A. Sadkowski and E. Yeager in B.E. Conway, J.O’M. Bock&, E. Yeager, S.U.M. Khan and R.E. White (Eds.), Comprehensive Treatise of Electrochemistry, Vol. 7, Plenum, New York, 1983, Ch. 6. 3 Z.B. Chou, C.S. Cha and Z.G. Wu, Chin. J. Appl. Chem., in press.