- Email: [email protected]
A solid-state potentiometric sensor for hydrogen detection in air
Sensors and Actuators B 98 (2004) 73–76
A solid-state potentiometric sensor for hydrogen detection in air N. Maffei∗ , A.K. Kuriakose Materials Technology Laboratory, CANMET, Natural Resources Canada, 405 Rochester St., Ottawa, Ont., Canada K1A 0G3 Received 14 February 2003; received in revised form 16 September 2003; accepted 22 September 2003
Abstract A solid-state potentiometric hydrogen gas sensor based on hydronium Nasicon, a hydrogen ion conducting solid electrolyte, is described. The device incorporates a patented silver-based reference electrode and a palladium working electrode. The sensor is robust, simple, fast, and capable of detecting hydrogen concentrations from at least 0.01 to 100%, a range of four orders of magnitude, in oxidizing and non-oxidizing atmospheres. The room temperature response characteristics of the sensor are reported. Crown Copyright © 2003 Published by Elsevier B.V. All rights reserved. Keywords: Hydrogen; Sensor; Solid electrolyte; Palladium
1. Introduction Many industrialized countries are becoming increasingly aware of factors that affect climate change, and of particular concern are increased emission of greenhouse gases. This issue, along with the inevitable depletion of fossil fuel reserves has resulted in a renewed interest in alternative and renewable energy sources. Fuel cells have recently seen an upsurge in interest for possible use in transportation and stationary power generation. Fuel cells are electrochemical devices that convert chemical energy directly into electrical energy, which when operated on pure hydrogen emit no greenhouse gases, the only waste product being pure water. The use of hydrogen as fuel for fuel cells, however, raises many safety issues; consequently detection of potentially hazardous concentrations of hydrogen gas in both commercial and residential environments is receiving increased attention. Several solid-state hydrogen gas sensors exploiting the ionic conduction of solid electrolytes have been reported [1–9]. This type of gas sensor has several advantages: the generated current or voltage can be easily measured with very high precision and since the potentials generated by these devices are independent of dimensions the sensor size can be tailored for any application. Hydrogen gas sensors are currently used extensively in a variety of areas, including aerospace and other industrial applications [10–13]. The use of hydrogen to power fuel cells for transportation and power ∗ Corresponding author. Tel.: +1-613-992-1391; fax: +1-613-992-9389. E-mail address: [email protected] (N. Maffei).
generation applications will further increase the demand for cheap, simple, reliable and low-cost hydrogen gas sensors. In an earlier publication [1], a potentiometric hydrogen sensor based on hydronium Nasicon was described. That sensor had a novel reference electrode that allowed it to operate without a standard reference gas [14]. That sensor used a platinum working electrode and was unable to measure hydrogen in air. Although, a previous publication did, however, report that a solid-state protonic conductor-based sensor, using a sputtered platinum working electrode, was able to detect hydrogen in air, albeit, over a limited range of hydrogen partial pressures [15]. In order to overcome this limitation in the present sensor, it was modified by changing the working electrode; the results of this modification on sensor performance are presented in this paper. 2. Experimental A bonded hydronium Nasicon disc (HyceramTM , a CANMET trade mark for a series of phosphate bonded hydronium ion conductors) was prepared as described previously [16]. The dried discs were parallel lapped with 120 grit abrasive paper, cleaned in 2-propanol, and dried. A silver nitrate solution in distilled water was applied to one side of the discs. The discs were dried for approximately 24 h, allowing the silver nitrate to diffuse into the discs thus forming a silver-Hyceram salt at and near the surface of the discs. The dry, silver nitrate treated surfaces were then covered with a commercially available electrically conducting silver-loaded epoxy resin [17]. A silver wire lead was attached with the same silver epoxy resin. The system was then cured at room
0925-4005/$ – see front matter. Crown Copyright © 2003 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2003.09.024
N. Maffei, A.K. Kuriakose / Sensors and Actuators B 98 (2004) 73–76
temperature for 24 h. The maximum working temperature of the silver epoxy is 100 ◦ C and consequently, environments exceeding this temperature may compromise the functionality of the reference electrode. The expected lifetime of the silver epoxy is not known, but sensors in our laboratory are still functional 1 year after fabrication. After curing, the untreated side of the electrolyte discs was slurry coated with a layer of palladium metal powder, using an organic medium (Engelhard A2466 Medium), and allowed to dry at room temperature for 24 h. A silver wire lead was attached to the dried palladium surface using the conducting silver epoxy resin. Measurement of the cell emf was made with a Keithley 619 electrometer, which was interfaced through an IEEE-488 device to a personal computer. The data were collected for equilibrium potentials and response times to equilibrium, as a function of hydrogen partial pressure. High purity dry hydrogen, hydrogen–nitrogen and hydrogen–air gas mixtures containing 0.01, 0.1, 1 and 10% hydrogen, at a flow rate of 200 cm3 min−1 , were used for the measurements.
1.0 0.9 0.8
100 % 10 %
0.7
1%
0.6
emf (V)
74
0.1 % 0.5 0.4
0.01 % 0.3 0.2 0.1 0.0 0
50
100
(a)
150
200
250
300
350
400
Time (s) 1.0 0.9
100 % 10 % 1% 0.1 %
0.8 0.7
3. Results and discussion
emf (V)
0.6
0.01 %
0.5 0.4
The sensor can be described in terms of the following galvanic cell:
0.3
H2 , Pd|Hyceram|Ag-Hyceram|Ag
0.1
0.2
0.0
The emf generated when the cell anode is exposed to atmospheres containing hydrogen, assuming that the concentration of all the species other than H2 are constant, may be represented by the Nernstian equation: E=k+
2.303RT log (pH2 ) 2nF
where E is the cell emf in volts, k the constant representing the emf at the reference electrode, R the gas constant, T the absolute temperature, n the number of electrons involved in the electrochemical process (1), F the Faraday’s constant and pH2 the partial pressure of hydrogen (atm) at the palladium electrode/Hyceram interface. Substitution for the various constants yields a value of 0.02957 V for the Nernstian slope at 25 ◦ C. The typical response of the sensor to various concentrations of hydrogen in nitrogen, is shown in Fig. 1a, where the lines represent the actual data points. Prior to the introduction of a new hydrogen gas mixture, the sensor was flushed with air to bring the cell emf back to the baseline level. Nitrogen was also used to flush the sensor, however, the time required to reduce the cell emf to the baseline level was too long, on the order of hours. The data in Fig. 1 were collected by exposing the sensor to 0.01% hydrogen and then going up to 100% hydrogen. This sequence was chosen purely for convenience. The sensor performance was also checked by exposing the sensor to 100% hydrogen and then going down to 0.01% hydrogen; here also, the
0
(b)
500
1000
1500
2000
2500
3000
3500
Time (s)
Fig. 1. Response time of the sensor to various concentrations of hydrogen in nitrogen: (a) for short exposure times and (b) for extended exposure times. The sensor was flushed with air prior to the introduction of each new hydrogen gas mixture.
sensor displayed behaviour similar to that shown in Fig. 1. The repeatability of the sensor output for a particular hydrogen concentration was good, the variation being typically less than ±5%. The zero point of the sensors was not calibrated in any way, but it could be adjusted electronically if required. The figure shows that the sensor response is extremely rapid for hydrogen concentrations between 1 and 100%. The response time is typically less than 10 s to achieve a signal level of 90%. As the concentration of hydrogen is lowered, however, the response time becomes longer. This is likely due to the presence of adsorbed oxygen on the working electrode surface, which has to be removed by the reaction with the sparsely available hydrogen in the system; only then the hydrogen equilibrium at the palladium electrode can be reached. Fig. 1b shows the sensor’s response for prolonged periods of time. The figure shows that the sensor output for hydrogen concentrations above 1% reaches equilibrium values very quickly and is very stable. It also indicates that the sensor does not reach equilibrium
N. Maffei, A.K. Kuriakose / Sensors and Actuators B 98 (2004) 73–76
75
0.9
0.9
100 %
0.8 0.7
10 %
0.8 0.6
emf (V)
emf (V)
1%
0.7
0.5 0.4
0.1 %
0.3 0.2
0.6
0.01 % 0.1 0.0 0
0.5 0.01
0.1
1
10
100
50
100
150
200
250
300
350
400
Time (s)
(a) 0.9
Percent Hydrogen
100 %
0.8
Fig. 2. Equilibrium emf of the sensor vs. the logarithm of the hydrogen partial pressure for hydrogen–nitrogen gas mixtures.
10 %
0.7
for very low hydrogen gas concentrations after 1 h. The response time for the palladium-based sensor, while fast, is however, not as short as that for a sensor that uses a platinum working electrode [1]. A plot of the equilibrium output voltage of the sensor versus the logarithm of the hydrogen partial pressure yielded a calibration curve (Fig. 2); the sensor response is linear over an extremely wide range of hydrogen partial pressures. A nonlinear least squares fit (NLLSF) to the data yielded a slope of 53 mV, while the Nernst equation predicts a value of 29 mV. Fig. 1b, however, clearly shows that the sensor output had not reached equilibrium for hydrogen concentrations of 0.01 and 0.1%. If these data points are discarded, a NLLSF to the data yields a value of 25 mV for the slope, in reasonable agreement with the theoretically predicted value. The linear response of the device thus allows one to determine the concentration of hydrogen in an unknown gas mixture by simply measuring the emf of the sensor. The emf values measured for a certain concentration of hydrogen did show some variation from sensor to sensor even though they were fabricated and tested under identical conditions. The variation was, however, less than ±10% on a set of 10 sensors. All sensors tested did, however, show comparable response times. The response of the palladium-based sensor to hydrogen– air mixtures is shown in Fig. 3a. The emf values are slightly less, especially for lower hydrogen concentrations, than the output of the sensor to hydrogen in inert atmospheres (Fig. 1a), but are still quite large. Fig 3b shows the sensor response to hydrogen–air mixtures for extended periods of time. The figure shows that the sensor reaches equilibrium for all the gas mixtures. This behaviour also shows that sensors with a palladium working electrode are quite responsive to hydrogen–air gas mixtures. The reasons why a similar sensor with a platinum working electrode [1] was unable to reliably detect hydrogen in air gas mixtures are unclear, but may be attributed to the generally high catalytic
emf (V)
0.6 0.5
1%
0.4
0.1 % 0.3 0.2
0.01 %
0.1 0.0 0
(b)
500
1000
1500
2000
2500
3000
3500
Time (s)
Fig. 3. Response time of the sensor to various concentrations of hydrogen in air: (a) for short exposure times and (b) for extended exposure times. The sensor was flushed with air prior to the introduction of each new hydrogen gas mixture.
activity of finely divided platinum for H2 –O2 combination reaction at the working electrode. It may also be due to the greater hydrogen permeability of palladium over platinum as suggested by Ammende et al. [9]. The cross sensitivity of the sensor to CO2 and NOx was not investigated and hence their influence on the sensors response is not known. Fig. 4 depicts the equilibrium potential values of the sensor to various hydrogen gas concentrations. The response of the sensor, while linear, is non-Nernstian, a NLLSF of the data yielded a slope of 166 mV, well above the value predicted by the Nernst equation. If the data for 0.01% hydrogen is discarded, the value of the resultant slope is 145 mV, still significantly greater than the expected theoretical value. The reasons for the non-Nernstian behaviour of the sensor to hydrogen–air mixtures are not clear. The fast, linear, reproducible and large, stable output of the sensor makes it very useful for detection of hydrogen leaks in air. The sensor performance thus suggests that it can be incorporated in fuel cell and other hydrogen process applications to detect potentially explosives levels of hydrogen concentrations.
76
N. Maffei, A.K. Kuriakose / Sensors and Actuators B 98 (2004) 73–76 0.9
References
0.8
[1] N. Maffei, A.K. Kuriakose, A hydrogen sensor based on a hydrogen ion conducting solid electrolyte, Sens. Actuators, B, Chem. 56 (1999) 243–246. [2] G. Alberti, A. Carbone, R. Palombari, Solid state potentiometric sensor at medium temperatures (150–300 ◦ C) for detecting oxidable gaseous species in air, Sens. Actuators, B, Chem. 75 (2001) 125–128. [3] R. Bouchet, S. Rosini, G. Vitter, E. Siebert, A solid-state potentiometric sensor based on polybenzimidazole for hydrogen determination in air, J. Electrochem. Soc. 149 (2002) H119–H122. [4] N. Miura, T. Harada, N. Yamazoe, Sensing characteristics and working mechanism of four-probe type solid-state hydrogen sensor using proton conductor, J. Electrochem. Soc. 136 (1989) 1215–1219. [5] R.V. Kumar, D.J. Fray, Development of solid-state hydrogen sensors, Sens. Actuators 15 (1988) 185–191. [6] S. Zhuiykov, Hydrogen sensor based on a new type of proton conductive ceramic, Int. J. Hydrogen Energy 21 (1996) 749–759. [7] S.F. Chehab, J.D. Canaday, A.K. Kuriakose, T.A. Wheat, A. Ahmad, A hydrogen sensor based on bonded hydronium NASICON, Solid State Ionics 45 (1991) 299–310. [8] W.F. Chu, V. Leonhard, H. Erdmann, M. Ilgenstein, Thick-film chemical sensors, Sens. Actuators, B, Chem. 4 (1991) 321–324. [9] S. Ammende, H. Erdmann, H.-W. Etzkorn, K. Zucholl, Sensor for Monitoring Hydrogen Concentration in Gases, US Patent No. 4,908,118. [10] G.W. Hunter, A Survey and Analysis of Experimental Hydrogen Sensors, NASA Technical Memorandum 106300, 1992. [11] G.W. Hunter, A Survey and Analysis of Commercially Available Hydrogen Sensors, NASA Technical Memorandum 105878, 1992. [12] G.W. Hunter, D.B. Makel, E.D. Jansa, G. Patterson, P.J. Cova, C.C. Liu, Q.H. Wu, W.T. Powers, A Hydrogen Leak Detection System for Aerospace and Commercial Applications, NASA Technical Memorandum 1070363, 1995. [13] B.H. Weiller, J.D. Barrie, K.A. Aitchison, P.D. Chaffee, Chemical microsensors for satellite applications, Mater. Res. Soc. Symp. Proc. 360 (1995) 535–540. [14] A.K. Kuriakose, N. Maffei, Hydrogen Sensor Using a Solid Hydrogen Ion Conducting Electrolyte, US Patent No. 6,073,478. [15] G. Alberti, R. Palombari, Solid-State Sensor for Determining the Concentration of a Gas with a Solid-State Reference Electrode, US Patent No. 5,453,172. [16] A.K. Kuriakose, T.A. Wheat, A. Ahmad, J.D. Canaday, A.J. Hanson, Bonded Hydrogen Conducting Solid Electrolytes, US Patent No. 4,724,191. [17] Circuit WorksTM Conductive Epoxy, Part No. CW2400, Available from Chemtronics® Inc., Kennesaw, GA 30152-4386, USA.
0.7
emf (V)
0.6 0.5 0.4 0.3 0.2 0.1 0.0 0.01
0.1
1
10
100
Percent Hydrogen Fig. 4. Equilibrium emf of the sensor vs. the logarithm of the hydrogen partial pressure for hydrogen–air gas mixtures.
4. Summary A solid-state hydrogen ion conducting solid electrolytebased hydrogen sensor, which has very fast response time and sensitivity over a wide range of hydrogen concentrations has been described. The sensor uses a palladium working electrode which allows it to sense hydrogen in air. The simple, robust construction of the sensor and its performance characteristics should allow the sensor to be used in applications where hydrogen gas is used, especially fuel cell vehicles and stationary power generation units.
Acknowledgements The authors acknowledge the financial support of this work by the Office of Energy Research and Development and CANMET, Natural Resources Canada.
A solid-state potentiometric sensor for hydrogen detection in air N. Maffei∗ , A.K. Kuriakose Materials Technology Laboratory, CANMET, Natural Resources Canada, 405 Rochester St., Ottawa, Ont., Canada K1A 0G3 Received 14 February 2003; received in revised form 16 September 2003; accepted 22 September 2003
Abstract A solid-state potentiometric hydrogen gas sensor based on hydronium Nasicon, a hydrogen ion conducting solid electrolyte, is described. The device incorporates a patented silver-based reference electrode and a palladium working electrode. The sensor is robust, simple, fast, and capable of detecting hydrogen concentrations from at least 0.01 to 100%, a range of four orders of magnitude, in oxidizing and non-oxidizing atmospheres. The room temperature response characteristics of the sensor are reported. Crown Copyright © 2003 Published by Elsevier B.V. All rights reserved. Keywords: Hydrogen; Sensor; Solid electrolyte; Palladium
1. Introduction Many industrialized countries are becoming increasingly aware of factors that affect climate change, and of particular concern are increased emission of greenhouse gases. This issue, along with the inevitable depletion of fossil fuel reserves has resulted in a renewed interest in alternative and renewable energy sources. Fuel cells have recently seen an upsurge in interest for possible use in transportation and stationary power generation. Fuel cells are electrochemical devices that convert chemical energy directly into electrical energy, which when operated on pure hydrogen emit no greenhouse gases, the only waste product being pure water. The use of hydrogen as fuel for fuel cells, however, raises many safety issues; consequently detection of potentially hazardous concentrations of hydrogen gas in both commercial and residential environments is receiving increased attention. Several solid-state hydrogen gas sensors exploiting the ionic conduction of solid electrolytes have been reported [1–9]. This type of gas sensor has several advantages: the generated current or voltage can be easily measured with very high precision and since the potentials generated by these devices are independent of dimensions the sensor size can be tailored for any application. Hydrogen gas sensors are currently used extensively in a variety of areas, including aerospace and other industrial applications [10–13]. The use of hydrogen to power fuel cells for transportation and power ∗ Corresponding author. Tel.: +1-613-992-1391; fax: +1-613-992-9389. E-mail address: [email protected] (N. Maffei).
generation applications will further increase the demand for cheap, simple, reliable and low-cost hydrogen gas sensors. In an earlier publication [1], a potentiometric hydrogen sensor based on hydronium Nasicon was described. That sensor had a novel reference electrode that allowed it to operate without a standard reference gas [14]. That sensor used a platinum working electrode and was unable to measure hydrogen in air. Although, a previous publication did, however, report that a solid-state protonic conductor-based sensor, using a sputtered platinum working electrode, was able to detect hydrogen in air, albeit, over a limited range of hydrogen partial pressures [15]. In order to overcome this limitation in the present sensor, it was modified by changing the working electrode; the results of this modification on sensor performance are presented in this paper. 2. Experimental A bonded hydronium Nasicon disc (HyceramTM , a CANMET trade mark for a series of phosphate bonded hydronium ion conductors) was prepared as described previously [16]. The dried discs were parallel lapped with 120 grit abrasive paper, cleaned in 2-propanol, and dried. A silver nitrate solution in distilled water was applied to one side of the discs. The discs were dried for approximately 24 h, allowing the silver nitrate to diffuse into the discs thus forming a silver-Hyceram salt at and near the surface of the discs. The dry, silver nitrate treated surfaces were then covered with a commercially available electrically conducting silver-loaded epoxy resin [17]. A silver wire lead was attached with the same silver epoxy resin. The system was then cured at room
0925-4005/$ – see front matter. Crown Copyright © 2003 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2003.09.024
N. Maffei, A.K. Kuriakose / Sensors and Actuators B 98 (2004) 73–76
temperature for 24 h. The maximum working temperature of the silver epoxy is 100 ◦ C and consequently, environments exceeding this temperature may compromise the functionality of the reference electrode. The expected lifetime of the silver epoxy is not known, but sensors in our laboratory are still functional 1 year after fabrication. After curing, the untreated side of the electrolyte discs was slurry coated with a layer of palladium metal powder, using an organic medium (Engelhard A2466 Medium), and allowed to dry at room temperature for 24 h. A silver wire lead was attached to the dried palladium surface using the conducting silver epoxy resin. Measurement of the cell emf was made with a Keithley 619 electrometer, which was interfaced through an IEEE-488 device to a personal computer. The data were collected for equilibrium potentials and response times to equilibrium, as a function of hydrogen partial pressure. High purity dry hydrogen, hydrogen–nitrogen and hydrogen–air gas mixtures containing 0.01, 0.1, 1 and 10% hydrogen, at a flow rate of 200 cm3 min−1 , were used for the measurements.
1.0 0.9 0.8
100 % 10 %
0.7
1%
0.6
emf (V)
74
0.1 % 0.5 0.4
0.01 % 0.3 0.2 0.1 0.0 0
50
100
(a)
150
200
250
300
350
400
Time (s) 1.0 0.9
100 % 10 % 1% 0.1 %
0.8 0.7
3. Results and discussion
emf (V)
0.6
0.01 %
0.5 0.4
The sensor can be described in terms of the following galvanic cell:
0.3
H2 , Pd|Hyceram|Ag-Hyceram|Ag
0.1
0.2
0.0
The emf generated when the cell anode is exposed to atmospheres containing hydrogen, assuming that the concentration of all the species other than H2 are constant, may be represented by the Nernstian equation: E=k+
2.303RT log (pH2 ) 2nF
where E is the cell emf in volts, k the constant representing the emf at the reference electrode, R the gas constant, T the absolute temperature, n the number of electrons involved in the electrochemical process (1), F the Faraday’s constant and pH2 the partial pressure of hydrogen (atm) at the palladium electrode/Hyceram interface. Substitution for the various constants yields a value of 0.02957 V for the Nernstian slope at 25 ◦ C. The typical response of the sensor to various concentrations of hydrogen in nitrogen, is shown in Fig. 1a, where the lines represent the actual data points. Prior to the introduction of a new hydrogen gas mixture, the sensor was flushed with air to bring the cell emf back to the baseline level. Nitrogen was also used to flush the sensor, however, the time required to reduce the cell emf to the baseline level was too long, on the order of hours. The data in Fig. 1 were collected by exposing the sensor to 0.01% hydrogen and then going up to 100% hydrogen. This sequence was chosen purely for convenience. The sensor performance was also checked by exposing the sensor to 100% hydrogen and then going down to 0.01% hydrogen; here also, the
0
(b)
500
1000
1500
2000
2500
3000
3500
Time (s)
Fig. 1. Response time of the sensor to various concentrations of hydrogen in nitrogen: (a) for short exposure times and (b) for extended exposure times. The sensor was flushed with air prior to the introduction of each new hydrogen gas mixture.
sensor displayed behaviour similar to that shown in Fig. 1. The repeatability of the sensor output for a particular hydrogen concentration was good, the variation being typically less than ±5%. The zero point of the sensors was not calibrated in any way, but it could be adjusted electronically if required. The figure shows that the sensor response is extremely rapid for hydrogen concentrations between 1 and 100%. The response time is typically less than 10 s to achieve a signal level of 90%. As the concentration of hydrogen is lowered, however, the response time becomes longer. This is likely due to the presence of adsorbed oxygen on the working electrode surface, which has to be removed by the reaction with the sparsely available hydrogen in the system; only then the hydrogen equilibrium at the palladium electrode can be reached. Fig. 1b shows the sensor’s response for prolonged periods of time. The figure shows that the sensor output for hydrogen concentrations above 1% reaches equilibrium values very quickly and is very stable. It also indicates that the sensor does not reach equilibrium
N. Maffei, A.K. Kuriakose / Sensors and Actuators B 98 (2004) 73–76
75
0.9
0.9
100 %
0.8 0.7
10 %
0.8 0.6
emf (V)
emf (V)
1%
0.7
0.5 0.4
0.1 %
0.3 0.2
0.6
0.01 % 0.1 0.0 0
0.5 0.01
0.1
1
10
100
50
100
150
200
250
300
350
400
Time (s)
(a) 0.9
Percent Hydrogen
100 %
0.8
Fig. 2. Equilibrium emf of the sensor vs. the logarithm of the hydrogen partial pressure for hydrogen–nitrogen gas mixtures.
10 %
0.7
for very low hydrogen gas concentrations after 1 h. The response time for the palladium-based sensor, while fast, is however, not as short as that for a sensor that uses a platinum working electrode [1]. A plot of the equilibrium output voltage of the sensor versus the logarithm of the hydrogen partial pressure yielded a calibration curve (Fig. 2); the sensor response is linear over an extremely wide range of hydrogen partial pressures. A nonlinear least squares fit (NLLSF) to the data yielded a slope of 53 mV, while the Nernst equation predicts a value of 29 mV. Fig. 1b, however, clearly shows that the sensor output had not reached equilibrium for hydrogen concentrations of 0.01 and 0.1%. If these data points are discarded, a NLLSF to the data yields a value of 25 mV for the slope, in reasonable agreement with the theoretically predicted value. The linear response of the device thus allows one to determine the concentration of hydrogen in an unknown gas mixture by simply measuring the emf of the sensor. The emf values measured for a certain concentration of hydrogen did show some variation from sensor to sensor even though they were fabricated and tested under identical conditions. The variation was, however, less than ±10% on a set of 10 sensors. All sensors tested did, however, show comparable response times. The response of the palladium-based sensor to hydrogen– air mixtures is shown in Fig. 3a. The emf values are slightly less, especially for lower hydrogen concentrations, than the output of the sensor to hydrogen in inert atmospheres (Fig. 1a), but are still quite large. Fig 3b shows the sensor response to hydrogen–air mixtures for extended periods of time. The figure shows that the sensor reaches equilibrium for all the gas mixtures. This behaviour also shows that sensors with a palladium working electrode are quite responsive to hydrogen–air gas mixtures. The reasons why a similar sensor with a platinum working electrode [1] was unable to reliably detect hydrogen in air gas mixtures are unclear, but may be attributed to the generally high catalytic
emf (V)
0.6 0.5
1%
0.4
0.1 % 0.3 0.2
0.01 %
0.1 0.0 0
(b)
500
1000
1500
2000
2500
3000
3500
Time (s)
Fig. 3. Response time of the sensor to various concentrations of hydrogen in air: (a) for short exposure times and (b) for extended exposure times. The sensor was flushed with air prior to the introduction of each new hydrogen gas mixture.
activity of finely divided platinum for H2 –O2 combination reaction at the working electrode. It may also be due to the greater hydrogen permeability of palladium over platinum as suggested by Ammende et al. [9]. The cross sensitivity of the sensor to CO2 and NOx was not investigated and hence their influence on the sensors response is not known. Fig. 4 depicts the equilibrium potential values of the sensor to various hydrogen gas concentrations. The response of the sensor, while linear, is non-Nernstian, a NLLSF of the data yielded a slope of 166 mV, well above the value predicted by the Nernst equation. If the data for 0.01% hydrogen is discarded, the value of the resultant slope is 145 mV, still significantly greater than the expected theoretical value. The reasons for the non-Nernstian behaviour of the sensor to hydrogen–air mixtures are not clear. The fast, linear, reproducible and large, stable output of the sensor makes it very useful for detection of hydrogen leaks in air. The sensor performance thus suggests that it can be incorporated in fuel cell and other hydrogen process applications to detect potentially explosives levels of hydrogen concentrations.
76
N. Maffei, A.K. Kuriakose / Sensors and Actuators B 98 (2004) 73–76 0.9
References
0.8
[1] N. Maffei, A.K. Kuriakose, A hydrogen sensor based on a hydrogen ion conducting solid electrolyte, Sens. Actuators, B, Chem. 56 (1999) 243–246. [2] G. Alberti, A. Carbone, R. Palombari, Solid state potentiometric sensor at medium temperatures (150–300 ◦ C) for detecting oxidable gaseous species in air, Sens. Actuators, B, Chem. 75 (2001) 125–128. [3] R. Bouchet, S. Rosini, G. Vitter, E. Siebert, A solid-state potentiometric sensor based on polybenzimidazole for hydrogen determination in air, J. Electrochem. Soc. 149 (2002) H119–H122. [4] N. Miura, T. Harada, N. Yamazoe, Sensing characteristics and working mechanism of four-probe type solid-state hydrogen sensor using proton conductor, J. Electrochem. Soc. 136 (1989) 1215–1219. [5] R.V. Kumar, D.J. Fray, Development of solid-state hydrogen sensors, Sens. Actuators 15 (1988) 185–191. [6] S. Zhuiykov, Hydrogen sensor based on a new type of proton conductive ceramic, Int. J. Hydrogen Energy 21 (1996) 749–759. [7] S.F. Chehab, J.D. Canaday, A.K. Kuriakose, T.A. Wheat, A. Ahmad, A hydrogen sensor based on bonded hydronium NASICON, Solid State Ionics 45 (1991) 299–310. [8] W.F. Chu, V. Leonhard, H. Erdmann, M. Ilgenstein, Thick-film chemical sensors, Sens. Actuators, B, Chem. 4 (1991) 321–324. [9] S. Ammende, H. Erdmann, H.-W. Etzkorn, K. Zucholl, Sensor for Monitoring Hydrogen Concentration in Gases, US Patent No. 4,908,118. [10] G.W. Hunter, A Survey and Analysis of Experimental Hydrogen Sensors, NASA Technical Memorandum 106300, 1992. [11] G.W. Hunter, A Survey and Analysis of Commercially Available Hydrogen Sensors, NASA Technical Memorandum 105878, 1992. [12] G.W. Hunter, D.B. Makel, E.D. Jansa, G. Patterson, P.J. Cova, C.C. Liu, Q.H. Wu, W.T. Powers, A Hydrogen Leak Detection System for Aerospace and Commercial Applications, NASA Technical Memorandum 1070363, 1995. [13] B.H. Weiller, J.D. Barrie, K.A. Aitchison, P.D. Chaffee, Chemical microsensors for satellite applications, Mater. Res. Soc. Symp. Proc. 360 (1995) 535–540. [14] A.K. Kuriakose, N. Maffei, Hydrogen Sensor Using a Solid Hydrogen Ion Conducting Electrolyte, US Patent No. 6,073,478. [15] G. Alberti, R. Palombari, Solid-State Sensor for Determining the Concentration of a Gas with a Solid-State Reference Electrode, US Patent No. 5,453,172. [16] A.K. Kuriakose, T.A. Wheat, A. Ahmad, J.D. Canaday, A.J. Hanson, Bonded Hydrogen Conducting Solid Electrolytes, US Patent No. 4,724,191. [17] Circuit WorksTM Conductive Epoxy, Part No. CW2400, Available from Chemtronics® Inc., Kennesaw, GA 30152-4386, USA.
0.7
emf (V)
0.6 0.5 0.4 0.3 0.2 0.1 0.0 0.01
0.1
1
10
100
Percent Hydrogen Fig. 4. Equilibrium emf of the sensor vs. the logarithm of the hydrogen partial pressure for hydrogen–air gas mixtures.
4. Summary A solid-state hydrogen ion conducting solid electrolytebased hydrogen sensor, which has very fast response time and sensitivity over a wide range of hydrogen concentrations has been described. The sensor uses a palladium working electrode which allows it to sense hydrogen in air. The simple, robust construction of the sensor and its performance characteristics should allow the sensor to be used in applications where hydrogen gas is used, especially fuel cell vehicles and stationary power generation units.
Acknowledgements The authors acknowledge the financial support of this work by the Office of Energy Research and Development and CANMET, Natural Resources Canada.