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A low temperature sensor for the detection of carbon monoxide in hydrogen
Solid State Ionics 175 (2004) 497 – 501 www.elsevier.com/locate/ssi
A low temperature sensor for the detection of carbon monoxide in hydrogen Rangachary Mukundan*, Eric L. Brosha, Fernando H. Garzon Los Alamos National Laboratory, MS D429, MST-11. Los Alamos, NM 87545, United States Received 1 May 2003; accepted 14 February 2004
Abstract A sensor to detect the carbon monoxide in a reformate stream that fuels a polymer electrolyte membrane (PEM) fuel cell is presented. This electrochemical sensor consists of a NafionR electrolyte and Pt- or Ru-based electrodes and works on the principle of differential CO poisoning of the various precious metal electrodes. Varying the composition and loading of the precious metal in the electrode optimized the response of the sensor. A sensor with a working electrode of Pt and a pseudo-reference electrode of Pt–Ru alloy and a precious metal loading of 10 mg/cm2 showed a stable and reproducible response to 10–200 ppm of CO at room temperature and 100–1000 ppm CO at 70 8C. D 2004 Elsevier B.V. All rights reserved. PACS: 07.07.D; 84.60.G Keywords: Sensors; Carbon monoxide; Fuel cells; Platinum and ruthenium
1. Introduction Polymer electrolyte membrane (PEM) fuel cells are currently being developed for transportation applications [1]. Since liquid hydrocarbons are the most widely available fuel for transportation, much research is being conducted to optimize low cost fuel reformer systems that convert this fuel to hydrogen gas containing streams [2]. This hydrogen gas typically feeds a PEM fuel cell stack utilizing platinum based anodes. It is well known that low concentrations of carbon monoxide (~10–100 ppm) impurities in hydrogen can severely degrade the performance of PEM fuel cell anodes due to the strong adsorption of carbon monoxide on the electro-active platinum surface sites where hydrogen is normally oxidized to protons [3]. Therefore, detection and measurement of carbon monoxide in high temperature reformate streams is of vital importance to the successful implementation of fuel cells for transportation [4,5].
* Corresponding author. Tel.: +1 505 665 8523; fax: +1 505 665 4292. E-mail address: [email protected] (R. Mukundan). 0167-2738/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2004.02.074
Fuel cell systems operating on reformate use air-bleeding methods to reduce the carbon monoxide poisoning of the Pt anode [6]. The quantity of air needed is dependent on the CO concentration and varies from 2% to 10% air (or 0.4– 2% oxygen) as the CO concentration varies from 5 to 100 ppm [6]. Since the air-bleeding systems lead to a decrease in the efficiency, the use of a CO sensor in a feedback loop can provide the appropriate amount of air depending on the CO concentration in the fuel. Currently, research on several technologies that measure the amount of CO in a reformate stream is on going [4,5,7,8]. The most advanced of these technologies utilizes the CO poisoning of a fuel cell to determine the amount of CO in the reformate stream [5,7]. These sensors involve the use of small PEM fuel cells in the reformate stream and pattern recognition software to calculate the amount of CO from the I–V curves of the fuel cells. Although these sensors have been shown to measure 50 ppm of CO in a reformate stream, they are complicated devices and are expensive to manufacture. In this paper, we present the development of a low temperature (b100 8C) CO sensor that is based on the
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R. Mukundan et al. / Solid State Ionics 175 (2004) 497–501
differential reversible carbon monoxide adsorptive poisoning of precious metal electrodes. The addition of metals such as ruthenium to the platinum electrode material greatly improves the hydrogen oxidation kinetics in the presence of CO [9]. An amperometric sensor that senses the differential CO inhibition of the hydrogen oxidation reaction has been fabricated from a platinum (Pt) electrode, a proton conductor (NafionR), and a platinum ruthenium alloy (Pt/ Ru) electrode. The Pt electrode serves as the sensing electrode whose current density is influenced by the surface coverage of carbon monoxide. The Pt/Ru alloy electrode is relatively unaffected by the presence of CO and serves as a pseudo-reference electrode. This sensor configuration eliminates the need for a cathode air reference and greatly simplifies the sensor design.
2. Experimental Several electrode compositions and loadings were prepared for sensor testing on NafionR (Du Pont) 117 and 1135 membranes. The catalyst layer (either unsupported or carbon-supported) was applied to the NafionR either by direct painting [10] or by a transfer decal process [11]. Typically, two different catalyst materials (Pt, Ru or Pt–Ru alloys from Johnson Matthey) of the same loading were applied to opposite sides of the NafionR membrane that was then cut into small (0.25–1 cm2) cells. The I–V characteristics of these cells were measured at various temperatures
in flowing H2 (saturated with H2O at 90 8C) with varying CO concentrations. The amperometric sensor response to CO concentration was then measured at the optimal voltages determined from the I–V characteristics.
3. Results Several sensor configurations with different catalyst loadings and various temperatures of operation were studied. Three different catalysts were tested including Pt, Ru and a Pt–Ru alloy with a nominal 1:1 Pt/Ru atomic ratio. The I–V characteristics at room temperature obtained from the sensor with a NafionR electrolyte and carbon-supported Pt (0.21 mg/ cm2) and Ru (0.12 mg/cm2) electrodes is shown in Fig. 1. It is seen that the Pt electrode is much better at hydrogen oxidation than the Ru electrode. For example, at a voltage of 0.8 V, the current at the Pt electrode was 40 mA whereas the current at the Ru electrode was only 2mA. Moreover, these carbonsupported catalysts with low (0.2 mg/cm2) loadings typical of current fuel cell membrane electrode assemblies (MEAs) showed an observable decrease in the current in the presence of CO. However, the CO poisoning of the Pt electrode was recoverable only after long (several hours) periods of equilibration in a CO-free atmosphere or short (several seconds) intervals of air bleeding. The Pt–Ru alloy electrode behaved very similar to the Pt electrode. The slow recovery times of these Pt- and Pt–Ru-based electrodes imply that these sensors are not conducive to be used as a simple
Fig. 1. I–V curve from a bPt (0.21 mg/cm2)–C/NafionR 117/Ru (0.12 mg/cm2)–CQ sensor.
R. Mukundan et al. / Solid State Ionics 175 (2004) 497–501
Fig. 2. I–V curve from a bPt (10 mg/cm2)/NafionR 117/Pt–Ru (10 mg/cm2)Q sensor.
Fig. 3. Sensor response from a bPt (10 mg/cm2)/NafionR 117/Pt–Ru (10 mg/cm2)Q sensor.
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amperometric CO sensor operating at the fuel inlet of a PEM fuel cell. Hence, only the Ru electrode where the CO could be cleaned by the application of a voltage b 0.7 V (see inset in Fig. 1) could be used as a CO sensor. However, the sensitivity of this sensor was very low since the Ru electrode was not efficient at hydrogen oxidation when compared to the Ptbased electrodes. The optimal performance was obtained for a bPt/ NafionR/Pt–RuQ sensor where the electrolyte used was NafionR 117, the sensing electrode was Pt black mixed with NafionR and painted at a 10 mg/cm2 loading, and the pseudo-reference electrode was a Pt–Ru alloy mixed with NafionR and painted at a 10 mg/cm2 loading. The I–V curve of this device at room temperature is shown in Fig. 2, where the Pt–Ru alloy electrode is relatively unaffected by the presence of 100 ppm of CO while the Pt electrode is poisoned significantly. Moreover, the CO from the Pt electrode can be easily cleaned by the application of a voltage N0.4 V. Therefore, this device, when biased at a voltage b0.4 V, can be used as a CO sensor. The CO response obtained at room temperature from this device at a bias of 0.3 V is illustrated in Fig. 3. For example, the current at a voltage of 0.3 V is decreased from 15 to 3 mA by the introduction of 100 ppm of CO. The recovery to baseline is slow for this sensor and can be accelerated using a short (b1 min) CO cleanup step by the application of 0.8 mV (Fig. 3). Moreover, there is a drift in the baseline voltage of these sensors over time, which could be due to the change in the water content of the NafionR membrane. To improve the response time of this sensor, it was operated at elevated temperatures (60–90 8C). The response of this sensor to 100–1000 ppm CO at 70 8C is shown in Fig. 4. Although the response time of the sensor was
significantly improved by the increase in temperature, the sensitivity was found to decrease due to the improved CO tolerance of the Pt electrode at elevated temperatures. For example, the percentage decrease in the current level of this sensor when exposed to 100 ppm of CO was 80% at room temperature compared to only 16% at 70 8C. Hence, the operating temperature of the sensor needs to be optimized to provide the desired sensor response time and sensor sensitivity.
4. Conclusions The reversible CO poisoning of precious metal catalysts could be potentially used to fabricate CO sensors for reformate streams. Electrodes with unsupported precious metals at high loadings (10 mg/cm2) showed reasonable sensitivity and response times. The response time improved with increasing temperature while the sensitivity decreased with increasing temperature. The recovery to baseline of these sensors can also be improved by the application of a voltage large enough to clean up the CO. The long-term stability of these sensors in reformate streams needs to be examined and the response time improved before these sensors could become practical.
Acknowledgements The authors acknowledge the DOE office of Energy Efficiency and Renewable Energy for funding this work. The authors would like to thank Francisco Uribe, Piotr Zelenay and Brian Pivovar for useful discussions.
Fig. 4. Sensor response from a bPt (10 mg/cm2)/NafionR 117/Pt–Ru (10 mg/cm2)Q sensor at 70 8C.
R. Mukundan et al. / Solid State Ionics 175 (2004) 497–501
References [1] D.P. Wilkinson, Electrochem. Soc. Interface 10 (1) (2001) 2. [2] L.J. Pettersson, R. Westerholm, Int. J. Hydrogen Energy 26 (2001) 243. [3] S. Gottesfeld, T. Zawodzinski, Adv. Electrochem. Sci. Eng. 5 (1998) 219. [4] A. Hashimoto, T. Hibino, M. Sano, Electrochem. Solid-State Lett. 5 (2) (2002) H1. [5] S.A. Grot, et al., US Patent No. 6063516, Method of monitoring CO concentrations in hydrogen feed to a PEM fuel Cell, May 16 (2000). [6] S. Gottesfeld, J. Pafford, J. Electrochem. Soc. 135 (1988) 2651.
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[7] K.W. Kirby, A.C. Chu, K.C. Fuller, Sens. Actuators, B, Chem. 95 (2003) 224. [8] C.T. Holt, A.-M. Azad, S.L. Swartz, R.R. Rao, P.K. Dutta, Sens. Actuators, B, Chem. 87 (2002) 414. [9] E. Auer, et al., US Patent No. 6007934, CO tolerant anode catalyst for PEM fuel cells and a process for its preparation, December 28 (1999). [10] S.C. Thomas, X. Ren, S. Gottesfeld, P. Zelanay, Electrochim. Acta 47 (2002) 3741. [11] X. Ren, M.S. Wilson, S. Gottesfled, J. Electrochem. Soc. 143 (1996) L12.
A low temperature sensor for the detection of carbon monoxide in hydrogen Rangachary Mukundan*, Eric L. Brosha, Fernando H. Garzon Los Alamos National Laboratory, MS D429, MST-11. Los Alamos, NM 87545, United States Received 1 May 2003; accepted 14 February 2004
Abstract A sensor to detect the carbon monoxide in a reformate stream that fuels a polymer electrolyte membrane (PEM) fuel cell is presented. This electrochemical sensor consists of a NafionR electrolyte and Pt- or Ru-based electrodes and works on the principle of differential CO poisoning of the various precious metal electrodes. Varying the composition and loading of the precious metal in the electrode optimized the response of the sensor. A sensor with a working electrode of Pt and a pseudo-reference electrode of Pt–Ru alloy and a precious metal loading of 10 mg/cm2 showed a stable and reproducible response to 10–200 ppm of CO at room temperature and 100–1000 ppm CO at 70 8C. D 2004 Elsevier B.V. All rights reserved. PACS: 07.07.D; 84.60.G Keywords: Sensors; Carbon monoxide; Fuel cells; Platinum and ruthenium
1. Introduction Polymer electrolyte membrane (PEM) fuel cells are currently being developed for transportation applications [1]. Since liquid hydrocarbons are the most widely available fuel for transportation, much research is being conducted to optimize low cost fuel reformer systems that convert this fuel to hydrogen gas containing streams [2]. This hydrogen gas typically feeds a PEM fuel cell stack utilizing platinum based anodes. It is well known that low concentrations of carbon monoxide (~10–100 ppm) impurities in hydrogen can severely degrade the performance of PEM fuel cell anodes due to the strong adsorption of carbon monoxide on the electro-active platinum surface sites where hydrogen is normally oxidized to protons [3]. Therefore, detection and measurement of carbon monoxide in high temperature reformate streams is of vital importance to the successful implementation of fuel cells for transportation [4,5].
* Corresponding author. Tel.: +1 505 665 8523; fax: +1 505 665 4292. E-mail address: [email protected] (R. Mukundan). 0167-2738/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2004.02.074
Fuel cell systems operating on reformate use air-bleeding methods to reduce the carbon monoxide poisoning of the Pt anode [6]. The quantity of air needed is dependent on the CO concentration and varies from 2% to 10% air (or 0.4– 2% oxygen) as the CO concentration varies from 5 to 100 ppm [6]. Since the air-bleeding systems lead to a decrease in the efficiency, the use of a CO sensor in a feedback loop can provide the appropriate amount of air depending on the CO concentration in the fuel. Currently, research on several technologies that measure the amount of CO in a reformate stream is on going [4,5,7,8]. The most advanced of these technologies utilizes the CO poisoning of a fuel cell to determine the amount of CO in the reformate stream [5,7]. These sensors involve the use of small PEM fuel cells in the reformate stream and pattern recognition software to calculate the amount of CO from the I–V curves of the fuel cells. Although these sensors have been shown to measure 50 ppm of CO in a reformate stream, they are complicated devices and are expensive to manufacture. In this paper, we present the development of a low temperature (b100 8C) CO sensor that is based on the
498
R. Mukundan et al. / Solid State Ionics 175 (2004) 497–501
differential reversible carbon monoxide adsorptive poisoning of precious metal electrodes. The addition of metals such as ruthenium to the platinum electrode material greatly improves the hydrogen oxidation kinetics in the presence of CO [9]. An amperometric sensor that senses the differential CO inhibition of the hydrogen oxidation reaction has been fabricated from a platinum (Pt) electrode, a proton conductor (NafionR), and a platinum ruthenium alloy (Pt/ Ru) electrode. The Pt electrode serves as the sensing electrode whose current density is influenced by the surface coverage of carbon monoxide. The Pt/Ru alloy electrode is relatively unaffected by the presence of CO and serves as a pseudo-reference electrode. This sensor configuration eliminates the need for a cathode air reference and greatly simplifies the sensor design.
2. Experimental Several electrode compositions and loadings were prepared for sensor testing on NafionR (Du Pont) 117 and 1135 membranes. The catalyst layer (either unsupported or carbon-supported) was applied to the NafionR either by direct painting [10] or by a transfer decal process [11]. Typically, two different catalyst materials (Pt, Ru or Pt–Ru alloys from Johnson Matthey) of the same loading were applied to opposite sides of the NafionR membrane that was then cut into small (0.25–1 cm2) cells. The I–V characteristics of these cells were measured at various temperatures
in flowing H2 (saturated with H2O at 90 8C) with varying CO concentrations. The amperometric sensor response to CO concentration was then measured at the optimal voltages determined from the I–V characteristics.
3. Results Several sensor configurations with different catalyst loadings and various temperatures of operation were studied. Three different catalysts were tested including Pt, Ru and a Pt–Ru alloy with a nominal 1:1 Pt/Ru atomic ratio. The I–V characteristics at room temperature obtained from the sensor with a NafionR electrolyte and carbon-supported Pt (0.21 mg/ cm2) and Ru (0.12 mg/cm2) electrodes is shown in Fig. 1. It is seen that the Pt electrode is much better at hydrogen oxidation than the Ru electrode. For example, at a voltage of 0.8 V, the current at the Pt electrode was 40 mA whereas the current at the Ru electrode was only 2mA. Moreover, these carbonsupported catalysts with low (0.2 mg/cm2) loadings typical of current fuel cell membrane electrode assemblies (MEAs) showed an observable decrease in the current in the presence of CO. However, the CO poisoning of the Pt electrode was recoverable only after long (several hours) periods of equilibration in a CO-free atmosphere or short (several seconds) intervals of air bleeding. The Pt–Ru alloy electrode behaved very similar to the Pt electrode. The slow recovery times of these Pt- and Pt–Ru-based electrodes imply that these sensors are not conducive to be used as a simple
Fig. 1. I–V curve from a bPt (0.21 mg/cm2)–C/NafionR 117/Ru (0.12 mg/cm2)–CQ sensor.
R. Mukundan et al. / Solid State Ionics 175 (2004) 497–501
Fig. 2. I–V curve from a bPt (10 mg/cm2)/NafionR 117/Pt–Ru (10 mg/cm2)Q sensor.
Fig. 3. Sensor response from a bPt (10 mg/cm2)/NafionR 117/Pt–Ru (10 mg/cm2)Q sensor.
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amperometric CO sensor operating at the fuel inlet of a PEM fuel cell. Hence, only the Ru electrode where the CO could be cleaned by the application of a voltage b 0.7 V (see inset in Fig. 1) could be used as a CO sensor. However, the sensitivity of this sensor was very low since the Ru electrode was not efficient at hydrogen oxidation when compared to the Ptbased electrodes. The optimal performance was obtained for a bPt/ NafionR/Pt–RuQ sensor where the electrolyte used was NafionR 117, the sensing electrode was Pt black mixed with NafionR and painted at a 10 mg/cm2 loading, and the pseudo-reference electrode was a Pt–Ru alloy mixed with NafionR and painted at a 10 mg/cm2 loading. The I–V curve of this device at room temperature is shown in Fig. 2, where the Pt–Ru alloy electrode is relatively unaffected by the presence of 100 ppm of CO while the Pt electrode is poisoned significantly. Moreover, the CO from the Pt electrode can be easily cleaned by the application of a voltage N0.4 V. Therefore, this device, when biased at a voltage b0.4 V, can be used as a CO sensor. The CO response obtained at room temperature from this device at a bias of 0.3 V is illustrated in Fig. 3. For example, the current at a voltage of 0.3 V is decreased from 15 to 3 mA by the introduction of 100 ppm of CO. The recovery to baseline is slow for this sensor and can be accelerated using a short (b1 min) CO cleanup step by the application of 0.8 mV (Fig. 3). Moreover, there is a drift in the baseline voltage of these sensors over time, which could be due to the change in the water content of the NafionR membrane. To improve the response time of this sensor, it was operated at elevated temperatures (60–90 8C). The response of this sensor to 100–1000 ppm CO at 70 8C is shown in Fig. 4. Although the response time of the sensor was
significantly improved by the increase in temperature, the sensitivity was found to decrease due to the improved CO tolerance of the Pt electrode at elevated temperatures. For example, the percentage decrease in the current level of this sensor when exposed to 100 ppm of CO was 80% at room temperature compared to only 16% at 70 8C. Hence, the operating temperature of the sensor needs to be optimized to provide the desired sensor response time and sensor sensitivity.
4. Conclusions The reversible CO poisoning of precious metal catalysts could be potentially used to fabricate CO sensors for reformate streams. Electrodes with unsupported precious metals at high loadings (10 mg/cm2) showed reasonable sensitivity and response times. The response time improved with increasing temperature while the sensitivity decreased with increasing temperature. The recovery to baseline of these sensors can also be improved by the application of a voltage large enough to clean up the CO. The long-term stability of these sensors in reformate streams needs to be examined and the response time improved before these sensors could become practical.
Acknowledgements The authors acknowledge the DOE office of Energy Efficiency and Renewable Energy for funding this work. The authors would like to thank Francisco Uribe, Piotr Zelenay and Brian Pivovar for useful discussions.
Fig. 4. Sensor response from a bPt (10 mg/cm2)/NafionR 117/Pt–Ru (10 mg/cm2)Q sensor at 70 8C.
R. Mukundan et al. / Solid State Ionics 175 (2004) 497–501
References [1] D.P. Wilkinson, Electrochem. Soc. Interface 10 (1) (2001) 2. [2] L.J. Pettersson, R. Westerholm, Int. J. Hydrogen Energy 26 (2001) 243. [3] S. Gottesfeld, T. Zawodzinski, Adv. Electrochem. Sci. Eng. 5 (1998) 219. [4] A. Hashimoto, T. Hibino, M. Sano, Electrochem. Solid-State Lett. 5 (2) (2002) H1. [5] S.A. Grot, et al., US Patent No. 6063516, Method of monitoring CO concentrations in hydrogen feed to a PEM fuel Cell, May 16 (2000). [6] S. Gottesfeld, J. Pafford, J. Electrochem. Soc. 135 (1988) 2651.
501
[7] K.W. Kirby, A.C. Chu, K.C. Fuller, Sens. Actuators, B, Chem. 95 (2003) 224. [8] C.T. Holt, A.-M. Azad, S.L. Swartz, R.R. Rao, P.K. Dutta, Sens. Actuators, B, Chem. 87 (2002) 414. [9] E. Auer, et al., US Patent No. 6007934, CO tolerant anode catalyst for PEM fuel cells and a process for its preparation, December 28 (1999). [10] S.C. Thomas, X. Ren, S. Gottesfeld, P. Zelanay, Electrochim. Acta 47 (2002) 3741. [11] X. Ren, M.S. Wilson, S. Gottesfled, J. Electrochem. Soc. 143 (1996) L12.