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Electrochemical carbon corrosion in high temperature proton exchange membrane fuel cells
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 0 8 4 4 e1 0 8 4 9
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Electrochemical carbon corrosion in high temperature proton exchange membrane fuel cells Hyung-Suk Oh, Jin-Hee Lee, Hansung Kim* Dept. of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 120-749, Republic of Korea
article info
abstract
Article history:
Electrochemical carbon corrosion occurring in a high temperature proton exchange
Received 8 March 2012
membrane fuel cell (HT-PEMFC) operating under non-humidification conditions was
Received in revised form
investigated by measuring CO2 generation using on-line mass spectrometry and comparing
9 April 2012
the results with a low-temperature proton exchange membrane fuel cell (LT-PEMFC)
Accepted 17 April 2012
operated under fully humidified conditions. The experimental results showed that more
Available online 15 May 2012
CO2 was measured for the HT-PEMFC, indicating that more electrochemical carbon corrosion occurs in HT-PEMFCs. This observation is attributed to the enhanced kinetics of
Keywords:
electrochemical carbon corrosion due to the elevated operating temperature in HT-
High temperature proton exchange
PEMFCs. Additionally, electrochemical carbon corrosion in HT-PEMFCs showed a strong
membrane fuel cell
dependence on water content. Therefore, it is critical to remove the water content in the
Carbon corrosion
supply gases to reduce electrochemical carbon corrosion.
Durability
Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Mass spectrometry
reserved.
Electrocatalysts
1.
Introduction
In recent years, there has been significant interest in the development of high-temperature proton exchange membrane fuel cells (HT-PEMFCs) operating at temperatures greater than 100 C [1e3]. Compared with low-temperature proton exchange membrane fuel cells (LT-PEMFCs) operating at approximately 60e80 C, HT-PEMFCs offer numerous advantages, such as the high tolerance of catalyst to contaminants, improved electrode kinetics, and simplified water and heat management [4e9]. To realize HT-PEMFCs, the largest challenge is replacing the perfluorosulfonic acid membranes (such as Nafion) that are currently used as the proton exchange membrane in LT-PEMFCs. Thus, tremendous effort has focused on the development of proton exchange membranes with high ionic conductivity and chemical and physical stability at elevated temperatures [10e12]. In recent
years, phosphoric acid-doped PBI (polybenzimidazole) has shown not only good proton conductivity but also excellent oxidative and thermal stability and almost zero electroosmotic drag [13e15]. In addition to developing a membrane for high temperature applications, it is critical to demonstrate the durability of HT-PEMFCs for commercial viability. The reduced life time of HT-PEMFCs is considered to be mainly due to the degradation of the electrode and the membrane such as the aggregation of catalyst particles [16] and the depletion of phosphoric acid [17]. However, compared to membranes, relatively little is reported about the degradation mechanism for the electrode, especially for catalyst supports associated with electrochemical carbon corrosion in HTPEMFCs. Recently, electrochemical carbon corrosion has received a considerable amount of attention because this is considered
* Corresponding author. Tel.: þ82 2 2123 5753; fax: þ82 2 312 6401. E-mail address: [email protected] (H. Kim). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2012.04.095
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 0 8 4 4 e1 0 8 4 9
one of the critical factors limiting the durability of PEMFCs [18e22]. The suggested mechanism of carbon corrosion involves carbon reacting with water and generating CO2, as shown in eq. (1). C þ 2H2 O/CO2 þ 4Hþ þ 4e
Eo ¼ 0:207 V vs: NHE
(1)
The equilibrium potential for carbon corrosion is 0.207 V vs. NHE at 25 C [23]. Although this implies that carbon is thermodynamically unstable to electrochemical carbon corrosion, the slow kinetics of this reaction makes it possible to use carbon in PEMFCs. However, it is well known that abnormal operating conditions, such as the air/fuel boundary created during the shut-down/start-up process at the anode, increase the cathodic potential to a level higher than the open circuit voltage [24e26]. Such a high potential quickly corrodes the carbon catalyst support, resulting in the severe loss of the active surface area of catalysts and rapid fuel cell performance degradation [27,28]. However, carbon corrosion research has mainly focused on LT-PEMFCs, and a detailed study of the carbon corrosion in HT-PEMFCs has not been conducted precisely. Our previous study about carbon corrosion in LT-PEMFCs revealed that carbon corrosion is accelerated as the humidity and the cell temperature increase [29]. HT-PEMFCs are generally operated under non-humidified conditions and at higher temperatures than are LT-PEMFCs. Therefore, increasing the cell temperature contributes to accelerating the carbon corrosion, while a non-humidified condition decelerates the carbon corrosion. As a result, it is difficult to predict the behavior of carbon corrosion in the HT-PEMFC. The focus of this work is to evaluate the dependence of electrochemical carbon corrosion in HT-PEMFCs and to compare the results with those of LT-PEMFCs. The influence of the humidity and cell temperature of HT-PEMFCs on the electrochemical carbon corrosion is studied systematically using on-line mass spectrometry and several electrochemical techniques.
2.
Experimental
2.1.
Preparation of the membrane electrode assemblies
High temperature polymer electrolyte membranes (Advent TPS), which were provided by the Advent Technologies Corporation, were used in this research. Catalyst ink was prepared by adding Pt/C catalyst (commercial 40 wt.% Pt from Johnson Matthey) into a solvent of n-methyl-2-pyrrolidone (NMP) containing Advent TPS solution (1.9 wt.% in NMP). After being dispersed in an ultrasonic bath for 30 min, the solution was stirred for 12 h with a magnetic stirrer. For preparation of the electrode, the ink was sprayed directly on the gas diffusion layer (SGL GDL 35BC, 350 mm thickness), dried at 80 C for 1 h, and then dried at 160 C for 2 h to remove NMP in a ventilated oven. The cell area was 5 cm2, and the amount of Pt loaded was 0.4 mg cm2 for both anode and cathode electrodes. The Advent TPS membranes were doped in 85% H3PO4 in a closed vessel at 140 C for 19 h. The doping level was calculated using the following equation and was fixed at 185%.
Doping level ¼
10845
Wdoped Wdry 100 Wdry
The completed membrane electrode assemblies (MEAs) were fabricated by sandwiching the doped membrane between two electrodes using an assembling torque of 3 N m. For comparison, the MEAs of LT-PEMFCs were prepared as follows. A commercial Pt/C catalyst from Johnson Matthey Co. was used for both the cathode and the anode. The catalysts were ultrasonically mixed with 5 wt.% Nafion ionomer in isopropanol and then sprayed directly on a Nafion 212 membrane to prepare the MEAs. The cell area was 5 cm2, and the amount of Pt loaded was 0.4 mg cm2.
2.2.
Electrochemical analysis and corrosion test
The polarization curve and cyclic voltammogram (CV) were conducted before and after the corrosion test to examine the effects associated with carbon corrosion. In the case of the LT-PEMFC, the polarization curves were measured at 75 C under 1 atm using humidified O2 (150 mL min1) and H2 (150 mL min1). For the HT-PEMFC, the polarization curves were obtained at a cell temperature of 150 C under 1 atm. The dry H2 was supplied to the anode at a flow rate of 50 mL min1, and dry O2 was supplied to the cathode at a flow rate of 50 mL min1. The CV was performed in the range of 0.05e1.2 V at a sweep rate 50 mV s1 to determine the electrochemically active surface area of Pt. For the carbon corrosion test, a constant potential between 1 V and 1.4 V with reference to the anode was applied to the cathode of the fuel cell for 30 min, and the amount of CO2 produced from the cathode of the fuel cell was monitored as a function of time using on-line mass spectrometry. Hydrogen was supplied to the anode at a flow rate of 20 mL min1, and oxygen was supplied to the cathode at a flow rate of 20 mL min1. In the humidified condition, humidified gases are delivered to the anode and the cathode after passing through a saturator at a different temperature to control the relative humidity (RH) during the carbon corrosion. In the non-humidified condition, gases are directly delivered to the anode and the cathode without passing through the saturator. The carbon corrosion tests were performed at 70 C in the LT-PEMFC and at 150 C in the HT-PEMFC.
3.
Results and discussion
To investigate the electrochemical carbon corrosion in the HTPEMFC, the potential of the oxygen electrode in the fuel cell was controlled from 1.0 V to 1.4 V using a potentiostat to simulate the abnormal operating conditions in fuel cells. As shown in equation (1), the generation of CO2 represents direct evidence of carbon corrosion. Therefore, electrochemical carbon corrosion could be evaluated by measuring the amounts of CO2 emitted during the corrosion test using online mass spectrometry as a function of time. Fig. 1 shows the CO2 concentration profiles during the corrosion test of the HT-PEMFC at different potentials. At 1.0 V, no CO2 gas was detected. CO2 gas started to be measured after 1.1 V. In general, as soon as the potential was applied, the
10846
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 0 8 4 4 e1 0 8 4 9
Fig. 1 e CO2 mass-spectrographic profiles of HT-PEMFCs. Carbon corrosion tests performed at 1.0 V, 1.1 V, 1.2 V, 1.3 V and 1.4 V for 30 min at a cell temperature of 150 C with non-humidified O2 delivered to the cathode.
concentration of CO2 increased initially and then gradually decreased, becoming zero after 30 min when the corrosion test was terminated. It is apparent that the amount of released CO2 increased significantly as the potential increased due to the enhanced overpotential of the carbon corrosion reaction. These observations strongly support that HT-PEMFCs also suffer from electrochemical carbon corrosion, even though they are operated under non-humidified conditions. Fig. 2 shows the polarization curves of the HT-PEMFC before and after the corrosion tests. When 1.0 V was applied to the cathode during the corrosion tests, the performance of the system remained essentially unaltered, whereas the cells tested at 1.1 V, 1.2 V, 1.3 V and 1.4 V showed considerable performance decays of approximately 15%, 22%, 44% and 52% at 0.6 V, respectively. These results are in good agreement with the results of CO2 formation shown in Fig. 1, indicating that electrochemical carbon corrosion is responsible for the decrease in the performance of HT-PEMFCs.
Fig. 2 e Comparison of the MEA performances before and after the corrosion tests at different potentials for the HTPEMFC.
To examine the effect of electrochemical carbon corrosion on the change in the active surface area Pt, CV measurements on MEAs were conducted before and after the corrosion tests. As shown in Fig. 3, the loss of active surface area tends to increase with increasing potential in the corrosion test. This occurs because electrochemical carbon corrosion converts carbon to CO2 gas, resulting in a decrease in the carbon surface available for Pt catalyst loading. Consequently, the corrosion of carbon supports forces Pt particles to aggregate and reduces the electrochemically active surface area of Pt. For comparison, the corrosion test using on-line mass spectrometry was conducted for the LT-PEMFC and is displayed in Fig. 4. Unlike the HT-PEMFC, the corrosion test of the LT-PEMFC was performed at 70 C under fully humidified conditions (RH 100%). The obtained CO2 concentration profiles look similar to those of the HT-PEMFC. However, the decreasing slope with time is lower in the case of the LT-PEMFC because water is supplied continuously through humidified gas during the corrosion test. Below 1.1 V, no CO2 was detected. This indicates that 1.1 V is not high enough for electrochemical carbon corrosion to occur in the LT-PEMFC during the period of the corrosion test. With further increased in the potential, the amount of released CO2 increased gradually. Fig. 5 shows the comparison of the total amount of CO2 production for the HT-PEMFC and the LT-PEMFC with respect to the potential. The figure clearly reveals that the CO2 emission for the HT-PEMFC under non-humidified conditions was higher than that of the LT-PEMFC under fully humidified conditions. The HT-PEMFC demonstrated a lower onset potential for carbon corrosion and a steeper slope with increasing potential. The total amount of CO2 produced at 1.4 V was three times higher in the HT-PEMFC than in the LT-PEMFC. This result suggests that the operating conditions of the HT-PEMFC are more vulnerable to carbon corrosion than those of the LT-PEMFC. According to our previous study, electrochemical carbon corrosion is affected by the water content and the cell temperature [29]. The presence of water is
Fig. 3 e Comparison of cyclic voltammograms before and after the corrosion tests at different potentials in the HTPEMFC. The cyclic voltammograms of the MEAs were measured at a scan rate of 50 mV sL1.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 0 8 4 4 e1 0 8 4 9
Fig. 4 e CO2 mass-spectrographic profiles of LT-PEMFCs. Carbon corrosion tests were performed at 1.1 V, 1.2 V, 1.3 V and 1.4 V for 30 min at a cell temperature of 70 C with humidified O2 delivered to the cathode.
indispensable for electrochemical carbon corrosion. Oxygen functional groups are created on the surface of a carbon support in contact with water and are ultimately converted into CO2. In this regard, the HT-PEMFC was expected to have less carbon corrosion than the LT-PEMFC because the HTPEMFC uses dry gases while the LT-PEMFC uses hydrated gases. However, it is notable that the kinetics of the electrochemical carbon corrosion occurring in the HT-PEMFC is much faster than that in the LT-PEMFC. Because the kinetics of electrochemical carbon corrosion is slow, a high temperature accelerates the rate of electrochemical carbon corrosion. Water exists in the HT-PEMFC as a result of the oxygen reduction reaction at the cathode in the vapor phase due to its high operating temperature. This water vapor reacts with carbon and produced CO2 at an enhanced rate during abnormal operating conditions. Therefore, it is more critical to
Fig. 5 e Comparison of the total amount of CO2 emitted as determined by on-line mass spectrometry at different potentials in both the HT-PEMFC and the LT-PEMFC, respectively.
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Fig. 6 e CO2 mass-spectrographic profiles of HT-PEMFCs. Carbon corrosion tests were performed at a cell temperature of 150 C under different humidification conditions. Humidified O2 (RH 1.6%) was delivered to the cathode after passing through a saturator at 40 C. The potential is fixed at 1.4 V.
control water in the HT-PEMFC to avoid electrochemical carbon corrosion. To investigate the effect of electrochemical carbon corrosion on water in the HT-PEMFC, oxygen gas was passed through the humidifier at 40 C. The relative humidity (RH) is calculated to be 1.6% based on the humidification conditions and the cell temperature. From the mass spectrometry results shown in Fig. 6, the CO2 production at RH 1.6% increases significantly, as high as 6 times compared with the nonhumidified conditions. These results suggest that a small change in water content plays a critical role in the electrochemical carbon corrosion in the HT-PEMFC. Fig. 7 shows the MEA performance before and after carbon corrosion tests with respect to humidification of the feed gases. As predicted from the mass spectrometry results shown in Fig. 6, an additional decrease in the MEA
Fig. 7 e Comparison of the MEA performances before and after the corrosion tests using non-humidified O2 and humidified O2 (RH 1.6%) at 40 C.
10848
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 0 8 4 4 e1 0 8 4 9
performance is observed at RH 1.6%. These results demonstrate that the amount of water supplied to the MEA is an important factor in the electrochemical carbon corrosion in the HT-PEMFC. To date, most durability studies of HT-PEMFCs have occurred under non-humidified conditions [30,31]. However, under actual circumstances, the supplied hydrogen gas could contain a small amount of water if it is produced from an on-site hydrogen generator (e.g., a reformer) [32]. For the cathode, the supplied ambient air can also contain a significant amount of water based on the weather conditions. Therefore, it is essential to remove water from the supply gases to decrease carbon corrosion and enhance the durability of HT-PEMFCs.
4.
Conclusion
The electrochemical corrosion of an HT-PEMFC under nonhumidification conditions was evaluated and compared with that of an LT-PEMFC under full humidification conditions by measuring the CO2 emission using an on-line mass spectrometer. As indicated by the results of the mass spectra, polarization curves and CV, the operating conditions of the HT-PEMFC are more vulnerable to electrochemical carbon corrosion than the operating conditions of the LT-PEMFC. Although the water content, which is indispensable for carbon corrosion, is lower in the HT-PEMFC compared with the LT-PEMFC, the enhanced kinetics due to the elevated temperature in the HT-PEMFC accelerates carbon corrosion significantly. Thus, it is critical to control the water content in the feed gases to reduce electrochemical carbon corrosion and increase the durability of HT-PEMFCs.
Acknowledgements This work was supported by the New & Renewable Energy of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Knowledge Economy (No. 20093021030021) and the Priority Research Centers Program through the National Research Foundation of Korea (2009-0093823) and the National Research Foundation of Korea (NRF-2009-C1AAA001-0092926) funded by the Ministry of Education, Science and Technology.
references
[1] Ong AL, Jung GB, Wu CC, Yan WM. Single-step fabrication of ABPBI-based GDE and study of its MEA characteristics for high-temperature PEM fuel cells. Int J Hydrogen Energy 2010; 35:7866e73. [2] Jun Y, Zarrin H, Fowler M, Chen ZW. Functionalized titania nanotube composite membranes for high temperature proton exchange membrane fuel cells. Int J Hydrogen Energy 2011;36:6073e81. [3] Wannek C, Konradi I, Mergel J, Lehnert W. Redistribution of phosphoric acid in membrane electrode assemblies for hightemperature polymer electrolyte fuel cells. Int J Hydrogen Energy 2009;34:9479e85.
[4] Wang JT, Savinell RF, Wainright J, Litt M, Yu H. A H2/O2 fuel cell using acid doped polybenzimidazole as polymer electrolyte. Electrochim Acta 1996;41:193e7. [5] Dhar HP, Christner LG, Kush AK, Maru HC. Performance study of a fuel cell Pt-on-C anode in presence of CO and CO2, and Calculation of Adsorption Parameters for CO Poisoning. J Electrochem Soc 1986;133:1574e82. [6] Zhang JL, Xie Z, Zhang JJ, Tanga YH, Song CJ, Navessin T, et al. High temperature PEM fuel cells. J Power Sources 2006; 160:872e91. [7] Zhang JL, Tang YH, Song CJ, Zhang JJ. Polybenzimidazolemembrane-based PEM fuel cell in the temperature range of 120-200 degrees C. J Power Sources 2007;172:163e71. [8] Verma A, Scott K. Development of high-temperature PEMFC based on heteropolyacids and polybenzimidazole. J Solid State Electrochem 2010;14:213e9. [9] Navessin T, Eikerling M, Wang QP, Song DT, Liu ZS, Horsfall J, et al. Influence of membrane ion exchange capacity on the catalyst layer performance in an operating PEM fuel cell. J Electrochem Soc 2005;152:A796e805. [10] Hu JW, Zhang HM, Zhai YF, Liu G, Hu J, Yi BL. Performance degradation studies on PBI/H3PO4 high temperature PEMFC and one-dimensional numerical analysis. Electrochim Acta 2006;52:394e401. [11] Oono Y, Fukuda T, Sounai A, Hori M. Influence of operating temperature on cell performance and endurance of high temperature proton exchange membrane fuel cells. J Power Sources 2010;195:1007e14. [12] Wannek C, Kohnen B, Oetien HF, Lippert H, Mergel J. Durability of ABPBI-based MEAs for high temperature PEMFCs at different operating conditions. Fuel Cells 2008;8:87e95. [13] Kongstein OE, Berning T, Børresen B, Seland F, Tunold R. Polymer electrolyte fuel cells based on phosphoric acid doped polybenzimidazole (PBI) membranes. Energy 2007;32: 418e22. [14] Seland F, Berning T, Børresen B, Tunold R. Improving the performance of high-temperature PEM fuel cells based on PBI electrolyte. J Power Sources 2006;160:27e36. [15] Boaventura M, Mendes A. Activation procedures characterization of MEA based on phosphoric acid doped PBI membranes. Int J Hydrogen Energy 2010;35:11649e60. [16] Lobato J, Can˜izares P, Rodrigo MA, Linares JJ. PBI-based polymer electrolyte membranes fuel cells: temperature effects on cell performance and catalyst stability. Electrochim Acta 2007;52:3910e20. [17] Zhai YF, Zhang HM, Liu G, Hu JW, Yi BL. Degradation study on MEA in H3PO4/PBI high-temperature PEMFC life test. J Electrochem Soc 2007;154:B72e6. [18] Cheng X, Chen L, Peng C, Chen Z, Zhang Y, Fan Q. Catalyst Microstructure Examination of PEMFC membrane electrode assemblies vs. Time. J Electrochem Soc 2004;151:A48e52. [19] Antolini E. Formation, microstructural characteristics and stability of carbon supported platinum catalysts for low temperature fuel cells. J Mater Sci 2003;38: 2995e3005. [20] Taniguchi A, Akita T, Yasuda K, Miyazaki Y. Analysis of degradation in PEMFC caused by cell reversal during air starvation. Int J Hydrogen Energy 2008;33:2323e9. [21] Wang J, Yin G, Shao Y, Zhang S, Wang Z, Gao Y. Effect of carbon black support corrosion on the durability of Pt/C catalyst. J Power Sources 2007;171:331e9. [22] Yu X, Ye S. Recent advances in activity and durability enhancement of Pt/C catalytic cathode in PEMFC Part II: degradation mechanism and durability enhancement of carbon supported platinum catalyst. J Power Sources 2007; 172:145e54. [23] Maass S, Finsterwalder F, Frank G, Hartmann R, Merten C. Carbon support oxidation in PEM fuel cell cathodes. J Power Sources 2008;176:444e51.
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[24] Meyers JP, Darling RM. Model of carbon corrosion in PEM fuel cells. J Electrochem Soc 2006;153:A1432e42. [25] Tang H, Qi Z, Ramani M, Elter JF. PEM fuel cell cathode carbon corrosion due to the formation of air/fuel boundary at the anode. J Power Sources 2006;158:1306e12. [26] Silva RA, Hashimoto T, Thompson GE, Rangel CM. Characterization of MEA degradation for an open air cathode PEM fuel cell. Int J Hydrogen Energy 2012;37: 7299e308. [27] Paik CH, Saloka GS, Graham GW. Influence of cyclic operation on PEM fuel cell catalyst stability. Electrochem Solid State Lett 2007;10:B39e42. [28] Zhang S, Yuan X, Wang H, Merida W, Zhu H, Shen J, et al. A review of accelerated stress tests of MEA durability in PEM fuel cells. Int J Hydrogen Energy 2009;34:388e404.
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[29] Lim KH, Oh HS, Jang SE, Ko YJ, Kim HJ, Kim H. Effect of operating conditions on carbon corrosion in polymer electrolyte membrane fuel cells. J Power Sources 2009;193: 575e9. [30] Chen C-Y, Lai W-H. Effects of temperature and humidity on the cell performance and resistance of a phosphoric acid doped polybenzimidazole fuel cell. J Power Sources 2010;195:7152e9. [31] Kwon K, Kim TY, Yoo DY, Hong S-G, Park JO. Maximization of high-temperature proton exchange membrane fuel cell performance with the optimum distribution of phosphoric acid. J Power Sources 2009;188:463e7. [32] Vagia EC, Lemonidou AA. Thermodynamic analysis of hydrogen production via autothermal steam reforming of selected components of aqueous bio-oil fraction. Int J Hydrogen Energy 2008;33:2489e500.
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Electrochemical carbon corrosion in high temperature proton exchange membrane fuel cells Hyung-Suk Oh, Jin-Hee Lee, Hansung Kim* Dept. of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 120-749, Republic of Korea
article info
abstract
Article history:
Electrochemical carbon corrosion occurring in a high temperature proton exchange
Received 8 March 2012
membrane fuel cell (HT-PEMFC) operating under non-humidification conditions was
Received in revised form
investigated by measuring CO2 generation using on-line mass spectrometry and comparing
9 April 2012
the results with a low-temperature proton exchange membrane fuel cell (LT-PEMFC)
Accepted 17 April 2012
operated under fully humidified conditions. The experimental results showed that more
Available online 15 May 2012
CO2 was measured for the HT-PEMFC, indicating that more electrochemical carbon corrosion occurs in HT-PEMFCs. This observation is attributed to the enhanced kinetics of
Keywords:
electrochemical carbon corrosion due to the elevated operating temperature in HT-
High temperature proton exchange
PEMFCs. Additionally, electrochemical carbon corrosion in HT-PEMFCs showed a strong
membrane fuel cell
dependence on water content. Therefore, it is critical to remove the water content in the
Carbon corrosion
supply gases to reduce electrochemical carbon corrosion.
Durability
Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Mass spectrometry
reserved.
Electrocatalysts
1.
Introduction
In recent years, there has been significant interest in the development of high-temperature proton exchange membrane fuel cells (HT-PEMFCs) operating at temperatures greater than 100 C [1e3]. Compared with low-temperature proton exchange membrane fuel cells (LT-PEMFCs) operating at approximately 60e80 C, HT-PEMFCs offer numerous advantages, such as the high tolerance of catalyst to contaminants, improved electrode kinetics, and simplified water and heat management [4e9]. To realize HT-PEMFCs, the largest challenge is replacing the perfluorosulfonic acid membranes (such as Nafion) that are currently used as the proton exchange membrane in LT-PEMFCs. Thus, tremendous effort has focused on the development of proton exchange membranes with high ionic conductivity and chemical and physical stability at elevated temperatures [10e12]. In recent
years, phosphoric acid-doped PBI (polybenzimidazole) has shown not only good proton conductivity but also excellent oxidative and thermal stability and almost zero electroosmotic drag [13e15]. In addition to developing a membrane for high temperature applications, it is critical to demonstrate the durability of HT-PEMFCs for commercial viability. The reduced life time of HT-PEMFCs is considered to be mainly due to the degradation of the electrode and the membrane such as the aggregation of catalyst particles [16] and the depletion of phosphoric acid [17]. However, compared to membranes, relatively little is reported about the degradation mechanism for the electrode, especially for catalyst supports associated with electrochemical carbon corrosion in HTPEMFCs. Recently, electrochemical carbon corrosion has received a considerable amount of attention because this is considered
* Corresponding author. Tel.: þ82 2 2123 5753; fax: þ82 2 312 6401. E-mail address: [email protected] (H. Kim). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2012.04.095
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 0 8 4 4 e1 0 8 4 9
one of the critical factors limiting the durability of PEMFCs [18e22]. The suggested mechanism of carbon corrosion involves carbon reacting with water and generating CO2, as shown in eq. (1). C þ 2H2 O/CO2 þ 4Hþ þ 4e
Eo ¼ 0:207 V vs: NHE
(1)
The equilibrium potential for carbon corrosion is 0.207 V vs. NHE at 25 C [23]. Although this implies that carbon is thermodynamically unstable to electrochemical carbon corrosion, the slow kinetics of this reaction makes it possible to use carbon in PEMFCs. However, it is well known that abnormal operating conditions, such as the air/fuel boundary created during the shut-down/start-up process at the anode, increase the cathodic potential to a level higher than the open circuit voltage [24e26]. Such a high potential quickly corrodes the carbon catalyst support, resulting in the severe loss of the active surface area of catalysts and rapid fuel cell performance degradation [27,28]. However, carbon corrosion research has mainly focused on LT-PEMFCs, and a detailed study of the carbon corrosion in HT-PEMFCs has not been conducted precisely. Our previous study about carbon corrosion in LT-PEMFCs revealed that carbon corrosion is accelerated as the humidity and the cell temperature increase [29]. HT-PEMFCs are generally operated under non-humidified conditions and at higher temperatures than are LT-PEMFCs. Therefore, increasing the cell temperature contributes to accelerating the carbon corrosion, while a non-humidified condition decelerates the carbon corrosion. As a result, it is difficult to predict the behavior of carbon corrosion in the HT-PEMFC. The focus of this work is to evaluate the dependence of electrochemical carbon corrosion in HT-PEMFCs and to compare the results with those of LT-PEMFCs. The influence of the humidity and cell temperature of HT-PEMFCs on the electrochemical carbon corrosion is studied systematically using on-line mass spectrometry and several electrochemical techniques.
2.
Experimental
2.1.
Preparation of the membrane electrode assemblies
High temperature polymer electrolyte membranes (Advent TPS), which were provided by the Advent Technologies Corporation, were used in this research. Catalyst ink was prepared by adding Pt/C catalyst (commercial 40 wt.% Pt from Johnson Matthey) into a solvent of n-methyl-2-pyrrolidone (NMP) containing Advent TPS solution (1.9 wt.% in NMP). After being dispersed in an ultrasonic bath for 30 min, the solution was stirred for 12 h with a magnetic stirrer. For preparation of the electrode, the ink was sprayed directly on the gas diffusion layer (SGL GDL 35BC, 350 mm thickness), dried at 80 C for 1 h, and then dried at 160 C for 2 h to remove NMP in a ventilated oven. The cell area was 5 cm2, and the amount of Pt loaded was 0.4 mg cm2 for both anode and cathode electrodes. The Advent TPS membranes were doped in 85% H3PO4 in a closed vessel at 140 C for 19 h. The doping level was calculated using the following equation and was fixed at 185%.
Doping level ¼
10845
Wdoped Wdry 100 Wdry
The completed membrane electrode assemblies (MEAs) were fabricated by sandwiching the doped membrane between two electrodes using an assembling torque of 3 N m. For comparison, the MEAs of LT-PEMFCs were prepared as follows. A commercial Pt/C catalyst from Johnson Matthey Co. was used for both the cathode and the anode. The catalysts were ultrasonically mixed with 5 wt.% Nafion ionomer in isopropanol and then sprayed directly on a Nafion 212 membrane to prepare the MEAs. The cell area was 5 cm2, and the amount of Pt loaded was 0.4 mg cm2.
2.2.
Electrochemical analysis and corrosion test
The polarization curve and cyclic voltammogram (CV) were conducted before and after the corrosion test to examine the effects associated with carbon corrosion. In the case of the LT-PEMFC, the polarization curves were measured at 75 C under 1 atm using humidified O2 (150 mL min1) and H2 (150 mL min1). For the HT-PEMFC, the polarization curves were obtained at a cell temperature of 150 C under 1 atm. The dry H2 was supplied to the anode at a flow rate of 50 mL min1, and dry O2 was supplied to the cathode at a flow rate of 50 mL min1. The CV was performed in the range of 0.05e1.2 V at a sweep rate 50 mV s1 to determine the electrochemically active surface area of Pt. For the carbon corrosion test, a constant potential between 1 V and 1.4 V with reference to the anode was applied to the cathode of the fuel cell for 30 min, and the amount of CO2 produced from the cathode of the fuel cell was monitored as a function of time using on-line mass spectrometry. Hydrogen was supplied to the anode at a flow rate of 20 mL min1, and oxygen was supplied to the cathode at a flow rate of 20 mL min1. In the humidified condition, humidified gases are delivered to the anode and the cathode after passing through a saturator at a different temperature to control the relative humidity (RH) during the carbon corrosion. In the non-humidified condition, gases are directly delivered to the anode and the cathode without passing through the saturator. The carbon corrosion tests were performed at 70 C in the LT-PEMFC and at 150 C in the HT-PEMFC.
3.
Results and discussion
To investigate the electrochemical carbon corrosion in the HTPEMFC, the potential of the oxygen electrode in the fuel cell was controlled from 1.0 V to 1.4 V using a potentiostat to simulate the abnormal operating conditions in fuel cells. As shown in equation (1), the generation of CO2 represents direct evidence of carbon corrosion. Therefore, electrochemical carbon corrosion could be evaluated by measuring the amounts of CO2 emitted during the corrosion test using online mass spectrometry as a function of time. Fig. 1 shows the CO2 concentration profiles during the corrosion test of the HT-PEMFC at different potentials. At 1.0 V, no CO2 gas was detected. CO2 gas started to be measured after 1.1 V. In general, as soon as the potential was applied, the
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Fig. 1 e CO2 mass-spectrographic profiles of HT-PEMFCs. Carbon corrosion tests performed at 1.0 V, 1.1 V, 1.2 V, 1.3 V and 1.4 V for 30 min at a cell temperature of 150 C with non-humidified O2 delivered to the cathode.
concentration of CO2 increased initially and then gradually decreased, becoming zero after 30 min when the corrosion test was terminated. It is apparent that the amount of released CO2 increased significantly as the potential increased due to the enhanced overpotential of the carbon corrosion reaction. These observations strongly support that HT-PEMFCs also suffer from electrochemical carbon corrosion, even though they are operated under non-humidified conditions. Fig. 2 shows the polarization curves of the HT-PEMFC before and after the corrosion tests. When 1.0 V was applied to the cathode during the corrosion tests, the performance of the system remained essentially unaltered, whereas the cells tested at 1.1 V, 1.2 V, 1.3 V and 1.4 V showed considerable performance decays of approximately 15%, 22%, 44% and 52% at 0.6 V, respectively. These results are in good agreement with the results of CO2 formation shown in Fig. 1, indicating that electrochemical carbon corrosion is responsible for the decrease in the performance of HT-PEMFCs.
Fig. 2 e Comparison of the MEA performances before and after the corrosion tests at different potentials for the HTPEMFC.
To examine the effect of electrochemical carbon corrosion on the change in the active surface area Pt, CV measurements on MEAs were conducted before and after the corrosion tests. As shown in Fig. 3, the loss of active surface area tends to increase with increasing potential in the corrosion test. This occurs because electrochemical carbon corrosion converts carbon to CO2 gas, resulting in a decrease in the carbon surface available for Pt catalyst loading. Consequently, the corrosion of carbon supports forces Pt particles to aggregate and reduces the electrochemically active surface area of Pt. For comparison, the corrosion test using on-line mass spectrometry was conducted for the LT-PEMFC and is displayed in Fig. 4. Unlike the HT-PEMFC, the corrosion test of the LT-PEMFC was performed at 70 C under fully humidified conditions (RH 100%). The obtained CO2 concentration profiles look similar to those of the HT-PEMFC. However, the decreasing slope with time is lower in the case of the LT-PEMFC because water is supplied continuously through humidified gas during the corrosion test. Below 1.1 V, no CO2 was detected. This indicates that 1.1 V is not high enough for electrochemical carbon corrosion to occur in the LT-PEMFC during the period of the corrosion test. With further increased in the potential, the amount of released CO2 increased gradually. Fig. 5 shows the comparison of the total amount of CO2 production for the HT-PEMFC and the LT-PEMFC with respect to the potential. The figure clearly reveals that the CO2 emission for the HT-PEMFC under non-humidified conditions was higher than that of the LT-PEMFC under fully humidified conditions. The HT-PEMFC demonstrated a lower onset potential for carbon corrosion and a steeper slope with increasing potential. The total amount of CO2 produced at 1.4 V was three times higher in the HT-PEMFC than in the LT-PEMFC. This result suggests that the operating conditions of the HT-PEMFC are more vulnerable to carbon corrosion than those of the LT-PEMFC. According to our previous study, electrochemical carbon corrosion is affected by the water content and the cell temperature [29]. The presence of water is
Fig. 3 e Comparison of cyclic voltammograms before and after the corrosion tests at different potentials in the HTPEMFC. The cyclic voltammograms of the MEAs were measured at a scan rate of 50 mV sL1.
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Fig. 4 e CO2 mass-spectrographic profiles of LT-PEMFCs. Carbon corrosion tests were performed at 1.1 V, 1.2 V, 1.3 V and 1.4 V for 30 min at a cell temperature of 70 C with humidified O2 delivered to the cathode.
indispensable for electrochemical carbon corrosion. Oxygen functional groups are created on the surface of a carbon support in contact with water and are ultimately converted into CO2. In this regard, the HT-PEMFC was expected to have less carbon corrosion than the LT-PEMFC because the HTPEMFC uses dry gases while the LT-PEMFC uses hydrated gases. However, it is notable that the kinetics of the electrochemical carbon corrosion occurring in the HT-PEMFC is much faster than that in the LT-PEMFC. Because the kinetics of electrochemical carbon corrosion is slow, a high temperature accelerates the rate of electrochemical carbon corrosion. Water exists in the HT-PEMFC as a result of the oxygen reduction reaction at the cathode in the vapor phase due to its high operating temperature. This water vapor reacts with carbon and produced CO2 at an enhanced rate during abnormal operating conditions. Therefore, it is more critical to
Fig. 5 e Comparison of the total amount of CO2 emitted as determined by on-line mass spectrometry at different potentials in both the HT-PEMFC and the LT-PEMFC, respectively.
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Fig. 6 e CO2 mass-spectrographic profiles of HT-PEMFCs. Carbon corrosion tests were performed at a cell temperature of 150 C under different humidification conditions. Humidified O2 (RH 1.6%) was delivered to the cathode after passing through a saturator at 40 C. The potential is fixed at 1.4 V.
control water in the HT-PEMFC to avoid electrochemical carbon corrosion. To investigate the effect of electrochemical carbon corrosion on water in the HT-PEMFC, oxygen gas was passed through the humidifier at 40 C. The relative humidity (RH) is calculated to be 1.6% based on the humidification conditions and the cell temperature. From the mass spectrometry results shown in Fig. 6, the CO2 production at RH 1.6% increases significantly, as high as 6 times compared with the nonhumidified conditions. These results suggest that a small change in water content plays a critical role in the electrochemical carbon corrosion in the HT-PEMFC. Fig. 7 shows the MEA performance before and after carbon corrosion tests with respect to humidification of the feed gases. As predicted from the mass spectrometry results shown in Fig. 6, an additional decrease in the MEA
Fig. 7 e Comparison of the MEA performances before and after the corrosion tests using non-humidified O2 and humidified O2 (RH 1.6%) at 40 C.
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performance is observed at RH 1.6%. These results demonstrate that the amount of water supplied to the MEA is an important factor in the electrochemical carbon corrosion in the HT-PEMFC. To date, most durability studies of HT-PEMFCs have occurred under non-humidified conditions [30,31]. However, under actual circumstances, the supplied hydrogen gas could contain a small amount of water if it is produced from an on-site hydrogen generator (e.g., a reformer) [32]. For the cathode, the supplied ambient air can also contain a significant amount of water based on the weather conditions. Therefore, it is essential to remove water from the supply gases to decrease carbon corrosion and enhance the durability of HT-PEMFCs.
4.
Conclusion
The electrochemical corrosion of an HT-PEMFC under nonhumidification conditions was evaluated and compared with that of an LT-PEMFC under full humidification conditions by measuring the CO2 emission using an on-line mass spectrometer. As indicated by the results of the mass spectra, polarization curves and CV, the operating conditions of the HT-PEMFC are more vulnerable to electrochemical carbon corrosion than the operating conditions of the LT-PEMFC. Although the water content, which is indispensable for carbon corrosion, is lower in the HT-PEMFC compared with the LT-PEMFC, the enhanced kinetics due to the elevated temperature in the HT-PEMFC accelerates carbon corrosion significantly. Thus, it is critical to control the water content in the feed gases to reduce electrochemical carbon corrosion and increase the durability of HT-PEMFCs.
Acknowledgements This work was supported by the New & Renewable Energy of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Knowledge Economy (No. 20093021030021) and the Priority Research Centers Program through the National Research Foundation of Korea (2009-0093823) and the National Research Foundation of Korea (NRF-2009-C1AAA001-0092926) funded by the Ministry of Education, Science and Technology.
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