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Anode and cathode degradation in a PEFC single cell investigated by electrochemical impedance spectroscopy
Electrochimica Acta 131 (2014) 245–249
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Anode and cathode degradation in a PEFC single cell investigated by electrochemical impedance spectroscopy Weiqi Zhang, Takahiro Maruta, Sayoko Shironita 1 , Minoru Umeda ∗,1 Department of Materials Science and Technology, Faculty of Engineering, Nagaoka University of Technology, 1603-1, Kamitomioka, Nagaoka, Niigata 940-2188, Japan
a r t i c l e
i n f o
Article history: Received 15 October 2013 Received in revised form 16 January 2014 Accepted 10 February 2014 Available online 2 March 2014 Keywords: anode degradation cathode degradation PEFC single cell potential cycling electrochemical impedance spectroscopy
a b s t r a c t The potential cycling of a polymer electrolyte fuel cell (PEFC) incorporating a reference electrode was carried out for 20 h. After the potential cycling, the maximum power density decreased from 56.12 mW cm−2 of the fresh membrane electrode assembly (MEA) to 47.64 mW cm−2 . Electrochemical impedance spectroscopy (EIS) was used to conduct an independent quantitative analysis of the anode and cathode degradation. The EIS results of the anode and cathode revealed the degradation properties after the potential cycling. For the anode, the charge transfer and mass transfer resistances significantly increased after the degradation procedure, indicating that a significant degradation occurred. However, for the cathode, it was demonstrated that a slight degradation occurred, because the charge transfer and mass transfer resistances slightly increased after the degradation procedure. The increase in the charge transfer resistance was attributed to the decrease in the Pt surface area, and the increased mass transfer resistance was considered to be due to the ionomer degradation. Meanwhile, the increase in the membrane resistance and decrease in the adsorption resistance were also observed after the degradation procedure. © 2014 Elsevier Ltd. All rights reserved.
1. Introduction Polymer electrolyte fuel cells (PEFCs) have attracted significant interest due to their high power density, a relatively simple design, quick startup, low noise emissions and high-energy conversion efficiency compared to traditional power sources [1–4]. The performance of the PEFC was drastically improved during the past two decades. Currently, the PEFC is close to the practical stage of commercialization. However, the two major factors limiting the PEFC commercialization are the price and the durability of the membrane electrode assemblies (MEAs) [5–7]. Several attempts have been made to reduce the cost of the MEA, including the synthesis of Pt alloy catalysts, invention of innovative polymer electrolyte membranes, and reduced use of the Pt catalyst via introduction of a novel MEA structure to the three-phase boundary [8–11]. On the other hand, numerous investigations of the MEA degradation have also been performed. Silva et al. [12] observed the thickness variation of the catalytic layer, and cracking, delamination, and catalyst migration. After a 1500-h operation,
∗ Corresponding author. Tel.: +81 258 47 9323; fax: +81 258 47 9323. E-mail address: [email protected] (M. Umeda). 1 ISE member. http://dx.doi.org/10.1016/j.electacta.2014.02.054 0013-4686/© 2014 Elsevier Ltd. All rights reserved.
the power performance decreased by about 34%. Wu et al. [13] found the degradation of the fuel cell performance was mainly attributed to catalyst decay in the first 800 h of the lifetime test, while the subsequent dramatic degradation is likely caused by membrane failure. Shao-Horn et al. [14] investigated the mechanisms of surface area loss of the supported Pt electrocatalysts in a low-temperature fuel cell. The reason was attributed to the loss of Pt from carbon and coarsening of the Pt nanoparticles on carbon. These reports analyzed the performance degradation by ex-situ methods such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). To use these methods, the MEA should be removed from the fuel cell and the results cannot tell us quantitatively which component of the fuel cell contributed on the performance degradation. Therefore, a quantitative study of the performance degradation of different components in the MEA is very important in order to obtain the clues to increase the durability. The electrochemical impedance spectroscopy (EIS) analyses on the PEFC single cell with a reference electrode have been reported in the previous papers [15–17]. As a powerful tool to nondestructively characterize fuel cell performance and provide quantitative information, the main advantage of the EIS is its ability to distinguish the individual contributions of the interfacial charge
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transfer and the mass transport resistances in the catalyst layer and diffusion layer [18]. In the present study, the performance degradation of the PEFC single cell containing a reference electrode was conducted by potential cycling and quantitatively analyzed by measuring the EIS of the anode and cathode to clarify the degradation mechanism.
2. Experimental 2.1. Preparation of the PEFC single cell The MEA (geometric electrode area: 5 cm2 ) used in this study was prepared as follows [19–22]: Nafion 117 (5 cm × 5 cm, DuPont) was used as the polymer electrolyte membrane. The membrane was boiled in a 0.5 mol dm−3 H2 SO4 solution, and then washed twice by boiling in distilled water for 1 h. Commercially available Pt/C (Pt loading: 45.7 wt%, Tanaka Kikinzoku Kogyo Co., Ltd., TEC10E50E) powder was used for both the anode and cathode catalysts. Three drops of Milli-Q water were added to the Pt/C powder and mixed using a ball mill for 5 min after which the mixture was diluted by a mixed solvent of methanol, 2-propanol, and Milli-Q water (weight ratio was 1:1:1). A 5 wt% Nafion 117 solution was added to the diluted mixture so that the weight ratio was 1:6 (Nafion: Pt/C powder), and dispersed using a ball mill for one day. The obtained catalyst slurry was sprayed over a piece of water repellent carbon paper (2.3 × 2.3 cm, TGP-H060, Toray Industries, Inc.), such that the amount of Pt was 1.0 mg cm−2 . Subsequently, the pretreated Nafion 117 membrane was sandwiched between two pieces of the catalyst-coated carbon paper and hot-pressed at 4.5 kN and 140 ◦ C for 10 min. The prepared MEA was installed in a single cell (ElectroChem, Inc., E3156) having a dynamic hydrogen electrode (DHE) as the reference electrode. The DHE consists of a Pt wire and uses the potential of the hydrogen evolution reaction. It was placed in a chamber inside of the separator flowing with the humidified hydrogen [23,24].
Fig. 1. I-V and I-P curves of the Pt/C-based MEA before and after the potential cycling for 20 h.
3. Results and discussion 3.1. MEA degradation under potential cycling Fig. 1 shows the I-V and I-P curves for the anode and cathode of the MEA before and after the potential cycling. The maximum power density decreased from 56.12 mW cm−2 to 47.64 mW cm−2 before and after the potential cycling of the fresh MEA for 20 h, respectively. For the anode, after the potential cycling, it showed a higher overpotential, indicating that degradation of the anode had certainly occurred. For the cathode, after the potential cycling, the onset potential decreased and when the potential is lower than 0.55 V vs. DHE, the current density is significantly lower than that of the fresh cathode, suggesting that the extent of cathode degradation is not as significant as that of the anode. 3.2. EIS measurements of the anode Fig. 2a shows the EIS measurement (symbol) and fitting (line) results before and after the potential cycling. The EIS measurement
2.2. Operation for the MEA degradation Humidified H2 (purity: 99.99%) and humidified O2 (purity: 99.5%) were supplied to the anode and cathode, respectively, at the flow rate of 50 cm3 min−1 by a PEFC power generation unit (HPR1000, FC Development Co., Ltd.) [19,25]. The cell was operated at 40 ◦ C during the experiments. For the MEA degradation, the potential cycling of the cathode was carried out in the potential range of 0.06–1.2 V vs. DHE for 20 h at the scan rate of 10 mV s−1 .
2.3. EIS measurements An impedance analyzer (NF Corporation AS-510-4) was employed to measure the impedance spectra in this study. In contrast to the conventional half cell EIS measurements, this equipment makes it possible to measure the impedance spectra between the anode and DHE, cathode and DHE, anode and cathode in a complete PEFC single cell at the same time. Thus, before and after the degradation, all the AC impedance spectra between the anode and DHE, cathode and DHE, were measured in the galvanostatic mode using by taking 10 points per decade in the frequency range between 100 kHz and 0.1 Hz. The amplitude of the AC signal was 2 mA cm−2 , so that the output AC voltage would be less than 5 mV. During the measurements, the humidified H2 (purity: 99.99%) and humidified O2 (purity: 99.5%) were supplied to the anode and cathode, respectively, at the flow rate of 50 cm3 min−1 .
Fig. 2. (a) Nyquist impedance spectra for the experimental data (symbol) and fit (line) of the anode at 60 mA cm−2 . (b) Equivalent circuit used for fitting.
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247
Table 1 Fitting results of the anode EIS measurements. R (m) R
before degradation
after degradation
Rs (membrane) Rct (charge transfer) Rmt (mass transfer) Rad (adsorption) Anode reaction resistance (combinetion of Rct , Rmt , Rad )
64.22 28.13 8.3 69.94 28.38
94 110.8 64.58 19.47 81.17
was carried out at a current density of 60 mA cm−2 . The equivalent circuit used for the fitting is shown in Fig. 2b. In this circuit, Rs includes the electrolyte membrane, gas diffusion layer (GDL) and interfacial resistances, in which the membrane resistance is dominant among these resistances. The constant phase element (CPE) is used in the model in place of a capacitor to compensate for the nonhomogeneity in the system [26]. The charge transfer resistance, Rct , is for the hydrogen oxidation reaction (HOR) process. The second branch consists of a capacitance, Cac , and resistance, Rmt , for the mass transfer process. A resistance Rad , as previously proposed is for the intermediates adsorption process [27–29]. An inductive loop was observed in the low frequency region, which was fitted by the element of L2 . Up to now, the inductive behavior is still controversial. It has been reported to be due to adsorption/desorption of the intermediate species [15,27,30,31]. Water transport in a proton exchange membrane has also been reported to lead to an inductive loop by simulation [29]. Naito pointed out the inductive loops in the low frequency range are related to the mobile carrier ratio of the anion and cation [32]. The fitting results of the resistances are listed in the Table 1. Significant changes are observed in Rct and Rmt ; Rct increased from 28.13 to 110.8 m and Rmt increased from 8.3 to 64.58 m after the potential cycling. In addition, Rs also shows a significant increase, indicating the degradation of the electrolyte membrane and GDL. Moreover, Rad dramatically decreased after the potential cycling. Subsequently, the anode reaction resistance (ARR) was calculated by [(Rct × Rad )/(Rct + Rad ) + Rmt ]. ARR increased from 28.38 to 81.17 m after the potential cycling, indicating a significant degradation of the anode. However, the significant degradation cannot be observed in the I-V curve (Fig. 1), suggesting that the EIS is an effective technique to investigate the electrode degradation in the PEFC single cell. The degradation mechanism of the anode is illustrated using the schematic figures of the anode before and after the degradation as shown in Fig. 3. After the potential cycling, the charge transfer resistance significantly increased, suggesting the decrease of the active Pt surface area due to the growth, dissolution and aggregation of the Pt particles in the catalyst layer [11]. Moreover, the Pt particles of the Pt/C electrocatalyst departed away and cracks are formed [11,12] in the catalyst layer also increased the charge transfer resistances of the anode. On the other hand, after potential
Fig. 3. Schematic figures of the anode before and after the degradation.
Fig. 4. (a) Nyquist impedance spectra for the experimental data (symbol) and fit (line) of the cathode at 60 mA cm−2 . Equivalent circuit used for fitting (b) before and (c) after the potential cycling.
cycling, the mass transfer resistance significantly increased, which is considered to be due to the degradation of the ionomer that affects the proton conductivity and results in a decrease of the corresponding fuel cell performance [33]. A degradation of the polymer electrolyte membrane is widely accepted; oxygen crossover from the cathode to anode leads to the formation of H2 O2 . The H2 O2 or OH radical then attacks the membrane and degradation of the membrane actually occurs [34]. After the degradation, the adsorption resistance decreased, suggesting that the intermediates are generated in a large amount and easily adsorb on the Pt surface.
3.3. EIS measurements of the cathode The EIS of the cathode was measured and Fig. 4a shows the EIS measurement (symbol) results and fitting (line) results at 60 mA cm−2 before and after the potential cycling. The two different equivalent circuits shown in Fig. 4b and Fig. 4c were used to obtain the best fits of the EIS spectra before and after the potential cycling, because an inductive behavior was observed before the potential cycling but disappeared after the potential cycling. In the circuit before the potential cycling (Fig. 4b), Rs is attributed to the membrane resistance. The charge transfer resistance, Rct is based on the oxygen reduction reaction (ORR) process. The CPEca is the capacitive component. A resistance Rad , as previously proposed, is for
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Table 2 Fitting results of the cathode EIS measurements.
4. Conclusions
R (m)
R
before degradation
after degradation
Rs (membrane) Rct (charge transfer) Rmt (mass transfer) Rad (adsorption) Cathode reaction resistance (combinetion of Rct , Rmt , Rad )
117 125.3 255.8 169.4 327.8
147 144.6 291.1 42.82 478.5
the intermediate adsorption process [27–29]. The second branch consists of a capacitance, Ccc , and resistance, Rmt , for the mass transfer process. An inductive loop was observed in the low frequency region, which was previously mentioned. In the circuit after the potential cycling (Fig. 4c), the first branch involves a branch for the adsorption process consisting of a capacitance, Cad and a resistance Rad . The fitting results of the resistances are listed in the Table 2. The charge transfer resistance of the ORR is much higher than that of the HOR due to the slower rate of the ORR. After the potential cycling, Rct slightly increased from 125.3 to 144.6 m, and Rmt increased from 255.8 to 291.1 m. In addition, Rs also showed a small increase. However, Rad dramatically decreased after the potential cycling. Subsequently, the cathode reaction resistance (CRR) was calculated by [(Rct × Rad )/(Rct + Rad ) + Rmt ] before the potential cycling and by (Rct + Rad + Rmt ) after the potential cycling. The CRR increased from 327.8 to 478.5 m after the potential cycling, indicating that degradation of the cathode had occurred. Fig. 5 shows schematic illustrations of the cathode before and after the degradation. As shown in Table 2, after the potential cycling, the charge transfer resistance increased, resulted from the degradation of the Pt/C catalyst. The electric charge transmission slows down due to diffusion of the Pt particles into the boundary surface of the electrode or electrolyte; this also affects the rate of the reduction reaction [35]. The nanoparticle catalysts tend to aggregate due to their high specific surface energy, and this tendency becomes clearer under more severe operating conditions [36]. Also, the growth and dissolution of the Pt particles in the catalyst layer resulted in a decreased active surface area for the electrochemical reaction [11]. Thus, the decrease in the electrocatalyst activation area increases the charge transfer resistance. The increase in the mass transfer resistance is due to the degradation of the ionomer [33]. Also, some other researchers reported that the water accumulation can block the electrocatalytic active sites, then the ORR may be suppressed. This effect increases the mass transfer resistance [37]. As is mentioned in 3.2, the degradation of the membrane is due to the attack of the H2 O2 or OH radical, which are generated during the ORR process. After the degradation, similar to the anode, the adsorption resistance decreased, suggesting that the intermediates are generated in a large amount and easily adsorbed on the Pt surface.
Fig. 5. Schematic figures of the cathode before and after the degradation.
The potential cycling of a PEFC single cell was carried out at 40 ◦ C for 20 h. The maximum power density decreased from 56.12 to 47.64 mW cm−2 . The EIS spectra at the anode and cathode were independently measured before and after the degradation procedure of the potential cycling. The EIS results demonstrated the degradation behavior of the anode and cathode. However, the extent of cathode degradation is not as significant as that of the anode. For the anode, the charge transfer and mass transfer resistances significantly increased after the degradation procedure, indicating that a significant degradation had occurred. However, for the cathode, it was demonstrated that a slight degradation occurred because the charge transfer and mass transfer resistances slightly increased after the degradation procedure. The increase in the charge transfer resistance is attributed to the decrease in the Pt surface area due to the fact that the Pt particles aggregate, diffuse, dissolve and grow larger. The increased mass transfer resistance is attributed to the ionomer degradation. Furthermore, the increase in the membrane resistance and the decrease in the adsorption resistance were also observed after the degradation procedure. Acknowledgment This work was supported by JSPS KAKENHI Grant Number 24350091. References [1] K.S. Jeong, B.S. Oh, J. Power Soures 105 (2002) 58. [2] P. Ekdunge, M. Raberg, Int. J. Hydrogen Energy 23 (1998) 385. [3] L.P.L. Carrette, K.A. Friedrich, M. Huber, U. Stimming, Phys. Chem. Chem. Phys. 3 (2001) 320. [4] C. Coutanceau, R.K. Koffi, J.M. Leger, K. Marestin, R. Mercier, C. Nayoze, P. Capron, J. Power Soures 160 (2006) 334. [5] X.G. Yang, F.Y. Zhang, A.L. Lubawy, C.Y. Wang, Electrochem, Solid State Lett. 7 (2004) A408. [6] S.G. Chalk, J.F. Milier, J. Power Sources 159 (2006) 73. [7] S. Mommura, K. Kawahara, T. Shimohira, Y. Teraoka, J. Electrochem. Soc. 155 (2008) A29. [8] H.A. Gasteiger, S.S. Kocha, B. Sompalli, F.T. Wagner, Appl. Catal. B. Environ. 56 (2005) 9. [9] K. Sawai, N. Suzuki, J. Electrochem. Soc. 151 (2004) A2132. [10] H. Zhong, H. Zhang, G. Liu, Y. Liang, J. Hu, B. Yi, Electrochem. Commun. 8 (2006) 707. [11] R. Lin, B. Li, Y.P. Hou, J.M. Ma, Int. J. Hydrogen Energy 34 (2009) 2369. [12] R.A. Silva, T. Hashimoto, G.E. Thompson, C.M. Rangel, Int. J. Hydrogen Energy 37 (2012) 7299. [13] J.F. Wu, X.Z. Yuan, J.J. Martin, H.J. Wang, D.J. Yang, J.L. Qiao, J.X. Ma, J. Power Sources 195 (2010) 1171. [14] Y. Shao-Horn, W.C. Sheng, S. Chen, P.J. Ferreira, E.F. Holby, D. Morgan, Top. Catal. 46 (2007) 285. [15] Y.C. Liu, X.P. Qiu, W.T. Zhu, G.S. Wu, J. Power Sources 114 (2003) 10. [16] G.C. Li, P.G. Pickup, Electrochim. Acta 49 (2004) 4119. [17] P. Hartmann, N. Zamel, D. Gerteisen, J. Power Sources 241 (2013) 127. [18] S. Asghari, A. Mokmeli, M. Samavati, Int. J. Hydrogen Energy 35 (2010) 9283. [19] S. Shironita, K. Karasuda, M. Sato, M. Umeda, J. Power Sources 228 (2013) 68. [20] S. Shironita, K. Karasuda, K. Sato, M. Umeda, J. Power Sources 240 (2013) 404. [21] M. Inoue, T. Iwasaki, K. Sayama, M. Umeda, J. Power Sources 195 (2010) 5986. [22] M. Umeda, K. Sayama, M. Inoue, J. Renewable Sustainable Energy 3 (2011) 043107. [23] M. Umeda, K. Sayama, T. Maruta, M. Inoue, Ionics 19 (2013) 623. [24] C. Guzman, A. Alvarez, S. Rivas, S.M. Duron-Torres, A.U. Chavez-Ramirez, J. Ledesma-Garcia, L.G. Arriaga, Int. J. Electrochem. Sci. 8 (2013) 8893. [25] M. Umeda, S. Kawaguchi, I. Uchida, Jpn. J. Appl. Phys. 45 (2006) 6049. [26] M.K. Jeon, J.Y. Won, K.S. Oh, K.R. Lee, S.I. Woo, Electrochim. Acta 53 (2007) 447. [27] H. Nakajima, T. Konomi, T. Kitahara, H. Tachibana, J. Fuel Cell Sci. Technol 5 (2008) 041013-1. [28] R.D. Armstrong, M. Henderson, J. Electroanal. Chem. Interfacial Electrochem. 39 (1972) 81. [29] D. Harrington, B. Conway, Electrochim. Acta 32 (1987) 1703. [30] J. Otomo, X. Li, T. Kobayashi, C.J. Wen, H. Nagamoto, H. Takahashi, J. Electroanal. Chem. 573 (2004) 99.
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Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Anode and cathode degradation in a PEFC single cell investigated by electrochemical impedance spectroscopy Weiqi Zhang, Takahiro Maruta, Sayoko Shironita 1 , Minoru Umeda ∗,1 Department of Materials Science and Technology, Faculty of Engineering, Nagaoka University of Technology, 1603-1, Kamitomioka, Nagaoka, Niigata 940-2188, Japan
a r t i c l e
i n f o
Article history: Received 15 October 2013 Received in revised form 16 January 2014 Accepted 10 February 2014 Available online 2 March 2014 Keywords: anode degradation cathode degradation PEFC single cell potential cycling electrochemical impedance spectroscopy
a b s t r a c t The potential cycling of a polymer electrolyte fuel cell (PEFC) incorporating a reference electrode was carried out for 20 h. After the potential cycling, the maximum power density decreased from 56.12 mW cm−2 of the fresh membrane electrode assembly (MEA) to 47.64 mW cm−2 . Electrochemical impedance spectroscopy (EIS) was used to conduct an independent quantitative analysis of the anode and cathode degradation. The EIS results of the anode and cathode revealed the degradation properties after the potential cycling. For the anode, the charge transfer and mass transfer resistances significantly increased after the degradation procedure, indicating that a significant degradation occurred. However, for the cathode, it was demonstrated that a slight degradation occurred, because the charge transfer and mass transfer resistances slightly increased after the degradation procedure. The increase in the charge transfer resistance was attributed to the decrease in the Pt surface area, and the increased mass transfer resistance was considered to be due to the ionomer degradation. Meanwhile, the increase in the membrane resistance and decrease in the adsorption resistance were also observed after the degradation procedure. © 2014 Elsevier Ltd. All rights reserved.
1. Introduction Polymer electrolyte fuel cells (PEFCs) have attracted significant interest due to their high power density, a relatively simple design, quick startup, low noise emissions and high-energy conversion efficiency compared to traditional power sources [1–4]. The performance of the PEFC was drastically improved during the past two decades. Currently, the PEFC is close to the practical stage of commercialization. However, the two major factors limiting the PEFC commercialization are the price and the durability of the membrane electrode assemblies (MEAs) [5–7]. Several attempts have been made to reduce the cost of the MEA, including the synthesis of Pt alloy catalysts, invention of innovative polymer electrolyte membranes, and reduced use of the Pt catalyst via introduction of a novel MEA structure to the three-phase boundary [8–11]. On the other hand, numerous investigations of the MEA degradation have also been performed. Silva et al. [12] observed the thickness variation of the catalytic layer, and cracking, delamination, and catalyst migration. After a 1500-h operation,
∗ Corresponding author. Tel.: +81 258 47 9323; fax: +81 258 47 9323. E-mail address: [email protected] (M. Umeda). 1 ISE member. http://dx.doi.org/10.1016/j.electacta.2014.02.054 0013-4686/© 2014 Elsevier Ltd. All rights reserved.
the power performance decreased by about 34%. Wu et al. [13] found the degradation of the fuel cell performance was mainly attributed to catalyst decay in the first 800 h of the lifetime test, while the subsequent dramatic degradation is likely caused by membrane failure. Shao-Horn et al. [14] investigated the mechanisms of surface area loss of the supported Pt electrocatalysts in a low-temperature fuel cell. The reason was attributed to the loss of Pt from carbon and coarsening of the Pt nanoparticles on carbon. These reports analyzed the performance degradation by ex-situ methods such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). To use these methods, the MEA should be removed from the fuel cell and the results cannot tell us quantitatively which component of the fuel cell contributed on the performance degradation. Therefore, a quantitative study of the performance degradation of different components in the MEA is very important in order to obtain the clues to increase the durability. The electrochemical impedance spectroscopy (EIS) analyses on the PEFC single cell with a reference electrode have been reported in the previous papers [15–17]. As a powerful tool to nondestructively characterize fuel cell performance and provide quantitative information, the main advantage of the EIS is its ability to distinguish the individual contributions of the interfacial charge
246
W. Zhang et al. / Electrochimica Acta 131 (2014) 245–249
transfer and the mass transport resistances in the catalyst layer and diffusion layer [18]. In the present study, the performance degradation of the PEFC single cell containing a reference electrode was conducted by potential cycling and quantitatively analyzed by measuring the EIS of the anode and cathode to clarify the degradation mechanism.
2. Experimental 2.1. Preparation of the PEFC single cell The MEA (geometric electrode area: 5 cm2 ) used in this study was prepared as follows [19–22]: Nafion 117 (5 cm × 5 cm, DuPont) was used as the polymer electrolyte membrane. The membrane was boiled in a 0.5 mol dm−3 H2 SO4 solution, and then washed twice by boiling in distilled water for 1 h. Commercially available Pt/C (Pt loading: 45.7 wt%, Tanaka Kikinzoku Kogyo Co., Ltd., TEC10E50E) powder was used for both the anode and cathode catalysts. Three drops of Milli-Q water were added to the Pt/C powder and mixed using a ball mill for 5 min after which the mixture was diluted by a mixed solvent of methanol, 2-propanol, and Milli-Q water (weight ratio was 1:1:1). A 5 wt% Nafion 117 solution was added to the diluted mixture so that the weight ratio was 1:6 (Nafion: Pt/C powder), and dispersed using a ball mill for one day. The obtained catalyst slurry was sprayed over a piece of water repellent carbon paper (2.3 × 2.3 cm, TGP-H060, Toray Industries, Inc.), such that the amount of Pt was 1.0 mg cm−2 . Subsequently, the pretreated Nafion 117 membrane was sandwiched between two pieces of the catalyst-coated carbon paper and hot-pressed at 4.5 kN and 140 ◦ C for 10 min. The prepared MEA was installed in a single cell (ElectroChem, Inc., E3156) having a dynamic hydrogen electrode (DHE) as the reference electrode. The DHE consists of a Pt wire and uses the potential of the hydrogen evolution reaction. It was placed in a chamber inside of the separator flowing with the humidified hydrogen [23,24].
Fig. 1. I-V and I-P curves of the Pt/C-based MEA before and after the potential cycling for 20 h.
3. Results and discussion 3.1. MEA degradation under potential cycling Fig. 1 shows the I-V and I-P curves for the anode and cathode of the MEA before and after the potential cycling. The maximum power density decreased from 56.12 mW cm−2 to 47.64 mW cm−2 before and after the potential cycling of the fresh MEA for 20 h, respectively. For the anode, after the potential cycling, it showed a higher overpotential, indicating that degradation of the anode had certainly occurred. For the cathode, after the potential cycling, the onset potential decreased and when the potential is lower than 0.55 V vs. DHE, the current density is significantly lower than that of the fresh cathode, suggesting that the extent of cathode degradation is not as significant as that of the anode. 3.2. EIS measurements of the anode Fig. 2a shows the EIS measurement (symbol) and fitting (line) results before and after the potential cycling. The EIS measurement
2.2. Operation for the MEA degradation Humidified H2 (purity: 99.99%) and humidified O2 (purity: 99.5%) were supplied to the anode and cathode, respectively, at the flow rate of 50 cm3 min−1 by a PEFC power generation unit (HPR1000, FC Development Co., Ltd.) [19,25]. The cell was operated at 40 ◦ C during the experiments. For the MEA degradation, the potential cycling of the cathode was carried out in the potential range of 0.06–1.2 V vs. DHE for 20 h at the scan rate of 10 mV s−1 .
2.3. EIS measurements An impedance analyzer (NF Corporation AS-510-4) was employed to measure the impedance spectra in this study. In contrast to the conventional half cell EIS measurements, this equipment makes it possible to measure the impedance spectra between the anode and DHE, cathode and DHE, anode and cathode in a complete PEFC single cell at the same time. Thus, before and after the degradation, all the AC impedance spectra between the anode and DHE, cathode and DHE, were measured in the galvanostatic mode using by taking 10 points per decade in the frequency range between 100 kHz and 0.1 Hz. The amplitude of the AC signal was 2 mA cm−2 , so that the output AC voltage would be less than 5 mV. During the measurements, the humidified H2 (purity: 99.99%) and humidified O2 (purity: 99.5%) were supplied to the anode and cathode, respectively, at the flow rate of 50 cm3 min−1 .
Fig. 2. (a) Nyquist impedance spectra for the experimental data (symbol) and fit (line) of the anode at 60 mA cm−2 . (b) Equivalent circuit used for fitting.
W. Zhang et al. / Electrochimica Acta 131 (2014) 245–249
247
Table 1 Fitting results of the anode EIS measurements. R (m) R
before degradation
after degradation
Rs (membrane) Rct (charge transfer) Rmt (mass transfer) Rad (adsorption) Anode reaction resistance (combinetion of Rct , Rmt , Rad )
64.22 28.13 8.3 69.94 28.38
94 110.8 64.58 19.47 81.17
was carried out at a current density of 60 mA cm−2 . The equivalent circuit used for the fitting is shown in Fig. 2b. In this circuit, Rs includes the electrolyte membrane, gas diffusion layer (GDL) and interfacial resistances, in which the membrane resistance is dominant among these resistances. The constant phase element (CPE) is used in the model in place of a capacitor to compensate for the nonhomogeneity in the system [26]. The charge transfer resistance, Rct , is for the hydrogen oxidation reaction (HOR) process. The second branch consists of a capacitance, Cac , and resistance, Rmt , for the mass transfer process. A resistance Rad , as previously proposed is for the intermediates adsorption process [27–29]. An inductive loop was observed in the low frequency region, which was fitted by the element of L2 . Up to now, the inductive behavior is still controversial. It has been reported to be due to adsorption/desorption of the intermediate species [15,27,30,31]. Water transport in a proton exchange membrane has also been reported to lead to an inductive loop by simulation [29]. Naito pointed out the inductive loops in the low frequency range are related to the mobile carrier ratio of the anion and cation [32]. The fitting results of the resistances are listed in the Table 1. Significant changes are observed in Rct and Rmt ; Rct increased from 28.13 to 110.8 m and Rmt increased from 8.3 to 64.58 m after the potential cycling. In addition, Rs also shows a significant increase, indicating the degradation of the electrolyte membrane and GDL. Moreover, Rad dramatically decreased after the potential cycling. Subsequently, the anode reaction resistance (ARR) was calculated by [(Rct × Rad )/(Rct + Rad ) + Rmt ]. ARR increased from 28.38 to 81.17 m after the potential cycling, indicating a significant degradation of the anode. However, the significant degradation cannot be observed in the I-V curve (Fig. 1), suggesting that the EIS is an effective technique to investigate the electrode degradation in the PEFC single cell. The degradation mechanism of the anode is illustrated using the schematic figures of the anode before and after the degradation as shown in Fig. 3. After the potential cycling, the charge transfer resistance significantly increased, suggesting the decrease of the active Pt surface area due to the growth, dissolution and aggregation of the Pt particles in the catalyst layer [11]. Moreover, the Pt particles of the Pt/C electrocatalyst departed away and cracks are formed [11,12] in the catalyst layer also increased the charge transfer resistances of the anode. On the other hand, after potential
Fig. 3. Schematic figures of the anode before and after the degradation.
Fig. 4. (a) Nyquist impedance spectra for the experimental data (symbol) and fit (line) of the cathode at 60 mA cm−2 . Equivalent circuit used for fitting (b) before and (c) after the potential cycling.
cycling, the mass transfer resistance significantly increased, which is considered to be due to the degradation of the ionomer that affects the proton conductivity and results in a decrease of the corresponding fuel cell performance [33]. A degradation of the polymer electrolyte membrane is widely accepted; oxygen crossover from the cathode to anode leads to the formation of H2 O2 . The H2 O2 or OH radical then attacks the membrane and degradation of the membrane actually occurs [34]. After the degradation, the adsorption resistance decreased, suggesting that the intermediates are generated in a large amount and easily adsorb on the Pt surface.
3.3. EIS measurements of the cathode The EIS of the cathode was measured and Fig. 4a shows the EIS measurement (symbol) results and fitting (line) results at 60 mA cm−2 before and after the potential cycling. The two different equivalent circuits shown in Fig. 4b and Fig. 4c were used to obtain the best fits of the EIS spectra before and after the potential cycling, because an inductive behavior was observed before the potential cycling but disappeared after the potential cycling. In the circuit before the potential cycling (Fig. 4b), Rs is attributed to the membrane resistance. The charge transfer resistance, Rct is based on the oxygen reduction reaction (ORR) process. The CPEca is the capacitive component. A resistance Rad , as previously proposed, is for
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Table 2 Fitting results of the cathode EIS measurements.
4. Conclusions
R (m)
R
before degradation
after degradation
Rs (membrane) Rct (charge transfer) Rmt (mass transfer) Rad (adsorption) Cathode reaction resistance (combinetion of Rct , Rmt , Rad )
117 125.3 255.8 169.4 327.8
147 144.6 291.1 42.82 478.5
the intermediate adsorption process [27–29]. The second branch consists of a capacitance, Ccc , and resistance, Rmt , for the mass transfer process. An inductive loop was observed in the low frequency region, which was previously mentioned. In the circuit after the potential cycling (Fig. 4c), the first branch involves a branch for the adsorption process consisting of a capacitance, Cad and a resistance Rad . The fitting results of the resistances are listed in the Table 2. The charge transfer resistance of the ORR is much higher than that of the HOR due to the slower rate of the ORR. After the potential cycling, Rct slightly increased from 125.3 to 144.6 m, and Rmt increased from 255.8 to 291.1 m. In addition, Rs also showed a small increase. However, Rad dramatically decreased after the potential cycling. Subsequently, the cathode reaction resistance (CRR) was calculated by [(Rct × Rad )/(Rct + Rad ) + Rmt ] before the potential cycling and by (Rct + Rad + Rmt ) after the potential cycling. The CRR increased from 327.8 to 478.5 m after the potential cycling, indicating that degradation of the cathode had occurred. Fig. 5 shows schematic illustrations of the cathode before and after the degradation. As shown in Table 2, after the potential cycling, the charge transfer resistance increased, resulted from the degradation of the Pt/C catalyst. The electric charge transmission slows down due to diffusion of the Pt particles into the boundary surface of the electrode or electrolyte; this also affects the rate of the reduction reaction [35]. The nanoparticle catalysts tend to aggregate due to their high specific surface energy, and this tendency becomes clearer under more severe operating conditions [36]. Also, the growth and dissolution of the Pt particles in the catalyst layer resulted in a decreased active surface area for the electrochemical reaction [11]. Thus, the decrease in the electrocatalyst activation area increases the charge transfer resistance. The increase in the mass transfer resistance is due to the degradation of the ionomer [33]. Also, some other researchers reported that the water accumulation can block the electrocatalytic active sites, then the ORR may be suppressed. This effect increases the mass transfer resistance [37]. As is mentioned in 3.2, the degradation of the membrane is due to the attack of the H2 O2 or OH radical, which are generated during the ORR process. After the degradation, similar to the anode, the adsorption resistance decreased, suggesting that the intermediates are generated in a large amount and easily adsorbed on the Pt surface.
Fig. 5. Schematic figures of the cathode before and after the degradation.
The potential cycling of a PEFC single cell was carried out at 40 ◦ C for 20 h. The maximum power density decreased from 56.12 to 47.64 mW cm−2 . The EIS spectra at the anode and cathode were independently measured before and after the degradation procedure of the potential cycling. The EIS results demonstrated the degradation behavior of the anode and cathode. However, the extent of cathode degradation is not as significant as that of the anode. For the anode, the charge transfer and mass transfer resistances significantly increased after the degradation procedure, indicating that a significant degradation had occurred. However, for the cathode, it was demonstrated that a slight degradation occurred because the charge transfer and mass transfer resistances slightly increased after the degradation procedure. The increase in the charge transfer resistance is attributed to the decrease in the Pt surface area due to the fact that the Pt particles aggregate, diffuse, dissolve and grow larger. The increased mass transfer resistance is attributed to the ionomer degradation. Furthermore, the increase in the membrane resistance and the decrease in the adsorption resistance were also observed after the degradation procedure. Acknowledgment This work was supported by JSPS KAKENHI Grant Number 24350091. References [1] K.S. Jeong, B.S. Oh, J. Power Soures 105 (2002) 58. [2] P. Ekdunge, M. Raberg, Int. J. Hydrogen Energy 23 (1998) 385. [3] L.P.L. Carrette, K.A. Friedrich, M. Huber, U. Stimming, Phys. Chem. Chem. Phys. 3 (2001) 320. [4] C. Coutanceau, R.K. Koffi, J.M. Leger, K. Marestin, R. Mercier, C. Nayoze, P. Capron, J. Power Soures 160 (2006) 334. [5] X.G. Yang, F.Y. Zhang, A.L. Lubawy, C.Y. Wang, Electrochem, Solid State Lett. 7 (2004) A408. [6] S.G. Chalk, J.F. Milier, J. Power Sources 159 (2006) 73. [7] S. Mommura, K. Kawahara, T. Shimohira, Y. Teraoka, J. Electrochem. Soc. 155 (2008) A29. [8] H.A. Gasteiger, S.S. Kocha, B. Sompalli, F.T. Wagner, Appl. Catal. B. Environ. 56 (2005) 9. [9] K. Sawai, N. Suzuki, J. Electrochem. Soc. 151 (2004) A2132. [10] H. Zhong, H. Zhang, G. Liu, Y. Liang, J. Hu, B. Yi, Electrochem. Commun. 8 (2006) 707. [11] R. Lin, B. Li, Y.P. Hou, J.M. Ma, Int. J. Hydrogen Energy 34 (2009) 2369. [12] R.A. Silva, T. Hashimoto, G.E. Thompson, C.M. Rangel, Int. J. Hydrogen Energy 37 (2012) 7299. [13] J.F. Wu, X.Z. Yuan, J.J. Martin, H.J. Wang, D.J. Yang, J.L. Qiao, J.X. Ma, J. Power Sources 195 (2010) 1171. [14] Y. Shao-Horn, W.C. Sheng, S. Chen, P.J. Ferreira, E.F. Holby, D. Morgan, Top. Catal. 46 (2007) 285. [15] Y.C. Liu, X.P. Qiu, W.T. Zhu, G.S. Wu, J. Power Sources 114 (2003) 10. [16] G.C. Li, P.G. Pickup, Electrochim. Acta 49 (2004) 4119. [17] P. Hartmann, N. Zamel, D. Gerteisen, J. Power Sources 241 (2013) 127. [18] S. Asghari, A. Mokmeli, M. Samavati, Int. J. Hydrogen Energy 35 (2010) 9283. [19] S. Shironita, K. Karasuda, M. Sato, M. Umeda, J. Power Sources 228 (2013) 68. [20] S. Shironita, K. Karasuda, K. Sato, M. Umeda, J. Power Sources 240 (2013) 404. [21] M. Inoue, T. Iwasaki, K. Sayama, M. Umeda, J. Power Sources 195 (2010) 5986. [22] M. Umeda, K. Sayama, M. Inoue, J. Renewable Sustainable Energy 3 (2011) 043107. [23] M. Umeda, K. Sayama, T. Maruta, M. Inoue, Ionics 19 (2013) 623. [24] C. Guzman, A. Alvarez, S. Rivas, S.M. Duron-Torres, A.U. Chavez-Ramirez, J. Ledesma-Garcia, L.G. Arriaga, Int. J. Electrochem. Sci. 8 (2013) 8893. [25] M. Umeda, S. Kawaguchi, I. Uchida, Jpn. J. Appl. Phys. 45 (2006) 6049. [26] M.K. Jeon, J.Y. Won, K.S. Oh, K.R. Lee, S.I. Woo, Electrochim. Acta 53 (2007) 447. [27] H. Nakajima, T. Konomi, T. Kitahara, H. Tachibana, J. Fuel Cell Sci. Technol 5 (2008) 041013-1. [28] R.D. Armstrong, M. Henderson, J. Electroanal. Chem. Interfacial Electrochem. 39 (1972) 81. [29] D. Harrington, B. Conway, Electrochim. Acta 32 (1987) 1703. [30] J. Otomo, X. Li, T. Kobayashi, C.J. Wen, H. Nagamoto, H. Takahashi, J. Electroanal. Chem. 573 (2004) 99.
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