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The effects of relative humidity on the performances of PEMFC MEAs with various Nafion® ionomer contents
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The effects of relative humidity on the performances of PEMFC MEAs with various Nafion ionomer contents Kun-Ho Kim a,b, Kwan-Young Lee b,**, Sang-Yeop Lee a, EunAe Cho a, Tae-Hoon Lim a, Hyoung-Juhn Kim a, Sung Pil Yoon a, Sae Hoon Kim c, Tae Won Lim c, Jong Hyun Jang a,* a
Fuel Cell Center, Korea Institute of Science and Technology, Seongbuk-gu, Seoul 136-791, Republic of Korea Department of Chemical and Biological Engineering, Korea University, Seongbuk-gu, Seoul 136-701, Republic of Korea c Fuel Cell Vehicle Team 1, Corporate Research and Development Division, Hyundai-Kia Motors, Gyeonggi-do 446-912, Republic of Korea b
article info
abstract
Article history:
For PEMFC operation, water management is very important to provide both sufficient
Received 24 December 2009
proton conductivity and mass transport. Therefore, in this study, the effect of the relative
Accepted 13 April 2010
humidity (38e87%) on cell performance is examined for PEMFC MEAs with various Nafion
Available online 21 May 2010
ionomer contents. The MEAs were fabricated using a CCM (catalyst-coated membrane) spraying method. As the relative humidity of the cathodes (RHC) increases, the cell volt-
Keywords:
ages at 0.4 and 1.2 A/cm2 increased for a MEA with 20 wt% ionomer. This can be explained
PEMFC (polymer electrolyte
in terms of the expansion of active sites with enhanced ionic conductivity (activation
membrane fuel cell)
overpotential). In contrast, with a higher RHC value, the cell voltages for 35 wt% ionomer
MEA (membrane-electrode
(more hydrophilic) gradually decreased as a result of slower gas transport (concentration
assembly)
overpotential). For MEAs with intermediate ionomer contents, 25 and 30 wt%, the cell
Relative humidity
voltages at 0.4 A/cm2 showed maximum values at a RHC of 67%, at which point the mass
Nafion ionomer
transport begins to be the more dominant factor. The highest unit cell performance was
CCM (catalyst-coated membrane)
observed in a MEA with an ionomer content of 25 wt% at a RHC of 59% and in a MEA at
spraying method
20 wt% ionomer content at a higher humidity of 87%. ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Proton exchange membrane fuel cells (PEMFCs) are considered to be clean and efficient electrochemical energy sources for mobile devices, residential power generation, and transportation applications due to their high energy density, zero emissions, quick start-up and system robustness. However, for the commercialization of PEMFCs, a number of technical problems should be overcome, including one related to water management [1,2]. In conventional PEMFCs, the perfluorosulfonic acid (PFSA) based membranes should be hydrated to provide sufficient proton conductivity. Therefore, gases are
usually humidified before supplied to the anodes and cathodes in order to maintain high levels of fuel cell performance and to avoid dehydration of the membrane and ionomers [3]. In addition, the water generated during the oxygen reduction reaction at cathodes should be properly removed to prevent flooding within the catalyst layers. Otherwise, the remaining liquid water in the pores of the catalyst layers and the gas diffusion layers (GDL) would slow down the gas transport, eventually decreasing the cell voltage due to the large concentration overpotential. As the water generation rate is proportional to the current density, a performance decrease with flooding would be more significant at a high current
* Corresponding author. Tel.: þ82 2 958 5287. ** Corresponding author. Tel.: þ82 2 3290 3299. E-mail addresses: [email protected] (K.-Y. Lee), [email protected] (J.H. Jang). 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.04.082
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Fig. 1 e Schematic procedures for the fabrication of PEMFC MEAs by a CCM (catalyst-coated membrane) spraying method: The MEAs were designated according to their Nafion ionomer contents, as follows: S20, S25, S30, and S35.
sprayed directly onto the Nafion membranes (Fig. 1). The catalyst inks were prepared by mixing a carbon-supported platinum catalyst (Tanaka K. K., Pt 45.5 wt%), isopropyl alcohol (Baker Analyzed HPLC Reagent), and Nafion ionomer dispersion in Hþ form (DuPont Inc., EW1100, 5 wt%). This was followed by sonication for 1 h. The ionomer content was controlled to be 20, 25, 30, and 35 wt% compared to the total solid content. The catalyst ink was then sprayed with an automated system (Gunman, Jeewon Hi-tech) onto a NRE-212 membrane (DuPont Inc.). After drying for 1 h, the catalyst ink was sprayed onto the other side of the membrane and the coated membrane was dried again for 8 h. Platinum loading was controlled at 0.4 mg/cm2 for both the anode and cathode layers. The active electrode area was 25 cm2.
2.2.
Single cell test of prepared MEAs
The prepared MEAs were assembled in a custom-made single cell frame with gas diffusion media (Sigracet 10BC, SGL
100
EIS
90 80
RHC / %
density [4,5]. Moreover, electro-osmosis under an applied electric field across the membrane, where protons move through the membrane to pull water molecules from the anode to the cathode, would contribute to the cathode flooding [6e8]. Therefore, to ensure the high performance of PEMFCs, the water generation rate and water removal rate should be suitably balanced to provide sufficient proton conductivity and gas transfer rates simultaneously. While the water generation rate is mainly determined by current density, the water removal rate depends on both the relative humidity (RH) of the supplied gases and the water retention properties of the catalyst layers. Regarding the effects of RH on PEMFC performance levels, several studies have been conducted for MEAs with a fixed ionomer content [9e13]. When oxygen or pressurized air was supplied to the cathodes, the cell performance gradually increased with the relative humidity of the cathode gas, giving maximum performance at a fully hydrated condition [9e11]. However, the use of ambient air would be more desirable for practical applications, such as fuel cell vehicles and residential power generators, despite the fact that the concentration overpotential would be more significant, especially at high current ranges. With unpressurized air as a cathode gas, it has been reported that the cell voltage was the highest at an intermediate humidity of the supplied air, and the performance drops near a fully humidified condition [12,13]. The water retention properties of catalyst layers, as well as the proton conductivity, depend on the amount of the Nafion type perfluorosulfonated ionomers included in catalyst layers. If the Nafion ionomer content is not sufficient to form a threedimensional network, some part of the catalyst layers will be inaccessible to protons, decreasing the number of active sites for electrochemical reactions. In contrast, if the ionomer content is high, electronic conduction paths (Pt/C) and gas transport channels (pores) would be blocked by ionomer material and/or by flooded water within the hydrophilic pores, respectively. To optimize the ionomer content, several studies have been carried out using oxygen or pressurized air as cathode reactants, where the oxygen supply to the active sites would not limit the cell performance [14e18]. Although some authors have examined the effect of different ionomer contents with atmospheric air, the PEMFC performances were characterized only under a fully or highly humidified gas condition [19,20,22]. However, with atmospheric air as a cathode gas, the highest cell performance can be obtained at a relative humidity of less than 100% [12,13], also depending on the ionomer content. In this study, the effect of the cathodic relative humidity was examined for PEMFC MEAs with various Nafion ionomer contents. Therefore, the combined effect of the activation overpotential and concentration overpotential on the fuel cell performance could be analyzed under practical fuel cell operation conditions while providing optimum ionomer content according to the relative humidity conditions.
70
IV, EIS, CV
60 50
Activation
40 1.2 A / cm
0.4 A / cm
30 20 10
2.
Experimental
2.1.
MEA fabrication by a CCM spraying method
The MEAs were fabricated using a CCM (catalyst-coated membrane) spraying method, where the catalyst inks were
0
20
40
60
80
100
120
140
160
180
Time / hr
Fig. 2 e Schematic of the relative humidity conditions during activation, initial characterization (polarization, EIS, and CV) and constant current operation at 0.4 and 1.2 A/cm2.
200
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Fig. 3 e (a) Cross-sectional and (b) surface SEM images of the S25 MEA.
Carbon Inc.) and Teflon gaskets. The single cell was then installed in a fuel cell test station (CES-F-S08008, CNL) equipped with an electronic loader (ESL-300Z, ELP Electronic), and operated at 80 C under atmospheric pressure conditions. Using bubbler-type humidifiers, the relative humidity of cathode gases (air and nitrogen) was held between 38 and 87%, whereas the relative humidity of the hydrogen was fixed at 84%. Fig. 2 shows the experimental sequence employed in this study to analyze the effect of the cathodic relative humidity (RHC). First, each MEA was activated under a constant voltage of 0.4 V for 20 h, where humidified air (RH 59%, 365 sccm) and hydrogen (RH 84%, 1160 sccm) were supplied to the cathodes and anodes, respectively. The initial performances were subsequently evaluated by recording the polarization curves with the current increase while the cathodic and anodic gas flow rates were controlled to make a stoichiometric ratio of 1.5 and 2.0, respectively. Electrochemical impedance spectroscopy (EIS) was carried out at 0.85 V in an ac frequency range of 10 kHze0.01 Hz and at an ac amplitude of 5 mV (IM6, Zahnerelektrik). After the cathode gas was switched to nitrogen, cyclic voltammetry (CV) was carried out between 0.05 and 1.2 V (scan rate: 50 mV/s) to measure the electrochemical active surface area (EAS) of the cathode catalyst layer. After the electrochemical characterizations, the single cell was operated under constant current mode. First, the current
1.0
0.8
E/V
0.7 0.6
Results and discussion
In this study, the effect of the cathodic relative humidity (RHC) on the fuel cell performance was evaluated for PEMFC MEAs with various Nafion ionomer contents. To do this, MEAs were fabricated by a CCM spray technique so as to form symmetrical porous catalyst layers (Fig. 3). The MEAs were designated according to the ionomer content: S20, S25, S30, and S35. For each MEA, the initial performance was evaluated by polarization experiments at a RHC 59%. Humidity dependency was then monitored under constant current operation at 0.4 and 1.2 A/cm2 (RHC: 38e87%). In Fig. 4, ieV curves (polarization) and cell voltage values at 0.4 and 1.2 A/cm2 (constant current operation) are presented for S20 and S35. For every MEAs, the open-circuit potential was higher than 0.96 V,
b
S20 RHC 38% RHC 48% RHC 52% RHC 59% RHC 66% RHC 87%
0.9
3.
0.8 0.7
0.5
0.6 0.5
0.4
0.4
0.3
0.3
0.2
0.2
0.1
0.0
0.2
0.4
0.6
0.8
1.0 -2
i / A cm
1.2
1.4
S35 RHC 38% RHC 48% RHC 52% RHC 59% RHC 66% RHC 87%
1.0 0.9
E/V
a
density was fixed at 0.4 A/cm2 and the relative humidity of air was decreased to 38%. After the single cell was stabilized for 12 h, the voltage value was recorded and the relative humidity was increased to 48%. In the same manner, stabilized voltage values at 0.4 A/cm2 were collected at various cathodic relative humidity levels of 38, 48, 52, 59, 66 and 87%. Second, the current density was increased to 1.2 A/cm2 and stabilized voltages were measured as a function of the RHC. Finally, EIS was carried out again at a relative humidity of 87%.
1.6
0.1
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
-2
i / A cm
Fig. 4 e Polarization curves at a RHC of 59% and at voltages values at 0.4 and 1.2 A/cm2 at a RHC between 38 and 87% for the (a) S20 and (b) S35 MEA.
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RHC 59% S20 S25 S30 S35
0.9 0.8
E/V
0.7 0.6 0.5 0.4 0.3 0.2 0.1
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
-2
i / A cm
Fig. 5 e Polarization curves at a RHC of 59% for S20, S25, S30, and S35.
0.76
0.74
S20 S25 S30 S35
b
0.6
0.5
S20 S25 S30 S35
-2
-2
a E @ 0.4 A cm / V
confirming that the unit cells were operating without significant gas crossover (Fig. 5). The cell performance levels of the four MEAs were similar at a low current density. However, as the current increased, the MEAs with larger amounts of Nafion ionomers (S30 and S35) showed more rapid voltage decreases. Generally, as the current increases, the single cell voltage drops due to the limited rate of electrochemical reactions (activation overpotential), voltage drops that typically occur with resistive components such as membranes and electric wires (Ohmic loss), and the limited transport rate of reactant gases to the electrochemical active sites within porous catalyst layers (concentration overpotential). As shown in Fig. 4, under constant current operation, the cell voltage of S20 gradually increased with the cathodic humidity as follows: 0.684 (38%) to 0.728 V (87%) at 0.4 A/cm2 and 0.368 (38%) to 0.435 V (87%) at 1.2 A/cm2. In contrast, S35 showed a voltage decrease as the humidity increased: 0.717 (38%) to 0.688 V (87%) at 0.4 A/cm2 and 0.423 (38%) to 0.232 V (87%) at 1.2 A/cm2. With higher cathodic humidity, the water content in the cathode electrodes will gradually increase as the rate of water evaporation decreases, whereas the effect of enhanced back diffusion is comparatively insignificant [21].
For a detailed analysis, the data from the constant current operations at 0.4 and 1.2 A/cm2 are summarized as a function of the relative humidity in Fig. 6. At 0.4 A/cm2, the cell voltage of S20 gradually increased as the relative humidity increased to 87% (0.684 / 0.728 V) while S25 and S30 showed maximum cell voltages (S25: 0.734 V; S30: 0.716 V) at a relative humidity of 66% (Fig. 6a). For S35, in contrast, at a higher humidity level the cell voltage gradually decreased from 0.717 V to 0.688 V. This different dependency on the relative humidity can be explained in terms of the combined effect of ionic resistance (activation overpotential) and flooding (concentration overpotential) within the catalyst layers. First, at a low humidity, due to the high ionic resistance of ionomer networks, some of the platinum catalysts would not be utilized for oxygen reduction reactions. As the humidity increases, the ionic resistance will decrease and, in turn, the active zone would expand to decrease the activation overpotential. Second, as the air humidity increases, the pores would be more flooded due to the lower water removal rate while the water generation rate is fixed (0.4 A/cm2). As a result, the concentration overpotential would be larger with the higher humidity. At a low ionomer content (S20), the activation overpotential seems to be a major factor as the ionic resistance is high, resulting in a gradual increase of the cell voltage with the humidity. For S35, due to high content of Nafion ionomer, ionic resistance is relatively low throughout the overall humidity range, but the flooding would be severe due to the hydrophilic characteristic. Therefore, the cell voltage gradually decreased with the humidity due to the increase of the concentration overpotential. In the case of the intermediate ionomer content (S25 and S30), a maximum voltage was observed at 67%, where the concentration overpotential begins to take precedence over the activation overpotential in terms of influencing the overall performance. At a higher current density, the effect of water flooding would be more significant because a larger amount of gas should be transported within the porous catalyst layer. Moreover, the water generation rate would increase in proportion to the current density. In Fig. 6b, the cell voltages at 1.2 A/cm2 are presented as a function of the relative humidity. The S20 MEA continues to show a gradual voltage increase
E @ 1.2 A cm / V
1.0
0.72
0.70
0.4
0.3
0.68 30
40
50
60
70
RHC / %
80
90
0.2 30
40
50
60
70
80
90
RHC / %
Fig. 6 e Variation of the cell voltages as a function of the relative humidity at a current density of (a) 0.4 A/cm2 and (b) 1.2 A/cm2.
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0.76
0.74
b
0.6
0.5
RHC 59% RHC 87%
-2
-2
RHC 59% RHC 87%
E @ 1.2 A cm / V
a E @ 0.4 A cm / V
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0.72
0.70
0.4
0.3
0.68 20
25
30
0.2
35
Ionomer content / wt. %
20
25
30
35
Ionomer content / wt. %
Fig. 7 e The cell voltages values at an intermediate humidity level (59%, closed circles) and at high humidity (87%, open circles) according to the Nafion ionomer content at a current density of (a) 0.4 A/cm2 and (b) 1.2 A/cm2.
with the relative humidity, confirming that the activation overpotential is more significant and showing only a minor effect of flooding in this case. However, for the S25 and S30 MEAs, the cell voltages gradually decreased as the humidity increased, showing that the concentration overpotential became the dominant factor that determines the overall humidity dependency of the MEAs. It should be noted that the degree of water flooding at various levels of relative humidity can be influenced by the characteristics of the micro-porous layers and the gas diffusion layers. In Fig. 7, the cell voltages are plotted as a function of the ionomer content. At a current density of 0.4 A/cm2, S25 showed the highest performance at an intermediate humidity of 59%, whereas at RHC 87%, the cell voltage was highest for S20 and gradually decreased with a higher ionomer content (Fig. 7a). This trend also would come from the different humidity effects, as explained earlier. For MEAs with a low ionomer content (S20 and S25), a humidity increase (59 / 87%) had a positive effect on the cell performance, lowering the activation overpotential. In contrast, at a higher ionomer content (S30 and S35), the concentration
a
1.4
2
1.0 0.8 0.6
0.8 0.6 0.4
0.2
0.2
0.2
0.4
0.6
0.8
1.0
1.2 2
ZRe / Ω cm
1.4
1.6
S20 S25 S30 S35
1.0
0.4
0.0 0.0
RHC 87%
1.2
- ZIm / Ω cm
2
1.2
- ZIm / Ω cm
b
S35 RHC 59% RHC 87%
1.4
overpotential would increase with a higher humidity level and the activation overpotential change would be comparatively insignificant. At a higher current density of 1.2 A/cm2, the negative effect of higher humidity, due to flooding, was more clearly observed. When the relative humidity was increased from 59% to 87%, the cell voltage dropped considerably for S25 (50 mV), S30 (83 mV), and S35 (126 mV), whereas the cell voltage of S20 was maintained at a nearly constant value. The performance decrease observed in S35 at a higher humidity was also confirmed by electrochemical impedance spectroscopy. The polarization resistance, as determined from the distance between the y-intercepts, increased with the relative humidity, as follows: 0.74 (59%) / 1.30 U cm2 (RHC 87%). Even in the high voltage region, the concentration overpotential was found to be significant, especially for S35 with a high ionomer content, as shown in Figs. 6 and 7. Therefore, it appears that the semi-circular shape is composed of resistive terms from both the charge transfer reaction and the mass transport, where the increased polarization resistance comes from the slowed gas transfer within
1.8
0.0 0.0
0.2
0.4
0.6
0.8
1.0
2
1.2
1.4
1.6
1.8
ZRe / Ω cm
Fig. 8 e Nyquist plots of impedance data measured at 0.85 V (a) for S35 at relative humidity levels of 59 and 87%, and (b) for S20, S25, S30, and S35 at a RHC of 87%.
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40
Acknowledgements
20
This work was supported by the New and Renewable Energy R&D Program and National RD&D Organization for Hydrogen and Fuel Cell under the Ministry of Knowledge Economy, Republic of Korea, as part of the “Development of 200 kW Class PEMFC System for Bus” (2005-N-F12-P-01). It was also supported by a grant from the Korea Institute of Science and Technology as part of its ‘‘Development of Fuel Cell PowerPacks for Portable Power Generation’’ program.
-2
i / mA cm
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0 -20 S20 S25 S30 S35
-40
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
E/V
references
Fig. 9 e Cyclic voltammograms for S20, S25, S30 and S35.
the catalyst layers. In addition, as shown in Fig. 8b, the polarization resistance levels gradually increased with the ionomer content, as follows: 0.85 (S20), 1.09 (S25), 1.24 (S30), and 1.29 U cm2 (S35). In Fig. 9, the cyclic voltammograms measured under nitrogen (cathodes) are presented for MEAs with various ionomer contents. When inert nitrogen gas instead of air or oxygen is supplied, an oxygen reduction reaction does not occur on the cathode sides. Instead, proton adsorption/ desorption on the platinum surface can be monitored at potentials lower than 0.4 V. From the integrated charge for hydrogen adsorption/desorption, the EAS (electrochemically active surface area) of the platinum catalysts can subsequently be calculated. The calculated EAS values were 87 (S20), 102 (S25), 98 (S30), and 82 m2/g (S35), which are inversely proportional to the charge transfer resistances. These CV results suggest that the polarization resistance in EIS was not determined solely by the charge transfer resistance, even at a high voltage of 0.85 V. Therefore, at a high humidity of 87%, the cell performance and the polarization resistance were influenced considerably by the concentration overpotentials.
4.
Conclusion
In initial polarization experiments (RHC 59%), a voltage drop due to the mass transport limitation commonly appeared for MEAs with high ionomer contents (30 and 35 wt%). Accordingly, the effect of the concentration overpotential with flooding was stronger with a higher ionomer content and at a higher relative humidity. A gradual voltage decrease with the humidity increase was observed for S35 at 0.4 A/cm2 and for S25, S30, and S35 at 1.2 A/cm2. Due to the combined effect of the relative humidity and Nafion ionomer content, the highest voltages were obtained with the S25 MEA at an intermediate humidity of 59% and with the S20 MEA at a higher humidity of 87%, at both 0.4 and 1.2 A/cm2. Therefore, the effect of relative humidity, which is dependent on the Nafion ionomer contents, should be importantly considered in developing MEAs.
[1] Litster S, Mclean G. PEM fuel cell electrodes. J Power Sources 2004;130:61e76. [2] Mehta V, Cooper JS. Review and analysis of PEM fuel cell design and manufacturing. J Power Sources 2003;114:32e53. [3] Zawodzinski Jr TA, Springer TE, Uribe F, Gottesfeld S. Characterization of polymer electrolytes for fuel cell applications. Solid State Ionics 1993;60:199e211. [4] Schmittinger W, Vahidi A. A review of the main parameters influencing long-term performance and durability of PEM fuel cells. J Power Sources 2008;180(1):1e14. [5] Li H, Tang Y, Wang Z, Shi Z, Wu S, Song D, et al. A review of water flooding issues in the proton exchange membrane fuel cell. J Power Sources 2008;178(1):103e17. [6] Park YH, Caton JA. An experimental investigation of electroosmotic drag coefficients in a polymer electrolyte membrane fuel cell. Int J Hydrogen Energy 2008;33:7513e20. [7] Neyerlin KC, Gasteiger HA, Mittelsteadt CK, Jorne J, Gu W. Effect of relative humidity on oxygen reduction kinetics in a PEMFC. J Electrochem Soc 2005;152(6):A1073e80. [8] Xu H, Song Y, Kunz HR, Fenton JM. Effect of elevated temperature and reduced relative humidity on ORR kinetics for PEM fuel cells. J Electrochem Soc 2005;152(9):A1828e36. [9] Wang L, Husar A, Zhou T, Liu H. A parametric study of PEM fuel cell performances. Int J Hydrogen Energy 2003;28: 1263e72. [10] Williams MV, Kunz HR, Fenton JM. Operation of Nafionbased PEM fuel cells with no external humidification: influence of operating conditions and gas diffusion layers. J Power Sources 2004;135:122e34. [11] Amirinejad M, Rowshanzamir S, Eikani MH. Effects of operating parameters on performance of a proton exchange membrane fuel cell. J Power Sources 2006;161:872e5. [12] Park GG, Sohn YJ, Yang TH, Yoon YG, Lee WY, Kim CS. Effect of PTFE contents in the gas diffusion media on the performance of PEMFC. J Power Sources 2004;131:182e7. [13] Yan Q, Toghiani H, Wu J. Investigation of water transport through membrane in a PEM fuel cell by water balance experiments. J Power Sources 2006;158:316e25. [14] Uchida M, Aoyama Y, Eda N, Ohta A. Investigation of the microstructure in the catalyst layer and effects of both perfluorosulfonate ionomer and PTFE-loaded carbon on the catalyst layer of polymer electrolyte fuel cells. J Electrochem Soc 1995;142(12):4143e9. [15] Paganin VA, Ticianelli EA, Gonzalez ER. Development and electrochemical studies of gas diffusion electrodes for polymer electrolyte fuel cells. J Appl Electrochem 1996;26: 297e304. [16] Passalacqua E, Lufrano F, Squadrito G, Patti A, Giorgi L. Nafion content in the catalyst layer of polymer electrolyte fuel cells: effects on structure and performance. Electrochim Acta 2001;46:799e805.
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[17] Song JM, Cha SY, Lee WM. Optimal composition of polymer electrolyte fuel cell electrodes determined by the AC impedance method. J Power Sources 2001;94:78e84. [18] Sasikumar G, Ihm JW, Ryu H. Optimum Nafion content in PEM fuel cell electrodes. Electrochim Acta 2004;50:601e5. [19] Qi Z, Kaufman A. Low Pt loading high performance cathodes for PEM fuel cells. J Power Sources 2003;113:37e43. [20] Frey T, Linardi M. Effects of membrane electrode assembly preparation on the polymer electrolyte
membrane fuel cell performance. Electrochim Acta 2004; 50:99e105. [21] Nguyen TV, White RE. A water and heat management model for proton-exchange-membrane fuel cells. J Electrochem Soc 1993;140:2178e86. [22] Kim K-H, Lee K-Y, Kim H-J, Cho EA, Lee S-Y, Lim T-H, et al. The effects of Nafion ionomer content in PEMFC MEAs prepared by a catalyst-coated membrane (CCM) spraying method. Int J Hydrogen Energy 2010;35:2119e26.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
The effects of relative humidity on the performances of PEMFC MEAs with various Nafion ionomer contents Kun-Ho Kim a,b, Kwan-Young Lee b,**, Sang-Yeop Lee a, EunAe Cho a, Tae-Hoon Lim a, Hyoung-Juhn Kim a, Sung Pil Yoon a, Sae Hoon Kim c, Tae Won Lim c, Jong Hyun Jang a,* a
Fuel Cell Center, Korea Institute of Science and Technology, Seongbuk-gu, Seoul 136-791, Republic of Korea Department of Chemical and Biological Engineering, Korea University, Seongbuk-gu, Seoul 136-701, Republic of Korea c Fuel Cell Vehicle Team 1, Corporate Research and Development Division, Hyundai-Kia Motors, Gyeonggi-do 446-912, Republic of Korea b
article info
abstract
Article history:
For PEMFC operation, water management is very important to provide both sufficient
Received 24 December 2009
proton conductivity and mass transport. Therefore, in this study, the effect of the relative
Accepted 13 April 2010
humidity (38e87%) on cell performance is examined for PEMFC MEAs with various Nafion
Available online 21 May 2010
ionomer contents. The MEAs were fabricated using a CCM (catalyst-coated membrane) spraying method. As the relative humidity of the cathodes (RHC) increases, the cell volt-
Keywords:
ages at 0.4 and 1.2 A/cm2 increased for a MEA with 20 wt% ionomer. This can be explained
PEMFC (polymer electrolyte
in terms of the expansion of active sites with enhanced ionic conductivity (activation
membrane fuel cell)
overpotential). In contrast, with a higher RHC value, the cell voltages for 35 wt% ionomer
MEA (membrane-electrode
(more hydrophilic) gradually decreased as a result of slower gas transport (concentration
assembly)
overpotential). For MEAs with intermediate ionomer contents, 25 and 30 wt%, the cell
Relative humidity
voltages at 0.4 A/cm2 showed maximum values at a RHC of 67%, at which point the mass
Nafion ionomer
transport begins to be the more dominant factor. The highest unit cell performance was
CCM (catalyst-coated membrane)
observed in a MEA with an ionomer content of 25 wt% at a RHC of 59% and in a MEA at
spraying method
20 wt% ionomer content at a higher humidity of 87%. ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Proton exchange membrane fuel cells (PEMFCs) are considered to be clean and efficient electrochemical energy sources for mobile devices, residential power generation, and transportation applications due to their high energy density, zero emissions, quick start-up and system robustness. However, for the commercialization of PEMFCs, a number of technical problems should be overcome, including one related to water management [1,2]. In conventional PEMFCs, the perfluorosulfonic acid (PFSA) based membranes should be hydrated to provide sufficient proton conductivity. Therefore, gases are
usually humidified before supplied to the anodes and cathodes in order to maintain high levels of fuel cell performance and to avoid dehydration of the membrane and ionomers [3]. In addition, the water generated during the oxygen reduction reaction at cathodes should be properly removed to prevent flooding within the catalyst layers. Otherwise, the remaining liquid water in the pores of the catalyst layers and the gas diffusion layers (GDL) would slow down the gas transport, eventually decreasing the cell voltage due to the large concentration overpotential. As the water generation rate is proportional to the current density, a performance decrease with flooding would be more significant at a high current
* Corresponding author. Tel.: þ82 2 958 5287. ** Corresponding author. Tel.: þ82 2 3290 3299. E-mail addresses: [email protected] (K.-Y. Lee), [email protected] (J.H. Jang). 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.04.082
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Fig. 1 e Schematic procedures for the fabrication of PEMFC MEAs by a CCM (catalyst-coated membrane) spraying method: The MEAs were designated according to their Nafion ionomer contents, as follows: S20, S25, S30, and S35.
sprayed directly onto the Nafion membranes (Fig. 1). The catalyst inks were prepared by mixing a carbon-supported platinum catalyst (Tanaka K. K., Pt 45.5 wt%), isopropyl alcohol (Baker Analyzed HPLC Reagent), and Nafion ionomer dispersion in Hþ form (DuPont Inc., EW1100, 5 wt%). This was followed by sonication for 1 h. The ionomer content was controlled to be 20, 25, 30, and 35 wt% compared to the total solid content. The catalyst ink was then sprayed with an automated system (Gunman, Jeewon Hi-tech) onto a NRE-212 membrane (DuPont Inc.). After drying for 1 h, the catalyst ink was sprayed onto the other side of the membrane and the coated membrane was dried again for 8 h. Platinum loading was controlled at 0.4 mg/cm2 for both the anode and cathode layers. The active electrode area was 25 cm2.
2.2.
Single cell test of prepared MEAs
The prepared MEAs were assembled in a custom-made single cell frame with gas diffusion media (Sigracet 10BC, SGL
100
EIS
90 80
RHC / %
density [4,5]. Moreover, electro-osmosis under an applied electric field across the membrane, where protons move through the membrane to pull water molecules from the anode to the cathode, would contribute to the cathode flooding [6e8]. Therefore, to ensure the high performance of PEMFCs, the water generation rate and water removal rate should be suitably balanced to provide sufficient proton conductivity and gas transfer rates simultaneously. While the water generation rate is mainly determined by current density, the water removal rate depends on both the relative humidity (RH) of the supplied gases and the water retention properties of the catalyst layers. Regarding the effects of RH on PEMFC performance levels, several studies have been conducted for MEAs with a fixed ionomer content [9e13]. When oxygen or pressurized air was supplied to the cathodes, the cell performance gradually increased with the relative humidity of the cathode gas, giving maximum performance at a fully hydrated condition [9e11]. However, the use of ambient air would be more desirable for practical applications, such as fuel cell vehicles and residential power generators, despite the fact that the concentration overpotential would be more significant, especially at high current ranges. With unpressurized air as a cathode gas, it has been reported that the cell voltage was the highest at an intermediate humidity of the supplied air, and the performance drops near a fully humidified condition [12,13]. The water retention properties of catalyst layers, as well as the proton conductivity, depend on the amount of the Nafion type perfluorosulfonated ionomers included in catalyst layers. If the Nafion ionomer content is not sufficient to form a threedimensional network, some part of the catalyst layers will be inaccessible to protons, decreasing the number of active sites for electrochemical reactions. In contrast, if the ionomer content is high, electronic conduction paths (Pt/C) and gas transport channels (pores) would be blocked by ionomer material and/or by flooded water within the hydrophilic pores, respectively. To optimize the ionomer content, several studies have been carried out using oxygen or pressurized air as cathode reactants, where the oxygen supply to the active sites would not limit the cell performance [14e18]. Although some authors have examined the effect of different ionomer contents with atmospheric air, the PEMFC performances were characterized only under a fully or highly humidified gas condition [19,20,22]. However, with atmospheric air as a cathode gas, the highest cell performance can be obtained at a relative humidity of less than 100% [12,13], also depending on the ionomer content. In this study, the effect of the cathodic relative humidity was examined for PEMFC MEAs with various Nafion ionomer contents. Therefore, the combined effect of the activation overpotential and concentration overpotential on the fuel cell performance could be analyzed under practical fuel cell operation conditions while providing optimum ionomer content according to the relative humidity conditions.
70
IV, EIS, CV
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40 1.2 A / cm
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2.
Experimental
2.1.
MEA fabrication by a CCM spraying method
The MEAs were fabricated using a CCM (catalyst-coated membrane) spraying method, where the catalyst inks were
0
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Fig. 2 e Schematic of the relative humidity conditions during activation, initial characterization (polarization, EIS, and CV) and constant current operation at 0.4 and 1.2 A/cm2.
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Fig. 3 e (a) Cross-sectional and (b) surface SEM images of the S25 MEA.
Carbon Inc.) and Teflon gaskets. The single cell was then installed in a fuel cell test station (CES-F-S08008, CNL) equipped with an electronic loader (ESL-300Z, ELP Electronic), and operated at 80 C under atmospheric pressure conditions. Using bubbler-type humidifiers, the relative humidity of cathode gases (air and nitrogen) was held between 38 and 87%, whereas the relative humidity of the hydrogen was fixed at 84%. Fig. 2 shows the experimental sequence employed in this study to analyze the effect of the cathodic relative humidity (RHC). First, each MEA was activated under a constant voltage of 0.4 V for 20 h, where humidified air (RH 59%, 365 sccm) and hydrogen (RH 84%, 1160 sccm) were supplied to the cathodes and anodes, respectively. The initial performances were subsequently evaluated by recording the polarization curves with the current increase while the cathodic and anodic gas flow rates were controlled to make a stoichiometric ratio of 1.5 and 2.0, respectively. Electrochemical impedance spectroscopy (EIS) was carried out at 0.85 V in an ac frequency range of 10 kHze0.01 Hz and at an ac amplitude of 5 mV (IM6, Zahnerelektrik). After the cathode gas was switched to nitrogen, cyclic voltammetry (CV) was carried out between 0.05 and 1.2 V (scan rate: 50 mV/s) to measure the electrochemical active surface area (EAS) of the cathode catalyst layer. After the electrochemical characterizations, the single cell was operated under constant current mode. First, the current
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E/V
0.7 0.6
Results and discussion
In this study, the effect of the cathodic relative humidity (RHC) on the fuel cell performance was evaluated for PEMFC MEAs with various Nafion ionomer contents. To do this, MEAs were fabricated by a CCM spray technique so as to form symmetrical porous catalyst layers (Fig. 3). The MEAs were designated according to the ionomer content: S20, S25, S30, and S35. For each MEA, the initial performance was evaluated by polarization experiments at a RHC 59%. Humidity dependency was then monitored under constant current operation at 0.4 and 1.2 A/cm2 (RHC: 38e87%). In Fig. 4, ieV curves (polarization) and cell voltage values at 0.4 and 1.2 A/cm2 (constant current operation) are presented for S20 and S35. For every MEAs, the open-circuit potential was higher than 0.96 V,
b
S20 RHC 38% RHC 48% RHC 52% RHC 59% RHC 66% RHC 87%
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density was fixed at 0.4 A/cm2 and the relative humidity of air was decreased to 38%. After the single cell was stabilized for 12 h, the voltage value was recorded and the relative humidity was increased to 48%. In the same manner, stabilized voltage values at 0.4 A/cm2 were collected at various cathodic relative humidity levels of 38, 48, 52, 59, 66 and 87%. Second, the current density was increased to 1.2 A/cm2 and stabilized voltages were measured as a function of the RHC. Finally, EIS was carried out again at a relative humidity of 87%.
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Fig. 4 e Polarization curves at a RHC of 59% and at voltages values at 0.4 and 1.2 A/cm2 at a RHC between 38 and 87% for the (a) S20 and (b) S35 MEA.
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RHC 59% S20 S25 S30 S35
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E/V
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Fig. 5 e Polarization curves at a RHC of 59% for S20, S25, S30, and S35.
0.76
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S20 S25 S30 S35
b
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a E @ 0.4 A cm / V
confirming that the unit cells were operating without significant gas crossover (Fig. 5). The cell performance levels of the four MEAs were similar at a low current density. However, as the current increased, the MEAs with larger amounts of Nafion ionomers (S30 and S35) showed more rapid voltage decreases. Generally, as the current increases, the single cell voltage drops due to the limited rate of electrochemical reactions (activation overpotential), voltage drops that typically occur with resistive components such as membranes and electric wires (Ohmic loss), and the limited transport rate of reactant gases to the electrochemical active sites within porous catalyst layers (concentration overpotential). As shown in Fig. 4, under constant current operation, the cell voltage of S20 gradually increased with the cathodic humidity as follows: 0.684 (38%) to 0.728 V (87%) at 0.4 A/cm2 and 0.368 (38%) to 0.435 V (87%) at 1.2 A/cm2. In contrast, S35 showed a voltage decrease as the humidity increased: 0.717 (38%) to 0.688 V (87%) at 0.4 A/cm2 and 0.423 (38%) to 0.232 V (87%) at 1.2 A/cm2. With higher cathodic humidity, the water content in the cathode electrodes will gradually increase as the rate of water evaporation decreases, whereas the effect of enhanced back diffusion is comparatively insignificant [21].
For a detailed analysis, the data from the constant current operations at 0.4 and 1.2 A/cm2 are summarized as a function of the relative humidity in Fig. 6. At 0.4 A/cm2, the cell voltage of S20 gradually increased as the relative humidity increased to 87% (0.684 / 0.728 V) while S25 and S30 showed maximum cell voltages (S25: 0.734 V; S30: 0.716 V) at a relative humidity of 66% (Fig. 6a). For S35, in contrast, at a higher humidity level the cell voltage gradually decreased from 0.717 V to 0.688 V. This different dependency on the relative humidity can be explained in terms of the combined effect of ionic resistance (activation overpotential) and flooding (concentration overpotential) within the catalyst layers. First, at a low humidity, due to the high ionic resistance of ionomer networks, some of the platinum catalysts would not be utilized for oxygen reduction reactions. As the humidity increases, the ionic resistance will decrease and, in turn, the active zone would expand to decrease the activation overpotential. Second, as the air humidity increases, the pores would be more flooded due to the lower water removal rate while the water generation rate is fixed (0.4 A/cm2). As a result, the concentration overpotential would be larger with the higher humidity. At a low ionomer content (S20), the activation overpotential seems to be a major factor as the ionic resistance is high, resulting in a gradual increase of the cell voltage with the humidity. For S35, due to high content of Nafion ionomer, ionic resistance is relatively low throughout the overall humidity range, but the flooding would be severe due to the hydrophilic characteristic. Therefore, the cell voltage gradually decreased with the humidity due to the increase of the concentration overpotential. In the case of the intermediate ionomer content (S25 and S30), a maximum voltage was observed at 67%, where the concentration overpotential begins to take precedence over the activation overpotential in terms of influencing the overall performance. At a higher current density, the effect of water flooding would be more significant because a larger amount of gas should be transported within the porous catalyst layer. Moreover, the water generation rate would increase in proportion to the current density. In Fig. 6b, the cell voltages at 1.2 A/cm2 are presented as a function of the relative humidity. The S20 MEA continues to show a gradual voltage increase
E @ 1.2 A cm / V
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Fig. 6 e Variation of the cell voltages as a function of the relative humidity at a current density of (a) 0.4 A/cm2 and (b) 1.2 A/cm2.
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0.76
0.74
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Fig. 7 e The cell voltages values at an intermediate humidity level (59%, closed circles) and at high humidity (87%, open circles) according to the Nafion ionomer content at a current density of (a) 0.4 A/cm2 and (b) 1.2 A/cm2.
with the relative humidity, confirming that the activation overpotential is more significant and showing only a minor effect of flooding in this case. However, for the S25 and S30 MEAs, the cell voltages gradually decreased as the humidity increased, showing that the concentration overpotential became the dominant factor that determines the overall humidity dependency of the MEAs. It should be noted that the degree of water flooding at various levels of relative humidity can be influenced by the characteristics of the micro-porous layers and the gas diffusion layers. In Fig. 7, the cell voltages are plotted as a function of the ionomer content. At a current density of 0.4 A/cm2, S25 showed the highest performance at an intermediate humidity of 59%, whereas at RHC 87%, the cell voltage was highest for S20 and gradually decreased with a higher ionomer content (Fig. 7a). This trend also would come from the different humidity effects, as explained earlier. For MEAs with a low ionomer content (S20 and S25), a humidity increase (59 / 87%) had a positive effect on the cell performance, lowering the activation overpotential. In contrast, at a higher ionomer content (S30 and S35), the concentration
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overpotential would increase with a higher humidity level and the activation overpotential change would be comparatively insignificant. At a higher current density of 1.2 A/cm2, the negative effect of higher humidity, due to flooding, was more clearly observed. When the relative humidity was increased from 59% to 87%, the cell voltage dropped considerably for S25 (50 mV), S30 (83 mV), and S35 (126 mV), whereas the cell voltage of S20 was maintained at a nearly constant value. The performance decrease observed in S35 at a higher humidity was also confirmed by electrochemical impedance spectroscopy. The polarization resistance, as determined from the distance between the y-intercepts, increased with the relative humidity, as follows: 0.74 (59%) / 1.30 U cm2 (RHC 87%). Even in the high voltage region, the concentration overpotential was found to be significant, especially for S35 with a high ionomer content, as shown in Figs. 6 and 7. Therefore, it appears that the semi-circular shape is composed of resistive terms from both the charge transfer reaction and the mass transport, where the increased polarization resistance comes from the slowed gas transfer within
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Fig. 8 e Nyquist plots of impedance data measured at 0.85 V (a) for S35 at relative humidity levels of 59 and 87%, and (b) for S20, S25, S30, and S35 at a RHC of 87%.
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40
Acknowledgements
20
This work was supported by the New and Renewable Energy R&D Program and National RD&D Organization for Hydrogen and Fuel Cell under the Ministry of Knowledge Economy, Republic of Korea, as part of the “Development of 200 kW Class PEMFC System for Bus” (2005-N-F12-P-01). It was also supported by a grant from the Korea Institute of Science and Technology as part of its ‘‘Development of Fuel Cell PowerPacks for Portable Power Generation’’ program.
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i / mA cm
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references
Fig. 9 e Cyclic voltammograms for S20, S25, S30 and S35.
the catalyst layers. In addition, as shown in Fig. 8b, the polarization resistance levels gradually increased with the ionomer content, as follows: 0.85 (S20), 1.09 (S25), 1.24 (S30), and 1.29 U cm2 (S35). In Fig. 9, the cyclic voltammograms measured under nitrogen (cathodes) are presented for MEAs with various ionomer contents. When inert nitrogen gas instead of air or oxygen is supplied, an oxygen reduction reaction does not occur on the cathode sides. Instead, proton adsorption/ desorption on the platinum surface can be monitored at potentials lower than 0.4 V. From the integrated charge for hydrogen adsorption/desorption, the EAS (electrochemically active surface area) of the platinum catalysts can subsequently be calculated. The calculated EAS values were 87 (S20), 102 (S25), 98 (S30), and 82 m2/g (S35), which are inversely proportional to the charge transfer resistances. These CV results suggest that the polarization resistance in EIS was not determined solely by the charge transfer resistance, even at a high voltage of 0.85 V. Therefore, at a high humidity of 87%, the cell performance and the polarization resistance were influenced considerably by the concentration overpotentials.
4.
Conclusion
In initial polarization experiments (RHC 59%), a voltage drop due to the mass transport limitation commonly appeared for MEAs with high ionomer contents (30 and 35 wt%). Accordingly, the effect of the concentration overpotential with flooding was stronger with a higher ionomer content and at a higher relative humidity. A gradual voltage decrease with the humidity increase was observed for S35 at 0.4 A/cm2 and for S25, S30, and S35 at 1.2 A/cm2. Due to the combined effect of the relative humidity and Nafion ionomer content, the highest voltages were obtained with the S25 MEA at an intermediate humidity of 59% and with the S20 MEA at a higher humidity of 87%, at both 0.4 and 1.2 A/cm2. Therefore, the effect of relative humidity, which is dependent on the Nafion ionomer contents, should be importantly considered in developing MEAs.
[1] Litster S, Mclean G. PEM fuel cell electrodes. J Power Sources 2004;130:61e76. [2] Mehta V, Cooper JS. Review and analysis of PEM fuel cell design and manufacturing. J Power Sources 2003;114:32e53. [3] Zawodzinski Jr TA, Springer TE, Uribe F, Gottesfeld S. Characterization of polymer electrolytes for fuel cell applications. Solid State Ionics 1993;60:199e211. [4] Schmittinger W, Vahidi A. A review of the main parameters influencing long-term performance and durability of PEM fuel cells. J Power Sources 2008;180(1):1e14. [5] Li H, Tang Y, Wang Z, Shi Z, Wu S, Song D, et al. A review of water flooding issues in the proton exchange membrane fuel cell. J Power Sources 2008;178(1):103e17. [6] Park YH, Caton JA. An experimental investigation of electroosmotic drag coefficients in a polymer electrolyte membrane fuel cell. Int J Hydrogen Energy 2008;33:7513e20. [7] Neyerlin KC, Gasteiger HA, Mittelsteadt CK, Jorne J, Gu W. Effect of relative humidity on oxygen reduction kinetics in a PEMFC. J Electrochem Soc 2005;152(6):A1073e80. [8] Xu H, Song Y, Kunz HR, Fenton JM. Effect of elevated temperature and reduced relative humidity on ORR kinetics for PEM fuel cells. J Electrochem Soc 2005;152(9):A1828e36. [9] Wang L, Husar A, Zhou T, Liu H. A parametric study of PEM fuel cell performances. Int J Hydrogen Energy 2003;28: 1263e72. [10] Williams MV, Kunz HR, Fenton JM. Operation of Nafionbased PEM fuel cells with no external humidification: influence of operating conditions and gas diffusion layers. J Power Sources 2004;135:122e34. [11] Amirinejad M, Rowshanzamir S, Eikani MH. Effects of operating parameters on performance of a proton exchange membrane fuel cell. J Power Sources 2006;161:872e5. [12] Park GG, Sohn YJ, Yang TH, Yoon YG, Lee WY, Kim CS. Effect of PTFE contents in the gas diffusion media on the performance of PEMFC. J Power Sources 2004;131:182e7. [13] Yan Q, Toghiani H, Wu J. Investigation of water transport through membrane in a PEM fuel cell by water balance experiments. J Power Sources 2006;158:316e25. [14] Uchida M, Aoyama Y, Eda N, Ohta A. Investigation of the microstructure in the catalyst layer and effects of both perfluorosulfonate ionomer and PTFE-loaded carbon on the catalyst layer of polymer electrolyte fuel cells. J Electrochem Soc 1995;142(12):4143e9. [15] Paganin VA, Ticianelli EA, Gonzalez ER. Development and electrochemical studies of gas diffusion electrodes for polymer electrolyte fuel cells. J Appl Electrochem 1996;26: 297e304. [16] Passalacqua E, Lufrano F, Squadrito G, Patti A, Giorgi L. Nafion content in the catalyst layer of polymer electrolyte fuel cells: effects on structure and performance. Electrochim Acta 2001;46:799e805.
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[17] Song JM, Cha SY, Lee WM. Optimal composition of polymer electrolyte fuel cell electrodes determined by the AC impedance method. J Power Sources 2001;94:78e84. [18] Sasikumar G, Ihm JW, Ryu H. Optimum Nafion content in PEM fuel cell electrodes. Electrochim Acta 2004;50:601e5. [19] Qi Z, Kaufman A. Low Pt loading high performance cathodes for PEM fuel cells. J Power Sources 2003;113:37e43. [20] Frey T, Linardi M. Effects of membrane electrode assembly preparation on the polymer electrolyte
membrane fuel cell performance. Electrochim Acta 2004; 50:99e105. [21] Nguyen TV, White RE. A water and heat management model for proton-exchange-membrane fuel cells. J Electrochem Soc 1993;140:2178e86. [22] Kim K-H, Lee K-Y, Kim H-J, Cho EA, Lee S-Y, Lim T-H, et al. The effects of Nafion ionomer content in PEMFC MEAs prepared by a catalyst-coated membrane (CCM) spraying method. Int J Hydrogen Energy 2010;35:2119e26.