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Preparation of water management layer and effects of its composition on performance of PEMFCs
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Energy Conversion and Management 49 (2008) 1500–1505 www.elsevier.com/locate/enconman
Preparation of water management layer and effects of its composition on performance of PEMFCs Tian Jian-hua, Shi Zhao-yuan, Shi Jin-song, Shan Zhong-qiang * School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China Received 24 August 2006; received in revised form 30 April 2007; accepted 22 December 2007 Available online 12 February 2008
Abstract A membrane electrode assembly (MEA) with a novel water management layer (WML) used in proton exchange membrane fuel cell (PEMFC) was prepared. The so called WML, which was located between the carbon paper and the catalyst layer, was a sublayer composed of carbon and hydrophobic PTFE. Various parameters of the WML, including carbon loading, PTFE content and species, sintering time and temperature and pore formers, were investigated in this study. As demonstrated in our experimental results, the performance of the membrane electrode assembly (MEA) PEMFC could be significantly improved by WML in the condition of operation with dry reactive gases. The MEA with the WML exhibited more stable performance than the situation of MEA without WML during a long time running period. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Proton exchange membrane fuel cell (PEMFC); Water management layer (WML); Pore former
1. Introduction The proton exchange membrane fuel cell (PEMFC) has attracted various attentions due to its high energy density, low operating temperature and excellent stability, and great improvement of the PEMFC’s performance has been witnessed in the past decades [1–4]. Perfluorosulfonic acid membranes, such as the Nafion membranes, are currently used as the electrolyte due to their favorable chemical and mechanical stabilities along with their high proton conductivity in the hydrated states [5]. However, these membranes need to be wet to provide satisfactory proton conductivity because of the hydrophilic nature of the sulfonic acid groups attached to the polymer backbone and the necessity to hydrate the ionic clusters [6,7]. Traditionally, for the PEMFC to function properly, the reactant gases are humidified, respectively, through
*
Corresponding author. Tel.: +86 22 27405457; fax: +86 22 27404208. E-mail address: [email protected] (S. Zhong-qiang).
0196-8904/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2007.12.020
an external humidifier before entering the fuel cell. Of course, this kind of operation increases the overall weight, balance of plant and operating range of the fuel cells, and besides, the membrane electrode assemblies (MEAs) can be in danger of water immersion as the cell runs at larger current densities because of the over production of water. Therefore, water management of a PEMFC, including humidification of the proton exchange membrane (PEM) and removal of product water, is considered to be of primary importance in obtaining high performances. Recently, a lot of work has been devoted to preparing MEAs with self humidification ability in order to improve the specific characteristics of the cell system [8]. Buchi and Srinivasan [9] operated the PEMFC without external humidification using Nafion 115 as the membrane and found that back diffusion of product water from the cathode to the anode is the dominant process for water management in the cell over a wide range of operating conditions. However, the cell performance obtained without humidification was much lower than that obtained with external humidification [5]. Watanabe and co-workers
T. Jian-hua et al. / Energy Conversion and Management 49 (2008) 1500–1505
[10,11] proposed a method of fabricating a self humidifying membrane with highly dispersed Pt and/or metal oxides, such as SiO2 and TiO2. The Pt particles catalyze the oxidation of the crossover hydrogen with oxygen to generate water and the membrane is, thus, humidified. The hydrophilic SiO2 or TiO2 was embedded into the membrane to preserve water [12]. The ultra-thin reinforced self humidifying membranes, as the electrolytes, were also used to decrease the back diffusion distance of water from the cathode to the anode [13,14]. A MEA with a sublayer composed of carbon and hydrophobic polytetrafluoroethylene (PTFE) have already been reported by the literature [15,16]. It is acceptable that a sublayer on the macro-porous carbon backing paper could reduce the influence of the qualitative difference among carbon papers on the MEA performance, but in fact, because of the strong hydrophobicity of PTFE, the sublayer could provide a barrier to liquid water, keeping the electrocatalytic layer partially hydrated and keeping the cathodic reaction water diffusion across the PEM to the anode. Therefore, the contribution of the sublayer to the water management ability of the MEA must be taken seriously. In this paper, the parameters of the sublayer, i.e. the so called water management layer (WML), were studied, and the optimum parameters of the WML are expected to bring significant improvement to the performance of the PEMFC with dry reactive gases. This method is of more practical importance because its preparation process is simple and easy to control. 2. Experimental
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2.2. Characterization of MEA The MEA was mounted in a single cell with stainless steel end plates and graphite collector plates, which were designed with parallel ribbed channels for gas manifolding. The total geometric reaction area of the MEA was 4 cm2. The reagent gases (hydrogen or oxygen) had a purity of 99.9%. A forced two step activation process was performed to make the cell run steadily. First, the pressures of hydrogen and oxygen were controlled at 0.10 and 0.12 MPa, respectively, and the two gases were separately humidified by passing through water at 75 °C and 65 °C, respectively. The humidification of the reactant gases was accomplished by diverting the gases through heated water containing bottles. The cell temperature was held at 50 °C. The forced current density was gradually adjusted to 0.3 A/cm2, and the cell kept running for 12 h under these conditions. In the second activation process, the pressure of H2 and O2 were increased 0.28 and 0.30 MPa, the cell temperature was held at 70 °C and the current density was adjusted up to 0.6 A/cm2, followed by running the cell for 4 h without humidification. When the two step activation process was finished, the I–E or I–t curves were measured with an electronic load. The fuel cell test set up is specially designed in this laboratory. With this device, it is possible to adjust and control the operations, such as current, cell voltage, temperature, gas pressure, humidification and gas rate of flow. The meters used in this device have a measurement precision of 0.5%. As-prepared, the MEA with the WML was analyzed by an environmental scanning electron microscope (ESEM, PHILIPS XL30).
2.1. Preparation of MEA 3. Results and discussion The electrode, anode or cathode, was composed of a WML and a catalyst layer. A homogeneous suspension, composed of PTFE (30 wt%), carbon black (Vulcan XC72R), soisopropanol alcohol and de-ionized water, was prepared. Having been stirred for 30 min by a supersonic mixer, the formed ‘‘ink” was dried at 70 °C in vacuum, until it became a porridge mixture. Then, the porridge mixture was brushed onto two pieces of 2 2 cm2 carbon paper (EC-TP1-060, Electrochem. Inc.). After being sintered for 30 min, the WMLs were obtained. The catalyst layer was adhered to the surface of the WML by brushing a porridge mixture of Pt/C catalyst (20 wt%, Johnson Mattey Corp.) and Nafion solution (5 wt %, Dupont Corp.), and the preparation method of the catalyst porridge was illustrated in the literature [1]. The ratio of dry Nafion to Pt/C was 1:3 by weight. The so obtained two pieces of electrodes were dried in air at room temperature and then hot pressed with a membrane Nafion 1135 (Dupont Corp.) for 60 s under 0.3 MPa and 140 °C. A pretreatment method of the Nafion membrane reported in the literature [17] was adopted. The Pt loading was fixed at 0.4 mg/cm2 in all situations.
3.1. Effects of carbon loading Carbon loading of the WML is an important factor that has a prominent effect on the performances of the MEA. The polarization curves of MEAs prepared with different carbon loadings in the WMLs are shown in Fig. 1. As demonstrated in this diagram, the carbon loading of 3 mg/cm2 was a good choice for the WML, and the MEA, prepared under this condition, performed obviously with better polarization characteristic than that of the other three MEAs. However, all too high carbon loadings (e.g. 4 mg/ cm2 in Fig. 1), which means a relatively thicker WML, will cause poor gas diffusivity due to the lengthened path in the WML. On the other hand, if the carbon loading of the WML is too low (e.g. 1 mg/cm2 or 2 mg/cm2 in Fig. 1), some other factors become predominant. Jordan et al. [18] suggested that the reason for lower cell performance at low carbon loadings was high electronic resistance in the electrode, since the electronic contact area between the catalyst and the WML was thought to be too small at low carbon loadings. It is also possible that when the
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T. Jian-hua et al. / Energy Conversion and Management 49 (2008) 1500–1505
1.0 carbon loadings in WML 2 0mg/cm 2 4mg/cm 2 3mg/cm 2 2mg/cm
Cell voltage/V
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0.6
0.4
0.2
0.0 0.0
0.2
0.4
0.6 0.8 1.0 2 Current density/(A /cm )
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1.4
1.6
Fig. 1. I–V curves of MEAs with different carbon loadings in WMLs pO2 ¼ 0:30 MPa, pH2 ¼ 0:28 MPa, T O2 ¼ T H2 ¼ 25 C, Tcell = 70 °C.
WML is too thin, it does not give a non-permeable support for the coating with the catalyst layer during the electrode fabrication process. This would result in the catalyst layer inserting, in part, into the pores of the carbon backing paper, where it will have poor ionic contact with the Nafion membrane. In either case, voltage losses are thought to occur in the cell due to ionic and/or electronic conductivity losses. Therefore, the optimum performance can be achieved with a medium carbon loading, e.g. 3 mg/cm2 in this experiment. 3.2. Effects of PTFE content Fig. 2 demonstrated the effects of PTFE content in the WML on the performances of the MEAs. The best performance was achieved when the PTFE content was 20% by weight, and increased PTFE content would cause deterio-
ration of the cell performance. Since PTFE is an electron insulator, excessive PTFE will cause an increase of electronic resistance in the WML, which is one of the important roots of a cell’s poor performance [19]. What is more, PTFE can hamper the diffusion of water out of the cathode due to its hydrophobicity, which may cause flooding of the cathode, especially at high current density conditions. The performances of the MEAs with PTFE contents in the WML of 30% and 40% were apparently inferior to the MEAs with relatively lower PTFE contents (10% or 20%). The reason for such phenomenon may lie in the above discussion. It was also noticeable that when the PTFE content was decreased from 20% to 10%, a corresponding slight decrease of cell voltage would happen. Since PTFE was used as a binding reagent, sufficient PTFE is needed to endow the WML with enough mechanical strength. So, when the PTFE content is too low, a cell voltage drop will be observed. 3.3. Effects of sintering temperature and time Figs. 3 and 4 show the influence of different sintering temperatures and time on cell performance, respectively. As can be seen from Fig. 3, an apparent cell voltage drop happened when the sintering temperature was 270 °C. Since the purpose of the sintering process is to form the uniform and cross linked distribution of PTFE, a too low sintering temperature may not be able to promote such transformation. 350 °C was chosen as the best sintering temperature. The effects of sintering time were not so notable, as shown in Fig. 4. There were no significant differences between the performances of the MEAs with different sintering times, but we can also see that the MEA with a sintering time of 0.5 h performed slightly better. So, the sintering time can be ascertained as 0.5 h.
1.0
0.8
Cell voltage/V
Cell voltage/V
0.8
sintered temperature: 270°C 350°C 430°C
0.9
10% PTFE 20% PTFE 30% PTFE 40% PTFE without WML
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-2
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Fig. 2. I–V curves of MEAs with different PTFE contents in WMLs pO2 ¼ 0:12 MPa, pH2 ¼ 0:10 MPa, T O2 ¼ T H2 ¼ 25 C, Tcell = 50 °C.
Fig. 3. I–V curves of MEAs prepared with WMLs sintered at different temperatures pO2 ¼ 0:30 MPa, pH2 ¼ 0:28 MPa, T O2 ¼ T H2 ¼ 25 C, Tcell = 70 °C.
T. Jian-hua et al. / Energy Conversion and Management 49 (2008) 1500–1505
1.0
sintering time: 0.2h 0.5h 1.0h
Cell voltage/V
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0.2
0.0 0.0
0.2
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Current density/A cm
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1.4
-2
Fig. 4. I–V curves of MEAs prepared with WMLs sintered for different times pO2 ¼ 0:30 MPa, pH2 ¼ 0:28 MPa, T O2 ¼ T H2 ¼ 25 C, Tcell = 70 °C.
3.4. Effects of carbon species The effects of the carbon species on cell performance were depicted in Fig. 5. Three kinds of carbon (Vulcan XC-72R, scale like graphite and acetylene black) were investigated. When the WML was prepared using Vulcan XC-72R, cell performance was much better than in the other two cases. When acetylene black was used as the preparation material for preparation of the WML, a sharp drop of cell voltage was observed at high current, which indicated the probable occurrence of flooding. 3.5. Effects of pore formers Since reactant gases have to diffuse through the WML before reaching the catalyst layer, a suitable porosity is very necessary for decreasing the concentration polariza-
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tion and obtaining good cell performances. In this study, (NH4)2SO4 was chosen as a pore former because of its high solubility. (NH4)2SO4 can be dissolved in water produced by the cathode reaction, and so, pores are formed in the WML. Fig. 6 demonstrated the improvement of cell performance caused by the addition of (NH4)2SO4 m[(NH4)2SO4]:m(carbon) = 1:1, as optimized by our experiment. There was an apparent difference between the polarization behavior of cells with and without pore formers in the high current density region, from which it can be concluded that the addition of a pore former could improve cell performances effectively. The ESEM is widely used for analyzing the morphology of MEAs, and hereby, it is used to measure the porosity of the WML in the as prepared MEA. After the measurement of the I–E curves were finished, the morphology of the WML surface was analyzed from the ESEM images, as shown in Fig. 7. The difference between the images with (a) and without (b) pore former can be obviously discriminated in Fig. 7. The WML with pore former exhibits a looser surface than that of the WML without pore former. 3.6. Long time performance of MEA Fig. 8 shows the long time performance of the MEA with and without the WML at 1 A/cm2. An enlarged MEA with an effective area of 24.6 cm2 was used in this experiment. The Pt loading was 0.2 mg/cm2 for both the cathode and anode, the Vulcan XC-72R loading of the WML was 3 mg/cm2, and the pore former (NH4)2SO4 of 3 mg/cm2 was used. The sintering conditions were 350 °C and 0.5 h. As shown in Fig. 8, a more stable performance was observed when the WML was applied. The flooding phenomenon was effectively avoided when the cell was run with humidified gases, and the decrease of power den-
1.1
1.0
1.0
Vulcan XC-72R Acetylene black Scalelike graphite
0.9 0.8
without pore former with pore former of (NH4)2SO4
0.9
Cell voltage/V
Cell voltage/V
0.8
0.7 0.6 0.5 0.4 0.3
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0.0 0.0
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Fig. 5. I–V curves of MEAs with different carbon materials pO2 ¼ 0:12 MPa, pH2 ¼ 0:10 MPa, T O2 ¼ T H2 ¼ 25 C, Tcell = 70 °C.
0.0 0.0
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Fig. 6. I–V curves of MEAs with and without pore formers in WML pO2 ¼ 0:30 MPa, pH2 ¼ 0:28 MPa, T O2 ¼ T H2 ¼ 25 C, Tcell = 70 °C.
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T. Jian-hua et al. / Energy Conversion and Management 49 (2008) 1500–1505
4. Conclusions In this study, MEAs with sublayers between carbon paper and a catalyst layer composed of carbon and PTFE were prepared. The effects of various WML parameters, including carbon loading, PTFE content and species, sintering time and temperature and pore former, on the cell performances were investigated. It was found that a suitable carbon loading of 3 mg/cm2 produced best results. The experimental results also revealed that when Vulcan XC-72R was used as the material for the WML preparation, good cell performance could be expected. The optimum sintering temperature and time were 350 °C and 30 min, respectively. PTFE content was fixed at 20% finally, as the cell performed best. Besides, it is concluded that pore formers can improve cell performances effectively. The experimental results revealed that the performance of the MEA with a WML was improved when there was no humidification of reactant gases, and a prolonged operation time of the MEA at larger current density was obtained at humidification conditions. References
Fig. 7. SEM images of WMLs with (a) and without (b) pore formers of (NH4)2SO4.
0.75 0.70 0.65
Cell Voltage/V
0.60 0.55 0.50 0.45 0.40 0.35 0.30 0.25 0.20 0
5
10
15
20
25
Time/h
Fig. 8. Cell voltage–time curves of MEAs with and without WML, I = 1 A/cm2, pO2 ¼ 0:30 MPa, pH2 ¼ 0:28 MPa, Tcell = 70 °C, dry gases (T O2 ¼ T H2 ¼ 25 C): with () and without (N) WML film; humidified gases (T O2 ¼ 85 C, T H2 ¼ 75 C): with () and without (M)WML.
sity was mitigated to a considerable degree when the cell was run with dry gases. This result could be mainly attributed to the water management capability of the WML.
[1] Yang TH, Yoon YG, Park GG, Lee WY, Kim CS. Fabrication of a thin catalyst layer using organic solvents. J Power Source 2004;127:230. [2] Litster S, McLean G. PEM fuel cell electrodes. J Power Source 2004;130:61. [3] Santarelli MG, Torchio MF. Experimental analysis of the effects of the operating variables on the performance of a single PEMFC. Energy Convers Manage 2007;48:40. [4] Cheddie D, Munroe N. Mathematical model of a PEMFC using a PBI membrane. Energy Convers Manage 2006;47:1490. [5] Yang B, Fu YZ, Manthiram A. Operation of thin Nafion-based selfhumidifying membranes in proton exchange membrane fuel cells with dry H2 and O2. J Power Source 2005;139:170. [6] Zawodzinski TA, Derouin C, Radzinski S, Sherman RJ, Smith VT, Springer TE, et al. Water uptake by and transport through NafionÒ 117 membranes. J Electrochem Soc 1993;140:1041. [7] Sone Y, Ekdunge P, Simonsson D. Proton conductivity of Nafion 117 as measured by a four-electrode AC impedance method. J Electrochem Soc 1996;143:1254. [8] Liu F, Yi BL, Xing DM, Yu JR, Hou ZJ, Fu YZ. Development of novel self-humidifying composite membranes for fuel cells. J Power Source 2003;124:81. [9] Buchi FN, Srinivasan S. Operating proton exchange membrane fuel cells without external humidification of the reactant gases. J Electrochem Soc 1997;144:2767. [10] Watanabe M, Uchida H, Emori M. Analyses of self-humidification and suppression of gas crossover in Pt-dispersed polymer electrolyte membranes for fuel cells. J Electrochem Soc 1998;145: 1137. [11] Watanabe M, Uchida H, Emori M. Self-humidifying polymer electrolyte membranes for fuel cells. J Electrochem Soc 1996;143:3847. [12] Yang T, Shi PF, Du CY. Study on self-humidified PEMFC with reactant circulation. Electrochim Acta 2006;51:5618. [13] Zhang Y, Zhang HM, Zhu XB, Liang YM. A low-cost PTFEreinforced Integral multilayered self-humidifying membrane for PEM fuel cells. Electrochem Solid State Lett 2006;9:A 332. [14] Wang C, Liu ZX, Mao ZQ, Xu JM, Ge KY. Preparation and evaluation of a novel self-humidifying Pt/PFSA composite membrane for PEM fuel cell. Chem Eng J 2005;112:87.
T. Jian-hua et al. / Energy Conversion and Management 49 (2008) 1500–1505 [15] Qi ZG, Kaufman A. Improvement of water management by a microporous sublayer for PEM fuel cells. J Power Source 2002;109:38. [16] Lee HK, Park JH, Kim DY. A study on the characteristics of the diffusion layerthicknessandporosityofthePEMFC.JPowerSource2004;131:200. [17] Denis MC, Ladande G, Guay D, Dodelet JP, Schulz R. High energy ball-milled Pt and Pt–Ru catalysts for polymer electrolyte fuel cells and their tolerance to CO. J Appl Electrochem 1999;29:951.
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[18] Jordan LR, Shukla AK, Behrsing T, Avery NR, Muddle BC, Forsyth M. Diffusion layer parameters influencing optimal fuel cell performance. J Power Source 2000;86:250. [19] Prasanna M, Ha HY, Cho EA, Hong SA, Oh IH. Influence of cathode gas diffusion media on the performance of the PEMFCs. J Power Source 2004;131:147.
Energy Conversion and Management 49 (2008) 1500–1505 www.elsevier.com/locate/enconman
Preparation of water management layer and effects of its composition on performance of PEMFCs Tian Jian-hua, Shi Zhao-yuan, Shi Jin-song, Shan Zhong-qiang * School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China Received 24 August 2006; received in revised form 30 April 2007; accepted 22 December 2007 Available online 12 February 2008
Abstract A membrane electrode assembly (MEA) with a novel water management layer (WML) used in proton exchange membrane fuel cell (PEMFC) was prepared. The so called WML, which was located between the carbon paper and the catalyst layer, was a sublayer composed of carbon and hydrophobic PTFE. Various parameters of the WML, including carbon loading, PTFE content and species, sintering time and temperature and pore formers, were investigated in this study. As demonstrated in our experimental results, the performance of the membrane electrode assembly (MEA) PEMFC could be significantly improved by WML in the condition of operation with dry reactive gases. The MEA with the WML exhibited more stable performance than the situation of MEA without WML during a long time running period. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Proton exchange membrane fuel cell (PEMFC); Water management layer (WML); Pore former
1. Introduction The proton exchange membrane fuel cell (PEMFC) has attracted various attentions due to its high energy density, low operating temperature and excellent stability, and great improvement of the PEMFC’s performance has been witnessed in the past decades [1–4]. Perfluorosulfonic acid membranes, such as the Nafion membranes, are currently used as the electrolyte due to their favorable chemical and mechanical stabilities along with their high proton conductivity in the hydrated states [5]. However, these membranes need to be wet to provide satisfactory proton conductivity because of the hydrophilic nature of the sulfonic acid groups attached to the polymer backbone and the necessity to hydrate the ionic clusters [6,7]. Traditionally, for the PEMFC to function properly, the reactant gases are humidified, respectively, through
*
Corresponding author. Tel.: +86 22 27405457; fax: +86 22 27404208. E-mail address: [email protected] (S. Zhong-qiang).
0196-8904/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2007.12.020
an external humidifier before entering the fuel cell. Of course, this kind of operation increases the overall weight, balance of plant and operating range of the fuel cells, and besides, the membrane electrode assemblies (MEAs) can be in danger of water immersion as the cell runs at larger current densities because of the over production of water. Therefore, water management of a PEMFC, including humidification of the proton exchange membrane (PEM) and removal of product water, is considered to be of primary importance in obtaining high performances. Recently, a lot of work has been devoted to preparing MEAs with self humidification ability in order to improve the specific characteristics of the cell system [8]. Buchi and Srinivasan [9] operated the PEMFC without external humidification using Nafion 115 as the membrane and found that back diffusion of product water from the cathode to the anode is the dominant process for water management in the cell over a wide range of operating conditions. However, the cell performance obtained without humidification was much lower than that obtained with external humidification [5]. Watanabe and co-workers
T. Jian-hua et al. / Energy Conversion and Management 49 (2008) 1500–1505
[10,11] proposed a method of fabricating a self humidifying membrane with highly dispersed Pt and/or metal oxides, such as SiO2 and TiO2. The Pt particles catalyze the oxidation of the crossover hydrogen with oxygen to generate water and the membrane is, thus, humidified. The hydrophilic SiO2 or TiO2 was embedded into the membrane to preserve water [12]. The ultra-thin reinforced self humidifying membranes, as the electrolytes, were also used to decrease the back diffusion distance of water from the cathode to the anode [13,14]. A MEA with a sublayer composed of carbon and hydrophobic polytetrafluoroethylene (PTFE) have already been reported by the literature [15,16]. It is acceptable that a sublayer on the macro-porous carbon backing paper could reduce the influence of the qualitative difference among carbon papers on the MEA performance, but in fact, because of the strong hydrophobicity of PTFE, the sublayer could provide a barrier to liquid water, keeping the electrocatalytic layer partially hydrated and keeping the cathodic reaction water diffusion across the PEM to the anode. Therefore, the contribution of the sublayer to the water management ability of the MEA must be taken seriously. In this paper, the parameters of the sublayer, i.e. the so called water management layer (WML), were studied, and the optimum parameters of the WML are expected to bring significant improvement to the performance of the PEMFC with dry reactive gases. This method is of more practical importance because its preparation process is simple and easy to control. 2. Experimental
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2.2. Characterization of MEA The MEA was mounted in a single cell with stainless steel end plates and graphite collector plates, which were designed with parallel ribbed channels for gas manifolding. The total geometric reaction area of the MEA was 4 cm2. The reagent gases (hydrogen or oxygen) had a purity of 99.9%. A forced two step activation process was performed to make the cell run steadily. First, the pressures of hydrogen and oxygen were controlled at 0.10 and 0.12 MPa, respectively, and the two gases were separately humidified by passing through water at 75 °C and 65 °C, respectively. The humidification of the reactant gases was accomplished by diverting the gases through heated water containing bottles. The cell temperature was held at 50 °C. The forced current density was gradually adjusted to 0.3 A/cm2, and the cell kept running for 12 h under these conditions. In the second activation process, the pressure of H2 and O2 were increased 0.28 and 0.30 MPa, the cell temperature was held at 70 °C and the current density was adjusted up to 0.6 A/cm2, followed by running the cell for 4 h without humidification. When the two step activation process was finished, the I–E or I–t curves were measured with an electronic load. The fuel cell test set up is specially designed in this laboratory. With this device, it is possible to adjust and control the operations, such as current, cell voltage, temperature, gas pressure, humidification and gas rate of flow. The meters used in this device have a measurement precision of 0.5%. As-prepared, the MEA with the WML was analyzed by an environmental scanning electron microscope (ESEM, PHILIPS XL30).
2.1. Preparation of MEA 3. Results and discussion The electrode, anode or cathode, was composed of a WML and a catalyst layer. A homogeneous suspension, composed of PTFE (30 wt%), carbon black (Vulcan XC72R), soisopropanol alcohol and de-ionized water, was prepared. Having been stirred for 30 min by a supersonic mixer, the formed ‘‘ink” was dried at 70 °C in vacuum, until it became a porridge mixture. Then, the porridge mixture was brushed onto two pieces of 2 2 cm2 carbon paper (EC-TP1-060, Electrochem. Inc.). After being sintered for 30 min, the WMLs were obtained. The catalyst layer was adhered to the surface of the WML by brushing a porridge mixture of Pt/C catalyst (20 wt%, Johnson Mattey Corp.) and Nafion solution (5 wt %, Dupont Corp.), and the preparation method of the catalyst porridge was illustrated in the literature [1]. The ratio of dry Nafion to Pt/C was 1:3 by weight. The so obtained two pieces of electrodes were dried in air at room temperature and then hot pressed with a membrane Nafion 1135 (Dupont Corp.) for 60 s under 0.3 MPa and 140 °C. A pretreatment method of the Nafion membrane reported in the literature [17] was adopted. The Pt loading was fixed at 0.4 mg/cm2 in all situations.
3.1. Effects of carbon loading Carbon loading of the WML is an important factor that has a prominent effect on the performances of the MEA. The polarization curves of MEAs prepared with different carbon loadings in the WMLs are shown in Fig. 1. As demonstrated in this diagram, the carbon loading of 3 mg/cm2 was a good choice for the WML, and the MEA, prepared under this condition, performed obviously with better polarization characteristic than that of the other three MEAs. However, all too high carbon loadings (e.g. 4 mg/ cm2 in Fig. 1), which means a relatively thicker WML, will cause poor gas diffusivity due to the lengthened path in the WML. On the other hand, if the carbon loading of the WML is too low (e.g. 1 mg/cm2 or 2 mg/cm2 in Fig. 1), some other factors become predominant. Jordan et al. [18] suggested that the reason for lower cell performance at low carbon loadings was high electronic resistance in the electrode, since the electronic contact area between the catalyst and the WML was thought to be too small at low carbon loadings. It is also possible that when the
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T. Jian-hua et al. / Energy Conversion and Management 49 (2008) 1500–1505
1.0 carbon loadings in WML 2 0mg/cm 2 4mg/cm 2 3mg/cm 2 2mg/cm
Cell voltage/V
0.8
0.6
0.4
0.2
0.0 0.0
0.2
0.4
0.6 0.8 1.0 2 Current density/(A /cm )
1.2
1.4
1.6
Fig. 1. I–V curves of MEAs with different carbon loadings in WMLs pO2 ¼ 0:30 MPa, pH2 ¼ 0:28 MPa, T O2 ¼ T H2 ¼ 25 C, Tcell = 70 °C.
WML is too thin, it does not give a non-permeable support for the coating with the catalyst layer during the electrode fabrication process. This would result in the catalyst layer inserting, in part, into the pores of the carbon backing paper, where it will have poor ionic contact with the Nafion membrane. In either case, voltage losses are thought to occur in the cell due to ionic and/or electronic conductivity losses. Therefore, the optimum performance can be achieved with a medium carbon loading, e.g. 3 mg/cm2 in this experiment. 3.2. Effects of PTFE content Fig. 2 demonstrated the effects of PTFE content in the WML on the performances of the MEAs. The best performance was achieved when the PTFE content was 20% by weight, and increased PTFE content would cause deterio-
ration of the cell performance. Since PTFE is an electron insulator, excessive PTFE will cause an increase of electronic resistance in the WML, which is one of the important roots of a cell’s poor performance [19]. What is more, PTFE can hamper the diffusion of water out of the cathode due to its hydrophobicity, which may cause flooding of the cathode, especially at high current density conditions. The performances of the MEAs with PTFE contents in the WML of 30% and 40% were apparently inferior to the MEAs with relatively lower PTFE contents (10% or 20%). The reason for such phenomenon may lie in the above discussion. It was also noticeable that when the PTFE content was decreased from 20% to 10%, a corresponding slight decrease of cell voltage would happen. Since PTFE was used as a binding reagent, sufficient PTFE is needed to endow the WML with enough mechanical strength. So, when the PTFE content is too low, a cell voltage drop will be observed. 3.3. Effects of sintering temperature and time Figs. 3 and 4 show the influence of different sintering temperatures and time on cell performance, respectively. As can be seen from Fig. 3, an apparent cell voltage drop happened when the sintering temperature was 270 °C. Since the purpose of the sintering process is to form the uniform and cross linked distribution of PTFE, a too low sintering temperature may not be able to promote such transformation. 350 °C was chosen as the best sintering temperature. The effects of sintering time were not so notable, as shown in Fig. 4. There were no significant differences between the performances of the MEAs with different sintering times, but we can also see that the MEA with a sintering time of 0.5 h performed slightly better. So, the sintering time can be ascertained as 0.5 h.
1.0
0.8
Cell voltage/V
Cell voltage/V
0.8
sintered temperature: 270°C 350°C 430°C
0.9
10% PTFE 20% PTFE 30% PTFE 40% PTFE without WML
1.0
0.6
0.4
0.7 0.6 0.5 0.4 0.3 0.2
0.2
0.0 0.0
0.1 0.0 0.0 0.2
0.4
0.6
0.8
1.0
1.2
1.4
0.2
0.4
0.6
0.8
1.0
Current density/A cm
1.2
1.4
1.6
-2
-2
Current density/A cm
Fig. 2. I–V curves of MEAs with different PTFE contents in WMLs pO2 ¼ 0:12 MPa, pH2 ¼ 0:10 MPa, T O2 ¼ T H2 ¼ 25 C, Tcell = 50 °C.
Fig. 3. I–V curves of MEAs prepared with WMLs sintered at different temperatures pO2 ¼ 0:30 MPa, pH2 ¼ 0:28 MPa, T O2 ¼ T H2 ¼ 25 C, Tcell = 70 °C.
T. Jian-hua et al. / Energy Conversion and Management 49 (2008) 1500–1505
1.0
sintering time: 0.2h 0.5h 1.0h
Cell voltage/V
0.8
0.6
0.4
0.2
0.0 0.0
0.2
0.4
0.6
0.8
Current density/A cm
1.0
1.2
1.4
-2
Fig. 4. I–V curves of MEAs prepared with WMLs sintered for different times pO2 ¼ 0:30 MPa, pH2 ¼ 0:28 MPa, T O2 ¼ T H2 ¼ 25 C, Tcell = 70 °C.
3.4. Effects of carbon species The effects of the carbon species on cell performance were depicted in Fig. 5. Three kinds of carbon (Vulcan XC-72R, scale like graphite and acetylene black) were investigated. When the WML was prepared using Vulcan XC-72R, cell performance was much better than in the other two cases. When acetylene black was used as the preparation material for preparation of the WML, a sharp drop of cell voltage was observed at high current, which indicated the probable occurrence of flooding. 3.5. Effects of pore formers Since reactant gases have to diffuse through the WML before reaching the catalyst layer, a suitable porosity is very necessary for decreasing the concentration polariza-
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tion and obtaining good cell performances. In this study, (NH4)2SO4 was chosen as a pore former because of its high solubility. (NH4)2SO4 can be dissolved in water produced by the cathode reaction, and so, pores are formed in the WML. Fig. 6 demonstrated the improvement of cell performance caused by the addition of (NH4)2SO4 m[(NH4)2SO4]:m(carbon) = 1:1, as optimized by our experiment. There was an apparent difference between the polarization behavior of cells with and without pore formers in the high current density region, from which it can be concluded that the addition of a pore former could improve cell performances effectively. The ESEM is widely used for analyzing the morphology of MEAs, and hereby, it is used to measure the porosity of the WML in the as prepared MEA. After the measurement of the I–E curves were finished, the morphology of the WML surface was analyzed from the ESEM images, as shown in Fig. 7. The difference between the images with (a) and without (b) pore former can be obviously discriminated in Fig. 7. The WML with pore former exhibits a looser surface than that of the WML without pore former. 3.6. Long time performance of MEA Fig. 8 shows the long time performance of the MEA with and without the WML at 1 A/cm2. An enlarged MEA with an effective area of 24.6 cm2 was used in this experiment. The Pt loading was 0.2 mg/cm2 for both the cathode and anode, the Vulcan XC-72R loading of the WML was 3 mg/cm2, and the pore former (NH4)2SO4 of 3 mg/cm2 was used. The sintering conditions were 350 °C and 0.5 h. As shown in Fig. 8, a more stable performance was observed when the WML was applied. The flooding phenomenon was effectively avoided when the cell was run with humidified gases, and the decrease of power den-
1.1
1.0
1.0
Vulcan XC-72R Acetylene black Scalelike graphite
0.9 0.8
without pore former with pore former of (NH4)2SO4
0.9
Cell voltage/V
Cell voltage/V
0.8
0.7 0.6 0.5 0.4 0.3
0.7 0.6 0.5 0.4 0.3
0.2
0.2
0.1
0.1
0.0 0.0
0.2
0.4
0.6
0.8
Current density/A cm
1.0
1.2
1.4
-2
Fig. 5. I–V curves of MEAs with different carbon materials pO2 ¼ 0:12 MPa, pH2 ¼ 0:10 MPa, T O2 ¼ T H2 ¼ 25 C, Tcell = 70 °C.
0.0 0.0
0.2
0.4
0.6
0.8
1.0
Current density/A cm
1.2
1.4
-2
Fig. 6. I–V curves of MEAs with and without pore formers in WML pO2 ¼ 0:30 MPa, pH2 ¼ 0:28 MPa, T O2 ¼ T H2 ¼ 25 C, Tcell = 70 °C.
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T. Jian-hua et al. / Energy Conversion and Management 49 (2008) 1500–1505
4. Conclusions In this study, MEAs with sublayers between carbon paper and a catalyst layer composed of carbon and PTFE were prepared. The effects of various WML parameters, including carbon loading, PTFE content and species, sintering time and temperature and pore former, on the cell performances were investigated. It was found that a suitable carbon loading of 3 mg/cm2 produced best results. The experimental results also revealed that when Vulcan XC-72R was used as the material for the WML preparation, good cell performance could be expected. The optimum sintering temperature and time were 350 °C and 30 min, respectively. PTFE content was fixed at 20% finally, as the cell performed best. Besides, it is concluded that pore formers can improve cell performances effectively. The experimental results revealed that the performance of the MEA with a WML was improved when there was no humidification of reactant gases, and a prolonged operation time of the MEA at larger current density was obtained at humidification conditions. References
Fig. 7. SEM images of WMLs with (a) and without (b) pore formers of (NH4)2SO4.
0.75 0.70 0.65
Cell Voltage/V
0.60 0.55 0.50 0.45 0.40 0.35 0.30 0.25 0.20 0
5
10
15
20
25
Time/h
Fig. 8. Cell voltage–time curves of MEAs with and without WML, I = 1 A/cm2, pO2 ¼ 0:30 MPa, pH2 ¼ 0:28 MPa, Tcell = 70 °C, dry gases (T O2 ¼ T H2 ¼ 25 C): with () and without (N) WML film; humidified gases (T O2 ¼ 85 C, T H2 ¼ 75 C): with () and without (M)WML.
sity was mitigated to a considerable degree when the cell was run with dry gases. This result could be mainly attributed to the water management capability of the WML.
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