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Determination of the pore size distribution of micro porous layer in PEMFC using pore forming agents under various drying conditions
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 1 1 1 4 8 e1 1 1 5 3
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Determination of the pore size distribution of micro porous layer in PEMFC using pore forming agents under various drying conditions Jeong Hwan Chun a, Ki Tae Park a, Dong Hyun Jo a, Ji Young Lee a, Sang Gon Kim a, Eun Sook Lee b, Jy-Young Jyoung b, Sung Hyun Kim a,* a b
Department of Chemical and Biological Engineering, Korea University, 1 Anam-Dong, Seongbuk-Ku, Seoul 136-713, South Korea Energy Research Center, HyupJin I&C Co., LTD, 143-1 Gwelang-Ri, Jungnam-Myun, Hwasung-Si, Kyunggi-Do, South Korea
article info
abstract
Article history:
In this paper, the effect of the pore size distribution of a micro-porous layer (MPL) on the
Received 17 May 2010
performance of polymer electrolyte membrane fuel cells (PEMFC) was investigated using
Received in revised form
self-made gas diffusion layers (GDLs) with different MPLs for which the pore size distri-
9 July 2010
bution was modified using pore forming agents under different drying conditions. When
Accepted 10 July 2010
MPL dried at high temperature, more macro pores, approximately 1,000e20,000 nm in
Available online 10 August 2010
diameter, and less micro pores, below 100 nm, were observed relative to when MPL was dried at low temperature. Self-made GDLs were characterized by a field-emission scanning
Keywords:
electron microscope (FE-SEM), mercury porosimetry and self-made gas permeability
Polymer electrolyte membrane
measurement equipment. The performance of the single cells was measured under two
fuel cell (PEMFC)
different humidification conditions. The results demonstrate that the optimum pore size
Micro porous layer (MPL)
distribution of MPL depended on the cell operating humidification condition. The MPL dried
Pore forming agent
at high temperature performed better than the MPL dried at low temperature under a low
Pore size distribution
humidification condition; however, MPL dried at low temperature performed better under a high humidification condition. ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Polymer electrolyte membrane fuel cells (PEMFCs) are attracting considerable interest as alternative power sources for stationary and mobile applications due to their high efficiency, low emission, and low noise. The most important component of PEMFC is the membrane electrode assembly (MEA), which consists of a polymer electrolyte membrane with catalyst layers on both sides and gas diffusion layers (GDL) [1e3]. The porous GDL plays an important role in the PEMFC. The most important function of the GDL is to distribute the reactant gas over the catalyst layer and to remove the generated product (liquid water) out of the cell.
The GDL offers a conductive path between the catalyst layer and the current collector, and provides a physical support for depositing the catalyst layer. The GDL consists of a macro porous substrate, which is called as the gas diffusion backing layer (GDBL), and a thin micro porous layer (MPL). The most promising candidates for use as the GDBL in PEMFCs are carbon fiber based products, such as non-woven papers, felts and woven clothes, due to their high porosity and good electrical conductivity. The MPL, which is composed of carbon powder and a hydrophobic agent, such as polytetrafluoroethylene (PTFE), is applied on one side of the GDBL [4]. Prior studies have been performed to understand the role of MPL. Passalacqua et al. [5] found that cells containing MPLs
* Corresponding author. Tel.: þ82 02 3290 3297; fax: þ82 02 926 6102. E-mail address: [email protected] (S.H. Kim). 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.07.056
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 1 1 1 4 8 e1 1 1 5 3
performed better than when they did not contain MPLs. The primary purpose of the MPL is for water management, since the MPL allows for effective wicking of liquid water from the cathode catalyst layer into the flow field. The MPL may also function in reducing electrical contact resistance with the adjacent catalyst layer. Recently, some researchers investigated the influences of the MPL components [6e12] and properties, especially porosity and pore size distribution [6,8,13e21], on PEMFC performance. Using numerical simulations [13e18] and experimental analysis [6,8,19e21], the porosity and pore size distribution were shown to be significantly associated with mass transport problems. In order to change the pore size distribution, various experimental methods were developed. Wang et al. [8] used carbon black, acetylene black carbon and black pearls 2000 carbon to fabricated MPLs that had various pore size distributions. Kong et al. [20] inserted Li2CO3, as a pore forming agent, into the MPL slurries to control the pore size distribution in the MPL and Tang et al. [21] used NH4Cl as a pore forming agent. Pore forming agents have been used by many fields [22,23] in addition to studies on MPL. Fig. 1 shows the mechanism of pore formation during the drying process. First, the pore forming agents must be dissolved in the MPL slurry. The MPL slurry containing the dissolved pore forming agents is then applied to the GDBL. During the drying process, the pore forming agents are solidified in MPL, since the solvent in the MPL slurry evaporates. The solidified pore forming agents are eliminated when the temperature is higher than the boiling point or decomposition temperature of the pore forming agent and at this point the pores are formed. In this study, MPLs with various pore size distributions were fabricated using pore forming agents under different drying conditions to investigate the effect of the pore size distribution in MPL on PEMFC performance. In general, contents of pore forming agent were changed to control pore size distribution in MPL, but this method caused changing of properties of MPL slurry. We controlled pore size distribution in MPL changing drying conditions. Single cell performance tests were carried out under two different humidification conditions to investigate the optimum pore size distribution in MPL for performances of PEMFC. The properties of the GDLs were characterized using FE-SEM, a self-made gas permeability measurement and mercury porosimetry.
2.
Experimental
2.1.
Preparation of MEA
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The wet-proofed carbon felt as GDBL was prepared by immersing commercial carbon felts in a 5.0 wt.% PTFE dispersion for 1 min, and then drying in air. The carbon slurry for the MPL was prepared using the following procedures. The carbon black, 60 wt.% PTFE dispersion, isopropyl alcohol (IPA), distilled water, and a pore forming agent, which was a type of ammonium salt, were mechanically mixed using a digital stirrer. The pore forming agent concentration was 20 wt.% of the carbon black. The slurries were applied to the wet-proofed carbon felts using a knife coating method and then dried at various temperatures. After the drying process was complete, the prepared GDL samples were sintered in air at 350 C for 30 min. The conditions used to fabricate the cathode GDL samples are shown in Table 1. The commercial carbon felt with MPL was used as the anode GDL and the commercial catalyst coated membranes with a 25 cm2 geometrically active area were used.
2.2.
Characterization of GDLs
The morphology of MPLs was characterized by FE-SEM (S-4300, Hitachi Co.). Images magnified by factors of 1,000 and 30,000 were captured to observe macro pores and micro pores, respectively. The pore size distribution of MPLs was measured using a mercury intrusion porosimetry (Autopore IV 9500, Micromeritics Inc.). Gas permeability was characterized by the Darcy coefficient, which can be estimated by applying a pressure drop and observing the flow [24]. The Darcy’s law is: V¼K
a DP ml
(1)
where V is the volumetric flow rate, K is the Darcy coefficient (gas permeability), a is the cross-sectional area through which the flow passes, m is the gas viscosity of the flow material (nitrogen), l is the length of the convective path (thickness of GDL sample), and DP is the pressure differential across the
Fig. 1 e Mechanism of pore formation in MPL using a pore forming agent.
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Table 1 e Fabrication conditions of the cathode GDLs. Samples
PF2-LT PF2-MT PF2-HT PFO-MT
Contents of pore forming agent (wt.%) 20 20 20 0
Drying conditions
60 C
80 C
24 min 3min
8min
3min
sample. Fig. 2 shows a diagram of the self-made gas permeability measurement equipment.
2.3.
Single cell test conditions
A single cell test was carried out using a 25 cm2 active area test cell in humidified H2/air gases. The operating temperature of the cell was 65 C and the test was operated under atmospheric pressure. The bubbling humidifier temperature of both cathode and anode were kept constant at 65 C (about RH 100%) under the high humidification condition and 45 C (about RH 40%) under the low humidification condition. The polarization curves were measured under constant current mode using a 4 A current step and a 2 min dwell time. The stoichiometries of hydrogen (lH2 ) and air (lair) were 1.5 and 2.0, respectively.
3.
Results and discussion
3.1. Characterization analysis of self-made GDLs with different MPLs The pore size distribution of self-made MPLs is shown in Fig. 3. The pores in the MPLs were divided according to size into three categories; macro pores e 5,000e20,000 nm, meso pores e 100e5,000 nm, and micro pores >100 nm. PF2-MT,
Fig. 2 e Self-made gas permeability measurement equipment.
Heat treatment
100 C
120 C
150 C
10 min 3min 10 min
3 min 10min 3 min
3 min
350 C 30 min
which was fabricated using the pore forming agent, had a large volume of macro, meso, and micro pores relative to PFO-MT, which was fabricated without the pore forming agent (Fig. 3). This indicates that the pore forming agent contributed to the formation of more pores in PF2-MT. It was also shown that the volume of the macro pores in MPL increased with increasing drying temperature, whereas the volume of the micro pores decreased. As described in the introduction, the pore forming agent in the MPL slurry produces pores by decomposing or evaporating during the drying process. In general, the rates of degradation and evaporation increase as the temperature increases. Therefore, rapid pore forming agent degradation, which is caused by a high drying temperature, results in the formation of several macro pores. In contrast, a low drying temperature results in the gradual degradation of the pore forming agent, and more micro pores are produced. The morphology of the MPL surface is shown in Fig. 4. SEM images, which were magnified by factors of 1,000 (Fig. 4(a), (c) and (e)) and 30,000 (Fig. 4(b), (d) and (f)), were captured to observe the macro and micro pores, respectively. As shown in Fig. 4, PF2-HT (Fig. 4(e), and (f)) had many more macro pores and less micro pores relative to PF2-LT (Fig. 4(a), and (b)). These images demonstrate that an increase in the drying temperature promotes formation of macro pores. Fig. 5 shows the gas permeability of self-made GDLs with different MPLs. The gas permeability of MPL dominated the overall gas permeability of GDL, because the mean pore
Fig. 3 e Effect of drying conditions on the pore size distribution of MPLs.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 1 1 1 4 8 e1 1 1 5 3
diameter of the MPL was small compared to the GDBL. According to the KozenyeCarman relation, the gas permeability is related to total porosity and pore diameter by the following equation: K¼
33 kc ð1 3Þ2
d2p
where kc is KozenyeCarman constant, 3 is total porosity, and dp is pore diameter [25]. According to the above equation, the gas permeability of GDL increases with an increase in the pore diameter of GDL. In Fig. 5, the gas permeability of PF2-MT was higher than PFO-MT, because the pore forming agent produced more macro, meso, and micro pores in PF2-MT. The gas permeability of GDL increased with increasing MPL drying temperature. The gas permeability of PF2-HT was higher than PF2-LT, because PF2-HT had more macro pores than PF2-LT.
3.2.
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Single cell performances
The polarization curves of single cells fabricated with various MPLs under different humidification conditions are shown in Figs. 6 and 7. The tests were performed under two different humidification conditions. Fig. 6 shows the single cell performance under the high humidification condition (cell: 65 C, the bubbling humidifier: 65 C). As shown in Fig. 6, the best performance under the high humidification condition was obtained when the PF2-LT was used. The difference in the cell performances of the four samples was especially evident in the high current density region. This difference resulted from the volume of the micro pores in MPLs. It is very important to remove water generated at the catalyst layer, particularly under high humidification conditions. The micro pores in MPL play an important role in the management of
Fig. 4 e The morphology of the self-made MPL surface; (a) PF2-LT, 1,000 magnification, (b) PF2-LT, 30,000 magnification, (c) PF2-MT, 1,000 magnification, (d) PF2-MT 30,000 magnification, (e) PF2-HT, 1,000 magnification, (f) PF2-HT, 30,000 magnification.
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Fig. 5 e Gas permeability of the self-made GDLs with different MPLs determined using self-made gas permeability measurement equipment.
water in the cathode GDLs. The generated water has to be emitted through the GDL to prevent water from flooding into the high current density region. Capillary-driven liquid flow through GDLs may predominate in fuel cell operation. According to the YoungeLaplace equation, the capillary pressure is related to the pore size by the following equation: Pc ¼
2gcosq r
where Pc is the capillary pressure, g is the surface energy of water, q is the contact angle of water with the surface of pore, and r is the radius of pores [8]. According to the above equation, the micro pore has an advantage over the macro pores in water removal, since the capillary pressure increases with decreasing pore diameter. PF2-LT had the best ability to remove the water produced at the catalyst layer, because PF2-LT had the largest volume of micro pores among the four samples. Single cell performances under the low humidification condition (cell: 65 C, the bubbling humidifier: 45 C) are shown
Fig. 7 e Polarization curves using the self-made GDLs with different MPLs under the low humidification condition; cell temperature 65 C, cathode and anode humidifiers temperature 45 C, lH2 [ 1.5, lair [ 2.0.
in Fig. 7. As the relative humidity of the inlet gas (hydrogen and air) decreased, the water content of the polymer electrolyte membrane decreased. This leads to a decrease in the proton conductivity of the polymer electrolyte membrane and an increase in ohmic loss. Thus, the current density at 0.6 V under the low humidification condition was 35 percent of the current density under the high humidification condition. More water, which was generated at the cathode catalyst layer, moved towards the dry polymer electrolyte membrane due to back diffusion under the low humidification condition. This indicates that there was a lower risk of flooding when the cell was operated under low humidification. Therefore, the sufficient supply of air and hydrogen was more important than water removal under the low humidification condition in terms of mass transport. The MPL must have high gas permeability in order to sufficiently supply hydrogen and air. As shown in Fig. 7, the best performance was obtained when the PF2-HT was used as the cathode GDL under the low humidification condition. PF2-HT had the greatest gas permeability among the four samples, since PF2-HT had the largest volume of macro pores. Thus, an increase in the volume of macro pores in the cathode MPL enhanced the cell performance under the low humidification condition. In short, using the GDL which had a lot of micro pores in MPL improved single cell performance under high humidification condition, while using the GDL which contained plenty of macro pores in MPL improved single cell performance under low humidification condition. The results of gas permeability of GDLs measurement and water removal analysis using YoungeLaplace equation support the single cell test results.
4. Fig. 6 e Polarization curves using self-made GDLs with different MPLs under the high humidification condition; cell temperature 65 C, cathode and anode humidifiers temperature 65 C, lH2 [ 1.5, lair [ 2.0.
Conclusions
The pore size distribution of MPLs was modified using a pore forming agent under different drying conditions to investigate the effect of the pore size distribution of MPL on cell performance. The pore size distribution, gas permeability, and
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 1 1 1 4 8 e1 1 1 5 3
surface morphology of the MPL were characterized and the performance of the single cell was measured under both high and low humidification conditions. As the drying temperature increased, formation of macro pores increased and micro pores were retrained. Therefore, PF2-HT dried at high temperature had the largest volume of macro pores but the smallest volume of micro pores. The single cell test results demonstrated that the optimum pore size distribution of MPL depended on the humidification condition. The best cell performance was obtained when the PF2-LT was used under the high humidification condition; however, the PF2-HT performed the best under the low humidification condition. MPL containing several micro pores (PF2-LT) can more efficiently remove water under high humidification conditions, which is beneficial to mass transport; however, the MPL containing several macro pores (PF2-HT) has the advantage of more efficiently supplying air and hydrogen under low humidification conditions.
Acknowledgement This work was supported by New & Renewable Energy R&D program (2008-N-FC12-J-01-2-100) under the Ministry of Knowledge Economy.
references
[1] Jung Un Ho, Park Ki Tae, Park Eun Hee, Kim Sung Hyun. Improvement of low-humidity performance of PEMFC by addition of hydrophilic SiO2 particles to catalyst layer. J Power Sources 2006;159:529. [2] Kim Taehee, Lee Ho, Sim Woojong, Lee Jonghyun, Kim Saehoon, Lim Taewon, et al. Degradation of proton exchange membrane by Pt dissolved/deposited in fuel cells. Korean J Chem Eng 2009;26:1265. [3] Sriring Noparat, Tantavichet Nisit, Pruksathorn Kejvalee. Preparation of Pt/C catalysts by electroless deposition for proton exchange membrane fuel cells. Korean J Chem Eng 2010;27:439. [4] Park Ki Tae, Jung Un Ho, Jeong Seong Uk, Kim Sung Hyun. Influence of anode diffusion layer properties on performance of direct borohydride fuel cell. J Power Sources 2006;162:192. [5] Passalacqua E, Lufrano F, Squadrito G, Patti A, Giorgi L. Influence of structure in low-Pt loading electrodes for polymer electrolyte fuel cells. Electrochim Acta 1998;43:3665. [6] Jordan LR, Shukla AK, Behrsing T, Avery NR, Muddle BC, Forsyth M. Effect of diffusion-layer morphology on the performance of polymer electrolyte fuel cells operating at atmospheric pressure. J Appl Electrochem 2000;30:641. [7] Jordan LR, Shukla AK, Behrsing T, Avery NR, Muddle BC, Forsyth M. Diffusion layer parameters influencing optimal fuel cell performance. J Power Sources 2000;86:250. [8] Wang Xiaoli, Zhang Huamin, Zhang Jianlu, Xu Haifeng, Zhu Xiaobing, Chen Jian, et al. A bi-functional micro-porous layer with composite carbon black for PEM fuel cells. J Power Sources 2006;162:474.
11153
[9] Giorgi L, Antolini E, Pozio A, Passalacqua E. Influence of the PTFE content in the diffusion layer of low-Pt loading electrodes for polymer electrolyte fuel cells. Electrochim Acta 1998;43:3675. [10] Park Sehkyu, Lee Jong-Won, Popov Branko N. Effect of PTFE content in microporous layer on water management in PEM fuel cells. J Power Sources 2008;177:457. [11] Qi Zhigang, Kaufman Arthur. Improvement of water management by a microporous sublayer for PEM fuel cells. J Power Sources 2002;109:38. [12] Moreira J, Ocampo AL, Sebastian PJ, Smit Mascha A, Salazar MD, del Angel P, et al. Influence of the hydrophobic material content in the gas diffusion electrodes on the performance of a PEM fuel cell. Int J Hydrogen Energy 2003; 28:625. [13] Gostick Jeffrey T, Fowler Michael W, Ioannidis Marios A, Pritzker Mark D, Volfkovich YM, Sakars A. Capillary pressure and hydrophilic porosity in gas diffusion layers for polymer electrolyte fuel cells. J Power Sources 2006;156:375. [14] Chu Hsin-Sen, Yeh Chung, Chen Falin. Effects of porosity change of gas diffuser on performance of proton exchange membrane fuel cell. J Power Sources 2003;123:1. [15] Zhan Zhigang, Xiao Jinsheng, Li Dayong, Pan Mu, Yuan Runzhang. Effects of porosity distribution variation on the liquid water flux through gas diffusion layers of PEM fuel cells. J Power Sources 2006;160:1041. [16] Zhan Zhigang, Xiao Jinsheng, Zhang Yongsheng, Pan Mu, Yua Runzhang. Gas diffusion through differently structured gas diffusion layers of PEM fuel cells. Int J Hydrogen Energy 2007;32:4443. [17] Roshandel R, Farhanieh B, Saievar-Iranizad E. The effects of porosity distribution variation on PEM fuel cell performance. Renew Energy 2005;30:1557. [18] Chen Falin, Chang Min-Hsing, Hsieh Ping-Tso. Two-phase transport in the cathode gas diffusion layer of PEM fuel cell with a gradient in porosity. Int J Hydrogen Energy 2008;33: 2525. [19] Han M, Xu JH, Chan SH, Jiang SP. Characterization of gas diffusion layers for PEMFC. Electrochim Acta 2008;53:5361. [20] Kong Chang Sun, Kim Do-Young, Lee Han-Kyu, Shul Young-Gun, Lee Tae-Hee. Influence of Pore-size distribution of diffusion layer on mass-transport problems f proton exchange membrane fuel cells. J Power Sources 2002;108:185. [21] Tang Haolin, Wang Shenlong, Pan Mu, Yuan Runzhang. Porosity-graded micro-porous layers for polymer electrolyte membrane fuel cells. J Power Sources 2002;108:185. [22] Engul Erce, Erkan Serdar, Erolu nc, Bac¸ Nurcan. Effect of gas diffusion layer characteristics and addition of pore-forming agents on the performance of polymer electrolyte membrane fuel cells. Chem Eng Comm 2008;196:161. [23] Song Ying, Wei Yu, Xu Hui, Williams Minkmas, Liu Yuxiu, Bonville Leonard J, et al. Fenton, Improvement in high temperature proton exchange membrane fuel cells cathode performance with ammonium carbonate. J Power Sources 2005;141:250. [24] Mathias MF, Roth J, Fleming J, Lehnert W. Diffusion media materials and characterization. In: Vielstich W, Lamm A, Gasteiger HA, editors. Handbook of Fuel Cells e Fundamentals Technology and applications 3. New York: Wiley; 2003. p. 517e37. [25] Hao Liang, Cheng Ping. Lattice Boltzmann simulations of anisotropic permeabilities in carbon paper gas diffusion layers. J Power Sources 2009;186:104e14.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Determination of the pore size distribution of micro porous layer in PEMFC using pore forming agents under various drying conditions Jeong Hwan Chun a, Ki Tae Park a, Dong Hyun Jo a, Ji Young Lee a, Sang Gon Kim a, Eun Sook Lee b, Jy-Young Jyoung b, Sung Hyun Kim a,* a b
Department of Chemical and Biological Engineering, Korea University, 1 Anam-Dong, Seongbuk-Ku, Seoul 136-713, South Korea Energy Research Center, HyupJin I&C Co., LTD, 143-1 Gwelang-Ri, Jungnam-Myun, Hwasung-Si, Kyunggi-Do, South Korea
article info
abstract
Article history:
In this paper, the effect of the pore size distribution of a micro-porous layer (MPL) on the
Received 17 May 2010
performance of polymer electrolyte membrane fuel cells (PEMFC) was investigated using
Received in revised form
self-made gas diffusion layers (GDLs) with different MPLs for which the pore size distri-
9 July 2010
bution was modified using pore forming agents under different drying conditions. When
Accepted 10 July 2010
MPL dried at high temperature, more macro pores, approximately 1,000e20,000 nm in
Available online 10 August 2010
diameter, and less micro pores, below 100 nm, were observed relative to when MPL was dried at low temperature. Self-made GDLs were characterized by a field-emission scanning
Keywords:
electron microscope (FE-SEM), mercury porosimetry and self-made gas permeability
Polymer electrolyte membrane
measurement equipment. The performance of the single cells was measured under two
fuel cell (PEMFC)
different humidification conditions. The results demonstrate that the optimum pore size
Micro porous layer (MPL)
distribution of MPL depended on the cell operating humidification condition. The MPL dried
Pore forming agent
at high temperature performed better than the MPL dried at low temperature under a low
Pore size distribution
humidification condition; however, MPL dried at low temperature performed better under a high humidification condition. ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Polymer electrolyte membrane fuel cells (PEMFCs) are attracting considerable interest as alternative power sources for stationary and mobile applications due to their high efficiency, low emission, and low noise. The most important component of PEMFC is the membrane electrode assembly (MEA), which consists of a polymer electrolyte membrane with catalyst layers on both sides and gas diffusion layers (GDL) [1e3]. The porous GDL plays an important role in the PEMFC. The most important function of the GDL is to distribute the reactant gas over the catalyst layer and to remove the generated product (liquid water) out of the cell.
The GDL offers a conductive path between the catalyst layer and the current collector, and provides a physical support for depositing the catalyst layer. The GDL consists of a macro porous substrate, which is called as the gas diffusion backing layer (GDBL), and a thin micro porous layer (MPL). The most promising candidates for use as the GDBL in PEMFCs are carbon fiber based products, such as non-woven papers, felts and woven clothes, due to their high porosity and good electrical conductivity. The MPL, which is composed of carbon powder and a hydrophobic agent, such as polytetrafluoroethylene (PTFE), is applied on one side of the GDBL [4]. Prior studies have been performed to understand the role of MPL. Passalacqua et al. [5] found that cells containing MPLs
* Corresponding author. Tel.: þ82 02 3290 3297; fax: þ82 02 926 6102. E-mail address: [email protected] (S.H. Kim). 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.07.056
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 1 1 1 4 8 e1 1 1 5 3
performed better than when they did not contain MPLs. The primary purpose of the MPL is for water management, since the MPL allows for effective wicking of liquid water from the cathode catalyst layer into the flow field. The MPL may also function in reducing electrical contact resistance with the adjacent catalyst layer. Recently, some researchers investigated the influences of the MPL components [6e12] and properties, especially porosity and pore size distribution [6,8,13e21], on PEMFC performance. Using numerical simulations [13e18] and experimental analysis [6,8,19e21], the porosity and pore size distribution were shown to be significantly associated with mass transport problems. In order to change the pore size distribution, various experimental methods were developed. Wang et al. [8] used carbon black, acetylene black carbon and black pearls 2000 carbon to fabricated MPLs that had various pore size distributions. Kong et al. [20] inserted Li2CO3, as a pore forming agent, into the MPL slurries to control the pore size distribution in the MPL and Tang et al. [21] used NH4Cl as a pore forming agent. Pore forming agents have been used by many fields [22,23] in addition to studies on MPL. Fig. 1 shows the mechanism of pore formation during the drying process. First, the pore forming agents must be dissolved in the MPL slurry. The MPL slurry containing the dissolved pore forming agents is then applied to the GDBL. During the drying process, the pore forming agents are solidified in MPL, since the solvent in the MPL slurry evaporates. The solidified pore forming agents are eliminated when the temperature is higher than the boiling point or decomposition temperature of the pore forming agent and at this point the pores are formed. In this study, MPLs with various pore size distributions were fabricated using pore forming agents under different drying conditions to investigate the effect of the pore size distribution in MPL on PEMFC performance. In general, contents of pore forming agent were changed to control pore size distribution in MPL, but this method caused changing of properties of MPL slurry. We controlled pore size distribution in MPL changing drying conditions. Single cell performance tests were carried out under two different humidification conditions to investigate the optimum pore size distribution in MPL for performances of PEMFC. The properties of the GDLs were characterized using FE-SEM, a self-made gas permeability measurement and mercury porosimetry.
2.
Experimental
2.1.
Preparation of MEA
11149
The wet-proofed carbon felt as GDBL was prepared by immersing commercial carbon felts in a 5.0 wt.% PTFE dispersion for 1 min, and then drying in air. The carbon slurry for the MPL was prepared using the following procedures. The carbon black, 60 wt.% PTFE dispersion, isopropyl alcohol (IPA), distilled water, and a pore forming agent, which was a type of ammonium salt, were mechanically mixed using a digital stirrer. The pore forming agent concentration was 20 wt.% of the carbon black. The slurries were applied to the wet-proofed carbon felts using a knife coating method and then dried at various temperatures. After the drying process was complete, the prepared GDL samples were sintered in air at 350 C for 30 min. The conditions used to fabricate the cathode GDL samples are shown in Table 1. The commercial carbon felt with MPL was used as the anode GDL and the commercial catalyst coated membranes with a 25 cm2 geometrically active area were used.
2.2.
Characterization of GDLs
The morphology of MPLs was characterized by FE-SEM (S-4300, Hitachi Co.). Images magnified by factors of 1,000 and 30,000 were captured to observe macro pores and micro pores, respectively. The pore size distribution of MPLs was measured using a mercury intrusion porosimetry (Autopore IV 9500, Micromeritics Inc.). Gas permeability was characterized by the Darcy coefficient, which can be estimated by applying a pressure drop and observing the flow [24]. The Darcy’s law is: V¼K
a DP ml
(1)
where V is the volumetric flow rate, K is the Darcy coefficient (gas permeability), a is the cross-sectional area through which the flow passes, m is the gas viscosity of the flow material (nitrogen), l is the length of the convective path (thickness of GDL sample), and DP is the pressure differential across the
Fig. 1 e Mechanism of pore formation in MPL using a pore forming agent.
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Table 1 e Fabrication conditions of the cathode GDLs. Samples
PF2-LT PF2-MT PF2-HT PFO-MT
Contents of pore forming agent (wt.%) 20 20 20 0
Drying conditions
60 C
80 C
24 min 3min
8min
3min
sample. Fig. 2 shows a diagram of the self-made gas permeability measurement equipment.
2.3.
Single cell test conditions
A single cell test was carried out using a 25 cm2 active area test cell in humidified H2/air gases. The operating temperature of the cell was 65 C and the test was operated under atmospheric pressure. The bubbling humidifier temperature of both cathode and anode were kept constant at 65 C (about RH 100%) under the high humidification condition and 45 C (about RH 40%) under the low humidification condition. The polarization curves were measured under constant current mode using a 4 A current step and a 2 min dwell time. The stoichiometries of hydrogen (lH2 ) and air (lair) were 1.5 and 2.0, respectively.
3.
Results and discussion
3.1. Characterization analysis of self-made GDLs with different MPLs The pore size distribution of self-made MPLs is shown in Fig. 3. The pores in the MPLs were divided according to size into three categories; macro pores e 5,000e20,000 nm, meso pores e 100e5,000 nm, and micro pores >100 nm. PF2-MT,
Fig. 2 e Self-made gas permeability measurement equipment.
Heat treatment
100 C
120 C
150 C
10 min 3min 10 min
3 min 10min 3 min
3 min
350 C 30 min
which was fabricated using the pore forming agent, had a large volume of macro, meso, and micro pores relative to PFO-MT, which was fabricated without the pore forming agent (Fig. 3). This indicates that the pore forming agent contributed to the formation of more pores in PF2-MT. It was also shown that the volume of the macro pores in MPL increased with increasing drying temperature, whereas the volume of the micro pores decreased. As described in the introduction, the pore forming agent in the MPL slurry produces pores by decomposing or evaporating during the drying process. In general, the rates of degradation and evaporation increase as the temperature increases. Therefore, rapid pore forming agent degradation, which is caused by a high drying temperature, results in the formation of several macro pores. In contrast, a low drying temperature results in the gradual degradation of the pore forming agent, and more micro pores are produced. The morphology of the MPL surface is shown in Fig. 4. SEM images, which were magnified by factors of 1,000 (Fig. 4(a), (c) and (e)) and 30,000 (Fig. 4(b), (d) and (f)), were captured to observe the macro and micro pores, respectively. As shown in Fig. 4, PF2-HT (Fig. 4(e), and (f)) had many more macro pores and less micro pores relative to PF2-LT (Fig. 4(a), and (b)). These images demonstrate that an increase in the drying temperature promotes formation of macro pores. Fig. 5 shows the gas permeability of self-made GDLs with different MPLs. The gas permeability of MPL dominated the overall gas permeability of GDL, because the mean pore
Fig. 3 e Effect of drying conditions on the pore size distribution of MPLs.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 1 1 1 4 8 e1 1 1 5 3
diameter of the MPL was small compared to the GDBL. According to the KozenyeCarman relation, the gas permeability is related to total porosity and pore diameter by the following equation: K¼
33 kc ð1 3Þ2
d2p
where kc is KozenyeCarman constant, 3 is total porosity, and dp is pore diameter [25]. According to the above equation, the gas permeability of GDL increases with an increase in the pore diameter of GDL. In Fig. 5, the gas permeability of PF2-MT was higher than PFO-MT, because the pore forming agent produced more macro, meso, and micro pores in PF2-MT. The gas permeability of GDL increased with increasing MPL drying temperature. The gas permeability of PF2-HT was higher than PF2-LT, because PF2-HT had more macro pores than PF2-LT.
3.2.
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Single cell performances
The polarization curves of single cells fabricated with various MPLs under different humidification conditions are shown in Figs. 6 and 7. The tests were performed under two different humidification conditions. Fig. 6 shows the single cell performance under the high humidification condition (cell: 65 C, the bubbling humidifier: 65 C). As shown in Fig. 6, the best performance under the high humidification condition was obtained when the PF2-LT was used. The difference in the cell performances of the four samples was especially evident in the high current density region. This difference resulted from the volume of the micro pores in MPLs. It is very important to remove water generated at the catalyst layer, particularly under high humidification conditions. The micro pores in MPL play an important role in the management of
Fig. 4 e The morphology of the self-made MPL surface; (a) PF2-LT, 1,000 magnification, (b) PF2-LT, 30,000 magnification, (c) PF2-MT, 1,000 magnification, (d) PF2-MT 30,000 magnification, (e) PF2-HT, 1,000 magnification, (f) PF2-HT, 30,000 magnification.
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 1 1 1 4 8 e1 1 1 5 3
Fig. 5 e Gas permeability of the self-made GDLs with different MPLs determined using self-made gas permeability measurement equipment.
water in the cathode GDLs. The generated water has to be emitted through the GDL to prevent water from flooding into the high current density region. Capillary-driven liquid flow through GDLs may predominate in fuel cell operation. According to the YoungeLaplace equation, the capillary pressure is related to the pore size by the following equation: Pc ¼
2gcosq r
where Pc is the capillary pressure, g is the surface energy of water, q is the contact angle of water with the surface of pore, and r is the radius of pores [8]. According to the above equation, the micro pore has an advantage over the macro pores in water removal, since the capillary pressure increases with decreasing pore diameter. PF2-LT had the best ability to remove the water produced at the catalyst layer, because PF2-LT had the largest volume of micro pores among the four samples. Single cell performances under the low humidification condition (cell: 65 C, the bubbling humidifier: 45 C) are shown
Fig. 7 e Polarization curves using the self-made GDLs with different MPLs under the low humidification condition; cell temperature 65 C, cathode and anode humidifiers temperature 45 C, lH2 [ 1.5, lair [ 2.0.
in Fig. 7. As the relative humidity of the inlet gas (hydrogen and air) decreased, the water content of the polymer electrolyte membrane decreased. This leads to a decrease in the proton conductivity of the polymer electrolyte membrane and an increase in ohmic loss. Thus, the current density at 0.6 V under the low humidification condition was 35 percent of the current density under the high humidification condition. More water, which was generated at the cathode catalyst layer, moved towards the dry polymer electrolyte membrane due to back diffusion under the low humidification condition. This indicates that there was a lower risk of flooding when the cell was operated under low humidification. Therefore, the sufficient supply of air and hydrogen was more important than water removal under the low humidification condition in terms of mass transport. The MPL must have high gas permeability in order to sufficiently supply hydrogen and air. As shown in Fig. 7, the best performance was obtained when the PF2-HT was used as the cathode GDL under the low humidification condition. PF2-HT had the greatest gas permeability among the four samples, since PF2-HT had the largest volume of macro pores. Thus, an increase in the volume of macro pores in the cathode MPL enhanced the cell performance under the low humidification condition. In short, using the GDL which had a lot of micro pores in MPL improved single cell performance under high humidification condition, while using the GDL which contained plenty of macro pores in MPL improved single cell performance under low humidification condition. The results of gas permeability of GDLs measurement and water removal analysis using YoungeLaplace equation support the single cell test results.
4. Fig. 6 e Polarization curves using self-made GDLs with different MPLs under the high humidification condition; cell temperature 65 C, cathode and anode humidifiers temperature 65 C, lH2 [ 1.5, lair [ 2.0.
Conclusions
The pore size distribution of MPLs was modified using a pore forming agent under different drying conditions to investigate the effect of the pore size distribution of MPL on cell performance. The pore size distribution, gas permeability, and
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 1 1 1 4 8 e1 1 1 5 3
surface morphology of the MPL were characterized and the performance of the single cell was measured under both high and low humidification conditions. As the drying temperature increased, formation of macro pores increased and micro pores were retrained. Therefore, PF2-HT dried at high temperature had the largest volume of macro pores but the smallest volume of micro pores. The single cell test results demonstrated that the optimum pore size distribution of MPL depended on the humidification condition. The best cell performance was obtained when the PF2-LT was used under the high humidification condition; however, the PF2-HT performed the best under the low humidification condition. MPL containing several micro pores (PF2-LT) can more efficiently remove water under high humidification conditions, which is beneficial to mass transport; however, the MPL containing several macro pores (PF2-HT) has the advantage of more efficiently supplying air and hydrogen under low humidification conditions.
Acknowledgement This work was supported by New & Renewable Energy R&D program (2008-N-FC12-J-01-2-100) under the Ministry of Knowledge Economy.
references
[1] Jung Un Ho, Park Ki Tae, Park Eun Hee, Kim Sung Hyun. Improvement of low-humidity performance of PEMFC by addition of hydrophilic SiO2 particles to catalyst layer. J Power Sources 2006;159:529. [2] Kim Taehee, Lee Ho, Sim Woojong, Lee Jonghyun, Kim Saehoon, Lim Taewon, et al. Degradation of proton exchange membrane by Pt dissolved/deposited in fuel cells. Korean J Chem Eng 2009;26:1265. [3] Sriring Noparat, Tantavichet Nisit, Pruksathorn Kejvalee. Preparation of Pt/C catalysts by electroless deposition for proton exchange membrane fuel cells. Korean J Chem Eng 2010;27:439. [4] Park Ki Tae, Jung Un Ho, Jeong Seong Uk, Kim Sung Hyun. Influence of anode diffusion layer properties on performance of direct borohydride fuel cell. J Power Sources 2006;162:192. [5] Passalacqua E, Lufrano F, Squadrito G, Patti A, Giorgi L. Influence of structure in low-Pt loading electrodes for polymer electrolyte fuel cells. Electrochim Acta 1998;43:3665. [6] Jordan LR, Shukla AK, Behrsing T, Avery NR, Muddle BC, Forsyth M. Effect of diffusion-layer morphology on the performance of polymer electrolyte fuel cells operating at atmospheric pressure. J Appl Electrochem 2000;30:641. [7] Jordan LR, Shukla AK, Behrsing T, Avery NR, Muddle BC, Forsyth M. Diffusion layer parameters influencing optimal fuel cell performance. J Power Sources 2000;86:250. [8] Wang Xiaoli, Zhang Huamin, Zhang Jianlu, Xu Haifeng, Zhu Xiaobing, Chen Jian, et al. A bi-functional micro-porous layer with composite carbon black for PEM fuel cells. J Power Sources 2006;162:474.
11153
[9] Giorgi L, Antolini E, Pozio A, Passalacqua E. Influence of the PTFE content in the diffusion layer of low-Pt loading electrodes for polymer electrolyte fuel cells. Electrochim Acta 1998;43:3675. [10] Park Sehkyu, Lee Jong-Won, Popov Branko N. Effect of PTFE content in microporous layer on water management in PEM fuel cells. J Power Sources 2008;177:457. [11] Qi Zhigang, Kaufman Arthur. Improvement of water management by a microporous sublayer for PEM fuel cells. J Power Sources 2002;109:38. [12] Moreira J, Ocampo AL, Sebastian PJ, Smit Mascha A, Salazar MD, del Angel P, et al. Influence of the hydrophobic material content in the gas diffusion electrodes on the performance of a PEM fuel cell. Int J Hydrogen Energy 2003; 28:625. [13] Gostick Jeffrey T, Fowler Michael W, Ioannidis Marios A, Pritzker Mark D, Volfkovich YM, Sakars A. Capillary pressure and hydrophilic porosity in gas diffusion layers for polymer electrolyte fuel cells. J Power Sources 2006;156:375. [14] Chu Hsin-Sen, Yeh Chung, Chen Falin. Effects of porosity change of gas diffuser on performance of proton exchange membrane fuel cell. J Power Sources 2003;123:1. [15] Zhan Zhigang, Xiao Jinsheng, Li Dayong, Pan Mu, Yuan Runzhang. Effects of porosity distribution variation on the liquid water flux through gas diffusion layers of PEM fuel cells. J Power Sources 2006;160:1041. [16] Zhan Zhigang, Xiao Jinsheng, Zhang Yongsheng, Pan Mu, Yua Runzhang. Gas diffusion through differently structured gas diffusion layers of PEM fuel cells. Int J Hydrogen Energy 2007;32:4443. [17] Roshandel R, Farhanieh B, Saievar-Iranizad E. The effects of porosity distribution variation on PEM fuel cell performance. Renew Energy 2005;30:1557. [18] Chen Falin, Chang Min-Hsing, Hsieh Ping-Tso. Two-phase transport in the cathode gas diffusion layer of PEM fuel cell with a gradient in porosity. Int J Hydrogen Energy 2008;33: 2525. [19] Han M, Xu JH, Chan SH, Jiang SP. Characterization of gas diffusion layers for PEMFC. Electrochim Acta 2008;53:5361. [20] Kong Chang Sun, Kim Do-Young, Lee Han-Kyu, Shul Young-Gun, Lee Tae-Hee. Influence of Pore-size distribution of diffusion layer on mass-transport problems f proton exchange membrane fuel cells. J Power Sources 2002;108:185. [21] Tang Haolin, Wang Shenlong, Pan Mu, Yuan Runzhang. Porosity-graded micro-porous layers for polymer electrolyte membrane fuel cells. J Power Sources 2002;108:185. [22] Engul Erce, Erkan Serdar, Erolu nc, Bac¸ Nurcan. Effect of gas diffusion layer characteristics and addition of pore-forming agents on the performance of polymer electrolyte membrane fuel cells. Chem Eng Comm 2008;196:161. [23] Song Ying, Wei Yu, Xu Hui, Williams Minkmas, Liu Yuxiu, Bonville Leonard J, et al. Fenton, Improvement in high temperature proton exchange membrane fuel cells cathode performance with ammonium carbonate. J Power Sources 2005;141:250. [24] Mathias MF, Roth J, Fleming J, Lehnert W. Diffusion media materials and characterization. In: Vielstich W, Lamm A, Gasteiger HA, editors. Handbook of Fuel Cells e Fundamentals Technology and applications 3. New York: Wiley; 2003. p. 517e37. [25] Hao Liang, Cheng Ping. Lattice Boltzmann simulations of anisotropic permeabilities in carbon paper gas diffusion layers. J Power Sources 2009;186:104e14.