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Effects of microstructure characteristics of gas diffusion layer and microporous layer on the performance of PEMFC
Energy Conversion and Management 51 (2010) 677–684
Contents lists available at ScienceDirect
Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Effects of microstructure characteristics of gas diffusion layer and microporous layer on the performance of PEMFC Chung-Jen Tseng *, Shih-Kun Lo Department of Mechanical Engineering, National Central University, Chungli, Taoyuan 320, Taiwan
a r t i c l e
i n f o
Article history: Received 6 April 2009 Accepted 7 November 2009 Available online 25 November 2009 Keywords: Gas diffusion layer Microporous layer Fuel cell
a b s t r a c t Water management is an important issue in proton exchange membrane (PEM) fuel cell design and operation. The purpose of this work is to investigate the effects of the microstructure characteristics of the gas diffusion layer (GDL) and microporous layer (MPL), including pore size distribution, hydrophobic treatment, gas permeability, and other factors, on the water management and performance of a PEM fuel cell. A commercial catalyst-coated membrane with an active area of 25 cm2 is used along with a GDL and an MPL for assembling a single cell. The effects of the MPL, the thickness of the MPL, the PTFE loading of carbon paper and MPL, and the baking time of the MPL have been investigated. Results show that the addition of MPL increases cell performance in the high current density region due to the elimination of mass transfer limitation. There exists an optimum thickness of MPL. Furthermore, increasing the MPL baking time enhances cell performance due to enlarged pore size and permeability. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Proton exchange membrane fuel cells (PEMFCs) convert the chemical energy of a fuel directly into electricity in an electrochemical reaction. PEMFCs are a viable alternative for environment-friendly and efficient power production and have a wide range of potential applications. In terms of environmental and global energy problems, the replacement of fossil fuels by renewable energy is being considered all over the world. The fuel cell as a revived energy technology is believed to be an ideal energy system because of its high conversion efficiency and excellent regenerative and zero-emission properties. However, like many emerging energy technologies, PEMFCs must overcome certain engineering and economic obstacles if they are to ever become commercially and popularly viable. Therefore, in order to ensure, without spatial concession, high power for a commercial-scale system, it is predicted that fuel cell operation at high current density conditions per unit electrode area will be necessary. Fig. 1 depicts the schematic diagram of the configuration and mass transport mechanism in the membrane electrode assembly (MEA) and flow channels. Water vapor or liquid water, along with hydrogen and oxygen gases, enters and exits through the diffusion path in the electrode. Electrodes for a PEMFC have, in general, twoor three-layered structures that can be divided into two parts, one catalytic and the other noncatalytic. The catalytic part is the catalyst layer. It is formed by depositing a mixture of a carbon-sup* Corresponding author. Tel.: +886 3 4267348. E-mail address: [email protected] (C.-J. Tseng). 0196-8904/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2009.11.011
ported platinum catalyst. A hydrophobic gas diffusion layer (GDL) and sometimes an additional microporous layer (MPL) are placed between the catalyst layer and the flow channel plate. Although the GDL and MPL have no electrochemical reaction sites, they are known to play an important role in providing the reactants a good access to the catalytic sites and in the effective removal of product water from the electrode. Mass transport problems in a PEMFC can be classified into three categories: (1) water flooding, i.e., the liquid water entrapped inside the electrodes or flow channels interrupts the flux of the reactant/ product gases; (2) dilution of oxidant concentration due to the use of air instead of pure oxygen; and (3) depletion of reactants along the flow channel, which results in a nonuniform current distribution over the whole electrode area and is particularly severe in a largescale fuel cell [1]. Appropriate water management is critical in order to obtain a stable cell performance of the PEMFC. On one hand, water is needed to hydrate the ionically conducting membrane and the ionic conductor in the catalyst layer for proton conductance. That is, the higher the water content, the better is the ionic conductivity. Water can be introduced by external humidification and as a product of the oxygen reduction reaction at the cathode side. On the other hand, this water can flood the pores in the catalyst layers as well as those in the gas diffusion layers, resulting in a higher mass transport resistance. That is, the less the water content, the lower is the resistance to reactant flow. Clearly, water plays a conflicting role; therefore, appropriate water management is required. The GDL is usually made of either a nonwoven carbon paper or a woven carbon cloth due to its high porosity and electric conductivity. Even if the MEA based on a carbon cloth has a higher
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Fig. 1. Schematic diagram of the configuration and mass transport mechanism in a fuel cell.
power performance than that based on a carbon paper, there are considerable theoretical and industrial interests in using carbon paper as there is a cost advantage associated with using a nonwoven substrate and it is more convenient to fabricate an MPL or catalyst layer onto it. The GDL is required to ensure efficient diffusion of each reactant gas to the catalyst and assists water management during the operation of the fuel cell [2]. Lee et al. [3] studied three types of GDLs, and they discovered that the optimum performance was related to the gasket thickness and the measured compression Ò pressure on the GDLs. In their results, the GDL of ELAT had better cell performance in 144 kgf cm/bolts. The cell performance may also be affected by the change in the porosity of the GDL, electric contact resistance, and water content in the membrane. For a slightly brittle and less compressible material, the experimental results showed that high cell performance was obtained with less torque, indicating that the excessive bolt torque might have damaged the GDLs in the fuel cell. Lim and Wang [4] examined a GDL by using the scanning electron microscopy (SEM) technique when a high compression pressure was applied on the GDL. The high compression of the fuel cell led to a remarkable decrease in the concentration limiting current density because of the hydrophilic broken fibers and cleavage defects generated during excessive compression. Giorgi et al. [5] investigated the influence of the polytetrafluoroethylene (PTFE) content of the MPL on the performance of a PEMFC cathode. The MPL was found to improve cell performance, and the optimum PTFE content was found to be 10 wt.% in the MPL. Song et al. [6] used AC impedance to determine the optimal composition of PEMFCs. In their investigation, they used an MPL to prevent the catalyst from falling into the pores of the carbon paper. Jordan et al. [7] used sintered GDLs made from acetylene black (AB) carbon that had a low pore volume. With an optimum thickness, the cell showed better performance than cells made with Vulcan XC-72R carbon. This was explained by the difference in the porosities of the two types of carbon. AB carbon, with a lower pore volume in the range of 10–100 lm, was thought to allow less liquid water to permeate into the diffusion layer. Yoon et al. [8] studied the effect of the pore structure of the cathode catalytic layer in a PEMFC on the performance of the cell. From the I–V characteristics of the catalytic layers, it was concluded that the pore structure was an important factor in determining the cell performance. Soler et al. [9] investigated the effect of both the permeability of the electrodes and the configuration of the gas flow distributor on
the cell performance of a PEMFC. For this purpose, MEAs with carbon paper and carbon cloth were characterized electrochemically by measuring the I–V characteristics curves. The experimental results showed that the fuel cell performance strongly depended on both the gas permeability and the type of flow distributor. Lee et al. [10] studied the effect of the fabrication method and the thickness of a GDL and the impregnation method of a Nafion solution on the cell performance. The GDL was prepared by rolling, spraying, and screen-printing methods, and the Nafion solution was impregnated using spraying and brushing methods. They found that the spraying method was better in terms of cell performance than the brushing method for the impregnation of the Nafion solution because it reduced the charge-transfer resistance of the solution and extended the three-phase region. In the same year, Lim and Wang [11] discussed the effects of the hydrophobic polymer content in the GDL. They compared the performance of fuel cells made from MEAs consisting of 10 wt.% fluorinated ethylene propylene (FEP)-impregnated GDL, 30 wt.% FEP-impregnated GDL, and 30 wt.% FEP-impregnated GDL coated with an MPL (40 wt.% PTFE + Vulcan carbon black). They found that the GDL coated with an MPL had the lowest limiting current density. They ascribed this result to the additional diffusion resistance to oxygen transport that the MPL might impose. However, they also noted that a properly designed and fabricated MPL should be beneficial at high current densities. Furthermore, they found that the surface contact angle of a wet-proof treated GDL generally decreased with temperature, and there was an insignificant difference in the contact angle on the carbon paper treated with different contents of PTFE. Yan et al. [12] studied the effects of electrode fabrication processes and material parameters on cell performance of PEMFC. They used FEP in the MPL. If the FEP content is too high, the pores in MPL are quite small; and the fuel gas is difficult to diffuse in the MPL. If the FEP content is too low, the water generated in the cell cannot be effectively removed. The best cell performance was found with a MPL using 10% FEP content in the carbon paper and 20% content in the micro porous layer. Jin et al. [13] studied the condensed water in the diffusion medium of PEMFC. He found that the local effective mass diffusivity of a fibrous diffusion medium is determined as a function of the local porosity and local water saturation. He also pointed out that the cell performance is affected by the capillary pressure, liquid water permeability, bulk porosity, and other factors. In the flow field design, Rajalakshmi et al. [14] reported the water transport in fuel cells with different flow field designs. The major factor which affects the performance of a fuel cell operated with dry gas is the transport of product water from cathode to anode, which in turn depends on the area of the electrode, electrode preparation and flow field design. Su et al. [15] proposed that the parallel and interdigitated flow channels are more easily flooded than serpentine flow channel. In the serpentine flow channel, the pressure in the upstream is larger than in the downstream, so the liquid water can be pushed into the downstream more easily. It also mentioned that flooding usually occurred in the corners of channels. The channel profile should be fitted to the streamline pattern in order to decrease the corner effect of flooding. Ying et al. [16] used the Kozeny–Carman equation to calculate gas permeability in the GDL. The K–C equation is a typical example of semi-empirical formula, which is
K¼
/nþ1 Cð1 /Þn
where / is the porosity, and the exponential n and constant C are called Kozeny–Carman constants. These two constants vary for different porous media. Thus far, few studies have reported on the effects of pore structure, pore size distribution, and other microscale characteristics of
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Table 1 Fuel cell properties and experimental conditions. Active area CCM Gas diffusion layer Thickness of GDL Catalyst loading (anode) Catalyst loading (cathode) Flow field structure Cell temperature Humidification temperature Fuel Oxidant
25 cm2 Gore 5621 TGPH-120 0.35 mm 0.45 mg Pt–Ru/cm2 0.60 mg Pt/cm2 Serpentine 60 °C 70 °C H2 O2
the MPL and GDL. The underlying physical mechanisms are not well explained. In the present study, the pore size range, pore size distribution, and pore structure are measured to investigate their effects on cell performance. 2. Experiments 2.1. Preparation of MPL The MPL is prepared by the following three-step process, which includes several pretreatment procedures. The first step is to deposit a diffusion layer on the carbon paper substrate. Carbon powder, Triton, PTFE, and isopropyl alcohol (IPA) as the pore former are mechanically mixed in a mixer. The resulting MPL ink is scraped on the wet-proofed carbon paper and then dried at 80 °C for 1 h. Next, the MPL and GDL is sintered in an oven at 350 °C for 4 h. The thickness of the MPL was measured using a Mitutoyo micrometer which has an accuracy of 2 lm. Finally, the dried MPL and GDL are assembled with a catalyst-coated membrane (CCM) to form an MEA. The catalyst loading is 0.45 mg/cm2 of Pt–Ru at the anode and 0.60 mg/ cm2 of Pt at the cathode (in this study we used Pt–Ru in the anode to avoid the effect of CO poisoning in the anode). Table 1 summarizes the data of the MEA used in this work. 2.2. Investigation of morphological characteristics The morphological characteristics of the MPL and GDL are measured using SEM, porosimetry, and a surface contact angle meter. SEM provides the image of the surface structure of the MPL and GDL. The information on the internal structure (i.e., pore size distribution and gas permeability) of the MPL and GDL is obtained using a capillary flow porometer (CFP-1100AE, Porous Materials, Inc., Ithaca, New York, USA). In this experiment structure, we used air with 100 psi pressure to deter the pore distribution and gas permeability of the sample [17]. 2.3. Cell operation and electrochemical analyses A single PEMFC is operated at a cell temperature of 60 °C, and the humidification temperatures of the anode and the cathode side are maintained at 70 °C. The active area of MEAs is 25 cm2 in this study. The flow field used is serpentine in both the anode and the cathode. During the experiment, the cell temperature and humidification temperature are kept constant. Nitrogen is used to purge the flow channels between measurements. The flow rates of the anode and cathode gases are set at 300 ml/min initially. All experiments are performed without external pressurization. The current density versus cell voltage curves of the fuel cell are obtained with a data acquisition system consisting of a personal computer, data acquisition board, electronic load, and other supporting devices.
Fig. 2. The SEM micrographs of the GDLs (a) without MPL and (b) with MPL.
3. Results and discussion 3.1. Effect of MPL on cell performance The effects of MPL on the performance of the unit cell are discussed first. In this section, we will discuss the surface structure, gas permeability, and pore size distribution for GDLs with and without an MPL. Fig. 2 shows the SEM images of the GDLs with and without an MPL. Fig. 2a is for the GDL (TGPH-120-20) without an MPL. We can see that the surface of the GDL is irregular and rough, and the pores are relatively large. The PTFE coating can be seen on the carbon fiber surface. In the GDL with an MPL case, Fig. 2b, the carbon particles are very close to each other and the pore sizes are very small. Generally speaking, adding the MPL may prevent the catalyst ink from falling into the pores of the carbon paper, and reduce the contact resistance between the catalyst layer and the GDL because the MPL has a smoother surface and a smaller pore size than the GDL. The pore size distributions of the two samples are plotted in Fig. 3a and b. The ordinate in both figures shows the percentage of the various pore sizes. For the carbon-paper GDL, most of the pores are in the range of 18–26 lm, whereas for the GDL with an MPL case, the primary pore sizes are shifted to the range of 0.32–12 lm. Pore size distribution is an important physical characteristic of the GDL and MPL, especially for the water management of the fuel cell. The pore size distribution affects the capillarity. A small pore has higher capillarity, and it can remove water more easily. However, small pores also reduce the gas permeability. On the contrary, a large pore provides better gas transport and lower mass transport resistance. For oxygen molecules at room temperature and atmospheric pressure, the mean free path is approximately 0.1 lm.
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Fig. 5. Effect of the MPL thickness on the cell performance.
Fig. 3. Pore size distribution in the GDLs (a) without MPL and (b) with MPL.
water transport from the GDL to the flow channels. However, it may not be very easy to remove the water that forms in the catalyst layer. For the sample with an MPL, the pores consist of mesopores and macropores. While the macropores provide efficient gas diffusion and water removal, the mesopores help in transporting water from the catalyst layer to the GDL. Finally, with the same MEA, the effect of the GDL with and without an MPL on the performance of a single cell is measured through a performance test using hydrogen and oxygen as the fuel and oxidant, respectively. The results are illustrated in Fig. 4. In the low and intermediate current density regions, the two setups show similar performances. However, in the high current density region, the influence of the MPL is very clear. The cell with the MPL does not show any sign of water flooding for current densities up to 1350 mA/cm2. On the other hand, the output voltage of the cell without the MPL drops very fast after 700 mA/cm2 due to mass transfer polarization. 3.2. Effect of MPL thickness on cell performance
Fig. 4. Effect of MPL on the cell performance of a single cell.
Therefore, pores larger than 5 lm may be classified as macropores; diffusion in these pores is mainly due to molecular collisions. On the other hand, pores smaller than 0.01 lm may be classified as micropores, and Knudsen diffusion is dominant in these pores. The pores in the range of 0.01–5 lm can be called mesopores and are a transition between the micropores and the macropores. As shown in Fig. 3, for the sample without MPL, most of the pores are macropores; these pores provide efficient gas diffusion and
Fig. 5 shows the effect of the thickness of an MPL on the performance of a cell. Measured after baking, the thicknesses of the MPL are varied as 38, 84, and 136 lm. There seems to exist an optimum thickness of 84 lm for the cases tested. An explanation for this value will be given after examining the pore characteristics. The pore size distributions for the three cases are presented in Fig. 6, and the pore characteristics are summarized in Table 2. We also used the K–C equation to calculate the theoretical gas permeability in MPL. And we in which C ¼ 180=k2mean , the mean pore diameter kmean is computed by experimental data from CFP. And the mean pore diameter increases with the thickness, the averaged gas permeability also. We can see that the gas permeability of 136-lm is the smallest. The difference in MPL thickness on the cell performance results as shown in Fig. 5. We can see the 84-lm sample has better performance than the 38-lm sample in all current density regions; the 136-lm sample has worse performance than the 84lm sample in the intermediate and high current density regions where mass transport dominates. A suitable MPL improves the reactant gas supply and facilitates the removal of the product water. An extremely thin MPL has smaller pores that may hinder gas supply and increase mass transport resistance. On the other hand, an extremely thick MPL has a higher
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C.-J. Tseng, S.-K. Lo / Energy Conversion and Management 51 (2010) 677–684 Table 3 Pore characteristics of MPL-coated GDLs with different PTFE loadings. Sample (lm)
Mean pore diameter (lm)
Smallest pore size (lm)
Average Darcy permeability (darcy)
K–C equation permeability (darcy)
TGPH-120-0-84 TGPH-120-20-84 TGPH-120-40-84
4.22 3.84 2.33
0.43 0.32 0.32
0.299 0.340 0.086
0.281 0.342 0.097
Fig. 6. Pore size distribution in an MPL with different thicknesses. MPL thickness: (a) 38 lm, (b) 84 lm, and (c) 136 lm.
Table 2 Pore characteristics for three different thicknesses of MPL. Sample (lm)
Mean pore diameter (lm)
Smallest pore size (lm)
Average Darcy permeability (darcy)
K–C equation permeability (darcy)
TGPH-120-20-38 TGPH-120-20-84 TGPH-120-20-136
3.57 3.84 7.84
0.39 0.32 0.49
0.442 0.340 0.205
0.403 0.342 0.217
Fig. 7. Pore size distributions in carbon paper. (a) 0 wt.% PTFE loading, (b) 20 wt.% PTFE loading, and (c) 40 wt.% PTFE loading.
3.3. Effect of PTFE loading of GDL electronic resistance because of the increased amount of PTFE. A thick layer also hampers the accessibility of the reactant gas due to the lengthened diffusion path.
In this section, we discuss the effect of PTFE loading of GDL on cell performance. The PTFE loadings in the GDL are 0 wt.%, 20 wt.%,
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Fig. 8. Polarization curves of cells with different PTFE loading in GDL.
Fig. 9. Comparison of polarization curves of cells with MPLs having different PTFE loadings.
and 40 wt.%. All three GDLs are coated with the same MPL. The mean pore diameter of the resulting MPL + GDL decreases with increasing PTFE loading in the GDL as shown in Table 3. The table also shows the gas permeability data. The carbon paper with 20 wt.% PTFE has the highest gas permeability in the three cases studied. The measured value of the gas permeability for the 40 wt.% sample is very low compared to that of the other samples and is only about 1/4 of the 20 wt.% sample. Fig. 7 presents the pore size distribution of the GDL with different PTFE loading. For the 40 wt.% PTFE loading sample, although the pore size range is from 0.32 to 24.59 lm, almost 70% of the pores are smaller than 4.0 lm. This may be the reason that it has the lowest gas permeability. Polarization curves of the cells made of the three GDLs are illustrated in Fig. 8. The 20 wt.% PTFE case has the best cell performance. This implies that 20 wt.% PTFE can facilitate the reactant gas and product water transport better than other PTFE loadings. In Fig. 8, the cell performance without any PTFE content in carbon paper is very poor, even with the MPL. The product water is removed quickly from the catalyst layer to the GDL but remains in
Fig. 10. Pore size distributions of MPLs with different PTFE loadings. (a) 10 wt.%, (b) 25 wt.%, (c) 40 wt.%, and (d) 55 wt.%.
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Fig. 11. Comparison of polarization curves of cells with MPLs with different baking times.
the pores of the GDL and hinders the reactant gas from entering into the MPL and the catalyst layer. Therefore, the presence of PTFE in a GDL is necessary for efficient removal of water. However, a higher PTFE loading in carbon paper will increase the electronic resistance and decrease the pore size and gas permeability in the GDL. Therefore, there exists an optimum amount of PTFE loading. 3.4. Effect of PTFE loading of MPL In this section, we will discuss the effect of the PTFE loading of an MPL. From Fig. 9, it is observed that the MPL that has 40 wt.% of PTFE has a better cell performance than others. The reason for this better performance is that using more PTFE loading in the MPL results in higher gas permeability and easier water removal. In other words, PTFE loading of 40 wt.% provides lower mass transport resistance in the high current density zone. Moreover, as shown in Fig. 10, the pore size distribution results also showed that the MPL with PTFE loading less than 25 wt.% had more small pores than the MPL with 25 and 40 wt.% PTFE. This may be due to the fact that during the baking process, part of PTFE vaporizes, and the remaining part cures, producing larger pores. Therefore, the MPL with a greater PTFE loading has less small pores and higher gas permeability. 3.5. Effect of the baking time of MPL From Fig. 11, it is observed that the MPL baked for 9 h has a better cell performance than that baked for 1 or 5 h. But in the baked for 13 h, the cell performance is almost the same with 9 h. Fig. 12 shows the pore size distribution with different baking times. In this experiment, we find that a longer baking time will result in larger pores and higher gas permeability. That is, when the baking time is 9 h, the fuel and oxidant can be transported more easily in the GDL and MPL. Therefore, when an MPL that was baked for 9 h is used, the cell performance is better than that of the other MPLs considered in this experiment. 4. Conclusions In this study, the effects of the physical characteristics of the GDL and MPL on the cell performance are investigated. A commercial CCM with an active area of 25 cm2 is used along with a GDL
Fig. 12. Pore size distributions of MPLs with different baking times. (a) 1 h, (b) 3 h, (c) 9 h, and (d) 13 h.
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and an MPL for assembling a single cell. The effects of the MPL, the thickness of the MPL, the PTFE loading of carbon paper, and the MPL have been investigated. The main findings based on the experimental results are summarized as follows: 1. Under light and intermediate load conditions, fuel cells with and without MPL show similar performances. However, in high current density region, the cell with an MPL has much better performance and does not show any sign of mass transfer limitation. 2. A suitable MPL improves the reactant gas supply and facilitates the removal of the product water. An extremely thin MPL has very small pores and low gas permeability that limits the gas supply and increases the mass transport resistance. An extremely thick MPL has a higher electronic resistance because of the increased amount of PTFE. 3. For the three PTFE loadings in the GDL studied in this work, the 20 wt.% case has the best cell performance. 4. The 40 wt.% PTFE loading in MPL case has the best cell performance. The highest PTFE loading in this experiment has the highest gas permeability. 5. A longer MPL baking time can provide higher gas permeability, and the cell performance is also better than that in the shorter baking time cases. Acknowledgements The authors would like to thank the National Science Council of Taiwan for the partial funding for this work through the Grant NSC93-2212-E-008-004. References [1] Kong CS, Kim DY, Lee HK, Shul YG, Lee TH. Influence of pore-size distribution of diffusion layer on mass-transport problems of proton exchange membrane fuel cells. J Power Sources 2002;108:185–91.
[2] K Kordesch, Simader G. Fuel cells and their applications. Weinheim: VCH Verlagsgesellschaft mbH; 1996. [3] Lee WK, Ho CH, Van Zee JW, Murthy M. The effects of compression and gas diffusion layers on the performance of a PEM fuel cell. J Power Sources 1999;84:45–51. [4] Lim C, Wang CY. Development of high-power electrodes for a liquid-feed direct methanol fuel cell. J Power Sources 2003;113:145–50. [5] 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;43:3675–80. [6] Song JM, Cha SY, Lee WM. Optimal composition of polymer electrolyte fuel cell electrodes determined by the AC impedance method. J Power Sources 2001;94:78–84. [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;86:250–4. [8] Yoon YG, Park GG, Yang TH, Han JN, Lee WY, Kim CS. Effect of pore structure of catalyst layer in a PEMFC on its performance. Int J Hydrogen Energy 2003;28:657–62. [9] Soler J, Hontanon E, Daza L. Electrode permeability and flow-field configuration: influence on the performance of a PEMFC. J Power Sources 2003;118:172–8. [10] Lee HK, Park JH, Kim DY, Lee TH. A study on the characteristics of the diffusion layer thickness and porosity of the PEMFC. J Power Sources 2004;131:200–6. [11] Lim C, Wang CY. Effects of hydrophobic polymer content in GDL on power performance of a PEM fuel cell. Electrochim Acta 2004;49:4149–56. [12] Yan WM, Hsueh CY, Soong CY, Chen F, Cheng CH, Mei SC. Effects of fabrication processes and material parameters of GDL on cell performance of PEM fuel cell. Int J Hydrogen Energy 2007;32:4452–8. [13] Jin HN, Massoud K. Effective diffusivity and water-saturation distribution in single- and two-layer PEMFC diffusion medium. Int J Heat Mass Transfer 2003;46:4595–611. [14] Rajalakshmi N, Jayanth TT, Thangamuthu R, Sasikumar G, Sridhar P, Dhathathreyan KS. Water transport characteristics of polymer electrolyte. Int J Hydrogen Energy 2004;29:1009–14. [15] Su A, Weng FB, Hsu CY, Chen YM. Studies on flooding in PEM fuel cell cathode channels. Int J Hydrogen Energy 2006;31:1031–9. [16] Shi Y, Xiao JS, Pan M, Yuan RZ. A fractal permeability model for the gas diffusion layer of PEM fuel cells. J Power Sources 2006;160:277–83. [17] The instruction manual for capillary flow porometer, PMI, Inc..
Contents lists available at ScienceDirect
Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Effects of microstructure characteristics of gas diffusion layer and microporous layer on the performance of PEMFC Chung-Jen Tseng *, Shih-Kun Lo Department of Mechanical Engineering, National Central University, Chungli, Taoyuan 320, Taiwan
a r t i c l e
i n f o
Article history: Received 6 April 2009 Accepted 7 November 2009 Available online 25 November 2009 Keywords: Gas diffusion layer Microporous layer Fuel cell
a b s t r a c t Water management is an important issue in proton exchange membrane (PEM) fuel cell design and operation. The purpose of this work is to investigate the effects of the microstructure characteristics of the gas diffusion layer (GDL) and microporous layer (MPL), including pore size distribution, hydrophobic treatment, gas permeability, and other factors, on the water management and performance of a PEM fuel cell. A commercial catalyst-coated membrane with an active area of 25 cm2 is used along with a GDL and an MPL for assembling a single cell. The effects of the MPL, the thickness of the MPL, the PTFE loading of carbon paper and MPL, and the baking time of the MPL have been investigated. Results show that the addition of MPL increases cell performance in the high current density region due to the elimination of mass transfer limitation. There exists an optimum thickness of MPL. Furthermore, increasing the MPL baking time enhances cell performance due to enlarged pore size and permeability. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Proton exchange membrane fuel cells (PEMFCs) convert the chemical energy of a fuel directly into electricity in an electrochemical reaction. PEMFCs are a viable alternative for environment-friendly and efficient power production and have a wide range of potential applications. In terms of environmental and global energy problems, the replacement of fossil fuels by renewable energy is being considered all over the world. The fuel cell as a revived energy technology is believed to be an ideal energy system because of its high conversion efficiency and excellent regenerative and zero-emission properties. However, like many emerging energy technologies, PEMFCs must overcome certain engineering and economic obstacles if they are to ever become commercially and popularly viable. Therefore, in order to ensure, without spatial concession, high power for a commercial-scale system, it is predicted that fuel cell operation at high current density conditions per unit electrode area will be necessary. Fig. 1 depicts the schematic diagram of the configuration and mass transport mechanism in the membrane electrode assembly (MEA) and flow channels. Water vapor or liquid water, along with hydrogen and oxygen gases, enters and exits through the diffusion path in the electrode. Electrodes for a PEMFC have, in general, twoor three-layered structures that can be divided into two parts, one catalytic and the other noncatalytic. The catalytic part is the catalyst layer. It is formed by depositing a mixture of a carbon-sup* Corresponding author. Tel.: +886 3 4267348. E-mail address: [email protected] (C.-J. Tseng). 0196-8904/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2009.11.011
ported platinum catalyst. A hydrophobic gas diffusion layer (GDL) and sometimes an additional microporous layer (MPL) are placed between the catalyst layer and the flow channel plate. Although the GDL and MPL have no electrochemical reaction sites, they are known to play an important role in providing the reactants a good access to the catalytic sites and in the effective removal of product water from the electrode. Mass transport problems in a PEMFC can be classified into three categories: (1) water flooding, i.e., the liquid water entrapped inside the electrodes or flow channels interrupts the flux of the reactant/ product gases; (2) dilution of oxidant concentration due to the use of air instead of pure oxygen; and (3) depletion of reactants along the flow channel, which results in a nonuniform current distribution over the whole electrode area and is particularly severe in a largescale fuel cell [1]. Appropriate water management is critical in order to obtain a stable cell performance of the PEMFC. On one hand, water is needed to hydrate the ionically conducting membrane and the ionic conductor in the catalyst layer for proton conductance. That is, the higher the water content, the better is the ionic conductivity. Water can be introduced by external humidification and as a product of the oxygen reduction reaction at the cathode side. On the other hand, this water can flood the pores in the catalyst layers as well as those in the gas diffusion layers, resulting in a higher mass transport resistance. That is, the less the water content, the lower is the resistance to reactant flow. Clearly, water plays a conflicting role; therefore, appropriate water management is required. The GDL is usually made of either a nonwoven carbon paper or a woven carbon cloth due to its high porosity and electric conductivity. Even if the MEA based on a carbon cloth has a higher
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Fig. 1. Schematic diagram of the configuration and mass transport mechanism in a fuel cell.
power performance than that based on a carbon paper, there are considerable theoretical and industrial interests in using carbon paper as there is a cost advantage associated with using a nonwoven substrate and it is more convenient to fabricate an MPL or catalyst layer onto it. The GDL is required to ensure efficient diffusion of each reactant gas to the catalyst and assists water management during the operation of the fuel cell [2]. Lee et al. [3] studied three types of GDLs, and they discovered that the optimum performance was related to the gasket thickness and the measured compression Ò pressure on the GDLs. In their results, the GDL of ELAT had better cell performance in 144 kgf cm/bolts. The cell performance may also be affected by the change in the porosity of the GDL, electric contact resistance, and water content in the membrane. For a slightly brittle and less compressible material, the experimental results showed that high cell performance was obtained with less torque, indicating that the excessive bolt torque might have damaged the GDLs in the fuel cell. Lim and Wang [4] examined a GDL by using the scanning electron microscopy (SEM) technique when a high compression pressure was applied on the GDL. The high compression of the fuel cell led to a remarkable decrease in the concentration limiting current density because of the hydrophilic broken fibers and cleavage defects generated during excessive compression. Giorgi et al. [5] investigated the influence of the polytetrafluoroethylene (PTFE) content of the MPL on the performance of a PEMFC cathode. The MPL was found to improve cell performance, and the optimum PTFE content was found to be 10 wt.% in the MPL. Song et al. [6] used AC impedance to determine the optimal composition of PEMFCs. In their investigation, they used an MPL to prevent the catalyst from falling into the pores of the carbon paper. Jordan et al. [7] used sintered GDLs made from acetylene black (AB) carbon that had a low pore volume. With an optimum thickness, the cell showed better performance than cells made with Vulcan XC-72R carbon. This was explained by the difference in the porosities of the two types of carbon. AB carbon, with a lower pore volume in the range of 10–100 lm, was thought to allow less liquid water to permeate into the diffusion layer. Yoon et al. [8] studied the effect of the pore structure of the cathode catalytic layer in a PEMFC on the performance of the cell. From the I–V characteristics of the catalytic layers, it was concluded that the pore structure was an important factor in determining the cell performance. Soler et al. [9] investigated the effect of both the permeability of the electrodes and the configuration of the gas flow distributor on
the cell performance of a PEMFC. For this purpose, MEAs with carbon paper and carbon cloth were characterized electrochemically by measuring the I–V characteristics curves. The experimental results showed that the fuel cell performance strongly depended on both the gas permeability and the type of flow distributor. Lee et al. [10] studied the effect of the fabrication method and the thickness of a GDL and the impregnation method of a Nafion solution on the cell performance. The GDL was prepared by rolling, spraying, and screen-printing methods, and the Nafion solution was impregnated using spraying and brushing methods. They found that the spraying method was better in terms of cell performance than the brushing method for the impregnation of the Nafion solution because it reduced the charge-transfer resistance of the solution and extended the three-phase region. In the same year, Lim and Wang [11] discussed the effects of the hydrophobic polymer content in the GDL. They compared the performance of fuel cells made from MEAs consisting of 10 wt.% fluorinated ethylene propylene (FEP)-impregnated GDL, 30 wt.% FEP-impregnated GDL, and 30 wt.% FEP-impregnated GDL coated with an MPL (40 wt.% PTFE + Vulcan carbon black). They found that the GDL coated with an MPL had the lowest limiting current density. They ascribed this result to the additional diffusion resistance to oxygen transport that the MPL might impose. However, they also noted that a properly designed and fabricated MPL should be beneficial at high current densities. Furthermore, they found that the surface contact angle of a wet-proof treated GDL generally decreased with temperature, and there was an insignificant difference in the contact angle on the carbon paper treated with different contents of PTFE. Yan et al. [12] studied the effects of electrode fabrication processes and material parameters on cell performance of PEMFC. They used FEP in the MPL. If the FEP content is too high, the pores in MPL are quite small; and the fuel gas is difficult to diffuse in the MPL. If the FEP content is too low, the water generated in the cell cannot be effectively removed. The best cell performance was found with a MPL using 10% FEP content in the carbon paper and 20% content in the micro porous layer. Jin et al. [13] studied the condensed water in the diffusion medium of PEMFC. He found that the local effective mass diffusivity of a fibrous diffusion medium is determined as a function of the local porosity and local water saturation. He also pointed out that the cell performance is affected by the capillary pressure, liquid water permeability, bulk porosity, and other factors. In the flow field design, Rajalakshmi et al. [14] reported the water transport in fuel cells with different flow field designs. The major factor which affects the performance of a fuel cell operated with dry gas is the transport of product water from cathode to anode, which in turn depends on the area of the electrode, electrode preparation and flow field design. Su et al. [15] proposed that the parallel and interdigitated flow channels are more easily flooded than serpentine flow channel. In the serpentine flow channel, the pressure in the upstream is larger than in the downstream, so the liquid water can be pushed into the downstream more easily. It also mentioned that flooding usually occurred in the corners of channels. The channel profile should be fitted to the streamline pattern in order to decrease the corner effect of flooding. Ying et al. [16] used the Kozeny–Carman equation to calculate gas permeability in the GDL. The K–C equation is a typical example of semi-empirical formula, which is
K¼
/nþ1 Cð1 /Þn
where / is the porosity, and the exponential n and constant C are called Kozeny–Carman constants. These two constants vary for different porous media. Thus far, few studies have reported on the effects of pore structure, pore size distribution, and other microscale characteristics of
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Table 1 Fuel cell properties and experimental conditions. Active area CCM Gas diffusion layer Thickness of GDL Catalyst loading (anode) Catalyst loading (cathode) Flow field structure Cell temperature Humidification temperature Fuel Oxidant
25 cm2 Gore 5621 TGPH-120 0.35 mm 0.45 mg Pt–Ru/cm2 0.60 mg Pt/cm2 Serpentine 60 °C 70 °C H2 O2
the MPL and GDL. The underlying physical mechanisms are not well explained. In the present study, the pore size range, pore size distribution, and pore structure are measured to investigate their effects on cell performance. 2. Experiments 2.1. Preparation of MPL The MPL is prepared by the following three-step process, which includes several pretreatment procedures. The first step is to deposit a diffusion layer on the carbon paper substrate. Carbon powder, Triton, PTFE, and isopropyl alcohol (IPA) as the pore former are mechanically mixed in a mixer. The resulting MPL ink is scraped on the wet-proofed carbon paper and then dried at 80 °C for 1 h. Next, the MPL and GDL is sintered in an oven at 350 °C for 4 h. The thickness of the MPL was measured using a Mitutoyo micrometer which has an accuracy of 2 lm. Finally, the dried MPL and GDL are assembled with a catalyst-coated membrane (CCM) to form an MEA. The catalyst loading is 0.45 mg/cm2 of Pt–Ru at the anode and 0.60 mg/ cm2 of Pt at the cathode (in this study we used Pt–Ru in the anode to avoid the effect of CO poisoning in the anode). Table 1 summarizes the data of the MEA used in this work. 2.2. Investigation of morphological characteristics The morphological characteristics of the MPL and GDL are measured using SEM, porosimetry, and a surface contact angle meter. SEM provides the image of the surface structure of the MPL and GDL. The information on the internal structure (i.e., pore size distribution and gas permeability) of the MPL and GDL is obtained using a capillary flow porometer (CFP-1100AE, Porous Materials, Inc., Ithaca, New York, USA). In this experiment structure, we used air with 100 psi pressure to deter the pore distribution and gas permeability of the sample [17]. 2.3. Cell operation and electrochemical analyses A single PEMFC is operated at a cell temperature of 60 °C, and the humidification temperatures of the anode and the cathode side are maintained at 70 °C. The active area of MEAs is 25 cm2 in this study. The flow field used is serpentine in both the anode and the cathode. During the experiment, the cell temperature and humidification temperature are kept constant. Nitrogen is used to purge the flow channels between measurements. The flow rates of the anode and cathode gases are set at 300 ml/min initially. All experiments are performed without external pressurization. The current density versus cell voltage curves of the fuel cell are obtained with a data acquisition system consisting of a personal computer, data acquisition board, electronic load, and other supporting devices.
Fig. 2. The SEM micrographs of the GDLs (a) without MPL and (b) with MPL.
3. Results and discussion 3.1. Effect of MPL on cell performance The effects of MPL on the performance of the unit cell are discussed first. In this section, we will discuss the surface structure, gas permeability, and pore size distribution for GDLs with and without an MPL. Fig. 2 shows the SEM images of the GDLs with and without an MPL. Fig. 2a is for the GDL (TGPH-120-20) without an MPL. We can see that the surface of the GDL is irregular and rough, and the pores are relatively large. The PTFE coating can be seen on the carbon fiber surface. In the GDL with an MPL case, Fig. 2b, the carbon particles are very close to each other and the pore sizes are very small. Generally speaking, adding the MPL may prevent the catalyst ink from falling into the pores of the carbon paper, and reduce the contact resistance between the catalyst layer and the GDL because the MPL has a smoother surface and a smaller pore size than the GDL. The pore size distributions of the two samples are plotted in Fig. 3a and b. The ordinate in both figures shows the percentage of the various pore sizes. For the carbon-paper GDL, most of the pores are in the range of 18–26 lm, whereas for the GDL with an MPL case, the primary pore sizes are shifted to the range of 0.32–12 lm. Pore size distribution is an important physical characteristic of the GDL and MPL, especially for the water management of the fuel cell. The pore size distribution affects the capillarity. A small pore has higher capillarity, and it can remove water more easily. However, small pores also reduce the gas permeability. On the contrary, a large pore provides better gas transport and lower mass transport resistance. For oxygen molecules at room temperature and atmospheric pressure, the mean free path is approximately 0.1 lm.
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Fig. 5. Effect of the MPL thickness on the cell performance.
Fig. 3. Pore size distribution in the GDLs (a) without MPL and (b) with MPL.
water transport from the GDL to the flow channels. However, it may not be very easy to remove the water that forms in the catalyst layer. For the sample with an MPL, the pores consist of mesopores and macropores. While the macropores provide efficient gas diffusion and water removal, the mesopores help in transporting water from the catalyst layer to the GDL. Finally, with the same MEA, the effect of the GDL with and without an MPL on the performance of a single cell is measured through a performance test using hydrogen and oxygen as the fuel and oxidant, respectively. The results are illustrated in Fig. 4. In the low and intermediate current density regions, the two setups show similar performances. However, in the high current density region, the influence of the MPL is very clear. The cell with the MPL does not show any sign of water flooding for current densities up to 1350 mA/cm2. On the other hand, the output voltage of the cell without the MPL drops very fast after 700 mA/cm2 due to mass transfer polarization. 3.2. Effect of MPL thickness on cell performance
Fig. 4. Effect of MPL on the cell performance of a single cell.
Therefore, pores larger than 5 lm may be classified as macropores; diffusion in these pores is mainly due to molecular collisions. On the other hand, pores smaller than 0.01 lm may be classified as micropores, and Knudsen diffusion is dominant in these pores. The pores in the range of 0.01–5 lm can be called mesopores and are a transition between the micropores and the macropores. As shown in Fig. 3, for the sample without MPL, most of the pores are macropores; these pores provide efficient gas diffusion and
Fig. 5 shows the effect of the thickness of an MPL on the performance of a cell. Measured after baking, the thicknesses of the MPL are varied as 38, 84, and 136 lm. There seems to exist an optimum thickness of 84 lm for the cases tested. An explanation for this value will be given after examining the pore characteristics. The pore size distributions for the three cases are presented in Fig. 6, and the pore characteristics are summarized in Table 2. We also used the K–C equation to calculate the theoretical gas permeability in MPL. And we in which C ¼ 180=k2mean , the mean pore diameter kmean is computed by experimental data from CFP. And the mean pore diameter increases with the thickness, the averaged gas permeability also. We can see that the gas permeability of 136-lm is the smallest. The difference in MPL thickness on the cell performance results as shown in Fig. 5. We can see the 84-lm sample has better performance than the 38-lm sample in all current density regions; the 136-lm sample has worse performance than the 84lm sample in the intermediate and high current density regions where mass transport dominates. A suitable MPL improves the reactant gas supply and facilitates the removal of the product water. An extremely thin MPL has smaller pores that may hinder gas supply and increase mass transport resistance. On the other hand, an extremely thick MPL has a higher
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C.-J. Tseng, S.-K. Lo / Energy Conversion and Management 51 (2010) 677–684 Table 3 Pore characteristics of MPL-coated GDLs with different PTFE loadings. Sample (lm)
Mean pore diameter (lm)
Smallest pore size (lm)
Average Darcy permeability (darcy)
K–C equation permeability (darcy)
TGPH-120-0-84 TGPH-120-20-84 TGPH-120-40-84
4.22 3.84 2.33
0.43 0.32 0.32
0.299 0.340 0.086
0.281 0.342 0.097
Fig. 6. Pore size distribution in an MPL with different thicknesses. MPL thickness: (a) 38 lm, (b) 84 lm, and (c) 136 lm.
Table 2 Pore characteristics for three different thicknesses of MPL. Sample (lm)
Mean pore diameter (lm)
Smallest pore size (lm)
Average Darcy permeability (darcy)
K–C equation permeability (darcy)
TGPH-120-20-38 TGPH-120-20-84 TGPH-120-20-136
3.57 3.84 7.84
0.39 0.32 0.49
0.442 0.340 0.205
0.403 0.342 0.217
Fig. 7. Pore size distributions in carbon paper. (a) 0 wt.% PTFE loading, (b) 20 wt.% PTFE loading, and (c) 40 wt.% PTFE loading.
3.3. Effect of PTFE loading of GDL electronic resistance because of the increased amount of PTFE. A thick layer also hampers the accessibility of the reactant gas due to the lengthened diffusion path.
In this section, we discuss the effect of PTFE loading of GDL on cell performance. The PTFE loadings in the GDL are 0 wt.%, 20 wt.%,
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Fig. 8. Polarization curves of cells with different PTFE loading in GDL.
Fig. 9. Comparison of polarization curves of cells with MPLs having different PTFE loadings.
and 40 wt.%. All three GDLs are coated with the same MPL. The mean pore diameter of the resulting MPL + GDL decreases with increasing PTFE loading in the GDL as shown in Table 3. The table also shows the gas permeability data. The carbon paper with 20 wt.% PTFE has the highest gas permeability in the three cases studied. The measured value of the gas permeability for the 40 wt.% sample is very low compared to that of the other samples and is only about 1/4 of the 20 wt.% sample. Fig. 7 presents the pore size distribution of the GDL with different PTFE loading. For the 40 wt.% PTFE loading sample, although the pore size range is from 0.32 to 24.59 lm, almost 70% of the pores are smaller than 4.0 lm. This may be the reason that it has the lowest gas permeability. Polarization curves of the cells made of the three GDLs are illustrated in Fig. 8. The 20 wt.% PTFE case has the best cell performance. This implies that 20 wt.% PTFE can facilitate the reactant gas and product water transport better than other PTFE loadings. In Fig. 8, the cell performance without any PTFE content in carbon paper is very poor, even with the MPL. The product water is removed quickly from the catalyst layer to the GDL but remains in
Fig. 10. Pore size distributions of MPLs with different PTFE loadings. (a) 10 wt.%, (b) 25 wt.%, (c) 40 wt.%, and (d) 55 wt.%.
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Fig. 11. Comparison of polarization curves of cells with MPLs with different baking times.
the pores of the GDL and hinders the reactant gas from entering into the MPL and the catalyst layer. Therefore, the presence of PTFE in a GDL is necessary for efficient removal of water. However, a higher PTFE loading in carbon paper will increase the electronic resistance and decrease the pore size and gas permeability in the GDL. Therefore, there exists an optimum amount of PTFE loading. 3.4. Effect of PTFE loading of MPL In this section, we will discuss the effect of the PTFE loading of an MPL. From Fig. 9, it is observed that the MPL that has 40 wt.% of PTFE has a better cell performance than others. The reason for this better performance is that using more PTFE loading in the MPL results in higher gas permeability and easier water removal. In other words, PTFE loading of 40 wt.% provides lower mass transport resistance in the high current density zone. Moreover, as shown in Fig. 10, the pore size distribution results also showed that the MPL with PTFE loading less than 25 wt.% had more small pores than the MPL with 25 and 40 wt.% PTFE. This may be due to the fact that during the baking process, part of PTFE vaporizes, and the remaining part cures, producing larger pores. Therefore, the MPL with a greater PTFE loading has less small pores and higher gas permeability. 3.5. Effect of the baking time of MPL From Fig. 11, it is observed that the MPL baked for 9 h has a better cell performance than that baked for 1 or 5 h. But in the baked for 13 h, the cell performance is almost the same with 9 h. Fig. 12 shows the pore size distribution with different baking times. In this experiment, we find that a longer baking time will result in larger pores and higher gas permeability. That is, when the baking time is 9 h, the fuel and oxidant can be transported more easily in the GDL and MPL. Therefore, when an MPL that was baked for 9 h is used, the cell performance is better than that of the other MPLs considered in this experiment. 4. Conclusions In this study, the effects of the physical characteristics of the GDL and MPL on the cell performance are investigated. A commercial CCM with an active area of 25 cm2 is used along with a GDL
Fig. 12. Pore size distributions of MPLs with different baking times. (a) 1 h, (b) 3 h, (c) 9 h, and (d) 13 h.
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and an MPL for assembling a single cell. The effects of the MPL, the thickness of the MPL, the PTFE loading of carbon paper, and the MPL have been investigated. The main findings based on the experimental results are summarized as follows: 1. Under light and intermediate load conditions, fuel cells with and without MPL show similar performances. However, in high current density region, the cell with an MPL has much better performance and does not show any sign of mass transfer limitation. 2. A suitable MPL improves the reactant gas supply and facilitates the removal of the product water. An extremely thin MPL has very small pores and low gas permeability that limits the gas supply and increases the mass transport resistance. An extremely thick MPL has a higher electronic resistance because of the increased amount of PTFE. 3. For the three PTFE loadings in the GDL studied in this work, the 20 wt.% case has the best cell performance. 4. The 40 wt.% PTFE loading in MPL case has the best cell performance. The highest PTFE loading in this experiment has the highest gas permeability. 5. A longer MPL baking time can provide higher gas permeability, and the cell performance is also better than that in the shorter baking time cases. Acknowledgements The authors would like to thank the National Science Council of Taiwan for the partial funding for this work through the Grant NSC93-2212-E-008-004. References [1] Kong CS, Kim DY, Lee HK, Shul YG, Lee TH. Influence of pore-size distribution of diffusion layer on mass-transport problems of proton exchange membrane fuel cells. J Power Sources 2002;108:185–91.
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