Hydrophilic Microporous Layer Research Articles

Water Transport in Ionomer and Ice Formation during Cold Startup with Supercooled State in PEFC

In cold regions, ensuring startup ability and durability of polymer electrolyte fuel cells (PEFCs) at subfreezing conditions is one of critical issues for the practical use of PEFC. At cold startup of PEFCs near 0 °C like –10 °C, the produced water remains as supercooled state before it freezes, and then the shutdown occurs with ice layer formation between the cathode catalyst layer (CL) and micro-porous layer (MPL) (1). It was estimated from our previous study (2) that water transport in the CL occurs by concentration gradient in ionomer. This study investigated the water transport in ionomer and the ice formation behavior by measurement of cold start characteristic and observation of membrane electrode assembly (MEA) by a cryo-SEM. To evaluate the water transport, we measured change in polymer membrane resistance corresponding to water content during the supercooling release. Further, to improve the cold start characteristic, we introduced a hydrophilic MPL and confirmed that the hydrophilic MPL makes the startup period longer and improves ice distribution. A single cell with an active area of 25 cm2(5 cm × 5 cm) was used in this study. After a preconditioning process to enhance the performance of a MEA, the initial conditions of the water in the cell were carefully controlled by the procedure described in Ref. 1. Then, the cell and the chamber were cooled to –10 °C, and cell operation was started and maintained at a constant current density. The anode and the cathode supplied gases were dry hydrogen and dry air. For the cryo-SEM observation, the sample of the MEA as removed from the cell after the stop of operation was immediately immersed in liquefied nitrogen, and the sample was set to a sample holder. The sample was cut by a cold knife in a vacuumed chamber, and the cut section area was coated with gold-palladium alloy (Au-Pd), and observed at –150 °C. To estimate the changes in the water content of a polymer electrolyte membrane from supercooled water to ice, we measured the change in the in-plane high frequency resistance of the polymer membrane. It is expected that when supercooled water changes to ice, the chemical potential of water decreases and the water content of the polymer membrane contacting with ice also decreases. Fig.1 shows change in resistance and temperature of the polymer electrolyte membrane. Supercooling release occurs after around 70 min at –7 °C, and the temperature slightly rises due to heat of solidification. Here, resistance increases even after the temperature becomes again –7 °C. This resistance increase suggests the decrease in the water content of the polymer electrolyte membrane due to ice formation. ΔR is the difference between the resistance contacting with ice and with supercooled water. Fig.2 shows relationship of ΔR and supercooling degree (ΔT). ΔR increases as ΔT increases, and this supports the above estimation that ice freezing from the supercooled state takes up water from the ionomer. Secondly, we introduced a hydrophilic MPL made by the gas diffusion electrode (GDE) method (3) to prevent water from freezing at the interface between the CL and MPL and to improve the cold start characteristics. In the previous study (1)(2), we observed the ice layer at the interface with a hydrophobic MPL. Fig.3 shows change in cell voltage and resistance at 0.02 A/cm2, –10 °C. This result suggests that hydrophilic MPL increases the startup period than conventional hydrophobic MPL. Fig.4 is observation results of cross-section of the cathode MEA with hydrophilic MPL after the 0.02 A/cm2shutdown. Figs.4(a) and (b) are the cryo-SEM images with low magnification and with high magnification. Fig.4(a) shows that there is no ice layer at the interface and water freezes in the MPL. Fig.4(b) shows that there is little ice in CL, and this suggests that the hydrophilic MPL enhances the migration of water through ionomer in the CL. These results also suggest that the hydrophilic MPL is effective to improve the characteristics at cold startup. Reference (1) Y. Tabe, et al. , Journal of Power Sources208 (2012), 366. (2) Y. Ishima, et al. , National Symposium on Power and Energy Systems(2015), 101. (3) Y. Aoyama, et al. , ECS Trans. 69 (17) (2015), 743. Figure 1

Effect of Properties of Hydrophilic Microporous Layer (MPL) on PEFC Performance

The effect of the microporous layer (MPL) on membrane electrode assemblies (MEAs) has been investigated. The performance of an MEA employing an MPL with a larger pore volume was better under 100%RH condition, which we reported in 2015 (1). Under 30%RH, no distinct relationship was found between the pore volume of the MPL and the MEA performance. However, the MEA employing an MPL with a larger median pore diameter showed higher cell voltage. We reported that the MEA using a hydrophilic MPL showed much better performance in a wide range of pressure and humidity conditions than that using a conventional hydrophobic MPL. We applied our technique of preparing a carbon fiber dispersion to the fabrication of MPLs using carbon particles and carbon fibers with various fiber diameters. Properties of carbon materials and MPLs are shown in Table 1. The pore size distribution (Fig. 1) was measured using mercury intrusion porosimetry. Figure 2 shows the surface SEM images of the MPLs: The MPL made of carbon black (Vulcan XC) has the smallest pore diameter (0.068 μm) and that made of carbon fiber (VGCF-H) has the largest pore diameter (0.77 μm). Although carbon fiber CF-X has the largest fiber diameter of 250 nm, the MPL made of it has the smallest pore volume. This is probably because fibers of CF-X are straight and longer than other carbon fibers, tending not to become tangled and form larger pores. MEAs having an MPL made of four different carbon materials (Vulcan, 24PS, CF-X, and VGCF-H) were prepared, and polarization curves were measured under the conditions of 80 ºC, 30%RH. The IR free cell voltage (@0.1, 0.5, 1.0 and 2.0 A/cm2) for the four MEAs are plotted against the median pore diameter of the MPL in Fig. 3. The larger the pore diameter of the MPL, the higher the cell voltage becomes. The MEA employing an MPL made of VGCF-H, which has the largest median pore diameter, shows the best performance under the dry condition of 30%RH. This is probably because there is little liquid water in the MPL under the dry condition, and the cell voltage is largely dependent on the oxygen diffusivity of the MPL: The larger median pore diameter of the MPL, the better oxygen diffusivity and the higher the cell voltage.

Water Transport in Ionomer and Ice Formation during Cold Startup with Supercooled State in PEFC

At cold startup of polymer electrolyte fuel cells near 0 °C like –10 °C, the produced water remains as supercooled state before it freezes, and then the shutdown occurs with ice layer formation between the cathode catalyst layer (CL) and micro-porous layer (MPL). It was estimated from our previous study that water transport in CL occurs by concentration gradient in ionomer. This study investigated the water transport in ionomer and the ice formation behavior by measurement of cold start characteristic and observation of cross-section of membrane electrode assembly by a cryo-SEM. To evaluate the water transport, we measured change in polymer membrane resistance corresponding to water content during the supercooling release. Further, to improve the cold start characteristic, we introduced a hydrophilic MPL and confirmed that the hydrophilic MPL makes the startup period until the shutdown longer, and improves the ice distribution.

Influence of Hydrophilic and Hydrophobic Triple MPL Coated GDL on the Oxygen Transport Resistance in a PEFC under High Humidity Conditions

Gas diffusion layers (GDLs) coated with a hydrophobic microporous layer (MPL) have been commonly used to improve the performance of polymer electrolyte fuel cells (PEFCs). In the present study, a hydrophilic and hydrophobic triple MPL coated GDL, in which a thin hydrophilic layer was coated on the hydrophobic double MPL, was developed to achieve further enhancement of the PEFC performance under high humidity conditions. The oxygen transport resistance was determined from the limiting current density values of polarization curves to evaluate the ability of the GDL to prevent flooding. A triple MPL coated GDL, in which the polytetrafluoroethylene (PTFE) content in the hydrophobic MPL in contact with the hydrophilic layer is 20 mass% and that in the MPL in contact with the carbon paper substrate is 10 mass%, exhibits much lower oxygen transport resistance than a conventional hydrophobic MPL coated GDL.

Effect of Pore Volume of Hydrophilic Microporous Layer (MPL) on PEFC Performance

For the better water management in polymer electrolyte fuel cells (PEFCs), microporous layers (MPLs) are generally used. In this paper, MPLs having various pore volumes were prepared using different carbon material. The effect of pore volume of the MPL was examined on cell performance, especially the degree of flooding at the cathode. It was confirmed that pore volume of the MPL has a significant impact on the cell performance and that the hydrophilic MPL with a large pore volume is effective in reducing water flooding in the cathode catalyst layer.

Effect of Hydrophilic Micro-Porous Layer Structure on Microscopic Water Distribution and Cell Performance in PEFC

This paper investigated the mechanism of the hydrophilic MPL with carbon fiber (CF) to increase cell performance by comparing the cell performance and water distribution inside the MPL with different pore size and thickness. The results show that the hydrophilic thick CFMPL improves cell performance by preventing the increase of concentration overvoltage under high current density region. The direct observations by the freezing method and cryo- SEM shows that water is observed in pores of the CFMPL at the catalyst layer (CL) side, while there is no water in the thick CFMPL at the gas diffusion layer (GDL) side. This suggests that the hydrophilic thick CFMPL evaporates water in the CFMPL before the water reaches to the GDL side by the large pores and the adequate thickness of the CFMPL, resulting in better anti-flooding performance.

Influence of Hydrophilic and Hydrophobic Triple MPL Coated GDL on the Oxygen Transport Resistance in a PEFC under High Humidity Conditions

Gas diffusion layers (GDLs) coated with a hydrophobic microporous layer (MPL) have been commonly used to improve water management characteristics of polymer electrolyte fuel cells (PEFCs). However, the appropriate design parameters for the MPL coated GDL are different under low and high humidity conditions [1]. An MPL coated GDL designed to prevent dehydration of the membrane electrode assembly (MEA) under low humidity conditions is generally inferior at reducing flooding under high humidity conditions. It is highly desirable to have a PEFC that can be operated under a wide range of conditions varying from low (or no) to high humidity. The authors have reported that a hydrophilic and hydrophobic double MPL coated GDLs was more effective at enhancing the PEFC performance under low humidity conditions than a hydrophobic MPL coated GDL [2,3]. This design involves the application of a thin hydrophilic layer on top of a hydrophobic MPL. The hydrophilic layer works to conserve the water content of the MEA, while the hydrophobic MPL between the hydrophilic layer and the carbon paper substrate prevents removal of water from the hydrophilic layer. This results in a significant enhancement of the ability to prevent dehydration of the MEA. In the present study, a hydrophilic and hydrophobic triple MPL coated GDL was developed to achieve further enhancement of the PEFC performance under high humidity conditions at a low temperature of 35°C. The active area of the MEA (GORE PRIMEA5580) was 1cm2. The relative humidity (RH) of the gas supplied to the anode and cathode was varied between 50 and 200%. The oxygen transport resistance [4] was measured using the limiting current density of polarization curves to evaluate the ability of the GDL to prevent flooding under high humidity conditions. Single (hydrophobic), double and triple MPL coated GDLs were used at the cathode. The hydrophobic MPL coated GDL consisted of a carbon paper substrate (SGL24BA) coated with an MPL of 20 mass% PTFE (polytetrafluoroethylene) and carbon black. For both the double MPL and the triple MPL coated GDLs, a hydrophilic layer of 25 mass% titanium dioxide, 5 mass% silicone, and carbon black was coated on the hydrophobic MPL coated GDL. The PTFE content in the hydrophobic intermediate MPL of the double MPL coated GDL was varied between 10 and 40 mass%. For the triple MPL coated GDL, the PTFE content in the hydrophobic MPL in contact with the hydrophilic layer was set to 20 mass% and that in contact with the substrate was set to 10 mass%. The contact angle for the hydrophilic MPL was 82°, while those for the hydrophobic MPLs were greater than 120°. When the PTFE content in the hydrophobic MPL was increased from 10 to 40 mass%, the hydrophobicity was enhanced, thereby increasing the contact angle. When the RH of the supplied gases increases from 50 to 100%, the oxygen transport resistance significantly increases due to flooding. However, the oxygen transport resistance becomes almost constant under high humidity conditions varying from 100 to 200 %RH. A double MPL coated GDL is effective to expel excess water from the catalyst layer [2], which results in lower oxygen transport resistance under high humidity conditions, compared with that for the hydrophobic MPL coated GDL. The performance is strongly dependent on the hydrophobic intermediate MPL in the double MPL. Increasing the PTFE content from 10 to 20 mass% represents an effective means of reducing flooding, thereby lowering the oxygen transport resistance. However, when the PTFE content becomes too high, such as 40 mass%, the discharge of excess water at the catalyst layer is inhibited, which increases the oxygen transport resistance. For the triple MPL coated GDL, the hydrophobic gradient promotes water transport from the hydrophobic MPL containing 20 mass% PFTE to the hydrophobic MPL containing 10 mass% PTFE, which causes a relatively large amount of water to be stored in the latter MPL in contact with the substrate, which is effective at expelling excess water from the catalyst layer, while maintaining the oxygen flow pathways inside the MPL from the substrate to the catalyst layer. This results in a significantly lower oxygen transport resistance than that obtained when employing the double MPL coated GDL.

Effect of Pore Volume of Hydrophilic Microporous Layer (MPL) on PEFC Performance

The effect of the microporous layer (MPL) on membrane electrode assemblies (MEAs) has been investigated. The performance of an MEA employing an MPL with a larger pore volume was better under both 100%RH and 30%RH conditions. We reported that the MEA using a hydrophilic MPL showed much better performance in a wide range of pressure and humidity conditions than that using a hydrophobic MPL (1, 2). The hydrophilic MPL consists of vapor-grown carbon fiber with a fiber diameter of 150 nm and ionomer. We applied our technique of preparing a carbon fiber dispersion to the fabrication of MPLs using carbon particles and carbon fibers with various fiber diameters. Properties of carbon materials and MPLs are shown in Table 1. The pore size distribution (Fig. 1) was measured using mercury intrusion porosimetry (Auto Pore IV 9500, Shimadzu Co.). Figure 2 shows the surface SEM images of the MPLs: The MPL made of carbon black (Vulcan XC) has the smallest pore diameter (0.068 μm) and that made of carbon fiber (VGCF-H) has the largest pore diameter (0.77 μm). Although carbon fiber CF-X has the largest fiber diameter of 250 nm, the MPL made of it has a smaller pore volume. This is probably because fibers of CF-X are straight and longer than other carbon fibers, tending not to become tangled and form larger pores. MEAs having an MPL made of four different carbon materials (Vulcan, 24PS, VGCF-H, and CF-X) were prepared, and polarization curves were measured under the conditions of 80 ºC, 100%RH & 30%RH. The estimated mass transfer loss based on the polarization curves (80 ºC, 100%RH) for the four MEAs are plotted against the pore volume of MPL in Fig. 3. The larger the pore volume of MPL, the smaller the mass transfer loss becomes. The MEA employing a hydrophilic MPL with a larger pore volume also showed higher cell voltage under the dry condition of 80 ºC, 30%RH. It was found that the pores of the hydrophilic MPL play a crucial role in achieving excellent MEA performance, such as mitigating cathode flooding under the wet condition and high cell voltage under the dry condition by preventing the membrane from drying.

Effect of Hydrophilic Micro-Porous Layer Structure on Microscopic Water Distribution and Cell Performance in PEFC

Micro porous layers (MPLs), which have hydrophobic characteristics with carbon powder and a hydrophobic polymer (PTFE), have been demonstrated to promote water removal from the reaction area and to improve the cell performance. Recently, some studies reported that the cell with hydrophilic MPL with micro-fibers show better cell performance than hydrophobic MPL in a wide range of humidity conditions (1). In this study, microscopic distribution of water in the cross-section of the cathode side hydrophilic MPL was observed by a freezing method and cryo-SEM to discuss the mechanism of the improved cell performance. A smaller size of single cell with an active area of 1.8 cm2 (0.9 cm × 2.0 cm) was used in this study because the cell must be frozen and disassembled rapidly for the freezing method. Two types of membrane electrode assembly (MEA) with different thickness of the hydrophilic MPL were used in this study: one with a thin MPL about 10μm and one with a thick MPL about 30μm. These hydrophilic MPL were composed of carbon fibers and ionomer. As the hydrophilic MPL composed of conventional carbon blacks did not improve the cell performance, carbon fibers were applied as the MPL material. The hydrophilic MPL was only applied at the cathode side. The interface between the cathode side MPL and the catalyst layer (CL) were fabricated by the gas diffusion electrode (GDE) method (1). These MEA were experimentally produced by Asahi Glass Co., Ltd. In this study, the experiment was consisted of the cell performance measurement and the observation of water distribution in the hydrophilic MPL. The freezing method and the cryo-SEM were used for the observation (2). The freezing method immobilizes the water in the cell as ice by rapidly freezing the cell just after steady-state operation is achieved, and the ice can be visualized with high spatial resolution by using the cryo-SEM. The polarization curves and resistances of the cell using the two types of the hydrophilic MPL are shown in Figure 1. The cell temperature and relative humidity were set at 70 ºC and 100%, respectively. The voltages of the cell using the thick hydrophilic MPL are more stable and higher than those using the thin hydrophilic MPL at high current densities. The limiting current densities of the cell using the thick hydrophilic MPL are also higher than those using the thin hydrophilic MPL. Figure 2 is the cryo-SEM images of the cathode hydrophilic MPL before operation. The structure of the hydrophilic MPL with carbon fibers is similar to that of the gas diffusion layer (GDL) rather than the conventional hydrophobic MPL with carbon blacks. Figure 3 is the cryo-SEM images of the whole thin hydrophilic MPL after the steady-state operation. Large amount of ice appears to be observed in the pores of the MPL, and the ice distributes uniformly inside the thin MPL from the CL side to the GDL side. Figures 4(a) and (b) are the cryo-SEM images of the thick hydrophilic MPL after the operation; (a) is inside the MPL at the CL side, and (b) is inside the MPL at the GDL side. In the case of the thick MPL, there is no ice at the GDL side (Fig. 4 (b)), while the ice distribution at the CL side is similar to that of the thin MPL (Fig. 4 (a)). These observation results suggest that the difference of the cell voltage and the water distribution in two types of MEA may be caused by the difference in the ability to evaporate water inside the MPL. The hydrophilic MPL with fiber structure may be considered to promote water evaporation because the fiber structure increases the surface area of water. In the case of thin MPL, water accumulates inside the whole MPL because the thickness of the MPL appears to be inadequate to evaporate water, and results in the blockage of the oxygen supply. In the case of thick MPL, water appears to evaporate inside the MPL before reaching to the GDL side because the thickness of the MPL appears to be adequate to evaporate water. It may be concluded that the thick hydrophilic MPL gives better anti-flooding performance at high current densities due to the high ability to evaporate water. References T. Tanuma and S. Kinoshita, J. Electrochem. Soc., 161, F94 (2014).Y. Aoyama, K. Suzuki, Y. Tabe, and T. Chikahisa, Electrochem. Commun., 41, 72 (2014). Figure 1

Effect of Wettability of Micro-Porous Layer on Microscopic Water Transport Phenomena in PEFC

The wettability of the micro-porous layer (MPL) is attracted as a key to improve the performance of a polymer electrolyte fuel cell (PEFC). This paper directly observed the cross-sectional distribution of liquid water inside the cathode MPL to elucidate the effect of wettability of the MPL on microscopic water transport phenomena in the PEFC. The performances of cells with hydrophobic and hydrophilic MPLs were measured, and the internal water distributions of the MPLs were observed by using the freezing method and cryo-scanning electron microscope (cryo-SEM). The results show that the water exists inside the hydrophilic MPL as liquid even during the high temperature operation while no liquid water exists inside the hydrophobic MPL under the same condition. It is estimated that the hydrophilic MPL drains liquid water from the catalyst layer (CL) and promote liquid water transport through the MPL.

(Invited) Development of PFSA Ionomers for the Membrane and the Electrodes

Perfluorosulfonic acid (PFSA) ionomer is one of the key materials used in polymer electrolyte fuel cell (PEFC). Most of the membranes used in high performance and high durability PEFC are made from PFSA ionomers. The ionomer for the membrane is required to perform well at high temperatures up to 120 °C at low relative humidity (RH). The ionomer for the electrode, besides transporting the protons to the membrane, its function in cathode of delivering the oxygen to the Pt surface is highlighted in the recent progress of high activity catalyst enabling low Pt loading. Asahi Glass Co., Ltd. (AGC) is also proposing the PFSA ionomer to be used for creating hydrophilic microporous layer (MPL) in between the gas diffusion media (GDM) and the catalyst layer to achieve better performance in low RH conditions. The monomer structures and their copolymer properties as well as their cell performance examined are presented in this paper.

Effect of Wettability of Micro-Porous Layer on Microscopic Water Transport Phenomena in PEFC

Water transport mechanism in the micro-porous layer (MPL) is reported from various viewpoints. Recently, the wettability of the MPL is attracted as a key to improve the performance of a polymer electrolyte fuel cell (PEFC). According to the earlier study investigating the influence of MPL wettability on cell performances, the cell with hydrophilic MPL showed better cell performance in a wide range of humidity conditions compared with the cell with hydrophobic MPL (1). In contrast, how MPL wettability affects water transport mechanism is not fully understood. In this study, we observed the cross-sectional distribution of liquid water inside both the hydrophobic and hydrophilic MPLs at the cathode side by using freezing method and cryo-SEM (2). From the observed results, we discuss the effect of the wettability on water transport phenomena in the vicinity of the MPL.A single cell with an active area of 1.8 cm2was used in the experiment. Both anode and cathode bipolar plates had straight flow channels. Hydrogen and air were used for the anode and the cathode side gases, respectively. In the experiments, the cell was set in a thermostatic chamber to keep the cell temperature constant. Two types of cathode side MPLs with different wettability were used in this study; the conventional hydrophobic MPL is composed of carbon black and PTFE, and the hydrophilic MPL is composed of carbon fiber and ionomer. The interface between the cathode side MPL and the catalyst layer (CL) were fabricated by the gas diffusion electrode (GDE) method (1). The GDE method improves interfacial contact between the MPL and the CL by directly coating the catalyst ink on the MPL. These membrane electrode assemblies (MEAs) were experimentally produced by Asahi Glass Co., Ltd.In this study, the experiment consisted of cell performance measurement and the observation of liquid water in the vicinity of MPLs. The freezing method and cryo-SEM were used for the observation (2). The method immobilizes the liquid water in the cell immediately after discontinuing operation by freezing them, and the cryo-SEM observation can visualize the liquid water distribution in the vicinity of the MPL at high spatial resolution without ice melting.Figure 1 is the cryo-SEM images of the cathode side MEA before operation. Figures 1(a) and 1(b) are the images of the hydrophobic and hydrophilic MPLs respectively. The structure of the hydrophilic MPL with the carbon fibers is clearly different form the conventional hydrophobic MPL with carbon blacks. The pore size of the hydrophilic MPL is larger than that of the hydrophobic MPL. Figures 1(c) and 1(d) are images of the hydrophobic and hydrophilic MPL/CL interfaces, respectively. The interface between the hydrophobic MPL and the CL can be hardly distinguished because both the hydrophobic MPL and the CL are composed of carbon black. In contrast, the interface between the hydrophilic MPL and the CL can be clearly observed due to the hydrophilic MPL with carbon fiber.Figure 2 is the cryo-SEM images of the cathode side MEA after the operation. Figures 2(a) and (b) are the hydrophobic and hydrophilic MPLs respectively. The cell was operated at 70 ºC, and constant current density was 0.7A/cm2. Relative humidity was set at 81% (MEA with hydrophobic MPL) and 100% (MEA with hydrophilic MPL). The ice distributions in the hydrophobic and hydrophilic MPLs are clearly different. There is no ice in the hydrophobic MPL (Fig. 2(a)). In contrast, much ice is observed in the hydrophilic MPL (Fig. 2(b)). The difference in the ice distributions is due to the wettability of the MPL. Figures 2(c) and (d) are the hydrophobic and hydrophilic MPL/CL interfaces, respectively. Although the further investigation is required under other conditions, e.g. dry condition, ice in the CL of the MEA having the hydrophilic MPL appears to be little as compared with ice in the CL of the MEA having the hydrophobic MPL. There might be a possibility that the hydrophilic MPL drains liquid water from the CL.ReferencesT. Tanuma and S. Kinoshita, J. Electrochem. Soc., 161, F94 (2014).Y. Aoyama, K. Suzuki, Y. Tabe, and T. Chikahisa, Electrochem. Commun., 41, 72 (2014).

Gas Diffusion Layer Fabrication with Coating of Carbon Fiber Dispersion

A new fabrication method of gas diffusion layer (GDL), with coating of carbon fiber dispersion, has been developed. The performance of a membrane electrode assembly (MEA) employing a dispersion-cast GDL was equal to that using a conventional GDL.The GDL is a critical component for water management in PEFCs (Polymer Electrolyte Fuel Cells). In the case of when identical catalyst-coated membranes are used, the performance of MEAs is largely dependent on the properties of the GDL, such as reactant permeability, product permeability, electronic conductivity, heat conductivity, and mechanical strength. However, according to the cost analysis by B. D. James (1), the GDL, as well as membranes, and catalyst ink application, is one of the largest cost contributors among the components to the 2012 cost of passenger bus fuel cell stacks. Because hydrophobicity is believed to be key to water management, both the cathode and anode GDL substrates are treated with a hydrophobic material such as PTFE, in order to improve water transport. In addition, a micro-porous layer (MPL), consisting of carbon black mixed with a PTFE dispersion, is typically applied to the substrates, followed by heat treatment at over 300 ºC to remove the dispersant used in the PTFE dispersion. These complicated processes result in increasing the production cost of the GDL.We reported that the MEA using a hydrophilic MPL showed much better performance in a wide range of pressure and humidity conditions than that using a hydrophobic MPL (2, 3). The hydrophilic MPL consists of carbon fiber and ionomer, and thus we applied our technique of preparing a carbon fiber dispersion to the GDL fabrication so that we could reduce manufacturing cost of GDLs.At first, we used carbon fiber MLD1000 (Toray) to find the best mixing ratio with ionomer and dispersing conditions, and then employed a different carbon fiber K223QG (Mitsubishi Plastics) to improve the performance when used in an MEA. Table 1 compares the properties of the carbon fibers and dispersion-cast GDL1, GDL2. Carbon fiber K223QG has a larger fiber diameter and lower electrical conductivity than MLD1000, which results in lower electric resistivity and better air permeability of the dispersion-cast GDL. Carbon fiber dispersions were coated onto a substrate film, and then dried in a convection oven at 80 ºC. Figure 1 and 2 show the surface photos of commercially available GDLs and the dispersion-cast GDLs, respectively.Figure 3 shows the polarization curves of MEAs using dispersion-cast GDL1, GDL2, and a reference GDL (SGL 25BC). The cell voltage of the MEA using dispersion-cast GDL2 is lower than those of the other two MEAs due to large high frequency resistance (HFR), which is attributed to the high electrical resistivity of carbon fiber MLD1000. On the other hand, the performance of the MEA using dispersion-cast GDL1 is on a par with that using a reference GDL. Although tensile strength of the dispersion-cast GDLs falls short of the reference GDL, it would be fair to say that dispersion-cast GDLs could work well if they are properly used, because they can be manufactured at less than 1/10 the cost of conventional GDLs.

Impact of Cathode Fabrication on Fuel Cell Performance

Hydrophobicity is generally believed to be key to water management in polymer electrolyte fuel cells (PEFCs). However, we reported that the membrane electrode assembly (MEA) using a hydrophilic micro porous layer (MPL) showed much better performance in a wide range of pressure and humidity conditions than that using a hydrophobic MPL. When operating an MEA, it usually needs to be humidified in order to maintain conductivity of the membrane. Operation without humidification is critical to the commercialization of PEFCs for backup power and automotive applications. We, therefore, evaluated MEAs at 60°C, at the H2/air stoichiometric ratio of 1.4/5.0, without humidification, in addition to the normal conditions of 80°C, 100 & 30%RH. We found that the MEA employing a gas diffusion electrode (GDE) method on the hydrophilic MPL showed the best performance under a very dry condition, suggesting that the firm interface between the catalyst layer and the MPL plays a crucial role in determining MEA performance, particularly under dry conditions

Impact of Cathode Fabrication on MEA Performance in PEFCs

We reported that the membrane electrode assembly (MEA) using a hydrophilic micro porous layer (MPL) showed much better performance in a wide range of pressure and humidity conditions than that using a hydrophobic MPL. When operating an MEA, it usually needs to be humidified in order to maintain conductivity of the membrane. However, operation without humidification is critical to the commercialization of polymer electrolyte fuel cells (PEFCs) for backup power and automotive applications. We, therefore, evaluated MEAs at 60 ºC, at the H2/air stoichiometric ratio of 1.4/5.0, without humidification, in addition to the normal conditions of 80 ºC, 100 & 30 %RH. We found that the MEA employing a Gas Diffusion Electrode (GDE) method on the hydrophilic MPL showed the best performance under a very dry condition, suggesting the importance of the interface between the cathode and the MPL.

Water Vapor Exchange Flow Channels to Enhance the Performance of Polymer Electrolyte Fuel Cells without Cathode Humidification

Polymer electrolyte fuel cells (PEFCs) generally have external humidifiers to feed humidified hydrogen and air for membrane hydration. The external humidifiers should be omitted to develop a very simplified PEFC system with increased total efficiency and reduced cost. In the present study, a water vapor exchange flow channels installed in the PEFC thus has been developed to enhance the performance without cathode humidification. A gas diffusion layer (GDL) coated with a hydrophilic microporous layer (MPL) consisting of polyvinyl alcohol (PVA) and carbon black is employed for the cathode water exchange flow channels to promote water transport from the cathode outlet wet gas to the anode inlet dry gas. This is effective for the membrane hydration, enhancing PEFC performance. Addition of hydrophilicity to the MPL is effective to water transport from the cathode to the anode.

Anode structure design for generating high stable power output for direct ethanol fuel cells

Three anode structures, one without a microporous layer (MPL), one with a hydrophilic MPL, and one with a hydrophobic MPL, were tested for direct ethanol fuel cells. The quick-scan polarization curve obtained from the membrane electrode assembly (MEA) without MPL exhibited a higher performance than those with MPL. From the anode potential measurement, electrochemical impedance spectroscopy (EIS) analysis and constant-current discharge measurement, it was proven that the existence of MPL at the anode diffusion layer could limit ethanol mass transport in the flow channel to the anode catalyst layer (CL), resulting in a low ethanol concentration in the anode CL, especially at higher current density operation. Moreover, the MEA with hydrophilic MPL showed lower cell performance than the one with hydrophobic MPL. The swelling of the Nafion ionomer at full hydration decreased pore size and porosity in the anode MPL, which then hindered the ethanol transport to the anode catalyst layer. Finally, it can be concluded that the negative effect from ethanol crossover seems to be less significant than the positive influence when keeping ethanol concentration high in the anode catalyst layer.

Effect of variation of hydrophobicity of anode diffusion media along the through-plane direction in direct methanol fuel cells

Reducing methanol crossover from the anode to cathode in direct methanol fuel cells (DMFCs) is critical for attaining high cell performance and fuel utilization, particularly when highly concentrated methanol fuel is fed into DMFCs. In this study, we present a novel design of anode diffusion media (DM) wherein spatial variation of hydrophobicity along the through-plane direction is realized by special polytetrafluoroethylene (PTFE) coating procedure. According to the capillary transport theory for porous media, the anode DM design can significantly affect both methanol and water transport processes in DMFCs. To examine its influence, three different membrane-electrode assemblies are fabricated and tested for various methanol feed concentrations. Polarization curves show that cell performance at high methanol feed concentration conditions is greatly improved with the anode DM design with increasing hydrophobicity toward the anode catalyst layer. In addition, we investigate the influence of the wettability of the anode microporous layer (MPL) on cell performance and show that for DMFC operation at high methanol feed concentration, the hydrophilic anode MPL fabricated with an ionomer binder is more beneficial than conventional hydrophobic MPLs fabricated with PTFE. This paper highlights that controlling wetting characteristics of the anode DM and MPL is of paramount importance for mitigating methanol crossover in DMFCs.

Influence of Hydrohilic and Hydrophobic Double MPL Coated GDL on PEFC Performance

Gas diffusion layers (GDLs) coated with a hydrophobic microporous layer (MPL) have been commonly used to improve water management properties of polymer electrolyte fuel cells (PEFCs). In the present study, a novel hydrophilic and hydrophobic double MPL coated GDL was developed to further enhance the PEFC performance under both low and high humidity conditions. Under low humidity conditions, a thin hydrophilic layer using titanium dioxide coated on the hydrophobic MPL is effective to conserve the humidity of the MEA, while a hydrophobic intermediate MPL between the hydrophilic layer and the GDL substrate prevents the removal of water in the hydrophilic layer. Under high humidity conditions, a double MPL coated GDL is also effective to reduce flooding on the cathode catalyst layer, which results in a higher PEFC performance compared with that for a conventional hydrophobic MPL coated GDL.

Water vapor exchange system using a hydrophilic microporous layer coated gas diffusion layer to enhance performance of polymer electrolyte fuel cells without cathode humidification

Polymer electrolyte fuel cells (PEFCs) generally have external humidifiers to supply humidified hydrogen and oxidant gases, which prevents dehydration of the membrane. If a PEFC could be operated without humidification, then external humidifiers could be removed, which would result in a simplified PEFC system with increased total efficiency and reduced cost. A water vapor exchange system installed in the PEFC was developed to enhance the performance without cathode humidification. A gas diffusion layer (GDL) coated with a hydrophobic microporous layer (MPL) was used at the active reaction area. A GDL coated with a hydrophilic MPL consisting of polyvinyl alcohol (PVA) and carbon black was used at the cathode water vapor exchange area to promote water transport from the cathode outlet wet gas to the anode inlet dry gas. This is effective for reducing the IR overpotential, which enhances the PEFC performance. Appropriate enhancement of hydrophilicity by increasing the PVA content in the MPL to 20mass% is effective to increase water transport from the cathode to anode. At the anode water exchange area, a GDL without the hydrophilic MPL is effective to promote water transport from the water exchange area to the active reaction area, which enhances the PEFC performance.