Hydrophilic Microporous Layer Research Articles

Examining Invasion‐Percolation Across the Cathode Layered Assembly in Polymer Electrolyte Membrane Fuel Cells: Oxygen Resistance, Wettability and Cracks

AbstractEfficient evacuation of water generated by oxygen reduction reaction is necessary to increase catalyst utilization in polymer electrolyte membrane Fuel Cells. However, analysis of two‐phase transport is challenged by the wide range of pore sizes present in the membrane electrode assembly (MEA), varying from 10–100 nm in the catalyst layer (CL), 10–1000 nm in the microporous layer (MPL) and 10 μm in the gas diffusion layer (GDL). In this work, a novel multiscale invasion‐percolation model accounting for the cathode CL, MPL and GDL is presented. Saturation in the macroporous GDL is modeled through an all‐or‐nothing invasion law (empty/filled), while a macroscopic description in terms of the capillary pressure curve is adopted for the MPL and the CL. The oxygen transport resistance across the cathode MEA is quantified using the computed saturation distributions. Among other conclusions, the results show that MPL addition is crucial to avoid local flooding at the CL/GDL interface, providing localized access points to the GDL rather than massively invading interfacial macropores. However, excessive MPL hydrophobicity can cause CL flooding. Water removal from the CL can be enhanced by using a more hydrophilic MPL, a hydrophobic CL or the addition of a moderate volume fraction of cracks. Oxygen transport can be further improved by modulating the arrangement of water in the GDL with patterned wettability, provided that the number of MPL cracks is low.

Some Improvements on Wettability and Internal Resistance of Modified Micro‐porous Layer for Direct Methanol Fuel Cell

Micro‐porous layer (MPL) is an essential component of the membrane electrode assembly in a direct methanol fuel cell (DMFC). It enables proton transport and maintains the stability of gas diffusion layers. The wettability and internal resistance of MPL directly affect the cell output power density. Herein, a hydrophilic MPL is developed on the anode side of DMFC by depositing Ag‐modified carbon nano‐fiber powder (CNFP). The modification improves the diffusion of methanol–water solution into the anode catalytic layer, leading to an increased catalytic reaction rate and output power density. Moreover, thanks to the lower electrical resistance of CNFP/Ag, the cell's charge transfer and mass transfer resistances can be reduced, which leverage the power densities at different concentrations. The experimental results show that the peak power density of the modified MPL is increased by 42.2% from to . The charge transfer resistance is reduced by 34.8% from to and a reduction of 43.1% from to for the mass transfer resistance.

Effect of Hydrophilic Layer in Double Microporous Layer Coated Gas Diffusion Layer on Performance of a Polymer Electrolyte Fuel Cell

Abstract Water flooding under high current and humidity conditions is a main barrier to enhancing the performance of polymer electrolyte fuel cells (PEFCs). This study evaluated a double microporous layer (MPL) coated gas diffusion layer (GDL) consisting of a thin hydrophilic layer coated on a hydrophobic MPL coated GDL. An accurate measurement of the contact angle was introduced to assess the wettability of the MPL. In addition, the water breakthrough pressure and water vapor permeance values were measured to evaluate the water transport ability of the MPL. The oxygen transport resistance was measured using the limiting current density in polarization curves. Appropriate hydrophilic MPL containing 5% Nafion, 25% TiO2, and carbon black in the double MPL enhanced the ability of the GDL to discharge water at the catalyst layer, effectively reducing water flooding. The total oxygen transport resistance obtained with the double MPL was reduced by about 20% compared to that obtained with a hydrophobic MPL. Moreover, the pressure-independent and pressure-dependent resistances were separated from the total oxygen transport resistance measured under various back pressure conditions. The double MPL exhibited a substantially reduced pressure-independent resistance at the interface between the MPL and the catalyst layer.

(Digital Presentation) Effect of the Hydrophilic Layer in Double Microporous Layer Coated Gas Diffusion Layer on PEFC Performance

The polymer electrolyte fuel cells (PEFCs) are commonly used in the vehicle industry, but the relatively higher costs of power generation limit the potential for further application. The PEFCs output power can be mainly undermined by the water management of the membrane electrode assembly (MEA). If the water content balance in the MEA is broken by fast water expelling, the dehydrated membrane significantly raises the proton transport resistance, which results in severe ohmic resistance loss. Oppositely, the slow water removal pace makes the production water stay at the catalyst layer (CL) surface, and the reactants are prohibited from accessing the reaction area and causing high reactants concentration overpotential during the high current density range. To elongate the limiting current density and raise the output power, beneficial water management should keep the MEA from dehydration by using the water generated from electrochemical reactions and can expel additional water from the CL and gas diffusion layer (GDL) interface. To implement the favorable water balance in the cell, the GDL with the microporous layer (MPL) is necessary.Traditional MPL mainly contains carbon black and hydrophobic binder Polytetrafluoroethylene (PTFE), which can guarantee water-unoccupied pathways for gas transport. The produced water can be expelled by the relatively large pores, and the water transportability is mainly controlled by the pore size and hydrophobicity. When the pore size is excessively narrow, the easily gathered water creates flooding between the CL and GDL. Large pores can solve this issue but bring the water stored in the MPL. Thus, a double MPL is developed to achieve better water management when it cannot forwardly enlarge the pore sizes under low and high humidity conditions. A commercially available hydrophobic MPL coated GDL (SGL 22BB) is the standard sample in this study. As for double MPL coated GDL, the hydrophilic layer is coated on the hydrophobic MPL coated GDL. One candidate composition uses Nafion as the hydrophilic binder, TiO2 as the hydrophilicity improvement particles, and the rest of the part is carbon black; the other way applies only Polyvinyl alcohol (PVA) as the hydrophilic binder and mixes with carbon black. Both types of hydrophilic slurry are directly coated on the 22BB, and the maximum pore diameter slightly changed from 45um (SGL 22BB) to a smaller size. These very thin hydrophilic layers modify the surface properties, which can help reduce the surface contact angle and make water easier to be introduced into the hydrophobic MPL.According to the tests, the performance of the double MPL containing PVA binder becomes worse than the Nafion-TiO2 double MPL, even than standard hydrophobic MPL. Due to the strong hydrophilicity of the PVA binder, even though a tiny amount of it is added to the top layer, water accumulation still occurs in the MPL, so the PVA binder is not suitable for the MPL property modification. The double MPL, which applied a Nafion-TiO2 hydrophilic layer, achieves lower oxygen transport resistance under high humidity conditions than the standard hydrophobic MPL. Besides, the appropriate composition of the hydrophilic contents is determined. With the increase of the TiO2 and Nafion content, the significantly enhanced hydrophilicity leads to more water absorption. However, it blocks the gas pathways, showing terrible reactants transport and high concentration over-potential. When the hydrophilic content becomes overly low, it is not enough to afford the water expelling, and water still occupies the MPL and CL interface. The thickness of the top hydrophilic MPL is another critical design parameter. A too thick hydrophilic layer can hold more water and cause a high risk of hampering reactants supply. The moderate thickness of the hydrophilic layer should be less than 5μm, which guarantees the function of the water transport and keeps away from severe water absorption.

N-Doped Colloid Imprinted Carbons As Promising ORR Catalysts for Alkaline Applications

The oxygen reduction reaction (ORR) has long been of interest in relation to its many energy applications and interesting multi-pathway mechanisms. The ORR is a key reaction in a range of electrochemical energy conversion and storage devices, such as hydrogen fuel cells and metal-air batteries, respectively. These devices are expected to play an ever-increasing role in the global transition to net zero emissions. Metal-air batteries, such as Zn/air batteries, can operate at room temperature, use recyclable materials, are environmentally friendly, and are preferred in relation to consumer safety than batteries relying on organic solvents and reactive electrode species.1 High rates of the ORR are crucial for the development of high performance Zn/air batteries and catalysts play a significant role, with lowering of the catalyst cost also of increasing importance.The costs of conventional Pt-based ORR catalysts are high. Therefore, metal-free carbon-based ORR electrocatalysts are viewed as increasingly promising alternatives, especially as they are lower in cost due to the availability of the precursor materials. Carbon is also a good electrical conductor and support material, is chemically stable, and can have large surface areas. However, the ORR kinetics are sluggish on carbon and chemical and physical modifications are required to enhance its activity. In typical Zn/air batteries, the ORR occurs at the three-phase boundary (TPB) formed between the solid electrode, liquid electrolyte, and gaseous oxygen. The porosity of the catalyst layer, the wettability of the catalyst/electrolyte interface, and the gas permeability and hydrophobicity of the gas diffusion layer (GDL) are thus also important, significantly influencing the cathode performance and durability. The catalyst layer (CL) must therefore be constructed with both a high-performance catalyst and an optimized TPB length to provide high performance without compromising durability.In the current work, we have doped nitrogen into the lattice of a family of nanoporous colloid imprinted carbon (CIC) powders to increase its ORR activity. The CICs are unique for their versatility in terms of pore size control and ease of surface functionalization.2 Pore sizes in the range of 12 to 100 nm were examined and their effect on the ORR activity and mass transport limitations were investigated. To carry out N-doping, the CICs were exposed to ammonia at 800 ˚C for 7 hr. Catalyst inks were then prepared by mixing the CICs with a binder in an isopropyl alcohol/water solution. Aliquots of the ink were drop-casted on the disc of an RRDE system, or were spray coated or drop-casted on a GDL to determine the ORR performance in an in-house Zn/air battery testing cell, with the N-doped CIC catalyst layer sandwiched between an electrolyte chamber and a graphite current collector. In this setup, O2 gas was flowed through the pores in the GDL to the catalyst/electrolyte interface, a Zn wire installed in the electrolyte chamber was used as the reference electrode, and a Ni sponge was used as the counter electrode.The RRDE experiments showed that, after N doping of the CIC powders, the production of peroxide decreased significantly and the ORR onset potential increased to a very respectable value of ca. 0.9 V vs RHE, indicating the successful activation of the ORR. Electron transfer numbers were found to be greater than 3.5, indicating that either a direct or pseudo- 4 electron transfer ORR pathway is dominant. In agreement with the literature, the ORR currents in the kinetic regions increased linearly with mass loading, expected to be proportional to the total active N-doped CIC surface area.3 The N-doped CIC samples retained excellent performance up to a loading of 0.350 μg/cm2 without losing mechanical stability.Similar N-doped CICs and binders of different hydrophobicity were tested in the Zn/air battery testing system. Electrodes made with a hydrophobic NCS microporous layer (MPL) showed much better ORR performance and durability than hydrophilic NCS MPLs. Although electrodes made using hydrophobic polytetrafluoroethylene (PTFE) as the binder in the catalyst layer showed a similar initial performance to those made using hydrophilic Nafion binders, the PTFE based electrodes exhibited better durability. Furthermore, the temperature and pressure used during electrode fabrication were also found to have a significant impact on the binder distribution and ORR performance. Once optimized, a very good correlation was obtained between the N-doped CIC catalyst performance in the RRDE setup and in the Zn/air battery testing system. References J. Pan et al., Adv. Sci., 5, 1700691 (2018).X. Li et al., ACS Appl. Mater. Interfaces, 10, 2130–2142 (2018).N. Gavrilov et al., J. Power Sources, 220, 306–316 (2012).

Hydrophilic and Hydrophobic Microporous Layer Coated Gas Diffusion Layer for Enhancing PEFC Performance

The 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). 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. Thus, it is important to develop an MPL coated GDL to enhance the PEFC performance under both low and high humidity conditions. To overcome the defects of pure hydrophobic MPL, several plans are raised to develop a composite MPL coated GDL which possesses both hydrophobic and hydrophilic structures in the same MPL to enhance PEFC performance under both low and high humidity conditions. One method is a hydrophilic and hydrophobic double MPL coated GDL. The hydrophobic MPL with 20mass% hydrophobic polytetrafluoroethylene (PTFE) and 80mass% carbon black is coated on the carbon paper substrate as the first layer, then the hydrophilic MPL with 20mass% titanium dioxide (TiO2) and 80 mass% carbon black is coated as a relatively thin second layer. The contact angle of the hydrophobic MPL is 134°, while that of the hydrophilic MPL is 82°. A thin hydrophilic layer is effective at conserving membrane humidity under low humidity, while a hydrophobic intermediate MPL between the hydrophilic layer and the carbon paper substrate prevents the removal of water from the hydrophilic layer. This results in a significant enhancement of PEFC performance under low humidity conditions. The oxygen transport resistance is measured using the limiting current density of polarization curves to evaluate the ability of the GDL to prevent flooding under high humidity conditions. This value obtained with a pure hydrophobic MPL coated GDL is 80 sm-1, while the double MPL coated GDL is 70 sm-1. In the case of the double MPL coated GDL, excess water can be expelled from the catalyst layer, thereby enhancing PEFC performance under high humidity conditions. Except for the double MPL coated GDL, directly changing the part of the hydrophobic carbon black to hydrophilic carbon black is another method. This MPL consists of 16mass% hydrophilic carbon black with 64mass% hydrophobic carbon black and 20mass% PTFE. The prepared MPL is expected to keep the hydrophilic and hydrophobic structures in the same layer. However, the contact angle of the composite MPL coated GDL is 131°. The cell performance obtained with the composite MPL is lower than that with the hydrophobic MPL. The reason is the carbon blacks with different properties in composite MPL coated GDL are completely bonding together and cannot generate hydrophobic and hydrophilic pores separately. As a result, under high humidity conditions, most pore structures of composite MPL are occupied by liquid water, and oxygen transportation is deteriorated, which results in low performance under high humidity conditions. Another designed plan is mixing hydrophilic carbon nanotubes (CNTs) with a hydrophobic MPL. The MPL that contains 4mass% CNTs, 76mass% carbon black, and 20mass% PTFE is coated on the carbon paper. Since it is difficult for the hydrophilic CNTs and PTFE to fully bond together, the hydrophobic pores and hydrophilic pores can be generated separately. In the case of a hydrophobic MPL, decreasing the mean flow pore diameter is effective at preventing membrane dehydration, which enhances cell performance under low humidity conditions. However, when the pore diameter decreases to 2 μm, the water breakthrough pressure significantly increases. This increases the oxygen transport resistance under high humidity conditions. In the case of an MPL with hydrophilic CNTs, the ability to retain membrane humidity is improved, which enhances cell performance under low humidity conditions. It is possible to decrease the pore diameter to 2 μm without raising the water breakthrough pressure. This decreases the oxygen transport resistance to 62 sm-1 under high humidity conditions. Both decreasing the pore diameter and using hydrophilic CNTs in the MPL allow much higher cell performance under both low and high humidity conditions compared to that obtained with a hydrophobic MPL coated GDL.

Managing Fuel Crossover in Direct Methanol Fuel Cells By Optimizing the Anode Microporous Layer

Direct methanol fuel cells (DMFCs) have been considered a promising candidate as a replacement for small scale and portable power applications, however, there are technical barriers that have prevented widespread use of DMFCs in commercial applications. Of these technical barriers, unreacted methanol crossing over from the anode to the cathode has been identified as a primary limiting factor of DMFCs as this phenomenon results in lowered efficiency and reduced open circuit voltages (OCVs) on the cathode, and ultimately limits the OCV of the cell itself.1 In this work, the authors used an anode design proposal provided by (Metzger et al, 2021, JEECS-20-1185) to create a strongly hydrophilic microporous layer (MPL) on the anode gas diffusion layer (GDL) to reduce methanol crossover by increasing the capillary pressure in the pores of the MPL required for the methanol to crossover prior to reacting.2 The authors fabricated three membrane electrode assemblies. (MEAs) MEA 1 was fabricated using a traditional hydrophobic MPL on the anode GDL, whereas MEAs 2 and 3 were fabricated using customized hydrophilic MPLs. The GDL for MEA 2 was heat treated at 165 F for one hour and the GDL for MEA 3 was dried in atmospheric conditions for 24 hours. A peak power density of 71 mW/cm2 was observed using 1M methanol solution with MEA 1. Using this approach, the authors were able to achieve a peak power density of 68.1 mW/cm2 and 70.1 mW/cm2 with MEAs 2 and 3, respectively, using 3M methanol solution. Additionally, the authors achieved a peak power density of 32 mW/cm2 and 30.7 mW/cm2 using 7.5M methanol solution. This study provides a framework for creating low-cost hydrophilic MPLs that can be utilized on the anode GDL to reduce fuel crossover in DMFCs. Figure 1

Investigation of effects of hydrophilic micro-porous layer on liquid water behavior by X-ray imaging

In this study, X-ray imaging and electrochemical measurements were simultaneously performed to clarify the effects of the properties of micro-porous layers (MPLs) on the i-V performance and the liquid water behavior (LWB) of polymer electrolyte fuel cells (PEFCs) operating at a cell temperature of 50 ∘ C and relative humidity of 80 %. To evaluate the effects of wettability, structure and thickness of MPLs, four types of MPLs were tested. The results demonstrate that LW accumulation (LWA) in the substrate part decreased in hydrophilic MPLs by a mechanism that sucks up liquid water and evaporates it. Moreover, MPLs using carbon fiber (CF) material demonstrated less LWA than those using carbon black (CB). Furthermore, the difference in MPL thickness significantly affects the cell performance and LWB, indicating that the MPL thickness optimization is necessary. These results indicates that hydrophilic MPLs effectively suppress LWA and enhance the oxygen transport. • The effects of hydrophilic MPLs are studied using in-situ X-ray visualization. • Hydrophilic MPL wicks liquid water from accumulated water in the substrate layer. • Carbon fiber MPL case demonstrates higher performance than carbon black MPL case. • Thicker MPL reduced liquid water accumulation compared to thinner MPL.

Effect of adding simultaneous hydrophilic and microporous layers on water transport in the gas diffusion layer of polymer electrolyte membrane fuel cell

In this study, the effect of location, wettability, and thickness of the hydrophilic layer and also the effect of microporous layer (MPL) thickness on the dynamic water transport in the gas diffusion layer (GDL) of polymer electrolyte membrane fuel cell (PEMFC) is investigated using the Lattice Boltzmann method. The effect of the presence of hydrophilic layer and MPL on the number of dominant flow paths, liquid water breakthrough sites, liquid water loops, liquid water pools, and time required to reach steady-state for 12 cases have been investigated. Results showed that if the hydrophilic layer is near the MPL/GDL interface, flooding will occur. Also, comparing with purely hydrophobic GDL, reduction of hydrophilic layer thickness in the GDL/GC interface reduces the water saturation level by 11.8% and the time required to reach steady-state by 8.88%. Moreover, this study indicated that adding a thin layer of MPL at the catalyst layer (CL)/GDL interface with present a hydrophilic thin layer at the GDL/gas channel (GC) interface reduces the liquid water saturation level by 50.1% and the steady-state time by 38.9%. Furthermore, the details of the dynamic behavior of water in the GDL under different hydrophilic conditions have been reported and discussed.

Ice Formation from a Supercooled State and Water Transport through Ionomers during PEFC Cold Startup

In startups of polymer electrolyte fuel cells at temperatures close to 0 °C below freezing, water produced by the cathode reaction is present in a supercooled state and may freeze. This study investigates the ice formation processes during isothermal operation at –10 °C. The observations using a cryo-scanning electron microscope show that an ice layer is formed at the interface between the cathode catalyst layer (CL) and the micro-porous layer (MPL), and that the layer becomes thicker during the startup. This suggests the possibility for produced water to be transported from the CL to the ice layer through the ionomer. To evaluate water transport phenomena, changes in water content of the polymer electrolyte membrane in contact with supercooled water are estimated by measurements of the high frequency resistance. The results show that the membrane resistance, corresponding to the water content, increases after freezing of the supercooled water, and the difference increases with the degree of supercooling. This indicates that the driving force of the water transport is the concentration gradient caused by the freezing, supporting the above suggestion. A hydrophilic MPL is introduced to enhance water removal from the interface into the MPL, resulting in improvements in the cold startup.

Highly Ordered Nanoporous Carbon Scaffold with Controllable Wettability as the Microporous Layer for Fuel Cells.

We introduce a novel self-standing, nanoporous carbon scaffold (NCS, 25 μm thick), with an ordered inverse opal pore structure (∼85 nm pore) as a microporous layer (MPL) in a polymer electrolyte membrane fuel cell. Unlike previous studies, through chemical functionalization of the pore surfaces, the wettability of the MPL is controllably modified without altering the pore structure. Ex situ environmental scanning electron microscopy experiments revealed water sorption in the hydrophilic NCS under moderate relative humidity (RH) conditions but not in the hydrophobic NCS, wherein water condensation on the surface was noted only at high RH. The influence of structure and wettability of different MPLs on cell performance was gleaned from steady-state cell polarization behavior. For cells operated under dry conditions (≤80% RH), the limiting current for cells with a hydrophilic NCS MPL was the highest while that for cells with a hydrophobic NCS MPL was the lowest regardless of the level of water saturation (RH).

Hydrophilic microporous layer coatings for polymer electrolyte membrane fuel cells operating without anode humidification

In this study, a hydrophilic microporous layer (MPL) coating was applied to a commercial hydrophobic bi-layer gas diffusion layer (GDL). We investigated the effect of the hydrophilic MPL coating on membrane hydration and liquid water distribution within the GDLs during fuel cell operation without anode humidification, using fuel cell performance monitoring and simultaneous synchrotron X-ray visualization. The application of the hydrophilic coating was found to enhance performance of the fuel cell. Specifically, the application of the hydrophilic MPL coating led to an increase in cell potential of up to 14% (0.07 V at 1.5 A/cm2) and a decrease in fuel cell membrane resistance. The decrease in membrane resistance was attributed to improved membrane hydration. This improvement in membrane hydration was caused by the increase in liquid water retention at the catalyst layer-MPL interfaces. At high current densities, the application of the hydrophilic MPL coating led to increased liquid water accumulation within the cathode GDL, which subsequently led to increased oxygen transport resistance. Our study demonstrates that the wettability of the GDL can be tailored to enhance fuel cell performance for a desired range of operating conditions by balancing membrane hydration and oxygen transport.

Analysis of Water Transport inside Hydrophilic Carbon Fiber Micro-Porous Layers with High-Performance Operation in PEFC

Polymer electrolyte membrane fuel cells using a hydrophilic micro-porous layer (MPL) consisting of carbon fiber (CF) and ionomer show better performance than those using conventional hydrophobic MPL with carbon black (CB) under a wide range of humidity conditions. This study investigates the effects of the wettability and structure of the MPL on cell performance and discusses the mechanism for improving cell performance by the hydrophilic CF-MPL at high humidity conditions. The water distribution inside the MPL is evaluated by a freezing method and cryo-SEM observations, and a comparison of the distribution and the cell performance of hydrophilic and hydrophobic MPL cells with various structures is made. The results show that the performance improvement is caused by preventing increases in concentration overvoltage at high current densities; where the fiber structure and thicker layers are important with hydrophilic MPL. In hydrophilic CF-MPL with better performance, liquid water accumulates in pores at the catalyst layer (CL) side, while there is no liquid water at the gas diffusion layer side. These results indicate that the hydrophilic CF-MPL enhances the liquid water transport from the CL due to the hydrophilic properties, and promotes water evaporation due to the large pores in the thick fiber structure.

Introducing Microporous Layer (MPL) Attached Catalyst Coated Membrane for PEFCs

A brand-new membrane electrode assembly (MEA) fabrication method was conceived. The performance of the MEA by using an MPL attached catalyst coated membrane (CCM) was equivalent to that by using the gas diffusion electrode (GDE) method, indicating that the coated interface between the catalyst layer and the MPL plays a significant role in achieving better MEA performance. For the last few years, we have been doing research on hydrophilic MPLs. We have already reported (1, 2) 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 also found that the MEA using the GDE method on the hydrophilic MPL showed the highest cell voltage compared to the MEAs using the decal method and the membrane coating method. In the case of the MEA using the GDE method on the hydrophilic MPL, the catalyst layer (CL) is integrated into the MPL to form a firm interface between the CL and the hydrophilic MPL. We suggested that this interface allows product water and oxygen gas to transport more smoothly (3).In order to examine the significance of the interface between the CL and the MPL in this work, we used an MPL attached CCM as illustrated in Fig.1, which was fabricated by hot-pressing a cathode CL coated hydrophilic MPL to the membrane with an anode CL. The hydrophilic MPL consists of carbon fiber and ionomer. The MEA employing the MPL attached CCM showed equivalent performance to that by using the gas diffusion electrode (GDE) method as shown in Fig. 2. This result clearly indicates that the coated interface between the CL and the MPL is more favorable than the hot-pressed interface. The MPL attached CCM is easy to handle because it shows little dimensional change despite employing a non-reinforced membrane. As any GDL substrate is applicable to this MPL attached CCM, this is a unique and promising method for MEA fabrication.

Analysis of Oxygen Transport Resistance Components and Water Transport Phenomena with Hydrophilic and Hydrophobic MPL in PEFC

This paper investigated the mechanism of water and oxygen transport phenomena in the vicinity of a micro-porous-layer (MPL). The influence of contact firmness between the MPL and a catalyst layer (CL) on the transport was evaluated quantitatively by analyzing oxygen transport resistance components. For the analysis, the limiting current method was extended to flooding conditions. These investigations were conducted with each of hydrophobic and hydrophilic MPLs. With low contact firmness, oxygen transport resistance inside the CL was high; there was ice layer at the MPL/CL interface, observed by the freezing method and cryo-SEM. These results suggested that the residual water at the MPL/CL interface blocks the oxygen transport and increases the oxygen transport resistance in the CL significantly. Furthermore, the analysis of oxygen transport resistance indicated that the hydrophilic MPL composed of carbon fiber has the role of draining condensed water from the CL to the MPL.

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

For better water management in polymer electrolyte fuel cells (PEFCs), microporous layers (MPLs) are generally used. In this paper, hydrophilic MPLs having various pore volumes and diameters were prepared using a range of carbon materials, and the effect of the MPL on the membrane electrode assembly (MEA) performance was investigated under dry and wet conditions. Under the dry condition (80°C, 30%RH), the MEA employing an MPL with a larger median pore diameter showed higher cell voltage, suggesting that the MPL with a larger pore diameter has better gas diffusivity, leading to better MEA performance. Under the wet condition (80°C, 100%RH), it was confirmed that pore volume of the MPL has a significant impact on the MEA performance and that the hydrophilic MPL with a large pore volume was effective in reducing water flooding in the cathode catalyst layer. When used in an MPL, VGCF-H (carbon fiber with a fiber diameter of 150 nm) gives the largest pore diameter and pore volume. This MEA with a hydrophilic MPL (made of VGCF-H and ionomer) showed the best MEA performance under both dry and wet operating conditions.

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.