Characteristics Of Gas Diffusion Layer Research Articles

고분자전해질형 연료전지의 단순 채널 리브 형상에서의 물방울 가시화 연구

The effective water management in a polymer electrolyte membrane fuel cell (PEMFC) is one of the key strategies for improving cell performance and durability. In this work, an ex situ measurement was carried out to understand the water droplet behavior on the surface of gas diffusion layer (GDL) as a fundamental study for establishing novel water management. For that purpose, simplified cell including one rib and two flow channels was designed and fabricated. Using this ex situ device, the water droplet emergence through the GDL of the PEMFC was emulated to understand liquid water transport through the porous diffusion medium. Through the visualization experiment, the emergence and growth of water droplets at the channel/GDL interface are mainly observed with the surface characteristics of GDL (SGL 10BA, 24BA) and rib when the liquid water passes through the GDL and is expelled to the flow channel. It is expected that the results obtained from this study can contribute to the better understanding on the water droplet behavior (emergence and removal) in the flow channels of PEMFC.

Improvement of oxygen diffusion characteristic in gas diffusion layer with planar-distributed wettability for polymer electrolyte fuel cell

Mass transfer characteristics of gas diffusion layer (GDL) are closely related to performance of polymer electrolyte fuel cells. Therefore, it is necessary to clarify the characteristics of water distribution relating to the microscopic conformation and oxygen diffusivity of GDL. A hybrid type carbon paper GDL with planar-distributed wettability is investigated for control of liquid water movement and distribution due to hydrophobic to hydrophilic areas that provide wettability differences in GDL and to achieve enhancement of both oxygen diffusion and moisture retention. Hybrid GDLs with different PTFE content were fabricated in an attempt to improve the oxygen diffusion characteristics. The effects of different PTFE contents on the oxygen diffusivity and water distribution were simultaneously measured and observed using galvanic cell oxygen absorber and X-ray radiography. The PTFE distribution was observed using scanning electron microscopy. The formation of oxygen diffusion paths was confirmed by X-ray radiography, where voids in the hybrid GDL were first formed in the hydrophobic regions and then spread to the untreated wetting region. Thus, the formation of oxygen diffusion paths enhanced the oxygen diffusion. In addition, the effects of local PTFE content in the hydrophobic region and the optimal amount of PTFE for hybrid GDL were elucidated.

Effect of local stress concentration near the rib edge on water and electron transport phenomena in polymer electrolyte fuel cell

The performance of polymer electrolyte fuel cell (PEFC) depends on transport phenomena in the gas diffusion layer (GDL). To elucidate the relationship between the local stress concentration near the rib edge and current density distribution, contact stress analysis and charge transport calculations were performed. Subsequently, the effect of GDL deformation caused by the local stress concentration on water transport was investigated. As a result, contact stress analysis showed that through-plane stress in GDL was concentrated near the rib edge. However, the stress was not concentrated on the catalyst layer even when the compression ratio and curvature radius of the rib were varied over a wide range. By solving the charge conservation equation, the potential and current density distribution were obtained. The current density, although not concentrated on the catalyst layer, was concentrated in GDL near the rib edge because the GDL conductivity near the rib edge increased locally from the stress concentration. Calculation showed that to decrease the effect of local stress, rounding off the rib is more effective than decreasing the compression ratio. From scanning electron microscopy (SEM) images of the GDL after clamping, the stress concentration caused the breakup of fibers and polytetrafluoroethylene (PTFE), which caused the hydrophobicity to deteriorate. Therefore, the preferential pathways of water behavior were observed near the rib edge. It was shown that local stress concentration near the rib edge affected the water behavior and GDL characteristics although it did not affect the catalyst layer.

Modeling of the anode of a liquid-feed DMFC: Inhomogeneous compression effects and two-phase transport phenomena

An isothermal two-phase 2D/1D across-the-channel model for the anode of a liquid-feed Direct Methanol Fuel Cell (DMFC) is presented. The model takes into account the effects of the inhomogeneous assembly compression of the Gas Diffusion Layer (GDL), including the spatial variations of porosity, diffusivity, permeability, capillary pressure, and electrical conductivity. The effective anisotropic properties of the GDL are evaluated from empirical data reported in the literature corresponding to Toray carbon paper TGP-H series. Multiphase transport is modeled according to the classical theory of porous media (two-fluid model), considering the effect of non-equilibrium evaporation and condensation of methanol and water. The numerical results evidence that the hydrophobic Leverett J-function approach is physically inconsistent to describe capillary transport in the anode of a DMFC when assembly compression effects are considered. In contrast, more realistic results are obtained when GDL-specific capillary pressure curves reflecting the mixed-wettability characteristics of GDLs are taken into account. The gas coverage factor at the GDL/channel interface also exhibits a strong influence on the gas-void fraction distribution in the GDL, which in turn depends on the relative importance between the capillary resistance induced by the inhomogeneous compression, Rc(∝∂pc/∂ε), and the capillary diffusivity, D¯c(∝∂pc/∂s).

기체확산층의 미세다공층 침투 깊이에 따른 고분자 전해질형 연료전지의 내구성능 저하 분석에 관한 연구

3 JNTG Co., Ltd., 235-36 Mannyun-ro, Jeongnam-myeon, Hwasung-si, Gyeonggi-do 445-961, Republic of Korea Abstract >> Durability characteristics of Gas Diffusion Layer(GDL) is one of the important issues for accomplishing commercialization of Proton Exchange Membrane Fuel Cell(PEMFC). It is strongly related to the performances of PEMFC because one of the main functions of GDL is to work as a path of fuel, air and water. When the GDL does not work on their proposed functions due to the degradation of durability, mass transfer in PEMFC is disturbed and it might cause the flooding phenomenon. Thus, investigating the durability of GDL is important and understanding the GDL degradation process is needed. In this study, electrochemical degradation with carbon corrosion is introduced. The carbon corrosion experiment is carried out with GDLs which have different MPL penetration thicknesses. After the experiment, the amount of degradation of GDL is measured with various properties of GDL such as weight, thickness and performance of the PEMFC. The degraded GDL shows loss of their properties.

GDL을 고려한 고분자전해질형 연료전지 모사 단위 유로 채널에서의 물방울 유동 특성에 대한 실험적인 고찰

Polymer electrolyte membrane fuel cells (PEMFCs) are regarded as a promising alternative to replace the existing automotive power sources. To get high performance and long-term durability for PEMFC systems, novel water management is essential. To this end, a comprehensive understanding of dynamics of the liquid water droplets within an operating PEMFC plays an important role. In this work, direct visualization of dynamic behaviors of the water droplet in the ex situ unit flow channel of a PEMFC including gas diffusion layer (GDL) is carried out as one of the fundamental studies for novel water management. Water droplet dynamics such as the movement and growth of liquid water droplets are mainly presented. Effects of GDL characteristics and inlet air flow rate on the water droplet transport and its removal from the flow channel are also discussed. The data obtained in this study can contribute to build up the fundamental operating strategy including balanced water removal capacity for automotive PEMFC systems.

Ex-situ Measurement of Properties of Gas Diffusion Layers of PEM Fuel Cells

The gas diffusion layer (GDL) of the proton exchange membrane (PEM) fuel cell is a vital component in water management since humidification and water removal are both achieved through the GDL. Capillary action in the GDL porous structure enhances water removal from the cathode catalyst layer, and hence prevents flooding which blocks the pathways of the reactants to the activation sites. To improve the transport of water and reactants, GDL properties are varied by changing the geometry and the PTFE loading of the carbon fiber paper (CFP), both changing the internal wettability of the GDL. In essence, the wettability describes the interaction of water with the porous structures inside the GDL. The knowledge of the surface properties and pore structure is important to enhance water management in the cell. In this work, two ex-situ techniques are used to measure transport characteristics of GDLs, such as the internal wettability, pore size distribution and permeability. These measurements have been applied to different types of GDLs with different structures and PTFE loadings. The comparison between the results will determine the effect of material and treatment on the properties of GDLs which can provide basic insight into the two-phase flow in this porous layer.

J081034 ガス流路形状及び内部液水状態が繊維質多孔体の伝熱特性に与える影響([J08103]燃料電池(3))

Polymer electrolyte fuel cells (PEFCs) are comparatively efficient, environmental friendly, and scalable energy converts suitable to a wide range of applications. Heat transport in PEFCs is the important factor to improve PEFCs performance. Especially, heat transport characteristics of gas diffusion layers (GDLs) have strong influence on PEFCs performance. In PEFCs, GDL is contacted separator that has gas channels, which causes a pressure distribution in GDL. Therefore, inner structure of GDL and condition of contact between GDL and separator might be changed. So, teat transport characteristics of GDL will change. Even when liquid water is in GDL, its heat transport characteristics will be change. However, they haven't investigated clearly, yet. In this study, we measured thermal conductivities and thermal resistances of GDLs in the through-plane direction using steady state method, and effects of gas channel geometry and inside water distribution are discussed. We found that contact condition between GDL and separator didn't change GDL-separator thermal contact resistance per unit contact area. And when there is water retention in GDL, we found that water distribution and degree of water content inside GDL influences the thermal conductivity of GDL and GDL-separator thermal contact resistance.

G203 繊維質多孔体における内部液水分布が伝熱特性に及ぼす影響(OS5 水素・燃料電池)

Polymer Electrolyte fuel cell (PEFC) are comparatively efficient, environmental friendly, and scalable energy converters suitable to a wide range of applications. Heat transport in PEFC is very important to improve cell performance. Especially, heat transport characteristics of gas diffusion layer (GDL) have strong influences on PEFC performance. However, characteristics of GDL for heat transport haven't been figured out yet, clearly. In this study, we measured thermal conductivity in the through-plane direction of GDL varying degree of water content in GDL by steady state method. In addition, we visualized water distribution inside GDL by simultaneous using X-ray radiography. As a result, we found that water distribution and degree of water content inside GDL strongly affect the thermal conductivity of GDL and thermal contact resistance between GDL and standard materials.

Simultaneous Measurement of Oxygen Diffusivity and Visualization of Moisture Distribution in Gas Diffusion Layer with Wettability Distribution for Improvement of Polymer Electrolyte Fuel Cell Performance

The mass transfer characteristics of gas diffusion layer (GDL) are closely related to cell performance in polymer electrolyte fuel cell (PEFC). It is necessary to characterize the liquid water distribution, the microscopic conformation, and the oxygen diffusion coefficient in a GDL containing moisture. The configurations of a novel hybrid type GDL, in which two porous media with different wettabilities are arranged alternately (1),(2) and two porous media with different pore size distributions were alternately arranged (3), have been proposed with the aim of improving the oxygen diffusivity by controlling the moisture distribution in GDL porous media. However, the basic principles of hybrid type GDLs were examined in the former reports as a pilot study by using thicker media than those used in practice. It is thus necessary to elucidate the performance of the hybrid type GDLs with thinner sizes for the practical use. The present study examined a hybrid type GDL to control the liquid water behavior in the porous media. A carbon paper GDL used in practice for PEFCs with a wettability distribution due to adjacent hydrophilic and hydrophobic region was used as a test material. The measurement of oxygen diffusivity and visualization of liquid water in the GDL with wettability distribution were performed simultaneously using a galvanic battery type oxygen absorber and X-ray radiography, respectively.

Investigation of Channel‐to‐Channel Cross Convection in Serpentine Flow Fields

AbstractChannel‐to‐channel cross convection in serpentine flow fields of polymer electrolyte fuel cells (PEFC) can influence the overall cell performance. The effect strongly depends on the gas transport properties of the gas diffusion layer (GDL). For the first time measured anisotropic, compression dependent permeability and effective diffusivity of GDLs are used to quantify the influence of cross convection on the local current distribution and performance. A model was developed to examine different channel‐rib geometries and GDL characteristics. The results show that cross convection can significantly increase the current density and consequently the power density of PEFCs. A strong sensitivity to GDL compression, flow velocity and rib width was found. As an optimised case the GDL thickness under the rib was increased resulting in about 20% higher current densities. Precise knowledge of the GDL characteristics and its compression are key to understand channel‐to‐channel cross convection and optimise perfomance.

固体高分子形燃料電池用ガス拡散層の液水制御 : X線ラジオグラフィ可視化法による新構造ミクロ多孔質体の液水分布特性(熱工学,内燃機関,動力など)

The mass transfer characteristics of gas diffusion layer (GDL) are closely related to cell performance in PEFC. In this study, the observation method of internal water distribution in microporous media for GDL by using X-ray radiography was examined to elucidate the enhancement mechanisms of oxygen diffusivity of new type microporous media shown in the previous report. It was shown that the distinction between the areas of void and water in microporous media was possible by using X-ray computer tomography. Further, the water distributions in the stracture of new type microporous media, in which two kinds of porous media having different wettability are arranged alternately (Hybrid type GDL), was visualized and the mechanism of high oxygen diffusivity of Hybrid structure of microporous media was clarified.

Evaluation of Factors Influencing Performance of PEM Fuel Cell with Thin Separator Channels

The authors developed PEM fuel cells consisting of thin separators with the thickness of 0.8mm. The paper investigates the dependence of cell performance on the gas diffusion layer (GDL) thickness, hydrophobic characteristics of GDL, binding contact pressure, channel depth, temperature and water content in the working gases. The result shows that the pressure losses increase inversely proportional to the third power of the cannel depth and it increases significantly for the channel depth less than 0.2mm. The best performance was obtained with a GDL which has relatively lower hydrophobic characteristics. It was also shown that better performance was obtained with relatively thinner GDL for the shallow channels with the depth of 0.3mm used in the experiment. The paper also proposes a simple estimation of liquid water amount in the cell, which may be useful for the water management control.

Effect of cathode GDL characteristics on mass transport in PEM fuel cells

The effect of the content of the hydrophobic agent in the cathode gas diffusion layer (GDL) on the mass transport in the proton exchange membrane fuel cells (PEMFCs) was studied using mercury porosimetry, scanning electron microscopy, and electrochemical polarization techniques. The mercury intrusion data and SEM micrograph indicated that the hydrophobic agent alters the surface and bulk structure of the GDL, thereby controlling gas-phase void volume and liquid water transport. The electrochemical polarization curves were measured and quantitatively analyzed to determine the oxygen transport limitation both in the catalyst layer and the GDL. Evaluation of the parameter ζ, which represents the cathode GDL characteristics for liquid water transport, indicated that the optimized content of the hydrophobic agent and effective water management results from a trade-off between the hydrophobicity and the absolute permeability for faster water drainage.

Method of measuring oxygen diffusivity in microporous media

It is known that the mass transfer characteristics of gas diffusion layers (GDLs) are closely related to cell performance in PEFCs. In this study, a method for measuring oxygen diffusivity in microporous media by means of an oxygen sensor based on a galvanic cell has been developed. This method is expected to be useful for measuring oxygen diffusivity in microporous media in a wet condition. Oxygen diffusivity in two kinds of microporous GDL media was evaluated. Experimental results indicate that diffusivity in microporous media cannot be determined on the basis of porosity alone, but that it may be influenced by some other factor, such as tortuosity. Additionally, there are some GDL materials that exhibit anisotropic diffusivity.

含水微細多孔質における有効酸素拡散係数測定(熱工学,内燃機関,動力など)

The mass transfer characteristics of gas diffusion layer (GDL) are closely related to cell performance in polymer electrolyte fuel cell (PEFC). In this study, the oxygen diffusivity of the paper type porous media used generally as GDL with respect to its liquid water content were measured by using an experimental apparatus consist of oxygen sensor based on Galvanic battery. Experimental results indicate that paper type GDLs have a relationship where the relative oxygen diffusivity increases nonlinearly as the water saturation decreases.

EFFECT OF GAS DIFFUSION LAYER CHARACTERISTICS AND ADDITION OF PORE-FORMING AGENTS ON THE PERFORMANCE OF POLYMER ELECTROLYTE MEMBRANE FUEL CELLS

The performance of five layer membrane electrode assemblies having different characteristics of gas diffusion layers was determined at 70°C cell temperature and ambient pressure. The maximum power density at 0.6 V was 0.36 W/cm2 for the membrane electrode assembly prepared with the gas diffusion layer having minimum thickness (SGL BC 30 type). On the other hand, the maximum power density at 0.5 V was 0.44 W/cm2 for the membrane electrode assembly prepared with SGL BC 34 type gas diffusion layer. It was found that resistance of a membrane electrode assembly is strongly dependent on gas diffusion layer thickness. Moreover, membrane electrode assemblies prepared with carbon paper gas diffusion layers resulted in higher performance than the assembly prepared with carbon cloth gas diffusion layer. Addition of pore-forming agents, which were ammonium carbonate, ammonium bicarbonate, ammonium sulfonate, and ammonium oxalate, to the catalyst ink lowered the performance.

Method for Measuring Oxygen Diffusivity of Microporous Media

It is known that the mass transfer characteristics of gas diffusion layers (GDL) are closely related to cell performance in PEFCs. In this study, a method for measuring oxygen diffusivity of microporous media by means of an oxygen sensor based on Galvanic battery has been developed. Furthermore, it is expected to be useful for measuring oxygen diffusivity in microporous media with moisture. The oxygen diffusivity of two kinds of GDL as microporous media was evaluated. Experimental results indicate that the diffusivity of microporous media cannot be specified by porosity alone, but that another factor, such as tortuosity, might be influential.

2118 含水状態における微細多孔体の酸素拡散特性 : 酸素拡散特性と液水分布の同時計測(J17-3 燃料電池(3) 固体高分子形燃料電池,ジョイントセッション,21世紀地球環境革命の機械工学:人・マイクロナノ・エネルギー・環境)

The mass transfer characteristics of gas diffusion layer (GDL) are closely related to cell performance in PEFC. In this study, following simultaneous measuring method was tried out, namely, the oxygen diffusivity of porous media with moisture was measured by using an experimental apparatus consist of oxygen sensor based on galvanic battery, and liquid water distribution in porous medium was visualized by using Neutron Radiography. It was confirmed that the method was effective in analyzing relationship between liquid water distribution and oxygen diffusivity. The relationship has been studied for hybrid type porous media.

Evaluation of Factors Influencing Performance of PEM Fuel Cell with Thin Separator Channels

The authors developed PEM fuel cells consisting of thin separators with the thickness of 0.8mm. The paper investigates the dependence of cell performance on the gas diffusion layer (GDL) thickness, hydrophobic characteristics of GDL, binding contact pressure, channel depth, temperature and water content in the working gases. The result shows that the pressure losses increase inversely proportional to the third power of the cannel depth and it increases significantly for the channel depth less than 0.2mm. The best performance was obtained with a GDL which has relatively lower hydrophobic characteristics. It was also shown that better performance was obtained with relatively thinner GDL for the shallow channels with the depth of 0.3mm used in the experiment. The paper also proposes a simple estimation of liquid water amount in the cell, which may be useful for the water management control.