X-ray imaging of water distribution in a polymer electrolyte fuel cell

At present, water management in a polymer electrolyte fuel cell (PEFC) is a major subject of research. In fact, proper water management is vital to achieve maximum performance and durability from a PEFC. Consequently, this study is conducted to visualize quantitatively the water distribution in a PEFC by means of an X-ray imaging technique. The X-ray images of the PEFC components with and without water are clearly distinguished. Reference to the visualized X-ray images, enables quantitative evaluation of the water distribution in the region between the separator and the gas-diffusion layer (GDL). Likewise, the meniscus of water in the channels of the PEFC is clearly observed.     

Investigation of the effects of the anisotropy of gas-diffusion layers on heat and water transport in polymer electrolyte fuel cells

The aim of this work is to study the effects of gas-diffusion layer (GDL) anisotropy and the spatial variation of contact resistance between GDLs and catalyst layers (CLs) on water and heat transfer in polymer electrolyte fuel cells (PEFCs). A three-dimensional, two-phase, numerical PEFC model is employed to capture the transport phenomena inside the cell. The model is applied to a two-dimensional cross-sectional PEFC geometry with regard to the in-plane and through-plane directions. A parametric study is carried out to explore the effects of key parameters, such as through-plane and in-plane GDL thermal conductivities, operating current densities, and electronic and thermal contact resistances. The simulation results clearly demonstrate that GDL anisotropy and the spatial variation of GDL/CL contact resistance have a strong impact on thermal and two-phase transport characteristics in a PEFC by significantly altering the temperature, water and membrane current density distributions, as well as overall cell performance. This study contributes to the identification of optimum water and thermal management strategies of a PEFC based on realistic anisotropic GDL and contact-resistance variation inside a cell.     

Tomographic analysis of porosity variation in gas diffusion layer under freeze-thaw cycles

Synchrotron X-ray micro-computed tomography (X-ray μCT) is employed to measure the volume variation of gas diffusion layer (GDL) of a polymer electrolyte fuel cell (PEFC). In the present study, 3D structures are reconstructed by merging orthogonal-plane images. Using the 3D reconstruction, the variation of structural parameters such as the porosity in GDL is investigated under freeze-thaw cycles. The freez-thaw cycles are established using cryo system and light source, respectively. As a result, a structural transformation is observed at the interface between GDL and micro porous layer (MPL). In addition, the porosity is critically changed with irreversible transition under freeze-thaw cycles.     

Super-cooled water behavior inside polymer electrolyte fuel cell cross-section below freezing temperature

This study investigated the phenomenon of water freezing below freezing point in polymer electrolyte fuel cells (PEFCs). To understand the details of water freezing phenomena inside a PEFC, a system capable of cross-sectional imaging inside the fuel cell with visible and infrared images was developed. Super-cooled water freezing phenomena were observed under different gas purge conditions. The present test confirmed that super-cooled water was generated on the gas diffusion layer (GDL) surface and that water freezing occurs at the interface between the GDL and MEA (membrane electrode assembly) at the moment cell performance deteriorates under conditions when remaining water was dry enough inside the fuel cell before cold starting. Moreover, using infrared radiation imaging, it was clarified that heat of solidification spreads at the GDL/MEA interface at the moment cell performance drops. Compared with a no-initial purge condition, liquid water generation was not confirmed to cause ice growth at the GDL/MEA interface after cell performance deterioration. Each condition indicated that ice formation at the GDL/MEA interface causes cell performance deterioration. Therefore, it is believed that ice formation between the GDL/MEA interface causes air gas stoppage and that this blockage leads to a drop in cell performance.     

Evaluation of the thickness of membrane and gas diffusion layer with simplified two-dimensional reaction and flow analysis of polymer electrolyte fuel cell

In the development of more efficient and stable polymer electrolyte fuel cell (PEFC), it is important to propose the optimal component shape that can generate high power and uniform the current density distribution in a single cell. In this study, our past model was improved, and simplified two-dimensional PEFC analysis model including flow and heat transfer of cooling water was made. And PEFC internal phenomenon, that is hardly measured experimentally, could be examined by using this model. The influence of changing the thickness of membrane and gas diffusion layer (GDL) on the cell performance was calculated. As a result, it was confirmed that it is possible to improve the cell output by thinning the GDL more than the membrane in case of low voltage and by thinning the membrane more than the GDL in case of high voltage, but thinning the membrane and the gas diffusion layer increased the current density distribution. In addition, by arranging the values of average current density and the current density distribution, the evaluation graphs were made, which became a help of the shape design in the membrane and the gas diffusion layer.     

Investigation of water accumulation and discharge behaviors with variation of current density in PEMFC by high-resolution soft X-ray radiography

The management of water is a challenging issue to achieving high power density, long-term operation, and increased robustness in PEMFCs. Development of in-situ diagnostic techniques to understand the dynamic behavior of liquid water is needed. In this study, visualization of liquid water across the membrane electrolyte assembly (MEA) of operating PEMFCs was performed by using high-resolution soft X-ray radiography, with which was possible to distinguish the catalyst layer (CL), polymer electrolyte membrane, and gas diffusion layer (GDL) of the MEA. Dynamic liquid water transport behavior in the cathode CL and GDL was observed at high spatial and temporal resolution (1 frame/s). At low current density, generation of liquid water was recognized under the rib, while at high current density, liquid water was observed both under the rib and the channel. Temporal behaviors of liquid water accumulation in GDL under the rib and discharge process of liquid water to under the channel were also visualized.     

High-resolution in-plane investigation of the water evolution and transport in PEM fuel cells

High-resolution synchrotron X-ray radiography is used to study the evolution of primary water clusters and the transport of liquid water from the catalyst layer through the gas diffusion layer (GDL) to the gas channels of a low temperature polymer electrolyte membrane (PEM) fuel cell. The liquid water content is quantified separately in the respective components; in the hydrophobic microporous layer (MPL) almost no liquid water can be observed. In the adjacent GDL, depending on the current density i₀ water clusters are formed which lead to a diffusion barrier for the reactant gases. Water transport dynamics are explained and a recently proposed eruptive mechanism describing the transport from the GDL to the gas channels is imaged in a pseudo three-dimensional representation [A. Bazylak, D. Sinton, Z.-S. Liu, N. Djilali, J. Power Sources 163 (2007) 784–792; S. Litster, D. Sinton, N. Djilali, J. Power Sources 154 (2006) 95–105; I. Manke, Ch. Hartnig, M. Grünerbel, W. Lehnert, N. Kardjilov, A. Haibel, A. Hilger, H. Riesemeier, J. Banhart, Appl. Phys. Lett. 90 (2007) 174105]. Based on a high temporal resolution the dynamics of the liquid water transport are observed; transient conditions resembling dynamic operation of the fuel cell are studied and an estimation of the time required to reach equilibrium conditions is given. The obtained spatial resolution of 3 μm is far below commonly used techniques such as neutron radiography or ¹H NMR. Fundamental aspects of cluster formation in hydrophobic/hydrophilic porous materials as well as processes of multi-phase flow are addressed.     

Development of simulated gas diffusion layer of polymer electrolyte fuel cells and evaluation of its structure

In polymer electrolyte fuel cell (PEFC), it is important to understand the behavior of liquid water in gas diffusion layer (GDL) which is one of the constructional elements so as to improve the output performance and the durability. As this behavior of liquid water is attributed to not only the hydrophilicity but also inhomogeneous structure, it is needed to examine in consideration of an actual GDL structure. In this study, as the basic examination of two-phase flow analysis in an actual GDL, a simulated GDL was made by numerical analysis considering the fiber placement. Furthermore, the prediction methods for pore size distribution, permeability and tortuosity of this simulated GDL were developed with the numerical analysis. These parameters of flow and mass transfer were compared with other studies, and the validity of this simulated GDL was confirmed. In addition, effective diffusion coefficient was calculated from tortuosity in simulated GDL, and PEFC output performance was evaluated by a simple model. Moreover, the optimal GDL was examined in consideration of the effect of porosity and fiber diameter at the fiber level.     

Measurement of liquid water content in cathode gas diffusion electrode of polymer electrolyte fuel cell

Water management in cathode gas diffusion electrode (GDE) of polymer electrolyte fuel cell (PEFC) is essential for high performance operation, because liquid water condensed in porous gas diffusion layer (GDL) and catalyst layer (CL) blocks oxygen transport to active reaction sites. In this study, the average liquid water content inside the cathode GDE of a low-temperature PEFC is experimentally and quantitatively estimated by the weight measurement, and the relationship between the water accumulation rate in the cathode GDE and the cell voltage is investigated. The liquid water behavior at the cathode is also visualized using an optical diagnostic, and the effects of operating conditions and GDL structures on the water transport in the cathode GDE are discussed. It is found that the liquid water content in the cathode GDE increases remarkably after starting the fuel cell operation due to the water production at the CL. At a high current density, the cell voltage drops suddenly after starting the operation in spite of a low water content in the cathode GDE. When the GDL thickness is increased, much water accumulates near the cathode CL and the fuel cell shuts down immediately after the operation. In the final section of this paper, the structure of cathode GDL that has several grooves for water removal is proposed to prevent water flooding and improve fuel cell performance. This groove structure is effective to promote the removal of the liquid water accumulated near the active catalyst sites.     

Two-phase flow and maldistribution in gas channels of a polymer electrolyte fuel cell

Liquid water transport in a polymer electrolyte fuel cell (PEFC) is a major issue for automotive applications. Mist flow with tiny droplets suspended in gas has been commonly assumed for channel flow while two-phase flow has been modeled in other cell components. However, experimental studies have found that two-phase flow in the channels has a profound effect on PEFC performance, stability and durability. Therefore, a complete two-phase flow model is developed in this work for PEFC including two-phase flow in both anode and cathode channels. The model is validated against experimental data of the wetted area ratio and pressure drop in the cathode side. Due to the intrusion of soft gas diffusion layer (GDL) material in the channels, flow resistance is higher in some channels than in others. The resulting flow maldistribution among PEFC channels is of great concern because non-uniform distributions of fuel and oxidizer result in non-uniform reaction rates and thus adversely affect PEFC performance and durability. The two-phase flow maldistribution among the parallel channels in an operating PEFC is explored in detail.     

Characteristics of oxygen diffusivity and water distribution by X-ray radiography in microporous media in alternate porous layers of different wettability for moisture control in gas diffusion layer of PEFC

The mass transfer characteristics of the gas diffusion layer (GDL) are closely related to the performance of polymer electrolyte fuel cells. This study investigates the configuration of a new GDL in which two porous media with different wettabilities are alternately arranged (hybrid GDL). The oxygen diffusivity characteristics with respect to water content (saturation) were measured using an experimental system that employs a galvanic oxygen sensor as an oxygen absorber. Furthermore, X-ray radiography was used to observe the internal water distribution in microporous media for GDL to elucidate the enhancement mechanisms of oxygen diffusivity of the new microporous media. It was possible to distinguish between voids and water in microporous media by using X-ray computer tomography. In addition, the water distributions in the new microporous media were visualized and the mechanisms for the high oxygen diffusivity of the hybrid structure of microporous media were clarified.     

Direct water balance analysis on a polymer electrolyte fuel cell (PEFC): Effects of hydrophobic treatment and micro-porous layer addition to the gas diffusion layer of a PEFC on its performance during a simulated start-up operation

Effects of hydrophobic treatment and micro-porous layer (MPL) addition to a gas diffusion layer (GDL) in a polymer electrolyte fuel cell (PEFC) have been investigated from water balance analysis at the electrode (catalyst layer), GDL and flow channel in the cathode after a simulated start-up operation. The water balance is directly analyzed by measuring the weight of the adherent water wiped away from each the component. As a result, we find that hydrophobic treatment without MPL leads to the increase in liquid water accumulation at the electrode which limits the oxygen transport to the catalyst and then lowers the cell voltage rapidly during start-up, whereas the treatment decreases the water at the GDL. The water accumulation at the electrode also decreases the cumulative current that represents the power generation and calorific power indispensable for warming up at a temperature below freezing point. On the other hand, we directly find that the hydrophobic treatment with MPL addition suppresses the water accumulation at the electrode, which increases the cumulative current. In addition, it is found that increase in air permeability of a GDL substrate by its coarser structure increases the cumulative current, which is explained by enhancing the exhaust of the product water vapor and liquid as well as by enhancing the oxygen transport directly. Thus, the hydrophobic treatment with MPL addition and larger air permeability of a GDL substrate improve the start-up performance of a PEFC.     

Proposal for an optimum water management method using two-pole simultaneous measurement

Most designers of Polymer Electrolyte Fuel Cells (PEFCs) supply the PEFC with humidified gas to prevent its membrane from drying. Because the steam generated by the electrochemical reaction is added to a humidified supply gas, the steam partial pressure in the cathode channel forces a supersaturated state. Therefore, the PEFC has water management issues, such as flooding and plugging. Many researchers have studied these issues in the cathode side using a visualization technique, and have introduced water repellency processing to the gas channel and GDL (gas diffusion layer) as a solution. However, the flooding/plugging phenomena in the cathode do not occur alone, and are influenced by the flooding/plugging phenomena in the anode channel through the membrane. Moreover, the water transport phenomenon through the membrane is affected by the locations of the flooding/plugging phenomena in each gas channel. Therefore, we aim to examine the water transport phenomenon through the membrane by the two-pole simultaneous image measurement, and to propose an optimum water management method. This work shows that the flooding/plugging phenomena on the anode side are clearly related to water transportation from the cathode side through the membrane.     

Diffusion parameter correlations for PEFC gas diffusion layers considering the presence of a water-droplet

A polymer electrolyte fuel cell (PEFC) produces electrical energy according to the electrochemical reactions carried out inside the cell. During the energy conversion, water molecules are also produced at the cathode side, which affects the gas diffusion layer (GDL) diffusion parameters. The generated water-drops from the reaction may give partial or total blockage of the reactant gases and also material oxidation. The mentioned phenomena influence the performance of the PEFCs. This paper aims to describe and quantify the impact on diffusion parameters of GDLs when the size of the formed water-drops inside the layer is varying. This study considers digitally generated GDLs, in which the porosity, gas-phase tortuosity and diffusibility are studied. The fluid flow behavior through the three-dimensional porous domain representing the GDL is obtained with the lattice Boltzmann method (LBM). Depending on the water-drop size, the impact of the mentioned parameters can be computed. For the current study a spherical water-drop whose radius varies between the 15 and 35% of the size of the domain was considered. The studied parameters showed a dependency of the water-drop radius, each changing independently and several correlations to predict the behavior of the mentioned diffusion transport parameters are proposed.     

The influence of polymer electrolyte fuel cell cathode degradation on the electrode polarization

The electrode of polymer electrolyte fuel cell (PEFC) consists of the porous catalyst layer and gas diffusion layer (GDL). Quantitative evaluation of the influence of these porous layers’ degradation on the cell performance was attempted. The cell was assembled by using the catalyst layer or GDL, which had been corroded ex situ, as the cathode and the cell performance was characterized. The oxygen diffusion polarizations of the catalyst layer and that of the GDL were evaluated from the polarization curves. The polarization curves before and after a long-term operation were also analyzed by the same way, and the influences of the degradation of catalyst layer and GDL were evaluated. The increase of the gaseous diffusion loss in the catalyst layer was found to cause the cell performance loss mainly from the analysis of the simulated corrosion test and the long-term operation cell.     

Measurement of effective oxygen diffusivity in microporous media containing moisture

The mass transfer characteristics of the gas diffusion layer (GDL) are closely related to the cell performance of a polymer electrolyte fuel cell (PEFC). The oxygen diffusivity of paper type porous media, which is generally used as a GDL, was measured with respect to its liquid water content using experimental apparatus consisting of an oxygen sensor based on the galvanic cell. A numerical method was established to obtain the effective oxygen diffusivity of microporous test materials by calculating the oxygen concentration distribution on both sides of the test material. Experimental results indicate that the relative oxygen diffusivity of paper type GDLs increases nonlinearly as the water saturation decreases. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20295     

In situ water distribution measurements in a polymer electrolyte fuel cell

The overall water vapor balance and concentration distribution in the flow channels is a critical phenomenon affecting polymer electrolyte fuel cell (PEFC) performance. This paper presents, for the first time, results of a technique to measure in situ water vapor, nitrogen and oxygen distribution within the gas channels of an operating PEFC. The use of a gas chromatograph (GC) to measure high levels of water saturation directly, without dehumidification of the flow stream, is a unique aspect of this work. Following careful calibration and instrumentation, a gas chromatograph (GC) was interfaced directly to the fuel cell at various locations along the serpentine anode and cathode flow paths of a specially designed fuel cell. The 50 cm² active area fuel cell also permits simultaneous current distribution measurements via the segmented collector plate approach. The on-line GC method allows discrete measurements of the water vapor content up to a fully saturated condition about every 2 minutes. Water vapor and other species distribution data are shown for several inlet relative humidities on the anode and cathode for different cell voltages. For the thin electrolyte membranes used (51 μm), there is little functional dependence of the anode gas channel water distribution on current output. For thin membranes, this indicates that there is little gradient in the water activity between anode and cathode, indicating diffusion can offset electro-osmotic drag under these circumstances (i<0.5 A/cm²). This technique can be used for detailed studies on water distribution and transport in the PEFC.     

Electrical/flow heterogeneity of gas diffusion layer and inlet humidity induced performance variation in polymer electrolyte fuel cells

A three-dimensional single-flow channel computational model is used to investigate the performance characteristics of polymer electrolyte fuel cells (PEFC). The combined influence of non-uniform interfacial contact resistance (ICR) and inlet relative humidity (RH), along with the heterogeneous flow properties of the gas diffusion layer (GDL) on the PEFC performance is evaluated. The study considers combinations of full and partial humidification of anode and cathode reactants. Results reveal heterogeneous GDL with non-uniform ICR distribution results in a slight ∼4.4% reduction in current density at 0.3V compared to the homogeneous case. However, under same electrical/flow heterogeneities, the current density is observed to increase by ∼19% to ∼1.3A/cm² under fully humidified anode and partially humidified cathode (i.e., RH_a|RH_c = 100%|60%) as compared to ∼1.1A/cm² under symmetric RH_a|RH_c = 100%|100%. Interesting observations are made on the temperature distribution and cathodic water fractions. The variation in anodic inlet humidity is observed to have no impact on temperature distribution in the membrane, whereas variation in cathodic inlet humidity is effective in reducing the temperature in the channel regime with a 4K (kelvin) difference among all the cases. It is noted here that the overpotential map is not an indicator for performance loss, at least at full inlet humidity. This parameter is observed to depend on water concentration in the cathode. The study provides a detailed analysis of the distribution of reactant mass fraction, water concentration, current density, temperature, cathodic overpotential, and cell performance for all the simulated cases.     

Visualization of the water distribution in perforated gas diffusion layers by means of synchrotron X-ray radiography

Perforated gas diffusion layers (GDLs) of polymer electrolyte membrane fuel cells (PEMFCs) were investigated by means of in-situ synchrotron X-ray radiography during operation. We found a strong influence of perforations on the water distribution and transport in the investigated Toray TGP-H-090 GDL. The water occurs mainly around the perforations, while the holes themselves show varying water distributions. Some remain dry, while most of them fill up with liquid water after a certain period or might serve as drainage volume for effective water transport.