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.     

Quantitative visualization of temporal water evolution in an operating polymer electrolyte fuel cell

Synchrotron X-ray radiography is employed to visualize the temporal evolution of water inside the gas diffusion layer (GDL) of an operating (in situ) polymer electrolyte fuel cell (PEFC). A single-cell PEFC test kit is specially designed for the convenient capture of X-ray images. X-ray images of water in the PEFC components, such as the polymer membrane, GDL, and end plate, are captured consecutively. The synchrotron X-ray radiography of high-spatial and high-temporal resolution is suitable for observing the transport of a liquid layer and for visualizing water distribution inside the PEFC. As a result, the spatial distribution of water in the PEFC components is clearly and quantitatively visualized. The temporal evolution of water in the anode GDL due to back diffusion effect is clearly observed by adopting the image normalization method. The water-saturation characteristics at the cathode GDL, including saturation time and speed, are quite different from those at the anode GDL.     

Mechanical behavior of the membrane during the polymer electrolyte fuel cell operation

A computational modeling framework is developed to represent the transport phenomena, electrochemistry and the mechanical stresses in a polymer electrolyte fuel cell (PEFC). The model is able to predict the mechanical stresses developed in the polymer electrolyte due to hydration changes, and restriction of the membrane swelling as a result of these hydration changes in the PEFC assembly. Anisotropy in the mechanical properties of the gas diffusion layers is accounted in the stress calculations. It is seen that hydration variations during the PEFC operation can cause significant mechanical stresses. The effects of operating voltage and relative humidities of reactants are investigated. It is observed that high inlet humidities result in a better performance; however, it can potentially cause the polymer electrolyte membrane to go through plastic deformation irreversibly. Thermal stresses due to temperature variations are also calculated and compared with hygral stresses; and it is found that thermal stresses are not negligible but are typically a fraction of the hygral stresses in a typical PEFC operation.     

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.     

Comparing through-plane diffusibility correlations in PEFC gas diffusion layers using the lattice Boltzmann method

One of the key elements in a polymer electrolyte fuel cell (PEFC) is the gas diffusion layer (GDL). The GDL offers mechanical support to the cell and provides the medium for diffusing the reactant gases from the flow plates to the electrolyte enabling the electrochemical reactions, and therefore the energy conversion. At the same time, it has the task of transporting the electrons from the active sites, near to the electrolyte, towards the flow plates.Describing the fluid flow and mass transport phenomena through the GDLs is not an easy task not only because of their complex geometries, but also because of these phenomena occur at microscale levels. Most of the PEFC models at cell scale make assumptions about certain microscale transport parameters, assumptions that can make a model less close to the reality. The purpose of this study is to analyze five different proposed correlations to estimate the through-plane (TP) diffusibility of digitally created GDLs and using lattice Boltzmann (LB) models. The correlations are ranked depending on their precision, accuracy and symmetry. The results show that the best estimation is given when the porosity and gas-phase tortuosity are taken into account in the correlation.     

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.     

Effect of gas-diffusion electrode material heterogeneity on the structural integrity of polymer electrolyte fuel cell

In polymer electrolyte fuel cell (PEFC), gas-diffusion electrode (GDE) plays very significant role in force transmission from bipolar plate to the membrane. This paper investigates the effects of material heterogeneities of gas-diffusion electrode layer (gas-diffusion layer (GDL) and catalyst layer (CL)) on the assembly stress levels of single PEFC stack. In addition, we adopt a force transfer mechanism in a single fuel cell stack based on material heterogeneities of GDL and CL to understand the limitations and advantages associated with it through numerical analyses. Nanoscale heterogeneities in GDE are effectively implemented in the simulation cases along with the membrane swelling. Influence of presence or absence of CL interlayer in the numerical environment is found to have significant impact on the adjacent layers as well as interfaces.     

Synchrotron X-ray tomography for investigations of water distribution in polymer electrolyte membrane fuel cells

Synchrotron X-ray tomography is used to visualize the water distribution in gas diffusion layers (GDL) and flow field channels of a polymer electrolyte membrane fuel cell (PEMFC) subsequent to operation. An experimental setup with a high spatial resolution of down to 10 μm is applied to investigate fundamental aspects of liquid water formations in the GDL substrate as well as the formation of water agglomerates in the flow field channels. Detailed analyses of water distribution regarding the GDL depth profile and the dependence of current density on the water amount in the GDL substrate are addressed. Visualizations of water droplets and wetting layer formations in the flow field channels are shown. The three-dimensional insight by means of this quasi in situ tomography allows for a better understanding of PEMFC water management at steady state operation conditions. The effect of membrane swelling as function of current density is pointed out. Results can serve as an essential input to create and verify flow field simulation outputs and single-phase models.     

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.     

Control-oriented modeling of gas purging process on the cathode of polymer electrolyte membrane fuel cell during shutting down

Gas purging process of cathode side during the shut-down procedure of a polymer electrolyte membrane fuel cell (PEMFC) system is of great importance for a successful cold start. This paper proposes a study on the modeling and control of the cathodic gas purging process, whose main purpose is to remove liquid water in the gas diffusion layer (GDL) and the membrane. The water removal process can be divided into three steps, which are called (a) the through-plane drying of the GDL, (b) the in-plane drying of the GDL, and (c) the vapor-transport from the membrane. A nonlinear model is firstly developed to describe the water removal process in the GDL and the membrane. It includes a one-dimensional three-step purging sub-model and an energy consumption sub-model considering the properties of the air compressor. Experiments are carried out to validate the water-remove model by using the membrane HFR. An optimal constant purging control strategy that minimizes energy consumption during the cathodic purging process is designed based on the model and verified in simulation.     

Quantitative analysis of oxygen-containing species adsorbed on the Pt surface of a polymer electrolyte fuel cell membrane electrode assembly electrode using stripping voltammetry

The quantity of oxygen-containing species adsorbed on Pt surface of a single-cell polymer electrolyte fuel cell membrane electrode assembly (PEFC MEA) in the gas-phase system was measured by stripping voltammetry (SV), of which the adsorbed amount is considered in terms of the quantity of electric charge required for stripping. The effect of different experimental parameters on the adsorption quantity was analyzed and an optimum condition for applying SV to a PEFC MEA electrode was then suggested. The electric charge required for stripping was observed to be linearly proportional to the potential and arose from 0.7 V vs. RHE. The adsorption amount of oxygen-containing species for the PEFC MEA at a cell temperature of 60 °C was 384 μC cm⁻²-Pt at a potential of 1.0 V vs. RHE. More importantly, considering the effect of O₂ partial pressure on the adsorption in the gas-phase PEFC MEAs, water is suggested to be the main source of the oxygen in adsorbed oxygen-containing species. The present method is well applicable to quantitative studies of the oxygen-containing species adsorbed on electrodes of PEFC MEAs.     

Neutron radiography measurements of in-situ PEMFC liquid water saturation in 2D & 3D morphology gas diffusion layers

This work uses neutron radiography to examine the through-plane liquid water distribution of an operating polymer electrolyte membrane fuel cell (PEMFC) with a Sigracet SGL 25BC, a 235 μm thick, 2D straight fiber Gas Diffusion Layer (GDL). This work is performed for interdigitated flow fields at several cross flow rates, and a parallel flow field for comparison. The effect on pressure drop, effective permeability and liquid water saturation is determined. These results are compared to the previously studied Sigracet SGL 10BC, a 420 μm thick, 3D “spaghetti” fiber GDL, for comparable cross flow rates. The SGL 25BC has a higher permeability than the SGL 10BC. The 25BC has a relatively constant saturation within the GDL, from 8% to 10%, which is higher than the 10BC saturation level, between 4 and 8%. This may indicate that 2D GDLs may be better suited to low pumping power situations, while 3D GDLs may be better suited to low liquid water situations.     

Effects of operating and design parameters on PEFC cold start

During startup from subzero temperatures the water produced in a polymer electrolyte fuel cell (PEFC) forms ice/frost in the cathode catalyst layer (CL), blocking the oxygen transport and causing cell shutdown once all CL pores are plugged with ice. This paper describes an experimental study on the effects of operating and design parameters on PEFC cold-start capability. The amount of total product water in mg cm⁻² during startup is used as an index to quantify the cold-start capability. The newly developed isothermal cold-start protocol is used to explore the basic physics of cold start, and the effects of purge methods prior to cold start, startup temperature and current density, and the membrane thickness are shown. The experimental data also confirm the current density effect predicted earlier by a multiphase model of PEFC cold start.     

Modeling the cation transport in an operating polymer electrolyte fuel cell (PEFC)

A computational fluid dynamics model is developed to investigate the multicomponent cation transport in polymer electrolyte membranes and to predict the performance degradation of the polymer electrolyte fuel cell (PEFC) due to the cationic contamination. A Maxwell–Stefan approach is implemented by modifying the Nernst–Planck equations to model the multicomponent cationic species transport in the membrane. Langmuir isotherms are used to model the non-ideal species adsorption in the membrane. Cation transport model shows good agreement with the experimental data found in the literature. Following the validation of the cation transport model, it is incorporated in a PEFC model framework, which solves for mass, momentum, species and charge conservations. Both fuel side and air side impurities are considered in analyses, which show that for air side impurities current density drops from 0.9 to 0.7 A/cm² whereas for fuel side impurities current density drops to impractical values as low as 0.05 A/cm², at the steady-state. Effect of cationic impurity on water transport in the membrane is also investigated and found that cathode dry-out occurs due to decreased water generation in case of fuel side contamination.     

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.     

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     

Sulfonated polyimide hybrid membranes for polymer electrolyte fuel cell applications

Sulfonated Si-MCM-41 (SMCM) with an ion exchange capacity (IEC) of 2.3 mequiv. g⁻¹ was used as a hydrophilic and proton-conductive inorganic component. Sulfonated polyimide (SPI) based on 1,4,5,8-naphthalene tetracarboxylic dianhydride and 2,2′-bis(3-sulfophenoxy) benzidine was used as a host membrane component. The SMCM/SPI hybrid membrane (H1) with 20 wt% loading of SMCM and an IEC of 1.90 mequiv. g⁻¹ showed the high mechanical tensile strength and the slightly higher water vapor sorption than the host SPI membrane (M1) with an IEC of 1.86 mequiv. g⁻¹. H1 and M1 showed anisotropic membrane swelling with about 10 times larger swelling in thickness direction than in plane one. The proton conductivity at 60 °C of H1 was lower in water than that of M1, but comparable at 30% RH. At 90 °C, H1 showed the rather lower performance of polymer electrolyte fuel cell (PEFC) at 82% RH than M1 and fairly better performance at 30% RH. On the other hand, at 110 °C and low humidity less than 50% RH, H1 showed the much better PEFC performance than M1 and Nafion 112. This was due to the promoted back diffusion of produced water by the superior water-holding capacity of SMCM. The SMCM/SPI hybrid membranes have high potential for PEFCs at higher temperatures and lower humidities.     

Gas-diffusion layer's structural anisotropy induced localized instability of nafion membrane in polymer electrolyte fuel cell

The design of robust polymer electrolyte fuel cell requires a thorough understanding of the materials' response of the cell components to the operational conditions such as temperature and hydration. As the electrolyte membrane's mechanical properties are temperature, hydration and rate dependent, its response under cyclic loading is of significant importance to predict the damage onset and thus the membrane lifetime. This article reports on the variation in stress levels in the membrane induced due to the gas-diffusion layer's (GDL) anisotropic mechanical properties while accurately capturing the membrane's mechanical response under time dependent hygrothermomechanical conditions. An observation is made on the evolution of negative strain in the membrane under the bipolar plate channel area, which is an indication of membrane thinning, and the magnitude of this strain found to depend upon the GDL's in-plane mechanical properties. In order to come up with a strategy that reduces the magnitude of tensile stresses evolved in the membrane during the hygrothermal unloading and to increase the membrane's lifetime, we numerically show that by employing a fast hygrothermal loading rate and unloading rate strategy, significant reduction (in this study, nearly 100%) in the magnitude of tensile stresses is achievable. The present study assists in understanding the relation between materials compatibility and durability of fuel cell components.     

Parametric study of the cathode and the role of liquid saturation on the performance of a polymer electrolyte membrane fuel cell—A numerical approach

A two-dimensional two-phase steady state model of the cathode of a polymer electrolyte membrane fuel cell (PEMFC) is developed using unsaturated flow theory (UFT). A gas flow field, a gas diffusion layer (GDL), a microporous layers (MPL), a finite catalyst layer (CL), and a polymer membrane constitute the model domain. The flow of liquid water in the cathode flow channel is assumed to take place in the form of a mist. The CL is modeled using flooded spherical agglomerate characterization. Liquid water is considered in all the porous layers. For liquid water transport in the membrane, electro-osmotic drag and back diffusion are considered to be the dominating mechanisms. The void fraction in the CL is expressed in terms of practically achievable design parameters such as platinum loading, Nafion loading, CL thickness, and fraction of platinum on carbon. A number of sensitivity studies are conducted with the developed model. The optimum operating temperature of the cell is found to be 80-85 °C. The optimum porosity of the GDL for this cell is in the range of 0.7-0.8. A study by varying the design parameters of the CL shows that the cell performs better with 0.3-0.35 mg cm⁻² of platinum and 25-30 wt% of ionomer loading at high current densities. The sensitivity study shows that a multi-variable optimization study can significantly improve the cell performance. Numerical simulations are performed to study the dependence of capillary pressure on liquid saturation using various correlations. The impact of the interface saturation on the cell performance is studied. Under certain operating conditions and for certain combination of materials in the GDL and CL, it is found that the presence of a MPL can deteriorate the performance especially at high current density.     

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.