Gas Diffusion Layer Interface Research Articles

The Effects of Contact By the Catalyst Layer and the Gas Diffusion Layer Interface on the Performance of a PEFC with Marimo-like Carbon

When assembling a proton exchange membrane fuel cell (PEMFC), a clamping pressure is applied to all the components. The catalyst layer (CL) and gas diffusion layer (GDL) are compressed to the gasket thickness by the clamping during assembly. This compression increases the conductivity of the protons and electrons while reduces the size of the pores, which are the removal channels for the generated water and the diffusion channels for the fuel and air gases. We evaluated the effect of the CL compression prepared with the Marimo-like carbon (MC) regarding the I-V performance. The MC is a higher order structure of carbon fibers. The maximum power density was the highest (237 mW cm-2) at the 22% compression ratio. The excessive compression was suggested to have a higher effect on blocking the diffusion channels of the fuel and air gases than on improving the conductivity.

Performance analysis and experimental study of titanium GDL in proton exchange membrane fuel cell

The gas diffusion layer (GDL) plays an important role in proton exchange membrane fuel cells (PEMFC) to support the catalytic layer, collect the current, conduct the gas and discharge the reaction product water. Titanium (Ti) can be one of the choices of GDL materials due to its high strength, low density and excellent corrosion resistance. Therefore, this paper investigated Ti substrate material for GDL and comparatively analyzed the changes in the contact angle of the GDL before and after hydrophobic treatment, analyzed the changes in the performance of the cell after different treatment processes, and measured the effect of compression force on the contact resistance of the cell. The results show that, after the selection of Ti felt as the substrate and the carbon plating process, the contact angle of the substrate layer and the microporous layer increased significantly, and the hydrophobicity of the GDL surface improved after carbon plating. The contact resistance of the Ti felt substrate was significantly reduced after carbon plating and was even lower than that of the C–H-060 substrate. Considering the importance of the hydrophobicity of GDL, acetylene black microporous layer coating is also needed to increase the contact angle of the GDL interface and improve the hydrophobicity of GDL. The hydrophobic treatment of MPL using Ti felt as the substrate can significantly improve the output performance of the cell at high current density, avoid flooding inside the cell, and increase the current density operating range of the cell.

A unique clamping mechanism for a cylindrical PEMFC for an enhanced performance

Polymer electrolyte membrane fuel cells with planar structures have been proven beneficial; however, cylindrical cells have higher gravimetric and volumetric power density with possible applications in aviation and portable power sources. In this study, a unique clamping mechanism for a cylindrically structured polymer electrolyte membrane fuel cell is developed to improve the performance of the cell through reduced contact resistance over the active area of the cell. The design is structurally modelled and simulated for the contact pressure distribution, followed by experimental validation of contact pressure distribution at the interface of the gas diffusion layer and current collector. The study results in a lower ohmic resistance due to the better electrical contact at the interface of gas diffusion layer and current collector using the new clamping system. The average contact pressure with the new system is observed to be 0.40 MPa, i.e. 33% higher than that of the strap clamping system. In addition, the contact resistance of the cell with new clamps is found to decrease by 18 %, thus resulting in 36 % better performance (55 mW/cm2 at 0.6 V) than that of the conventional clamping system (40 mW/cm2 at 0.6 V). Such enhancement is also attributed to more oxidant supply (12%) at the cathode current collectors due to better cell clamping design.

Improving Water Management and Reactant Distribution in PEM Fuel Cells Via Flow Fields with Biomimetic Auxiliary Channels

The bipolar plate, a component of the polymer electrolyte membrane (PEM) fuel cell is responsible for reactant delivery, product removal, and mechanical stability of PEM fuel cell stacks. Bipolar plates also account for 70-90% of the weight and volume, and 18-28% of the production cost of stacks (1). Improving the function of the flow fields embedded in the bipolar plates can significantly impact the energy density of PEM fuel cells by improving liquid water management and reactant distribution. Previous works show that water preferentially accumulates under the lands of flow fields in the gas diffusion layer (GDL) for various GDL materials, creating a heterogeneous distribution of water and reactants, thereby increasing cathode mass transport overpotential(2–4). Biomimetic channel architectures have been shown to enhance preferential water flow; however, these designs have not been tailored to control water accumulation for improved reactant distribution (5,6). Targeting areas of known water accumulation such as the under-land regions of GDLs, could have a profound impact on reducing mass transport losses and improving reactant homogeneity, thereby improving the power density and efficiency of the PEM fuel cell.In this work, biomimetic auxiliary channels were laser-cut into the lands of a parallel PEM fuel cell flow field to enhance the liquid water removal and reactant distribution in the under-land region of the GDL. Constant-current electrochemical testing and electrochemical impedance spectroscopy revealed a 29% increase in peak power density resulting from a 54% decrease in oxygen mass transport overpotential using the biomimetic flow field (BFF) compared to a baseline trapezoidal flow field (TFF). Operando synchrotron X-ray radiography revealed reduced GDL water accumulation when using the BFF compared to the TFF. Water accumulation under the BFF flow fields was more homogenous especially near the catalyst layer (CL) – GDL interface, indicating enhanced reactant distribution near the CL reaction sites. Therefore, the reduced mass transport overpotential and corresponding power density increase were attributed to enhanced liquid water removal and reactant distribution due to the biomimetic auxiliary channels. These results demonstrate that significant performance enhancements can be realized by embedding alternate pathways for air and water in the lands of flow field components. Ultimately, this work can be used to further optimize the design of bipolar plates for more efficient stacks and accelerate the commercialization of PEM fuel cells. Y. Wang, D. F. Ruiz Diaz, K. S. Chen, Z. Wang, and X. C. Adroher, Materials Today, 32, 178–203 (2020).N. Ge et al., Electrochim Acta, 328, 135001 (2019).D. Muirhead et al., Int J Hydrogen Energy, 42, 29472–29483 (2017).S. Chevalier et al., J Electrochem Soc, 164, F107–F114 (2017)N. Guo, M. C. Leu, and U. O. Koylu, Int J Hydrogen Energy, 39, 21185–21195 (2014).S. Feng et al., Science, 373, 1344–1348 (2021).

(Digital Presentation) Evaluating the Performance of Multi-Walled Carbon Nanotube Composite Microporous Layers Deposited on Carbon Felt Gas Diffusion Layers

Electrochemical energy storage devices (EESDs) such as metal-air batteries (MABs) and alkaline electrolyte membrane fuel cells (AEMFCs) have begun to gain vast amounts of attention due to their high theoretical energy densities and open-circuit voltage values. Although these devices present promising aspects for the environment and sustainability, they suffer from the slow reaction kinetics and degradation effects that occur at the catalyst layer interface of the air electrode. Typically, these effects are due to the accumulation of molecular species such as water or oxygen at catalyst active sites because of poor electrode hydrophobicity and fluid management. Herein, we perform experimentation to mitigate these effects by implementing mixed carbon composite microporous layers onto porous carbon felt based air electrodes. This adaptation of the air cathode was predominantly based on the aforementioned downfalls and the lack of availability of commercial carbon felts that have adequate wet proofing and conductivity like some of the related carbon paper gas diffusion layers. Microporous layers (MPLs) enhance electrical conductivity, surface area, and adhesion at the catalyst layer and gas diffusion layer interface. These layers typically consist of a hydrophobic polymer such as polytetrafluoroethylene (PTFE) and a conductive carbon black substrate such as Ketjen Black 600JD (KJB). The incorporation of multi-walled carbon nanotube (MWCNT) and carbon black composites into the structure of the MPLs drastically decreases ohmic polarization with minimal increases in specific resistance. The electrical resistance of the synthesized carbon substrate decreases at high rates of MWCNT dispersion, which corresponds directly to a low MWCNT weight percentage within the composite. The MPL increases the mass transport through the electrode by mitigating flooding effects that typically occur within the pores near the surface of the catalyst layer, ultimately aiding in charge transfer between the gas diffusion layer and catalyst active sites. Preliminary data from AEMFC symmetric cell tests suggests that a microporous layer consisting of a 10 wt.% MWCNT/KJB (m/m) substrate is the optimal loading of carbon nanotubes and leads to better cell performance. There is an increase in current density of 60 mA/cm2 compared to an air electrode with no MPL within the ohmic region of the polarization curve corresponding to approximately 0.7 V, while only an increase of 18 mA/cm2 is seen for an MPL consisting of a 30 wt.% MWCNT/KJB (m/m) carbon substrate. The increase of charge transfer by the 10 wt.% synthesized carbon substrate MPL was attributed to the ability of the nanotubes at high rates of dispersion to act as molecular wires within the electrode. Additionally, zinc-air battery tests using the optimally configured air electrode, determined from previous experiments, show that the current density at an operating voltage of 1.1 V was increased by 2.3 times that of an electrode without an MPL using the same catalyst material and loading. Significantly, this research will lead to a better understanding of optimization of the air electrode used within electrochemical devices and how to eliminate some of the adverse effects associated with it. Figure 1

Pore-scale simulation of two-phase flow and oxygen reactive transport in gas diffusion layer of proton exchange membrane fuel cells: Effects of nonuniform wettability and porosity

Enhancing oxygen transport and reducing water flooding in the gas diffusion layer (GDL) of proton exchange membrane fuel cells are important for improving cell performance. In this study, a pore-scale model based on the lattice Boltzmann method is proposed, which considers two-phase flow, oxygen diffusion and electrochemical reaction in the GDL. The invasion speed of the water into the GDL is determined by the water generation rate and correspondingly the oxygen consumption rate. The model is then adopted to study effects of wettability and porosity distribution on the liquid water saturation, oxygen concentration and current density. The results demonstrate that while reducing the total saturation in the GDL is important, decreasing the local saturation near the microporous layer (MPL)/GDL interface is also crucial for enhancing cell performance. It is found that GDL with locally enhanced hydrophobicity at the MPL/GDL interface or gradually increased porosity from the GDL bottom to the GDL top can improve cell performance. Particularly, by delicately designing the GDL porosity, the current density can be considerably increased by 201%. The developed pore-scale model provides a useful tool for understanding the underlying multiphase reactive transport processes in GDL and designing the microscopic structures of GDL.

Porous media flow field for polymer electrolyte membrane fuel cell: Depression of gas diffusion layer intrusion, deformation, and delamination

Porous media flow fields have received a growing attention for polymer electrolyte membrane (PEM) fuel cell application due to their multiple benefits, including enhancements in thermal removal, electron conduction, and two-phase flow control. This study numerically investigates the intrusion of the gas diffusion layer (GDL) into the gas flow channel (GFC) and potential delamination at the GDL and microporous layer (MPL) interface upon land compression and evaluates their depressions by adoption of the porous media flow field. Finite element simulation is conducted to predict the GDL deformation and stresses for both porous media and traditional hollow GFCs. The model predictions are compared with other literature data of GDL intrusion/deformation for traditional hollow GFCs. The results show that GDL intrusion may significantly reduce the GFC space and GDL deformation causes a large spatial variation of porosity and permeability of traditional GFCs. Adoption of the porous media GFCs significantly depresses the GDL intrusion and deformation and hence the porosity/permeability spatial variation. In addition, the shear and tensile stresses at the GDL-MPL interface are evaluated, which are major causes for delamination. It is found that both compressive and tensile stresses may be induced by assembly pressure for traditional GFCs. The shear stress can be significantly depressed and tensile stress can be avoided by porous media GFCs, thereby mitigating the risk of GDL/MPL delamination.

Capillary Pressure Evolution in Operating Polymer Electrolyte Fuel Cells

Capillary pressure driven transport of liquid water through the gas diffusion layer has been studied in various ex-situ, in-situ and operando setups. So far, insights into the actual capillary pressure has been limited to ex-situ experiments. Advanced image analysis tools like 3D interfacial curvature analysis, that have been developed to determine the capillary pressure in digital rock physics investigations [1], were recently applied to ex-situ X-ray tomographic microscopy (XTM) imaging experiments of the droplet release cycle in polymer electrolyte fuel cells [2]. Within this presentation, we will give insights into the water cluster formation using operando XTM imaging at a frequency of 1 Hz from liquid water emergence at the catalyst layer – gas diffusion layer interface until liquid water breakthrough and droplet formation in the gas channel of the flow field. With the help of interfacial curvature analysis as well as volume of fluid simulations [3], it was possible to obtain information about the capillary pressure evolution in the water phase [4]. We will explain the nuanced interactions of water volume and pressure evolution during the growth of the percolation network within the GDL and the droplet formation and comment on the observed distinct differences to ex-situ pressure evaluations.

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.

Numerical investigation on the sensitivity of endplate design and gas diffusion material models in quantifying localized interface and bulk electrical resistance

A localized non-intuitive relationship between electrical interface contact resistance and bulk properties such as bulk electrical resistance and permeability in the fuel cell gas diffusion layer (GDL) is reported. A numerical method is adopted to investigate contact pressure and hence the interface contact resistance at the interfaces of bipolar plate (BPP)|GDL and GDL|Polymer electrolyte membrane (PEM). The results are observed to be sensitive to GDL material models as well as endplate designs. This means, endplates designed to improve the electrical contact resistance or contact pressure at the BPP|GDL interface may not necessarily assure an improvement in bulk properties, in fact, it is observed in this study that these properties are inversely related. Further, a differential deformation in GDL along with consolidation effect is predicted with compressible version of hyperelastic material model. More importantly, it is revealed that the selection of material models plays a significant role in the deformation behaviour of the GDLs irrespective of the clamping design adopted.

Optimizing the architecture of lung-inspired fuel cells

A finite-element model of a polymer electrolyte membrane fuel cell (PEMFC) with fractal branching, lung-inspired flow-field is presented. The effect of the number of branching generations N on the thickness of the gas diffusion layer (GDL) and fuel cell performance is determined. Introduction of a fractal flow-field to homogenize reactant concentration at the flow-field | GDL interface allows for the use of thinner GDLs. The model is coupled with an optimized cathode catalyst layer microstructure with respect to platinum utilization and power density, revealing that the 2020 DoE target of ~8 kW/gPt is met at N = 4 generations, and a platinum utilization of ~36 kW/gPt is achieved at N = 6 generations. In terms of the overall fuel cell stack architecture, our results indicate that either the platinum loading or the number of cells in the stack can be reduced by ~75%, the latter option of which, when combined with a 100 µm GDL, can lead to >80% increase in the volumetric power density of the fuel cell stack.

The effects of water accumulation at the interface between gas diffusion layer and gas supplying channel on the water distribution in polymer electrolyte membrane fuel cells

The effects of water accumulation at the interface of gas diffusion layer (GDL) and gas supplying channel on the water distribution in polymer electrolyte membrane fuel cells (PEMFCs) is analyzed. The amount of water at the interface and in the GDL are quantified using X-ray. Quantitative analyses show that the value of the criterion of water accumulation that can affect the water distribution is in the water saturation range of 0.22–0.24. The amount of water in the GDL increases with the water accumulated at the beginning of the water generation cycle. However, it remains constant after the water accumulation exceeds a criterion value. The result shows that the water accumulation at the interface should be investigated to understand the water distribution in PEMFC. The water distribution in PEMFC cannot be analyzed based on the steady state concept but can be analyzed based on the concept of cyclic process.

Effects of assembly pressure on PEM fuel cell performance by taking into accounts electrical and thermal contact resistances

In this paper, a three-dimensional multiphase model of the polymer exchange membrane (PEM) fuel cell is simulated to study the effect of assembly pressure on the contact resistance between the gas diffusion layer (GDL) and bipolar plate (BP) interface. The results reveal that the increase of assembly pressure is associated with a decrease in the contact resistance between the GDL and BP interface, which results in reaching an ideal fuel cell performance. The performance improves until the assembly pressure of 4.5 MPa and it slightly drops with a clamping pressure of 5.5 MPa in the ohmic loss region of the polarization curve. Additionally, the variation of the electrical field in a cross-section of the channel length shows that the intrusion of GDL into the flow channel increases with increasing assembly pressure; consequently, the maximum electrical current will increase. The cell temperature rises at higher assembly pressure when considering the thermal contact resistance. This increase is higher on the cathode side because of the existence of the reaction heat source. Additionally, it is found that the distribution of electrical potential and oxygen concentration is more uniform at higher clamping pressure. This results in the development of the PEM fuel cell life cycle.

Numerical analysis of parallel flow fields improved by micro-distributor in proton exchange membrane fuel cells

The flow field on bipolar plates of proton exchange membrane fuel cells (PEMFCs) is critical to flow distribution and overall pressure drop, both of which have great influence on cell performance. A micro-distributor was proposed to improve the uniformity of flow distribution in parallel flow field. The effects of micro-distributor size on output cell performance and pressure drop were systematically studied with computational fluid dynamics (CFD) simulations. To evaluate the competence of the modified parallel flow field, a serpentine flow field was also considered for comparison. The results suggest that the modification of the flow field is able to enhance the uniformity of flow distribution significantly. The maximum output power density increases when decreasing the size of micro-distributor. Compared with the serpentine flow field, the modified parallel flow field with micro-distributor enables the fuel cell to give nearly comparable output performance, but requires two orders of magnitude lower pressure. More generally, it was found that the flow uniformity could be simply and conveniently improved by enhancing the mean pressure drop in the sub-channels. The origin of non-uniform distribution of oxygen concentration in the interface of gas diffusion layer (GDL) and catalyst layer (CL) was also discussed. This work provides an alternative to conventional parallel and serpentine flow fields, and is helpful for the designing of high output PEMFCs.

The analysis of adhesion force at the interface of gas diffusion layer and channel in polymer electrolyte membrane fuel cell

The adhesion force of water droplet on the gas diffusion layer (GDL) is modeled based on the droplet deformation. The deformed droplet is represented as an ovoid shape. The adhesion force is calculated based on it and verified by the surface tilting experiment. The model predicts the shape of deformed droplet and adhesion force within 30% error, whereas previous models predict adhesion force with error larger than 30%. The modified model is used to compare the adhesion force among 3 types of GDL having pore gradient. The comparison result is well matched with the water distribution in polymer electrolyte membrane fuel cell (PEMFC) and water detachment phenomena at the GDL. High adhesion force makes more water accumulation at the interface of GDL and gas supplying channel. This makes different boundary condition and changes the water distribution in PEMFC.

Coupling of Lattice Boltzmann and Volume of Fluid Approaches to Study the Droplet Behavior at the Gas Diffusion Layer/Gas Channel Interface

With the increased concern about energy security, global warming, and air pollution, the possibility of using polymer electrolyte fuel cells (PEFCs) in approaching renewable and sustainable energy systems has reached significant momentum [1,2]. A typical PEFC flow field consists of a series of micro/minichannels. The continuous removal of liquid water from the cathode channels is a critical issue, as water droplets forming in the channels can block the transport of gaseous oxygen to the active sites. This phenomenon gives not only an uneven current distribution, substantial loss of performance, but also, unstable operation and increased degradation rates [1]. Water generated by the electrochemical reactions often condenses into liquid form, potentially flooding the GC (gas channel), the GDL (gas diffusion layer), the micro porous layer (MPL) and the catalyst layer. Insight into the fundamental processes of liquid water transport and evolution is still not complete, preventing further FC development [1]. Computational fluid dynamics (CFD) models make it possible to reduce the number of experiments needed for cell design and development [1,3]. The aim of this work is to obtain an increased understanding of the droplet behavior at the GDL/GC interface by coupling of Lattice Boltzmann and Volume of Fluid (VOF) approaches, for gas channels relevant for PEFCs. The VOF model is an interface-resolving method and has become a widespread methodology for solving two-phase flow phenomena inside PEFCs. The volume fraction of liquid water at the computational cell volume schemes, are used to track the interface between the phases [1,3]. Input parameters in the VOF model are extracted from our in-house Lattice Boltzmann calculations [4], i.e., establishing a multiscale environment. It is clear that the contact angle as well as the size of the liquid droplet vary with positions at the interface, depending on the stochastic GDL geometry. A model describing one straight channel with one gas inlet, one liquid inlet (at the GDL surface) and one two-phase outlet is developed. The operating parameters belong to the laminar flow regime. Notice that a developing flow region covers the entire channel. The uniqueness of this work includes a calculation of contact angles at the GDL interface as well as the size of the liquid inlet, by the authors, with Lattice Boltzmann calculations (3D nineteen-velocity (D3Q19) pseudopotential multiphase multicomponent isothermal LBM) [4].

Interface resolving two-phase flow simulations in gas channels relevant for polymer electrolyte fuel cells using the volume of fluid approach

With the increased concern about energy security, air pollution and global warming, the possibility of using polymer electrolyte fuel cells (PEFCs) in future sustainable and renewable energy systems has achieved considerable momentum. A computational fluid dynamic model describing a straight channel, relevant for water removal inside a PEFC, is devised. A volume of fluid (VOF) approach is employed to investigate the interface resolved two-phase flow behavior inside the gas channel including the gas diffusion layer (GDL) surface. From this study, it is clear that the impact on the two-phase flow pattern for different hydrophobic/hydrophilic characteristics, i.e., contact angles, at the walls and at the GDL surface is significant, compared to a situation where the walls and the interface are neither hydrophobic nor hydrophilic (i.e., 90° contact angle at the walls and also at the GDL surface). A location of the GDL surface liquid inlet in the middle of the gas channel gives droplet formation, while a location at the side of the channel gives corner flow with a convex surface shape (having hydrophilic walls and a hydrophobic GDL interface). Droplet formation only observed when the GDL surface liquid inlet is located in the middle of the channel. The droplet detachment location (along the main flow direction) and the shape of the droplet until detachment are strongly dependent on the size of the liquid inlet at the GDL surface. A smaller liquid inlet at the GDL surface (keeping the mass flow rates constant) gives smaller droplets.

Determining the Impact of Dynamic Load Conditions on Interfacial Liquid Water Accumulation in Polymer Electrolyte Membrane Fuel Cell Gas Diffusion Layers Using Synchrotron X-Ray Radiography

Automakers around the world are investing in polymer electrolyte membrane (PEM) fuel cells for the next generation drivetrains for their low emission automobiles. For PEM fuel cell vehicles to become commercially competitive, fuel cell efficiency and durability must increase. However, in order to advance the performance of PEM fuel cells, the dynamic behaviour of liquid water transport must be taken into consideration, particularly because fuel cell cars will be driven with dynamic load (power draw) conditions1. The relationship between PEM fuel cell performance and dynamic load must be well understood so that next generation fuel cells can be tailored for optimal dynamic operation. Effective water management is key to obtaining the optimal performance of fuel cells2,3. It has also been shown that the transient response of two-phase flows of water is significantly longer compared to the electrochemical response1. Therefore, the changes in two-phase flows in the fuel cell can have a larger impact on the dynamic performance of fuel cells4,5. Liquid water evolves in the porous layers of the fuel cell until it reaches a steady state for the current density of operation6. However, there is a scarcity of experimental observations focused on GDL liquid water saturation as a function of time with changes in operational current density. In this work, the transient response of the cell potential and the dynamic change in water saturation of the gas diffusion layer (GDL) are investigated. The current density of the cell was increased from 0 A/cm2 to prescribed values at various rates of increasing current density (ramp rates), while the response of the cell potential was concurrently measured. Figure 1 shows the transient response of the cell potential due to a change in the current density from 0 to 1.2 A/cm2 at various rates of increase. With a step change in the current density, the voltage fluctuated for a few minutes before the potential of the cell dropped, leading to performance failure that was indicative of flooding. However, when a ramp was applied to gradually reach the same current density as the step change, the cell was able to maintain a steady potential. This shows that the ramp rate of increasing current density has a direct impact on the cell performance and its transient response. In addition, X-ray radiographic evidence of water in the porous layers of the PEM fuel cell during transient operation will be presented. The dynamic water evolution at the microporous layer (MPL)|catalyst layer interface and the MPL|GDL interface are highly influenced by the rate of increasing current density. The cell performance was correlated to the time-dependent water evolution patterns in order to identify the interfacial liquid water accumulation as a function of changing current density.

(Keynote) Multiscale Modeling of Polymer-Electrolyte-Fuel-Cell Components

Water management in proton-exchange membrane fuel cells plays an important role in device operation, especially at lower temperatures where liquid water exists. Key to understanding water management is determining the mechanisms of water movement through the various fuel-cell components. Traditionally, continuum-scale mathematical modeling has been used to optimize and understand fuel-cell water and thermal management. However, it is increasingly become apparent that there is a need for more detailed lower lengthscale information to feed into the models due to microscale and mesoscale heterogeneities. In this talk, we will examine these issues with a focus on liquid-water movement in fuel-cell diffusion media and in water and ion transport in fuel-cell membranes including both proton and alkaline varieties. It is well known that continuum models use a volume-averaged approach to model liquid-water transport. This strategy is suitable for higher operating temperatures and homogeneous gas-diffusion-layer (GDL) morphologies, but, such structures are not realistic. To handle the complexity of the GDL morphology and access the expected capillary-fingering type of water transport in these porous media, pore-network models (PNMs) are utilized. However, while PNMs models capture the water transport through the GDL, they do not contain suitable descriptions of the other layers, complex nonlinear transport, and electrochemical phenomena. Here, we combine the advantage of both models in an iterative scheme integrating continuum and PN models to describe the multiphase and multiscale water transport properly. The coupling is achieved through an iterative scheme wherein the 2-D continuum PEFC model passes the calculated, heat and mass fluxes to the PNM, where they act as spatially varying source terms. Subsequently, the PNM returns to the continuum model a set of effective properties, such as thermal conductivity, effective diffusivity, and permeability, which are discretized along the electrode|GDL interface. This talk will discuss the work-flow between the coupled models including utilization of X-ray CT inputs and how such an approach can explain nonintuitive experimental data. For an ion-exchange membrane, mathematical modeling is ideally suited to explore the genesis of the observed water profiles and subsequently the mechanisms of water movement. Recently, such modeling has been receiving increased attention as new knowledge on the impact and control of the membrane/environment interface is elucidated. To model such effects requires the use of a multiscale framework involving mesoscale networks and microscale ion-transport information. The linking between these scales will be discussed including appropriate ways to incorporate molecular-level information into a physically consistent framework that still allows for easy incorporation into cell-level fuel-cell models. Acknowledgement I would like to acknowledge the various graduate students, postdocs, and collaborators who have worked on this research over the years. This work was supported by The Office of Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

One-dimensional comparison of numerical approaches on two-phase flow in the membrane electrode assembly of PEMFC

Numerical simulation of two-phase flow is important for the performance evaluation of proton exchange membrane fuel cell (PEMFC). Exchange of water between vapor and liquid phases in the gas diffusion layer (GDL) was considered by Nguyen and co-workers (2000) using the evaporation and condensation rate constants. Hsuen and Yin (2011), in which a pseudo-phase-equilibrium function approximates the vapor/liquid phase equilibrium, proposed another two-phase flow approach. Both methods are capable of predicting the cell performance and the species concentration profiles in the membrane electrode assembly (MEA) without the need to track liquid water front in the GDL explicitly. The present study compares the numerical results of these two approaches in detail. In addition, one-dimensional simplified semi-analytical solution based on the instantaneous phase equilibrium assumption is used to validate the numerical calculations. The evaporation/condensation approach allows the existence of super-saturated vapor near the interface of cathode GDL and membrane. Thus, membrane can be better hydrated near the interface once liquid water emerges in the GDL as compared to the pseudo-phase equilibrium method. Consequently, smaller ohmic resistance and hence better cell performance are observed with Nguyen's prediction once liquid water appears in the GDL.