Gas Diffusion Layer Compression Research Articles

Analytical Modeling of Water Droplet Behavior at the Gas Channel Corner for Proton Exchange Membrane Fuel Cells

Abstract Water management is of significant importance to achieving high performance of proton exchange membrane fuel cells. In recent years, droplets emerged from the rib surface and accumulated at the channel corner have been found to be a crucial part of water flooding. In this study, an analytical model is first proposed to quantitatively estimate the variation in the morphology and dynamic behavior of growing droplets with consideration of the channel sidewall interaction. In order to predict the water geometry, the flow channel with compressed gas diffusion layer (GDL) is described mathematically, and water behavior at steady-state and dynamic state are both evaluated through the geometric and force analysis. The model results indicate that the droplet profile transforms from concave to convex when its size grows, in which process contact angles and channel shape play an important role. Compared with the graphite channel, the droplet in the metallic channel is more inclined to be adsorbed on the sidewall and GDL, resulting in a higher adhesion force and a lower gas shear force. The critical gas velocities for the detachment of droplets are quantitatively predicted to avoid water flooding. The model is helpful to understand the droplet behavior in the presence of channel sidewall interaction.

Transient characteristics of in-plane water transport in gas diffusion layers of PEM fuel cells

During the operation of proton exchange membrane (PEM) fuel cells, water and heat are produced as two byproducts. A reliable and continuous operation of a PEM fuel cell requires an efficient water removal from the porous structure of the electrodes. In PEM fuel cells, gas diffusion layers (GDL) are used to effectively remove the produced water from the electrodes and also to supply reactant gases across them. The focus of this study is to characterize liquid water percolation within the plane of the GDL by utilizing a nondimensional energy ratio-time scale model (Ce-t∗). The transport phenomena are investigated for various GDL compressions, GDL samples with the microporous layer (MPL), and a degraded GDL sample. The transition between flow regimes is characterized based on the variation of slopes in Ce-t∗ curves. The flow regimes were observed to change from fingering and channeling in the capillary fingering regime to macro-transport along with uniform percolation in the stable displacement regime. It was observed that GDL compression can expedite flow regime transition and can cause faster energy dissipation during steady state periods. When MPLs were added to the GDL, the wetted areas were observed to occur at more dispersed locations. Finally, the experiment conducted with a degraded GDL without an MPL demonstrated higher slopes in the Ce-t∗ curve, indicating more frequent liquid water flooding. This could ultimately result in reduced cell performance.

Modeling and performance investigation on the deformed gas diffusion layer of PEM fuel cell

Simulation of a proton exchange membrane fuel cell (PEMFC) in its assembled state has been a long-standing challenge, with one factor being the influence of gas diffusion layer compression resulting from heat and mass transfer on the effective proton conductivity of the proton exchange membrane. Due to the lack of in-situ data, it is customary to utilize an empirical formula as a conventional model for determining the effective proton conductivity. However, significant deviations (>10%) have been observed between simulated and experimental data for fuel cells, mainly when the fuel cell is assembled. The assembly of PEMFC caused a shift of effective proton conductivity, leading to a significant deviation. To address this issue, this study proposes a model using COMSOL that integrates mechanics, electrochemistry, heat, and mass transfer of the fuel cell. To decrease the deviation between simulation and experiment, the effective proton conductivity of the proposed model is corrected by the reference proton conductivity. Specifically, an adjustment factor is introduced to the reference proton conductivity to correct the shift of effective proton conductivity caused by the compression. As a result, the average deviation of the proposed model is decreased from 10.44% to 2.25%, compared to a traditional model. As a case study, the optimal compression ratio of 20% is obtained by heat and mass transfer analysis, in which peak power density is increased from 6611.2 to 7466.6 W m−2. This study highlights the importance of membrane proton conductivity for the output performance of PEMFC.

Pore-Scale Modeling of Liquid Water Transport in Compressed Gas Diffusion Layer of Proton Exchange Membrane Fuel Cells Considering Fiber Anisotropy.

Water management of the gas diffusion layer (GDL) is crucial to the performance of proton exchange membrane fuel cells (PEMFCs). Appropriate water management ensures efficient transport of reactive gases and maintains wetting of the proton exchange membrane to enhance proton conduction. In this paper, a two-dimensional pseudo-potential multiphase lattice Boltzmann model is developed to study liquid water transport within the GDL. Liquid water transport from the GDL to the gas channel is the focus, and the effect of fiber anisotropy and compression on water management is evaluated. The results show that the fiber distribution approximately perpendicular to the rib reduces liquid water saturation within the GDL. Compression significantly changes the microstructure of the GDL under the ribs, which facilitates the formation of liquid water transport pathways under the gas channel, and the increase in the compression ratio leads to a decrease in liquid water saturation. The performed microstructure analysis and the pore-scale two-phase behavior simulation study comprise a promising technique for optimizing liquid water transport within the GDL.

Effect of assembly pressure on the performance of proton exchange membrane fuel cell

The assembly force is a crucial factor in the process of proton exchange membrane fuel cell (PEMFC) stacking, and has significant effects on the fluid flow, mass transfer, and water and thermal management, which affect the fuel cell performance. In this study, from the most deformable component, the gas diffusion layer (GDL), combining with a finite-element analysis, and computational fluid dynamic method, the impact of the assembly force on a single-channel PEMFC is analyzed. A nonlinear stress–strain curve obtained from a microanalysis is creatively introduced into the two-dimensional compression model. The gas diffusion coefficient in the three-dimensional model is also obtained from the microscopic simulation. The simulated effective oxygen diffusion coefficient of the compressed GDL is approximately 0.86 times the Bruggemann estimated value. When the contact resistance is ignored, the output voltage at 2.5 MPa is decreased by approximately 15.4% at 1.7 A·cm−2 compared with that at 0.5 MPa. After the contact resistance is considered, the effects of the assembly pressure on the cell performance (V–I curve) are qualitatively different. The pressure drop of the 2.5 MPa case is 20% higher than that of the 1.4 MPa case at 1.7 A cm−2. O2 is hard to flow into the region under the rib where the porosity and permeability are lower. The results indicate that both liquid water and membrane water contents increase when the assembly force increases. The effect of the assembly force on the temperature is also analyzed.

Experimental and simulation analysis of liquid capillary fingering process in the gas diffusion layer

In a proton exchange membrane fuel cell (PEMFC), water movement through the gas diffusion layer (GDL) is essential for water management. Using a combination of experiments and simulations, this paper investigates the capillary fingering process of liquid water within the GDL and analyzes the effects of flow field type, GDL compression ratio, and contact angle on the capillary fingering of liquid water within the GDL. The results reveal finger-stopping, splitting, and fusion during the injection of non-wetting liquid, which is not conducive to rapid water movement within the GDL. When water breaks through the top layer of the fibers, a finger path between the inlet and the gas channel is formed, and the liquid preferentially migrates along this path. Due to the existence of air cross-flow, the GDL water saturation for the serpentine/cross-finger flow field is 17.2%. The water preferentially produces directional flow in the through-plane direction as the compression ratio rises, and the GDL water saturation falls to 24.18%. The distribution of anisotropic contact angles determines the flow state of liquid water, making water removal from the GDL easier. The results and findings here are expected to lead to more reliable water management and related high-power technologies of PEMFCs.

Optimization of the Cathode Gas Diffusion Layer Also Matters for Water Electrolyzers

Polymer electrolyte water electrolyzers (PEMWEs) are a promising technology for the storage of energy from intermittent renewable sources such as wind and solar. PEMWEs split water into hydrogen and oxygen electrochemically. Under typical operating conditions, the hydrogen evolution reaction (HER) in the cathode is not limited by reactant transport, since it is supplied by the rapid transport of protons from the polymer electrolyte and electrons from the external circuit. There are very limited studies on the role of the cathode gas diffusion layer (GDL), typically a carbon-paper based layer. In this study, we investigated the effect of thickness, presence of a microporous layer, and wettability of the cathode GDL. Results show that cathode GDL properties have a significant effect on the performance of the PEMWE cells. Figure 1a shows the importance of the GDL thickness with the optimized GDL compression. The thickest GDL, MGL370 (i.e. 370µm thick ) has the best performance compared to MGL280 (280µm thick) and MGL190 (190µm thick). Compression of the GDL also affects the performance: when the compression is increased from 12% to 25% of the thickness, a performance loss is observed possibly due to damaged carbon fiber network and collapse the pores of the GDL (Figure 1b). Acknowledgment: Financial support from the US Department of Energy through the Office of Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cells Technology Office is gratefully acknowledged. Figure 1

Microstructures and electrical conductivity properties of compressed gas diffusion layers using X-ray tomography

Bulk resistance of the gas diffusion layer (GDL) plays an important role in the ohmic loss of proton exchange membrane fuel cells (PEMFCs). This work represents the first direct experimental study and comparison of the bulk resistances during the compression process of commonly used commercial GDLs, carbon paper and carbon felt, from the perspective of the microstructure mechanisms. The in-situ X-ray tomography and a fiber tracing algorithm are used to obtain the microstructure characteristics of compressed GDLs. The through-plane (T-P) and in-plane (I-P) bulk resistances of GDLs are measured by the microelectrode probe measurement method and the four-point probe method, respectively. Then, their resistivities are calculated. For the T-P direction, the arrangement of fibers gradually becomes compact during the compression, arousing the nearly linear increase of fiber contact points. Thus, more and more conductive paths are constructed, inducing a rapid decline of the T-P bulk resistivity. Since the fibers in carbon felt are entangled together, the fibers in the edge areas are arranged loosely at low compression. This uneven distribution of contact points in carbon felt results in much larger T-P bulk resistivity than that of carbon paper at low compressive strain. For carbon paper, binders contribute significantly to the smaller T-P resistivity because they tend to gather at the fiber intersections, which forms more conductive paths with better conductivity. For the I-P direction, the bulk resistivities of GDLs decrease linearly during compression. The nearly horizontal fibers are the key to the I-P bulk resistivity, and both smaller fiber density and higher fiber tortuosity lead to the increase of the I-P bulk resistivity. This study helps to understand the microstructure mechanisms of the bulk resistances of various GDLs during the compression process, which can guide the design and fabrication process of GDLs.

The influence of baffle geometry on mass transfer and performance of proton exchange membrane fuel cells considering the compression of gas diffusion layer

In this paper, the effects of baffle geometric parameters on fuel cell performance and mass transfer are studied considering the geometric deformation of the gas diffusion layer and the changes in porosity and permeability caused by the assembly force of the stack. The parameters include the windward part length, the middle section length, the leeward part length, the height and the number. We find that changing the windward or leeward part length of baffles will not impact the net power density significantly; short leeward part will produce bimodal streamlines and relatively more power; the baffle height close to blockage is undesirable. In addition, we observe that the trapezoidal baffles can not improve the net power density and the distribution uniformity of the physical quantities inside the fuel cell at the same time.

Analysis of compression in uniform and non-uniform GDL microstructures on water transport

ABSTRACT A 3D compressed gas diffusion layer (GDL) microstructure is developed based on the finite volume method (FVM) and used to evaluate water transport behavior. The compressed GDL microstructure and two-phase flow VOF model are developed and validated on the OpenFOAM CFD platform. The models are compared to experimental data, with good agreement. Consequently, the reconstructed GDL microstructures are subjected to compressive stresses. The water uptake behavior in the compressed samples with different (CR) compression ratios (10% CR, 20% CR, and 40% CR) is compared to that in an uncompressed GDL microstructure. Also, the effects of GDL wettability, water pressure, and non-uniform fiber diameter arrangement in GDLs are investigated. In GDL microstructures, two-phase interaction is influenced by the GDL contact angle, compression ratio, water inlet pressure and capillary pressure. It is found that excessive compression on GDLs constricts the pores, thereby restricting access of water through the pores. As such, 10% CR was found to be the safe limit with just 8% saturation drop. On the other hand, altering the fiber structural arrangement through unequal-sized fiber diameter had very little impact on water saturation in the through-plane direction of both the uncompressed and compressed GDL microstructures. As such, the excessive introduction of the unequal-sized fibers may not be the right choice, especially under compressive forces. Thus care must be taken in the treatment of GDL carbon papers and clamping force during assembly since these significantly influences water patterns in them.

Numerical investigation on the improved reactant mass transport with depth-dependent flow fields in polymer electrolyte fuel cell under inhomogeneous gas diffusion layer compression

In this work, a numerical model is developed to analyse the effects of depth-dependent reactant flow field geometry under inhomogeneous gas diffusion layer (GDL) compression on the mass transport process and performance of polymer electrolyte fuel cell (PEFC). The types of depth-dependent flow channels considered in this study are: converging channel (depth continuously decreasing) and diverging channel (depth continuously increasing), and the conventional flow field designs. The model is investigated for local and global inhomogeneity due to GDL compression. The localized inhomogeneity is introduced in the flow-field rib as well as channel regions. The results are compared for reactant concentration, water concentration, local current density, and the polarization curve for different flow channel combinations. It is observed that the availability of reactants is higher in case of converging channel design, which leads to an increase in cell performance at higher currents. However, this is subjected to GDL inhomogeneity in compression. We observe in this study that such inhomogeneity, instead of having a significant impact on cell performance, lead to minimal influence in terms of reduction in cell performance. This we observe is due to improved H2 availability at anode and reduced O2 distribution at cathode that ultimately impacts respective hydrogen oxidation reaction (HOR) and reduction in oxygen reduction reaction (ORR). This study aims to investigate the cases for altered variation in cell performance due to change in depth-dependent flow fields.

Numerical investigations of vapor condensation and water transport in gas diffusion layers of PEMFC

Water transport in vapor/liquid form in gas diffusion layers (GDL) of proton exchange membrane fuel cell (PEMFC) are numerically investigated in this study. A stochastic method is employed to reconstruct the 3D structured GDL, then a validated phase change solver is developed and implemented in the OpenFOAM platform to simulate the vapor condensation process. Comprehensive simulations are carried out to analyze the GDL porosity distribution, compressed deformation, mass flow rate, local temperature and contact angle effects on the vapor condensation and transport dynamics. Two GDL with gradient porosities and one with TGP-H-060 type are reconstructed to compare the vapor/liquid-water transport behaviors. It is found that the spatial distribution of the condensate water is comparatively different from the liquid water transport patterns. It reveals that the water transport in liquid form was much more sensitive to the GDL design, while it is more applicable in vapor form. Due to the GDL compression, the vapor/liquid water transport are impeded, especially in the middle layers of the GDL and large velocity separations are detected during the vapor transport. Meanwhile, the results of the parametric analysis indicate that water transport in vapor form provides more options in avoiding water flooding. This study is significant to deepen our understanding on the vapor condensation process and water transport dynamics in GDL, which can further guide the GDL design and optimization.

An analytical model for hydrogen and nitrogen crossover rates in proton exchange membrane fuel cells

In this paper, the hydrogen and nitrogen crossover through the membrane in proton exchange membrane fuel cells, are investigated by developing a semi-empirical analytical model. Different factors that affect the gas crossover rates were considered including pressure drop in gas diffusion layer (GDL) and catalyst layer (CL), operating temperature, relative humidity (RH) of the reactants, GDL compression, and the current density effect on the membrane temperature. The model is validated by published experimental data. It is found that RH is the most important parameter, followed by temperature. The hydrogen pressure drop through GDL and CL greatly depends on the GDL substrate properties, microporous layer (MPL) and CL. When permeability is low, an increase in current density reduces gas crossover. GDL compression, when MPL is used, was found to have a low impact on gas crossover. Gas crossover is improved with current density due to an increase in membrane temperature.

Multi-scale pore morphologies of a compressed gas diffusion layer for polymer electrolyte fuel cells

Understanding the pore morphologies of a compressed gas diffusion layer is critical to improve the performance of polymer electrolyte fuel cells. In this study, the effect of compression on the cell performance was investigated. Increasing the gas diffusion layer compression increases oxygen transport resistance. Moreover, the pore morphologies of the compressed gas diffusion layer were investigated using mercury intrusion porosimetry with a simple compression device and synchrotron X-ray computed micro-tomography. The average pore diameter of the fibrous substrate reduced applying compression pressure, whereas that of the micro-porous layer remained unchanged even at high compression (38.6%). In addition, the oxygen transport resistance calculated from the structural parameters of a compressed gas diffusion layer, where porosity and pore diameter are explanatory factors, was in good agreement with the oxygen transport resistance obtained by fuel cell testing.

Effect of PEM fuel cell porous media compression on in-plane transport phenomena

Abstract Liquid-gas two-phase flow in the gas diffusion layer (GDL) of proton exchange membrane fuel cells is investigated using an ex-situ experimental setup. The mass transport phenomena is investigated in carbon paper and carbon cloth GDLs and at different compressions. Water percolation within the plane of the GDL is visualized with a CCD camera while its injection pressure is measured. Similarly, air percolation within the plane of GDL samples which were initially saturated with water is investigated. Experiments are conducted for the three flow regimes of stable displacement, capillary fingering, and viscous fingering. Images are analyzed to obtain the normalized wetted area. It is observed that while the GDL compression directly affects normalized wetted area for stable displacement and viscous fingering flow regimes, it has no impact on this parameter for capillary fingering flow regime. For stable displacement flow regime and for both carbon paper and carbon cloth samples, water percolation pressure increases with GDL compression. However, the water percolation pressure data obtained for capillary fingering flow regime does not suggest any discernible trend as a function of GDL compression. The findings in this study can be used to validate percolation models proposed by different schemes such as pore-network models.

Effects of surface wettability on two-phase flow in the compressed gas diffusion layer microstructures

The surface wettability plays a significant role in two-phase flow in the gas diffusion layer (GDL). However, few reported researches have considered directional single gradient polytetrafluoroethylene (PTFE) distribution, PTFE immersion depth, and the GDL compression. In this study, effects of various surface wettability distribution schemes on two-phase flow in the GDL microstructures are investigated. In addition, the GDL deformation is also considered via clamping pressure simulation based on the finite element method (FEM). Two-phase flow in the compressed GDL microstructures is modeled using the volume of fluid (VOF) model. The results show that the direction of single gradient of the surface wettability is influential in two-phase flow, especially for the compressed GDL. Moreover, thicker PTFE immersion depth contributes to water removal from the GDL. Finally, novel surface wettability distribution schemes are first proposed, and they are demonstrated as important references for controllable water transport in the GDL.

Effect of clamping pressure on liquid-cooled PEMFC stack performance considering inhomogeneous gas diffusion layer compression

Gas diffusion layer (GDL) is a critical component of fuel cells, since it directly determines the transfer rate of mass, charge and heat that in turn affects the cell performance and stability. Within a fuel cell stack, assembled by over hundreds of single cells, GDL plays a more dominant role since the GDL deformation caused by assembly pressure would result in significant changes in its thermal, electrical and transport properties, thus impacting the uniformity, consistency and overall performance of the stack. Here, a numerical study is performed to investigate the effect of clamping pressure on the performance of liquid-cooled PEMFC stacks, which can be utilized as a guidance for stack assembly process in practical application. In particular, inhomogeneous compression as well as nonlinear strain-stress behavior of GDL are taken into consideration in the fuel-cell stack model, thus the GDL at under-rib and under-channel regions owns different physical properties, i.e., thickness, porosity, effective diffusivity, permeability, thermal conductivity, electrical conductivity, and contact resistance. It is found that the stack power output can be maximized when clamping pressure ranging from 1.5 MPa and 3.5 MPa owing to the balanced mass transport and contact resistance, meanwhile, 1.5 MPa enables the lowest voltage difference among different single cells that is beneficial in cell-to-cell consistency. Moreover, the inhomogeneous compression of GDL is found to be capable of facilitating water removal via enhanced convection at both in-plane and through-plane directions, and avoiding local hotspots with the increased in-plane thermal conductivity.

Investigation of two-phase flow in the compressed gas diffusion layer microstructures

This study aims to investigate how multiple parameters affect the two-phase flow in compressed gas diffusion layer (GDL). A stochastic model is adopted to reconstruct the GDL microstructures. Solid mechanics simulations on the reconstructed GDL microstructures are performed, based on the finite element method (FEM). Various pore morphologies and distributions of compressed GDLs are observed. Two-phase flow in GDL is simulated using a volume of fluid (VOF) model. Corner droplet (on the GDL surface) and water flow (emerging from GDL bottom) are considered. It is found that two-phase flow in the GDL is highly influenced by compression, fiber diameter, porosity, and GDL thickness. The results indicate that a larger fiber diameter or higher porosity contributes to the water transport due to larger average pore size. Furthermore, water removal from a thicker GDL is more difficult, whereas water transport in the lower part of a compressed thick GDL is easy.

Two-phase flow in compressed gas diffusion layer: Finite element and volume of fluid modeling

In this study, a stochastic model is used to reconstruct the uncompressed gas diffusion layer (GDL) microstructures. Subsequently, the finite element method (FEM) is conducted for assembly pressure simulation to generate the compressed GDL microstructures. The effects of assembly pressure on GDL deformation are investigated. It is found that assembly pressure causes non-uniform deformation of the GDL along the thickness direction. Finally, a volume of fluid (VOF) model is developed to investigate two-phase flow in the compressed GDL. The results show that when the capillary pressure is higher than 4 kPa, the water saturation decreases as the compression ratio increases. But when the capillary pressure is below 3 kPa, compression has little effect on water saturation. Based on the above findings, three regions namely weak deformation region (WDR), moderate deformation region (MDR), and strong deformation region (SDR) are defined. Impacts of compression on water saturation differ in these three regions. Moreover, compression increases the pressure of water breakthrough, but has minor effects on preferential pathways of water breakthrough. Quantitative correlations between water saturation and capillary pressure in the uncompressed and compressed GDL microstructures are also concluded.

Three-dimensional modeling of a PEMFC with serpentine flow field incorporating the impacts of electrode inhomogeneous compression deformation

The effects of compression deformation of gas diffusion layer (GDL) on the performance of a proton exchange membrane fuel cell (PEMFC) with serpentine flow field were numerically investigated by coupling two-dimensional GDL mechanical deformation model based on Finite Element Analysis and three-dimensional two-phase PEMFC model with incorporating the deformation impacts. Emphasis is located on exploring the influences of assembly pressure on the non-uniform geometric deformation and distributions of transport properties in the GDL, flow behaviors and local distributions of oxygen and current density, cell polarization curves and net power densities of the PEMFC. It was indicated that the non-uniform deformation of GDL results in inhomogeneous distributions of porosity and permeability in the GDL due to the presence of rib-channel pattern, and the transport properties in the under-rib region are greatly reduced with increasing the assembly pressure, consequently weakening the gas flow and oxygen transport in the under-rib region and increasing the non-uniformity of local current density distribution. As for the overall cell performance, however, attributed to the tradeoff between the adverse impacts of GDL compression on mass transport loss and positive effects on reducing ohmic loss, the overall cell performance is firstly increased and then decreased with increasing assembly pressure from 0 MPa to 5.0 MPa, and the maximum cell performance can be achieved at the assembly pressure of about 1.0 MPa for all cases studied. As compared with the case for zero assembly pressure, the maximum net power density of the cell can be improved by about 7.7%, 9.9%, 10.5% and 10.7% for the cathode stoichiometry ratios of 2.0, 3.0, 4.0 and 5.0@iref = 1 A·cm−2, respectively. Practically, it is suggested that the assembly pressure is controlled in an appropriate range of 0.5 MPa–1.5 MPa such that the cell net power can be boosted and pressure head requirement for the pump can be maintained in a appropriate level.