Gas Diffusion Layer Deformation Research Articles

Effects of Inhomogeneous Gas Diffusion Layer Properties on the Transportation Phenomenon and Performances of Proton-Exchange Membrane Fuel Cells.

The encapsulation of the proton-exchange membrane fuel cell (PEMFC) is an essential step of fuel cell stack assembly. The selection of the assembly pressure is very important to the stack performance and life. Based on that, this paper presents a method to describe the internal physical deformation of the gas diffusion layer (GDL) after inhomogeneous pressure by using user-defined functions (UDFs). The results show that the transmission difference caused by the nonuniform GDL deformation can be clearly seen by the UDFs method, and there is an obvious transition of GDL at the interface of the channel and the rib. A three-dimensional single-channel PEMFC model is established, and an optimal assembly pressure range is obtained, between 1.5 and 2.0 MPa. The maximum thermal stress inside the cell occurs in the middle of the membrane electrode assembly and decreases as the assembly pressure increases. Furthermore, the influence of rib-channel ratios is discussed. Compared to the fuel cells with ratios of 2:1, 2:3, and 1:2, the one with 1:1 exhibits the maximum current density and the highest power density.

(Digital Presentation) A Constant Deformation Modulus for the Simulation of Gas Diffusion Layer

The deformation of gas diffusion layer(GDL) directly affects gas diffusion and electron transfer in the fuel cell. The modulus of stress-strain relationship is the basic parameter in the process of deformation. In this paper, based on beam bending theory and geometric probability analysis, a nonlinear analytic model of stress-strain with physical meaning is established, and a constant deformation modulus of GDL is given. Based on the deformation modulus, we further simulate the inhomogeneous deformation of GDL within a half channel-rib model. The results indicate that the uneven deformation with deformation modulus is more significant than that with linear elastic modulus in the same condition. Finally, a shape-dependent bending radius design for the rib edge is proposed, in which the inhomogeneous deformation of GDL becomes smoother as the radius increases, thereby improving the service condition of GDL and prolonging its service life.

Combined effect of channel to rib width ratio and gas diffusion layer deformation on high temperature – Polymer electrolyte membrane fuel cell performance

The present study investigates the combined influence of Channel to Rib Width (CRW) ratio and clamping pressure on the structure and performance of High Temperature-Polymer Electrolyte Membrane Fuel Cell (HT-PEMFC) using a three-dimensional numerical model developed previously. It also considers the impact of interfacial contact resistance between the Gas Diffusion Layer (GDL) and Bipolar Plate (BPP). The structural analysis of the single straight channel HT-PEMFC geometry shows that the von-Mises stress greatly increases in the GDL under the ribs as the CRW ratio increases resulting in considerably high deformation. The cell performance analysis depicts the significance of ohmic resistance and concentration polarization for different CRW ratios, particularly at higher operating current densities. However, in low to medium current density regions, the CRW ratio has little influence on cell performance. A substantial impact on the species, overpotential, and current distributions is observed. The findings also reveal that the CRW ratio significantly affects the temperature distribution in the cell.

A novel constitutive stress-strain law for compressive deformation of the gas diffusion layer

Substantial compressive deformation occurs in the gas diffusion layer (GDL) under the pressure applied during the fuel cell assembly. The GDL deformation has a direct impact on the efficiency and performance of the fuel cell since it leads to the alteration of the GDL microstructure and porosity. This makes the accurate characterization of the GDL compressive behavior crucial for analyzing the fuel cell performance and its optimal design. In this paper, analytical, experimental, and numerical methods have been employed to comprehensively study the constitutive law of the GDL under compression. Starting from the recently developed stress-density relations, the constitutive stress-strain equations are derived for the GDL and the relation between the stress-density and stress-strain laws are revealed. Experimental compression tests have been performed on GDL samples and the capability of the proposed constitutive law in capturing the real behavior of the material has been proved. It has been observed that the simplifying assumption of constant zero Poisson's ratio in the through-plane direction made in many previous studies cannot accurately represent the GDL material behavior and a modification is proposed. The developed constitutive law has been successfully implemented in a finite element model of the GDL-bipolar plate assembly in the fuel cell structure and the variations of the GDL porosity, density, and through-plane Young's modulus and Poisson's ratio have been investigated for different vertical displacements of the bipolar plate.

(Digital Presentation) A Constant Deformation Modulus for the Simulation of Gas Diffusion Layer

The deformation of gas diffusion layer(GDL) directly affects gas diffusion and electron transfer in the fuel cell. The modulus of stress-strain relationship is the basic parameter in the process of deformation. In this paper, based on beam bending theory and geometric probability analysis, a nonlinear analytic model of stress-strain with physical meaning is established, and a constant deformation modulus of GDL is given. Based on the deformation modulus, we further simulate the inhomogeneous deformation of GDL within a half channel-rib model. The results indicate that the uneven deformation with deformation modulus is more significant than that with linear elastic modulus in the same condition. Finally, a shape-dependent bending radius design for the rib edge is proposed, in which the inhomogeneous deformation of GDL becomes smoother as the radius increases, thereby improving the service condition of GDL and prolonging its service life.

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.

Numerical study of inhomogeneous deformation of gas diffusion layers on proton exchange membrane fuel cells performance

Gas diffusion layers play a critical role in the operation of proton exchange membrane fuel cells. As the most compressible component in proton exchange membrane fuel cells, the non-uniform deformation mainly occurs on the interface between bipolar plates and gas diffusion layers caused by the special channel-rib geometry of the flow field, which results in a non-uniform variation of physical properties of gas diffusion layers, such as porosity, effective electrical conductivity, and gas diffusivity, consequently affects the cell performance. In this paper, a two-dimensional, across-the-channel, multi-physics and two-phase flow model based on the spherical agglomerate assumption is developed to investigate the complicated relationships between the non-uniform deformation and variation of physical properties of the gas diffusion layers, as well as the cell performance. A modified diffusion coefficient is introduced to describe the effect of the variation of species concentration on the effective diffusion coefficient based on the Bruggeman formula. Simulation results show that an optimal cell performance can be achieved by balancing the variation of porosity, effective electrical conductivity, and effective gas diffusion coefficient with respect to different degrees of deformation of gas diffusion layers.

The influences of gas diffusion layer material models and parameters on mechanical analysis of proton exchange membrane fuel cell

AbstractThe deformation of gas diffusion layer (GDL) under compacting pressure has a close relationship with transport properties, and electrical and thermal conductivity of proton exchange membrane fuel cell (PEMFC). In this paper, two kinds of GDL material models with different material parameters are studied with the finite element method (FEM). Taguchi orthogonal experiments and analysis of variance (ANOVA) are performed to determine their effects on mechanical analysis of GDL deformation in PEMFC. Finally, the effects of material parameters on GDL diffusion and electrical performance are discussed. The results reveal that elastic modulus (E) is the only factor affecting the maximum displacement for GDL with isotropic model when the Poisson's ratio (ν) is in the range of 0–0.1. For orthotropic model, the through‐plane elastic modulus (Ez) has a great impact on both the deformation and contact pressure. In‐plane elastic modulus (Ex) and through‐plane shear modulus (Gxz) have significant influences on contact pressure. Ez has a great impact on diffusion and electrical performance of GDL, while the effects of Ex, Gxz and ν are negligible. In addition, orthotropic GDL material model and the parameters utilized are effective in analyzing the deformation of GDL.

Transport properties of gas diffusion layer of proton exchange membrane fuel cells: Effects of compression

The gas diffusion layer (GDL) is an essential component for a proton exchange membrane (PEM) fuel cell because it provides pathways for different transport processes, including gas diffusion, liquid permeation, and thermal/electrical conduction. However, the compact pressure applied on the fuel cell stack can lead to GDL deformation and thus a change of the transport properties. Therefore, this study numerically estimated the effect of compression on different transport properties of GDL. The anisotropic fibrous structure of GDL is first reconstructed by a stochastic orientation method. Then, the compression process is simulated by a finite element method to export a deformed GDL geometry for transport models. The variations of stress, porosity, and the transport properties with compression are validated. It is found that the compression reduces the oxygen diffusivity and intrinsic permeability, while improves the thermal and electrical conductivity of GDL. The transport property changes are proved larger in the in-plane direction than through-plane direction. By fitting the correlations of compression ratio with the transport properties, several correction factors are proposed. Furthermore, the sensitivity of different transport properties to compression level is analyzed. These results provide a deeper understanding and practical approach for the modification of GDL and PEM fuel cells.

A comprehensive two-phase proton exchange membrane fuel cell model coupled with anisotropic properties and mechanical deformation of the gas diffusion layer

Adequate mechanical compression applied to a proton exchange membrane fuel cell is essential for enhancing performance and durability. The compression not only improves the intimate contact between each layer component, but also causes significant deformation of the gas diffusion layer (GDL). As a result, the level of compression results in changes of electrical and thermal contact resistances, and anisotropic distribution of porosity and tortuosity in the GDL. To improve the fundamental understanding of the compression effect, a comprehensive two-phase model coupling solid mechanics, heat and mass transfer, and electrochemical reactions is developed. First, the model results are validated and show good agreement with the experimental data at baseline operating conditions of 80 °C, 200 kPa, 80% relative humidity, and 20% compression strain ratio with hydrogen and air. Then, oxygen, temperature and liquid saturation distribution at low, intermediate and high current densities are studied at the baseline compression level (20% strain). Finally, five compression strain ratios ranging from 5%, 10%, 15%, 20% and 25% are investigated to study the effect on stress distribution, transport properties, and fuel cell performance. Our results indicate that significant GDL deformation and channel intrusion are caused by land and rib distribution. As a combined effect from heat and mass transport, the liquid water condensation tends to initiate near the rib/GDL interface and can reach near 30% saturation at high current density. In addition, significant difference of in-plane temperature, oxygen, current, and load distributions can be observed due to the geometric aspect ratio and anisotropic GDL properties. A compression strain ratio of 20% is found to have an optimal performance balancing between high contact resistance caused by low compression and high transport resistance due to high compression. The findings of this study provide a new insight to the understanding the complicated coupling effect between mechanical, thermal, mass transport, and electrochemical multiphysics.

Compression properties of gas diffusion layers and its constitutive model under cyclic loading

The compressive deformation of gas diffusion layer (GDL), which is highly nonlinear and related to the loading history, affects the performance of PEM fuel cell stacks. However, linear elastic models are widely used. In this study, a new nonlinear constitutive model is proposed to describe the compression properties. Macroscopic studies reveal that GDL has different mechanical properties during the first and repeated compression stages. Besides, the tangent modulus has a significant linear relationship with stress. The constitutive model can be rebuilt using the micro-mechanical theory of fiber assemblies by considering the bending of carbon fibers. Furthermore, a prediction method is proposed to describe cyclic compression behavior. The prediction results fit well with the test results with an average and maximum relative error of less than 5.30% and 18.13%, respectively. These conclusions are beneficial to the design of GDL with specific mechanical properties and the real-time analysis of PEM fuel cell.

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.

A parametric study on the deformation of gas diffusion layer in PEM fuel cell

A well-designed proton exchange membrane (PEM) fuel cell requires a robust design of cell components. Therefore, the understanding of the deformation mechanism of each component is essential for a successful design. This study deals with the induced deformation of gas diffusion layer (GDL) due to the deformation of the membrane during fuel cell operation. A parametric study with six cases is constructed and the corresponding responses of a stress variation, distribution of contact pressure and a channel intrusion are selected to assess the effect of design parameters on the deformation of the GDL. The variation in the thickness of the GDL is considered as a parameter of the study in addition to the material model of the membrane. The variation in stress levels in the GDL induced due to the cyclically deformed membrane is calculated with different material models of the membrane. Through-plane stiffness of the GDL represented by the thickness of the GDL affects the three responses of the model considered. However, the material model of the membrane influences the planned responses selectively. An observation of contact loss at the interface is a strong indication of the importance of through-plane stiffness of the GDL on the deformation between the GDL and the membrane. In order to come up with a way that improves the cell’s performance, we numerically show that the manipulation of through-plane stiffness of the GDL is needed by employing an appropriate thickness of the GDL. The results from this study assist in understanding the interrelation between the GDL and the membrane for the deformation of 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.

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.

Mass transport in anode gas diffusion layer of direct methanol fuel cell derived from compression effect

The anode gas diffusion layer plays an important role on mass transport. By adjusting the assembly pressure, the structure of gas diffusion layer can be controlled to investigate the two-phase behavior. The electrochemical test indicates that increasing the assembly pressure causes increase in mass transport resistance. Inhomogeneous compression of gas diffusion layer is observed by three-dimensional imaging technology. Based on the experimental data, a two-phase model is established. The simulated results show that the increment of surface roughness and the reduction of porosity, both derived from compression of gas diffusion layer, exacerbate CO2 blockage of the cell. Lowering polytetrafluoroethylene content of anode gas diffusion layer alleviates CO2 blockage by reducing the difference of surface tension between liquid-solid and gas-solid interface as well as the negative capillary pressure of gas diffusion layer. Thus, the peak power density climbs from 60.61 to 49.94 mW cm−2 to 74.79 and 70.47 mW cm−2 at the assembly pressure of 1.00 and 2.00 MPa, respectively. The optimal assembly pressure locates at where good contact between membrane electrode assembly and bipolar plate is achieved, with the least gas diffusion layer deformation possible.

An integrated model of the water transport in nonuniform compressed gas diffusion layers for PEMFC

Abstract Water transport in gas diffusion layer (GDL) is a very important issue for high power density Proton Exchange Membrane Fuel Cell (PEMFC). During the GDL and bipolar plate (BPP) assembly process, the water transport behavior is greatly influenced by the nonuniform compression on the GDL, which leads to uneven distribution of the internal mass transport pores. In this study, an integrated model is developed to predict the water transport in nonuniform compressed GDL. Firstly, a GDL compression deformation model is built to obtain the relationship between the GDL deformation and assembly clamping force based on energy method. Then, a water transport model is established by considering the probability density function (PDF) of the pore size for the compressed GDL. The accuracy of the integrated model has been verified by comparing with the finite element method (FEM) and the computational fluid dynamics (CFD) simulation results. The influence of assembly clamping force, GDL thickness and channel geometry are analyzed based on the integrated model. Drainage pressure increases monotonically with the assembly clamping force and is divided into three stages. For the baseline case, 0.2 mm of GDL thickness and small rib-channel ratio is conducive to improving drainage capacity. It provides the guidance for matching of GDL/BPP assembly condition and performance prediction of PEMFC.

Study on Assembly Mechanics and Its Effect on the Performance of Proton Exchange Membrane Fuel Cell

Proton exchange membrane fuel cell (PEMFC) is an energy conversion device, which converts the chemical energy of fuels into electrical energy directly through electrochemical reactions, it has been considered as one of the most promising clean energy conversion devices widely because of its wide range of fuel sources, clean and free pollution, high operating current, can start-up quickly at room temperature and other advantages. All of components of a PEM fuel cell need to be assembled by exerting pressure from the outside, since the gas diffusion layer (GDL) is a porous structure and its elastic modulus is relatively small, during the process of exerting pressure, if the assembly pressure is too large, it will cause the GDL over deformation or even irreversible damage, thus reducing the gas transmission channel, increasing the mass transfer resistance, and even will damage the fuel cell components, shorten the lifetime of the fuel cell; in contrast, if the assembly pressure is too small, due to the poor contact between the bipolar plate and GDL, the contact resistance will increase, which will lead the reduce of the fuel cell efficiency, and the leak tightness of gas will not be guaranteed, there will be a risk of fuel gas leakage. In this paper, the numerical simulation method is used to simulatethe deformation of GDL under different assembly pressures, and the influence of transmission parameters such as GDL porosity and permeability caused by assembly pressure and its influence on the performance of PEMFC are studied in order to get the optimum pressure, making the best performance of PEMFC. The main work and achievements are as follows: The single channel PEMFC geometric model is established, and the numerical simulation is used to simulate, and the parameters such as porosity and permeability of PEMFC under different assembly pressures are calculated according to the empirical formula. When the assembly pressure from 0 to 3.0 MPa, the porosity decreased from the initial value of 0.78 to about 0.38; permeability also has an order of magnitude change; and the contact resistance decreases as the assembly pressure increases, but it decreases gradually; The finite element analysis (FEA) is used to analyze the effect of different assembly pressures on GDL deformation. The results show that, under the action of assembly pressure, the part of GDL, which under the bipolar plate ribs, will be deformed in different degrees, and the deformation on both sides is similar and obvious, the degree of deformation increases as the assembly pressure increases, while the part of GDL, which under the channel, is almost unchanged; Through the simulation, the distribution of the components concentration in the single channel PEMFC flow channel and the polarization curves under the assembly pressure are analyzed, and compares with different assembly pressures. Since the gas in the flow channel reacts in the catalyst layer, the gas concentration decreases along the flow direction under different assembly pressures. For the polarization curves, when the assembly pressure is between 0.5 to 1.0 MPa, the same operating voltage, the fuel cell current density is greater than the case of other pressures, and the fuel cell with the highest power under this pressure. Therefore, when the assembly pressure between 0.5-1.0 MPa, the single channel PEMFC performance is optimal.

Thermal and electrochemical performance analysis of a proton exchange membrane fuel cell under assembly pressure on gas diffusion layer

In this study, the effect of clamping pressure on the performance of a proton exchange membrane fuel cell (PEMFC) is investigated for three different widths of channel. The deformation of gas diffusion layer (GDL) due to clamping pressure is modeled using a finite element method, and the results are applied as inputs to a CFD model. The CFD analysis is based on finite volume method in non-isothermal condition. Also, a comparison is made between three cases to identify the geometry that has the best performance. The distribution of temperature, current density and mole fraction of oxygen are investigated for the geometry with best performance. The results reveal that by decreasing the width of channel, the performance of PEMFC improves due to increase of flow velocity. Also, it is found that intrusion of GDL into the gas flow channel due to assembly pressure deteriorates the PEMFC performance, while decrease of GDL thickness and GDL porosity have smaller effects. It is shown that assembly pressure has a minor effect on temperature profile in the membrane-catalyst interface at cathode side. Also, assembly pressure has a significant effect on ohmic and concentration losses of PEMFC at high current densities.