Numerical study of assembly pressure effect on the performance of proton exchange membrane fuel cell

The performance of the fuel cell is affected by many parameters. One of these parameters is assembly pressure that changes the mechanical properties and dimensions of the fuel cell components. Its first duty, however, is to prevent gas or liquid leakage from the cell and it is important for the contact behaviors of fuel cell components. Some leakage and contact problems can occur on the low assembly pressures whereas at high pressures, components of the fuel cell, such as bipolar plates (BPP), gas diffusion layers (GDL), catalyst layers, and membranes, can be damaged. A finite element analysis (FEA) model is developed to predict the deformation effect of assembly pressure on the single channel PEM fuel cell in this study. Deformed fuel cell single channel model is imported to three-dimensional, computational fluid dynamics (CFD) model which is developed for simulating proton exchange membrane (PEM) fuel cells. Using this model, the effect of assembly pressure on fuel cell performance can be calculated. It is found that, when the assembly pressure increases, contact resistance, porosity and thickness of the gas diffusion layer (GDL) decreases. Too much assembly pressure causes GDL to destroy; therefore, the optimal assembly pressure is significant to obtain the highest performance from fuel cell. By using the results of this study, optimum fuel cell design and operating condition parameters can be predicted accordingly.     

Analyses of the fuel cell stack assembly pressure

A proper stacking design and cell assembly are important to the performance of fuel cells. The cell assembly will affect the contact behavior of the bipolar plates with the membrane electrode assembly (MEA). Not enough assembly pressure may lead to leakage of fuels, high contact resistance and malfunctioning of the cells. Too much pressure, on the other hand, may result in damage to the gas diffusion layer and/or MEA. The stacking design may affect the pressure distribution within the fuel cell stack and thus the interfacial contact resistance. Uneven distribution of the contact pressure will result in hot spots which may have a detrimental effect on fuel cell life.In this study, finite element analysis (FEA) procedures were established for a PEM single cell with point stack assembly method. The mechanical properties and geometrical dimensions of all the fuel cell components, such as bipolar plates, membrane, gas diffusion layer and end plates were collected for accurate simulation. From the FEAs, the compliance as well as the pressure distribution of the single cell was calculated. In order to verify the results of the analysis, experimental tests, with a pressure film inserted between the bipolar plates and the MEA, were conducted to establish the actual pressure distribution. Color variations of the pressure film could be calibrated to obtain pressure distribution. Compliance of the gas diffusion layer was also measured. The analysis procedures for the fuel cell stacking assembly were established by comparing the simulation results with those of the experimental data at various levels of assembly pressures. They can help determine the proper stacking parameters such as stacking design, bipolar plate thickness, sealing size and assembly pressure, and are important in obtaining a consistent fuel cell performance.     

Multi-scale dimensionless prediction model of PEMFC sealing interface leakage rate based on fractal geometry

The performance of the sealing is closely related to the working efficiency and safety of PEMFCs and the interfacial leaking behavior is heavily influenced by the heterogeneous rough surfaces. And the BPPs and Gaskets of the fuel cell sealing structure are ultra-thin, which makes obtaining leakage rate by experiments difficult. In this paper, a highly efficient method to calculate the PEMFC sealing interface leakage rate is proposed based on the fractal geometry and numerical simulations. FEM and LBM methods are used to analyze the nonlinear contact and microscopic gas flow behaviors in the numerical simulation processes. Furthermore, a multi-scale dimensionless model of PEMFC interface leaking is established by numerical simulated results. The multi-scale dimensionless model can significantly reduce the negative influence of the single observation scale on interface leakage rate prediction accuracy and solve the problem of the inability to analyze interface leakage rates at different scales simultaneously. By comparison with the Roth's theory, the traditional model that only considers the small elastic deformation has a large deviation in the evaluation of the leakage rate of the rubber seal interface, while the multi-scale model can accurately predict the interface leakage rate under different PEMFC Gasket compression rates.     

Assembly pressure and membrane swelling in PEM fuel cells

Assembly pressure and membrane swelling induced by elevated temperature and humidity cause inhomogeneous compression and performance variation in proton exchange membrane (PEM) fuel cells. This research conducts a comprehensive analysis on the effects of assembly pressure and operating temperature and humidity on PEM fuel cell stack deformation, contact resistance, overall performance and current distribution by advancing a model previously developed by the authors. First, a finite element model (FEM) model is developed to simulate the stack deformation when assembly pressure, temperature and humidity fields are applied. Then a multi-physics simulation, including gas flow and diffusion, proton transport, and electron transport in a three-dimensional cell, is conduced. The modeling results reveal that elevated temperature and humidity enlarge gas diffusion layer (GDL) and membrane inhomogeneous deformation, increase contact pressure and reduce contact resistance due to the swelling and material property change of the GDL and membrane. When an assembly pressure is applied, the fuel cell overall performance is improved by increasing temperature and humidity. However, significant spatial variation of current distribution is observed at elevated temperature and humidity.     

Effect of nonuniformity of the contact pressure distribution on the electrical contact resistance in proton exchange membrane fuel cells

Electrical contact resistance (ECR) is one of the most important factors affecting the ohmic loss in proton exchange membrane (PEM) fuel cells. Dominated by the contact pressure at the interface of two neighboring components, the ECR can be reduced by increasing the clamping force applied on fuel cell stack. However, too large a clamping force will result in excessive resistance to the transport of reactants in the gas diffusion layer (GDL) and even damage to the fuel cell components. Therefore, for a given clamping force, the minimum ECR is expected by making the pressure distribution as uniform as possible. This paper investigates two questions: (a) how to evaluate the distribution of non-uniform pressure based on the ECR, and (b) in what situation will a uniform pressure distribution reduce the ECR obviously, i.e., the sensitivity of the contact resistance to the pressure distribution.     

The effects of compression and gas diffusion layers on the performance of a PEM fuel cell

Changes in the performance of a PEM fuel cell are presented as a function of the compression pressure resulting from torque on the bolts that clamp the fuel cell. Three types of gas diffusion layers were studied at 202 kPa and 353°K. An optimum bolt torque was observed when ELAT® or a combination of CARBEL® and TORAY™ gas diffusion media were used as diffusion layers. The optimum is related to the gasket thickness and the measured compression pressure on the diffusion layer. The performance changes may also be related to changes in the porosity, the electrical contact resistance, and the excluded water at the membrane.     

Optimization of contact resistance with better gasketing for a unitized regenerative fuel cell

Unitized regenerative fuel cells, as being able to use and regenerate the hydrogen, seem to be compact solution for the standalone systems. The cells with smaller active areas (<50 cm²) have better contact between electrode and bipolar plate due to their smaller sizes. It therefore results in very low resistance at the interface owing to high performance. However, the power produced by the cells is not generally observed to linearly follow the changes in the size of electrodes. Such losses in the performance for scaled up cells is due to the high interfacial contact resistance incurred at the interface. Such resistance could be lowered using optimized gaskets as well as applied torque. The present study evaluates different gaskets for a scaled-up version of the cell (300 cm²) as a measure of increased contact resistance when operated at high pressures during electrolysis mode of operation. The cell is modelled structurally and simulated for most available gasket materials i.e. silicon and Teflon. Average contact pressure at the interface of electrode and bipolar plate is considered as the parameter to estimate the interfacial contact resistance. Silicon is evaluated better material than Teflon and is observed to hold almost 4.5 bar of gas in electrolysis mode when clamped with 8 Nm of torque. The cell is observed to perform close to state of art system and delivered 125 A at 0.5 V during fuel cell mode and generate 500 and 250 mL/min of hydrogen and oxygen during electrolysis mode of operation.     

Stress response and contact behavior of PEMFC during the assembly and working condition

Mechanical behavior of proton exchange membrane fuel cell (PEMFC) is closely related to its service life. Stress response and contact behavior of PEMFC during the assembly and working condition were investigated. Effects of clamping force, steel bands number and width on the mechanical behavior of PEMFC were discussed. Thermal-mechanical coupling was considered during the PEMFC working for thermal effect caused by chemical reaction. The results show that stress distribution of multi-cells is more uniform after assembly. Stress and contact pressure distributions of single-cell and multi-cells are similar after assembly. During the assembly process, the average contact pressure between the MEA and other parts increases with the increasing of clamping force, steel band width and number. With the steel bandwidth increases, the contact pressure distribution of MEA is more uniform. And the chemical reaction inside the fuel cell is more favorable. Stress concentration is prone to appear on the corners of GDL on the MEA, corners of sealing gasket, and edges of bipolar plate ribs, which may cause seal failure after long-term work. Under working condition, thermal expansion can enhance the sealing performance, but affect stress of bipolar plate and MEA. Those results can be used for design, manufacturing, maintenance and evaluation of PEMFC.     

A micro-scale model for predicting contact resistance between bipolar plate and gas diffusion layer in PEM fuel cells

Contact resistance between the bipolar plate (BPP) and the gas diffusion layer (GDL) in a proton exchange membrane (PEM) fuel cell constitutes a significant portion of the overall fuel cell electrical resistance under the normal operation conditions. Most current methods for contact resistance estimation are experimental and there is a lack of well developed theoretical methods. A micro-scale numerical model is developed to predict the electrical contact resistance between BPP and GDL by simulating the BPP surface topology and GDL structure and numerically determining the status for each contact spot. The total resistance and pressure are obtained by considering all contact spots as resistances in parallel and summing the results together. This model shows good agreements with experimental results. Influences of BPP surface roughness parameters on contact resistance are also studied. This model is beneficial in understanding the contact behavior between BPP and GDL and can be integrated with other fuel cell simulations to predict the overall performance of PEM fuel cells.     

Contact resistance between bipolar plate and gas diffusion layer in high temperature polymer electrolyte fuel cells

One of the major contributors to the ohmic loss in fuel cells originates at the interface between adjacent cell components. The compressive pressure used to achieve contact in cells should be carefully estimated to ensure that resistive losses arising from contact behavior remain minimal. In present work, a generic model is developed, capable of estimating contact resistance as a function contact pressure at the interface of graphite bipolar plate and carbon fiber based gas diffusion layer at different temperatures. A good agreement is observed between the results obtained from the model and experiments. Compressive pressure in the ranges of 3–4 MPa is found optimum for achieving low contact resistance. The contact resistance obtained for carbon paper and BPP while using recommended pressure lies between ∼9 and 4 mΩ cm² considering the operating regime of HT-PEMFC (120–180 °C). Operating under similar conditions, the contact resistance values for carbon cloth and BPP is ∼13 to 7 mΩ cm².     

An improved model for predicting electrical contact resistance between bipolar plate and gas diffusion layer in proton exchange membrane fuel cells

Electrical contact resistance between bipolar plates (BPPs) and gas diffusion layers (GDLs) in PEM fuel cells has attracted much attention since it is one significant part of the total contact resistance which plays an important role in fuel cell performance. This paper extends a previous model by Zhou et al. [Y. Zhou, G. Lin, A.J. Shih, S.J. Hu, J. Power Sources 163 (2007) 777–783] on the prediction of electrical contact resistance within PEM fuel cells. The original microscale numerical model was based on the Hertz solution for individual elastic contacts, assuming that contact bodies, GDL carbon fibers and BPP asperities are isotropic elastic half-spaces. The new model features a more practical contact by taking into account the bending behavior of carbon fibers as well as their anisotropic properties. The microscale single contact process is solved numerically using the finite element method (FEM). The relationship between the contact pressure and the electrical resistance at the GDL/BPP interface is derived by multiple regression models. Comparisons of the original model by Zhou et al. and the new model with experimental data show that the original model slightly overestimates the electrical contact resistance, whereas a better agreement with experimental data is observed using the new model.     

Estimation of contact resistance in proton exchange membrane fuel cells

The contact resistance between the bipolar plate (BPP) and the gas diffusion layer (GDL) is an important factor contributing to the power loss in proton exchange membrane (PEM) fuel cells. At present there is still not a well-developed method to estimate such contact resistance. This paper proposes two effective methods for estimating the contact resistance between the BPP and the GDL based on an experimental contact resistance–pressure constitutive relation. The constitutive relation was obtained by experimentally measuring the contact resistance between the GDL and a flat plate of the same material and processing conditions as the BPP under stated contact pressure. In the first method, which was a simplified prediction, the contact area and contact pressure between the BPP and the GDL were analyzed with a simple geometrical relation and the contact resistance was obtained by the contact resistance–pressure constitutive relation. In the second method, the contact area and contact pressure between the BPP and GDL were analyzed using FEM and the contact resistance was computed for each contact element according to the constitutive relation. The total contact resistance was then calculated by considering all contact elements in parallel. The influence of load distribution on contact resistance was also investigated. Good agreement was demonstrated between experimental results and predictions by both methods. The simplified prediction method provides an efficient approach to estimating the contact resistance in PEM fuel cells. The proposed methods for estimating the contact resistance can be useful in modeling and optimizing the assembly process to improve the performance of PEM fuel cells.     

Enhanced reliability of planar-type solid oxide fuel cell stack incorporating leakage gas induction channels

Planar type solid oxide fuel cell stacks have been studied for use as commercial products because of their high energy density and environment-friendly operation. They employ a gasket as a sealant as it enables easy assembly and repair. However, its imperfect sealing property, detrimental to long-term operation, has limited its introduction. Here, a stack design with a gasket sealant including a leakage gas induction channel is developed. The leakage gas induction channel within the gasket sealant prevents gas leakage of fuel and air to the opposite electrode and ensures the stack's stable operation. The stack design's reliability is confirmed via a heating-cycle test and an open circuit voltage to a 0.7 V voltage control test. In-situ gas chromatography analysis shows that the diffusion of gases effectively reduces by releasing them outside the stack through the leakage gas induction channel.     

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.     

A novel technique for measuring current distributions in PEM fuel cells

A novel and simple technique was developed to measure current distribution in PEM fuel cells with serpentine flow fields. In this technique, a specially designed measuring gasket was inserted between the flow field plate and the gas diffusion layer, and the current at each sub-area of the fuel cell was measured by each of the current collecting strips on the measuring gasket. The current distribution measurement gasket was independent of PEM fuel cells, and can be used in any fuel cell without the need of a special fuel cell or modification of any component of an existing fuel cell. More importantly, this technique can be easily used to measure current density distribution in any cell or every cell in a fuel cell stack. In addition, this technique is very inexpensive, with the only additional cost being that of the measuring gasket. In this work, this measurement gasket technique was used to study the influences of humidification temperatures, cell operating temperatures, reactant flow rates, and operating pressures on current distributions in a PEM fuel cell. Local membrane hydration, reactant depletion and possible cathode flooding can be deduced from the measurement results, and some potential improvements in fuel cell designs are suggested.     

X-ray tomography and modelling study on the mechanical behaviour and performance of metal foam flow-fields for polymer electrolyte fuel cells

Porous metal foams have been used as alternative flow-fields in proton exchange membrane fuel cells (PEMFCs), exhibiting improved performance compared to conventional ‘land and channel’ designs. In the current work, the mechanical behaviour of PEMFCs using metal foam flow-fields is investigated across different length scales using a combination of electrochemical testing, X-ray computed tomography (CT), compression tests, and finite element analysis (FEA) numerical modelling.Fuel cell peak power was seen to improve by 42% when foam compression was increased from 20% to 70% due to a reduction in the interfacial contact resistance between the foam and GDL. X-ray CT scans at varying compression levels reveal high levels of interaction between the metal foam and gas diffusion layer (GDL), with foam ligaments penetrating over 50% of the GDL thickness under 25% cell compression. The interfacial contact area between the foam and GDL were seen to be 10 times higher than between the foam and a stainless-steel plate. Modelling results demonstrate highly uniform contact pressure distribution across the cell due to plastic deformation of the foam. The effect of stack over-tightening and operating conditions are investigated, demonstrating only small changes in load distribution when paired with a suitable sealing gasket material.     

A two-fluid model for two-phase flow in PEMFCs

In this study, a two-fluid (TF) model is developed for two-phase flows in proton exchange membrane fuel cells (PEMFCs). The drag force and lift force between gas and liquid phase are considered in N-S equations. In addition, a simplified model is introduced to obtain the liquid water droplet detachment diameter on the gas diffusion layer (GDL)/channel interface which involves the properties of the GDL/channel interface (contact angle and surface tension). The TF model and the simplified model for the prediction of water droplet detachment diameter on GDL/channel interface are validated by the comparison between the experimental data and the model results, respectively. The effect of the properties of GDL/channel interface (contact angle and surface tension) on two-phase behavior in PEMFCs is investigated, The results show that a high contact angle and a low surface tension are advantageous for liquid water removal in the gas channel and the GDL even though a low surface tension will lead to a low capillary force in the GDL.     

Treatment of two-phase flow in cathode gas channel for an improved one-dimensional proton exchange membrane fuel cell model

It has been reported recently that water flooding in the cathode gas channel has significant effects on the characteristics of a proton exchange membrane fuel cell. A better understanding of this phenomenon with the aid of an accurate model is necessary for improving the water management and performance of fuel cell. However, this phenomenon is often not considered in the previous one-dimensional models where zero or a constant liquid water saturation level is assumed at the interface between gas diffusion layer and gas channel. In view of this, a one-dimensional fuel cell model that includes the effects of two-phase flow in the gas channel is proposed. The liquid water saturation along the cathode gas channel is estimated by adopting Darcy’s law to describe the convective flow of liquid water under various inlet conditions, i.e. air pressure, relative humidity and air stoichiometry. The averaged capillary pressure of gas channel calculated from the liquid water saturation is used as the boundary value at the interface to couple the cathode gas channel model to the membrane electrode assembly model. Through the coupling of the two modeling domains, the water distribution inside the membrane electrode assembly is associated with the inlet conditions. The simulation results, which are verified against experimental data and simulation results from a published computational fluid dynamics model, indicate that the effects of relative humidity and stoichiometry of inlet air are crucial to the overall fuel cell performance. The proposed model gives a more accurate treatment of the water transport in the cathode region, which enables an improved water management through an understanding of the effects of inlet conditions on the fuel cell performance.     

Effects of assembling method and force on the performance of proton-exchange membrane fuel cells with metal foam flow field

Recently, highly porous metal foams have been used to replace the traditional open-flow channels to improve gas transport and distribution in the cells. Deformation of flow plate, gas diffusion layer (GDL), and metal foam may occur during assembling. When the cell size is small, the deformation may not be significant. For large area cells, the deformation may become significant to affect the cell performance. In this study, an assembling device that is capable of applying uniform clamping force is built to facilitate fuel cell assembling and alleviate the deformation. A compressing plate that is the same size of the active area is used to apply uniform clamping force before surrounding bolts are fastened. Therefore, bending of the flow plate and deformation of GDL and metal foam can be minimized. Effects of the clamping force on the microstructures of GDL and metal foam, various resistances, pressure drops, and cell performance are investigated. Distribution of the contact pressure between metal foam and GDL is measured by using pressure sensitive films. Field-emission scanning electron microscope is used to observe the microstructures. Electrochemical impedance spectroscopy analysis is used measure resistances. The fuel cell performance is measured by using a fuel cell test system. For the cell design used in this study, the optimum clamping force is found to be 200 kgf. Using this optimum clamping force, the cell performance can be enhanced by 50%, as compared with that of the cell assembled without using clamping plates. With appropriate clamping force, the compression force distribution across the entire cell area can approach uniform. This enables uniform flow distribution and reduces mass transfer resistance. Good contact between GDL and metal foam also lowers the interface resistance. All these factors contribute to the enhanced cell performance.     

Three-dimension simulation of two-phase flows in a thin gas flow channel of PEM fuel cell using a volume of fluid method

This study investigates the two-phase flow in a thin gas flow channel of PEM fuel cells and wall contact angle's impact using the volume of fluid (VOF) method with tracked two-phase interface. The VOF results are compared with experimental data, theoretical solution and analytical data in terms of flow pattern, pressure drop and water fraction. Stable film flow is predicted, as observed experimentally, for the contact angle ranging from 5° to 40° including varying contact angles at different walls of a channel. The contact angle is found to have small impact on the gas pressure drop for the stratified flow regime, but it determines the meniscus of the two-phase interface, which affects the optical detection of the liquid thickness in experiment. The work is important to study of two-phase flow dynamics, multichannel design, experimental design and control of two-phase flows in thin gas flow channels for PEM fuel cells.