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Sealing performance between two contacting surfaces is of significant importance to stable operation of proton exchange membrane (PEM) fuel cells. In this work, an analytical micro-scale approach is first established to predict the gas leakage in fuel cells. Gas pressure and uneven pressure distribution at the interface are also included in the model. At first, the micro tortuous leakage path at the interface is constructed by introducing contact modelling and fractal porous structure theory. In order to obtain the leakage at the entire surface, contact pressure distribution is predicted based on bonded elastic layer model. The gas leakage through the discontinuous interface can be obtained with consideration of convection and diffusion. Then, experiments are conducted to validate the numerical model, and good agreement is obtained between them. Finally, influences of surface topology, gasket compression and gasket width on leakage are studied based on the model. The results show that gas leakage would be greatly amplified when the asperity standard deviation of surface roughness exceeds 1.0 μm. Gaskets with larger width and smaller thickness are beneficial to sealing performance. The model is helpful to understand the gas leakage behavior at the interface and guide the gasket design of fuel cells. [Display omitted] • An analytical model is built to predict gas leakage rate in fuel cells. • Uneven pressure distribution at the interface is considered in the model. • Leakage is amplified when asperity height standard deviation exceeds 1.0 μm. • Leakage becomes serious when gasket compression ratio is lower than 8%. • Gaskets with larger width and smaller thickness are beneficial to sealing. [ABSTRACT FROM AUTHOR]

Uniformity of fluid distribution in the manifold is extremely essential to enhance the output performance and prolong the lifetime of high power proton exchange membrane (PEM) fuel cell stack. The entrance effects are usually ignored in the existing studies focusing on the fluid distribution at stack level, which cannot thoroughly guide the high-power fuel cell stack development. In this study, the effects of entrance geometry on the fluid distribution in manifold for a high-power fuel cell stack are investigated using computational fluid dynamics (CFD) method. Optimizations of the intermediate zone configuration in upper endplate and inlet tube diameter are conducted under different current densities. Results show that the fluid distribution in manifold is strongly influenced by entrance geometry which determines the generation of vortexes. The mass flow rates in unit cells near the entrance of the stack with diffuser-type intermediate zone are enhanced compared to the non-diffuser-type intermediate zone. The coefficient of variation (CV) of mass flow rate decreases dramatically as the ratio of inlet tube to manifold hydraulic diameter (RITMHD) increases and then raises. The experimental and simulation results are useful in guiding the design of high-power PEM fuel cell stacks to seek higher power density. • Optimization of intermediate zone configurations and inlet tube diameters are conducted. • Fluid distribution in manifold is strongly influenced by entrance geometry. • Diffuser-type intermediate zone promotes enhanced unit cell flow near the entrance. • Flow maldistribution decreases as the inlet tube diameter increases and then raises. [ABSTRACT FROM AUTHOR]

Increasing the conductivity of gas diffusion layers (GDLs) is an important way to improve the output performance of polymer electrolyte membrane fuel cells (PEMFCs). However, the complex porous fiber structures of GDLs significantly enhances the difficulty of quantitatively altering their conductivity which is determined by the carbon fibers and the conduction characteristics between fibers. In addition, the microstructures of various types of GDLs are different. Thus, it is a considerable challenge to explore the conductive mechanisms of these porous materials and optimize their structures to reduce their bulk resistances. In this work, a mathematical graph theory model that applies to the through-plane (T-P) bulk resistance prediction of two types of commonly used GDLs, carbon paper and carbon felt, is established to explain their different micro conduction mechanisms in depth. In addition to the number of fiber contact points, their distribution, as well as the resistance of the carbon fibers, are all important factors affecting the T-P conductivity. Optimizing fiber density and fiber diameter can significantly improve the T-P conductivity of carbon paper. In comparison, making the structure of carbon felt more compact so that the distribution of its contact points in the T-P direction can be more uniform will be more effective for the reduction of its T-P bulk resistance. Meanwhile, the T-P bulk resistance of carbon paper can also be effectively improved by optimizing the content and distribution of the binders. A method to decline the bulk resistance of carbon paper by aggregating the binders in the in-plane (I P) direction is proposed. The simulation results show that it can reduce the T-P bulk resistance of carbon paper by about 19.9% at a compressive stress of 1.5 MPa. This study provides further guidance for optimizing the structural designs of GDLs to optimize their conduction performance. • T-P bulk resistance prediction model suitable for different GDLs is constructed. • Micro conductivity mechanisms of carbon paper and carbon felt are compared. • Increasing fiber density and diameter improves carbon paper's T-P conductivity. • Making carbon felt's structure more compact can optimize its T-P conductivity. • To promote carbon paper's T-P conductivity by optimizing the binders inside. [ABSTRACT FROM AUTHOR]

Summary: The mechanical degradation of the proton exchange membrane (PEM) is one of the main aspects affecting the lifetime of proton exchange membrane fuel cells (PEMFCs). It was observed in our previous study that the stress/strain distribution in the PEM of fuel cells with metallic bipolar plates (BPPs) is more complex, owing to manufacturing and assembly errors of the BPPs. The present study further concentrates on the stress/strain evolution in the membrane of fuel cells throughout the assembly and running processes by a finite element model. In membranes at the joint area between the gasket and gas diffusion layers, a serious stress concentration aggravated as the misalignment displacement increases. As for the membrane in reaction area, the plastic strain reaches highest level at the center of the groove after hygrothermal loading. The maximum stress is mainly relevant to the temperature and humidity and has little concern with the misalignment. The model and results of this study offer guidance regarding the design of PEMFC. Owing to the stress concentration, an additional protection should be set in the joint area, and the assembly error should be limited within 0.05 mm. [ABSTRACT FROM AUTHOR]

Reliable sealing is necessary for the stable operation of proton exchange membrane fuel cell (PEMFC). In practical application, various materials have been tried in PEMFC sealing. However, the mechanical properties of these sealing materials, which play a key role in the sealing stability, have not been fully understood in PEMFC environment, especially after long-term operation. In this paper, according to the operating environment of PEMFC, sealing material experiments are carried out to explore the differences in mechanical behaviors of sealing materials, including silicone rubber (SR), fluororubber (FR), nitrile rubber (NBR) and ethylene-propylene-diene-terpolymer rubber (EPDM) and the variation of mechanical properties of these sealing materials is predicted as time goes on. The results indicate that compression rate has a great influence on sealing contact stress. SR and EPDM, with the variation of 0.15 MPa and 0.45 MPa in stress, show the best and worst mechanical stability at different compression rates, respectively. In terms of temperature, it is found that SR can adapt to different operating temperature of PEMFC and only 18% variation is found from 20 °C to 100 °C. Finally, based on Time-Temperature Superposition (TTS), high temperature experiments are conducted to predict long-term relaxation stress under PEMFC working condition. The analysis results are beneficial for choosing suitable sealing material, and it can also be applied to predict sealing ability in PEMFC. • Mechanical behavior differences of different sealing materials were compared. • Influences of compression rate and temperature on stress were discussed. • The better gasket material was selected for proton exchange membrane fuel cell. • High temperature experiments were conducted to predict long-term stress relaxation. [ABSTRACT FROM AUTHOR]

Air-cooled proton exchange membrane fuel cell (PEMFC) is a promising electrochemical device in fields of unmanned aerial and light-duty ground vehicles， with the virtues of simple system, low parasitic power and low cost. However, the significantly uneven distribution of internal reaction state of air-cooled PEMFC greatly limits the output performance and stability of the cells. In this study, a PCB test board for the in-situ observation of cell temperature and current density is designed and applied in fuel cells. The result shows that when the average current density is 500 mA/cm2，the difference of temperature and current density reach 20 °C and 400 mA/cm2, respectively. For the optimization of uniformity of reaction state, the effects of hydrogen pulsing interval, hydrogen supply mode, fan configuration and air inlet speed are experimentally explored. Based on the observed results of the PCB test board, optimization proposals of control strategy for air-cooled PEMFC are given: 1) A 30-s pulsing interval of hydrogen is preferred due to its little changes on current density distribution while significant increasement of hydrogen utilization rate. 2) The proposed bidirectional hydrogen flow improves the reaction uniformity, and reduces the fluctuation of current density during hydrogen pulsing discharge; 3) Compared to two paralleled fans, employing a single fan helps to improve the temperature uniformity, with the standard deviation of temperature from 8.9 °C to 6.7 °C; 4) Medium velocity of air inlet speed is preferred due to poor temperature uniformity at low velocity, and low water content at high velocity. • A PCB test board is designed to obtain the internal reaction state of fuel cell. • The uneven distributed temperature and current density in fuel cell is analyzed. • Optimization proposals of control strategy of air-cooled PEMFC are raised. [ABSTRACT FROM AUTHOR]

Abstract Proton exchange membrane (PEM) fuel cells usually operate in dead-ended anode mode due to a comparatively simple system. Nevertheless, flooding in the anode channels in dead-ended mode is severe than that in flow-through configuration, which causes cell performance degradation and durability decrease. In this study, water droplet dynamics in the anode channel is investigated numerically using volume of fluid method to study water accumulation and drainage in the PEM fuel cell with the dead-ended anode. Simulations are divided into a dead-ended stage and a purge stage to study the two-phase flow behaviors. Impact of water accumulating volume is taken into consideration and, cases of different wettability of the gas diffusion layer surface and the bipolar plate surface are compared. The numerical results reveal that water droplets emerge from the water inlets and then accumulate and coalesce in the dead-ended stage. Most water droplets are drained out of the gas channel along the channel corners in the purge stage. It is found that larger water accumulating volume results in higher eliminating rate. The total purge time is mainly affected by the wettability of the bipolar plate surface. Highlights • Water droplet dynamics in a dead-ended anode channel is investigated numerically. • The majority of water droplets drain away along the channel corners. • Most water flows away from the gas channel in the middle period of the purge. • More accumulated water results in higher drainage rate. • The total purge time is highly affected by the wettability of the BPP surface. [ABSTRACT FROM AUTHOR]

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. • Nonuniform compression induces different water transport from the uncompressed GDL. • An integrated model is developed to predict the water transport. • The probability density function (PDF) of the pore size is considered. • Evaluated the influence of assembly force, GDL thickness and the channel geometry. • Rapidly applied in flow field design and GDL assembly matching. [ABSTRACT FROM AUTHOR]

Abstract It is necessary to obtain internal heat and water distribution of proton exchange membrane fuel cells in order to perform water-heat management. Most of the existing works focus on the measurement in small experimental fuel cells, which cannot effectively guide the development of large-area fuel cells. In this study, an in-situ measurement method using micro-sensors is developed to observe the temperature and relative humidity in a commercial-size fuel cell with parallel flow fields and an active area of 250 cm2. A sensor array is incorporated into the cathode flow field, and well-designed waterproof and sealing structures protect sensors from liquid water. The sensors are verified with accuracy and response speed. The fuel cell performance and electrochemical impedance spectroscopy under co-flow and counter-flow configurations considering anode gas humidification are investigated. The results show that the temperature distribution is more uneven in the cases with lower output voltage, and the fuel cell performance is significantly affected by the humidity near the air inlet area. As current density increases, the relative humidity drops while dew point temperature keeps almost unchanged. The experimental method and analysis are beneficial to the understanding of the water-heat state and optimization of fuel cell stack design. Highlights • Measurement with micro-sensors for the commercial-size fuel cell is effective. • Uneven temperature and humidity distribution in the cathode flow field is obtained. • Performance and resistance are greatly related to temperature-humidity distribution. • Counter-flow without humidification gets good performance and water-heat balance. [ABSTRACT FROM AUTHOR]

Abstract Mechanical degradation, caused by local stress concentration and variation, significantly affects the lifetime of proton exchange membrane fuel cells. This study constitutes the first numerical investigation of stress evolution in the membrane between the frame of the membrane exchange assembly (MEA) and gas diffusion layer (GDL) throughout the processes of assembly, operation and gas filling in fuel cells. A finite element model is outlined to determine mechanical deformation of the membrane by exerting assembly displacement, hygrothermal conditions and gas pressure in turn. It is observed that severe stress concentration and bending deformations occur in the joint-area membrane. The results show that a plastic deformation occurs after the temperature and water content are increased, and would be substantially enhanced by the gas pressure difference between the anode and cathode. The in-plane stress may throw some light on the rapid degradation of the membrane between the frame and GDL. The gas pressure difference, which exceeds 10 kPa, leads to a rapid increase in the in-plane stress and plastic deformation. Decreasing the joint width may not be a good approach for reducing the stress/strain concentration. It is suggested that additional gasket seals or adhesive protection layers are helpful in joining frame and GDL. Highlights • Numerical model of fuel cells is developed to investigate membrane degradation. • Stress concentration and plastic deformation are caused in the joint-area membrane. • As much as 70% increase is found in the in-plane stress after gas filling. • Gas pressure difference exceeding 10 kPa leads to a rapid increase in deformation. • The maximum stress and plastic strain are obtained if joint width is 0.05–0.15 mm. [ABSTRACT FROM AUTHOR]

Highlights • Gas diffusion layer accounts for as much as 20% of the resistance in fuel cells. • Parallel fiber microstructure leads to lower electrical resistance. • Resistance of carbon paper and carbon cloth declines as the load cycles increases. • Fiber crack and loss in resistance occur when exceeding "break stress" of 2.0 MPa. • Carbon felt has the highest stability in electric resistance and microstructure. Abstract Electrical resistance accounts for a significant part of the performance loss in proton exchange membrane fuel cells. To the best of the authors' knowledge, this work represents the first direct experimental investigation and comparison of the bulk resistance and microstructure of commercially available gas diffusion layers, carbon paper, carbon cloth and carbon felt, under cyclic and steady loads, which are typical compression conditions in the fuel cell. It was found that with the improvement of contact conductivity between gas diffusion layer and bipolar plate, the bulk resistance of gas diffusion layer accounts for as much as 20% of the resistance in the fuel cell, especially when the assembly pressure is high enough. Experimental results indicate that three kinds of gas diffusion layers show various electrical behaviors under compression due to their different fiber structures. For carbon paper, the resistance displays a gradual decline as the load cycles increases. A reduction in the resistance and obvious fiber cracks are observed when the compression pressure exceeds the "break stress" of 2.0 MPa. For woven carbon cloth, more uniform decline of the resistance is caused by the increasing fiber cracks, which are pulled and bent in the middle of a weave. Although felt gas diffusion layer features the lowest electrical conductivity, its tortuous and thick fibers lead to higher stability in electric resistance and microstructure than bonded carbon paper and woven carbon cloth. This study is helpful for enhancing our understanding of the relationship between electrical resistance and compression loads in the fuel cell with various gas diffusion layers. [ABSTRACT FROM AUTHOR]

Highlights • Design and fabrication of BPP is further introduced based on our previous study. • Round corner and clearance of moulds play important role in maximum channel height. • Channel dimensions are selected by formability model and reaction efficiency. • Channel height, channel thickness and effect of blank holder are discussed. • Performance of single cell and short stack is tested to evaluate method reliability. Abstract This study offers an efficient design method of flow channels of metallic bipolar plates (BPPs) to improve manufacturing technique of BPPs and maximize power density in proton exchange membrane (PEM) fuel cells. Stamped thin metallic BPPs with anticorrosive and conductive coating are promising candidates for replacing conventional carbon-based BPPs. Nevertheless, unlike carbon-based BPPs, the flow channel design of metallic BPPs should take into account not only the reaction efficiency, but also formability due to the possible rupture of the metallic channel during the micro-forming process. In our previous study, a forming limit model was first proposed to predict the maximum allowable channel height by the forming process. This study is conducted to further propose the method of the design and fabrication of metallic BPPs based on the numerical model. In order to determine channel geometry design from formability perspective, response surface method is utilized to build a formability model. Combining the formability model and reaction efficiency, flow field design for metallic BPPs (channel width of 0.9 mm, rib width of 0.9 mm, channel depth of 0.4 mm and radius of 0.15 mm) is proposed. Experiments on BPP fabrication and assembled 20-cell fuel cell testing are conducted to observe forming quality of micro channel and output performance on the real fuel cell. It is shown that the stamping force grows with increasing channel depth in a nonlinear manner and a blank holder is needed to eliminate the sheet wrinkle in the forming process. The uniformity of the voltage distribution in the 1000 W-class stack further proves the reliability of metallic BPPs designed by our method. The methodology developed is beneficial to the fabrication management of metallic BPPs and effective supplement to the channel design principle for PEM fuel cells. [ABSTRACT FROM AUTHOR]

This study offers an efficient method to improve manufacturing technique of metallic bipolar plates (BPPs) so as to simultaneously reduce contact resistance (CR) and maximize power density in proton exchange membrane fuel cell (PEMFC). CR plays an important role in the energy conversion in the cell and can be normally reduced by coating on the BPP surface. Nevertheless, the effect of fabrication process on the CR has not been revealed, especially for the welding and forming characteristics. In this paper, a comprehensive three-dimensional finite element model of BPP/gas diffusion layer (GDL) assembly was established to investigate the influences of coating, weld and dimensional error on the CR, which are produced during the fabrication process of metallic BPPs. Experiments were carried out to validate the accuracy of the model. The results indicate that direction and distribution of the current in the cell change significantly with altering the weld path of metallic BPPs, which are different from graphite BPPs. 47% CR reduction is observed for the case of dense weld arrangement. For the coating process, it is found that the necessity of coating on both sides of single BPP is quite low if channel number is less than 20. Statistic simulation was conducted to investigate the effect of dimensional error on CR. Specially, 14.5% increment in CR is found when the dimensional error exceeds 30 μm. The methodology developed is beneficial to the fabrication management of metallic BPPs and the efficiency improvement of PEMFC. [ABSTRACT FROM AUTHOR]

Contact behavior between the gas diffusion layer (GDL) and bipolar plate (BPP) is of significant importance for proton exchange membrane fuel cells. Most current studies on contact behavior utilize experiments and finite element modelling and focus on fuel cells with graphite BPPs, which lead to high costs and huge computational requirements. The objective of this work is to build a more effective analytical method for contact behavior in fuel cells and investigate the size effect resulting from configuration alteration of channel and rib (channel/rib). Firstly, a mathematical description of channel/rib geometry is outlined in accordance with the fabrication of metallic BPP. Based on the interface deformation characteristic and Winkler surface model, contact pressure between BPP and GDL is then calculated to predict contact resistance and GDL porosity as evaluative parameters of contact behavior. Then, experiments on BPP fabrication and contact resistance measurement are conducted to validate the model. The measured results demonstrate an obvious dependence on channel/rib size. Feasibility of the model used in graphite fuel cells is also discussed. Finally, size factor is proposed for evaluating the rule of size effect. Significant increase occurs in contact resistance and porosity for higher size factor, in which channel/rib width decrease. [ABSTRACT FROM AUTHOR]

Exposed to severe loading condition in fuel cells, the perfluorinated sulfonic-acid (PFSA) membrane degrades over time, which becomes the primary cause of the increasing transmembrane gas permeation during the service. As a result, the hydrogen crossover rate increases with the damage propagation, leading to loss in power and life. However, the effect of mechanical degradation on gas permeability of membrane cannot be quantitatively estimated due to difficulties in damage observation and modeling. This paper first establishes a numerical technique to predict the gas permeation through membrane with consideration of mechanical degradation. Blister test as well as gas permeation test are conducted for the verification of the numerical model. The model shows great prediction accuracy, based on which the uneven distributed damage and its effect on gas permeability are analyzed. Result shows that the gas permeation rate grows with pressure and mechanical degradation has a remarkable contribution, accounting for about 7.3% of total permeation rate at testing pressure of 130 kPa. Finally, the varied membrane shape and gas permeability, which all accelerate the gas permeation, are further analyzed based on the numerical model. • Accumulated damage before fracture of PFSA membrane is investigated. • Quantitative relationship between mechanical damage and gas permeability is built. • The mechanical damage in PFSA membrane is modeled by Gurson damage model. • Numerical model of gas permeation is built and matches well with experiment. [ABSTRACT FROM AUTHOR]

Water management is the primary factor affecting the output performance of proton exchange membrane fuel cells. Aiming to effectively deal with the flooding problem during the cell operation in a cathode flow field, this paper proposes a novel sub-channel design based on a wave-like flow field. The effects of sub-channel wall properties and channel geometry on the water removal capacity of the sub-channel are numerically evaluated by using the volume of fluid (VOF) method. Verification experiments are carried out to observe the water dynamics from droplets accumulation to detachment. The results reveal that the sub-channel achieves drainage performance by enhancing the gas pressure difference to overcome the wall adhesion. A wall contact angle of not less than 110° and an included angle to the main channel of not more than 60° are the prerequisites for the effective water removal of the sub-channel. The optimal design of the sub-channel is obtained to achieve better drainage performance. In addition, it is found that the effects of the sub-channel on water droplets will be weakened when the main channel period is densified to 1.5 mm or less, which is usually adopted to improve the mass transfer capacity under the ribs. • Water droplet dynamics in a novel sub-channel design are investigated numerically. • Effects of sub-channel wall properties and channel geometry are explored. • Sub-channel achieves drainage performance by enhancing the gas pressure difference. • Hydrophobic walls and small included angles (≤60°) are the prerequisites. • Water removal capacity of sub-channel is weakened in micro and dense flow fields. [ABSTRACT FROM AUTHOR]

• The concept of bifunctional channel is proposed to balance water–gas management. • New bifunctional flow channel design index is proposed. • A kW-grade URFC stack is revealed with the RTE over 45%. • Good bifunctional performance is due to the improvement of mass transfer. Unitized regenerative fuel cell (URFC) is a promising electrochemical device, which can function either in fuel cell (FC) mode or water electrolysis (WE) mode. However, few applications of kW-grade URFC stack are available due to their low round-trip efficiency. In this study, a new approach to URFC stack design is provided to break the limitations of round-trip efficiency (RTE) and output power, by introducing the concept of bifunctional flow channels for the first time. Hence, a two-phase flow model based on the volume of fluid (VOF) method for bifunctional flow channels is implemented first. It reveals that URFC suffers from the issues of water adhesion on the channel wall in FC mode and the problem of air film coverage on the GDL surface in WE mode. To address this, water volume fraction in FC mode and gas coverage fraction of GDL in WE mode are proposed as new bifunctional flow channel design index, and the optimized configuration of the bifunctional flow channel is provided to balance the drainage and exhaust capabilities. Eventually, a high power-density URFC stack is demonstrated to achieve a high RTE of 45 %, with an output power of 6.01 kW in FC mode and a power requirement of 18.16 kW in WE mode. Moreover, validation experiments show that the good bifunctional performance is since the little water/gas transport problem of the URFC at high current densities. Further sensitivity experiments of the operating conditions also show that the mass transfer capacity of URFC is well adapted to various operating conditions, which implies that the bifunctional flow channel successfully balances good drainage and exhaust capabilities. [ABSTRACT FROM AUTHOR]

Fuel cell assembly plays a dominant role in performance and lifetime of proton exchange membrane (PEM) fuel cell. Most current methods for assembly design are based on experiment and finite element (FE) model, which need high cost and huge computation for the whole fuel cell. Unfortunately, there are only few theoretical methods which contribute to the whole stack assembly, especially those in 3D model. This study develops a comprehensive methodology to simulate the stack assembly and to design the clamping displacement/pressure. At first, contact pressure field on the GDL is predicted with the use of the continuous equivalent model. The total contact resistance and porosity of fuel cell are proposed as the evaluation indexes combined by the desirability of function method. Then, in order to validate the contact pressure prediction, experiments with dimensional-error metallic bipolar plate are carried out and the numerical results show good agreements with experimental results. At last, clamping pressure of the endplate is calculated to optimize the assembly process. The methodology in this study is beneficial to the understanding of the internal contact behavior of the whole stack and helpful to guide the assembling of PEM fuel cell. [ABSTRACT FROM AUTHOR]

• A type of gradient wettability cathode CL with three sub-layers is designed. • Gradient CL's performance (895 mW/cm2@0.6 V) is higher than normal (721 mW/cm2@0.6 V). • This CL can adapt high current density, cathode humidity and air stoichiometry. • Gradient CL with best performance is found through comparison of 18 structures. The design of cathode catalyst layer (CL) is essential to improve the mass transfer capacity of proton exchange membrane (PEM) fuel cell and increase power density. In this work, cathode CL is divided into three sub-layers, and each sub-layer is added nano-particles with different wettability. The gradient CL with hydrophilic SiO 2 particles at inner layer and hydrophobic polytetrafluoroethylene (PTFE) particles at outer layer significantly enhances the performance of membrane exchange assembly (MEA). Its performance is 895 mW/cm2@0.6 V, which is 24.1 %higher than CL without any particles (721 mW/cm2@ 0.6 V). Under operating conditions of high current density, high cathode humidity and high air stoichiometry, the gradient CL has only a little voltage loss. Through Electrochemical Impedance Spectroscopies (EIS) impedance analysis under high current density (1.8A/cm2), mass transfer resistance of gradient CL is 25.4 Ω, and is much smaller than the mass transfer resistance of the homogeneous CL of 35.1 Ω, which reflects the significant enhancement in mass transfer capacity of gradient CL. The gradient catalyst layer is suitable for a wider range of current density, humidity, and stoichiometry, but excessive cathode gas stoichiometry causes a decrease in performance, which is caused by excessive drainage capacity. In addition, 18 different gradient CLs are designed and manufactured, and the gradient CL with catalyst coated membrane (CCM) structure has the best performance. In gradient CL, increasing the capillary pressure difference between sublayers is the key to performance improvement. It is confirmed that the property of MEA with appropriate wettability gradient design can be significantly improved. [ABSTRACT FROM AUTHOR]

• Down-the-channel performance model and genetic algorithm are used to optimize flow channel. • The local current density of PEMFC is measured by experiments. • Optimal constant and variable optimal flow channel designs are analyzed. • Optimal variable flow channel designs benefit output performance, especially at low output voltages and large SRCWs. It is necessary to establish an effective method to guide the flow channel design of the bipolar plate (BPP) to obtain the optimal performance of the proton exchange membrane fuel cells (PEMFC). Most of the existing works focused on the optimization of constant channel dimensions and structures without considering achieving the flow channel's optimal local performance. In this study, an approach combining the down-the-channel performance model and genetic algorithm (GA) is developed to optimize BPP channel/rib patterns for fuel cells. A flow channel is divided into several segments, and the channel dimensions of each segment are brought into the down-the-channel performance model as variables to obtain the optimal parameter design by GA. The model and the optimization results are validated by the CFD and local current density experiment. The performance results of both CFD and local current density experiments are in good agreement with the results of the down-the-channel performance model. It is found that the cell performance of variable rib to channel width ratio (RCWR) design along the flow channel is better than the performance of constant RCWR design. Performance improvements are more significant when the sum of rib and channel width (SRCW) is higher, and the output voltage is lower. Smaller SRCW design can effectively reduce the ohmic losses and concentration polarization which is beneficial to cell performance, and as the SRCW is reduced, the performance improvement of variable RCWR design along the flow channel is less necessary. A guide for the design of the cathode flow channel RCWR from upstream to downstream is provided. [Display omitted] [ABSTRACT FROM AUTHOR]

Thin metallic bipolar plate (BPP), due to mechanical strength, thermal conductivity, high power density, and relatively low cost, is considered to be an alternative to graphite BPP in proton exchange membrane (PEM) fuel cell. However, shape error of thin metallic BPPs is not avoidable due to its flexibility and springback in stamping process, as well as deformation resulted from thermal stress in welding process. In this study, fluctuation analysis is conducted and response surface methodology (RSM) is adopted to establish the relationship between shape error and contact pressure distribution on gas diffusion layer (GDL). Thin metallic BPPs made of stainless steel (SS) 304 sheets are fabricated and shape error is defined. Two types of specimens are selected and assembled with GDL. Effects of assembly force, BPP size and shape error are systematically investigated and a response surface model is developed to predict the effect on contact pressure distribution resulted from the shape error of BPP. The methodology in this study is beneficial to understand the effect of the shape error and predict the acceptable shape error. Based on the model, tolerance of the shape error of BPP is given to guide the manufacturing process of the thin metallic BPP. [ABSTRACT FROM AUTHOR]

Proton exchange membrane (PEM) fuel cells are ideal power sources with great potential for automobiles, backup power systems and stationary applications, owing to high efficiency, zero emissions and high power density. For these devices with large power consumption, many unit cells are assembled in series to construct a stack to provide the required voltage and power. However, the assembly process remains a major obstacle to the large-scale deployment of high-power stack. The performance and durability of stacks are greatly affected by the assembly procedures, and the impact mechanism and assembly technique need to be fully understanding. This paper presents an overview of important issues related to the assembly process of fuel cell stacks, providing a basis for engineers and researchers to improve stack performance. It begins with a description of quality evaluation of the stack assembly, followed by assembly methods to clarify the history of the development of stack design. The main contributions to in-situ behavior of stack during the assembly compression and dynamic compression is presented in detail. Numerical methods and optimization techniques are analyzed to guide assembly process. Finally, novel stack designs involving the assembly process are sorted out. A summary of the key points in this area is also provided as a direction for future work. The aim of this paper is to evaluate which factors affect the cell performance during assembly process and how adverse effects should be mitigated via mechanism analysis, quality evaluation, assembly method selection, process optimization and novel stack structure design. • Optimization method should be proposed to balance eight quality evaluation metrics. • Stack adaptability and endplate integration are orientation of assembly methods. • Micro scale model should be improved with the consideration of GDL fiber structure. • Complete stack model combining material features and multiscale dimensions is lack. • Accumulated manufacturing errors cause uneven performance of unit cells. [ABSTRACT FROM AUTHOR]

The flow distribution in manifold has significant effect on the output performance of the stack according to the Cannikin Law, especially for multi-cell proton exchange membrane fuel cell (PEMFC) stacks. This study presents an analytical model which simultaneously considered local pressure losses, pressure recovery phenomenon, electrochemical reactions, and liquid water to calculate the flow distribution using the flow network method. Both U-shape and Z-shape flow configuration stack are investigated. The analytical model is simultaneously and quantitatively validated with three distinct variables (the dimensionless mass flow rates, the stack pressure drop and the pressure distribution in manifold) to ensure the accurateness of modelling results. Effects of reactant consumption and two-phase flow in unit cell on the flow distribution are quantitatively investigated. Results show that the flow maldistribution is overestimated when ignoring gas consumption and two-phase flow for both 200-cell U-shape and Z-shape stack. Balance the pressure drop of inlet and outlet manifold significantly improves the flow distribution for U-shape stack. Finally, the differentiated assembly strategy is first proposed to improve the flow distribution in manifold for multi-cell PEMFC stacks. Compared to the original design, the ratios of the improvements for 200-cell U-shape and Z-shape stack are 85% and 25%, respectively. • New analytical model is developed to evaluate flow distribution in manifold. • Analytical model is simultaneously and quantitatively validated with three distinct variables. • Flow maldistribution is overestimated when ignoring gas consumption and two-phase flow. • Balance pressure drop of inlet and outlet manifold significantly improves flow distribution. • Uniform flow distribution cell to cell is obtained by differentiated assembly strategy. [ABSTRACT FROM AUTHOR]

Air-cooled proton exchange membrane (PEM) fuel cells simplify fuel cell design by combining oxygen supply and air cooling in open cathode channels. Their performance is sensitive to the structure of cathode channels, which significantly affects distribution of temperature, relative humidity and mass transfer in the cells. This study offers a three-dimensional air-cooled fuel cell model with consideration of electrochemistry simulation to investigate the effect of cathode channel design. Experiments are conducted to validate the model. It is observed that obvious gradient in the distributions of temperature, humidity and oxygen concentration lies in the membrane exchange assembly (MEA) between the channel and rib owing to air dual functions in distributing oxygen and cooling the stack. For models with fixed rib-channel ratio of 1.0, the performance is better when channel width is smaller. Considering the effect of contact resistance when the ratio is small, rib-channel ratio within a reasonable range of around 3.0 is preferred in order to enhance the performance. Channels with curved features improve the mass transfer from channel to catalyst layer, thus increasing the cell performance. This study is helpful for enhancing our understanding of the relationship between cell performance and cathode channel design in the air-cooled fuel cell. • A validated three-dimensional model is built to investigate air-cooled fuel cells. • Larger channel width induces uneven distribution of RH and oxygen concentration. • Rib-channel ratio around 3.0 is preferred in order to enhance the performance. • RH rises with regular fluctuation along the direction of cathode channel. • Channels with curved features improve mass transfer and cell performance. [ABSTRACT FROM AUTHOR]

• Method combining computational fluid dynamics model and empirical model is proposed. • Voltage distribution of unit cells shows the similar trend as the flow distribution in manifold. • U-type configuration stack promotes more uniform voltage among unit cells than Z-type. • The voltage unevenness of unit cells climbs dramatically as the cell number increased. This study offers an efficient method for commercial-size proton exchange membrane fuel cell (PEMFC) stack performance evaluation to improve designing of fuel cell and maximize power density in PEMFC stack. The flow distribution in the manifold of the stack is critical to the energy conversion of the assembled unit cells in series due to the typical short-board effect. Most existing works studying on the flow distribution focus on small fuel cell stack and does not establish a clear relationship to performance, which are not able to thoroughly guide commercial-size PEMFC stack design and development. In the present study, an effective method combining computational fluid dynamics (CFD) model and empirical model is proposed to evaluate the performance of commercial-size stack considering air flow distribution in the manifold. Firstly, the air flow distribution in the manifold is predicted by a CFD model. A performance evaluation empirical model is developed by a series of experiments to evaluate the effects of flow maldistribution on the performance of PEMFC stack. Then, the predicted flow distribution and performance are respectively validated by a novel experimental setup. Finally, the effects of stack configuration, cell number, and current density on flow distribution and performance of PEMFC stacks are discussed. The results show that U-type configuration promotes more uniform voltage among unit cells than Z-type. The voltage unevenness of unit cells caused by flow maldistribution climbs dramatically as the cell number and current density increase. The methodology developed is beneficial to the energy management and the efficiency improvement of commercial-size PEMFC stack. [ABSTRACT FROM AUTHOR]

Amorphous carbon (a-C) films exhibit promising application in the field of Proton Exchange Membrane Fuel Cells (PEMFCs) due to their unique properties and have been coated on the metallic bipolar plates (BPPs) of PEMFCs to improve the interfacial electrical conductivity and corrosion resistance of BPPs. However, a-C is still gradually oxidized in the harsh environments of PEMFCs and the performance of a-C needs to be further improved to achieve the lifetime target of PEMFCs. In this paper, nitrogen with different content is doped in a-C and the further understanding of corresponding mechanism on the structure and performance of a-C is revealed via first principles calculation and several experimental methods. The results indicate that doping nitrogen promotes the formation of stable CNx phase with better corrosion resistance and improves the compactness of the films. In addition, moderate content of nitrogen can improve the sp2 fraction of a-C. Furthermore, the film doped with about 10 at. % nitrogen exhibits excellent improvement of the electrical conductivity, corrosion resistance, durability and stability, especially the corrosion resistance under start-up/shut-down operating condition of PEMFCs. This study is beneficial to enhance the lifetime of a-C films and promote the commercialization of PEMFCs. Image 1 [ABSTRACT FROM AUTHOR]

Abstract Proton exchange membrane fuel cells (PEMFCs) have been promoted more than 100 years and are in the forefront of the large-scale commercial application with the technology breakthrough of key components and stack. As a key component in PEMFCs, bipolar plates (BPPs) can distribute reaction gases, collect current, remove product water, and cool the stack. Metallic BPPs have superior manufacturability and cost effectiveness, higher levels of power density, and high mechanical strength, and have been regarded as an alternative to graphite BPPs. Surface coatings are essential to metallic BPPs because they enhance corrosion resistance and electrical conductivity. Carbon-based coatings have attracted considerable attention from both academia and industry owing to their merits of high performance and low cost. In this paper, a comprehensive survey is presented on the recent progress in carbon-based coatings in terms of evaluation methods, material design, deposition process, and coating performance. Pure amorphous carbon (a-C), metal-doped a-C film, and metal carbide (Me C) are summarised. Carbon nanotubes (CNTs), graphene, and C 60 are discussed as well. Finally, technical barriers and developing trends are presented in the application of carbon-based coatings for metallic BPPs in PEMFCs. Highlights • Latest developments on carbon-based coatings used on metallic BPPs in PEMFCs. • General evaluation methods for coatings used on metallic BPPs are summarised. • Carbon-based coatings are divided into four categories and reviewed separately. • Coating designs, microstructures, and performances are included in each category. • Technical barriers and developing trends of carbon-based coatings are discussed. [ABSTRACT FROM AUTHOR]

[Display omitted] • In-situ monitoring technology for the fuel cells is proposed. • A method for decoupling the three kinds of voltage losses in fuel cells is built. • The internal states and voltage losses in large area fuel cells are analyzed. • The effect of operating conditions on the voltage losses is investigated. Understanding the local operating state and voltage loss distributions are of great importance to guide the performance improvement of proton exchange membrane fuel cells (PEMFCs). Especially when the active area is large, the uneven electrochemical reaction becomes more severe. However, the current research does not enable the simultaneous monitoring of the multiple operating states and decoupling of the voltage losses in the fuel cell. In this study, a method is established that can obtain the distribution of activation polarization, ohmic loss, and concentration polarization by analyzing the measured data of temperature, humidity, current density, and high-frequency resistance (HFR), which are obtained inside a 320 cm2 and 3 kW level fuel cell stack by using the multilayer printed circuit board (PCB) technology with designed isolation segments. The effects of humidity, operating temperature, and current density on the distributions of output voltage and voltage losses are discussed. Results show that the potential distribution on the surface of membrane exchange assembly (MEA) isn't uniform in the large active area. The activation polarization gradually increases in the direction of coolant flow and the direction of the cathode flow and the ohmic loss and concentration polarization have opposite distribution characteristics. In addition, when the humidity rises to full humidity, the inhomogeneity of ohmic loss distribution becomes higher with three local increased areas of 20 mV and one decreased area of −20 mV. Higher temperatures can significantly reduce the flooding near the outlet of the active area. When the current density increases, the inhomogeneity of the ohmic loss distribution increases, and the concentration polarization near the cathode outlet of the active area become more severe. [ABSTRACT FROM AUTHOR]

Metallic bipolar plates (BPPs) are promising candidates for replacing conventional carbon-based BPPs. However, manufacturing errors are unavoidable on metallic BPPs due to the characteristics of fabrication processes. This study is conducted to further develop tolerance design method for dimensional error and assembly error based on the contact behavior model proposed in our previous work. Contact pressure distribution, contact resistance and gas diffusion layer (GDL) porosity are used as the evaluation indexes combined by desirability function method. Based on the contact behavior model, effect of dimensional error is substituted into the model by Monte Carlo Simulation, and misalignment between the upper plate and lower plate is included in the model to obtain the effect of assembly error. It is found that dimensional error and assembly error contribute to uneven pressure distribution, larger contact resistance and higher GDL porosity. The acceptable maximum dimensional error of 0.007, 0.012, 0.010 mm and assembly error of 0.080, 0.10, and 0.060 mm are recommended for the fuel cells with the channel widths of 1.0, 2.0 and 4.0 mm, respectively. This methodology developed is beneficial to the understanding of the effect of dimensional error and assembly error, and effective supplement to fabrication management of metallic BPPs. Image 1 • Tolerance design method for fabrication error of metallic BPPs is introduced. • Response variables are almost in linear rise with increasing the dimensional error. • Assembly error smaller than 0.05 mm contributes little to the response variables. • Dimensional error of 7, 12, 10 μm considered acceptable for channels of 1, 2, 4 mm. • Assembly error of 80, 100, 60 μm considered acceptable for channels of 1, 2, 4 mm. [ABSTRACT FROM AUTHOR]

It is necessary to obtain the current density distribution in order to understand local reactions inside the proton exchange membrane (PEM) fuel cells. Enlarging the active area to commercial-size makes it difficult to maintain uniform assembly pressure, gas supply and heat dissipation inside the fuel cell stack. Existing studies for laboratory-level fuel cells cannot effectively guide the development of large-area fuel cells. In this study, current density distribution inside a 250 cm2 and 3 kW level fuel cell stack is measured and analyzed using the printed circuit board (PCB) technology. A four-layer PCB sensor plate covering 144 current collecting segments and shunt resistors for current measurement is well-designed. The in-house developed measurement system is proved with good function, and the accuracy of current density measurement is within 2%. Results show that the flow field configuration and stack assembly condition greatly affects the uniformity of local current densities. Higher current passes through the area with larger contact pressure, and the current density is low in the poor contact area. The local current density has larger deviations for higher output current, but the normalized current is more uniform. Effects of operating conditions including stoichiometric ratio and flow direction are investigated. • A large-area segmented PCB current sensor plate is designed and tested. • The current distribution of a commercial-size fuel cell stack is investigated. • Nonuniformity contact pressure has a great impact on current density distribution. [ABSTRACT FROM AUTHOR]

Multi-stage forming of the ultra-thin metallic sheets is a promising process for manufacturing fine width and high aspect ratio bipolar plates in proton exchange membrane fuel cell. Correlations between the processing parameters and the channel quality evaluation indicators are investigated experimentally and numerically. A novel parameter optimization method and stage number design guidance are established with the consideration of the indicator priorities and material properties. The forming depth can be increased by larger Stage II punch stroke along with smaller clearance and radius. Increasing or decreasing the Stage II punch stroke and radius simultaneously contributes to larger draft angle. Small Stage II punch stroke and radius are beneficial to greater rib width. More uniform thickness distribution can be achieved by using smaller Stage II punch stroke and clearance. Two-stage forming is enough for soft materials and three-stage forming is suitable for relatively hard materials. Both the output power density and voltage distribution uniformity of the proton exchange membrane fuel cell are improved by employing multi-sage formed bipolar plates. The relationship between the parameters and indicators, optimization method, stage number design guidelines, all serve as the design principles of the micro channel's multi-stage forming for bipolar plates with ultra-thin metallic sheets. Image 1 • High performance BPPs are fabricated by multi-stage forming in ultra-thin sheets. • Influences of processing parameters on quality evaluation indicators are studied. • Novel parameter optimization method is established in a quantitative manner. • Guidance of the stage number design for different material properties is proposed. [ABSTRACT FROM AUTHOR]