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Metallic bipolar plates (BPPs) are prospective candidates for BPPs in PEMFCs due to their lower cost and higher power density than traditional graphite. Coatings are used to enhance the electrical conductivity and anti-corrsive quality of BPPs, and amorphous carbon (a-C) films have attracted broad attentions from both industry and academia. In this study chromium was incorporated into a-C to further enhance conductivity. A series of Cr-doped a-C films (a-C:Cr) with different doping contents were prepared by CFUBMSIP. Moreover, Microstructure analyses and composition characterizations were performed to explore the mechanism. The results show that Cr decreases the interfacial resistance with satisfying the anti-corrosive behavior. XRD patterns indicate that Cr will form chromium carbide phase and pure metal phase. Moreover, TEM results also show that Cr atoms bond with C atoms. SEM photographs show that Cr atoms refine the grain size and compact the film all the same. C 1s spectrum of XPS shows that C-sp2/C-sp3 increases first and then decreases, reaching the maximum level of 1.708 at Cr 0.205 , indicating the resistance drops first and then increases. In addition, Cr doping will also bring changes in nano-hardness and surface roughness. (a) Interfacial contact resistance and contact resistance of samples series, indicating the mechanism of interfacial contact resistance differs from that of contact resistance; (b) Bulk resistance of samples series, indicating resistance in-plane differ from resistance through –plane; (c) Potentiodynamic polarization curves of samples series, indicating anti-corrosive behavior increase first then decrease as the doping content increases; (d) Raman results of samples series, indicating Cr's incorporation would change the micro-structure of a-C phase of the coatings. [Display omitted] • The regular and principle of ICR/IR/BR for series doping are explored. • A-C:Cr films with excellent conductivity and anti-corrosive property are acquired. • The research discover the oxygen act the dominant impact factor for ICR/IR. • Material analyses and resistance test support each other very well. [ABSTRACT FROM AUTHOR]

Stainless-steel bipolar plates (BPPs) are widely used in place of graphite bipolar plates in proton exchange membrane fuel cells (PEMFCs). Amorphous hydrogenated carbon (a-C:H) coatings are widely used to improve the conductivity and corrosion resistance of metal bipolar plates. However, a-C:H coatings prepared by the sputtering method cannot be applied to quantity production on account of its low deposition rate. Our paper focuses on a-C:H coatings applied at metallic BPPs for PEMFCs produced by direct-current plasma-enhanced chemical vapor deposition (DC-PECVD) with high deposition rates. The effects of adjusting deposition pressures on the structure and properties of coatings have been investigated. The results show that the a-C:H coating deposited at 8 Pa deposition pressure have high stability, with a high deposition rate of 37.5 nm/min. As the deposition pressure increased, sp2-hybridized carbon atoms increased, the larger microcrystalline carbon clusters are found, and the structure will undergo different structural transformations in a-C:H coatings. Overall, the a-C:H coatings deposited at 8 Pa are attractive to be applied in metallic bipolar plate with have high deposition rates, dense coating structure, and proper carbon structure. Image 1 • The a-C:H films deposited by DC-PECVD with high deposition rates. • The coating's structure is changed from amorphous to microcrystal as pressure increased. • The a-C:H films are a dense and low C –H bond coating with high corrosion resistance. • DC-PECVD method have attractive to be applied in metal bipolar plate's production. [ABSTRACT FROM AUTHOR]

Amorphous carbon (a-C) films exhibit properties that make them attractive for a wide range of applications in automotive components. Proton exchange membrane fuel cells (PEMFCs) assembled by metallic bipolar plates (BPPs) are promising power sources for new energy vehicles. Harsh environments of PEMFCs require preparing coatings on metallic BPPs to achieve high corrosion resistance and electrical conductivity, and a-C based coatings have the great potential. However, their low stability and high internal stress significantly damage their durability. This work aims to develop a fundamental understanding of the effect of doping Ag and Cr on the performance and durability of a-C based coatings. Experimental results revealed that a-C films doped with Ag and Cr (a-C:Ag:Cr) simultaneously achieve ultra-low interfacial contact resistance (ICR), low internal stresses, and higher stabilities. We also developed an atomistic model by molecular dynamics (MD) to understand certain behaviors of a-C:Ag:Cr. The ultra-low ICR was mainly attributed to Ag nanoclusters embedded in a-C phases, low hardness, and a more graphite-like structure. The enhanced stability was attributed to denser structures and lower internal stresses. MD simulations suggested that the effect of doping Ag or Cr on the surface morphology and internal stress of a-C films may have different mechanisms. 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]

Graphical abstract Highlights • The model is used to predict the lifetime of coated metallic bipolar plates. • The predicted results show good agreement with the experimental results. • The model is universal and can be applied to metallic bipolar plates coated with different films. • The model can provide theoretical guidance for the rapid development of coatings. Abstract Metallic bipolar plates are considered to be cost-effective and feasible alternative to graphite bipolar plates for high power density of Proton Exchange Membrane Fuel Cells. However, the lifetime of metallic bipolar plates is determined by surface coating and the durability evaluation is extremely costly and time-consuming. It is urgent to establish a method for the lifetime evaluation and quick development of coated metallic bipolar plates. In this study, a lifetime prediction model for coated metallic bipolar plates is proposed and verified. Firstly, the model is established based on the electrochemical reaction kinetics of Proton Exchange Membrane Fuel Cells. Secondly, the operating conditions for automotive application are equivalent to the ex-situ accelerated tests for the coated metallic bipolar plates and the model is formulated based on the ex-situ accelerated tests. Finally, 2500 h in-situ test of stacks assembled with carbon-coated metallic bipolar plates is carried out to verify the accuracy of the model and the relative prediction error is 8.68%. The universality of the model has been evaluated by applying it to gold-coated metallic bipolar plates, which were tested in-situ in stacks up to 3000 h. This study is beneficial to the quick evaluation of lifetime for coated metallic bipolar plates and provides a theoretical guidance for the rapid development of coatings. [ABSTRACT FROM AUTHOR]

Abstract Metallic bipolar plates (BPPs) are a promising candidate to replace conventional graphite BPPs due to higher power density and lower cost in proton exchange membrane fuel cells (PEMFCs). Surface coating is essential to enhance interfacial conductivity and corrosion resistance. Amorphous carbon (a-C) films have attracted broad attention from both academia and industry. This study incorporated Niobium (Nb) into a-C to further enhance its performance. First principle calculation was introduced to investigate the evolution mechanism of bond and phase by Nb doping and instruct the coating design. Then, a series of Nb-doped a-C samples were deposited by closed field unbalanced magnetron sputtering ion plating (CFUBMSIP) method. The microstructure was systematically characterized by SEM, XRD, Raman spectrum, and XPS. Interfacial contact resistance (ICR) and electrochemical corrosion were also tested to evaluate the effect of Nb doping. A higher C-sp2/C-sp3 ratio was observed in a-C film with moderate Nb doping. As a result, the ICR decreased to 1.22 mΩ•cm2 from initial value of 4.41 mΩ•cm2. Besides, the doped Nb also refined grain size and increased the film compactness, which was beneficial for corrosion resistance. Rct, which reflects the relative anti-corrosive property, was increased from 1.8 × 106 Ω•cm2 to 3.29 × 106 Ω•cm2. Moreover, the current density in 0.84 V (vs. SHE) potentiodynamic polarization is 3.59 × 10−7 A/cm2, lower than 4.59 × 10−7 A/cm2 of a-C films. Besides, the current density in 0.84 V (vs. SHE) potentiostatic polarization shows the same tendency. The enhanced performance of Nb-doped a-C coatings would advance the commercialization of metallic BPPs for PEMFCs. Graphical abstract Image 1 Highlights • First principle is used to survey the doping mechanism and instruct film design. • A-C film with excellent conductivity and anti-corrosive property are obtained. • Nb content will be weighed to balance the conductivity and corrosion resistance. • Theory calculation, material characterization and experiments support each other. [ABSTRACT FROM AUTHOR]

Abstract Proton exchange membrane fuel cells (PEMFCs) are deemed to be a promising renewable energy for variety of applications. However, metallic bipolar plates, one of key components in PEMFCs, still suffer from severe corrosion and degradation of interfacial conductivity under the humid and acid operating condition. Herein we proposed a novel strategy to enhance interfacial conductivity and corrosion resistance of TiCx/a-C coatings for metallic bipolar plates by alternating substrate bias voltage during the deposition process. The effects of the alternating substrate bias voltage strategy on the composition and morphology of the multilayered TiCx/a-C coatings had been explored. Both the corrosion resistance and the interfacial conductivity of the multilayered TiCx/a-C coatings were improved with more alternating periods of bias voltage. The effects of the enhanced performance had been discussed, and it was found that the alternating bias voltage strategy will restrain the columnar structures in the a-C layers and promote the generation of sp2-rich clusters on the surface. This versatile strategy based on moderately alternating cycles of substrate bias voltage exhibits great potential in many applications. Highlights • A novel strategy is proposed to deposit multilayer TiCx/a-C films. • Both corrosion resistance and interfacial conductivity are enhanced. • The interfacial contact resistance is 3.58 mΩ cm2 at 1.4 MPa. • The corrosion current density is 0.297 μA/cm2 at 0.6 V (vs. Ag/AgCl). [ABSTRACT FROM AUTHOR]

Ti6Al4V alloy has been widely used as a suitable material for surgical implants such as artificial hip joints. In this study, a series of multilayered gradient TiAlN coatings were deposited on Ti6Al4V substrate using closed field unbalanced magnetron sputter ion plating (CFUBMSIP) process. Taguchi design of experiment approach was used to reveal the influence of depositing parameters to the film composition and performance of TiAlN coatings. The phase structure and chemical composition of the TiAlN films were characterized by X-ray diffractometry (XRD) and X-ray photoelectron spectroscopy (XPS). Mechanical properties, including hardness, Young's modulus, friction coefficient, wear rate and adhesion strength were systematically evaluated. Potentiodynamic tests were conducted to evaluate the corrosion resistance of the coated samples in Ringer's solution at 37 °C to simulate human body environment. Comprehensive performance of TiAlN films was evaluated by assigning different weight according to the application environment. S8, deposited by Ti target current of 8 A, Al target current of 6 A, bias voltage of − 60 V and nitrogen content with OEM (optical emission monitor) value of 45%, was found to achieve best performance in orthogonal experiments. Depositing parameters of S8 might be practically applied for commercialization of surgical implants. [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]

Interfacial conductivity and corrosion resistance of bipolar plates are two significant parameters affecting the performance and durability of proton exchange membrane fuel cells. This study designs to investigate the effects of Al incorporation on the interfacial conductivity and corrosion resistance of CrN film coated on bipolar plates, ternary Cr–Al–N films with different Al content have been deposited on SS316L samples by closed unbalanced magnetron sputter ion plating (CFUBMSIP). Al content was adjusted by altering magnetron sputtering current of Al target. Scanning electron microscopy (SEM) results show that the deposited films are dense and continuous. The phase structures and bonding types before and after Al incorporation have also been investigated by X-ray diffractometry (XRD) and X-ray photoelectron spectroscopy (XPS). Interfacial contact resistance (ICR) between gas diffusion layer (GDL) and coated samples increases with the increase of Al content doped in CrN film, and the lowest ICR value is 5.1 mΩ cm 2 at 1.4 MPa. The incorporation of Al has influence on the interfacial conductivity of CrN films by combining two competitive aspects actual contact area and the conductivity of the sample surface. Potentiodynamic polarization tests in the simulated corrosive circumstance of PEMFCs reveal that the corrosion potential of coated sample become more positive after Al incorporation and the corrosion current density obtained from Al doped CrN film after potentiostatic tests in cathode PEMFCs environment is 0.021 μA cm −2 , which witnesses nearly one order of magnitude decrease compared with CrN without Al content. Electrochemical corrosion tests and inductively coupled plasma-mass spectrometry (ICP-MS) detection disclose that Al doped CrN film can improve the durability of bipolar plates by forming a dense passive film in real PEMFCs environments and reduce the metal ion contamination of membrane. Based on the results of ICR and electrochemical corrosion tests, it is demonstrated that low content of Al incorporation in CrN film can combine the two aspects, namely, better corrosion resistance and high interfacial conductivity which are beneficial for the commercial application of metallic bipolar plates. [ABSTRACT FROM AUTHOR]

Laser welding is the most effective joining method for thin metallic bipolar plates (BPPs) to achieve reliable seal and fulfill the requirements for fuel cell stacks. However, distortion caused by the welding heat will produce shape error in BPPs. The shape error will cause uneven assembly stress distribution and unacceptable contact resistance between BPPs and the gas diffusion layer (GDL), eventually affect the fuel cell performance. In this study, transverse deformation and angular distortion are considered to be the main sources of BPPs shape error and studied by modeling and experimental methods. An analytical model based on inherent strain theory is established to predict the transverse deformation of BPPs. Meanwhile, a model based on the weld pool geometry is proposed to evaluate the angular distortion. Experiments are performed to validate the models by welding 316L stainless steel sheets of 0.1 mm and 0.2 mm thick with a multi-mode fiber laser system. A good correlation is found between experimental results and the prediction models. Finally, a formula based on the former prediction models is proposed to calculate the welding induced shape error of BPPs with two pass welds. The formula is validated by experiments. The methodology in this study can be applied to guide the laser welding process design and manufacturing of the metallic BPPs. [ABSTRACT FROM AUTHOR]

Stainless steel bipolar plates (BPPs) are regarded as promising alternatives to traditional graphite BPPs in proton exchange membrane fuel cells (PEMFCs). This technology has experienced more than 20 years development and has been partially applied in industrial production. This review surveys recent progress of entire development process for stainless steel BPPs in terms of flow field design, microforming process, joining process and coating process. Besides, assembly process considering dimensional error, shape error and assembly error are comprehensively summarized as well. Finally, technical challenges and future trends are presented for the application of stainless steel BPPs for PEMFCs. [ABSTRACT FROM AUTHOR]

Highlights: [•] The morphological features of the SSSFFs were inspected and RVE was established. [•] The elastic and plastic constitute behaviors were derived by micromechanics. [•] The elastic and plastic relations were unified by statistical theory. [•] The constitutive model showed good agreements with experimental results. [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]

Abstract: To combine the advantages of chromium nitride (CrN) and amorphous carbon (a-C) film, this study proposes a novel Cr–N–C multilayer film on 316L stainless steel (SS316L) as bipolar plates for proton exchange membrane fuel cells (PEMFCs) using closed field unbalanced magnetron sputter ion plating (CFUBMSIP) method. The characterizations of Cr–N–C film are analyzed by X-ray photoelectron spectroscopy (XPS), X-ray diffractometry (XRD), and scanning electron microscopy (SEM). Scratch tests indicate that the adhesion strength between the film and SS316L substrate has been greatly improved which is beneficial to prevent the multilayer film from spalling. Interfacial contact resistance (ICR) between coated SS316L sheets and simulated gas diffusion layer (GDL) decreases to 2.64 mΩ cm2 at 1.4 MPa. Potentiodynamic results reveal that the anodic corrosion potential of coated samples is more positive than the operation potential and the cathodic passivation current density is only 0.61 μA cm−2 at 0.6 V. Potentiostatic test, contamination analysis and surface morphology results reveal that the substrate is well protected by the Cr–N–C film. This research demonstrates that the novel Cr–N–C film exhibits excellent ex-situ performance including strong adhesion strength, high corrosion resistance and low ICR. [Copyright &y& Elsevier]

Abstract: In this study, 304 stainless steel (SS) bipolar plates are fabricated by flexible forming process and an amorphous carbon (a-C) film is coated by closed field unbalanced magnetron sputter ion plating (CFUBMSIP). The interfacial contact resistance (ICR), in-plane conductivity and surface energy of the a-C coated 304SS samples are investigated. The initial performance of the single cell with a-C coated bipolar plates is 923.9mWcm−2 at a cell voltage of 0.6V, and the peak power density is 1150.6mWcm−2 at a current density of 2573.2mAcm−2. Performance comparison experiments between a-C coated and bare 304SS bipolar plates show that the single cell performance is greatly improved by the a-C coating. Lifetime test of the single cell over 200h and contamination analysis of the tested membrane electrode assemble (MEA) indicate that the a-C coating has excellent chemical stability. A 100W-class proton exchange membrane fuel cell (PEMFC) short stack with a-C coated bipolar plates is assembled and shows exciting initial performance. The stack also exhibits uniform voltage distribution, good short-term lifetime performance, and high volumetric power density and specific power. Therefore, a-C coated 304SS bipolar plates may be practically applied for commercialization of PEMFC technology. [Copyright &y& Elsevier]

The membrane electrode assembly (MEA) in the proton exchange membrane (PEM) fuel cell needs to be encapsulated by a frame to improve its assembly strength and sealing performance. However, irreversible degradation usually occurs to invalidate the frame and cause the fuel cell fail. In this study, a series of experiments are conducted to simulate operating environment in the fuel cell, and degradation mechanism of the frame is explored. Changes of tensile strength, peeling strength, shearing strength, and bending strength are adopted to evaluate the frame stability, and effects of temperature, water, and acid on these indexes are quantified. It is found that the peeling strength has the most significant decline, which is the primary failure form of the frame. Acid solution provides the main contribution to this failure. In acid solution, hydrogen ions and water molecules permeate the frame, resulting in rapid degradation and separation of the bonding interface. This study reveals the degradation process of the frame for the first time, and helps enhance our understanding of the frame failure. • Ex-situ accelerated experiment is designed to simulate the degradation process of frame after long time. • The mechanical strength of proton exchange membrane fuel cell's frame decreases rapidly due to operating conditions, and peel strength reduces the most significantly. • The bonding interface of frame generate cavitation under operating conditions, which cause peel strength reduce by 62.14%. • Cavitation is formed at bonding interface because of the water molecules' and protons' penetration through substrate layer of frame. • Compared with polyethylene terephthalate, polyethylene naphthalate can reduce the penetration. [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]

Highlights • Micro/meso scale hydroforming experiments of sheet metals were conducted. • The forming pressure and limit height decreases with the increase of grain size. • The GTN–Thomason model was utilized in hydroforming process design. • Bipolar plate sample was successfully hydroformed after process optimization. ABSTRACT The geometry and grain size effects have been revealed to affect the formability of metal materials during the micro/meso scale forming processes. To understand and characterize how these size effects influence practical forming process and further to utilize those knowledge in process design and optimization, an experimental and numerical investigation on a micro/meso scale hydroforming process of pure copper sheet metals was conducted as a case study. A hydroforming process experimental setup was first developed to form long multi-channel features with different dimensions. The experimental results reveal evident size effect: The pressure and maximum height onset of failure decrease as the grain size approaches the thickness. The size effect on the pressure was identified to be attribute to the reduction of flow stress of material as the grain size increases. The surface layer model has been employed to explain the mechanism. On the other hand, the decrease of ultimate height was revealed to be affected by the reduction of forming limit of sheet metals as the size effect becomes more significant. The interaction of geometry and grain sizes and the evolution of micro voids were discussed and a modified GTN–Thomason model with the consideration of grain size effect was employed in FE simulations to estimate the forming results. A reasonable agreement between the numerical and experimental results was observed. After that the method was further utilized in the process optimization for the fabrication of a fuel cell bipolar plate with typical micro/meso scale channel features. The dangerous area with a high risk of failure was predicted based on simulations. After optimizing the process parameters, the satisfying simulation result was obtained. Hence the hydroforming apparatus was developed accordingly and the bipolar plate was successfully fabricated with high quality, which verifies the applicability of the method in the present work. Graphical abstract Image, graphical abstract [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]

The demands for lightweight, low cost and high power density of proton exchange membrane fuel cells (PEMFCs) have made stainless steel coated with amorphous carbon (a-C) film an ideal alternative material to replace graphite for bipolar plates in automotive application. However, the existing a-C film cannot resist high potentials generated during vehicle operating conditions such as start-stop process, high power loading condition, which result in carbon corrosion and undoubtedly decrease the lifetime of fuel cells. Therefore in this study, we designed three types of a-C films comprising Cr, Ti and Nb seed layers based on the materials E-pH diagrams. The a-C films were deposited through closed unbalanced magnetron sputtering and characterized by scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS) and Raman methods. The durability of a-C films were evaluated through potentiostatic polarization tests by applying different high potentials. When applied potential exceeds 1.2 V SHE , the a-C film with Cr seed layer will be delaminated from the substrate due to the dissolution of Cr element. While the a-C films with Ti or Nb layer exhibit an excellent corrosion resistance even the applied potential as high as 1.6 V SHE . After potentiostatic polarization tests, the interfacial contact resistance (ICR) values increased mechanism were determined through Raman and XPS detection comprehensively. This present study will shed new light on solving the high potential impact and improving the lifetime of fuel cells vehicles. [ABSTRACT FROM AUTHOR]

Electrical contact resistance (ECR) at the interface is of significant importance in many fields of science and engineering. Current methods for contact resistance estimation are based on the typical nearly incompressible rough surfaces, which is not suitable for porous material with large deformation in the compression process. The objective of this work is to build an analytical model for ECR between solid material and porous material, for example, which could be used to predict power loss between carbon fiber paper and bipolar plate in the fuel cell. First, mathematical description of solid roughness surface is built by classic Greenwood and Williamson model. Considering the porous structure, carbon fiber paper is modeled by multi-layer construction based on random line network model. Effect of large compression of carbon paper on contact behavior is furtherly given necessary attention in this study. Contact pressure and resistance are calculated based on statistical methods with consideration of multi-deformation states. Then, experiments are carried out to validate the numerical model. The results show good agreements with the numerical model. Finally, influences of carbon paper compression and main parameters are systematically discussed based on the numerical model. The model developed will enhance our understanding regarding the relation between contact pressure and contact resistance at the interface for solid material and fiber-structure material. [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]

Thin stamped bipolar plate (BPP) is promising for proton exchange membrane fuel cell (PEMFC). However, design laws in conventional method based on graphite BPP are not suitable for the design of stamped BPP since the tapered channel shape influences the flow distribution. By considering the channel cross-section on the stamped BPP, a formula for the pipe resistance is proposed to describe the flow resistance characteristics by introducing a shape factor. The shape factor is calculated by the computational fluid dynamics (CFD) simulation. Then, a theoretical flow network model based on mass and momentum conservation is developed to predict the flow distribution in Z-type flow field configuration. The accuracy of the model has been verified by comparing with CFD results. It is found that the aspect ratio and the base angle have significant influences on the resistance characteristic and flow distribution. And the results indicate that the homogeneity can be improved by increasing the proportion of flow resistance in the side channel. The flow network model provides an easy-to-use method to calculate the flow distribution and the results can be used as guidance for the design and optimization of the flow field for stamped BPP. [ABSTRACT FROM AUTHOR]

Stainless steel (SS) has received much attention in recent years as an alternative material for bipolar plates (BPPs) in proton exchange membrane fuel cells (PEMFCs). However, stainless steel BPPs are still required to combat corrosion in the acid environment without forming oxidants, passive layers and metal ions. In this study, multilayered chromium-carbon (Cr-C) film is deposited on SS316L sheet as BPPs using closed field unbalanced magnetron sputter ion plating (CFUBMSIP). Five films with different composition are deposited and the influence of chromium content adjusted by sputtering current is characterized by X-ray diffractometry (XRD) and X-ray photoelectron spectroscopy (XPS). Interfacial contact resistance (ICR) between coated SS316L sheets and gas diffusion layer (GDL) is measured. Potentiodynamic and potentiostatic tests are conducted to evaluate the corrosion resistance of the coated samples. Experimental results show that Cr-C film with content of Cr 0.75 C 5 presents the optimal performance, i.e. the interfacial contact resistance(ICR) reduces to 1.4 mΩ cm 2 under a compaction pressure of 1.4 MPa, and the corrosion current density reaches 1.046 μA cm −2 in the PEMFCs cathodic environment (0.5 M H 2 SO 4 + 5 ppm HF solution at 70 °C, 0.6 V vs. SCE). The performance variation of five films can be preliminarily explained by the crystallographic structure and chemical composition analysis. [ABSTRACT FROM AUTHOR]

• Quasi in-situ observation of catalyst-coated membrane cracks was achieved. • A freeze–thaw method under saturated humidity condition was established. • Cracks tend to extend along the larger pores formed between agglomerates. • Cracks in the catalyst layer led to the appearance of cracks in the membrane. • The membrane cracks can even traverse the entire membrane. The evolution of catalyst-coated membrane (CCM) under saturated humidity conditions during freeze–thaw cycling was investigated, with a particular focus on the evolution of initial cracks within the catalyst layer (CL) and their impact on the proton exchange membrane (PEM). A quasi in-situ fuel cell fixture was designed to enable direct observation of the CCM via scanning electron microscopy (SEM). As the number of freeze–thaw cycles increases, the initial cracks in the CL gradually propagated, with new cracks appearing around them. These CL cracks tend to extend along the larger pores, which essentially represent gaps between agglomerates. The overall trajectory of formed cracks appears to be influenced by the flow channel direction. Additionally, under freeze–thaw cycling, cracks in the CL led to the formation of cracks in the PEM. A strong correlation is found between the direction of membrane cracks and CL cracks, suggesting a direct impact of the CL defects on the structural integrity of the PEM. Furthermore, some cracks at the tips of CL cracks even penetrate through the entire membrane. This study provides valuable insights into the degradation mechanisms of CCMs under low-temperature environmental conditions and could inform future design improvements for enhanced durability. [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]

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]

Fracture of metallic thin films during deformation hampers extensive applications of coated engineering components. In this study, the resistance to micro-crack initiation of Niobium (Nb) coatings deposited on stainless steel substrates is improved remarkably by tuning the substrate bias voltages during unbalanced magnetron sputtering. With the tailored nanocrystalline microstructure, the failure strain for coating fracture is increased to 30 %, which is far greater than that of previous metallic coatings (below 10 %). Microstructure characterizations reveal that intergranular nano-pinholes and amorphous phases can be eliminated effectively with a moderate ions bombardment effect to avoid brittle film fracture. At the same time, compact and homogenous nanocrystalline grains with appropriate intragranular defects are obtained via enhanced atom diffusivity during deposition. Therefore, large plastic deformation can be accommodated through both stress-driven grain coarsening and continuous dislocation plasticity, avoiding fracture of the Nb coatings. [Display omitted] [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]

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]

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]

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]

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]