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The membrane usually breaks down at a specific local position due to the mechanical degradation caused by nonuniform hygrothermal conditions in proton exchange membrane fuel cells. Many studies have been carried out analyzing the stress and strain on membrane along thickness direction, but few of them considered the stress along the surface. By imposing uneven temperature and water profiles according to experiments and simulation, this study systematically investigated effects of varying temperatures, relative humidity, and gas flow directions on the membrane stress/strain in a comprehensive 3D model. The results proved that nonuniform temperature and water content affect the response of the membrane a lot. Although the membrane at the inlet of the flow field suffers higher stress, the membrane at the outlet is easier to fail because higher humidity leads to lower yield stress. For the operating condition, the stress range of cells under the counter‐flow reactant gas is 0.2 MPa less than those under co‐flow direction. And increasing humidity to near‐saturated condition would reduce the stress range from 1.2 to 0.49 MPa. The study contributes to achieving better fatigue resistance for membranes in terms of controlling anisotropic heat and relative humidity for fuel cells. [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]

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]

Abstract: The design of the flow field greatly affects the flow distribution and the final performance of the proton exchange membrane fuel cell (PEMFC) system. The clamping force between the gas diffusion layer (GDL) and the flow field plate (FFP) will cause the inhomogeneous compression of the GDL. Then the GDL will be intruded into the reactant gas channels and eventually change the flow distribution. This paper presents a study on the effect of the intrusion of the GDL on the flow field in a PEMFC, and tries to explain the reason for poor performance of the previous design of the flow field other than those have been studied in other papers such as GDL roughness, porosity, etc. First of all, a linear analytical model is used to analyze the sensitivities of the flow field to the GDL intrusion, and then used to estimate the effect of the GDL intrusion on the flow field distribution. Secondly, a multi-objective optimization model is proposed to eliminate the nonuniform distribution in the flow field with the GDL intrusion taken into consideration. Subsequently, three different designs are analyzed and compared with each other as a demonstration to show the effect of the GDL intrusion. From the analysis results, it is recommended that the effect of the GDL intrusion on the multi-depth flow field should be taken into consideration and the flow field should be insensitive to the GDL intrusion to obtain high robust performance. The results obtained in the study provide the designer some useful guidelines in the concept design of flow field configurations. [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]

Abstract: Bipolar plate is one of the most important and costliest components of polymer electrolyte membrane (PEM) fuel cells. Micro-hydroforming is a promising process to reduce the manufacturing cost of PEM fuel cell bipolar plates made of metal sheets. As for hydroformed bipolar plates, the main defect is the rupture because of the thinning of metal sheet during the forming process. The flow channel section decides whether high quality hydroformed bipolar plates can be successively achieved or not. Meanwhile, it is also the key factor that is related with the reaction efficiency of the fuel cell stacks. In order to obtain the optimum flow channel section design prior the experimental campaign, some key geometric dimensions (channel depth, channel width, rib width and transition radius) of flow channel section, which are related with both reaction efficiency and formability, are extracted and parameterized as the design variables. By design of experiments (DOE) methods and an adoptive simulated annealing (ASA) optimization method, an optimization model of flow channel section design for hydroformed metal bipolar plate is proposed. Optimization results show that the optimum dimension values for channel depth, channel width, rib width and transition radius are 0.5, 1.0, 1. 6 and 0.5mm, respectively with the highest reaction efficiency (79%) and the acceptable formability (1.0). Consequently, their use would lead to improved fuel cell efficiency for low cost hydroformed metal bipolar plates. [Copyright &y& Elsevier]

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]

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]

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]

Proton exchange membrane (PEM) fuel cells are promising zero-emission power source for automobiles, portable devices, backup power system and stationary applications. However, their relatively short lifespan remains a major obstacle to the commercial deployment of this type of fuel cell. The membrane's mechanical degradation is the main cause of early-stage failure in fuel cell lifetimes. In order to provide engineers and researchers with a basis for extending fuel cell durability, this paper presents an overview of important issues relating to mechanical failure and mitigation strategies for PEM fuel cell membranes, drawing on a survey of the existing literature. This review begins with a sketch of failure mechanisms in an effort to establish an unambiguous definition of membrane degradation in each stage of its lifespan. The material properties of typical membranes are outlined below to illustrate the fundamentals of their mechanical behavior and cell degradation. Following the lifespan of a membrane, the causes and mechanisms of mechanical degradation in the fabrication process, cell assembly process, short-term phase and long-term phase of cell operation are discussed in detail. Practical strategies for reducing the degradation rate are introduced to each process. Finally, in-situ and ex-situ methods for the evaluation and characterization of mechanical durability are summarized to pursue the measurement methods and protocols of membranes. The aim is to assess which mechanisms affect the mechanical failure of membranes and how degradation should be mitigated across the entire lifetime of fuel cells. A summary of further work in this area is also provided to give a direction to future research. • Mechanical degradation of membranes across fuel cell's lifetime are discussed. • Mitigated strategies for failure are proposed for each stage of membrane's lifespan. • A standard set of evaluation method and protocol for membrane durability is discussed. • A summary of key points and research interests is given based on our understanding. [ABSTRACT FROM AUTHOR]

Stainless steel sheets with a higher Cr content feature better corrosion resistance but lower ductility and formability, leading to the trade-off of performance and fabrication complexity/cost in many industrial scenarios, such as fuel cell bipolar plates and microreactors. To address that, thin stainless steel (SS) plates with a Cr gradient composition are fabricated with laser powder bed fusion technology in this work. Combining a Cr-rich (30 wt% Cr content) stainless steel composition with SS 316 L along the thickness direction using a gradient transition layer, the prepared gradient sheet achieves an elongation higher than 45%, which far exceeds the 19.9 ± 1.1% of the freestanding Cr-rich composition. Furthermore, detailed mechanical tests and microstructure characterizations are performed to analyze the mechanism for significant ductility improvement. The restraining function of high ductility composition promotes the multi-axial stress state in the low ductility Cr-rich composition and gradient layers, eventually activating more slip systems and dislocation entanglement. The resulting extra-hardening is accompanied by a significant increase in plastic deformation, leading to the fracture morphology with more pronounced large deformation characteristics. • Cr-rich composition elongation is improved from 19.9% to 47.6% with a gradient. • High ductility layers delay the formation of strain concentration zone. • The constraining effect of gradient promotes dislocation slip activation by grain rotation. • Ductility improvement leads to extra hardening and fracture morphology plasticization. [ABSTRACT FROM AUTHOR]

Metallic bipolar plates are important components of the proton exchange membrane fuel cell. To achieve higher performance of fuel cells, dozens of fine flow channels with the width of approximately 1 mm and a high aspect ratio of over 0.3 are required for bipolar plates. Those functional demands push the forming process of bipolar plates to the limit of material and tools, making the rupture sensitive to even a tiny deviation of the tools' geometric dimensions and material properties. To address that, a robust design method is established in this work to analyze the rupture probability by introducing the stochastic variation of material properties and tools' geometric dimensions. Utilizing the method, the rupture probability is decreased from 19.14% to 0.18% during the practical forming of metallic bipolar plates by parameters control. The yield rate is also verified to be over 99.5%. [Display omitted] • A robust design method is established for forming metallic bipolar plates. • The stochastic deviation of material properties and tool dimensions are considered. • Criteria of those factors are provided for the fracture-free stamping process design. • High-quality metallic bipolar plates are fabricated with a yield rate of over 99.5%. [ABSTRACT FROM AUTHOR]

[Display omitted] • An in-situ dynamic experiment on a 320 cm2 PEMFC is conducted. • An in-situ monitoring method and a voltage loss decoupling method is utilized. • The mechanisms of the voltage undershoot of fuel cells are thoroughly studied. • The in-plane heterogeneity of the voltage undershoot in large area PEMFC is analyzed. • Suggestions for improving the dynamic performance of fuel cells are proposed. The voltage undershoot is one of the main dynamic problems of proton exchange membrane fuel cells (PEMFCs) under dynamic loadings. However, its mechanisms remain poorly understood due to its complex and coupled influencing factors. Besides, PEMFCs with a large active area are more prone to local dynamic failures for their pronounced in-plane (I-P) heterogeneity, while few studies have focused on the I-P heterogeneity of the dynamic response of large-area fuel cells. In this work, the mechanisms and the I-P heterogeneity of the voltage undershoot of a 320 cm2 PEMFC are thoroughly studied based on an in-situ monitoring method and a voltage loss decoupling method. Results show that the voltage overshoot is primarily driven by the ohmic loss (OL) overshoot and the concentration polarization (CP) overshoot. Results also prove that the OL overshoot is mainly caused by the instantaneous water shortage in the proton exchange membrane (PEM) which depends on the initial water content in the PEM. Besides, the CP overshoot is due to the instantaneous reactant shortage at the catalyst, and it is much sensitive to the change in the oxygen diffusion coefficient of the ionomer caused by the change in its water content. Meanwhile, both OL overshoot and CP overshoot demonstrate significant I-P heterogeneities under poor dynamic operating conditions, especially in cases of high load step and low inlet air humidity. Finally, the influences of operating conditions on the voltage undershoot and its I-P heterogeneity are discussed, and suggestions for improving the dynamic performance of PEMFCs are proposed. [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]

• 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]

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]

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]

The results show that the Schottky barrier depends on the charge distribution induced by graphite. When the charge depletion and accumulation layer are mainly distributed in the space-charge region, the contact resistance is determined by the conductivity of the doped TiO 2. For example, the n-type Schottky barrier is formed between graphite and the n-type of Sb-doped TiO 2. In the contrast, if the charge rearrangement occurs in the heterojunction as a whole, the contact resistance relies on the electronic injection of graphite. For instance, the n-type Schottky barrier is formed between graphite and the p-type of Ge-doped TiO 2. We hope that the formation mechanisms and the control strategies of the contact resistance between titanium oxides and graphite can be used to develop new materials with a high conductivity. [Display omitted] • The effects of thirty-five metallic dopants on the interfacial contact resistance and electrical conductivity of TiO 2 /graphite were studied. • The effects of charge rearrangement on interfacial contact resistance of TiO 2 /graphite is revealed. The contact resistance is significantly affected by the charge rearrangement. • Graphite has a good conductivity because the Fermi-Dirac cone is not destroyed in the heterojunctions. Titanium bipolar plates have been widely employed in fuel cells, due to the high resistance achieved by the oxide film (TiO 2). However, that also results in high interfacial contact resistance between the titanium and the gas diffusion layer of graphite, reducing the cells' efficiency significantly. To improve the conductivity properties, the effects of thirty-five metallic dopants on the contact resistance and electrical conductivity were studied based on the first-principles merged with the Schottky-Mott theory and Boltzmann transport equation. The results show that the contact resistance depends on the charge distribution induced by graphite. The contact resistance depends on the conductivity when the charge depletion and accumulation layer are distributed in the space-charge region. If the charge rearrangement occurs in the heterojunction as a whole, the contact resistance is determined by the electronic injection. In addition, the single conduction path between titanium, oxygen and graphite atoms disappears in the TiO 2 doped with Zn, Cd, etc., because those dopants have stronger electron affinity in the heterojunction. While a stronger built-in electric field is formed that can further enhance the electronic conductivity. [ABSTRACT FROM AUTHOR]

Stainless steel bipolar plates possess good manufacturability, low costs, but inadequate interfacial conductivity and corrosion resistance in proton exchange membrane fuel cells (PEMFCs). Amorphous carbon films have been deposited on stainless steel as a function of bias voltage through closed field unbalanced magnetron sputtering ion plating system (CFUMSIP) to enhance the interfacial conductivity and corrosion resistance. Surface and cross-section morphologies, hybridization, interfacial conductivity, corrosion resistance and degradation mechanisms of a-C films were systemically investigated and results are very sensitive to the substrate bias voltage. The compactness and hybridization sp 2 /sp 3 ratio have parabolic relation with the substrate bias voltage and the a-C film deposited with 120 V has the densest cross structure and maximum sp 3 percentage. Various electrochemical corrosion tests in the simulated PEMFCs cathode environment confirm the fact corrosion resistance is closely related to the film compactness and hybridization. Then a-C film prepared at bias voltage 120 V has the lowest corrosion current density. The initial interfacial contact resistance (ICR) of a-C is a combination result of sp 2 percentage and film compactness and the samples deposited with bias voltage 60 V and 300 V have much lower ICR values than DOE target. Afterwards, the ICR increase mechanism of a-C film after electrochemical corrosion tests is illustrated through Raman and XPS detection, and the results reveal that increased oxygen content adsorbed on the film surface contributes to the increase of ICR value. [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]

• 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]

Abstract: The membrane electrode assembly (MEA) pressure distribution is an important factor that affects the performance of polymer membrane electrolyte fuel cell (PEMFC) stack. However, the general rules for assembly parameters that affect the MEA pressure distribution are hardly reported. In this study, a robust design analysis based on response surface methodology (RSM) was performed on a simplified fuel cell stack in order to identify the effect of assembly parameters on the MEA pressure distribution. The assembly pressure and bolt position were considered as randomly varying parameters with given probabilistic property and acted as the design variables. The max normal stress and normal stress uniformity of the MEA were determined in terms of the probabilistic design variables. The reliability of the robust design has been verified by comparing the robust solution with the optimal solution and an arbitrary solution. [Copyright &y& Elsevier]

• Methodology using composited equivalent is built to evaluate stack mechanical state. • The cells approaching to mid-stack have more uniform pressure. • Numerical results show good agreements with experimental results. • Nonlinear compression behavior of MEA is applied in high-power PEM fuel cell stack. • Equivalent model has higher computational efficiency than traditional finite element model. A high-power proton exchange membrane fuel cell (PEMFC) stack usually composes many cells, which induce high difficulty in evaluating its mechanical state of stack assembly. A methodology based on composite model and material property equivalent is developed to predict the mechanical state in PEMFC stack. In this methodology, the stack system is modeled based on a finite element model (FEM), in which bipolar plate (BPP) and membrane exchange assembly (MEA) are combined into a composite component. Experiments with stamped BPP are carried out to validate the FEM of the stack, and both the predicted clamping force and endplates deformation of FEM have a great agreement with the experimental results. Based on the methodology, it is found that more uniform pressure distribution can be generated when high-stiffness endplates are applied and cell number of the stack increases. The cells approaching to mid-stack have more uniform pressure. The pressure distribution of the stack is very sensitive to the compression ratio. High compression ratios leads to large endplate deformation, which increases the average pressure deviation between simulated value and design value, and also increases non-uniformity of pressure distribution. This methodology offers the possibility of evaluating the mechanical state of high-power fuel cell stack and greatly improves the computational efficiency. [ABSTRACT FROM AUTHOR]