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A compact cell structure is highly desirable for next-generation high-power-density proton exchange membrane (PEM) fuel cell (>9.0 kW L−1). A novel integrated bipolar plate (BP) – gas diffusion layer (GDL) structure utilizing porous material is a promising configuration, which not only improves output power due to the reduced transport resistance but also reduces cell thickness. Herein, we propose a structured mesh material with gradient surface wettability for this cell structure, which is characterized by an ordered skeleton in the three-dimensional (3D) space with high structure design flexibility. It is experimentally found that the structured mesh material shows higher performance than serpentine and parallel flow fields. Meanwhile, 3D modeling work on an automobile fuel cell (cell area: 245.76 cm2) with the distribution zones of dot matrix structure for hydrogen, air and coolant flow, is conducted, and it is found that the structured mesh material effectively decreases the concentration and electric ohmic losses in comparison with the traditional "BP/flow field + GDL" structure, improving power density with uniform distribution characteristics. Moreover, the gradient wettability on the mesh surface helps promote the water detach the GDL and hold the residual liquid water in the top region, optimizing the water management. [ABSTRACT FROM AUTHOR]

Next‐generation ultrahigh power density proton exchange membrane fuel cells rely not only on high‐performance membrane electrode assembly (MEA) but also on an optimal cell structure. To this end, this work comprehensively investigates the cell performance under various structures, and it is revealed that there is unexploited performance improvement in structure design because its positive effect enhancing gas supply is often inhibited by worse proton/electron conduction. Utilizing fine channel/rib or the porous flow field is feasible to eliminate the gas diffusion layer (GDL) and hence increase the power density significantly due to the decrease of cell thickness and gas/electron transfer resistances. The cell structure combining fine channel/rib, GDL elimination and double‐cell structure is believed to increase the power density from 4.4 to 6.52 kW L−1 with the existing MEA, showing nearly equal importance with the new MEA development in achieving the target of 9.0 kW L−1. [ABSTRACT FROM AUTHOR]

The internal water and thermal state and performance of an air-cooled proton exchange membrane (PEM) fuel cell stack are strongly determined by the fan arrangement and operational characteristics. This study presents an innovative method to investigate the influence of fan on stack performance through integration of fan's full morphology, including the fan blade, grill between the fan and stack, etc. into a three-dimensional (3D) multiphase stack model. The multiple reference frame (MRF) model is implemented to simulate the air flow field caused by the rotation of fan and the multiple transfers coupled with electrochemical reactions is considered in the 3D stack model in detail. Through this integrated model, the influence of the air supply mode, fan number and duty ratio on stack performance and the distribution characteristics are investigated in detail. It is found that the air flowing into each cell in the fan-cooled stack is highly non-uniform, which increases the variations of oxygen concentration, temperature, and voltage in the stack. In comparison with the blowing mode, the suction mode helps to promote the uniformity of air supply with a lower air flow velocity. [ABSTRACT FROM AUTHOR]

• A novel airfoil flow field is proposed for fuel cells. • Airfoil flow field can enhance mass transfer while maintain low pressure drop. • Fuel cell with airfoil flow field is evaluated via experimental test and simulation. • Airfoil flow field increases the fuel cell's maximum power density effectively. • Airfoil flow field promotes the uniform distributions in fuel cells. Optimal configuration of bipolar plate (BP) embedding flow field is of great significance in improving the power density and durability of proton exchange membrane fuel cells (PEMFCs). Inspired by the airfoil in aircraft dividing the airflow into two streams causing pressure difference, a novel airfoil flow field (AFF) is designed to enhance the mass transfer capacity in channel and across the rib between two adjacent channels, which hardly increases the pressure drop in channel, unlike many baffle designs. The performance improvement achieved by implementing AFF in PEMFC is compared with common parallel flow field (PFF) and the wave flow field (WFF) by experimental test with respect to polarization curve and electrochemical loss and numerical simulation via a three-dimensional (3D) PEMFC model. The experimental results indicate that the novel AFF exhibits best fuel cell performance at various operation conditions (e.g. relative humidity, air stoichiometric ratio) mainly benefitted from the enhanced oxygen transfer, which contributes to the improvement of maximum power density by 8.85 % and 14.37 % compared to PFF and WFF, respectively. The modeling results indicate that the pressure drop increase in AFF is negligible compared with PFF, and is even slightly lower than WFF, indicating high practicability. Moreover, the utilization of AFF also helps the uniformity of oxygen concentration, current density and temperature in PEMFC. [ABSTRACT FROM AUTHOR]

Segmented proton exchange membrane (PEM) fuel cells are widely implemented in measuring their internal spatially distributed variables (e.g. current density and temperature). However, the difference between the segmented and ordinary fuel cells in spatial distribution characteristics has not been fully understood. In this work, we studied the spatial distribution characteristics in segmented fuel cells (cell area: 366.6 cm2) utilizing a three-dimensional (3D) large-scale fuel cell model that considers the full morphologies of metallic bipolar plates (BPs) and dot-matrix hydrogen, air and coolant flow distribution zones, including the current density, temperature, water content, etc., and compared with those in ordinary fuel cells. It is found that placing a segmented print circuit board (PCB) next to the bipolar plate (BP) component has little influence on fuel cell performance and internal distribution characteristics, whereas the measured current density distribution in the PCB still has inconsistency with that in the ordinary fuel cell, indicating that the influence of segmentation on electron conduction cannot be neglected. In the fuel cell with segmented BP and PCB, the fuel cell performance decreases significantly mainly due to increased electrical and ionic ohmic loss. In addition, the BP's segmentation significantly influences on the water and thermal state inside the whole fuel cell, leading to inconsistency in the measured temperature, relative humidity (RH) and current density distributions with ordinary fuel cells. [ABSTRACT FROM AUTHOR]

It is well-known that flow field design is of primary importance to optimization of proton exchange membrane (PEM) fuel cell. Traditional channel-rib flow fields, e.g. parallel or serpentine channels, always lead to non-uniform distributions of reactant gas, liquid, current density and so on between the channel and rib regions. Metal foam materials with high porosity (>90%) have been proposed as alternative flow fields for PEM fuel cells. In this study, influences of metal foam flow field on the transport phenomena coupled with the electrochemical reactions in PEM fuel cell are investigated using a three-dimensional (3D) multi-phase non-isothermal model. Specifically, the full morphology of metal foam flow field is taken into account in the 3D simulation after validated against experimental permeability data. The full morphology inclusion enables capture of the detailed gas flow from the flow field into the gas diffusion layer (GDL) and the current collection at the metal foam/GDL interface. In addition, compared with the conventional channel-rib flow fields, the metal foam design greatly increases fuel cell performance in the high current density regime. In addition, the oxygen and current density distributions in PEM fuel cell with the metal foam flow field are more uniform than those in the conventional one. Though the current collection area at the GDL surface is much smaller in the metal foam flow field, the relevant Ohmic loss won't increase significantly due to the improved physical contact by the fine pore structure of metal foam over the GDL. Image 1 • Full morphology of a foam flow field is included in 3D PEM fuel cell simulation. • Using metal foam flow field improves oxygen and current density distributions. • Metal foam flow field enhances gas flow towards CL due to the solid ligaments. • Small-pore metal foams help reduce the Ohmic loss at the GDL/metal foam interface. [ABSTRACT FROM AUTHOR]

Flow field design and optimization are critical for the proton exchange membrane (PEM) fuel cells to meet the requirement of ultra-high power density commercial applications. However, the multi-physics transport mechanism inside such large-scale PEM fuel cells has not been fully understood. In this study, we have developed a three-dimensional (3D) PEM fuel cell model. It is first validated against the experimental data obtained from a small-size cell. Thereafter, based on this model, the performance characteristics of a large fuel cell (cell area: 245.76 cm2) assembled with a partially narrow flow field are elucidated, while the full bipolar plate (BP) and cell morphology are also considered. It is found that the partially narrow flow field can not only significantly increase the net power density of PEM fuel cells, but also effectively improve the uniform distribution of species with the staggered partially narrow (SPN) zone arrangement between adjacent channels. While this flow field can improve the cell performance, the blindly increasing of the narrow zone numbers may play a negative impact on the net power density due to the dramatic increase in pressure drop. Finally, the partially narrow channel can indeed provide improved drainage capacity. • Liquid water cooling is considered in the large-scale 3D model of PEM fuel cell. • Comprehensive comparison between the partially narrow and parallel flow fields. • The partially narrow channel has the advantage of enhancing oxygen convection. • The drainage mechanism of the partially narrow channel is clearly presented. [ABSTRACT FROM AUTHOR]

• The full-morphology simulation of the large-scale automobile PEM fuel cell is proposed. • All components, the realistic morphology of distribution area and the coolant flow is fully considered. • The advantages and disadvantages and its influence on physical and chemical processes of double-cell structure are deeply analyzed. • Novel double-cell structure can significantly improve power density (almost 20%) under actual operating conditions. The structural design of PEM fuel cell is crucial to improving its power density, and the cell structure largely depends on the bipolar plate (BP). This study compares the conventional single-cell structure and double-cell structure through three-dimensional (3D) large-scale (cell area: 312 cm2) full-morphology simulation. As for the double-cell structure, the channels are arranged in a dislocation manner and there is one cooling flow field per two cells. For the structure of the BP, we also fully considered the realistic morphology of distribution area and the coolant flow. It is found that the heat dissipation effect of double-cell structure is worse but the performance is very close compared to single-cell structure. Moreover, its volumetric power density is significantly improved (∼ 20%) due to the reduction in height. It is also found that increasing the velocity of coolant in the double-cell structure increases the performance first and then decreases. [ABSTRACT FROM AUTHOR]

Highlights • Comfort index is proposed to evaluate water and thermal characteristics. • Comfort index is helpful to improve the water and thermal management. • Cathode water flooding and anode membrane drying are determined quantitatively. • Various operation conditions and design parameters are studied by comfort index. Abstract Water and thermal management is of great importance to proton exchange membrane fuel cell. A comfort index is proposed to comprehensively evaluate the water and thermal characteristics in proton exchange membrane fuel cell. It refers to the concept of comfort index in meteorology and selects the cathode liquid water accumulation and anode membrane drying as two basic factors so as to prevent water flooding and low proton conductivity simultaneously. The anode and cathode comfort degrees corresponding to the anode membrane drying and cathode liquid water accumulation, respectively, are quantified and fitted at various operation conditions, of which the parameters in the calculation process are determined utilizing a carefully validated quasi-two-dimensional model. The influences of cathode stoichiometric ratio, operating pressure, coolant temperature difference, cathode relative humidity, coolant temperature, anode recirculating ratio, current density and membrane thickness at wide-range temperature are studied in detail by the comfort index. It is found that the comfort index usually first increases and then decreases with the increment of temperature, because the low water saturation pressure at low temperature makes the water condensation easier and thus leads to water flooding. Compared to analytical simulation models, the comfort index is able to instantly obtain the quantified water and thermal characteristics in proton exchange membrane fuel cell, which is of great significance to the stack design and in-situ system controlling in practical design. [ABSTRACT FROM AUTHOR]

Highlights • Large-scale multi-phase simulations of PEMFC are conducted. • Realistic flow fields with distribution zone next to inlet and outlet are designed. • Effects of surface tension, wall adhesion and gravity in flow fields are included. • Counter-flow is more helpful to PEMFC compared to co-flow arrangement. • Coolant flow direction is suggested to be the same with air under counter-flow. Abstract Limited by the computational efficiency and stability, traditional 3D (three-dimensional) CFD (computational fluid dynamics) simulations of PEMFC (proton exchange membrane fuel cell) are always in single-channel scale, which neglect the realistic flow field structures in commercial PEMFC. In this study, a large-scale PEMFC (109.93 cm2), which is a repeated unit in commercial stacks and includes realistic anode and cathode flow fields, is investigated in detail utilizing a comprehensive 3D multi-phase model. In particular, the Eulerian-Eulerian model is chosen for the solution of gas and liquid two-phase flow in flow fields and the surface tension, wall adhesion, drag force and gravity are all taken into consideration. The gas concentration and liquid water amount in each channel of flow field are studied to test the influence of flow field. Moreover, it is proved that increasing operating pressure is helpful to improve PEMFC performance by increasing the reactant gas concentration and membrane water content significantly. Besides, counter-flow arrangement of hydrogen and air facilitates uniform distribution of membrane content and electrochemical reaction. And in this case, the coolant flow direction designed to be the same with that of air is beneficial to PEMFC performance. [ABSTRACT FROM AUTHOR]

Summary: A three‐dimensional (3D) multi‐phase numerical model of proton exchange membrane fuel cell (PEMFC) is built. The catalyst layer (CL) spherical agglomerate model is used to replace traditional homogenous model, which can predict the concentration loss in PEMFC more accurately. Utilizing this multi‐phase model, the PEMFC with 3D fine mesh flow field is investigated at length, and the liquid water distribution in 3D flow field is qualitatively compared with the experimental image in previous literature. It is found that the 3D fine mesh flow field can improve the reactant gas supply from flow field to porous electrodes significantly and facilitate liquid water removal in PEMFC simultaneously. Therefore, it reduces the concentration loss of PEMFC effectively without increasing the pumping power loss thanks to the greatly increased mass transfer area between gas diffusion layer (GDL) and flow field and vertical flow design of hydrogen and air, which also make the reaction rate distribution in CL more uniform. However, the decreased contact area between GDL and bipolar plate in 3D flow field may decrease PEMFC performance at the current densities where ohmic loss is dominated, but its effect is insignificant. A comprehensive three‐dimensional (3D) multi‐phase computational fluid dynamics numerical model is developed, in which the spherical agglomerate model of catalyst layer is introduced to better reflect the concentration loss in proton exchange membrane fuel cell (PEMFC). Utilizing this model, the PEMFC with 3D fine mesh flow field is simulated in detail, and the liquid water distribution in 3D fine mesh flow field is qualitatively compared with the experimental image in previous literature. [ABSTRACT FROM AUTHOR]

Highlights • A 3D multi-phase model of PEMFC coupling E-E model and two-fluid model is developed. • The water condensation and evaporation in PEMFC channels is taken into account. • The gravity force in the whole PEMFC is also incorporated in this model. • Partial low temperature and velocity may result in local liquid water accumulation. • The wave-like channel can improve PEMFC performance due to the enhanced convection. Abstract A 3D (three-dimensional) multi-phase model of PEMFC (proton exchange membrane fuel cell) is developed, in which the Eulerian-Eulerian model is utilized to solve the gas and liquid two-phase flow in channels, while the two-fluid model is adopted in porous electrodes. Hence, the surface tension, wall adhesion and drag force in channels are all included. Besides, the gravity effects in the whole PEMFC are also taken into consideration. It is found that the water vapor concentration in cathode channel at high inlet humidity (e.g. 1.0) will be much higher than the saturation concentration if neglecting the water vapor condensation. In addition, the liquid water condensed from vapor in channel is mainly blown to side walls, rather than only exist on the bottom surface. The effect of water condensation and evaporation in channel is found to be lower than that in porous electrodes because of the much higher gas velocity in channel. Besides, the partial low temperature is likely to cause local liquid water accumulation in both anode and cathode channels and porous electrodes. Meanwhile, the wave-like channel is found to be able to remove the liquid water effectively due to the enhanced convection effect. Meanwhile, the simulation results in this study show that the serpentine flow field is much more beneficial to the liquid water removal and reactant gas distribution than parallel flow field, which results in the much higher performance. [ABSTRACT FROM AUTHOR]

The 3D (three-dimensional) multi-phase CFD (computational fluid dynamics) model is widely utilized in optimizing water and thermal management of PEM (proton exchange membrane) fuel cell. However, a satisfactory 3D multi-phase CFD model which is able to simulate the detailed gas and liquid two-phase flow in channels and reflect its effect on performance precisely is still not developed due to the coupling difficulties and computation amount. Meanwhile, the agglomerate model of CL (catalyst layer) should also be added in 3D CFD model so as to better reflect the concentration loss and optimize CL structure in macroscopic scale. Besides, the effect of thermal management is perhaps underestimated in current 3D multi-phase CFD simulations due to the lack of coolant channel in computation domain and constant temperature boundary condition. Therefore, the 3D CFD simulations in cell and stack levels with convection boundary condition are suggested to simulate the water and thermal management more accurately. Nevertheless, with the rapid development of PEM fuel cell, current 3D CFD simulations are far from practical demand, especially at high current density and low to zero humidity and for the novel designs developed recently, such as: metal foam flow field, 3D fine mesh flow field, anode circulation etc. [ABSTRACT FROM AUTHOR]

A comprehensive 3D (three-dimensional) multiphase model of PEMFC (proton exchange membrane fuel cell) is developed, in which the gas and liquid two-phase flow in channel and porous electrodes are investigated in detail. In the simulation of gas and liquid two-phase flow in channels, the effect of surface tension, wall adhesion and gravity is taken into account, including the influence of pressure difference between the inlet and outlet on inlet reactant gas concentration; while in porous electrodes, the anisotropy of GDL (gas diffusion layer) and liquid saturation jump at the interface of two different porous layers (e.g. GDL and MPL (micro-porous layer)) are also considered in this model. It is found that the amount of liquid water in channels increases with the increment of current density. In addition, increasing the contact angle at GDL/channel interface is found to be able to improve the performance of PEMFC by facilitating the water removal process in channels. Moreover, it can be concluded that adding baffles in cathode channel not only increases the oxygen concentration in porous electrodes but also facilitates the water removal process, both of which prevent PEMFC from concentration loss effectively. [ABSTRACT FROM AUTHOR]

A three-dimensional multiphase numerical model for proton exchange membrane fuel cell (PEMFC) is developed to study the fuel cell performance and water transport properties with low external humidification. The results show that the sufficient external humidification is necessary to prevent the polymer electrolyte dehydration at low current density, while at high current density, the water produced in cathode CL is enough to humidify the polymer electrolyte instead of external humidification by flowing back and forth between the anode and cathode across the membrane. Furthermore, a steady anode circulation status without external humidification is demonstrated in this study, of which the detailed internal water transfer path is also illustrated. Additionally, it is also found that the water balance under the counter-flow arrangement is superior to co-flow at low anode external humidification. [ABSTRACT FROM AUTHOR]

• Full-scale 3D simulation for air-cooled PEM fuel cell stack was presented; • The stack polarization curve and temperature distribution were validated; • Oxygen content becomes lower at a higher temperature site in the stack; • Thermal behavior has a decisive effect on the distributions of temperature, gas, etc.; • Increasing the air flow rate benefits the uniform distribution of oxygen, water, etc. This study presents one of the first full-scale stack three-dimensional (3D) simulations for proton exchange membrane (PEM) fuel cell to investigate temperature distribution, cell/stack performance, and impact of air cooling. The stack consists of 30 cells with the active area of 50 mm × 200 mm for each fuel cell. The manifold and all the basic components are included in the computational domain, while the flow field structure is treated as porous media in order to reduce the computational burden. The model predictions are validated against the experimentally measured voltage and detailed temperature distribution under different currents. It is found that significant differences in temperature are present between the fuel cells at the two sides and that in the middle, and the oxygen content becomes lower at a higher temperature site due to the local increased water vapor concentration. As a consequence, thermal behavior has a decisive effect on the distributions of temperature, reactant concentration, membrane hydration, current density, etc., inside this air-cooled stack. It is also found that increasing the cooling air flow rate not only decreases the stack temperature, but also reduces the temperature variation, which further benefit the uniform distributions of oxygen, water vapor, etc. in the stack. [ABSTRACT FROM AUTHOR]

• Thermal analysis of fuel cells, e.g. temperature changes & time constants, are present. • Cooling methods are summarized, including those in fuel cell electric vehicles. • Ice formation and impacts, and thermal management in cold start are discussed. • Durability issues related to thermal management are reviewed. • Fundamental, model, data-driven & practical studies on thermal management are needed. Proton exchange membrane (PEM) fuel cells are a promising electrochemical energy converter with an energy efficiency as high as 60%. Thermal management plays an important role in PEM fuel cell operation. In this review, recent progress in thermal management is summarized with in-depth discussion on the waste heat generation mechanisms, thermal analysis, non-isothermal two-phase flow, cooling methods, cold starts, and relevant material properties and durability. Due to low operating temperature (~80 °C), cooling PEM fuel cell stacks is much more challenging than traditional combustion engines. Various cooling methods are discussed and compared, along with the cooling strategies implemented in fuel cell electric vehicles (FCEVs). A challenging subject is to couple the thermal and water managements, such as non-isothermal two-phase flow, heat pipe effect, and two-phase interface dynamics. Cold start, i.e. startup from subfreezing condition, is discussed with in-depth analysis of key parameters regarding ice formation/melt and cold-start capability. Degradation, such as delamination and electrochemical active surface area (ECSA) loss during thermal, humidity and freeze/thaw cycles, is also briefly reviewed. Future work is proposed to further advance thermal management for PEM fuel cells. [ABSTRACT FROM AUTHOR]

• A data-driven surrogate model is used in optimization of porous media flow field. • The surrogate model predicts similar results with a validated 3D physical model. • The optimal values obtained by the data-driven model are verified by the 3D model. • The porous media flow field's porosity significantly influences cell performance. • The Ohmic resistance increases with the porous media flow field's porosity. Flow field design plays a key role in proton exchange membrane (PEM) fuel cells due to its decisive influence on reactant gas transfer, liquid water discharge, electron conductance, and heat transfer. Porous media flow field (e.g. metal foam) shows promise to improve cell performance in the concentration polarization regime and to benefit uniform distributions of oxygen, liquid, current density, and temperature, etc. In this study, a data-driven surrogate model based on support vector machine (SVM) is applied to optimize porous media flow field geometry. The training data is obtained from a validated three-dimensional (3D), multi-phase non-isothermal model. For better characterization, the complex structure is simplified as a 3D structured mesh consisting of a set of fibers with each fiber perpendicular to each other at the joint. The results show that the data-driven surrogate model based on SVM predicts similar results as the 3D physical model. The genetic algorithm (GA) is further used for optimization, in which the data-driven surrogate model is selected as a fitness evaluation function. The optimal values obtained by the surrogate model are verified by the 3D physical model, indicating that the proposed data-driven surrogate model is effective in the design and optimization of porous media flow field. [ABSTRACT FROM AUTHOR]

In this study, a three-dimensional (3D) multi-phase non-isothermal model of proton exchange membrane (PEM) fuel cell is present to investigate the current density spatial distributions under different output voltages, temperatures, current densities and relative humidities (RH). Two sets of experimental data are selected for validation, including those from the Los Alamos National Laboratory in the United States and the University of Waterloo in Canada. Reasonable agreements are achieved between the model prediction and experimental measurements, indicative of the validity of this 3D model. In addition, it is found that under a higher RH of 50%, most electric current is produced near the cathode inlet, which is primarily due to the local availability of abundant oxygen and hence small transport polarization. Low current occurs near the outlet of the cathode air flow, as a result of oxygen consumption by the oxygen reduction reaction (ORR). Under a lower RH of 25%, the high current density region shifts to the middle of the fuel cell, which is primarily attributed to hydration of the dry membrane by water production. In our case study, the operating temperature has little impact on the current density distribution. [ABSTRACT FROM AUTHOR]

• A new cooling strategy is investigated, which simplifies stack design. • 3D multiphase simulation of a 5-cell stack with new cooling strategy was conducted. • Two conditions were studied: finite vs. infinite convective heat transfer cooling. • A maximum temperature variation of ~30 K is predicted in the stack. • The new strategy needs to increase heat transfer coefficient for stack applications. In this study, a new cooling strategy for a proton exchange membrane (PEM) fuel cell stack is investigated using a three-dimensional (3D) multiphase non-isothermal model. The new cooling strategy follows that of the Honda's Clarity design and further extends to a cooling unit every five cells in stacks. The stack consists of 5 fuel cells sharing the inlet and outlet manifolds for reactant gas flows. Each cell has 7-path serpentine flow fields with a counter-flow configuration arranged for hydrogen and air streams. The coolant flow fields are set at the two sides of the stack and are simplified as the convective heat transfer thermal boundary conditions. This study also compares two thermal boundary conditions, namely limited and infinite coolant flow rates, and their impacts on the distributions of oxygen, liquid water, current density and membrane hydration. The difference of local temperature between these two cooling conditions is as much as 6.9 K in the 5-cell stack, while it is only 1.7 K in a single cell. In addition, the increased vapor concentration at high temperature (and hence water saturation pressure) dilutes the oxygen content in the air flow, reducing local oxygen concentration. The higher temperature in the stack also causes low membrane hydration, and consequently poor cell performance and non-uniform current density distribution, as disclosed by the simulation. The work indicates the new cooling strategy can be optimized by increasing the heat transfer coefficient between the stack and coolant to mitigate local overheating and cell performance reduction. [ABSTRACT FROM AUTHOR]

• An improved agglomerate sub-model of catalyst layer is developed. • Oxygen local transport resistance is clarified with three individual parts. • Effects of Pt loading and Pt particle dispersion are considered. • The CL agglomerate sub-model is incorporated into 3D PEM fuel cell model. • Fine channel geometry helps improve gas distribution and cell performance. An improved agglomerate sub-model of catalyst layer (CL) involving actual agglomerate size and oxygen local transport characteristics is developed and incorporated into a three-dimensional (3D) multi-phase model of proton exchange membrane (PEM) fuel cell. This makes it capable to consider the effect of platinum (Pt) loading on oxygen transport and fuel cell performance more accurately. Oxygen local transport resistance near the catalyst surface is divided into three parts caused by liquid water blockage, ionomer coverage and Pt/carbon agglomeration, respectively. The resistances caused by ionomer coverage and Pt/carbon agglomeration are two major sources of oxygen local transport resistance. They have opposite variation trends as Pt loading changes. However, the ionomer resistance increases dramatically when Pt loading is lower than 0.1 mg cm−2 because of the much harder transport process through a relatively heavier ionomer coating. The simulation results agree with the experimental data reasonably under different cathode Pt loadings (from 0.3 to 0.025 mg cm−2), for both polarization curves and local transport resistance. In addition, a transport dominance parameter is defined to judge whether the concentration loss predominates the electrochemical reaction. A value greater than 10% can be seen as a symbol of local oxygen starvation. Using this model, fine channel geometry with extremely small channel and rib widths is investigated, and the highest net output power in this study is corresponding to 0.2 and 0.6 mm for channel (rib) width and height. [ABSTRACT FROM AUTHOR]

Reducing Pt catalysts amount without sacrificing the performance and durability is critical for the commercialization of proton exchange membrane fuel cells (PEMFCs). Pt degradation in the cathode catalyst layer (CCL) decreases the electrochemically active surface area (ECSA), and Pt precipitation, especially Pt band formation, has a significant effect on proton exchange membrane (PEM) stability, resulting in severe PEMFC degradation during long-term operation. In this study, a one-dimensional model is developed focusing on Pt catalyst degradation and precipitation, including Ostwald-ripening, mass loss, precipitation in the PEM from nucleation, particle size growth, to Pt band formation. The model accuracy is comprehensively validated against the corresponding experimental data, including ECSA loss and particle size distribution (PSD) evolution in the CCL under different operating conditions (temperature, relative humidity, and loading mode), as well as the precipitated Pt size distribution characteristics and local potential in the PEM. Using this model, the Pt band formation process is comprehensively analyzed, and the effects of operating conditions on Pt degradation and Pt band formation are investigated in detail. It is found that the nucleation stage is brief, and then the significant inhomogeneity at the size growth stage causes Pt band formation. In addition, the high activation enthalpy of dissolution under low humidity can effectively inhibit Pt degradation, and Ostwald-ripening is the mean cause of Pt degradation, which is likely to occur during the rapid voltage change period. [ABSTRACT FROM AUTHOR]

To meet the demands for high power density, enhancing the water discharge capacity of proton exchange membrane fuel cells (PEMFCs) is imperative. The microporous layer (MPL), placed between a catalyst layer and gas diffusion layer, plays a crucial role in water management for PEMFCs. In this study, we fabricated electrospun MPLs of different pore sizes and characterized their morphology and pore structures. The mercury intrusion test showed that the mean pore sizes of electrospun MPLs are about 1.2–2.3 μm, much larger than commercial MPL's (about 70 nm). The electrospun MPLs were assembled in single PEMFCs to test their performance and Electrochemical Impedance Spectroscopy (EIS). The results demonstrate that the electrospun MPLs with a pore size exceeding 1.9 μm significantly enhance water discharge and oxygen transport at the cathode relative humidity (RH) of 50 % and 100 %. Moreover, the EIS testing and fitting outcomes indicate that when the PEMFC performance is dominated by mass transport under high current density and 100 % cathode RH, the electrospun MPLs with a pore size exceeding 1.9 μm exhibit a better water discharge capacity and lower mass transport resistance. • Electrospun MPLs have larger pore sizes (>1 μm) than commercial ones (∼100 nm). • Electrospun MPLs of a pore size exceeding 1.9 μm enhance water discharge. • Fuel cell performance was improved at cathode relative humidity >50 %. [ABSTRACT FROM AUTHOR]

The gas diffusion layer (GDL) plays a vital role in water management of proton exchange membrane fuel cells. In this study, GDLs with nanoscale gradient pore sizes (nano-GDLs) are fabricated using electrospinning to explore the influence of the gradient layer number and degree of the pore size gradient on the water transport performance. The breakthrough pressure and water saturation tests indicate that nano-GDLs with uniform and gradient structures have higher breakthrough pressures and lower water saturation compared with those of the commercial Toray GDL, which facilitates back diffusion of water through the membrane and water discharge in the GDL. Specifically, among the nano-GDLs with a gradient structure, the three-layer medium-gradient nano-GDL sample (3-MG) possesses the maximum breakthrough pressure and minimum water saturation ratio. The fuel cell test determined that the maximum power density of the 3-MG nano-GDL sample is 14% higher than that with commercial Toray GDL without cathode humidification. • Electrospinning can fabricate gas diffusion layers with gradient pore sizes. • The nano-GDL pore size is much smaller than that of commercial Toray GDL. • The 3-MG nano-GDL has higher breakthrough pressure and lower water saturation. • The power density of 3-MG nano-GDL is 14% higher than commercial Toray GDL. [ABSTRACT FROM AUTHOR]

Ultrahigh power density is of primary importance to next-generation polymer electrolyte membrane fuel cell (PEMFC) for commercialization, which demands an in-depth insight into the intricate multi-physics transports, particularly for large-size PEMFCs in automobile application. In this study, a three-dimensional two-phase model is extended to study an automobile fuel cell with the active area of 366.6 cm2, including full morphologies of metallic bipolar plate, flow field, and distribution zones for hydrogen, air and coolant flow. It is found that the cross flow inside membrane electrode assembly under the rib between two adjacent channels promoted by the distribution zone can effectively mitigate local oxygen starvation and water flooding, increasing PEMFC performance at high current density, in comparison with conventional single-channel or no-dot-matrix models. Theoretical analysis indicates that the dot matrix distribution zone induces a larger pressure gradient and hence the cross flow than the commonly-used channel & rib distribution zones. Additionally, the counter-flow configuration has a higher cell performance and more uniform current density than the co-flow one due to internal humidification. When the internal humidification effect is weak, for example, in anode/cathode gas co-flow configuration, coolant flow direction becomes more important for the temperature profile and prevent severe dehydration. • A full-scale 366.6 cm2 automobile PEMFC model with full morphologies is presented. • Cross flow inside MEA effectively mitigates oxygen starvation and water flooding. • Dot matrix enables a stronger cross flow than that of channel & rib structure. • Counter-flow achieves uniform multi-physics transfer by internal humidification. [ABSTRACT FROM AUTHOR]

Two novel cathode channel designs (multi-plates structure channel and integrated structure channel) are proposed and investigated by a three-dimensional multiphase numerical model. The numerical results indicate that the PEMFC with 30° angle, 0.5 mm width and 6.0 mm distance of air-guide plates exhibits the best performance, and the effect of plates angle on PEMFC performance is most significant. Compared with the conventional channel, the novel channel designs are found to be capable of forcing more oxygen towards cathode catalyst layer (CCL) to improve the electrochemical reaction rate. In addition, these two novel channel designs are able to remove more liquid water from PEMFC to effectively avoid from water flooding. According to the simulation results, the maximum improvement of PEMFC net power densities for the multi-plates structure and integrated structure are 4.7% and 7.5%, respectively. [ABSTRACT FROM AUTHOR]

• A comprehensive validation for three-dimensional simulation is implemented. • The combined influence of ohmic and concentration voltage losses is analyzed. • The validation methodology is clarified with details. • The liquid water in gas channel has a double effect on cell performance. For modeling and simulation of proton exchange membrane (PEM) fuel cell, validation has been an essential and challenging task. This study implements a comprehensive validation including both overall cell performance and local distribution characteristics under different operating conditions with experimental data from two public sources. Polarization curve, cell ohmic resistance, current density distribution and temperature distribution are all involved. A "three dimensional + one dimensional" ("3D+1D") model is adopted which simplifies part of cell components in order to boost the calculation efficiency. The validation methodology is clarified by listing those undetermined model parameters and analyzing their "accessibility" as well as correlations with the three kinds of voltage losses (activation, ohmic and mass transfer). It is found that the control regions of ohmic voltage loss and concentration voltage loss overlap among a wide current density range, which may lead to misjudgment in the validation process. The details of parameter adjustment are also shared. Simulation results of the two validation tests both obtain decent agreement with the experiments and reflect consistent variation trends as the condition changes. The liquid water in gas channel is proved to have a double effect on cell performance and should be taken into careful consideration especially under low humidification and high current density working conditions. [ABSTRACT FROM AUTHOR]

[Display omitted] • Electrochemical active surface area is affected by humidity and temperature. • A fitting equation of electrochemical active surface area has been proposed. • The equation can be universally applied in different catalyst layer compositions. • Validations are completed on data measured in low humidity operations. • Including the equation in models helps predict more accurate cell performances. The electrochemical active surface area has a vital influence on the performance of proton exchange membrane fuel cells. In this study, a universal semi-empirical equation of the electrochemical active surface area in the catalyst layer of fuel cells is proposed based on self-designed experiments, aiming to improve the predictive accuracy of numerical models under low humidity conditions. The generalization of this equation is also verified with the other nine catalyst layers of different compositions. At very low water content corresponding to low inlet humidification operations, the electrochemical active surface area is generally low and increases sharply with the increment of water content. When the water content exceeds a certain threshold value, the electrochemical active surface area becomes independent of water content. In addition, the electrochemical active surface area significantly decreases as the operating temperature increases. Consequently, including this newly proposed electrochemical active surface area equation in conventional full-cell models is of significant importance to accurately predict the activation loss and performance of fuel cells under both cold-start and normal operation conditions, especially at low humidification, which is validated against experimental data. In contrast with the conventional constant electrochemical active surface area assumption, this equation improves the model accuracy from about 70% to be higher than 90% under low humidity conditions. [ABSTRACT FROM AUTHOR]

In this study, the use of metal foam as a flow distributor at cathode is evaluated numerically by a comprehensive three-dimensional solid oxide fuel cell (SOFC) model. The results show that the adoption of metal foam improves the power density by 13.74% at current density of 5000 A m−2 in comparison with conventional straight channel design. It is found that electronic overpotential, oxygen concentration and reaction rates distribute more uniformly without the restriction of ribs. The effects of cathode thickness on the two different flow distributors are compared. Compared with conventional straight channel, the metal foam is found to be more suitable as a distributor for anode supported SOFC with thin cathode gas diffusion layer. Moreover, when metal foam is applied to the fuel cell with a larger reaction area, a more uniform velocity distribution and a lower temperature distribution can be achieved. It is also found that an appropriate permeability coefficient should offer a reasonable pressure drop, which is beneficial for the fuel cell system performance improvement. • Metal foam is compared with straight channel for SOFC cathode. • Using metal foam can decrease concentration loss and ohmic loss. • Metal foam is more suitable with thin cathode gas diffusion layer. • Effect of permeability on metal foam is revealed. • Metal foam offers more uniform oxygen distribution and lower temperature distribution. [ABSTRACT FROM AUTHOR]

The effect of micro-porous layer (MPL) on proton exchange membrane (PEM) fuel cell cold start is investigated experimentally with theoretical analysis. Under normal condition, the anode and cathode MPLs can improve the independency on inlet gas humidification on the corresponding side. For potentiostatic startup, the humidity independence enhanced by anode MPL improves the cell performance at around −7 °C. Cathode MPL can promote the water back diffusion and hinder the ice formation on the catalyst layer (CL) surface at low temperature (e.g. −10 °C). Besides, anode MPL also contributes to reducing the blockage tendency at very low temperature (e.g. −15 °C or lower). For galvanostatic startup, water electrolysis and carbon corrosion may occur which leads to the voltage reversal when the cell cannot achieve the desired current density, which may produce more waste heat. Low water content will aggravate the voltage reversal and carbon corrosion. [ABSTRACT FROM AUTHOR]

[Display omitted] • Method of coupling FEM model with "3D + 1D" model for large-scale PEMFC is proposed. • Non-uniform contact resistance distribution is investigated utilizing this method. • Performance reduces slightly when considering non-uniform contact resistance. • Current density gets more uneven when considering non-uniform contact resistance. • Deformed GDL under assembly affects both concentration loss and ohmic loss. The assembly of proton exchange membrane (PEM) fuel cell has a significant influence on contact pressure of BP and GDL, which affects key parameters of GDL including contact resistance, porosity, and permeability. In this study, a large-scale finite element method (FEM) PEM fuel cell model is developed considering realistic assembly, which is assembled by 14 bolts. The contact pressure at the interface of GDL and BP is converted into non-uniform contact resistance incorporated into a self-developed "3D + 1D" full PEM fuel cell multi-phase model with the active area of 108 cm2. The "3D + 1D" model simplifies part of components along through-plane direction into 1D model to improve calculation efficiency and stability. The results show that the performance reduces slightly when considering non-uniform contact resistance compared with that considering uniform contact resistance. The uniformity of current density and temperature distributions reduces evidently when considering non-uniform contact resistance affecting the performance and durability, which demonstrates the necessity of treatment of non-uniform contact resistance. Furthermore, the effects of porosity and permeability of deformed GDL under different preloading torques on performance are investigated. It is found that the contact resistance reduces first and then flats as the preloading torques increase, which results in the same trend of electronic ohmic loss, while the concentration loss nearly linearly increases. The simulation shows that the PEM fuel cell shows the best performance when the preloading torque is 3 Nm. This study provides some meaningful guidance for PEM fuel cell stack assembly. [ABSTRACT FROM AUTHOR]

• A self-adaptive water transition mechanism in catalyst layer is proposed. • Data from commercial-level laboratory is comprehensively validated. • On-board various operating conditions are considered. • Influence of simulation scale is investigated. It is of great significance to gain deeper and clearer understanding of the transport mechanism inside proton exchange membrane (PEM) fuel cell under on-board conditions to propel its commercialization process. This study investigates detailed water transition mechanism in PEM fuel cell catalyst layer from the perspective of macroscale performance model. Full layout of single cell structure and variable operating conditions are considered. Six combinations of water transition mechanism are analyzed and a self-adaptive mechanism related with local vapor saturation state is proposed, which is determined and validated by comparing the simulation results with experimental data from commercial-level laboratory. The influence of simulation scale is also investigated by comparing calculation results of typical single-channel domain with practical single-cell domain. Results show that appropriate water transition mechanism equips the model with decent adaptability to multi-condition prediction. Single-channel simulation domain tends to gain misjudgment on water removal capability and could be used for preliminary evaluation. Full-scale single-cell simulation domain should be the first choice for structure designing work, especially under practical working condition. The proposed method serves as a potential solution to multi-condition simulation with good adaptability and fidelity, which is one of the urgent requirements for PEM fuel cell R&D. [ABSTRACT FROM AUTHOR]

• Feasibility of MF in AAEMFC is investigated by numerical study for the first time. • The model is validated with experimental data from both the literature and this study. • The MF improves the performance significantly, especially at high current density. • Water transport and distribution are analyzed to explain the performance improvement. • MF prevents the AAEMFC from flooding in anode and drying out in cathode. Metal foam (MF) material is a promising alternative in fuel cell for its extremely porous structure, high electrical conductivity, controllable permeability and strong mechanical strength. However, there are seldom applications of MF flow field in alkaline anion exchange membrane (AAEM) fuel cell so far. Therefore, a three-dimensional (3D) multi-phase numerical model is implemented to investigate the feasibility of MF flow field in AAEM fuel cell and validated with experimental data from both the literature and this study. The performance of AAEM fuel cell with MF flow field is compared with that with traditional serpentine flow field. The simulation results show that MF flow field is able to improve the performance of AAEM fuel cell significantly, especially at higher current density where the concentration loss is dominant. According to analysis of transports and distributions of reactants and water (including liquid water, membrane water, and water vapor), the MF flow field is proven to be beneficial to membrane hydration, anode water removal, cathode water utilization, and reactant distribution. [ABSTRACT FROM AUTHOR]

A novel dot matrix and sloping baffle flow field plate for proton exchange membrane fuel cell (PEMFC) cathode is designed. The plate consists of dispersive and arrayed blocks with sloping angles as shoulders. Features of the plate include a large fluid domain, and air guidance in two directions is achieved by sloping sides of the block. The cell output performance, internal transport process and liquid water removal process of the PEMFC with the matrix flow field are numerically investigated by three-dimensional two-phase full cell model and volume of fluid (VOF) model. The simulation results show that compared with the parallel and serpentine flow field, the matrix flow field can achieve high cell output performance by both improving oxygen supply to gas diffusion layer (GDL) and uniform distribution. Comparing five matrix flow fields with different block sizes and numbers shows that adequate contact area between the plate and GDL for current conductor is critical. For liquid water removal process in the matrix flow field, liquid water is hardly blocked by arrayed blocks and can leave GDL quickly. In summary, the matrix flow field fits high current density demand of PEMFC well, and some new perspectives on flow field design are presented. • A novel dot matrix and sloping baffle cathode flow field of PEMFC is designed. • The features of the plate include a large fluid domain and air guidance. • The matrix flow field is evaluated by full cell model and VOF model. • The matrix flow field fits high current density demand of PEMFC well. • Adequate contact area between the plate and GDL for current conductor is critical. [ABSTRACT FROM AUTHOR]