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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.

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]

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]

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]

• 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]

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]

The effect of ionomer/carbon (I/C) ratio on proton exchange membrane (PEM) fuel cell cold start is investigated experimentally with theoretical water transport analysis. The scanning electron microscope (SEM) images show larger agglomerates and smaller effective reaction area by increasing the I/C ratio from 0.7 to 1.7. For normal operation, increasing the I/C ratio can improve the humidity tolerance, especially in the cathode. For cold start >−10 °C, a lower I/C ratio leads to better performance because the core reaction area is shifted towards the membrane, leading to more membrane water absorption and slower ice formation. For <−15 °C, the total water production is low and almost the same for the different I/C ratios because the ice formation takes place before effective membrane water absorption; and although the cathode catalyst layer (CL) and micro-porous layer (MPL) can provide sufficient space to store all the ice, higher I/C ratios (e.g. 1.2) still cause more ice formation in GDL and flow channel because the core reaction area becomes closer to GDL. The results show that the CL design has significant effect on the cold start performance, and there is a potential for further improvement. [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]

• 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]

• 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]

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]

• 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]