•I/C ratio effect is experimentally studied with theoretical analysis for PEMFC.•Increasing I/C ratio enhances humidity tolerance for normal operation.•For >−10 °C, properly decreasing I/C ratio enhances membrane water absorption.•For <−15 °C, total water production is low and similar for various I/C ratios.•For <−15 °C, increasing I/C ratio may lead to more water removal to flow channel.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.

•A multiphase model for PEMFC cold start is developed considering MPL effect.•Effect of MPL hydrophobicity is more significant for startup from −10 °C or higher.•MPL effect is weak for startup from −15 °C or lower because less liquid presents.•MPL hydrophobicity needs to be designed according to CL and GDL properties.•PEMFC with thinner membrane is more sensitive to MPL hydrophobicity change.A transient multiphase model for cold start process is developed considering micro-porous layer (MPL), super-cooled water freezing mechanism and ice formation in cathode channel. The effect of MPL's hydrophobicity on the output performance and ice/water distribution is investigated under various startup temperatures, structural properties, membrane thicknesses and surrounding heat transfer coefficients. Under the maximum power startup mode, it is found that the hydrophobicity disparity of MPL has negligible influences when started from −15 °C, but it strongly affects the overall performance when started from −10 °C, especially after the cell survives the cold start. Decreasing the MPL's hydrophobicity leads to higher current density, meanwhile, it facilitates the super-cooled water's removal, which in turn reduces the ice formation in catalyst layer. However, excessive water accumulation happens if the generated water is hindered from getting into gas diffusion layer (GDL) due to the significant hydrophobicity gap. Weakening the GDL's hydrophobicity contributes to the water removal since the generated water is easier to diffuse out. A thinner membrane benefits the cold start owing to the reduction of ohmic loss and improvement of membrane hydration, and is more sensitive to the hydrophobicity of MPL. Ice formation in cathode channel is identified under various surrounding heat transfer coefficients.

•PEMFC with low humidity and high current density is studied by numerical simulation.•At high current density, water production lowers external humidification requirement.•A steady anode circulation status without external humidification is demonstrated.•The corresponding detailed internal water transfer path in the PEMFC is illustrated.•Counter-flow is superior to co-flow at low anode external humidification.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.

•PEM fuel cell cold start tests compare metal foam (MF) and parallel flow fields.•MF produces higher max power than parallel considering pumping loss.•MF produces slightly lower power than parallel at low current operation.•Galvanostatic MF cold start is better due to improved ice storage and gas flow.•Ice blockage occurs in both MF and parallel channel in potentiostatic mode.Metal foam has been regarded as one of the most important replacement for the conventional flow distributor of commercial fuel cells in recent years. One critical issue for the commercialization of proton exchange membrane (PEM) fuel cell is the successful startup from subzero temperatures. In this study, experimental tests on a PEM fuel cell using nickel metal foam as the cathode flow distributor are carried out to investigate the cold start performance. The cold start performance is also compared to a PEM fuel cell with parallel flow channels. Both galvanostatic and potentiostatic control are considered. The results show that under normal operating conditions the metal foam PEM fuel cell exhibits higher maximum net power density than the cell with parallel flow channels, whereas the parallel channel case exhibits slightly better performance at lower current densities. For cold start tests, metal foam is superior to the conventional parallel flow channel under galvanostatic control, due to its extremely porous structure, uniform mass and heat distribution. It is more difficult for PEM fuel cell under potentiostatic control to successfully start up due to possible ice blockage at the outlet.

•DNS for diesel injection is conducted with real turbulence inlet profile.•Jet breakup differs from uniform velocities and sinusoidal perturbation injection.•Droplets are detected from long-narrow, flat and curly ligaments along flow.•Surface wave motion, entrainment and shear stress result in jet fragment.•Effects of liquid-gas density ratio and nozzle size are analyzed.Fuel atomization which begins with turbulence perturbation has a significant influence on engine performance. The effect of turbulence on near nozzle field characteristics under various liquid-gas density ratios is performed in present models. First, using three-dimensional DNS (3-D Direct Numerical Simulation) method, a time-varying single-phase fully developed turbulent pipe flow is generated to store time-varying outlet velocity databases which are then mapped as the two-phase jet inlet velocities. After that, the characteristics of near nozzle diesel jet evolution are studied by solving the two-phase turbulent flows and tracking the two-phase interfaces based on DNS and VOF (Volume of Fluid) methods. It shows that with fully developed turbulent inject velocities, the topology of the jet evolves gradually from violent wavy surface, ligaments, to droplet parcels with different dimensions and shapes, which are considered more realistic and different from uniform/sinusoidal perturbation inject velocities, especially in the near nozzle field. Jets surface distortion, stretched and sheared interface are captured. Three different separation processes in different downstream positions are detected. In the initial stage of the jet, long and narrow ligaments act as primary breakup. In the later stage, the whole breakup process evolves more disorderly and more intensively, so that flat and curly ligaments are observed. Also, higher gas densities and narrower nozzle sizes can accelerate the jet break-up process. Furthermore, the present work is reasonably compared with experimental and numerical modeling results.Download high-res image (92KB)Download full-size image

•An analytical model for hydrogen AEMFC is formulated with saturation jump.•More hydrophilic cathode CL improves cell performance by retaining more water.•Proper liquid water injection into cathode further improves the cell performance.•Anode MPL is generally useful for the AEMFC, by promoting water back-diffusion.•Use of MPL needs to fit different humidification strategies.Alkaline anion exchange membrane fuel cell (AEMFC) has been recognized as a promising zero-emission power source for portable, mobile and stationary application in recent years. To ensure high ionic conductivity and efficient reactants delivery, water management is regarded as one of the most critical issues for AEMFC. In this study, an analytical model is formulated to investigate the effect of electrode wettability on the water transport and resultant AEMFC performance. The pressure continuity method is considered to simulate liquid saturation jump on the interfaces of adjacent electrode layers. The results show that decreasing the cathode catalyst layer (CL) contact angle improves the performance because more water can be kept in the cathode CL decreasing polarization losses. The anode micro porous layer (MPL) is generally helpful, by forcing the liquid water to back-diffuse to the cathode. However, cathode MPL hinders the water transport to the cathode CL, leading to a lower reaction rate and membrane conductivity. The liquid water injection into the cathode has great potential to further improve the performance of AEMFC, however it may cause flooding in the flow channel and GDL. The cathode reaction kinetics should be considered as one of the most significant factors dragging the cell performance.

•A 3D electrochemical-thermal coupling model of LiPeO4 battery is developed.•Core reaction area changes differently on both sides in constant current mode.•Current density reduces faster for constant voltage than that for overpotential.•Thick electrode has positive effect in maximum output power discharge mode.A three-dimensional electrochemical-thermal coupling model of LiFePO4 battery was developed based on the real multi-layer structure. The model was developed to describe the discharge process of the battery in four modes: the constant current, constant overpotential, constant voltage and maximum power discharge processes. The electric conductivity of the electrode, the electric conductivity of the current collector, the ionic conductivity of the electrolyte and the diffusion coefficient of the electrolyte were considered. The results show that: (1) In the constant current discharge mode, the core electrochemical reaction area of the positive electrode moved towards the separator while that of the negative electrode moved towards the current collector due to the change of the electrolyte concentration. (2) In the constant overpotential discharge process, the change of the average chemical reaction rate and the decreasing rate of the current density increased with increasing overpotential. (3) In the constant voltage discharge process, the current density and the temperature increased when the electrode thickness was increased. (4) In the maximum power discharge process, the output power increased with increasing electrode thickness but decreased with increasing contact resistance at the same current density.

•A comprehensive 3D multiphase anisotropic model of PEMFC is developed.•Effects of surface tension, wall adhesion and gravity in channel are included.•Anisotropy of GDL and liquid saturation jump are taken into account in this model.•Contact angle at GDL/channel interface can affect the performance of PEMFC.•Adding baffles in channel can prevent PEMFC from concentration loss effectively.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.

•A HT-PEMFC dynamic start-up model is developed.•Both constant voltage and constant current density start-up methods are investigated.•There is liquid water generated during the start-up process.•A novel variable start-up voltage strategy is proposed and proved.In this study, a three-dimensional dynamic multi-phase model is developed for the high temperature proton exchange membrane fuel cell (HT-PEMFC) to study its start-up processes. Two different kinds of start-up methods which are constant voltage and constant current density are investigated and compared with each other under the same conditions. It is found that the constant voltage mode can make HT-PEMFC reach its normal operating temperature faster than the constant current density. During both the constant voltage and constant current density conditions, there is liquid water produced in the catalyst layer of the cathode zone because the cell temperature is under 100 °C. The effects of surrounding environment are also analyzed and investigated. It is found that the heat convection coefficient has an important influence on the HT-PEMFC start-up process. The final cell temperatures under different conditions are obtained and the start-up durations are compared. To shorten the start-up time, a new non-heating method, variable voltage start-up strategy, is proposed and compared with the traditional assisting heating method. It is shown that the HT-PEMFC start-up performance is improved significantly by using this new variable voltage start-up strategy.

•A transient model for passive DMFC is developed to study cell orientation effect.•Vertical orientation has higher energy density than horizontal except high current.•Roles of AMPL and CMPL are different with different cell orientations.•Horizontal orientation is more sensitive to methanol crossover.A transient model for passive direct methanol fuel cell (DMFC) is developed to investigate the effect of cell orientation and operating condition. The results show that the passive DMFC with vertical orientation has better performance than the horizontal one, except the case of high current density, because a large amount of water produced in cathode is hard to be removed in vertical orientation, which is easier for horizontal orientation due to gravity. The passive DMFC with horizontal orientation is sensitive to methanol crossover, and moderate current density or voltage is necessary to ensure high energy efficiency. The anode micro-porous layer (MPL) plays an important role in reducing the rate of methanol crossover by providing flow resistance. The MPL in cathode has a significant effect on water transport by enhancing the water back-flow from cathode to anode, which prevents water removal. Therefore, the anode MPL and cathode MPL have different effects on horizontal orientation and vertical orientation. Additionally, the size of fuel tank can improve the energy density by providing more fuel, and the effect on fuel efficiency and energy efficiency is a bit obvious in vertical orientation than horizontal orientation.

•A turbulent two-phase DNS model is developed for fuel cell flow channel.•The two-phase transport of DNS is different from k−εk−ε and laminar models.•Turbulence has significant effect on water droplet movement and deformation.For high-performance low-temperature fuel cells (e.g. hydrogen proton exchange membrane fuel cell for powering vehicles), significant amount of reactant needs to be supplied, leading to turbulent two-phase flow, which is largely ignored in previous studies. In this study, a direct numerical simulation (DNS) model of the two-phase turbulent flow in fuel cell flow channel is developed with a modified volume-of-fluid (VOF) approach for tracking the air/water interface. The turbulent flow inlet of the two-phase DNS model is obtained from a validated single-phase DNS model. By resolving the whole range of spatial and temporal scales of turbulence, the results of the two-phase DNS model show that the deformation of water droplet is asymmetric and broken into small pieces/films, and is significantly different from the laminar and the corresponding k−εk−ε models. It is suggested that the turbulence effect on the two-phase transport in fuel cell flow channel is significant and needs to be considered for water management by using the DNS model.

•A comprehensive numerical model for passive AAEM-DMFC is developed.•Role of MPL and effects of porous media wettability are investigated.•AMPL can act as an effective methanol diffusion barrier.•Influence of MPL on water transport depends on current density.•Preferred water distribution and lower methanol crossover can be achieved.In this study, a two-dimensional two-phase model is developed for a passive alkaline anion exchange membrane direct methanol fuel cell (AAEM-DMFC) to understand the role of micro-porous layer (MPL) and the effect of porous media wettability on species transport. The results indicate that different regions of polarization curve exhibit different dependence on the methanol feed concentration. Anode MPL can act as the methanol diffusion barrier to retard the methanol mass transport and thus mitigate the methanol crossover. This effect becomes more significant by increasing anode MPL hydrophobicity, which facilitates the use of highly concentrated methanol fuel. However, the insertion of cathode MPL and changes in the wettability of cathode porous layers show insignificant effects on the methanol crossover. Moreover, the influence of MPL on the water transport depends on the current density. Less water crossover can be achieved by reducing the water diffusion or enhancing the back-diffusion through the membrane. Ultimately, a favorable water distribution and lower methanol crossover might be achieved by designing porous layers with desired properties. The simulation results presented in this study may help guide the optimization of water management and the mitigation of methanol crossover in passive AAEM-DMFC.

•Catalytic H2–O2 reaction assisted cold start of PEMFC is investigated.•Anode catalytic reaction is more effective than cathode.•Startup modes with high current are favorable for rapid cold start.•Startup modes with low current improve survivability.•Anode catalytic reaction decreases cell resistance and increases heat generation.Fuel cell vehicles (FCVs) have shown the potential of commercialization in recent years. The concerns on the startup ability of proton exchange membrane (PEM) fuel cell stack from subfreezing temperature have risen. The hydrogen–oxygen catalytic reactions assisted cold start method is developed and analyzed in this study. It utilizes a small amount of hydrogen/air mixture to react at low temperature in the catalyst layers (CLs) through platinum catalyst. The interactions between this assisted method and various startup modes are the major issue to be discussed. Anode catalytic reaction with air mole fraction higher than 16% is effective to assist a 30-cell stack starting from −25 °C within 13 s in maximum power mode. However, cathode catalytic reaction cannot sustain a successful startup. The anode humidification effect plays an important role to reduce the stack resistance, and to increase the inherent heat generation rate. In maximum power mode and high current density constant power mode, anode catalytic reaction assisted cold start can be achieved within 10–20 s from −40 °C. Anode air mole fraction must be higher than 18% to ensure the successful cold start in these two modes. For constant power mode, the operating power must be lower than 12 W per cell. In constant current mode, when the current density is low, there would be less demand for anode catalytic reaction to achieve successful startup from −40 °C, indicating that lower current density operations have better survivability in low temperature. Nevertheless, much longer start duration is required for lower operating current. Generally, high current density operating mode with high air mole fraction is a more practical and energy efficient cold start strategy, as the startup time can be reduced significantly. Cold start from about −20 °C without ice accumulation is feasible using this method, which may have reduced concern about degradation. Increasing the volume of CL (porosity and thickness) also helps reduce the ice formation.

•An analytical model for alkaline anion exchange membrane fuel cell is developed.•Cathode humidification is critical, especially at low operating temperatures.•Proper Liquid humidification for cathode improves the performance.•Increasing operating pressure leads to less cathode water vapor supply.•Decreasing membrane or CL thickness reduces both ohmic and activation losses.An analytical model for hydrogen alkaline anion exchange membrane fuel cell (AAEMFC) is developed in this study. The results show that due to both the electrochemical reaction and electro-osmotic drag, water in cathode is consumed faster than oxygen. Proper liquid humidification in cathode is favorable for performance improvement, especially at low operating temperatures; on the other hand, without liquid humidification, high reactant flow rate is needed. If there is no liquid humidification and the oxygen stoichiometry ratio is fixed, a higher operating pressure increases both the activation loss and ohmic loss, leading to lower cell performance. With the increment of catalyst layer (CL) thickness, the reactant concentration in CL decreases, and the ohmic resistance of electron and ion increases. Decreasing the membrane thickness reduces both the ohmic resistance and activation loss, because more water can transfer from anode to cathode.

•A transient multiphase model for passive DMFC is developed.•Effects of current, voltage, MPL and methanol feeding condition are investigated.•Voltage needs to be moderate to ensure energy efficiency and density.•AMPL and CMPL can all reduce methanol crossover, but AMPL is more important.•Fuel tank size has considerable effect on energy density, but not energy efficiency.By developing a transient multiphase model for passive direct methanol fuel cell (DMFC), the effects of operating current density, voltage, micro-porous layer (MPL) and methanol feeding condition are comprehensively investigated for the whole operating processes (fuel tank from full to empty). It is found that for all the operating conditions, it is necessary to operate at moderate current density or voltage to limit the methanol crossover and ensure the energy conversion efficiency. The MPL in anode is needed to provide sufficient flow resistance at the MPL/gas diffusion layer (GDL) interface to improve the fuel efficiency. Although the cathode MPL can strengthen the convective transport of methanol from cathode to anode, its effect on reducing methanol crossover is less significant than the anode MPL. If the energy density is the most important factor, it is suggested to operate with sufficiently high methanol feeding concentration; and if the fuel and energy efficiencies have the priority, the methanol feeding concentration needs to be moderate. Increasing the size of fuel tank generally improves the energy density, but has negligible effect on the fuel and energy efficiencies.

•A comprehensive multiphase model for passive DMFC is developed.•Variable contact angle effect of electrode due fabrication is studied.•Increasing contact angle along flow direction of methanol reduces crossover.•DL/MPL and MPL/CL interfacial transport resistance is critical.•Results shed light on development of novel electrode with variable contact angle.In this study, a multiphase passive direct methanol fuel cell (DMFC) model is developed to study the effect of micro porous layer (MPL) insertion, and the variable contact angle effects due to the different fabrication processes of diffusion layer (DL) and MPL are also investigated in details. It is found that with the contact angle increasing along the flow direction of methanol in anode DL (ADL), the liquid always needs to flow from a more hydrophilic region to a more hydrophobic region, leading to less methanol crossover. A higher ADL contact angle generally leads to more methanol crossover, although it increases the flow resistance at the inlet, the mass transport resistance at the DL/MPL interface is reduced, and the capillary driven flow is also enhanced inside the DL. However, since MPL is often a very thin layer compared with GDL, it could act as a mass transfer barrier mainly because of the contact angle differences across the MPL/DL and MPL/CL interfaces, which is the dominating factor determining the MPL function. The results also shed the light on the development of novel electrode with variable contact angle based on different fabrication processes.

•A PEMFC cold start stack model is developed.•A novel variable heating and load control (VHLC) method is proposed.•Dead individual cells can be activated again by other cells with VHLC.•Proper VHLC improves cold start performance significantly.In this study, a cold start model for proton exchange membrane fuel cell (PEMFC) stacks is developed, and a novel start-up method, variable heating and load control (VHLC), is proposed and evaluated. The main idea is to only apply load to the neighboring still-active cells, and to apply external heating to certain cells inside the stack simultaneously (load is not applied to the cells fully blocked by ice, although these cells can gain heat from neighboring cells). With the VHLC method, it is found that the stack voltage first increases, then decreases due to the full blockage of ice in some of the individual cells, and finally the dead cells are heated by the other active cells and activated again one by one. Based on this method, the external heating power and the stack self-heating ability are utilized more efficiently. With proper implementation of the VHLC method, it is demonstrated that the cold stat performance can be improved significantly, which is critically important for PEMFC in automotive applications.

•A 3D model of high temperature proton exchange membrane fuel cell is developed.•The model is based on sulfonated polybenzimidazole membranes.•Conductivity of sulfonated polybenzimidazole membrane is validated experimentally.•Model prediction of cell performance is validated by experimental data.In this study, a three-dimensional, steady-state, non-isothermal numerical model of high temperature proton exchange membrane fuel cells (HT-PEMFCs) operating with novel sulfonated polybenzimidazole (SPBI) membranes is developed. The proton conductivity of the phosphoric acid doped SPBI membranes with different degrees of sulfonation is correlated based on experimental data. The predicted conductivity of SPBI membranes and cell performance agree reasonably with published experimental data. It is shown that a better cell performance is obtained for the SPBI membrane with a higher level of phosphoric acid doping. Higher operating temperature or pressure is also beneficial for the cell performance. Electrochemical reaction rates under the ribs of the bipolar plates are larger than the values under the flow channels, indicating the importance and dominance of the charge transport over the mass transport.

•Maximum power cold start mode of PEMFC is investigated based on a stack model.•Performance is continuously degrading before melting point for maximum power mode.•Maximum power mode can better balance heat and ice than the other modes.•Once survivability is ensured, increasing purging is unsuitable for fast start-up.Successful and fast cold start is important for proton exchange membrane (PEM) fuel cell in vehicular applications in addition to the desired maximum power in any case. In this study, the maximum power cold start mode is investigated in details and compared with other cold start modes based on a multiphase stack model. It is found that for the maximum power cold start mode, the current density is generally kept at high levels, and the performance improvement caused by the membrane hydration and temperature increment may not be observable. Therefore, before the melting point, the performance drops continuously. The maximum power cold start mode could better balance the heat generation and ice formation, leading to improved cold start survivability than that in the constant voltage and constant current modes, with a fast start-up generally guaranteed. Once the survivability can be ensured, the initial water content needs to be higher for fast cold start, suggesting that over purging should be avoided. The maximum power mode is suggested to be optimal for PEM fuel cell cold start based on the modeling results.

•A 3D multiphase model of AAEM fuel cell is developed.•Water is removed in different phases with different anode humidification levels.•Liquid water supply to cathode is needed, especially for high current densities.•Liquid water transport direction is affected by humidification level in cathode.•Decreasing membrane thickness reduces both ohmic and mass transport losses.Alkaline anion exchange membrane (AAEM) fuel cell is becoming more attractive because of its outstanding merits, such as fast electrochemical kinetics and low dependence on non-precious catalyst. In this study, a three-dimensional multiphase non-isothermal AAEM fuel cell model is developed. The modeling results show that the performance is improved with more anode humidification, but the improvement becomes less significant at higher humidification levels. The humidification level of anode can change the water removal mechanisms: at partial humidification, water is removed as vapor; and for full humidification, water is removed as liquid. Cathode humidification is even more critical than anode. Liquid water supply in cathode has a positive effect on performance, especially at high current densities. With more liquid water supply in cathode, liquid water starts moving from channel to CL, rather than being removed from CL. Liquid water supply in cathode is needed to balance the water amounts in anode and cathode. Decreasing the membrane thickness generally improves the cell performance, and the improvement is even enhanced with thinner membranes, due to the faster water diffusion between anode and cathode, which reduces the mass transport losses.

•A 3D two-phase model of GDL microstructure with variable contact angle is developed.•Water flows through GDL first in “fingering” mode and then in “steady” mode.•Increasing pressure or decreasing contact angle accelerates water transport.•Transport with variable contact angle is similar to the average contact angle case.For a gas diffusion layer (GDL) with hydrophobic treatment, its hydrophobicity (contact angle) may change along the through-plane direction, and lead to different two-phase transport characteristics. In this study, such variable contact angle is implemented in a three-dimensional unsteady two-phase model based on the microstructure of GDL to study the liquid water transport characteristics along the in-plane direction caused by cross flow. It is found that during a liquid water intrusion process, the liquid water first moves through some of the pores that are easy to penetrate, forming a “fingering transport” mode; and after that, with more liquid water accumulated, the rest of the pores can also be filled, forming a “steady transport” mode. Increasing the differential pressure or decreasing the contact angle of GDL accelerates the liquid water intrusion, and this effect is weakened at higher differential pressures and contact angles. For a GDL with variable contact angle, the water transport characteristics in different cross sections normal to the through-plane direction with different contact angles are similar to the corresponding fixed contact angle cases in these cross sections, and the overall process of water intrusion with variable contact angle is similar to its corresponding average fixed contact angle case.

To comprehensively understand the cold start processes of proton exchange membrane fuel cell (PEMFC) stack which is important for the automotive applications, a three-dimensional multiphase PEMFC stack model is developed in this study. The detailed analysis of the cold start processes shows that for the stacks with more cells, the voltage decreases more slowly due to the lower ice formation rates. The temperature increases faster for a stack with more cells, and a higher temperature can be reached at the end of the cold start process. No apparent difference in voltage exists among the different individual cells in a stack when the reactant gases are evenly supplied to each cell. The temperature in the individual cell in the middle of a stack is higher and more evenly distributed than those on the sides and single cells, due to weakened cooling effect of the bi-polar plate (BP) on the membrane electrode assembly (MEA), and the ice formation rate is also lower in the middle cell. At a lower current density, the ice in the cathode catalyst layer (CL) is formed faster at the section close to the BP, and it is close to the membrane at a higher current density.Highlights► A 3D multiphase model is developed to study the cold start process of PEMFC stacks. ► Stacks with more cells can reach higher temperature with better performance. ► Ice formation in middle cells is slower.

Thermal management is critically important to maintain the performance of lithium ion battery stacks. In this study, a numerical model and an analytical model for the thermal management of lithium ion battery stacks are developed to investigate the thermal behaviors of flat-plate and cylindrical stacks during discharging processes. It is found that for the same volume ratio of cooling channel and battery of flat-plate design, changing the channel size and the number of channels results in similar average battery temperatures, however, increasing the channel size improves the cooling energy efficiency but leads to more unevenly distributed temperature, and vice versa. The volume ratio of cooling channel to battery needs to be higher than 0.014 for flat-plate design when the Reynolds number of cooling air is around 2000 or higher with a high discharging rate of 2 C. The cylindrical battery stacks considered in this study are generally less compact and more energy-efficient in cooling than the flat-plate battery stacks, and the general thermal behaviors are similar between these two designs. A counter-flow arrangement of the cooling channels or changing the flow direction of the co-flow arrangement periodically may also help the thermal management.Highlights► Numerical and analytical models are developed for thermal management of Li battery. ► Same cooling channel to battery volume ratio leads to same average temperature. ► Larger channel improves evenness of temperature distribution and energy efficiency.

Catalytic hydrogen–oxygen reaction is a potentially effective way to help start up proton exchange membrane fuel cells (PEMFCs) from sub-zero temperatures. In this study, the anode hydrogen–oxygen catalytic reaction is implemented in a three-dimensional multiphase cold start model. It is found that successful cold start from −20 °C can be achieved with the assist of the catalytic reaction in galvanostatic mode. With anode catalytic reaction, the start-up current density must be moderate, because a high current density lowers the assisted heating effect, and a low current density slows down the start-up process. The temperature difference between the anode and cathode catalyst layers (CLs) is negligible, which indicates that the heating location in the electrodes for the catalytic reaction makes no significant difference. The humidification of anode due to the catalytic reaction also reduces the ohmic resistance of the membrane, leading to enhanced performance during the start-up processes.Highlights► A 3D PEMFC cold start model with anode catalytic reaction is developed. ► Successful cold start from −20 °C can be achieved with anode catalytic reaction. ► The start-up current density must be moderate with anode catalytic reaction. ► Humidification of anode due to catalytic reaction improves performance.

•A transient water management model for AAEM fuel cell anode is developed.•The dynamic responses to various operating parameters are investigated.•More time is needed to reach steady when current decreases rather than increases.•Time needed to reach steady is reduced with a higher current.•Overshoot and undershoot of water diffusion through membrane are observed.Alkaline anion exchange membrane (AAEM) fuel cell has attracted increasing attention in recent years due to its several outstanding advantages over proton exchange membrane (PEM) fuel cell such as fast electrochemical kinetics and friendly alkaline environment for catalysts. In this study, a three-dimensional (3D) half-cell transient model is developed to study the dynamic characteristics of AAEM fuel cell under different step changes of operating conditions. It is found that the current density has significant effects on the distribution of liquid water, while the anode stoichiometric ratio effect is insignificant. More time is needed to reach a steady state when the current density decreases rather than increases, and the similar phenomenon also occurs when the operating temperature decreases rather than increases, however, this effect within a low temperature range becomes insignificant. Moreover, the overshoot and undershoot of water diffusion through membrane can also be observed with the step change of the anode stoichiometric ratio and anode inlet relative humidity. The model prediction also has reasonable agreement with published experimental data. The dynamic behaviors observed in this study are of significant importance to the development of AAEM fuel cells for portable and automotive applications.

Water management is an important issue for alkaline anion exchange membrane fuel cell (AAEMFC) due to its significant role in the energy conversion processes. In this study, a numerical model is developed to investigate the water transport in AAEMFC anode. The gas and liquid transport characteristics in the gas diffusion layer (GDL) and catalyst layer (CL) with different designs and under various operating conditions are discussed. The results show that the current density affects the liquid water distribution in anode most significantly, and the temperature is the second considerable factor. The stoichiometry ratio of the supplied reactant has insignificant effect on the liquid water transport in anode. The change of liquid water amount in anode with cathode relative humidity follows a similar trend with anode inlet relative humidity. Some numerical results are also explained with published experimental and modeling data with reasonable agreement.Highlights► A steady-state water management model for AAEMFC anode is developed. ► The transport mechanisms with various designs and conditions are investigated. ► Current density affects the liquid water transport in anode most significantly. ► The effects of relative humidity of anode and cathode are similar. ► Water amount decreases almost linearly with increment of GDL or CL porosity.

•A model for hydrogen AAEM fuel cell with saturation jump in electrode is developed.•Effects of MPL, anode back pressure and membrane thickness are investigated.•Anode MPL, anode pressurization and membrane thickness reduction are favorable.•Cathode MPL shows insignificant influence due to cathode dryout.In this study, a whole-cell 3D multiphase non-isothermal model is developed for hydrogen alkaline anion exchange membrane (AAEM) fuel cell, and the interfacial effect on the two-phase transport in porous electrode is also considered in the model. The results show that the insertion of anode MPL, slight anode pressurization and reduction of membrane thickness generally improve the cell performance because the water transport from anode to cathode is enhanced, which favors both the mass transport and membrane hydration. The effect of cathode MPL is generally insignificant because liquid water rarely presents in cathode. It is demonstrated that slight pressurization of anode, which might not lead to apparent damage to the membrane, can effectively solve the anode flooding and cathode dryout issues.

•The VOF model is utilized to simulate the two phase flow in fuel cell anode channel.•The differences of liquid water transport in anode and cathode channel are discussed.•Suggestions on how to improve the liquid water removal in anode channel and advice for channel design are given.In this study, the gas liquid two-phase flow of low-temperature fuel cells is simulated to study the water removal process in the anode channel utilizing VOF (volume of fluid) method. It is found that the water removal process in the cathode channel is easier compared to that in the anode under the same operating condition and inlet velocity. Increasing the humidification rate and contact angle is helpful for water removal. Moreover, further simulations show that in the semicircle-U channel, the water can be removed successfully, but it will be stuck in the corner in the rectangular-U channel, which can be prevented by changing the contact angle of specific walls into extremely hydrophilic or hydrophobic. Extremely hydrophilic wall would make the entire water pave on the wall and smash into little droplets which is not only helpful in water removal and evaporation but also prevents the water from blocking the holes on the Gas Diffusion Layer (GDL). And different hydrophilic/hydrophobic levels also influence the water removal.

•A transient model for passive vapor-feed DMFC is developed.•Cathode water recovery is critical when neat methanol vapor is supplied.•Cathode MPL is beneficial to water recovery from cathode to anode.•Increasing current and membrane thickness help reduce methanol crossover.•Decreasing vaporizer open ratio generally has positive impact.A transient model is presented to investigate the transport phenomena for passive vapor-feed direct methanol fuel cell (DMFC). The pervaporation membrane and vapor transport layer are considered for the formation and transport of methanol vapor, respectively. We attempt to provide insight into the transient mass transport characteristics of DMFCs by testing different operation conditions, including current density, open area ratio of the vaporizer, and membrane thickness. The results show that the methanol crossover rate and water transport from the cathode to the anode are the key factors for improving the cell performance, and indicate that fuel efficiency, energy efficiency and energy density of the DMFCs are improved by increasing current density, decreasing open ratio of the vaporizer or increasing membrane thickness due to the reduced methanol crossover rate. The cathode micro-porous layer (MPL) is useful in enhancing water recovery flux and decreasing water losses.

With performance improvement of low-temperature fuel cell (FC), high reactant supply and water generation rates may induce air-water turbulence in the FC flow channel. In this research, an air-water turbulent direct numerical simulation (DNS) model is developed to simulate different droplet sizes, locations and interactions in the air-water transport processes comprehensively. It is found that a larger droplet breaks up more easily in turbulence, and a smaller droplet tends to keep lumped. The droplet at corner does not break up because it is away from channel center. The droplet interaction simulations show that the small droplets merge to form slugs, but still keep lumped in turbulence. It is suggested that two conditions need to be satisfied for droplet break up in FC flow channel, one is turbulent flow, and another is that the droplet needs to be large enough and occupy the center region of flow channel to suffer sufficient turbulence fluctuations. The DNS results illustrate some unique phenomena in turbulent flow, and show that the turbulence has significant effect on the air-water flow behavior in FC flow channel.Download high-res image (195KB)Download full-size image