Two-phase flow pressure drop hysteresis in parallel channels of a proton exchange membrane fuel cell

Two-phase flow pressure drop hysteresis was studied in a non-operational PEM fuel cell to understand the effect of stoichiometry, GDL characteristics, operating range, and initial conditions (dry vs. flooded) for flow conditions typical of an operating fuel cell. This hysteresis is noted when the air and water flow rates are increased and then decreased along the same path, exhibiting different pressure drops. When starting from dry conditions, the descending pressure drop tended to be higher than the ascending pressure drop at lower simulated current densities. The hysteresis effect was noted for stoichiometries of 1-4 and was eliminated at a stoichiometry of 5. It was found that the hysteresis was greater when water breakthrough occurred at higher simulated current densities, which is a function of GDL properties. The operating range had to reach a critical simulated current density (800 mA cm⁻² in this case) between the ascending and descending approach to create a pressure drop hysteresis zone. The descending step size does not change the size of the hysteresis effect, but a larger step size leads to lower fluctuations in the pressure drop signal. An initially flooded condition also showed hysteresis, but the ascending approach tended to have a higher pressure drop than the descending approach.     

In situ-polymerized wicks for passive water management in proton exchange membrane fuel cells

Air-delivery is typically the largest parasitic loss in PEM fuel cell systems. We develop a passive water management system that minimizes this loss by enabling stable, flood-free performance in parallel channel architectures, at very low air stoichiometries. Our system employs in situ-polymerized wicks which conform to and coat cathode flow field channel walls, thereby spatially defining regions for water and air transport. We first present the fabrication procedure, which incorporates a flow field plate geometry comparable to many state-of-the-art architectures (e.g., stamped metal or injection molded flow fields). We then experimentally compare water management flow field performance versus a control case with no wick integration. At the very low air stoichiometry of 1.15, our system delivers a peak power density of 0.68 W cm⁻². This represents a 62% increase in peak power over the control case. The open channel and manifold geometries are identical for both cases, and we demonstrate near identical inlet-to-outlet cathode pressure drops at all fuel cell operating points. Our water management system therefore achieves significant performance enhancement without introducing additional parasitic losses.     

A critical review of two-phase flow in gas flow channels of proton exchange membrane fuel cells

Water management in PEM fuel cells has received extensive attention due to its key role in fuel cell performance. The unavoidable water, from humidified gas streams and electrochemical reaction, leads to gas-liquid two-phase flow in the flow channels of fuel cells. The presence of two-phase flow increases the complexity in water management in PEM fuel cells, which remains a challenging hurdle in the commercialization of this technology. Unique water emergence from the gas diffusion layer, which is different from conventional gas-liquid two-phase flow where water is introduced from the inlet together with the gas, leads to different gas-liquid flow behaviors, including pressure drop, flow pattern, and liquid holdup along flow field channels. These parameters are critical in flow field design and fuel cell operation and therefore two-phase flow has received increasing attention in recent years. This review emphasizes gas-liquid two-phase flow in minichannels or microchannels related to PEM fuel cell applications. In situ and ex situ experimental setups have been utilized to visualize and quantify two-phase flow phenomena in terms of flow regime maps, flow maldistribution, and pressure drop measurements. Work should continue to make the results more relevant for operating PEM fuel cells. Numerical simulations have progressed greatly, but conditions relevant to the length scales and time scales experienced by an operating fuel cell have not been realized. Several mitigation strategies exist to deal with two-phase flow, but often at the expense of overall cell performance due to parasitic power losses. Thus, experimentation and simulation must continue to progress in order to develop a full understanding of two-phase flow phenomena so that meaningful mitigation strategies can be implemented.     

Water permeation through gas diffusion layers of proton exchange membrane fuel cells

Water transport through gas diffusion layer of proton exchange membrane fuels cells is investigated experimentally. A filtration cell is designed and the permeation threshold and the apparent water permeability of several carbon papers are investigated. Similar carbon paper with different thicknesses and different Teflon loadings are tested to study the effects of geometrical and surface properties on the water transport. Permeation threshold increases with both GDL thickness and Teflon loading. In addition, a hysteresis effect exists in GDLs and the permeation threshold reduces as the samples are retested. Moreover, several compressed GDLs are tested and the results show that compression does not affect the breakthrough pressure significantly. The measured values of apparent permeability indicate that the majority of pores in GDLs are not filled with water and the reactant access to the catalyst layer is not hindered.     

Liquid water flooding process in proton exchange membrane fuel cell cathode with straight parallel channels and porous layer

Liquid water management plays a significant role in proton exchange membrane fuel cell (PEMFC) performance, especially when the PEMFC is operating with high current density. Therefore, understanding of liquid water behavior and flooding process is a critical challenge that must be addressed. To overcome PEMFC durability problems, a liquid water flooding process is studied in the cathode side of a PEMFC with straight parallel channels and a porous layer using FLUENT^® v6.3.26 software with a volume-of-fluid (VOF) algorithm and user-defined-function (UDF). The general process of liquid water flooding within this type of PEMFC cathode is investigated by analyzing the behavior of liquid water in porous layer and gas flow channels. Two important phenomena, the “first channel phenomenon” and the “last channel phenomenon”, and their effects on the flow distribution along different parallel channels are discussed.     

Comparison of water management between two bipolar plate flow-field geometries in proton exchange membrane fuel cells at low-density current range

This experimental research studies some aspects of water formation and management in polymer electrolyte membrane fuel cells (PEMFCs). To this end, two different single cells of 49 cm² active area have been tested, the first one with a serpentine-parallel geometry and the second with a cascade-type flow-field topology. In order to visualize the processes, flow-field channels have been machined on transparent plastic. Experiments have consisted in both image acquisition using a CCD camera, and simultaneous measurements of pressure drop in both hydrogen and oxygen gas flow paths. It has been observed that with the cascade-type flow-field geometry, water produced in the cathode does not flood the gas flow channels and, consequently, can be drained in an easy way. On the other hand, it has also been verified that saturated condition for the hydrogen gas flow at the anode side produces water condensation and channel flooding for the serpentine-parallel flow-field topology. Time fluctuations in the pressure drop of the gas flow have been detected and are associated to some transient process inherent to water formation and management.     

质子交换膜燃料电池双极板的设计与模拟分析

质子交换膜燃料电池是直接将化学能转换为电能的装置,双极板上的流道结构对燃料电池的工作性能具有较大的影响。根据应用要求设计了具有平行流道、蛇形流道及希尔伯特分形流道的双极板结构,模拟计算了氢气在不同类型的流道和气体扩散层中的分布状态,分析了燃料电池的输出电流密度和功率密度随电极间电压的变化特点,比较了不同的流道结构对燃料电池输出电流密度的影响,以及不同的工作温度及气体压强的情况下,燃料电池输出电流密度随温度及压强的变化规律。     

Two-phase flow pressure drop hysteresis in an operating proton exchange membrane fuel cell

Two-phase flow pressure drop hysteresis was studied in an operating PEM fuel cell. The variables studied include air stoichiometry (1.5, 2, 3, 4), temperature (50, 75, 90 °C), and the inclusion of a microporous layer. The cathode channel pressure drops can differ in PEM fuel cells when the current density is increased along a path and then decreased along the same path (pressure drop hysteresis). Generally, the descending pressure drop is greater than the ascending pressure drop at low current densities (<200 mA cm⁻²), and the effect is worse at low stoichiometries and low temperatures. The results show that the hysteresis occurs with or without the inclusion of a microporous layer. Initial results show a modified Lockhart-Martinelli approach seems to be able to predict the two-phase flow pressure drop during the ascending path. The results compare well with photographs taken from the cathode flow field channel of a visualization cell.     

In-plane gas permeability of proton exchange membrane fuel cell gas diffusion layers

A new analytical approach is proposed for evaluating the in-plane permeability of gas diffusion layers (GDLs) of proton exchange membrane fuel cells. In this approach, the microstructure of carbon papers is modeled as a combination of equally-sized, equally-spaced fibers parallel and perpendicular to the flow direction. The permeability of the carbon paper is then estimated by a blend of the permeability of the two groups. Several blending techniques are investigated to find an optimum blend through comparisons with experimental data for GDLs. The proposed model captures the trends of experimental data over the entire range of GDL porosity. In addition, a compact relationship is reported that predicts the in-plane permeability of GDL as a function of porosity and the fiber diameter. A blending technique is also successfully adopted to report a closed-form relationship for in-plane permeability of three-directional fibrous materials.     

Performance prediction of plate-fin radiator for low temperature preheating system of proton exchange membrane fuel cells using CFD simulation

The objective of this paper is to analyze the heat transfer characteristics of plate-fin radiator for the cold air heating system of a PEMFC engine and to find the optimal parameter combination in order to reduce the power consumption. The effect of the coolant mass flow and temperature on the heat exchange performance of the radiator was investigated based on 3D porous medium model. The results, including the amount of heat transferred and temperature change and heat exchanger effectivity with the increasing of the air flow rate at different coolant flow rate were obtained using CFD method. Good agreement is found by comparing the simulation values with the test data and the deviation is less than 7% which indicate simulation model validation and research method feasibility used in this study. The simulation results indicate that bigger coolant flow rate and temperature result in higher outlet air temperature and the amount of heat transferred. The variation of the heat exchanger effectivity is predicted for different working conditions. Based on the Taguchi method, the influence of structural parameters of the corrugated fins on the heat transfer and pressure drop of the radiator is analyzed qualitatively. It is shown that fin length has the greatest impact on the comprehensive heat transfer performance of the radiator. This research provides a guide for optimizing the air preheating system and improving the amount of heat transferred.     

Characterisation of porous carbon electrode materials used in proton exchange membrane fuel cells via gas adsorption

Porous carbon materials are typically used in both the substrate (typically carbon paper) and the electrocatalyst supports (often platinised carbon) within proton exchange membrane fuel cells. Gravimetric nitrogen adsorption has been studied at a carbon paper substrate, two different Pt-loaded carbon paper electrodes and three particulate carbon blacks. N₂ BET surface areas and surface fractal dimensions were determined using the fractal BET and Frenkel–Halsey–Hill models for all but one of the materials studied. The fractal dimensions of the carbon blacks obtained from gas adsorption were compared with those obtained independently by small angle X-ray scattering and showed good agreement. Density functional theory was used to characterise one of the carbon blacks, as the standard BET model was not applicable.     

Understanding porous water-transport plates in polymer-electrolyte fuel cells

In this article, numerical simulations are used to compare polymer-electrolyte fuel cells (PEFCs) with porous flow-field plates to cells with conventional solid plates. The model clarifies the role of the porous plates in humidifying dry reactant streams and managing liquid water. The influence of gas-diffusion-layer (GDL) and bipolar-plate properties and coolant vacuum on the behavior of the cell is investigated.     

Performance prediction of proton exchange membrane fuel cells using a three-dimensional model

In this study a steady-state three-dimensional computational fluid dynamics (CFD) model of a proton exchange membrane fuel cell is developed and presented for a single cell. A complete set of conservation equations of mass, momentum, species, energy transport, and charge is considered with proper account of electrochemical kinetics based on Butler–Volmer equation. The catalyst layer structure is considered to be agglomerate. This model enables us to investigate the flow field, current distribution, and cell voltage over the fuel cell which includes the anode and cathode collector plates, gas channels, catalyst layers, gas diffusion layers, and the membrane. The numerical solution is based on a finite-volume method in a single solution domain. In this investigation a CFD code was used as the core solver for the transport equations, while mathematical models for the main physical and electrochemical phenomena were devised into the solver using user-developed subroutines. Three-dimensional results of the flow structure, species concentrations and current distribution are presented for bipolar plates with square cross section of straight flow channels. A polarization curve is obtained for the fuel cell under consideration. A comparison between the polarization curves obtained from the current study and the corresponding available experimental data is presented and a reasonable agreement is obtained. Such CFD model can be used as a tool in the development and optimization of PEM fuel cells.     

Liquid water breakthrough pressure through gas diffusion layer of proton exchange membrane fuel cell

The dynamic behavior of liquid water transport through the gas diffusion layer (GDL) of the proton exchange membrane fuel cell is studied with an ex-situ approach. The liquid water breakthrough pressure is measured in the region between the capillary fingering and the stable displacement on the drainage phase diagram. The variables studied are GDL thickness, PTFE/Nafion content within the GDL, GDL compression, the inclusion of a micro-porous layer (MPL), and different water flow rates through the GDL. The liquid water breakthrough pressure is observed to increase with GDL thickness, GDL compression, and inclusion of the MPL. Furthermore, it has been observed that applying some amount of PTFE to an untreated GDL increases the breakthrough pressure but increasing the amount of PTFE content within the GDL shows minimal impact on the breakthrough pressure. For instance, the mean breakthrough pressures that have been measured for TGP-060 and for untreated (0 wt.% PTFE), 10 wt.% PTFE, and 27 wt.% PTFE were 3589 Pa, 5108 Pa, and 5284 Pa, respectively.     

Transport phenomena within the porous cathode for a proton exchange membrane fuel cell

A two-phase, one-dimensional steady model is developed to analyze the coupled phenomena of cathode flooding and mass-transport limiting for the porous cathode electrode of a proton exchange membrane fuel cell. In the model, the catalyst layer is treated not as an interface between the membrane and gas diffusion layer, but as a separate computational domain with finite thickness and pseudo-homogenous structure. Furthermore, the liquid water transport across the porous electrode is driven by the capillary force based on Darcy's law. And the gas transport is driven by the concentration gradient based on Fick's law. Additionally, through Tafel kinetics, the transport processes of gas and liquid water are coupled. From the numerical results, it is found that although the catalyst layer is thin, it is very crucial to better understand and more correctly predict the concurrent phenomena inside the electrode, particularly, the flooding phenomena. More importantly, the saturation jump at the interface of the gas diffusion layer and catalyst layers is captured, when the continuity of the capillary pressure is imposed on the interface. Elsewise, the results show further that the flooding phenomenon in the CL is much more serious than that in the GDL, which has a significant influence on the mass transport of the reactants. Moreover, the saturation level inside the cathode is determined, to a great extent, by the surface overpotential, the absolute permeability of the porous electrode, and the boundary value of saturation at the gas diffusion layer-gas channel interface. In order to prevent effectively flooding, it should remove firstly the liquid water accumulating inside the CL and keep the boundary value of liquid saturation as low as possible.     

Anode water removal and cathode gas diffusion layer flooding in a proton exchange membrane fuel cell

Anode water removal (AWR) is studied as a diagnostic tool to assess cathode gas diffusion layer (GDL) flooding in PEM fuel cells. This method uses a dry hydrogen stream to remove product water from the cathode, showing ideal fuel cell performance in the absence of GDL mass transfer limitations related to water. When cathode GDL flooding is limiting, the cell voltage increases as the hydrogen stoichiometry is increased. Several cathode GDLs were studied to determine the effect of microporous layer (MPL) and PTFE coating. The largest voltage gains occur with the use of cathode GDLs without an MPL since these GDLs are prone to higher liquid water saturation. Multiple GDLs are studied on the cathode side to exacerbate GDL flooding conditions to further confirm the mechanism of the AWR process. Increased temperature and lower cathode RH allow for greater overall water removal so the voltage improvement occurs faster, though this leads to quicker membrane dehydration.     

Water removal characteristics of proton exchange membrane fuel cells using a dry gas purging method

Water removal from proton exchange membrane fuel cells (PEMFC) is of great importance to improve start-up ability and mitigate cell degradation when the fuel cell operates at subfreezing temperatures. In this study, we report water removal characteristics under various shut down conditions including a dry gas-purging step. In order to estimate the dehydration level of the electrolyte membrane, the high frequency resistance of the fuel cell stack was observed. Also, a novel method for measuring the amount of residual water in the fuel cell was developed to determine the amount of water removal. The method used the phase change of liquid water and was successfully applied to examine the water removal characteristics. Based on these works, the effects of several parameters such as purging time, flow rate of purging gas, operation current, and stack temperature on the amount of residual water were investigated.     

Bipolar plate research using Computational Fluid Dynamics and neutron radiography for proton exchange membrane fuel cells

This work presents the development of liquid-cooled industry-scale bipolar plates for improved water management in PEM Fuel Cells. The methods used for the design development are based on Computational Fluid Dynamics (CFD) modelling and simulation, and Neutron Radiography experiments to analyse liquid water distributions within the cell for different operating conditions.A novel 140 cm² bipolar plate was designed and manufactured on 0.1 mm thick stainless steel using pre-coated strip steel. CFD modelling carried out for the novel design predicted a significant improvement in terms of cell performance, as well as a more uniform temperature distribution within the membrane. Liquid water distributions were later analysed by neutron radiography experiments, defining a set of different operating conditions (current density, stoichiometry, inlet gases dew point, and cell temperature).Electrochemical and neutron radiography results are presented for all cases and the influence of the operating conditions is discussed. Liquid water distributions within the cell are also analysed and compared against the CFD model results obtained. The influence of the gas flow configuration (reactant gases and cooling water) is clearly observable in the results.     

Passive water management at the cathode of a planar air-breathing proton exchange membrane fuel cell

Water management is a significant challenge in portable polymer electrolyte membrane (PEM) fuel cells and particularly in proton exchange membrane (PEM) fuel cells with air-breathing cathodes. Liquid water condensation and accumulation at the cathode surface is unavoidable in a passive design operated over a wide range of ambient and load conditions. Excessive flooding or dry out of the open cathode can lead to a dramatic reduction of fuel cell power. We report a water management design based on a hydrophilic and electrically conductive wick. A prototype air-breathing fuel cell with the proposed water management design successfully operated under severe flooding conditions, ambient temperature 10 °C and relative humidity of 80%, for up to 6 h with no observable cathode flooding or loss of performance.     

Experimental study on the two phase flow behavior in PEM fuel cell parallel channels with porous media inserts

In this study, the air–water two phase flow behavior in PEM fuel cell parallel channels with porous media inserts was experimentally investigated using a self-designed and manufactured transparent assembly. The visualization images of the two phase flow in channels with porous media inserts were presented and three patterns were summarized. Compared with the traditional hollow channel design, the novel configuration featured less severe two phase flow mal-distribution and self-adjustment to water amount in channels, although a higher pressure drop was introduced due to the porous media inserts. The dominant frequency of pressure drop signal was found to be a diagnostic tool for water behavior in channels. The novel flow channel design with porous media inserts may become a solution to the water management problem in PEM fuel cells.