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.     

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.     

Experimental validation of two-phase pressure drop multiplier as a diagnostic tool for characterizing PEM fuel cell performance

Water management is a key area of interest in improving the performance of Proton Exchange Membrane fuel cells. Cell flooding and membrane dehydration are two extreme conditions arising from poor water management. Pressure drop has been recognized as a good diagnostic tool to determine the presence of liquid water in the reactant channels. Presence of liquid water in the channels increases the mass transport resistances and therefore reduces the cell performance, which is quantified by the cell voltage at a set current density. Since the two-phase pressure drop multiplier is uniquely related to the water content in the channel, it serves as a good diagnostic tool for directly predicting the cell performance. Experiments are carried out to establish the relationship between the pressure drop multiplier and cell voltage at different operating conditions. Cell temperature was varied from 30 °C to 80 °C and the inlet RH was varied from 0 to 95%. At the lower temperatures, a two-phase multiplier below 1.5 reduces flooding in the flow field. However, at the higher temperatures, a two-phase flow multiplier above 1.2 is preferred as it indicates the membrane remains hydrated for improved performance from the cell. The two-phase pressure drop multiplier has been successfully demonstrated as a diagnostic tool to predict cell flooding and membrane dehydration.     

Effect of wettability on water removal from the gas diffusion layer surface in a novel proton exchange membrane fuel cell flow channel

Effective water removal from the proton exchange membrane fuel cell (PEMFC) surface exposed to the flow channel is critical to the operation and water management in PEMFCs. In this study, the water removal process is investigated numerically for a novel flow channel formed by inserting a hydrophilic needle in the conventional PEMFC flow channel, and the effect of the surface wettability of the membrane electrode assembly (MEA) and the inserted needle on the water removal process is studied. The results show that the liquid water can be more effectively removed from the MEA surface for larger MEA surface contact angles and smaller needle surface contact angles. The pressure drop for the flow in the channel is also examined and it is seen to be indicative of the liquid water flow and transport in the flow channel, suggesting that pressure drop is a useful parameter for the investigation of water transport and dynamics in the flow channel.     

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.     

Numerical study of water management in the air flow channel of a PEM fuel cell cathode

The water management in the air flow channel of a proton exchange membrane (PEM) fuel cell cathode is numerically investigated using the FLUENT software package. By enabling the volume of fraction (VOF) model, the air–water two-phase flow can be simulated under different operating conditions. The effects of channel surface hydrophilicity, channel geometry, and air inlet velocity on water behavior, water content inside the channel, and two-phase pressure drop are discussed in detail. The results of the quasi-steady-state simulations show that: (1) the hydrophilicity of reactant flow channel surface is critical for water management in order to facilitate water transport along channel surfaces or edges; (2) hydrophilic surfaces also increase pressure drop due to liquid water spreading; (3) a sharp corner channel design could benefit water management because it facilitates water accumulation and provides paths for water transport along channel surface opposite to gas diffusion layer; (4) the two-phase pressure drop inside the air flow channel increases almost linearly with increasing air inlet velocity.     

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.     

A comparison on the dynamical performance of a proton exchange membrane fuel cell (PEMFC) with traditional serpentine and an open pore cellular foam material flow channel

Minimising the pressure drop in flow channels, ensuring high efficiency and utilisation of open pore cellular foam (OPCF) material in place of a traditional serpentine channel are investigated in this work. The paper establishes novel mathematical model that takes into account the effect of pressure drop in the flow channel and compares the dynamics of a porous flow channel with those of the traditional serpentine flow channel. The performance of a Polymer Electrolyte Membrane fuel cell with porous foam flow channel is analysed under static and transient conditions. The fuel cell mass transport equations are used in the model that also takes into account the effect of varying the current on the stack temperature. The membrane water content and IV-curves are analysed and simulation results are presented based on the mathematical models of the proposed system using the MATLAB®/Simulink® environments. The effect of varying pore diameter, porosity, and the flow velocity on pressure drop are also investigated using sensitivity analysis. Due to the lower pressure drop provided by the uniform distribution of reactants in OPCF channel, an improvement of approximately 55% is observed in current density when compared with that of the serpentine channel under the same operating conditions. The investigation further concluded that a higher pore diameter can have a lower drop in pressure provided the flow velocity of the reactant does not exceed 6 m/s.     

Experimental investigations on liquid water removal from the gas diffusion layer by reactant flow in a PEM fuel cell

The cross flow from channel to channel through gas diffusion layer (GDL) under the land could play an important role for water removal in proton exchange membrane (PEM) fuel cells. In this study, characteristics of liquid water removal from GDL have been investigated experimentally, through measuring unsteady pressure drop in a cell which has the GDL initially wet with liquid water. The thickness of GDL is carefully controlled by inserting various thicknesses of metal shims between the plates. It has been found that severe compression of GDL could result in excessive pressure drop from channel inlet to channel outlet. Removing liquid water from GDL by cross flow is difficult for GDL with high compression levels and for low inlet air flow rates. However, effective water removal can still be achieved at high compression levels of GDL if the inlet air flow rate is high. Based on different compressed GDL thicknesses, different GDL porosities and permeabilities were calculated and their effects on the characteristics of liquid water removal from GDL were evaluated. Visualization of liquid water transport has been conducted by using transparent flow channel, and liquid water removal from GDL under the land was observed for all the tested inlet air flow rates, which confirms that cross flow is practically effective to remove the liquid water accumulated in GDL under the land area.     

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.     

Diagnosis of PEM fuel cell stack dynamic behaviors

In this study, the steady-state performance and dynamic behavior of a commercial 10-cell Proton Exchange Membrane (PEM) fuel cell stack was experimentally investigated using a self-developed PEM fuel cell test stand. The start-up characteristics of the stack to different current loads and dynamic responses after current step-up to an elevated load were investigated. The stack voltage was observed to experience oscillation at air excess coefficient of 2 due to the flooding/recovery cycle of part of the cells. In order to correlate the stack voltage with the pressure drop across the cathode/anode, fast Fourier transform was performed. Dominant frequency of pressure drop signal was obtained to indicate the water behavior in cathode/anode, thereby predicting the stack voltage change. Such relationship between frequency of pressure drop and stack voltage was found and summarized. This provides an innovative approach to utilize frequency of pressure drop signal as a diagnostic tool for PEM fuel cell stack dynamic behaviors.     

Degradation mitigation effects of pressure swing in proton exchange membrane fuel cells with dead-ended anode

High cost remains to be one of the primary obstacles for the commercialization of proton exchange membrane fuel cells (PEMFCs). To simplify the fuel cell system and reduce cost, dead-ended anode (DEA) is widely used. However, water and nitrogen can accumulate in the dead-ended anode, resulting in cell performance decrease and severe cell degradation. Anode pressure swing supply is a new technology which has been shown to be effective in reducing local water and nitrogen accumulation in the anode channel. In this work, the effects of pressure swing supply on fuel cell degradation have been experimentally studied. Two sets of experiments on the same fuel cell are conducted, one under conventional constant pressure operation and the other under pressure swing operation. Polarization curves show that pressure swing supply can significantly mitigate cell degradation during DEA operations. Electrochemical characterizations are performed to study the mechanisms of mitigations in cell degradation. The cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) results show that pressure swing supply can significantly reduce electrolyte membrane degradation, but has no significant mitigation effect on the cathode catalyst degradation during DEA operation. Further examinations of the membrane-electrode-assembly (MEA) by scanning electron microscope (SEM) confirm the significant difference in membrane degradations since there is a very large difference in average thickness of the membranes after the degradation tests.     

Multiple concentric spirals for the flow field of a proton exchange membrane fuel cell

The present analysis considers a three-dimensional non-isothermal model in a single phase of a PEM fuel cell with a flow field path in the shape of 1, 2, 3, 4, 6, and 8 concentric spirals. The current density contours, the water content and the entropy generated in all zones of the fuel cell are predicted. The analysis of the three-dimensional model includes the gas flow channels in the six geometric shapes mentioned above, the current collectors, gas diffusion layers, catalyst layers on both sides of the model, anode and cathode, and a proton exchange membrane in between. The energy equation, mass conservation, and transport of species equations are solved, including source terms that take into account the electrochemical effects occurring inside the cell. Also, the entropy generation equation is added to the governing equations of the model. The results allow a comparison to help to decide which of the 6 analyzed configurations improve the performance of the fuel cell, increasing the current density produced, reducing the pressure drop and producing the most uniform current density. The entropy generation analysis reveals the effects that cause the most significant losses (irreversibilities) in the cell. The Bejan number and the Π number are used to compare the irreversibilities produced by the matter flow and by the heat transfer for each one of the six models.     

Transient behavior of water generation in a proton exchange membrane fuel cell

The effect of water generation on the performance of proton exchange membrane fuel cell (PEMFC) was investigated by using a periodical linear sweep method. Three different kinds of I–V curves were obtained, which reflected different amount of water uptake in the fuel cell. The maximum water uptake that could avoid flooding in the fuel cell and the hysteresis of water diffusion were also discussed. Quantitative analysis of water uptake and water transport phenomena in this study were conducted both experimentally and theoretically. Results showed that the water uptake capacity for the fuel cell under no severe flooding was 27.837 mg cm⁻². The transient response of the internal resistance indicated that the high frequency resistance (HFR) lagged the current with a value of about 20 s. The effect of purging operation on the internal resistance of the fuel cell was also explored. Experimental data showed that the cell experienced a continuous 8-min purging process can maintain at a relatively steady and dry state.     

The statistical relationship between flow channel geometry and pressure drop in a direct methanol fuel cell with parallel channels

In this study, the relationship between the pressure drop on the channels due to the methanol flow and the geometry of the flow channels on the anode side of a direct methanol fuel cell (DMFC) has been investigated. Parallel type channels are used as flow channels. The active area of the fuel cell is 5 × 5 cm². The system consists of channels that are optimally placed in the active area, with channel widths and distance of the channels kept constant. Combinations of 1, 1.5, 2, 2.5, 3 mm measurements were used for flow channel width and distance between channels. The ratio of the area created by the prepared geometries to the active area (percentage of contact area) is defined as a new parameter. The main motivation of this study is to be able to determine the effect of the geometric measurements of the designed flow channels on the intra-channel pressure drop by statistical method. There was a statistically significant difference between the flow channel widths and the distance between the channels and the pressure. Among the selected parameters, the effect of the channel width on the pressure drop was highest but it had a statistically moderate relationship. However, there was no significant relationship between the distance between channels and the pressure drop.     

Parallel serpentine-baffle flow field design for water management in a proton exchange membrane fuel cell

In a proton exchange membrane fuel cell (PEMFC) water management is one of the critical issues to be addressed. Although the membrane requires humidification for high proton conductivity, water in excess decreases the cell performance by flooding. In this paper an improved strategy for water management in a fuel cell operating with low water content is proposed using a parallel serpentine-baffle flow field plate (PSBFFP) design compared to the parallel serpentine flow field plate (PSFFP). The water management in a fuel cell is closely connected to the temperature control in the fuel cell and gases humidifier. The PSBFFP and the PSFFP were evaluated comparatively under three different humidity conditions and their influence on the PEMFC prototype performance was monitored by determining the current density–voltage and current density–power curves. Under low humidification conditions the PEMFC prototype presented better performance when fitted with the PSBFFP since it retains water in the flow field channels.     

A generalized numerical model for liquid water in a proton exchange membrane fuel cell with interdigitated design

A new approach to numerical simulation of liquid water distribution in channels and porous media including gas diffusion layers (GDLs), catalyst layers, and the membrane of a proton exchange membrane fuel cell (PEMFC) was introduced in this study. The three-dimensional, PEMFC model with detailed thermo-electrochemistry, multi-species, and two-phase interactions. Explicit gas-liquid interface tracking was performed by using Computational Fluid Dynamics (CFD) software package FLUENT^® v6.2, with its User-Defined Functions (UDF) combined with volume-of-fluid (VOF) algorithm. The liquid water transport on a PEMFC with interdigitated design was investigated. The behavior of liquid water was understood by presenting the motion of liquid water droplet in the channels and the porous media at different time instants. The numerical results show that removal of liquid water strongly depends on the magnitude of the flow field. Due to the blockage of liquid water, the gas flow is unevenly distributed, the high pressure regions takes place at the locations where water liquid appears. In addition, mass transport of the species and the current density distribution is significantly degraded by the presence of liquid water.     

Droplet and slug formation in polymer electrolyte membrane fuel cell flow channels: The role of interfacial forces

A microfluidic device is employed to emulate water droplet emergence from a porous electrode and slug formation in the gas flow channel of a PEM fuel cell. Liquid water emerges from a 50 μm pore forming a droplet; the droplet grows to span the entire cross-section of a microchannel and transitions into a slug which detaches and is swept downstream. Droplet growth, slug formation, detachment, and motion are analyzed using high-speed video images and pressure-time traces. Slug volume is controlled primarily by channel geometry, interfacial forces, and gravity. As water slugs move downstream, they leave residual micro-droplets that act as nucleation sites for the next droplet-to-slug transition. Residual liquid in the form of micro-droplets results in a significant decrease in slug volume between the very first slug formed in an initially dry channel and the ultimate “steady-state” slug. A physics-based model is presented to predict slug volumes and pressure drops for slug detachment and motion.     

Measurement of the water transport rate in a proton exchange membrane fuel cell and the influence of the gas diffusion layer

Water management in a PEM fuel cell significantly affects the fuel cell performance and durability. The gas diffusion layer (GDL) of a PEM fuel cell plays a critical role in the water management process. In this short communication, we report a simple method to measure the water transport rate across the GDL. Water rejection rates across a GDL at different cathode air-flow rates were measured. Based on the measurement results, the fuel cell operating conditions, such as current density, temperature, air stoichiometry and relative humidity, corresponding to membrane drying and flooding conditions were identified for the particular GDL used. This method can help researchers develop GDLs for a particular fuel cell design with specific operating conditions and optimize the operation conditions for the given PEM fuel cell components.     

Design of a novel high-efficiency water separator for proton exchange membrane fuel cell system

In the fuel cell system, hydrogen recirculation subsystem is usually used to increase efficiency of hydrogen usage. While the hydrogen recirculation subsystem is a closed circuit that the water might be accumulated, water separator is used necessarily to separate the water and gas at the anode side. As the poor swirling effect caused by the guide vane in commercial separator, a novel water separator for proton exchange membrane fuel cell system is designed and the flow field characteristics of the separator are gained by computational fluid dynamics. The structure of volute inlet and overflow pipe in the novel separator can enhance the swirling flow and increase the tangential velocity. Based on the results, the separation efficiency and steady performance throughout the flow-rate range can be improved by the novel water separator.