Water transport characteristics in the gas diffusion media of proton exchange membrane fuel cell - Role of the microporous layer

Water transport through the gas diffusion media of a proton exchange membrane fuel cell (PEMFC) was investigated with a focus on the role of the microporous layer (MPL) coated on the cathode gas diffusion layer (GDL). The capillary pressure of the MPL and GDL, which plays a significant role in water transport, is derived as a function of liquid saturation using a pore size distribution (PSD) model. PSD functions are derived with parameters that are determined by fitting to the measured total PSD data. Computed relations between capillary pressure and liquid saturation for a GDL and a double-layered GDL (GDL + MPL) show good agreement with the experimental data and proposed empirical functions. To investigate the role of the MPL, the relationship between the water withdrawal pressure and liquid saturation are derived for a double-layered GDL. Water transport rates and cell voltages were obtained for various feed gas humidity using a two-dimensional cell model, and are compared with the experimental results. The calculated results for the net drag with application of the capillary pressure derived from the PSD model show good agreement with the experimental values. Furthermore, the results show that the effect of the MPL on the cell output voltage is significant in the range of high humidity operation.     

Water management studies in PEM fuel cells,part III: Dynamic breakthrough and intermittent drainage characteristics from GDLs with and without MPLs

The transport of liquid water and gaseous reactants through a gas diffusion layer (GDL) is one of the most important water management issues in a proton exchange membrane fuel cell (PEMFC). In this work, the liquid water breakthrough dynamics, characterized by the capillary pressure and water saturation, across GDLs with and without a microporous layer (MPL) are studied in an ex-situ setup which closely simulates a real fuel cell configuration and operating conditions. The results reveal that recurrent breakthroughs are observed for all of the GDL samples tested, indicating the presence of an intermittent water drainage mechanism in the GDL. This is accounted for by the breakdown and redevelopment of the continuous water paths during water drainage as demonstrated by Haines jumps. For GDL samples without MPL, a dynamic change of breakthrough locations is observed, which originates from the rearrangement of the water configuration in the GDL following the drainage. For GDL samples with MPL, no dynamic change of breakthrough location can be found and the water saturation is significantly lower than the samples without MPL. These results suggest that the MPL not only limits the number of water entry locations into the GDL (such that the water saturation is drastically reduced), but also stabilizes the water paths (or morphology). The effect of MPL on the two-phase flow dynamics in gas channels is also studied with multi-channel flow experiments. The most important result is that GDL without MPL promotes film flow and shifts the slug-to-film flow transition to lower air flow rates, compared with the case of GDL with MPL. This is closely related to the larger number of water breakthrough locations through GDL without MPL, which promotes the formation of water film.     

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 pore network study on the role of micro-porous layer in control of liquid water distribution in gas diffusion layer

In proton exchange membrane fuel cell (PEMFC), a hydrophobic micro-porous layer (MPL) is usually placed between catalyst layer (CL) and gas diffusion layer (GDL) to reduce flooding. Recent experimental studies have demonstrated that liquid water saturation in GDL is drastically decreased in the presence of MPL. However, theoretical studies based on traditional continuum two-phase flow models suggest that MPL has no effect on liquid water distribution in GDL. In the present study, a pore network model with invasion percolation algorithm is developed and used to investigate the impacts of the presence of MPL on liquid water distribution in GDL from the viewpoint at the pore level. A uniform pressure and uniform flux boundary conditions are considered for liquid water entering the porous layer in PEMFC. The simulation results reveal that liquid water saturation in GDL is reduced in the presence of MPL, but the reduction depends on the condition of liquid water entering the porous layer in PEMFC.     

Effects of an MPL on water and thermal management in a PEMFC

In this study, the effects of adding a microporous layer (MPL) as well as the impact of its physical properties on polymer electrolyte fuel cell (PEMFC) performance with serpentine flow channels were investigated. In addition, numerical simulations were performed to reveal the effect of relative humidity and operating temperature. It is indicated that adding an extra between the gas diffusion layer (GDL) and catalyst layer (CL), a discontinuity in the liquid saturation shows up at their interface because of differences in the wetting properties of the layers. In addition, results show that a higher MPL porosity causes the liquid water saturation to decrease and the cell performance is improved. A larger MPL thickness reduces the cell performance. The effects of MPL on temperature distribution and thermal transport of the membrane prove that the MPL in addition to being a water management layer also improves the thermal management of the PEMFC.     

Effect of the micro porous layer design on the dynamic performance of a proton exchange membrane fuel cell

The mass transport characteristics of a gas diffusion layer (GDL) predominantly affect the performance of a proton exchange membrane (PEM) fuel cell. However, studies examining the transient response related to the GDL are insufficient, although the dynamic behavior of a PEM fuel cell is an important issue. In this study, the effects of the design of a micro porous layer (MPL) on the transient response of a PEM fuel cell are investigated. The MPL slurry density and multiple functional layers are treated as the variable design parameter. The results show that the transient response is determined by the capillary pressure gradient through the GDL. The trade-off relation for the PEM fuel cell performance under low and high humidity conditions due to the hydrophobic GDL is mitigated by designing a reverse capillary pressure gradient in the MPL.     

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.     

Demonstration of water management role of microporous layer by similarity model experiments

The water management role of a microporous layer (MPL) in a polymer electrolyte membrane fuel cell (PEMFC) is demonstrated experimentally by visualizing the drainage behaviors of a non-wetting fluid through multiple porous layers. An intermediate layer inserted between a fine layer and a coarse layer is observed to reduce the number of (the non-wetting fluid) breakthrough sites towards the coarse layer by merging many transport paths. Then, the reduced number of the breakthrough sites decreases the non-wetting fluid saturation in the coarse layer by minimizing capillary fingering process. These results clearly demonstrate the water management role of an MPL: An MPL reduces the liquid water breakthrough into a gas diffusion layer (GDL) by merging many paths from a catalyst layer (CL), and thereby reduces the liquid water saturation in the GDL.     

Liquid transport in gas diffusion layer of proton exchange membrane fuel cells: Effects of micro-porous layer cracks

Micro porous layer (MPL) is a carbon layer (～15 μm) that coated on the gas diffusion layer (GDL) to enhance the electrical conduction and membrane hydration of proton exchange membrane fuel cell (PEMFC). However, the liquid transport behavior from MPL to GDL and its impact on water management remain unclear. Thus, a three-dimensional volume of fluid (VOF) model is developed to investigate the effects of MPL crack properties on liquid water saturation, liquid pathway formation, and the two-phase mass transport mechanism in GDL. Firstly, a stochastic orientation method is used to reconstruct the fibrous structure of the GDL. After that, the liquid water saturation calculated from the numerical results agrees well with the experimental data. With considering the full morphology of the overlap between MPL and GDL, it's found that this overlap determines the preferred liquid emerging port of both MPL and GDL. Three crack design shapes in MPL are proposed on the base of the similarity crack formation processes of soil mud. In addition, the effects of crack shape, distance between cracks, and crack number on liquid water transport from MPL to GDL are investigated. It is found that the liquid water saturation of GDL increases with crack number and the distance between cracks, while presents little correlation to the crack shape. Hopefully, these results can help the development of PEMFC models without reconstructing full MPL morphology.     

Effects of porosity distribution variation on the liquid water flux through gas diffusion layers of PEM fuel cells

Flooding of the membrane electrode assembly (MEA) and dehydrating of the polymer electrolyte membrane have been the key problems to be solved for polymer electrolyte membrane fuel cells (PEMFCs). So far, almost no papers published have focused on studies of the liquid water flux through differently structured gas diffusion layers (GDLs). For gas diffusion layers including structures of uniform porosity, changes in porosity (GDL with microporous layer (MPL)) and gradient change porosity, using a one-dimensional model, the liquid saturation distribution is analyzed based on the assumption of a fixed liquid water flux through the GDL. And then the liquid water flux through the GDL is calculated based on the assumption of a fixed liquid saturation difference between the interfaces of the catalyst layer/GDL and the GDL/gas channel. Our results show that under steady-state conditions, the liquid water flux through the GDL increases as contact angle and porosity increase and as the GDL thickness decreases. When a MPL is placed between the catalyst layer and the GDL, the liquid saturation is redistributed across the MPL and GDL. This improves the liquid water draining performance. The liquid water flux through the GDL increases as the MPL porosity increases and the MPL thickness decreases. When the total thickness of the GDL and MPL is kept constant and when the MPL is thinned to 3 μm, the liquid water flux increases considerably, i.e. flooding of MEA is difficult. A GDL with a gradient of porosity is more favorable for liquid water discharge from catalyst layer into the gas channel; for the GDLs with the same equivalent porosity, the larger the gradient is, the more easily the liquid water is discharged. Of the computed cases, a GDL with a linear porosity 0.4x + 0.4 is the best.     

Effects of basic gas diffusion layer components on PEMFC performance with capillary pressure gradient

The gas diffusion layer (GDL) is composed of a substrate and a micro-porous layer (MPL), and is treated with polytetrafluoroethylene (PTFE) to promote water discharge. Additionally, the MPL mainly consists of carbon black and PTFE. In other words, the optimal design of these elements has a dominant effect on the polymer electrolyte membrane fuel cell (PEMFC) performance. For the GDL, it is crucial to prevent water flooding, and the water flux within the GDL is strongly affected by the capillary pressure gradient. In this study, the PEMFC performance was systematically investigated by varying the substrate PTFE content, MPL PTFE content, and MPL carbon loading per unit area. The effects of each experimental variable on the PEMFC performance and especially on the capillary pressure gradient were quantitatively analyzed when the GDLs were manufactured by the doctor blade manufacturing method. The experimental results indicated that as the PTFE content of the anode and cathode GDL increased, the PEMFC performance deteriorated due to the deformation of the porosity and tortuosity of the GDL. Additionally, the PEMFC performance improved as the MPL PTFE content of the cathode GDL increased at low relative humidity (RH), but the PEMFC performance tendency was reversed at high RH. Further, the MPL carbon loading of 2 mg/${\text{cm}}^2$ demonstrated the best performance, and the advantages and disadvantages of the MPL carbon loading were identified. In addition, the effects of each experimental variable on liquid water, water vapor, and gas permeability were investigated.     

Experimental investigation of liquid water formation and transport in a transparent single-serpentine PEM fuel cell

Liquid water formation and transport were investigated by direct experimental visualization in an operational transparent single-serpentine PEM fuel cell. We examined the effectiveness of various gas diffusion layer (GDL) materials in removing water away from the cathode and through the flow field over a range of operating conditions. Complete polarization curves as well as time evolution studies after step changes in current draw were obtained with simultaneous liquid water visualization within the transparent cell. The level of cathode flow field flooding, under the same operating conditions and cell current, was recognized as a criterion for the water removal capacity of the GDL materials. When compared at the same current density (i.e. water production rate), higher amount of liquid water in the cathode channel indicated that water had been efficiently removed from the catalyst layer.

Visualization of the anode channel was used to investigate the influence of the microporous layer (MPL) on water transport. No liquid water was observed in the anode flow field unless cathode GDLs had an MPL. MPL on the cathode side creates a pressure barrier for water produced at the catalyst layer. Water is pushed across the membrane to the anode side, resulting in anode flow field flooding close to the H₂ exit.     

Gas diffusion through differently structured gas diffusion layers of PEM fuel cells

Proton exchange membrane fuel cell (PEMFC) gas diffusion layers (GDLs) play important parts in diffusing gas, discharging liquid water, and conducting electricity, etc. When liquid water is discharged through GDL to gas channel, there will be some pores of GDLs occupied by liquid water. In this study, based on a one-dimensional model, the distribution of liquid water phase saturation is analyzed for different GDL structures including GDL with uniform porosity, GDL with sudden change porosity (GDL with microporous layer (MPL)) and GDL with gradient porosity distribution. The effect on gas diffusion of the changes of porosity and liquid saturation due to water remaining in GDL pores is calculated. The conclusions are that for uniform porosity GDL, the gas diffusion increases with the increase of porosity and contact angle and increases with the decrease of the thickness of GDL; for GDL with MPL, the larger the MPL porosity and the thinner the MPL thickness are, the stronger the gas diffusion is; for gradient change porosity GDL with the same average equivalent porosity, the larger the porosity gradient is, the more easily the gas diffuses. The optimization for GDL gradient structure shows that the GDL with a linear porosity distribution of 0.4x+0.4$0.4x+0.4$ is the best of the computed cases.     

Determination of effective water vapor diffusion coefficient in pemfc gas diffusion layers

The primary removal of product water in proton exchange membrane (PEM) fuel cells is through the cathode gas diffusion layer (GDL) which necessitates the understanding of vapor and liquid transport of water through porous media. In this investigation, the effect of microporous layer (MPL) coatings, GDL thickness, and polytetrafluorethylene (PTFE) loading on the effective water vapor diffusion coefficient is studied. MRC Grafil, SGL Sigracet, and Toray TGP-H GDL samples are tested experimentally with and without MPL coatings and varying PTFE loadings. A dynamic diffusion test cell is developed to produce a water vapor concentration gradient across the GDL and induce diffusion mass transfer. Tests are conducted at ambient temperature and flow rates of 500, 625, and 750 sccm. MPL coatings and increasing levels of PTFE content introduce significant resistance to diffusion while thickness has negligible effects.     

Effects of ratio variation in substrate and micro porous layer penetration on polymer exchange membrane fuel cell performance

A gas diffusion layer (GDL) facilitates the diffusion of reactant gas and the discharge of the generated water. The GDL performs various functions, such as conducting heat and electrons generated by electrochemical reactions and providing mechanical support for the catalyst layer. In this study, the effects of ratio variation in the substrate and microporous layer (MPL) penetration region on the proton exchange membrane fuel cell (PEMFC) performance were investigated. Furthermore, the reasons for these performance tendencies are explained based on the thermogravimetric analysis, contact angle, scanning electron microscopy, mercury porosimetry, electrical resistance, electrochemical impedance spectroscopy, and capillary pressure gradient. The experimental results indicate that the MPL penetration ratio within 15–20% of the total GDL thickness and the combined ratio of the MPL and MPL penetration within 35–40% is the best for the overall PEMFC performance. In addition, when the substrate ratio is excessively low, water flooding substantially occurs in the substrate, and this accumulated water functions as a back pressure, causing severe capillary condensation in the MPL penetration region and thus depriving the supply of the reactant gas.     

Numerical modeling and analysis of micro-porous layer effects in polymer electrolyte fuel cells

It is well known that a micro-porous layer (MPL) plays a crucial role in the water management of polymer electrolyte fuel cells (PEFCs), and thereby, significantly stabilizes and improves cell performance. To ascertain the exact roles of MPLs, a numerical MPL model is developed in this study and incorporated with comprehensive, multi-dimensional, multi-phase fuel-cell models that have been devised earlier. The effects of different porous properties and liquid-entry pressures between an MPL and a gas diffusion layer (GDL) are examined via fully three-dimensional numerical simulations. First, when the differences in pore properties and wettability between the MPL and GDL are taken into account but the difference in the entry pressures is ignored, the numerical MPL model captures a discontinuity in liquid saturation at the GDL|MPL interface. The simulation does not, however, capture the beneficial effects of an MPL on cell performance, predicting even lower performance than in the case of no MPL. On the other hand, when a high liquid-entry pressure in an MPL is additionally considered, the numerical MPL model predicts a liquid-free MPL and successfully demonstrates the phenomenon that the high liquid-entry pressure of the MPL prevents any liquid water from entering the MPL. Consequently, it is found from the simulation results that a liquid-free MPL significantly enhances the back-flow of water across the membrane into the anode, which, in turn, helps to avoid membrane dehydration and alleviate the level of GDL flooding. As a result, the model successfully reports the beneficial effects of MPLs on PEFC performance and predicts higher performance in the presence of MPLs (e.g., an increase of 67 mV at 1.5 A cm⁻²). This study provides a fundamental explanation of the function of MPLs and quantifies the influence of their porous properties and the liquid-entry pressure on water transport and cell performance.     

Parametric study of the cathode and the role of liquid saturation on the performance of a polymer electrolyte membrane fuel cell—A numerical approach

A two-dimensional two-phase steady state model of the cathode of a polymer electrolyte membrane fuel cell (PEMFC) is developed using unsaturated flow theory (UFT). A gas flow field, a gas diffusion layer (GDL), a microporous layers (MPL), a finite catalyst layer (CL), and a polymer membrane constitute the model domain. The flow of liquid water in the cathode flow channel is assumed to take place in the form of a mist. The CL is modeled using flooded spherical agglomerate characterization. Liquid water is considered in all the porous layers. For liquid water transport in the membrane, electro-osmotic drag and back diffusion are considered to be the dominating mechanisms. The void fraction in the CL is expressed in terms of practically achievable design parameters such as platinum loading, Nafion loading, CL thickness, and fraction of platinum on carbon. A number of sensitivity studies are conducted with the developed model. The optimum operating temperature of the cell is found to be 80-85 °C. The optimum porosity of the GDL for this cell is in the range of 0.7-0.8. A study by varying the design parameters of the CL shows that the cell performs better with 0.3-0.35 mg cm⁻² of platinum and 25-30 wt% of ionomer loading at high current densities. The sensitivity study shows that a multi-variable optimization study can significantly improve the cell performance. Numerical simulations are performed to study the dependence of capillary pressure on liquid saturation using various correlations. The impact of the interface saturation on the cell performance is studied. Under certain operating conditions and for certain combination of materials in the GDL and CL, it is found that the presence of a MPL can deteriorate the performance especially at high current density.     

An analytical model for hydrogen and nitrogen crossover rates in proton exchange membrane fuel cells

In this paper, the hydrogen and nitrogen crossover through the membrane in proton exchange membrane fuel cells, are investigated by developing a semi-empirical analytical model. Different factors that affect the gas crossover rates were considered including pressure drop in gas diffusion layer (GDL) and catalyst layer (CL), operating temperature, relative humidity (RH) of the reactants, GDL compression, and the current density effect on the membrane temperature. The model is validated by published experimental data. It is found that RH is the most important parameter, followed by temperature. The hydrogen pressure drop through GDL and CL greatly depends on the GDL substrate properties, microporous layer (MPL) and CL. When permeability is low, an increase in current density reduces gas crossover. GDL compression, when MPL is used, was found to have a low impact on gas crossover. Gas crossover is improved with current density due to an increase in membrane temperature.     

Direct water balance analysis on a polymer electrolyte fuel cell (PEFC): Effects of hydrophobic treatment and micro-porous layer addition to the gas diffusion layer of a PEFC on its performance during a simulated start-up operation

Effects of hydrophobic treatment and micro-porous layer (MPL) addition to a gas diffusion layer (GDL) in a polymer electrolyte fuel cell (PEFC) have been investigated from water balance analysis at the electrode (catalyst layer), GDL and flow channel in the cathode after a simulated start-up operation. The water balance is directly analyzed by measuring the weight of the adherent water wiped away from each the component. As a result, we find that hydrophobic treatment without MPL leads to the increase in liquid water accumulation at the electrode which limits the oxygen transport to the catalyst and then lowers the cell voltage rapidly during start-up, whereas the treatment decreases the water at the GDL. The water accumulation at the electrode also decreases the cumulative current that represents the power generation and calorific power indispensable for warming up at a temperature below freezing point. On the other hand, we directly find that the hydrophobic treatment with MPL addition suppresses the water accumulation at the electrode, which increases the cumulative current. In addition, it is found that increase in air permeability of a GDL substrate by its coarser structure increases the cumulative current, which is explained by enhancing the exhaust of the product water vapor and liquid as well as by enhancing the oxygen transport directly. Thus, the hydrophobic treatment with MPL addition and larger air permeability of a GDL substrate improve the start-up performance of a PEFC.     

Liquid and oxygen transport through bilayer gas diffusion materials of proton exchange membrane fuel cells

In proton exchange membrane fuel cells (PEMFCs), a hydrophobic micro-porous layer (MPL) is usually placed between the catalyst layer (CL) and the conventional gas diffusion layer (GDL) to relieve the flooding. In this paper, a pore network model is developed to investigate how the MPL structure affects the liquid and oxygen transport properties of the bilayer gas diffusion material (GDM) consisting of fine MPL and coarse GDL. The regular three-dimensional pore network constructed to represent the bilayer GDM are composed of the cubic pores that are connected by the narrow throats of square cross section. Based on this model, the capillary pressure, liquid permeability, and oxygen effective diffusivity as a function of GDM liquid saturation are determined. Parameter studies are performed to elucidate the influences of MPL thickness and of MPL crack width. Also analyzed are the liquid distributions in different structural GDMs at the moment of breakthrough. The results reveal a liquid saturation jump at the MPL/GDL interface in the plain bilayer GDM, but a liquid saturation drop in the defective bilayer GDM.