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.     

An experimental study on water transport through the membrane of a PEFC operating in the dead-end mode

Water transport through the membrane of a polymer electrolyte fuel cell (PEFC) was investigated by not only measuring the voltage variation but also visualizing the accumulation of water at the anode for various values of operating parameters, such as the humidity, current density, stoichiometry, location of humidification, and membrane properties. The PEFC was operated in the dead-end mode to prevent the discharge of water from the anode. The water transport in the PEFC was characterized by the elapsed time for the voltage to reach its limit. Anode visualization showed water transport under various conditions. In addition, the mass balance of water at the anode of the PEFC was considered. The variations of water diffusion and electro-osmotic drag were analyzed based on the experimental results.     

质子交换膜燃料电池内部传热传质三维模拟

针对常规流场质子交换膜燃料电池提出了三维非等温数学模型。模型考虑了电化学反应动力学以及反应气体在流道和多孔介质内的流动和传递过程,详细研究了水在质子膜内的电渗和扩散作用。计算结果表明,反应气体传质的限制和质子膜内的水含量直接决定了电极局部电流密度的分布和电池输出性能;在电流密度大于0.3~0.4A/cm2时开始出现水从阳极到阴极侧的净迁移;高电流密度时膜厚度方向存在很大的温度梯度,这对膜内传递过程有较大影响。     

Experimental study on water transport coefficient in Proton Exchange Membrane Fuel Cell

Water transport within Proton Exchange Membrane Fuel Cell (PEMFC) is investigated by systematic measurements of the water transport coefficient, defined as the net water flux across the membrane divided by the water production. It is recorded for various operating conditions (current density, gas stoichiometry, air inlet relative humidity, temperature, pressure) in a fuel cell stack fed by dry hydrogen. The measurement of the water transport coefficient shows that a significant fraction of water is collected at the anode while water is produced or injected at the cathode. Moreover, in usual operating conditions, liquid water is present at the cell outlet not only in the cathode but also in the anode. Contrary to the electrical performances, ageing has no influence on the water transport coefficient, which allows the comparison between data collected at different periods of the fuel cell lifetime. From this comparison, it was found that the hydrogen flow rate, the amount of vapor injected at cathode inlet, and the temperature are the main parameters influencing the water transport coefficient. It is shown that air and hydrogen stoichiometry present significant effects on water transport but only through these parameters.     

Computational study of water transport in proton exchange membrane fuel cells

A computational fuel cell dynamics framework is used to develop a unified water transport equation for a proton exchange membrane fuel cell (PEMFC). Various modes of water transport, i.e., diffusion, convection and electro-osmotic drag, are incorporated in the unified water transport equation. The water transport model is then applied to elucidate water management in three-dimensional fuel cells with dry-to-low humidified inlet gases after its validation against available experimental data for dry oxidant and fuel streams. An internal circulation of water with the aid of counter-flow design is found to be of vital importance for low-humidity operation, for example, in the portable application of a PEMFC without an external humidifier. The general features of water transport in PEMFCs are discussed to show various water transport regimes of practical interest, such as anode water loss, cathode flooding, and the equilibrium condition of water at the channel outlets, particularly for limiting situations where anode and cathode water profiles acquire an equilibrium state. From the practical point of view, the effects of the flow arrangement, membrane thickness, and inlet gas humidity as important determinants of fuel cell performance are also analyzed to elucidate fuel cell water transport characteristics.     

Water transport in the proton-exchange-membrane fuel cell: measurements of the effective drag coefficient

The water transport in proton-exchange-membrane fuel cells has been experimentally investigated by measurements of the effective or net drag coefficient. Results are presented for a wide range of operating conditions (current density, temperature, pressure, stoichiometry and humidity of the inlet gases), as well as for different types of membrane-electrode-assemblies. It was found that the humidity and the stoichiometry of the inlet gases had a large effect on the drag. Of the material properties investigated, the membrane thickness was found to be the most significant parameter. Inspection of the cell performances showed that drying of the cathode was much more detrimental for the cell performance than drying of the anode. This was ascribed to increased activation losses, which turned out to be extremely sensitive to the type of cathode used.     

The effect of relative humidity of the cathode on the performance and the uniformity of PEM fuel cells

The effect of relative humidity of the cathode (RHC) on proton exchange membrane (PEM) fuel cells has been studied focusing on automotive operation. Computational fluid dynamics (CFD) simulations were performed on a 300-cm² serpentine flow-field configuration at various RHC levels. The dependency of current density, membrane water contents, net water flux on the performance and the uniformity was investigated. The uniformity of current density and temperature was evaluated by employing standard deviation. The water balance inside a fuel cell was examined by describing electro-osmotic drag and back diffusion. It was concluded that the RHC has strong effect on the cell performance and uniformity. The dry RHC showed low cell voltage and non-uniform distributions of current density and temperature, whereas high RHC presented increased cell performance and uniform distributions of current density and temperature with well-hydrated membrane electrode assembly (MEA). Also the local current density distribution was strongly dependent on the local membrane water contents distribution that has complex phenomena of water transport. The elimination of external humidifier is desirable for the automotive operation, but it could degrade cell performance and durability due to dehydration of the MEA. Therefore a proper humidification of the reactant is necessary.     

In situ measurements of water crossover through the membrane for direct methanol fuel cells

We show analytically that the water-crossover flux through the membrane used for direct methanol fuel cells (DMFCs) can be in situ determined by measuring the water flow rate at the exit of the cathode flow field. This measurement method enables investigating the effects of various design and geometric parameters as well as operating conditions, such as properties of cathode gas diffusion layer (GDL), membrane thickness, cell current density, cell temperature, methanol solution concentration, oxygen flow rate, etc., on water crossover through the membrane in situ in a DMFC. Water crossover through the membrane is generally due to electro-osmotic drag, diffusion and back convection. The experimental data showed that diffusion dominated the total water-crossover flux at low current densities due to the high water concentration difference across the membrane. With the increase in current density, the water flux by diffusion decreased, but the flux by back convection increased. The corresponding net water-transport coefficient was also found to decrease with current density. The experimental results also showed that the use of a hydrophobic cathode GDL with a hydrophobic MPL could substantially reduce water crossover through the membrane, and thereby significantly increasing the limiting current as the result of the improved oxygen transport. It was found that the cell operating temperature, oxygen flow rate and membrane thickness all had significant influences on water crossover, but the influence of methanol concentration was negligibly small.     

Water transport characteristics of a PEM fuel cell at various operating pressures and temperatures

In this investigation, water in a single-cell proton exchange membrane (PEM) fuel cell was managed using saturated hydrogen and dry air. The experiment was conducted at temperatures of 40, 50 and 60 °C and pressures of 1 and 1.5 bar at both the anode and cathode gas inlets. The feed velocities of hydrogen and air were fixed at 3 and 6 L min⁻¹, respectively. After reaching steady-state conditions, the relative humidity along the single serpentine gas channel was measured. From the experimental data, water transport properties were characterized based on a membrane hydration model. The electro-osmotic drag coefficient, water diffusion coefficient, membrane ionic conductivity and water back-diffusion flux were significantly influenced by the water content in the membrane of the PEM fuel cell. The water content depended on the relative humidity profile along the gas channel. In this investigation, a negative value for the water back-diffusion flux was measured; thus, the transport of water from the cathode to the anode did not occur. This phenomenon was due to the large water concentration gradient between the anode and cathode. Therefore, this strategy successfully prevented flooding in the PEM fuel cell.     

Water transport in the proton exchange-membrane fuel cell: Comparison of model computation and measurements of effective drag

Water transport through the membrane of a PEMFC was investigated by measurement of the net drag under various feed gas humidity. Measured data were compared with computed results obtained using a two-dimensional cell model. Considering the change in the gas content related to the flow configuration, the humidity of the supply gas, reaction rates, and the mass balance of each gas species were derived at five sections along the flow channels. By solving these mass balance equations, the water transport rates and current density distribution were obtained along the flow channels for various feed gas humidity. The results for net drag computed from the model show a similar tendency, but are slightly higher than the measured values. This suggests that there is a certain resistance related to water transport at the cathode membrane interface in association with water production. The cause of this water transport resistance is discussed.     

Comparison of current distributions in proton exchange membrane fuel cells with interdigitated and serpentine flow fields

Current distributions in a proton exchange membrane fuel cell (PEMFC) with interdigitated and serpentine flow fields under various operating conditions are measured and compared. The measurement results show that current distributions in PEMFC with interdigitated flow fields are more uniform than those observed in PEMFC with serpentine flow fields at low reactant gas flow rates. Current distributions in PEMFC with interdigitated flow fields are rather uniform under any operating conditions, even with very low gas flow rates, dry gas feeding or over-humidification of reactant gases. Measurement results also show that current distributions for both interdigitated and serpentine flow fields are significantly affected by reactant gas humidification, but their characteristics are different under various humidification conditions, and the results show that interdigitated flow fields have stronger water removal capability than serpentine flow fields. The optimum reactant gas humidification temperature for interdigitated flow fields is higher than that for serpentine flow fields. The performance for interdigitated flow fields is better with over-humidification of reactant gases but it is lower when air is dry or insufficiently humidified than that for serpentine flow fields.     

Characteristics of water transport through the membrane in direct methanol fuel cells operating with neat methanol

The water required for the methanol oxidation reaction in a direct methanol fuel cell (DMFC) operating with neat methanol can be supplied by diffusion from the cathode to the anode through the membrane. In this work, we present a method that allows the water transport rate through the membrane to be in-situ determined. With this method, the effects of the design parameters of the membrane electrode assembly (MEA) and operating conditions on the water transport through the membrane are investigated. The experimental data show that the water flux by diffusion from the cathode to the anode is higher than the opposite flow flux of water due to electro-osmotic drag (EOD) at a given current density, resulting in a net water transport from the cathode to the anode. The results also show that thinning the anode gas diffusion layer (GDL) and the membrane as well as thickening the cathode GDL can enhance the water transport flux from the cathode to the anode. However, a too thin anode GDL or a too thick cathode GDL will lower the cell performance due to the increases in the water concentration loss at the anode catalyst layer (CL) and the oxygen concentration loss at the cathode CL, respectively.     

Application of water barrier layers in a proton exchange membrane fuel cell for improved water management at low humidity conditions

This study discusses the use of an additional layer in the cathode side of a proton exchange membrane fuel cell (PEMFC) for improved water management at dry conditions. The performance of fuel cells deteriorates significantly when low to no gas humidification is used. This study demonstrates that adding a non-porous material with perforations, such as stainless steel, between the cathode flow field plate and the gas diffusion layer (GDL) improves the water saturation in the cathode GDL and catalyst layer, increases the water content in the anode, and keeps the membrane hydrated. The slight voltage drop in the performance as a result of transport limitations is justifiable since the overall durability of the cell at these extreme conditions is enhanced. The results show that the perforated layer(s) enhances the operational life of the PEMFC under completely dry conditions. These extreme conditions (dry gases without humidification, 90 kPa, 75 °C) were used to accelerate the failure modes in the fuel cells.     

Analysis of water transport in a high pressure PEM electrolyzer

This paper analyzes, through experimental data and a transport model, the water transported through the membrane under different operating conditions in a on a Proton Exchange Membrane (PEM) electrolyzer operating with a high-pressure gradient across the membrane from the cathode (high-pressure) side to the anode (nearly ambient-pressure) side. The phenomena involved in this movement are described and analyzed, with a focus on the electro-osmotic drag coefficient, n_eo. We have observed that the behavior of the hydraulic percolation determines the results obtained for the electro-osmotic drag, while the contribution of the water diffusion is negligible. In general, the cathode pressure significantly reduces the water transport (a positive effect). Also, operation at lower current density reduces the net electro-osmotic drag coefficient, n_g; therefore, the best operation strategy for obtaining dried hydrogen at the cathode is to impose high cathode pressure and low current density.     

Three-dimensional two-phase flow model of proton exchange membrane fuel cell with parallel gas distributors

A non-isothermal, steady-state, three-dimensional (3D), two-phase, multicomponent transport model is developed for proton exchange membrane (PEM) fuel cell with parallel gas distributors. A key feature of this work is that a detailed membrane model is developed for the liquid water transport with a two-mode water transfer condition, accounting for the non-equilibrium humidification of membrane with the replacement of an equilibrium assumption. Another key feature is that water transport processes inside electrodes are coupled and the balance of water flux is insured between anode and cathode during the modeling. The model is validated by the comparison of predicted cell polarization curve with experimental data. The simulation is performed for water vapor concentration field of reactant gases, water content distribution in the membrane, liquid water velocity field and liquid water saturation distribution inside the cathode. The net water flux and net water transport coefficient values are obtained at different current densities in this work, which are seldom discussed in other modeling works. The temperature distribution inside the cell is also simulated by this model.     

Effect of primary parameters on the performance of PEM fuel cell

A one-dimensional, steady-state and isothermal model for a proton exchange membrane (PEM) fuel cell has been developed to investigate the effects of various parameters such as the molar fraction of nitrogen gas, relative humidity, temperature, pressure, membrane thickness, anode and cathode stoichiometric flow ratio and the distribution of oxygen in the cathode catalyst while water transfer in membrane is produced by diffusion, pressure gradient and electro-osmotic drag. The most critical problems to overcome in the proton exchange membrane (PEM) fuel cell technology are the water and thermal management. The results show that the cell performance increases as operating pressure and temperature are increased. The performance of cell can decrease by decreasing the relative humidity of inlet gases and increasing the membrane thickness. Increasing the anode and cathode stoichiometric flow ratio can also improve the cell performance. As the oxygen concentration becomes zero in about 8 percent depth of cathode catalyst layer, the thickness of cathode catalyst layer can be reduced 92 percent without any potential loss in output voltage. The cathode activation loss also becomes high by increasing the molar fraction of nitrogen gas. The modeling results agree very well with experimental results.     

The effect of low humidity on the uniformity and stability of segmented PEM fuel cells

The performance and stability of a PEMFC depends on many operating parameters. The measurement of local currents in PEMFC cells is an important tool for diagnoses and development of fuel cells. In this study, a segmented cell was developed, which could serve as an essential instrument to investigate the different operating conditions in the cells and stacks of technical relevance. In addition, the effects of different feed gas humidity and temperatures were investigated to analyze the steady-state performance, uniformity, and the local stability of PEMFC with the use of eight segmented regions. With this research method, the resistance in each segment could be measured by ac impedance as well as make a comparison between Nafion^® 117 and 112 membranes in PEMFC. In the experiment, by probing into the high frequency internal resistance and performance of this cell, the effects of flow rates of fuels, oxidants, relative humidity, and directional channel flows were investigated for performance and stability of local segmented regions. The results of the experiments demonstrate that the local current distribution is strongly influenced by the relative humidity of fuel, the stoichiometric of the processed air, and the mode of operation. The cell was operated at a cell temperature of 50 °C with low relative humidity of 33% and 0%, causing the drying of the membrane (and increase of its resistance) at the top-stream path. The membrane conductivity was enhanced due to the water product increase by the reaction in the middle- and down-stream paths, because the down-stream has higher current than the top-stream. The relative humidity of the air increased along the path due to the product water, therefore, the current density increased as well. The local segmented cell could maintain stable performance at low hydrogen stoichiometry of 1.05 for low humidity gases. As the counter-flow and inverse gravity direction of hydrogen fuel was operated, the fuel cell showed the much more stable and uniform local performance.     

Effects of operating parameters on the transient response of proton exchange membrane fuel cells subject to load changes

Water management is critical to the steady state and transient performance of proton exchange membrane (PEM) fuel cell systems. The dynamic behavior of PEM fuel cells is profoundly related to the dynamics of water transport in the fuel cell system, which include electro-osmotic drag and back-diffusion of water through membrane and rate at which water is generated, supplied and removed from the electrodes. In particular, for low humidity operations, water transport dynamics plays a dominant role in determining the time taken to reach steady state. Toward an understanding of the fuel cell dynamics for dry operations, numerical simulations are carried out for a single channel polymer electrolyte (PEM) fuel cell undergoing step change in cell voltage over wide range of operating conditions. A detailed model-based parametric study is carried out to analyze the effects of operating conditions on membrane water content and the time taken by the single cell system to reach steady state. Based on the studies, design windows are presented that limit the time taken to reach steady state to a desired value. Optimum parameter combinations that minimize the transient time are also identified from the studies.     

Design of low-humidification PEMFC by using cell simulator and its power generation verification test

As long as the perfluorinated proton exchange membrane (PEM) is used for the electrolyte, both the cell performance and life are highly dependent upon the water content in the electrolyte. On the other hand, pre-humidification of fuel and oxidant gases complicates the PEMFC system and prevents it from possible cost reduction measures. In this study, in order to maintain a membrane electrode assembly (MEA) with a satisfactory water content by only the water produced in catalyst layer through the electrode reaction without prior humidification of both the fuel and oxidant gases, a novel gas diffusion layer (GDL) was fabricated. This was achieved by coating a water management layer (WML) onto a traditional GDL in order to place the WML between the traditional GDL and the catalyst layer of the PEMFC. This study describes the significant balance of water with WML in the fuel cell using both simulation and experimental analysis.     

The effect of fuel cell operational conditions on the water content distribution in the polymer electrolyte membrane

Models play an important role in fuel cell design and development. One of the critical problems to overcome in the proton exchange membrane (PEM) fuel cells is the water management. In this work a steady state, two-dimensional, isothermal model in a single PEM fuel cell using individual computational fluid dynamics code was presented. Special attention was devoted to the water transport through the membrane which is assumed to be combined effect of diffusion, electro-osmotic drag and convection. The effect of current density variation distribution on the water content (λ) in membrane/electrode assembly (MEA) was determined. In this work the membrane heat conductivity is considered as a function of water content and the effect of temperature distribution in membrane is also analyzed. After that detail distributions of oxygen concentration, water content in membrane, net water flux and different overpotentials were calculated. Our simulation results show the reduction of reactant concentration in flow channels has a significant effect on electrochemical reaction in the gas diffusion and catalyst layer. Different fluxes are compared to investigate the effect of operating condition on the water fluxes in membrane. The amounts of different fluxes are strong function of current density, which is related to external load. The model also can use for simulating different kind of membranes. The model prediction of water content curves are compared with one-dimensional model predictions data reported in the validated open literature and a good compatibility were observed.