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.     

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.     

Synchrotron X-ray tomography for investigations of water distribution in polymer electrolyte membrane fuel cells

Synchrotron X-ray tomography is used to visualize the water distribution in gas diffusion layers (GDL) and flow field channels of a polymer electrolyte membrane fuel cell (PEMFC) subsequent to operation. An experimental setup with a high spatial resolution of down to 10 μm is applied to investigate fundamental aspects of liquid water formations in the GDL substrate as well as the formation of water agglomerates in the flow field channels. Detailed analyses of water distribution regarding the GDL depth profile and the dependence of current density on the water amount in the GDL substrate are addressed. Visualizations of water droplets and wetting layer formations in the flow field channels are shown. The three-dimensional insight by means of this quasi in situ tomography allows for a better understanding of PEMFC water management at steady state operation conditions. The effect of membrane swelling as function of current density is pointed out. Results can serve as an essential input to create and verify flow field simulation outputs and single-phase models.     

Assisted water management in a PEMFC with a modified flow field and its effect on performance

This work designed and tested innovative flow channels in order to improve water management in a polymer electrolyte membrane fuel cell (PEMFC). The design employed slanted channels with an angle of 20° in a flow plate to collect the liquid water that permeated from the gas diffusion layers. The effects of orientations of the slanted channels in up-slanted and down-slanted directions and relative humidity levels on the cell performance were investigated. The experimental results showed that modifying the anode flow field using down-slanted channels provided higher cell performance. Water concentration at the gas diffusion layer is reduced resulting in more back diffusion of water from the cathode to anode, thus inducing membrane hydration and improving the conductivity. Promotion of water removal by applying down-slanted channels in the cathode side did not improve the performance. This work has demonstrated that channel cross-section design alone could improve the PEM fuel cell performance. The anode down-slanted cell indeed improved the performances at extremely wet condition and the power was equally good as that without modified flow channel at less wet condition.     

CFD investigating the effects of different operating conditions on the performance and the characteristics of a high-temperature PEMFC

The effects of different operating conditions on the performance and the characteristics of a high-temperature proton exchange membrane fuel cell (PEMFC) are investigated using a three-dimensional (3-D) computational fluid dynamics (CFD) fuel-cell model. This model consists of the thermal-hydraulic equations and the electrochemical equations. Different operating conditions studied in this paper include the inlet gas temperature, system pressure, and inlet gas flow rate, respectively. Corresponding experiments are also carried out to assess the accuracy of this CFD model. Under the different operating conditions, the PEMFC performance curves predicted by the model correspond well with the experimentally measured ones. The performance of PEMFC is improved as the increase in the inlet temperature, system pressure or flow rate, which is precisely captured by the CFD fuel cell model. In addition, the concentration polarization caused by the insufficient supply of fuel gas can be also simulated as the high-temperature PEMFC is operated at the higher current density. Based on the calculation results, the localized thermal-hydraulic characteristics within a PEMFC can be reasonably captured. These characteristics include the fuel gas distribution, temperature variation, liquid water saturation distribution, and membrane conductivity, etc.     

Predicting current density distribution of proton exchange membrane fuel cells with different flow field designs

In a proton exchange membrane fuel cell (PEMFC), flow field design is an important factor that influences the distributions of current density and water accumulation. The segmented model developed in prior study is used to investigate the effect of flow field patterns on current density distribution. This model predicts the distributed characteristics of water content in the membrane, relative humidity in the flow channels, and water accumulation in the gas diffusion layers (GDLs).Three single cells with different flow field patterns are designed and fabricated. These three flow field designs are simulated using the segmented model and the predicted results are compared and validated by experimental data. This segmented model can be used to predict the effect of flow field patterns on water and current distributions before they are machined.     

Basic evaluation of separator type specific phenomena of polymer electrolyte membrane fuel cell by the measurement of water condensation characteristics and current density distribution

This paper investigates phenomena related to water condensation behavior inside a polymer electrolyte membrane fuel cell (PEMFC), and analyzes the effects of liquid water and gas flow on the performance of the fuel cell. A method for simultaneous measurements of the local current density across the reaction area and direct observation of the phenomena in the cell are developed. Experimental results comparing separator types indicate the effect of shortcut flow in the gas diffusion layer (GDL) under the land areas of serpentine separators, and also show the potential of straight channel separators to achieve a relatively uniform current density distribution. To evaluate shortcut flows under the land areas of serpentine separators, a simple circuit model of the gas flow is presented. The analysis shows that slight variations in oxygen concentration caused by the shortcut flows under the land areas affect the local and overall current density distributions. It is also shown that the establishment of gas paths under the water in channels filled with condensed water is effective for stable operation at low flow rates of air in the straight channels.     

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.     

A three-dimensional full-cell CFD model used to investigate the effects of different flow channel designs on PEMFC performance

A three-dimensional “full-cell” computational fluid dynamics (CFD) model is proposed in this paper to investigate the effects of different flow channel designs on the performance of proton exchange membrane fuel cells (PEMFC). The flow channel designs selected in this work include the parallel and serpentine flow channels, single-path and multi-path flow channels, and uniform depth and step-wise depth flow channels. This model is validated by the experiments conducted in the fuel cell center of Yuan Ze University, showing that the present model can investigate the characteristics of flow channel for the PEMFC and assist in the optima designs of flow channels. The effects of different flow channel designs on the PEMFC performance obtained by the model predictions agree well with those obtained by experiments. Based on the simulation results, which are also confirmed by the experimental data, the parallel flow channel with the step-wise depth design significantly promotes the PEMFC performance. However, the performance of PEMFC with the serpentine flow channel is insensitive to these different depth designs. In addition, the distribution characteristics of fuel gases and current density for the PEMFC with different flow channels can be also reasonably captured by the present model.     

Water distribution measurement for a PEMFC through neutron radiography

Neutron radiography has been used for in situ and non-destructive visualization and measurement technique for liquid water in a working proton exchange membrane fuel cell (PEMFC). In an attempt to differentiate water distribution in the anode side from that in the cathode side, a specially designed cell was machined and used for the experiment. The major difference between our design and traditional flow field design is the fact the anode channels and cathode channels were shifted by a channel width, so that the anode and cathode channels do not overlap in the majority of the active areas.

The neutron radiography experiments were performed at selected relative humidities, and stoichiometry values of cathode inlet. At each operating condition, the water distribution in anode/cathode gas diffusion layers (GDLs) was obtained. Image processing with four different spatial masks was applied to those images to differentiate liquid water in four different types of areas. Results indicate that the reactant gas relative humidity and stoichiometry significantly influence current density distribution and water distribution.     

Experimentally and numerically investigating cell performance and localized characteristics for a high-temperature proton exchange membrane fuel cell

This paper is to experimentally and numerically investigate the cell performance and the localized characteristics associated with a high-temperature proton exchange membrane fuel cell (PEMFC). Three experiments are carried out in order to study the performance of the PEMFC with different operating conditions and to validate the numerical simulation model. The model proposed herein is a three-dimensional (3-D) computational fluid dynamics (CFD) non-isothermal model that essentially consists of thermal–hydraulic equations and electrochemical model. The performance curves of the PEMFC predicted by the present model agree with the experimental measured data. In addition, both the experiments and the predictions precisely demonstrate the enhanced effects of inlet gas temperature and system pressure on the PEMFC performance. Based on the simulation results, the localized characteristics within a PEMFC can be reasonably captured. These parameters include the fuel gas distribution, liquid water saturation distribution, membrane conductivity distribution, temperature variation, and current density distribution etc. As the PEMFC is operated at the higher current density, the fuel gas would be insufficiently supplied to the catalyst layer, consequently causing the decline in the generation of power density. This phenomenon is so called mass transfer limitation, which can be precisely simulated by the present CFD model.     

Optimization of blocked flow field performance of proton exchange membrane fuel cell with auxiliary channels

The flow field optimization design is one of the important methods to improve the performance of proton exchange membrane fuel cell (PEMFC). In this study, a new structure with staggered blocks on the parallel flow channels of PEMFC and auxiliary flow channels under the ribs is proposed. Through numerical calculation method, the effect of blocks auxiliary flow field (BAFF) on pressure drop, reactant distribution and liquid water removal in the fuel cells are investigated. The results show that when the operating voltage is 0.5 V, the current density of BAFF is 21.74% higher than that of the straight parallel flow field (SPFF), and the power density reaches 0.65 W cm^?2. BAFF improves performance by equalizing the pressure drop across sub-channels, promoting the uniform distribution of reactant, and enhancing transport across the ribs. In addition, through parameter analysis, it is found that BAFF can discharge liquid water in time at the conditions of high humidification, high current density and low temperature, which ensures the output performance of the fuel cell and improves the durability of the fuel cell. This paper provides new ideas for the improvement of PEMFC flow field design, which is beneficial to the development of PEMFC with high current density.     

Analyze the effects of flow mode and humidity on PEMFC performance by equivalent membrane conductivity

A three‐dimensional and two‐phase numerical model is developed for a 25‐cm² proton exchange membrane fuel cell (PEMFC) to investigate the effects of flow mode (coflow and counterflow) and relative humidity (anode 0%/100%; cathode 60%/100%) on the cell performance. Experimental studies are performed to validate this developed model. An equivalent membrane conductivity is proposed to describe the match level between current flux and membrane conductivity. It is found that the cell performance is enhanced under low relative humidity conditions because of the optimized equivalent membrane conductivity. More specifically, the voltage is improved from 0.611 to 0.637 V, and the equivalent membrane conductivity is enhanced from 10.35 to 11.11 S m^?1 by replacing the coflow mode with counterflow mode at 1000 mA cm^?2 when anode gas is dry and cathode gas is 100% hydrated. Both the anode and cathode relative humidities show an obvious influence on the PEMFC performance, and a suitable inlet humidity could ensure adequate hydration of membrane and avoid water flooding in gas diffusion layers simultaneously.     

Detection of liquid water in the flow channels of PEM fuel cell using an optical sensor

An optical sensor was developed with the capability of detecting liquid water in the flow channels of a proton exchange membrane fuel cell (PEMFC) as well as simultaneously measuring temperature. This work is an extension of previous research in which an optical temperature sensor was developed for measuring the in situ temperature of PEM fuel cells based on the principles of phosphor thermometry. The optical sensor was installed in the cathode flow channel of a 5 cm² proton exchange membrane fuel cell. The fuel cell was tested under both dry and humid conditions. Liquid water formation in the flow channels was quantitatively measured from the experimental data. An observed time fraction value was estimated for characterizing flow channel flooding. The observed time fraction of liquid water in the flow channel was found to be closely related to the relative humidity of reactants and the operating current of the fuel cell.     

Liquid water preferential accumulation in channels of PEM fuel cells with multiple serpentine flow fields

This work presents an experimental investigation on the preferential accumulation of liquid water in the channels of a multiple serpentine PEMFC with 50 cm² active area. Neutron imaging was used for visualizing the liquid water distribution during the cell operation for a wide range of operating conditions. Liquid water accumulation in the cathode channels was observed for most of the operating conditions, with a preferential accumulation in certain channels of the flow field. A statistical analysis was performed in order to determine the main characteristics of this accumulation (i.e. channel number and degree of accumulation). As cathode channels were positioned in vertical direction, it was found that gravity effects had an important influence in the accumulation, as well as the relative position of the channel with respect to the inlet and outlet locations. The gas flow direction had also a major impact on the water accumulation within the channels, with significantly more water accumulated in channels with upwards gas flow.     

Numerical investigation of innovative 3D cathode flow channel in proton exchange membrane fuel cell

Two kinds of innovative 3‐dimensional (3D) proton exchange membrane fuel cell (PEMFC) cathode flow channel designs were proposed to improve the water removal on the surface of gas diffusion layer and enhance mass transfer between flow channel and gas diffusion layer. A validated 2‐phase volume of fluid model was used to investigate different water removal behaviors in flow channel. The optimal length of water baffle and other parameters of the proposed designs were determined. A validated 3D PEMFC performance model was adopted to assess the new designs. The results suggest that these 2 designs can improve PEMFC performance as to 9% when operating at the high current density because of the significant enhancement of mass transfer induced by air baffles.     

不同型式流场的PEMFC内部传质分析

对采用不同型式流场的PEMFC进行建模,并用控制容积法对控制方程进行离散,求解得到PEMFC内部各物理量的分布以及综合水拖带系数、质子交换膜平均电导率等。分析了采用交趾型流场和常规流场时PEMFC的内部传质以及阴极性能、电池性能和膜性能,结果认为采用交趾型流场时,PEMFC阴极性能高于采用常规流场的PEMFC阴极性能,但质子交换膜的平均电导率低于采用常规流场时。在没有液态水产生时常规流场PEMFC性能高于交趾型流场PEMFC。     

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.     

Numerical simulation of exchange membrane fuel cells in different operating conditions

A two-dimensional, steady state model for proton exchange membrane fuel cell (PEMFC) is presented. The model is used to describe the effect operation conditions (current density, pressure and water content) on the water transport, ohmic resistance and water distribution in the membrane and performance of PEMFC. This model considers the transport of species and water along the porous media: gas diffusion layers (GDL) anode and cathode, and the membrane of PEMFC fuel cell.     

Innovative gas diffusion layers and their water removal characteristics in PEM fuel cell cathode

Liquid water transport is one of the key challenges regarding the water management in a proton exchange membrane (PEM) fuel cell. Conventional gas diffusion layers (GDLs) do not allow a well-organized liquid water flow from catalyst layer to gas flow channels. In this paper, three innovative GDLs with different micro-flow channels were proposed to solve liquid water flooding problems that conventional GDLs have. This paper also presents numerical investigations of air–water flow across the proposed innovative GDLs together with a serpentine gas flow channel on PEM fuel cell cathode by use of a commercial computational fluid dynamics (CFD) software package FLUENT. The results showed that different designs of GDLs will affect the liquid water flow patterns significantly, thus influencing the performance of PEM fuel cells. The detailed flow patterns of liquid water were shown. Several gas flow problems for the proposed different kinds of innovative GDLs were observed, and some useful suggestions were given through investigating the flow patterns inside the proposed GDLs.