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.     

Numerical investigation of the impact of two-phase flow maldistribution on PEM fuel cell performance

Flow maldistribution usually happens in PEM fuel cells when using common inlet and exit headers to supply reactant gases to multiple channels. As a result, some channels are flooded with more water and have less air flow while other channels are filled with less water but have excessive air flow. To investigate the impact of two-phase flow maldistribution on PEM fuel cell performance, a Volume of Fluid (VOF) model coupled with a 1D MEA model was employed to simulate two parallel channels. The slug flow pattern is mainly observed in the flow channels under different flow maldistribution conditions, and it significantly increases the gas diffusion layer (GDL) surface water coverage over the whole range of simulated current densities, which directly leads to poor fuel cell performance. Therefore, it is recommended that liquid and gas flow maldistribution in parallel channels should be avoided if possible over the whole range of operation. Increasing the gas stoichiometric flow ratio is not an effective method to mitigate the gas flow maldistribution, but adding a gas inlet resistance to the flow channel is effective in mitigating maldistribution. With a carefully selected value of the flow resistance coefficient, both the fuel cell performance and the gas flow distribution can be significantly improved without causing too much extra pressure drop.     

Model prediction of effects of operating parameters on proton exchange membrane fuel cell performance

The performance of a proton exchange membrane (PEM) fuel cell is greatly affected by the operating parameters. Appropriate operating parameters are necessary for PEM fuel cells to maintain stable performance. A three-dimensional multi-phase fuel cell model (FCM) is developed to predict the effects of operating parameters (e.g. operating pressure, fuel cell temperature, relative humidity of reactant gases, and air stoichiometric ratio) on the performance of PEM fuel cells. The model presented in this paper is a typical nine-layer FCM that consists of current collectors, flow channels, gas diffusion layers, catalysts layers at the anode and the cathode as well as the membrane. A commercial Computational Fluid Dynamics (CFD) software package Fluent is used to solve this predictive model through SIMPLE algorithm and the modeling results are illustrated via polarization curves including I–V and I–P curves. The results indicate that the cell performance can be enhanced by increasing operating pressure and operating temperature. The anode humidification has more significant influences on the cell performance than the cathode humidification, and the best performance occurs at moderate air relative humidity while the hydrogen is fully humidified. In addition, the cell performance proves to be improved with the increase of air stoichiometric ratio. Based on these conclusions, several suggestions for engineering practice are also provided.     

In situ investigation of water transport in an operating PEM fuel cell using neutron radiography: Part 2 – Transient water accumulation in an interdigitated cathode flow field

An interdigitated cathode flow field has been tested in situ with neutron radiography to measure the water transport through the porous gas diffusion layer in a PEM fuel cell. Constant current density to open circuit cycles were tested and the resulting liquid water accumulation and dissipation rates with in-plane water distributions are correlated to measured pressure differential between inlet and outlet gas streams. The effect of varying the reactant gas relative humidity on liquid water accumulation is also demonstrated. These results provide evidence that the reactant gas establishes a consistent in-plane transport path through the diffusion layer, leaving stagnant regions where liquid water accumulates. A simplified permeability model is presented and used to correlate the relative permeability to varying gas diffusion layer liquid water saturation levels.     

Treatment of two-phase flow in cathode gas channel for an improved one-dimensional proton exchange membrane fuel cell model

It has been reported recently that water flooding in the cathode gas channel has significant effects on the characteristics of a proton exchange membrane fuel cell. A better understanding of this phenomenon with the aid of an accurate model is necessary for improving the water management and performance of fuel cell. However, this phenomenon is often not considered in the previous one-dimensional models where zero or a constant liquid water saturation level is assumed at the interface between gas diffusion layer and gas channel. In view of this, a one-dimensional fuel cell model that includes the effects of two-phase flow in the gas channel is proposed. The liquid water saturation along the cathode gas channel is estimated by adopting Darcy’s law to describe the convective flow of liquid water under various inlet conditions, i.e. air pressure, relative humidity and air stoichiometry. The averaged capillary pressure of gas channel calculated from the liquid water saturation is used as the boundary value at the interface to couple the cathode gas channel model to the membrane electrode assembly model. Through the coupling of the two modeling domains, the water distribution inside the membrane electrode assembly is associated with the inlet conditions. The simulation results, which are verified against experimental data and simulation results from a published computational fluid dynamics model, indicate that the effects of relative humidity and stoichiometry of inlet air are crucial to the overall fuel cell performance. The proposed model gives a more accurate treatment of the water transport in the cathode region, which enables an improved water management through an understanding of the effects of inlet conditions on the fuel cell performance.     

Effects of operating parameters on transport phenomena and cell performance of PEM fuel cells with conventional and contracted flow field designs

A contracted parallel flow field design was developed to improve fuel cell performance compared with the conventional parallel flow field design. A three-dimensional model was used to compare the cell performance for both designs. The effects of the cathode reactant inlet velocity and cathode reactant inlet relative humidity on the cell performance for both designs were also investigated. For operating voltages greater than 0.7 V because the electrochemical reaction rates are lower with less oxygen consumption and less liquid water production, the cell performance is independent of the flow field designs and operating parameters. However, for lower operating voltages where the electrochemical reaction rates gradually increase, the oxygen transport and the liquid water removal efficiency differ for the various flow field designs and operating parameters; therefore, the cell performance is strongly dependent on both the design and operating parameters. For lower operating voltages, the cell performance for the contracted design is better than for the conventional design because the reactant flow velocities in the contracted region significantly increase, which enhances liquid water removal and reduces the oxygen transport resistance. For lower operating voltages, as the cathode reactant inlet velocity increases and the cathode reactant inlet relative humidity decreases, the cell performance for both designs improves.     

Effects of electrode wettabilities on liquid water behaviours in PEM fuel cell cathode

Liquid water transport is one of the key challenges for water management in a proton exchange membrane (PEM) fuel cell. Investigation of the air–water flow patterns inside fuel cell gas flow channels with gas diffusion layer (GDL) would provide valuable information that could be used in fuel cell design and optimization. This paper presents numerical investigations of air–water flow across an innovative GDL with catalyst layer and serpentine channel on PEM fuel cell cathode by use of a commercial Computational Fluid Dynamics (CFD) software package FLUENT. Different static contact angles (hydrophilic or hydrophobic) were applied to the electrode (GDL and catalyst layer). The results showed that different wettabilities of cathode electrode could affect liquid water flow patterns significantly, thus influencing on the performance of PEM fuel cells. The detailed flow patterns of liquid water were shown, several gas flow problems were observed, and some useful suggestions were given through investigating the flow patterns.     

A hybrid model of cathode of PEM fuel cell using the interdigitated gas distributor

A two-dimensional (2D), single- and two-phase, hybrid multi-component transport model is developed for the cathode of PEM fuel cell using interdigitated gas distributor. The continuity equation and Darcy's law are used to describe the flow of the reactant gas and production water. The production water is treated as vapor when the current density is small, and as two-phase while the current density is greater than the critical current density. The advection–diffusion equations are utilized to study species transport of multi-component mixture gas. The Butler–Volmer equation is prescribed for the domain in the catalyst layer. The predicted results of the hybrid model agree well with the available experimental data. The model is used to investigate the effects of operating conditions and the cathode structure parameters on the performance of the PEM fuel cell. It is observed that liquid water appears originally in the cathodic catalyst layer over outlet channel under intermediate current and tends to be distributed uniformly by the capillary force with the increase of the current. It is found that reduction of the width of outlet channel can enhance the performance of PEM fuel cell via the increase of the current density over this region, which has, seemingly, not been discussed in previous literatures.     

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.     

An experimental and numerical investigation on the cross flow through gas diffusion layer in a PEM fuel cell with a serpentine flow channel

A serpentine flow channel is one of the most common and practical channel layouts for a polymer electrolyte membrane (PEM) fuel cell since it ensures the removal of water produced in a cell with acceptable parasitic load. During the reactant flows along the flow channel, it can also leak or cross to neighboring channel via the porous gas diffusion layer due to the high pressure gradient caused by the short distance. Such a cross flow leads to a larger effective flow area altering reactant flow in the flow channel so that the resultant pressure and flow distributions are substantially different from that without considering cross flow, even though this cross flow has largely been ignored in previous studies. In this work, a numerical and experimental study has been carried out to investigate the cross flow in a PEM fuel cell. Experimental measurements revealed that the pressure drop in a PEM fuel cell is significantly lower than that without cross flow. Three-dimensional numerical simulation has been performed for wide ranges of flow rate, permeability and thickness of gas diffusion layer to analyze the effects of those parameters on the resultant cross flow and the pressure drop of the reactant streams. Considerable amount of cross flow through gas diffusion layer has been found in flow simulation and its effect on pressure drop becomes more significant as the permeability and the thickness of gas diffusion layer are increased. The effects of this phenomenon are also crucial for effective water removal from the porous electrode structure and for estimating pumping energy requirement in a PEM fuel cell, it cannot be neglected for the analysis, simulation, design, operation and performance optimization of practical PEM fuel cells.     

Assembly pressure and membrane swelling in PEM fuel cells

Assembly pressure and membrane swelling induced by elevated temperature and humidity cause inhomogeneous compression and performance variation in proton exchange membrane (PEM) fuel cells. This research conducts a comprehensive analysis on the effects of assembly pressure and operating temperature and humidity on PEM fuel cell stack deformation, contact resistance, overall performance and current distribution by advancing a model previously developed by the authors. First, a finite element model (FEM) model is developed to simulate the stack deformation when assembly pressure, temperature and humidity fields are applied. Then a multi-physics simulation, including gas flow and diffusion, proton transport, and electron transport in a three-dimensional cell, is conduced. The modeling results reveal that elevated temperature and humidity enlarge gas diffusion layer (GDL) and membrane inhomogeneous deformation, increase contact pressure and reduce contact resistance due to the swelling and material property change of the GDL and membrane. When an assembly pressure is applied, the fuel cell overall performance is improved by increasing temperature and humidity. However, significant spatial variation of current distribution is observed at elevated temperature and humidity.     

A numerical study of orientated-type flow channels with porous-blocked baffles of proton exchange membrane fuel cells

Orientated-type flow channels having porous blocks enhance the reactant transfer into gas diffusion layers of proton exchange membrane fuel cells. However, because of the blockages accounted by baffles and porous blocks in channel regions, pumping power increases. In this study, with the aim of further reducing the pumping power in flow channels with porous-blocked baffles, an orientated-type flow channel with streamline baffles having porous blocks is proposed. By employing an improved two-fluid model, cell performance, liquid water distribution and pumping power in a single flow channel are numerically studied. The simulation results show that the baffles with porous blocks increase the cell performance, and the streamline baffles with larger volumes further increase the performance; the produced water in porous regions is ejected more under inertial effect, especially at the regions near to baffles in gas diffusion layers and inside porous blocks. In addition, by using the streamline baffles, the excessive increase in power loss is further reduced. Moreover, the location and porosity effects of baffles with porous blocks are analyzed. Simulation results show that the location exhibits obscure effects on reactant transfer and cell performance, while the liquid water can be removed more when the porous blocked baffles are concentrated at downstream. The net power is enhanced more when using three porous blocks with the porosity of 0.00.     

Characterization of flooding and two-phase flow in polymer electrolyte membrane fuel cell stacks

A partially flooded gas diffusion layer (GDL) model is proposed and solved simultaneously with a stack flow network model to estimate the operating conditions under which water flooding could be initiated in a polymer electrolyte membrane (PEM) fuel cell stack. The models were applied to the cathode side of a stack, which is more sensitive to the inception of GDL flooding and/or flow channel two-phase flow. The model can predict the stack performance in terms of pressure, species concentrations, GDL flooding and quality distributions in the flow fields as well as the geometrical specifications of the PEM fuel cell stack. The simulation results have revealed that under certain operating conditions, the GDL is fully flooded and the quality is lower than one for parts of the stack flow fields. Effects of current density, operating pressure, and level of inlet humidity on flooding are investigated.     

Water and pressure effects on a single PEM fuel cell

A fuel cell is a promising energy conversion system that will eventually become the first-choice for producing power because of its clean or zero-emission nature. A steady-state, two-dimensional mathematical model with pressure and phase change effects for a single PEM fuel cell was developed to illustrate the inlet humidification and pressure effects on proton exchange membrane (PEM) fuel cell performance. This model considers the transport of species along the channel as well as water transfer through the membrane. It can be used to predict trends of the following parameters along the fuel cell channels: mole number of liquid water and water vapor, pressure, temperature, density, viscosity, velocity, saturation pressure, pressure drop, vapor mole fraction, volume flow rate, required pumping power and current density.     

Experimental investigation on PEM fuel cell using serpentine with tapered flow channels

The performance comparison of various flow fields for practical application can be better understood when loading the cell at a lower voltage region for an operational duration of more than an hour. In this paper, the performance of serpentine and serpentine with tapered channels are compared by loading the Polymer Electrolyte Membrane (PEM) fuel cell at a constant voltage of 0.5 V for an operational duration of 5 h. Purging with nitrogen at the inlets is done to recover the drop in current over the period of operation and flush out water. The presence of tapered flow fields in a serpentine flow channel shows an improvement of 15% in the current obtained due to better reactant gas transport in the gas diffusion layer. The performance of both flow fields were studied with polarization and power density curve obtained after 5 h of testing. Further, to confirm the performance improvement at lower voltage region, impedance study is done and obtained Nyquist plot confirms the better transport phenomenon of reactant gases to catalyst site and better removal of water in Gas Diffusion Layer (GDL).     

Water balance for the design of a PEM fuel cell system

The design of a proton exchange membrane (PEM) fuel cell system is important for the optimization of the function of supporting parameters in the fuel cell. The water balance in a PEM fuel cell is investigated based on the water transport phenomena. In this investigation, the diffusion of water from the cathode side to the anode side of the cell is observed to not occur at 20% relative humidity at the cathode (RHC) and 58% relative humidity at the anode (RHA). The minimum concentration of condensed water at the cathode side is observed at a cathode gas inlet relative humidity of 40% RHC–92% RHC and at temperatures between 343 K and 363 K. RHC operating conditions that are greater than 90% and at a temperature of 363 K increased the concentration of condensed water and occurred quickly, which result in a water balance that became difficult to control. On the anode side, the condensation of water is observed at operating temperatures of 353 K and 363 K.     

Effects of inlet humidification on PEM fuel cell dynamic behaviors

The dynamic behaviors of a proton exchange membrane (PEM) fuel cell have been studied both experimentally and numerically. The objective of this paper is to investigate the effects of cathode inlet humidification on PEM fuel cell load change operations and the fuel cell performance during a simulated start‐up process. The PEM fuel cell was found to respond quickly and reproducibly to load changes. It was also found that an increase in the cathode inlet humidification significantly influences the start‐up performance of a PEM fuel cell. The cathode inlet relative humidity (RH) under 30% significantly dropped the cell dynamic performance. Extensive numerical simulations, with the transient processes of load jump and gradual changes considered, were performed to characterize dynamic responses of a singe‐channel PEM fuel cell under different inlet humidification levels. The results showed that the response time for a fuel cell to reach steady state depends on water accumulation in the membrane, which is consistent with the experimental results. Copyright © 2010 John Wiley & Sons, Ltd.     

Along‐channel flooding prediction of polymer electrolyte membrane fuel cells

Non‐uniform current distribution in polymer electrolyte membrane (PEM) fuel cells results in local over‐heating, accelerated ageing, and lower power output than expected. This issue is quite critical when a fuel cell experiences water flooding. In this study, the performance of a PEM fuel cell is investigated under cathode flooding conditions. A two‐dimensional approach is proposed for a single PEM fuel cell based on conservation laws and electrochemical equations to provide useful insight into water transport mechanisms and their effect on the cell performance. The model results show that inlet stoichiometry and humidification, and cell operating pressure are important factors affecting cell performance and two‐phase transport characteristics. Numerical simulations have revealed that the liquid saturation in the cathode gas distribution layer (GDL) could be as high as 20%. The presence of liquid water in the GDL decreases oxygen transport and surface coverage of active catalyst, which in turn degrades the cell performance. The thermodynamic quality in the cathode flow channel is found to be greater than 99.7%, indicating that liquid water in the cathode gas channel exists in very small amounts and does not interfere with the gas phase transport. A detailed analysis of the operating conditions shows that cell performance should be optimized based on the maximum average current density achieved and the magnitude of its dispersion from its mean value. Copyright © 2010 John Wiley & Sons, Ltd.     

Three dimensional,two phase flow mathematical model for PEM fuel cell: Part II. Analysis and discussion of the internal transport mechanisms

The internal transport mechanisms, which were acquired from the modeling results in Part I of this series, are discussed and compared for PEM fuel cells with a conventional flow field and an interdigitated flow field. The modeling results show that the oxygen concentration fraction in an interdigitated flow field is higher than that in a conventional flow field to increase the reaction rate, and the liquid water saturation in the former flow field is much less than that in the latter one at the cathode side to reduce the concentration overpotential largely. However, if the cathode inlet air in a PEM fuel cell with interdigitated flow field is not humidified, the performance of this fuel cell is inferior to that of a PEM fuel cell with conventional flow field because of a larger ohmic overpotential. As a result, the humidification is important for an interdigitated flow field to acquire a much better performance than a conventional flow field.     

Water management in alkaline anion exchange membrane fuel cell anode

Water management is an important issue for alkaline anion exchange membrane fuel cell (AAEMFC) due to its significant role in the energy conversion processes. In this study, a numerical model is developed to investigate the water transport in AAEMFC anode. The gas and liquid transport characteristics in the gas diffusion layer (GDL) and catalyst layer (CL) with different designs and under various operating conditions are discussed. The results show that the current density affects the liquid water distribution in anode most significantly, and the temperature is the second considerable factor. The stoichiometry ratio of the supplied reactant has insignificant effect on the liquid water transport in anode. The change of liquid water amount in anode with cathode relative humidity follows a similar trend with anode inlet relative humidity. Some numerical results are also explained with published experimental and modeling data with reasonable agreement.