A systematic approach for matching simulated and experimental polarization curves for a PEM fuel cell

Matching simulated and experimental polarization curves is an essential step in the modelling of polymer electrolyte membrane (PEM) fuel cells, but the numerical values of many input parameters like exchange current densities, charge transfer coefficients, protonic conduction coefficient and water removal coefficient are hard to be found experimentally. In this paper, the influence of these input parameters on the performance of PEM fuel cells has been investigated using the ANSYS PEM Fuel Cell Module. The simulation results show how the exchange current densities and charge transfer coefficients influence the activation losses; membrane resistance and contact resistance between the different components of a fuel cell contribute to the ohmic losses; and the coefficient of liquid water removal affects the concentration losses. A systematic procedure to match a simulated polarization curve with an experimental curve is presented and illustrated by application to an experimental PEM fuel cell with 5 cm² active area.     

Numerical studies of liquid water behaviors in PEM fuel cell cathode considering transport across different porous layers

In this paper, a two-phase two-dimensional PEM fuel cell model, which is capable of handling liquid water transport across different porous materials, is employed for parametric studies of liquid water transport and distribution in the cathode of a PEM fuel cell. Attention is paid particularly to the coupled effects of two-phase flow and heat transfer phenomena. The effects of key operation parameters, including the outside cell boundary temperature, the cathode gas humidification condition, and the cell operation current, on the liquid water behaviors and cell performance have been examined in detail. Numerical results elucidate that increasing the fuel cell temperature would not only enhance liquid water evaporation and thus decrease the liquid saturation inside the PEM fuel cell cathode, but also change the location where liquid water is condensed or evaporated. At a cell boundary temperature of 80 °C, liquid water inside the catalyst layer and gas diffusion media under the current-collecting land would flow laterally towards the gas channel and become evaporated along an interface separating the land and channel. As the cell boundary temperature increases, the maximum current density inside the membrane would shift laterally towards the current-collecting land, a phenomenon dictated by membrane hydration. Increasing the gas humidification condition in the cathode gas channel and/or increasing the operating current of the fuel cell could offset the temperature effect on liquid water transport and distribution.     

Degradation behavior of a proton exchange membrane fuel cell stack under dynamic cycles between idling and rated condition

The durability of proton exchange membrane (PEM) fuel cells is a key factor which prevents its commercial application on the vehicle. Dynamic current cycle is one of the most common conditions for PEM fuel cells, especially varying the currents between the idling and the rated condition. To investigate the degradation behavior of fuel cells under this kind of dynamic cycles, a PEM fuel cell stack with 330 cm² active area is operated under 10,000 dynamic cycles with the cycling current density ranging from 25 mAcm^?2 to 600 mAcm^?2, which simulates common operating conditions in a vehicle cycle from the idling condition to the rated condition. Polarization curves, the high-frequency resistance (HFR), the uniformity of the individual cells, the performance degradation of PEM fuel cell stack at 25 mAcm^?2 and 600 mAcm^?2 are characterized to investigate the performance degradation over cycling. In addition, scanning electron microscopy (SEM) of the surface and the cross-section of the tested membrane electrode assemblies (MEAs) are compared with different single-cell samples. The results indicate that the degradation rate of the stack is 1.0 μVcycle^?1 at 25 mAcm^?2 under the idling condition. A more severe performance degradation of about 2.0 μVcycle^?1 is detected at 600 mAcm^?2 under the rated condition. The individual cell near the coolant outlet of the PEM fuel cell stack shows a more serious degradation caused by the HFR increase, which is also proved by the SEM analysis. The cross-section SEM analysis indicates that the dynamic cycle has a significantly negative effect on the catalyst layer, resulted in an obvious decrease on the thickness of the catalyst layer.     

Proton transfer mechanism in perfluorinated sulfonic acid polytetrafluoroethylene

The proton exchange membrane (PEM) fuel cell is one of the most promising fuel cells for wide applications, and the proton exchange membrane is one of its key components. However, the proton transfer mechanism in perfluorinated sulfonic acid polytetrafluoroethylene remains unclear for the research on PEM fuel cells. In this paper, the model of the proton transfer mechanism in perfluorinated sulfonic acid polytetrafluoroethylene is developed based on the fundamentals of the molecular dynamics, particularly the principle of energy and radial distribution function. The proton transfer process in perfluorinated sulfonic acid polytetrafluoroethylene is simulated, whereas the effects driven by the water content in the membrane and fuel cell temperature are analyzed. The results show that the water bridges developed by free water are the passage for proton transfer from one sulfonic group to its adjacent sulfonic group in perfluorinated sulfonic acid polytetrafluoroethylene. The proton transfers along the water bridge by the formation and cleavage of the H–O bond between the water from the water bridge and the proton; the increases in the water content in the membrane and the fuel cell temperature speed up the proton transfer, causing the decrease in the resistance of proton transfer. These findings are remarkably helpful to understand the working mechanism of PEM fuel cells.     

In situ measurements of water transfer due to different mechanisms in a proton exchange membrane fuel cell

Water management is of critical importance in a proton exchange membrane (PEM) fuel cell, in particular, those based on a sulfonic acid polymer, which requires water to conduct protons. Yet there are limited in situ studies of water transfer through the membrane and no data are available for water transfer due to individual mechanisms through the membrane in an operational fuel cell. Thus it is the objective of this study to measure water transfer through the membrane due to each individual mechanism in an operational PEM fuel cell. The three different mechanisms of water transfer, i.e., electro-osmotic drag, diffusion and hydraulic permeation are isolated by specially imposed boundary conditions. Therefore water transfer through the membrane due to each mechanism is measured separately. In this study, all the data is collected in an actual assembled operational fuel cell. The experimental results show that water transfer due to hydraulic permeation, i.e. the pressure difference between the anode and cathode is at least an order of magnitude lower than those due to the other two mechanisms. The data for water transfer due to diffusion through the membrane are in good agreement with some of the ex situ data in the literature. The data for electro-osmosis show that the number of water molecules dragged per proton increases not only with temperature but also with current density, which is different from existing data in the literature. The methodology used in this study is simple and can be easily adopted for in situ water transfer measurement due to different mechanisms in other PEM fuel cells without any cell modifications.     

Numerical simulation of laminar flow development with heat and mass transfer in PEM fuel cell flow channels having oxygen and hydrogen suction at one channel wall

Numerical simulation has been carried out of the fluid flow, heat and mass transfer for the developing laminar flow in polymer electrolyte membrane (PEM) fuel cell cathode and anode flow channels, respectively. Each flow channel is considered to be composed of two parallel walls, one porous (simulating electrode surface) and one non‐porous, or impermeable, wall (simulating bipolar plate surface). Various flow situations have been analyzed, and the local and the averaged friction coefficient, Nusselt number for heat transfer and Sherwood number for mass transfer are determined for various flow conditions corresponding to different stoichiometries, operating current densities and operating pressures of the cell. The effect of suction or injection (blowing) wall boundary condition has also been investigated, corresponding to the oxygen consumption in the cathode and hydrogen consumption in the anode. Correlations for the averaged friction coefficient, Nusselt and Sherwood numbers are developed, which can be useful for PEM fuel cell modeling and design calculations. Copyright © 2010 John Wiley & Sons, Ltd.     

Thermal-fluid transports in a five-layer membrane-electrode assembly of a PEM fuel cell

Thermal-fluid transport phenomena in a membrane-electrode assembly (MEA) of a polymer electrolyte membrane (PEM) fuel cell attached to interdigitated gas distributors are studied numerically. The MEA consists of two porous catalyst layers, two porous gas diffusion layers, and an impermeable PEM. In the catalyst layers, the overpotential heating by the electrochemical reaction under thermal equilibrium conditions produces heat that is removed by the fluids as well as the solid matrices. In the diffusion layers, the difference in the heat conductivities between the solid matrices and the fluids causes a thermal non-equilibrium in the porous medium. A two-equation approach is used to resolve the temperature difference between the solid matrices and the fluids. The effects of the porous Reynolds number, interfacial heat transfer coefficient, and overpotential heating are examined. It is found that the local maximum temperature occurs inside the cathodic catalyst layer. In addition, the temperature difference between the solid matrices and the fluids in the diffusion layers decreases with increasing the non-dimensional interfacial heat transfer coefficient. The present results have provided comprehensive heat transfer information that is helpful in understanding of the mechanisms responsible for thermal pathways in a PEM fuel cell.     

Study of the characteristics of temperature rise and coolant flow rate control during malfunction of PEM fuel cells

In actual PEM fuel cell systems, the coolant flow rate is generally controlled to maintain a preset temperature at the coolant outlet. This implies that a change in coolant supply flow rate is a good early indicator of a malfunctioning PEM fuel cell stack and system components. In this study, various fuel cell malfunctions are simulated based on the practical coolant flow control strategy by using a three-dimensional, two-phase, multiscale PEM fuel cell model developed in our previous studies. The focus is on analysis of the characteristics of coolant flow rate change along with voltage degradation in various fuel cell malfunction cases. The model predictions show that in general, the coolant flow rate tends to increase proportionally with the degree of voltage degradation, but the increase in temperature inside the membrane electrode assembly (MEA) is not always related to the voltage drop and is influenced more directly by local current density distribution. Although the present numerical comparison between the normal and malfunctioning cases is conducted at the low current density of 0.3 A cm^?2, the general cell behavior will not be altered at higher current densities due to inverse relationship between cell performance and waste heat generation. The present work elucidates the complex interplay among increase in coolant flow rate, increase in MEA temperature, voltage drop, and change in local current density distribution when a PEM fuel cell malfunctions.     

An experimental study and model validation of a membrane humidifier for PEM fuel cell humidification control

This paper presents an experimental study and model validation of an external membrane humidifier for PEM fuel cell humidification control. Membrane humidification behavior was investigated with steady-state and dynamic tests. Steady-state test results show that the membrane vapor transfer rate increases significantly with water channel temperature, air channel temperature, and air flow rate. Water channel pressure has little effect on the vapor transfer rate and thus can be neglected in the system modeling. Dynamic test results reveal that the membrane humidifier has a non-minimum phase (NMP) behavior, which presents extra challenges for control system design. Based on the test data, a new water vapor transfer coefficient for Nafion membrane was obtained. This coefficient increases exponentially with the membrane temperature. The test results were also used to validate a thermodynamic model for membrane humidification. It is shown that the model prediction agrees well with the experimental results. The validated model provides an important tool for external humidifier design and fuel cell humidification control.     

Numerical study of PEMFC heat and mass transfer characteristics based on roughness interface thermal resistance model

The thermal contact resistance (TCR) is the main component of proton exchange membrane fuel cell (PEMFC) thermal resistance due to the existence of surface roughness between the components of PEMFC, and the influence of TCR is often ignored in traditional three dimensional PEMFC simulations. In this paper, the heat and mass transfer characteristics including polarization curve, power density curve, temperature distribution, membrane water content distribution, membrane current density are studied under different component surface roughness conditions, and finally the effect of each TCR on the PEMFC performance is studied. It is found that under the same operating conditions, the TCR makes the radial heat transfer of the PEMFC decrease, and the temperature of the membrane electrode and the temperature difference of each component of the PEMFC is higher than that of the model without TCR. When the surface roughness of components in the PEMFC equals 1 μm, 2 μm, 3 μm, the cell current density decreases by 6.56%, 12.46% and 17.17% respectively when the output cell voltage equals 0.3 V, and the cell power density decreases by 3.64%, 7.54%, 13.14% respectively when the cell current density equals 1.2 A·cm^?2. When the TCR between the CL and PEM equals 0.003 K·m²·W^?1, 0.005 K·m²·W^?1, 0.01 K·m²·W^?1, the cell current density is increased by 2.30%, 3.65%, 6.74% respectively under the condition that the output cell voltage equals 0.3 V, and the cell power density is increased by 1.24%, 1.85%, 3.10% respectively when the cell current density equals 1.2 A·cm^?2. The results show that the numerical simulation of PEMFC cannot ignore the effect of TCR.     

The effect of La oxide additive on the solubility of NiO in molten carbonates

A comprehensive analysis on a novel energy recovery system for reformate-based proton exchange membrane (PEM) fuel cell systems is presented. The energy recovery system includes a throttling valve, a heat exchanger, a compressor, and is coupled with a coolant loop for the fuel cell stack. The feed stock of the fuel reformer, which is primarily a mixture of water and fuel, is vaporized in the heat exchanger and is then compressed to a sufficiently high pressure before it is ducted into the fuel reformer. The analysis includes the throttling of two-phase fuel/water mixture and vaporization in the heat exchanger to obtain the temperature and pressure of the mixture at the inlet of the compressor. The results indicate that the power plant efficiency with the energy recovery system can be increased by more than 20% compared to that of a fuel cell power plant without the energy recovery system. Additionally, more than 25% of the waste heat generated by the fuel cell stack can be removed due to the energy recovery system, and the fuel burned for the fuel reforming purpose is reduced by more than 70%.     

Combined heat and power generation via hybrid data center cooling-polymer electrolyte membrane fuel cell system

Although there has been a lot of waste heat utilization studies for the air-cooled data center (DC) systems, the waste heat utilization has not been studied for the liquid-cooled DC systems, which have been rapidly gaining importance for the high-performance Information and Communication Technology facilities such as cloud computing and big data storage. Compared to the air-cooled systems, higher heat removal capacity of the liquid-cooled DC systems provides better heat transfer performance; and therefore, the waste heat of the liquid-cooled DC systems can be more efficiently utilized in the low-temperature and low-carbon energy systems such as electricity generation via polymer electrolyte membrane (PEM) fuel cells. For this purpose, the current study proposes a novel hybrid system that consists of the PEM fuel cell and the two-phase liquid-immersion DC cooling system. The two-phase liquid immersion DC cooling system is one of the most recent and advanced DC cooling methods and has not been considered in the DC waste heat utilization studies before. The PEM fuel cell unit is operated with the hydrogen and compressed air flows that are pre-heated in the DC cooling unit. Due to its original design, the hybrid system brings its own original design criteria and limitations, which are taken into account in the energetic and exergetic assessments. The power density of the PEM fuel cell reaches up to 0.99 kW/m² with the water production rate of 0.0157 kg/s. In the electricity generation case, the highest energetic efficiency is found as 15.8% whereas the efficiency increases up to 96.16% when different multigeneration cases are considered. The hybrid design deduces that the highest exergetic efficiency and sustainability index are 43.3% and 1.76 and they are 9.4% and 6.6% higher than exergetic and sustainability performances of the stand-alone PEM fuel cell operation, respectively.     

Experimental analysis of SiO2-Distilled water nanofluids in a Polymer Electrolyte Membrane fuel cell parallel channel cooling plate

An experimental report on the thermal performance of Silicone Dioxide (SiO₂) nanofluid coolants based on a PEM fuel cell cooling system is presented. The aim of this study is to evaluate the feasibility of applying these nanofluids coolants as an alternative to conventional distilled water through detailed analysis of thermofluids behaviour in a simulated cooling plate environment. SiO₂ nanoparticles were dispersed in distilled water at 0.1%, 0.3% and 0.5% volume concentrations and tested in a parallel channel cooling plate system. A constant heat load was supplied to simulate a fuel cell stack thermal condition. At inlet flow conditions from 750 to 900 Reynolds number, the SiO₂ nanofluids reduced the average plate temperatures by 15%–20% compared to conventional water coolant. The nanofluids also increased the cooling effectiveness by a similar margin, as well as improving the bulk heat transfer coefficient to a range between 2700 and 4400 W m⁻². ^oC⁻¹. However, the required pumping power was also increased due to the added viscous effect. Through the Advantage Ratio (AR) analysis, it was concluded that the enhancement in heat transfer mechanics was more significant than the penalties in fluid flow dynamics. Thus, the SiO₂ nanofluids and the cooling plate design are possible options for advanced PEM fuel cell thermal management practice in future stack designs.     

A flow channel design procedure for PEM fuel cells with effective water removal

Proper water management in polymer electrolyte membrane (PEM) fuel cells is critical to achieve the potential of PEM fuel cells. Membrane electrolyte requires full hydration in order to function as proton conductor, often achieved by fully humidifying the anode and cathode reactant gas streams. On the other hand, water is also produced in the cell due to electrochemical reaction. The combined effect is that liquid water forms in the cell structure and water flooding deteriorates the cell performance significantly. In the present study, a design procedure has been developed for flow channels on bipolar plates that can effectively remove water from the PEM fuel cells. The main design philosophy is based on the determination of an appropriate pressure drop along the flow channel so that all the liquid water in the cell is evaporated and removed from, or carried out of, the cell by the gas stream in the flow channel. At the same time, the gas stream in the flow channel is maintained fully saturated in order to prevent membrane electrolyte dehydration. Sample flow channels have been designed, manufactured and tested for five different cell sizes of 50, 100, 200, 300 and 441 cm². Similar cell performance has been measured for these five significantly different cell sizes, indicating that scaling of the PEM fuel cells is possible if liquid water flooding or membrane dehydration can be avoided during the cell operation. It is observed that no liquid water flows out of the cell at the anode and cathode channel exits for the present designed cells during the performance tests, and virtually no liquid water content in the cell structure has been measured by the neutron imaging technique. These measurements indicate that the present design procedure can provide flow channels that can effectively remove water in the PEM fuel cell structure.     

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.     

Effect of humidified water vapor on heat balance management in a proton exchange membrane fuel cell stack

Thermal management has been considered as one of the most important issues for the operation of proton exchange membrane fuel cells (PEMFCs). Phase change affects the performance and even the heat balance of the stack during operation. A 46 single cell PEM stack with anode and cathode humidification is developed to investigate, both theoretically and experimentally, the effect of phase change on the heat generation and removal characteristics of the stack. The results show that the heat removed by the coolant water is greater than that generated by the electrochemistry reaction, and heat released due to the phase change of water vapor cannot be neglected. Heat generated in the stack can be removed completely by the coolant water, which need to be forced cooling for recycling use when the current density reaches 1000 mA·cm^?2. The arithmetic product of the specific heat capacity and mass of the stack can be used as a novel criterion to evaluate the validity of the heat balance in the system. The exothermic reaction is very fast in the stack, which consequently requires bipolar plates with high heat conductivity coefficient to improve the temperature uniformity at the elevated operational current density. Copyright © 2014 John Wiley & Sons, Ltd.     

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.     

Simulation of the water and heat management in proton exchange membrane fuel cells

A non-isothermal and three-dimensional numerical model of a PEM fuel cell was developed to compute the water and heat management. Transport of water in the polymer membrane, phase change of water in the cathode porous medium and capillary flow to the gas channels were determined. Influences of these phenomena on fuel cells and conditions that may affect their performance have been numerically evaluated. Output variables are velocity, temperature, mass fraction, current density, voltage loss, water content of the polymer membrane, saturation and liquid flow fields. Cell voltage and total current density of PEM fuel cell were computed as well. Results show that there may be severe mass transfer limitations depending either on the design or on the water management of the cell. For the chosen conditions, the polymer membrane can keep and even increase its water content, as long as inlet flows are injected at 100% relative humidity. In case the fuel cell is operated under dehydrating conditions, the decrease of the water content of the polymer electrolyte may affect the performance. The variations of temperature were small. However, temperature plays an important role in the cathode reaction rate of the cell and in the dehydration of the polymer membrane. Numerical results and experimental data were found to be in good agreement.     

Exergetic performance analysis of a PEM fuel cell

In this paper we investigate the effects of thermodynamic irreversibilities on the exergetic performance of proton exchange membrane (PEM) fuel cells as a function of cell operating temperature, pressures of anode and cathode, current density, and membrane thickness. The practical operating conditions are selected to be 3–5 atm for anode and cathode pressures, and 323–353 K for the cell temperatures, respectively. In addition, the membrane thicknesses are chosen as 0.016, 0.018 and 0.02 cm, respectively. Moreover, the current density range of the PEM fuel cell is selected to be 0.01–2.0 A cm^?2. It is concluded that exergy efficiency of PEM fuel cell decreases with a rise in membrane thickness and current density, and increases with a rise of cell operating pressure and with a decrease of current density for the same membrane thickness. Thus, it can be said that, in order to increase the exergetic performance of PEM fuel cell, the lower membrane thickness, the lower current density and the higher cell operating pressure should be selected in case PEM fuel cell is operated at constant cell temperature. Copyright © 2005 John Wiley & Sons, Ltd.     

Effect of flow orientation on thermal-electrochemical transports in a PEM fuel cell

A non-isothermal model of a proton exchange membrane (PEM) fuel cell in contact with interdigitated gas distributors has been performed. The model accounts for the major transports of convective and diffusive heat and mass transfer, electrode kinetics, and potential fields. The effects of flow orientation and total overpotential across a five-layer membrane-electrode assembly on the thermal behaviors in a PEM fuel cell are examined. A unique feature of the model is the implementation of a thermal-electrochemical algorithm to predict the fluid-phase temperature as well as the solid-matrix temperature in a PEM fuel cell. The simulation results reveal both the solid-matrix temperature and the fluid-phase temperature are increased with increasing total overpotential. Moreover, the fluid-phase and solid-matrix temperature distributions are significantly affected by the flow orientation in the PEM fuel cell. Replacing the parallel-flow geometry by the counter-flow geometry has an advantage of reducing the local maximum temperature inside the fuel cell. Thermal effects on the active material degradation and hence fuel cell durability will be incorporated in the future work.