Validation of a three dimensional PEM fuel cell CFD model using local liquid water distributions measured with neutron imaging

This work presents the validation carried out for a three dimensional CFD 50 cm² PEM fuel cell model, particularly focus on the prediction of liquid water distributions within the cell. The CFD model was previously validated against a set of experimental polarization curves, where model results adequately matched the experimental curves. An extension of the validation is presented in this work, by performing a comparison of the local liquid water distributions predicted by the model with the liquid water distributions of the real cell. The experimental measurements were obtained by means of Neutron Imaging, where a set of different cell operating conditions was tested. Although the exact quantitative results are not directly comparable due to differences in the cell setup, qualitative results show a very good agreement between the model results and the water distributions observed in the neutron radiographs. A model validation approach using local variable distributions (such as liquid water in this case) in addition to the integral quantities (i.e. polarization curves) is necessary to ensure the validity of models.     

In situ diagnostics for water transport in proton exchange membrane fuel cells

Proton exchange membrane fuel cells (PEMFCs) have attracted considerable attention as energy-conversion systems for future applications in vehicles and for on-site power generation. Major technical challenges exist in achieving a high cell performance over a wide range of operating conditions, such as various cell current densities, operating temperatures, and relative humidities of the supplied gases. Correct water management is critical to achieving a high power density, long-term operation, and increased robustness in PEMFCs. Aspects such as the swelling of the membrane by water, the generation and accumulation of liquid water inside the fuel cells, and the discharge of accumulated water need to be clarified to ensure a fundamental understanding of water transport in PEMFCs. In this article, we examine the state of art regarding in situ diagnostics, particularly visualization techniques, for probing the behaviour of water in PEMFCs, with attention to neutron radiography, X-ray imaging, magnetic resonance imaging, and optical visualization techniques. The recent rapid development of in situ imaging techniques with high spatial and temporal resolutions provides a novel platform for the development of PEMFCs.     

Dynamic fuel cell gas humidification system

Water management is one of the crucial factors regarding the performance and durability of low temperature PEM fuel cells. Amongst other factors, the water balance in an operating fuel cell can be influenced by the humidification of the reaction gases. For transient response investigations of the fuel cell behavior under fast humidification changes a system is needed which is able to humidify the supplied gases in a highly dynamic and reproducible way. Exact knowledge of the water content of the supplied gases is of utmost importance to study humidification effects. In this contribution, a dynamic fuel cell humidification system is presented. Reliability of the concept is proven by using three different methods: straightforward dew point measurements, electrochemical impedance spectroscopy (EIS) and in situ neutron radiography. The test setup is able to provide dew point temperatures with a tolerance range of 1–3 K leading to a highly reproducible fuel cell performance and water content of the complete cell.     

In situ investigation of water transport in an operating PEM fuel cell using neutron radiography: Part 1 – Experimental method and serpentine flow field results

Effective management of liquid water produced in the cathodic reaction of a polymer electrolyte membrane (PEM) fuel cell is essential to achieve high cell efficiency. Few experimental methods are available for in situ measurements of water transport within an operating cell. Neutron radiography is a useful tool to visualize water within a cell constructed of many common materials, including metals. The application of neutron radiography to measurements of water content within the flow field channels of an operating 50 cm² PEM fuel cell is described. Details of the experimental apparatus, image processing procedure and quantitative analysis are provided. It is demonstrated that water tends to accumulate in the 180° bends of the serpentine anode and cathode flow fields used in this study. Moreover, the effects of both the current density and cathode stoichiometric ratio on the quantity of accumulated water are discussed.     

Experimental fuel cell performance analysis under different operating conditions and bipolar plate designs

This work presents experimental performance results for a 50 cm² Polymer Electrolyte Membrane (PEM) Fuel Cell, including polarization curves and Electrochemical Impedance Spectroscopy (EIS) analysis of the Fuel Cell. EIS results were used for the determination of the cell ohmic resistance as well as charge transfer resistances under different operating conditions. Different combinations of operating conditions and bipolar plate designs were analysed. In particular, the effect of the cathode oxygen concentration, reactant gases humidification, and bipolar plate (BP) design were assessed. Butler–Volmer (BV) kinetic parameters such as the charge transfer coefficient were also determined from Tafel plots. The electronic contact resistances were measured for both Bipolar Plate designs, and the membrane protonic resistances were calculated. Its dependence on the BP flow field design and operating conditions is addressed. The results obtained in this work are aimed both at gaining insight into the fundamental processes determining the fuel cell performance, and at determining parameters needed for Computational Fuel Cell Dynamics (CFCD) numerical simulations.     

Numerical model for the performance prediction of a PEM fuel cell. Model results and experimental validation

This work presents a Computational Fluid Dynamics (CFD) model developed for a 50 cm² fuel cell with parallel and serpentine flow field bipolar plates, and its validation against experimental measurements. The numerical CFD model was developed using the commercial ANSYS FLUENT software, and the results obtained were compared with the experimental results in order to perform a model validation. A single parameter, namely the reference exchange current density, was fitted to calibrate the model results. All other model parameters were determined from technical data sheets, literature survey, or experimental measurements. A discussion on different validation issues and model parameters is provided. The results of the numerical model show a good agreement with the experimental measurements for the different bipolar plates and range of operating conditions analysed. However, inaccuracies in the results in the mass-transport polarization region were observed, presumably when liquid water in the channels produces a blockage effect that cannot be modelled with the multiphase flow model currently implemented.     

Water profile determination in a running PEMFC by small-angle neutron scattering

The water management in an operating polymer electrolyte fuel cell was studied by small-angle neutron scattering (SANS). The experiments were conducted at 80 °C under pressure (1.5 bar) using non-humidified gases and porous gas distributors. The parameters under study were the gas distributor porosity, the membrane thickness, the nature of the electrodes and the gas flow. SANS is shown to be a powerful technique to determine in situ the amount of water in the cell, to differentiate the water located within and outside of the membrane and its distribution across the membrane thickness. The data analysis leads to the determination of water concentration profiles across the membrane that could be used to validate mass transfer models. It is shown that a significant current can be extracted with a dry membrane, porous gas distributors and dry gases and that the membrane does not significantly swell whatever the electric load.     

Simulation of an innovative polymer electrolyte membrane fuel cell design for self-control thermal management

Two novel fuel cell designs attempt to improve efficiency and reduce the balance of plant weight by implementing a square hole through the center of the bipolar plates. Air is forced through the square hole for the purpose of oxygen delivery, water removal, and stack cooling. This study demonstrates, for the two novel designs, a more even temperature distribution and hot spots away from the center of the bipolar plates. This reduces the number and size of components required to effectively run the system, thus reducing the weight of the balance of plant. Four simulations are presented in this paper, with inlet gases and initial cell temperature set to 333 K. The maximum temperature for case 1 without cooling is 347.97 K, case 1 with water cooling is 335.29 K, case 2 with forced air cooling is 339.42 K, and case 3 with forced air cooling is 335.13 K.     

Voltage instability in a simulated fuel cell stack correlated to cathode water accumulation

Single fuel cells running independently are often used for fundamental studies of water transport. It is also necessary to assess the dynamic behavior of fuel cell stacks comprised of multiple cells arranged in series, thus providing many paths for flow of reactant hydrogen on the anode and air (or pure oxygen) on the cathode. In the current work, the flow behavior of a fuel cell stack is simulated by using a single-cell test fixture coupled with a bypass flow loop for the cathode flow. This bypass simulates the presence of additional cells in a stack and provides an alternate path for airflow, thus avoiding forced convective purging of cathode flow channels. Liquid water accumulation in the cathode is shown to occur in two modes; initially nearly all the product water is retained in the gas diffusion layer until a critical saturation fraction is reached and then water accumulation in the flow channels begins. Flow redistribution and fuel cell performance loss result from channel slug formation. The application of in-situ neutron radiography affords a transient correlation of performance loss to liquid water accumulation. The current results identify a mechanism whereby depleted cathode flow on a single cell leads to performance loss, which can ultimately cause an operating proton exchange membrane fuel cell stack to fail.     

Quantification of liquid water accumulation and distribution in a polymer electrolyte fuel cell using neutron imaging

Operating parameters, material properties and flow field geometry have a deterministic role on the water storage and distribution within the flow channels and porous media in a fuel cell. However, their effects are not yet precisely understood. In this study, extensive neutron imaging experiments were conducted to visualize and quantify the amount of liquid water in the fuel cell channels and diffusion media as a function of inlet gas flow rate, cell pressure and inlet relative humidity. A seven-channel parallel flow configuration PEFC was used to isolate these parameters from flow field switchback interaction effects.

The neutron imaging experiments were performed at different inlet gas flow rates, operating cell pressures and inlet relative humidities. At each operating condition, the distribution of liquid water in the diffusion media under the lands, and in or under the channels was obtained. Furthermore, at three different cell pressures (0.2 MPa, 0.15 MPa and 0.1 MPa), liquid water distribution and quantification was obtained. The liquid water mass in the cell decreased with increasing pressure for over-humidified anode inlet conditions. Comparison of the fuel cell performance with the total liquid water mass in the cell indicates a non-monotonic relationship between liquid water content and performance. Furthermore, cell performance was highly sensitive to incremental changes in the membrane liquid water content.     

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.     

Two-phase flow in a proton exchange membrane electrolyzer visualized in situ by simultaneous neutron radiography and optical imaging

In proton exchange membrane (PEM) electrolyzers, oxygen evolution in the anode and flooding due to water cross-over in the cathode yields two distinct two-phase transport conditions which strongly affect the performance. Two-phase transport in an electrolyzer cell is visualized by simultaneous neutron radiography and optical imaging. Optical and neutron data are used in a complementary manner to aid in understanding the two-phase flow behavior. Two different patterns of gas-bubble evolution and departure are identified: periodic growth/removal of small bubbles vs. prolonged blockage by stagnant large bubbles. In addition, the bubble distribution across the active area is not uniform due to combined effects of buoyancy and proximity to the inlet. The effects of operating parameters such as current density, temperature and water flow rate on the two-phase distribution are investigated. Higher water accumulation is detected in the cathode chamber at higher current density, even though the cathode is purged with a high flow rate of N₂.     

The effect of pH and halides on the corrosion process of stainless steel bipolar plates for proton exchange membrane fuel cells

Stainless steel is attractive as material for bipolar plates in proton exchange membrane fuel cells, due to its high electrical conductivity, high mechanical strength and relatively low material and processing cost. Potentiostatic and potentiodynamic tests were performed in H₂SO₄ solutions on AISI 316L stainless steel bipolar plates with etched flow fields. The effect of pH and presence of small amounts of fluoride and chloride on the corrosion rate and interfacial contact resistance of the stainless steel bipolar plate were investigated. The tests performed in electrolytes with various pH values revealed that the oxide layer was thinner and more prone to corrosion at pH values significantly lower than the pH one expects the bipolar plate to experience in an operating proton exchange membrane fuel cells. The use of solutions with very low pH in such measurements is thus probably not the best way of accelerating the corrosion rate of stainless steel bipolar plates. By use of strongly acidic solutions the composition and thickness of the oxide layer on the stainless steel is probably altered in a way that might never have happened in an operating proton exchange membrane fuel cell. Additions of fluoride and chloride in the amounts expected in an operating fuel cell (2 ppm F⁻ and 10 ppm Cl⁻) did not cause significant changes for neither the polarization- nor the contact resistance measurements. However, by increasing the amount of Cl⁻ to 100 ppm, pitting was initiated on the stainless steel surface.     

Isolation of transport mechanisms in PEFCs using high resolution neutron imaging

Liquid water saturation profiles were determined using high resolution neutron radiography for commercially available fuel cell materials and hardware. Temperature, pressure, and relative humidity (concentration) gradients were imposed on the cell to determine individual influences on water content for each gradient. The asymmetric anode/cathode channel/land architecture used in this work results in significant water accumulation in the anode diffusion media with saturation values of up to ∼50%. Anode water content was found to change substantially with imposed pressure or concentration gradient, whereas the cathode saturation profile remained relatively consistent, indicating the channel/land ratio and thickness have a determinant role in diffusion media retention. The data generated in this work has been made publicly available through www.pemfcdata.org, and should be useful for computational modelers seeking validation data.     

Water management studies in PEM fuel cells,Part I: Fuel cell design and in situ water distributions

A proton exchange membrane fuel cell (PEMFC) must maintain a balance between the hydration level required for efficient proton transfer and excess liquid water that can impede the flow of gases to the electrodes where the reactions take place. Therefore, it is critically important to understand the two-phase flow of liquid water combined with either the hydrogen (anode) or air (cathode) streams. In this paper, we describe the design of an in situ test apparatus that enables investigation of two-phase channel flow within PEMFCs, including the flow of water from the porous gas diffusion layer (GDL) into the channel gas flows; the flow of water within the bipolar plate channels themselves; and the dynamics of flow through multiple channels connected to common manifolds which maintain a uniform pressure differential across all possible flow paths. These two-phase flow effects have been studied at relatively low operating temperatures under steady-state conditions and during transient air purging sequences.     

Diagnostic analysis of a single-cell Proton Exchange Membrane unitised regenerative fuel cell using numerical simulation

Fabrication and testing of Proton Exchange Membrane (PEM) fuel cells to improve performance is an expensive and time-consuming process. This paper presents a novel procedure for using computer simulation – namely the ANSYS PEM Fuel Cell Module – to identify key performance limiting factors in fuel cell mode of a PEM Unitised Regenerative Fuel cell (URFC) fabricated at RMIT by comparing its performance with a higher performing URFC reported in the literature. The diagnostic analysis is performed in two steps: firstly, changing operating conditions to ensure both cells are compared based on the same conditions; secondly identifying differences in cell properties, specifically catalyst exchange current densities and membrane conductivity. The simulation results show that applying the more optimal operating conditions of the higher performing cell doubled the maximum power of the RMIT cell (from 0.163 W/cm² to 0.327 W/cm²). To overcome the remaining performance deficit in the ohmic polarization region, the value of the protonic conduction coefficient in the modelled RMIT cell had to be increased. Overall the study indicates that computer simulation modelling, in conjunction with carefully focussed experiments, can be a very useful tool in diagnosing fuel-cell performance problems.     

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.     

Simulation of the performance for the direct internal reforming molten carbonate fuel cell. Part I: distributions of temperature,energy transfer and current density

This study examined the temperature distributions of the anode and cathode gases of the cell body as well as the current density distributions at each point of the direct internal reforming molten carbonate fuel cell (DIR‐MCFC) using numerical modelling. The model was based on assumptions and experimental data from a 5 cm × 5 cm sized unit cell operation. The results showed there was an approximately 13°C temperature difference between the initial point (0, 0) and end point (1, 1) of the cell body and the temperature increased steadily along with the direction of the anode gas flow. The temperature distribution of the anode gases showed a similar trend to those of the cell body. The temperature of the anode gases was an average 11°C lower than that of the cell body. The temperature distributions of cathode gases were relatively higher than those of the anode gases and the cell body. The temperature distributions at each point of the cell body, including the anode and cathode gases, could be explained by the different rates of the electrochemical, methane steam reforming and water–gas shift reactions at each point in the cell body. The current density distribution at the entrance of the cell was the highest at 290 mA cm^?2, and decreased steadily to 150 mA cm^?2 at the exit. These results were also confirmed by the amount of hydrogen reacted in the electrochemical reaction (referred to Part II). Finally, modelling simulations showed a non‐uniform distribution of the temperature and current density throughout the DIR‐MCFC were observed. In addition, it was confirmed that the distributions of the reaction rates and gas compositions at each point of the cell also showed a great deal of difference throughout the DIR‐MCFC. The non‐uniformity of these temperature distributions can lead to deterioration in the cell performance. These might provide the necessary information for solving these problems. Copyright © 2005 John Wiley & Sons, Ltd.     

Modelling of polymer electrolyte membrane fuel cell stacks based on a hydraulic network approach

Polymer electrolyte membrane (PEM) fuel cells convert the chemical energy of hydrogen and oxygen directly into electrical energy. Waste heat and water are the reaction by‐products, making PEM fuel cells a promising zero‐emission power source for transportation and stationary co‐generation applications. In this study, a mathematical model of a PEM fuel cell stack is formulated. The distributions of the pressure and mass flow rate for the fuel and oxidant streams in the stack are determined with a hydraulic network analysis. Using these distributions as operating conditions, the performance of each cell in the stack is determined with a mathematical, single cell model that has been developed previously. The stack model has been applied to PEM fuel cell stacks with two common stack configurations: the U and Z stack design. The former is designed such that the reactant streams enter and exit the stack on the same end, while the latter has reactant streams entering and exiting on opposite ends. The stack analysed consists of 50 individual active cells with fully humidified H₂ or reformate as fuel and humidified O₂ or air as the oxidant. It is found that the average voltage of the cells in the stack is lower than the voltage of the cell operating individually, and this difference in the cell performance is significantly larger for reformate/air reactants when compared to the H₂/O₂ reactants. It is observed that the performance degradation for cells operating within a stack results from the unequal distribution of reactant mass flow among the cells in the stack. It is shown that strategies for performance improvement rely on obtaining a uniform reactant distribution within the stack, and include increasing stack manifold size, decreasing the number of gas flow channels per bipolar plate, and judicially varying the resistance to mass flow in the gas flow channels from cell to cell. Copyright © 2004 John Wiley & Sons, Ltd.     

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.