Characterization of water management in metal foam flow-field based polymer electrolyte fuel cells using in-operando neutron radiography

Metal foam flow-fields have shown great potential in improving the uniformity of reactant distribution in polymer electrolyte fuel cells (PEFCs) by eliminating the ‘land/channel’ geometry of conventional designs. However, a detailed understanding of the water management in operational metal foam flow-field based PEFCs is limited. This study aims to provide the first clear evidence of how and where water is generated, accumulated and removed in the metal foam flow-field based PEFCs using in-operando neutron radiography, and correlate the water ‘maps’ with electrochemical performance and durability. Results show that the metal foam flow-field based PEFC has greater tolerance to dehydration at 1000 mA cm⁻², exhibiting a ~50% increase in voltage, ~127% increase in total water mass and ~38% decrease in high frequency resistance (HFR) than serpentine flow-field design. Additionally, the metal foam flow-field promotes more uniform water distribution where the standard deviation of the liquid water thickness distribution across the entire cell active area is almost half that of the serpentine. These superior characteristics of metal foam flow-field result in greater than twice the maximum power density over serpentine flow-field. Results suggest that optimizing fuel cell operating condition and foam microstructure would partly mitigate flooding in the metal foam flow-field based PEFC.     

Investigation of the effects of the anisotropy of gas-diffusion layers on heat and water transport in polymer electrolyte fuel cells

The aim of this work is to study the effects of gas-diffusion layer (GDL) anisotropy and the spatial variation of contact resistance between GDLs and catalyst layers (CLs) on water and heat transfer in polymer electrolyte fuel cells (PEFCs). A three-dimensional, two-phase, numerical PEFC model is employed to capture the transport phenomena inside the cell. The model is applied to a two-dimensional cross-sectional PEFC geometry with regard to the in-plane and through-plane directions. A parametric study is carried out to explore the effects of key parameters, such as through-plane and in-plane GDL thermal conductivities, operating current densities, and electronic and thermal contact resistances. The simulation results clearly demonstrate that GDL anisotropy and the spatial variation of GDL/CL contact resistance have a strong impact on thermal and two-phase transport characteristics in a PEFC by significantly altering the temperature, water and membrane current density distributions, as well as overall cell performance. This study contributes to the identification of optimum water and thermal management strategies of a PEFC based on realistic anisotropic GDL and contact-resistance variation inside a cell.     

Numerical simulation of convective heat transfer between air flow and ceramic foams to optimise volumetric solar air receiver performances

Porous ceramic foams are used to achieve high performance in solar heat recovery systems. Understanding the convective heat transfer between the air flow and the ceramic foam is of great importance when optimising the volumetric air receiver. In this work, the convective heat transfer was numerically studied. The present approach was designed to compute the local convective heat transfer coefficient between the air flow and a porous ceramic foam. For that purpose, the energy balance and the flow inside the porous ceramic foam were solved. In addition, a detailed geometry of the porous ceramic foam was considered. The ceramic foams were represented by idealised packed tetrakaidecahedron structures. The numerical simulations were based on the three dimensional Reynolds-averaged Navier–Stokes (RANS) equations. A sensitivity study on the heat transfer coefficient was conducted with the porosity, velocity and mean cell size as parameters. Based on the numerical simulation results, a correlation for the volumetric local convective heat transfer coefficient between air and ceramic foams was developed. The resulting correlation covers a wide range of porosities, velocities, cell sizes and temperatures. The correlation results were compared with experimental data from the literature, and the comparison shows good agreement. The correlation is intended to be used in the design of volumetric solar air receivers.     

Thermo-fluidic characteristics of open cell metal foam as an anodes for DCFC,part I: Head loss coefficient of metal foam

A porous metal was suggested to be used for the anode of a DCFC. Thermo-fluidic characteristics of fuel-electrolyte mixture in the porous metal should be known in order for proper design of the anode. Previous researchers investigated pressure drop and heat transfer performance of fluids flowing through metal foams that have different pore densities and porosities. Various characteristic length scales were used for the Reynolds number and friction factor of metal foams in the previous works. In the present study, we propose a realistic definition of the characteristic length scale that is applicable to pressure drop evaluation in metal foams regardless of pore density and porosity. A series of experiments was conducted to obtain friction factor of metal foam. An equivalent diameter based on permeability and porosity appeared to best fit the experimental data produced using various metal foams. The relationship between the friction factor and Reynolds number through metal foams can be classified into three regimes. In Re < 20, ln f is inversely proportion to ln 16/Re. In Re > 2000, the value of friction factor approaches 0.17. The relationship between friction factor and Reynolds number of foam materials appeared to be similar trend to that of Moody chart.     

Review on thermal transport in high porosity cellular metal foams with open cells

Thermal transport in metal foams has received growing attention in both academic research and industrial applications. In this paper the recent research progress of thermal transport in metal foams has been reviewed. This paper aims to provide the comprehensive state-of-the-art knowledge and research results of thermal transport in open celled cellular metal foams, which covers the effective thermal conductivity, forced convection, natural convection, thermal radiation, pool boiling and flow boiling heat transfer, solid/liquid phase change heat transfer and catalytic reactor. The forced convection and thermal conductivity have been extensively investigated, while less research were performed on two-phase (boiling and solid/liquid phase change heat transfer) and thermal radiation in metal foams. Also most research still treats the metal foam as one type of effective continuous porous media, very few researchers investigated the detailed thermal behaviours at the pore level either by numerical or experimental approaches.     

Performance enhancement of a direct absorption solar collector using copper oxide porous foam and nanofluid

The current research proposes the idea of using water-saturated metal oxide foams and water-based nanofluids as solar absorber in the direct absorption solar collectors (DASCs). Specifically, the novel solar collector design utilizes copper oxide (CuO) porous foam and nanoparticle with high optical properties and is expected to have enhanced thermal performance than the conventional collectors utilizing pure water. The finite volume technique is used to solve the governing equations of flow and heat transfer in the radiative participating media. Also, to establish the reliability and accuracy of numerical solutions, the obtained results are compared with the corresponding numerical and experimental data. The computations are carried out for different nanoparticle volume fractions, foam pore sizes, working fluid mass flow rates, and both porous layer thicknesses and positions (inserted at the lower or upper wall of the collector). It is found that the efficiency of DASC partially/fully filled with metal oxide foam is maximized when the collector is completely filled with it. Compared with the water flow, the numerical results show that the collector efficiency using CuO nanofluid and metal oxide foam is improved by up to 26.8% and 23.8%, respectively. Moreover, considering the second law of thermodynamics, the use of CuO nanofluids in the DASC seems to be more effective than the use of CuO porous foam.     

Numerical evaluation of a dual-function microporous layer under subzero and normal operating temperatures for use in automotive fuel cells

In a previous study, we proposed a dual-function microporous layer (MPL) to improve the cold-start capability of polymer electrolyte fuel cells (PEFCs). The conceptual MPL design is to use an ionomer-based binder with low Pt loading, thereby allowing the MPL to provide additional volume for ice storage during cold-start PEFC operations. Although the benefit of using a dual-function MPL was numerically elucidated in our previous study, the question regarding the use of this new MPL under normal PEFC operation remains to be addressed. In this paper, we extend our discussion to the effects of a dual-function MPL under subzero to normal operating temperatures. The three-dimensional (3D) cold-start PEFC model developed in our previous study is modified for transient PEFC simulations to consider a wide range of operating temperatures from −20 °C to 80 °C. Simulation results show a negligible performance drop at the normal PEFC temperature of 80 °C, because of the presence of the dual-function MPL in a PEFC membrane electrode assembly. In addition, water back flow from the cathode to anode is reduced on using the dual-function MPL, owing to the additional water uptake driven by its ionomer content. This study clearly demonstrates that this dual-function MPL technology may be applied to automotive PEFC stack development without sacrificing fabrication cost and cell performance during normal PEFC operations.     

Numerical Simulations of Fluid Flow and Heat Transfer through Aluminum and Copper Metal Foam Heat Exchanger – A Comparative Study

Abstract

This article discusses about a numerical simulation of a metal foam heat exchanger system carried out by a commercial software. A metal foam layer is attached to the bottom of the heat exchanger to absorb heat from the exhaust hot gas leaving the system. Two types of metal foams with two different pores per inch (PPI) values are considered for heat transfer enhancement. Similarly, two different materials Aluminum and copper, that poses high thermal conductivity, metal foams are considered for the present numerical simulations. The heat exchanger system is simulated over a range of 6–30?m/s fluid velocity. The proposed simulations are compared with theoretical and experimental data available in the literature. The goal is to improve the thermal performance of the heat exchanger by decreasing the pressure drop and maximizing the heat transfer rate. Finally, it has been noticed that the velocity of the fluid decreases as PPI increases at the expense of its pressure drop. The copper metal foam gives a maximum increase of 4–10% heat transfer rate compared to aluminum metal foams for a fluid velocity of 30?m/s.     

Analyzing oxygen transport resistance and Pt particle growth effect in the cathode catalyst layer of polymer electrolyte fuel cells

The oxygen transport resistance in the cathode catalyst layer (CL) of polymer electrolyte fuel cells (PEFCs) has been reported to be significantly higher than expected, especially when the platinum (Pt) loading is low and/or the degree of CL degradation is severe. In this paper, the oxygen transport resistance behavior in the cathode CL is numerically analyzed under various CL design and operating conditions. Particular emphasis is placed on the aged CL wherein Pt particle growth and active Pt surface area loss are observed. For this study, a previously developed micro-scale catalyst model is improved upon to account for Pt particle size. The new model includes calculations of catalyst activity and electrochemically active surface areas, as well as various transport resistances through the ionomer and liquid films. After coupling the micro-scale CL model with a three-dimensional PEFC model, multi-scale simulations are carried out under various PEFC catalyst designs (varying Pt loading, ionomer fraction, oxygen permeation rate through the ionomer film) and operating conditions (drying or flooding of the electrode, high or lower current density). The simulation results agree well with experimental oxygen transport resistance data and further indicate that CL design with low Pt loading is more susceptible to degradation. Providing extensive multi-dimensional contours of species concentration, temperature, and current density inside the PEFC, this study provides a comprehensive understanding of oxygen transport resistance in the cathode CL in different PEFC situations.     

A numerical investigation of the effects of GDL compression and intrusion in polymer electrolyte fuel cells (PEFCs)

The purpose of this work is to numerically investigate the effects of non-uniform compression of the gas diffusion layer (GDL) and GDL intrusion into a channel due to the channel/rib structure of the flow-field plate. The focus is placed on accurately predicting two-phase transport between the compressed GDL near the ribs and uncompressed GDL near the channels, and its associated effects on cell performance. In this paper, a GDL compression model is newly developed and incorporated into a comprehensive three-dimensional, two-phase PEFC model developed earlier. To assess solely the effects of GDL compression and intrusion, the new fuel cell model is applied to a simple single-straight channel fuel cell geometry. Numerical simulations with different levels of GDL compression and intrusion are carried out and simulation results reveal that the effects of GDL compression and intrusion considerably increase the non-uniformity, particularly, the in-plane gradient in liquid saturation, oxygen concentration, membrane water content, and current density profiles that in turn results in significant ohmic and concentration polarizations. The present three-dimensional GDL compression model yields realistic species profiles and cell performance that help to identify the optimal MEA, gasket, and flow channel designs in PEFCs.     

Modeling of the heat transfer in a portable PEFC system within MATLAB-Simulink

A dynamic model of a (portable) polymer electrolyte fuel cell system including the heat transfer between stack and periphery – here stack and metal hydride storage – has been developed within MATLAB-Simulink. The implemented equations describe the steady-state as well as the dynamic operation of the PEFC system with sufficient accuracy, although considerable simplifications have been made to keep model complexity and computing time low. As simulations are performed in 100-fold real time, different operating conditions and control strategies for PEFCs can be analyzed and evaluated in short-time. Here, emphasis is given to the operation limits of the PEFC system, i.e. the conditions for trouble-free operation at different loads and high or low ambient temperature are established. The model has been validated by experiments on a home-made portable system.     

Metal foams as a gas diffusion layer in direct borohydride fuel cells

Renewable energy sources have become an important issue due to global warming and the diminishing of fossil fuel resources that have become a major problem for the world. Fuel cells are one of the most important renewable energy systems. The use of metal foams within the scope of commercialization and cost reduction of fuel cells attracts attention today. Metal foams are a material that provides excellent performance in many engineering applications, especially in fuel cells, thanks to the high permeability provided by the porous structure, narrow flow channels, large specific surface area, capillary, and diffusive forces. Among the metallic foams, the focus is on the use of aluminum foam, due to its low cost, superior strength, and thermal conductivity performance properties as well as ease of production. As an alternative to gas diffusion layers, the use of different metallic foam materials in fuel cell systems and performance improvements were analyzed. The use of aluminum foams as flow distributors and electrodes in the anode diffusion layer, which is one of the DBHYP components, provides significant increases in cell performance, cell weight, and volume. It has been found that open-cell metallic foams used in fuel cell systems cause an increase in performance due to their positive effects on the amount of catalyst coating, flow characteristics, and electrical properties.     

Numerical analysis of gas crossover effects in polymer electrolyte fuel cells (PEFCs)

The gas crossover phenomenon in polymer electrolyte fuel cells (PEFCs) is an indicator of membrane degradation. The objective of this paper is to numerically investigate the effects of hydrogen and oxygen crossover through the membrane in PEFCs. A gas crossover model is newly developed and implemented in a comprehensive multi-dimensional, multi-phase PEFC model developed earlier. A parametric study is carried out to investigate the effects of the crossover diffusion coefficients for hydrogen and oxygen as well as the membrane thickness. The simulation results demonstrate that the hydrogen crossover induces an additional oxygen reduction reaction (ORR) and consequently causes an additional voltage drop, while the influence of oxygen crossover on PEFC performance is relatively insignificant because it leads to the hydrogen/oxygen chemical reaction at the anode side. Finally, using the time-dependent gas crossover data that are available in the literature (measured in days), we conduct gas crossover simulations to examine the effects of increased gas crossover due to membrane degradation on PEFC performance and successfully demonstrate decaying polarization curves with respect to time. This study clearly elucidates the detailed mechanisms of the hydrogen and oxygen crossover phenomena and their effect on PEFC performance and durability.     

Materials and design development for bipolar/end plates in fuel cells

Bipolar/end plate is one of the most important and costliest components of the fuel cell stack and accounts to more than 80% of the total weight of the stack. In the present work, we focus on the development of alternative materials and design concepts for these plates. A prototype one-cell polymer electrolyte membrane (PEM) fuel cell stack made out of SS-316 bipolar/end plate was fabricated and assembled. The use of porous material in the gas flow-field of bipolar/end plates was proposed, and the performance of these was compared to the conventional channel type of design. Three different porous materials were investigated, viz. Ni–Cr metal foam (50 PPI), SS-316 metal foam (20 PPI), and the carbon cloth. It was seen that the performance of fuel cell with Ni–Cr metal foam was highest, and decreased in the order SS-316 metal foam, conventional multi-parallel flow-field channel design and carbon cloth. This trend was explained based on the effective permeability of the gas flow-field in the bipolar/end plates. The use of metal foams with low permeability values resulted in an increased pressure drop across the flow-field which enhanced the cell performance.     

质子交换膜水电解中气-液两相传输的模拟研究

两相传输是影响质子交换膜水电解系统性能的关键因素。为掌握质子交换膜水电解单元中多孔扩散层内气液两相传输规律,基于数值重构的三维多孔扩散层结构,采用格子Boltzmann两相流动模型模拟研究了扩散层内两相传输过程,详细分析了扩散层孔隙率和表面接触角对气泡传输与分布的影响。数值模拟结果表明：孔隙率减小会明显降低气体渗透率,从而导致气泡难以在扩散层内找到有效传输通道。接触角的增大不仅增加了气泡在界面堆积的风险,也减缓了气泡在孔隙内的传输速度。从孔隙尺度水平初步掌握了质子交换膜水电解单元多孔扩散层内两相传输规律,可为高性能水电解系统设计和优化提供理论支撑。     

Metal foams as flow field and gas diffusion layer in direct methanol fuel cells

Metal foams are routinely used in structures to enhance stiffness and reduce weight over a range of platforms. In direct methanol fuel cells, the controlled porosity and high electrical conductivity of metal foams provide additional benefits. Performance studies were conducted with direct methanol fuel cells incorporating metal foams as the flow field. The influence of the foam pore size and density on cell performance was investigated. The performance of similar density metal foams but with different pore sizes was non-monotonic due to the opposing trends of electrical contact and CO₂ removal with pore size. In contrast, for metal foams with the same in-plane pore size, the performance improved with increasing density. Because the cell operates in a diffusion-dominated regime, its performance showed a strong dependence on methanol concentration and a moderate dependence on methanol flow rate. The feasibility of using metal foams as a gas diffusion layer (GDL) was also explored.     

Microporous layer coated gas diffusion layers for enhanced performance of polymer electrolyte fuel cells

The influence of microporous layer (MPL) design parameters for gas diffusion layers (GDLs) on the performance of polymer electrolyte fuel cells (PEFCs) was clarified. Appropriate MPL design parameters vary depending on the humidification of the supplied gas. Under low humidification, decreasing both the MPL pore diameter and the content of polytetrafluoroethylene (PTFE) in the MPL is effective to prevent drying-up of the membrane electrode assembly (MEA) and enhance PEFC performance. Increasing the MPL thickness is also effective for maintaining the humidity of the MEA. However, when the MPL thickness becomes too large, oxygen transport to the electrode through the MPL is reduced, which lowers PEFC performance. Under high humidification, decreasing the MPL mean flow pore diameter to 3 μm is effective for the prevention of flooding and enhancement of PEFC performance. However, when the pore diameter becomes too small, the PEFC performance tends to decrease. Both reduction of the MPL thickness penetrated into the substrate and increase in the PTFE content to 20 mass% enhance the ability of the MPL to prevent flooding.     

多孔泡沫陶瓷中预混火焰燃烧速率的试验研究

本文对在多孔泡沫陶瓷中的甲烷／空气预混燃烧的燃速特性进行了实验研究，用一专用燃烧器对两种材质不同孔径尺寸的多孔介质分别测定了它们的预混燃烧速率。所得结果表明，其燃速与层流无多孔介质的自由火焰相比有显著的提高，并且受到材质和孔径大小的影响。同时，当量皆可燃稳定上下界限也有相应扩大。     

Numerical simulation of liquid water and gas flow in a channel and a simplified gas diffusion layer model of polymer electrolyte membrane fuel cells using the lattice Boltzmann method

Numerical simulations using the lattice Boltzmann method (LBM) are developed to elucidate the dynamic behavior of condensed water and gas flow in a polymer electrolyte membrane (PEM) fuel cell. Here, the calculation process of the LBM simulation is improved to extend the simulation to a porous medium like a gas diffusion layer (GDL), and a stable and reliable simulation of two-phase flow with large density differences in the porous medium is established. It is shown that dynamic capillary fingering can be simulated at low migration speeds of liquid water in a modified GDL, and the LBM simulation reported here, which considers the actual physical properties of the system, has significant advantages in evaluating phenomena affected by the interaction between liquid water and air flows. Two-phase flows with the interaction of the phases in the two-dimensional simulations are demonstrated. The simulation of water behavior in a gas flow channel with air flow and a simplified GDL shows that the wettability of the channel has a strong effect on the two-phase flow. The simulation of the porous separator also indicates the possibility of controlling two-phase distribution for better oxygen supply to the catalyst layer by gradient wettability design of the porous separator.     

Natural convection in metal foams with open cells

This paper presents a combined experimental and numerical study on natural convection in open-celled metal foams. The effective thermal conductivities of steel alloy (FeCrAlY) samples with different relative densities and cell sizes are measured with the guarded-hot-plate method. To examine the natural convection effect, the measurements are conducted under both vacuum and ambient conditions for a range of temperatures. The experimental results show that natural convection is very significant, accounting for up to 50% of the effective foam conductivity obtained at ambient pressure. This has been attributed to the high porosity (ε > 0.9) and inter-connected open cells of the metal foams studied.Morphological parameters characterizing open-celled FeCrAlY foams are subsequently identified and their cross-relationships are built. The non-equilibrium two-equation energy transfer model is employed, and selected calculations show that the non-equilibrium effect between the solid foam skeleton and air is significant. The study indicates that the combined parameter, i.e., the porous medium Rayleigh number, is no longer appropriate to correlate natural convection by itself when the Darcy number is sufficiently large as in the case of natural convection in open-celled metal foams. Good agreement between model predictions and experimental measurements is obtained.