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.     

Effects of assembling method and force on the performance of proton-exchange membrane fuel cells with metal foam flow field

Recently, highly porous metal foams have been used to replace the traditional open-flow channels to improve gas transport and distribution in the cells. Deformation of flow plate, gas diffusion layer (GDL), and metal foam may occur during assembling. When the cell size is small, the deformation may not be significant. For large area cells, the deformation may become significant to affect the cell performance. In this study, an assembling device that is capable of applying uniform clamping force is built to facilitate fuel cell assembling and alleviate the deformation. A compressing plate that is the same size of the active area is used to apply uniform clamping force before surrounding bolts are fastened. Therefore, bending of the flow plate and deformation of GDL and metal foam can be minimized. Effects of the clamping force on the microstructures of GDL and metal foam, various resistances, pressure drops, and cell performance are investigated. Distribution of the contact pressure between metal foam and GDL is measured by using pressure sensitive films. Field-emission scanning electron microscope is used to observe the microstructures. Electrochemical impedance spectroscopy analysis is used measure resistances. The fuel cell performance is measured by using a fuel cell test system. For the cell design used in this study, the optimum clamping force is found to be 200 kgf. Using this optimum clamping force, the cell performance can be enhanced by 50%, as compared with that of the cell assembled without using clamping plates. With appropriate clamping force, the compression force distribution across the entire cell area can approach uniform. This enables uniform flow distribution and reduces mass transfer resistance. Good contact between GDL and metal foam also lowers the interface resistance. All these factors contribute to the enhanced cell performance.     

Review on current research of materials,fabrication and application for bipolar plate in proton exchange membrane fuel cell

Bipolar plate (BP) in proton exchange membrane (PEM) fuel cells provides conducting paths for electrons between cells, distributes and blocks the reactant gases, removes waste heat, and provides stack structural integrity. It is a key component to ensure the aforementioned functions while maintaining a low cost of fuel cell stack. This paper presents a comprehensive review about the BP materials (metallic, non-porous graphite and composite materials) and the corresponding fabrication methods, flow field layouts, and PEM fuel cells applications. Among the materials, the metallic BP has attracted high attention in automotive application due to its superior mechanical and physical properties, competitive cost compared with non-porous graphite and composite materials, but the fabrication technology and corrosion resistance are the major concerns for metallic bipolar plates. In recent studies, the protective coatings reported such as the conductive polymer, metal nitride/carbide and noble coatings have become the hot topics. They have been widely applied in different kinds of metallic bipolar plates, and the metal nitride coatings exhibit relatively low corrosion current and moderate interfacial contact resistance in comparison to other coatings. In future, developing excellent corrosion resistance and electrical conductivity coatings or novel metallic materials for bipolar plates will greatly enhance PEM fuel cells application in transportation field.     

Effects of metal foam properties on flow and water distribution in polymer electrolyte fuel cells (PEFCs)

Using a two-phase polymer electrolyte fuel cell (PEFC) model, we numerically investigated the influence of metal foam porous properties and wettability on key species and current distributions within a PEFC. Three-dimensional simulations were conducted under practical low humidity inlet hydrogen and air gases, and numerical comparisons were made for different metal foam design variables. These simulations were conducted to elucidate the detailed operating characteristics of PEFCs using metal foams as a flow distributor, and the simulation results showed that two-phase transport and the resulting flooding behavior in a PEFC are influenced by both the metal foam porous properties and the porous properties of an adjacent layer (e.g., the gas diffusion layer). This paper offers basic directions to design metal foams for optimal water management of PEFCs.     

Thermo-hydraulic study of a single row heat exchanger consisting of metal foam covered round tubes

Open cell metal foam is a novel engineering material that offers an interesting combination of material properties from a heat exchanger point of view such as a high specific surface area, tortuous flow paths for flow mixing and low weight. A new heat exchanger design with metal foams is studied in this work, aimed at low airside pressure drop. It consists of a single row of aluminum tubes covered with thin layers (4–8 mm) of metal foam. Through wind tunnel testing the impact of various parameters on the thermo-hydraulic performance was considered, including the Reynolds number, the tube spacing, the foam height and the type of foam. The results indicated that providing a good metallic bonding between the foam and the tubes can be achieved, metal foam covered tubes with a small tube spacing, small foam heights and made of foam with a high specific surface area potentially offer strong benefits at higher air velocities (>4 m/s) compared to helically finned tubes. The bonding was done by conductive epoxy glue and was found to have a strong impact on the final results, showing a strong need for a cost-effective and efficient brazing process to connect metal foams to the tube surfaces.     

Performance of a metallic gas diffusion layer for PEM fuel cells

A novel metallic porous medium with improved thermal and electrical conductivities and controllable porosity was developed based on micro/nano technology for its potential application in PEM fuel cells. In this work to demonstrate its applicability, the gas diffusion medium, made of 12.5 μm thick copper foil, was tested in an operational fuel cell. The small thickness and straight-pore feature of this novel material provides improved water management even at low flow rates. The performance does not decline at lower flow rates, unlike conventional gas diffusion layers. It has been shown that the performance can be further enhanced by increasing the in-plane transport. The improvements of such gas diffusion layer, including pore shape, porosity, and surface properties, are fully discussed.     

Achieving breakthrough performance caused by optimized metal foam flow field in fuel cells

Enhanced mass transport in polymer electrolyte membrane fuel cells (PEMFCs) is required for achieving high performance because concentration losses dominate cell performance. In particular, the flow field is crucial for mass transport. Recently, metal foam has been proposed as an alternative flow field owing to its three-dimensional pores, high porosity, and enhanced electrical conductivity. Here, we inspect the microstructure of various copper foams and investigate its effect as a flow field on PEMFCs. The PEMFCs with the optimized foam flow field deliver the highest performance reported to date. A large contact area and small ribs of the optimized foam flow field are advantageous for mass transfer and ohmic resistance. In addition, the internally generated pressure increases the partial pressure of the reactant, which leads to increased performance. This foam flow field has a significant potential for achieving high cell performance by enhancing the electrochemical reaction of the catalyst.     

Progress in poly(phenylene oxide) based cation exchange membranes for fuel cells and redox flow batteries applications

In fuel cell technologies, low-temperature proton exchange membrane fuel cells (LT-PEMFC), high-temperature proton exchange membrane fuel cells (HT- PEMFC), and direct methanol fuel cells (DMFC) are gained significant attention as a promising energy system for practical applications. The developments of cost-effective membrane materials with excellent physicochemical properties are indispensable for replacing the high cost of commercial membranes and achieving the higher performance of fuel cell systems. This review focuses on the developments and modifications of cost-effective poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) as a cation exchange membrane for LT-PEMFC, HT- PEMFC and DMFC. Notably, this review bridges the understanding of PPO based membranes, current advancements, structure, physicochemical properties and fuel cell performances. Progressive developments and a systematic overview of PPO-based membrane developments are explained in detail in terms of functionalization, blend, composite, acid-base, cross-linking, copolymerization, coated and reinforcement. Moreover, the changes in physicochemical properties and fuel cell performances in the membrane are deeply reviewed. Additionally, the utilization of PPO based membranes in different kinds of redox flow battery systems are reviewed. Overall, this review provides an exclusive vision and perspectives to develop the PPO based advanced, cost-effective, and high-performance membranes for fuel cell technologies and redox flow battery systems.     

X-ray tomography and modelling study on the mechanical behaviour and performance of metal foam flow-fields for polymer electrolyte fuel cells

Porous metal foams have been used as alternative flow-fields in proton exchange membrane fuel cells (PEMFCs), exhibiting improved performance compared to conventional ‘land and channel’ designs. In the current work, the mechanical behaviour of PEMFCs using metal foam flow-fields is investigated across different length scales using a combination of electrochemical testing, X-ray computed tomography (CT), compression tests, and finite element analysis (FEA) numerical modelling.Fuel cell peak power was seen to improve by 42% when foam compression was increased from 20% to 70% due to a reduction in the interfacial contact resistance between the foam and GDL. X-ray CT scans at varying compression levels reveal high levels of interaction between the metal foam and gas diffusion layer (GDL), with foam ligaments penetrating over 50% of the GDL thickness under 25% cell compression. The interfacial contact area between the foam and GDL were seen to be 10 times higher than between the foam and a stainless-steel plate. Modelling results demonstrate highly uniform contact pressure distribution across the cell due to plastic deformation of the foam. The effect of stack over-tightening and operating conditions are investigated, demonstrating only small changes in load distribution when paired with a suitable sealing gasket material.     

Effect of oscillatory frequency on heat transfer in metal foam heat sinks of various pore densities

In this paper, an experimental investigation was performed to study the heat transfer performance of metal foam heat sinks of different pore densities subjected to oscillating flow under various oscillatory frequencies. The variations of pressure drop and flow velocity along the kinetic Reynolds number of oscillating flow through aluminum foams were compared. The measured pressure drops, velocities and surface temperatures of oscillating flow through aluminum 10, 20 and 40 PPI foams were presented in detail. The calculated cycle-averaged local temperature and Nusselt number for different kinetic Reynolds numbers were analyzed and compared with finned heat sinks. The results of length-averaged Nusselt number for both oscillating and steady flows indicate that higher heat transfer rates can be obtained in metal foams subjected to oscillating flow. For the purpose of designing a novel heat sink using metal foam, the characteristics of the pumping power of the cooling system for aluminum foam with different pore densities were also analyzed.     

Three-dimensional multi-phase simulation of PEM fuel cell considering the full morphology of metal foam flow field

It is well-known that flow field design is of primary importance to optimization of proton exchange membrane (PEM) fuel cell. Traditional channel-rib flow fields, e.g. parallel or serpentine channels, always lead to non-uniform distributions of reactant gas, liquid, current density and so on between the channel and rib regions. Metal foam materials with high porosity (>90%) have been proposed as alternative flow fields for PEM fuel cells. In this study, influences of metal foam flow field on the transport phenomena coupled with the electrochemical reactions in PEM fuel cell are investigated using a three-dimensional (3D) multi-phase non-isothermal model. Specifically, the full morphology of metal foam flow field is taken into account in the 3D simulation after validated against experimental permeability data. The full morphology inclusion enables capture of the detailed gas flow from the flow field into the gas diffusion layer (GDL) and the current collection at the metal foam/GDL interface. In addition, compared with the conventional channel-rib flow fields, the metal foam design greatly increases fuel cell performance in the high current density regime. In addition, the oxygen and current density distributions in PEM fuel cell with the metal foam flow field are more uniform than those in the conventional one. Though the current collection area at the GDL surface is much smaller in the metal foam flow field, the relevant Ohmic loss won't increase significantly due to the improved physical contact by the fine pore structure of metal foam over the GDL.     

Chemical-foam cleaning processes for the control of power plant particle erosion

When exposed to high-temperature steam over extended time periods, heat transfer surfaces in power plants can be degraded by corrosive deposits of various oxidation products commonly known as scale. Solid particles can exfoliate from these surfaces, move downstream, and erode costly power-generation equipment, including turbine valve stems, nozzle blocks, turbine diaphragms, turbine buckets etc. In addition, the heat transfer efficiency is substantially reduced. Such effects can be severe and can significantly increase capital expenses, fuel and operating costs unless abated by various techniques, among which the most common are mechanical and chemical cleaning. In this paper we analyse the use of foams as an alternative chemical cleaning method, and use a network of parallel tubes to stimulate key process and performance variables in chemical and foam cleaning. Among the process (or control) variables are: rheological properties of the foams, foam quality, inlet flow rates, network geometry, and injection strategy. Among the performance variables are: distributions of residence times, of flow, and of quality, pressure drops, and energy requirements. From this analysis we conclude that chemical-foam cleaning offers technical as well as economic advantages over conventional cleaning methods.     

Experimental study on non-uniform arrangement of 3D printed structure for cathodic flow channel in PEMFC

Enhancing mass transfer capability of flow channel is important subject to improve fuel cell performance. In this study, an experimental study about non-uniform arrangement of metallic structures in cathodic flow channel on polymer electrolyte membrane fuel cell is conducted. Metallic 3D printer is used to manufacture 3D mesh with complex geometry. Channel width and bottom-rear channel depth are selected to change the geometry of structure. Assuming that accelerating flow velocity and reinforcing flow direction to gas diffusion layer can improve fuel cell performance, eight basic arrangements are inserted into cathodic carbon bipolar plate to measure fuel cell performance. With I–V curve, the unit cells with non-uniform arrangements of width and tapered structure show performance enhancement of 12.8% in maximum power density, compared with conventional parallel flow channel. According to electrochemical impedance spectroscopy results, performance enhancement by non-uniform arrangement is mainly occur in high current density operation due to low mass transfer resistance.     

New electroplated aluminum bipolar plate for PEM fuel cell

Further improvement in the performance of the polymer electrolyte membrane fuel cells as a power source for automotive applications may be achieved by the use of a new material in the manufacture of the bipolar plate. Several nickel alloys were applied on the aluminum substrate, the use of aluminum as a bipolar plate instead of graphite is to reduce the bipolar plate cost and weight and the ease of machining. The electroplated nickel alloys on aluminum substrate produced a new metallic bipolar plate for PEM fuel cell with a higher efficiency and longer lifetime than the graphite bipolar plate due to its higher electrical conductivity and its lower corrosion rate. Different pretreatment methods were tested; the optimum method for pretreatment consists of dipping the specimen in a 12.5% NaOH for 3 min followed by electroless zinc plating for 2 min, then the specimen is dipped quickly in the electroplating bath after rinsing with distilled water. The produced electroplate was tested with different measurement techniques, chosen based on the requirement for a PEM fuel cell bipolar plate, including X-ray diffraction, EDAX, SEM, corrosion resistance, thickness measurement, microhardness, and electrical conductivity.     

Air-cooled micro-porous heat exchangers for thermal management of fuel cells

This paper presents 3D numerical simulation of an air-cooled metal foam heat exchanger with potential application in thermal management of fuel cell systems in general and Proton Exchange Membrane Fuel Cells, PEMFCs, in particular. It has been shown that the new design can lead to a uniform temperature distribution for the heated plate especially at higher air flow speeds. The heat transfer enhancement because of the foams leads to an increase in the pressure drop which is, interestingly, comparable to that of water-cooled PEMFCs. Other potential benefits of the application of metal foams for fuel cell thermal management are briefly discussed and estimated.     

Performance of miniaturized direct methanol fuel cell (DMFC) devices using micropump for fuel delivery

A fuel cell is a device that can convert chemical energy into electricity directly. Among various types of fuel cells, both polymer electrolyte membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs) can work at low temperature (<80 °C). Therefore, they can be used to supply power for commercial portable electronics such as laptop computers, digital cameras, PDAs and cell phones. The focus of this paper is to investigate the performance of a miniaturized DMFC device using a micropump to deliver fuel. The core of this micropump is a piezoelectric ring-type bending actuator and the associated nozzle/diffuser for directing fuel flow. Based on the experimental measurements, it is found that the performance of the fuel cell can be significantly improved if enough fuel flow is induced by the micropump at anode. Three factors may contribute to the performance enhancement including replenishment of methanol, decrease of diffusion resistance and removal of carbon dioxide. In comparison with conventional mini pumps, the size of the piezoelectric micropump is much smaller and the energy consumption is much lower. Thus, it is very viable and effective to use a piezoelectric valveless micropump for fuel delivery in miniaturized DMFC power systems.     

Performance improvement of proton exchange membrane fuel cells with compressed nickel foam as flow field structure

In order to improve the performance of proton exchange membrane fuel cell (PEMFC), the compressed nickel foam as flow field structure was applied to the fuel cell. The fuel cell test system was built and the performance of fuel cells with nickel foam flow field with different thicknesses were tested and analyzed by electrochemical active surface area (EASA), electrochemical impedance and polarization curve. And its operating parameters were optimized to improve the performance of PEMFC. Our results show that the membrane electrode assembly (MEA) can show a larger catalytic active area and uniformity of gas diffusion can be improved by using the nickel foam flow field instead of the conventional graphite serpentine flow field, and the impedance characteristic of 110PPI nickel foam can be improved by increasing the compression ratio of the original material. What's more, the polarization characteristic and power output performance of PEMFC with nickel foam flow field were improved by optimizing the operating parameters. Using the optimized operating parameters (cell temperature = 80 °C; humidification temperature = 75 °C; stoichiometric ratio = 2; back pressure = 0.23 Map), a peak power density of 1.89 W cm⁻² was obtained with an output voltage of 0.46 V.     

A Comparison of Metal-Foam Heat Exchangers to Compact Multilouver Designs for Air-Side Heat Transfer Applications

High-porosity metal foams, with novel thermal, mechanical, electrical, and acoustic properties, are being more widely used in various industrial applications. In this paper, open-cell aluminum foam is considered as a highly compact replacement for conventional louver fins in brazed aluminum heat exchangers. A model based on the ?-N_TU method is developed to compare the flat-tube, serpentine louver-fin heat exchanger to the flat-tube metal-foam heat exchanger. The two heat exchangers are subjected to identical thermal-hydraulic requirements, and volume, mass, and cost of the metal-foam and louver-fin designs are compared. The results show that the same performance is achieved using the metal-foam heat exchanger but a lighter and smaller heat exchanger is required. However, the cost of the metal-foam heat exchanger is currently much higher than that of the louver-fin heat exchanger, because of the high price of metal foams. If the price of metal foam falls to equal that of louver-fin stock (per unit mass), then the metal-foam heat exchanger will be less expensive, smaller, and lighter than the louver-fin heat exchanger, with identical thermal performance.     

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.     

Two/three-dimensional reduced graphene oxide coating for porous flow distributor in polymer electrolyte membrane fuel cell

There are drawbacks to use stainless-steel plates as a flow distributor plate in fuel cell, due to some of their properties being inferior to graphite flow distributor plates in terms of electrical conductivity and corrosion resistance. To overcome these problems, many researches have been conducted to improve the properties of stainless-steel flow distributor plates through coating of carbon materials. Herein, two-dimensional Web-like graphene (WG) and self-assembled three-dimensional graphene (STG) are coated through superheat vaporization of micro-droplet method. WG is coated in porous Ni–Cr foam and STG is constructed on the flat flow distributor plate, and they exhibit the feasibility to be applied in flow distributors. Compared to uncoated Ni–Cr foam, the performance of the PEMFC system with the graphene coated foam is enhanced remarkably. Furthermore, the flow distributor plate with the STG exhibits potential to be used directly to flow distributor.