Numerical study on applicability of metal foam as flow distributor in polymer electrolyte fuel cells (PEFCs)

In this paper, the effects of using porous metal foam based bipolar plates (BPs) are investigated under practical automotive fuel cell operations with low humidification reaction gases. Particular emphasis is placed on evaluating water management capabilities of metal foam based BP designs, compared to the traditional serpentine flow field BP designs. A three-dimensional, two-phase fuel cell model developed in a previous study is applied to 25cm² real-scale fuel cell geometries with metal foam and serpentine flow modes, and then successfully validated against the experimental data measured under different operating pressures and current densities. The detailed simulation results clearly elucidate advantages of using metal foam as flow distributor through extensive multidimensional contours of flow velocity, species, and current density.     

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.     

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.     

Development of porous carbon foam polymer electrolyte membrane fuel cell

In order to prove the feasibility of using porous carbon foam material in a polymer electrolyte membrane fuel cell (PEMFC), a single PEMFC is constructed with a piece of 80PPI (pores per linear inch) Reticulated Vitreous Carbon (RVC) foam at a thickness of 3.5 mm employed in the cathode flow-field. The cell performance of such design is compared with that of a conventional fuel cell with serpentine channel design in the cathode and anode flow-fields. Experimental results show that the RVC foam fuel cell not only produces comparative power density to, but also offers interesting benefits over the conventional fuel cell. A 250 h long term test conducted on a RVC foam fuel cell shows that the durability and performance stability of the material is deemed to be acceptable. Furthermore, a parametric study is conducted on single RVC foam fuel cells. Effect of geometrical and material parameters of the RVC foam such as PPI and thickness and operating conditions such as pressure, temperature, and stoichiometric ratio of the reactant gases on the cell performance is experimentally investigated in detail. The single cell with the 80PPI RVC foam exhibits the best performance, especially if the thinnest foam (3.5 mm) is used. The cell performance improves with increasing the operating gauge pressure from 0 kPa to 80 kPa and the operating temperature from 40 °C to 60 °C, but deteriorates as it further increases to 80 °C. The cell performance improves as the stoichiometric ratio of air increases from 1.5 to 4.5; however, the improvement becomes marginal when it is raised above 3.0. On the other hand, changing the stoichiometric ratio of hydrogen does not have a significant impact on the cell performance.     

Multi-pass serpentine flow-fields to enhance under-rib convection in polymer electrolyte membrane fuel cells: Design and geometrical characterization

The flow-field for reactant distribution is an important design factor that influences the performance of polymer electrolyte membrane fuel cells (PEMFCs). Under-rib convection between neighboring channels has been recognized to enhance the performance of PEMFCs with serpentine flow-fields. This study presents a simple design method to generate multi-pass serpentine flow-fields (MPSFFs) that can maximize under-rib convection in a given cell area. Geometrical characterization indicates that MPSFFs lead to significantly higher under-rib convection intensities and more uniform conditions, such as reactant concentrations, temperature, and liquid water saturation, compared with conventional serpentine flow-fields. The implications of the enhanced under-rib convection due to MPSFFs on the performance of PEMFCs are discussed.     

Synchrotron X-ray tomography for investigations of water distribution in polymer electrolyte membrane fuel cells

Synchrotron X-ray tomography is used to visualize the water distribution in gas diffusion layers (GDL) and flow field channels of a polymer electrolyte membrane fuel cell (PEMFC) subsequent to operation. An experimental setup with a high spatial resolution of down to 10 μm is applied to investigate fundamental aspects of liquid water formations in the GDL substrate as well as the formation of water agglomerates in the flow field channels. Detailed analyses of water distribution regarding the GDL depth profile and the dependence of current density on the water amount in the GDL substrate are addressed. Visualizations of water droplets and wetting layer formations in the flow field channels are shown. The three-dimensional insight by means of this quasi in situ tomography allows for a better understanding of PEMFC water management at steady state operation conditions. The effect of membrane swelling as function of current density is pointed out. Results can serve as an essential input to create and verify flow field simulation outputs and single-phase models.     

Performance enhancement of air-cooled open cathode polymer electrolyte membrane fuel cell with inserting metal foam in the cathode side

In this study, a novel way to improve performance of the air-cooled open cathode polymer electrolyte membrane fuel cell is introduced. We suggest using a metal foam in the cathode side of the planar unit fuel cell for the solution to conventional problems of the open cathode fuel cell such as excessive water evaporation from the membrane and poor transportation of air. We conduct experiment and the maximum power density of the fuel cell with metal foam increases by 25.1% compared with the conventional fuel cell without metal foam. The open cathode fuel cell with metal foam has smaller ohmic losses and concentration losses. In addition, we prove that the open cathode fuel cell with metal foam prevents excessive water evaporation and membrane drying out phenomena with numerical approach. Finally, we apply the metal foam to the air-cooled open cathode fuel cell stack as well as the planar unit cell.     

Reticulated porous and parallel channel cathode flow fields in real scale polymer electrolyte membrane fuel cells: A comparative experimental study

This paper focuses on understanding the effect of reticulated porous cathode flow fields in real scale close and open cathode polymer electrolyte membrane fuel cells (PEMFCs) in terms of their thermo-electrical performance. This research contributes to addressing challenges with PEMFCs linked to oxygen supply to the cathode and proper mixing of gasses as well as water removal issues. Parallel channel and porous cathode flow fields in both open and closed cathode PEMFCs of medium scale (active area of 15 × 15 cm²) have been investigated. The porous material consisted of 20 pores per inch with a porosity level of 80%. The cells’ polarisation and impedance characteristics have been analysed. The porous flow field has been found providing better electrical performance in closed cathode PEMFC compared to the open cathode. Improvements in gas diffusion and temperature uniformity were observed with porous flow field; however, water removal has been observed challenging, which need to be addressed before the benefits of using porous flow field are fully realised.     

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.     

Three-dimensional performance simulation of PEMFC of metal foam flow plate reconstructed with improved full morphology

Recently, Proton Exchange Membrane Fuel Cell (PEMFC) with metal foam flow field has attracted extensive attention from scholars. In the present work, the full morphological reconstruction method of metal foam is improved and is successfully applied to a three-dimensional simulation of a metal foam PEMFC. The numerical predictions of metal foam PEMFC before and after reconstruction improvement are compared. The numerical results of the traditional channel flow field and two metal foam flow fields with pore sizes (40PPI and 100PPI) are compared. It is found that the application of metal foam can greatly improve the performance of PEMFC at higher current densities, and the smaller pore size (100PPI) of the metal foam makes the performance better. In addition, the numerical results of the oxygen concentration, ohmic loss, intra-membrane ionic current density and pressure drop of the cathode component are elaborated to explain the phenomenon.     

Numerical study to examine the performance of multi-pass serpentine flow-fields for cooling plates in polymer electrolyte membrane fuel cells

Temperature is an important factor that impacts the performance of polymer electrolyte membrane fuel cells (PEMFCs). Proper cooling systems are indispensable for heat management. Cooling plates with coolant flow channels are mainly used to release the reaction heat in PEMFCs and thus control their operating temperature. In this study, several multi-pass serpentine flow-field (MPSFF) designs are studied in order to achieve better heat management by using cooling plates. Based on computational fluid dynamics (CFD) simulations of fluid flow and heat transfer in the cooling plates, the cooling performance of the six serpentine channel designs is evaluated. The results demonstrate that MPSFFs lead to better cooling performance compared with a conventional serpentine flow-field, in terms of both the maximum temperature and temperature uniformity. The effect of the Reynolds number and heat flux on the cooling performance exhibited by the six designs is also investigated.     

The effects of water and microstructure on the performance of polymer electrolyte fuel cells

In this paper, we present a comprehensive non-isothermal, one-dimensional model of the cathode side of a Polymer Electrolyte Fuel Cell. We explicitly include the catalyst layer, gas diffusion layer and the membrane. The catalyst layer and gas diffusion layer are characterized by several measurable microstructural parameters. We model all three phases of water, with a view to capturing the effect that each has on the performance of the cell. A comparison with experiment is presented, demonstrating excellent agreement, particularly with regard to the effects of water activity in the channels and how it impacts flooding and membrane hydration. We present several results pertaining to the effects of water on the current density (or cell voltage), demonstrating the role of micro-structure, liquid water removal from the channel, water activity, membrane and gas diffusion layer thickness and channel temperature. These results provide an indication of the changes that are required to achieve optimal performance through improved water management and MEA-component design. Moreover, with its level of detail, the model we develop forms an excellent basis for a multi-dimensional model of the entire membrane electrode assembly.     

Examining the effect of the secondary flow-field on polymer electrolyte fuel cells using X-ray computed radiography and computational modelling

Flow-fields are key factors in determining the operation of fuel cells. While extensive work has been conducted to develop and optimise the reactant flow and current collection performance of polymer electrolyte membrane fuel cell (PEMFC) components, there is a factor that remains largely unaccounted for. Depending on how a membrane electrode assembly (MEA) is incorporated into a cell, there will often be a small gap between the edge of the gas diffusion layer (GDL) and the seal or bipolar plate. This gap acts as a ‘secondary flow-field’ (SFF) that can bypass or affect/augment the conventional or ‘primary flow-field’. Understanding how this affects performance (either positively or adversely) is essential for holistic flow-field design. This paper describes the issues associated with the SFF, examines how cell compression affects its width due to lateral expansion of the GDL and discusses the results of a 3-D computational model that investigates the effect of the SFF during dead-ended anode (DEA) operation for a fuel cell without a macroscopic (conventional) anode flow-field.     

A numerical investigation of the effects of compression force on PEM fuel cell performance

In the present study we report on numerical investigations into the effects of compression on the performance of a unit cell. The focus of this study is how the transport properties of the gas diffusion layer (GDL) material, specifically porosity and permeability, affect numerical predictions of cell performance. Experimental data of porosity and permeability of uncompressed and compressed GDLs were obtained using a porometer, and used in numerical simulations. A 3D model with two parallel channels and an membrane electrode assembly (MEA) is constructed for the calculations. Three different configurations of transport properties were tested, i.e. uniform uncompressed GDL properties, uniform compressed GDL properties, and non-homogeneous GDL properties. It is found that the non-homogeneous case shows noticeable differences in predicted cell performance. For the non-homogenous case, simulations with a pressure difference between two cathode channels were carried out to gain insight into the effect of cross-channel flow on the overall prediction of cell performance. We found that the cross-channel flow changes local current density distribution primarily on the high-pressure channel. The present study demonstrates the importance of the proper use of transport properties for the compressed portion of the GDL.     

An experimental study on the bubble humidification method of polymer electrolyte membrane fuel cells

A necessary requirement for polymer electrolyte membrane fuel cell (PEMFC) performance is providing sufficient water content in the membrane. The bubble humidifier is the simplest and inexpensive method for PEMFC humidification. In this study, a prototype of bubble humidifier is designed, fabricated, and tested. The effects of water temperature in the reservoir, water level inside the reservoir and inlet air flow on the humidifier performance are investigated. The results show that the outlet air relative humidity decreases (about 6% - 11%) with an increase in the inlet air flow rate from 1 m³ h^?1 to 3 m³ h^?1 at four different water temperatures. The increase in the water temperature and water level inside the reservoir lead to the better humidifier performance. At the water temperature of 20^°C, increasing water level from 5 cm to 7.5 cm has a significant effect on humidifier performance but increasing water level from 7.5 cm to 15 cm does not offer any advantage.     

Effect of height/width-tapered flow fields on the cell performance of polymer electrolyte membrane fuel cells

Water flooding at the cathode is a critical issue in polymer electrolyte membrane fuel cells, since it will block the oxygen transport, resulting in a large concentration loss. To address this issue, two tapered flow fields with varying height or width are proposed in this study, as the tapered channels can gradually increase flow velocity which is beneficial in water removal. To confirm the proposed function, both experiments and simulation are performed. The results prove that the tapered flow fields with high velocity at the downstream region can significantly enhance water removal in flow channels, which avoids the mass transport limitation and improves the cell performance at high current densities. Additionally, the tapered design can also enhance the under-rib convection between adjacent channels, which helps to remove accumulated water in the gas diffusion layer.     

Electrocatalytic reduction of dioxygen on PdCu for polymer electrolyte membrane fuel cells

The present research is aimed to study the oxygen reduction reaction (ORR) on a PdCu electrocatalyst synthesized through reduction of PdCl₂ and CuCl with NaBH₄ in a THF solution. Characterization of PdCu electrocatalyst was performed by X-ray diffraction (XRD), transmission electron microscopy (TEM) and energy-dispersive X-ray (EDX) spectroscopy. Characterization results showed that the synthesis method produced spherical agglomerated nanocrystalline PdCu particles of about 10 nm size. The electrochemical activity was evaluated using cyclic voltammetry (CV), rotating disc electrode (RDE) and electrochemical impedance spectroscopy (EIS) in a 0.5 M H₂SO₄ electrolyte at 25 °C. The onset potential for ORR on PdCu is shifted by ca. 30 mV to more positive values and enhanced catalytic current densities were observed, compared to that of pure Pd catalyst. The synthesized PdCu electrocatalyst dispersed on a carbon black support was tested as cathode electrode in a membrane-electrode assembly (MEA) achieving a power density of 150 mW cm⁻² at 0.38 V and 80 °C.     

Characterisation of mechanical behaviour and coupled electrical properties of polymer electrolyte membrane fuel cell gas diffusion layers

Local compression distribution in the gas diffusion layer (GDL) of a polymer electrolyte membrane fuel cell (PEMFC) and the associated effect on electrical material resistance are examined. For this purpose a macroscopic structural material model is developed based on the assumption of orthotropic mechanical material behaviour for the fibrous paper and non-woven GDLs. The required structural material parameters are measured using depicted measurement methods. The influence of GDL compression on electrical properties and contact effects is also determined using specially developed testing tools. All material properties are used for a coupled 2D finite element simulation approach, capturing structural as well as electrical simulation in combination. The ohmic voltage losses are evaluated assuming constant current density at the catalyst layer and results are compared to cell polarisation measurements for different materials.     

X-ray computed tomography reconstruction and analysis of polymer electrolyte membrane fuel cell porous transport layers

A commercially available porous transport layer (SGL carbon group Sigracet^® GDL 30BA), is investigated using X-ray computed tomography reconstruction. A novel aspect of this study is an investigation of the effects of non-homogeneous compression of the GDL 30BA sample including effective transport properties. Non-homogeneous compression is typical in polymer electrolyte fuel cells as the flow field plates consist of a series of lands and channels which apply an uneven loading to the porous transport layers. The X-ray computed tomography technique provides input data for the computer reconstruction procedures integrating image post-processing and iso-surface reconstruction. The resulting tomographic and surface reconstruction is converted into the computational volume/grid for microstructural and computational fluid dynamics (CFD) analysis. The heterogeneous compression effects on effective geometric and transport properties are investigated for various compression levels and effective transport properties are compared to theoretical studies such as Bruggeman [1] and Tomadakis and Sotirchos [2]. The effects of non-homogeneous compression are significant, with the transport properties differing by a factor of about 2 between the land and the channel regions. It is found that the effective transport properties are significantly lower than predicted by commonly used relations, with the lowest values representing only 15% of the predictions from the Bruggeman relation.     

Two-phase flow behaviour and performance of polymer electrolyte membrane electrolysers: Electrochemical and optical characterisation

Understanding gas evolution and two-phase flow behaviour are critical for performance optimization of polymer electrolyte membrane water electrolysers (PEMWEs), particularly at high current densities. This study investigates the gas-bubble dynamics and two-phase flow behaviour in the anode flow-field of a PEMWE under different operating conditions using high-speed optical imaging and relates the results to the electrochemical performance. Two types of anode flow-field designs were investigated, the single serpentine flow-field (SSFF) and parallel flow-field (PFF). The results show that the PFF design yielded a higher cell performance than the SSFF design at identical operating conditions. Optical visualization shows a strong relationship between the flow path length and the length of gas slugs produced, which in turn influences the flow regime of operation. Longer flow path length in the SSFF results in annular flow regime at a high current density which degrades cell performance. The annular flow regime was absent in the PFF design. It was found the effect of flow rate on performance depends strongly on operating temperature in both flow patterns. Results of this study indicate that long channel length promotes gas accumulation and channel-blocking which degrades performance in PEMWEs.