Steady saturation distribution in hydrophobic gas-diffusion layers of polymer electrolyte membrane fuel cells: A pore-network study

A pore-network model is developed to simulate liquid water transport in a hydrophobic gas-diffusion layer (GDL) during the operation of polymer electrolyte membrane fuel cells (PEMFCs). The steady saturation distribution in GDLs is determined through a numerical procedure using a pore-network model combined with invasion-percolation path-finding and subsequent viscous two-phase flow calculation. The simulation results indicate that liquid water transport in hydrophobic GDLs is a strongly capillary-driven process that almost reaches the pure invasion-percolation limit with zero capillary number. A uniform flux condition is found to better reflect the actual phenomenon occurring at the inlet boundary for liquid water entering a GDL than a uniform pressure condition. The simulation further clarifies the effect of the invaded pore fraction at a uniform-flux inlet boundary in modifying water transport in GDLs. Finally, the effect of the GDL thickness on the steady saturation distribution is investigated.     

Effect of gas-diffusion electrode material heterogeneity on the structural integrity of polymer electrolyte fuel cell

In polymer electrolyte fuel cell (PEFC), gas-diffusion electrode (GDE) plays very significant role in force transmission from bipolar plate to the membrane. This paper investigates the effects of material heterogeneities of gas-diffusion electrode layer (gas-diffusion layer (GDL) and catalyst layer (CL)) on the assembly stress levels of single PEFC stack. In addition, we adopt a force transfer mechanism in a single fuel cell stack based on material heterogeneities of GDL and CL to understand the limitations and advantages associated with it through numerical analyses. Nanoscale heterogeneities in GDE are effectively implemented in the simulation cases along with the membrane swelling. Influence of presence or absence of CL interlayer in the numerical environment is found to have significant impact on the adjacent layers as well as interfaces.     

Study on water transport in hydrophilic gas diffusion layers for improving the flooding performance of polymer electrolyte fuel cells

For hydrogen-based polymer electrolyte fuel cells (PEFCs), water transport control in gas diffusion layers (GDLs) by wettability distribution is useful to suppress the flooding problem. In this study, the water transport of a novel GDL with hydrophilic-hydrophobic patterns was investigated. First, we clarified that the water motion in the hydrophilic GDL with microstructures could be reproduced by the enlarged scale model. The scale model experiment also showed that the same water behavior in hydrophilic GDL can be obtained from Capillary numbers (Ca) in a range of Ca ~ 10^?5 to 10^?3. As the computational load is inversely proportional to Ca, the computational load could be reduced by 1/100th by using Ca ~ 10^?3, which is 100 times higher than PEFC operation (Ca ~ 10^?5). Finally, the simulation with Ca ~ 10^?3 was performed, and we showed that the GDL with straight region of contact angle 50° minimized the water accumulation.     

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.     

Modeling of two-phase transport in the diffusion media of polymer electrolyte fuel cells

A three-dimensional model of polymer electrolyte fuel cells (PEFCs) is developed to investigate multiphase flows, species transport, and electrochemical processes in fuel cells and their interactions. This two-phase model consists of conservation principles of mass, momentum, species concentration and charges, and elucidates the key physicochemical mechanisms in the constituent components of PEFCs that govern cell performance. Efforts are made to formulate two-phase transport in the anode diffusion media and its coupling with cathode flooding as well as the interaction between single- and two-phase flows. Numerical simulations are carried out to investigate multiphase flow, electrochemical activity, and transport phenomena and the intrinsic couplings of these processes inside a fuel cell at low humidity. The results indicate that multiphase flows may exist in both anode and cathode diffusion media at low-humidity operation, and two-phase flow emerges near the outlet for co-flow configuration while is present in the middle of the fuel cell for counter-flow one. The validated numerical tools can be applied to investigate vital issues related to anode performance and degradation arising from flooding for PEFCs.     

Numerical analyses on oxygen transport resistances in polymer electrolyte membrane fuel cells using a novel agglomerate model

Characterizing oxygen transport resistances in different components of a polymer electrolyte membrane fuel cell (PEMFC) is essential to achieve better cell performance at high current under low Pt loading. In this work, a macroscopic three-dimensional model, together with a novel agglomerate model was proposed to analyze impacts of operating conditions on these resistances via limiting current strategy. By introducing a focusing factor obtained with lattice Boltzmann method at mesoscopic level, the structure-dependent local transport resistance in ionomer thin-film of the electrode was comprehensively captured and validated by existing experimental studies. Contributions of the cell components to the total transport resistance were dissected. Results show that the present agglomerate model could well reproduce the local transport behaviors of oxygen in catalyst layer by fully considering the detailed nanoscale diffusion and adsorption processes. A small mass fraction of oxygen was favored to minimize the relative deviation of the local transport resistance from its intrinsic one due to the water production and heat generation, which can reach 7% for the mass fraction of oxygen of 1%. Contribution of the in-plane diffusion of oxygen in the inactive electrode is around 1%. The total transport resistance increased with the absolute pressure, mainly due to the dominated molecular diffusion mechanism in gas channel and gas diffusion layer. Gas convection accounted for 26% of the oxygen transport resistance originated from gas channel. The transport resistance of catalyst layer increased significantly with the reduction of Pt loading, and decreased with relative humidity and operating temperature, particularly at high Pt loading.     

Comparison between numerical simulation and visualization experiment on water behavior in single straight flow channel polymer electrolyte fuel cells

A relationship between a flooding and a cell voltage drop for polymer electrolyte fuel cell was investigated experimentally and numerically. A visualization cell, which has single straight gas flow channel (GFC) and observation window, was fabricated to visualize the flooding in GFC. We ran the cell with changing operation condition, and measured the time evolution of cell voltage and took the images of cathode GFC. Considering the operation condition, we executed a developed numerical simulation, which is based on multiphase mixture model with a formulation on water transport through the surface of polymer electrolyte membrane and the interface of gas diffusion layer/GFC. As a result in experiment, we found that the cell voltage decreased with time and this decrease was accelerated by larger current and smaller air flow rate. Our simulation succeeded to demonstrate this trend of cell voltage. In experiment, we also found that the water flushing in GFC caused an immediate voltage change, resulting in voltage recovery or electricity generation stop. Although our simulation could not replicate this immediate voltage change, the supersaturated area obtained by our simulation well corresponded to fogging area appeared on the window surface in the GFC.     

Influence of anisotropic bending stiffness of gas diffusion layers on the electrochemical performances of polymer electrolyte membrane fuel cells

The effects of gas diffusion layer’s (GDL’s) anisotropic bending stiffness on the electrochemical performances of polymer electrolyte membrane fuel cells have been investigated for carbon fiber-felt and -paper GDLs. The bending stiffness values of all GDLs in the machine direction are higher than those in the cross-machine direction. We have prepared GDL sheet samples such that the machine direction of GDL roll is aligned with the major flow field direction of a metallic bipolar plate at angles of 0° (parallel: ‘0° GDL’) and 90° (perpendicular: ‘90° GDL’). The I–V performances of all the 5-cell stacks with 90° GDLs are higher than those with 0° GDLs. All the 5-cell stacks with 90° GDLs show lower values of high-frequency resistance (HFR) than those with 0° GDLs. However, the gas pressure differences at both anode and cathode of 5-cell stacks with 90° GDLs appear to be similar to or slightly lower than those with 0° GDLs, making the lower HFR as a dominant factor for the improved I–V performances. This may result from the reduced intrusion of 90° GDLs into gas channels than 0° GDLs as observed by less thickness reduction under compression of 90° GDLs. A 45° GDL (skew alignment) also shows better performances than the 0° GDL.     

Porosity-graded micro-porous layers for polymer electrolyte membrane fuel cells

In this paper, the effect of porosity-graded micro-porous layer (GMPL) on the performance of polymer electrolyte membrane fuel cells (PEMFCs) was studied in detail. The GMPL was prepared by printing micro-porous layers (MPL) with different content of NH₄Cl pore-former and the porosity of the GMPL decreased from the inner layer of the MPLs at the membrane/MPL interface to the outer layer of the MPLs at the gas diffusion electrode/MPL interface. The morphology and porosity of the GMPLs were characterized and the performance of the cell with GMPLs was compared with those having conventional homogeneous MPLs. The result demonstrates that the fuel cells consisting of GMPL have better performance than those consisting of conventional homogeneous MPLs, especially at high current densities. Micro-porous layer with graded porosity is beneficial for the electrode process of fuel cell reaction probably by facilitating the liquid water transportation through large pores and gas diffusion via small pores in the GMPLs.     

The effect of diffusioosmosis on water transport in polymer electrolyte fuel cells

An analytical study of the effect of diffusioosmosis caused by the concentration gradient of hydrogen ions on the isothermal transport of water in a fully hydrated membrane of a polymer electrolyte fuel cell (PEFC) is presented. A capillary tube or slit with a negatively charged wall is chosen to model the nanopores of the membrane. The electric double layer adjacent to the capillary wall may have an arbitrary thickness relative to the capillary radius and its electrostatic potential distribution is determined as the solution of the Poisson–Boltzmann equation. Solving a modified Navier–Stokes equation, the fluid velocity in the axial direction of the capillary induced by the macroscopic electric field and protonic concentration gradient is obtained as a function of the radial position in closed forms. The results for the local and averaged electrokinetic velocities in the capillary show that the effect of diffusioosmosis on the water transport in the membrane of a PEFC can be significant in comparison with that of electroosmosis under low-potential-difference operations.     

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.     

Electronic conductivity of catalyst layers of polymer electrolyte membrane fuel cells: Through-plane vs. in-plane

In this work, novel procedures are developed to measure in-plane and through-plane electronic conductivities of catalyst layers (CLs) for polymer electrolyte membrane fuel cells. The developed procedures are used in a parametric study on different CL designs to investigate effects of different composition and fabrication parameters, including ionomer to carbon weight ratio (I/C ratio), dry milling time of the catalyst powder, and drying temperature of the catalyst ink. Results show that CLs have anisotropic electronic conductivity with through-plane values being three orders of magnitude lower than the in-plane values. The reason for this anisotropy is speculated to be alignment of fibrillar nanostructures of ionomer by large shear forces during coating, which could result in better carbon-carbon contact in the in-plane direction. A simple order of magnitude analysis shows the significance of poor through-plane conduction for fuel cell performance.     

Modelling of coupled electron and mass transport in anisotropic proton-exchange membrane fuel cell electrodes

As one of the key components of proton-exchange membrane fuel cells, the gas-diffusion layer (GDL) that is made of carbon fibres usually exhibits strong structural anisotropy. Nevertheless, the GDL has traditionally been simplified as a homogeneous porous structure in modelling the transport of species through the GDL. In this work, a coupled electron and two-phase mass transport model for anisotropic GDLs is developed. The effects of anisotropic GDL transport properties due to the inherent anisotropic carbon fibres and caused by GDL deformations are studied. Results indicate that the inherent structural anisotropy of the GDL significantly influences the local distribution of both cathode potential and current density. Simulation results further indicate that a GDL with deformation results in an increase in the concentration polarization due to the increased mass-transfer resistance in the deformed GDL. On the other hand, the ohmic polarization is found to be smaller in the deformed GDL as the result of the decreased interfacial contact resistance and electronic resistance in the GDL. This result implies that an optimum deformation needs to be achieved so that both concentration and ohmic losses can be minimized.     

The impact of rib structure on the water transport behavior in gas diffusion layer of polymer electrolyte membrane fuel cells

Using the multiphase lattice Boltzmann method (LBM), the liquid water transport dynamics is simulated in a gas diffusion layer (GDL) of polymer electrolyte membrane fuel cells (PEMFCs). The effect of rib structure on the water invasion process in the micro-porous GDL is explored by comparing the two cases, i.e., with rib and without rib structures. The liquid water distribution and water saturation profile are presented to determine the wetting mechanism in the GDL. The results show that the liquid water transport in the GDL is strongly governed by capillary force and the rib structure plays a significant role on water distribution and water transport behavior in the GDL. Comparison of two cases confirms that the rib structure influences on the location of water breakthrough. The liquid water distribution and water saturation profile indicate that the high resistance force underneath the rib suppresses the growth of water cluster, resulting in the change of flow path. After water breakthrough, the liquid water distribution under the channel has little variation, whereas that under the rib continues to change. The predicted value of effective permeability is in good agreement with Carman-Kozeny correlation and experimental results in the literature. The results suggest that the LBM approach is an effective tool to investigate the water transport behavior in the GDL.     

Macroscopic analysis of characteristic water transport phenomena in polymer electrolyte fuel cells

Comprehensive analytical and numerical analyses were performed, focusing on anode water loss, cathode flooding, and water equilibrium for polymer electrolyte fuel cells. General features of water transport as a function of membrane thickness and current density were presented to illustrate the net effect of back-diffusion of water from the cathode to anode over a polymer electrolyte fuel cell domain. First, two-dimensional numerical simulation were performed, showing that the difference in molar concentration of water at the channel outlet is widened as the operating current density increases with a thin membrane (Nafion^®${\mathrm{Nafion}}^{\circledR }$ 111), which was verified by Dong et al. [Distributed performance of polymer electrolyte fuel cells under low-humidity conditions. J Electrochem Soc 2005; 152: A2114–22]. Then, analytical solutions were compared with computational results in predicting those characteristics of water transport phenomena. It was theoretically estimated that the high pressure operation of fuel cells expedites water condensing and results in shorter anode water loss and cathode flooding locations. In this study, it was also found that a thin membrane (Nafion^®${\mathrm{Nafion}}^{\circledR }$ 111) facilitates water transport in the through-membrane direction and therefore water concentration at the anode and cathode channel outlets reaches an equilibrium state particularly at low operating current densities. Moreover, the difference in the anode water concentration between Nafion^®${\mathrm{Nafion}}^{\circledR }$ 111 and Nafion^®${\mathrm{Nafion}}^{\circledR }$ 115 membranes becomes intensified in the in-plane direction under the same water production condition, while the cathode water concentration profiles remains almost same.     

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.     

Influence of anisotropic bending stiffness of gas diffusion layers on the degradation behavior of polymer electrolyte membrane fuel cells under freezing conditions

The effects of gas diffusion layer’s (GDL’s) anisotropic bending stiffness on the degradation behavior of polymer electrolyte membrane fuel cells have been investigated under freezing conditions. We have prepared GDL sheet samples such that the higher stiffness direction of GDL roll is aligned with the major flow field direction of a metallic bipolar plate at angles of 0° (parallel: ‘0° GDL’) and 90° (perpendicular: ‘90° GDL’). The I-V performances before and after 1000 temperature cycles between −10 and 1 °C of 90° GDL stack are higher than those of 0° GDL stack, and the voltages of 90° GDL stack are decreased slower than those of 0° GDL stack, indicating a higher durability of 90° GDL stack. Furthermore, the values and increasing rates of high-frequency resistance of 90° GDL stack are lower than those of 0° GDL stack. However, the H₂ and air pressure differences before and after 1000 temperature cycles of 90° GDL stack are very similar to those of 0° GDL stack. The surface of anode catalyst layer (CL) of membrane-electrode assembly (MEA) with catalyst-coated membrane type in 0° GDL stack appears to be more severely damaged than that in 90° GDL stack, especially under the channels, whereas the surfaces of cathode CLs of MEAs in both 0° and 90° GDL stacks are slightly damaged after 1000 temperature cycles.     

Water transport in polymer electrolyte membrane fuel cells

Polymer electrolyte membrane fuel cell (PEMFC) has been recognized as a promising zero-emission power source for portable, mobile and stationary applications. To simultaneously ensure high membrane proton conductivity and sufficient reactant delivery to reaction sites, water management has become one of the most important issues for PEMFC commercialization, and proper water management requires good understanding of water transport in different components of PEMFC. In this paper, previous researches related to water transport in PEMFC are comprehensively reviewed. The state and transport mechanism of water in different components are elaborated in detail. Based on the literature review, it is found that experimental techniques have been developed to predict distributions of water, gas species, temperature and other parameters in PEMFC. However, difficulties still remain for simultaneous measurements of multiple parameters, and the cell and system design modifications required by measurements need to be minimized. Previous modeling work on water transport in PEMFC involves developing rule-based and first-principle-based models, and first-principle-based models involve multi-scale methods from atomistic to full cell levels. Different models have been adopted for different purposes and they all together can provide a comprehensive view of water transport in PEMFC. With the development of computational power, application of lower length scale methods to higher length scales for more accurate and comprehensive results is feasible in the future. Researches related to cold start (startup from subzero temperatures) and high temperature PEMFC (HT-PEMFC) (operating at the temperatures higher than 100 °C) are also reviewed. Ice formation that hinders reactant delivery and damages cell materials is the major issue for PEMFC cold start, and enhancing water absorption by membrane electrolyte and external heating have been identified as the most effective ways to reduce ice formation and accelerate temperature increment. HT-PEMFC that can operate without liquid water formation and membrane hydration greatly simplifies water management strategy, and promising performance of HT-PEMFC has been demonstrated.     

Effects of gas-diffusion layer properties on the performance of the cathode for high-temperature polymer electrolyte membrane fuel cell

The role of the gas-diffusion layer (GDL) in high-temperature polymer electrolyte fuel cell (HT-PEMFC) differs from that in low-temperature PEMFC GDL due to operating conditions and environment. Determining the GDL's structural parameters that affect its transport properties, and how these properties impact HT-PEMFC performance was urgently required. Four commercial GDLs were employed in HT-PEMFC cathode's GDE and was examined using X-μCT, mercury intrusion porosimetry, and an optical microscope to analyze structural parameters and characteristics. Fractal theory was applied to comprehend the gas transmission property of GDL, and the validity of the theory was confirmed through ex-situ through-plane gas permeability measurement. The analysis indicated that the porosity of GDL influenced by the crack region of the MPL has more impact on the GDL's gas transmission than its thickness. After that, we established a correlation between HT-PEMFC cathode performance and GDL porosity and theoretical gas transmission properties using R² coefficient of determination.     

Numerical analysis on performances of polymer electrolyte membrane fuel cells with various cathode flow channel geometries

This paper investigated numerically the effect of cathode channel shapes on the local transport characteristics and cell performance by using a three-dimensional, two-phase, and non-isothermal polymer electrolyte membrane (PEM) fuel cell model. The cells with triangle, trapezoid, and semicircle channels were examined using that with rectangular channel as comparison basis. At high operating voltages, the cells with various channel shapes would have similar performance. However, at low operating voltages, the fuel cell performance would follow: triangle > semicircle > trapezoid > rectangular channel. Analyses of the local transport phenomena in the cell indicate that triangle, trapezoid, and semicircle channel designs increase remarkably flow velocity of reactant, enhancing liquid water removal and oxygen utilization. Thus, these designs increase the limiting current density and improve the cell performance relative to rectangular channel design.