Numerical modeling and analysis of micro-porous layer effects in polymer electrolyte fuel cells

It is well known that a micro-porous layer (MPL) plays a crucial role in the water management of polymer electrolyte fuel cells (PEFCs), and thereby, significantly stabilizes and improves cell performance. To ascertain the exact roles of MPLs, a numerical MPL model is developed in this study and incorporated with comprehensive, multi-dimensional, multi-phase fuel-cell models that have been devised earlier. The effects of different porous properties and liquid-entry pressures between an MPL and a gas diffusion layer (GDL) are examined via fully three-dimensional numerical simulations. First, when the differences in pore properties and wettability between the MPL and GDL are taken into account but the difference in the entry pressures is ignored, the numerical MPL model captures a discontinuity in liquid saturation at the GDL|MPL interface. The simulation does not, however, capture the beneficial effects of an MPL on cell performance, predicting even lower performance than in the case of no MPL. On the other hand, when a high liquid-entry pressure in an MPL is additionally considered, the numerical MPL model predicts a liquid-free MPL and successfully demonstrates the phenomenon that the high liquid-entry pressure of the MPL prevents any liquid water from entering the MPL. Consequently, it is found from the simulation results that a liquid-free MPL significantly enhances the back-flow of water across the membrane into the anode, which, in turn, helps to avoid membrane dehydration and alleviate the level of GDL flooding. As a result, the model successfully reports the beneficial effects of MPLs on PEFC performance and predicts higher performance in the presence of MPLs (e.g., an increase of 67 mV at 1.5 A cm⁻²). This study provides a fundamental explanation of the function of MPLs and quantifies the influence of their porous properties and the liquid-entry pressure on water transport and cell performance.     

Quantitative visualization of temporal water evolution in an operating polymer electrolyte fuel cell

Synchrotron X-ray radiography is employed to visualize the temporal evolution of water inside the gas diffusion layer (GDL) of an operating (in situ) polymer electrolyte fuel cell (PEFC). A single-cell PEFC test kit is specially designed for the convenient capture of X-ray images. X-ray images of water in the PEFC components, such as the polymer membrane, GDL, and end plate, are captured consecutively. The synchrotron X-ray radiography of high-spatial and high-temporal resolution is suitable for observing the transport of a liquid layer and for visualizing water distribution inside the PEFC. As a result, the spatial distribution of water in the PEFC components is clearly and quantitatively visualized. The temporal evolution of water in the anode GDL due to back diffusion effect is clearly observed by adopting the image normalization method. The water-saturation characteristics at the cathode GDL, including saturation time and speed, are quite different from those at the anode GDL.     

Prevention of the water flooding by micronizing the pore structure of gas diffusion layer for polymer electrolyte fuel cell

In polymer electrolyte fuel cells, high humidity must be established to maintain high proton conductivity in the polymer electrolyte. However, the water that is produced electrochemically at the cathode catalyst layer can condense in the cell and cause an obstruction to the diffusion of reaction gas in the gas diffusion layer and the gas channel. This leads to a sudden decrease of the cell voltage. To combat this, strict water management techniques are required, which usually focus on the gas diffusion layer. In this study, the use of specially treated carbon paper as a flood-proof gas diffusion layer under extremely high humidity conditions was investigated experimentally. The results indicated that flooding originates at the interface between the gas diffusion layer and the catalyst layer, and that such flooding could be eliminated by control of the pore size in the gas diffusion layer at this interface.     

Flooding of the diffusion layer in a polymer electrolyte fuel cell: Experimental and modelling analysis

Water management is widely investigated because it affects both the performance and the lifetime of polymer electrolyte fuel cells. Membrane hydration is necessary to ensure the high proton conductivity, but too much water can cause flooding and pore obstruction within the cathode gas diffusion layer and the electrode. Experimental studies prove that the characteristics of the diffusion layer have great influence on water transport; the introduction of a micro-porous layer between the gas diffusion layer and the electrode reduces flooding and stabilizes the performance of the fuel cell, although the reason is not fully explained. A quantitative method to characterize water transport through the diffusion layers was proposed in our previous work, and the present work aims to further understand the flooding phenomenon and the role of the micro-porous layer. The improved experimental setup and methodology allow an accurate and reliable evaluation of water transport through the diffusion layer in a wide range of operating conditions. The proposed 1D + 1D model faithfully reproduces the experimental data adopting effective diffusivity values in agreement with literature. The presented experimental and modelling analysis allows us to evaluate the influence of pore obstruction on the effective diffusivity, the overall transport coefficient and water flow through the diffusion layer, elucidating the effect of the micro-porous layer on fuel cell performance and operation stability.     

The use of a novel water porosimeter to predict the water handling behaviour of gas diffusion media used in polymer electrolyte fuel cells

A novel water porosimeter and its use in determining the capillarity of gas diffusion layers are described. It is found that, in accordance with the Washburn equation, the pressure required to force water into the gas diffusion layer depends on the cosine of the contact angle of water with the surface of the pore. Negative pressure is required to withdraw water from the gas diffusion layer, even when the surface is hydrophobic. The negative pressure required is found to be independent of surface contact angle. It is shown that the performance of gas diffusion layers in an operating fuel cell can be qualitatively predicted from the capillary pressure curves obtained. The advantages of the use of water porosimetry over the use of either mercury porosimetry or porosimetry using wetting fluids are discussed.     

Mechanical behavior of the membrane during the polymer electrolyte fuel cell operation

A computational modeling framework is developed to represent the transport phenomena, electrochemistry and the mechanical stresses in a polymer electrolyte fuel cell (PEFC). The model is able to predict the mechanical stresses developed in the polymer electrolyte due to hydration changes, and restriction of the membrane swelling as a result of these hydration changes in the PEFC assembly. Anisotropy in the mechanical properties of the gas diffusion layers is accounted in the stress calculations. It is seen that hydration variations during the PEFC operation can cause significant mechanical stresses. The effects of operating voltage and relative humidities of reactants are investigated. It is observed that high inlet humidities result in a better performance; however, it can potentially cause the polymer electrolyte membrane to go through plastic deformation irreversibly. Thermal stresses due to temperature variations are also calculated and compared with hygral stresses; and it is found that thermal stresses are not negligible but are typically a fraction of the hygral stresses in a typical PEFC operation.     

Ex situ measurements of through-plane thermal conductivities in a polymer electrolyte fuel cell

In this paper thermal properties for materials typically used in the proton exchange membrane fuel cell (PEMFC) are reported. Thermal conductivities of Nafion membranes were measured ex situ at 20 °C to be 0.177 ± 0.008 and 0.254 ± 0.016 W K⁻¹ m⁻¹ for dry and maximally wetted membranes respectively. This paper also presents a methodology to determine the thermal conductivity of compressible materials as a function of applied load. This technique was used to measure the thermal conductivity of an uncoated SolviCore porous transport layer (PTL) at various compaction pressures. For the dry PTL at 4.6, 9.3 and 13.9 bar compaction pressures, the thermal conductivity was found to be 0.27, 0.36 and 0.40 W K⁻¹ m⁻¹ respectively and the thermal contact resistivity to the apparatus was determined to be 2.1, 1.8 and 1.1 × 10⁻⁴ m² K W⁻¹, respectively. It was shown that the thermal contact resistance between two PTLs is negligible compared to the apparatus’ thermal contact resistivity. For a humidified PTL, the thermal conductivity increases by up to 70% due to a residual liquid saturation of 25%.     

Neutron images of the through-plane water distribution of an operating PEM fuel cell

Neutron imaging has proven an invaluable tool for water metrology in operating proton exchange membrane fuel cells. Due to limitations in scintillator-based detector resolution, neutron imaging has been applied only to assessing the in-plane water distribution, without being able to distinguish water in the anode from the cathode. A new detector technology, based on micro-channel plates, enables a near order of magnitude improvement in the image resolution. This new detector technology will enable direct measurement of the through-plane water distribution in the gas diffusion layer, and enable the determination of the relative water content on the anode and cathode sides of a proton exchange membrane fuel cell. We report on the initial measurements with this new detector and discuss future measurement possibilities.     

Measurement of liquid water content in cathode gas diffusion electrode of polymer electrolyte fuel cell

Water management in cathode gas diffusion electrode (GDE) of polymer electrolyte fuel cell (PEFC) is essential for high performance operation, because liquid water condensed in porous gas diffusion layer (GDL) and catalyst layer (CL) blocks oxygen transport to active reaction sites. In this study, the average liquid water content inside the cathode GDE of a low-temperature PEFC is experimentally and quantitatively estimated by the weight measurement, and the relationship between the water accumulation rate in the cathode GDE and the cell voltage is investigated. The liquid water behavior at the cathode is also visualized using an optical diagnostic, and the effects of operating conditions and GDL structures on the water transport in the cathode GDE are discussed. It is found that the liquid water content in the cathode GDE increases remarkably after starting the fuel cell operation due to the water production at the CL. At a high current density, the cell voltage drops suddenly after starting the operation in spite of a low water content in the cathode GDE. When the GDL thickness is increased, much water accumulates near the cathode CL and the fuel cell shuts down immediately after the operation. In the final section of this paper, the structure of cathode GDL that has several grooves for water removal is proposed to prevent water flooding and improve fuel cell performance. This groove structure is effective to promote the removal of the liquid water accumulated near the active catalyst sites.     

Effect of capillary pressure on liquid water removal in the cathode gas diffusion layer of a polymer electrolyte fuel cell

In order to investigate the effect of capillary pressure on the transport of liquid water in the cathode gas diffusion layer (GDL) of a polymer electrolyte fuel cell, a one-dimensional steady-state mathematical model was developed, including the effect of temperature on the capillary pressure. Numerical results indicate that the liquid water saturation significantly increases with increases in the operating temperature of the fuel cell. An elevated operating temperature has an undesirable influence on the removal of liquid water inside the GDL. A reported peculiar phenomenon in which the flooding of the fuel cell under a high operating temperature and an over-saturated environment is more serious in a GDL combined with a micro-porous layer (MPL) than in a GDL without an MPL [Lim and Wang, Electrochim. Acta 49 (2004), 4149–4156] is explained based on the present analysis.     

Gas diffusion layer design focusing on the structure of the contact face with catalyst layer against water flooding in polymer electrolyte fuel cell

High humidity must be maintained inside polymer electrolyte fuel cells to achieve high ion conductivity. However, water condensation blocks the diffusion of the reaction gas in the gas diffusion layer under water saturation conditions which are produced by the product water. This effect is known as flooding and causes a sudden drop in the cell voltage. Therefore, advanced water management is required in such fuel cells. Internal water management is generally carried out by making adjustments to the gas diffusion layer. This study reports that the extremely highly flood-resistant gas diffusion layer has been developed, based on simple carbon paper. It was experimentally revealed that flooding is controlled by a gas diffusion layer with a smaller pore-structure facing the catalyst layer and it is one of the governing factors for flooding in the gas diffusion layer.     

Capillaries for water management in polymer electrolyte membrane fuel cells

Some of the new liquid water management systems in polymer electrolyte membrane (PEM) fuel cells hold great potential in providing flood-free performance and internal humidification. However, current water management systems entail major setbacks, which either inhibit implementation into state-of-the-art architectures, such as stamped metal flow-fields, or restrict their application to certain channel configurations. Here, a novel water management strategy is presented that uses capillary arrays to control liquid water in PEMFCs. These capillaries are laser-drilled into the land of the flow-fields and allow direct removal (wicking) or supply of water (evaporation), depending on the local demand across the electrode. For a 6.25 cm² active area parallel flow-field, a ～92% improvement in maximum power density from capillary integration was demonstrated. The proposed mechanism serves as a simple and effective means of achieving robust and reliable fuel cell operation, without incurring additional parasitic losses due to the high pressure drop associated with conventional serpentine flow-fields.     

Pore network modeling of fibrous gas diffusion layers for polymer electrolyte membrane fuel cells

A pore network model of the gas diffusion layer (GDL) in a polymer electrolyte membrane fuel cell is developed and validated. The model idealizes the GDL as a regular cubic network of pore bodies and pore throats following respective size distributions. Geometric parameters of the pore network model are calibrated with respect to porosimetry and gas permeability measurements for two common GDL materials and the model is subsequently used to compute the pore-scale distribution of water and gas under drainage conditions using an invasion percolation algorithm. From this information, the relative permeability of water and gas and the effective gas diffusivity are computed as functions of water saturation using resistor-network theory. Comparison of the model predictions with those obtained from constitutive relationships commonly used in current PEMFC models indicates that the latter may significantly overestimate the gas phase transport properties. Alternative relationships are suggested that better match the pore network model results. The pore network model is also used to calculate the limiting current in a PEMFC under operating conditions for which transport through the GDL dominates mass transfer resistance. The results suggest that a dry GDL does not limit the performance of a PEMFC, but it may become a significant source of concentration polarization as the GDL becomes increasingly saturated with water.     

Lattice Boltzmann simulations of water transport in gas diffusion layer of a polymer electrolyte membrane fuel cell

The effect of wettability on water transport dynamics in gas diffusion layer (GDL) is investigated by simulating water invasion in an initially gas-filled GDL using the multiphase free-energy lattice Boltzmann method (LBM). The results show that wettability plays a significant role on water saturation distribution in two-phase flow in the uniform wetting GDL. For highly hydrophobicity, the water transport falls in the regime of capillary fingering, while for neutral wettability, water transport exhibits the characteristic of stable displacement, although both processes are capillary force dominated flow with same capillary numbers. In addition, the introduction of hydrophilic paths in the GDL leads the water to flow through the hydrophilic pores preferentially. The resulting water saturation distributions show that the saturation in the GDL has little change after water breaks through the GDL, and further confirm that the selective introduction of hydrophilic passages in the GDL would facilitate the removal of liquid water more effectively, thus alleviating the flooding in catalyst layer (CL) and GDL. The LBM approach presented in this study provides an effective tool to investigate water transport phenomenon in the GDL at pore-scale level with wettability distribution taken into consideration.     

Influence of humidification on deterioration of gas diffusivity in catalyst layer on polymer electrolyte fuel cell

The effect of water on polymer electrolyte fuel cell degradation was examined with humidity as a parameter. Polymer electrolyte fuel cells were subjected to long-term operation of 10 000 h to examine the relation between decline in cell voltage and degradation of the catalyst layers or gas diffusion layers. The diffusion overpotential increased during long-term operation at relatively high humidification of 81% RH, but only in the catalyst layer and not in the gas diffusion layer. At low humidification of 52% RH, the increase in diffusion overpotential was small, indicating that the increase was more likely to occur under high humidification. Post-analysis of the catalyst layer revealed that the membrane electrode assembly had increased diffusion overpotential during operation under high humidification, as a result of the sharp decline in porosity. The increase of diffusion overpotential in the catalyst layer was also investigated by the observation of the degradation due to the oxidation of the Pt-carbon supports. However, it was found that the oxidation of carbon support which had increased diffusion overpotential was small.     

Stability and performance improvement of a polymer electrolyte membrane fuel cell stack by laser perforation of gas diffusion layers

The performance and stability of a hydrogen-driven polymer electrolyte membrane fuel cell stack (6-cell PEFC stack) are investigated with regard to pore flooding within the gas diffusion layers (GDLs). Two short stacks with various GDLs (Toray TGP-H-060 untreated and laser-perforated) were characterized at different operating conditions by several characterization techniques such as constant current load, polarization curve, chronoamperometry and chronovoltammetry. The experimental results reveal that the perforation of the cathode GDLs improves the water transport in the porous media and thus the performance as well as the stability of the operating stack in medium and high current density range. A reduced pore flooding is verified when using the customized laser-perforated GDLs. The GDL perforation has a huge potential to balance the inhomogeneous in-plane saturation conditions between the inlet and outlet area of the cell and to compensate to a certain degree the effects of temperature distribution within a stack regarding the water management.     

Modeling the cation transport in an operating polymer electrolyte fuel cell (PEFC)

A computational fluid dynamics model is developed to investigate the multicomponent cation transport in polymer electrolyte membranes and to predict the performance degradation of the polymer electrolyte fuel cell (PEFC) due to the cationic contamination. A Maxwell–Stefan approach is implemented by modifying the Nernst–Planck equations to model the multicomponent cationic species transport in the membrane. Langmuir isotherms are used to model the non-ideal species adsorption in the membrane. Cation transport model shows good agreement with the experimental data found in the literature. Following the validation of the cation transport model, it is incorporated in a PEFC model framework, which solves for mass, momentum, species and charge conservations. Both fuel side and air side impurities are considered in analyses, which show that for air side impurities current density drops from 0.9 to 0.7 A/cm² whereas for fuel side impurities current density drops to impractical values as low as 0.05 A/cm², at the steady-state. Effect of cationic impurity on water transport in the membrane is also investigated and found that cathode dry-out occurs due to decreased water generation in case of fuel side contamination.     

Basic evaluation of separator type specific phenomena of polymer electrolyte membrane fuel cell by the measurement of water condensation characteristics and current density distribution

This paper investigates phenomena related to water condensation behavior inside a polymer electrolyte membrane fuel cell (PEMFC), and analyzes the effects of liquid water and gas flow on the performance of the fuel cell. A method for simultaneous measurements of the local current density across the reaction area and direct observation of the phenomena in the cell are developed. Experimental results comparing separator types indicate the effect of shortcut flow in the gas diffusion layer (GDL) under the land areas of serpentine separators, and also show the potential of straight channel separators to achieve a relatively uniform current density distribution. To evaluate shortcut flows under the land areas of serpentine separators, a simple circuit model of the gas flow is presented. The analysis shows that slight variations in oxygen concentration caused by the shortcut flows under the land areas affect the local and overall current density distributions. It is also shown that the establishment of gas paths under the water in channels filled with condensed water is effective for stable operation at low flow rates of air in the straight channels.     

Utilization of 3D printed carbon gas diffusion layers in polymer electrolyte membrane fuel cells

3D printing and carbonisation is used to produce designed gas diffusion layer materials for polymer electrolyte membrane fuel cells (PEMFC). Using a desktop UV 3D printer, designed porous microstructures are printed with micro and macro-scale features. Successful improvement of the pyrolysis process maintains the structural accuracy during carbonisation, reducing the material to electrically conductive carbon. The size of the material allows for testing in a lab scale fuel cell with 1.5 × 1.5 cm electrode size, which shows lower but interesting electrochemical performance (power density of 205 mW cm^?2). Challenges associated with integration of a 3D printed structure into a membrane electrode assembly are highlighted, including the low open circuit voltage caused by large amounts of membrane deformation and subsequent hydrogen crossover. This study shows that it is possible to design and manufacture a gas diffusion layer for fuel cells. Numerical simulation on the new GDL structure shows that advective-diffusive transport of oxygen in the 3D printed design is superior to conventional carbon paper. This study serves as the first attempt to implement 3D printed microstructures as GDL into PEMFC.     

The effects of non-uniform distribution of catalyst loading on polymer electrolyte membrane fuel cell performance

The catalyst layers are the most important part of the polymer electrolyte membrane (PEM) fuel cells, and the cell performance is highly related to its structure. The gas diffusion layers (GDLs) are also the essential components of the PEM fuel cell since the reactants should pass through these layers. Model prediction shows that electrical current in catalyst layer is non-uniform, influenced by the channel-land geometry. In addition, the compression effect of GDLs and water generation due to the electrochemical reaction may cause non-uniformity in porosity and, therefore, increases the non-uniformity in reactant concentration in GDL/catalyst layer interface. Simulation results suggest that non-uniform catalyst loading distribution in the catalyst layer will improve the performance of the whole catalyst layer by diminishing the variation in current density.