Effects of liquid water on the pore structure and transport coefficients in the cathode catalyst layer of PEM fuel cells

We present a pore-scale simulation of the capillary condensation of water in the cathode catalyst layer (CCL) of proton exchange membrane fuel cells by the lattice Boltzmann method. Based on the reconstructed CCL, the capillary condensation process in CCL is simulated under different humidity conditions, and the effects of porosity and especially wettability on the liquid water distribution in CCL are studied. The influence of liquid water on the void pore size distribution and pore connectivity in CCL is evaluated, and the results show that the hydrophilic CCL is more prone to be flooded. Subsequently, the effective transport coefficients of oxygen and proton in partially saturated CCL are investigated. The results reveal that the hydrophobic CCL is beneficial for reducing the gas transport tortuosity but simultaneously causes a higher Knudsen diffusion resistance. By comprehensively considering the changes in tortuosity and Knudsen resistance caused by liquid water, a more practical correlation of effective diffusivity for the partially saturated CCL is proposed. Moreover, this work proves the vital role of liquid water in the proton conduction in CCL. The simulated effective proton conductivity in CCL is more agree with the measurements if the contribution of liquid water to proton transport is considered.     

Pore network modeling of cathode catalyst layer of proton exchange membrane fuel cell

In this paper, a pore network model is developed to investigate the coupled transport and reaction processes in the cathode catalyst layer (CCL) of proton exchange membrane fuel cell (PEMFC). The developed model is validated by comparing the predicted polarization curve with the experimental data, and the parametric studies are carried out to elucidate the effects of CCL design parameters. With the decrease of the CCL thickness and the Nafion content, the cell voltage reduces at the low current density but increases when the current density is higher. The cell performance is also improved by increasing the proton conductivity of the Nafion film in the CCL. As compared to the CCL of uniformly distributed Nafion, the CCL with the Nafion volume decreasing along the thickness direction exhibits better performance at the high current density.     

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.     

Preparation of water management layer and effects of its composition on performance of PEMFCs

A membrane electrode assembly (MEA) with a novel water management layer (WML) used in proton exchange membrane fuel cell (PEMFC) was prepared. The so called WML, which was located between the carbon paper and the catalyst layer, was a sublayer composed of carbon and hydrophobic PTFE. Various parameters of the WML, including carbon loading, PTFE content and species, sintering time and temperature and pore formers, were investigated in this study. As demonstrated in our experimental results, the performance of the membrane electrode assembly (MEA) PEMFC could be significantly improved by WML in the condition of operation with dry reactive gases. The MEA with the WML exhibited more stable performance than the situation of MEA without WML during a long time running period.     

Analysis of capillary flow driven model for water transport in PEFC cathode catalyst layer: Consideration of mixed wettability and pore size distribution

The solid matrix of the porous cathode catalyst layer (CCL) of a polymer electrolyte fuel cell is made of two different materials (carbon with supported Pt and ionomer), which are characterized by different wettability (i.e. contact angles). This paper discusses the need for considering the combined consideration of the mixed wettability and the distributed pore structure of CCL in modelling the transport of liquid water and oxygen gas. A simple 1-D model that considers two different pore size distributions, derived from experimental capillary pressure–saturation literature data, for the hydrophobic and hydrophilic pores is presented. The results indicate that for water to be transported in liquid-state through the CCL, the liquid saturation is such that only very small hydrophobic pores remain available for gas transport such that Knudsen diffusion will dominate and must be considered in CCL models.     

Numerical analysis of catalyst agglomerates and liquid water transport in proton exchange membrane fuel cells

A two-dimensional computational fluid dynamics model of a proton exchange membrane fuel cell (PEMFC) was formulated by taking into account the liquid water transport through the membrane electrode assembly (MEA) on the basis of the agglomerate catalyst framework. Various modes of water transport in the PEMFC, including electro-osmotic drag, back diffusion, water generation by oxygen reduction reaction, etc., were considered in the presence of three-phase water, i.e., gas-, liquid- and dissolved-phase water. The presented model was validated with cell polarization data taken from experimental data prepared by the authors, and the agreement was good. Various parameters were investigated to analyze the influence on cell polarization and are summarized as follows: i) various compositions of Pt/C, Nafion and void in the catalyst layer and ii) various channel-to-shoulder ratios. Consequently, detailed investigations on the degree of flooding, the potential field, and polarizations were fully discussed.     

Performance analysis of a PEM fuel cell cathode with multiple catalyst layers

The cathode catalyst layer (CL) of a PEM fuel cell (PEMFC) plays an important role in the performance of the cell because of the rate limiting mechanisms that take place in it. For enhancing the performance of a PEMFC, the use of multiple, ultra thin CLs instead of a single CL is considered in the present work. Since the concentration of oxygen decreases in a CL from the diffusion medium-CL interface towards the polymer membrane, the CL adjacent to the diffusion medium should be of higher porosity than the other CLs. Similarly, the CL adjacent to the polymer membrane should contain more ionomer than the other CLs. Furthermore, liquid water should be removed without causing significant mass transport and/or ohmic losses. Therefore, the design parameters of a CL can be varied spatially to minimize losses in a PEMFC. However, such a continuously graded CL is difficult to manufacture due to lack of commercially available techniques and associated costs. As an alternative, a combination of layers can be synthesized where each layer is manufactured with different design parameters. This approach provides the opportunity to optimize the design parameters of each layer. With this objective in mind, a detailed steady state model of a PEMFC cathode with multiple layers is developed. The model considers liquid water in all the layers. The catalyst layer microstructure is modeled as a network of spherical agglomerates. For improved water management, a thin micro-porous layer is considered between the gas diffusion layer (GDL) and the first catalyst layer. The performance curves for various combinations of the design parameters are shown and the results are analyzed. The results show that there exists an optimum combination of design parameters for each catalyst layer that can significantly improve the performance of a PEMFC.     

Effect of catalyst layer mesoscopic pore-morphology on cold start process of PEM fuel cells

Water transport is of paramount importance to the cold start of proton exchange membrane fuel cells (PEMFCs). Analysis of water transport in cathode catalyst layer (CCL) during cold start reveals the distinct characteristics from the normal temperature operation. This work studies the effect of CCL mesoscopic pore-morphology on PEMFC cold start. The CCL mesoscale morphology is characterized by two tortuosity factors of the ionomer network and pore structure, respectively. The simulation results demonstrate that the mesoscale morphology of CCL has a significant influence on the performance of PEMFC cold start. It was found that cold-starting of a cell with a CCL of less tortuous mesoscale morphology can succeed, whereas starting up a cell with a CCL of more tortuous mesoscale morphology may fail. The CCL of less tortuous pore structure reduces the water back diffusion resistance from the CCL to proton exchange membrane (PEM), thus enhancing the water storage in PEM, while reducing the tortuosity in ionomer network of CCL is found to enhance the water transport in and the water removal from CCL. For the sake of better cold start performance, novel preparation methods, which can create catalyst layers of larger size primary pores and less tortuous pore structure and ionomer network, are desirable.     

Single-walled carbon nanotube interlayer modified gas diffusion layers to boost the cell performance of self-humidifying proton exchange membrane fuel cells

A tradeoff between the low humidity and the high performance remains a key challenge for the proton exchange membrane fuel cell (PEMFC). In this work, a novel self-humidifying gas diffusion layer (GDL) with a single-walled carbon nanotube (SWCNT) nonwoven layer between the gas diffusion substrate and the hydrophobic microporous layer is controllably prepared to elevate the cell performance under dry conditions. The membrane electrode assembly (MEA) with 0.25 mg cm⁻² SWCNT loading exhibits a current density of 0.69 A cm⁻² at 0.6 V, which is 392.8% higher than that of the counterpart without the SWCNT interlayer at the same relative humidity. Moreover, the SWCNT interlayer with rational pore structure and proper wettability dramatically improves the water retention capacity of MEA, thus enhancing the low-humidity performance of MEA. The structure design of GDL provides an effective strategy for self-humidifying PEMFC control optimization.     

Study of current distribution and oxygen diffusion in the fuel cell cathode catalyst layer through electrochemical impedance spectroscopy

In this study, an analysis of the current distribution and oxygen diffusion in the Polymer Electrolyte Fuel Cell (PEFC) Cathode Catalyst Layer (CCL) has been carried out using Electrochemical Impedance Spectroscopy (EIS) measurements. Cathode EIS measurements obtained through a three-electrode configuration in the measurement system are compared with simulated EIS data from a previously validated numerical model, which subsequently allows the diagnostics of spatio-temporal electrochemical performance of the PEFC cathode. The results show that low frequency EIS measurements commonly related to mass transport limitations are attributed to the low oxygen equilibrium concentration in the CCL–Gas Diffusion Layer (GDL) interface and the low diffusivity of oxygen through the CCL. Once the electrochemical and diffusion mechanisms of the CCL are calculated from the EIS measurements, a further analysis of the current density and oxygen concentration distributions through the CCL thickness is carried out. The results show that high ionic resistance within the CCL electrolyte skews the current distribution towards the membrane interface. Therefore the same average current density has to be provided by few catalyst sites near the membrane. The increase in ionic resistance results in a poor catalyst utilization through the CCL thickness. The results also show that non-steady oxygen diffusion in the CCL allows equilibrium to be established between the equilibrium oxygen concentration supplied at the GDL boundary and the surface concentration of the oxygen within the CCL. Overall, the study newly demonstrates that the developed technique can be applied to estimate the factors that influence the nature of polarization curves and to reveal the effect of kinetic, ohmic and mass transport mechanisms on current distribution through the thickness of the CCL from experimental EIS measurements.     

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.     

A pore network study on the role of micro-porous layer in control of liquid water distribution in gas diffusion layer

In proton exchange membrane fuel cell (PEMFC), a hydrophobic micro-porous layer (MPL) is usually placed between catalyst layer (CL) and gas diffusion layer (GDL) to reduce flooding. Recent experimental studies have demonstrated that liquid water saturation in GDL is drastically decreased in the presence of MPL. However, theoretical studies based on traditional continuum two-phase flow models suggest that MPL has no effect on liquid water distribution in GDL. In the present study, a pore network model with invasion percolation algorithm is developed and used to investigate the impacts of the presence of MPL on liquid water distribution in GDL from the viewpoint at the pore level. A uniform pressure and uniform flux boundary conditions are considered for liquid water entering the porous layer in PEMFC. The simulation results reveal that liquid water saturation in GDL is reduced in the presence of MPL, but the reduction depends on the condition of liquid water entering the porous layer in PEMFC.     

A pore network study on water distribution in bi-layer gas diffusion media: Effects of inlet boundary condition and micro-porous layer properties

Water flooding in gas diffusion material (GDM) is an important limit in performance of proton exchange membrane fuel cell (PEFMC). Some efforts, such as modifying the pore structures in the GDM, have been made in order to facilitate water transport and to reduce flooding in PEMFC. Recent experimental studies have demonstrated that using a bi-layer GDM, consisting of a fine micro-porous layer (MPL) and a coarse gas diffusion layer (GDL), can be advantageous for water management in PEMFC. In this work, a pore network model with an invasion percolation algorithm is developed and used to investigate the effects of MPL properties, including thickness, wettability and connectivity, on water distribution in the bi-layer GDM from the viewpoint at the pore level. Furthermore, a reasonable inlet boundary condition is proposed to describe the actual phenomenon that the CL surface is covered with many independent water droplets which are much larger than pore sizes in MPL. Influences of water droplet size and coverage fraction are also clarified in the present study.     

Fractal model for prediction of effective hydrogen diffusivity of gas diffusion layer in proton exchange membrane fuel cell

This paper presents a novel prediction model of the effective hydrogen diffusivity for the gas diffusion layer (GDL) in proton exchange membrane fuel cell (PEMFC) by using fractal theory to characterize microstructure. With the consideration of pore-size distribution and Knudsen diffusion effect, a relationship between micro-structural parameters and effective hydrogen diffusivity of GDL is deduced. The prediction of effective hydrogen diffusivities of two samples shows that Knudsen diffusion effect makes the effective diffusivity value decrease, and after being treated with polytetrafluoroethylene (PTFE), carbon paper, a basal material of the GDL, exhibits a lower effective diffusivity value due to the decrease in the pore space and porosity. From the parametric effect study, it can be concluded that effective diffusivity has a positive correlation with pore area fractal dimension D_p or porosity ?, whereas it has a negative correlation with tortuosity fractal dimension D_t.     

Ionomer content in the catalyst layer of polymer electrolyte membrane fuel cell (PEMFC): Effects on diffusion and performance

The percolating paths of the carbons and electrolytes in a cathode catalyst layer (CCL) could be successfully visualized in three-dimensions in order to investigate both the electronic and ionic connectivity by modeling a three-dimensional (3-D), meso-scale CCL of a polymer electrolyte membrane fuel cell (PEMFC). The effective Knudsen diffusion coefficients could also be obtained by computing pore tortuosity values. Electrochemical simulation studies were carried out by feeding air at 70 °C. Low platinum (Pt) loading (0.1 mg cm⁻²) catalysts with ionomer contents ranging from 14 to 50% were studied. The performance of a PEMFC electrode was affected by the ionomer content which is optimal at about 33%. In this case, both electronic and ionic connectivity produced the broadest active surface area of the Pt catalyst. The polarization drop tendency was in good agreement with the experiment, and this percolation study could successfully explain the existence of an optimum amount of ionomer.     

A Pt content and pore structure gradient distributed catalyst layer to improve the PEMFC performance

The catalyst layer is a key component in the proton exchange membrane fuel cell (PEMFC) for it is where the conversion of fuel into electricity takes place. Traditionally, electrocatalyst is uniformly distributed in the catalyst layers of the membrane electrode assembly (MEA) and the high Pt consumption in catalyst layers blocks the widely use of PEMFC. Here we proposed a Pt content and pore structure gradient distributed, two-layer catalyst layer for PEMFC to improve the MEA performance. Energy-dispersive X-ray (EDX) spectroscopy results show Pt nanoparticles gradient distributed on the vertical direction of catalyst layer. The pore size in the Pt poor layer is larger than that in the Pt rich layer, and this structure can improve the Pt utilization and enhance the mass transfer in the catalyst layer. The single cell test result shows this new MEA has a better performance (11%) than the traditional MEA.     

Liquid and oxygen transport in defective bilayer gas diffusion material of proton exchange membrane fuel cell

In this paper, pore network simulations are carried out to explore the effects of micro porous layer (MPL) and its crack location on the liquid and oxygen transport in the gas diffusion material (GDM) of proton exchange membrane fuel cell (PEMFC). The constructed network is composed of cubic pores connected by throats of square cross section. The GDM is partially screened by the land, and the MPL is assumed to have a crack. When the MPL crack is considered under the land in the model, the predicted results agree with experimental findings regarding the effect of MPL on the liquid saturation and distribution in the GDM. This indicates that the liquid may prefer to flow through the MPL crack under the land. The role of MPL in the fuel cell performance is revealed to be dependent on the oxygen effective diffusivity of MPL and GDL. Therefore, caution should be taken before employing the MPL to improve the cell performance. Based on the present studies, some guidelines are gained for the GDM design and optimization.     

Water transport characteristics in the gas diffusion media of proton exchange membrane fuel cell - Role of the microporous layer

Water transport through the gas diffusion media of a proton exchange membrane fuel cell (PEMFC) was investigated with a focus on the role of the microporous layer (MPL) coated on the cathode gas diffusion layer (GDL). The capillary pressure of the MPL and GDL, which plays a significant role in water transport, is derived as a function of liquid saturation using a pore size distribution (PSD) model. PSD functions are derived with parameters that are determined by fitting to the measured total PSD data. Computed relations between capillary pressure and liquid saturation for a GDL and a double-layered GDL (GDL + MPL) show good agreement with the experimental data and proposed empirical functions. To investigate the role of the MPL, the relationship between the water withdrawal pressure and liquid saturation are derived for a double-layered GDL. Water transport rates and cell voltages were obtained for various feed gas humidity using a two-dimensional cell model, and are compared with the experimental results. The calculated results for the net drag with application of the capillary pressure derived from the PSD model show good agreement with the experimental values. Furthermore, the results show that the effect of the MPL on the cell output voltage is significant in the range of high humidity operation.     

Measurement of effective oxygen diffusivity in microporous media containing moisture

The mass transfer characteristics of the gas diffusion layer (GDL) are closely related to the cell performance of a polymer electrolyte fuel cell (PEFC). The oxygen diffusivity of paper type porous media, which is generally used as a GDL, was measured with respect to its liquid water content using experimental apparatus consisting of an oxygen sensor based on the galvanic cell. A numerical method was established to obtain the effective oxygen diffusivity of microporous test materials by calculating the oxygen concentration distribution on both sides of the test material. Experimental results indicate that the relative oxygen diffusivity of paper type GDLs increases nonlinearly as the water saturation decreases. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20295     

Enhanced diffusion in polymer electrolyte membrane fuel cells using oscillating flow

This study investigates the enhancement of the oxygen diffusion rate at the cathode of a proton exchange membrane fuel cell (PEMFC) due to pure oscillating flow. A unit cell of PEMFC using hydrogen fuel and oscillating air was tested. The experimental results show that the non-dimensional effective diffusivity varies linearly with the square of the Womersley number, when the Womersley number is close to unity. The non-dimensional effective diffusivity varies linearly with the Womersley number itself when the Womersley number is much larger than unity. Similar trend has been confirmed from the theoretical approach. Under the experimental conditions in this study, the reaction rate of oxygen increased linearly with respect to the sweep distance. The experimental results showed that a power density of 115.4 mW/cm² was obtained from the unit cell with oscillating flow, which is comparable to that obtained with forced flow. Therefore, an oscillating flow is found to be able to increase the concentration of the oxygen in the channel of PEMFCs, and consequently enhances mass-transfer, similarly to the use of forced flow using blowers or compressors.