Compressive cyclic response of PEM fuel cell gas diffusion media

The fuel cell gas diffusion media (GDM) is a highly porous carbon-fiber-reinforced thin composite layer. The experimental response of these materials is observed to be highly nonlinear at low-stress levels. The cyclic mechanical response of GDM is investigated in terms of stiffness and damage parameters. The prediction of the state of deformation in GDM is vital in relating GDM's properties to ohmic and transport losses. To this end, a compressible form of the phenomenological model is proposed to capture the experimental cyclic response accurately. The model is constituent dependent; that is, the cumulative cyclic stress-strain response of GDM is a function of individual constituent phases present in the material. These individual constituents are porous matrix and reinforced fibers. The model hence derived for a typical GDM material, can predict residual strain, hysteresis, and damage quotient associated with the stress softening. This advanced model is implemented in the numerical domain to evaluate the response of the polymer electrolyte fuel cell (PEFC) unit cell. The stress-strain distribution fields are analyzed and compared with those of conventional GDM models. The results point to a remarkable deviation from the conventional notion of structural analysis.     

Channel intrusion of gas diffusion media and the effect on fuel cell performance

The effect of gas diffusion medium (GDM) intrusion on the performance of proton exchange membrane (PEM) fuel cells is investigated. The mechanical behaviors of various GDM are characterized in compressive, flexural, and shear tests. The results are used in a numerical model to calculate the channel intrusion of GDM. The intrusion calculation from the numerical model agrees well with the measurements from an intrusion measurement setup and a pressure drop measurement device for various GDM. A simplified reactant flow redistribution model of parallel channels developed in this study suggests that a 5% variation in GDM intrusion can result in a 20% reduction of reactant flow in the most intruded channel. The GDM intrusion and intrusion variation are found to induce significant performance discrepancy among cells of a 30-cell automotive fuel cell stack consisting of two different production lots of commercial GDM. The study suggests that in the future mass production of fuel cell stacks, GDM manufacturers need to greatly tighten their product variations in mechanical property and thickness to ensure reliable PEM fuel cell operations.     

Modeling the original and cyclic compression behavior of non-woven gas diffusion layers for fuel cells

It is established that the compression behavior of gas diffusion layers (GDL) is dependent on the level of the mechanical stress it experienced during its lifetime. As a matter of fact, every cycle of compression induces damages in the GDL, including fibers breakage and/or their spatial reorganization. As observed in the experimental work, the first cycle of compression of GDLs as received from the suppliers is already altered by a previous compression that is applied during the manufacturing process. This paper then presents a model able to predict the cyclic behavior of GDL, considering the existence of this compressive stress applied during the manufacturing process. The experimental mechanical properties of the three main types of non-woven GDL (rolls, sheets and felts) were first measured and then predicted using the proposed model, thereby allowing to separate the influences of the manufacturing process, the type of fibers, the presence of a micro-porous layer and a hydrophobic treatment on the GDL.     

Analysis of liquid water transport in fuel cell gas diffusion media using two-mobile phase pore network simulations

Transport of liquid water in a fuel cell gas diffusion layer is analyzed using capillary network modeling in which the mobility of both liquid and gas phases is considered to examine two distinct multiphase flow regimes: displacement and co-current flows. The simulations utilize a modified invasion percolation with trapping algorithm, and the capillary network consists of throats of different radii to account for the local heterogeneities of the porous media. Both displacement and two-mobile phase flow are solved, with inlet boundary condition for two-mobile phase flow prescribed through a discrete sequence of alternating phases entering the network. For both flow types (displacement and two-mobile phase), the cases studied range from capillary force controlled, where the maximum distance between two throats filled consecutively is equal to the network size, to viscous force controlled, where the maximum distance is set so as not to exceed some predefined value that is less than the network size. The maximum distance determines the distribution of phases; phase entrapment, percolation, and channeling are observed during the spread of phases for distinct flow conditions. Once a distribution of phases is obtained, we calculate saturation, relative permeabilities, and the capillary pressure at the interface between the phases; we also determine the dependence of these transport parameters on medium heterogeneity and cluster size. Finally, the changes of relative permeability and capillary pressure as a function of saturation are compared for displacement and two-mobile phase flow.     

Measurement of internal wettability of gas diffusion porous media of proton exchange membrane fuel cells

One of the major problems of current proton exchange membrane (PEM) fuel cells is water management. The gas diffusion layer (GDL) of the fuel cell plays an important role in water management since humidification and water removal are both achieved through the GDL. Various numerical models developed to illustrate the multiphase flow and transport in the fuel cell. The accuracy of these models depends on the accurate measurement of the GDL properties such as wettability, surface energy, and porosity. Most of the studies conducted for measuring the wettability of the GDL are based on the external contact angle measurements. However, the external contact angle does not describe adequately capillary forces acting on the water inside the GDL pores. In a recent study, the capillary penetration technique has been used to measure indirectly the wettability of the GDL based on the experimental weight increase due to penetration of the liquid into the porous sample. In essence, the mass penetration technique was used along with the Washburn's equation. The shortcoming of this method is that the external factors such as the mass of the meniscus formed outside the sample as well as evaporation occurring during the experiment were not considered. It was found that these factors affect the wettability measurements of the GDL, especially for a hydrophilic sample. In this paper, the experimental setup of the capillary penetration method has been modified to control the evaporation rate as the liquid is penetrated into the sample. Also, the capillary penetration technique which was initially used based on mass penetration has been modified to the height penetration method to eliminate the effect of the weight of the meniscus formed outside the sample. The experiments were performed for a time period of 10 s. For this time period, it was found that the Washburn's equation is not an accurate model since it does not include the frictional work effects that are significant at the first few seconds of the experiments. Therefore, the Washburn's equation was replaced by a more general form. Using the Levenberg-Marquardt optimization technique, the experimental data obtained from the height penetration technique is fitted to the theoretical curve to find the internal contact angles of a sample GDL. Finally, these contact angle results are used to determine the surface tension of the GDL using two approaches: the Owens-Wendt surface tension components and the equation-of-state models.     

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.     

Operative conditions effect on PEM-FC performance by in-situ and ex-situ analysis of gas diffusion media with different bulk textile structure

A set of four Gas Diffusion Layers (GDLs) having different textile properties (i.e. density, warp, weft and weight), produced by an Italian Company (SAATI), and coated with the same Microporous Layer (MPL), was investigated with the aim of assessing the influence of the Gas Diffusion Media (GDM) on the FC performance. Ex-situ compression measurements of the Gas Diffusion Media, i.e. GDL + MPL, were performed. Then, single cell assemblies, mounting the same Catalyst Coated Membrane (Nafion^® 112; Pt load 0.5/0.5 mg cm⁻²) but different GDM, were tested by means of steady-state polarization measurements combined with Electrochemical Impedance Spectroscopy (EIS). The FC was run under different operating conditions (i.e. humidity and gases flow rate) at constant temperature (60 °C): three RHs were selected at the cathode side (i.e. 100, 80 and 60% RH), while keeping constant the RH at the anode side (100% RH). Moreover, all experiments were carried out at constant gas flow rate: for the anode 0.2/0.25 Nl/min (Hydrogen) and for the cathode 1.0/1.6 Nl/min (Air).     

Development and impact of sandwich wettability structure for gas distribution media on PEM fuel cell performance

Water management is one of the key issues related to the performance, durability and cost of polymer electrolyte membrane fuel cells (PEMFCs); and the wettability of gas distribution media (GDM) is critical to the water management. In this study, a novel design is developed for GDM, referred to as sandwich wettability GDM. After being coated with a silica particle/poly(dimethylsiloxane) (PDMS) composite, the GDM has superhydrophobic surfaces with a contact angle of 162 ± 2°, but hydrophilic internal pores. Water droplets (10 μl) can roll off the tilted surface of the coated GDM at an angle of 5°, and can also be drawn into the pores of the coated GDM in 10 min when it is horizontal. The surface morphology, roughness and pore structures of GDMs are characterized by profilometry, scanning electron microscopy, and porosimetry. The measured internal pore size of the coated GDM is around 7.1 μm, and shows low energy resistance to gas transport. Performance testing indicates that the PEMFC equipped with sandwich wettability GDMs offers the best performance compared to those with raw GDM (untreated with surface coating), conventional GDM (with microporous layer) coated with PTFE or hydrophilic GDM (coated with silica particles).     

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.     

Scale effect and two-phase flow in a thin hydrophobic porous layer. Application to water transport in gas diffusion layers of proton exchange membrane fuel cells

Pore network simulations are performed to study water transport in gas diffusion layers (GDLs) of polymer electrolyte membrane fuel cells (PEMFCs). The transport and equilibrium properties are shown to be scale dependent in a thin system like a GDL. A distinguishing feature of such a thin system is the lack of length scale separation between the system size and the size of the representative elementary volume (REV) over which are supposed to be defined the macroscopic properties within the framework of the continuum approach to porous media. Owing to the lack of length scale separation, two-phase flow traditional continuum models are expected to offer poor predictions of water distribution in a GDL. This is illustrated through comparisons with results from the pore network model. The influence of inlet boundary conditions on invasion patterns is studied and shown to affect greatly the saturation profiles. The effects of GDL differential compression and partial coverage of outlet surface are also investigated.     

Experimental investigation of current distributions in a direct methanol fuel cell with various gas diffusion layers and Nafion membranes

The influences of the gas diffusion layer (GDL) properties on the current distributions of a direct methanol fuel cell are investigated. Cathode GDLs with different hydrophobicity/hydrophilicity, air permeability, microporous layer (MPL), thickness, and texture properties are examined. Among the GDLs examined, a thin hydrophobic GDL with an MPL has the most homogeneous current distribution, which is primarily ascribed to the better water management capabilities of the cathode GDL properties. The differences in the current distribution among the different GDLs are more apparent when the air flow rate and loaded current are lower. The effect of the membrane thickness on the current distributions is also investigated. Among the membranes examined, Nafion^® 112 has different current distributions from the others, whereas there is no noticeable difference between the current distributions with Nafion^® 115 and Nafion^® 117. The current distribution with Nafion^® 112 is most affected by the enhanced methanol crossover and the high mixed potential.     

Analysis of surface and interior degradation of gas diffusion layer with accelerated stress tests for polymer electrolyte membrane fuel cell

The effects of surface and interior degradation of the gas diffusion layer (GDL) on the performance and durability of polymer electrolyte membrane fuel cells (PEMFCs) have been investigated using three freeze-thaw accelerated stress tests (ASTs). Three ASTs (ex-situ, in-situ, and new methods) are designed from freezing ?30 °C to thawing 80 °C by immersing, supplying, and bubbling, respectively. The ex-situ method is designed for surface degradation of the GDL. Change of surface morphology from hydrophobic to hydrophilic by surface degradation of GDL causes low capillary pressure which decreased PEMFC performance. The in-situ method is designed for the interior degradation of the GDL. A decrease in the ratio of the porosity to tortuosity by interior degradation of the GDL deteriorates PEMFC performance. Moreover, the new method showed combined effects for both surface and interior degradation of the GDL. It was identified that the main factor that deteriorated the fuel cell performance was the increase in mass transport resistance by interior degradation of GDL. In conclusion, this study aims to investigate the causes of degraded GDL on the PEMFC performance into the surface and interior degradation and provide the design guideline of high-durability GDL for the PEMFC.     

The effect of Nafion ionomer loading coated on gas diffusion electrodes with in-situ grown Pt nanowires and their durability in proton exchange membrane fuel cells

The effect of varying Nafion^® ionomer loadings coated on the surface of gas diffusion electrodes (GDEs) with in-situ grown single crystal Pt nanowires and their durability in Proton Exchange Membrane Fuel Cells (PEMFCs) were investigated. GDEs were fabricated by growing Pt nanowires directly onto the Gas Diffusion Layer (GDL) surface with a simple one-step wet chemical approach at room temperature, as reported in our previous studies. The samples were then coated with various Nafion^® ionomer loadings and tested as cathodes in a 25 cm² PEMFC hardware with Hydrogen/Air. The data were compared to commercial GDEs (E-TEK ELAT^® GDE LT 120E-W). Performance results showed that the as-prepared GDEs with Pt nanowires required higher Nafion^® ionomer loading coating compared to the commercial ones. Accelerated ageing tests (500 cycles of voltage scan) were performed in views of evaluating the as-prepared GDE durability. The experimental data showed that the as-prepared GDEs exhibited much larger current densities at 0.7 V but higher degradation rates compared to commercial GDEs, indicating that the as-prepared GDEs gave poor durability. This was due to the difference in GDE surface nanostructures influenced by the electrolyte ionomer loading coating. This effect is further discussed in this paper.     

Carbon nano-chain and carbon nano-fibers based gas diffusion layers for proton exchange membrane fuel cells

Gas diffusion layers (GDL) for proton exchange membrane fuel cell have been developed using a partially ordered graphitized nano-carbon chain (Pureblack^® carbon) and carbon nano-fibers. The GDL samples’ characteristics such as, surface morphology, surface energy, bubble-point pressure and pore size distribution were characterized using electron microscope, inverse gas chromatograph, gas permeability and mercury porosimetry, respectively. Fuel cell performance of the GDLs was evaluated using single cell with hydrogen/air at ambient pressure, 70 °C and 100% RH. The GDLs with combination of vapor grown carbon nano-fibers with Pureblack carbon showed significant improvement in mechanical robustness as well as fuel cell performance. The micro-porous layer of the GDLs as seen under scanning electron microscope showed excellent surface morphology showing the reinforcement with nano-fibers and the surface homogeneity without any cracks.     

An analytical model for the transverse permeability of gas diffusion layer with electrical double layer effects in proton exchange membrane fuel cells

An analytical model is presented for the transverse permeability of gas diffusion layer (GDL) based on an ordered array of parallel charged circular cylinders at the steady state. The formula of calculating the permeability of the transverse direction is given by solving the fluid momentum equation in a unit cell. In the present approach, the proposed model is explicitly related to the porosity and fiber radius of fibrous porous media, the zeta potential, and the physical properties of the electrolyte solution. Besides, the effects of these parameters (the porosity, unit cell aspect ratio, fiber radius, and molar concentration) on the transverse permeability are discussed detailedly. The model predictions are compared with the previous studies in the available literature, and good agreement is found.     

Characterization of transport properties in gas diffusion layers for proton exchange membrane fuel cells: 1. Wettability (internal contact angle to water and surface energy of GDL fibers)

This is the first in a series of papers in which we present state-of-the-art methods demonstrated at Case for the estimation of transport properties in gas diffusion layers (GDLs) for proton exchange membrane fuel cells (PEMFCs). Most of the methods used today for measuring wettability properties of GDLs are related to the external contact angle to water. The external contact angle however does not describe adequately capillary forces acting on the water inside the GDL pores. We show as well that the direct method of estimation of the internal contact angle using goniometry on micrographs is impractical. We propose and describe in this paper a method for estimating the internal contact angle to water and the surface energy of hydrophobic and hydrophilic gas diffusion media. The method was applied to GDLs having different contents of hydrophobic agent and carbon types. The method can be applied separately to different components of the GDL including macro-porous substrates and micro-porous layers. The uncertainty estimates using this method are usually within 3% of the measured value.     

Estimation of through-plane and in-plane gas permeability across gas diffusion layers (GDLs): Comparison with equivalent permeability in bipolar plates and relation to fuel cell performance

The present work focusses on measuring the permeability across gas diffusion layers (GDLs) first in a dedicated cell and later in PEM fuel cell configuration with varying bi-polar plate designs. Eight carbon paper-based GDLs with and without the microporous layer (MPL), have been tested. An in-house designed dedicated cell allowed measuring pressure drop depending on flow rate, for i) through-plane and ii) in-plane direction. Further, transport measurements were conducted in 25 cm² bi-polar plates (BPs) in fuel cell configuration having single or multiple serpentine channels, by stacking the GDL inside. The results show that gas permeability in the dedicated cell for through-plane and in-plane can be estimated by using Darcy's law. However, for BPs, the flow is affected additionally by inertial contribution (Darcy-Forchheimer). Finally, the efficiency allowed by selected GDLs installed in a fuel cell under operation shows a relationship between the equivalent permeability and the fuel cell performance.     

Performance loss of proton exchange membrane fuel cell due to hydrophobicity loss in gas diffusion layer: Analysis by multiscale approach combining pore network and performance modelling

Loss of hydrophobicity in the gas diffusion layers (GDL) is sometimes suggested as a potential mechanism to explain in part the performance loss of PEMFC. The present study proposes a numerical methodology to analyse this effect by combining pore network modelling (PNM) and performance modelling (PM): the PNM/PM approach. PNM allows simulating the decrease of through-plane gas diffusion coefficient in the GDL as a function of the hydrophobicity loss, which is taken into account through the increase in the fraction of hydrophilic pores in GDL. Then PM based on Darcy equations allows simulating performance loss of PEMFC as a function of gas diffusion decay. This coupling shows that the loss of hydrophobic treatment increases flooding, decreases performance, and increases current density heterogeneities between inlet and outlet of the cell. Interestingly, this degradation is found to be highly non-linear, mainly because of the non-linear influence of the fraction of hydrophilic pores on gas diffusion (this is due to the existence of a percolation threshold associated with the hydrophilic pore sub-network) as well as the non-linear behaviour of electrochemistry with gas diffusion. This study also shows that the loss of hydrophobicity in a GDL is a very suitable candidate to explain performance loss rates that are classically observed during long-term tests. The proposed methodology may also help linking other local properties of components to fuel cell global performance.     

The comparison of direct borohydride-hydrogen peroxide fuel cell performance with membrane electrode assembly prepared by catalyst coated membrane method and catalyst coated gas diffusion layer method using Ni@Pt/C as anodic catalyst

Carbon supported Ni@Pt nanoparticles are synthesized using sodium dodecyl sulphate (SDS) and sodium borohydride (NaBH₄) as a structure-directing and reducing agents, respectively. The metal loading in synthesized nanocatalyst is 20 wt% and the ratio of Ni:Pt in the nanocatalyst is 1:1. The structural characterizations and morphologies of Ni@Pt/C nanocatalyst are investigated by field emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HR-TEM) and X-ray diffraction (XRD). The electrocatalytic activity of Ni@Pt/C catalyst toward borohydride $\left({\text{BH}}_4^{?}\right)$ oxidation in alkaline medium is studied by means of cyclic voltammetry (CV), chronopotentiometry (CP) and chronoamperometry (CA). The results show that Ni@Pt/C catalyst has superior catalytic activity toward borohydride oxidation (8825.38 mA mg Pt^?1). The Membrane Electrode Assembly (MEA) used in fuel cell set-up is fabricated with catalyst-coated membrane (CCM) and catalyst coated gas diffusion medium (CCG) techniques. The effect of two MEA performances on current–voltage (I–V) and current–power density (I–P) curves in the direct borohydride-hydrogen peroxide fuel cell was investigated using Pt/C 0.5 mg cm^?2 as cathode catalyst and Ni@Pt/C 1 mg cm^?2 as anode catalyst. The influence of cell temperature, sodium borohydride and hydrogen peroxide concentration on the I–V and I–P is determined. The results show that the maximum power density in MEA prepared using CCM method (CCM-MEA) is 68.64 mW cm^?2 at 60 °C, 1 M sodium borohydride and 2 M hydrogen peroxide (H₂O₂) that is higher than MEA prepared using CCG method (CCG-MEA). The impedance results show that with increasing temperature and discharging current, the overall anodic and cathodic charge transfer resistances reduce.