Optimization of cathode microporous layer materials for proton exchange membrane fuel cell

The microporous layer (MPL) of diffusion medium has an important impact on the water management ability of proton exchange membrane fuel cells. In this study, six kinds of carbon black were used to prepare the cathode MPL. The thickness, conductivity, pore structure, hydrophobicity, and surface microstructure of MPL were characterized. The single cell was prepared and electrochemical tests were performed. The results showed that the single cell prepared by Acetylene black (ACET) and Vulcan XC-72R has a considerable power generation performance. In addition, polyvinylidene fluoride hexafluoropropylene copolymer P(VDF-HFP) was used to replace Polytetrafluoroethylene (PTFE) as hydrophobic binder. MPL with different P(VDF-HFP) contents were prepared, and the single cell performance was investigated. The results showed that all the single cells prepared by P(VDF-HFP) were worse than that of PTFE. This study provides an important reference for further improving the performance of fuel cells from the perspective of material optimization with MPL.     

Carbon film coating on gas diffusion layer for proton exchange membrane fuel cells

This study discusses a novel process to increase the performance of proton exchange membrane fuel cells (PEMFC). In order to improve the electrical conductivity and reduce the surface indentation of the carbon fibers, we modified the carbon fibers with pitch-based carbon materials (mesophase pitch and coal tar pitch). Compared with the gas diffusion backing (GDB), GDB-A240 and GDB-MP have 32% and 33% higher current densities at 0.5 V, respectively. Self-made carbon paper with the addition of a micro-porous layer (MPL) (GDL-A240 and GDL-MP) show improved performance compared with GDB-A240 and GDB-MP. The current densities of GDL-A240 and GDL-MP at 0.5 V increased by 37% and 31% compared with GDL, respectively. This study combines these two effects (carbon film and MPL coating) to promote high current density in a PEMFC.     

Enhancement of proton exchange membrane fuel cell performance by titanium-coated anode gas diffusion layer

Titanium was coated onto an anode gas diffusion layer (GDL) by direct current sputtering to improve the performance and durability of a proton exchange membrane fuel cell (PEMFC). Scanning electron microscopy (SEM) images showed that the GDLs were thoroughly coated with titanium, which showed angular protrusion. Single-cell performance of the PEMFCs with titanium-coated GDLs as anodes was investigated at operating temperatures of 25 °C, 45 °C, and 65 °C. Cell performances of all membrane electrode assemblies (MEAs) with titanium-coated GDLs were superior to that of the MEA without titanium coating. The MEA with titanium-coated GDL, with 10 min sputtering time, demonstrated the best performance at 25 °C, 45 °C, and 65 °C with corresponding power densities 58.26%, 32.10%, and 37.45% higher than that of MEA without titanium coating.     

Fabrication of a carbon nanofiber sheet as a micro-porous layer for proton exchange membrane fuel cells

A carbon nanofiber sheet (CNFS) has been prepared by electrospinning, stabilisation and subsequent carbonisation processes. Imaging with scanning electron microscope (SEM) indicates that the CNFS is formed by nonwoven nanofibers with diameters between 400 and 700 nm. The CNFS, with its three-dimensional pores, shows excellent electrical conductivity and hydrophobicity. In addition, it is found that the CNFS can be successfully applied as a micro-porous layer (MPL) in the cathode gas diffusion layer (GDL) of a proton exchange membrane fuel cell (PEMFC). The GDL with the CNFS as a MPL has higher gas permeability than a conventional GDL. Moreover, the resultant cathode GDL exhibits excellent fuel cell performance with a higher peak power density than that of a cathode GDL fabricated with a conventional MPL under the same test condition.     

Gas diffusion layer for proton exchange membrane fuel cells—A review

Gas diffusion layer (GDL) is one of the critical components acting both as the functional as well as the support structure for membrane-electrode assembly in the proton exchange membrane fuel cell (PEMFC). The role of the GDL is very significant in the H₂/air PEM fuel cell to make it commercially viable. A bibliometric analysis of the publications on the GDLs since 1992 shows a total of 400+ publications (>140 papers in the Journal of Power Sources alone) and reveals an exponential growth due to reasons that PEMFC promises a lot of potential as the future energy source for varied applications and hence its vital component GDL requires due innovative analysis and research. This paper is an attempt to pool together the published work on the GDLs and also to review the essential properties of the GDLs, the method of achieving each one of them, their characterization and the current status and future directions. The optimization of the functional properties of the GDLs is possible only by understanding the role of its key parameters such as structure, porosity, hydrophobicity, hydrophilicity, gas permeability, transport properties, water management and the surface morphology. This paper discusses them in detail to provide an insight into the structural parts that make the GDLs and also the processes that occur in the GDLs under service conditions and the characteristic properties. The required balance in the properties of the GDLs to facilitate the counter current flow of the gas and water is highlighted through its characteristics.     

Effects of a microporous layer on the performance degradation of proton exchange membrane fuel cells through repetitive freezing

The gas diffusion layer (GDL) covered with a microporous layer (MPL) is being widely used in proton exchange membrane fuel cells (PEMFCs). However, the effect of MPL on water transport is not so clear as yet; hence, many studies are still being carried out. In this study, the effect of MPL on the performance degradation of PEMFCs is investigated in repetitive freezing conditions. Two kinds of GDL differentiated by the existence of MPL are used in this experiment. Damage on the catalyst layer due to freezing takes place earlier when GDL with MPL is used. More water in the membrane and catalyst layer captured by MPL causes permanent damage on the catalyst layer faster. More detailed information about the degradation is obtained by electrochemical impedance spectroscopy (EIS). From the point of view that MPL reduces the ohmic resistance, it is effective until 40 freezing cycles, but has no more effect thereafter. On the other hand, from the point of view that MPL enhances mass transport, it delays the increase in the mass transport resistance.     

Novel single-layer gas diffusion layer based on PTFE/carbon black composite for proton exchange membrane fuel cell

A series of poly(tetrafluoroethylene)/carbon black composite-based single-layer gas diffusion layers (PTFE/CB-GDLs) for proton exchange membrane fuel cell (PEMFC) was successfully prepared from carbon black and un-sintered PTFE, which included powder resin and colloidal dispersion, by a simple inexpensive method. The scanning electron micrographs of PTFE/CB-GDLs indicated that the PTFE resins were homogeneously dispersed in the carbon black matrix and showed a microporous layer (MPL)-like structure. The as-prepared PTFE/CB-GDLs exhibited good mechanical property, high gas permeability, and sufficient water repellency. The best current density obtained from the PEMFC with the single-layer PTFE/CB-GDL was 1.27 and 0.42 A cm⁻² for H₂/O₂ and H₂/air system, respectively.     

Optimization of polytetrafluoroethylene content in cathode gas diffusion layer by the evaluation of compression effect on the performance of a proton exchange membrane fuel cell

This research investigates the optimal polytetrafluoroethylene (PTFE) content in the cathode gas diffusion layer (GDL) by evaluating the effect of compression on the performance of a proton exchange membrane (PEM) fuel cell. A special test fixture is designed to control the compression ratio, and thus the effect of compression on cell performance can be measured in situ. GDLs with and without a microporous layer (MPL) coating are considered. Electrochemical impedance spectroscopy (EIS) is used to diagnose the variations in ohmic resistance, charge transfer resistance and mass transport resistance with compression ratio. The results show that the optimal PTFE content, at which the maximum peak power density occurs, is about 5 wt% with a compression ratio of 30% for a GDL without an MPL coating. For a GDL with an MPL coating, the optimal PTFE content in the MPL is found to be 30% at a compression ratio of 30%.     

Study on ice-melting performance of gradient gas diffusion layer in proton exchange membrane fuel cell

In this study, a three-dimensional model was established using the lattice Boltzmann method (LBM) to study the internal ice melting process of the gas diffusion layer (GDL) of the proton exchange membrane fuel cell (PEMFC). The single-point second-order curved boundary condition was adopted. The effects of GDL carbon fiber number, growth slope of the number of carbon fibers and carbon fiber diameter on ice melting were studied. The results were revealed that the temperature in the middle and lower part of the gradient distribution GDL is significantly higher than that of the no-gradient GDL. With the increase of the growth slope of the number of carbon fiber, the temperature and melting rate gradually increase, and the position of the solid-liquid interface gradually decreases. The decrease in the number of carbon fibers has a similar effect as the increase in the growth slope of the number of carbon fibers. In addition, as the diameter of the carbon fiber increases, the position of the solid-liquid interface gradually decreases first and then increases.     

Effect of water distribution in gas diffusion layer on proton exchange membrane fuel cell performance

Water management of proton exchange membrane fuel cells remains a prominent issue in research concerning fuel cells. In this study, the gas diffusion layer (GDL) of a fuel cell is partially treated with a hydrophobic agent, and the effect of GDL hydrophobicity on the water distribution in the fuel cell is examined. First, the effect of the position of the cathode GDL hydrophobic area relative to the channel on the fuel cell performance is investigated. Then, the water distribution in the fuel cell cathode GDL is observed using X-ray imaging. The experimental results indicate that when the hybrid GDL's hydrophobic area lies on the channel, water tends to accumulate under the rib, and the water content in the channel is low; this improves the fuel cell performance. When the hydrophobic area is under the rib, the water distribution is more uniform, but the performance deteriorates.     

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.     

Fractal model for prediction of effective thermal conductivity of gas diffusion layer in proton exchange membrane fuel cell

The process of heat transfer within porous media is usually considered as a transport through large numbers of straight channels with uniform pore sizes. For the prediction of effective thermal conductivity of gas diffusion layer (GDL), morphological properties such as the tortuosity of channels and pore-size distribution of this porous layer should be considered. Thus in this article, novel parallel and series-parallel prediction models of effective thermal conductivity for the GDL in proton exchange membrane fuel cell (PEMFC) have been derived by fractal theoretical characterization of the real microstructure of GDL. The prediction of fractal parallel model for carbon paper, a basal material of the GDL, is in good agreement with the reference value supplied by Toray Inc. The prediction results from the proposed models are also reasonable because they are distributed between the upper and lower bounds. Parametric effect has been investigated by using the presented models in dimensionless formalism. It can be concluded that dimensionless effective thermal conductivity (k^′eff${k^{\prime }}_{\text{eff}}$) has a positive correlation with effective porosity (?) or the pore-area fractal dimension (D_p) when k_s/k_g < 1; whereas it has a negative correlation with ? or D_p when k_s/k_g > 1 and with tortuous fractal dimension (D_t) whether k_s/k_g < 1 or not. Furthermore, these fractal models have been modified by considering the effect of polytetrafluoroethylene (PTFE) incorporated into the pore spaces of carbon paper, and the corresponding model prediction shows that there is an increase in the effective thermal conductivity due to the filling of PTFE that has high thermal conductivity.     

The effect of materials on proton exchange membrane fuel cell electrode performance

This paper describes the optimisation in the fabrication materials and techniques used in proton exchange membrane fuel cell (PEMFC) electrodes. The effect on the performance of membrane electrode assemblies (MEAs) from the solvents used in producing catalyst inks is reported. Comparison in MEA performances between various gas diffusion layers (GDLs) and the importance of microporous layers (MPLs) in gas diffusion electrodes (GDEs) are also shown. It was found that the best performances were achieved for GDEs using tetrahydrofuran (THF) as the solvent in the catalyst ink formulation and Sigracet 10BC as the GDL. The results also showed that our in-house painted GDEs were comparable to commercial ones (using Johnson Matthey HiSpec™ and E-TEK catalysts).     

Liquid transport in gas diffusion layer of proton exchange membrane fuel cells: Effects of micro-porous layer cracks

Micro porous layer (MPL) is a carbon layer (～15 μm) that coated on the gas diffusion layer (GDL) to enhance the electrical conduction and membrane hydration of proton exchange membrane fuel cell (PEMFC). However, the liquid transport behavior from MPL to GDL and its impact on water management remain unclear. Thus, a three-dimensional volume of fluid (VOF) model is developed to investigate the effects of MPL crack properties on liquid water saturation, liquid pathway formation, and the two-phase mass transport mechanism in GDL. Firstly, a stochastic orientation method is used to reconstruct the fibrous structure of the GDL. After that, the liquid water saturation calculated from the numerical results agrees well with the experimental data. With considering the full morphology of the overlap between MPL and GDL, it's found that this overlap determines the preferred liquid emerging port of both MPL and GDL. Three crack design shapes in MPL are proposed on the base of the similarity crack formation processes of soil mud. In addition, the effects of crack shape, distance between cracks, and crack number on liquid water transport from MPL to GDL are investigated. It is found that the liquid water saturation of GDL increases with crack number and the distance between cracks, while presents little correlation to the crack shape. Hopefully, these results can help the development of PEMFC models without reconstructing full MPL morphology.     

Investigation of the effect of microporous layers on water management in a proton exchange membrane fuel cell using novel diagnostic methods

Water management remains a significant challenge for the Proton Exchange Membrane Fuel Cell (PEMFC) with respect to performance, lifetime and operational flexibility. In recent years, microporous layers (MPL) have been widely used on the cathode side of the PEMFC in order to improve fuel cell performance and water management capabilities. Many modeling and experimental studies have with limited success attempted to analyze the underlying mechanisms that are responsible for the performance improvement due to the MPL. In this study, porous inserts along with various in-situ experimental techniques are used to investigate the MPLs. It was observed that the anode pressure drop increased when a cathode MPL was present, indicating water cross-over from the cathode towards the anode side. Further testing identified that the MPL improved cell performance due to the reduction of water saturation in the cathode catalyst layer, which resulted in enhanced oxygen diffusion. The influence of the MPL on the anode side was also studied with the aid of porous inserts and other techniques, and it was observed that the anode MPL improves cell voltage stability and reduces water accumulation in the anode catalyst layer. The present investigation provides further important information on the critical role of the MPL in the PEMFC.     

Optimal microporous layer for proton exchange membrane fuel cell

This study elucidates how fabrication processes (screen-printing and spraying) and constituent materials (carbon paper as backing, Acetylene Black (AB) carbon (42 nm), XC-72R carbon (30 nm) or BP2000 (15 nm) as carbon powders, and 10-50% fluorinated ethylene propylene (FEP) as hydrophobic substances) for microporous layers (MPLs) affect the performance of proton exchange membrane fuel cells. The screen-printing process produces MPLs with smaller surface fractures than does the spraying process. The effect of optimal FEP content on cell performance is noted. The presence of an optimal FEP content is due to the counterbalance between enhanced performance produced with increased gas permeability and decreased performance yielded with small contact area and electrical conductivity with excess FEP. The MPL with large carbon powders is preferred when oxygen supply is limited; otherwise, small carbon powders should be utilized. Optimal MPL design should address negative effects possibly associated with contact resistance, gas permeation resistance, and excess water resistance.     

Effects of the carbon black properties in gas diffusion layer on the performance of proton exchange membrane fuel cells

The microporous layer (MPL) as a part of diffusion medium has an important impact on mass transfer of proton exchange membrane fuel cell (PEMFC). In this study, MPLs of gas diffusion layers (GDLs) are prepared with different carbon blacks, and the properties of carbon blacks and their effects as MPLs on cell performance are systematically investigated. The results show that the GDL prepared by Acetylene Black (ACET) exhibits the best performance with a maximum power density up to 2.05 W cm⁻². Moreover, it still maintains extremely high performance with increasing current density even at humidity condition of 100% relative humidity, which means its excellent water/gas transportation capacity. This study contributes to deeply understanding the correlations between the properties of MPL material itself and their corresponding performance exhibited in cell. It also provides an important reference for enhancing cell performance and further advancing the practical applications of MPLs in PEMFC field.     

Electrochemical durability of gas diffusion layer under simulated proton exchange membrane fuel cell conditions

An effective ex-situ method for characterizing electrochemical durability of a gas diffusion layer (GDL) under simulated polymer electrolyte membrane fuel cell (PEMFC) conditions is reported in this article. Electrochemical oxidation of the GDLs are studied following potentiostatic treatments up to 96 h holding at potentials from 1.0 to 1.4 V (vs.SCE) in 0.5 mol L⁻¹ H₂SO₄. From the analysis of morphology, resistance, gas permeability and contact angle, the characteristics of the fresh GDL and the oxidized GDLs are compared. It is found that the maximum power densities of the fuel cells with the oxidized GDLs hold at 1.2 and 1.4 V (vs.SCE) for 96 h decreased 178 and 486 mW cm⁻², respectively. The electrochemical impedance spectra measured at 1500 mA cm⁻² are also presented and they reveal that the ohmic resistance, charge-transfer and mass-transfer resistances of the fuel cell changed significantly due to corrosion at high potential.     

Thermal and electrochemical performance analysis of a proton exchange membrane fuel cell under assembly pressure on gas diffusion layer

In this study, the effect of clamping pressure on the performance of a proton exchange membrane fuel cell (PEMFC) is investigated for three different widths of channel. The deformation of gas diffusion layer (GDL) due to clamping pressure is modeled using a finite element method, and the results are applied as inputs to a CFD model. The CFD analysis is based on finite volume method in non-isothermal condition. Also, a comparison is made between three cases to identify the geometry that has the best performance. The distribution of temperature, current density and mole fraction of oxygen are investigated for the geometry with best performance. The results reveal that by decreasing the width of channel, the performance of PEMFC improves due to increase of flow velocity. Also, it is found that intrusion of GDL into the gas flow channel due to assembly pressure deteriorates the PEMFC performance, while decrease of GDL thickness and GDL porosity have smaller effects. It is shown that assembly pressure has a minor effect on temperature profile in the membrane-catalyst interface at cathode side. Also, assembly pressure has a significant effect on ohmic and concentration losses of PEMFC at high current densities.