Preparation of microporous layer for proton exchange membrane fuel cell by using polyvinylpyrrolidone aqueous solution

A new method of preparing microporous layer (MPL) for proton exchange membrane fuel cell (PEMFC) was presented in this paper. Considering the bad dispersion of PTFE aqueous suspension in the carbon slurry based on ethanol, polyvinylpyrrolidone (PVP) aqueous solution was used to prepare carbon slurry for microporous layer. The prepared gas diffusion layers (GDLs) were characterized by scanning electron microscopy, contact angle system and pore size distribution analyzer. It was found that the GDL prepared with PVP aqueous solution had higher gas permeability, as well as more homogeneous hydrophobicity. Moreover, the prepared GDLs were used in the cathode of fuel cell and evaluated with fuel cell performance and EIS analysis, and the GDL prepared with PVP aqueous solution indicated better fuel cell performance and lower ohmic resistance and mass transfer resistance.     

Effect of PTFE content in microporous layer on water management in PEM fuel cells

The effect of hydrophobic agent (PTFE) concentration in the microporous layer on the PEM fuel cell performance was investigated using mercury porosimetry, water permeation experiment, and electrochemical polarization technique. The mercury porosimetry and water permeation experiments indicated that PTFE increases the resistance of the water flow through the GDL due to a decrease of the MPL porosity and an increase of the volume fraction of hydrophobic pores. When air was used as an oxidant, a maximum fuel cell performance was obtained for a PTFE loading of 20 wt.%. The experimental polarization curves were quantitatively analyzed to determine the polarization resistances resulting from different physical and electrochemical processes in the PEM fuel cell. The polarization analysis indicated that the optimized PTFE content results in an effective water management (i.e., a balancing of water saturations in the catalyst layer and the gas diffusion layer), thereby improving the oxygen diffusion kinetics in the membrane-electrode assembly.     

Degradation of gas diffusion layers through repetitive freezing

This work investigates the degradation of an individual gas diffusion layer (GDL) by repeated freezing cycles. The pore size distribution, gas permeability, surface structure, and contact angle on the surface of the GDL were measured in four different types of GDL: SGL paper with a microporous layer (MPL); SGL paper with 5 wt% of polytetrafluoroethylene (PTFE) loading; Toray paper without PTFE loading; and Toray paper with 20 wt% of PTFE loading. After repeated freezing cycles, the porosity of the GDL without PTFE was reduced by 27.2% due to the volumetric expansion of the GDL. The peak of the log differential intrusion moved toward a smaller pore diameter slightly because of the repeated freezing process. The crack of the MPL increased in its width and length after repeated freezing cycles. The through-plane gas permeability of the GDL with the MPL doubled after repeated freezing cycles due to the growth of the crack in the MPL, but was very small for the GDLs with Toray paper. Besides, the GDLs with PTFE loading showed a relatively larger decrease in the contact angle on the surface than the GDL without PTFE loading due to the separation of PTFE from the carbon fiber during the repeated freezing process.     

Experimental study of micro-porous layers for PEMFC with gradient hydrophobicity under various humidity conditions

The effect of micro-porous layer (MPL) with hydrophobic gradient design on fuel cell performance and stability is investigated under various relative humidity (RH) conditions. Experimental results show that when such MPL is used between catalyst layer and gas diffusion layer, the membrane may retain more water and stay well humidified, and cell performance is increased at low RH conditions. On the other hand, at high RH conditions, the gradient MPL is able to efficiently remove water from the electrode, achieving maximum performance under these conditions. It is found that the design of hydrophobic gradient must take into consideration factors including gas permeability, electronic resistance and hydrophobic characteristics, because excessive hydrophobicity gradient in the MPL may result into high mass transfer resistance, which causes performance degradation.     

Liquid and oxygen transport through bilayer gas diffusion materials of proton exchange membrane fuel cells

In proton exchange membrane fuel cells (PEMFCs), a hydrophobic micro-porous layer (MPL) is usually placed between the catalyst layer (CL) and the conventional gas diffusion layer (GDL) to relieve the flooding. In this paper, a pore network model is developed to investigate how the MPL structure affects the liquid and oxygen transport properties of the bilayer gas diffusion material (GDM) consisting of fine MPL and coarse GDL. The regular three-dimensional pore network constructed to represent the bilayer GDM are composed of the cubic pores that are connected by the narrow throats of square cross section. Based on this model, the capillary pressure, liquid permeability, and oxygen effective diffusivity as a function of GDM liquid saturation are determined. Parameter studies are performed to elucidate the influences of MPL thickness and of MPL crack width. Also analyzed are the liquid distributions in different structural GDMs at the moment of breakthrough. The results reveal a liquid saturation jump at the MPL/GDL interface in the plain bilayer GDM, but a liquid saturation drop in the defective bilayer GDM.     

Development of a novel hydrophobic/hydrophilic double micro porous layer for use in a cathode gas diffusion layer in PEMFC

In this study, a gas diffusion layer (GDL) was modified to improve the water management ability of a proton exchange membrane fuel cell (PEMFC). We developed a novel hydrophobic/hydrophilic double micro porous layer (MPL) that was coated on a gas diffusion backing layer (GDBL). The water management properties, vapor and water permeability, of the GDL were measured and the performance of single cells was evaluated under two different humidification conditions, R.H. 100% and 50%. The modified GDL, which contained a hydrophilic MPL in the middle of the GDL and a hydrophobic MPL on the surface, performed better than the conventional GDL, which contained only a single hydrophobic MPL, regardless of humidity, where the performance of the single cell was significantly improved under the low humidification condition. The hydrophilic MPL, which was in the middle of the modified GDL, was shown to act as an internal humidifier due to its water absorption ability as assessed by measuring the vapor and water permeability of this layer.     

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.     

The influence of anode gas diffusion layer on the performance of low-temperature DMFC

The influence of the anode gas diffusion layers (GDLs) on the performances of low-temperature DMFCs, and the properties of mass transport and CO₂ removal on these anode GDLs were investigated. The membrane electrode assembly (MEA) based on the hydrophilic anode GDL, which consisted of the untreated carbon paper and hydrophilic anode micro-porous layer (comprised carbon black and 10 wt.% Nafion), showed the highest power density of 13.4 mW cm⁻² at 30 °C and ambient pressure. The performances of the MEAs tended to decline with the increase of the PTFE content in the anode GDLs due to the difficulty of methanol transport. The contact angle measurements revealed that the wettabilities of the anode GDLs decreased as the increase of PTFE content. The wettabilities of the GDLs were improved by addition of hydrophilic Nafion ionomer to the GDLs. From the visualizations of CO₂ gas bubbles dynamics on the anodes using a transparent cell, it was observed that uniform CO₂ gas bubbles with smaller size formed on hydrophilic anode GDLs. And bubbles with larger size were not uniform over the hydrophobic anode GDLs. It was believed that adding PTFE to the anode GDL was not helpful for improving the CO₂ gas transport in the anode GDL of the low-temperature DMFC.     

Determination of the pore size distribution of micro porous layer in PEMFC using pore forming agents under various drying conditions

In this paper, the effect of the pore size distribution of a micro-porous layer (MPL) on the performance of polymer electrolyte membrane fuel cells (PEMFC) was investigated using self-made gas diffusion layers (GDLs) with different MPLs for which the pore size distribution was modified using pore forming agents under different drying conditions. When MPL dried at high temperature, more macro pores, approximately 1,000–20,000 nm in diameter, and less micro pores, below 100 nm, were observed relative to when MPL was dried at low temperature. Self-made GDLs were characterized by a field-emission scanning electron microscope (FE-SEM), mercury porosimetry and self-made gas permeability measurement equipment. The performance of the single cells was measured under two different humidification conditions. The results demonstrate that the optimum pore size distribution of MPL depended on the cell operating humidification condition. The MPL dried at high temperature performed better than the MPL dried at low temperature under a low humidification condition; however, MPL dried at low temperature performed better under a high humidification condition.     

Numerical modeling and experimental study of the influence of GDL properties on performance in a PEMFC

This work is to study the effect of properties of gas diffusion layer (GDL) on performance in a polymer electrolyte membrane fuel cell (PEMFC) by both numerical simulation and experiments. The 1-dimension numerical simulation using the mixture-phase model is developed to calculate polarization curve. We are able to estimate optimum GDL properties for cell performance from numerical simulation results. Various GDLs which have different properties are prepared to verify accuracy of the simulation results. The contact angle and gas permeability of GDLs are controlled by polytetrafluoroethylene (PTFE) content in micro-porous layers (MPLs). MPL slurry is prepared by homogeneous blending of carbon powder, PTFE suspension, isopropyl alcohol and glycerol. Then the slurry is coated on gas diffusion mediums (GDMs) surface with controlled thickness by blade coating method. Non-woven carbon papers which have different thicknesses of 200 μm and 380 μm are used as GDMs. The prepared GDLs are measured by surface morphology, contact angle, gas permeability and through-plane electrical resistance. Moreover, the GDLs are tested in a 25 cm² single cell at 70 °C in humidified H₂/air condition. The contact angle of GDL increases with increasing PTFE content in MPL. However, the gas permeability and through-plane electrical conductivity decrease with increasing PTFE content and thickness of GDM. These changes in properties of GDL greatly influence the cell performance. As a result, the best performance is obtained by GDL consists of 200 μm thick non-woven carbon paper as GDM and MPL contained 20 wt.% PTFE content.     

Through-plane gas permeability of gas diffusion layers and microporous layer: Effects of carbon loading and sintering

Knowledge of the absolute permeability for the various porous layers is necessary to obtain accurate profiles for water saturation within the membrane electrode assembly (MEA) in a two-phase model of a polymer electrolyte membrane fuel cell (PEMFC). In this paper, the gas permeability of gas diffusion layers (GDLs) coated with microporous layers (MPLs) of various carbon loadings for two different carbon blacks have been experimentally measured. The permeability of the GDL was found to decrease by at least one order of magnitude after the MPL-coating. Also, the permeability of the MPLs was shown to be lower than that of the carbon substrate by 2–3 orders of magnitude. Further, it was found that the gas permeability of the MPLs changes significantly from one carbon loading to another despite the use of a single weight composition for all the MPLs coated, namely 20% PTFE and 80% carbon black. This signifies the possible inaccuracy in estimating the MPL permeability through employing the cross-section SEM images as they do not resolve the MPL penetration into the carbon substrate. Finally, the MPL sintering was found to slightly decrease the permeability of the GDL.     

Effect of polytetrafluoroethylene-treatment and microporous layer-coating on the in-plane permeability of gas diffusion layers used in proton exchange membrane fuel cells

The in-plane permeability has been experimentally estimated for a number of carbon substrates and microporous layer (MPL)-coated gas diffusion layers (GDLs) as used in proton exchange membrane (PEM) fuel cells. The results show that the in-plane permeability of the tested carbon substrates decreases with increasing polytetrafluoroethylene (PTFE) loading and, in contrast, the greater is the PTFE loading in the MPL, the greater is the permeability. It has been shown that the in-plane permeability of the carbon substrates is reduced by an order of magnitude if they are coated with MPLs. Further, the permeability is different from one in-plane principal direction to another by a factor of about two. Finally, ignoring the inertial terms (for the reported flow rates) and the compressibility of the flowing air results in significant errors in the obtained values of the permeability.     

Liquid water breakthrough pressure through gas diffusion layer of proton exchange membrane fuel cell

The dynamic behavior of liquid water transport through the gas diffusion layer (GDL) of the proton exchange membrane fuel cell is studied with an ex-situ approach. The liquid water breakthrough pressure is measured in the region between the capillary fingering and the stable displacement on the drainage phase diagram. The variables studied are GDL thickness, PTFE/Nafion content within the GDL, GDL compression, the inclusion of a micro-porous layer (MPL), and different water flow rates through the GDL. The liquid water breakthrough pressure is observed to increase with GDL thickness, GDL compression, and inclusion of the MPL. Furthermore, it has been observed that applying some amount of PTFE to an untreated GDL increases the breakthrough pressure but increasing the amount of PTFE content within the GDL shows minimal impact on the breakthrough pressure. For instance, the mean breakthrough pressures that have been measured for TGP-060 and for untreated (0 wt.% PTFE), 10 wt.% PTFE, and 27 wt.% PTFE were 3589 Pa, 5108 Pa, and 5284 Pa, respectively.     

Effect of variation of hydrophobicity of anode diffusion media along the through-plane direction in direct methanol fuel cells

Reducing methanol crossover from the anode to cathode in direct methanol fuel cells (DMFCs) is critical for attaining high cell performance and fuel utilization, particularly when highly concentrated methanol fuel is fed into DMFCs. In this study, we present a novel design of anode diffusion media (DM) wherein spatial variation of hydrophobicity along the through-plane direction is realized by special polytetrafluoroethylene (PTFE) coating procedure. According to the capillary transport theory for porous media, the anode DM design can significantly affect both methanol and water transport processes in DMFCs. To examine its influence, three different membrane-electrode assemblies are fabricated and tested for various methanol feed concentrations. Polarization curves show that cell performance at high methanol feed concentration conditions is greatly improved with the anode DM design with increasing hydrophobicity toward the anode catalyst layer. In addition, we investigate the influence of the wettability of the anode microporous layer (MPL) on cell performance and show that for DMFC operation at high methanol feed concentration, the hydrophilic anode MPL fabricated with an ionomer binder is more beneficial than conventional hydrophobic MPLs fabricated with PTFE. This paper highlights that controlling wetting characteristics of the anode DM and MPL is of paramount importance for mitigating methanol crossover in DMFCs.     

Sublayers for Pt catalyst electrodeposition electrodes in PEMFC

The effect of sublayers on the deposited platinum (Pt) catalyst layer fabricated by electrodeposition, and on the resulting fuel cell performance, was investigated. The substrate was prepared by applying a hydrophobic sublayer, composed of polytetrafluoroethylene (PTFE) and carbon black, and a hydrophilic sublayer, composed of Nafion and glycerol, onto an uncatalyzed gas diffusion layer prior to the electrodeposition of the Pt catalyst. The hydrophilic sublayer was found to play a substantial role in the Pt electrodeposition, since the structure of the resultant Pt catalyst significantly depended on the presence of the hydrophilic sublayer, the total loading amount and the Nafion to glycerol weight ratio, which in turn affected the fuel cell performance. The hydrophobic sublayer, which did not directly contact with the plating solution, did not show as marked an effect on the Pt deposit structure compared to that of the hydrophilic sublayer, except at high PTFE to carbon black weight ratios (≥70:30). However, the suitable PTFE to carbon black weight ratio in the hydrophobic sublayer was still important for the water management, mass transport of reactant gases and ohmic resistance of the membrane electrode assembly (MEA) during fuel cell operation. In this study, a total hydrophilic loading of 0.8 mg cm⁻² with a Nafion to glycerol weight ratio of 50:50, and a PTFE to carbon black weight ratio in the hydrophobic sublayer of 30:70 was found to yield the best Pt catalyst layer for PEMFC.     

Effect of cathode gas diffusion layer on water transport and cell performance in direct methanol fuel cells

The optimal design of the cathode gas diffusion layer (GDL) for direct methanol fuel cells (DMFCs) is not only to attain better cell performance, but also to achieve better water management for the DMFC system. In this work, the effects of both the PTFE loading in the cathode backing layer (BL) as well as in the micro-porous layer (MPL) and the carbon loading in the MPL on both water transport and cell performance were investigated experimentally. The experimental data showed that with the presence of a hydrophobic MPL in the GDL, the water-crossover flux through the membrane decreased slightly with increasing the PTFE loading in the BL. However, a higher PTFE loading in the BL not only lowered cell performance, but also resulted in an unstable discharging process. It was also found that the PTFE loading in the MPL had little effect on the water-crossover flux, but its effect on cell performance was substantial: the 40-wt% PTFE loading in the MPL was found to be the optimal value to achieve the best performance. The experimental results further showed that increasing the carbon loading in the MPL significantly lowered the water-crossover flux, but a too high carbon loading would decrease the cell performance as the result of the increased oxygen transport resistance; the 2.0-mg C cm⁻² carbon loading was found to exhibit the best performance.     

Effects of basic gas diffusion layer components on PEMFC performance with capillary pressure gradient

The gas diffusion layer (GDL) is composed of a substrate and a micro-porous layer (MPL), and is treated with polytetrafluoroethylene (PTFE) to promote water discharge. Additionally, the MPL mainly consists of carbon black and PTFE. In other words, the optimal design of these elements has a dominant effect on the polymer electrolyte membrane fuel cell (PEMFC) performance. For the GDL, it is crucial to prevent water flooding, and the water flux within the GDL is strongly affected by the capillary pressure gradient. In this study, the PEMFC performance was systematically investigated by varying the substrate PTFE content, MPL PTFE content, and MPL carbon loading per unit area. The effects of each experimental variable on the PEMFC performance and especially on the capillary pressure gradient were quantitatively analyzed when the GDLs were manufactured by the doctor blade manufacturing method. The experimental results indicated that as the PTFE content of the anode and cathode GDL increased, the PEMFC performance deteriorated due to the deformation of the porosity and tortuosity of the GDL. Additionally, the PEMFC performance improved as the MPL PTFE content of the cathode GDL increased at low relative humidity (RH), but the PEMFC performance tendency was reversed at high RH. Further, the MPL carbon loading of 2 mg/${\text{cm}}^2$ demonstrated the best performance, and the advantages and disadvantages of the MPL carbon loading were identified. In addition, the effects of each experimental variable on liquid water, water vapor, and gas permeability were investigated.     

Measurement of water transport rates across the gas diffusion layer in a proton exchange membrane fuel cell,and the influence of polytetrafluoroethylene content and micro-porous layer

Water management in a proton exchange membrane (PEM) fuel cell is one of the critical issues for improving fuel cell performance and durability, and water transport across the gas diffusion layer plays a key role in PEM fuel cell water management. In this work, we investigated the effects of polytetrafluoroethylene (PTFE) content and the application of a micro-porous layer (MPL) in the gas diffusion layer (GDL) on the water transport rate across the GDL. The results show that both PTFE and the MPL play a similar role of restraining water transport. The effects of different carbon loadings in the MPL on water transport were also investigated. The results demonstrate that the higher the carbon loading in the MPL, the more it reduces the water transport rate. Using the given cell hardware and components, the optimized operation conditions can be obtained based on a water balance analysis.     

Determination of effective water vapor diffusion coefficient in pemfc gas diffusion layers

The primary removal of product water in proton exchange membrane (PEM) fuel cells is through the cathode gas diffusion layer (GDL) which necessitates the understanding of vapor and liquid transport of water through porous media. In this investigation, the effect of microporous layer (MPL) coatings, GDL thickness, and polytetrafluorethylene (PTFE) loading on the effective water vapor diffusion coefficient is studied. MRC Grafil, SGL Sigracet, and Toray TGP-H GDL samples are tested experimentally with and without MPL coatings and varying PTFE loadings. A dynamic diffusion test cell is developed to produce a water vapor concentration gradient across the GDL and induce diffusion mass transfer. Tests are conducted at ambient temperature and flow rates of 500, 625, and 750 sccm. MPL coatings and increasing levels of PTFE content introduce significant resistance to diffusion while thickness has negligible effects.     

Correlations between the catalytic layer composition,the relative humidity and the performance for PEMFC carbon aerogel based membrane electrode assemblies

The parameters influencing the water management or the gas diffusion have a large impact on the performances of the PEMFC. In this work, we focused on the influence of the composition of the cathode catalytic layer on the performance of the MEA in relation to the relative humidity. A carbon aerogel was synthesized as catalyst support with a texture minimizing diffusive losses. We studied the influence of the Nafion content by preparing various cathode catalytic layers whose hydric properties were modified by adding PTFE as a hydrophobic agent. We evaluated the impact of the cathodic relative humidity by testing MEAs on a single cell test bench. We showed that modifying the composition of the catalytic layers playing on the content of both Nafion and PTFE, enables to improve the performance due to a better water management. The best performance was obtained with a Nafion/Carbon mass ratio (N/C) of 0.5 and a PTFE/Carbon (PTFE/C) mass ratio of 0.35 allowing 40% more power at 0.4 V and 75%RH. A first positive evaluation of the impact of PTFE is also done with TEC10E40E.