Facilitation of water management in low Pt loaded PEM fuel cell by creating hydrophobic microporous layer with PTFE,FEP and PDMS polymers: Effect of polymer and carbon amounts

Microporous layers (MPLs) were prepared with different hydrophobic polymers to establish water management in polymer electrolyte membrane (PEM) fuel cells. Besides conventionally used polymers polytetrafluoroethylene (PTFE) and fluorinated ethylene propylene (FEP), two different molecular weights (MW) of polydimethylsiloxane (PDMS) polymer were used as hydrophobic materials in MPL. Membrane electrode assemblies (MEAs) having MPLs with low MW PDMS polymer exhibited the best fuel cell performance compared to the PTFE and FEP based ones. Thus it is concluded that PDMS polymer has a great potential to be used as hydrophobic material for MPL to reduce flooding phenomena in PEM fuel cell.     

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.     

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.     

Improving proton exchange membrane fuel cell performance with carbon nanotubes as the material of cathode microporous layer

The cathode microporous layer (MPL) is fabricated by various multiwall carbon nanotubes (CNTs), and its influence on the performance of a proton exchange membrane fuel cell (PEMFC) is evaluated. Three types of CNT with different dimensions are employed in the experiments, and the conventional MPL made by acetylene black (AB) is also considered for the purpose of comparison. The results show that the employment of CNT as MPL composition indeed may improve fuel cell performance significantly in comparison with the case of AB. The type of CNT with the largest tube diameter and straight cylinder in shape exhibits the highest cell performance. The corresponding optimal CNT loading and polytetrafluoroethylene (PTFE) content in the MPL are also evaluated. Results show that the case of cathode MPL composed of 1.5 mg cm^?2 CNT and 20 wt% PTFE exhibits the best performance in all the experimental cases. The present data reveal that the application of CNT for MPL fabrication is beneficial to promote PEMFC performance. Copyright © 2015 John Wiley & Sons, Ltd.     

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.     

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.     

The influence of anode diffusion layer on the performance of direct dimethyl ether fuel cell

The effects of the parameters of the anode gas diffusion layer (GDL), including the PTFE content in the backing layer (BL), the PTFE content in the microporous layer (MPL), and the carbon black loading on the performance of a liquid‐feed direct dimethyl ether fuel cell (DDFC), were experimentally investigated. The results indicated that increase in the PTFE content can produce more cracks across the whole surface of the MPL. These cracks were benefit to the anode two‐phase mass transport. The optimal PTFE content in anode BL and MPL was 18 and 40 wt%, respectively. The performances of the DDFCs tended to decline with the increase in the carbon black loading in the anode GDLs due to the difficult long path of mass transport. The maximum power density was obtained with 18 wt% PTFE in BL and 0 mg cm^?2 carbon black loading, the optimal result, was 76.6 mW cm^?2 at ambient pressure. Copyright © 2011 John Wiley & Sons, Ltd.     

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.     

Effects of anode microporous layers made of carbon powder and nanotubes on water transport in direct methanol fuel cells

The effects of the design parameters of the anode diffusion layer (DL), including the PTFE loading in the backing layer (BL), and the carbon and PTFE loading in the microporous layer (MPL), on water transport through the membrane and the performance of a liquid-feed direct methanol fuel cell (DMFC) are experimentally investigated. The results indicate that increasing the PTFE loading in the BL and introducing a MPL could decrease water crossover through the membrane without sacrificing cell performance when the feed methanol concentration is increased. It is also found that changing the PTFE loading in the MPL has little effect on water crossover, whereas increasing the carbon loading in the MPL could noticeably decrease the water-crossover flux. Nevertheless, the ability of the MPL to reduce water crossover is limited by the presence of a number of mud cracks. To reduce further the water-crossover flux, a crack-free MPL made of multi-walled carbon nanotubes (MWCNTs) and PTFE is proposed. Tests indicate that the DMFC with the nanotube MPL results in a much lower water-crossover flux than a conventional carbon-powder MPL. More importantly, the use of the nanotube MPL allows the DMFC to be operated with a higher methanol concentration, and thereby increases the fuel cell system energy density.     

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%.     

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.     

Novel superhydrophobic microporous layers for enhanced performance and efficient water management in PEM fuel cells

Microporous layers (MPLs) obtained from inks containing three fluorinated polymers (perfluoroalcoxy, fluorinated ethylene propylene and a fluorinated polyurethane based on perfluoropolyether blocks) replacing conventional PTFE were prepared. Inks composition and rheological behaviour were fixed in order to apply the blade coating technique for MPL deposition. Superhydrophobic layers were obtained since contact angles higher than 150° were measured. The samples, tested in a single fuel cell at lab scale, at 60 °C and different relative humidity (RH 80/100 and 80/60, hydrogen/air) evidenced that new polymers are able to improve electrical performances reaching maximum power density values higher than those showed by conventional MPLs based fuel cells. Electrochemical impedance spectroscopy (EIS) was also carried out on the running cell using a Frequency Response Analyzer to assess the different dissipation phenomena and related losses.     

Freezing characteristics of supercooled water in gas diffusion layer of proton exchange membrane fuel cells

The freezing characteristics of supercooled water in a gas diffusion layer (GDL), which are the bases for the cold start-up of proton exchange membrane fuel cells (PEMFCs), were investigated. An experimental apparatus for noncontact temperature measurement and observation systems was developed. GDL and GDL with a microporous layer (MPL) were prepared, and freezing experiments using a water-containing GDL under various cooling rates were performed with variations in polytetrafluoroethylene (PTFE) content and water saturation. Furthermore, based on the experimental results, the freezing initiation probability was theoretically investigated to elucidate the freezing characteristics. Results showed that, with increasing supercooling of water in GDL, the freezing probability of water increased abruptly. The effect of saturation showed a different trend depending on PTFE addition. For the GDL without PTFE, the freezing initiations occurred at approximately 6 °C of supercooling degree, and the probability approached 1.0 at approximately 9.5–11.5 °C, with saturation dependency. In contrast, for both GDL and GDL + MPL containing PTFE, the initiation temperature characteristics were relatively similar, which were approximately 8–12 °C, regardless of the saturation and PTFE content. In these cases, the ice-nucleating activity of water in the GDL was possibly stronger than that in the MPL.     

Porosity-graded micro-porous layers for polymer electrolyte membrane fuel cells

In this paper, the effect of porosity-graded micro-porous layer (GMPL) on the performance of polymer electrolyte membrane fuel cells (PEMFCs) was studied in detail. The GMPL was prepared by printing micro-porous layers (MPL) with different content of NH₄Cl pore-former and the porosity of the GMPL decreased from the inner layer of the MPLs at the membrane/MPL interface to the outer layer of the MPLs at the gas diffusion electrode/MPL interface. The morphology and porosity of the GMPLs were characterized and the performance of the cell with GMPLs was compared with those having conventional homogeneous MPLs. The result demonstrates that the fuel cells consisting of GMPL have better performance than those consisting of conventional homogeneous MPLs, especially at high current densities. Micro-porous layer with graded porosity is beneficial for the electrode process of fuel cell reaction probably by facilitating the liquid water transportation through large pores and gas diffusion via small pores in the GMPLs.     

Microporous layer coated gas diffusion layers for enhanced performance of polymer electrolyte fuel cells

The influence of microporous layer (MPL) design parameters for gas diffusion layers (GDLs) on the performance of polymer electrolyte fuel cells (PEFCs) was clarified. Appropriate MPL design parameters vary depending on the humidification of the supplied gas. Under low humidification, decreasing both the MPL pore diameter and the content of polytetrafluoroethylene (PTFE) in the MPL is effective to prevent drying-up of the membrane electrode assembly (MEA) and enhance PEFC performance. Increasing the MPL thickness is also effective for maintaining the humidity of the MEA. However, when the MPL thickness becomes too large, oxygen transport to the electrode through the MPL is reduced, which lowers PEFC performance. Under high humidification, decreasing the MPL mean flow pore diameter to 3 μm is effective for the prevention of flooding and enhancement of PEFC performance. However, when the pore diameter becomes too small, the PEFC performance tends to decrease. Both reduction of the MPL thickness penetrated into the substrate and increase in the PTFE content to 20 mass% enhance the ability of the MPL to prevent flooding.     

Effect of PTFE loading of gas diffusion layers on the performance of proton exchange membrane fuel cells running at high‐efficiency operating conditions

In this paper, the effects of microporous layer (MPL) addition and polytetrafluoroethylene (PTFE) loading of gas diffusion layers (GDLs) on the overall performance of the proton exchange membrane fuel cell have been investigated. The focus was on fuel cells that operate at relatively low current densities where the power demand is low, but the efficiency is of concern. The results show that, in the activation loss‐controlled region, the performance of the fuel cell operating with moderately PTFE‐treated carbon substrates is superior over that operating with coated GDLs. This is due to the addition of the MPL which lengthens the diffusion paths and significantly reduces the mass transport properties. Conversely, in the ohmic loss‐controlled region, the fuel cell with coated GDLs performs better than those with carbon substrates. This is explained by the enhanced contact of the GDL with the adjacent components after the MPL addition, which outweighs the negative effects associated with the activation loss‐controlled region. Also, it was found that the fuel cell performance becomes lower if the GDL is treated with a relatively high PTFE loading in either the carbon substrate (due to the decrease in the porosity of the GDL) or the MPL. Copyright © 2012 John Wiley & Sons, Ltd.     

Influence of properties of gas diffusion layers on the performance of polymer electrolyte-based unitized reversible fuel cells

Polymer electrolyte-based unitized reversible fuel cells (URFCs) combine the functionality of a fuel cell and an electrolyzer in a single device. In a URFC, titanium (Ti)-felt is used as a gas diffusion layer (GDL) of the oxygen electrode, whereas typical carbon paper is used as a GDL of the hydrogen electrode. Different samples of Ti-felt with different structural properties (porosity and fiber diameter) and PTFE content were prepared for use as GDLs of the oxygen electrode, and the relation between the properties of the GDL and the fuel cell performance was examined for both fuel cell and electrolysis operation modes. Experimental results showed that the cell with a Ti-felt GDL of 80 μm fiber diameter had the highest round-trip efficiency due to excellent fuel cell operation under relatively high-humidity conditions despite degradation in performance in the electrolysis mode.     

Influence of micro-porous layer and operating conditions on the fluoride release rate and degradation of PEMFC membrane electrode assemblies

In this study, the influence of micro-porous layers (MPL) on polymer electrolyte membrane fuel cell (PEMFC) durability was investigated. Two fuel cells were built, one with a micro-porous layer on the anode side, and a second cell with MPL on both sides. Experiments were conducted by varying operational parameters such as current density, reactant stoichiometry, and inlet relative humidity. Fuel cell degradation was evaluated by measuring fluoride release rates. The largest factor determining fluoride release rate was found to be the presence of MPL; the cell with MPLs on both sides exhibited significantly reduced fluoride release rates compared to that of cell with one MPL. Increasing the current density also reduced the fluoride release rate for cells with only one MPL whereas there was only a moderate effect on cells with two MPLs. Microscopy analysis showed small but significant changes in ionomer layer thickness. Polarization measurements indicated that there was little change in the performance for both cells over the test period.     

Performance of proton exchange membrane fuel cells with microporous layer hydrophobized by polyphenylene sulfide at conventional temperature and cold start

Polyphenylene sulfide (PPS), a thermoplastic polymer with excellent chemical and thermal stability, has properties similar to those of polytetrafluoroethylene (PTFE) but is less expensive. In this study, the feasibility of using PPS to replace conventional PTFE as a hydrophobic agent for microporous layers (MPLs) in proton exchange membrane fuel cells (PEMFCs) is explored. First, PTFE-MPL and PPS-MPL with 30 wt% hydrophobic agent were prepared. The pore size, porosity, contact angle, and microstructure of the two MPL samples were measured and analyzed. Subsequently, PEMFCs with the two MPL samples were tested for their operational performance at a conventional temperature of 70 °C and cold-start capability at ?10 °C. The performances of PTFE-MPL and PPS-MPL at conventional temperatures were similar, but PPS-MPL showed obvious advantages in cold-start performance. In addition, the operating performance of PEMFC at the conventional temperature and cold-start capability at ?10 °C were investigated for PPS mass fractions of 10%, 20%, and 30% in MPL, and for PPS particle sizes of 10 and 30 μm. The results indicate that the optimal performance can be attained when the PPS particle size is 10 μm and the mass fraction is 20%.     

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.