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.     

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.     

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.     

Effect of the cathode gas diffusion layer on the water transport behavior and the performance of passive direct methanol fuel cells operating with neat methanol

The passive operation of a direct methanol fuel cell with neat methanol requires the water that is produced at the cathode to diffuse through the membrane to the anode to compensate the methanol oxidation reaction (MOR). Hence, the anode performance of this type of fuel cell can be limited by the water transport rate from the cathode to the anode. In this work we theoretically show that the water transport from the cathode to the anode depends primarily on the design of the cathode gas diffusion layer (GDL). We investigate experimentally the effects of the design parameters of the cathode GDL, including the PTFE (polytetrafluoroethylene) content in the backing layer (BL), and the carbon loading and the PTFE content in the microporous layer (MPL) on the water transport and the performance of the passive DMFC with the help of a reference electrode. The results indicate that on one hand, these parameters can be adjusted to decrease the water concentration loss of the anode performance, but on the other hand, they can also cause an increase in the oxygen concentration loss of the cathode performance. Hence, an optimal balance in minimizing the both concentration losses is the key to maximize the cell performance.     

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%.     

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.     

In situ measurements of water crossover through the membrane for direct methanol fuel cells

We show analytically that the water-crossover flux through the membrane used for direct methanol fuel cells (DMFCs) can be in situ determined by measuring the water flow rate at the exit of the cathode flow field. This measurement method enables investigating the effects of various design and geometric parameters as well as operating conditions, such as properties of cathode gas diffusion layer (GDL), membrane thickness, cell current density, cell temperature, methanol solution concentration, oxygen flow rate, etc., on water crossover through the membrane in situ in a DMFC. Water crossover through the membrane is generally due to electro-osmotic drag, diffusion and back convection. The experimental data showed that diffusion dominated the total water-crossover flux at low current densities due to the high water concentration difference across the membrane. With the increase in current density, the water flux by diffusion decreased, but the flux by back convection increased. The corresponding net water-transport coefficient was also found to decrease with current density. The experimental results also showed that the use of a hydrophobic cathode GDL with a hydrophobic MPL could substantially reduce water crossover through the membrane, and thereby significantly increasing the limiting current as the result of the improved oxygen transport. It was found that the cell operating temperature, oxygen flow rate and membrane thickness all had significant influences on water crossover, but the influence of methanol concentration was negligibly small.     

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.     

Numerical study of the effect of the GDL structure on water crossover in a direct methanol fuel cell

A two-dimensional two-phase non-isothermal mass transport model is developed to numerically investigate the behavior of water transport through the membrane electrode assembly (MEA) of a direct methanol fuel cell. The model enables the visualization of the distribution of the liquid saturation through the MEA and the analysis of the distinct effects of the three water transport mechanisms: diffusion, convection and electro-osmotic drag, on the water-crossover flux through the membrane. A parametric study is then performed to examine the effects of the structure design of the gas diffusion layer (GDL) on water crossover. The results indicate that the flow-channel rib coverage on the GDL surface and the deformation of the GDL can cause an uneven distribution of the water-crossover flux along the in-plane direction, especially at higher current densities. It is also found that both the contact angle and the permeability of the cathode GDL can significantly influence the water-crossover flux. The water-crossover flux can be reduced by improving the hydrophobicity of the cathode GDL.     

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.     

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.     

Parametric study of the cathode and the role of liquid saturation on the performance of a polymer electrolyte membrane fuel cell—A numerical approach

A two-dimensional two-phase steady state model of the cathode of a polymer electrolyte membrane fuel cell (PEMFC) is developed using unsaturated flow theory (UFT). A gas flow field, a gas diffusion layer (GDL), a microporous layers (MPL), a finite catalyst layer (CL), and a polymer membrane constitute the model domain. The flow of liquid water in the cathode flow channel is assumed to take place in the form of a mist. The CL is modeled using flooded spherical agglomerate characterization. Liquid water is considered in all the porous layers. For liquid water transport in the membrane, electro-osmotic drag and back diffusion are considered to be the dominating mechanisms. The void fraction in the CL is expressed in terms of practically achievable design parameters such as platinum loading, Nafion loading, CL thickness, and fraction of platinum on carbon. A number of sensitivity studies are conducted with the developed model. The optimum operating temperature of the cell is found to be 80-85 °C. The optimum porosity of the GDL for this cell is in the range of 0.7-0.8. A study by varying the design parameters of the CL shows that the cell performs better with 0.3-0.35 mg cm⁻² of platinum and 25-30 wt% of ionomer loading at high current densities. The sensitivity study shows that a multi-variable optimization study can significantly improve the cell performance. Numerical simulations are performed to study the dependence of capillary pressure on liquid saturation using various correlations. The impact of the interface saturation on the cell performance is studied. Under certain operating conditions and for certain combination of materials in the GDL and CL, it is found that the presence of a MPL can deteriorate the performance especially at high current density.     

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.     

Effects of cathode gas diffusion layer design on polymer electrolyte membrane fuel cell water management and performance

The effects of a microporous layer (MPL) on performance and water management of polymer electrolyte fuel cells are investigated. The presence of an MPL on the cathode side is found to slightly improve performance, although the voltage gain is less significant than that obtained by wetter reactants. The effect of the MPL on water management depends on the cathode inlet-gas humidity. Differences in water crossover rate are insignificant for wet cathode feed (RH = 75%), while they are significant for dry feed (RH = 25%). A model based on transport resistance of the MPL is proposed to explain the experimental trends observed. Modeling results suggest that the presence of the MPL on the cathode side causes a reduction of the water flux from the cathode catalyst layer to the flow channels, effectively promoting water back diffusion through the membrane. Higher cathode humidity reduces the driving force for water transport from the electrode to the gas channels, also reducing the importance of the water transport resistance due to the presence of the MPL.     

Anode water removal and cathode gas diffusion layer flooding in a proton exchange membrane fuel cell

Anode water removal (AWR) is studied as a diagnostic tool to assess cathode gas diffusion layer (GDL) flooding in PEM fuel cells. This method uses a dry hydrogen stream to remove product water from the cathode, showing ideal fuel cell performance in the absence of GDL mass transfer limitations related to water. When cathode GDL flooding is limiting, the cell voltage increases as the hydrogen stoichiometry is increased. Several cathode GDLs were studied to determine the effect of microporous layer (MPL) and PTFE coating. The largest voltage gains occur with the use of cathode GDLs without an MPL since these GDLs are prone to higher liquid water saturation. Multiple GDLs are studied on the cathode side to exacerbate GDL flooding conditions to further confirm the mechanism of the AWR process. Increased temperature and lower cathode RH allow for greater overall water removal so the voltage improvement occurs faster, though this leads to quicker membrane dehydration.     

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.     

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.     

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.     

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.     

Experimental investigation of the role of a microporous layer on the water transport and performance of a PEM fuel cell

A highly reliable experimental system that consistently closed the overall water balance to within 5% was developed to study the role of a microporous layer (MPL), attached to carbon paper porous transport layer (PTL), on the water transport and performance of a standard 100 cm² active area PEM fuel cell. Various combinations of cells were built and tested with PTLs at the electrodes using either carbon fibre paper with a MPL (SGL 10BB) or carbon fibre paper without a MPL (SGL 10BA). The net water drag coefficient at three current densities (0.3, 0.5 and 0.7 A cm⁻²) for two combinations of anode/cathode relative humidity (60/100% and 100/60%) and stoichiometric ratios of H₂/air (1.4/3 and 1.4/2) was determined from water balance measurements. The addition of a MPL to the carbon fibre paper PTL at the cathode did not cause a statistically significant change to the overall drag coefficient although there was a significant improvement to the fuel cell performance and durability when a MPL was used at the cathode. The presence of a MPL on either electrode or on both electrodes also exhibited similar performance compared to when the MPL was placed at the cathode. These results indicate that the presence of MPL indeed improves the cell performance although it does not affect the net water drag coefficient. The correlation between cell performance and global water transport cannot be ascertained and warrants further experimental investigation.