The use of a novel water porosimeter to predict the water handling behaviour of gas diffusion media used in polymer electrolyte fuel cells

A novel water porosimeter and its use in determining the capillarity of gas diffusion layers are described. It is found that, in accordance with the Washburn equation, the pressure required to force water into the gas diffusion layer depends on the cosine of the contact angle of water with the surface of the pore. Negative pressure is required to withdraw water from the gas diffusion layer, even when the surface is hydrophobic. The negative pressure required is found to be independent of surface contact angle. It is shown that the performance of gas diffusion layers in an operating fuel cell can be qualitatively predicted from the capillary pressure curves obtained. The advantages of the use of water porosimetry over the use of either mercury porosimetry or porosimetry using wetting fluids are discussed.     

In situ accelerated degradation of gas diffusion layer in proton exchange membrane fuel cell: Part I: Effect of elevated temperature and flow rate

Past studies have shown that both the substrate and microporous layer of the gas diffusion layer (GDL) significantly affect water balance and performance of a proton exchange membrane (PEM) fuel cell. However, little effort has been made to investigate the importance of GDL properties on the durability of PEM fuel cells. In this study, the in situ degradation behaviour of a commercial GDL carbon fiber paper with MPL was investigated under a combination of elevated temperature and elevated flow rate conditions. To avoid the possible impact of the catalyst layer during degradation test, different barriers without catalyst were utilized individually to isolate the anode and cathode GDLs. Three different barriers were evaluated on their ability to isolate GDL degradation and their similarity to a fuel cell environment, and finally a novel Nafion/MPL/polyimide barrier was chosen. Characterization of the degraded GDL samples was conducted through the use of various diagnostic methods, including through-plane electrical resistivity measurements, mercury porosimetry, relative humidity sensitivity, and single-cell performance curves. Noticeable decreases in electrical resistivity and the hydrophobic properties were detected for the degraded GDL samples. The experimental results suggested that material loss plays an important role in GDL degradation mechanisms, while excessive mechanical stress prior to degradation weakens the GDL structure and changes its physical property, which consequently accelerates the material loss of the GDL during aging.     

Understanding the role of cathode structure and property on water management and electrochemical performance of a PEM fuel cell

Water management is vital for the successful development of PEM fuel cells. Water should be carefully balanced within a PEM fuel cell to meet the conflicting requirements of membrane hydration and cathode anti-flooding. In order to understand the key factors that can improve water management and fuel cell performance, the cathodes with different structures and properties are prepared and tested in this study. The experimental results show that even though no micro-porous layer (MPL) is placed between the cathode catalyst layer (CCL) and macro-porous substrate (MaPS), a hydrophobic CCL is effective to prevent cathode flooding and keep membrane hydrated. The impedance study and the analysis of the polarization curves indicate that the optimized hydrophobic micro-porous structure in the MPL or the hydrophobic CCL could be mainly responsible for the improved water management in PEM fuel cells, which functions as a watershed to provide wicking of liquid water to the MaPS and increase the membrane hydration by enhancing the back-diffusion of water from the cathode side to the anode side through the membrane.     

Impact of PTFE distribution across the GDL on the water droplet removal from a PEM fuel cell electrode containing binder

By progress of the new generation of electrical powertrains and reducing of the fossil fuel resources, vehicle industry becomes more interested in utilizing proton exchange membrane (PEM) fuel cells. However, practical utilization of them is faced with some challenges including liquid water accumulation in the porous electrodes. The common belief for mitigating this issue is the treatment of electrodes' gas diffusion layers (such as carbon papers consisting of carbon fibers and binder for binding fibers) with a highly hydrophobic material such as poly-tetra-fluoro-ethylene (PTFE). In the current investigation, 3D stochastic reconstructions and 3D lattice Boltzmann simulations are employed to discover the impact of PTFE distribution as well as the role of binder content on the removal process of a water droplet from a PEM fuel cell electrode for the first time. Nine different simulations with three dissimilar PTFE distributions and three dissimilar binder contents are implemented. The results demonstrate that the PTFE distribution and the existence of binder can greatly affect the removal efficiency of water droplet from gas diffusion layer. Unexpectedly, for higher binder contents, the uniform distribution of PTFE is less effective. Besides, for a specific PTFE distribution adding binder can effectively hinder the removal process of droplet.     

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.     

Effects of electrode wettabilities on liquid water behaviours in PEM fuel cell cathode

Liquid water transport is one of the key challenges for water management in a proton exchange membrane (PEM) fuel cell. Investigation of the air–water flow patterns inside fuel cell gas flow channels with gas diffusion layer (GDL) would provide valuable information that could be used in fuel cell design and optimization. This paper presents numerical investigations of air–water flow across an innovative GDL with catalyst layer and serpentine channel on PEM fuel cell cathode by use of a commercial Computational Fluid Dynamics (CFD) software package FLUENT. Different static contact angles (hydrophilic or hydrophobic) were applied to the electrode (GDL and catalyst layer). The results showed that different wettabilities of cathode electrode could affect liquid water flow patterns significantly, thus influencing on the performance of PEM fuel cells. The detailed flow patterns of liquid water were shown, several gas flow problems were observed, and some useful suggestions were given through investigating the flow patterns.     

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.     

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.     

Effect of the micro porous layer design on the dynamic performance of a proton exchange membrane fuel cell

The mass transport characteristics of a gas diffusion layer (GDL) predominantly affect the performance of a proton exchange membrane (PEM) fuel cell. However, studies examining the transient response related to the GDL are insufficient, although the dynamic behavior of a PEM fuel cell is an important issue. In this study, the effects of the design of a micro porous layer (MPL) on the transient response of a PEM fuel cell are investigated. The MPL slurry density and multiple functional layers are treated as the variable design parameter. The results show that the transient response is determined by the capillary pressure gradient through the GDL. The trade-off relation for the PEM fuel cell performance under low and high humidity conditions due to the hydrophobic GDL is mitigated by designing a reverse capillary pressure gradient in the MPL.     

Effects of ratio variation in substrate and micro porous layer penetration on polymer exchange membrane fuel cell performance

A gas diffusion layer (GDL) facilitates the diffusion of reactant gas and the discharge of the generated water. The GDL performs various functions, such as conducting heat and electrons generated by electrochemical reactions and providing mechanical support for the catalyst layer. In this study, the effects of ratio variation in the substrate and microporous layer (MPL) penetration region on the proton exchange membrane fuel cell (PEMFC) performance were investigated. Furthermore, the reasons for these performance tendencies are explained based on the thermogravimetric analysis, contact angle, scanning electron microscopy, mercury porosimetry, electrical resistance, electrochemical impedance spectroscopy, and capillary pressure gradient. The experimental results indicate that the MPL penetration ratio within 15–20% of the total GDL thickness and the combined ratio of the MPL and MPL penetration within 35–40% is the best for the overall PEMFC performance. In addition, when the substrate ratio is excessively low, water flooding substantially occurs in the substrate, and this accumulated water functions as a back pressure, causing severe capillary condensation in the MPL penetration region and thus depriving the supply of the reactant gas.     

Water permeation analysis on gas diffusion layers of proton exchange membrane fuel cells for Teflon-coating annotation

This study presents an analysis of water permeation of a polytetrafluoroethylene (PTFE)-coated gas diffusion layer (GDL) to determine the influence of hydrophobic treatment on the GDL for diagnosis of water flooding. It is found that the behaviour of water drainage is controlled by the pore configuration instead of the hydrophobicity in GDL. Better water drainage is achieved by the action of the Teflon coating in modulating the GDL pore configuration to give both a larger average pore size and a wider distribution of pore size. The results show that water penetration through the GDL must overcome a threshold surface tension defined by the largest pore range. A 30 wt.% PTFE coating of a GDL is shown to generate a satisfactory pore configuration, explaining the improved cell polarization performance with a lower driven pressure (∼1.91 kPa) and a higher rate of water drainage.     

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%.     

A study on polarization hysteresis in PEM fuel cells by galvanostatic step sweep

In this work, the galvanostatic step sweep was used to investigate the hysteresis phenomenon found in the polarization measurements in proton exchange membrane (PEM) fuel cells. Three different diffusion media including Toray, E-TEK and SGL as well as the PTFE content from 10% to 30% in the diffusion media were carefully examined. Based on these results along with the comparison between results using dry and fully humidified air, the reason for the formation of such hysteresis in polarization curves was clarified. The generation and storage of liquid water was correlated with the materials of diffusion layers as well as the cross point in polarization curves during the forward and backward sweeps. Experimental results on the effect of operation conditions, i.e. flow rates, cell temperature and the time at each current density were also explored to illuminate the transient behavior within fuel cells behind such hysteresis phenomenon.     

Membrane electrode assemblies for unitised regenerative polymer electrolyte fuel cells

Membrane electrode assemblies for regenerative polymer electrolyte fuel cells were made by hot pressing and sputtering. The different MEAs are examined in fuel cell and water electrolysis mode at different pressure and temperature conditions. Polarisation curves and ac impedance spectra are used to investigate the influence of the changes in coating technique. The hydrogen gas permeation through the membrane is determined by analysing the produced oxygen in electrolysis mode. The analysis shows, that better performances in both process directions can be achieved with an additional layer of sputtered platinum on the oxygen electrode. Thus, the electrochemical round-trip efficiency can be improved by more than 4%. Treating the oxygen electrode with PTFE solution shows better performance in fuel cell and less performance in electrolysis mode. The increase of the round-trip efficiency is negligible. A layer sputtered directly on the membrane shows good impermeability, and hence results in high voltages at low current densities. The mass transportation is apparently constricted. The gas diffusion layer on the oxygen electrode, in this case a titanium foam, leads to flooding of the cell in fuel cell mode. Stable operation is achieved after pretreatment of the GDL with a PTFE solution.     

PEMFC catalyst layer modification with the addition of different amounts of PDMS polymer in order to improve water management

The balance between preventing water flooding and adequate humidification of the membrane will provide a significant contribution to proton exchange membrane (PEM) fuel cell performance. For this purpose, polydimethylsiloxane (PDMS), a hydrophobic polymer, was added to the catalyst layer of the fuel cell at different amounts including 5, 10, and 20 wt%. The performances of the fuel cells including PDMS were compared with the commercial catalyst. Morphological changes of the gas diffusion electrodes (GDEs) were confirmed by using scanning electron microscopy (SEM). Fourier transformation infrared spectroscopy (FTIR) was used to determine the functional groups and contact angle measurements were used to determine the hydrophobic characteristics. Cyclic voltammetry (CV), impedance, and oxygen reduction reaction (ORR) analysis were performed for electrochemical characterization and degradation behaviors. In situ PEM fuel cell tests were performed in order to define the best catalyst ink combination that include PDMS. The results of the cyclic voltammograms proved that the electrochemical surface area (ECSA) increased with the increasing amount of PDMS. The highest ECSA of 53.84 m² g^?1 was calculated for catalyst ink with 20‐wt% PDMS. The lowest ECSA loss after aging was observed in the catalyst ink with 10‐wt% PDMS. As a result, the catalyst layer having 10‐wt% PDMS showed the best polymer electrolyte membrane fuel cells (PEMFC) performance.     

Effect of liquid water on transport properties of the gas diffusion layer of polymer electrolyte membrane fuel cells

Water management in polymer electrolyte membrane (PEM) fuel cells is of importance due to its impact on the performance, durability and ultimately the cost of the cell. In the gas diffusion layer (GDL), liquid water has a direct effect on species and heat transport. The amount of liquid water in the GDL affects the relative permeability and capillary pressure, which govern the convective and diffusive transport of liquid water. Liquid water acts as a barrier to the diffusion of gases through the void region and facilitates in heat transfer. In this study, the full morphology model was used in order to investigate the effects of liquid water presence on the transport properties of the carbon paper GDL and examine the applicability of using various laws to estimate the transport properties in the presence of liquid water. The numerical results were compared against published experimental data. Further, the method of standard porosimetry was used to experimentally measure the effect of Teflon treatment on the capillary pressure of carbon paper. It was found that the addition of PTFE to the GDL results in the increase of capillary pressure; however, further increases to the PTFE loading did not result in additional changes to the capillary pressure.     

Computational modeling and experimental verification of cathode catalyst layer on PEM fuel cells

Fuel cell systems are environmentally friendly energy converters that directly transform the chemical energy of the fuel to electricity. The proton exchange membrane (PEM) fuel cells are one of the most common type of fuel cells since they deliver high power density and are lighter and smaller when compared to the other cells. However, commercialization of the PEM fuel cells is challenging due to the high cost of its components. In addition to high catalyst costs, the problem of poor water management is also a vital issue that needs to be overcome. While the gas diffusion layer of a fuel cell is essential for removing the by-product water, the Nafion solution contained in the catalyst layer has hydrophobic properties and is crucial for both preventing the water accumulation and increasing the effectiveness of the fuel cell. In this study, the effects of Carbon:Nafion ratio on the reduction potential was investigated. The cyclic voltammograms (CV) was produced for each ratio, and it was shown that the CVs exhibit characteristics of hydrogen adsorption/desorption peaks. All the linear sweep voltammogram (LSV) curves revealed well distinguished regions of kinetic, mixed and diffusion limited reaction rate. As a result, it was observed that the ratio of 1:5 resulted higher reduction potential compared to 1:3 and 1:7. Finally, a mathematical model was purposed, in which related the rotation rate and platinum coating with the current density, in order to gain insight about the responses of the fuel cell system. The constructed model is tested and validated experimentally for various parameters that are present in the system, and it may be utilized to determine oxygen reaction activities of the catalysts without performing any unnecessary electrochemical tests.     

Experimental study of the effect of dissolution on the gas diffusion layer in polymer electrolyte membrane fuel cells

The gas diffusion layer (GDL) is important for maintaining the performance of polymer electrolyte membrane (PEM) fuel cells, as its main function is to provide the cells with a path for fuel and water. In this study, the mechanical degradation process of the GDL was investigated using a leaching test to observe the effect of water dissolution. The amount of GDL degradation was measured using various methods, such as static contact angle measurements and scanning electron microscopy. After 2000 h of testing, the GDL showed structural damage and a loss of hydrophobicity. The carbon-paper-type GDL showed weaker characteristics than the carbon-felt-type GDL after dissolution because of the structural differences, and the fuel cell performance of the leached GDL showed a greater voltage drop than that of the fresh GDL. Contrary to what is generally believed, the hydrophobicity loss of GDL was not caused by the decomposition of polytetrafluoroethylene (PTFE).     

Application of Open Pore Cellular Foam for air breathing PEM fuel cell

Open Pore Cellular Foam (OPCF) has received increased attention for use in Proton Exchange Membrane (PEM) fuel cells as a flow plate due to some advantages offered by the material, including better gas flow, lower pressure drop and low electrical resistance.In the present study, a novel design for an air-breathing PEM (ABPEM) fuel cell, which allows air convection from the surrounding atmosphere, using OPCF as a flow distributor has been developed. The developed fuel cell has been compared with one that uses a normal serpentine flow plate, demonstrating better performance.A comparative analysis of the performance of an ABPEM and pressurised air PEM (PAPEM) fuel cell is conducted and poor water management behaviour was observed for the ABPEM design.Thereafter, a PTFE coating has been applied to the OPCF with contact angle and electrochemical polarisation tests conducted to assess the capability of the coating to enhance the hydrophobicity and corrosion protection of metallic OPCF in the PEM fuel cell environment. The results showed that the ABPEM fuel cell with PTFE coated OPCF had a better performance than that with uncoated OPCF.Finally, OPCF was employed to build an ABPEM fuel cell stack where the performance, advantages and limitations of this stack are discussed in this paper.     

Measurement of the water transport rate in a proton exchange membrane fuel cell and the influence of the gas diffusion layer

Water management in a PEM fuel cell significantly affects the fuel cell performance and durability. The gas diffusion layer (GDL) of a PEM fuel cell plays a critical role in the water management process. In this short communication, we report a simple method to measure the water transport rate across the GDL. Water rejection rates across a GDL at different cathode air-flow rates were measured. Based on the measurement results, the fuel cell operating conditions, such as current density, temperature, air stoichiometry and relative humidity, corresponding to membrane drying and flooding conditions were identified for the particular GDL used. This method can help researchers develop GDLs for a particular fuel cell design with specific operating conditions and optimize the operation conditions for the given PEM fuel cell components.