Visualization of unstable water flow in a fuel cell gas diffusion layer

Modeling two-phase flow in proton exchange membrane (PEM) fuel cells is hampered by a lack of conceptual understanding of flow patterns in the gas diffusion layer (GDL). In this paper, pore-scale visualizations of water in different types of GDLs were used to improve current understanding of flow and transport phenomena in PEM fuel cells. Confocal microscopy was used to capture the real-time transport of water, and pressure micro-transducers were installed to measure water breakthrough pressures. Three types of fuel cell GDLs were examined: TO series (Toray Corp., Tokyo, Japan), SGL series (SGL Carbon Group, Wiesbaden, Germany), and MRC series (Mitsubishi Rayon Corp., Otake City, Japan). The visualizations and pressure measurements revealed that despite difference in “pore” structures in the three types of GDLs, water followed distinct flow paths spanning several pores with characteristics similar to the “column flow” phenomena observed previously in hydrophobic or coarse-grained hydrophilic soils. The results obtained from this study can aid in the construction of theories and models for optimizing water management in fuel cells.     

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.     

Uneven gas diffusion layer intrusion in gas channel arrays of proton exchange membrane fuel cell and its effects on flow distribution

Intrusion of the gas diffusion layer (GDL) into gas channels due to fuel cell compression has a major impact on the gas flow distribution, fuel cell performance and durability. In this work, the effect of compression resulting in GDL intrusion in individual parallel PEMFC channels is investigated. The intrusion is determined using two methods: an optical measurement in both the in-plane and through-plane directions of GDL, as well as an analytical fluid flow model based on individual channel flow rate measurements. The intrusion measurements and estimates obtained from these methods agree well with each other. An uneven distribution of GDL intrusion into individual parallel channels is observed. A non-uniform compression force distribution derived from the clamping bolts causes a higher intrusion in the end channels. The heterogeneous GDL structure and physical properties may also contribute to the uneven GDL intrusion. As a result of uneven intrusion distribution, severe flow maldistribution and increased pressure drop have been observed. The intrusion data can be further used to determine the mechanical properties of GDL materials. Using the finite element analysis software program ANSYS, the Young's modulus of the GDL from these measurements is estimated to be 30.9 MPa.     

Implications of non-uniform porosity distribution in gas diffusion layer on the performance of a high temperature PEM fuel cell

The present study discusses a detailed investigation on the implications of non-uniform porosity distribution in the gas diffusion layer (GDL) on the performance of proton exchange membrane fuel cell (PEMFC). A three-dimensional, single-phase, isothermal model of high-temperature PEMFC is employed to study the effect of non-uniform porosity distribution in GDL. The different porosity configurations with stepwise, sinusoidal, and logarithmic variation in porosity along the streamwise direction of GDL are considered. The numerical experiments are performed, keeping average porosity as constant in the GDL. The electrochemical characteristics such as the oxygen molar concentration, power density, current density, total power dissipation density, average diffusion coefficient, vorticity magnitude, and overpotential are studied for a range of porosity distributions. Furthermore, the variations of oxygen concentration, average diffusion coefficient, and vorticity magnitude are also discussed to showcase the influence of non-uniform porosity distribution. Our study reveals that the PEM fuel cell performance is the best when the porosity of the GDL decreases logarithmically in the streamwise direction. On the contrary, the performance deteriorates when the GDL porosity decreases sinusoidally. Also, it has been observed that the effects of non-uniform porosity distribution are more pronounced, especially at higher current densities. The outcomes of present investigation have potential utility in GDL fabrication and membrane assembly's sintering process for manufacturing high valued PEMFC products.     

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.     

Validity of two-phase polymer electrolyte membrane fuel cell models with respect to the gas diffusion layer

A dynamic two-phase model of a proton exchange membrane fuel cell with respect to the gas diffusion layer (GDL) is presented and compared with chronoamperometric experiments. Very good agreement between experiment and simulation is achieved for potential step voltammetry (PSV) and sine wave testing (SWT). Homogenized two-phase models can be categorized in unsaturated flow theory (UFT) and multiphase mixture (M²) models. Both model approaches use the continuum hypothesis as fundamental assumption. Cyclic voltammetry experiments show that there is a deterministic and a stochastic liquid transport mode depending on the fraction of hydrophilic pores of the GDL. ESEM imaging is used to investigate the morphology of the liquid water accumulation in the pores of two different media (unteflonated Toray-TGP-H-090 and hydrophobic Freudenberg H2315 I3). The morphology of the liquid water accumulation are related with the cell behavior. The results show that UFT and M² two-phase models are a valid approach for diffusion media with large fraction of hydrophilic pores such as unteflonated Toray-TGP-H paper. However, the use of the homgenized UFT and M² models appears to be invalid for GDLs with large fraction of hydrophobic pores that corresponds to a high average contact angle of the GDL.     

Effect of electrochemical aging on the interaction between gas diffusion layers and the flow field in a proton exchange membrane fuel cell

Gas diffusion layer (GDL) is a porous medium placed between the flow field and the catalyst layer in a proton exchange membrane fuel cell (PEMFC), and experiences electrochemical aging and mechanical stresses during usage. In the present work carbon cloth and carbon paper, two commonly used GDLs in PEMFC, are electrochemically aged in a simulated PEMFC environment. The results indicate that carbon paper is less prone to oxidation when compared to carbon cloth, which can be attributed to higher degree of graphitization of carbon fibers in the paper. However, carbon paper suffers greater loss of structural stability due to the adverse effects of aging on the fiber matrix interface. Increased weakening of paper when compared to cloth, after electrochemical aging, results in higher residual strain when subjected to cyclic compression and an increased intrusion of paper into the flow field channel when compared to cloth GDL.     

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.     

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.     

Platinum sonoelectrodeposition on glassy carbon and gas diffusion layer electrodes

The electrodeposition of Pt on glassy carbon (GC) and gas diffusion layer (GDL) surfaces in dilute chloroplatinic acid solutions (10 mM PtCl₄²⁻ in 0.5 M NaCl) was performed potentiodynamically in the absence and presence of ultrasound (20 kHz) at various ultrasonic powers (up to 6 W) respectively and at (313 ± 2) K. In our conditions, it was found that platinum electrodeposition is an irreversible process which requires a substantial overpotential to drive the formation of Pt nuclei on the GC and GDL surfaces; however, under sonication Pt electrodeposition becomes more facile due to lower concentration and nucleation overpotentials and overall currents are significantly increased compared to silent conditions. It was also observed that the specific electrochemical surface area (SECSA) was significantly affected for Pt/GC and Pt/GDL electrodes prepared in the presence of rotation (GC only) and under sonication compared to those prepared under silent conditions. This finding was explained to be due to both larger and agglomerated platinum nanoparticles formed on the GC and GDL surface caused by forced convection. It was also found that ultrasound produced larger Pt nanoparticles on GC electrodes than those on GDL electrodes.     

Increasing the performance of gas diffusion layer by insertion of small hydrophilic layer in proton-exchange membrane fuel cells

The present study applied Lattice Boltzmann method (LBM) for examining the transport of liquid water in a GDL carbonic paper of polymer electrolyte membrane (PEM) fuel cells. The stochastic method is used for GDL carbonic paper reconstruction. In order to study the behavior of liquid water, different simulations are carried out on the reconstructed GDL. While removing from the GDL of a PEM fuel cell, the dynamics of liquid water is simulated by LBM in this study. The effects that the wettability of GDL imposes on the removal process and liquid water distribution are investigated. In addition, the dynamic behaviors and the saturation process of the liquid water in GDL in a steady state and a transient mode are also explored. The effects of surface wettability on the effective clusters in GDL, merging of different clusters and the loops developed by the fingers are investigated. Moreover, the effects of mixed wettability on the liquid water dynamic behavior and liquid water saturation within the GDL are studied in detail. The results show that the best location for insertion of the hydrophilic layer inside the GDL is near the GDL-GC interface. In this case, the time required for liquid water to reach the GDL/GC interface is reduced about 17% than purely hydrophobic GDL. A decrease of 18.7% in the steady-state saturation level is also observed by insertion of hydrophilic layer; therefore, use of hydrophilic layer near GDL-GC interface is more effective than increasing the contact angle of GDL-fibers. Different validation studies are also reported to show the accuracy of the model.     

Numerical study on the compression effect of gas diffusion layer on PEMFC performance

A numerical method is developed to study the effect of the compression deformation of the gas diffusion layer (GDL) on the performance of the proton exchange membrane fuel cell (PEMFC). The GDL compression deformation, caused by the clamping force, plays an important role in controlling the performance of PEMFC since the compression deformation affects the contact resistance, the GDL porosity distribution, and the cross-section area of the gas channel. In the present paper, finite element method (FEM) is used to first analyze the ohmic contact resistance between the bipolar plate and the GDL, the GDL deformation, and the GDL porosity distribution. Then, finite volume method is used to analyze the transport of the reactants and products. We investigate the effects of the GDL compression deformation, the ohmic contact resistivity, the air relative humidity, and the thickness of the catalyst layer (CL) on the performance of the PEMFC. The numerical results show that the fuel cell performance decreases with increasing compression deformation if the contact resistance is negligible, but an optimal compression deformation exists if the contact resistance is considerable.     

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.     

Embedded flexible micro-sensors in MEA for measuring temperature and humidity in a micro-fuel cell

In this investigation, flexible sensors embedded in a membrane electrode assembly (MEA) are fabricated to measure the temperature and humidity of a micro-fuel cell. Fuel cell performance was determined by the temperature and humidity of the MEA. Restrictions on the sensor volume are such that in previous investigations the temperature and the humidity of the MEA have been measured only at the fuel inlet and outlet. Hence, flexible micro-thin film sensors were fabricated using micro-electro-mechanical systems (MEMS) fabrication technology.     

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.     

Surface roughness dominated wettability of carbon fiber in gas diffusion layer materials revealed by molecular dynamics simulations

High water contact angle in carbon fiber can facilitate water removal ability of gas diffusion layer (GDL) in proton exchange membrane fuel cells (PEMFCs). Water contact angle is intensively dependent on the surface hydrophobicity of carbon fiber in GDL. In this study, the hydrophobicity of commercial GDL is enhanced through the immersion and hydrothermal methods. The porosity decreases slightly while the surface roughness and surface topology diversity increase significantly in hydrothermal GDL compared with commercial reference and immersion GDL samples. The molecular dynamics simulations show that the water contact angle increases significantly with the increasing surface roughness but varies slightly with different surface topology, indicating that the water contact angle is dominated by the surface roughness. This study's findings are expected to offer an approach that can effectively enhance the water removal capacity by tailoring the surface roughness of carbon fibers in GDL materials.     

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.     

Effect of deformation on electrical properties of carbon fibers used in gas diffusion layer of proton exchange membrane fuel cells

In proton exchange membrane fuel cells, the stacks of anode, cathode, and membrane layers including gas diffusion layer (GDL) are held together by a compressive force applied through a bipolar plate. In this work, we studied the electrical properties of a carbon fiber of a GDL under deformation using four-point measurement methods inside a scanning electron microscope (SEM). We found out that through bending deformation the electrical resistivity of carbon fibers will be reduced. The drop in resistance during deformation may be the result of increasing conduction channels in the carbon fiber and parallel transport through them. Our finding offers a new insight on the effect of deformation on tuning the electrical properties of GDL materials.