Effects of gas-diffusion layer properties on the performance of the cathode for high-temperature polymer electrolyte membrane fuel cell

The role of the gas-diffusion layer (GDL) in high-temperature polymer electrolyte fuel cell (HT-PEMFC) differs from that in low-temperature PEMFC GDL due to operating conditions and environment. Determining the GDL's structural parameters that affect its transport properties, and how these properties impact HT-PEMFC performance was urgently required. Four commercial GDLs were employed in HT-PEMFC cathode's GDE and was examined using X-μCT, mercury intrusion porosimetry, and an optical microscope to analyze structural parameters and characteristics. Fractal theory was applied to comprehend the gas transmission property of GDL, and the validity of the theory was confirmed through ex-situ through-plane gas permeability measurement. The analysis indicated that the porosity of GDL influenced by the crack region of the MPL has more impact on the GDL's gas transmission than its thickness. After that, we established a correlation between HT-PEMFC cathode performance and GDL porosity and theoretical gas transmission properties using R² coefficient of determination.     

Enhancement of proton exchange membrane fuel cell performance by titanium-coated anode gas diffusion layer

Titanium was coated onto an anode gas diffusion layer (GDL) by direct current sputtering to improve the performance and durability of a proton exchange membrane fuel cell (PEMFC). Scanning electron microscopy (SEM) images showed that the GDLs were thoroughly coated with titanium, which showed angular protrusion. Single-cell performance of the PEMFCs with titanium-coated GDLs as anodes was investigated at operating temperatures of 25 °C, 45 °C, and 65 °C. Cell performances of all membrane electrode assemblies (MEAs) with titanium-coated GDLs were superior to that of the MEA without titanium coating. The MEA with titanium-coated GDL, with 10 min sputtering time, demonstrated the best performance at 25 °C, 45 °C, and 65 °C with corresponding power densities 58.26%, 32.10%, and 37.45% higher than that of MEA without titanium coating.     

Effect of cyclic compression on structure and properties of a Gas Diffusion Layer used in PEM fuel cells

Proton Exchange Membrane Fuel Cell (PEMFC) components are known to deteriorate during use, in time scales much shorter than that required for commercially successful deployment of this technology. Therefore, servicing operations such as identifying and replacing poorly performing components will likely be required to extend the operational lifetime of PEMFC stacks. During such servicing operations fuel cell components are subjected to cyclic compression and expansion due to opening and rebuilding of the fuel cell stack. This cyclic compression may further contribute to the deterioration of PEMFC components. There are several reports in the literature showing the effect of static compression on change in GDL properties, however the present work focuses on the effect of cyclic compression on GDL properties, an aspect that has not been reported in detail elsewhere. This paper focuses on the impact of cyclic compression on the Gas Diffusion Layer (GDL). The results indicate that cyclic compression causes significant and irreversible changes to the structure and properties of the GDL such as surface morphology, surface roughness, pore size, void fraction, thickness, electrical resistance, contact angle, water uptake and in-plane permeability. The implications of these results are considered.     

Reduced graphene oxide–coated anode gas diffusion layer for performance enhancement of proton‐exchange membrane fuel cell

In this study, we produced reduced graphene oxide (RGO) by reduction of graphene oxide (GO) in Teflon‐lined autoclave, maintained at 100°C for 12 hours, and coated on the anode gas diffusion layer (GDL) of a proton‐exchange membrane fuel cell (PEMFC) to improve the cell performance. Fourier transform infrared spectroscopy and X‐ray photoelectron spectroscopy analysis showed the presence of residual oxygen‐containing functional groups in RGO. Field‐emission scanning electron microscopy images revealed the uniform and adequate coating of the GDLs with RGO. The wettability of RGO‐coated GDL was determined by the sessile drop method and has optimum contact angle 117°. The power densities for the membrane electrode assembly (MEA) with RGO coated on the anode GDL were 30.92%, 41%, and 36.20% higher than those for the MEA without the RGO coating at anode gas humidified temperatures of 25°C, 45°C, and 65°C, respectively.     

Accelerated stress testing of PUREBLACK® carbon-based gas diffusion layers with pore forming agent for proton exchange membrane fuel cells

Gas diffusion layer (GDL) configurations containing PUREBLACK® and VULCAN® carbons with 30 wt % polyethylene glycol as pore forming agent are evaluated under two ex-situ methods of accelerated stress testing (AST), in water and hydrogen peroxide (30%), for 1000 and 24 h, respectively. The samples are characterized via contact angle, scanning electron microscopy (SEM), porosity and pore size distribution and the fuel cell performance and durability are also evaluated, before and after the ASTs. Contact angle and SEM demonstrate extensive degradation impact on VULCAN® carbon, especially in hydrogen peroxide, with cracked surface due to carbon corrosion and wash-off, complete hydrophobicity loss, along with porosity increase. The fuel cell performance is evaluated at 60 and 100% RH at 70 °C, using O₂ and air as oxidants, and the degraded VULCAN® carbon-based GDLs after the ASTs, result in significant performance loss and durability in air (~20% after 50 h) test shows non uniform gas distribution to the catalyst layer after ~30 h of continuous operation under constant current density 600 mA.cm⁻², especially under high RH conditions. On the other hand, PUREBLACK®-based GDLs demonstrate superior durability in air (~12% after 50 h), possibly attributed to its graphitized carbon structure, as evident from the stable durability for 50 h.     

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.     

Utilization of 3D printed carbon gas diffusion layers in polymer electrolyte membrane fuel cells

3D printing and carbonisation is used to produce designed gas diffusion layer materials for polymer electrolyte membrane fuel cells (PEMFC). Using a desktop UV 3D printer, designed porous microstructures are printed with micro and macro-scale features. Successful improvement of the pyrolysis process maintains the structural accuracy during carbonisation, reducing the material to electrically conductive carbon. The size of the material allows for testing in a lab scale fuel cell with 1.5 × 1.5 cm electrode size, which shows lower but interesting electrochemical performance (power density of 205 mW cm^?2). Challenges associated with integration of a 3D printed structure into a membrane electrode assembly are highlighted, including the low open circuit voltage caused by large amounts of membrane deformation and subsequent hydrogen crossover. This study shows that it is possible to design and manufacture a gas diffusion layer for fuel cells. Numerical simulation on the new GDL structure shows that advective-diffusive transport of oxygen in the 3D printed design is superior to conventional carbon paper. This study serves as the first attempt to implement 3D printed microstructures as GDL into PEMFC.     

Evaluating Breakthrough Pressure in Gas Diffusion Layers of Proton Exchange Membrane Fuel Cells

This paper studied the breakthrough pressure for liquid water to penetrate the gas diffusion layer(GDL) of a proton exchange membrane fuel cell(PEMFC).An ex-situ testing was conducted on a transparent test cell to visualize the water droplet formation and detachment on the surface of different types of GDLs through a CCD camera.The breakthrough pressure,at which the liquid water penetrates the GDL and starts to form a droplet,was measured.The breakthrough pressure was found to be different for the GDLs with...     

Study on the performance of a proton exchange membrane fuel cell related to the structure design of a gas diffusion layer substrate

Although characteristics of the gas diffusion layer (GDL) affect the performance of a proton exchange membrane fuel cell (PEMFC), mass transfer mechanisms inside the GDL and the performance of the PEMFC have not been directly correlated. To determine the design parameters of the GDL, the effects of substrate design of the GDL on performance of a PEMFC are investigated. By adding an active carbon fiber (ACF), which has a high surface area, the substrate is designed to have a different pore size structure. The results show that steady-state and transient responses are determined by capillary pressure gradient characteristics of the GDL made by pore size distribution of the substrate. The small macro-pore functions as water-retaining passage and the large macro-pore functions as water-removal passage. It is concluded that both small and large macro-pore must be present on the substrate to facilitate its function in a wide range of operating conditions.     

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.     

Measurement of flow maldistribution in parallel channels and its application to ex-situ and in-situ experiments in PEMFC water management studies

Uniform flow distribution is critical to obtaining high performance in many heat and mass transfer devices. It also plays an important role in the effective operation of a proton exchange membrane fuel cell (PEMFC). Presently there are a few theoretically based models available for predicting flow distribution in individual fuel cell channels and across fuel cell stacks in PEMFCs, but little or no experimental data has been published on the actual flow rates measured in individual channels. This is mainly because of the lack of experimental techniques available to measure the instantaneous flow rates in parallel channels. In this work, a novel technique based on the entrance region pressure drop measurements is presented for monitoring fluid flow maldistribution in individual channels. The method is validated using liquid water flow in a test section with four tubes in parallel, and then applied to assess the air flow maldistribution in PEMFCs using (a) an ex-situ experimental setup simulating the two-phase flow in parallel channels, and (b) an in-situ experimental setup with an operating fuel cell. While an almost uniform air distribution is obtained for the parallel channels with an impermeable backing (plastic sheet), severe maldistribution is observed for the same channels with porous GDL backing. The maldistribution caused by the water blockage in an ex-situ test setup is further investigated and the results are verified by the high-speed images of the two-phase flow in channels. The technique has also been applied in an in-situ experimental setup to obtain the flow maldistribution under electrochemical reaction conditions in the presence of two-phase flow in the cathode side gas channels.     

Enhanced low-humidity performance in a proton exchange membrane fuel cell by developing a novel hydrophilic gas diffusion layer

An ultrathin layer of hydrophilic titanium dioxide (TiO₂) is coated on the gas diffusion layer (GDL) to enhance the performance of a proton exchange membrane fuel cell (PEMFC) at low relative humidity (RH) and high cell temperature. Both of the modified and unmodified GDLs are characterized using contact angles, and the cell performance is evaluated at various RHs and cell temperatures. It is found that the modified GDL, which contains a hydrophilic TiO₂ layer between the microporous layer (MPL) and the gas diffusion-backing layer (GDBL), exhibits better self-humidification performance than a conventional GDL without the TiO₂ layer. At 12% RH and 65 °C cell temperature, the current density is 1190 mA cm⁻² at 0.6 V, and it maintains 95.8% of its initial performance after 50 h of continuous testing. The conventional GDL, however, exhibits 55.7% (580 mA cm⁻²) of its initial performance (1040 mA cm⁻²) within 12 h of testing. The coated hydrophilic TiO₂ layer acts as a mini humidifier retaining sufficient moisture for a PEMFC to function at low humidity conditions.     

Degradations in porous components of a proton exchange membrane fuel cell under freeze-thaw cycles: Morphology and microstructure effects

In this study, porous components of a proton exchange membrane (PEM) fuel cell, i.e., single-layer gas diffusion layer (GDL, carbon paper), double-layer GDL (microporous layer (MPL) deposited carbon papers), and catalyzed electrodes, are subjected to 60 repetitive freeze-thaw cycles between −40 °C and 30 °C under water-submerged conditions; and their morphological and microstructural characteristics are investigated at each 15 cycles and compared with those of virgin materials. The results indicate that consecutive cycling of temperature causes different degradation patterns in different components. The single-layer GDL shows a unique degradation mechanism, in which macro-scale pores volumetrically expand, and relatively small-scale hollows and cracks form on the polymeric binder and carbon fiber interfaces, respectively. For the double-layer GDL, large-scale surface cracks form on the MPL surface after 15 cycles, and the morphology and microstructure degradation gains momentum with the formation of these cracks, and upon completion of 30 cycles, large-scale carbon/hydrophobic agent flakes start to detach from the surface. For the catalyzed electrodes, due to their inherently cracked surface, the catalyst layers (CLs) degrade first through expansion of the cracks in the in- and through-plane directions, and then through swelling and agglomeration of the ionomer; and combination of these two patterns triggers detachment of large CL flakes from the surface, negatively affecting the microstructure.     

Numerical study for in-plane gradient effects of cathode gas diffusion layer on PEMFC under low humidity condition

A numerical study about in-plane porosity and contact angle gradient effects of cathode gas diffusion layer (GDL) on polymer electrolyte membrane fuel cell (PEMFC) under low humidity condition below 50% relative humidity is performed in this work. Firstly, a numerical model for a fuel cell is developed, which considers mass transfer, electrochemical reaction, and water saturation in cathode GDL. For water saturation in cathode GDL, porosity and contact angle of GDL are also considered in developing the model. Secondly, current density distribution in PEMFC with uniform cathode GDL is scrutinized to design the gradient cathode GDL. Finally, current density distributions in PEMFC with gradient cathode GDL and uniform cathode GDL are compared. At the gas inlet side, the current density is higher in GDL with a gradient than GDL with high porosity and large contact angle. At the outlet side, the current density is higher in GDL with a gradient than GDL with low porosity and small contact angle. As a result, gradient cathode GDL increases the maximum power by 9% than GDL with low porosity and small contact angle. Moreover, gradient cathode GDL uniformizes the current density distribution by 4% than GDL with high porosity and large contact angle.     

Effect of hydrophilic pipe structure of proton exchange membrane fuel cell on water removal from the gas diffusion layer surface

In a proton exchange membrane fuel cell (PEMFC), effective GDL surface water elimination is significant to water management. This paper used the volume-of-fluid method (VOF) method to carry out simulation research on transferring liquid water in the flow channel with a hydrophilic pipe. The findings indicated that compared with a straight channel, a hydrophilic pipe structure could effectively remove water from the gas diffusion surface (GDL) and reduce the surface water coverage of the GDL. With the increase in the diameter and height of the pipe structure, the GDL surface's water coverage first increased and then decreased, and it was less with the pipe structure than with the direct flow channel. The removal rate of water on the GDL surface was accelerated. The spacing of hydrophilic pipes has a significant impact on the transportation of water. As the spacing increases, the removal rate of water on the GDL surface slowed. A hydrophilic pipe structure with a diameter of 75 μm, a height of 400 μm, and spacing of 300 μm has good water removal performance on the GDL surface. This research work proposes a new internal structure design of the flow channel, which has specific implications for removing water on the GDL surface.     

Water management in a novel proton exchange membrane fuel cell stack with moisture coil cooling

Water flooding causes severe degradation of the performance and lifetime of proton exchange membrane fuel cell (PEMFC). In this study, a novel PEMFC stack with in-built moisture coil cooling was designed and the effects of moisture coil cooling on water management in the new PEMFC stack under various operating conditions were investigated. The result showed that the performance of the PEMFC stack was significantly improved due to the moisture condensation under high current density, high operating temperature, high relative humidity and high operating pressure. The output power was increases by 21.62% (525.71 W) at 1600·mA cm⁻² while the increased parasitic power was no more than 35W. Moreover, degradation of the cathode catalyst layer after 100 h operation was also reduced by using moisture coil cooling. Compared with the situation without moisture condensation, the maximum decay rate of the cathode catalyst layer thickness after 100 h operation was reduced by 13.01%. Accordingly, the novel design is valuable and can be widely used in the future design of PEMFC.     

CF4 plasma treatment for preparing gas diffusion layers in membrane electrode assemblies

The hydrophobic properties of carbon fibers improved by a CF₄ plasma treatment were used to fabricate gas diffusion layers (GDLs) for use in proton exchange membrane fuel cells. The water contact angle of the CF₄ plasma treated GDL was measured as 132.8 ± 0.2° at 45 °C and very few surface gas diffusion pores were either sealed or blocked by the excessive hydrophobic material residuals. Polarization measurements verified that the CF₄ plasma treated modules can indeed enhance fuel cell performance, compared to the membrane electrode assemblies (MEAs) with a non-wet-proofed GDL, 10 wt% PTFE dip-coated GDL, and commercially available GDL (10 wt% PTFE).     

Investigation of sintered stainless steel fiber felt as gas diffusion layer in proton exchange membrane fuel cells

The feasibility of using sintered stainless steel fiber felt (SSSFF) as gas diffusion layer (GDL) in proton exchange membrane fuel cells (PEMFCs) is evaluated in this study. The SSSFF is coated with an amorphous carbon (a-C) film by closed field unbalanced magnetron sputter ion plating (CFUBMSIP) to enhance the corrosion resistance and reduce the contact resistance. The characteristics of treated SSSFF, including microscopic morphology, mechanical properties, electrical conductivity, electrochemical behavior and wettablity characterization, are systematically investigated and summarized according to the requirements of GDL in PEMFC. A membrane electrode assembly (MEA) with a-C coated SSSFF-15 GDL is fabricated and assembled with a-C coated stainless steel bipolar plates in a single cell. The initial peak power density of the single cell is 877.8 mW cm⁻² at a current density of 2324.9 mA cm⁻². Lifetime test of the single cell over 200 h indicates that the a-C coating protects the SSSFF-15 GDL from corrosion and decreases the performance degradation from 30.6% to 6.3%. The results show that the SSSFF GDL, enjoying higher compressive modulus and ductility, is a promising solution to improve fluid permeability of GDL under compression and PEMFC durability.     

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.     

Effects of needle orientation and gas velocity on water transport and removal in a modified PEMFC gas flow channel having a hydrophilic needle

Water management is critical to the performance and operation of the proton exchange membrane fuel cell (PEMFC). Effective water removal from the gas diffusion layer (GDL) surface exposed to the gas flow channel in PEMFC mitigates the water flooding of and improves the reactants transport into the GDL, hence benefiting the PEMFC performance. In this study, a 3D numerical investigation of water removal from the GDL surface in a modified PEMFC gas flow channel having a hydrophilic needle is carried out. The effects of the needle orientation (inclination angle) and gas velocity on the water transport and removal are investigated. The results show that the water is removed from the GDL surface in the channel for a large range of the needle inclination angle and gas velocity. The water is removed more effectively, and the pressure drop for the flow in the channel is smaller for a smaller needle inclination angle. It is also found that the modified channel is more effective and viable for water removal in fuel cells operated at smaller gas velocity.