Unmasking the reactants inhomogeneity in gas diffusion layer and the performances of PEMFC induced by assembly pressure.

The gas diffusion layer (GDL), as the bridge to reactants and electrons in PEMFC, is a carbon-based porous component and would be deformed under compression to induce changes in the distributions of reactants and the corresponding performances of PEMFC; therefore, unmasking the impacts of assembly pressure on the distribution of the reactants in GDL is significant to improve the performance of PEMFC. In the present article, the structural response of GDL to assembly pressure was first studied; the variations in transport properties of GDL and the reactant distributions induced by assembly pressure were then discussed; the impacts on the dynamic performances of PEMFC were finally investigated. From the results, assembly pressure was found to have different effects on the regions of GDL under the rib and channel, significant gaps in GDL porosity and/or GDL permeability existed near the rib/channel transition region to worsen the inhomogeneity of reactants. Suffering assembly pressure, the distribution of current density became uneven, and the current density near the rib-channel border seriously rose to the aggravated risk of MEA thermal damage. Furthermore, the power density at specific efficiencies was raised under certain assembly pressures, which meant suitable assembly pressure(s) existed for better output performances of PEMFC.

Characterisation of hydraulic properties of commercial gas diffusion layers: Toray, SGL, MGL, woven carbon cloth.

This study utilises computational fluid dynamics simulations with the OpenFOAM computational framework to investigate and compare the in-plane and through-plane permeability properties of four different gas diffusion layers (GDLs). Also the through-plane water and air relative permeability values and water saturations at different rates were simulated. Permeability analysis enhances our understanding of fluid flow, ways to decrease pressure loss in the GDL, and methods to enhance oxygen concentration at the catalyst layer interface through convection. The analysis reveals that the investigated GDL materials have spatial heterogeneity of porosity and permeability, especially in the Sigracet SGL 25 BA GDL. However, the porosity and permeability of the Toray TGP-H 060 and AvCarb 370 MGL GDLs exhibit less variations. The two-phase flow studies on GDL saturation show that at the same water injection flowrate, the AvCarb 370 MGL GDL has the largest remaining water saturation, with Sigracet SGL 25 BA GDL being the less saturated GDL among the four investigated GDLs. The compression from the ribs significantly affected the in-plane permeabilities of both Toray TGP-H 060 and especially impacted Sigracet SGL 25 BA GDL. This impact was expected as the pore size distribution varied significantly in the areas under the ribs versus the channel.

Comprehensive Analysis of Wettability in Waterproofed Gas Diffusion Layers for Polymer Electrolyte Fuel Cells.

In polymer electrolyte fuel cells (PEFCs), the gas diffusion layer (GDL) is crucial for managing the flooding tolerance, which is the ability to remove the water produced during power generation from the assembled cell. However, an improved understanding of the properties of GDLs is required to develop effective waterproofing strategies. This study investigated the influence of the polytetrafluoroethylene (PTFE) content on the pore diameter, porosity, wettability, water saturation, and flooding tolerance of waterproofed carbon papers as cathode GDLs in PEFCs. The addition of minimal PTFE (∼6 wt %) to carbon paper provided external waterproofing, whereas internal waterproofing was achieved at a higher PTFE content (∼13 wt %). However, excessive PTFE (∼37 wt %) led to macropore collapse within the carbon paper, reducing fuel cell performance. Although PTFE addition was expected to improve the flooding tolerance, operando synchrotron X-ray radiography revealed that the water saturation level in carbon paper increased with increasing PTFE content. These findings provide a benchmark for assessing whether GDLs meet the flooding tolerance requirements of PEFCs and may be applicable to waterproofed GDLs in electrochemical devices for water and CO2 electrolysis.

Similarities and Differences between Gas Diffusion Layers Used in Proton Exchange Membrane Fuel Cell and Water Electrolysis for Material and Mass Transport.

Proton-exchange membrane fuel cells (PEMFCs) and water electrolysis (PEMWE) are rapidly developing hydrogen energy conversion devices. Catalyst layers and membranes have been studied extensively and reviewed. However, few studies have compared gas diffusion layers (GDLs) in PEMWE and PEMFC. This review compares the differences and similarities between the GDLs of PEMWE and PEMFC in terms of their material and mass transport characteristics. First, the GDL materials are selected based on their working conditions. Carbon materials are prone to rapid corrosion because of the high anode potential of PEMWEs. Consequently, metal materials have emerged as the primary choice for GDLs. Second, the mutual counter-reactions of the two devices result in differences in mass transport limitations. In particular, water flooding and the effects of bubbles are major drawbacks of PEMFCs and PEMWE, respectively; well-designed structures can solve these problems. Imaging techniques and simulations can provide a better understanding of the effects of materials and structures on mass transfer. Finally, it is anticipated that this review will assist research on GDLs of PEMWE and PEMFC.

Manufacturing protocol and post processing of ultra-thin gas diffusion layer using advanced scanning techniques.

The typical commercial size of a Gas Diffusion Layer (GDL) for Proton Exchange Membrane Fuel Cell (PEMFC) application is around 180 µm up to 290 µm. GDL facilitates the diffusion of reactants to the catalyst layers and liquid removal from the membrane to the flow field. In this regard, GDL should be a porous region with conductive materials as thin as possible to reduce the size and the costs. Lowering the thickness of the GDL also results in better performance of the stack since it increases the speed of reactants to reach the catalysts. However, the main obstacle is the formation of ultra-thin porous GDL, which can be also named as standalone microporous layer (MPL). The novelty of this study is the manufacturing process and production of ultra-thin porous GDL with carbon and Polytetrafluoroethylene (PTFE) as the main materials. The produced GDL has the thickness of 28.9 µm, which has been measured using microscope imaging. This novel GDL can be used as the conductive diffusive region inside the PEM fuel cells, Alkaline fuel cells, and the cathode of PEM and Alkaline electrolyzers. Additionally, the novel invention can be considered as a 2D membrane for carbon capture purposes after being functionalized.

Effect of grayscale threshold on X-ray computed tomography reconstruction of gas diffusion layers in polymer electrolyte membrane fuel cells.

In X-ray computed tomography (CT) reconstructions of gas diffusion layers (GDLs), grayscale threshold selection is a critical issue. Although various selection methods exist, they all have their own drawbacks. This study investigates the influence of grayscale threshold on GDL properties and compares Otsu and porosity-adaptive thresholds. We utilized X-ray CT to reconstruct a Toray carbon paper sample (TGP-H-060) at a resolution of 2 µm. Using reconstructed 3D models generated under different grayscale thresholds, we performed structural analysis, computational fluid dynamics simulation, and compression simulation. We subsequently calculated porosity, tortuosity, permeability, and macroscopic stress-strain relationships, quantitatively analyzing the sensitivity of these parameters to the change of grayscale threshold. The results indicated that small change in the grayscale threshold can significantly impact the transport and mechanical properties of reconstructed GDLs. The difference between Otsu and porosity-adaptive thresholds is notable, and the porosity-adaptive threshold appears to be less accurate than the Otsu threshold.

Quantitative measurement and comparison of breakthroughs inside the gas diffusion layer using lattice Boltzmann method and computed tomography scan.

In Proton Exchange Membrane Fuel Cells (PEMFCs), the presence of residual water within the Gas Diffusion Layer (GDL) poses challenges during cold starts and accelerates degradation. A computational model based on the Lattice Boltzmann Method (LBM) was developed to consider the capillary pressure inside the PEMFC and to analyze the exact geometries of the GDLs, which were obtained using the Computed Tomography scan. The novelty of this study is to suggest a methodology to compare the quantitative water removal performance of the GDLs without long-term experimental testing. Two different samples of GDLs were considered, pristine and aged. The results of quantitative measurements revealed the amount of water columns (breakthroughs) inside each sample. Considering the volume of 12,250,000 µm3 for each sample, the pristine and the aged samples are prone to have 774,200 µm3 (6.32%) and 1,239,700 µm3 (10.12%) as water columns in their porous domain. Micro-structural properties such as connectivity, mean diameter, effective diffusivity, etc. were also compared to observe the impacts of aging on the properties of the GDL.

Efficient photoelectrocatalytic degradation of pollutants over hydrophobic carbon felt loaded with Fe-doped porous carbon nitride via direct activation of molecular oxygen.

Developing a photoelectric cathode capable of efficiently activating molecular oxygen to degrade pollutants is a coveted yet challenging goal. In pursuit of this, we synthesize a Fe doped porous carbon nitride catalyst (Fe-CN) using an ionothermal strategy and subsequently loaded it on the hydrophobic carbon felt (CF) to fabricate the Fe-CN/CF photoelectric cathode. This cathode benefits from the synergistic effects between the porous CN support and the highly dispersed Fe species, which enhance O2 absorption and activation. Additionally, the hydrophobic CF serves as a gas diffusion layer, accelerating O2 mass transfer. These features enable the Fe-CN/CF cathode to demonstrate notable photoelectrocatalytic (PEC) degradation efficiency. Specifically, under optimal conditions (cathodic bias of -0.3 VAg/AgCl, pH 7, and a catalyst loading of 3 mg/cm2), the system achieves a 76.4% removal rate of tetracycline (TC) within 60 min. The general application potential of this system is further underscored by its ability to remove approximately 98% of 4-chlorophenol (4-CP) and phenol under identical conditions. Subsequent investigations into the active species and degradation pathways reveal that 1O2 and h+ play dominant role during the PEC degradation process, leading to gradually breakdown of TC into less toxicity, smaller molecular intermediates. This work presents a straightforward yet effective strategy for constructing efficient PEC systems that leverage molecular oxygen activation to degrade pollutants.

A Nanofiber-Based Gas Diffusion Layer for Improved Performance in Air Cathode Microbial Fuel Cells.

This work investigates a new nanostructured gas diffusion layer (nano-GDL) to improve the performance of air cathode single-chamber microbial fuel cells (a-SCMFCs). The new nano-GDLs improve the direct oxygen reduction reaction by exploiting the best qualities of nanofibers from electrospinning in terms of high surface-area-to-volume ratio, high porosity, and laser-based processing to promote adhesion. By electrospinning, nano-GDLs were fabricated directly by collecting two nanofiber mats on the same carbon-based electrode, acting as the substrate. Each layer was designed with a specific function: water-resistant, oxygen-permeable polyvinylidene-difluoride (PVDF) nanofibers served as a barrier to prevent water-based electrolyte leakage, while an inner layer of cellulose nanofibers was added to promote oxygen diffusion towards the catalytic sites. The maximum current density obtained for a-SCMFCs with the new nano-GDLs is 132.2 ± 10.8 mA m-2, and it doubles the current density obtained with standard PTFE-based GDL (58.5 ± 2.4 mA m-2) used as reference material. The energy recovery (EF) factor, i.e., the ratio of the power output to the inner volume of the device, was then used to evaluate the overall performance of a-SCMFCs. a-SCMFCs with nano-GDL provided an EF value of 60.83 mJ m-3, one order of magnitude higher than the value of 3.92 mJ m-3 obtained with standard GDL.

Elucidating the Mass Transportation Behavior of Gas Diffusion Layers via a H2 Limiting Current Test.

The gas diffusion layer (GDL), as a key component of proton exchange membrane fuel cells (PEMFCs), plays a crucial role in PEMFC's polarization performance, particularly in mass transport properties at high current densities. To elucidate the correlation between GDLs' structure and their mass transport properties, a limiting current test with the H2 molecular probe was established and employed to investigate three representative GDLs with and without the microporous layer (MPL). By varying humidity and back pressure, the mass transport resistance of three GDLs was measured in an operating fuel cell, and an elaborate analysis of H2 transport was conducted. The results showed that the transport resistance (RDM) of GDLs was affected by the thickness and pore size distribution of the macroporous substrate (MPS) and the MPL. In the process of gas transport, the smaller pore size and thicker MPL increase the force of gas on the pore wall, resulting in an increase in transmission resistance. Through further calculation and analysis, the total transport resistance can be divided into pressure-related resistance (RP) and pressure-independent resistance (RNP). RP mainly originates from the transport resistance in both MPLs and the substrate layers of GDLs, exhibiting a linear relationship to the pressure; RNP mainly originates from the transport resistance in the MPLs. 29BC with thick MPL shows the largest RNP, and T060 without MPL shows the RNP = 0. This methodology enables in situ measurements of mass transport resistances for gas diffusion media, which can be easily applied for developing and deploying PEMFCs.

A Super Uniform Hydrophobic Gas Diffusion Layer for a Proton Exchange Membrane Fuel Cell.

The design and optimization of the gas diffusion layer (GDL) play a crucial role in the improvement of proton exchange membrane fuel cell performance. Hydrophobic treatment of a GDL is an important method for facilitating mass transfer, while conventional Teflon treatment is not uniform and leads to an increase in ohmic and heat resistance. Herein, a homogeneous molecular hydrophobic GDL was prepared by liquid phase synthesis, and a two-dimensional non-isothermal model was developed to investigate the transfer mechanism. The peak power density of cells with the GDL described above was improved by 46% compared to that of the conventional GDL. The ohmic and mass transport resistance decreased by 15% and 52%, respectively, under a current density of 1 A cm-2 using the uniform hydrophobic GDL. The simulation results proved that the uniform hydrophobic GDL eliminates the hydrophilic dots, which prevents the formation of water pools and reduces the resistance to gas flow. The water saturation of the conventional GDL reaches 0.19 at a current density of 1 A cm-2, and the saturation of a modified GDL under the same conditions is only 0.13. A dimensionless parameter, Tf, is proposed to characterize the resistance of oxygen diffusion. In conclusion, molecular-level uniform hydrophobic treatment can effectively reduce the ohmic and mass transfer resistance of a GDL and effectively improve the performance of fuel cells.

Pore-Scale Modeling of Liquid Water Transport in Compressed Gas Diffusion Layer of Proton Exchange Membrane Fuel Cells Considering Fiber Anisotropy.

Water management of the gas diffusion layer (GDL) is crucial to the performance of proton exchange membrane fuel cells (PEMFCs). Appropriate water management ensures efficient transport of reactive gases and maintains wetting of the proton exchange membrane to enhance proton conduction. In this paper, a two-dimensional pseudo-potential multiphase lattice Boltzmann model is developed to study liquid water transport within the GDL. Liquid water transport from the GDL to the gas channel is the focus, and the effect of fiber anisotropy and compression on water management is evaluated. The results show that the fiber distribution approximately perpendicular to the rib reduces liquid water saturation within the GDL. Compression significantly changes the microstructure of the GDL under the ribs, which facilitates the formation of liquid water transport pathways under the gas channel, and the increase in the compression ratio leads to a decrease in liquid water saturation. The performed microstructure analysis and the pore-scale two-phase behavior simulation study comprise a promising technique for optimizing liquid water transport within the GDL.

Two-Stage Microporous Layers with Gradient Pore Size Structure for Improving the Performance of Proton Exchange Membrane Fuel Cells.

In this paper, we report the preparation of a gas diffusion layer (GDL) with different gradient pore size structures. The pore structure of microporous layers (MPL) was controlled by the amount of pore-making agent sodium bicarbonate (NaHCO3). We investigated the effects of the two-stage MPL and the different pore size structures in the two-stage MPL on the performance of proton exchange membrane fuel cells (PEMFC). The conductivity and water contact angle tests showed that the GDL had outstanding conductivity and good hydrophobicity. The results of the pore size distribution test indicated that introducing a pore-making agent altered the pore size distribution of the GDL and increased the capillary pressure difference within the GDL. Specifically, there was an increase in pore size within the 7-20 µm and 20-50 µm ranges, which improved the stability of water and gas transmission within the fuel cell. The maximum power density of the GDL03 was increased by 37.1% at 40% humidity, 38.9% at 60% humidity, and 36.5% at 100% humidity when compared to the commercial GDL29BC in a hydrogen-air environment. The design of gradient MPL ensured that the pore size between carbon paper and MPL changed from an initially abrupt state to a smooth transition state, which significantly improved the water and gas management capabilities of PEMFC.

Accelerating Gas Escape in Anion Exchange Membrane Water Electrolysis by Gas Diffusion Layers with Hierarchical Grid Gradients.

At high current densities, gas bubble escape is the critical factor affecting the mass transport and performance of the electrolyzer. For tight assembly water electrolysis technologies, the gas diffusion layer (GDL) between the catalyst layer (CL) and the flow field plate plays a critical role in gas bubble removal. Herein, we demonstrate that the electrolyzer's mass transport and performance can be significantly improved by simply manipulating the structure of the GDL. Combined with 3D printing technology, ordered nickel GDLs with straight-through pores and adjustable grid sizes are systematically studied. Using an in situ high-speed camera, the gas bubble releasing size and resident time have been observed and analyzed upon the change of the GDL architecture. The results show that a suitable grid size of the GDL can significantly accelerate mass transport by reducing the gas bubble size and the bubble resident time. An adhesive force measurement has further revealed the underlying mechanism. We then proposed and fabricated a novel hierarchical GDL, reaching a current density of 2 A/cm2 at a cell voltage of 1.95 V and 80 °C, one of the highest single-cell performances in pure-water-fed anion exchange membrane water electrolysis (AEMWE).

Effects of Compression and Porosity Gradients on Two-Phase Behavior in Gas Diffusion Layer of Proton Exchange Membrane Fuel Cells.

Water management within the gas diffusion layer (GDL) plays an important role in the performance of the proton exchange membrane fuel cell (PEMFC) and its reliability. The compression of the gas diffusion layer during fabrication and assembly has a significant impact on the mass transport, and the porosity gradient design of the gas diffusion layer is an essential way to improve water management. In this paper, the two-dimensional lattice Boltzmann method (LBM) is applied to investigate the two-phase behavior in gas diffusion layers with different porosity gradients under compression. Compression results in an increase in flow resistance below the ribs, prompting the appearance of the flow path of liquid water below the channel, and liquid water breaks through to the channel more quickly. GDLs with linear, multilayer, and inverted V-shaped porosity distributions with an overall porosity of 0.78 are generated to evaluate the effect of porosity gradients on the liquid water transport. The liquid water saturation values within the linear and multilayer GDLs are significantly reduced compared to that of the GDL with uniform porosity, but the liquid water within the inverted V-shaped GDL accumulates in the middle region and is more likely to cause flooding.

Performance Enhancement of Proton Exchange Membrane Fuel Cell through Carbon Nanofibers Grown In Situ on Carbon Paper.

We developed an integrated gas diffusion layer (GDL) for proton exchange membrane (PEM) fuel cells by growing carbon nanofibers (CNFs) in situ on carbon paper via the electro-polymerization of polyaniline (PANI) on carbon paper followed by a subsequent carbonization treatment process. The CNF/carbon paper showed a microporous structure and a significantly increased pore volume compared to commercial carbon paper. By utilizing this CNF/carbon paper in a PEM fuel cell, it was found that the cell with CNF/carbon paper had superior performance compared to the commercial GDL at both high and low humidity conditions, and its power density was as high as 1.21 W cm-2 at 100% relative humidity, which is 26% higher than that of a conventional gas diffusion layer (0.9 W cm-2). The significant performance enhancement was attributed to a higher pore volume and porosity of the CNF/carbon paper, which improved gas diffusion in the GDL. In addition, the superior performance of the cell with CNF/carbon paper at low relative humidity demonstrated that it had better water retention than the commercial GDL. This study provides a novel and facile method for the surface modification of GDLs to improve the performance of PEM fuel cells. The CNF/carbon paper with a microporous structure has suitable hydrophobicity and lower through-plane resistance, which makes it promising as an advanced substrate for GDLs in fuel cell applications.

Effects of Hydrophobicity Treatment of Gas Diffusion Layers on Ice Crystallization in Polymer Electrolyte Fuel Cells.

The unassisted cold-start capability of polymer electrolyte fuel cells (PEFCs) remains challenging for large-scale automotive applications. Various studies have shown that the freezing of produced water at the cathode catalyst layer (CL) and gas diffusion layer (GDL) interface blocks the oxidant gas and leads to a cold-start failure. However, the impact of GDL properties, including substrate, size, and hydrophobicity, on the freezing behavior of supercooled water is yet to be thoroughly investigated. We use differential scanning calorimetry to perform non-isothermal calorimetric measurements on untreated and waterproofed GDLs (Toray TGP-H-060, Freudenberg H23). By conducting a large number of experiments (>100) for each type of GDL, we obtained the corresponding distribution of onset freezing temperature (Tonset) and found noticeable sample-to-sample variations in both untreated and waterproofed GDLs. Furthermore, ice crystallization is affected by GDL wettability, coating load, coating distribution, and GDL size, whereas the impact of the GDL substrate and saturation level is not apparent. The Tonset distribution allows for predicting the capability of PEFC freeze-start and the freezing probability of residual water at a given subzero temperature. Our work paves the way for GDL modifications toward the improved cold-start capability of PEFC by identifying and avoiding the features that systematically trigger the freezing of supercooled water with high probability.

Gas Diffusion Layer for Proton Exchange Membrane Fuel Cells: A Review.

Proton exchange membrane fuel cells (PEMFCs) are an attractive type of fuel cell that have received successful commercialization, benefitted from its unique advantages (including an all solid-state structure, a low operating temperature and low environmental impact). In general, the structure of PEMFCs can be regarded as a sequential stacking of functional layers, among which the gas diffusion layer (GDL) plays an important role in connecting bipolar plates and catalyst layers both physically and electrically, offering a route for gas diffusion and drainage and providing mechanical support to the membrane electrode assemblies. The GDL commonly contains two layers; one is a thick and rigid macroporous substrate (MPS) and the other is a thin microporous layer (MPL), both with special functions. This work provides a brief review on the GDL to explain its structure and functions, summarize recent progress and outline future perspectives.

Research Progress, Trends, and Current State of Development on PEMFC-New Insights from a Bibliometric Analysis and Characteristics of Two Decades of Research Output.

The consumption of hydrogen could increase by sixfold in 2050 compared to 2020 levels, reaching about 530 Mt. Against this backdrop, the proton exchange membrane fuel cell (PEMFC) has been a major research area in the field of energy engineering. Several reviews have been provided in the existing corpus of literature on PEMFC, but questions related to their evolutionary nuances and research hotspots remain largely unanswered. To fill this gap, the current review uses bibliometric analysis to analyze PEMFC articles indexed in the Scopus database that were published between 2000-2021. It has been revealed that the research field is growing at an annual average growth rate of 19.35%, with publications from 2016 to 2012 alone making up 46% of the total articles available since 2000. As the two most energy-consuming economies in the world, the contributions made towards the progress of PEMFC research have largely been from China and the US. From the research trend found in this investigation, it is clear that the focus of the researchers in the field has largely been to improve the performance and efficiency of PEMFC and its components, which is evident from dominating keywords or phrases such as 'oxygen reduction reaction', 'electrocatalysis', 'proton exchange membrane', 'gas diffusion layer', 'water management', 'polybenzimidazole', 'durability', and 'bipolar plate'. We anticipate that the provision of the research themes that have emerged in the PEMFC field in the last two decades from the scientific mapping technique will guide existing and prospective researchers in the field going forward.

Au-TiO2/Ti Hybrid Coating as a Liquid and Gas Diffusion Layer with Improved Performance and Stability in Proton Exchange Membrane Water Electrolyzer.

The liquid and gas diffusion layer is a key component of proton exchange membrane water electrolyzer (PEMWE), and its interfacial contact resistance (ICR) and corrosion resistance have a great impact on the performance and durability of PEMWE. In this work, a novel hybrid coating with Au contacts discontinuously embedded in a titanium oxidized layer was constructed on a Ti felt via facile electrochemical metallizing and followed by a pre-oxidization process. The physicochemical characterizations, such as scanning electron microscopy, energy dispersive spectrometer, and X-ray diffraction results confirmed that the distribution and morphology of the Au contacts could be regulated with the electrical pulse time, and a hybrid coating (Au-TiO2/Ti) was eventually achieved after the long-term stability test under anode environment. At the compaction force of 140 N cm-2, the ICR was reduced from 19.7 mΩ cm2 of the P-Ti to 4.2 mΩ cm2 of the Au-TiO2/Ti. The corrosion current density at 1.8 V (RHE) is 0.689 µA cm-2. Both the ICR and corrosion resistance results showed that the prepared protective coating could provide comparable ICR and corrosion resistance to a dense Au coating.