Porous Gas Diffusion Layer Research Articles

An improved hydrophobic coating for the porous gas diffusion layer in a PEM-based electrochemical hydrogen pump to mitigate anode flooding

Anode flooding is a critical issue in proton exchange membrane (PEM)-based electrochemical hydrogen pumps. Water-proofing the gas diffusion layer (GDL) on the anode side is a highly effective approach to mitigate the flooding problem and to enhance hydrogen pump performance. In this study, we report a low-cost, wet chemical process for water-proofing a metallic GDL. Silver particles, used as roughening and bonding agents, were first deposited on the porous metallic gas diffusion medium, and then heptadecafluoro-1-decanethiol (HDFT) was applied as a hydrophobic surface modifier. The result was a super-hydrophobic porous metallic GDL. Real electrochemical pump cell tests under simulated flooding conditions revealed that the water management performance of the coated GDL was dramatically improved.

Directly Grown Aligned Carbon Nanotubes As a Novel Microporous Layer for Proton Exchange Membrane Fuel Cells

Proton Exchange Membrane Fuel Cells (PEMFC) convert electrochemical energy into electricity. Nevertheless, the system still needs to be improved in order to be economically relevant. In that purpose, it is needed to produce more current for a reduced use of materials. Working on developing new materials for PEMFCs, managing both high gas diffusion and adequate water transport, is necessary to achieve these goals. One of the most important materials used in the fuel cell is the gas diffusion layer (GDL). A microporous layer (MPL) is generally deposited on top of the gas diffusion layer to improve gas transport and water management, and thus enhance the cell performances and stability.Carbon nanotubes interest as a MPL or as catalyst support has already been demonstrated in several works [Kannan (2009); Tang (2011); Xie (2015)]. In the literature, most of works about CNTs as microporous layers are using bulk carbon nanotubes integrated in a carbon black MPL. A few works are displacing a different microporous layer structure where carbon nanotubes are obtained by direct growth and are forming a foam around the gas diffusion layer fibers. In this work, a layer made of vertically aligned CNTs was grown in-situ on the fibers of a gas diffusion layer by a hot filaments assisted chemical vapor deposition (HFCVD). It has been possible to successfully grow aligned CNTs on different commercial supports. Functional properties of the GDL with CNTs were characterized with SEM imaging, contact angle and electrical conductivity measurements. A variety of growth time, filaments temperature, chamber temperature and gas flows have been tested. These parameters influence the carbon nanotubes length, thickness, and the CNTs forest density. At last, electrochemical characterizations were conducted in a differential single cell. This cell allows a better homogeneity of flows on the surface. Several cell configurations were studied: gas diffusion layer with carbon nanotubes on the anode, on the cathode, and on both electrodes, either in dry conditions (no condensed water) or wet conditions (with condensed water). Polarization curves have been measured to study the electrochemical phenomena leading to the variation of performances of the various cell configurations. Impedance spectroscopy has been done to measure the overall cell resistance and its intrinsic composition such as the protonic resistance or the transport resistance.SEM imaging has shown that the CNTs layers are distributed all along the carbon fibers of the GDL, also inside the GDL pores. The overall structure strictly differs from conventional microporous layers as well as other CNT-made MPLs in the literature. 10-25 µm-long multiwall carbon nanotubes with a diameter ranging between 7-10nm were obtained. CNTs as a microporous layer gives better performances (10% in dry conditions, 25% in wet conditions) than commercial MPLs. Contact angle measurements indicates that the obtained gas diffusion media (GDL+MPL) is hydrophilic, which also differs from commercial hydrophobic GDLs. Although carbon nanotubes are individually expected to be hydrophobic, the CNT forest structure and its combination with the former GDL porosity have a different property. The measurement of electrical resistance of the global fuel cell with a commercial MPL or with CNT layers shows that carbon nanotubes don’t provide a better electrical transport.Electrochemical measurements gave access to the fuel cell performance in operating conditions. The results have been compared to the reference SGL 29BC and to the Department of Energy expectations for the fuel cell performances for the next years. In dry state, GDL with CNTs work as well as the reference gas diffusion media. In wet conditions, a dramatic improvement is obtained, especially at low current density. These results are all the more interesting as the gas diffusion layer with carbon nanotubes is hydrophilic, but can compete with the hydrophobic commercial GDLs. The arrangement of carbon nanotubes might also be important as it is aligned in the direction of gas flows and water flows in the fuel cell.In closing, the results of this work show the need to develop nanostructured materials for energy applications. Carbon nanotubes proved their interest as a material for fuel cells. Even with a hydrophilic and a strictly different structure of microporous layer, the fuel cell performances with carbon nanotubes compete with the best commercial reference results. This could lead to a better understanding of the flows phenomena in fuel cells on one side, and of the predominant factors inhibiting a dramatic improvement of the PEMFCs performances. Figure 1

Investigation of Transition Metal-Based (Mn, Co, Ni, Fe) Trimetallic Oxide Nanoparticles on N-doped Carbon Nanotubes as Bifunctional Catalysts for Zn-Air Batteries

Various transition metal-based bimetallic and trimetallic oxides on N-CNTs were successfully synthesized in a one-pot process. Porous gas diffusion layers (GDLs) were simultaneously impregnated with the catalysts during synthesis, resulting in an efficient and scalable preparation technique for nano-composite air electrodes. NiMnOx/N-CNT, NiFeOx/N-CNT, and (Co,Fe)3O4/N-CNT catalysts were the highest performing bimetallic oxides based on battery rate test results. Therefore, Ni-Co-Fe, Ni-Mn-Fe, and Mn-Co-Fe systems were investigated as combined trimetallic oxide/N-CNT catalysts for Zn-air batteries. Trimetallic oxides on N-CNTs exhibited improved OER activity and comparable ORR activity relative to the bimetallic oxide catalysts in linear sweep voltammetry (LSV) tests. Discharge/charge efficiencies for tri-metallic oxides on N-CNTs calculated from battery rate test measurements at a current density of 20 mA cm−2 were 60.5%, 60.7%, and 60.0% for NCFO/N-CNT, NMFO/N-CNTs and MCFO/N-CNT, respectively. Bifunctional cycling of all trimetallic oxide/N-CNT electrodes at 10 mA cm−2 for 100 h showed excellent stability, particularly the NCFO/N-CNT catalyst. Additionally, the efficiencies for NCFO/N-CNT, NMFO/N-CNT and MCFO/N-CNT samples were 58.5%, 57.9% and 57.2%, respectively, after cycling, which compare favorably with that of Pt-Ru/C (55.3%). Trimetallic oxides on N-CNTs are, therefore, excellent candidates as high performing, non-precious metal catalysts for Zn-air batteries.

(Co,Fe)3O4 Decorated Nitrogen‐Doped Carbon Nanotubes in Nano‐Composite Gas Diffusion Layers as Highly Stable Bifunctional Catalysts for Rechargeable Zinc‐Air Batteries

Abstract(Co,Fe)3O4 nanoparticles are decorated onto N‐doped carbon nanotubes at room temperature through a simple mixing process and are simultaneously deposited within a porous gas diffusion layer (GDL) by an impregnation technique. The (Co,Fe)3O4 nanoparticles are identified as the spinel phase through transmission electron microscopy (TEM) and X‐ray photoelectron spectroscopy (XPS) analysis. The composite GDL is used as the air electrode for Zn‐air batteries and shows excellent performance as a bifunctional catalyst with initial discharge and charge potentials of 1.19 V and 2.00 V, respectively, at 20 mA cm−2. Cycling performance of the impregnated electrode compares favourably with benchmark Pt‐RuO2 catalysts at both 10 mA cm−2 and 20 mA cm−2. The (Co,Fe)3O4/N‐CNT impregnated GDL had a final discharge/charge efficiency of 58.5 % after 100 h (200 cycles) of bifunctional cycling at 10 mA cm−2, which is superior to that of Pt‐RuO2 (55.3 % efficiency). The cycling efficiency for (Co,Fe)3O4/N‐CNT impregnated GDL at 20 mA cm−2 is also better than that for Pt−Ru (53.5 % vs 41.3 % after 50 h (100 cycles)). The result is a simple and easily scalable one‐pot electrode synthesis method for high performing bi‐functional air electrodes for Zn‐air batteries.

Pairing Asymmetric Gas Diffusion Media for High-Power Fuel Cell Operation

Proton exchange membrane fuel cell (PEMFC) power production is highly influenced by the properties of the gas diffusion media (GDM). The GDM typically comprise a mesoporous carbon layer (MPL) on top of a porous carbon-fiber gas diffusion layer (GDL) which are both wetted with polytetrafluoroethylene (PTFE) to modify their water retention. In a PEMFC, the GDM are compressed to either side of the catalyst coated membrane. The GDM are active in the fuel cell transport processes including electron and heat conduction, water removal, and distributing reactant gases to the catalyst layers, which affects the electrochemistry in the catalyst layers. These transport phenomena rely on both the properties of the GDM solid phase and its voids, making optimization of GDM material properties challenging. The majority of reports in the PEMFC literature addressing GDM focus solely on the cathode, since water is produced at this electrode and its ineffective removal can block active sites and occlude O2 diffusion. Most research reports use symmetrical anode and cathode GDM. To date, there has been less discussion on the role of the anode GDM or how anode and cathode GDM properties can be selected in concert to increase fuel cell power output, despite that a significant amount of water in a fuel cell is rejected to the anode.We consider that anode and cathode GDM properties, such as air permeability, must be chosen in unison to improve cell water management. In our previous work, we used X-ray computed tomography to observe that dry-laid, non-woven, Freudenberg GDM maintain large void volume while permitting high compressive stress, ultimately resulting in lower contact and ohmic resistances and better mass transport compared to another distinct class of GDM (wet-laid, SGL).1 In this work, we make a range of PEMFCs using a suite of Freudenberg GDM to study the influence of anode and cathode GDM air permeability and how these properties may be paired to improve cell hydration and increase fuel cell power production in a broad range of operating conditions. Fuel cells are characterized with cyclic voltammetry, electrochemical impedance spectroscopy (EIS), gravimetric analysis, and limiting current measurements to quantify the sources of mass transport resistance. Gradients of GDM material properties from anode to cathode improve cell hydration and facilitate water removal to reduce mass transport resistances and enable higher power operation. 1. “The Role of Compressive Stress on Gas Diffusion Media Morphology and Fuel Cell Performance,” R. W. Atkinson III, Y. Garsany, B. D. Gould, K. E. Swider-Lyons, I. V Zenyuk, ACS Applied Energy Materials, 2017, 1 (1), 191–201.

Evaluation of passive and active lattice Boltzmann method for PEM fuel cell modeling

Modeling of multi-component gas transport ( O2, N2, H2O) in a Proton exchange membrane fuel cell (PEMFC) cathode channel with heterogeneous porous gas diffusion layer (GDL) has been carried out using a two different lattice Boltzmann models. Generally, two passive and active methods are used for modeling single-phase, multi-component fluid flows in the Lattice Boltzmann method. In this study, the implementation of both models is described completely and their results are compared with experimental study. The results demonstrate that maximum error in active approach is only 2.5% at the common operating voltage of PEMFC and this model reaches to steady solution 33.3% faster than passive model. Also, that is found that the active method is more appropriate for complex geometry by using bounce back boundary condition on the obstacles in GDL and modify new bounce back on the reaction surface where the implementation of this boundary condition is so easy for complicated geometries.

Construction of mass-transfer channel in air electrode with bifunctional catalyst for rechargeable zinc-air battery

For developing high performance zinc-air battery, an integrated air electrode with rational design of the catalyst layer and gas diffusion layer (GDL) was proposed using a facile and scalable method, in which an in-situ grown Co3O4 nanosheets was employed to tailor the pore sizes and properties of the GDL with efficient oxygen-transfer channels as well as more triple-phase boundaries. The Co3O4 nanosheets were directly grown both on the surface of nickel foam and the large pores of GDL. Therefore, this approach avoided blocking active sites due to a binder, and enabled tailoring of the architecture and properties of the GDL, resulting in more exposed active sites, fast oxygen transport capability, and abundant triple-phase boundaries. Remarkably, benefitting from their unique architecture, the Co3O4/Ni/GDL electrodes exhibited high activity toward both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Application of the Co3O4/Ni/GDL electrodes as bifunctional electrodes for zinc–air batteries resulted in an ultrahigh capacity of 806 mAh/gZn, high maximum power density of 162 mW/cm2, and superior cycling stability over 425 h at 10 mA/cm2 with a high energy efficiency of 68%. These properties were much better than those of Pt/C-based electrodes. Furthermore, the Co3O4/Ni/GDL-based flexible zinc-air battery also exhibited high mechanical flexibility and stability. Our findings thus open up a path for the design of highly efficient integrated air electrodes for metal-air batteries.

Pore-Scale Liquid Water Visualization for Understanding Water Transport in Operating Fuel Cells

To implement successful water management strategies in polymer electrolyte membrane (PEM) fuel cells, we need to understand how the complex heterogenous nature of the gas diffusion layer (GDL) impacts the pore-scale transport of liquid water. Here, we investigated the 3-D pore-scale liquid water distribution within the cathode GDL via in operando synchrotron X-ray tomography during low current density fuel cell operation to capture the early appearance of liquid water pathways. We found that the invasion of liquid water into the GDL only partially filled certain GDL pores. Liquid water wicked along some GDL fibers, which was attributed to the hydrophilic nature of carbon fiber and the presence of pore-scale mixed wettability within the GDLs. Mixed wettability and partial pore filling should be considered in the pore-scale modeling and design of next-generation GDLs.

Three-dimensional simulation of a PEM fuel cell with experimentally measured through-plane gas effective diffusivity considering Knudsen diffusion and the liquid water effect in porous electrodes

A fundamental understanding of the gas diffusion properties in porous electrodes, such as catalyst layers (CLs), micro porous layers (MPLs) and gas diffusion layers (GDLs) is necessary for the optimal design of polymer electrolyte membrane (PEM) fuel cell. The conventional Bruggeman correlation has been proven to overestimate through-plane gas effective diffusivity (TP-GED) in GDLs; however, few studies have shed light on the influence of gas diffusion in MPLs and CLs in simulations of PEM fuel cells. In this study, to accurately predict the gas diffusion process and cell performance, a three-dimensional, two-phase and non-isothermal PEM fuel cell model was developed with experimentally measured TP-GED considering Knudsen diffusion and the liquid water effect in porous electrodes. The numerical results indicate that the conventional Bruggeman correlation greatly overpredicts TP-GED in porous electrodes and the oxygen reduction reaction (ORR) within the cathode CL and thus cannot accurately reflect the mass transport loss at high current density. The gas diffusion properties in the cathode porous electrode play a dominant role in determining cell performance. Furthermore, MPLs and CLs exhibit lower TP-GED than GDLs due to Knudsen diffusion; therefore, the distinction among the gas diffusion properties of GDLs, MPLs and CLs must be considered in quantitative modeling and optimization of PEM fuel cells.

Investigating effect of different gas diffusion layers on water droplet characteristics for proton exchange membrane (PEM) fuel cells

In this work, advanced x-ray radiographic techniques available at the Canadian Light Source (CLS) were utilized to study water droplet dynamics in a serpentine flow channel mimicking a proton exchange membrane fuel cell (PEMFC). High spatial and temporal resolution coupled with high energy photons of an x-ray beam provided high-resolution images of water droplets. This technique solved the problem caused by the opaqueness of fuel cell materials including the gas diffusion layer by providing a unique way to study water droplet dynamics at different operating conditions. From the captured images, droplet emergence and formation on porous gas diffusion layers (GDLs) were analyzed. Three commercially available GDLs (Sigracet AA, Sigracet BA, and Sigracet BC) were used and droplet detachment height was found to decrease in the following order AA < BA < BC under the same flow condition. Increasing the superficial gas velocity was found to decrease the droplet detachment height for all GDLs tested. Average droplet cycle for various operating conditions was obtained. It was found that humidified air did not show a difference in droplet dimensions at detachment compared to dry air used at the inlet gas. However, it did show an impact on droplet cycle time, which might be due to condensation.

Hydrogen mass transport resistance changes in a high temperature polymer membrane fuel cell as a function of current density and acid doping

High temperature polymer electrolyte membrane fuel cells (HT-PEMFC) have phosphoric acid doped membranes. Acid in the membrane is mobile and tends to move out of the membrane depending on the acid doping. The migration of acid (when the doping is high) towards the anode at high current density >0.4Acm−2 causes gas diffusion layer (GDL) and catalyst flooding which thereby results higher hydrogen transport resistance. Thus, it is important to determine the acid doping level, which is optimal. In this study, transient changes in hydrogen mass transport is investigated as a function of doping level and current density. Three doping levels 11, 8.3 and 7 molecules of H2PO4 per PBI repeat unit are investigated. Electrochemical impedance spectroscopy (EIS) was modified to a single frequency measurement and time constant are calculated for resistance change with current density using a linear fit. The time constants are 2.0 ± 0.5, 3.4 ± 0.3, 8.2 ± 0.2 min for low and 2.5 ± 0.8, 4.9 ± 0.3 and 4.5 ± 0.2 min for high current densities, for the respective doping levels. The resistance decreases at high and increases at low current densities for all the doping levels with a varying time constant. This change in time constant is attributed to low doping level having lower capillary pressure to push the acid from reaching GDL pores from the membrane and/or catalyst layer.

Novel Electrospun Gas Diffusion Layers with Spatially Graded Pore Sizes for Polymer Electrolyte Membrane Fuel Cells

High current density performance of a polymer electrolyte membrane (PEM) fuel cell is severely limited by the accumulation of liquid water in the gas diffusion layer (GDL) of the cell. The accumulation of water blocks pathways for oxygen transport and leads to reduced cell performance. Graded GDL properties, such as increasing porosity and decreasing hydrophobicity are strategies for expelling excess water and improving cell performance [1,2]. A powerful tool to manufacture GDL materials with tailored, graded properties is electrospinning [3]. In this study, novel electrospun, nano-fibrous, graphitic, and hydrophobic GDLs with increasing pore-sizes from the catalyst layer (CL) to the flow fields were designed and fabricated to improve the high current density performance of PEM fuel cells. The fiber diameters, pore-sizes, material composition, and electrical conductivity of the novel materials where quantified using a combination of scanning electron microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, and 4-point probe measurements. The effects of the electrospun GDLs (eGDLs) with graded pore-sizes on cell performance were evaluated via fuel cell performance testing and electrochemical impedance spectroscopy (EIS). A solution of polyacrylonitrile (PAN) and N,N-dimethylformamide was used to electrospin nano-fibrous polymer substrates. Higher PAN concentrations in the polymer solution led to larger fibre diameters and pore sizes. Graded pore structures were manufactured by spinning subsequent layers with increasing PAN concentrations. The electrospun substrates were converted to graphitic carbon substrates via heat treatment at 1400 oC and rendered hydrophobic without structural morphological alterations via an oxygen plasma and trichloro(1H,1H,2H,2H-perfluorooctyl)silane treatment. Two uniform eGDLs were manufactured for base line comparison, and two graded eGDLs were manufactured to test the effect of graded pore-sizes on cell performance. The two uniform eGDLs were called s-eGDL and l-eGDL and comprised of small fibres and large fibres, respectively. The two graded structures were called the Bi-Layer eGDL and Tri-Layer eGDL and had increasing fibre diameters and pore-sizes from the catalyst layer to the flow fields. The fuel cell tests were conducted at 60oC and 50, 75,and 100% RH conditions. The graded eGDL materials led to improved high current density performance compared to the uniform eGDLs at all test conditions with the most significant improvements at 100% RH (≥ 27% increase in peak power output). EIS measurements at 2.0 A/cm2 and 100% RH indicated a substantial decrease (≥ 23%) in mass transport resistance for the graded eGDLs compared to the uniform eGDLs. The reduction in mass transport losses is attributed to enhanced liquid water removal due to the graded structures. The increasing pore-sizes from the CL to the flow fields leads to a capillary pressure gradient favourable for liquid water removal which leads to improved oxygen transport. Additionally, the high frequency resistance (HFR) of the cell at 100% RH did not vary significantly between the s-eGDL and the graded eGDL materials at 2.0 A/cm2 (≤ 8%). However, the l-eGDL material, which had relatively larger pores at the CL compared to the other three materials, had a significantly higher measured HFR value compared to the s-eGDL (31% increase) at the same conditions. The higher HFR value for the l-eGDL material may be attributed to reduced diffusion of water from the CL towards the membrane due to the larger pore-sizes, leading to reduced membrane hydration and increased ohmic losses. In this study, novel electrospun GDLs with graded pore sizes were manufactured for PEM fuel cells. The results provide valuable insight into the effects of GDL pore structure and material properties on cell performance. The work presented can aid in the design of the next generation of GDL materials for improved PEM fuel cell performance. REFERENCES Kitahara, T., Nakajima, H., Inamoto, M., Shinto, K. (2014). Triple microporous layer coated gas diffusion layer for performance enhancement of polymer electrolyte fuel cells under both low and high humidity conditions. Journal of Power Sources, Vol. 248, 1256-1263. doi:https://doi.org/10.1016/j.jpowsour.2013.10.066Ko, D., Doh, S., Park, H. S., Kim, M. H. (2018). The effect of through plane pore gradient GDL on the water distribution of PEMFC. International Journal of Hydrogen Energy, Vol. 42, 2369-2380. doi: https://doi.org/10.1016/j.ijhydene.2017.12.007Chevalier, S., Lavielle, N., Hatton, B. D., Bazylak, A. (2017). Novel electrospun gas diffusion layers for polymer electrolyte membrane fuel cells: Part I. Fabrication, morphological characterization, and in situ performance. Journal of Power Sources, Vol. 352, 272–280. doi: http://dx.doi.org/10.1016/j.jpowsour.2017.03.098

A NOVEL FRACTAL MODEL FOR RELATIVE PERMEABILITY OF GAS DIFFUSION LAYER IN PROTON EXCHANGE MEMBRANE FUEL CELL WITH CAPILLARY PRESSURE EFFECT

The investigation of fluids transport through porous gas diffusion layer (GDL) is very important to improve the performance of proton exchange membrane fuel cell (PEMFC). In this paper, the fractal analytical models of capillary pressure for water phase and water and gas relative permeabilities of fibrous GDL are obtained. The determined capillary pressure and relative permeabilities based on the present models are in good agreement with the available experimental data and existing models reported in the literature. Thus the validity of the proposed models for the capillary pressure and relative permeabilities is verified. It is found that the capillary pressure increases with the increase of fiber volume fraction and tortuosity fractal dimension. On the other hand, it is seen that the capillary pressure decreases with increasing water saturation. It is seen that the water relative permeability decreases with increasing tortuosity fractal dimension. On the contrary, it is found that the gas relative permeability increases with tortuosity fractal dimension. Besides, it is observed that the porosity has little effect on the water and gas relative permeabilities.

Comprehensive statistical analysis of heterogeneous transport characteristics in multifunctional porous gas diffusion layers using lattice Boltzmann method for fuel cell applications

Comprehensive computational modeling based on a full statistical approach is performed to investigate the heterogeneous multi-transport characteristics in the gas diffusion layers of fuel cells. For the purposes, a series of carbon paper gas diffusion layers are randomly generated at a 95% confidence level to reflect the heterogeneous microstructures. A representative element volume is determined based on the relative porosity gradient errors to minimize the uncertainty in the statistical analysis. Subsequently, a single-phase three-dimensional lattice Boltzmann method is applied to obtain the velocity distribution throughout the porous layers, enabling to calculate the average tortuosity. The effective mass diffusivity in the diffusion layers is then derived from the tortuosity factor. Additionally, three directional permeabilities are derived from the pressure gradient to account for the anisotropic characteristics of the porous diffusion layers. The relationship between the permeability and porosity is found to match the modified Kozeny–Carman equations. Further, a path-finding algorithm based on the percolation theory is developed to simulate electron and thermal conduction along the carbon fibers in the in-plane and through-plane directions. The present model can be utilized to investigate the heterogeneous transport characteristics of fibrous porous diffusion media for various electrochemical systems.

Imaging of the desaturation of gas diffusion layers by synchrotron computed tomography

The 4-D imaging and quantification of the desaturation of an initially flooded gas diffusion layer (GDL) with a serpentine gas flow channel by synchrotron radiography is presented. An imaging area with a diameter of 10 mm allows for the study of how the natural anisotropy of a GDL affects the desaturation profile. The GDL is progressively spatially segmented from the overall domain down to sections of the individual channels and ribs. Temporal saturation profiles and desaturation rates are presented for each step of this progressive segmentation. Although the desaturation of the overall domain initially appears to be a constant and steady process with a desaturation rate of 0.0030 μL cm−2 s−1, segmented results display heterogeneous behavior with over two-fold differences between areas of the flow field. Segmented saturation surface plots are presented that spatially show how this heterogeneity occurs within the entire domain. Results indicate that initial conditions, anisotropic material properties, and flow field geometry all play a key role in determining local desaturation behavior. The larger domain considered here provides valuable data for future modeling studies that combine continuum studies of flow in the gas flow channels with GDL pore network models that consider two-phase flow behavior.

Development of a hydrophobic coating for the porous gas diffusion layer in a PEM-based electrochemical hydrogen pump to mitigate anode flooding

Anode flooding is one of the critical issues in developing a proton exchange membrane (PEM)-based electrochemical hydrogen pump. Improving the hydrophobicity of the gas diffusion layer (GDL) has been studied as an approach to mitigating anode flooding in electrochemical pumps. A mixture of Nafion™ and oxidized carbon nanotubes (O-CNT) has been applied to the porous gas diffusion medium in the hydrogen pump cell. The coating renders the GDL hydrophobic with an effective contact angle of 130°. Electrochemical pump testing has shown that, with the help of the coating, the flood-recovery performance of the hydrogen pump was greatly improved. A hydrogen pump cell with an uncoated GDL was not able to recover from a flooded state, while a hydrogen pump cell with a coated GDL was able to recover its performance in about 100 s.

Selective Visualization of Water in Fuel Cell Gas Diffusion Layers with Neutron Dark-Field Imaging

Visualizing the water distribution in porous gas diffusion layers (GDLs) of operating polymer electrolyte fuel cells (PEFCs) is indispensable to understand the impact of water management on performance. For this purpose, neutron and X-ray transmission imaging have been used for nearly two decades. Certain limitations inherent to attenuation based imaging methods can be overcome by applying neutron dark-field imaging, which has the ability to selectively visualize structures in the micrometer size range. In this study, we compare dark-field images and transmission images of GDLs filled with water through an injection channel. The high contrast of the dark-field value between a heavy water filled and a dry GDL is suitable to reveal water distribution patterns in the GDL. The water present in the 1 mm wide water injection channel of the test device does not alter the dark-field signal, as this technique is selectively sensitive to microstructures. Therefore, neutron dark-field imaging can be applied for the selective analysis of the water distribution in the GDL overlapping with channel water. In addition to the selective visualization of water distributed in a GDL, we show that neutron dark-field imaging can also be used to visualize GDL damages.

Saturation Behavior of Atomic Layer Deposition MnOx from Bis(Ethylcyclopentadienyl) Manganese and Water: Saturation Effect on Coverage of Porous Oxygen Reduction Electrodes for Metal–Air Batteries

MnOx deposits on porous gas diffusion layer (GDL) substrates for application as catalysts for the oxygen reduction reaction (ORR) in metal–air batteries have been prepared using atomic layer deposition (ALD). The saturation behavior of the bis(ethylcyclopentiadienyl) manganese ((EtCp)2Mn) and H2O ALD system has been investigated. The observed saturation behavior is in disagreement with previous reports in literature. (EtCp)2Mn + H2O depositions exhibited non-ALD behavior, as demonstrated by a high growth per cycle (GPC) at substrate temperatures (Tsub) = 40–50 °C and nonsaturating reactions at Tsub ≥ 60 °C. The introduction of a forming gas (FG) (95% N2 + 5% H2) plasma between the (EtCp)2Mn and H2O doses promoted precursor saturation with a constant GPC (1.15 A/cy) in the Tsub range of 100–200 °C. The effect of saturation behavior on porosity coverage was investigated by coating porous carbon electrodes with ALD MnOx. Energy dispersive X-ray (EDX) spectroscopy, electrochemical surface area measurements, a...

FRACTAL ANALYSIS OF GAS FLOW THROUGH THE GAS DIFFUSION LAYER IN PROTON EXCHANGE MEMBRANE FUEL CELLS WITH ROUGHENED MICRO-CHANNELS

The effective permeability is one of the key parameters for porous fibrous gas diffusion layer (GDL), which plays a crucial role in the proton exchange membrane fuel cells (PEMFCs). However, the effect of the surface morphology of porous fibrous GDL on gas transport behaviors is so far been neglected. In order to take that into consideration, a new analytical model is presented for gas flow in porous fibrous GDL with roughened micro-channels based on the fractal scaling laws. Due to the existence of very small pores in the porous fibrous GDL, gas slippage effects through the small pores are also taken into account. The fractal gas permeability is expressed in terms of the relative roughness, other microstructural parameters as well as gas properties. The proposed fractal model is validated by comparing the model prediction with available experimental data. The effect of relative roughness and other micro-structural parameters of GDL on the permeability is analyzed in detail.

A pore-level direct numerical investigation of water evaporation characteristics under air and hydrogen in the gas diffusion layers of polymer electrolyte fuel cells

During the operation of polymer electrolyte fuel cells (PEFC) liquid water clusters generated due to electrochemical reactions in the gas diffusion layers (GDL) create a resistance against gas transport towards the catalyst layers (CL), and hence must be efficiently removed to maintain the cell performance, a process often realized by means of forced evaporation. Water accumulation is of interest not only for the cathode GDL but also for the anode GDL, particularly under thermoneutral cell operation whereby liquid water can be added to either anode or cathode channel flows. Although many works studied water evaporation in air and especially at moderate temperatures, less effort has been devoted in investigating evaporation at elevated temperatures and at the anode side, where hydrogen flows in the gas channels. In this work, direct numerical simulation is used to characterize evaporation at pore level for both anode and cathode sides and at different temperatures (up to 80 °C), for which experimental data are rare and micro-scale transport is often difficult to assess in the laboratory. Realistic water distribution and porous GDL geometry, acquired from X-ray tomography, have been used while regularized distributions of liquid water are further considered in order to investigate evaporation at various saturation levels. Key results indicate that (a) the water evaporation rate in hydrogen flows can be up to 4 times larger than the corresponding one in air flows at the same gas stream velocity and temperature, (b) the predicted vertical evaporation-induced velocities under hydrogen are an order of magnitude bigger than the corresponding ones under air and they can grow large enough at elevated temperatures to potentially hamper hydrogen transport towards the CLs.