In-plane gas permeability of proton exchange membrane fuel cell gas diffusion layers

A new analytical approach is proposed for evaluating the in-plane permeability of gas diffusion layers (GDLs) of proton exchange membrane fuel cells. In this approach, the microstructure of carbon papers is modeled as a combination of equally-sized, equally-spaced fibers parallel and perpendicular to the flow direction. The permeability of the carbon paper is then estimated by a blend of the permeability of the two groups. Several blending techniques are investigated to find an optimum blend through comparisons with experimental data for GDLs. The proposed model captures the trends of experimental data over the entire range of GDL porosity. In addition, a compact relationship is reported that predicts the in-plane permeability of GDL as a function of porosity and the fiber diameter. A blending technique is also successfully adopted to report a closed-form relationship for in-plane permeability of three-directional fibrous materials.     

Characterization of internal wetting in polymer electrolyte membrane gas diffusion layers

Capillary pressure vs. saturation (P_C(S_L)) curves are fundamental to understanding liquid water transport and flooding in PEM gas diffusion layers (GDLs). P_C(S_L) curves convolute the influence of GDL pore geometry and internal contact angles at the three-phase liquid/solid/gas boundary. Even simple GDL materials are a spatially non-uniform mixture of carbon fiber and binder, making a Gaussian distribution of contact angles likely, based on the Cassie–Baxter equation. For a given Gaussian contact angle distribution with mean (θ_Mean) and standard deviation (σ), a realistic P_C(S_L) curve can be computed using a bundle of capillaries model and GDL pore size distribution data. As expected, computed P_C(S_L) curves show that θ_Mean sets the overall hydrophilic (θ_Mean < 90°) or hydrophobic (θ_Mean > 90°) character of the GDL (i.e., liquid saturation level at a given capillary pressure), and σ affects the slope of the P_C(S_L) curve. The capillary bundle model also can be used with (θ_Mean, σ) as unknown parameters that are best-fit to experimentally acquired P_C(S_L) and pore size distribution data to find (θ_Mean, σ) values for actual GDL materials. To test this, pore size distribution data was acquired for Toray TGP-H-090 along with hysteretic liquid and gas intrusion capillary pressure curve data. High quality best-fits were found between the model and combined datasets, with GDL liquid intrusion showing fairly neutral internal surface wetting properties (θ_Mean = 92° and σ = 10°) whereas gas intrusion displayed a hydrophilic character (θ_Mean = 52° and σ = 8°). External liquid advancing and receding contact angles were also measured on this same material and they also showed major hysteresis. The new methods described here open the door for better understanding of the link between GDL material processing and the wetting properties that affect flooding.     

Performance enhancement of membrane electrode assemblies with plasma etched polymer electrolyte membrane in PEM fuel cell

In this work, a surface modified Nafion 212 membrane was fabricated by plasma etching in order to enhance the performance of a membrane electrode assembly (MEA) in a polymer electrolyte membrane fuel cell. Single-cell performance of MEA at 0.7 V was increased by about 19% with membrane that was etched for 10 min compared to that with untreated Nafion 212 membrane. The MEA with membrane etched for 20 min exhibited a current density of 1700 mA cm⁻² at 0.35 V, which was 8% higher than that of MEA with untreated membrane (1580 mA cm⁻²). The performances of MEAs containing etched membranes were affected by complex factors such as the thickness and surface morphology of the membrane related to etching time. The structural changes and electrochemical properties of the MEAs with etched membranes were characterized by field emission scanning electron microscopy, Fourier transform-infrared spectrometry, electrochemical impedance spectroscopy, and cyclic voltammetry.     

Highly efficient single-layer gas diffusion layers for the proton exchange membrane fuel cell

In the present study, a series of highly efficient single-layer gas diffusion layers (SL-GDLs) was successfully prepared by the addition of a vapor grown carbon nanofiber (VGCF) in the carbon black/poly(tetrafluoroethylene) composite-based SL-GDL through a simple and inexpensive process. The scanning electron micrographs of the as-prepared VGCF-containing SL-GDLs (SL-GDL-CFs) showed that the GDLs had a microporous layer (MPL)-like structure, while the wire-like VGCFs were well dispersed and crossed among the carbon black particles in the SL-GDL matrix. Utilization of the SL-GDL-CFs for MEA fabrication was also done by direct coating with the catalyst layer. Due to the presence of VGCFs, the properties of the SL-GDL-CFs, including electronic resistivity, mechanical characteristic, gas permeability, and water repellency, varied with the VGCF content, with the overall effect beneficial to the performance of the proton exchange membrane fuel cell (PEMFC). The best performances obtained from the PEMFC with VGCFs at 15 wt.% was approximately 63% higher than those without VGCFs, and about 85% as efficient as ELAT GDL, a commercial dual-layer GDL, for both the H₂/O₂ and H₂/air systems.     

Study of membrane electrode assemblies for direct methanol fuel cells

At DLR, membrane electrode assemblies (MEA) for direct methanol fuel cells (DMFC), are produced with the company’s own dry production technique. For improving this production technique, the MEAs in fuel cells are characterized electrochemically in fuel cell test facilities as well as physically by scanning electron microscopy (SEM).In order to measure the local current densities in polymer electrolyte membrane fuel cells, a method has been developed at DLR and tested in fuel cells supplied with hydrogen as fuel. For the DMFC, a measuring cell with 16 segments was built for examining MEAs with an overall active electrode area of 25 cm². With a sufficient resolution of location and time, simultaneous measurement of different local current densities in the cell can be carried out thus accelerating and improving operating parameter studies. This new tool is used at DLR for characterizing and developing improved MEAs and for examining the cell design (e.g. flow fields) and operating conditions of DMFC. In the measuring cell with its segments, the local mass conversion rates in the DMFC for liquid methanol–water mixtures are examined.     

Pore network modeling of fibrous gas diffusion layers for polymer electrolyte membrane fuel cells

A pore network model of the gas diffusion layer (GDL) in a polymer electrolyte membrane fuel cell is developed and validated. The model idealizes the GDL as a regular cubic network of pore bodies and pore throats following respective size distributions. Geometric parameters of the pore network model are calibrated with respect to porosimetry and gas permeability measurements for two common GDL materials and the model is subsequently used to compute the pore-scale distribution of water and gas under drainage conditions using an invasion percolation algorithm. From this information, the relative permeability of water and gas and the effective gas diffusivity are computed as functions of water saturation using resistor-network theory. Comparison of the model predictions with those obtained from constitutive relationships commonly used in current PEMFC models indicates that the latter may significantly overestimate the gas phase transport properties. Alternative relationships are suggested that better match the pore network model results. The pore network model is also used to calculate the limiting current in a PEMFC under operating conditions for which transport through the GDL dominates mass transfer resistance. The results suggest that a dry GDL does not limit the performance of a PEMFC, but it may become a significant source of concentration polarization as the GDL becomes increasingly saturated with water.     

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 pro- ton exchange membrane fuel cell （PEMFC）. An ex-situ testing was conducted on a transparent test cell to visu- alize the water droplet formation and detachment on the surface of different types of GDLs through a CCD cam- era. 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 different porosities and thick- nesses. The equilibrium pressure, which is defined as the minimum pressure required maintaining a constant flow through the GDL, was also recorded. The equilibrium pressure was found to be much lower than the breakthrough pressure for the same type of GDL. A pore network model was modified to further study the relationship between the breakthrough pressure and the GDL properties and thicknesses. The breakthrough pressure increases for the thick GDL with smaller micro-pore size.     

Selective membrane electrode assemblies for bipolar plate-free mixed-reactant fuel cells

Membrane electrode assemblies (MEAs) with PtRu/C and Pt/C as both sides electrocatalysts have been fabricated and used in a mixed-reactant alkaline fuel cell without bipolar plate. It has been found that the reactant selectivity of the electrocatalyst is tunable by changing the electrode hydrophobicity. The measurement on a three-cell stack with MEAs using a more hydrophobic PtRu/C electrode and a more hydrophilic Pt/C electrode in an ethanol/oxygen mixed solution shows that the PtRu/C electrode serves as a cathode and Pt/C electrode performs as an anode. The steady-state power density of 31 mW cm⁻² is achieved in an ethanol solution by continuously bubbling oxygen at ambient temperature and atmosphere pressure.     

Water permeation through gas diffusion layers of proton exchange membrane fuel cells

Water transport through gas diffusion layer of proton exchange membrane fuels cells is investigated experimentally. A filtration cell is designed and the permeation threshold and the apparent water permeability of several carbon papers are investigated. Similar carbon paper with different thicknesses and different Teflon loadings are tested to study the effects of geometrical and surface properties on the water transport. Permeation threshold increases with both GDL thickness and Teflon loading. In addition, a hysteresis effect exists in GDLs and the permeation threshold reduces as the samples are retested. Moreover, several compressed GDLs are tested and the results show that compression does not affect the breakthrough pressure significantly. The measured values of apparent permeability indicate that the majority of pores in GDLs are not filled with water and the reactant access to the catalyst layer is not hindered.     

Development of a self-supported single-wall carbon nanotube-based gas diffusion electrode with spatially well-defined reaction and diffusion layers

This work reports on the development of a solvent-free method for the fabrication of a self-supported single-wall carbon nanotubes electrode, which is based on successive sedimentation of both SWCNT/surfactant and PtRu-SWCNT/surfactant suspensions followed by a thermal treatment at 130 °C. The as-prepared self-supported electrode showed sufficient mechanical strength for half-cell investigation and membrane-electrodes assembly fabrication. By using a Pt catalyst loading of 1 mg cm⁻², the overall thickness of the gas diffusion electrode reached 95 μm. Its electrochemical activity towards methanol oxidation was investigated by means of cyclic voltammetry and current-voltage polarisation measurements under half-cell and direct methanol fuel cell conditions.     

Carbon nano-chain and carbon nano-fibers based gas diffusion layers for proton exchange membrane fuel cells

Gas diffusion layers (GDL) for proton exchange membrane fuel cell have been developed using a partially ordered graphitized nano-carbon chain (Pureblack^® carbon) and carbon nano-fibers. The GDL samples’ characteristics such as, surface morphology, surface energy, bubble-point pressure and pore size distribution were characterized using electron microscope, inverse gas chromatograph, gas permeability and mercury porosimetry, respectively. Fuel cell performance of the GDLs was evaluated using single cell with hydrogen/air at ambient pressure, 70 °C and 100% RH. The GDLs with combination of vapor grown carbon nano-fibers with Pureblack carbon showed significant improvement in mechanical robustness as well as fuel cell performance. The micro-porous layer of the GDLs as seen under scanning electron microscope showed excellent surface morphology showing the reinforcement with nano-fibers and the surface homogeneity without any cracks.     

Wire rod coating process of gas diffusion layers fabrication for proton exchange membrane fuel cells

Gas diffusion layers (GDLs) were fabricated using non-woven carbon paper as a macro-porous layer substrate developed by Hollingsworth & Vose Company. A commercially viable coating process was developed using wire rod for coating micro-porous layer by a single pass. The thickness as well as carbon loading in the micro-porous layer was controlled by selecting appropriate wire thickness of the wire rod. Slurry compositions with solid loading as high as 10 wt.% using nano-chain and nano-fiber type carbons were developed using dispersion agents to provide cohesive and homogenous micro-porous layer without any mud-cracking. The surface morphology, wetting characteristics and pore size distribution of the wire rod coated GDLs were examined using FESEM, Goniometer and Hg porosimetry, respectively. The GDLs were evaluated in single cell PEMFC under various operating conditions (temperature and RH) using hydrogen and air as reactants. It was observed that the wire rod coated micro-porous layer with 10 wt.% nano-fibrous carbon based GDLs showed the highest fuel cell performance at 85 °C using H₂ and air at 50% RH, compared to all other compositions.     

Effects of heat treatment time on electrochemical properties and electrode structure of polytetrafluoroethylene-bonded membrane electrode assemblies for polybenzimidazole-based high-temperature proton exchange membrane fuel cells

To improve cell performance, the effects of heat treatment time on the electrochemical properties and electrode structure of PTFE-bonded membrane electrode assemblies for PBI-based high-temperature proton exchange membrane fuel cells are investigated. The cell performance is observed to decrease in the high-current-density region rather than in the low-current-density region with increasing heat treatment time at 350 °C from 1 to 30 min. Microscopic studies reveal remarkable differences in the electrode structure by the agglomeration of dispersed PTFE and adjacent catalyst particles, depending on the heat treatment time. As the heat treatment time increases, only the large pore (secondary pore) volume in the electrode decreases, resulting in increase in mass transport resistance and concentration overpotential in the high-current-density region. Cell performance is not measured without heat treatment because the electrodes are not formed. When the electrodes are heat treated for 1 min at 350 °C, the best cell performance is obtained, 0.67 V at 200 mA cm⁻².     

A novel process to fabricate membrane electrode assemblies for proton exchange membrane fuel cells

A new fabrication method of membrane electrode assembly (MEA) for proton exchange membrane fuel cells is developed by using perfluorosulfonyl fluoride copolymer powder and Pt/C catalyst. The perfluorosulfonyl fluoride copolymer powder is pressed into a sheet at 230°C by hot pressing. The Pt/C catalyst is then coated on to either side of the sheet by screen printing, followed by hot pressing. During this process, due to the melt-fabricable property of the pre-formed sheet, the coated catalyst layer is embedded into the membrane. The resultant MEA is converted into perfluorosulfonate polymer by hydrolysis of NaOH solution. The thermal property of the copolymer powder has been analyzed by DTA-TGA, and the interfacial contact of the catalyst with the membrane has been also investigated by SEM. The performance characteristics of the MEA have been evaluated in a single cell.     

Sputtered Pt loadings of membrane electrode assemblies in proton exchange membrane fuel cells

The fabrication of electrodes use in proton exchange membrane fuel cells (PEMFCs) by Pt sputter deposition has great potential to increase Pt utilization and reduce Pt loading without loss of cell performance. A radio frequency (RF) magnetron sputter deposition process (RF power = 100 W and argon pressure = 10^?3 Torr) was adopted to prepare Pt catalyst layers of PEMFC electrodes. The effects of cathode Pt and Nafion loadings on membrane electrode assembly (MEA)/cell performance were investigated using cell polarization, cyclic voltammetry, AC impedance, and microstructure analysis. Among the tested MEAs with various cathode Pt loadings (0.02–0.4 mg cm^?2), the one with 0.1 mg‐Pt cm^?2 (grain size = 3.90 nm, mainly Pt(111)) exhibited the best cell performance (320 and 285 mW cm^?2 at 0.44 and 0.60 V, respectively), which was similar to or better than those of some commercial nonsputtered/sputtered electrodes with the same or higher Pt loadings. The electrode Pt utilization efficiency increased as the Pt loading decreased. A Pt loading of greater than or lower than 0.1 mg cm^?2 yielded a lower electrode electrochemical active surface (EAS) area but a higher charge transfer and diffusion resistance. Nafion impregnation (0.1 to 0.3 mg cm^?2) into the sputtered Pt layer (Pt = 0.1 mg cm^?2) noticeably increased the EAS area, consistent with the decrease of the capacitance of the electrode double layer, but did not improve MEA/cell performance, mainly because of the increase in the kinetic and mass transfer resistances associated with oxygen reduction on the cathode. Copyright © 2011 John Wiley & Sons, Ltd.     

High performance proton exchange membrane fuel cell electrode assemblies

The conventional 5-layer membrane electrode assembly (MEA) consists of a proton exchange membrane (PEM) locating at its center, two layers of Pt-C-40 (Pt content 40 wt%) locating next on both surfaces of PEM, and two gas diffusion layers (GDL) locating next on the outer surfaces of Pt-C layers (structure-a MEA). In this paper, we report three modified MEAs consisting of Pt-C-40 (Pt content 40 wt%) and Pt-C-80 (Pt content 80 wt%) catalysts. These are: (1) 7-layer structure-b MEA with a thin Pt-C-80 layer locating between Pt-C-40 layer and PEM; (2) 7-layer structure-c MEA with a thin Pt-C-80 layer locating between Pt-C-40 layer and GDL; and (3) 5-layer structure-d MEA with Pt-C-40 and Pt-C-80 mixing homogeneously and locating between PEM and GDL. Under a fixed Pt loading, we find structure-b, -c, and -d MEAs with 20-40 wt% Pt contributed from Pt-C-80 have better fuel cell performance than structure-a MEA consisting only of Pt-C-40. The reasons for the better fuel cell performance of these modified MEAs are attributed to the better feasibility for O₂ gas to reach cathode Pt particles and lower proton transport resistance in catalyst layers of the modified MEAs than structure-a MEA.     

Superhydrophobic PAN nanofibers for gas diffusion layers of proton exchange membrane fuel cells for cathodic water management

Proton exchange membrane (PEM) fuel cells are considered to be promising alternatives to natural resources for generating electricity and various other powers. Optimal water management in the gas diffusion layer (GDL) is critical to the high performance of fuel cells. The basic function of the GDL includes transporting the reactant gas from flow channels to the catalyst effectively, draining liquid water from the catalyst layer to the flow channels, and conducting electrons with low humidity. In this study, poly-acrylonitrile (PAN) was dissolved in a solvent and electrospun at various conditions to produce PAN nanofibers prior to their stabilization at atmospheric pressure at 280 °C for 1 h and carbonization at 850 °C for one more hour. The surface hydrophobicity of the carbonized PAN nanofibers were adjusted using superhydrophobic and hydrophilic agents. The thermal, mechanical, and electrical properties of the new GDLs showed better results than the conventional ones. Water condensation tests (superhydrophobic and hydrophilic) on the surfaces of the GDLs showed a crucial step towards improved water management in fuel cells. This study may open up new possibilities for developing high-performing GDL materials for future PEM fuel cell applications.     

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

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

Low temperature decal transfer method for hydrocarbon membrane based membrane electrode assemblies in polymer electrolyte membrane fuel cells

Decal transfer is an effective membrane electrode assembly (MEA) fabrication method known for its low interfacial resistance and suitability for mass processing. Previously decal transfer for hydrocarbon membranes was performed at temperatures above 200 °C. Here a novel low temperature decal transfer (LTD) method for hydrocarbon membranes is introduced. The new method applies a small amount (2.2 mg cm⁻²) of liquid (1-pentanol) onto the membrane separator before decal transfer to lower the T_g of the membrane and achieves complete decal transfer at 110 °C and 6 MPa. Nafion binder amount in the catalyst layer and catalyst layer annealing temperature is controlled to optimize the fuel cell performance. Compared to conventional decal transfer (CDT), the novel LTD method shows enhancement in energy efficiency, simplicity in the process scheme, and improvement in fuel cell performance.     

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.