Gas Diffusion Media Research Articles

Development of an optimal gas diffusion medium for polymer electrolyte membrane fuel cells and assessment of its degradation mechanisms

Currently, Gas Diffusion Media for Polymer Electrolyte Membrane Fuel Cells are made hydrophobic by using mainly PTFE. In this work, PTFE was replaced with a less conventionally employed polymer (i.e. fluorinated ethylene propylene, FEP) and carbon nanotubes partially substituted carbon black in the formulation for preparing MPLs. Three inks with variable CNTs concentration were prepared. The presence of FEP allowed to decrease the treatment temperature (260 °C) with respect to the one necessary with GDMs based on PTFE (350 °C). Performances of the cells assembled with FEP-based samples were better than those obtained with conventional GDMs. GDM containing the highest amount of CNTs (10% wt) showed the best results in terms of output power density and cell efficiency. Durability of such a sample was also assessed. Both chemical and mechanical ad-hoc accelerated stress tests were developed. Such tests showed that the main degradation for MPL is due to mechanical stresses, which led to detachment of part of the micro-porous surface and this caused a dramatic mass transfer resistance increase which means a reduction of capability of managing water. Whereas, chemical AST did not show substantial changes in GDMs surface resulting in more restrained diffusion resistance increases.

Double-layer gas diffusion media for improved water management in polymer electrolyte membrane fuel cells

Water management is an important consideration to improve the performance of polymer electrolyte membrane fuel cells (PEMFCs). Reactant gases are supplied to the catalyst layer through gas diffusion media (GDM) in PEMFCs. Under high current draw conditions, the GDM can flood with liquid water which is detrimental to fuel cell operation. In this paper, we investigate the effectiveness of a double-layer GDM with different polytetrafluoroethylene (PTFE) loadings to mitigate saturation with liquid water under flooding conditions. We developed a numerical model to predict the saturation field in the double-layer GDM, and showed that the overall saturation level is decreased when the layer with higher PTFE loading is placed adjacent to the catalyst layer. It was found that the interface between the two GDM layers plays a significant role in regulating the saturation level by lowering the driving saturation level for water transport inside the GDM. The numerical predictions of the model were validated by conducting experiments with the double-layer GDM which showed a higher limiting current density and more stable performance, thereby confirming that the new GDM architecture can improve water management in PEMFCs.

Synthesis, Pore and PTFE Distribution Studies of Monolithic Gas Diffusion Media for PEMFC By Varying Solvent

Proton Exchange Membrane Fuel cells (PEMFC) are the focus of research internationally, due to their potential in solving energy problems by enabling energy conversion in a clean manner. Gas Diffusion Medium (GDM) is a vital component of a PEMFC which controls the transport of gas and water molecules. The performance of a fuel cell is impacted by mass transport losses. Existing GDMs are based on carbon paper and carbon cloth structures that are commercially available and have specific structures. These commercially available starting materials therefore restrict the overall design of the GDM to work within the constraints of their structure. In the present work the focus is to make a combined porous self standing GDM constituting meso and macro pores using Carbon black (CB) and PTFE. PTFE content as well as its distribution play a prominent role in wicking out water from the reaction site. Method for preparation of the self standing GDM is tailored with starting materials such as CB, Graphite, and Teflon. Factors affecting the uniform distribution of PTFE are identified. Pore size, uniformity and pore distribution obtained through the use of four different solvents – IsoPropylAlcohol (IPA), Glycerol, Ethanol, and Water, are studied. The GDMs prepared are characterized using SEM and Mercury Intrusion Porosimetry. Pore distribution, sizes and composition are examined using SEM images and composition maps on a comparative basis. Optimization of conductivity, hydrophobicity and water transport is carried out. The results obtained are presented and discussed and compared. The advantages a particular solvent and factor contributing an uniform PTFE distribution is recognized as a part of this approach.

Optimization of catalyst layer composition for PEMFC using graphene-based oxygen reduction reaction catalysts

The focus in recent years is on developing high performance non-precious metal catalysts (NPMCs) to reduce the catalyst cost in fuel cells. However, little attention has been paid to improve the utilization of NPMCs. Thus, this study focuses on the optimization of electrode component, particularly the Nafion content. With the synthesized graphene based oxygen reduction reaction (ORR) catalyst, the catalyst inks were prepared at various Nafion contents with suitable amounts of catalysts sprayed on the gas diffusion media. Twenty different single cells were assembled and measured for polarization, resistance and electrochemical impedance. Electrodes of 66.7 and 50.0% Nafion contents showed the highest performance for hydrogen/oxygen and hydrogen/air operation, respectively. These results were explained using the electrochemical impedance spectra, where the highest performance electrode resulted with the lowest charge transfer resistance. Moreover, negligible change in performance was observed during the 80 h of stability test. The optimization compositions of NPMC-based MEAs were very different to Pt-based MEAs, indicating the importance of optimization studies for the practical use of NPMCs.

Micro-porous layer stochastic reconstruction and transport parameter determination

The Micro-Porous Layer (MPL) is a porous, thin layer commonly used in fuel cells at the interfaces between the catalyst layers and gas diffusion media. It is generally made from spherical carbon nanoparticles and PTFE acting as hydrophobic agent. The scale and brittle nature of the MPL structure makes it challenging to study experimentally. In the present work, a 3D stochastic model is developed to virtually reconstruct the MPL structure. The carbon nanoparticle and PTFE phases are fully distinguished by the algorithm. The model is shown to capture the actual structural morphology of the MPL and is validated by comparing the results to available experimental data. The model shows a good capability in generating a realistic MPL successfully using a set of parameters introduced to capture specific morphological features of the MPL. A numerical model that resolves diffusive transport at the pore scale is used to compute the effective transport properties of the reconstructed MPLs. A parametric study is conducted to illustrate the capability of the model as an MPL design tool that can be used to guide and optimize the functionality of the material.

In-Plane Distribution Analysis in the Land-Channel Direction of a Proton Exchange Membrane Fuel Cell (PEMFC)

A novel design was implemented to segment a proton exchange membrane fuel cell (PEMFC) in the land-channel direction to measure the current density distribution at sub-millimeter resolution of 340 μm. This design was validated by performing an ex-situ electrical resistance test, in-situ hydrogen pumping test, and by comparing performances between non-segmented and segmented cells at two different conditions. Further, this design was utilized to measure the current density distribution in wet conditions, in extremely dry condition, and in moderate condition. In wet conditions current densities are found to be higher in the channel areas than in the land areas. To differentiate between the effects of transport distances from channel to the catalyst layer and non-uniform water saturation distribution in gas diffusion medium, oxygen concentration ratios were estimated in wet conditions. Predicted oxygen ratio distribution showed that the effect of water accumulation under the land in gas diffusion medium is more significant than the effect of transport distance which can be well supported by in-situ microscopy study published in literature. However, in dry condition, current density was higher in the land areas than in the channel while in moderate condition current density was found to be uniform.

Hydrophobic Gas-Diffusion Media for Polymer-Electrolyte Fuel Cells by Direct Fluorination

A polymer-electrolyte fuel cell depends on proper water management to obtain high performance. During operation, liquid water is generated in the cell. When it is not properly and adequately removed, accumulation leads to poor fuel-cell performance by reducing and blocking the gas pores in the catalyst and gas-diffusion media. To address this problem, gas-diffusion media are often coated with a wet-proofing agent. This approach results in reduced pore size and volume resulting in lower transport properties, as well as inducing durability and performance issues due to the inherent non-uniformity. To overcome these issues, an alternative wet-proofing process called direct fluorination was developed. In this approach, fluorine gas reacts with carbon to create a more uniform, durable, and consistent wet-proof surface without affecting the morphology of the media. The fluorinated media showed capillary pressure properties that are more suitable for fuel-cell application. Fuel cells with fluorinated materials in the cathodes showed better performance, lower ohmic resistance, and lower liquid water amount in the cathode. These advantages are attributed to having a better wet-proofed fluorinated media at the cathode that forces water back to the anode, thereby keeping the membrane more hydrated and reducing the amount of water in and transported out of the cathode.

Influence of local carbon fibre orientation on the water transport in the gas diffusion layer of polymer electrolyte membrane fuel cells

We used synchrotron X-ray imaging to investigate the influence of local fibre structures of gas diffusion layers (GDLs) in polymer electrolyte membrane fuel cells on the transport of water. Two different measurement techniques, namely in-situ radiography and ex-situ tomography, were combined to reveal the structure–properties relationships between the three-dimensional fibre arrangement and the water flow. We found that the orientation of the local carbon fibres strongly affects the direction of liquid water transport. The carbon fibres act as guiding rails for the water droplets. These findings provide completely new ideas on how gas diffusion media in various types of fuel cells could be designed, in order to optimise transport pathways for liquid water and therefore increase cell performance.

Novel Superhydrophobic Gas Diffusion Media for PEM Fuel Cells: Evaluation of Performance and Durability

Micro-porous layer (MPL) is a crucial component for an efficient water management in Polymer Electrolyte Membrane Fuel Cells (PEMFCs). Nowadays, MPLs are obtained by depositing a dispersion, commonly known as ink, containing carbon black particles and PTFE onto a carbon cloth substrate. In this work PTFE was replaced by an innovative fluorinated polymer in order to improve hydrophobic properties of MPLs and consequently the water management of the whole system. Moreover, carbon nanotubes (CNTs) partially substituted carbon black in ink formulation. The new polymer allowed to decrease considerably the treatment temperature (260 °C) with respect to the one necessary with conventional samples based on PTFE (350 °C). Fuel cells assembled with the sample containing CNTs showed better performances than those obtained with traditional CNTs-free MPLs. Durability of the most performant sample was assessed. Ad-hoc Accelerated Stress Tests (ASTs) were developed; they showed that the main degradation for MPL is due to mechanical stresses, caused by reactants and water flow, which leads to detachment of materials and to a consequent dramatic mass transfer resistance increase.

(Invited) Development of PFSA Ionomers for the Membrane and the Electrodes

Perfluorosulfonic acid (PFSA) ionomer is one of the key materials used in polymer electrolyte fuel cell (PEFC). Most of the membranes used in high performance and high durability PEFC are made from PFSA ionomers. The ionomer for the membrane is required to perform well at high temperatures up to 120 °C at low relative humidity (RH). The ionomer for the electrode, besides transporting the protons to the membrane, its function in cathode of delivering the oxygen to the Pt surface is highlighted in the recent progress of high activity catalyst enabling low Pt loading. Asahi Glass Co., Ltd. (AGC) is also proposing the PFSA ionomer to be used for creating hydrophilic microporous layer (MPL) in between the gas diffusion media (GDM) and the catalyst layer to achieve better performance in low RH conditions. The monomer structures and their copolymer properties as well as their cell performance examined are presented in this paper.

Investigation of Interfacial Water Transport at the Gas Diffusion Media by Neutron Radiography

For cost reduction of fuel cell, it is necessary to realize high current density operation, which requires a thorough understanding of mass transport. However, especially interfacial liquid water transport between carbon paper (CP), micro porous layer (MPL), and catalyst layer (CL) are not fully understood. Coupled cell performance evaluation, liquid water visualization by neutron radiography and numerical modeling were performed with three types of gas diffusion media (GDM): MPL free; CP with MPL; and CP free. It revealed that the presence of a gap at the CL-GDM interface has a significant effect on water accumulation and that the MPL could minimize this effect. At the same time, CP free case showed the best performance and the lowest liquid water content due to multiple impacts of interfacial liquid water transport both at CL-MPL and MPL-channel interfaces. These results confirm the importance of interfacial design between each component of fuel cell for further cost reduction.

Investigation of Interfacial Water Transport at the Gas Diffusion Media by Neutron Radiography

Cost reduction is the most important issue for commercialization of Fuel Cell Electric Vehicle (FCEV)., and high current density operation is one of the solutions that is currently being considered. In order to realize high current density operation, however it is necessary to reduce mass transport resistance. Carbon paper (CP) is commonly used as the gas diffusion layer (GDL), and is shown to dominate the mass transport resistance in fuel cells (1, 2), Tabuchi et al. revealed that it is possible to achieve higher cell performance by removing carbon paper (3). However, mass transport phenomena, especially liquid water transport at the interface between CP, micro porous layer (MPL), and catalyst layer (CL) are yet to be fully understood. In this study, coupled cell performance evaluation, liquid water visualization by neutron radiography (NRG) method and numerical modeling based on multiphase mixture (M2) model(4)were performed to investigate the liquid water transport at the interfaces of components and understand its effect on cell performance.All experiments were conducted with three types of gas diffusion media (GDMs): MPL free; CP with MPL; and CP free. CPs without PTFE treatment were utilized for all these analyses. Operating conditions are summarized in Table 1. NRG was carried out to evaluate liquid water distribution in-situ in each cell during operation. Parameters for NRG were same with T. Kotaka et al. (5) Additionally, numerical analysis with M2 model, which included sub models accounting interface effect of each component on water transport(5), was conducted for thorough understanding of the effect of the interfaces on liquid water transport and cell performance.Figure 1 shows the polarization curves of each cell. CP free cell showed much better performance rather than cells with CPs. The liquid water distributions of each cell at IR-free voltage of around 0.65 V are shown in Figure 2. In case of MPL free cell, liquid water accumulated at the gap between CL and CP, and this liquid water film was seen as a peak of water thickness toward to CL. This liquid water film caused significant increase in oxygen transport resistance and resulted in poor performance. By placing MPL at the interface, this liquid water accumulation was prevented, as MPL is easily deformed by compression and fills the gap, significantly improving the cell performance compared to MPL free case. However, there was still large amount liquid water present inside the CP, which hampers the oxygen transport. In CP free case, the effect is removed. Additionally, due to strong hydrophobicity and much finer structure of MPL, the liquid water coverage on GDM surface is small and liquid water removal is enhanced in CP free case. Thus CP free showed lower water content and better performance. These facts indicated that interfacial design is important to achieve high current density operation. References (1) D. R. Baker, D. A. Caulk, K. C. Neyerlin, and M. W. Murphy, J. Electrochem. Soc, 156(9), B991(2009).(2) R. Jiang, C. K. Mittelsteadt, and C. S. Gittleman, J. Electrochem. Soc.,156(12), B1440(2009).(3) Y. Tabuchi, T. Shiomi, Y. Fukuyama, K. Sato, et al., 222th ECS Meeting , No. 1496(2012).(4) Y. Wang and C. Y. Wang, J. Electrochem. Soc., 154, B636 (2007).(5) T. Kotaka, Y. Tabuchi, U. Pasaogullari, and C. Y. Wang, ECS Trans., 58(1), 1033 (2013). Acknowledgement The authors acknowledge D. S. Hussy, E. Baltic and D. L. Jacobson of the NIST for technical assistance for carrying out NRG experiments and valuable discussion.

Quality Control Diagnostics for Mass Produced Fuel Cell Gas Diffusion Electrodes

With fuel cell systems striving into the market place at increasingly larger numbers, production methods need to begin to rely on cost effective methods to maximize yield and efficiency and minimize waste. The fuel cell industry has identified quality control as a critical barrier for continuous production of MEA components, i.e. membranes, electrodes, and GDLs. One of the critical manufacturing tasks is developing and deploying techniques to provide in-process measurement of fuel cell components for quality control, i.e. quality control (QC) diagnostics that allow for 100% areal inspection of moving fuel cell materials / components.The National Renewable Energy Laboratory has developed a QC diagnostic method for gas diffusion electrodes that is applicable to roll-to-roll fuel cell materials during production. The experimental setup of this Infrared Reactive Impinged Flow (IR/RIF) method is shown in Figure 1. It provides three main functionalities: (i) gas supply and delivery, (ii) GDE material transport, and (iii) IR thermographic imaging. A non-flammable gas mixture that contains small amounts of hydrogen and oxygen (typically 2% and 1%, respectively) is impinged on a gas diffusion media in an ambient environment. The hydrogen reacts in an exothermic reaction at the platinum catalyst of the electrode with the oxygen. The resulting heat signature contains information about the homogeneity of the sample and is observed with an infrared sensing device.Results will be presented that demonstrate the feasibility of the IR/RIF method using NREL’s industrial webline at 10 ft min-1and higher. Production defects were intentionally created in-house using various scratch and/or spray coating methods. Figure 2 shows a sample set consisting of 100% loading reduction defects placed at various locations. Defect sizes of this sample were 5 mm x 5 mm and 2 mm x 2 mm. Results will be discussed using false color temperature movies and images as well as raw and processed temperature data.Figure 1: Schematic of Reactive Impinged Flow Method for detecting defects in the catalyst layer coating of a fuel cell gas diffusion electrode.Figure 2: Example of GDE with catalyst loading defects: 100% loading reduction, 2 mm x 2 mm and 5 mm x 5 mm.Figure 3: False color image of the sample’s heat signature with the sample moving at 10 ft min-1.Figure 4: Temperature data along the dashed line in Figure 3 indicating the detection of two coating defects.AcknowledgmentsThe authors would like to acknowledge funding from the U.S. DOE Fuel Cell Technology Office under contract number DE-AC36-08GO28308. The authors would further like to thank D. Bittinat and J. Porter of Colorado School of Mines for participation in the early development stages of this method.

Invited: Corrugated Cell Structures in Membrane Electrode Assemblies

The traditional Proton Exchange membrane fuel cell stack with its bipolar plates with grooved flow channels,flat membrane electrode assemblies (MEAs), seals and end-plates has been the dominantmethod of construction of multi kW fuel cells for the past 40 years. Smaller sub-watt andportable applications have explored novel cell design variations such as “jelly roll”concepts but none have been able to achieve the power density of the traditionalstacked plate design. The primary reason for this is the efficient collection of the currentand the compact packing of the MEAs. However at the present time one of the most challenging aspects of traditional PEM fuelcell stacks is the difficulty to achieve the platinum catalyst utilization target of 0.2 gPt/kWe set forth by DOE. Good catalyst utilization can be achieved with state-of-theartcatalyst coated membranes (CCM) when ultra-low catalyst loadings (< 0.1 mg/cm2)are used at lower current. Unfortunately when ultra-low loadings are used, the peakpower density is lower as compared to conventional loadings. Thus a larger totalactive area and a larger bipolar plate is needed which results in a lower overall stackpower density and as such the DOE targets of power density can not be achievedsimultaneouslyWe thus now introduce in this proposal, our novel concept to increase the available MEAactive area by 2x or more while maintaining the same bipolar plate size. This isachieved via a corrugated MEA sandwiched between bipolar plates. This is much likecorrugated cardboard, where the center corrugation (or flute) is the MEA. Thecorrugations then form the gas channels thus there is no need for forming flow fieldchannels in the bipolar plates. Thus the gas diffusion media as well as the membrane needs to be formed in the corrugated structure. We will describe the state of our development on this approach.

Design of properties and performances of innovative gas diffusion media for polymer electrolyte membrane fuel cells

• Gas diffusion media for PEM fuel cells were prepared via blade coating technique. • Innovative fluorinated polymers were employed in GDMs preparation. • Novel samples were found to be superhydrophobic. • FEP-based GDMs reduced mass transfer resistance. Gas diffusion medium (GDM) is a crucial carbon-based component for water management in polymer electrolyte membrane fuel cells (PEMFCs). GDM consists of a macro-porous substrate, which is usually a carbon cloth or carbon paper, coated with a thin micro-porous layer (MPL). Traditionally, PTFE is used as hydrophobic agent and binder. In this work, three different GDMs were prepared using fluorinated polymers (polytetrafluoroethylene-co-hexafluoro-propylene (FEP), polytetrafluoroethylene-co-perfluoroalcoxy vinyl ether (PFA) and a fluorinated polyurethane based on perfluoropolyether blocks (PFPE)) in order to replace conventional PTFE as hydrophobic agent, which was used as reference. Inks composition and rheological behaviour were fixed in order to apply the blade coating technique for MPL deposition. These polymers allowed to decrease temperature during thermal treatments with respect to the one necessary for treating PTFE-based MPLs. Moreover, superhydrophobic layers were obtained for all the samples and the final electrical performances of the whole fuel cell system were improved with respect to the ones reached with PTFE-based samples. In particular, FEP allowed to reduce significantly mass transfer resistances in the operating conditions of the fuel cell.

Methodic aspects of XPS depth profiling for investigations of fuel cell components

The focus of this work is to obtain detailed information about the physical composition of the individual fuel cell components in order to optimize them and increase cell durability. An approach for the ex‐situ characterization of fuel cell components, such as gas diffusion media or catalyst layers is by using XPS. In this work, we address methodic aspects of depth profiling of direct methanol fuel cell components (microporous layers and catalyst layers) using adhesive tape peel‐off with subsequent XPS analysis. By using this approach, we have successfully demonstrated cross‐over of Ru ions and possibly Pt ions from the anodic catalyst to the cathodic catalyst layer and even to the microporous layer. Moreover, our experimental results allow determining where the dissolved catalyst ions redeposit within the membrane electrode assembly. Finally, our findings suggest that cross‐over of Pt and Ru ions, observed under different operation conditions, are uncoupled. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Effect of the hydrophilic and hydrophobic characteristics of the gas diffusion medium on polymer electrolyte fuel cell performance under non-humidification condition

Water is a significant component of polymer electrolyte fuel cells, affecting the proton conductivity in the membrane electrolyte. Therefore, polymer electrolyte fuel cells are generally operated with a humidifier to maintain a high relative humidity of the supplied gases; however, the humidifier contributes additional weight and cost. Although many studies have attempted to develop polymer electrolyte fuel cells without a humidifier, the studies have been mainly focused on the self-humidified membrane electrolyte and catalyst layer. In this paper, the author investigates the effect of polytetrafluoroethylene coated gas diffusion medium on the water content in the membrane electrolyte. The water condensation on the surfaces of the gas diffusion medium is visualised when the polymer electrolyte fuel cell is operated under non-humidification conditions. Numerical simulation suggests that the optimum water saturation is between 0.1 and 0.3 at the gas diffusion medium to hydrate the membrane electrolyte sufficiently without significantly blocking the diffused species under non-humidification conditions.

CNT Membrane as a Free Standing Electrode for PEM Fuel Cell

The prospects of using multiwalled carbon nanotube (MWCNT) membrane as a freestanding gas diffusion media (GDM) in hydrogen proton exchange membrane fuel cells (PEMFCs) has been explored over the conventional GDM of carbon black coated on carbon fiber support. This can have significant implication in developing lightweight, thin, and low cost fuel cells – a step toward increasing its energy and power density. Four different types of membranes are studied for their structure, electrical conductivity, surface area, porous network etc. These were further correlated to their performance in unit PEMFC operated at room temperature.

Investigation of Liquid Water Accumulation in Operating PEM Fuel Cells With/Without MPL and Its Effect on Cell Performance

In situ visualization of liquid water distribution in PEM fuel cell with and without MPL and its effect on the fuel cell performance was investigated. The liquid water accumulation at the interface of catalyst layer (CL) and gas diffusion media (GDM) was observed to have large impact on the performance of fuel cell without MPL. The MPL reduces the liquid water at the interface of CL and GDM and enhance the performance of the fuel cell with MPL. The effect of gas flow rate on the liquid water distribution and performance of the fuel cell with MPL was also studied. The higher water content at the cathode MPL/GDL boundary was observed and corresponds for the large decreasing in cell voltage when lower the gas flow rate.

Influence of the gas diffusion cathode structure on the performance of an air-breathing proton exchange membrane fuel cell

This study presents a comprehensive analysis regarding the relevance of the cathode electrode structure on the performance of an air-breathing proton exchange membrane fuel cell, for which water management is a critical issue. In the light of the obtained results, general guidelines for designing a high-performance cathode for air-breathing operation are provided. Selecting adequate characteristics for the gas diffusion electrode in the cathode is shown to be crucial for optimizing performance. Sufficiently thin catalytic layers increase impressively Pt utilization by favouring the access of oxygen and protons to the electroactive centres. The properties of the gas diffusion medium for air-breathing cathodes have been also observed to be critical. The study reveals that selection criteria for gas diffusion layers based on high air permeability and good hydrophobicity does not ensure optimal performance. The ability of the GDL macroporous structure to expel water from the cathode is decisive for improving oxygen access to the catalytic layer and obtaining high-performance air-breathing fuel cells.