Carbon film coating on gas diffusion layer for proton exchange membrane fuel cells

This study discusses a novel process to increase the performance of proton exchange membrane fuel cells (PEMFC). In order to improve the electrical conductivity and reduce the surface indentation of the carbon fibers, we modified the carbon fibers with pitch-based carbon materials (mesophase pitch and coal tar pitch). Compared with the gas diffusion backing (GDB), GDB-A240 and GDB-MP have 32% and 33% higher current densities at 0.5 V, respectively. Self-made carbon paper with the addition of a micro-porous layer (MPL) (GDL-A240 and GDL-MP) show improved performance compared with GDB-A240 and GDB-MP. The current densities of GDL-A240 and GDL-MP at 0.5 V increased by 37% and 31% compared with GDL, respectively. This study combines these two effects (carbon film and MPL coating) to promote high current density in a PEMFC.     

Synthesis and characterization of carbon nanotubes supported platinum nanocatalyst for proton exchange membrane fuel cells

Multi-walled carbon nanotubes (MWCNTs) were used as catalyst support for depositing platinum nanoparticles by a wet chemistry route. MWCNTs were initially surface modified by citric acid to introduce functional groups which act as anchors for metallic clusters. A two-phase (water-toluene) method was used to transfer PtCl₆²⁻ from aqueous to organic phase and the subsequent sodium formate solution reduction step yielded Pt nanoparticles on MWCNTs. High-resolution TEM images showed that the platinum particles in the size range of 1-3 nm are homogeneously distributed on the surface of MWCNTs. The Pt/MWCNTs nanocatalyst was evaluated in the proton exchange membrane (PEM) single cell using H₂/O₂ at 80 °C with Nafion-212 electrolyte. The single PEM fuel cell exhibited a peak power density of about 1100 mW cm⁻² with a total catalyst loading of 0.6 mg Pt cm⁻² (anode: 0.2 mg Pt cm⁻² and cathode: 0.4 mg Pt cm⁻²). The durability of Pt/MWCNTs nanocatalyst was evaluated for 100 h at 80 °C at ambient pressure and the performance (current density at 0.4 V) remained stable throughout. The electrochemically active surface area (64 m² g⁻¹) as estimated by cyclic voltammetry (CV) was also similar before and after the durability test.     

Single-walled carbon nanotube interlayer modified gas diffusion layers to boost the cell performance of self-humidifying proton exchange membrane fuel cells

A tradeoff between the low humidity and the high performance remains a key challenge for the proton exchange membrane fuel cell (PEMFC). In this work, a novel self-humidifying gas diffusion layer (GDL) with a single-walled carbon nanotube (SWCNT) nonwoven layer between the gas diffusion substrate and the hydrophobic microporous layer is controllably prepared to elevate the cell performance under dry conditions. The membrane electrode assembly (MEA) with 0.25 mg cm⁻² SWCNT loading exhibits a current density of 0.69 A cm⁻² at 0.6 V, which is 392.8% higher than that of the counterpart without the SWCNT interlayer at the same relative humidity. Moreover, the SWCNT interlayer with rational pore structure and proper wettability dramatically improves the water retention capacity of MEA, thus enhancing the low-humidity performance of MEA. The structure design of GDL provides an effective strategy for self-humidifying PEMFC control optimization.     

Functionalizing carbon nanotubes for proton exchange membrane fuel cells electrode

In recent years, carbon nanotubes (CNTs) have been increasingly considered as an advanced metal catalyst support for proton exchange membrane fuel cells (PEMFCs), owing to their outstanding physical and mechanical characteristics. However, the effective attachment of metal catalysts, uniformly dispersed onto the CNT surface, remains a formidable challenge because of the inertness of the CNT walls. Therefore, the surface functionalization of CNTs seems necessary in most cases in order to enable a homogeneous metal deposition. This review presents the different surface functionalization approaches that provide efficient avenues for the deposition of metal nanoparticles on CNTs, for the application of catalyst supports in PEMFCs with improved reactivity.     

Anode water removal and cathode gas diffusion layer flooding in a proton exchange membrane fuel cell

Anode water removal (AWR) is studied as a diagnostic tool to assess cathode gas diffusion layer (GDL) flooding in PEM fuel cells. This method uses a dry hydrogen stream to remove product water from the cathode, showing ideal fuel cell performance in the absence of GDL mass transfer limitations related to water. When cathode GDL flooding is limiting, the cell voltage increases as the hydrogen stoichiometry is increased. Several cathode GDLs were studied to determine the effect of microporous layer (MPL) and PTFE coating. The largest voltage gains occur with the use of cathode GDLs without an MPL since these GDLs are prone to higher liquid water saturation. Multiple GDLs are studied on the cathode side to exacerbate GDL flooding conditions to further confirm the mechanism of the AWR process. Increased temperature and lower cathode RH allow for greater overall water removal so the voltage improvement occurs faster, though this leads to quicker membrane dehydration.     

Effect of deformation on electrical properties of carbon fibers used in gas diffusion layer of proton exchange membrane fuel cells

In proton exchange membrane fuel cells, the stacks of anode, cathode, and membrane layers including gas diffusion layer (GDL) are held together by a compressive force applied through a bipolar plate. In this work, we studied the electrical properties of a carbon fiber of a GDL under deformation using four-point measurement methods inside a scanning electron microscope (SEM). We found out that through bending deformation the electrical resistivity of carbon fibers will be reduced. The drop in resistance during deformation may be the result of increasing conduction channels in the carbon fiber and parallel transport through them. Our finding offers a new insight on the effect of deformation on tuning the electrical properties of GDL materials.     

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.     

Modeling an ordered nanostructured cathode catalyst layer for proton exchange membrane fuel cells

A 3D mathematical model of an ordered nanostructured cathode catalyst layer (CCL) has been developed for proton exchange membrane (PEM) fuel cells. In an ordered nanostructured CCL, carbon nanotubes (CNTs) are used as support material for Pt catalyst, upon which a thin layer of proton-conducting polymer (Nafion) is deposited, which are then aligned along the main transport direction (perpendicular to the membrane) of various species. The model considers all the relevant processes in different phases of an ordered nanostructured CCL. In addition, the effect of Knudsen diffusion is accounted in the model. The model can predict not only the performance of an ordered nanostructured CCL at various operating and design conditions but also can predict the distributions of various fields in different phases of an ordered nanostructured CCL. The predicted nanostructured CCL performance with estimated membrane overpotential is validated with measured data found in the literature, and a good agreement is obtained between the model prediction and measured result. Moreover, a parametric study is conducted to investigate the effect of key design parameters on the performance of an ordered nanostructured CCL. In the absence of liquid water, it is found that oxygen diffusion in the pore phase is not the limiting factor for the performance of an ordered nanostructured CCL, owing to its parallel gas pores and high porosity. However, the transport of dissolved oxygen through the Nafion phase has a significant effect on the performance of an ordered nanostructured CCL. Further, it is found that increasing the spacing between CNTs results in a considerable drop in the performance of an ordered nanostructured CCL at the base case conditions considered in the simulation.     

A review of platinum-based catalyst layer degradation in proton exchange membrane fuel cells

Catalyst layer degradation has become an important issue in the development of proton exchange membrane (PEM) fuel cells. This paper reviews the most recent research on degradation and durability issues in the catalyst layers including: (1) platinum catalysts, (2) carbon supports, and (3) Nafion ionomer and interfacial degradation. The review aims to provide a clear understanding of the link between microstructural/macrostructural changes of the catalyst layer and performance degradation of the PEM fuel cell fueled with hydrogen under normal operating or accelerated stress conditions. In each section, different degradation mechanisms and their corresponding representative mitigation strategies are presented. Also, general experimental methods are classified and various investigation techniques for evaluating catalyst degradation are discussed.     

Effect of conductive carbon material content and structure in carbon fiber paper made from carbon felt on the performance of a proton exchange membrane fuel cell

This study concerns the use of conductive carbon material with different content and structure to produce carbon fiber paper for use in proton exchange membrane fuel cells, and investigates how changes in the content and structure of the conductive carbon material influence fuel cell performance.In this study, phenolic resin is used as a conductive carbon material, and is subjected to heat treatment at temperatures of 700 °C, 1000 °C, and 1400 °C, which changes its structure. Before carbon fiber paper is prepared from carbon felt, the felt is treated with phenolic resin solutions with resin content of 5, 10, 15, 20, 25, and 30 wt%. During fuel cell testing, torsion of 40, 60, 80, 100, and 120 kgf-cm is applied. The study found that when the phenolic resin content is 15 wt%, the heat treatment temperature 1400 °C, the test area 25 cm², and the test temperature 65 °C, a fuel cell can achieve a current density of 2020 mA cm⁻² at 0.5 V and torque of 120 kgf-cm.     

Liquid water breakthrough pressure through gas diffusion layer of proton exchange membrane fuel cell

The dynamic behavior of liquid water transport through the gas diffusion layer (GDL) of the proton exchange membrane fuel cell is studied with an ex-situ approach. The liquid water breakthrough pressure is measured in the region between the capillary fingering and the stable displacement on the drainage phase diagram. The variables studied are GDL thickness, PTFE/Nafion content within the GDL, GDL compression, the inclusion of a micro-porous layer (MPL), and different water flow rates through the GDL. The liquid water breakthrough pressure is observed to increase with GDL thickness, GDL compression, and inclusion of the MPL. Furthermore, it has been observed that applying some amount of PTFE to an untreated GDL increases the breakthrough pressure but increasing the amount of PTFE content within the GDL shows minimal impact on the breakthrough pressure. For instance, the mean breakthrough pressures that have been measured for TGP-060 and for untreated (0 wt.% PTFE), 10 wt.% PTFE, and 27 wt.% PTFE were 3589 Pa, 5108 Pa, and 5284 Pa, respectively.     

Effects of the carbon black properties in gas diffusion layer on the performance of proton exchange membrane fuel cells

The microporous layer (MPL) as a part of diffusion medium has an important impact on mass transfer of proton exchange membrane fuel cell (PEMFC). In this study, MPLs of gas diffusion layers (GDLs) are prepared with different carbon blacks, and the properties of carbon blacks and their effects as MPLs on cell performance are systematically investigated. The results show that the GDL prepared by Acetylene Black (ACET) exhibits the best performance with a maximum power density up to 2.05 W cm⁻². Moreover, it still maintains extremely high performance with increasing current density even at humidity condition of 100% relative humidity, which means its excellent water/gas transportation capacity. This study contributes to deeply understanding the correlations between the properties of MPL material itself and their corresponding performance exhibited in cell. It also provides an important reference for enhancing cell performance and further advancing the practical applications of MPLs in PEMFC field.     

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.     

Investigation of fabrication of gas diffusion substrate for proton exchange membrane fuel cells

The gas diffusion substrate (GDS) is essential in the proton exchange membrane fuel cells. Its fabrication techniques affect the performance significantly and are worthy of investigation. In this study, a manufacturing process of the GDS is proposed to understand the formation process of GDS and promote its structure and performance more pertinently. Different states during the preparation process, raw carbon paper, pre-curing, curing, carbonation, and graphitization, are characterized and measured. Experimental and numerical methods are employed to determine the relationships between microstructure, transport, and mechanical performance variation with the fabricating processes. The results show that its porosity, average pore size, and effective diffusivity decrease first and increase after curing. These parameters after graphitization are lower than that of the carbon paper (CP). The electrical resistivity increases dramatically while pre-curing and decreases gradually after curing, carbonation, and graphitization, and it is much reduced after graphitization. Moreover, mechanical measurement results show that both the picks of tensile strength and flexural modulus occur after curing. Its tensile strength shows little change after graphitization compared to the initial paper's. In contrast, the flexural modulus is improved significantly.     

An improved model for predicting electrical contact resistance between bipolar plate and gas diffusion layer in proton exchange membrane fuel cells

Electrical contact resistance between bipolar plates (BPPs) and gas diffusion layers (GDLs) in PEM fuel cells has attracted much attention since it is one significant part of the total contact resistance which plays an important role in fuel cell performance. This paper extends a previous model by Zhou et al. [Y. Zhou, G. Lin, A.J. Shih, S.J. Hu, J. Power Sources 163 (2007) 777–783] on the prediction of electrical contact resistance within PEM fuel cells. The original microscale numerical model was based on the Hertz solution for individual elastic contacts, assuming that contact bodies, GDL carbon fibers and BPP asperities are isotropic elastic half-spaces. The new model features a more practical contact by taking into account the bending behavior of carbon fibers as well as their anisotropic properties. The microscale single contact process is solved numerically using the finite element method (FEM). The relationship between the contact pressure and the electrical resistance at the GDL/BPP interface is derived by multiple regression models. Comparisons of the original model by Zhou et al. and the new model with experimental data show that the original model slightly overestimates the electrical contact resistance, whereas a better agreement with experimental data is observed using the new model.     

Synthesis and characterization of platinum nanoparticles on in situ grown carbon nanotubes based carbon paper for proton exchange membrane fuel cell cathode

Multi-walled carbon nanotubes (MWCNTs) based micro-porous layer on the carbon paper substrates was prepared by in situ growth in a chemical vapor deposition setup. Platinum nanoparticles were deposited on in situ grown MWCNTs/carbon paper by a wet chemistry route at <100 °C. The in situ MWCNTs/carbon paper was initially surface modified by silane derivative to incorporate sulfonic acid–silicate intermediate groups which act as anchors for metal ions. Platinum nanoparticles deposition on the in situ MWCNTs/carbon paper was carried out by reducing platinum (II) acetylacetonate precursor using glacial acetic acid. High resolution TEM images showed that the platinum particles are homogeneously distributed on the outer surface of MWCNTs with a size range of 1–2 nm. The Pt/MWCNTs/carbon paper electrode with a loading of 0.3 and 0.5 mg Pt cm⁻² was evaluated in proton exchange membrane single cell fuel cell using H₂/O₂. The single cells exhibited a peak power density of 600 and 800 mW cm⁻² with catalyst loadings of 0.3 and 0.5 mg Pt cm⁻², respectively with H₂/O₂ at 80 °C, using Nafion-212 electrolyte. In order to understand the intrinsically higher fuel cell performance, the electrochemically active surface area was estimated by the cyclic voltammetry of the Pt/MWCNTs/carbon paper.     

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

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

A novel catalyst layer with carbon matrix for Pt nanowire growth in proton exchange membrane fuel cells (PEMFCs)

Novel catalyst layers for proton exchange membrane fuel cells (PEMFCs) were investigated by in-situ growing of Pt nanowires (Pt-NWs) on carbon matrix. The Pt-NWs grew on the matrix along the thickness direction with a length of 10–20 nm and a diameter of 4 nm. In-situ cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and polarization experiments were employed to characterize the electrochemical performance of the Pt-NWs electrodes. The results showed that the predominantly {111}-oriented facets and oxygen access of the Pt-NWs structure contribute to the higher performance in comparison with that of the conventional catalyst layers. This work is advantageous for fuel cell catalyst layer design by allowing the controlled modification of both Pt distribution and pore size.     

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.     

Effect of water distribution in gas diffusion layer on proton exchange membrane fuel cell performance

Water management of proton exchange membrane fuel cells remains a prominent issue in research concerning fuel cells. In this study, the gas diffusion layer (GDL) of a fuel cell is partially treated with a hydrophobic agent, and the effect of GDL hydrophobicity on the water distribution in the fuel cell is examined. First, the effect of the position of the cathode GDL hydrophobic area relative to the channel on the fuel cell performance is investigated. Then, the water distribution in the fuel cell cathode GDL is observed using X-ray imaging. The experimental results indicate that when the hybrid GDL's hydrophobic area lies on the channel, water tends to accumulate under the rib, and the water content in the channel is low; this improves the fuel cell performance. When the hydrophobic area is under the rib, the water distribution is more uniform, but the performance deteriorates.