Hygrothermal effects on properties of highly conductive epoxy/graphite composites for applications as bipolar plates

The hygrothermal effects on mechanical, thermal, and electrical properties of highly conductive graphite-based epoxy composites were investigated. The highly conductive graphite-based epoxy composites were found to be suitable for applications as bipolar plates in proton exchange membrane (PEM) fuel cells. The hygrothermal aging experiments were designed to simulate the service conditions in PEM fuel cells. Specifically, the composite specimens were immersed in boiling water, aqueous sulphuric acid solution, and aqueous solution of hydrogen peroxide. The water uptake, changes in surface appearance and dimensions, glass transition behavior and thermal stability, and electrical and mechanical properties were evaluated. The water uptake at short time increased linearly with the square root of time as in linear Fickian diffusion. The presence of graphite significantly reduced both the rate and extent of water uptake. No discernible changes in specimen dimensions, surface appearance, and morphology of the composites were observed. The electrical conductivity and mechanical properties remained almost unchanged. The wet specimens showed slight reduction of glass transition temperature (T_g) due to plasticization of epoxy networks by absorbed water, while the re-dried specimens showed small increase of T_g. The composites maintained high electrical conductivity of about 300–500 S cm⁻¹ and good mechanical properties and showed thermal stability up to 350 °C.     

On the mesoporous TiN catalyst support for proton exchange membrane fuel cell

A mesoporous TiN structure with high surface area and excellent electrical conductivity was fabricated for application as a catalyst support in proton exchange membrane fuel cell (PEMFC). Pt nanoparticles were then uniformly deposited on the TiN porous support by wet chemical reduction. The performances of PEMFC using Pt@TiN electrodes were evaluated by a single cell test station. The membrane electrode assembly using Pt@TiN for both anode and cathode exhibited 70%–120% higher specific power densities than that of commercial E-Tek due to higher electrical conductivity and porosity of the catalyst support and higher Pt utilization efficiency.     

Influence of the gas diffusion layer on the performance of an open cathode polymer electrolyte membrane fuel cell

The performance of an open cathode fuel cell was investigated as a function of the type of gas diffusion layer used in its membrane electrode assemble (MEA) preparation. In this context, a new diffusion layer developed in our laboratory called eCoCell was studied, in comparison with the reputed commercial one Sigracet 38BC.Thus, the characterization of both gas diffusion layers was carried out through the measurement of the following properties: porosity, electrical conductivity, thermal conductivity and hydrophobicity. Finally, the performance of a single open cathode proton exchange membrane fuel cell was carried using both gas diffusion layers. From the results obtained in this work, we conclude that this new gas diffusion layer called eCoCell shows an optimum performance for the whole range of temperatures studied due to its high hydrophobicity, bimodal pore distribution, low thermal conductivity and high electrical conductivity.     

Phosphoric acid‐doped electrodes for a PBI polymer membrane fuel cell

A study of a phosphoric acid (PA)‐doped polybenzimidazole (PBI) membrane fuel cell is reported. The fuel cell used polytetrafluoroethylene (PTFE) in the catalyst layer of the membrane electrode assembly to act as a binder and did not use PBI. The PTFE provided an amorphous phase to hold the PA added to the catalyst layers. The study investigated several parameters of the fuel cell electrode, catalyst layer including: PA loading, PTFE content and catalyst loading and wt% of Pt in the carbon supported catalysts and doping of the PBI membrane. There was a minimum amount of acid doping that gave good cell performance for oxygen reduction in the cathode layer. Good performance of the fuel cell was achieved at 120°C with air of 0.27 W cm^?2 using a 0.51 mg_Pt cm^?2 loading of catalyst. Peak power of 0.4 W cm^?2 was achieved with air at 150°C using a membrane doping of PA of 5.6 PRU (doped acid molecules per repeat polymer unit). Heat treatment of the PTFE‐bonded electrodes to increase hydrophobicity did not improve the cell performance. The effect of a perfluorinated surfactant although reported to enhance oxygen solubility in the catalyst layer led to a poorer cell performance. Copyright © 2010 John Wiley & Sons, Ltd.     

Role of electrical resistance and geometry of porous electrodes in the performance of microfluidic fuel cells

Significant electrical resistance is observed in porous electrodes of microfluidic fuel cell due to the size limitation of this energy system. In this work, role of electrical resistance and geometry of porous electrodes in the performance of microfluidic fuel cells is studied with a three‐dimensional numerical model. Parametric simulations are performed to find proper ways to reduce the electrical resistance, including increasing the electrical conductivity of the electrode, changing the electrode geometry, and optimizing the current collector design. The results indicate that the cell cannot fully get rid of the negative influences of the electrical resistance by increasing the electrical conductivity due to the material restriction. Decreasing the electrode length or increasing the electrode width is also not feasible due to the trade‐off between current and current density. Optimization of the aspect ratio of the electrode active region is proved effective in realizing the enhancement of both current and current density. Extending the current collector area from the exposed end to the active region of the porous electrode is also promising as it can decrease the electrical resistance and boost the cell performance simultaneously. The present findings are generally applicable to various miniaturized fuel cell types using porous electrodes.     

Parametric study of the porous cathode in the PEM fuel cell

The electrochemical behavior and the reactant transport in the porous gas diffusion layer (GDL) and catalyst layer (CL) are controlled by a large number of parameters such as porosity, permeability, conductivity, catalyst loading, and average pore size, etc. A three‐dimensional polymer electrolyte membrane fuel cell model is developed. The model accounts for the mass, fluid, and thermal transport processes as well as the electrochemical reaction. Using this model, the effects of the various porous electrode design parameters including porosity, solid electronic conductivity, and thermal conductivity of cathode GDL, and the catalyst loading, average pore size of cathode CL are investigated through parametric study. The model is shown to agree well with the experimental data of some porous electrode specifications. In addition, the model shows promise as a tool for optimizing the design of fuel cells. Copyright © 2008 John Wiley & Sons, Ltd.     

Enhanced transport properties of graphene‐based,thin Nafion® membrane for polymer electrolyte membrane fuel cells

A polymer electrolyte membrane fuel cell (PEMFC) is one of the promising renewable energy conversion systems; however, its performance is considerably limited by the sluggish transport properties and/or reaction kinetics of the catalyst layers, especially at a high current density. In this study, graphene‐based, thin Nafion® membranes are prepared using 0 to 4 wt% of graphene nanoflakes, and the effects of the graphene are examined for enhanced transport properties. The electrical conductivity and dielectric constant are drastically enhanced to 0.4 mS/cm and 26 at 4 wt% of graphene nanoflakes, respectively, while the thermal conductivity linearly increases to 3 W/m‐K. The proton conductivity also significantly increases with the aid of graphene nanoflakes at >2 wt% of graphene nanoflakes, and the enhancement doubles compared with those of the carbon‐black (CB)‐based and carbon nanotube (CNT)‐based, thin Nafion® membranes, perhaps due to unique graphene structures. Additionally, the quasi‐steady‐state water contact angle increases from 113° to ~130° with the addition of graphene nanoflakes, showing that a hydrophobic‐like water wetting change may be related to the significant proton conductivity enhancement. This work provides an optimal material design guideline for the transport‐enhanced cathode catalyst layer using graphene‐based materials for polymer electrolyte membrane fuel cell applications.     

Electrode in direct methanol fuel cells

Nanotechnology has recently generated a lot of attention and high expectations not only in the academic community but also among investors, scientists and researchers in both government and industry sectors. Its unique capability to fabricate new structures at the atomic scale has already produced novel materials and devices with great potential applications in a wide number of fields. Up to now, the electrodes in direct methanol fuel cells (DMFCs) have generally been based on the porous carbon gas diffusion electrodes that are employed in proton exchange membrane fuel cells. Typically, the structure of such electrodes is comprised of a catalyst layer and a diffusion layer, the latter being carbon cloth or carbon paper. It is a challenge to develop an electrode with high surface area, good electrical conductivity and suitable porosity to allow good reactant flux and high stability in the fuel cell environment. This paper presents an overview of electrode structure in general and recent material developments, with particular attention paid to the application of nanotechnology in DMFCs.     

A study on novel pulse preparation and electrocatalytic activities of Pt/C-Nafion electrodes for proton exchange membrane fuel cell

To aim at reducing the platinum loading and increasing the utilization of platinum in PEMFC electrode, a new pulse electrodeposition technique for preparing proton exchange membrane fuel cell (PEMFC) electrodes has been developed in this paper. This method combines coating Pt seeds on the C-Nafion substrate and introducing polyethylene glycol (PEG) into the deposition solution. SEM images of the samples show that Pt seeds and PEG take an important role in the morphology of the Pt deposit. The surface area and average particle size of Pt were determined by charge integration under the hydrogen desorption peaks of cyclic voltammetry. The electrocatalytic activities of these electrodes towards oxygen reduction reaction (ORR) were investigated by using rotating disc electrode (RDE). The Pt catalyst which was prepared by Pt seeds and PEG, its active surface area and electrocatalytic activity towards ORR were improved remarkably. And the optimized electrode displayed higher catalytic activity than a conventional electrode made from commercial Pt/C catalyst. The possible reasons for the effects of Pt seeds and PEG on the higher catalytic activity of prepared Pt catalysts have been preliminarily discussed.     

Single-step fabrication of ABPBI-based GDE and study of its MEA characteristics for high-temperature PEM fuel cells

Single-step fabrication of a Poly(2,5-benzimidazole) (ABPBI)-based gas diffusion electrode (GDE) by directly adding a carbon-supported-catalyst to a homogeneous ABPBI solution prior to deposition and its membrane electrode assembly (MEA) were investigated for high-temperature proton exchange membrane (PEM) fuel cell applications. The ABPBI and LiCl dosages of the catalyst layer were varied. The characterizations of the resulting electrodes and/or MEA for the gas permeability, electrical resistance, specific electrochemical surface area, AC impedance, cyclic voltammetry and high-temperature PEM fuel cell performance were carried out. The high-temperature PEM fuel cell was successfully demonstrated at temperatures of up to 180 °C under ambient pressure operation. The fuel cell performance was evaluated by using dry hydrogen/oxygen gases, which added the advantage of eliminating the complicated humidification system of Nafion cells. The obtained results revealed that a catalyst layer with an ABPBI content of 15 wt.% and an ABPBI/LiCl ratio of 1:2 was sufficient to obtain the optimal cell performance with better electrochemical properties of low cell impedance, high electrochemical activity, low contact resistance and short activation time.     

The effects of Nafion ionomer content in PEMFC MEAs prepared by a catalyst-coated membrane (CCM) spraying method

We analyzed the effects of ionomer content on the proton exchange membrane fuel cell (PEMFC) performance of membrane electrode assemblies (MEAs) fabricated by a catalyst-coated membrane (CCM) spraying method in partially humidified atmospheric air and hydrogen. When high loading Pt/C catalysts (45.5 wt.%) were used, we observed that catalytic activity was not directly proportional to electrochemical active surface area (EAS). This suggests that ionic conductivity through ionomers in catalyst layers is also an important factor affecting MEA performance. In addition, the effects of mass transport were experimentally evaluated by manipulating the air stoichiometry ratio at the cathodes. MEA performance was more sensitive to flow rates under conditions of higher ionomer content. Due to the combined effect of EAS, ionic conductivity, and mass transfer characteristics (all of which varied according to the ionomer content), an MEA with 30 wt.% ionomer content at the cathode (25 wt.% at the anode) was shown to yield the best performance.     

Power generation using a mesoscale fuel cell integrated with a microscale fuel processor

An integrated fuel reformer and fuel cell system for microscale (10–500 mW) power generation is being developed and demonstrated as an alternative to conventional batteries. In this system, thermal energy is transformed to electricity by stripping the hydrogen from the hydrocarbon fuel (reforming) and converting the hydrogen to electricity in a proton exchange membrane (PEM) fuel cell. The fabrication and operation of a mesoscale fuel cell based on phosphoric acid doped polybenzimidazole (PBI) technology is discussed, along with tests integrating the methanol processor with the fuel cell. The PBI membrane had high ionic conductivity at high temperatures (>150 °C), and sustained the high conductivity at low relative humidity at these temperatures. This high-temperature stability and high ionic conductivity enabled the membrane to tolerate extremely high levels of carbon monoxide up to 10% without significant degradation in performance. The combined fuel cell/reformer system was successfully operated to enable the production of 23 mW of electrical power.     

Electrochemical characterization of binary carbon supported electrode in polymer electrolyte fuel cells

In this paper, we report the use of binary carbon supports for fabricating electrodes of polymer electrolyte fuel cells and their detailed electrochemical characterization. The introduction of a secondary support in electrode is shown to provide a combination of high conductivity and good surface morphology. Cyclic voltammetry and cell polarization test indicate that the electrode prepared by binary carbon supports offers more catalytic sites and thus gives a better performance than that of single support. Comparison of kinetic parameters obtained from the model fitting shows that the improvement is not only due to the increased Pt active surface area, but also attributed to the enhanced kinetics, which is further supported by the decreased activation energy for ORR on binary-support electrode. Through the determination of pressure effect, reaction order of ORR with respect to O₂ is identified to be unity, which is in agreement with other researcher’s result.     

Pore scale modeling of a proton exchange membrane fuel cell catalyst layer: Effects of water vapor and temperature

A pore scale model of a polymer electrolyte membrane (PEM) fuel cell cathode catalyst layer is developed which accounts for species transport, electrochemical reactions and thermal transport. Effective transport parameters are computed over a range of operating conditions including the effective oxygen diffusivity, effective water vapor diffusivity, effective proton conductivity, effective electron conductivity and the effective thermal conductivity. In addition, the total amount of oxygen consumption is computed for different operating conditions. Finally, a critical assessment of the impact of assumptions made in the absence of detailed morphological data is presented.     

Effect of matrix content on the performance of carbon paper as an electrode for PEMFC

Porous conducting carbon fiber‐based composite paper is used as an electrode backing in the fuel cell assembly. It not only acts as a channel through which the reactant and product gases pass to and from the bipolar plate and the catalyst site but also helps in the flow of electrons. In order to perform its role efficiently, it should have sufficient strength, high electrical conductivity, and ideal porous structure. Carbon paper has been fabricated, which builds up the required composite properties. Studies have been conducted to optimize the fiber/matrix ratio in the carbon paper, while ensuring the perfect combination of porosity, mechanical strength, and electrical conductivity for an electrode in a proton electrolyte membrane fuel cells. Detail physico‐mechanical and electrochemical characterizations further ascertain that the fiber/matrix ratio plays an important role in tuning the composite properties. The polarization curve of the unit proton exchange membrane (PEM) fuel cell (with an effective electrode area 4 cm²) shows a peak power density of 916 mW/cm² for the sample with fiber/matrix ratio of 65:35, which is almost the same as the commercially available sigracet gas diffusion layer (SGL) carbon paper tested under similar conditions. Further, proportionally enlarging the electrode area to 100 cm² shows that the carbon paper not only shows almost repeatable results in a given set up but also scales up.     

Catalyst layer formulations for slot-die coating of PEM fuel cell electrodes

The series of electrodes were fabricated by the scalable and manufacturable slot-die coating method for proton exchange membrane fuel cell (PEMFC) application. The inks with different amounts of solids were studied by rheological methods in order to establish a coating window with minimum manufacturing defects. The obtained electrodes were characterized by SEM, AFM, and optical microscopy, which showed that they were uniform and homogeneous with minimum defects. The electrochemical evaluation of the manufactured gas diffusion electrodes (GDE) showed that the main characteristics of the electrodes, like electrochemical surface area, proton resistivity, and double layer capacitance, were found to be close for all samples confirming the reproducibility of the slot-die process. Additionally, we studied the effects of membrane thickness on the performance of the GDE membrane electrode assemblies and determined that a decrease in membrane thickness favored the performance. The obtained results clearly demonstrated the applicability and feasibility of the approach for the Manufacturing of catalyst layers for the fuel cell application with potential for future mass production.     

Sol-gel based silica electrodes for inorganic membrane direct methanol fuel cells

Inorganic glass electrodes are of interest for use with inorganic proton exchange membranes for direct methanol fuel cells. Platinum-ruthenium glass electrodes (PtRu/C-SiO₂) have been prepared by incorporating the PtRu/C nanoparticles into a silica-based matrix. The SiO₂ matrix was synthesized through the sol-gel reaction of 3-(trihydroxysilyl)-1-propanesulfonic acid (3TPS) and 3-glycidoxypropyltrimethoxysilane (GPTMS). The distribution of the PtRu/C particles can be controlled by changing the properties of the gel matrix. The effect of gelation time, mole fraction of reactants within the sol, curing temperature, and glass ionomer content were investigated. The adhesion of the catalyst layer on the membrane, catalytic activity for methanol oxidation, and inhibition of methanol permeation through the membrane have been characterized and optimized. The electroless deposition of PtRu onto the PtRu/C nanoparticles was performed to increase the sheet conductivity of the electrode. It was found that the electrolessly deposited metal improved the catalytic activity for methanol oxidation and decreased the methanol cross-over. The methanol fuel cell performance using the inorganic membrane electrode assembly was 236 μA cm⁻² at 0.4 V and was stable for more than 10 days.     

Degradations in the surface wettability and gas permeability characteristics of proton exchange membrane fuel cell electrodes under freeze-thaw cycles: Effects of ionomer type

Current state-of-the-art proton exchange membrane (PEM) fuel cell electrodes are typically comprised of either short-side-chain (SSC) or long-side-chain (LSC) ionomers, owing to their proven success in the electrode performance and durability under regular cell operation. However, the electrodes based on these two prominent ionomers have not been sufficiently investigated under sub-freezing conditions. In this study, the impact of ionomer type on the degradations of the surface wettability and gas permeability characteristics has been investigated for PEM fuel cell electrodes under freeze-thaw (F-T) cycles between 30 °C and −40 °C. The electrodes comprised of either SSC or LSC ionomers are manufactured with different catalyst loadings. It is found that the F-T cycles induce severe degradations in the electrodes, and the resulting surface morphologies differ greatly, depending on the ionomer type and catalyst loading. For a given catalyst loading, the SSC electrodes degrade more heavily than the LSC ones. Further, independent of the ionomer type, the high catalyst loading electrodes tend to degrade slower than their low catalyst loading counterparts. The SSC catalyst layers peel off from the electrodes virtually completely with the microporous layer largely degraded, inducing a highly corroded and heterogeneous surface morphology. The LSC electrodes experience relatively less degradations, thus the resulting surface morphologies are less corroded and more homogeneous. For all the electrodes, the morphological degradations cause a substantial increase in the gas permeability coefficients, but a decrease in the static contact angles. These increments and decrements correlate well with the severity of the surface degradations, and they are rapid and more substantial for the SSC electrodes.     

Issues of using inorganic proton conductor in the electrodes of polymer electrolyte fuel cells

We clarify the issues to be addressed when inorganic materials are employed in the electrodes of low temperature fuel cells, which conventionally operate around 80 °C. The employed inorganic proton conductor is zirconium sulfophenyl phosphonate (ZrSPP), which is incorporated as an ionomer in the electrode catalyst layers by coating the Pt-supported carbon nanotubes with a ZrSPP layer (ZrSPP–Pt/CNTs). Compared with an MEA with an electrode comprising Nafion and Pt/CNT without a ZrSPP coating, a membrane–electrode assembly (MEA) with an electrode comprising ZrSPP–Pt/CNT exhibits an improved performance at elevated temperatures of 90 °C and 100 °C, illustrating an advantage of the inorganic proton conductors. However, a ZrSPP coating on the Pt/CNTs decreases the cell performance at 80 °C. A detailed in situ analysis using limiting-current measurements reveals that the oxygen transport resistance through the solid ionomers increases by approximately six times with the incorporation of the ZrSPP layer. These results indicate that the mass transport through the inorganic materials should be addressed when they are employed in electrodes.     

Study of the influence of the amount of PBI–H3PO4 in the catalytic layer of a high temperature PEMFC

The influence of the amount of polybenzimidazole (PBI)-H₃PO₄ (normalized with respect to the PBI loading, which expressed as C/PBI weight ratio) content in both the anode and cathode has been studied for a PBI-based high temperature proton exchange membrane (PEM) fuel cell. The electrodes prepared with different amounts of PBI have been characterized physically, by measuring the pore size distribution, and visualizing the surface microstructure. Afterwards, the electrochemical behaviour of the electrodes has been evaluated. The catalytic electrochemical activity has been measured by voltamperometry for each electrode prepared with a different PBI content, and the cell performance results have been studied, supported by the impedance spectra, in order to determine the influence of the PBI loading in each electrode. The best results have been achieved with a C/PBI weight ratio of 20, for both the anode and the cathode. A lower C/PBI weight ratio (larger amount of PBI in the catalytic layer) reduced the electrocatalytic activity, and impaired the mass transport processes, due to the large amount of polymer covering the catalyst particle, lowering the cell performance. A higher C/PBI weight ratio (lower amount of PBI in the catalytic layer) reduced the electrocatalytic activity, and slightly increased the ohmic resistance. The low amount of the polymeric ionic carrier PBI–H₃PO₄ limited the proton mobility, despite of the presence of large amounts of “free” H₃PO₄ in the catalytic layer.