Effect of chemical oxidation of CNFs on the electrochemical carbon corrosion in polymer electrolyte membrane fuel cells

Effect of chemical oxidation of carbon nanofibers (CNFs) on the electrochemical carbon corrosion in polymer electrolyte membrane (PEM) fuel cells is examined. With increasing time of chemical oxidation treatment using an acidic solution, more oxygen functional groups are formed on the surface of CNF resulting in an increasingly hydrophilic carbon surface. This effect contributes to improvements in Pt loading and the distribution of Pt particles on carbon supports. However, the chemical oxidation treatment is found to accelerate electrochemical carbon corrosion. The oxygen functional group and the hydrophilic nature of CNFs after chemical oxidation treatment are believed to encourage the formation of CO₂, which is a product of carbon corrosion. From the observed results, it can be concluded that the chemical oxidation of CNFs is beneficial for catalyst loading and distribution. On the other hand, however, it reduces the durability of the PEM fuel cells caused by the electrochemical carbon corrosion.     

Electrocatalytic activity and durability study of carbon supported Pt nanodendrites in polymer electrolyte membrane fuel cells

In this paper, Pt nanodendrites are synthesized, and their use as an oxygen reduction catalyst in polymer electrolyte membrane fuel cells is examined. When the Pt nanoparticles are shape-controlled in a dendritic form, the Pt nanoparticles exhibit a high mass activity that is nearly twice as high as the commercial Pt/C catalyst for the oxygen reduction reaction. This high activity is only achieved when the Pt nanodendrites are supported on carbon. The unsupported Pt nanodendrites exhibit very poor catalytic activity due to the limited accessibility of the active sites in the catalyst layer of the fuel cells. Based on the durability study of Pt nanodendrites, however, the dendritic structure is not stable during repeated potential cycling test and its structure collapse is the primary reason for the performance loss in the fuel cells.     

Effect of operating conditions on carbon corrosion in polymer electrolyte membrane fuel cells

The influence of humidity, cell temperature and gas-phase O₂ on the electrochemical corrosion of carbon in polymer electrolyte membrane fuel cells is investigated by measuring CO₂ emission at a constant potential of 1.4 V for 30 min using on-line mass spectrometry. Carbon corrosion shows a strong positive correlation with humidity and cell temperature. The presence of water is indispensable for electrochemical carbon corrosion. By contrast, the presence of gas-phase O₂ has little effect on electrochemical carbon corrosion. With increased carbon corrosion, changes in fuel cell electrochemical characteristics become more prominent and thereby indicate that such corrosion significantly affects fuel cell durability.     

New nitrogen-containing carbon supports with improved corrosion resistance for proton exchange membrane fuel cells

In the present study the effect of the modification of commercial carbon black KetjenBlack DJ-600 with nitrogen and carbon containing compounds on the electrochemical stabilities has been investigated. The modification has been carried out by high temperature decomposition of acetonitrile, pyridine vapors and ethylene on the support surface. The corrosion stability of the KetjenBlack DJ-600 carbon black has been found to be improved upon modification.     

Experimental study on carbon corrosion of the gas diffusion layer in polymer electrolyte membrane fuel cells

Generally, the GDL of a PEM fuel cell experiences three external attacks: dissolution of water, erosion of gas flow, and corrosion of electric potential. Of these degradation factors, this study focuses on the carbon corrosion of electric potential and investigates its impact through the accelerated carbon corrosion test. This study confirms that carbon corrosion occurs at the GDL, which decreases the operating fuel cell’s performance. To discover the effects of carbon corrosion, the GDL property changes are measured through various devices, including a scanning electron microscopy, a thermo gravimetric analyzer, and a tensile stress test. Carbon corrosion causes not only loss of weight and thickness but also degradation of mechanical strength in the GDL. In addition, the GDL shows serious damage in its center.     

Non-kinetic losses caused by electrochemical carbon corrosion in PEM fuel cells

This paper presented non-kinetic losses in PEM fuel cells under an accelerated stress test of catalyst support. A cathode with carbon-supported Pt catalyst was prepared and characterized during potential hold at 1.2 V vs. SHE in a PEM fuel cell. Irreversible losses caused by carbon corrosion were evaluated using a variety of electrochemical characterizations including cyclic voltammetry, linear sweep voltammetry, electrochemical impedance spectroscopy, and polarization technique. Ohmic losses at the cathode during potential hold were determined using its capacitive responses. Concentration losses in the PEM fuel cell were analyzed in terms of Tafel behavior and thin film/flooded-agglomerate dynamics.     

Polypyrrole-modified hydrophobic carbon nanotubes as promising electrocatalyst supports in polymer electrolyte membrane fuel cells

As an alternative to oxidative acid treatment, a hydrophobic graphitized carbon nanotube (CNT) was functionalized with 1-4 nm thick polypyrrole (PPy) prior to application as catalyst supports in polymer electrolyte membrane (PEM) fuel cells. Unlike oxidative acid treatment, the PPy coating method converts the hydrophobic surface of a CNT to a hydrophilic one without creating defects on the surface of the CNT. As a result, Pt nanoparticles deposited on the PPy-coated CNTs showed an improved distribution, which significantly enhanced the fuel cell performance while preserving the intrinsic properties of the CNTs, i.e., resistance to electrochemical carbon corrosion. An additional advantage of PPy coating is that it prevents Pt nanoparticles from agglomerating on the CNT surface. These results indicated that PPy-coated CNTs are a promising catalyst support to improve both the performance and durability of PEM fuel cells.     

Simulation of nanostructured electrodes for polymer electrolyte membrane fuel cells

Aligned carbon nanotubes (CNTs) with Pt uniformly deposited on them are being considered in fabricating the catalyst layer of polymer electrolyte membrane (PEM) fuel cell electrodes. When coated with a proton conducting polymer (e.g., Nafion) on the Pt/CNTs, each Pt/CNT acts as a nanoelectrode and a collection of such nanoelectrodes constitutes the proposed nanostructured electrodes. Computer modeling was performed for the cathode side, in which both multicomponent and Knudsen diffusion were taken into account. The effect of the nanoelectrode lengths was also studied with catalyst layer thicknesses of 2, 4, 6, and 10 μm. It was observed that shorter lengths produce better electrode performance due to lower diffusion barriers and better catalyst utilization. The effect of spacing between the nanoelectrodes was studied. Simulation results showed the need to have sufficiently large gas pores, i.e., large spacing, for good oxygen transport. However, this is at the cost of obtaining large electrode currents due to reduction of the number of nanoelectrodes per unit geometrical area of the nanostructured electrode. An optimization of the nanostructured electrodes was obtained when the spacing was at about 400 nm that produced the best limiting current density.     

Mesoporous iridium oxide/Sb-doped SnO2 nanostructured electrodes for polymer electrolyte membrane water electrolysis

In proton exchange membrane (PEM) water electrolysis, iridium oxide (IrO₂) has often been utilized as a main catalyst for the oxygen evolution reaction (OER) as a rate-determining step. In general, the performance of PEM water electrolysis is dominantly affected by the specific surface area and the porous structure of the IrO₂ catalyst. Thus, in this study, IrO₂ and antimony-doped tin oxide (ATO) nanostructures with high specific surface areas were synthesized through the Adams fusion method. The as-prepared samples showed well-defined porous high-crystalline nanostructures. The ATO nanoparticles as a support were surrounded by IrO₂ nanoparticles as a catalyst without serious agglomeration, indicating that the IrO₂ catalyst was uniformly distributed on the ATO support. Compared to pure IrO₂, the IrO₂/ATO mixture electrodes showed superior OER properties because of their increased electrochemical active sites.     

Evaluation of electrochemical performance for surface-modified carbons as catalyst support in polymer electrolyte membrane (PEM) fuel cells

The electrochemical performance of platinum (Pt) catalyst deposited on various functionalized carbon supports was investigated and compared with that of a commercial catalyst, Pt on Vulcan XC-72 carbon. The supports employed were graphitic or amorphous with a wide range of surface areas. Cyclic voltammetry (CV) and rotating disk electrode (RDE) studies on the supported catalysts indicated equivalent platinum catalyst activities. Fuel cell performance was determined for membrane electrode assemblies (MEA) fabricated from the supported catalysts. The use of high surface area supports did not necessarily translate into a higher electrochemical utilization of platinum. Electrochemical impedance spectroscopy (EIS) measurements indicated lower ohmic losses for low surface area carbon MEAs. This is explained by the supported catalyst electrode microstructures and their intrinsic resistivities. Correlation of all data indicates that for low surface carbons, nature of the support does not significantly affect the Pt catalytic activity. The influence of the support is more critical when high surface area carbons are used because of the vastly different electrode morphology and resistivity.     

Deuterium isotope separation by polymer electrolyte fuel cells connected in series

The separation of hydrogen isotopes has important applications for fundamental science and nuclear engineering. This study investigates isotope separation by stacked polymer electrolyte fuel cells that form part of a combined electrolysis and fuel cell (CEFC) system. Fuel gas containing deuterium (D) was generated by water electrolysis and passed through three fuel cells (FCs) connected in series. Increasing the number of operating FCs in the series greatly improved D separation, but had only a modest impact on power consumption. When all three FCs were individually controlled, the separation efficiency depended on the power condition in each FC. At high current the separation factor of the CEFC system reached over 100 owing to the relationship between fuel gas utilization and separation efficiency.     

Bipolar plate made of carbon fiber epoxy composite for polymer electrolyte membrane fuel cells

Polymer electrolyte membrane fuel cells (PEMFCs) are able to efficiently generate high power densities, making the technology potentially attractive for certain mobile and portable applications. Since the bipolar plate is a major part of the PEMFC stack both in weight and volume, the bipolar plate should be developed with its weight and thickness in mind.     

Capillaries for water management in polymer electrolyte membrane fuel cells

Some of the new liquid water management systems in polymer electrolyte membrane (PEM) fuel cells hold great potential in providing flood-free performance and internal humidification. However, current water management systems entail major setbacks, which either inhibit implementation into state-of-the-art architectures, such as stamped metal flow-fields, or restrict their application to certain channel configurations. Here, a novel water management strategy is presented that uses capillary arrays to control liquid water in PEMFCs. These capillaries are laser-drilled into the land of the flow-fields and allow direct removal (wicking) or supply of water (evaporation), depending on the local demand across the electrode. For a 6.25 cm² active area parallel flow-field, a ～92% improvement in maximum power density from capillary integration was demonstrated. The proposed mechanism serves as a simple and effective means of achieving robust and reliable fuel cell operation, without incurring additional parasitic losses due to the high pressure drop associated with conventional serpentine flow-fields.     

Preparation and properties of carbon nanotube/polypropylene nanocomposite bipolar plates for polymer electrolyte membrane fuel cells

This study aims at the fabrication of lightweight and high performance nanocomposite bipolar plates for the application in polymer electrode membrane fuel cells (PEMFCs). The thin nanocomposite bipolar plates (the thickness <1.2 mm) consisting of multiwalled carbon nanotubes (MWCNTs), graphite powder and PP were fabricated by means of compression molding. Three types of polypropylene (PP) with different crystallinities including high crystallinity PP (HC-PP), medium crystallinity PP (MC-PP), low crystallinity PP (LC-PP) were prepared to investigate the influence of crystallinity on the dispersion of MWCNTs in PP matrix. The optimum composition of original composite bipolar plates was determined at 80 wt.% graphite content and 20 wt.% PP content based on the measurements of electrical and mechanical properties with various graphite contents. Results also indicate that MWCNTs was dispersed better in LC-PP than other PP owing to enough dispersed regions in nanocomposite bipolar plates. This good MWCNT dispersion of LC-PP would cause better bulk electrical conductivity, mechanical properties and thermal stability of MWCNTs/PP nanocomposite bipolar plates. In the MWCNTs/LC-PP system, the bulk electrical conductivities with various MWCNT contents all exceed 100 S cm⁻¹. The flexural strength of the MWCNTs/LC-PP nanocomposite bipolar plate with 8 phr of MWCNTs was approximately 37% higher than that of the original nanocomposite bipolar plate and the unnotched Izod impact strength of MWCNTs/LC-PP nanocomposite bipolar plates was also increased from 68.32 J m⁻¹ (0 phr) to 81.40 J m⁻¹ (8 phr), increasing 19%. In addition, the coefficient of thermal expansion of MWCNTs/LC-PP nanocomposite bipolar plate was decreased from 32.91 μm m⁻¹ °C⁻¹ (0 phr) to 25.79 μm m⁻¹ °C⁻¹ (8 phr) with the increasing of MWCNT content. The polarization curve of MWCNTs/LC-PP nanocomposite bipolar plate compared with graphite bipolar plate was also evaluated. These results confirm that the addition of MWCNTs in LC-PP leads to a significant improvement on the cell performance of the nanocomposite bipolar plate.     

Performance of laboratory polymer electrolyte membrane hydrogen generator with sputtered iridium oxide anode

The continuous improvement of the anode materials constitutes a major challenge for the future commercial use of polymer electrolyte membranes (PEM) electrolyzers for hydrogen production. In accordance to this direction, iridium/titanium films deposited directly on carbon substrates via magnetron sputtering are operated as electrodes for the oxygen evolution reaction interfaced with Nafion 115 electrolyte in a laboratory single cell PEM hydrogen generator. The anode with 0.2 mg cm⁻² Ir catalyst loading was electrochemically activated by cycling its potential value between 0 and 1.2 V (vs. RHE). The water electrolysis cell was operated at 90 °C with current density 1 A cm⁻² at 1.51 V without the ohmic contribution. The corresponding current density per mgr of Ir catalyst is 5 A mg⁻¹. The achieved high efficiency is combined with sufficient electrode stability since the oxidation of the carbon substrate during the anodic polarization is almost negligible.     

New materials for polymer electrolyte membrane fuel cell current collectors

Polymer Electrolyte Membrane Fuel cells for automotive applications need to have high power density, and be inexpensive and robust to compete effectively with the internal combustion engine. Development of membranes and new electrodes and catalysts have increased power significantly, but further improvements may be achieved by the use of new materials and construction techniques in the manufacture of the bipolar plates. To show this, a variety of materials have been fabricated into flow field plates, both metallic and graphitic, and single fuel cell tests were conducted to determine the performance of each material. Maximum power was obtained with materials which had lowest contact resistance and good electrical conductivity. The performance of the best material was characterised as a function of cell compression and flow field geometry.     

Effect of heat-treatment temperature on carbon corrosion in polymer electrolyte membrane fuel cells

This study examines the effect of heat-treatment temperature on the electrochemical corrosion of carbon nanofibers (CNFs) in polymer electrolyte membrane (PEM) fuel cells. Corrosion is investigated by monitoring the generation of CO₂ using an on-line mass spectrometer at a constant potential of 1.4 V for 30 min. The experimental results show that the generation of CO₂ decreases with increasing heat-treatment temperature, indicating that less electrochemical carbon corrosion occurs. In particular, when the heat-treatment temperature is 2400 °C, the change intensifies. X-ray photoelectron spectroscopic analysis shows that oxygen functional groups on the carbon surface decrease with increasing heat-treatment temperature. A reduction in oxygen functional groups increases the hydrophobic nature of the carbon surface, which is responsible for the increased corrosion resistance of CNFs.     

Evaluation of coated metallic bipolar plates for polymer electrolyte membrane fuel cells

Metallic bipolar plates for polymer electrolyte membrane (PEM) fuel cells typically require coatings for corrosion protection. Other requirements for the corrosion protective coatings include low electrical contact resistance, good mechanical robustness, low material and fabrication cost. The authors have evaluated a number of protective coatings deposited on stainless steel substrates by electroplating and physical vapor deposition (PVD) methods. The coatings are screened with an electrochemical polarization test for corrosion resistance; then the contact resistance test was performed on selected coatings. The coating investigated include Gold with various thicknesses (2 nm, 10 nm, and 1 μm), Titanium, Zirconium, Zirconium Nitride (ZrN), Zirconium Niobium (ZrNb), and Zirconium Nitride with a Gold top layer (ZrNAu). The substrates include three types of stainless steel: 304, 310, and 316. The results show that Zr-coated samples satisfy the DOE target for corrosion resistance at both anode and cathode sides in typical PEM fuel cell environments in the short-term, but they do not meet the DOE contact resistance goal. Very thin gold coating (2 nm) can significantly decrease the electrical contact resistance, however a relatively thick gold coating (>10 nm) with our deposition method is necessary for adequate corrosion resistance, particularly for the cathode side of the bipolar plate.     

Electromagnetic-carbon surface treatment of composite bipolar plate for high-efficiency polymer electrolyte membrane fuel cells

Polymer electrolyte membrane (PEM) or proton-exchange membrane fuel cell systems are environmentally friendly power sources for many applications. Bipolar plates are essential components of a PEM fuel cell. Recently, composite bipolar plates have received considerable interest due to their superior performance. The most important properties of bipolar plates are electrical resistance and contact resistance, which are largely dependent on the surface morphology of the bipolar plate, because low electrical resistance improves the efficiency of PEM fuel cells. In this study, a selective surface preparation technology is developed using an electromagnetic field and carbon black (electromagnetic-carbon surface treatment). The carbon black is heated by an electromagnetic field on the surface of the bipolar plate with a high rate of temperature rise. The non-electrically conducting surface resin is removed, without damaging the carbon fibre to give a low electrical resistance. It is found that the surface-treated composite bipolar plate has a lower electrical resistance than those of conventional composite bipolar plates, and that the electromagnetic-carbon surface treatment can be applied for production of the composite bipolar plates in a fast and efficient process.     

Investigation on the corrosion resistance of carbon black-graphite-poly(vinylidene fluoride) composite bipolar plates for polymer electrolyte membrane fuel cells

The goal of the present work was to evaluate the corrosion resistance of carbon black (CB)-synthetic graphite (SG)-poly(vinylidene fluoride) (PVDF) composites using electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization curves. The tests were conducted in 0.5 M H₂SO₄ + 2 ppm HF solution at 70 °C to simulate the typical environment of polymer electrolyte membrane fuel cells. The fracture surface of the specimens was characterized by scanning electron microscopy. The through-plane electrical conductivity was also determined. The corrosion resistance decreased as the carbon black content increased up to 5 wt.%. The highest electrical conductivity was achieved for the composition CB = 5 wt.%, PVDF = 15 wt.%, SG = 80 wt.%. A detailed discussion of the EIS data is given. This approach is unprecedented in the current literature. EIS has proven to be a valuable tool to the design of electrically efficient bipolar plates.