Hydrogen For Fuel Cell Applications Research Articles

Advances in structural and chemical analysis of catalystcoated membranes for hydrogen fuel cell applications

Catalyst-coated membranes are a key element of advanced membrane electrode assembly designs for automotive, hydrogen-based fuel cell systems. The morphology of the catalyst layers of these membranes should provide a structure that is optimised for maximum catalyst utilisation; water management at a wide range of operational temperatures and relative humidity; fuel (hydrogen) and oxidant (oxygen in air) mass-transfer; and optimum electronic conductivity. Because of the multi-component nature of the layers (catalyst, ionomer and catalyst support) and resulting hierarchy in the layered structure at different spatial scales, new tools for their characterisation are required. Such a development should lead to an improved understanding of the links between fuel cell performance and structure of the catalyst layers under various operating conditions. This feature article presents soft X-ray spectromicroscopy as a tool for such studies and shows how it can probe the structural and chemical properties of the membrane and the catalyst layers of membrane electrode assemblies.

Advanced DC–DC converter for power conditioning in hydrogen fuel cell systems

The fuel cell (FC) generators can produce electric energy directly from hydrogen and oxygen. The DC voltage generated by FC is generally low amplitude and it is not constant, depending on the operating conditions. Furthermore, FC systems have dynamic response that is slower than the transient responses typically requested by the load. For this reason, in many applications the FC generators must be interfaced with other energy/power sources by means of an electronic power converter.An advanced full-bridge (FB) DC–DC converter, which effectively achieves zero-voltage switching and zero-current switching (ZVS–ZCS), is proposed for power-conditioning (PC) in hydrogen FC applications. The operation and features of the converter are analyzed and verified by simulations results. The ZVS–ZCS operation is obtained by means of a simple auxiliary circuit. Introduction of the soft-switching operation in PC unit brings improvements not only from the converter efficiency point of view, but also in terms of increased converter power density. Quantitative analysis of hard and soft-switching operating of the proposed converter is also made, bringing in evidence the benefits of soft-switching operation mode. The proposed converter can be a suitable solution for PC in hydrogen FC systems, especially for the medium to high-power applications.

Mesoscale Organization of Nearly Monodisperse Flowerlike Ceria Microspheres

Nearly monodisperse flowerlike CeO2 microspheres were synthesized via a simultaneous polymerization-precipitate reaction, metamorphic reconstruction, and mineralization under hydrothermal condition as well as subsequent calcination. The obtained CeO2 microsphere consists of 20-30 nm thick nanosheets as petals. It has an open three-dimensional (3D) porous and hollow structure and possesses high surface area, large pore volume, and marked hydrothermal stability. It can be doped easily after synthesis, and the initial 3D texture is maintained. The controlling factors and a possible formation mechanism are discussed in detail. This novel material can be used as a support for catalysts with various purposes. With CuO loaded on flowerlike CeO2, the catalytic activities and hydrothermal stability of Cu/CeO2 for ethanol stream reforming were examined. At 300 degrees C, the H2 selectivity reached a maximum value of 74.9 mol %, while CO was not detected within the precision of the gas chromatogram. It produced a hydrogen-rich gas mixture in the wide temperature range (300-500 degrees C) and showed excellent hydrothermal stability at high temperature (550 degrees C), which is a good choice for ethanol processors for hydrogen fuel cell applications.

Synthesis and characterization of P 2O 5–SiO 2–X (X = phosphotungstic acid) glasses as electrolyte for low temperature H 2/O 2 fuel cell application

A new class of proton conducting glass membranes for hydrogen fuel cell applications are being developed using phosphotungstic acid. These glasses are being design to yield high proton conductivities could be potential substitutes for electrolytes in H 2/O 2 fuel cell. P 2O 5–SiO 2–PWA glasses have been non-crystalline phases confirmed by structural studies. The glass materials showed good mechanical and thermal stability, and also found a maximum proton conductivity of 9.1 × 10 −2 S/cm at 90 °C and 30% RH. The average pore size less than 5 nm was determined by Barrett–Joyner–Halenda (BJH) desorption method. The electrochemical activity was investigated by polarization curves and current–voltage profiles. A maximum power density value of 10.2 mW/cm 2 was obtained using 0.15 mg/cm 2 of Pt/C loaded on electrode and 5P 2O 5–87SiO 2–8PWA glasses at 30 °C and 30% humidity.

Near-surface alloys for hydrogen fuel cell applications

Near-surface alloys (NSAs) possess a variety of unusual catalytic properties that could make them useful candidates for improved catalysts in a variety of chemical processes. It is known from previous work, for example, that some NSAs bind hydrogen very weakly while, at the same time, permitting facile H 2 activation. These NSAs could, potentially, facilitate highly selective hydrogenation reactions at low temperatures. In the present work, the suitability of NSAs for use as hydrogen fuel cell anodes has been evaluated; the combination of properties, possessed by selected NSAs, of weak binding of CO with relatively facile H 2 activation is nearly ideal for this application. We suggest that, as nanoscale materials synthesis techniques improve, it will become feasible to reproducibly prepare NSAs with highly specified surface structures, resulting in the design and manufacture of a wide variety of such materials for use in fuel cells and in an ever-increasing range of catalytic applications. Furthermore, we introduce a new concept for NSA-defect sites, which could be responsible for the promotional catalytic effects of a second metal added, even in minute quantities, to a host metal catalyst.

Structural and proton conductivity study of P 2O 5-TiO 2-SiO 2 glasses

Proton conducting membranes for hydrogen fuel cell applications are being developed using sol–gel-derived P 2O 5-TiO 2-SiO 2 glasses. The present work is devoted to the structural analysis of P 2O 5-TiO 2-SiO 2 glasses containing controlled pore sizes and the surface area of the glasses reaching a maximum 410 m 2/g, while the average pore diameter is about 2.4 nm. They were characterized by nitrogen adsorption–desorption measurements, thermal measurement as well as FTIR spectroscopy. The structural studies were based on the evolution of the intensities and profiles of certain characteristics bands in the FTIR spectra. In order to increase proton conductivity at low relative humidity, the result suggests that the mobility of proton in the P 2O 5-TiO 2-SiO 2 glasses increases with decreasing O H bonding strength. An H 2/O 2 fuel cell was constructed using the P 2O 5-TiO 2-SiO 2 glasses as electrolyte and Pt/C-loaded carbon paper sheets as electrodes. Results show that the performance of the cell is an output power of 88 μW/cm 2 at 30 °C with humidity.

Hydrogen production from methanol by oxidative steam reforming carried out in a membrane reactor

The aim of this work is to study from an experimental point of view the oxidative steam reforming of methanol by investigating the behaviour of a dense Pd/Ag membrane reactor (MR) in terms of methanol conversion as well as hydrogen production. The main parameters considered are the operating temperature and the O 2/CH 3OH feed ratio. This is a pioneer work in the application of MR to this kind of reaction, whose goal should be to produce a CO-free hydrogen stream suitable for hydrogen fuel cell applications. The experimental results show that the MR gives methanol conversions higher than traditional reactors (TRs) at each temperature investigated, confirming the good potential of the membrane reactor device for this interesting reaction system.

Quantitative investigation of catalytic natural gas conversion for hydrogen fuel cell applications

Hydrogen generation from natural gas for driving proton exchange membrane fuel cells in residential small-scale combined heat and power (CHP) applications is investigated by a series of computer simulations. Natural gas is converted into hydrogen either by the combined oxidation–steam reforming, i.e. indirect partial oxidation mechanism on Pt-Ni catalyst or by the direct, one-step partial oxidation mechanism on Pt monoliths. A water–gas shift converter and a catalytic selective carbon monoxide oxidation unit are used for reducing carbon monoxide levels to a value which the anode of proton exchange membrane fuel cell (PEMFC) can tolerate. Unconverted hydrocarbons and hydrogen rejected from the fuel cell are considered to be oxidized in a Pt catalyst packed afterburner in order to supply energy to the system. Reactor simulations based on available kinetic data together with energy integration calculations indicate direct partial oxidation to give higher hydrogen yields corresponding to increased electrical power outputs and elevated efficiencies. Indirect partial oxidation has the advantage of operating simplicity, since the direct route runs only at millisecond level residence times and high temperatures. In both mechanisms, water injection and energy integration are critical issues in adjusting product yields and in temperature control. The simulation outputs are compared and validated by the results based on the thermodynamics of the pertinent mechanism.

Oxidation of CO over Au/MO x/Al 2O 3 multi-component catalysts in a hydrogen-rich environment

Au/MgO/Al 2O 3 is able to oxidize CO selectively in hydrogen-rich gases (∼70 vol.%) at the temperatures relevant to hydrogen fuel cell applications (70–100 °C). The presence of MgO enables the preparation of small, stable Au particles on γ-Al 2O 3 with high surface area and, in this way, improves both low-temperature CO and H 2 oxidation by O 2 compared to Au/Al 2O 3. Addition of MnO x and FeO x to Au/MgO/Al 2O 3 further enhances low-temperature CO oxidation with improved CO 2 selectivity. The increase in CO oxidation activity is attributed to the implementation of new routes for supplying active oxygen, e.g., via lattice oxygen. The better CO 2 selectivity also probably results from suppression of H 2 oxidation at low temperatures. The results support a model in which CO is adsorbed onto metallic Au or at the Au/MO x perimeter interface and reacts with oxygen also present at the Au/MO x perimeter interface. A focus on decreasing the H 2 oxidation activity is recommended for further catalyst improvement.

On the development of proton conducting polymer membranes for hydrogen and methanol fuel cells

The transport properties and the swelling behaviour of NAFION and different sulfonated polyetherketones are explained in terms of distinct differences on the microstructures and in the p K a of the acidic functional groups. The less pronounced hydrophobic/hydrophilic separation of sulfonated polyetherketones compared to NAFION corresponds to narrower, less connected hydrophilic channels and to larger separations between less acidic sulfonic acid functional groups. At high water contents, this is shown to significantly reduce electroosmotic drag and water permeation whilst maintaining high proton conductivity. Blending of sulfonated polyetherketones with other polyaryls even further reduces the solvent permeation (a factor of 20 compared to NAFION), increases the membrane flexibility in the dry state and leads to an improved swelling behaviour. Therefore, polymers based on sulfonated polyetherketones are not only interesting low-cost alternative membrane material for hydrogen fuel cell applications, they may also help to reduce the problems associated with high water drag and high methanol cross-over in direct liquid methanol fuel cells (DMFC). The relatively high conductivities observed for oligomers containing imidazole as functional groups may be exploited in fully polymeric proton conducting systems with no volatile proton solvent operating at temperatures significantly beyond 100°C, where methanol vapour may be used as a fuel in DMFCs.