Thin Catalyst Layer Research Articles

Investigation of the local catalyst distribution in an aged direct methanol fuel cell MEA by means of differential synchrotron X-ray absorption edge imaging with high energy resolution

Synchrotron X-ray absorption edge imaging with high energy resolution was applied to study aging of fuel cell catalyst materials. The combination of an imaging and a high X-ray energy resolution set-up allows acquiring spatially resolved XAS (XANES and EXAFS) spectra. We analyzed the two-dimensional distribution of Pt and Ru in fresh and aged fuel cell catalysts. Spatially resolved XAS images were taken at the RuK edge and at the PtL3 edge. Taking radiographs above and below the absorption edges provides quantitative information about the thickness of the catalytic materials and additional chemical information. A strong influence of the flow field channels and the structure of the gas diffusion layers on the thicknesses of the catalytic elements were found: a thinner catalyst layer was found below the ribs of the flow field geometries as well as under crossing points of fiber bundles of the woven gas diffusion layers.

VISUAL OPTICAL HYDROGEN SENSOR ON THE BASIS OF WO3 FILM AND A CATALYST LAYER

The cathodic deposition of WO3 film by pulsed mode makes for decrease in the size of nanoclusters in the structure of the film and facilitates the shift of its coloration maximum from the IR region of the optical spectrum to the visible region on reduction. This made it possible to increase the sensitivity of optical hydrogen sensor with such a film and a thin layer of catalyst (Pt or Pd) to 1-2 vol % H2. It has been shown that using a thicker fine-grained catalyst layer in visual sensor with planar SnO2 — WO3 — Sb2O5 — Pd structure, one can reduce the sensor coloration time from 3 minutes to 1–3 seconds.

Growth of carbon nanotube forests between a bi-metallic catalyst layer and a SiO2 substrate to form a self-assembled carbon–metal heterostructure

A special nanostructure was formed by the growth of carbon nanotubes (CNTs) between a substrate and a thin bi-metallic catalyst layer using a thermal chemical vapor deposition process. The catalyst layer is composed of adjacently disposed Cr and Ni phases formed prior to CNT growth. The Cr/Ni layer serves as a bi-metallic catalyst layer, which is pushed away from the substrate as a thin and continuous nanomembrane with the growth of CNTs. The self-assembled CNT–catalyst heterostructure possesses a smooth surface (RMS=2.9nm) with a metallic shine. Directly interlinked to the Cr/Ni layer, dense and vertically aligned multi-walled CNTs are found. Compared to conventional CNT films, the structure has significant advantages for CNT integration. From technology point of view, the structure allows further processing without impact on the CNTs as well as transfer of pristine vertically aligned CNTs to arbitrary substrates. Moreover, the as-grown CNT films provide an interface ideal for further electrical, thermal and mechanical contacting of CNT films. We present structural investigations of this special CNT–metal heterostructure. Furthermore, we discuss possible interface mechanisms during catalyst layer formation and CNT growth.

Thin catalyst layer deposited on proton-exchange membrane by self-breathing method

Catalyst layers at membrane–electrode interfaces play an important role in the performance of proton-exchange membrane fuel cells. Increasing the effective interface area and reducing the interface charge transfer resistance are key factors in enhancing cell performance. This study reports on a self-breathing process that has been developed to coat a thin catalyst layer on a membrane surface prior to application of Pt/C slurry. To further improve the breathing efficiency, oxygen plasma treatment was applied. The morphology and surface bonding of the processed membranes were characterized by scanning electron microscopy, atomic force microscopy, and X-ray photoelectron spectroscopy. The interface charge transfer resistances of the membrane electrode assemblies were measured by electrochemical impedance spectroscopy. The results show that the charge transfer resistance of the cell with the self-breathing membrane is one order lower than that of a pristine cell.

H2 production by methane decomposition: Catalytic and technological aspects

Abstract Ni and Co supported on SiO 2 and Al 2 O 3 silica cloth thin layer catalysts have been investigated in the catalytic decomposition of natural gas (CDNG) reaction. The influence of carrier nature and reaction temperature was evaluated with the aim to individuate the key factors affecting coke formation. Both Ni and Co silica supported catalysts, due to the low metal support interaction (MSI), promote s the formation of carbon filament with particles at tip. On the contrary, in case alumina was used as support, metals strongly interact with surface thus depressing both the metal sintering and the detachment of particles from catalyst surface. In such cases, carbon grows on metal particle with a “base mechanism” while particles remain well anchored on the catalyst surface. This allowed to realize a cyclic dual-step process based on methane decomposition and catalyst oxygen regeneration without deactivation of catalyst. Technological considerations have led to conclude that the implement of a process based on decomposition and regeneration of catalyst by oxidation requires the development of a robust catalytic system characterized by both a strong MSI and a well defined particle size distribution. In particular, the catalyst should be able to operate at high temperature, necessary to reach high methane conversion values (> 90%), avoiding at the same time the formation of both the carbon filaments with metal at tip or the encapsulating carbon which drastically deactivate the catalyst.

Combined steam and dry reforming of methane in narrow channel reactors

The performance of methane reforming reactions in narrow channel reactors has been investigated. Two types of reactors (Diffusion Bonded Reactor and Demountable Reactor) were utilized and two forms of catalysts were prepared by the sol-gel method with different additives. The sol-gels were prepared to have desirable rheological properties for coating onto stainless steel substrates, which after calcining formed an adherent thin catalyst layer. Employing the catalyst as a thin layer (<50μm) coated on the channel surface reduces mass and heat transfer restrictions compared with pellet catalysts and can improve the effectiveness factor. Carbon deposition is known to be rapid in the case of the CO2 reforming alone. In this study, carbon deposition was reduced drastically when CO2 reforming is carried out simultaneously with the steam reforming reaction in narrow channels coated by thin layers of catalyst (≤50μm) prepared using the sol-gel method. It has been shown that the stability and coking resistance of Ni/Al2O3 catalyst are increased by the addition of Ba, Cr and La2O3 in combined steam reforming of methane with carbon dioxide reforming. This process is an attractive approach for improving catalyst stability and offers the possibility of obtaining H2/CO ratio close to 2, which is suitable for Fischer–Tropsch and methanol synthesis.

Effect of Surface Hydrophilicity on the Formation of Nafion Thin Film Inside PEMFC Catalyst Layers: A Computational Study

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Computational Fluid Dynamics in chemical reactor analysis and design: Application to the ZoneFlow™ reactor for methane steam reforming

The ZoneFlow reactor (Tribute Creations, LLC) is a tubular reactor with two types of internals: a core type and adjacent to the wall, a casing type, both covered with a thin layer of catalyst. It aims for higher energy efficiency and lower steam-to-carbon ratios in methane steam reforming by a judicious design of the relative position of the internals and their geometrical characteristics and a higher catalyst effectiveness.The performance of this novel structured catalytic reactor for synthesis gas production by methane steam reforming was simulated using a reactor model accounting in detail for the flow pattern by means of Computational Fluid Dynamics (CFD). A Reynolds Averaged Navier Stokes (RANS) approach was taken. Turbulence, heat transfer by convection and radiation, detailed reaction kinetics including coking, intra-particle diffusion limitations, and the compressibility of the gas phase were accounted for. Simulations were carried out to investigate the ZoneFlow reactor performance under typical commercial operating conditions and compare it with a conventional packed bed reactor using the same Ni catalyst.

OPTICAL HYDROGEN SENSOR WITH INCREASED SENSITIVITY BASED ON TUNGSTEN OXIDE FILM

A computer simulation of the effect of frustration of total internal reflection in the region of the interface between glass and WO3 film with thin catalyst (Pt, Pd) layer by hydrogen with different concentration in the hydrogen-air mixture has been performed. A design of optical hydrogen sensor has been developed, and its laboratory model has been made on the basis of this effect with high hydrogen sensitivity in the range of low hydrogen concentration in the hydrogen-air mixture (down to under 0.01vol. % [H2]). The substantial increase in the hydrogen sensitivity of the sensor has been achieved through the multiple reflection by the glass/WO3 interface of IR light with 940 nm wavelength in its propagation through glass light guide.

Cathode catalyst layer design for proton exchange membrane fuel cells

To improve water management and enhance the catalyst utilization of the cathode catalyst layer of proton exchange membrane (PEM) fuel cells, the effects of polytetrafluoroethylene (PTFE) addition in the catalyst ink and the loading pattern of the catalyst layer were investigated. Two types of catalyst ink were used: a typical one without PTFE (Pt on carbon support+Nafion) and another type added with PTFE (Pt on carbon support+Nafion+PTFE). In exploring the effect of PTFE addition into the conventional full loading pattern of catalyst layer, the presence of 10% PTFE in the catalyst layer improved the cell performance (34% increase of maximum power density) and the optimum Pt loading for the PTFE-added catalyst layer was 0.25mg/cm2. Two catalyst layer loading patterns created in this work were the strip and chess patterns. Each pattern consists of equal areas of several hydrophilic and hydrophobic segments. The hydrophilic segments were formed by using the ink with PTFE while the hydrophilic had no PTFE. For the catalyst loading pattern effect, the cell achieved the highest performance with the chess pattern, followed by the strip and full loading pattern for the case of 0.5mg/cm2 Pt loading having a thick catalyst layer of 50-μm thickness. On the other hand, for the case of 0.25mg/cm2 Pt loading forming a thin catalyst layer of ∼30-μm thickness, the catalyst loading pattern had no effect on the cell’s performance.

The effect of electrode parameters on the performance of anion exchange polymer membrane fuel cells

An investigation of several electrode parameters on performance of an alkaline membrane fuel cell is described. The studied parameters were: ionomer content, anode and cathode catalyst layer thickness, electrode aminating agent and membrane thickness. It was found that an optimum ionomer content depended on a balance between the OH − ion/water mobility and the oxygen solubility/diffusivity through it and which varied with temperature. Thick catalyst layers were necessary for the anode as thin anode catalyst layers suffered from flooding. 40%Pt/C provided the best thickness (with loading of 0.4 mg Pt cm −2) for cathodes operating with air. An aminated low density poly(ethylene- co-vinyl benzyl chloride) (LDPE-VBC) membrane was shown to be a good membrane for an alkaline membrane fuel cell, giving conductivities up to 0.13 S cm −1 at 80 °C. A Membrane Electrode Assembly (MEA) utilizing this membrane with fully hydrated thickness of 57 μm produced good peak power density, at a high potential of 500 mV, of 337 mW cm −2 with air (1 bar gauge) at 60 °C.

CH 4 decomposition on Ni and Co thin layer catalysts to produce H 2 for fuel cell

Ni and Co supported on SiO 2 and Al 2O 3 Silica Cloth Thin Layer Catalysts (TLC) were investigated in the Catalytic Decomposition of Natural Gas (CDNG) reaction using a Multilayer Catalytic Reactor (MCR). The influence of support and reaction temperature was evaluated with the aim to design a novel catalyst suitable to develop a dual-step process to produce pure hydrogen for fuel cell. Ni and Co silica supported catalyst, due to the low metal supported interaction (MSI) characterizing such systems, were not found suitable to efficiently perform CDNG reaction. On the contrary in case alumina was used as support both metals strongly interact with surface becoming so more resistant to sintering. In addition in case the metals (both Ni and Co) strongly interact with support surface, the formation of encapsulating carbon is depressed and only filamentous carbon forms. Using Ni and Co TLC supported on Al 2O 3 samples it has been possible to realize a cyclic dual-step process based on reaction and regeneration in oxygen stream without any catalyst detriment. A deactivation modelling to discriminate the role of sintering and coking on the deactivation pattern is proposed.

Electrochemical Activity and Catalyst Utilization of Low Pt and Thickness Controlled Membrane Electrode Assemblies

An improved catalyst deposition methodology based on a piezo-electric printing technique has been developed and used to fabricate catalyst coated membranes (CCM) with thin catalyst layers (1−5 μm) and ultra-low Pt loadings (0.02−0.12 mgPt/cm2). The performance of these CCMs was examined in proton exchange membrane fuel cells (PEMFCs). The catalyst utilization was observed to increase with decreasing catalyst layer thickness (decreasing Pt loading). The printed CCM with two layers containing an ultra-low Pt loading (0.02 mgPt/cm2) exhibited Pt utilizations of 100%. Neglecting the anode contributions, the mass activity at 850 mV for the printed CCM is nearly 76.5 mA/mgPt which is 3.5 times higher than that for the CCM fabricated by conventional spraying method (22.5 mA/mgPt).

Performance of TS-1-Coated Structured Packing Materials for Styrene Oxidation Reaction

Titanium silicalite-1 (TS-1)-coated on stainless steel packing rings of complex geometries provided a large interfacial contact area for improving mass transport processes. Zeolite seeds were assembled using organic linker, and a thin uniform layer of TS-1 catalyst was deposited on the surface by hydrothermal regrowth. The chemical, structural, and catalytic properties of the TS-1 layer were examined. An order of magnitude higher styrene conversion rate was obtained from the TS-1-coated substrates compared with powder. The catalyst was sensitive to pretreatment, and heat treatment in nitrogen provided a 30% higher conversion rate than the air-calcined samples. The N2-pretreated catalyst displayed lower activity loss and better regenerability.

Selective Growth of Carbon Nanotubes using Two-step Etch Scheme for Semiconductor Via Interconnects

In the present work, a new approach is proposed for via interconnects of semiconductor devices, where multi-wall carbon nanotubes (MWCNTs) are used instead of conventional metals. In order to implement a selective growth of carbon nanotubes (CNTs) for via interconnect, the buried catalyst method is selected which is the most compatible with semiconductor processes. The cobalt catalyst for CNT growth is pre-deposited before via hole patterning, and to achieve the via etch stop on the thin catalyst layer (ca. 3nm), a novel 2-step etch scheme is designed; the first step is a conventional oxide etch while the second step chemically etches the silicon nitride layer to lower the damage of the catalyst layer. The results show that the 2-step etch scheme is a feasible candidate for the realization of CNT interconnects in conventional semiconductor devices.

Fuzzy-based Field-programmable Gate Array Implementation of a Power Quality Enhancement Strategy for ac-ac Converters

In the present work, a new approach is proposed for via interconnects of semiconductor devices, where multi-wall carbon nanotubes (MWCNTs) are used instead of conventional metals. In order to implement a selective growth of carbon nanotubes (CNTs) for via interconnect, the buried catalyst method is selected which is the most compatible with semiconductor processes. The cobalt catalyst for CNT growth is pre-deposited before via hole patterning, and to achieve the via etch stop on the thin catalyst layer (ca. 3nm), a novel 2-step etch scheme is designed; the first step is a conventional oxide etch while the second step chemically etches the silicon nitride layer to lower the damage of the catalyst layer. The results show that the 2-step etch scheme is a feasible candidate for the realization of CNT interconnects in conventional semiconductor devices.

Integration of Carbon Nanotube Interconnects for Full Compatibility with Semiconductor Technologies

We report the latest works on via interconnects of future memory or LSI devices, where multi-wall carbon nanotubes (MWCNTs) are used instead of conventional metals. MWCNTs are grown vertically in 80 nm via holes using plasma-enhanced chemical vapor deposition. The carbon nanotube (CNT) via interconnects are integrated into an 8-inch Si wafer in full compatibility with conventional semiconductor processes. We have used buried catalyst method for the catalyst layer deposition, two-step etch method for achieving via etch stop on the thin catalyst layer (ca. 3 nm), and the chemical mechanical polishing (CMP) process for cutting CNT. The two-step etch method is composed of two consecutive etch steps: the first step is a conventional oxide etch while the second step chemically etches the silicon nitride layer to relieve the damage of the catalyst layer. After a full integration, a resistance of 293 ∼ 493 Ω and a CNT density of about 4 × 1011/cm2 have been achieved for the 80 nm via. These results show that the 2-step etch scheme is a promising candidate for the realization of CNT interconnects in conventional semiconductor devices.

Highly exposed and activity modulated sandwich type Pt thin layer catalyst with enhanced utilization

A Pt thin layer catalyst supported on an in situ prepared ‘RuO2–carbon–RuO2’ sandwich type hybrid support is presented. This is achieved by the extensive functionalization of a hollow carbon nanofiber support to introduce oxygen containing functional groups (FCNF) with the specific aim to accomplish the exclusive adsorption of Ru ions along its inner cavity and outer surfaces. Preferential adsorption of Ru ions from a mixture of Pt and Ru with sufficient time for adsorption and reorganization of ions on the carbon nanofiber surface leads to the in situ renovation of FCNFs to form a hybrid ‘RuO2–carbon–RuO2’ sandwich type support followed by Pt nanoparticle decoration. While the selective exposure of Pt on the hybrid support surface is confirmed from the HRTEM analysis, the electronic changes effected in the CNF support are evident from the XPS and XRD analysis. Finally, the potential benefit of such a design is also demonstrated using electrochemical studies, where the three-fold increase in the electrochemically active surface area from cyclic voltammetric analysis, a four-fold improvement in the limiting current density coupled with a 80 mV gain in onset potential from rotating disc electrode studies for the oxygen reduction reaction, and a drastic reduction in the CO poisoning for methanol oxidation reaction underlines the superb performance of this material. Such an exceptionally high performance can be attributed to the strong electronic perturbations occurring in the Pt and the FCNF support due to the presence of a continuous RuO2 layer in between. Such a high aspect ratio core–shell type design with an unusual enhancement in the Pt utilization establishes the roles of both the hybrid support and active catalyst to address the future challenges in the area of utilization improvement.

Fabrication of Silicon Nanowires with Precise Diameter Control Using Metal Nanodot Arrays as a Hard Mask Blocking Material in Chemical Etching

We report on a simple and cost-effective method to fabricate high density silicon nanowires (SiNWs) through catalytic chemical wet etching. Metallic chromium/gold (Cr/Au) nanodots were first deposited onto the silicon wafer using an anodic aluminum oxide (AAO) template. The AAO template was then removed before a thin blanket layer of gold catalyst was evaporated onto the sample surface. The gold-assisted chemical wet etching was carried out in a solution consisting of deionized water, hydrogen peroxide, and hydrofluoric acid to produce well-aligned silicon nanowires of uniform diameters. We demonstrate that the diameter of the silicon nanowires can be precisely controlled to a precision of 10 nm in the range of 40 to 80 nm through fine-tuning of the pore diameter of the AAO template. The reported fabrication procedure therefore gives a highly repeatable method to form well-aligned, uniform, and crystalline SiNWs of high density with controllable diameters below 100 nm. The use of Cr/Au as a hard mask blocking material will also be of great interest for the fabrication of other Si nanostructures using the catalytic etching process.

Methane decomposition over Co thin layer supported catalysts to produce hydrogen for fuel cell

Co/Al 2O 3/Silica Cloth Thin Layer Catalysts (CoAS) for Catalytic Decomposition of Natural Gas (CDNG) were investigated using a new Multilayer Catalytic Reactor (MCR). The influence of Co loading and reaction temperature was evaluated. Irrespective of Co loading, initial CH 4 conversion rises with T R, while for lifetime and carbon capacity (C/Ni, number of CH 4 molecules decomposed for Ni atom until complete deactivation) it is not possible to find a direct relationship with other operating parameters or catalyst characteristics. However, Co loading significantly affects the catalytic activity: Co loading as high as 20 wt.% ensures both long lifetime and high H 2 productivity. On the contrary of what occurs using Ni-based catalysts and irrespective of reaction temperature investigated, filamentous coke forms with a “base” reaction mechanism and Co catalyst can be regenerated in oxygen without any problem, thus allowing to realize a dual-step process for the production of “CO x-free” hydrogen stream.