Highly porous activated glassy carbon film sandwich structure for electrochemical energy storage in ultracapacitor applications: Study of the porous film structure and gradient

Glassy carbon plates were thermochemically gas phase oxidized to obtain monolithic sandwichlike electrode assemblies with high surface area porous films for electrochemical energy storage applications. Film thicknesses were varied by variation of oxidation parameters time, temperature, and oxygen concentration and measured with electron microscopy. The mass density of the porous carbon film material was estimated by fitting a geometrical model to experimental gravimetric data. Optical Raman spectroscopy line scans suggest that the porosity has a gradient between the surface and the film/bulk interface, which is supported by pore-size distribution data obtained from small-angle x-ray scattering (SAXS) on slightly oxidized and fully oxidized samples. Detailed inspection of the power law behavior of SAXS data suggests that the internal surface area of well-oxidized glassy carbon (GC) is compact and extends over the entire probed volume and thus has optimal pore connectivity. This effect goes along with pore enlargement and a relative decrease of internal surface area per volume. Slightly oxidized carbon has no pore space with a compact, high connectivity internal surface area. The corresponding SAXS power law and the x-ray density suggest that this high volumetric surface area must be interpreted as a result of surface roughness, rather than true geometric or volumetric surface area. In consequence, is this surface area of limited use for electrochemical energy storage

Investigation of the Interaction between Nafion Ionomer and Surface Functionalized Carbon Black Using Both Ultrasmall Angle X-ray Scattering and Cryo-TEM

In making a catalyst ink, the interactions between Nafion ionomer and catalyst support are the key factors that directly affect both ionic conductivity and electronic conductivity of the catalyst layer in a membrane electrode assembly. One of the major aims of this investigation is to understand the behavior of the catalyst support, Vulcan XC-72 (XC-72) aggregates, in the existence of the Nafion ionomer in a catalyst ink to fill the knowledge gap of the interaction of these components. The dispersion of catalyst ink depends not only on the solvent but also on the interaction of Nafion and carbon particles in the ink. The interaction of Nafion ionomer particles and XC-72 catalyst aggregates in liquid media was studied using ultrasmall-angle X-ray scattering and cryogenic TEM techniques. Carbon black (XC-72) and functionalized carbon black systems were introduced to study the interaction behaviors. A multiple curve fitting was used to extract the particle size and size distribution from scattering data. The results suggest that the particle size and size distribution of each system changed significantly in Nafion + XC-72 system, Nafion + NH2-XC72 system, and Nafion + SO3H-XC-72 system, which indicates that an interaction among these components (i.e., ionomer particles and XC-72 aggregates) exists. The cryogenic TEM, which allows for the observation the size of particles in a liquid, was used to validate the scattering results and shows excellent agreement

Effects of Ink Formulation on Construction of Catalyst Layers for High-Performance Polymer Electrolyte Membrane Fuel Cells

Rational design of catalyst layers in a membrane electrode assembly (MEA) is crucial for achieving high-performance polymer electrolyte membrane fuel cells. Establishing a clear understanding of the property (catalyst ink)-structure (catalyst layer)-performance (MEA) relationship lays the foundation for this rational design. In this work, a synergistic approach was taken to correlate the ink formulation, the microstructure of catalyst layers, and the resulting MEA performance to establish such a property-structure-performance relationship. The solvent composition (n-PA/H2O mixtures) demonstrated a strong influence on the performance of the MEA fabricated with an 830-EW (Aquivion) ionomer, especially polarization losses of cell activation and mass transport. The performance differences were studied in terms of how the solvent composition affects the catalyst/ionomer interface, ionomer network, and pore structure of the resulting catalyst layers. The ionomer aggregates mainly covered the surface of catalyst aggregates acting as oxygen reduction reaction active sites, and the aggregate sizes of the ionomer and catalyst (revealed by ultrasmall angle X-ray scattering and cryo-transmission electron microscopy) were dictated by tuning the solvent composition, which in turn determined the catalyst/ionomer interface (available active sites). In n-PA/H2O mixtures with 50∼90 wt % H2O, the catalyst agglomerates could be effectively broken up into small aggregates, leading to enhanced kinetic activities. The boiling point of the mixed solvents determined the pore structure of ultimate catalyst layers, as evidenced by mercury porosimetry and scanning electron microscopy. For mixed solvents with a higher boiling point, the catalyst-ionomer aggregates in the ink tend to agglomerate during the solvent evaporation process and finally form larger catalyst-ionomer aggregates in the ultimate catalyst layer, resulting in more secondary pores and thus lower mass transport resistance. Both the enlarged catalyst/ionomer interface and appropriate pore structure were achieved with the catalyst layer fabricated from an n-PA/H2O mixture with 90 wt % H2O, leading to the best MEA performance

Quantification of Thermal Oxidation in Metallic Glass Powder using Ultra-small Angle X-ray Scattering

In this paper, the composition, structure, morphology and kinetics of evolution during isothermal oxidation of Fe 48Cr 15Mo 14Y 2C 15B 6 metallic glass powder in the supercooled region are investigated by an integrated ex-situ and in-situ characterization and modelling approach. Raman and X-ray diffraction spectra established that oxidation yielded a hierarchical structure across decreasing length scales. At larger scale, Fe 2O 3 grows as a uniform shell over the powder core. This shell, at smaller scale, consists of multiple grains. Ultra-small angle X-ray scattering intensity acquired during isothermal oxidation of the powder over a wide Q-range delineated direct quantification of oxidation behavior. The hierarchical structure was employed to construct a scattering model that was fitted to the measured intensity distributions to estimate the thickness of the oxide shell. The relative gain in mass during oxidation, computed theoretically from this model, relatively underestimated that measured in practice by a thermogravimetric analyzer due to the distribution in sizes of the particles. As a whole, this paper introduces the first direct quantification of oxidation in metallic glass powder by ultra-small angle X-ray scattering. It establishes novel experimental environments that can potentially unfold new paradigms of research into a wide spectrum of interfacial reactions in powder materials at elevated temperatures

Modeling Structural Colors from Disordered One-Component Colloidal Nanoparticle-based Supraballs using Combined Experimental and Simulation Techniques

Bright, saturated structural colors in birds have inspired synthesis of self-assembled, disordered arrays of assembled nanoparticles with varied particle spacings and refractive indices. However, predicting colors of assembled nanoparticles, and thereby guiding their synthesis, remains challenging due to the effects of multiple scattering and strong absorption. Here, we use a computational approach to first reconstruct the nanoparticles' assembled structures from small-angle scattering measurements and then input the reconstructed structures to a finite-difference time-domain method to predict their color and reflectance. This computational approach is successfully validated by comparing its predictions against experimentally measured reflectance and provides a pathway for reverse engineering colloidal assemblies with desired optical and photothermal properties.Comment: 14 pages, 3 figures, 1 ToC figur

Experimental and Computational Investigation of Subcritical Near-Nozzle Spray Structure and Primary Atomization in the Engine Combustion Network Spray D

[EN] In order to improve understanding of the primary atomization process for diesel-like sprays, a collaborative experimental and computational study was focused on the near-nozzle spray structure for the Engine Combustion Network (ECN) Spray D single-hole injector. These results were presented at the 5th Workshop of the ECN in Detroit, Michigan. Application of x-ray diagnostics to the Spray D standard cold condition enabled quantification of distributions of mass, phase interfacial area, and droplet size in the near-nozzle region from 0.1 to 14 mm from the nozzle exit. Using these data, several modeling frameworks, from Lagrangian-Eulerian to Eulerian-Eulerian and from Reynolds-Averaged Navier-Stokes (RANS) to Direct Numerical Simulation (DNS), were assessed in their ability to capture and explain experimentally observed spray details. Due to its computational efficiency, the Lagrangian-Eulerian approach was able to provide spray predictions across a broad range of conditions. In general, this "engineering-level" simulation was able to reproduce the details of the droplet size distribution throughout the spray after calibration of the spray breakup model constants against the experimental data. Complementary to this approach, higher-fidelity modeling techniques were able to provide detailed insight into the experimental trends. For example, interface-capturing multiphase simulations were able to capture the experimentally observed bimodal behavior in the transverse interfacial area distributions in the near-nozzle region. Further analysis of the spray predictions suggests that peaks in the interfacial area distribution may coincide with regions of finely atomized droplets, whereas local minima may coincide with regions of continuous liquid structures. The results from this study highlight the potential of x-ray diagnostics to reveal salient details of the near-nozzle spray structure and to guide improvements to existing primary atomization modeling approaches.Battistoni, M.; Magnotti, GM.; Genzale, CL.; Arienti, M.; Matusik, KE.; Duke, DJ.; Giraldo-Valderrama, JS.... (2018). Experimental and Computational Investigation of Subcritical Near-Nozzle Spray Structure and Primary Atomization in the Engine Combustion Network Spray D. SAE International Journal of Fuel and Lubricants. 11(4):337-352. https://doi.org/10.4271/2018-01-0277S33735211

Nanoassemblies of ultrasmall clusters with remarkable activity in carbon dioxide conversion into C1 fuels

Cu nanoassemblies formed transiently during reaction from size-selected subnanometer Cu4 clusters supported on amorphous OH-terminated alumina convert CO2 into methanol and hydrocarbons under near-atmospheric pressure at rates considerably higher than those of individually standing Cu4 clusters. An in situ characterization reveals that the clusters self-assemble into 2D nanoassemblies at higher temperatures which then disintegrate upon cooling down to room temperature. DFT calculations postulate a formation mechanism of these nanoassemblies by hydrogen-bond bridges between the clusters and H2O molecules, which keep the building blocks together while preventing their coalescence