Harry J. Ploehn

Ultrathin, Molecular-Sieving Graphene Oxide Membranes for Selective Hydrogen Separation

Gas Separations When gas separation membranes are made thinner, they usually allow permeating gases to pass through faster. However, a thinner membrane may be poorer at separating between gas species. Kim et al. (p. 91) examined the permeability and selectivity of layered graphene and graphene oxide membranes. Gas molecules diffuse through defective pores and channels that form between the layers. Controlling these structures tuned the properties of the membranes to allow the extraction of carbon dioxide from other gases. Li et al. (p. 95) describe membranes as thin as 1.8 nanometers made from only two to three layers of graphene oxide. Small defects within the layers allowed hydrogen to pass through, separating it from carbon dioxide and nitrogen. Ultrathin graphene oxide membranes show enhanced separation selectivity for hydrogen gas. Ultrathin, molecular-sieving membranes have great potential to realize high-flux, high-selectivity mixture separation at low energy cost. Current microporous membranes [pore size < 1 nanometer (nm)], however, are usually relatively thick. With the use of current membrane materials and techniques, it is difficult to prepare microporous membranes thinner than 20 nm without introducing extra defects. Here, we report ultrathin graphene oxide (GO) membranes, with thickness approaching 1.8 nm, prepared by a facile filtration process. These membranes showed mixture separation selectivities as high as 3400 and 900 for H2/CO2 and H2/N2 mixtures, respectively, through selective structural defects on GO.

Polymer Composite and Nanocomposite Dielectric Materials for Pulse Power Energy Storage

This review summarizes the current state of polymer composites used as dielectric materials for energy storage. The particular focus is on materials: polymers serving as the matrix, inorganic fillers used to increase the effective dielectric constant, and various recent investigations of functionalization of metal oxide fillers to improve compatibility with polymers. We review the recent literature focused on the dielectric characterization of composites, specifically the measurement of dielectric permittivity and breakdown field strength. Special attention is given to the analysis of the energy density of polymer composite materials and how the functionalization of the inorganic filler affects the energy density of polymer composite dielectric materials.

Solvent Diffusion Model for Aging of Lithium-Ion Battery Cells

This work presents a rigorous continuum mechanics model of solvent diffusion describing the growth of solid-electrolyte interfaces ~SEIs! in Li-ion cells incorporating carbon anodes. The model assumes that a reactive solvent component diffuses through the SEI and undergoes two-electron reduction at the carbon-SEI interface. Solvent reduction produces an insoluble product, resulting in increasing SEI thickness. The model predicts that the SEI thickness increases linearly with the square root of time. Experimental data from the literature for capacity loss in two types of prototype Li-ion cells validates the solvent diffusion model. We use the model to estimate SEI thickness and extract solvent diffusivity values from the capacity loss data. Solvent diffusivity values have an Arrhenius temperature dependence consistent with solvent diffusion through a solid SEI. The magnitudes of the diffusivities and activation energies are comparable to literature values for hydrocarbon diffusion in carbon molecular sieves and zeolites. These findings, viewed in the context of recent SEI morphology studies, suggest that the SEI may be viewed as a single layer with both micro- and macroporosity that controls the ingress of electrolyte, anode passivation by the SEI, and cell perfor

AC Impedance Studies on Metal Hydride Electrodes

The metal hydride (MH{sub x}) electrode is the negative electrode in one of the most advanced rechargeable batteries (i.e. nickel/metal hydride). The objective of this study is to obtain insight on the mechanism of the hydriding/dehydriding reaction in the battery, using the electrochemical impedance spectroscopy (EIS) technique. An equivalent circuit for the MH{sub x} electrode reaction is proposed. The rate capabilities of charge/discharge reaction of MH{sub x} electrode are determined by the kinetics of charge transfer reaction at the alloy surface. Transient and pseudo steady-state analyses (cyclic voltammetry and potential vs. current density behavior) qualitatively and quantitatively support the EIS results. EIS studies on electrodes with (i) three types of binding additives, (ii) varying amounts of active material, and (iii) two types of alloys as active materials demonstrate the usefulness of this technique to develop electrodes with the optimum compositions and structures.

Modeling the Effects of Electrode Composition and Pore Structure on the Performance of Electrochemical Capacitors

This work presents a mathematical model for charge/discharge of electrochemical capacitors that explicitly accounts for particlepacking effects in a composite electrochemical capacitor consisting of hydrous RuO2 nanoparticles dispersed within porous activated carbon. The model is also used to investigate the effect of nonuniform distributions of salt in the electrolyte phase of the electrode in the context of dilute solution theory. We use the model to compare the performance of capacitors with electrodes made from different activated carbons and to investigate the effects of varying carbon content and discharge current density. Even at low discharge current density, concentration polarization in the electrodes results in underutilization of the electrodes’ charge-storage capability, and thus decreased performance. Among various types of activated carbons, those with large micropore surface areas and low meso- and macropore surface areas are preferred because they give high double-layer capacitance and favor efficient packing of RuO2 nanoparticles, thus maximizing faradaic pseudocapacitance. Increasing the electrode carbon content decreases the delivered charge and energy density, but the reductions are not severe at moderate carbon content and high discharge current. This suggests the possibility of optimizing the carbon content to minimize cost while achieving acceptable discharge performance.

New Magnetic Field-Enhanced Process for the Treatment of Aqueous Wastes

ABSTRACT A new magnetic adsorbent material, called magnetic polyamine-epichlorohydrin (MPE) resin, was prepared by attaching activated magnetite to the outer surface of polyamine-epichlorohydrin resin beads. Experiments were carried out in the presence of a 0.3-tesla magnetic field to investigate the removal of actinides (plutonium and americium) from pH 12 wastewater using this new resin. The results demonstrated that the MPE resin has a significantly enhanced capacity for actinides over conventional ferrite-based surface complexation adsorption processes (where no field is applied) and over traditional high-gradient magnetic separation (HGMS) processes that remove suspended particles. This enhancement was attributed to the presence and subsequent removal of suspended actinide nanoparticles through an HGMS effect, with the magnetite acting as a very effective HGMS element. A theoretical analysis verified this supposition by showing that under adequate pHs and particle-particle separations, the attractive...

Dynamic Mechanical Analysis of the Effect of Water on Glass Bead–Epoxy Composites

Dynamic mechanical analysis (DMA) has been used to investigate the effect of water and glass bead surface treatment on the properties of glass bead–epoxy composites. By treating or not treating the glass beads with a silane coupling agent, we fabricated composites with ostensibly good or poor interfacial adhesion. SEM images of fracture surfaces and water uptake data confirmed this picture. We used dynamic mechanical tests to measure the material properties of dry and wet specimens. Temperature sweep tests of atmosphere-conditioned specimens indicated that the value of the loss tangent at the temperature of the α-α-relaxation peak was most sensitive to interfacial adhesion. For wet specimens, the magnitude of an additional relaxation process, denoted as the ω-relaxation, correlated strongly with water uptake and, indirectly, interfacial adhesion. Master curves constructed from frequency sweep tests also manifested differences among dry and wet specimens, but shift factor data suggested that these tests were more prone to complications due to water loss. Apparent activation energies of α- and β-relaxation processes were statistically significant indicators of interfacial adhesion in dry and wet composites, respectively.

Shrinking Core Model for the Discharge of a Metal Hydride Electrode

Metal hydride particles are used to make negative electrodes 1-3 in nickel/metal hydride batteries. The performance of these electrodes is affected by both the kinetics of the processes occurring at the metalelectrolyte interface and the hydrogen diffusion within the bulk of the metal alloy particle. Two different phases exist in the metal hydride particles. The hydriding or charging process of metal hydride particles was discussed in detail in Ref. 4 where equations governing the diffusion of hydrogen in the particle during charging (hydriding) were derived from the fundamental laws of mass and momentum transfer. Zhang et al. 4 were the first to develop rigorous boundary conditions based on jump balances. They provided a closed-form solution for the charging of metal hydride electrodes assuming a known (constant) concentration at the surface. They derived expressions describing the motions of the ab interface and the weight fraction of hydrogen entering the electrode particle from the electrolyte. They predicted that for particles of smaller radius and smaller diffusion coefficients, the pseudosteady-state (PSS) solution 5-12 does not provide an accurate solution of the governing equations. Unfortunately, their model cannot be used to predict the effect of applied current density on the concentration profiles and charge/discharge curves for the metal hydride electrodes. The discharge process of a metal hydride particle includes a phase change as shown schematically in Fig. 1. In the fully charged state (Fig. 1a) the metal hydride particle is in the b phase. The discharge process begins when the adsorbed hydrogen (Hads) at the surface of the particle reacts electrochemically with hydroxide ions as follows discharge

Sustainable thermoplastic elastomers derived from renewable cellulose, rosin and fatty acids

Two series of graft copolymers, cellulose-g-poly(n-butyl acrylate-co-dehydroabietic ethyl methacrylate) (Cell-g-P(BA-co-DAEMA)) and cellulose-g-poly(lauryl methacrylate-co-dehydroabietic ethyl methacrylate) (Cell-g-P(LMA-co-DAEMA)), were prepared by “grafting from” atom transfer radical polymerization (ATRP). In these novel graft copolymers, cellulose, DAEMA (derived from rosin), and LMA (derived from fatty acids) are all sourced from renewable natural resources. The “grafting from” ATRP strategy allows the preparation of high molecular weight graft copolymers consisting of a cellulose main chain with acrylate copolymer side chains. By manipulating the monomer ratios in the P(BA-co-DAEMA) and P(LMA-co-DAEMA) side chains, graft copolymers with varying glass transition temperatures (−50–60 °C) were obtained. Tensile stress–strain and creep compliance testing were employed to characterize mechanical properties. These novel graft copolymers did not exhibit linear elastic properties above about 1% strain, but they did manifest remarkable elasticity at strains of 500% or more. These results suggest that these cellulose-based, acrylate side-chain polymers are potential candidates for service as thermoplastic elastomers materials in applications requiring high elasticity without rupture at high strains.

Feasibility and limitations of nanolevel high gradient magnetic separation

This work proposes a new separation concept denoted as nanolevel high gradient magnetic separation (HGMS) or magnetic adsorption. A magnetic heteroflocculation model describes the magnetic forces between two spherical particles with different sizes and magnetic properties, and reveals the feasibilities and limitations of nanolevel HGMS. The adsorbent particles, composed of antiferromagnetic magnetite, are modeled as large, immobile spheres on the order of 100–500 nm in radius. The adsorbate, paramagnetic colloidal Fe(OH)2 particles, are treated as freely diffusing small spheres on the order of 20–80 nm in radius. The model assumes that the magnetite particles are dispersed throughout a porous, nonmagnetic, solid matrix and that they are free of convective forces. The model also assumes that magnetic forces alone act on the Fe(OH)2 particles, opposed only by Brownian motion. When the magnetic force is attractive and overwhelms the randomizing Brownian force, adsorption occurs. The results from this model show the importance of the external field strength, the sizes of the adsorbent and adsorbate particles, and their magnetic properties in developing a practical nanolevel HGMS process.