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Hierarchically-structured, electro-catalysts consisting of a titanium core with controlled macro-porosity and a thin titania surface-layer with controlled nano-porosity were synthesized via combination of additive manufacturing and in-situ anodization. The electrodes were tested for hydrogen evolution reaction and depicted competitive electro-catalytic activities, with Tafel slopes between 40 and 56 mV dec-1. The electrodes also depicted competitive onset and over potentials when compared with other electrode alternatives. The synthesis approach for the electro-catalysts reported in this work is being extended to other metal/metal-oxide pairs and applications.

Naturally-occurring gas hydrates (in permafrost and marine sediments) have the potential to contribute as a carbon-based source to the increasing energy demand. Precise estimates of gas-hydrate global inventory, development of strategies for their exploitation, and evaluation of their environmental impact require models that accurately describe gas-hydrate stability in marine sediments. A model for gas-hydrate equilibrium in porous media, developed from fundamental thermodynamic principles, is proposed and validated against available experimental data. The derivation of the model allowed for the natural incorporation of sediment properties into equilibrium conditions, such as interfacial energies and contact angles between different phases. Model parameters were obtained from independent experiments and fundamental calculations reported in the literature. For the range of pore sizes (3.4-24.75 nm) of different materials reported in the literature, the absolute average deviations (AAD%) between the model predictions and the experimental data are between 0.04% and 2.07%. The wettability of the pore surface affects the shape of the hydrate phase inside the pore and consequently influences the equilibrium pressures of gas-hydrate formed in porous media.

The electrical double layer (EDL) profiles of electrical potential within charged slit-type pores is calculated in this work via a combination of grand canonical Monte Carlo (GCMC) simulations and electrodynamics concepts. The electrical potential distribution inside the pore is calculated with respect to field-free virtual bulk electrolyte solution implicit in the grand canonical ensemble. The GCMC simulations for slit-type pores performed in this work show that entropy effects lead to the exclusion of co-ions from the pore to allow densely packed counterions to efficiently neutralize surface charge. This phenomenon, in conjunction with the fact that electrical effects are long range with respect to the source charge, leads to a semioscillatory behavior of the potential profiles not predicted by classical theory. While bulk conditions in terms of ion concentrations are achieved in the center of the pore for most pore sizes studied in this work, electrical bulk conditions are not achieved except for the l...

Molecular modeling of electrical double layer (EDL) structure and electrosorption of ions in pores predicts the on-set of competitive entropic and energetic effects, such as electrosorption capacity mainly determined by size exclusion effects rather than applied electrical potential. Classical EDL Theory does not predict these effects. Although size-exclusion effects have been used to explain discrepancies between experimental observations and Classical EDL Theory, direct experimental validation of molecular modeling predictions have yet to be achieved. The main objective of the present work was to devise and test a model system that can be used to experimentally measure size-exclusion effects for future validation of molecular simulations. This work is a first attempt to bridge the gap between molecular modeling predictions of EDL structure and experimentally-measured macroscopic properties of the EDL. Highly-uniform, titanium dioxide nanotubes with three pore sizes (36.8, 41.4 and 44.9 nm) were used as model electrodes to study the interactive effects of pore diameter, applied potential and ion size on the electrosorption of three monovalent alkaline-metal cations (Li + , Na + , Cs + ). The proportionality of EDL electrical capacitance and ion-electrosorption capacity to surface area and applied potential predicted by Classical EDL Theory did not hold for all combinations of pore sizes and applied potentials examined in this work, and electrosorption capacity clearly depended on pore diameter rather than specific surface area, and on hydrated-ion radius. The measured electrosorption capacity trend in most cases corresponded to Cs + > Na + > Li + . This trend follows the hydrated radius size. Size exclusion effects on EDL capacitance and electrosorption capacity triggered by hydrated ion-size and pore diameter observed in this work were in qualitative agreement with molecular modeling predictions.

Publisher's version (útgefin grein), Highly ordered titanium dioxide nanotubes (TiO2 NTs) were fabricated through anodization and tested for their applicability as model electrodes in electrosorption studies. The crystalline structure of the TiO2 NTs was changed without modifying the nanostructure of the surface. Electrosorption capacity, charging rate, and electrochemical active surface area of TiO2 NTs with two different crystalline structures, anatase and amorphous, were investigated via chronoamperometry, cyclic voltammetry, and electrochemical impedance spectroscopy. The highest electrosorption capacities and charging rates were obtained for the anatase TiO2 NTs, largely because anatase TiO2 has a reported higher electrical conductivity and a crystalline structure that can potentially accommodate small ions within. Both electrosorption capacity and charging rate for the ions studied in this work follow the order of Cs+ > Na+ > Li+, regardless of the crystalline structure of the TiO2 NTs. This order reflects the increasing size of the hydrated ion radii of these monovalent ions. Additionally, larger effective electrochemical active surface areas are required for larger ions and lower conductivities. These findings point towards the fact that smaller hydrated-ions experience less steric hindrance and a larger comparative electrostatic force, enabling them to be more effectively electrosorbed.

Although some differences have been observed, the transport behavior of radioactive aerosol particles has often been assumed to be analogous to the behavior of nonradioactive aerosols in dispersion models. However, radioactive particles can become electrostatically charged as a result of the decay process. Theories have been proposed to describe this self-charging phenomenon, which may have a significant effect on how these particles interact with one another and with charged surfaces in the environment. In this study, atomic force microscopy (AFM) was employed to quantify surface forces between a particle and a planar surface and to compare measurements with and without the involvement of radioactivity. The main objective of this work is to assess directly the effects of radioactivity on the surface interactions of radioactive aerosols via the measurement of the adhesion force. The adhesion force between a silicon nitride AFM tip and an activated gold substrate was measured so that any possible effects due to radioactivity could be observed. The adhesion force between the tip and the gold surface increased significantly when the gold substrate (25 mm{sup 2} surface area) was activated to a level of approximately 0.6 mCi. The results of this investigation will prompt further work into themore » effects of radioactivity in particle-surface interactions.« less

A continuous-jet hydrate reactor originally developed to generate a CO2 hydrate stream has been modified to continuously produce CH4 hydrate. The reactor has been tested in the Seafloor Process Simulator (SPS), a 72-L pressure vessel available at Oak Ridge National Laboratory. During experiments, the reactor was submerged in water inside the SPS and received water from the surrounding through a submersible pump and CH4 externally through a gas booster pump. Thermodynamic conditions in the hydrate stability regime were employed in the experiments. The reactor produced a continuous stream of CH4 hydrate, and based on pressure values and amount of gas injected, the conversion of gas to hydrate was estimated. A conversion of up to 70% was achieved using this reactor.

A three-phase, pilot-scale continuous-jet hydrate reactor (CJHR) has been developed for the production of gas hydrates. The reactor receives water and a hydrate-forming species to produce the solid gas hydrate. The CJHR has been tested for the production of CO{sub 2} hydrate for the purpose of ocean carbon sequestration. Formation of CO{sub 2} hydrate was investigated using various reactor/injector designs in a 72-l high-pressure vessel. Designs of the CJHR varied from single-capillary to multiple-capillary injectors that dispersed (1) liquid CO{sub 2} into water or (2) water into liquid CO{sub 2}. The novel injector is designed to improve the dispersion of one reactant into the other and, thus, eliminate mass transfer barriers that negatively affect conversion. An additional goal was an increase in production rates of two orders of magnitude. The designed injectors were tested in both distilled and saline water. Hydrate production experiments were conducted at different CO{sub 2} and water flow rates and for pressures and temperatures equivalent to intermediate ocean depths (1100-1700 m). The pilot-scale reactor with the novel injection system successfully increased hydrate production rates and efficiency.

A pilot-scale, three-phase continuous-jet hydrate reactor, developed to produce CO 2 hydrate for ocean sequestration, was tested both in the laboratory and at sea. A 72-L pressure vessel was used for laboratory tests; field experiments were performed with a remotely operated vehicle at depths between 1200 and 2000 m off the coast of Monterey, CA. Rapid production of a consolidated sinking CO 2 -hydrate composite paste was achieved in both settings. The vertical and lateral movement of the extruded hydrate was monitored by the high-definition television camera mounted on the vehicle and with a 675-kHz scanning sonar, along with dissolution rates and associated temperature and pH changes during the injection operations. It was observed that globules of unconverted liquid CO 2 occluded in the structure of the hydrate composite largely determine the hydrate composite behavior in the ocean by providing sites for accelerated dissolution, thereby affecting the CO 2 -hydrate particle orientation, shape, lifetime, and sinking rate. Model calculations predict that large-scale releases of these particles (at a CO 2 injection rate of ∼100 kg/s) should show a descent depth of nearly 1000 m below their release point, as a result of plume dynamics and the increase in density caused by the CO 2 dissolution into the surrounding ocean water.

A nonlinear solution of the Poisson−Boltzmann equation between two interacting surfaces was used to model the interaction force between a gold electrode and a standard silicon−nitride cantilever tip employed in atomic force microscopy (AFM). AFM measurements were used to calculate the effective gold electrode/electrolyte solution interface potential via minimization of the error between predicted interaction force values and those measured via AFM. Analysis of the data reveals that an effective electrode/electrolyte potential, rather than the applied potential to the electrode, is responsible for the interaction forces observed in this work. Further examination of the gold electrode/electrolyte interface via cyclic voltammetry reveals that, despite the fact that the gold electrode is considered inert, some degree of association of ions present in the solution with the gold electrode occurs. Accumulation of different ions in the solution at the electrode/electrolyte interface determines the magnitude of th...

The study of aggregation of colloidal particles involves assessment of the thermodynamics and kinetics of the process and the characteristics of the phase formed. In this article, recent contributions to the understanding of aggregation are discussed from two different perspectives: (1) developments following the classical treatment and (2) new molecular approaches.

Unfavorable aggregation and deposition of colloidal particles in natural and engineered systems is still a subject of debate. Complicating factors such as surface roughness, secondary minimum aggregation, and the nature of discrete surface charge and surface potential make it difficult to attribute a specific cause to these phenomena. The presence of surface charge heterogeneity and its influence on interaction forces, which are responsible for aggregation and deposition, are studied in this work through the application of atomic force microscopy (AFM). Force-volume-mode AFM was used to map interaction forces on a surface and relate them to surface charge heterogeneities. The experimental system consisted of a silica plate and a standard silicon nitride AFM tip. Copper ions were used for sorption on the silica surface in order to modify the surface charge and cause charge reversal. Different concentrations of copper ions were selected to identify conditions of partial coverage of the silica surface. The pH and ionic strength of the solutions were varied, and the extension of the surface charge modification and its influence on the resulting interaction forces were monitored via AFM force measurements. Depending on the pH and ionic strength, the interaction force was found to change at certain regions on the surface from attraction to either weak or strong repulsion. Force imaging allowed the visual localization of zones of strong repulsive interaction that diminished in size with increasing ionic strength. X-ray photoelectron spectroscopy analysis was used to confirm the presence of copper on the surface. Local charge differences on a surface result in local differences in surface forces, not only in magnitude but also in direction. This behavior may explain the aggregation, deposition, and transport of colloidal particles under unfavorable chemical conditions.

Canonical Monte Carlo simulations of the interaction between a uniformly charged spherical particle and a discretely charged planar surface in solutions of symmetric and asymmetric electrolytes were performed. To assess the nature of the interactions, the force exerted on the colloidal particle perpendicular to the planar surface was calculated. Attractive minima in the interaction force between the similarly charged surfaces reveal the occurrence of two phenomena: long-range attraction of electrostatic origin and short-range attraction due to depletion effects. The degree of electrostatic coupling determines the magnitude and range of like-charge attraction between the two surfaces.

Canonical Monte Carlo (CMC) simulations are employed in this work in order to study the structure of the electrical double layer (EDL) near discretely charged planar surfaces in the presence of symmetric and asymmetric indifferent electrolytes within the framework of a primitive model. The effects of discreteness and strength of surface charge, charge asymmetry, and size asymmetry are specific focuses of this work. The CMC simulation protocol is initially tested against the classical theory, the modified Gouy-Chapman (GC) theory, in order to assess the reliability of the simulation results. The CMC simulation results and the predictions of the classical theory show good agreement for 1:1 electrolytes and low surface charge, at which conditions the GC theory is valid. Simulations with symmetric and asymmetric electrolytes and mixtures of the two demonstrate that size plays an important role in determining the species present in the EDL and how the surface charge is screened. A size-exclusion effect could be consistently detected. Although it is energetically favorable that higher-valence ions screen the surface charge, their larger size prevents them from getting close to the surface. Smaller ions with lower valences perform the screening of the charge, resulting in higher local concentrations of small ions close to the surface. The simulations also showed that the strength of the surface charge enhances the size-exclusion effect. This effect will definitely affect the magnitude of the forces between interacting charged surfaces.

Grand canonical Monte Carlo (GCMC) simulations are employed to study the selective electrosorption of ions from a mixture of symmetric and asymmetric electrolytes confined in pores and results are compared to experimental observations obtained via cyclic voltammetry and batch electrosorption equilibrium experiments. GCMC simulations have the advantage over other Monte Carlo methods to unambiguously quantify the total number of ions in the pore solution. The exclusion parameter and selectivity factor are used to evaluate the selective capacity of pores toward different ionic species under various conditions. The number of coions inside the pore solution is determined by the proportion of different counterions present in the double-layer region. Because of the competitive effects resulting from asymmetries in charge and size associated with different ions, the electrosorption selectivity of small monovalent over large divalent counterions first decreases with increasing surface charge, passes through a minimum, and then increases with further increase in surface charge. At low and moderate surface charge densities, the fact that large divalent counterions preferentially screen the surface charge has a strong effect on pore occupancy; whereas at a very high surface charge density, size-exclusion effects dominate and determine the accessibility of different ions into the pores. Therefore, electrosorption selectivity of ions from a mixture of electrolytes could, in principle, be achieved via tuning the electrical double-layer formation inside the pores through the regulation of surface charge tailored for different ion characteristics. The findings of this work provide important information relevant to ion selectivity during separation processes and energy storage in supercapacitors.