Gennady Gor: Research

Our primary area of expertise is modeling of chemical and mechanical processes in porous materials, ranging from nanoporous adsorbents and polymer membranes to geological materials. Our toolkit spans various modeling techniques that cover both the micro and macro scales. We use molecular modeling: Monte Carlo and molecular dynamics simulations, classical density functional theory (cDFT); additionally we employ electronic structure DFT and finite element methods (FEM). Specifically, we are interested in the mechanical effects of fluids sorption by porous and soft materials. These effects are manifested in numerous applied areas of research: membrane separations, high energy storage batteries, chemical sensing, shale gas recovery, carbon sequestration, etc.

The current projects are:

The detailed description of each of the projects is given below.

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Exploring Physisorption-Induced Stresses in Nanoporous Materials for Sensing of Chemical Warfare Agents

Organophosphorous compounds (sarin, soman, tabun, etc.) are among the most potent chemical warfare agents (CWAs), with very small concentrations being lethal. Therefore, rapid and sensitive methods for their detection is a high priority for national security. To date, there have been several sensing mechanisms proposed for this purpose: chemielectrical sensors based on semiconducting metal oxides (SMO), surface acoustic wave sensors and micro-cantilevers. Among those, SMO is the only technology which became commercialized due to its fast response time relative to its competitors. However, a number of limitations (e.g. a high operation temperature) prevents it from widespread use and drives the development of alternative sensing technologies.
Figure 1: Nanoporous PILTf2N/C-pillar[5]arene membrane actuator: SEM of the structure (reproduced from Zhao et al. Nat. Commun., 2014, 5, 4293).
Figure 2: The actuator shows unprecedentedly fast response to acetone vapor and as fast recovery (reproduced from Zhao et al. Nat. Commun., 2014, 5, 4293).
The working principle of the chemomechanical micro-cantilever sensors is based on the change in surface stress induced by adsorption of guest molecules on the surface. The change in surface stress causes bending of a microcantilever, which can be monitored. The drawbacks of this technology are the microscopic deflection of the cantilever (on the order of nm), which is hard to monitor, and long response times (on the order of 103 s) due to the slow kinetics of chemisorption. The same physical principle, i.e. adsorption-induced stress, but applied to a different type of material has a potential to substantially improve current sensing technologies. Nanoporous materials have high surface areas and relatively low elastic moduli, therefore the surface stresses induced by adsorption in nanoporous materials can produce easily-measurable macroscopic strains. Moreover, the response times for such systems can be on the order of seconds (See Figure 2). Finally, the surface properties of nanoporous materials can be tuned to make them selective for adsorption of certain compounds (e.g. CWAs). The objective of this research project is to evaluate a new sensing mechanism based on adsorption-induced stresses in nanoporous materials. This project will involve development of molecular and continuum model for predicting elastic response of functionalized porous materials to organophosphorous compounds. Due to extreme toxicity, the experiments on CWAs are generally performed on simulants, less toxic compounds with the structure and properties similar to that of the CWAs'; the most typical simulant being dimethyl methylphosphonate (DMMP). Unlike experiments, simulations can be done both with DMMP molecules and molecules of sarin.

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Effects of Electrolyte Solvents on Soft Components of Secondary Batteries

The development of cleaner renewable energy sources for the grid (wind, solar, etc.) as well as cleaner ways to power vehicles requires reliable and economical energy storage with high capacity and long lifetime. Currently the leading contenders are lithium-ion batteries (LIBs) , which provide the highest energy and power per unit mass. Although it is already a well-developed technology, it still has two weak points: (1) the price of large-scale LIBs remains too high given their short lifetime and (2) LIBs failure may cause thermal runaway and a battery can catch fire, e.g. the notorious Boeing 787 case.

Figure 3: Structure of a lithium battery cell (left). Two challenging problems related to the properties of the separator: lithium intercalation into the electrode particles causes noticeable expansion of the anode particles. Expanding anode compresses the separator, decreasing the ion transport through its pores (center). Lithium dendrite can grow through the separator and cause short circuit (right).
A lithium battery cell consists of two electrodes and a separator between them (Figure 3, left). The role of the separator is to electrically isolate the electrodes, but allow ion transport between them. While the performance characteristics of the batteries (e.g. specific power or specific energy) are determined by the electrode materials, the battery lifespan and safety strongly depend on the battery separator. During battery operation lithium ions intercalate into electrode particles causing their expansion, e.g. conventional graphite anode particles expand up to 10 %. Since the expanding electrodes are constrained in the battery cell, they compress the porous separator between them (Figure 3, center). Experiments show that compression of the separator causes its creep, closure of its pores, and local reduction of the ion transport. With cycling such local defects grow and lead to the aging and deterioration of the cells. The possibility of this aging scenario strongly depends on the mechanical properties of the separator. The knowledge of the mechanical properties of the separators is also necessary to assess the possibility of a lithium dendrite piercing through it (Figure 3, right). If a dendrite pierces through the separator, it causes a short circuit and an inevitable failure of the lithium-ion cell. Typical separators for liquid electrolyte batteries are made of porous polyolefins, polyethylene (PE) and polypropylene (PP). Although these are relatively simple and abundant polymers, the mechanical properties of the separator are determined by its complex porous microstructure (Figure 4). In addition, these polymers are semi-crystalline, which adds complexity on the molecular level. Moreover, recent experiments showed that the mechanical properties of the commercial polyolefin separators immersed in electrolyte solvents used in LIBs noticeably differ from the properties of the dry materials, e.g. tensile Young's modulus of the PP separator gets 50 % lower upon immersion. Interestingly, such significant softening of the separators is not accompanied with a noticeable change in its volume, and therefore cannot be explained by the rubber swelling phenomenon. Since the mechanical properties of the separator strongly affect the battery lifespan and safety, understanding of the effects of the solvents is of utmost importance for the battery industry.
Figure 4: Microstructure of a polypropylene membrane, typical commercial battery separator, Celgard 3501 (from Gor et al., J. Electrochem. Soc. 2014, 161(11), F3065-F3071, DOI: 10.1149/2.0111411jes).
The objective of this project is to develop a model for the interactions between the electrolyte solvent and soft components of the lithium-ion batteries (separators and binders) to predict the mechanical behavior of these components relevant to the batteries degradation and failure. This project has two specific aims. The first is to get the fundamental understanding of interactions between the carbonate solvents and semicrystalline polyolefins on the molecular level, which would explain the mechanism of experimentally observed softening with negligible swelling. And second, to apply this knowledge to get the quantitative predictions for the in situ behavior of various LIB separators with respect to compression and Li dendrite piercing.

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Unveiling the Mechanism of Restructuring of Soot Nanoparticles: Insight from Molecular Modeling

Soot is a major environmental pollutant with impacts ranging from air quality and human health to climate. The extent of these impacts depends on the microstructure of soot nanoparticles and their surface properties. The soot microstructure is complex, with nanoparticles being fractal aggregates of graphitic spherules mixed with organic and inorganic combustion products or other atmospheric chemicals. Soot nanoparticles often change structure when interacting with chemicals adsorbed on their surface. Recent experiments carried out by Khalizov's group showed that combustion products with similar molecular structures, e.g. various polycyclic aromatic compounds, behave qualitatively different with respect to restructuring (Figure 5).
SEM micrographs of (a) fresh soot particle, (b) soot particle coated with anthracene, where original morphology is sustained, and (c) soot particle coated with phenanthrene, where the fractal particle collapsed into a globule. (From Dr. Alexei Khalizov, unpublished).
The main goal of this project is to develop a molecular-based model for soot nanoparticles restructuring (Gor's group) and verify it against experimental measurements (Khalizov's group).

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Chemomechanical Effects on the Zeolite Membrane Permeation

Since their appearance in the 1940-s, zeolites have found applications in many chemical engineering processes: adsorption, ion-exchange, catalysis, etc. Uniform structure and small size of the zeolites' pores suggest that zeolites could be applied as separation membrane materials. In the last two decades the substantial progress towards this goal has been made, so that to date some of the zeolite membranes are already used on industrial scale.
Figure 6: Top and side views (SEM) of 0.5 um MFI membrane (from Rangnekar et al., Chem. Soc. Rev., 2015, 44, 7128-7154. ).
Zeolite membranes are synthesized by introducing the ex situ prepared crystal seeds on a porous substrate and further in situ growth of the zeolite layer. This procedure leads to the formation of a structure composed of multiple micrometer sized crystallites (Figure 6). This composite structure inevitably has numerous defects -- intercrystalline spaces, which are larger than the zeolite pores. A thorough characterization of the zeolite membranes showed that these defects can noticeably contribute to the zeolite membrane transport. Moreover, recent in situ X-ray diffraction and optical microscopy studies have shown that adsorption of many molecules in the zeolite pores cause the swelling of the crystallites and change of the defects sizes in the membranes (Figure 7). This effect alters the permeation and separation properties of the membranes, so that the membrane transport fails to obey the Maxwell-Stefan law. Since this chemomechanical behavior has a dramatic effect on the membrane transport, it has to be well understood and incorporated into the adsorption-transport models for zeolite membrane separation. Since the chemomechanical behavior of zeolite membranes is qualitatively different depending on the adsorbing species, it requires the development of the molecular-level model. Although the techniques for molecular modeling of adsorption and transport in zeolites are well-developed, most of the research is done for rigid zeolite frameworks. Some works investigated the effects of the framework flexibility on the heat of adsorption and Henry constant and also on diffusion coefficient in zeolites pores. None of these studies focused on the zeolite membranes and their transport properties altered by chemomechanical effects. Therefore, there is an obvious demand for development of the model for adsorption in flexible framework with the main focus on the elastic response of the framework itself.
Figure 7: Schematic of adsorption-induced swelling of zeolite crystallites and consequent effect on the defects (from Yu et al., Acc. Chem. Res., 2011, 44, 1196-1206)
The objective of this project is to develop a chemomechanical model for behavior of zeolite membranes in the course of adsorption and transport of various gases of industrial relevance: alkanes, CO2, xylene, alcohols, etc. On the one hand, this model has to capture the specifics of zeolite-adsorbate interactions on molecular level. On the other hand, the resulting model has to be capable of macroscopic predictions with a limited numbers of parameters, so that it could be employed for simulations of the membrane transport process. The project has the following specific aims: the first is the development of an atomistic models for MFI and NaA zeolites, which capture not just the adsorption potential, but also the elastic response of zeolites to adsorption. The second is the development a theoretical framework for predicting the deformation induced by adsorption of mixtures. This research will be performed in the following three stages.

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Thermodynamic and Transport Properties of Nanoconfined Hydrocarbons and Carbon Dioxide

Fossil fuels are currently the main source of energy for mankind, and according to the DOE perspective even in 30 years we will still be satisfying 80% of our energy needs by use of fossil fuels. Use of fossil fuels has an inherent waste product - carbon dioxide. Within the last decade, a significant effort has been expended on identifying ways to avoid CO2 release in the atmosphere, which is the domain of CO2 sequestration. Various geological formations are considered as options for long-term storage of CO2: depleted oil reservoirs, unmineable coal seams, deep saline aquifers, etc. So far the commercially viable strategies are the ones related to the use of CO2 for enhanced recovery of hydrocarbons - oil or coalbed methane. In this case carbon dioxide is injected into an oil well or a coal seam, displacing hydrocarbons, increasing the production, and then remaining trapped in the geological formation.

Figure 8: Shale has porous structure with nanopores of different sizes and morphologies, SEM image. (From Loucks et al., AAPG bulletin, 2012, 96, 1071-1098).

In recent years a new source of fossil fuels became promising - natural gas from shale formations. The current technology for shale gas recovery based on hydraulic fracturing extracts only about 25% of the gas. Therefore, performing enhanced recovery of shale gas after hydraulic fracturing is highly desirable. Several recent studies have shown big promise for adapting techniques for enhanced recovery of hydrocarbons by CO2 injection for shale gas reservoirs. However, this technology has a number of fundamental issues which are not yet well understood.

Shale has porous structure (see Figure) and substantial amount of natural gas is present inside these nanopores. Solid-fluid molecular attraction between shale and CO2 is stronger than between shale and methane (the main component of shale gas). Therefore, if CO2 is injected in the shale, it displaces methane; methane gets extracted (desorbed) from pores, while CO2 is adsorbed. However, stronger intermolecular forces cause adsorption-induced deformation effects that cause the shale to swell. Swelling will result in the narrowing and even closing of the macropores and fractures that are providing the permeability of the shale formation. Decrease of permeability due to swelling has been previously observed in coalbed methane recovery operations and still remains a challenging problem. These effects can substantially affect the transport of fluids in nanoporous media and its poromechanical response, so development of the chemomechanical model is crucial for estimating the feasibility of the enhanced shale gas recovery operations and CO2 sequestration.

The objective of this project is to develop a chemomechanical model for behavior of hydrocarbons and carbon dioxide in nanopores, in order to shed light on CO2 sequestration in shales and enhanced recovery of shale gas. Since the thermodynamic properties of nano-confined fluids significantly differ from that of the bulk, it requires the development of a molecular-level model. However, the resulting model has to be capable of macroscopic predictions with a limited number of parameters, so that it can be employed for continuum simulations on the geological scale. The project has the following specific aims: identify the spectrum of the characteristic atomistic structures of shale surfaces and their properties with respect to interactions with alkanes and carbon dioxide; develop a model for equilibrium adsorption properties for the shale pores and predict the adsorption-induced stresses and strains; calculate the main parameters necessary for predicting the transport of alkanes and CO2 in shale.

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