A Study of Fluid-Membrane Transport Processes and Their Applications: A Molecular Dynamics Approach
Hinkle, Kevin R.
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Selectively permeable membranes perform important roles in a wide range of systems including naturally occurring lipid membranes in biological systems to engineered polymeric membranes in filtration and energy technologies. In order to design technologies that incorporate such membranes it is crucial to understand the behavior of these systems on the molecular level so that optimal performance and maximum efficiencies can be achieved. One of the main focuses of this thesis is how ionic hydration affects the transport of electrolyte species through porous membranes. Molecular dynamics simulations are used to examine this in detail both for model systems as well as for actual industrial membranes including zeolite materials as potential ion exchange membranes (IEMs) in energy systems such as redox flow batteries. In addition, model selective membranes are used to computationally predict the phase equilibrium behavior of various gas/liquid systems at experimentally difficult conditions. Because of their safety, capacity, and small environmental footprint, redox flow batteries (RFBs) have become an attractive form of energy storage. However, this technology is not yet widely used commercially due to inefficiencies in the ion-exchange membrane. The current technology widely utilizes polymeric membranes that have stability problems in the highly reactive environment of the RFB and tend to break down, shortening the life of the battery. Also, they present less than desirable selectivity for proton transport, which is crucial to the overall efficiency of the battery. It has been proposed that thin zeolite membranes will provide both the stability and the selectivity to improve the performance of RFBs and make their wide-scale application more feasible. A molecular dynamics study of six zeolite framework types and the ions present in the vanadium-RFB has been undertaken to determine their transport behavior and investigate at the molecular level the requirements for suitability in IEM applications. In addition to investigating different zeolite frameworks, the effect of composition was examined by introducing different levels of aluminum substitution into the crystalline structure of one specific framework, namely MFI. By investigating two characteristics, membrane loading and intramembrane diffusion, it is possible to predict the overall ion permeability with the goal of optimizing the amount of substitution for high proton permeability while maintaining selectivity to undesirable ions. Beyond these specific applications, model selective membranes were also used in molecular dynamics simulations to predict phase behavior of various gas/liquid at conditions difficult to achieve in experiments. This research was carried out to fill data gaps that are urgently needed for the design of industrial processes for gas capture/ storage or separation. By validating our models against limited experimental data available, we show that molecular modeling can be a useful predictive tool for industrial applications. Simulations are currently not widely used in industry for data prediction even though they can be carried out at a small fraction of the cost of experimental studies.
Redox Flow Batteries