Modeling And Understanding Surface Electrolyte Interactions In Mxene Supercapacitors
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Modeling and Understanding Surface/electrolyte Interactions in MXene Supercapacitors
With the surge in the use of renewable resources, devices for energy storage have gained critical importance. Supercapacitors exhibit properties that provide high power-density and quick charging times, but their energy-density is significantly lower than batteries, demanding further research into the materials components of these supercapacitors. A candidate class of materials for bridging this gap in properties are MXenes, which are novel two-dimensional materials that integrate high electrical conductivity and hydrophilicity. These are carbides and nitrides of early transition metals with a variety of functionalizations, allowing a greater flexibility in tailoring their properties. Due to the complex surface/electrolyte interactions in the interlayers, a thorough understanding of the structure and dynamics of the intercalants is needed for its optimal configuration. Computational tools, such as atomic-scale molecular dynamics (MD) and grand canonical Monte Carlo (GCMC) simulations with the ReaxFF reactive potential, and mesoscale simulations to capture the complex flow field information in nanoconfinement, are ideal tools to provide an atomistic and nanoscale understanding to elucidate the key phenomena and interactions. In this dissertation, we focus on the investigations needed to improve the charge storage capability in supercapacitors and develop tools to enable simulations on a larger length and timescale. First, we investigate the heterostructure of 2D-TiO2 with Ti3C2 MXenes. Most battery electrode materials, such as 2D-TiO2, suffer from poor electronic conductivity and intercalant dynamics, but exhibit better charge storage (cation adsorption) capability than MXenes. Studying the interlayer structure and dynamics with ReaxFF MD/GCMC and quasi elastic neutron scattering (QENS), we find that the MXenes promote water intercalation and faster transport of intercalated water than 2D-TiO2, with the heterostructure sharing properties of both. However, cations (Li+,Na+, and K+) are bound more readily at the 2D-TiO2 surface than the MXenes, exhibiting the expected higher charge storage capability. Interestingly, in addition to finding that the intercalation of Li+ promotes water diffusion in the heterostructure, we find that the protons on the OH terminated MXenes are highly mobile. We observe a consistent transfer of protons from the Ti3C2(OH)2 surface to 2D-TiO2, indicating that the 2D-TiO2 is capable of storing H+ better than the MXenes in addition to the other cations considered. Therefore, the synergy between these materials in the heterostructure provides a favorable combination of properties between a battery and a supercapacitor. The higher activity of protons, and its importance in the charge storage capability, in the interlayer warrants an accurate representation of its dynamics under confinement. While often ignored in simulations, nuclear quantum effects (NQEs), e.g. tunneling on light nuclei such as H+, can be significant under confinement even at room temperature. We investigate the role of NQEs on proton and water transport in bulk and under confinement in Ti3C2{O,OH}2 MXenes at two extents of hydration. Using path integral molecular dynamics (PIMD) with ReaxFF force field, we find NQEs promote proton transfer in both bulk and confined environments. The water self diffusion coefficient decreases in bulk, matching the experimental values, but is amplified under confinement. Therefore, the NQEs lead to contrasting behavior in bulk and confinement. However, the exceptions to the qualitative trend -- Li+-MXene system for proton transport and Na+-MXene system for water transport -- indicate the dominant parameter is the interlayer hydration and configuration. While NQEs only have a small impact on the structure of the system, they can alter the qualitative trend between the systems for transport under confinement. Therefore, the results suggest the need for NQEs to be considered to simulate aqueous systems under confinement for both qualitative and quantitative accuracy. However, these ReaxFF simulations only span a few nanoseconds and a few nanometers in time and length scale, respectively. To understand the complexity of the flow at a scale comparable to the cell-level, a mesoscopic method capturing flow field beyond the continuum regime is required. The Boltzmann Bhatnagar-Gross-Krook equation provides a formulation to capture the non-continuum effects in the flow field while being computationally efficient to scale up data from atomistic simulations to larger domains and geometries. Here, we introduce a discontinuous Galerkin finite element method for spatial discretization of the discrete Boltzmann equation for isothermal flows with high Knudsen numbers [Kn∼O(1)]. In conjunction with a high-order Runge--Kutta time marching scheme, this method is capable of achieving high-order accuracy in both space and time, while maintaining a compact stencil. We validate the spatial order of accuracy of the scheme on a two-dimensional Couette flow with Kn=1 and the D2Q16 velocity discretization. We then apply the scheme to lid-driven micro-cavity flow at Kn=1,2,and 8, and we compare the ability of Gauss--Hermite (GH) and Newton--Cotes (NC) velocity sets to capture the high non-linearity of the flow-field. While the GH quadrature provides higher integration strength with fewer points, the NC quadrature has more uniformly distributed nodes with weights greater than machine-zero, helping to avoid the so-called ray-effect. Broadly speaking, we anticipate that the insights from this work will help facilitate the efficient implementation and application of high order numerical methods for complex high Knudsen number flows. Furthermore, we investigate the capacitive mechanism in birnessite MnO2 -- a cost-effective material for electrochemical energy storage -- which has been described using electric double layer (EDL) and pseudocapacitive models. In our multi-scale theory (density functional theory in addition to ReaxFF) and experiment (X-ray diffraction, Raman spectroscopy, electrochemical quartz crystal microbalance, and atomic force microscope dilatometry) study, we show that the degree of interaction between the intercalants and the surface is dependent on the local environment. Therefore, indicating that the interaction must be considered as an amalgamation of both processes. Additionally, we develop an accelerated MD framework for ReaxFF combining parallel replica dynamics and collective variable hyperdynamics to tackle the requirement of long timescales in a variety of applications, e.g., pyrolysis in combustion chemistry, and dissolution mechanisms in energy storage devices. We show that this combination helps in sampling chemistry at microsecond timescales and demonstrate the capability of this method in capturing high barrier transition events (pyrolysis; barrier of ~60 kcal/mol) and low barrier transition events (ethylene carbonate ring opening; ~10 kcal/mol). Overall, the insights, fundamental understanding, and the novel methods developed in this work can be useful in improving the charge storage capability in materials for supercapacitors and provide a framework for scalable and efficient simulations at a larger length and time domain.
Fundamental Aspects and Perspectives of MXenes
This book presents the fundamental aspects, recent developments in fabrication and characterization techniques, structure, properties, and emerging applications of MXenes. It shows the advancement in scale-up, challenges, and their futuristic perspectives. An overview of all the latest developments in energy storage and conversion applications, catalysis, environmental remediation, and radiation shielding, etc is reported.
Transition Metal Carbides and Nitrides (MXenes) Handbook
A comprehensive overview of the synthesis of high-quality MXenes In Transition Metal Carbides and Nitrides (MXenes) Handbook: Synthesis, Processing, Properties and Applications, a team of esteemed researchers provides an expert review encompassing the fundamentals of precursor selection, MXene synthesis, characterizations, properties, processing, and applications. You’ll find detailed discussions of the selection of MXene members for specific applications, as along with summaries of the physical and chemical properties of MXenes, including electrical, mechanical, optical, electromechanical, electrochemical, and electromagnetic properties. The authors delve into both successful and unsuccessful synthesis examples, offering detailed explanations of various failures to facilitates a comprehensive understanding of the reasons behind unsuccessful syntheses. Additionally, they provide detailed examinations on the characterizations of MXenes, empowering readers to develop a sophisticated understanding of how to achieve optimal quality, flake size, oxidation states, and more. You’ll also find: A thorough review of common applications of MXenes, including electrochemical applications, electromagnetic interference shielding, communications devices, and more Comprehensive explorations of solution and non-solution processing of MXenes Practical discussions of the synthesis of high-quality MXene powders, colloidal solutions and flakes, including information about MXene precursors Fulsome treatments of MXene precursor selection and their impact on MXene quality Tailored to meet the needs of graduate students, researchers, and scientists in the areas of materials science, inorganic chemistry, and physical chemistry, the Transition Metal Carbides and Nitrides (MXenes) Handbook will also benefit biochemists and professionals working in drug delivery.