Uptake and Diffusion of Ions in Organically Synthesized Porous Carbon for Battery Anode Applications

Organically synthesized porous carbon (OSPC-1) has a high lithium uptake of 748 mA h g-1, demonstrating that it is a strong contender as an anode material for lithium-ion batteries. Simulations of the lithium uptake on models generated of OSPC-1 gave values close to the experimentally obtained data. Thus, we assess the potential of OSPC-1 for use as an anode material in batteries of sodium, potassium, magnesium, and calcium. We find ion uptakes of 770, 386, 158, and 774 mA h g-1 for Li+, Na+, K+, and Ca2+, respectively. We also study the diffusive capabilities of ions through the OSPC-1 structure via means of active diffusion. The lithium ions were able to diffuse at a greater rate, followed by the divalent ions, Mg2+ and Ca2+, and the monovalent ions, Na+ and K+. All these ions were able to diffuse completely through the OSPC-1 structure with the diffusion rate being dependent on the ionic radius of the ion, coupled with the valency of the ion. Therefore, we show that OSPC-1 also has great potential as an anode material for Na+, K+, Mg2+, and Ca2+ batteries

Aspects of biomacromolecular dynamics at different scales

Biological functions of biomacromolecules are often indispensably linked to their internal dynamics. To investigate the dynamic nature of biomolecules, molecular dynamics (MD) simulation offers unique advantages by providing high spatial and temporal resolution over orders of magnitude in time- and length scales. Here, simulations at two different scales are used to investigate different aspects of biomolecular dynamics. At the atomistic scale, the first study investigates the relationship between the axial methyl group order parameter and the corresponding entropy in protein side chains. Three classes of methyl group are characterized based on the methyl group’s “topological distance” from the backbone (that is the number of bonds between the methyl group axis and the closest backbone atom) even when direct effects of the topological distance are removed. This distinction implies that methyl groups at the same topological position share similar nonbonded environments. Furthermore, consideration of these classes of methyl group improves the accuracy of entropy-estimates based upon changes in order parameter. The second study investigates the deconstruction of crystalline cellulose, a problem relevant to bioenergy research. The large size of crystalline cellulose together with the associated long-time dynamics exceeds the capabilities of atomistic simulation. Thus, a residue-scale, coarse-grained model of cellulose is calculated using the REACH (Realistic Extension Algorithm via Covariance Hessian) method. The model is successfully validated against experiment using Young’s moduli and the velocity of sound. The coarse-grained analysis of the cellulose fibril suggests that the intrinsic dynamics facilitates deconstruction of the crystalline cellulose fibril from the hydrophobic surface. Both applications share the same concept of approach (that is, computational modeling and simulation at an appropriate scale), which reveals key insights into biomolecules by investigating their dynamic behavior

CARBON-BASED COMPLEX STRUCTURE AND MODEL DEVELOPMENT

Growing concerns about the environment and energy crisis prompt a search for effective carbon-based materials due to their low cost, renewability, sustainability, easy accessibility and excellent properties. We study the model development, structure and properties of graphene oxide, cellulose and their nanocomposites in order to obtain a better fundamental understanding of carbon complex materials and construct a structure-property relationship via reactive molecular dynamics simulations. In chapter 3, the model development of GO is studied. Theoretical GO models developed so far present a good description of its chemical structure. However, when it comes to the structural properties, such as the size and distribution of vacancy defects, the curvature (or roughness), there exist significant gaps between computational models and experimentally synthesized GO materials. We carry out reactive molecular dynamics simulations and use experimental characteristics to fine tune theoretical GO models. Attentions have been paid to the vacancy defects, the distribution and hybridization of carbon atoms, and the overall C/O ratio of GO. The GO models proposed in this work have been significantly improved to represent quantitative structural details of GO materials synthesized via the modified Hummers method. The temperature-programmed protocol and the computational post analyses of Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, vacancy size and curvature distribution, are of general interest to a broad audience working on GO structures from other synthesis methods and other two-dimensional materials and their composites. In Chapter 4, we outline the state-of-the-art understanding of cellulose structures, and discuss in details cellulose interactions, dissolutions and decompositions via computational methods of molecular dynamics (MD) and reactive molecular dynamics (RxMD) simulations. In addition, cellulose characterizations, beneficial to validate and support computational results, are also briefly summarized. Such a state-of-the-art account of atomistic computational studies could inspire interdisciplinary collaborations, optimize process design, promote cellulose-based materials for emerging important applications and shed a light on fundamental understandings of similar systems of biomolecules, polymers and surfactants. In Chapter 5, we investigate the fundamental mechanism of how cellulose structure transforms under pyrolysis conditions and the practical guideline of how cellulose properties are fined tuned accordingly. A series of reactive molecular dynamics calculations has been designed to reveal the structural evolution of crystalline cellulose under pyrolysis treatments. Through the detailed analysis of cellulose configuration change, hydrogen bonding network variation, reaction and redistribution of carbon, oxygen and hydrogen elements, and Young’s modulus, a molecule level insight of crystalline cellulose and its structural evolution under pyrolysis conditions has been constructed via reactive molecular dynamics simulations. We anticipate those theoretical results could effectively promote the design, the manufacture, and the optimization of cellulose based materials for relevant emerging applications. In Chapter 6, we combined the results from previous chapters and explore a new composite material that incorporating amorphous cellulose chains on GO surface, which is barely reported by recent publications. A series of RxMD simulations have been carried out to reveal the mechanical properties of pure GO and cellulose-GO nanocomposites. Two different cellulose-GO composites are constructed, namely, cellulose (monolayer)-GO model and cellulose (multilayer)-GO model. The tensile deformation, Young’s modulus and mechanical strength of GO and cellulose-GO composites have been recorded and calculated under the temperature of 300, 500 800 K, with two strain rates of 10-4/fs and 10-5/fs. We hope the GO model with the simultaneously description to both structural and chemical properties can provide a new fundamental understanding of the mechanical performance of GO and cellulose-GO composites, and could add some advancement to existing knowledge of carbon-based materials