Browsing by Author "Liu, Guoliang"

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    Aligned Continuous Cylindrical Pores Derived from Electrospun Polymer Fibers in Titanium Diboride
    Hicks, David Cyprian (Virginia Tech, 2019-02-01)
    The use of electrospun polystyrene (PS) fibers to create continuous long range ordered multi-scale porous structures in titanium diboride (TiB2) is investigated in this work. The introduction of electrospun PS fibers as a sacrificial filler into a colloidal suspension of TiB2 allows for easy control over the pore size, porosity, and long range ordering of the porous structures of the sintered ceramic. Green bodies were formed by vacuum infiltrating an electrospun-fiber-filled mold with the colloidal TiB2 suspension. The size, volume, distribution, and dispersion of the pores were optimized by carefully selecting the sacrificial polymer, the fiber diameter, the solvent, and the solid content of TiB2. The green bodies were partially sintered at 2000 C in argon to form a multiscale porous structure via the removal of the PS fibers. Aligned continuous cylindrical pores were derived from the PS fibers in a range of ~5 - 20 μm and random porosity was revealed between the ceramic particles with the size of ~0.3 - 1 μm. TiB2 near-net-shaped parts with the multi-scale porosities (~50 to 70%) were successfully cast and sintered. The multi-scale porous structure produced from electrospun fibers was characterized both thermally and mechanically, at room temperature. The conductivity ranged from 12-31 W m^(-1) K^(-1) at room temperature and the compressive strength ranged from 2-30 MPa at room temperature. Analytical thermal and mechanical models were employed to understand and verify he processing-structure-properties relationship. Finally, a method was devised for estimating the effective thermal conductivity of candidate materials for UHTC applications at relevant temperatures using a finite difference model and a controlled sample environment. This low-cost processing technique facilitates the production of thermally and mechanically anisotropic structures into near-net shape parts, for extreme environment applications, such as ultra high temperature insulation and active cooling components.
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    Antioxidant studies of fullerene and metallofullerene derivatives and fluorescence studies of terbium-containing metallofullerene derivatives
    Huang, Rong (Virginia Tech, 2022-02-10)
    Fullerenes and metallofullerenes have been discovered to have a lot of applications in the biomedical area, for instance, they have been shown to have antioxidant, anti-virus, anti-cancer, immunological properties, etc. However, the hydrophobicity nature of fullerenes and metallofullerenes raises the need for functionalized hydrophilic fullerenes and metallofullerenes. Also, the advancement of the purification techniques of fullerenes and metallofullerenes makes the isolation of new fullerenes and metallofullerenes possible. Therefore, discovering the biomedical applications of these newly found fullerenes and metallofullerenes is also of vital importance. In Chapter 1, we provided a comprehensive background on the history of fullerenes and metallofullerenes, synthesis and purification methods of fullerenes and metallofullerenes, and some of their biological applications, including antioxidant applications and fluorescence applications. Some important fullerene and metallofullerenes and milestones in this area were also discussed. In Chapter 2, we demonstrated the antioxidant and anti-inflammation ability of a conjugate, FIFIFK(Cy5)PEG24K(NH2)CONH2-C60, that consisted of a peptide that binds specifically to a formyl peptide receptor-1 (FPR-1), which expresses on activated macrophages, and a carboxyl-group-functionalized C60, which is the first discovered and most used fullerene is history. We showed that the fullerene-peptide conjugate had great ability as a radical scavenger and to reduce the volume of inflammatory tissue. In Chapter 3, we demonstrated the antioxidant and anti-inflammation ability of several metallofullerene derivatives, including amino-Gd3N@C80, amino-Sc3N@C80, carboxy-Gd3N@C80, and carboxyl-Sc3N@C80. Amino-group-functionalized metallofullerenes were found to have higher radical scavenging ability and anti-inflammation ability. In Chapter 4, we developed fluorescent metallofullerene derivatives, including Tb3N@C80(NH2)9(OH)4NO2 and Tb3N@C80(CH2CH2COOH)21(OH)18. The fluorescence properties of the derivatives of fullerenes or metallofullerenes were normally realized by the addition of fluorescent moieties on the carbon cage of fullerenes or metallofullerenes. However, the fluorescence of our newly developed metallofullerene derivative was realized by the tri-terbium nitride cluster inside the carbon cage. This saved the multi-step synthesis of the fluorescence probe and maintained the simplicity of the structure of the metallofullerene derivative. These derivatives of terbium-containing metallofullerene also showed radical scavenging ability towards hydroxyl radicals. In Chapter 5, we developed another fluorescent Tb3N@C80 derivative, Tb3N@C80[DiPEG2000]. The advantage of this fluorescent metallofullerene derivative was that it was easier to purify compared to small-functional-groups-functionalized Tb3N@C80 derivatives. This newly developed fluorescent Tb3N@C80 derivative also showed great fluorescent ability and radical scavenging ability. In Chapter 6, we provided a summary of the studies on the antioxidant and fluorescent properties of fullerenes and metallofullerenes' derivatives that were discussed in this dissertation.
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    Block Copolymer Derived Porous Carbon Fiber for Energy and Environmental Science
    Serrano, Joel Marcos (Virginia Tech, 2022-04-26)
    As the world population grows, a persistent pressure on natural resources remains. Resource requirements have extensively expanded due to industrialization. Several technological advancements continually aim to alleviate these resource shortages by targeting existing shortcomings in effective and efficient material design. Practical, high-performing, and economical materials are needed in several key application areas, including energy storage, energy harvesting, electronics, catalysis, and water purification. Further development into high-performing and economical materials remain imperative. Innovators must seek to develop technologies that overcome fundamental limitations by designing materials and devices which address resource challenges. Carbon serves as a versatile material for a wide range of applications including purification, separation, and energy storage owing to excellent electrical, physical, and mechanical properties. One-dimensional (1D) carbon fiber in particular is renowned for excellent strength with high surface-to-volume ratio and is widely commercially available. Although an exceptional candidate to address current energy and environmental needs, carbon fibers require further investigation to be used to their full potential. Emerging strategies for carbon fiber design rely on developing facile synthetic routes for controlled carbon structures. The scientific community has shown extensive interest in porous carbon fabrication owing to the excellent performance enhancement in separation, filtration, energy storage, energy conversion, and several other applications. This dissertation both reviews and contributes to the recent works of porous carbon and their applications in energy and environmental sciences. The background section shows recent development in porous carbon and the processing methods under investigation and current synthetic methods for designing porous carbon fibers (PCF). Later sections focus on original research. A controlled radical polymerization method, reversible addition-fragmentation chain transfer (RAFT), enabled a synthetic design for a block copolymer precursor, poly(methyl methacrylate) (PMMA) and polyacrylonitrile (PAN). The block copolymer (PMMA-b-PAN) possesses a unique microphase separation when electrospun and develop narrowly disperse mesopores upon carbonization. The PMMA and PAN domains self-assemble in a kinetically trapped disordered network whereby PMMA decomposes and PAN cross-links into PCF. The initial investigation highlights the block copolymer molecular weight and compositional design control for tuning the physical and electrochemical properties of PCF. Based on this study, mesopore (2 – 50 nm) size can be tuned between 10 – 25 nm while maintaining large surface areas, and the PAN-derived micropores (< 2 nm). The mesopores and micropores both contribute to the development of the unique hierarchical porous carbon structure which brings unprecedented architectural control. The pore control greatly contributes to the carbon field as the nano-scale architecture significantly influences performance and functionality. The next section uses PCF to clean water sources that are often tainted with undesirable ions such as salts and pollutants. Deionization or electrosorption is an electrochemical method for water purification via ion removal. I employed the PCFs as an electrode for deionization because of their high surface area and tunable pore size. Important for deionization, the adsorption isotherms and kinetics highlight the capacity and speed for purification of water. I studied PCF capacitive filtration on charged organic salts. Because PCF have both micropores and mesopores, they were able to adsorb ions at masses exceeding their own weight. The PFC adsorption efficiency was attributed to the diffusion kinetics within the hierarchical porous system and the double layer capacitance development on the PCF surface. In addition, based on the mechanism of adsorption, the PCFs showed high stability and reusability for future adsorption/desorption applications. The PCF performance as an electrosorption material highlights the rational design for efficient electrodes by hierarchical interconnected porosity. Another application of PFCs is updating evaporative desalination methods for water purification. Currently distillation is not widely used as a source of potable water owing to the high cost and energy requirement. Solar desalination could serve as a low-cost method for desalination; however, the evaporation enthalpy of water severely limits practical implementation. Here I apply the pore design of PCF as a method for water nano-confinement. Confinement effects reduce water density and lowers evaporation enthalpy. Desalination in PCF were studied in pores < 2 nm to 22 nm. The PCF pore size of ~ 10 nm was found to be the peak efficiency and resulted in a ~ 46% reduction in enthalpy. Interestingly, the PCF nano-confinement also contributed to the understanding in competing desorption energy for evaporation in micropores. The pore design in PCF also shows confinement effects that can be implemented in other environmental applications. Lastly, the block copolymer microphase morphology was explored in a vapor induced phase separation system. The resulting PCF properties showed a direct influence from the phase separation caused by nonsolvent. At low nonsolvent vapor, a disordered microphase separation occurred, however upon application of nonsolvent vapor, the polymer chains reorganized. The reorganization initially improved mechanical properties by developing more long-range ordered graphic chains in the PAN-derived carbon. However, at higher nonsolvent vapor concentrations, the fibers experienced polymer precipitation which resulted in bead and clump formation in the fiber mats. The beads and clumps lowered both mechanical properties and electrochemical performance. The vapor induced phase separation showed a method for enhancing mechanical properties without compromising electrochemical performance in flexible carbon fibers.
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    Block copolymer derived uniform mesopores enable ultrafast electron and ion transport at highmass loadings
    Liu, Tianyu; Zhou, Zhenping; Guo, Yichen; Guo, Dong; Liu, Guoliang (Nature Research, 2019)
    High mass loading and fast charge transport are two crucial but often mutually exclusive characteristics of pseudocapacitors. On conventional carbon supports, high mass loadings inevitably lead to sluggish electron conduction and ion diffusion due to thick pseudocapacitive layers and clogged pores. Here we present a design principle of carbon supports, utilizing self-assembly and microphase-separation of block copolymers. We synthesize porous carbon fibers (PCFs) with uniform mesopores of 11.7 nm, which are partially filled with MnO2 of <2 nm in thickness. The uniform mesopores and ultrathin MnO2 enable fast electron/ion transport comparable to electrical-double-layer-capacitive carbons. At mass loadings approaching 7mg cm−2, the gravimetric and areal capacitances of MnO2 (~50% of total mass) reach 1148 F g−1 and 3141 mF cm−2, respectively. Our MnO2-coated PCFs outperform other MnO2-based electrodes at similar loadings, highlighting the great promise of block copolymers for designing PCF supports for electrochemical applications.
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    Block Copolymer-Derived Porous Carbon Fibers Enable High MnO2 Loading and Fast Charging in Aqueous Zinc-Ion Battery
    Guo, Dong; Zhao, Wenqi; Pan, Fuping; Liu, Guoliang (Wiley-V C H Verlag, 2022-04)
    Rechargeable aqueous Zn MnO2 batteries are promising for stationary energy storage because of their high energy density, safety, environmental benignity, and low cost. Conventional gravel MnO2 cathodes have low electrical conductivity, slow ion (de-)insertion, and poor cycle stability, resulting in poor recharging performance and severe capacity fading. To improve the rechargeability of MnO2, strategies have been devised such as depositing micrometer-thick MnO2 on carbon cloth and blending nanostructured MnO2 with additives and binders. The low electrical conductivity of binders and sluggish ion (de)insertion in micrometer-thick MnO2, however, still limit the fastcharging performance. Herein, we have prepared porous carbon fiber (PCF) supported MnO2 cathodes (PCF@MnO2), comprised of nanometer-thick MnO2 uniformly deposited on electrospun block copolymer-derived PCF that have abundant uniform mesopores. The high electrical conductivity of PCF, fast electrochemical reactions in nanometer-thick MnO2, and fast ion transport through porous nonwoven fibers contribute to a high rate capability at high loadings. PCF@MnO2, at a MnO2 loading of 59.1 wt%, achieves a MnO2-based specific capacity of 326 and 184 mAhg(-1) at a current density of 0.1 and 1.0 Ag-1, respectively. Our approach of block copolymer-based PCF as a support for zinc-ion cathode inspires future designs of fastcharging electrodes with other active materials.
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    Block Copolymer-derived Porous Polyimides and Carbon for High-Performance Energy Storage
    Guo, Dong (Virginia Tech, 2022-05-12)
    Block copolymer-derived nanoporous materials are featured with microstructures defined by the microphase separation of constituent blocks, enabling various applications in energy storage. Dictated by the molecular weights and volume fractions of constituent blocks, the microphase separation forms nanoscale microstructures of 1-100 nm. Selective removal of a sacrificial phase produces nanopores with tailored pore width, continuity, and tortuosity. The remaining phase customizes the properties of resulting nanoporous materials, including specific surface area, electrical conductivity/insulation, and mechanical performance. Therefore, block copolymer-derived porous materials are felicitous for use in high-performance energy storage. This dissertation presents the utilization of block copolymers to derive nanoporous materials: i) high-modulus polyimide separators for lithium-metal batteries, and ii) high-surface-area carbon electrodes for fast-charging zinc-ion batteries. In lithium-metal batteries, the dendritic growth of lithium leads to deteriorating performance and severe safety concerns. Suppressing lithium dendrites is imperative to guarantee both high performance and safe cycling. Mesoporous polyimide separators are promising for dendrite suppression: i) the mesopores are smaller than the width of lithium dendrites, preventing lithium dendrites from penetrating the separator. ii) The high-modulus polyimide ceases the growth of lithium dendrites. Herein, this dissertation reports a mesoporous polyimide separator produced by thermalizing polylactide-b-polyimide-b-polylactide at 280 °C. The mesoporous polyimide separator exhibits a median pore width of 21 nm and a storage modulus of 1.8 GPa. When serving as a dendrite-suppressing separator in lithium-metal batteries, the mesoporous polyimide separator enables safe cycling for 500 hours at a current density of 4 mA/cm2. In zinc-ion batteries, developing cathodes compatible with fast charging remains a challenge. Conventional MnO2 gravel cathodes suffer from low electrical conductivity and slow ion (de-)insertion, resulting in poor recharging performance. In this dissertation, porous carbon fiber (PCF) supported MnO2 (PCF@MnO2), comprising nanometer-thick MnO2 deposited on block copolymer-derived PCF, serves as a fast-charging cathode. The high electrical conductivity of PCF and fast ion (de-)insertion in nanometer-thick MnO2 both contribute to a high rate capability. The PCF@MnO2 cathode, with a MnO2 loading of 59.1 wt%, achieves a MnO2-based specific capacity of 326 and 184 mAh/g at a current density of 0.1 and 1.0 A/g, respectively. This dissertation investigates approaches to utilizing block copolymers-derived nanoporous materials for high-performance energy storage. Those approaches are envisaged to inspire the design of block copolymer-derived nanoporous materials, and advance the development of "beyond Li-ion" energy storage.
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    Block copolymer–based porous carbon fibers
    Zhou, Zhengping; Liu, Tianyu; Khan, Assad U.; Liu, Guoliang (American Association for the Advancement of Science, 2019-02-01)
    Carbon fibers have high surface areas and rich functionalities for interacting with ions, molecules, and particles. However, the control over their porosity remains challenging. Conventional syntheses rely on blending polyacrylonitrile with sacrificial additives, which macrophase-separate and result in poorly controlled pores after pyrolysis. Here, we use block copolymermicrophase separation, a fundamentally disparate approach to synthesizing porous carbon fibers (PCFs) with well-controlledmesopores (~10 nm) and micropores (~0.5 nm).Without infiltrating any carbon precursors or dopants, poly(acrylonitrile-block-methyl methacrylate) is directly converted to nitrogen and oxygen dual-doped PCFs. Owing to the interconnected network and the highly optimal bimodal pores, PCFs exhibit substantially reduced ion transport resistance and an ultrahigh capacitance of 66 µF cm⁻² (6.6 times that of activated carbon). The approach of using block copolymer precursors revolutionizes the synthesis of PCFs. The advanced electrochemical properties signify that PCFs represent a new platform material for electrochemical energy storage.
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    Cascade degradation and upcycling of polystyrene waste to high-value chemicals
    Xu, Zhen; Pan, Fuping; Sun, Mengqi; Xu, Jianjun; Munyaneza, Nuwayo Eric; Croft, Zacary L.; Cai, Gangshu; Liu, Guoliang (National Academy of Sciences, 2022-08-23)
    Plastic waste represents one of the most urgent environmental challenges facing humankind. Upcycling has been proposed to solve the low profitability and high market sensitivity of known recycling methods. Existing upcycling methods operate under energy-intense conditions and use precious-metal catalysts, but produce low-value oligomers, monomers, and common aromatics. Herein, we report a tandem degradation-upcycling strategy to exploit high-value chemicals from polystyrene (PS) waste with high selectivity. We first degrade PS waste to aromatics using ultraviolet (UV) light and then valorize the intermediate to diphenylmethane. Low-cost AlCl3 catalyzes both the reactions of degradation and upcycling at ambient temperatures under atmospheric pressure. The degraded intermediates can advantageously serve as solvents for processing the solid plastic wastes, forming a self-sustainable circuitry. The low-value-input and high-value-output approach is thus substantially more sustainable and economically viable than conventional thermal processes, which operate at high-temperature, high-pressure conditions and use precious-metal catalysts, but produce low-value oligomers, monomers, and common aromatics. The cascade strategy is resilient to impurities from plastic waste streams and is generalizable to other high-value chemicals (e.g., benzophenone, 1,2-diphenylethane, and 4-phenyl-4-oxo butyric acid). The upcycling to diphenylmethane was tested at 1-kg laboratory scale and attested by industrial-scale techno-economic analysis, demonstrating sustainability and economic viability without government subsidies or tax credits.
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    Chemical and Physical Modifications of Semicrystalline Gels to Achieve Controlled Heterogeneity
    Anderson, Lindsey J. (Virginia Tech, 2019-02-07)
    Sulfonated polyaromatic hydrocarbon membranes have emerged as desirable candidates for proton exchange membranes (PEMs) due to their excellent mechanical properties, high thermal and chemical stability, and low cost. Specifically, sulfonated multiblock copolymers are attractive because their phase-separated morphologies aide in facile proton transport. In this work, the functionalization of semicrystalline gels of poly(ether ether ketone) (PEEK) is explored as a novel post-polymerization method to prepared blocky copolymers, and the effect of copolymer architecture on membrane physical properties, structure, and performance is extensively investigated. First, the blocky sulfonation of PEEK was explored to prepare blocky copolymers (SPEEK) with densely sulfonated domains and unfunctionalized, crystallizable domains. Compared to random SPEEK ionomers at similar ion content, blocky SPEEK exhibited enhanced crystallizability, decreased melting point depression, and faster crystallization kinetics. Phase separation between the hydrophilic sulfonated blocks and hydrophobic PEEK blocks, aided by polymer crystallization, resulted in enhanced water uptake, superior proton conductivity, and more closely associated ionic domains than random SPEEK. Furthermore, the random and blocky bromination of PEEK was investigated to prepare PEEK derivatives (BrPEEK) with reactive aryl-bromides. Spectroscopic evidence revealed long domains of unfunctionalized homopolymer for blocky BrPEEK, and this translated to an increased degree of crystallinity, higher melting temperature, and more rapid crystallization kinetics than random BrPEEK at similar degrees of bromination. The subsequent sulfonation of blocky BrPEEK resulted in a hydrophilic-hydrophobic blocky copolymer with clear multi-phase behavior. The phase-separated morphology contributed to decreased water uptake and areal swelling compared to random SPEEK and resulted in considerably higher proton conductivity at much lower hydration levels. Moreover, Ullmann coupling introduced superacidic perfluorosulfonic acid side chains to the BrPEEK backbone, which yielded membranes with less water content and less dimensional swelling than random SPEEK. Superior proton transport than random SPEEK was observed due to the superacid side chain and wider hydrophilic channels within the membranes, resulting in more continuous pathways for proton transport. Overall, this work provided a novel platform for the preparation of functionalized PEEK membranes using a simple post-polymerization functionalization procedure. The established methods produced blocky-type copolymers with properties reminiscent of multiblock copolymers prepared by direct polymerization from monomers/oligomers.
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    Colloidal Processing, Microstructural Evolution, and Anisotropic Properties of Textured Ultra-High Temperature Ceramics Prepared Using Weak Magnetic Fields
    Shiraishi, Juan Diego (Virginia Tech, 2024-02-09)
    The texturing of ultra-high temperature ceramics (UHTCs) using weak magnetic fields is studied and developed for the first time. Textured UHTCs were prepared by magnetically assisted slip casting (MASC) in weak magnetic field (B ~ 0.5 T). Analytical calculations describing the balance of torques acting on the suspended particles suggested that texture would form at such low magnetic fields. The calculations include a novel contribution of Stokes drag arising from the inhomogeneous velocity profile of the fluid during slip casting. Experimental proof-of-concept of the theoretical calculations was successfully demonstrated. Calculations of Lotgering orientation factor (LOF) based on the intensities of the (00l) family of peaks measures by XRD revealed strong c-axis crystalline texture in TiB2 (LOF = 0.88) and ZrB2 (LOF = 0.79) along the direction of the magnetic field. Less texture was achieved in HfB2 (LOF = 0.39). In all cases, the density of the textured materials was less than that of control untextured materials, indicating that texturing hindered the densification. The findings from this work confirm the potential for more cost-effective, simple, and flexible processes to develop crystalline texture in UHTCs and other advanced ceramics and give new insight into the mechanisms of magnetic alignment of UHTCs under low magnetic fields. The microstructural evolution during slip casting and pressureless sintering is investigated. The interplay between magnetic alignment and particle packing was investigated using XRD and SEM. During MASC, the suspended particles rotate into their aligned configuration. Particles that deposit at the bottom of the mold near the plaster of Paris substrate have their alignment slightly disrupted over a ~220 μm-thick region. The aligned suspended particles lock into an aligned configuration as they consolidate, leading to a uniform degree of texturing across the entire sample height of several millimeters upon full consolidation of the particle network. If the magnetic field is removed before the particles fully consolidate, the suspended particles re-randomize their orientation. Grain size measurements done using the ASTM E112 line counting method on SEM images revealed anisotropic microstructures in green and sintered textured ZrB2 materials. Smaller effective grain sizes were observed in the direction of c-axis texture than the directions perpendicular to the texture. Grain aspect ratios of 1.20 and 1.13 were observed in materials where the c-axis texture directions were parallel (PAR) and perpendicular (PERP) to the slip casting direction, respectively. Constraint of the preferred a-axis grain growth direction in the textured materials inhibited their densification compared to the untextured material. The PERP material with the preferred grain growth direction constrained along the casting direction had smaller average grain sizes than the PAR material which contained the preferred grain growth directions in the circular plane normal to the casting direction. Compression testing suggests a trend towards higher strength and stiffness in materials with higher density. Classical catastrophic brittle failure was observed in the untextured materials, but in the textured materials some samples exhibited a multiple failure mode. The PERP material tended to exhibit superior strength and stiffness to the PAR material in the classical brittle failure mode due to the orientation of the stiffer a-axis along the loading direction and smaller average grain size in the plane normal to the loading direction in the PERP condition. In the multiple failure mode, the PAR material tended to reach higher strength values after the initial failure and reach slightly higher strains before ultimate failure due to the orientation of the compliant c-axis along the loading direction and ability of the grains elongated in the plane normal to the loading direction to rearrange themselves after initial failure(s). Regardless of density or texture condition, all ZrB2 samples survived thermal shock resistance (TSR) testing. Samples were heated to 1500°C in air, held for 30 minutes, then quenched in room temperature air. After TSR testing, oxide layers formed on the surface of the materials. The specific mass gain and oxide layer thickness tended to increase with increasing porosity and were dramatically increased when open porosity was dominant as in the CTRL 1900 condition. After TSR testing, the compressive strength and strain at failure were both higher compared to the as-sintered materials. The increases in the average compressive strength were 20%, 76%, and 57% in the CTRL, PAR, and PERP conditions, respectively. The combination of the presence of the oxide layer shifting the onset of macroscale damage to higher strain values, the dissipation of load in the more porous region near the oxide layer, and the constraining effect of the oxide layer acting against the expansion of the material contributed to reinforcement of the samples after TSR testing. The CTRL material outperformed the textured materials on average in terms of strength and stiffness due to the higher density. The results suggest that reinforcement was more effective in the PAR condition than the PERP, which may be caused by the formation of a homogenous oxide layer on the PAR while the PERP formed an anisotropic layer. The work presented in this dissertation lays the foundation for affordable, energy efficient preparation of UHTCs and other ceramic materials. Equipment costs are reduced by 3 orders of magnitude, and the operating costs and energy consumption are greatly reduced. Facilitation of the preparation of textured materials opens the door to renewed investigations into their processing and performance. This work describes in detail for the first time the relationships between processing, microstructure, and properties of a textured UHTC part, providing a model for future research. Finally, the findings in this work can be used to guide process optimization, exploration of complex shapes and microstructures, and design of manufacturing schemes to create specialty textured parts for demanding structural and functional applications.
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    Comparison of Cation-Anion Oxidizer Pairings in Electrically Controllable Solid Propellants
    Sellards, Emily Rose (Virginia Tech, 2024-02-13)
    Electrically controllable solid propellants are an area of interest as a viable solution to the lack of throttle-ability in solid propellant rocket motors. Existing studies have focused on propellants compositions using hydroxyl-ammonium nitrate, ammonium nitrate, or lithium perchlorate as oxidizers. Additionally, the thermochemical and electrochemical reaction mechanisms have not yet been fully defined. The research in this thesis explores the nitrate and perchlorate oxidizer families to compare their cation-anion relationships. Using these oxidizers, pseudo electrically controllable solid propellant compositions were created with the addition of multi-wall carbon nanotubes to enhance ohmic heating capabilities. These additives were selected based on theory that with a non-complexing polymer, an oxidizer melt layer is required for ions to dissociate and electrically controlled ignition to occur. Using an applied voltage, ignition delay and current draw experiments were performed to expand on prior findings that ignition delay follows oxidizer melt temperature while mobility is associated with the size of the ionic radii. Additionally, neat oxidizer pellets were electrically decomposed to determine their linear regression rate. These results help to characterize the mechanism of reaction. This advances the knowledge of oxidizers in electrically controllable applications.
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    Computational Studies of Polyetherimides: Beyond All-Atom Molecular Dynamics Simulations
    Wen, Chengyuan (Virginia Tech, 2020-01-24)
    Polyetherimides are an important class of engineering thermoplastics used in a broad range of industries and applications because of their high heat resistance and stability, high strength and moduli, excellent electrical properties over a wide range of temperatures and frequencies, good processability, good adhesive properties, and chemical stability. All-atom molecular dynamics (MD) simulation is a useful tool to study polymers, but the accessible length and time scales are limited. In this thesis, we explore several computational methods that go beyond all-atom MD simulations to investigate polyetherimides. First, we have developed a transferable coarse-grained MD model of polyetherimides that captures their mechanical and thermal expansion properties. Our results show that in order to make the model transferable, it is critical to include an entropic correction term in the coarse-grained force field and require the coarse-grained model to capture the thermal expansion property of polyetherimides. Secondly, we have constructed a predictive model of the glass transition temperature (Tg) for polyimides by using machine-learning algorithms to analyze existing data on Tg reported in the literature. The predictive model is validated by comparing its predictions to experimental data not used in the training process of the model. We further demonstrate that the diffusion coefficients of small gas molecules can be quickly computed with all-atom MD simulations and used to determine Tg. Finally, we have developed a Monte Carlo (MC) program to model the polymerization process of branched polyetherimides and to compute their molecular weight distribution for a wide range of systems, including fully reacted, partially reacted, stoichiometric, and nonstoichiometric ones. The MC results are compared to the predictions of the Flory-Stockmayer theory of branched polymers and an excellent agreement is found below the gel point of the system under consideration. Above the gel point, the Flory- Stockmayer theory starts to fail but the MC method can still be used to quickly determine the molecular weight distribution of branched polyetherimides under very general conditions.
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    Design and Fabrication of Polymer/Graphene Laminate Thin Films
    Croft, Zacary Lane (Virginia Tech, 2024-09-05)
    The development of graphene-based electronics may produce a new generation of electronic devices with enhanced performance over traditional materials. However, quality graphene electronics will require large-area, continuous graphene film produced by chemical vapor deposition (CVD), which is typically not stable when free-standing. Instead, CVD graphene may be coupled with polymer thin films to produce polymer/graphene laminates (PGLs), which show improved mechanical stability and good electrical conductivity over large areas. For example, single-layer graphene (SLG) has been coupled with polyetherimide (PEI) to produce thin film audio speakers with significantly enhanced energy efficiency compared to traditional speaker designs. However, the poor design and fabrication of PGLs may degrade the interfacial interactions of polymers with graphene and make the interface more susceptible to deformation. Interfacial weakening then limits their long-term reliability and performance in micro- and nano-electromechanical systems (MEMS/NEMS), in which materials are constantly subjected to dynamic loads. In this dissertation, we construct a framework for developing PGL thin films with controlled interfacial properties based on the rational design of polymer substrates and careful consideration of fabrication-induced defects. We evaluate this framework from several angles. First, the design of PEI/SLG film thickness is explored for controlling mechanical mixing and composite Young's modulus. Then, the role of thermal annealing during PEI/SLG fabrication is examined and its effect of mechanical mixing studied. Finally, we investigate mechanical fatigue in PEI/SLG thin films under dynamic loading with different pre-tensions. The design of mechanical properties in PGLs is crucial for ensuring long-term stability and operational performance. The Young's modulus is an important mechanical parameter governed by mechanical mixing in PGLs, and better mechanical mixing is facilitated by improved stress transfer efficiency at the polymer/graphene interface. Thus, the control of mechanical mixing through PGL design is an important research topic for developing PGL thin films with good mechanical performance. To this end, we evaluated the design of PEI/SLG thin films with precisely controlled thicknesses to afford control over mechanical mixing and the composite Young's modulus. PEI concentrations of 3–6 wt% were used to spin-coat PEI/SLG films at precisely controlled thicknesses of ~200–1000 nm, which enabled the design of PEI/SLG thin films based on thickness control. The design of PEI/SLG film thickness afforded control over the volume fraction of SLG, which is related to the composite Young's modulus of the films via the well-known mixing rule. To validate this approach, we measured the composite Young's modulus of PEI/SLG films and modelled the relationship between modulus and film thickness (i.e., volume fraction) using the mixing rule. However, we found that linear regression analysis yielded an unexpectedly high effective Young's modulus of 1.12 ± 0.05 TPa for SLG. Further investigation revealed the larger-than-expected value was due to the gradual deterioration of mechanical mixing between SLG and PEI at film thicknesses > 250 nm. Overall, our results demonstrate that the control of mechanical mixing in PGL thin films is achievable through the physical design of film thickness, which fits well within the proposed framework for PGL development. The fabrication of PGL materials is another important topic for controlling PGL properties. For example, the transfer process required for removing PGL thin films from a CVD substrate is known to introduce mechanical defects and electronic doping if improper processing conditions are used. However, little is known about the impact of thermal annealing on the properties of PGL materials. Normally, thermal annealing is thought to aid in the removal solvent and stress after polymer deposition on SLG, but we show that thermal annealing instead induces substantial structural damage and reduced film properties when poor conditions are used. Specifically, thermally annealing PEI/SLG in air past the Tg of PEI (~217 °C) caused widespread structural damage due to oxidation of the underlying Cu substrate. This conclusion was supported through an in-depth mechanistic investigation of annealing-induced deformation. First, changing the annealing atmosphere to high-purity nitrogen prevented widespread structural deformation from occurring during annealing, regardless of temperature. Second, the onset of film deformation during annealing in air was strongly associated with temperatures above the glass transition temperature of PEI. Finally, Arrhenius analysis yielded an activation energy of 159 kJ/mol for the deformation process, almost twice that associated with Cu oxidation and instead closer in scale to that of glass transitions in amorphous polymers. A physical failure mechanism was proposed based on the in-plane shrinkage of PEI during the glass-to-rubber transition caused by the release of internal stress induced by spin-coating. In-plane contraction of PEI would then cause compression of SLG as internal stress was released, which we confirmed from a significant blue-shift in the 2D band of SLG by ~30 cm-1 after annealing both with and without visible deformation. Importantly, we demonstrate that common secondary processing steps, like thermal annealing, will have critical effects on film properties if conditions are not properly controlled. The mechanical fatigue and cycling stability of PGL thin films is another topic of interest for developing reliable PGLs with long operational lifetimes. Mechanical failure analysis might be used to evaluate failure mechanisms governing interfacial fatigue in PGL thin films. However, little work has been done understanding mechanical fatigue in PGLs. To this end, we explore the use of mechanical bulge testing for the evaluation of cycling fatigue in PGLs and construct a custom instrument, known as a bulge test apparatus (BTA), to perform fatigue measurements. In general, our BTA test platform provides a means of analyzing different fabrication/design conditions. To test its use, we prepared PEI/SLG thin films with different tensioning weights between 0 and 30 g. Then, the well-established methods for bulge testing were applied to the dynamic loading of PEI/SLG using the BTA. Specifically, the evolution of bulge-test-derived pre-stress in PEI/SLG was monitored as a function of cycle number and tensioning weight, which revealed large fluctuations in pre-stress with mechanical cycling up to ~200k cycles, which was unexpected for these films. To investigate the underlying mechanism, we further employed Raman spectroscopy, optical microscopy, and scanning electron microscopy (SEM) to determine the strain state of SLG before and after cycling and probe for structural changes. Raman measurements revealed that mechanical cycling induced a large redshift in the 2D and G peak positions of SLG by ~25.4 and ~10.1 cm-1, respectively, for 30 g-tensioned PEI/SLG, with similar shifts observed for 20 g-tensioned films. Optical and SEM images showed possible changes in the surface structure of SLG after cycling accompanying the shift in Raman characteristics. We discuss the possibility of interfacial failures involving in-plane slippage and out-of-plane buckling of SLG based on our results. If validated, the use of BTA may provide future insight regarding mechanical fatigue and failure at the PGL interface during dynamic loading. The practical relevance of methods for determining the influence of design and fabrication characteristics in PGLs will no doubt be invaluable for further development. Additional work in this area will be necessary to connect our analytical approach by BTA to underlying failure mechanisms in PGLs.
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    Development of Controlled Ring-Opening Polymerization of  O-Carboxyanhydrides
    Zhong, Yongliang (Virginia Tech, 2020-10-27)
    The aim of my Ph.D. thesis is to summarize my research on the development of ring-opening polymerization (ROP) of O-carboxyanhydrides (OCAs) to synthesize functionalized, degradable polyesters. Biodegradable polyesters are promising alternatives to conventional petroleum-based non-degradable polyolefins and they are widely used in everyday applications ranging from clothing and packaging to agriculture and biomedicine. Commercially available polyesters, such as poly(lactic-co-glycolic acid), poly(lactic acid), and polycaprolactone, hydrolyze in physicochemical media. They have been approved by FDA and widely used for medical applications. However, the lack of side-chain functionality in polyesters and in corresponding monomers greatly plagues their utility for applications that demand physicochemical properties such as high stiffness, tensile strength and elasticity. Increasing efforts have been devoted to the introduction of pendant groups along the polymer chain in order to modify and modulate the physicochemical properties of polyesters and thereby to expand their applications. Over the last decade, OCAs have emerged as an alternative class of highly active monomers for polyester polymerization. OCAs are prepared from amino acids and thus have a richer range of side chain functionalities than lactone or lactide. Like lactones, OCAs can undergo ROP to obtain polyesters. Unfortunately, current ROP methods, especially those involving organocatalysts, result in uncontrolled polymerization including epimerization for OCAs bearing electron-withdrawing groups, unpredictable molecular weights (MWs), or slow polymerization kinetics. Based on our recent success of Ni/Ir photoredox catalysis allowing for rapid synthesis of high-MWs polyesters, we further explore new polymerization chemistry to use earth-abundant metal complexes to replace expensive rare-earth metal photocatalysts, and practice the polymerization in moderate and energy-efficient reaction conditions. This thesis introduces novel photoredox and electrochemical earth-abundant metal catalysts that overcome above difficulties in the ROP chemistry of OCAs, and allow for the preparation of stereoregular polyesters bearing abundant side-chain functionalities in a highly controlled manner. Specifically, various highly active metal complexes have been developed for stereoselective ROP of OCAs, either using light or electricity, to synthesize syndiotactic or stereoblock copolymers with different thermal properties. Additionally, simple purification protocols of OCAs have also been initially studied, which potentially paves the way to bulk production of functional monomers. In this thesis, I first describe newly-developed photoredox Co/Zn catalysts to achieve a controlled ROP of enantiopure OCAs under mild reaction conditions (Chapter 2). Such discovery is extended to the combination use of Co catalysts with various Zn/Hf complexes that enable stereoselective controlled ROP of racemic OCAs for the preparation of stereoregular polyesters (Chapter 3). The mechanistic studies of the aforementioned developments lead to the application of such a catalytic system in controlled electrochemical ROP of OCAs (Chapter 4). Such chemistry can also be translated to stereoselectively electrochemical ROP of racemic OCAs to either syndiotactic or stereoblock polyesters, allowing precise control of polyester's tacticity and sequence (Chapter 5). An overview future work has been summarized (Chapter 6).
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    The Effect of Ionomer Architecture on the Morphology in Gel State Functionalized Sulfonated Syndiotactic Polystyrene
    Fahs, Gregory Bain (Virginia Tech, 2020-03-04)
    This dissertation presents a discussion of blocky and randomly functionalized sulfonated syndiotactic polystyrene copolymers. These copolymers have been prepared over a range of functionalization (from 2% to 10%) in order to assess the effect of the incorporation of these polar side groups on both the thermal behavior and morphology of these polymer systems. The two different architectures are achieved by conducting the reaction in both the heterogeneous gel-state to obtain blocky copolymers and in the homogeneous solution state to obtain randomly functionalized copolymers. In order to compare both the thermal properties and morphology of these two systems several sets of samples were prepared at comparable levels of sulfonation. Thermal analysis of these two systems proved that the blocky functionalized copolymers provided superior properties with regard to the speed and total amount of the crystalline component of sulfonated syndiotactic polystyrene. Above 3% functionalizion the randomly functionalized copolymer was no longer able to crystallize, whereas, the blocky functionalized copolymer is able to crystallize even at a functionalization level of 10.5% sulfonate groups. When considering the morphology of these systems even at low percentages of sulfonation it is clear that the distribution of these groups is different based on the amplitude of the signal measured by small angle x-ray scattering. Additionally, methods were developed to describe both the distribution of ionic multiplets, which varies between blocky and randomly functionalized systems, but also the distribution of crystals. At a larger scale ultra-small angle x-ray scattering was employed to attempt to understand the clustering of ionic multiplets in these systems. Randomly functionalized polymers should a peak that is attributed to ion clusters, whereas blocky polymers show no such peak. Additional studies have also been done to look at the analysis of crystallite sizes in these systems when there are multiplet polymorphs present, it was observed the polymorphic composition is drastically different. All of these studies support that these systems bear vastly different thermal behavior and possess significantly different morphologies. This supports the hypothesis that this gel-state heterogeneous functionalization procedure produces a much different chain architecture compared to homogeneous functionalization in the solution-state.
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    Exceptional capacitive deionization rate and capacity by block copolymer–based porous carbon fibers
    Liu, Tianyu; Serrano, Joel; Elliott, John; Yang, Xiaozhou; Cathcart, William; Wang, Zixuan; He, Zhen; Liu, Guoliang (American Association for the Advancement of Science, 2020-04-17)
    Capacitive deionization (CDI) is energetically favorable for desalinating low-salinity water. The bottlenecks of current carbon-based CDI materials are their limited desalination capacities and time-consuming cycles, caused by insufficient ion-accessible surfaces and retarded electron/ion transport. Here, we demonstrate porous carbon fibers (PCFs) derived from microphase-separated poly(methyl methacrylate)-block-polyacrylonitrile (PMMA-b-PAN) as an effective CDI material. PCF has abundant and uniform mesopores that are interconnected with micropores. This hierarchical porous structure renders PCF a large ion-accessible surface area and a high desalination capacity. In addition, the continuous carbon fibers and interconnected porous network enable fast electron/ion transport, and hence a high desalination rate. PCF shows desalination capacity of 30 mgNaCl g⁻¹ PCF and maximal time-average desalination rate of 38.0 mgNaCl g⁻¹ PCF min⁻¹, which are about 3 and 40 times, respectively, those of typical porous carbons. Our work underlines the promise of block copolymer–based PCF for mutually high-capacity and high-rate CDI.
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    Fabrications and Applications of  Protein-based Bionanocomposites
    Li, Yunhua (Virginia Tech, 2020-06-26)
    Stabilization of highly sensitive noble metal nanoparticles is essential for their practical application. Bionanocomposites in which various types of noble metal nanoparticles, especially anisotropic noble metal nanoparticles, are immobilized into a macroscopic biomaterial membrane show promising applications in biomedical, catalytic, and environmental fields. This research focuses on developing two fabrication methods to generate novel bionanocomposite materials by immobilizing gold (Au) or silver (Ag) nanoparticles onto a "green" biomaterial, namely an eggshell membrane (ESM). Furthermore, the applications of the resulting bionanocomposite materials were demonstrated by studying their use as catalysts for environmental pollutant conversion and for the detection of two pollutant chemicals. The first fabrication method immobilizes ex situ synthesized nanoparticles onto a chemically modified ESM. Disulfide originating from the ESM was reduced by dithiothreitol into free thiol groups for binding to Au nanoparticles with different morphologies. The immobilization of Au nanoparticles greatly enhances their stability, making it possible to apply the resulting bionanocomposites for catalyzing the reduction reaction to convert pollutant p-nitrophenol (PNP) to p-aminophenol (PAP), with a great increase in their lifetime use from 2 to 10 reaction cycles. The second fabrication method utilizes the zwitterionic property of the protein based ESM for binding with Ag nanoparticles to form bionanocomposites. A seed mediated nanoparticle synthesis method originally performed in suspension was modified and adapted for the in situ synthesis of Ag nanodisks in this research. Ag nanoseeds were first immobilized onto an eggshell membrane using the static interaction between the nanoseeds and the membrane. Subsequently, Ag nanodisks were further grown directly on the Ag nanoseeds on the ESM. The final distribution density of Ag nanodisks can be adjusted by tuning the distribution density of Ag nanoseeds immobilized on the ESM. The performance of the resulting bionanocomposites were evaluated for both catalysis, and their application as substrates for surface enhanced Raman spectroscopy (SERS). The material performance was found to depend on the final distribution density of the Ag nanodisks on the ESM, offering the possibility to optimize bionanocomposite material performance by adjusting this density. A SERS based technique was further developed for detecting pollutant chemical species using the Ag nanodisks/ESM bionanocomposite material as a SERS substrate. Direct detection of thiram, a commonly used pesticide, was achieved at a concentration that is lower than that regulated by the US EPA. By using crystal violet as a SERS probe molecule, mercury, a heavy metal without an intrinsic Raman fingerprint, was indirectly detected not only at a limit of detection lower than most reported in the scientific literature, but also with a selectivity against a group of metal ions including Ba, Cu, Ca, Co, Mg, Mn, Ni, and Zn. It was also found that the detection sensitivity can be optimized by adjusting the Ag nanodisk distribution density on the ESM. The development of the fabrication approach and the use of ESM as a matrix material for immobilizing noble metal nanoparticles to form bionanocomposite materials demonstrates a novel strategy for meeting the needs of a variety of applications. The development of bionanocomposites for detecting pollutant species with different SERS activities by simply tuning the nanoparticle distribution density on the surface of the substrate, is a novel discovery, as it does not appear to have been previously reported in the literature.
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    Functionalized Metal-Organic Frameworks for Water Oxidation Catalysis
    Lin, Shaoyang (Virginia Tech, 2019-05-02)
    Increasing energy demand will not only aggravate global warming, but also cause fossil fuels shortage in the near future. Solar energy is an infinite green energy resource that can potentially satisfy our energy usage. By utilizing solar energy to drive reactions like water splitting, solar fuels system are able to produce valuable energy resource. Catalysts for water oxidation are the essential component of water splitting cells which have been intensively studied. As a solid state porous crystalline material with synthetic tunability, Metal-organic framework (MOF) is a promising platform for water oxidation catalysis due to its outstanding properties. Herein, we aimed to develop molecular catalysts incorporated MOF for water oxidation and study the reaction mechanism. Chapter 1 introduces the background of water oxidation and previous research on ruthenium nuclear water oxidation catalysts (WOCs). The reaction mechanism of binuclear and mononuclear ruthenium WOCs was briefly summarized. Opportunities for the design and the synthesis of MOF based WOCs were then discussed. Lastly, studies about MOF based WOCs were categorized based on the difference of the WOCs active site location in frameworks. Water oxidation catalyst [Ru(dcbpy)(tpy)OH2]2+ (RuTB) was incorporated into UiO-67 MOF (resulting materials denoted as RuTB-UiO-67) for chemical water oxidation in Chapter 2. Differences of catalytic reaction behavior between homogeneous RuTB and RuTB incorporated in MOF were examined. Based on MOF particle size dependent catalysis reaction experiments, in-MOF reactivity was anticipated to be primarily arose from redox hopping between RuTB active sites in the framework. In Chapter 3, RuTB-UiO-67 MOF thin films grown on conducting FTO substrate (RuTB-UiO-67/FTO) were synthesized to test their catalytic activity of electrochemical water oxidation. Electrochemical behavior of RuTB-UiO-67/FTO was found to be consistent with homogeneous RuTB by various electrochemistry study and in-situ X-ray absorption spectroscopy characterization. Scan-rate-dependent voltammetry study demonstrated the homogeneous distribution of electrochemical active sites throughout the MOF thin film. Diffusion controlled redox hopping was attributed to be the main charge transfer pathway during catalysis. In order to pursue photo-induced water splitting system, we further our study by investigating MOF based photoelectrochemical catalysis in Chapter 4. Photoelectrochemical alcohol oxidation was chosen as the preliminary-stage study towards the more challenging goal, photoelectrochemical water oxidation. Electron transfer processes of the photosensitizer ([Ru(bpy)2(dcbpy)]2+) and the catalyst (RuTB) doped UiO-67 MOF were investigated with transient absorption spectroscopy analysis. Finally, the role of redox hopping in electrocatalysis by MOF was reviewed in Chapter 5. Pathways of charge transfer in electroactive MOF were first summarized. Redox hopping in MOF was then compared with previous studies on redox active polymer thin films. Lastly, factors that will affect the rate of redox hopping of MOF electrocatalyst were discussed.
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    Influence of Sidechain Structure and Interactions on the Physical Properties of Perfluorinated Ionomers
    Orsino, Christina Marie (Virginia Tech, 2020-10-19)
    The focus of this dissertation was to investigate the influence of sidechain structure and sidechain content on the morphology and physical properties of perfluorosulfonic acid ionomer (PFSA) membranes. One of the primary objectives was to characterize the thermomechanical relaxations for short sidechain PFSAs developed by 3M and Solvay, as well as a new multi-acid sidechain perfluoroimide acid ionomer (PFIA) from 3M. Partial neutralization experiments played a key role in systematically manipulating the strength of the electrostatic interactions between proton exchange groups on each sidechain, leading to the elucidation of the molecular-level motions associated with multiple thermal relaxations observed by dynamic mechanical analysis (DMA). Particularly, 3M PFSA and Solvay Aquivion lack an observable β-relaxation in the sulfonic acid-form that is observed in the long sidechain PFSA, Nafion. By varying the strength of the physically-crosslinked network through exchanging the proton on the sulfonic acid groups for large counterions, we were able to conclude that the shorter sidechain length and increase in ion content in the 3M PFSA and Solvay Aquivion serves to restrict the mobility of the polymer backbone such that the onset of segmental motions of the main chains is not observed at temperatures below the α-relaxation temperature, where destabilization of the physically crosslinked network occurs. As a complementary technique to DMA for probing the relaxations in PFSAs, we introduced a new pretreatment method for differential scanning calorimetry (DSC) measurements that uncover a thermal transition in H+-form 3M PFSA, Aquivion, and Nafion membranes. This thermal transition was determined to be of the same molecular origin as the dynamic mechanical α-relaxation temperature in H+-form PFSAs, and the β-relaxation temperature in tetrabutylammonium (TBA+)-form PFSAs. The thermomechanical relaxations in multi-acid sidechain 3M PFIA were also investigated. Interestingly, the additional acidic site on PFIA led to unexpected differences in thermal and mechanical properties, including the appearance of a distinct glass transition temperature otherwise not seen in PFSA ionomers. We utilized small-angle X-ray scattering (SAXS) studies to probe the differences in aggregate structure between the PFIA and PFSA membranes in order to uncover the morphological origin of the anomalous thermomechanical behavior in PFIA membranes. Larger aggregate structures for PFIA, compared to PFSA, incorporate intervening fluorocarbon chains within the aggregate, resulting in increased spacing between ions that reduce the collective electrostatic interactions between ions such that the onset of chain mobility occurs at lower temperatures than the α-relaxation for PFSA. The SAXS profiles of PFSAs showed two scattering features resulting from scattering between crystalline domains and ionic domains distributed throughout the polymer matrix. In order to fit the "ionomer peak" to models used for the PFIA and PFSA aggregate structure determination, we presented a method of varying the electron density of the ionic domains by using different alkali metal counterions as a tool to make the intercrystalline feature indistinguishable. This allows for isolation of the ionomer peak for better fits to scattering models without any interference from the intercrystalline peak. Lastly, an investigation of annealing PFSAs of different sidechain structures in the tetramethylammonium (TMA+) counterion form above their α-relaxation showed a profound crystalline-like ordering of the TMA+ counterions within the ionic domains. This ordering is maintained after reacidification and leads to improved proton conductivity, which indicates that this method can be used as a simple processing method for obtaining improved morphologies in proton exchange membranes for fuel cell applications.
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    Investigating Cathode–Electrolyte Interfacial Degradation Mechanism to Enhance the Performance of Rechargeable Aqueous Batteries
    Zhang, Yuxin (Virginia Tech, 2023-12-04)
    The invention of Li-ion batteries (LIBs) marks a new era of energy storage and allows for the large-scale industrialization of electric vehicles. However, the flammable organic electrolyte in LIBs raises significant safety concerns and has resulted in numerous fires and explosion accidents. In the pursuit of more reliable and stable battery solutions, interests in aqueous batteries composed of high-energy cathodes and water-based electrolytes are surging. Limited by the narrow electrochemical stability window (ESW) of water, conventional aqueous batteries only achieve inferior energy densities. Current development mainly focuses on manipulating the properties of aqueous electrolytes through introducing excessive salts or secondary solvents, which enables an unprecedentedly broad ESW and more selections of electrode materials while also resulting in some compromises. On the other hand, the interaction between electrodes and aqueous electrolytes and associated electrode failure mechanism, as the key factors that govern cell performance, are of vital importance yet not fully understood. Owing to the high-temperature calcination synthesis, most electrode materials are intrinsically moisture-free and sensitive to the water-rich environment. Therefore, compared to the degradation behaviors in conventional LIBs, such as cracking and structure collapse, the electrode may suffer more severe damage during cycling and lead to rapid capacity decay. Herein, we adopted multi-scale characterization techniques to identify the failure modes at cathode–electrolyte interface and provide strategies for improving the cell capacity and life during prolonged cycling. In Chapter 1, we first provide a background introduction of conventional non-aqueous and aqueous batteries. We then show the current development of modern aqueous batteries through electrolyte modification and their merits and drawbacks. Finally, we present typical electrode failure mechanism in non-aqueous electrolytes and discuss how water can further impact the degradation behaviors. In Chapter 2, we prepare three types of aqueous electrolytes and systematically evaluate the electrochemical performance of LiNixMnyCo1-x-yO2, LiMn2O4 and LiFePO4 in the aqueous electrolytes. Combing surface- and bulk-sensitive techniques, we identify the roles played by surface exfoliation, structure degradation, transition metal dissolution and interface formation in terms of the capacity decay in different cathode materials. We also provide fundamental insights into the materials selection and electrolyte design in the aqueous batteries. In Chapter 3, we select LiMn2O4 as the material platform to study the transition metal dissolution behavior. Relying on the spatially resolved X-ray fluorescence microscopy, we discover a voltage-dependent Mn dissolution/redeposition (D/R) process during electrochemical cycling, which is confirmed to be related to the Jahn–Teller distortion and surface reconstruction at different voltages. Inspired by the findings, we propose an approach to stabilize the material performance through coating sulfonated tetrafluoroethylene (i.e., Nafion) on the particle, which can regulate the proton diffusion and Mn dissolution behavior. Our study discovers the dynamic Mn D/R process and highlights the impact of coating strategy in the performance of aqueous batteries. In Chapter 4, we investigate the diffusion layer formed by transition metals at the electrode–electrolyte interface. With the help of customized cells and XFM technique, we successfully track the spatiotemporal evolution of the diffusion layer during soaking and electrochemical cycling. The thickness of diffusion layer is determined to be at micron level, which can be readily diminished when gas is generated on the electrode surface. Our approach can be further expanded to study the phase transformation and particle agglomeration at the interfacial region and provide insights into the reactive complexes. In Chapter 5, we reveal the correlation between the electrolytic water decomposition and ion intercalation behaviors in aqueous batteries. In the Na-deficient system, we discover that overcharging in the formation process can introduce more cyclable Na ions into the full cell and allows for a boosted performance from 58 mAh/g to 124 mAh/g. The mechanism can be attributed to the water oxidation on the cathode and Na-ion intercalation on the anode when the charging voltage exceeds the normal oxidation potential of cathode. We emphasize the importance of unique formation process in terms of the cell performance and cycle life of aqueous batteries. In Chapter 6, we summarize the results of our work and propose perspectives of future research directions.