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As energy demands increase, development of energy storage and conversion systems becomes critical to meet the needs of society. The performance and lifetime of energy devices are highly dependent on the material chemistry, processing, and nanostructure. 1D nanomaterials show great promise for advanced energy materials owing to their high aspect ratios, mechanical flexibility, among other unique properties. Electrospun nanofibers are particularly advantageous owing to the versatility, in which control of the nanoarchitecture and composition is easily achievable. Electrospinning is a relatively simple fiber formation technique that uses a strong electric field to rapidly stretch and elongate a polymer-based solution or melt jet, creating ultrathin fibers with diameters on the order of 50 – 500 nm. The nanofibers form continuous into a non-woven fiber mat that is inherently free-standing which allows for direct usage without additional processing with insulating, inactive binder materials. My dissertation will focus on the synthesis and characterization of electrospun nanofibers materials and will establish key process-structure-property relationships for creating simple materials for organic photovoltaics and lithium-sulfur batteries. The nanomaterials for these systems have very different requirements, thus, these studies focused on the challenges associated with each application such that the nanofiber synthesis and types of characterization techniques were tailored to each application. The lithium-sulfur (Li-S) rechargeable battery is an emerging, close-to-market chemistry with several key advantages over lithium-ion: high energy density ~5 times greater than lithium-ion, high theoretical capacity of 1,675 mA h g-1 (vs < 300 mA h g-1 of li-ion), and sulfur is inexpensive, non-toxic, and environmentally-benign. However, several challenges must be addressed to develop high performance Li-S batteries. The electronically insolating nature of sulfur and the dissolution and shuttling of soluble reaction intermediates (Li2Sn, 4 ≤ n ≤ 8) into the electrolyte significantly reduce performance, such as rapid capacity fade. Thus, research intensely focuses on sulfur cathodes with conductive components that confine sulfur to hinder polysulfide intermediate shuttling. Complex conductive nanoarchitectures to encapsulate or trap sulfur to the cathode require length methods of sulfur diffusion (~8-10 hours) and require binders and heavy current collectors to integrate the composites in a final electrode. We developed a rapid (5-second) sulfur deposition technique is demonstrated on electrospun carbon nanofibers to fabricate binder-free, free-standing cathodes for lithium-sulfur batteries. The 5-second procedure melts sulfur into carbon nanofiber mats, which play a significant role as a built-in conductive matrix without the hindrance of conventional insulating binding agents. Meanwhile, the large inter-fiber spacing facilitates electrolyte diffusion. The cathodes thus obtained deliver 550 mA h g-1, effectively an effective discharge capacity of ~ 997 mA h g-1 owing to their simple construction, with 100% capacity retention at 0.5C rate over 150 cycles. The excellent stability could be attributed to the functional groups along the CNF surface which showed evidence in post-mortem FT-IR analysis of binding with polysulfides. The novel time-efficient sulfur-deposition technique combined with the free-standing CNF eliminates the need for lengthy sulfur incorporation (i.e. > 8 hours) and slurry processing with insulating binders, toxic solvents, and heavy current collectors which can make up 30-50% of the electrode weight. While cathodes are critical in confining polysulfides to prevent shuttling, the electrolyte plays an equally important role in redox chemistry and overall battery performance and understanding fundamental interactions between intermediate redox species and electrolyte is critical for rational design of electrolyte/cathode systems for Li-S batteries. New electrolyte additives are under development with the aim of suppressing polysulfide shuttling. Although better performance is observed, there is a lack in fundamental evidence of physical and chemical interactions to understand the underlying mechanism of the additive. The freestanding, binder-free sulfur cathodes developed in our previous work facilitated this electrolyte study through in situ infrared spectro-electrochemistry. Owing to the free-standing nature of the cathode, the lack of current collector in this cathode simplified the optical physics in FT-IR with attenuated total reflection (ATR), such that an in situ cell could be developed that closely replicated a coin cell without altering the key battery components, such as electrolyte volume, cathode, sulfur loading, etc., which was made possible with the ATR accessory. In this work, we demonstrate in situ ATR FT-IR to monitor both polysulfide speciation (Sx2-, 2 ≤ x ≤ 8) and triflate anion (electrolyte) coordination states while simultaneously running cyclic voltammetry (CV). Polysulfide evolution was monitored with IR via the S-S vibrational frequency around ~500 cm-1, which shifts with polysulfide order (chain length). While most in situ techniques only monitor polysulfide s, we found molecular-level changes occurred in the salt anion in response to polysulfide speciation. During CV, polysulfide dissolution increases total solute concentration, inducing anion interactions between low coordination state complexes– ion-pairs and free ions– to form aggregate complexes. Under fast CV sweep, less progressive formation of all polysulfides, due to diffusion limitations, resulting in higher concentrations of aggregates and polysulfide even upon completion of discharge. This new application of in situ FT-IR offers direct insight on dynamic interactions between electrolyte salt and polysulfide fundamental in developing Li-S systems and would be particularly useful to study electrolytes, additives, and new systems. With the development of energy technologies, green and renewable resources provide attractive alternatives to depleting fossil fuel supplies. Organic photovoltaics (OPV) offer several advantages over conventional inorganic PV: low weight, flexibility, semi-transparency, simple integration into other products, significantly lower manufacturing costs, short energy payback time, low environmental impact processing, as well as the potential for new markets, such as wearable PV. However, significantly lower efficiencies, due to poor separation and transport of free charge carriers in the photoactive layer, hinder widespread use of OPV technology. The photoactive layer consists of a spin-coated film of electron donor and acceptor materials, typically poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), respectively, with tortuous, discontinuous donor/acceptor phases. Alternatively, methods that control phase separation on the order of the exciton (photo-generated electron-hole pair) diffuse length scale (~10-20 nm) with continuous, ordered donor and acceptor pathways are desirable because these design elements will lead to a highly probability of separation of free carriers at donor/acceptor interface as well as limited recombination while charges transport through each phase to be collected. We development a single-step monoaxial electrospinning of P3HT/PCBM to understand impact of extensional flow on controlling the nanostructure. This conventional method of spinning has yet to be applicable to this blend owing to the rigidity of the P3HT rod-like polymer, which hinders polymer chain-entanglements needed for electrospinning; therefore, coaxial spinning and auxiliary polymer methods have been employed in literature. Nevertheless, a specific solution concentration was determined to successfully fabricate pure P3HT/PCBM nanofibers, without the need to remove auxiliary polymer or shell material, possible by PCBM molecules or aggregates serving as nodes for temporary networks between P3HT chains to enhance the material elasticity. Nanofibers were directly compared to films (as made and annealed) to reveal that the elongational flow during electrospinning improved crystallinity and reduced length scale of co-continuous P3HT and PCBM phases. These morphological characteristics are necessary for designing OPV materials and electrospinning offers a facile route to controlling the photoactive layer at the nanoscale. |
