Browsing by Author "Li, Xin"
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Item Novel electrochemical capacitors based on free-standing single-walled carbon nanotube films(University of Delaware, 2012) Li, XinIn order to effectively store the energy from the renewable sources and release them when needed, energy storage devices play an important role. Among all the energy storage devices, the electrochemical capacitor currently holds the highest power density and modest energy density. Nanostructured materials have shown promising electrochemical performance as electrode materials for electrochemical capacitor and different carbon nanostructured materials are under close scrutiny. Among those, carbon nanotube films draw great attention owing to their extraordinary properties, such as high surface area, excellent electrical conductivity, and mechanical robustness. In this thesis, the electrochemical capacitors based on freestanding single-walled carbon nanotube (SWNT) film are investigated from three perspectives. First, the effect of compressive stress on the electrochemical behavior of flexible supercapacitors based on the freestanding SWNT film electrodes and 1 M aqueous electrolytes with different anions and cations were investigated. The results demonstrated that the specific capacitance increased firstly and saturated in corresponding to decreases of the series resistance, the charge transfer resistance, and the Warburg diffusion resistance under an increased pressure from 0 to 1723.96 kPa. The wettability and the ion-size effect play important roles to determine the pressure dependence behavior of the supercapacitors. An improved high frequency capacitive response with 1172 Hz "knee" frequency was observed under the compressive pressure of 1723.96 kPa. Second, a dynamically stretchable supercapacitor (DSS) with highly reversible stretchability was demonstrated based on the elastomeric electrospun polyurethane separator, the buckled SWNT macrofilms electrodes, and organic electrolyte. The electrochemical behavior of the stretchable cells was instantaneously characterized under both dynamic stretch/release (DSR) and fixed static strain modes. The interesting capacitance variation was observed under the DSR mode, which corresponds with the state of the strain and strain rate applied. The capacitance variation is mainly attributed to the changes of charge transfer resistance and Warburg diffusion resistance during the stretching/releasing. The cells have shown excellent cycling stabilities under cyclic stretching/releasing. Third, the SWNT/MnO2 hybrid films as supercapacitor electrode materials were synthesized using a new and facile approach, which is binder-free, robust and with pre-formed electrical pathways. The one-step precipitation process is facile, cost-effective and scalable for mass production of the electrode materials with enhanced electrochemical performances. The supercapacitors demonstrate a much improved energy density and equally high power density, as well as an ultra-high frequency response and excellent long-cycle stability. The energy density of 70 Wh kg-1 with power density of 77.3 W kg-1 shows its high competitiveness compared with the current Ni-MH battery systems, thus makes the supercapacitors very promising to be implemented as the electric energy storage devices for hybrid vehicles.Item Using priming to promote neuroplasticity and motor learning post-stroke(University of Delaware, 2018) Li, XinThe majority of stroke survivors experience persistent motor impairments even with rehabilitation treatments. An underlying mechanism for this is the decreased motor cortical excitability in the lesioned hemisphere after stroke. Priming techniques, such as acute exercise and transcranial direct current stimulation (tDCS), can increase motor cortical excitability and enhance motor learning in healthy individuals. But whether they have the same effects in people with stroke is unclear. Selective serotonin-reuptake inhibitors, a type of antidepressant medication, can change motor cortical excitability in healthy individuals and in acute stroke survivors. Moreover, they can interact with tDCS, changing the effects of tDCS in healthy individuals. Given that up to 30% of stroke survivors take antidepressant medications, this is an important factor to consider when evaluating the effects of tDCS in stroke. The overall purpose of this dissertation was to investigate the neurophysiological effects of exercise priming and tDCS (with chronic antidepressant intake as a factor), and to investigate the effects of tDCS on locomotor learning in people with chronic stroke. ☐ In Aim 1, we showed that exercise priming, in the form of 5 minutes of high-intensity walking, induced increased motor cortical excitability in the lesioned hemisphere, as measured in a resting upper extremity muscle. This finding is significant because it provides evidence on the effectiveness of a clinically feasible exercise priming paradigm to induce broad excitability changes in the brain. ☐ In Aim 2, we showed that stroke survivors taking antidepressant medications had higher motor cortical excitability in the non-lesioned hemisphere compared to those not on antidepressants. We also found that application of anodal tDCS as a primer over the lesioned hemisphere produced differential effects on excitability in the unstimulated, non-lesioned hemisphere, depending on antidepressant-taking status. In antidepressant-takers, motor cortical excitability in the non-lesioned hemisphere increased, while it decreased compared to sham in those not taking antidepressants. These findings draw attention to the fact that stroke survivors may not respond in the same way to tDCS as healthy individuals, and that antidepressants, and potentially other medications and stroke-related factors, must be considered and their effects investigated before providing tDCS as a clinical treatment. ☐ Finally, in Aim 3, we showed that anodal tDCS over the lesioned hemisphere did not have any effect on split-belt treadmill locomotor learning and retention in chronic stroke survivors. We speculate that split-belt adaptation may not be sensitive to modulation by tDCS. Future studies should investigate whether tDCS affects other types of locomotor learning. ☐ Overall, this work demonstrates the potential of exercise priming for stroke recovery, and highlights the complexity of tDCS usage in people with chronic stroke. Future studies should focus on how individual differences affect priming in stroke.