Synthesis and systematic study of Co₃O₄-based catalysts for oxygen reduction and oxygen evolution reactions

textCo₃O₄-based composite materials are good electrocatalysts for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) in alkaline solutions. Here, this thesis first investigated the individual functionality of Co₃O₄ and the N-doped carbon nanoweb (CNW) in ORR and OER. The Co₃O₄/CNW bifunctional catalysts were synthesized by an in situ growth of Co precursors onto CNW followed by a controlled heat treatment. Rotating disk electrode measurements were utilized to provide insight into the specific functions of Co₃O₄ and CNW in the composite material during catalysis. It was found that Co₃O₄ alone exhibited poor ORR catalytic activity. However, in the presence of CNW, Co₃O₄ assisted the selective four-electron oxygen reduction over the two-electron pathway. Co₃O₄ acted as the primary catalytic site for OER and CNW improved the electronic conduction between Co₃O₄ and the current collector. CNW underwent serious degradation at the high potential of the OER, but its stability improved greatly upon the deposition of Co₃O₄. Two possible mechanisms for the improved catalytic stability are discussed. The findings demonstrate the specific functions of Co₃O₄ and CNW in catalyzing the OER and ORR and further establish an understanding of the synergy of the composite in electrocatalysis. Based on the critical functionality of Co₃O₄ in stabilizing carbon materials in the OER potential region, it is of interest to investigate novel synthesis methods to prepare nano-sized Co₃O₄ that can provide more active sites for catalytic reactions and thus, improve the OER kinetics. Here, in situ electrochemical generation of 2-dimensional Co₃O₄ (2D-Co₃O₄) nanoplates were achieved by scanning CoO[subscript x]/Co precursors in 1 M KOH solution. X-ray diffraction characterization suggested that CoO[subscript x]/Co precursors were oxidized to Co₃O₄ before the onset potential of OER. Scanning electron microscopy showed that oxidation from CoO[subscript x]/Co to 2D-Co₃O₄ was associated with the formation of hexagonal nanoplates. The 2D-Co₃O₄ exhibited excellent OER catalytic activity and stability probably due to the effective mass transfer through the 2D structure.Materials Science and Engineerin

Convergence of summation-by-parts finite difference methods for the wave equation

In this paper, we consider finite difference approximations of the second order wave equation. We use finite difference operators satisfying the summation-by-parts property to discretize the equation in space. Boundary conditions and grid interface conditions are imposed by the simultaneous-approximation-term technique. Typically, the truncation error is larger at the grid points near a boundary or grid interface than that in the interior. Normal mode analysis can be used to analyze how the large truncation error affects the convergence rate of the underlying stable numerical scheme. If the semi-discretized equation satisfies a determinant condition, two orders are gained from the large truncation error. However, many interesting second order equations do not satisfy the determinant condition. We then carefully analyze the solution of the boundary system to derive a sharp estimate for the error in the solution and acquire the gain in convergence rate. The result shows that stability does not automatically yield a gain of two orders in convergence rate. The accuracy analysis is verified by numerical experiments.Comment: In version 2, we have added a new section on the convergence analysis of the Neumann problem, and have improved formulations in many place

Cascaded Segmentation-Detection Networks for Word-Level Text Spotting

We introduce an algorithm for word-level text spotting that is able to accurately and reliably determine the bounding regions of individual words of text "in the wild". Our system is formed by the cascade of two convolutional neural networks. The first network is fully convolutional and is in charge of detecting areas containing text. This results in a very reliable but possibly inaccurate segmentation of the input image. The second network (inspired by the popular YOLO architecture) analyzes each segment produced in the first stage, and predicts oriented rectangular regions containing individual words. No post-processing (e.g. text line grouping) is necessary. With execution time of 450 ms for a 1000-by-560 image on a Titan X GPU, our system achieves the highest score to date among published algorithms on the ICDAR 2015 Incidental Scene Text dataset benchmark.Comment: 7 pages, 8 figure

Design and fabrication of a micro Coulter counter with thin film electrodes

We report a new approach for micro Coulter counter with thin film electrodes. Traditional Coulter counter measures DC resistance change to sense particles passing through an aperture between two flow chambers. One of the, biggest challenges to miniaturize the device is to overcome the large resistance encountered. We propose a new approach to use thin film metal electrodes in micron size range for sensing. Resolution can be improved and it's easy for system integration. A device based on this approach was built and tested