Design and Assembly of Hybrid Nanomaterial Systems for Energy Storage and Conversion

<p>Energy storage systems are critically important for many areas in modern society including consumer electronics, transportation and renewable energy production. This dissertation summarizes our efforts on improving the performance metrics of energy storage and conversion devices through rational design and fabrication of hybrid nanomaterial systems. </p><p>This dissertation is divided into five sections. The first section (chapter 2) describes comparison of graphene and carbon nanotubes (CNTs) on improving the specific capacitance of MnO2. We show that CNTs provided better performance when used as ultrathin electrodes but they both show similar performance with rapid MnO2 specific capacitance decrease as electrodes become thicker. We further designed ternary composite electrodes consisting of CNTs, graphene and MnO2 to improve thick electrode performance (chapter 3). We demonstrate that these electrodes were flexible and mechanically strong, had high electrical conductivity and delivered much higher capacity than electrodes made without CNTs. </p><p>Chapter 4 describes assembly of flexible asymmetric supercapacitors using a graphene/MnO2/CNTs flexible film as the positive electrode and an activated carbon/CNTs flexible film as the negative electrode. The devices were assembled using roll-up approach and can operate safely with 2 V in aqueous electrolytes. The major advantage of these devices is that they can deliver much higher energy under high power conditions compared with those designed by previous studies, reaching a specific energy of 24 Wh/kg at a power density of 7.8 kW/kg. </p><p>Chapter 5 describes our approach to improve the energy and power densities of nickel hydroxides for supercapacitors. This was done by assembling CNTs with Co-Ni hydroxides/graphene nanohybrids as freestanding electrodes. The assembled electrodes have dramatically improved performance metrics under practically relevant mass loading densities (~6 mg/cm2), reaching a specific capacitance of 2360 F/g at 0.5 A/g and 2030 F/g even at 20 A/g (~86% retention). </p><p>Finally, we discuss our efforts on designing highly active electrocatalysts based on winged nanotubes for oxygen reduction reactions (ORR). The winged nanotubes were prepared through controlled oxidization and exfoliation of stacked-cup nanotubes. When doped with nitrogen, they exhibited strong activity toward catalyzing ORR through the four-electron pathway with excellent stability and methanol/carbon monoxide tolerance owning to their unique carbon structure.</p>Dissertatio

Low-Dimensional Gradient Helps Out-of-Distribution Detection

Detecting out-of-distribution (OOD) samples is essential for ensuring the reliability of deep neural networks (DNNs) in real-world scenarios. While previous research has predominantly investigated the disparity between in-distribution (ID) and OOD data through forward information analysis, the discrepancy in parameter gradients during the backward process of DNNs has received insufficient attention. Existing studies on gradient disparities mainly focus on the utilization of gradient norms, neglecting the wealth of information embedded in gradient directions. To bridge this gap, in this paper, we conduct a comprehensive investigation into leveraging the entirety of gradient information for OOD detection. The primary challenge arises from the high dimensionality of gradients due to the large number of network parameters. To solve this problem, we propose performing linear dimension reduction on the gradient using a designated subspace that comprises principal components. This innovative technique enables us to obtain a low-dimensional representation of the gradient with minimal information loss. Subsequently, by integrating the reduced gradient with various existing detection score functions, our approach demonstrates superior performance across a wide range of detection tasks. For instance, on the ImageNet benchmark, our method achieves an average reduction of 11.15% in the false positive rate at 95% recall (FPR95) compared to the current state-of-the-art approach. The code would be released

SEM-CL and LA-ICP-MS trace element analysis of cassiterite in Gejiu tin district, SW China

Scanning electron microscope-cathodoluminescence images of cassiterite from various ore deposits in the Gejiu tin disrict, SW China reveal microstructures that differ among samples from different mineralizing environments. Cassiterite grains derived from granites and granite-proximal skarns and greisens are commonly larger in size, with clear euhedral oscillatory growth zones. In vein, stratiform oxidized and stratiform semioxidized samples, which are distal to the source granites, cassiterites are generally smaller, anhedral, and zonings are less evident. LA-ICP-MS trace element analyses indicate Ti, Fe and W are the most abundant trace elements in the samples of this study and Al, Sc, V, Ga, Zr, Nb, and Sb are commonly in lower concentrations. V, Nb, W and U have significant variations in concentration in a single sample. The skarn, greisen and tin granite samples have higher Ti, V and Al concentrations than vein-hosted, stratiform semioxidized and stratiform oxidized cassiterite. Trace element variations may reflect the distance between the granite-derived source fluids and the hydrothermal orebodies. In a tin granite cassiterite grain, the concentrations of Nb, U, Sb, W, Sc, V, Ti and Fe increase from core to rim of the crystal, but the greisen cassiterite does not show the same element zonation

Organic solar cells using few-walled carbon nanotubes electrode controlled by the balance between sheet resistance and the transparency

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Yiyu Feng, Xiaohui Ju, Wei Feng, Hongbo Zhang, Yingwen Cheng, Jie Liu, Akihiko Fujii, Masanori Ozaki, and Katsumi Yoshino, Appl. Phys. Lett. 94, 123302 (2009) and may be found at https://doi.org/10.1063/1.3103557

Removal of ammonium from aqueous solutions using alkali-modified biochars

Biochars converted from agricultural residuals can effectively remove ammonium from water. This work further improved the sorption ability of biochars to aqueous ammonium through alkali modification. Three modified biochars were prepared from agricultural residuals pre-treated with NaOH solution through low-temperature (300 °C) slow pyrolysis. The modified biochars effectively removed ammonium ions from water under various conditions with relatively fast adsorption kinetics (reached equilibrium within 10 h) and extremely high adsorption capacity (>200 mg/g). The Langmuir maximum capacity of the three modified biochars were between 313.9 and 518.9 mg/g, higher than many other ammonium adsorbents. Although the sorption of ammonium onto the modified biochar was affected by pH and temperature, it was high under all of the tested conditions. Findings from this work indicated that alkali-modified biochars can be used as an alternative adsorbent for the removal of ammonium from wastewater

The climate backgrounds of urban migrants affect thermal response

Human thermal responses can vary based on individuals' thermal histories. Although comparative studies on thermal comfort have been conducted in different regions, research is still limited regarding the differences in thermal responses among people living in the same city but having emigrated from diverse climates. Addressing this gap, this study, conducted in Beijing between 2021 and 2022, involved on-site meteorological measurements and questionnaire surveys, collecting a total of 1,370 questionnaires. The objective was to examine the differences in thermal response among individuals from different climatic backgrounds post-migration and assess the implications for urban design improvements. The study findings revealed the following: 1) Southern migrants exhibited a higher neutral Universal Thermal Climate Index (UTCI) compared to northern migrants (20.2 °C vs. 18.7 °C). 2) In terms of the acceptable temperature range (TAR) for 1 h, southern migrants showed a greater tolerance for hot conditions compared to northern migrants (4.4–30.8 °C vs. 0.6–28.6 °C), but less tolerant for cold conditions. 3) The transient acceptable range was considerably wider than the 1-h TAR. 4) Residents feeling thermally neutral at present had lower demands for urban design improvements. Our results highlight the importance of considering diversity of thermal response and implementing targeted management and planning interventions. The study is expected to provide practical recommendations for creating more livable and sustainable urban environments

Understanding the mechanisms of silica nanoparticles for nanomedicine

As a consequence of recent progression in biomedicine and nanotechnology, nanomedicine has emerged rapidly as a new discipline with extensive application of nanomaterials in biology, medicine, and pharmacology. Among the various nanomaterials, silica nanoparticles (SNPs) are particularly promising in nanomedicine applications due to their large specific surface area, adjustable pore size, facile surface modification, and excellent biocompatibility. This paper reviews the synthesis of SNPs and their recent usage in drug delivery, biomedical imaging, photodynamic and photothermal therapy, and other applications. In addition, the possible adverse effects of SNPs in nanomedicine applications are reviewed from reported in vitro and in vivo studies. Finally, the potential opportunities and challenges for the future use of SNPs are discussed. This article is categorized under: Nanotechnology Approaches to Biology > Nanoscale Systems in Biology Therapeutic Approaches and Drug Discovery > Emerging Technologies

Significantly Improved Long-Cycle Stability in High-Rate Li–S Batteries Enabled by Coaxial Graphene Wrapping over Sulfur-Coated Carbon Nanofibers

Long-term instability of Li–S batteries is one of their major disadvantages compare to other secondary batteries. The reasons for the instability include dissolution of polysulfide intermediates and mechanical instability of the electrode film caused by volume changes during charging/discharging cycles. In this paper, we report a novel graphene–sulfur–carbon nanofibers (G-S-CNFs) multilayer and coaxial nanocomposite for the cathode of Li–S batteries with increased capacity and significantly improved long-cycle stability. Electrodes made with such nanocomposites were able to deliver a reversible capacity of 694 mA h g^–1 at 0.1C and 313 mA h g^–1 at 2C, which are both substantially higher than electrodes assembled without graphene wrapping. More importantly, the long-cycle stability was significantly improved by graphene wrapping. The cathode made with G-S-CNFs with a initial capacity of 745 mA h g^–1 was able to maintain ∼273 mA h g^–1 even after 1500 charge–discharge cycles at a high rate of 1C, representing an extremely low decay rate (0.043% per cycle after 1500 cycles). In contrast, the capacity of an electrode assembled without graphene wrapping decayed dramatically with a 10 times high rate (∼0.40% per cycle after 200 cycles). These results demonstrate that the coaxial nanocomposites are of great potential as the cathode for high-rate rechargeable Li–S batteries. Such improved rate capability and cycle stability could be attributed to the unique coaxial architecture of the nanocomposite, in which the contributions from graphene and CNFs enable electrodes with improved electrical conductivity, better ability to trap soluble the polysulfides intermediate and accommodate volume expansion/shrinkage of sulfur during repeated charge/discharge cycles