Advanced Carbon Materials Based Electrodes for High Performance Symmetric Supercapacitors

Supercapacitors have received considerable attention due to their high energy density, long life time, rapid charge/discharge rate, and because they are environmental friendly technology. Electrode materials play a key role in the final performance of supercapacitors. Carbon, usually used as symmetric supercapacitors electrode materials, exhibit extraordinary stability in harsh electrolyte and electrochemical performance owing to its physical and chemical properties. In addition, porous structure originated from activation, excellent electrical conductivity, sustainability, wide availability and low cost further offering the improvement in electrochemical performance and make it to be a promising electrode material for symmetric supercapacitors. Considering the factors affected electrochemical performance, such as surface area, pore size and functional groups, several carbon materials are developed and used as high performance symmetric supercapacitors electrodes. In this work, porous carbon derived from different biomass are prepared via catalytic graphitization, KOH activation, and plasma treatment. Depending on detailed physical characterizations, porous structure with high surface area and porosity are presented for prepared carbon materials, which serves as reservoir and ion channels for electrolyte and consequently resulted in high capacitance performance for assembled supercapacitors evaluated by electrochemical measurements

Analysis and mitigation of dead time harmonics in the single-phase full-bridge PWM converters with repetitive controllers

In order to prevent the power switching devices (e.g., the Insulated-Gate-Bipolar-Transistor, IGBT) from shoot through in voltage source converters during a switching period, the dead time is added either in the hardware driver circuits of the IGBTs or implemented in software in Pulse-Width Modulation (PWM) schemes. Both solutions will contribute to a degradation of the injected current quality. As a consequence, the harmonics induced by the dead time (referred to as "dead time harmonics" hereafter) have to be compensated in order to achieve a satisfactory current quality as required by standards. In this paper, the emission mechanism of dead time harmonics in single-phase PWM inverters is thus presented considering the modulation schemes in details. More importantly, a repetitive controller has been adopted to eliminate the dead time effect in single-phase grid-connected PWM converters. The repetitive controller has been plugged into a proportional resonant-based fundamental current controller so as to mitigate the dead time harmonics and also maintain the control of the fundamental frequency grid current in terms of dynamics. Simulations and experiments are provided, which confirm that the repetitive controller can effectively compensate the dead time harmonics and other low-order distortions, and also it is a simple method without hardware modifications

Tightly bound excitons in monolayer WSe2

Exciton binding energy and excited states in monolayers of tungsten diselenide (WSe2) are investigated using the combined linear absorption and two-photon photoluminescence excitation spectroscopy. The exciton binding energy is determined to be 0.37eV, which is about an order of magnitude larger than that in III-V semiconductor quantum wells and renders the exciton excited states observable even at room temperature. The exciton excitation spectrum with both experimentally determined one- and two-photon active states is distinct from the simple two-dimensional (2D) hydrogenic model. This result reveals significantly reduced and nonlocal dielectric screening of Coulomb interactions in 2D semiconductors. The observed large exciton binding energy will also have a significant impact on next-generation photonics and optoelectronics applications based on 2D atomic crystals.Comment: 19 pages, 4 figures, to appear in PR

Fractional-Order Time Delay Compensation in Deadbeat Control for Power Converters

Deadbeat control scheme is widely implemented in the control of power electronics and electrical drives, which is of simplification, rapidity and flexibility. However, owing to its sensitive to model uncertainties and unmodeled dynamics, the practical control performance is severely degraded and sometimes even unstable. Uncertain time delay is a typical case of model uncertainties, which severely deteriorates the control accuracy and dramatically reduce the system stability margin of deadbeat control. In this paper, the time delay effects on the control performance and system stability are investigated. A fractional-order Smith Predictor based solution is proposed to compensate arbitrary time delay with high accuracy, simple structure, and good robustness. The composite control scheme offers accurate time delay compensations in digital implementation and considerably enhances the robustness of the control system, which will effectively promote widespread applications of the deadbeat scheme. An application example of three-phase inverter system is explored to comprehensively illustrate the feasibility and effectiveness of the proposed scheme

Palygorskite hybridized carbon nanocomposite as a high-performance electrocatalyst support for formic acid oxidation

A nanocomposite, in which acid-treated palygorskite was hybridized with carbon, was prepared and designed as an efficient support for electrocatatlysts. Pd nanoparticles were deposited on the hybrid support as an electrocatalyst for formic acid oxidation. The hybrid supports and electrocatalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS).TEMandXRDresults showed that acid treatment had an effective impact on the morphology of palygorskite, but did not destroy its architecture. XPS results indicated that the introduction of palygorskite resulted in a negative shift of binding energy of Pd deposited on it. The electrochemical results showed that the addition of palygorskite into the carbon facilitated the formation ofOHads orOads on the surface of Pd/C-PLS, and further improved the formic acid electrooxidation activity. Therefore, considerable improvements in electrocatalytic activity toward formic acid oxidation was achieved by using this hybrid support when compared with conventional carbon support, suggesting that the introduction of SiO2-based porous palygoriskite was an excellent and cost-efficient way to improve the electrocatalytic performance of carbon support.Web of Scienc

Virtual variable sampling discrete fourier transform based selective odd-order harmonic repetitive control of DC/AC converters

This paper proposes a frequency adaptive discrete Fourier transform (DFT) based repetitive control (RC) scheme for DC/AC converters. By generating infinite magnitude on the interested harmonics, the DFT-based RC offers a selective harmonic scheme to eliminate waveform distortion. The traditional DFT-based selective harmonic RC, however, is sensitive to frequency fluctuation since even very small frequency fluctuation leads to a severe magnitude decrease. To address the problem, virtual variable sampling method, which creates an adjustable virtual delay unit to closely approximate a variable sampling delay, is proposed to enable the DFT-based selective harmonic RC to be frequency adaptive. Moreover, a selective odd-order harmonic DFT filter is developed to deal with the dominant odd order harmonic. Because it halves the number of sampling delays in the DFT filter, the system transient response gets nearly 50% improvement. A comprehensive series of experiments of the proposed VVS DFT-based selective odd-order harmonic RC controlled programmable AC power source under frequency variations are presented to verify the effectiveness of the proposed method

LncRNA-p21 alters the antiandrogen enzalutamide-induced prostate cancer neuroendocrine differentiation via modulating the EZH2/STAT3 signaling

While the antiandrogen enzalutamide (Enz) extends the castration resistant prostate cancer (CRPC) patients' survival an extra 4.8 months, it might also result in some adverse effects via inducing the neuroendocrine differentiation (NED). Here we found that lncRNA-p21 is highly expressed in the NEPC patients derived xenograft tissues (NEPC-PDX). Results from cell lines and human clinical sample surveys also revealed that lncRNA-p21 expression is up-regulated in NEPC and Enz treatment could increase the lncRNA-p21 to induce the NED. Mechanism dissection revealed that Enz could promote the lncRNA-p21 transcription via altering the androgen receptor (AR) binding to different androgen-response-elements, which switch the EZH2 function from histone-methyltransferase to non-histone methyltransferase, consequently methylating the STAT3 to promote the NED. Preclinical studies using the PDX mouse model proved that EZH2 inhibitor could block the Enz-induced NED. Together, these results suggest targeting the Enz/AR/lncRNA-p21/EZH2/STAT3 signaling may help urologists to develop a treatment for better suppression of the human CRPC progression