Advances of Strategies to Increase the Surface Charge Density of Triboelectric Nanogenerators: A Review.

Triboelectric nanogenerators (TENGs) have manifested a remarkable potential for harvesting environmental energy and have the prospects to be utilized for various uses, for instance, self-powered sensing devices, flexible wearables, and marine corrosion protection. However, the potential for further development of TENGs is restricted on account of their low output power that in turn is determined by their surface charge density. The current review majorly focuses on the selection and optimization of triboelectric materials. Subsequently, various methods capable of enhancing the surface charge density of TENGs, including environmental regulation, charge excitation, charge pumping, electrostatic breakdown, charge trapping, and liquid-solid structure are comprehensively reviewed. Lastly, the review is concluded by highlighting the existing challenges in enhancing the surface charge density of TENGs and exploring potential opportunities for future research endeavors in this area.

Improving the Surface Oxygen Vacancy Concentration of Bi2O4 through the Pretreatment of the NaBiO3·2H2O Precursor as a High-Performance Visible Light Photocatalyst.

The oxygen-deficient bismuth oxide, Bi2O4, synthesized by a typical hydrothermal method using commercial NaBiO3·2H2O as a raw material only has a relatively low concentration of surface oxygen vacancies (OVs). How to improve the visible light photocatalytic performance of Bi2O4 via tuning its surface OV concentration is still a huge challenge. In this study, improving the surface OVs of Bi2O4 was successfully realized through the pretreatment of commercial NaBiO3·2H2O, including thermal treatment in air and hydrothermal treatment in 10 M NaOH solution, forming NaBiO3·xH2O intermediate products first, and then hydrothermal preparation of Bi2O4 target products using NaBiO3·xH2O instead of commercial NaBiO3·2H2O as the precursor. The enhanced surface OV content not only narrows the band gap of Bi2O4 and thus extends its optical response range but also captures more photoexcited electrons and thus increases the charge carriers' separation efficiency and prolongs the charge carriers' lifetime of Bi2O4. Among the above-mentioned two pretreatment methods, the effects of the hydrothermal pretreatment are superior to those of the thermal treatment, involving the increase of surface OVs, the optical harvesting capacity, and the charge carriers' separation efficiency. Accordingly, Bi2O4 prepared by the hydrothermal pretreatment route exhibits the optimal visible light catalytic performance toward the removal of methyl orange (MO) and phenol due to its most abundant surface OV concentration, which is 2.59 times and 4.26 times higher than that of Bi2O4 synthesized directly by the commercial NaBiO3·2H2O route, respectively. Holes (h+) and superoxide radicals (•O2-) are identified as the main active species, while singlet oxygen (1O2) and hydroxyl radicals (•OH) are verified as the second and third important active species for organic pollutant removal, respectively. This work has developed a novel strategy to promote the catalytic performance of single Bi2O4 induced by the enhanced surface OV concentration through the pretreatment of the precursor, commercial NaBiO3·2H2O.

An Energy Harvester Coupled with a Triboelectric Mechanism and Electrostatic Mechanism for Biomechanical Energy Harvesting.

Energy-harvesting devices based on a single energy conversion mechanism generally have a low output and low conversion efficiency. To solve this problem, an energy harvester coupled with a triboelectric mechanism and electrostatic mechanism for biomechanical energy harvesting is presented. The output performances of the device coupled with a triboelectric mechanism and electrostatic mechanism were systematically studied through principle analysis, simulation, and experimental demonstration. Experiments showed that the output performance of the device was greatly improved by coupling the electrostatic induction mechanisms, and a sustainable and enhanced peak power of approximately 289 µW was produced when the external impedance was 100 MΩ, which gave over a 46-fold enhancement to the conventional single triboelectric conversion mechanism. Moreover, it showed higher resolution for motion states compared with the conventional triboelectric nanogenerator, and can precisely and constantly monitor and distinguish various motion states, including stepping, walking, running, and jumping. Furthermore, it can charge a capacitor of 10 µF to 3 V within 2 min and light up 16 LEDs. On this basis, a self-powered access control system, based on gait recognition, was successfully demonstrated. This work proposes a novel and cost-effective method for biomechanical energy harvesting, which provides a more convenient choice for human motion status monitoring and can be widely used in personnel identification systems.

Design on a Novel Titanium Dioxide Irregularly Distributed Bragg Reflector for Thin Film Silicon Solar Cells.

Titanium dioxide, which leads an excellent optical performance, is proposed to design irregularly distributed Bragg reflector (IDBR) through theoretical simulation as well as experimental verification. Firstly, a primary distributed Bragg reflector (DBR) model with the titanium dioxide serving as low reflection layer in, and amorphous silicon as high reflection layer is analyzed. The titanium dioxide DBR shows much enhanced reflection bandwidth relative to the DBR with silicon dioxide. A further study suggests that a traditional titanium dioxide IDBR demonstrate much enhanced performance versus the silicon dioxide IDBR with similar structure. Besides, the reflection bandwidth of the IDBR, especially in the high wavelength range, is dramatically promoted with respect to the DBR. Finally, a novel gradient IDBR model is developed. The simulation results reveal a higher reflection bandwidth of the titanium dioxide gradient IDBR than the silicon dioxide one. The reflectance of the titanium dioxide gradient IDBR is up to 90% in a range by 300 to 1450 nm. And, the reflection bandwidth of the gradient IDBR is much improved respect to the traditional IDBR. It seems that the titanium dioxide gradient IDBR could be an efficient selection for the thin film silicon solar cells. Finally, the gradient IDBR were fabricated via plasma enhanced chemical vapor deposition (PECVD) on a silicon wafer. A further test demonstrates a reflectance over 95% in the range from 400 to 1400 nm, and verifies the simulation results.

Inverted Planar Perovskite Solar Cells Based on NiOx Nano Film with Enhanced Efficiency and Stability.

The organometal halide perovskite (OHP) materials have attracted much attention throughout the world due to their superb optoelectronic properties. Tremendous progress has been made in the OHP based solar cells with increased efficiency from 3.8% to 24.2% within the last decade, benefiting from efforts in the photovoltaic field. However, all the OHP solar cells with highest efficient are based on a normal mesoporous structure with TiO2 at the bottom, which needs high temperature process. The inverted planar structure OHP solar cells based on PEDOT:PSS suffer from low efficiency (lower than 15%) and inferior stability due to degradation of PEDOT:PSS in ambient air. Herein, we employed sol-gel method to fabricate a NiOx nano film as the hole transporting layer for inverted OHP solar cells. The device performance based on PEDOT:PSS and NiOx were systematically investigated. It was found that the perovskite films on NiOx film had larger grain size and thus lower defects) density. The Capacitance-Voltage measurement indicated that the device based on NiOx exhibited larger built-in potential, which significantly enhanced the open-circuit potential of the OHP solar cells. Furthermore, the solar cell based on NiOx nano film exhibited excellent stability compared with the PEDOT:PSS based device, due to robust property of NiOx in ambient air.

An Accurate and Efficient Time Delay Estimation Method of Ultra-High Frequency Signals for Partial Discharge Localization in Substations.

Partial discharge (PD) localization in substations based on the ultra-high frequency (UHF) method can be used to efficiently assess insulation conditions. Localization accuracy is affected by the accuracy of the time delay (TD) estimation, which is critical for PD localization in substations. A review of existing TD estimation methods indicates that there is a need to develop methods that are both accurate and computationally efficient. In this paper, a novel TD estimation method is proposed to improve both accuracy and efficiency. The TD is calculated using an improved cross-correlation algorithm based on full-wavefronts of array UHF signals, which are extracted using the minimum cumulative energy method and zero-crossing points searching methods. The cross-correlation algorithm effectively suppresses the TD error caused by differences between full-wavefronts. To verify the method, a simulated PD source test in a laboratory and a field test in a 220 kV substation were carried out. The results show that the proposed method is accurate even in case of low signal-to-noise ratio, but with greatly improved computational efficiency.