Quantifying and understanding the triboelectric series of inorganic non-metallic materials

Metal-support interactions in metal oxide-supported atomic, cluster, and nanoparticle catalysis

Supported metal catalysts are essential to a plethora of processes in the chemical industry. The overall performance of these catalysts depends strongly on the interaction of adsorbates at the atomic level, which can be manipulated and controlled by the different constituents of the active material (i.e., support and active metal). The description of catalyst activity and the relationship between active constituent and the support, or metal-support interactions (MSI), in heterogeneous (thermo)catalysts is a complex phenomenon with multivariate (dependent and independent) contributions that are difficult to disentangle, both experimentally and theoretically. So-called "strong metal-support interactions" have been reported for several decades and summarized in excellent review articles. However, in recent years, there has been a proliferation of new findings related to atomically dispersed metal sites, metal oxide defects, and, for example, the generation and evolution of MSI under reaction conditions, which has led to the designation of (sub)classifications of MSI deserving to be critically and systematically evaluated. These include dynamic restructuring under alternating redox and reaction conditions, adsorbate-induced MSI, and evidence of strong interactions in oxide-supported metal oxide catalysts. Here, we review recent literature on MSI in oxide-supported metal particles to provide an up-to-date understanding of the underlying physicochemical principles that dominate the observed effects in supported metal atomic, cluster, and nanoparticle catalysts. Critical evaluation of different subclassifications of MSI is provided, along with discussions on the formation mechanisms, theoretical and characterization advances, and tuning strategies to manipulate catalytic reaction performance. We also provide a perspective on the future of the field, and we discuss the analysis of different MSI effects on catalysis quantitatively.

Strategies to Improve the Output Performance of Triboelectric Nanogenerators

Triboelectric nanogenerators (TENGs) can collect and convert random mechanical energy into electric energy, with remarkable advantages including broadly available materials, straightforward preparation, and multiple applications. Over the years, researchers have made substantial advancements in the theoretical and practical aspects of TENG. Nevertheless, the pivotal challenge in realizing full applications of TENG lies in ensuring that the generated output meets the specific application requirements. Consequently, substantial research is dedicated to exploring methods and mechanisms for enhancing the output performance of TENG devices. This review aims to comprehensively examine the influencing factors and corresponding improvement strategies of the output performance based on the contact electrification mechanism and operational principles that underlie TENG technology. This review primarily delves into five key areas of improvement: materials selection, surface modification, component adjustments, structural optimization, and electrode enhancements. These aspects are crucial in tailoring TENG devices to meet the desired performance metrics for various applications.

Triboelectric Nanogenerator Made with Stretchable, Antibacterial Hydrogel Electrodes for Biomechanical Sensing

Triboelectric nanogenerators (TENGs) have attracted widespread attention as a promising candidate for energy harvesting due to their flexibility and high power density. To meet diverse application scenarios, a highly stretchable (349%), conductive (1.87 S m-1), and antibacterial electrode composed of carbon quantum dots/LiCl/agar-polyacrylamide (CQDs/LiCl/agar-PAAm) dual-network (DN) hydrogel is developed for wearable TENGs. Notably, the concentration of agar alters the pore spacing and pore size of the DN hydrogel, thereby impacting the network cross-linking density and the migration of conductive ions (Li+ and Cl-). This variation further affects the mechanical strength and conductivity of the hydrogel electrode, thus modulating the mechanical stability and electrical output performance of the TENGs. With the optimal agar content, the tensile strength and conductivity of the hydrogel electrode increase by 211 and 719%, respectively. This enhancement ensures the stable output of TENGs during continuous operation (6000 cycles), with open-circuit voltage, short-circuit current, and transferred charge increasing by 200, 530, and 155%, respectively. Additionally, doping with CQDs enables the hydrogel electrode to effectively inhibit the Gram-negative bacterium Escherichia coli. Finally, the TENGs are utilized as a self-power smart ring for efficient and concise information transmission via Morse code. Consequently, this study introduces a creative approach for designing and implementing multifunctional, flexible wearable devices.

A review of material design for high performance triboelectric nanogenerators: performance improvement based on charge generation and charge loss

As a type of innovative device, triboelectric nanogenerators (TENGs) are capable of converting mechanical energy into electrical energy through the triboelectric effect. Based on the working mechanism, the output performance of TENGs heavily relies on the triboelectric materials used. The modification of triboelectric materials is the most efficient way to improve the output performance of TENGs. Herein, this review focuses on the recent progress in triboelectric material design for high-performance TENGs. First, the basic theory of TENGs is introduced. Second, the relationship between the triboelectric materials and the output performance of TENGs is summarized in detail based on a theoretical model of the triboelectric charge dynamic equilibrium. Furthermore, the relevant strategies are analyzed in detail. Finally, challenges and shortcomings of the triboelectric materials for high-performance TENGs are discussed. This review provides a basis for the research status and future development of triboelectric materials.

Electrostatic energy-driven contact electrification mechanism from the ReaxFF molecular dynamics perspective

Understanding the underlying principles of contact electrification is critical for more efficient triboelectric nanogenerator (TENG) development. Herein, we use ReaxFF molecular dynamics simulations in conjunction with a charge equilibration method to investigate the contact electrification mechanism in polyisoprene (PI), a natural rubber polymer, when it comes into contact with copper (Cu) and polytetrafluoroethylene (PTFE). The simulations reveal that the charge transfer directions in the PI/Cu and PI/PTFE contact models are opposite, and the amount of charge transfer in the former is substantially less than that in the latter, which are consistent with our TENG measurements. Contact electrification is revealed to be a spontaneous process that occurs to lower electrostatic energy, and the electrostatic energy released during contact electrification of PI/PTFE is greater than that of PI/Cu, which can be correlated with the relative strength of triboelectric charging observed for the two systems. A compression simulation of the PI/Cu contact model reveals that the quantity of charge transfer grows exponentially as compressive strain increases. Despite increasing the total energy of the system due to densification and distortion of the polymer structure, the applied deformation results in an energetically more stable electrostatic arrangement. We also find that the incorporation of a carbonaceous material into a polyisoprene matrix causes a faster increase in the amount of charge transfer with compressive strain, which is governed by a steeper electrostatic energy profile. This study provides an alternative perspective on the contact electrification mechanism, which could be beneficial for the development of energy harvesting devices.

Development of flexible high-performance PDMS-based triboelectric nanogenerator using nanogratings

This article investigates the performance of a contact-mode Triboelectric Nanogenerator (TENG) utilizing polydimethylsiloxane (PDMS) with nano gratings as a dielectric in a metal-dielectric configuration. The evaluation encompasses the impact of gratings, tapping frequency, various load conditions, and contact area on the TENG performance. The fabrication involves spin-coating PDMS onto a master mold to create the device. Experimental measurements reveal a significant enhancement of 97% in open-circuit voltage by introducing gratings on PDMS. Furthermore, as the tapping frequency increases from 1 Hz to 3 Hz, there is a corresponding rise of 108% in output voltage. The influence of load resistance on TENG output performance demonstrates its ability to drive different loads efficiently. Moreover, enlarging the contact area of the device substantially increases the open-circuit voltage. A device with a 400 mm2 contact area can generate a voltage of 80 V at a low frequency of 3 Hz, indicating the importance of considering device size and contact area for specific applications. A practical circuit integrating a TENG with a full-wave bridge rectifier demonstrates energy harvesting capabilities by successfully illuminating a light-emitting diode (LED) and charging various capacitors. The fabricated devices exhibit better performance along with a cost-effective and easy fabrication process.

Flexoelectricity, Triboelectricity, and Free Interfacial Charges

Triboelectricity has been a topic of some confusion for many years, probably because it is very diverse and some of the fundamental science has not been clear. This is now starting to change. A few years ago, the importance of flexoelectricity at asperities is pointed out. That paper exploited the established physics of compensation of bound surface or interfacial charges without going into detail. The purpose of this paper is to expand further on this, mapping from the established physics of electrostatics with contact potentials and Maxwell's displacement field to the underlying fundamentals of charge transfer in triboelectricity. Examples from the published literature are used to illustrate this. In the discussion, some of the open questions and challenges to the community are mentioned.

Tailoring morphological and chemical contributions of nanoscale charge transfer for enhanced triboelectric nanogenerators

Triboelectric devices, operating through contact electrification (CE) and electrostatic induction, have shown great promise in energy harvesting applications. However, optimizing charge transfer at the interface remains crucial for enhancing device performance. This study introduces a novel approach to harnessing CE by employing morphological and chemical modifications of polymers. Our strategy involves adjusting the elastomer base to curing agent ratio to fine-tune the chemical properties of polydimethylsiloxane (PDMS) and introducing morphological modifications through a peeling and flipping (P/F) process of PDMS off the Si-substrate. Unlike conventional methods, the P/F-method minimally alters the intrinsic properties of PDMS, creating nanoscale surface corrugations adiabatically. We explore the mechanical, tribological, and electrical properties of the surface at the nano-scale and demonstrate that our approach allows for precise control of energy dissipation and electric potential at the surface, thereby optimizing charge transfer. Furthermore, we show that using a plasma-treated Si-substrate can further increase device performance up to 80% without affecting other properties. This study presents a comprehensive strategy for fine-tuning CE to enhance the performance of triboelectric nanogenerators.

Wearable Smart Silicone Belt for Human Motion Monitoring and Power Generation

Human physical activity monitoring plays a crucial role in promoting personalized health management. In this work, inspired by an ancient Chinese belt, a belt-type wearable sensor (BWS) based on a triboelectric nanogenerator (TENG) is presented to monitor daily movements and collect the body motion mechanical energy. The developed BWS consists of a soft silicone sheet and systematically connected sensing units made from triboelectric polymer materials including polytetrafluoroethylene (PTFE) and polyamide (PA). A parameter study of the sensing units is firstly conducted to optimize the structure of BWS. The experimental studies indicate that the parameter-optimized BWS unit achieves a maximum output voltage of 47 V and a maximum current of 0.17 μA. A BWS with five sensing units is manufactured to record body movements, and it is able to distinguish different physical activities including stillness, walking, running, jumping, normal breathing, cessation of breathing, and deep breathing. In addition, the developed BWS successfully powers electronic devices including a smartphone, digital watch, and LED lights. We hope this work provides a new strategy for the development of wearable self-powered intelligent devices.

The Interaction and Lift-Off Forces of an Atomic Force Microscope Tip from Single Fibers Extracted from Protective Clothing Fabric

The widespread use of engineered nanoparticles (ENPs) poses a potential health hazard to humans, especially to those involved in either nanoparticle manufacturing or the usage and assembly of a final product. In this study, we performed systematic force vs distance experiments (F(z)) using an atomic force microscope (AFM) on fibers commonly used in street clothing and protective laboratory clothing to better characterize the relevant interaction forces between engineered nanoparticles (ENPs) and the contacted fabric fibers. The intent of this study is to identify those factors that influence the interaction of ENPs with fabrics with an aim to improve the efficacy of protective clothing against ENP exposure and mitigate potential health risks. A ∼14 nm diameter AFM SiOx tip (with nanoscale radius of curvature) is considered as an effective oxide ENP. Features present (or absent) in a well-executed F(z) AFM experiment provide a fingerprint that distinguishes the relevant forces and interaction mechanisms in play. Measurements of F(z) as a function of relative humidity were also performed to assess the importance of thin surface water layers in binding nanometer-size oxide ENPs to a fabric fiber. The F(z) data indicate the dominant mechanism for adhesion of the oxide tip to the various fabric fibers (cotton, Tyvek (HD polyethylene), polypropylene, and polyester) can be attributed to a van der Waals interaction. The analysis provides no evidence for long-range electrostatic forces or capillary-induced adhesion of the AFM tip to the fibers studied.

Bionic e-skin with precise multi-directional droplet sliding sensing for enhanced robotic perception

Electronic skins with deep and comprehensive liquid information detection are desired to endow intelligent robotic devices with augmented perception and autonomous regulation in common droplet environments. At present, one technical limitation of electronic skins is the inability to perceive the liquid sliding information as realistically as humans and give feedback in time. To this critical challenge, in this work, a self-powered bionic droplet electronic skin is proposed by constructing an ingenious co-layer interlaced electrode network and using an overpass connection method. The bionic skin is used for droplet environment reconnaissance and converts various dynamic droplet sliding behaviors into electrical signals based on triboelectricity. More importantly, the two-dimensional sliding behavior of liquid droplets is comprehensively perceived by the e-skin and visually fed back in real-time on an indicator. Furthermore, the flow direction warning and intelligent closed-loop control of water leakage are also achieved by this e-skin, achieving the effect of human neuromodulation. This strategy compensates for the limitations of e-skin sensing droplets and greatly narrows the gap between artificial e-skins and human skins in perceiving functions.

Visualization and standardized quantification of surface charge density for triboelectric materials

Triboelectric nanogenerator (TENG) operates on the principle of utilizing contact electrification and electrostatic induction. However, visualization and standardized quantification of surface charges for triboelectric materials remain challenging. Here, we report a surface charge visualization and standardized quantification method using electrostatic surface potential measured by Kevin probe and the iterative regularization strategy. Moreover, a tuning strategy on surface charge is demonstrated based on the corona discharge with a three-electrode design. The long-term stability and dissipation mechanisms of the injected negative or positive charges demonstrate high dependence on deep carrier traps in triboelectric materials. Typically, we achieved a 70-fold enhancement on the output voltage (~135.7 V) for the identical polytetrafluoroethylene (PTFE) based TENG (neg-PTFE/PTFE or posi-PTFE/PTFE triboelectric pair) with stable surface charge density (5% decay after 140 days). The charged PTFE was demonstrated as a robot e-skins for non-contact perception of object geometrics. This work provides valuable tools for surface charge visualization and quantification, giving a new strategy for a deeper understanding of contact electrification.

Triboelectric Nanogenerators: State of the Art

The triboelectric nanogenerator (TENG), as a novel energy harvesting technology, has garnered widespread attention. As a relatively young field in nanogenerator research, investigations into various aspects of the TENG are still ongoing. This review summarizes the development and dissemination of the fundamental principles of triboelectricity generation. It outlines the evolution of triboelectricity principles, ranging from the fabrication of the first TENG to the selection of triboelectric materials and the confirmation of the electron cloud overlapping model. Furthermore, recent advancements in TENG application scenarios are discussed from four perspectives, along with the research progress in performance optimization through three primary approaches, highlighting their respective strengths and limitations. Finally, the paper addresses the major challenges hindering the practical application and widespread adoption of TENGs, while also providing insights into future developments. With continued research on the TENG, it is expected that these challenges can be overcome, paving the way for its extensive utilization in various real-world scenarios.

Work Function-Guided Electrocatalyst Design

The development of high-performance electrocatalysts for energy conversion reactions is crucial for advancing global energy sustainability. The design of catalysts based on their electronic properties (e.g., work function) has gained significant attention recently. Although numerous reviews on electrocatalysis have been provided, no such reports on work function-guided electrocatalyst design are available. Herein, a comprehensive summary of the latest advancements in work function-guided electrocatalyst design for diverse electrochemical energy applications is provided. This includes the development of work function-based catalytic activity descriptors, and the design of both monolithic and heterostructural catalysts. The measurement of work function is first discussed and the applications of work function-based catalytic activity descriptors for various reactions are fully analyzed. Subsequently, the work function-regulated material-electrolyte interfacial electron transfer (IET) is employed for monolithic catalyst design, and methods for regulating the work function and optimizing the catalytic performance of catalysts are discussed. In addition, key strategies for tuning the work function-governed material-material IET in heterostructural catalyst design are examined. Finally, perspectives on work function determination, work function-based activity descriptors, and catalyst design are put forward to guide future research. This work paves the way to the work function-guided rational design of efficient electrocatalysts for sustainable energy applications.

Directional Moisture-Wicking Triboelectric Materials Enabled by Laplace Pressure Differences

Wearable sensors are experiencing vibrant growth in the fields of health monitoring systems and human motion detection, with comfort becoming a significant research direction for wearable sensing devices. However, the weak moisture-wicking capability of sensor materials leads to liquid retention, severely restricting the comfort of the wearable sensors. This study employs a pattern-guided alignment strategy to construct microhill arrays, endowing triboelectric materials with directional moisture-wicking capability. Within 2.25 s, triboelectric materials can quickly and directionally remove the droplets, driven by the Laplace pressure differences and the wettability gradient. The directional moisture-wicking triboelectric materials exhibit excellent pressure sensing performance, enabling rapid response/recovery (29.1/37.0 ms), thereby achieving real-time online monitoring of human respiration and movement states. This work addresses the long-standing challenge of insufficient moisture-wicking driving force in flexible electronic sensing materials, holding significant implications for enhancing the comfort and application potential of electronic skin and wearable electronic devices.

Triboelectric-Responsive Drug Delivery Hydrogel for Accelerating Infected Wound Healing

Electrotherapy is of great interest in the field of tissue repair as an effective, well-tolerated, and noninvasive treatment. Triboelectric nanogenerator (TENG) has shown advantages in promoting wound healing due to its peak output characteristic and low Joule heating effect. However, it is limited in infected wound healing due to poor antimicrobial capacity. Here, a wearable triboelectric stimulator (WTS) is developed that consists of a flexible TENG (F-TENG) and a triboelectric-responsive drug delivery hydrogel (TR-DDH) for healing of bacterium-infected wounds. F-TENG can generate pulsed current to wounds by converting mechanical energy from body movements. Polypyrrole is prone to reduction and volume contraction under electrical stimulation, resulting in desorption of curcumin nanoparticles (CUR NPs) from the polypyrrole in TR-DDH. Therefore, the highly efficient and controllable release of CUR NPs can be achieved by triboelectric stimulation. According to the in vitro and in vivo experiments, WTS has the greatest antimicrobial effect and the fastest promotion of infected wound healing compared to treatment with electrical stimulation or curcumin. Finally, the safety assessment demonstrates that the WTS has excellent tissue safety for chronic wound healing. Synergistic therapy with WTS provides an efficient strategy for chronic wound healing and smart-responsive drug delivery systems.

Contact-electro-catalysis (CEC)

Contact-electro-catalysis (CEC) is an emerging field that utilizes electron transfer occurring at the liquid-solid and even liquid-liquid interfaces because of the contact-electrification effect to stimulate redox reactions. The energy source of CEC is external mechanical stimuli, and solids to be used are generally organic as well as in-organic materials even though they are chemically inert. CEC has rapidly garnered extensive attention and demonstrated its potential for both mechanistic research and practical applications of mechanocatalysis. This review aims to elucidate the fundamental principle, prominent features, and applications of CEC by compiling and analyzing the recent developments. In detail, the theoretical foundation for CEC, the methods for improving CEC, and the unique advantages of CEC have been discussed. Furthermore, we outline a roadmap for future research and development of CEC. We hope that this review will stimulate extensive studies in the chemistry community for investigating the CEC, a catalytic process in nature.

Free-Standing Electrode and Fixed Surface Tiny Electrode Implemented Triboelectric Nanogenerator with High Instantaneous Current

Conventional triboelectric nanogenerators (TENGs) face challenges pertaining to low output current density at low working frequencies and high internal impedance. While strategies, such as surface modification to enhance surface charge density, permittivity regulation of materials, and circuit management, have partially mitigated these issues. However, they have also resulted in increased complexity in the fabrication process. Therefore, there is an urgent demand for a universal and simplified approach to address these challenges. To fulfill this need, this work presents a free-standing electrode and fixed surface tiny electrode implemented triboelectric nanogenerator (FFI-TENG). It is fabricated by a straightforward yet effective method: introducing a tiny electrode onto the surface of the tribo-negative material. This approach yields substantial enhancements in performance, notably a more than tenfold increase in output current density, a reduction in effective working frequencies, and a decrease in matching resistance as compared to vertical contact-separation TENGs (CS-TENGs) or single-electrode TENGs (SE-TENGs). Simultaneously, a comprehensive examination and proposition regarding the operational mechanism of FFI-TENG, highlighting its extensive applicability are also offered. Significantly, FFI-TENG excels in mechanical energy harvesting even under ultra-low working frequencies (0.1 Hz), outperforming similar contact-separation models. This innovation positions it as a practical and efficient solution for the development of low-entropy energy harvesters.

Gross morphology and adhesion-associated physical properties of Drosophila larval salivary gland glue secretion

One of the major functions of the larval salivary glands (SGs) of many Drosophila species is to produce a massive secretion during puparium formation. This so-called proteinaceous glue is exocytosed into the centrally located lumen, and subsequently expectorated, serving as an adhesive to attach the puparial case to a solid substrate during metamorphosis. Although this was first described almost 70 years ago, a detailed description of the morphology and mechanical properties of the glue is largely missing. Its main known physical property is that it is released as a watery liquid that quickly hardens into a solid cement. Here, we provide a detailed morphological and topological analysis of the solidified glue. We demonstrated that it forms a distinctive enamel-like plaque that is composed of a central fingerprint surrounded by a cascade of laterally layered terraces. The solidifying glue rapidly produces crystals of KCl on these alluvial-like terraces. Since the properties of the glue affect the adhesion of the puparium to its substrate, and so can influence the success of metamorphosis, we evaluated over 80 different materials for their ability to adhere to the glue to determine which properties favor strong adhesion. We found that the alkaline Sgs-glue adheres strongly to wettable and positively charged surfaces but not to neutral or negatively charged and hydrophobic surfaces. Puparia formed on unfavored materials can be removed easily without leaving fingerprints or cascading terraces. For successful adhesion of the Sgs-glue, the material surface must display a specific type of triboelectric charge. Interestingly, the expectorated glue can move upwards against gravity on the surface of freshly formed puparia via specific, unique and novel anatomical structures present in the puparial's lateral abdominal segments that we have named bidentia.

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.

High Performance and Reprocessable In Situ Generated Nanofiber Reinforced Composites Based on Liquid Crystal Polyarylate

Polymers are playing important roles in the rapid development of triboelectric nanogenerators (TENGs); However, most polymers cannot meet the high requirements of thermomechanical performance; Thus, various polymeric composites are developed for triboelectric layer. These composites are hardly recycled since their reinforcements are unevenly distributed after reprocessing, which limits the sustainable development of TENGs. To solve the above challenges, in situ generated nanofiber reinforced composites (NFRCs) based on single-component liquid crystal polyarylate (LCP) are designed and prepared via a one-step polycondensation. Nonlinear naphthalene (NDA) widens the processing window of LCP without destabilizing the liquid crystal phase. The NDA-rich domains act as a matrix while the NDA-poor domains with higher rigidity form oriented nanofibers to achieve self-reinforcement. The resultant NFRCs possess high glass transition temperature (Tg > 220 °C) and storage modulus (E' = 0.1 GPa at 350 °C), which are far beyond existing triboelectric polymers, typically Tg < 110 °C and E' < 0.1 MPa (flowable) at 350 °C. Furthermore, NFRC-based TENG exhibits superior electrical output performance and retention rate (>90%) after reprocessing; Overall, this work offers a new design principle to prepare self-reinforced composites, which paves a way to explore high performance materials.

Charges Transfer in Interfaces for Energy Generating

Under the threat of energy crisis and environmental pollution, the technology for sustainable and clean energy extraction has received considerable attention. Owing to the intensive exploration of energy conversion strategies, expanded energy sources are successfully converted into electric energy, including mechanical energy from human motion, kinetic energy of falling raindrops, and thermal energy in the ambient. Among these energy conversion processes, charge transfer at different interfaces, such as solid-solid, solid-liquid, liquid-liquid, and gas-contained interfaces, dominates the power-generating efficiency. In this review, the mechanisms and applications of interfacial energy generators (IEGs) with different interface types are systematically summarized. Challenges and prospects are also highlighted. Due to the abundant interfacial interactions in nature, the development of IEGs offers a promising avenue of inexhaustible and environmental-friendly power generation to solve the energy crisis.

Quantifying the interfacial triboelectricity in inorganic-organic composite mechanoluminescent materials

Mechanoluminescence (ML) sensing technologies open up new opportunities for intelligent sensors, self-powered displays and wearable devices. However, the emission efficiency of ML materials reported so far still fails to meet the growing application requirements due to the insufficiently understood mechano-to-photon conversion mechanism. Herein, we propose to quantify the ability of different phases to gain or lose electrons under friction (defined as triboelectric series), and reveal that the inorganic-organic interfacial triboelectricity is a key factor in determining the ML in inorganic-organic composites. A positive correlation between the difference in triboelectric series and the ML intensity is established in a series of composites, and a 20-fold increase in ML intensity is finally obtained by selecting an appropriate inorganic-organic combination. The interfacial triboelectricity-regulated ML is further demonstrated in multi-interface systems that include an inorganic phosphor-organic matrix and organic matrix-force applicator interfaces, and again confirmed by self-oxidization and reduction of emission centers under continuous mechanical stimulus. This work not only gives direct experimental evidences for the underlying mechanism of ML, but also provides guidelines for rationally designing high-efficiency ML materials.

Advanced Dielectric Materials for Triboelectric Nanogenerators: Principles, Methods, and Applications

Triboelectric nanogenerator (TENG) manifests distinct advantages such as multiple structural selectivity, diverse selection of materials, environmental adaptability, low cost, and remarkable conversion efficiency, which becomes a promising technology for micro-nano energy harvesting and self-powered sensing. Tribo-dielectric materials are the fundamental and core components for high-performance TENGs. In particular, the charge generation, dissipation, storage, migration of the dielectrics, and dynamic equilibrium behaviors determine the overall performance. Herein, a comprehensive summary is presented to elucidate the dielectric charge transport mechanism and tribo-dielectric material modification principle toward high-performance TENGs. The contact electrification and charge transport mechanism of dielectric materials is started first, followed by introducing the basic principle and dielectric materials of TENGs. Subsequently, modification mechanisms and strategies for high-performance tribo-dielectric materials are highlighted regarding physical/chemical, surface/bulk, dielectric coupling, and structure optimization. Furthermore, representative applications of dielectric materials based TENGs as power sources, self-powered sensors are demonstrated. The existing challenges and promising potential opportunities for advanced tribo-dielectric materials are outlined, guiding the design, fabrication, and applications of tribo-dielectric materials.

Recent Progress of Solid-Liquid Interface-Mediated Contact-Electro-Catalysis

Contact electrification (CE) is a common physical process by which triboelectric charges are generated through the mutual contact between two objects. Despite the ongoing debates on CE's mechanism, recent advancements in technology have elucidated the primary role of electron transfer in most CE processes. This discovery leads to the spawning of an emerging field, known as contact-electro-catalysis (CEC), which utilizes the electron transfer phenomenon during CE to initiate CEC. In this work, we provide the first comprehensive review of the recent progress of the solid-liquid interface-mediated CEC process, including its working principles, relationship with surface science, recent breakthroughs in applications, and future challenges. We aim to provide fundamental guidance for researchers to understand the reaction mechanism of the CEC process and to propose potential pathways to enhance CEC efficiency from a surface and interfacial science perspective. Later, recent application scenarios using the novel CEC techniques are summarized, including wastewater treatment, efficient generation of hydrogen peroxide (H2O2), lithium-ion battery recycling, and CO2 reduction. In general, CEC technology has opened a new avenue for catalysis, effectively expanding the range of catalyst options and holding promise as a solution to a variety of complex catalytic challenges in the future.

A Review of Polymer-Based Environment-Induced Nanogenerators: Power Generation Performance and Polymer Material Manipulations

Natural environment hosts a considerable amount of accessible energy, comprising mechanical, thermal, and chemical potentials. Environment-induced nanogenerators are nanomaterial-based electronic chips that capture environmental energy and convert it into electricity in an environmentally friendly way. Polymers, characterized by their superior flexibility, lightweight, and ease of processing, are considered viable materials. In this paper, a thorough review and comparison of various polymer-based nanogenerators were provided, focusing on their power generation principles, key materials, power density and stability, and performance modulation methods. The latest developed nanogenerators mainly include triboelectric nanogenerators (TriboENG), piezoelectric nanogenerators (PENG), thermoelectric nanogenerators (ThermoENG), osmotic power nanogenerator (OPNG), and moist-electric generators (MENG). Potential practical applications of polymer-based nanogenerator were also summarized. The review found that polymer nanogenerators can harness a variety of energy sources, with the basic power generation mechanism centered on displacement/conduction currents induced by dipole/ion polarization, due to the non-uniform distribution of physical fields within the polymers. The performance enhancement should mainly start from strengthening the ion mobility and positive/negative ion separation in polymer materials. The development of ionic hydrogel and hydrogel matrix composites is promising for future nanogenerators and can also enable multi-energy collaborative power generation. In addition, enhancing the uneven distribution of temperature, concentration, and pressure induced by surrounding environment within polymer materials can also effectively improve output performance. Finally, the challenges faced by polymer-based nanogenerators and directions for future development were prospected.

Energy-efficient electronics with an air-friction-driven rotating gate transistor using tribotronics

Concern for the environment is one of the main factors that are increasing the demand for compact and energy-efficient electronic devices. Recent research has made advances in reducing the power consumption of field-effect transistors, including the use of high-dielectric insulators, low-voltage operation, and selective power-conservation strategies. This paper introduces a revolutionary air-friction-driven rotating gate transistor that operates without the need for a conventional gate voltage. This new device offers the advantages of wear resistance, a slim and flexible design (achieved through low-temperature solution processing), and a simplified three-layer structure that streamlines manufacturing and reduces potential carbon emissions. This device's wear resistance and ease of fabrication render the device a promising technology with applications in various fields, including electronics, vehicles, aviation, and wearable devices. This study provides evidence of the device's feasibility for use in real-world vehicular scenarios, underscoring its potential for future innovation and widespread adoption.

Enhanced tribocatalytic degradation performance of organic pollutants by Cu1.8S/CuCo2S4 p-n junction

Tribocatalysis research, leveraging the triboelectric effect, presents significant potential for environmental water pollution control. However, there is a notable scarcity of studies pertaining to tribocatalysis involving heterojunctions, particularly in the context of p-n junction tribocatalysis. In this study, we employed a one-step solvothermal method to synthesize a Cu1.8S/CuCo2S4 p-n junction composite catalyst. Subsequently, we explored the tribocatalytic degradation performance of organic pollutants facilitated by the Cu1.8S/CuCo2S4 catalyst. The findings reveal that, under simple magnetic stirring conditions, the degradation rates achieved by the Cu1.8S/CuCo2S4 catalyst for tetracycline (TC), methylene blue (MB), and methyl orange (MO) are remarkably high, reaching 99.9 %, 99.7 %, and 94.0 %, respectively. This underscores the broad applicability of the Cu1.8S/CuCo2S4 catalyst for the tribocatalytic degradation of diverse organic pollutants. Experimental evidence establishes that friction occurring between the polytetrafluoroethylene (PTFE) magnet rod, the beaker, and the catalyst induces charge transfer at their interfaces, generating highly oxidized active species that effectively decompose pollutants. Through free radical capture and electron spin resonance (ESR) tests, it was empirically determined and validated that the principal active species involved in tribocatalytic degradation are holes (h+) and superoxide radicals (O2-). Incorporating insights from the experimental characterization of p-n junctions and density functional theory (DFT) theoretical calculations, we propose a plausible tribocatalytic mechanism for Cu1.8S/CuCo2S4. This research not only contributes novel findings but also serves as a reference for the exploration of innovative heterojunction tribocatalysts.

A contact-electro-catalysis process for producing reactive oxygen species by ball milling of triboelectric materials

Ball milling is a representative mechanochemical strategy that uses the mechanical agitation-induced effects, defects, or extreme conditions to activate substrates. Here, we demonstrate that ball grinding could bring about contact-electro-catalysis (CEC) by using inert and conventional triboelectric materials. Exemplified by a liquid-assisted-grinding setup involving polytetrafluoroethylene (PTFE), reactive oxygen species (ROS) are produced, despite PTFE being generally considered as catalytically inert. The formation of ROS occurs with various polymers, such as polydimethylsiloxane (PDMS) and polypropylene (PP), and the amount of generated ROS aligns well with the polymers' contact-electrification abilities. It is suggested that mechanical collision not only maximizes the overlap in electron wave functions across the interface, but also excites phonons that provide the energy for electron transition. We expect the utilization of triboelectric materials and their derived CEC could lead to a field of ball milling-assisted mechanochemistry using any universal triboelectric materials under mild conditions.

Interfacial Triboelectricity Lights Up Phosphor-Polymer Elastic Composites: Unraveling the Mechanism of Mechanoluminescence in Zinc Sulfide Microparticle-Embedded Polydimethylsiloxane Films

Composites comprising copper-doped zinc sulfide phosphor microparticles embedded in polydimethylsiloxane (ZnS:Cu-PDMS) have received significant attention over the past decade because of their bright and durable mechanoluminescence (ML); however, the underlying mechanism of this unique ML remains unclear. This study reports empirical and theoretical findings that confirm this ML is an electroluminescence (EL) of the ZnS:Cu phosphor induced by the triboelectricity generated at the ZnS:Cu microparticle-PDMS matrix interface. ZnS:Cu microparticles that exhibit bright ML are coated with alumina, an oxide with strong positive triboelectric properties; the contact separation between this oxide coating and PDMS, a polymer with strong negative triboelectric properties, produces sufficient interfacial triboelectricity to induce EL in ZnS:Cu microparticles. The ML of ZnS:Cu-PDMS composites varies on changing the coating material, exhibiting an intensity that is proportional to the amount of interfacial triboelectricity generated in the system. Finally, based on these findings, a mechanism that explains the ML of phosphor-polymer elastic composites (interfacial triboelectric field-driven alternating-current EL model) is proposed in this study. It is believed that understanding this mechanism will enable the development of new materials (beyond ZnS:Cu-PDMS systems) with bright and durable ML.

Construction of Triboelectric Series and Chirality Detection of Amino Acids Using Triboelectric Nanogenerator

Triboelectrification necessitates a frictional interaction between two materials, and their contact electrification is characteristically based on the polarity variance in the triboelectric series. Utilizing this fundamental advantage of the triboelectric phenomenon, different materials can be identified according to their contact electrification capability. Herein, an in-depth analysis of the amino acids present in the stratum corneum of human skin is performed and these are quantified regarding triboelectric polarization. The principal focus of this study lies in analyzing and identifying the amino acids present in copious amounts in the stratum corneum to explain their positive behavior during the contact electrification process. Thus, an augmented triboelectric series of amino acids with quantified triboelectric charging polarity by scrutinizing the transfer charge, work function, and atomic percentage is presented. Furthermore, the chirality of aspartic acid as it is most susceptible to racemization with clear consequences on the human skin is detected. The study is expected to accelerate research exploiting triboelectrification and provide valuable information on the surface properties and biological activities of these important biomolecules.

Electrospun PVDF/aromatic HBP of 4th gen based flexible and self-powered TENG for wearable energy harvesting and health monitoring

In recent times, high-performance wearable electronic devices that can transform mechanical force into electrical energy for biomedical monitoring applications are receiving an increasing amount of attention. In the present study, we focused on a flexible, self-powered and wearable triboelectric nanogenerator (TENG) based on electrospun polyvinylidene fluoride (PVDF)/aromatic hyperbranched polyester of 4th generation (Ar.HBP-G4, 0-40 wt.-% w.r.t. PVDF content) blend nanoweb as tribo-negative layer and melt-blown thermoplastic polyurethane (TPU) as tribo-positive layer for energy harvesting and human health monitoring applications. Among the varying Ar.HBP-G4 content used, incorporation of Ar.HBP-G4 (10 wt.-%) in PVDF (P-Ar.HBP-G4-10) showed higher increase in the triboelectric output voltage when compared to pristine PVDF and other Ar.HBP-G4 weight ratios. The optimized P-Ar.HBP-G4-10/TPU based TENG exhibited a peak-to-peak voltage (Vp-p) of 124.4 V under an applied load of 9.8 N and frequency 1 Hz which is superior to many other TENGs reported elsewhere. Higher triboelectric performance of P-Ar.HBP-G4 blend based TENG compared to that of neat PVDF is attributed to the effect of Ar.HBP-G4-10 in enhancing the degree of crystallinity and polar β-crystalline phase content (98.3%) in PVDF. The ability of the TENG to power up portable electronic devices is demonstrated when it is powered for 750 s while connected through a capacitor and a rectifier, and the TENG was able to operate 45 light-emitting diodes directly. Evaluation of the triboelectric output of the TENG device attached to different parts of the human body reveal significantly better output voltage and sensitivity for human health monitoring. The results of this work pave a new way to develop TENG based on P-Ar.HBP-G4 nanowebs for sustainable energy generation and wearable healthcare monitoring systems.

Micro-Droplets Parameters Monitoring in a Microfluidic Chip via Liquid-Solid Triboelectric Nanogenerator

The monitoring of micro-droplets parameters is significant to the development of droplet microfluidics. However, existing monitoring methods have drawbacks such as high cost, interference with droplet movement, and even the potential for cross-contamination. Herein, a micro-droplets monitoring method (MDMM) based on liquid-solid triboelectric nanogenerator (LS-TENG) is proposed, which can realize non-invasive and self-powered monitoring of micro-droplets in a microfluidic chip. The droplet frequency is monitored by voltage pulse frequency and a mathematical model is established to monitor the droplet length and velocity. Furthermore, this work constructs micro-droplets sensor (MDS) based on the MDMM to carry out the experiment. The coefficients of determination (R2 ) of the fitting curves of the micro-droplets frequency, length, and velocity monitoring are 0.998, 0.997, and 0.995, respectively. To prove the universal applicability of the MDMM, the micro-droplets generated by different liquid media and channel structures are monitored. Eventually, a micro-droplet monitoring system is built, which can realize the counting of micro-droplets and the monitoring of droplet frequency and length. This work provides a novel approach for monitoring micro-droplets parameters, which holds the potential to advance developments in the field of microfluidics.

Comparison of Contact Electrification Mechanisms of Selected Polymers and Surface-Functionalized Molecules

Among the possible alternatives for the improvement of contact electrification for triboelectric energy harvesting purposes, the functionalization of contact surfaces has attracted wide attention due to its versatility and cost-efficiency. Similarly, low-stiffness polymeric materials such as poly(dimethylsiloxane) (PDMS) are regarded as a promising choice of contact material for the same purpose. However, for defining the most efficient combinations of materials of the aforementioned types, a number of theoretical questions still frequently pose difficulties for practical implementation-related tasks. In this regard, the presented study theoretically assesses the possibilities of consistently selecting optimum performance combinations of contact materials. Here, the optimum is defined as the minimum energy of the charge transfer reaction and, consequently, the maximum density of the predicted triboelectric surface charge. With this aim, the most promising combinations in terms of electron-transfer energies were identified among the candidates of functionalized molecules and polymers. Based on the ordering of materials according to the basic characteristics of charge-transfer reactions─electron and hole affinities─certain differences were observed. These findings indicate that for the materials under consideration, it is not possible to establish a single triboelectric series solely based on a single characteristic. Furthermore, to evaluate the potential compatibility of charge-transfer reaction mechanisms based on electron and material transfer, molecular dynamics simulations were conducted using structures that depict pairs of polymers and self-assembled monolayers of functionalized molecules in contact and separated types of operations. The obtained results indicate that the formation of equally charged free fragments of polymer chains is likely taking place in the contact electrification for N-(2-aminoethyl)-3-aminopropyl trimethoxysilane/PDMS interfaces. At variance, a contact electrification mechanism by charge-dependent material transfer may occur for 1H, 1H, 2H, 2H-perfluorooctyl trimethoxysilane/PDMS interfaces.

Boosting Output Performance of Sliding Mode Triboelectric Nanogenerator by Shielding Layer and Shrouded-Tribo-Area Optimized Ternary Electrification Layered Architecture

Sliding mode triboelectric nanogenerator (S-TENG) is effective for low-frequency mechanical energy harvesting owing to their more efficient mechanical energy extraction capability and easy packaging. Ternary electrification layered (TEL) architecture is proven useful for improving the output performance of S-TENG. However, the bottleneck of electric output is the air breakdown on the interface of tribo-layers, which seriously restricts its further improvement. Herein, a strategy is adopted by designing a shielding layer to prevent air breakdown on the central surface of tribo-layers. And the negative effects of air breakdown on the edge of sliding layer are averted by increasing the shrouded area of tribo-layers on slider. Output charge of this shielding-layer and shrouded-tribo-area optimized ternary electrification layered triboelectric nanogenerator (SS-TEL-TENG) achieves 3.59-fold enhancement of traditional S-TENG and 1.76-fold enhancement of TEL-TENG. Furthermore, even at a very low speed of 30 rpm, output charge, current, and average power of the rotation-type SS-TEL-TENG reach 4.15 µC, 74.9 µA, and 25.4 mW (2.05 W m-2 Hz-1 ), respectively. With such high-power output, 4248 LEDs can be lighted brightly by SS-TEL-TENG directly. The high-performance SS-TEL-TENG demonstrated in this work will have great applications for powering ubiquitous sensor network in the Internet of Things (IoT).

Removal of Particulate Matter by a Non-Powered Brush Filter Using Electrostatic Forces

In urban areas, a major source of harmful particulate matter is generated by vehicles. In particular, bus stops, where people often stay for public transportation, generate high concentrations of particulate matter compared to the general atmosphere. In this study, a non-powered type brush filter that generates electrostatic force without using a separate power source was developed to manage the concentration of particulate matter exposed at bus stops, and the removal performance of particulate matter was evaluated. The dust collection performance of the non-motorized brush filter varied by material, with particle removal efficiencies of 82.1 ± 3.4, 76.1 ± 4.7, and 73.7 ± 4.5% for horse hair, nylon, and stainless steel, respectively. In conditions without the fan running to see the effect of airflow, the particle removal efficiency was relatively low at 58.2 ± 8.4, 53.6 ± 9.2, and 58.0 ± 7.3%. Then, to check the dust collection performance according to the density, the number of brushes was increased to densify the density, and the horse hair, nylon, and stainless steel brush filters showed a maximum dust collection performance of 89.6 ± 2.2, 88.3 ± 3.2, and 82.1 ± 3.8%, respectively. To determine the replacement cycle of the non-powered brush filter, the particulate removal performance was initially 88.0 ± 3.2% when five horse hair brushes were used. Over time, particulate matter tended to gradually decrease, but after a period of time, particulate matter tended to increase again. The purpose of this study is to evaluate the particulate matter removal performance using a brush filter that generates electrostatic force without a separate power source. This study's brush filter is expected to solve the maintenance problems caused by the purchase and frequent replacement of expensive HEPA filters that occur with existing abatement devices, and the ozone problems caused by abatement devices that use high voltages.

Advances in Bioresorbable Triboelectric Nanogenerators

With the growing demand for next-generation health care, the integration of electronic components into implantable medical devices (IMDs) has become a vital factor in achieving sophisticated healthcare functionalities such as electrophysiological monitoring and electroceuticals worldwide. However, these devices confront technological challenges concerning a noninvasive power supply and biosafe device removal. Addressing these challenges is crucial to ensure continuous operation and patient comfort and minimize the physical and economic burden on the patient and the healthcare system. This Review highlights the promising capabilities of bioresorbable triboelectric nanogenerators (B-TENGs) as temporary self-clearing power sources and self-powered IMDs. First, we present an overview of and progress in bioresorbable triboelectric energy harvesting devices, focusing on their working principles, materials development, and biodegradation mechanisms. Next, we examine the current state of on-demand transient implants and their biomedical applications. Finally, we address the current challenges and future perspectives of B-TENGs, aimed at expanding their technological scope and developing innovative solutions. This Review discusses advancements in materials science, chemistry, and microfabrication that can advance the scope of energy solutions available for IMDs. These innovations can potentially change the current health paradigm, contribute to enhanced longevity, and reshape the healthcare landscape soon.

Unique Contact Point Modification Technique for Boosting the Performance of a Triboelectric Nanogenerator and Its Application in Road Safety Sensing and Detection

A triboelectric nanogenerator (TENG) is a potential technique that can convert waste kinetic energy to electrical energy by contact separation followed by electrostatic induction. Herein, a unique contact point modification technique has been reviewed carefully via the enlargement of the effective surface area of the tribo layer by using a simple and scalable printing method. In this study, the zinc sulfide (ZnS) nanostructure morphology has been introduced directly on an aluminum electrode (Al) as a tribo positive layer by a modified hydrothermal method and different line patterns directly printed on overhead projector (OHP) transparent sheets by a monochrome laser printer as a tribo negative layer to increase the effective contact area and work-function difference between two tribo layers. This dual parameter results in ∼11 times increment in the open-circuit output voltage (∼420 V) and ∼17 times increment in the short-circuit current density (∼83.33 mA m^-2) compared to the normal one. Furthermore, with the proposed surface modification technique, an ultrahigh instantaneous output power density of ∼3.9 W m^-2 at a load resistance of 2 MΩ was easily achieved. The direct energy conversion efficiency reached up to 66.67% at 2 MΩ load, which is very high compared to other traditional TENGs. Further, the fabricated TENG demonstrated efficacy in novel road safety sensing applications in hilly areas to control vehicle movement. Therefore, the current idea of surface engineering using a laser printer will be helpful for energy-harvesting enthusiasts to develop more efficient nanogenerators for higher energy conversions.

Chicken skin based Milli Watt range biocompatible triboelectric nanogenerator for biomechanical energy harvesting

This work reports a high-performance, low-cost, biocompatible triboelectric nanogenerator (TENG) using chicken skin (CS). The device is suitable to power wearable devices, which is critical to adapt electronics in monitoring, predicting, and treating people. It also supports sustainability by providing a cost-effective way to reduce the poultry industry's waste. It has been shown here that CS-derived biowaste is an effective means of generating tribopositive material for TENGs. The CS contains amino acid functional groups based on (Glycine, Proline, and Hydroxyproline), which are essential to demonstrate the electron-donating ability of collagen. The skin was cut into 3 × 3 cm2 and used as the raw material for fabricating the TENG device with a stacking sequence of Al/Kapton/spacing/CS/Al. The chicken skin-based TENG (CS-TENG) is characterized at different frequencies (4-14 HZ) using a damping system. The CS-TENG produces an open-circuit voltage of 123 V, short-circuit current of 20 µA and 0.2 mW/cm2 of a power density at 20 MΩ. The biocompatible CS-TENG presents ultra-robust and stable endurance performance with more than 52,000 cycles. The CS-TENG is impressively capable of scavenging energy to light up to 55 commercial light-emitting diodes (LEDs), a calculator, and to measure the physiological motions of the human body. CS-TENG is a step toward sustainable, battery-less devices or augmented energy sources, especially when using traditional power sources, such as in wearable devices, remote locations, or mobile applications is not practical or cost-effective.

Large and Tunable Ranking Shift in Triboelectric Series of Polymers by Introducing Phthalazinone Moieties

Regulating the ranking of polymer in triboelectric series over a wide range is of great help for material's selection of triboelectric nanogenerators (TENGs). Herein, fluorinated poly(phthalazinone ether)s (FPPEs) with tunable molecular structure and aggregate structure are synthesized by co-polycondensation, while the large positive ranking shift in the triboelectric series can be achieved by introducing phthalazinone moieties with strong electron donating capability. FPPE-5, which includes abundant phthalazinone moieties, is more positive than all of the previously reported triboelectric polymers. Hence, the regulating range of FPPEs in this work updates a new record in triboelectric series, which is wider than that of previous works. A peculiar crystallization behavior, capable of trapping and storing more electrons, has been observed in FPPE-2 with 25% phthalazinone moieties. Correspondingly, FPPE-2 is more negative than FPPE-1 without a phthalazinone moiety, which is an unexpected shift against the common changing tendency in triboelectric series. With FPPEs films as the probing material, a tactile TENG sensor is applied to enable material identification via electrical signal polarity. Hence, this study demonstrates a strategy to regulate the series of triboelectric polymers by copolymerization using monomers with distinct electrification capabilities, where both the monomer ratio and the peculiar nonlinear behavior can control triboelectric performance.

3D printed triboelectric nanogenerator for underwater ultrasonic sensing

The underwater ultrasound power measurement has become necessary due to the rapid development of sonochemistry and sonocatalysis. This article presents construction of novel triboelectric nanogenerator (TENG) and its application for a detection of ultrasonic waves in water. The device was 3D printed using widely available and cost-effective materials. TENG consisted of the device housing and movable polymer pellets confined between flat electrodes. The device housing and pellets were 3D printed via stereolithography (SLA) and fused deposition modelling (FDM) methods, respectively. The pellets moved periodically driven by the ultrasonic waves leading to generation of an alternating voltage signal. The electric response of TENG was calibrated using a commercially available ultrasonic power sensor. The open-circuit voltage output of TENG was registered in different sections of the ultrasonic bath in order to determine the distribution of the acoustic power. TENG electric responses were analyzed by applying the fast Fourier transform (FFT) and fitting the theoretical dependence to experimental data. The main peaks in the frequency spectra of the voltage waveforms corresponded to the fundamental excitation frequency of the ultrasonic bath. TENG device, presented in this paper, can be successfully applied as a self-powered sensor for detection of ultrasonic waves. It enables precise control of the sonochemical process and reduction of power losses of the ultrasonic reactor. 3D printing technology has been confirmed to be fast, easy, and scalable method of fabrication of the ultrasonic sensors.

Rational Design of Cellulosic Triboelectric Materials for Self-Powered Wearable Electronics

With the rapid development of the Internet of Things and flexible electronic technologies, there is a growing demand for wireless, sustainable, multifunctional, and independently operating self-powered wearable devices. Nevertheless, structural flexibility, long operating time, and wearing comfort have become key requirements for the widespread adoption of wearable electronics. Triboelectric nanogenerators as a distributed energy harvesting technology have great potential for application development in wearable sensing. Compared with rigid electronics, cellulosic self-powered wearable electronics have significant advantages in terms of flexibility, breathability, and functionality. In this paper, the research progress of advanced cellulosic triboelectric materials for self-powered wearable electronics is reviewed. The interfacial characteristics of cellulose are introduced from the top-down, bottom-up, and interfacial characteristics of the composite material preparation process. Meanwhile, the modulation strategies of triboelectric properties of cellulosic triboelectric materials are presented. Furthermore, the design strategies of triboelectric materials such as surface functionalization, interfacial structure design, and vacuum-assisted self-assembly are systematically discussed. In particular, cellulosic self-powered wearable electronics in the fields of human energy harvesting, tactile sensing, health monitoring, human-machine interaction, and intelligent fire warning are outlined in detail. Finally, the current challenges and future development directions of cellulosic triboelectric materials for self-powered wearable electronics are discussed.

An Advanced Strategy to Enhance TENG Output: Reducing Triboelectric Charge Decay

The Internet of Things (IoT) is poised to accelerate the construction of smart cities. However, it requires more than 30 billion sensors to realize the IoT vision, posing great challenges and opportunities for industries of self-powered sensors. Triboelectric nanogenerator (TENG), an emerging new technology, is capable of easily converting energy from surrounding environment into electricity, thus TENG has tremendous application potential in self-powered IoT sensors. At present, TENG encounters a bottleneck to boost output for large-scale commercial use if just by promoting triboelectric charge generation, because the output is decided by the triboelectric charge dynamic equilibrium between generation and decay. To break this bottleneck, the strategy of reducing triboelectric charge decay to enhance TENG output is focused. First, multiple mechanisms of triboelectric charge decay are summarized in detail with basic theoretical principles for future research. Furthermore, recent advances in reducing triboelectric charge decay are thoroughly reviewed and outlined in three aspects: inhibition and application of air breakdown, simultaneous inhibition of air breakdown and triboelectric charge drift/diffusion, and inhibition of triboelectric charge drift/diffusion. Finally, challenges and future research focus are proposed. This review provides reference and guidance for enhancing TENG output.

A Liquid Triboelectric Series

The triboelectric series is a generally accepted method for describing the triboelectric effect. It provides a way to control the double face of the ubiquitous triboelectric effect: causes of unpredictable accidents and the resultant surface charge as energy sources. However, previous studies have been biased in solids despite being observed in liquids (liquid-solid contact electrification). Therefore, a liquid triboelectric series is necessary to be established to manipulate the liquid triboelectric effect according to the appropriate goal. In this study, a liquid triboelectric series is first established to describe the triboelectric properties of each liquid when contact electrification occurs with a solid surface. The liquid triboelectric series covers electrolytes, organic solvents, oxidants, and higher sugar alcohols. Common chemical groups can be derived from the liquid triboelectric series that hydroxyl groups enhance, and benzene groups suppress the liquid triboelectric effect. The results are demonstrated by the amplified efficiency of an energy harvester and particle contamination after surface washing. This study will play a pivotal role in understanding the liquid-solid contact electrification phenomenon and providing new perspectives on the applications of the liquid triboelectric effect.

Improving the Performance of a Triboelectric Nanogenerator by Using an Asymmetric TiO2/PDMS Composite Layer

To improve the output power of the polydimethylsiloxane (PDMS)-based triboelectric nanogenerators (TENGs), we fabricated an asymmetric TiO₂/PDMS composite film in which a pure PDMS thin film was deposited as a capping layer on a TiO₂ nanoparticles (NPs)-embedded PDMS composite film. Although in the absence of the capping layer, the output power decreased when the content of TiO₂ NPs exceeded a certain value, the asymmetric TiO₂/PDMS composite films showed that the output power increased with increasing content. The maximum output power density was approximately 0.28 W/m² at a TiO₂ content of 20 vol.%. The capping layer could be responsible not only for maintaining the high dielectric constant of the composite film but also for suppressing interfacial recombination. To further improve the output power, we applied a corona discharge treatment to the asymmetric film and measured the output power at a measurement frequency of 5 Hz. The maximum output power density was approximately 78 W/m². The idea of the asymmetric geometry of the composite film should be applicable to various combinations of materials for TENGs.

Large Harvested Energy by Self-Excited Liquid Suspension Triboelectric Nanogenerator with Optimized Charge Transportation Behavior

To enhance the durability of triboelectric nanogenerator (TENG), liquid lubrication has been used to reduce mechanical abrasion. However, as the charge transportation behavior in dielectric liquid is not clearly understood, the output energy is still low although some improvements have been reported. Herein, the charge transportation behaviors in dielectric liquid by self-excited liquid suspension triboelectric nanogenerator (LS-TENG) are systematically investigated. The important role of solid-liquid triboelectrification effect, charge-liquid transmission and dissipation effect, and the homogeneous dielectric induction effect in promoting its output performance is found. The LS-TENG with a dual dielectric tribolayer has advantages of slight driving force and long lifetime for harvesting micro energy. The output of LS-TENG remains almost constant for more than 234 k operating cycles. A high charge density of 704 µC m^-2 is obtained, 2.7 times as much as that of the current highest record in non-contact TENG. Additionally, the rotary LS-TENG lights up 4200 LEDs and continuously powers a variety of wireless sensors by harvesting wind energy at low wind speed. This work provides an important insight toward the charge transportation mechanism in dielectric liquid, and a prospective strategy for achieving highly robust TENG in micro energy harvesting for practical applications.

High-fidelity and clean nanotransfer lithography using structure-embedded and electrostatic-adhesive carriers

Metallic nanostructures are becoming increasingly important for both fundamental research and practical devices. Many emerging applications employing metallic nanostructures often involve unconventional substrates that are flexible or nonplanar, making direct lithographic fabrication very difficult. An alternative approach is to transfer prefabricated structures from a conventional substrate; however, it is still challenging to maintain high fidelity and a high yield in the transfer process. In this paper, we propose a high-fidelity, clean nanotransfer lithography method that addresses the above challenges by employing a polyvinyl acetate (PVA) film as the transferring carrier and promoting electrostatic adhesion through triboelectric charging. The PVA film embeds the transferred metallic nanostructures and maintains their spacing with a remarkably low variation of <1%. When separating the PVA film from the donor substrate, electrostatic charges are generated due to triboelectric charging and facilitate adhesion to the receiver substrate, resulting in a high large-area transfer yield of up to 99.93%. We successfully transferred the metallic structures of a variety of materials (Au, Cu, Pd, etc.) with different geometries with a <50-nm spacing, high aspect ratio (>2), and complex 3D structures. Moreover, the thin and flexible carrier film enables transfer on highly curved surfaces, such as a single-mode optical fiber with a curvature radius of 62.5 μm. With this strategy, we demonstrate the transfer of metallic nanostructures for a compact spectrometer with Cu nanogratings transferred on a convex lens and for surface-enhanced Raman spectroscopy (SERS) characterization on graphene with reliable responsiveness.

2.11 - Accurate characterization of indoor photovoltaic performance

Ambient energy harvesting has great potential to contribute to sustainable development and address growing environmental challenges. Converting waste energy from energy-intensive processes and systems (e.g. combustion engines and furnaces) is crucial to reducing their environmental impact and achieving net-zero emissions. Compact energy harvesters will also be key to powering the exponentially growing smart devices ecosystem that is part of the Internet of Things, thus enabling futuristic applications that can improve our quality of life (e.g. smart homes, smart cities, smart manufacturing, and smart healthcare). To achieve these goals, innovative materials are needed to efficiently convert ambient energy into electricity through various physical mechanisms, such as the photovoltaic effect, thermoelectricity, piezoelectricity, triboelectricity, and radiofrequency wireless power transfer. By bringing together the perspectives of experts in various types of energy harvesting materials, this Roadmap provides extensive insights into recent advances and present challenges in the field. Additionally, the Roadmap analyses the key performance metrics of these technologies in relation to their ultimate energy conversion limits. Building on these insights, the Roadmap outlines promising directions for future research to fully harness the potential of energy harvesting materials for green energy anytime, anywhere.

A Review of Perovskite-Based Photodetectors and Their Applications

Perovskite photodetectors have attracted much research and attention because of their outstanding photoelectric characteristics, such as good light harvesting capability, excellent carrier migration behavior, tunable band gap, and so on. Recently, the reported studies mainly focus on materials synthesis, device structure design, interface engineering and physical mechanism analysis to improve the device characteristics, including stability, sensitivity, response speed, device noise, etc. This paper systematically summarizes the application fields and device structures of several perovskite photodetectors, including perovskite photoconductors, perovskite photodiodes, and perovskite phototransistors. Moreover, based on their molecular structure, 3D, 2D, 1D, and 0D perovskite photodetectors are introduced in detail. The research achievements and applications of perovskite photodetectors are summarized. Eventually, the future research directions and main challenges of perovskite photodetectors are prospected, and some possible solutions are proposed. The aim of the work is to provide a new thinking direction for further improving the performance of perovskite photodetectors.