Mechanism Of Contact Electrification Research Articles

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.

Strategies to Improve the Output Performance of Triboelectric Nanogenerators

AbstractTriboelectric 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.

Contact electrification controlled by material deformation-induced electronic structure changes

The effective control of triboelectricity through material development is crucial for the safety of electronics and human well-being. However, owing to the absence of fundamental research on the mechanism of triboelectricity, material development has stagnated in this regard. In this study, we investigated the deformation effect on triboelectric behavior to enable a comprehensive understanding of the contact electrification mechanism. Theoretical results demonstrated that deformation leads to a reduction in the energy level associated with electron-accepting properties, thereby enhancing the tribonegative behavior of tribonegative materials. Conversely, the energy level related to electron-donating properties decreases, resulting in a waning of the tribopositive behavior of tribopositive materials. The experimental results further demonstrated that, consistent with the theoretical results, increased deformation induces an enhancement in the output of tribonegative materials while decreasing the output of tribopositive materials. Our findings provide a fundamental basis for controlling triboelectricity and advancing the development of stagnant triboelectric materials.

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.

Effect of PV panel electric field on the wind speed required for dust removal from the PV panels

Abstract Methods to remove dust deposits by high-speed airflow have significant potential applications, with optimal design of flow velocity being the core technology. In this paper, we discuss the wind speed required for particle removal from photovoltaic (PV) panels by compressed air by analyzing the force exerted on the dust deposited on inclined photovoltaic panels, which also included different electrification mechanisms of dust while it is in contact with the PV panel. The results show that the effect of the particle charging mechanism in the electric field generated by the PV panel is greatly smaller than the effect of the Van der Waals force and gravity, but the effect of the particle charged by the contact electrification mechanism in the electrostatic field is very pronounced. The wind speed required for dust removal from the PV panel increases linearly with the PV panel electric field, so we suggest that the nighttime, when the PV electric field is relatively small, would be more appropriate time for dust removal. The above results are of great scientific importance for accurately grasping the dust distribution law and for achieving scientific removal of dust on PV panels.

On the contact electrification mechanism in semiconductor–semiconductor case by vertical contact-separation triboelectric nanogenerator

With the speed of industrialization accelerating, the traditional energy is in the predicament of being exhausted. Humans urgently need a clean energy to maintain the peace and development. Triboelectric nanogenerator (TENG) is a tiny device that collects and converts the renewable energy, such as wind, vibration and tidal/blue energy, into electrical energy. As the most significant working principle of TENG, contact electrification (CE) has been broadly studied since it was documented thousands of years ago. A large number of related researches are reported. However, most of them are focused on the polymer materials, device structures and potential applications. There are few literatures about the mechanism of CE, especially in the semiconductor–semiconductor case. Semiconductor–semiconductor CE is a promising method to generate electricity, which has been used in many fields, such as the photodetector and displacement sensor. Therefore, it is necessary to establish a serious and detailed theory in order to deeply explain the underlying mechanisms of semiconductor–semiconductor CE. In this work, a novel Fermi level model based on energy band theory is proposed to illustrate the semiconductor–semiconductor CE mechanism. By assembling a ZnO/Si vertical contact-separation (CS) mode TENG, the charge transfer introduced by CE is systematically measured. According to the energy band theory and TENG governing equation, the experimental data is qualitatively and quantitatively analyzed. Moreover, the effects of different concentrations of growth solutions on the morphology of ZnO nanowires and the Fermi level difference between ZnO and Si are explored as well. Results show that it is the Fermi level difference that dominates the short circuit transfer charge amount and direction of semiconductor–semiconductor CE mechanism. Our work can be applied to understand the CE mechanism in semiconductor–semiconductor case and broaden the application prospects of semiconductor-based TENG.

Contact Electrification Characteristics of Polytetrafluoroethylene and Metal

In industrial production, the contact electrification phenomenon between materials will cause equipment damage and bring potential safety hazards to personnel, so it is necessary to study the contact electrification mechanism of materials. In order to explore the contact electrification characteristic mechanism between polytetrafluoroethylene and brass, aluminum alloy, a contact separation electrification test platform was built to pay attention to the change rule of surface potential of composite materials under contact pressure, environmental relative humidity, temperature and material temperature difference. The test results show that the surface potential first increases and then decreases slightly with the increase of contact pressure, and the surface potential is stable when the contact pressure is greater than 7 KN; The contact electrification of materials decreases with the increase of relative humidity; At 15°C ~35°C, the material surface potential is proportional to the temperature; With the temperature difference between the two test materials becoming larger, the contact potential shows a law of first increasing and then decreasing. Through the analysis of the contact electrification characteristics of materials under different conditions, it can provide a direction for further research on the contact electrification mechanism and industrial electrostatic protection.

On the temperature and humidity effects of contact electrification in semiconductor-semiconductor case: An energy band model for electron transfer in triboelectrification

Contact electrification (CE) (or triboelectrication) is a well-known phenomenon, but the identity of the charge carriers and their charge transfer mechanism have been under controversy. In recent years, some progress has been made in the study of CE mechanism in metal/dielectric-dielectric cases (in which cases the charge transfer is mainly dominated by the electron transfer mechanism), but the mechanism of CE in semiconductor-semiconductor (S-S) cases is still not well known. In this study, we chose ZnO-Si as friction pairs and conducted a series of studies on the effects of the conductivity type and doping concentration of Si wafer and further investigated the ambient temperature and humidity effects on CE in S-S cases. The results show that the CE in S-S cases is a charge transfer process dominated by the electron transfer mechanism. We describe the electron transfer in the contact-separation (CS) process with an energy band model, and explain the following principle more intuitively: the larger the Fermi energy level difference between the semiconductor friction layers is, the larger the number of transferred electrons are in CS process and the higher the output of CS-TENG is; the main influence mechanism of temperature on CE in S-S cases is thermal electron emission; and the influence of humidity on CE in S-S cases is mainly caused by the change of the surface state density of the friction layers by the adsorption of water molecules. This work not only clarifies that the charge transfer in S-S CE is dominated by the electron transfer mechanism originated from the Fermi energy level difference, but also proposes a more complete energy band model to explain the semiconductor-semiconductor CE mechanism, including the mechanism of the effect of temperature and humidity on the output performance of S-S CS-TENG, which is of great significance to improve the application potential of S-S TENG.

Triboelectric Mechanism of Oil-Solid Interface Adopted for Self-Powered Insulating Oil Condition Monitoring.

The liquid-solid contact electrification mechanism has been explored in the aqueous solution system, but there are few systematic studies on oil-solid triboelectrification. Herein, an oil droplet triboelectric nanogenerator (Oil-droplet TENG) is designed as the probe to investigate the charge transfer properties at oil-solid interface. The charge transfer kinetics process is disclosed by the electrical signals produced, showing that the electron species initially predominated the oil-solid triboelectrification. The molecular structure and electronic properties of oil also affect triboelectric performance. Further, the charge transfer principle in multi-component liquid mixture during the electric double layer (EDL) development process is proposed to explain the component competition effect. As a proof of concept, a tubular-TENG is designed as a self-powered sensor for transformer oil trace water detection. The device demonstrates high water sensitivity with a detection limit of 10µL L-1 and a response range of 10-100µL L-1 . This work not only reveals the oil-solid triboelectric and charge transfer competition mechanism in EDL, but also open up a new channel for real-time online monitoring of trace water in transformer oil, which holds promise for information perception and intelligent operation of transformers in the power industry.

Effective interfacial energy band engineering strategy toward high-performance triboelectric nanogenerator

Provided that electron transition ascribed to overlapping electron dominates the mechanism of contact electrification of triboelectric nanogenerators (TENGs), the electron transfer process occurs due to the built-up electric field between the triboelectrification surface and bottom electrode. In this work, we embed a solution-processed high-permittivity electron blocking layer (HPEBL) of LaZrO into the classical contact-separation mode TENG to build an energy barrier (ΔE > 1.3 eV) between ITO and PDMS, which could effectively block the transportation of electrons towards the bottom electrode. The surface potential decline rate of PDMS is obviously suppressed, extending the half-charge decay time by ∼3.1 times. A La0.1Zr0.9Ox HPEBL based TENG (H-TENG) demonstrates the electrical outputs with 215 V, 96.3 mA m−2 and 243.3 μC/m−2. Under the external load of 100 MΩ, the average gravimetric/volumetric power density and energy conversion efficiency can be calculated to be 59.34 μW g−1, 152.5 μW cm−3 and 39.2%, respectively. Furthermore, the dielectric behaviors of LaZrO thin films are investigated by metal–insulator-metal (MIM) devices. The Poole-Frenkel (PF) emission is found to dominate the leakage mechanism of LaZrO during operation. Consequently, increasing the relative permittivity and reducing the trap density could be indicated as the optimization orientation of HPEBL.

Revisiting Contact Electrification at Polymer-Liquid Interfaces.

Contact electrification (CE) occurs naturally at all interfaces between solids and solids, solids and liquids, solids and gasses, and so forth. It has been extensively studied for decades. While CE at a solid-solid interface has been demonstrated to be primarily caused by electron transfer, the underlying mechanism of CE at a liquid-solid interface remains controversial. In this paper, the CE process between polyethylene terephthalate (PET) and different inorganic solutions at different temperatures is studied to investigate the charge transfer mechanism. The observed temperature-CE charge relationship falls into two categories, that is, the general case and the special case. In the general case, the CE charge first increases negatively and then positively with the temperature. The CE charge increasing negatively could result from enhanced electron transfer at the interface, while the CE charge increasing positively may be caused by increasing adsorption of cations, which neutralize the negative charges on the PET surface. In contrast, the CE charge first increases positively and then negatively with the temperature in the special case. The CE charge increasing positively could be attributed to more cations being attracted to the negatively charged PET surface, while the charge increasing negatively may be caused by more anions being attracted to the PET due to enhanced cation adsorption. Supported by the surface charge and dynamic charge transfer at different PET-solution interfaces and solution temperatures, our study provides a plausible interpretation of the temperature-dependent CE at the polymer-liquid interfaces.

A Review: Contact Electrification on Special Interfaces

The contact electrification of materials plays an important role in developing and applying triboelectric nanogenerators (TENGs). By exploring the contact electrification phenomena at different interfaces, we can improve the understanding of the electrification mechanism and expand the application field of TENGs. In this way, the rate of energy utilization can be improved while the harm caused by the electrostatic effect is reduced. This article systematically summarized the different interface contacts between the research status quo of electricity. This article expounds the solid–solid interface, liquid–solid interface, and liquid–liquid interface, as well as the gas and other interface contact electrification mechanism, and the research and application of these are introduced; finally, it prospects the contact between the different interfaces of electric potential applications as well as the challenge.

Probing Polymer Contact Electrification by Gamma-Ray Radiation

Triboelectric Nanogenerators (TENGs) have been regarded as an effective method to solve the energy problem since they were first demonstrated in 2012. Due to their high-power generation and low cost, TENGs have been widely applied in the fields of energy, security, biomedicine, the environment, and so on. For now, many researchers are focusing on contact electrification (CE) and surface modification in order to discover the fundamental CE mechanism and approaches to further enhance the performance of TENG devices. In this work, we employ gamma radiation (γ ray) to induce surface modifications on the dielectric materials (polymers in this work) and study its influence on CE. It is found that, due to the high energy of γ ray, some chemical bonds in polymers are destroyed and reformed. This changes the electron density of the polymer molecule, and thus varies the electron transfer ability of the dielectric materials. Afterward, polytetrafluoroethylene (PTFE) and polyvinyl chloride (PVC) become more negative, and polyethylene terephthalate (PET) become the opposite. Kapton, in particular, remains stable even after 1 MGy-dose radiation. This study reconfirms that electron transfer is the dominant process for polymers-related CE. It also suggests that triboelectric nanogenerators could be fabricated with materials possessing a high anti-radiation ability, and used for sensing or energy generation in space or other environments where radiation exists.

Bio‐Inspired Macromolecular Ordering of Elastomers for Enhanced Contact Electrification and Triboelectric Energy Harvesting

AbstractTriboelectrification of polymers enables mechanical energy harvesting in triboelectric generators, droplet generators, and ferroelectrets. Herein, triboelectric polymers, inspired by the ordering in spider‐silk, with strongly enhanced contact electrification are presented. The ordering in polyether block amide (PEBA) is induced by the addition of inorganic goethite (α‐FeOOH) nanowires that form H‐bonds with the elastomeric matrix. The addition of as little as 0.1 vol% of α‐FeOOH into PEBA increases the surface charge by more than order of magnitude (from 0.069 to 0.93 nC cm–2). The H‐bonds between α‐FeOOH and PEBA promote the formation of inclusions with higher degree of macromolecular ordering, analogous to the structure of spider silk. The formation of these inclusions is proven via nanoindentation hardness measurements and correlated with H‐bond‐induced chemical changes by Fourier transform infrared spectroscopy and direct scanning calorimetry. Theoretical studies reveal that the irregularity in hardness provides stress accumulation on the polymer surface during contact‐separation. Subsequent molecular dynamic studies demonstrate that stress accumulation promotes the mass‐transfer mechanism of contact electrification. The proposed macromolecular structure design provides a new paradigm for developing materials for applications in mechanical energy harvesting.

A Theoretical Model to Predict Contact Electrification

Abstract Triboelectric nanogenerators (TENGs) are widely applied to latest nanogenerators which are based on contact electrification (CE). In order to predict the working process of TENGs effectively, this paper proposes a dynamics model to simulate the dynamic mechanism of CE. Taking Fe and polytetrafluoroethylene (PTFE) as contact materials, the field emission under a strong electric field could occur when the interface distance is small enough. The transferred charges in steady state and the differences of interface barrier are calculated by self-consistent field (SCF) method. The molecular dynamics (MD) simulation is carried out under the contact pressure and electric field. The structure results of MD simulation and the electric fields obtained by SCF calculation are the input parameters for each other which run alternately. According to dynamic interface distances and the differences of interface barrier, the dynamic transferred charges can be finally predicted by tunnel current density. The results reveal that the amounts of transferred electrons are positively corresponded to the external pressure and the energy conversion efficiency will reach the extreme value when the pressure is near 130 MPa.

Evolution of surfaces and mechanisms of contact electrification between metals and polymers

Contact electrification (CE) is a pretty common phenomenon, but still is poorly understood. The long-standing controversy over the mechanisms of CE related to polymers is particularly intense due to their complexity. In this paper, the CE between metals and polymers is systematically studied, which shows the evolution of surfaces is accompanied by variations of CE outputs. The variations of CE charge quantity are closely related to the creep and deformation of the polymer and metal surfaces. Then the relationship between CE and polymer structures is put forward, which is essentially determined by the electronegativity of elements and the functional groups in the polymers. The effects of load and contact frequency on the CE process and outputs are also investigated, indicating the increase of CE charge quantity with load and frequency. Material transfer from polymer to metal is observed during CE while electrons transfer from metal to polymer, both of which are believed to have an influence on each other. The findings advance our understanding of the mechanism of CE between metal and polymers, and provides insights into the performance of CE-based application in various conditions, which sheds light on the design and optimization of CE-based energy harvest and self-powered sensing devices.

Understanding Contact Electrification at Water/Polymer Interface.

Contact electrification (CE) involves a complex interplay of physical interactions in realistic material systems. For this reason, scientific consensus on the qualitative and quantitative importance of different physical mechanisms on CE remains a formidable task. The CE mechanism at a water/polymer interface is a crucial challenge owing to the poor understanding of charge transfer at the atomic level. First-principle density functional theory (DFT), used in the present work, proposes a new paradigm to address CE. Our results indicate that CE follows the same trend as the gap between the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO) of polymers. Electron transfer occurs at the outmost atomic layer of the water/polymer interface and is closely linked to the functional groups and atom locations. When the polymer chains are parallel to the water layer, most electrons are transferred; conversely, if they are perpendicular to each other, the transfer of charges can be ignored. We demonstrate that a decrease in the interface distance between water and the polymer chains leads to CE in quantitative agreement with the electron cloud overlap model. We finally use DFT calculations to predict the properties of CE materials and their potential for triboelectric nanogenerator energy harvesting devices.

Hard coating films of fluorine-containing ladder-like structured polysilsesquioxane as negative triboelectric materials for high-performance triboelectric generators

Triboelectric nanogenerators (TENGs) have attracted considerable attention owing to their versatile potential to meet the needs of renewable energy sources. TENG materials have both electrical and mechanical excellence, considering the common mechanism of contact electrification. In this study, ladder-like structured poly(1H,1H,2H,2H-nonafluorohexyl-co-2-(3,4-epoxycyclohexyl)ethyltrimethoxy silsesquioxane)s (LPEFSQs) were synthesized as a hard coating material in TENG applications. LPEFSQs contain the UV-curable functional group and halogen elements and can be coated easily on various polymeric films. Such hard coating films of LPEFSQs enhance the electrical and mechanical properties of any polymer substrate. A LPEFSQ-based TENG exhibits superior performance in both aspects than polytetrafluoroethylene, an extreme triboelectric material commonly used as a negative side. In a large-scale demonstration, the open-circuit voltage and short-circuit current outputs reached 1140 V and 23μA, respectively. Therefore, this hard coating material will provide a facile but efficient method to increase the electrical performance of TENGs in harsh mechanical environments.

Effects of Oxygen Vacancies and Cation Valence States on the Triboelectric Property of Substoichiometric Oxide Films.

Temperature effects on the contact electrification (CE) is of great interest. Here, different kinds of substoichiometric oxide films, such as TiO2-x, Al2O3-x, Ta2O5-x, and Cr2O3-x, are deposited and annealed at different temperatures, and the CE between the films and a Pt-coated tip is performed by using Kelvin probe force microscopy (KPFM). An intriguing finding is that the polarity on the TiO2-x surface changes from negative to positive with the increase of the sample annealing temperature in air atmosphere. Such a result is attributed to the fact that annealing under an oxidative atmosphere repairs oxygen vacancies and helps upgrade the low valency of Ti3+ to a stable high valency of Ti4+. On the contrary, after annealing occurs in an Ar/H2 atmosphere, the polarity on the TiO2-x surface reverses from positive to negative. This is mainly due to the increase of oxygen vacancies after annealing in reducing atmosphere. Through the KPFM results of Al2O3-x, Ta2O5-x, and Cr2O3-x films, the effect of oxygen vacancies is further confirmed, that is, the decrease of oxygen vacancies eases the films at capturing positive charges. Based on this, TiO2-x-based identical material triboelectric nanogenerators (IM-TENGs) are designed and prepared for the first time to control the current direction. Moreover, a surface state model for explaining the CE mechanism between the metal and annealed dielectric is proposed. This study is conducive to the development of the IM-TENGs which regulate the current direction or voltage output accurately in the future and also provides a further understanding of the dominant mechanism of electron transfer in the CE.

Reversal of triboelectric charges on sol–gel oxide films annealed at different temperatures

Using Kelvin probe force microscopy, we studied the contact-electrification (CE) induced charge density on the surfaces of organic–inorganic composite oxide films, such as Nb2O5, ZrO2, and HfO2 films, prepared by a sol–gel method and annealed at different temperatures. The results show that positive triboelectric charges on the film surface gradually turn into negative ones with the increase in the annealing temperature. This phenomenon is attributed to the broken valence bonds between organics and organic–inorganic composites in the film at high temperature, which causes oxygen vacancy defects and consequently changes the surface states of the film. A surface states model is established to understand the process of CE, which further confirms that electron transfer is the dominant mechanism of CE. Moreover, it indicates that in addition to oxygen vacancies, there are also many other factors that can influence CE, including but not limited to non-stoichiometric ratios, impurities, and other types of defects. These factors will modify the surface states through changing the potential barrier energy of the bound electrons on the surface and thus impact the amount and even the direction of electron transfer.