Understanding Contact Electrification at Water/Polymer Interface

How liquids charge the superhydrophobic surfaces

Liquid-solid contact electrification (CE) is essential to diverse applications. Exploiting its full implementation requires an in-depth understanding and fine-grained control of charge carriers (electrons and/or ions) during CE. Here, we decouple the electrons and ions during liquid-solid CE by designing binary superhydrophobic surfaces that eliminate liquid and ion residues on the surfaces and simultaneously enable us to regulate surface properties, namely work function, to control electron transfers. We find the existence of a linear relationship between the work function of superhydrophobic surfaces and the as-generated charges in liquids, implying that liquid-solid CE arises from electron transfer due to the work function difference between two contacting surfaces. We also rule out the possibility of ion transfer during CE occurring on superhydrophobic surfaces by proving the absence of ions on superhydrophobic surfaces after contact with ion-enriched acidic, alkaline, and salt liquids. Our findings stand in contrast to existing liquid-solid CE studies, and the new insights learned offer the potential to explore more applications.

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.

Thermodynamic driving forces in contact electrification between polymeric materials

Contact electrification, or contact charging, refers to the process of static charge accumulation after rubbing, or even simple touching, of two materials. Despite its relevance in static electricity, various natural phenomena, and numerous technologies, contact charging remains poorly understood. For insulating materials, even the species of charge carrier may be unknown, and the direction of charge-transfer lacks firm molecular-level explanation. Here, we use all-atom molecular dynamics simulations to investigate whether thermodynamics can explain contact charging between insulating polymers. Based on prior work suggesting that water-ions, such as hydronium and hydroxide ions, are potential charge carriers, we predict preferred directions of charge-transfer between polymer surfaces according to the free energy of water-ions within water droplets on such surfaces. Broad agreement between our predictions and experimental triboelectric series indicate that thermodynamically driven ion-transfer likely influences contact charging of polymers. Furthermore, simulation analyses reveal how specific interactions of water and water-ions proximate to the polymer-water interface explain observed trends. This study establishes relevance of thermodynamic driving forces in contact charging of insulators with new evidence informed by molecular-level interactions. These insights have direct implications for future mechanistic studies and applications of contact charging involving polymeric materials.

Water-Assisted Contact Electrification Properties of Selected Polymers and Surface Functionalization Molecules: A Computational Study

Motivated by the requirements of performance stability in environments of variable humidity, the focus of this study is on the effects and role of humidity-induced water molecules and ions in the contact electrification (CE) mechanisms of triboelectric materials. In particular, the compatibility of direct charge transfer-based CE and other generally known or proposed water molecules or OH/H3O ion-facilitated CE mechanisms was assessed for a set of high-performance polymeric materials and functionalization molecules. The first set of test mechanisms included OH/H3O ion adsorption at the low-humidity limit. The adsorption resulted in physisorption or H transfer involving reactions that were not fully compatible with charge affinity-driven CE reactions on the considered contact surfaces for both ions in terms of the potential increase of the resultant density of surface charge. An alternative mechanism, which yielded compatibility at a large humidity limit, consisted of free energy-driven segregation and separation of the ions. Further test mechanisms included water adsorption-induced charge transfer and two mechanisms pertinent to charged material transfer: adsorption modulation due to formation of water monolayers and water solvation-induced separation of polymer fragments. According to the obtained results, both mechanisms could be verified as viable contributors to enhanced charge transfer. Consequently, the results allowed for conclusions regarding the general applicability of different, water-assisted CE mechanisms and the selection of particular pairs of contact materials of similar type for optimum performance in humid environments.

Triboelectric Nanogenerators Based on Fluid Medium: From Fundamental Mechanisms toward Multifunctional Applications

Fluid-based triboelectric nanogenerators (FB-TENGs) are at the forefront of promising energy technologies, demonstrating the ability to generate electricity through the dynamic interaction between two dissimilar materials, wherein at least one is a fluidic medium (such as gas or liquid). By capitalizing on the dynamic and continuous properties of fluids and their interface interactions, FB-TENGs exhibit a larger effective contact area and a longer-lasting triboelectric effect in comparison to their solid-based counterparts, thereby affording longer-term energy harvesting and higher-precision self-powered sensors in harsh conditions. In this review, various fluid-based mechanical energy harvesters, including liquid-solid, gas-solid, liquid-liquid, and gas-liquid TENGs, have been systematically summarized. Their working mechanism, optimization strategies, respective advantages and applications, theoretical and simulation analysis, as well as the existing challenges, have also been comprehensively discussed, which provide prospective directions for device design and mechanism understanding of FB-TENGs.

Additional kinetic energy harvesting with extra electrodes by single electrode droplet-based electricity generator (SE-DEG)

The utilization of water energy through the Single Electrode Droplet-Based Electricity Generator (SE-DEG) represents a universal and high-efficiency method for water energy harvesting. Previous research has extensively elucidated the working principle of SE-DEG based on bulk effect. However, scant attention has been paid to the investigation of the electrical characteristics surrounding the SE-DEG. Remarkably, the electrical characteristics around the SE-DEG can be exploited to generate electricity and harvest corresponding energy. Here we evaluate the electrical characteristics around the SE-DEG by arranging extra electrodes. An interesting phenomenon is found that, on the premise of no contact between extra electrodes and the droplet, there is opposite electricity output from extra electrodes synchronously when the droplet contacts on the PTFE film and SE-DEG electrode and outputs the electricity. This phenomenon is comprehensively explained and verified from working mechanism, the impacts of different arrangements and the array design of extra electrodes. Significantly, utilizing the electrical characteristics could harvest additional kinetic energy with extra electrodes in SE-DEG. This investigation is expected to provide new insights into the future harnessing of water kinetic energy within the SE-DEG framework.

A wave energy driven high-performance self-powered oil spill positioner

The oil spill positioner is capable of real-time monitoring oil films on the sea surface. However, the lack of high-performance power supply methods greatly restricts the application of oil spill positioner. In this research, we design a high-performance self-powered oil spill positioner based on a soft-contact-triboelectric-nanogenerator (SC-TENG). This device achieves soft-contact by attaching rabbit fur to the rotor, which can effectively reduce frictional resistance, quickly transfer charge to the electrode, and improve the durability of the parts. First, we calculate the highest occupied molecular orbital and the lowest unoccupied molecular orbital of polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF) molecules through first-principles simulations, and compared the ease of electron excitation between the two materials. The results show that the performance of SC-TENG with PVDF as dielectric material is significantly better than that of PTFE. At the same time, this phenomenon has been confirmed by experiments. On the basis of experimental and simulate research on two types of power management circuits, a bridge rectifier circuit with the function of converting alternating current to direct current is selected to realize the self-power supply of the oil spill positioner. Additionally, by optimizing the structure of the SC-TENG and employing a bridge rectifier circuit, the SC-TENG can achieve a maximum open-circuit voltage of 1400 V and a short-circuit current of 3.49μA, which are enough to light up 200 light-emitting diodes and power the oil spill positioner. Finally, we simulate the open-circuit voltage and short-circuit current of the SC-TENG on a six-degree-of-freedom platform and test its durability under real-world ocean wave conditions, all of which show excellent performance. This work develops an efficient wave energy conversion mechanism and successfully realizes the high-performance self-powering of the oil spill positioner, making oil spill monitoring more flexible and reliable.

Mechanism for Generating H2 O2 at Water-Solid Interface by Contact-Electrification

The recent intensification of the study of contact-electrification at water-solid interfaces and its role in physicochemical processes lead to the realization that electron transfers during water-solid contact-electrification can drive chemical reactions. This mechanism, named contact-electro-catalysis (CEC), allows chemically inert fluorinated polymers to act like single electrode electrochemical systems. This study shows hydrogen peroxide (H2 O2 ) is generated from air and deionized water, by ultrasound driven CEC, using fluorinated ethylene propylene (FEP) as the catalyst. For a mass ratio of catalyst to solution of 1:10000, at 20 °C, the kinetic rate of H2 O2 evolution reaches 58.87 mmol L-1  gcat -1  h-1 . Electron paramagnetic resonance (EPR) shows electrons are emitted in the solution by the charged FEP, during ultrasonication. EPR and isotope labelling experiments show H2 O2 is formed from hydroxyl radicals (HO• ) or two superoxide radicals (O2 •- ) generated by CEC. Finally, it is traditionally believed such radicals migrate in the solution by Brownian diffusion prior to reactions. However, ab-initio molecular dynamic calculations reveal the radicals can react by exchanging protons and electrons through the hydrogen bonds network of water, i.e., owing to the Grotthuss mechanism. This mechanism can be relevant to other systems, artificial or natural, generating H2 O2 from air and water.

Prediction of the Effective Work Function of Aspirin and Paracetamol Crystals by Density Functional Theory-A First-Principles Study

Crystals of active pharmaceutical ingredients (API) are prone to triboelectric charging due to their dielectric nature. This characteristic, coupled with their typically low density and often large aspect ratio, poses significant challenges in the manufacturing process. The pharmaceutical industry frequently encounters issues during the secondary processing of APIs, such as particle adhesion to walls, clump formation, unreliable flow, and the need for careful handling to mitigate the risk of fire and explosions. These challenges are further intensified by the limited availability of powder quantities for testing, particularly in the early stages of drug development. Therefore, it is highly desirable to develop predictive tools that can assess the triboelectric propensity of APIs. In this study, Density Functional Theory calculations are employed to predict the effective work function of different facets of aspirin and paracetamol crystals, both in a vacuum and in the presence of water molecules on their surfaces. The calculations reveal significant variations in the work function across different facets and materials. Moreover, the adsorption of water molecules induces a shift in the work function. These findings underscore the considerable impact of distinct surface terminations and the presence of molecular water on the calculated effective work function of pharmaceuticals. Consequently, this approach offers a valuable predictive tool for determining the triboelectric propensity of APIs.

Degradation of methyl orange by dielectric films based on contact-electro-catalysis

Contact-electro-catalysis (CEC) has been recently proposed for the effective degradation of methyl orange, but the reactivity of catalysts in the CEC process needs further investigation. Here, we have used dielectric films, such as fluorinated ethylene propylene (FEP), modified by inductively coupled plasma (ICP) etching with argon, to replace the previously employed micro-powder due to their potential scalability, facile recycling process, and possible lower generation of secondary pollution. It has been found that ICP creates cone-like micro/nano structures on the surface, and thus changes the contact angle and specific surface area. The value of the contact angle varies non-linearly with etching time and attains a maximum after 60 seconds of etching. Concurrently, an increased electron transfer is observed, as well as an enhanced degradation efficiency, thus suggesting a special role of the surface structure. Finally, KPFM measurements show a lower electron affinity at the summit of the nanocones. This observation suggests that the structures are endowed with higher charge transfer ability. In addition, this film-based CEC has been observed in several polymer materials, such as PET, PTFE, and PVC. We view this work as a stepping stone to develop CEC into scalable applications, based on film technologies.

Washable and Breathable Electret Sensors Based on a Hydro-Charging Technique for Smart Textiles

Flexible electromechanical sensors based on electret materials have shown great application potential in wearable electronics. However, achieving great breathability yet maintaining good washability is still a challenge for traditional electret sensors. Herein, we report a washable and breathable electret sensor based on a hydro-charging technique, namely, hydro-charged electret sensor (HCES). The melt-blown polypropylene (MBPP) electret fabric can be charged while washing with water. The surface potential of MBPP electret fabric can be improved by optimizing the type of water, water pressure, water temperature, drying temperature, drying time, ambient air pressure, and ambient relative humidity. It is proposed that the single fiber has charges of different polarities on the upper and lower surfaces due to contact electrification with water, thereby forming electric dipoles between fibers, which can lead to better surface potential stability than the traditional corona-charging method. The HCES can achieve a high air permeability of ∼215 mm/s and sensitivity up to ∼0.21 V/Pa, with output voltage remaining stable after over 36,000 working cycles and multiple times of water washing. As a demonstration example, the HCES is integrated into a chest strap to monitor human respiration conditions.