Power generation from the interaction of a liquid droplet and a liquid membrane

Wetting Behavior-Induced Interfacial transmission of Energy and Signal: Materials, Mechanisms, and Applications

Wetting behaviors can significantly affect the transport of energy and signal (E&S) through vapor, solid, and liquid interfaces, which has prompted increased interest in interfacial science and technology. E&S transmission can be achieved using electricity, light, and heat, which often accompany and interact with each other. Over the past decade, their distinctive transport phenomena during wetting processes have made significant contributions to various domains. However, few studies have analyzed the intricate relationship between wetting behavior and E&S transport. This review summarizes and discusses the mechanisms of electrical, light, and heat transmission at wetting interfaces to elucidate their respective scientific issues, technical characteristics, challenges, commonalities, and potential for technological convergence. The materials, structures, and devices involved in E&S transportation are also analyzed. Particularly, harnessing synergistic advantages in practical applications and constructing advanced, multifunctional, and highly efficient smart systems based on wetted interfaces is the aim to provide strategies.

Zebra-Patterned Stretchable Helical Yarn for Triboelectric Self-Powered Multifunctional Sensing

Smart textiles capable of both energy harvesting and multifunctional sensing are highly desirable for next-generation portable electronics. However, there are still challenges that need to be conquered, such as the innovation of an energy-harvesting model and the optimization of interface bonding between fibers and active materials. Herein, inspired by the spiral structure of natural vines, a highly stretchable triboelectric helical yarn (TEHY) was manufactured by twisting the carbon nanotube/polyurethane nanofiber (CNT/PU NF) Janus membrane. The TEHY had a zebra-stripe-like design that was composed of black interval conductive CNTs and white insulative PU NFs. Due to the different electron affinity, the zebra-patterned TEHY realized a self-frictional triboelectric effect because the numerous microscopic CNT/PU triboelectric interfaces generated an alternating current in the external conductive circuit without extra external friction layers. The helical geometry combined with the elastic PU matrix endowed TEHY with superelastic stretchability and outstanding output stability after 1000 cycles of the stretch-release test. By virtue of the robust mechanical and electrical stability, the TEHY can not only be used as a high-entropy mechanical energy harvester but also serve as a self-powered sensor to monitor the stretching or deforming stimuli and human physiological activities in real time. These merits manifested the versatile applications of TEHY in smart fabrics, wearable power supplies, and human-machine interactions.

Aqueous-Aqueous Triboelectric Nanogenerators Empowered Multifunctional Wound Healing System with Intensified Current Output for Accelerating Infected Wound Repair

Triboelectric nanogenerators (TENGs) have emerged as promising devices for generating self-powered therapeutic electrical stimulation over multiple aspects of wound healing. However, the challenge of achieving full 100% contact in conventional TENGs presents a substantial hurdle in the quest for higher current output, which is crucial for further improving healing efficacy. Here, a novel multifunctional wound healing system is presented by integrating the aqueous-aqueous triboelectric nanogenerators (A-A TENGs) with a functionalized conductive hydrogel, aimed at advancing infected wound therapy. The A-A TENGs are founded on a principle of 100% contact interface and efficient post-contact separation of the immiscible interface within the aqueous two-phase system (ATPS), enhancing charge transfer and subsequently increasing current performance. Leveraging this intensified current output, this system demonstrates efficient therapeutic efficacies over infected wounds both in vitro and in vivo, including stimulating fibroblast migration and proliferation, boosting angiogenesis, enhancing collagen deposition, eradicating bacteria, and reducing inflammatory cells. Moreover, the conductive hydrogel ensures the uniformity and integrity of the electric field covering the wound site, and exhibits multiple synergistic therapeutic effects. With the capability to realize accelerated wound healing, the developed "A-A TENGs empowered multifunctional wound healing system" presenting an excellent prospect in clinical wound therapy.

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.

Droplet-Based Direct-Current Electricity Generation Induced by Dynamic Electric Double Layers

Harvesting energy from water droplets has received tremendous attention due to the pursuit of sustainable and green energy resources. The droplet-based electricity generator (DEG) provides an admirable strategy to harvest energy from droplets into electricity. However, most of the DEGs merely generate electricity of alternating current (AC) output rather than direct current (DC) without the utilization of rectifiers, impeding its practical applications in energy storage and power supply. Here, a direct current droplet-based electricity generator (DC-DEG) is developed by the simple configuration of the electrodes. The DC output originates from the dynamical electric double layer (EDL) formed at two electrodes and droplet interfaces where the charging/discharging process of EDL capacitance occurs. Several experiments are exhibited to demonstrate the rationality of the proposed principle. The influence of some factors on the output is investigated for further insight into the DC-DEG device. This work provides a novel strategy to harvest energy from water droplets directly into DC electricity and may expand the application of DEGs in powering electronic devices without the help of rectifiers.

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.

Energy harvesting from water streaming at charged surface

Water flowing at a charged surface may produce electricity, known as streaming current/potentials, which may be traced back to the 19th century. However, due to the low gained power and efficiencies, the energy conversion from streaming current was far from usable. The emergence of micro/nanofluidic technology and nanomaterials significantly increases the power (density) and energy conversion efficiency. In this review, we conclude the fundamentals and recent progress in electrical double layers at the charged surface. We estimate the generated power by hydrodynamic energy dissipation in multi-scaling flows considering the viscous systems with slipping boundary and inertia systems. Then, we review the coupling of volume flow and current flow by the Onsager relation, as well as the figure of merits and efficiency. We summarize the state-of-the-art of electrokinetic energy conversions, including critical performance metrics such as efficiencies, power densities, and generated voltages in various systems. We discuss the advantages and possible constraints by the figure of merits, including single-phase flow and flying droplets.

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.

Self-Powered Sensing in Wearable Electronics─A Paradigm Shift Technology

With the advancements in materials science and micro/nanoengineering, the field of wearable electronics has experienced a rapid growth and significantly impacted and transformed various aspects of daily human life. These devices enable individuals to conveniently access health assessments without visiting hospitals and provide continuous, detailed monitoring to create comprehensive health data sets for physicians to analyze and diagnose. Nonetheless, several challenges continue to hinder the practical application of wearable electronics, such as skin compliance, biocompatibility, stability, and power supply. In this review, we address the power supply issue and examine recent innovative self-powered technologies for wearable electronics. Specifically, we explore self-powered sensors and self-powered systems, the two primary strategies employed in this field. The former emphasizes the integration of nanogenerator devices as sensing units, thereby reducing overall system power consumption, while the latter focuses on utilizing nanogenerator devices as power sources to drive the entire sensing system. Finally, we present the future challenges and perspectives for self-powered wearable electronics.

A Review of Contact Electrification at Diversified Interfaces and Related Applications on Triboelectric Nanogenerator

The triboelectric nanogenerator (TENG) can effectively collect energy based on contact electrification (CE) at diverse interfaces, including solid-solid, liquid-solid, liquid-liquid, gas-solid, and gas-liquid. This enables energy harvesting from sources such as water, wind, and sound. In this review, we provide an overview of the coexistence of electron and ion transfer in the CE process. We elucidate the diverse dominant mechanisms observed at different interfaces and emphasize the interconnectedness and complementary nature of interface studies. The review also offers a comprehensive summary of the factors influencing charge transfer and the advancements in interfacial modification techniques. Additionally, we highlight the wide range of applications stemming from the distinctive characteristics of charge transfer at various interfaces. Finally, this review elucidates the future opportunities and challenges that interface CE may encounter. We anticipate that this review can offer valuable insights for future research on interface CE and facilitate the continued development and industrialization of TENG.

Low-grade wind-driven directional flow in anchored droplets

Low-grade wind with airspeed Vwind < 5 m/s, while distributed far more abundantly, is still challenging to extract because current turbine-based technologies require particular geography (e.g., wide-open land or off-shore regions) with year-round Vwind > 5 m/s to effectively rotate the blades. Here, we report that low-speed airflow can sensitively enable directional flow within nanowire-anchored ionic liquid (IL) drops. Specifically, wind-induced air/liquid friction continuously raises directional leeward fluid transport in the upper portion, whereas three-phase contact line (TCL) pinning blocks further movement of IL. To remove excessive accumulation of IL near TCL, fluid dives, and headwind flow forms in the lower portion, as confirmed by microscope observation. Such stratified circulating flow within single drop can generate voltage output up to ~0.84 V, which we further scale up to ~60 V using drop "wind farms". Our results demonstrate a technology to tap the widespread low-grade wind as a reliable energy resource.

Solid-Liquid Triboelectric Nanogenerator Based on Vortex-Induced Resonance

Energy converters based on vortex-induced vibrations (VIV) have shown great potential for harvesting energy from low-velocity flows, which constitute a significant portion of ocean energy. However, solid-solid triboelectric nanogenerators (TENG) are not wear-resistant in corrosive environments. Therefore, to effectively harvest ocean energy over the long term, a novel solid-liquid triboelectric nanogenerator based on vortex-induced resonance (VIV-SL-TENG) is presented. The energy is harvested through the resonance between VIV of a cylinder and the relative motions of solid-liquid friction pairs inside the cylinder. The factors that affect the output performance of the system, including the liquid mass ratio and the deflection angle of the friction plates, are studied and optimized by establishing mathematical models and conducting computational fluid dynamics simulations. Furthermore, an experimental platform for the VIV-SL-TENG system is constructed to test and validate the performance of the harvester under different conditions. The experiments demonstrate that the energy harvester can successfully convert VIV energy into electrical energy and reach maximum output voltage in the resonance state. As a new type of energy harvester, the presented design shows a promising potential in the field of 'blue energy' harvesting.

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.

A Wind-Driven Rotating Micro-Hybrid Nanogenerator for Powering Environmental Monitoring Devices

In recent years, environmental problems caused by natural disasters due to global warming have seriously affected human production and life. Fortunately, with the rapid rise of the Internet of Things (IoT) technology and the decreasing power consumption of microelectronic devices, it is possible to set up a multi-node environmental monitoring system. However, regular replacement of conventional chemical batteries for the huge number of microelectronic devices still faces great challenges, especially in remote areas. In this study, we developed a rotating hybrid nanogenerator for wind energy harvesting. Using the output characteristics of triboelectric nanogenerator (TENG) with low frequency and high voltage and electromagnetic generator (EMG) with high frequency and high current, we are able to effectively broaden the output voltage range while shortening the capacitor voltage rising time, thus obtaining energy harvesting at wide frequency wind speed. The TENG adopts the flexible contact method of arch-shaped film to solve the problem of insufficient flexible contact and the short service life of the rotating triboelectric generator. After 80,000 cycles of TENG operation, the maximum output voltage drops by 7.9%, which can maintain a good and stable output. Through experimental tests, the maximum output power of this triboelectric nanogenerator is 0.55 mW at 400 rpm (wind speed of about 8.3 m/s) and TENG part at an external load of 5 MΩ. The maximum output power of the EMG part is 15.5 mW at an external load of 360 Ω. The hybrid nanogenerator can continuously supply power to the anemometer after running for 9 s and 35 s under the simulated wind speed of 8.3 m/s and natural wind speed of 5.6 m/s, respectively. It provides a reference value for solving the power supply problem of low-power environmental monitoring equipment.

Chemo-Mechanical Energy Harvesters with Enhanced Intrinsic Electrochemical Capacitance in Carbon Nanotube Yarns

Predicting and preventing disasters in difficult-to-access environments, such as oceans, requires self-powered monitoring devices. Since the need to periodically charge and replace batteries is an economic and environmental concern, energy harvesting from external stimuli to supply electricity to batteries is increasingly being considered. Especially, in aqueous environments including electrolytes, coiled carbon nanotube (CNT) yarn harvesters have been reported as an emerging approach for converting mechanical energy into electrical energy driven by large and reversible capacitance changes under stretching and releasing. To realize enhanced harvesting performance, experimental and computational approaches to optimize structural homogeneity and electrochemical accessible area in CNT yarns to maximize intrinsic electrochemical capacitance (IEC) and stretch-induced changes are presented here. Enhanced IEC further enables to decrease matching impedance for more energy efficient circuits with harvesters. In an ocean-like environment with a frequency from 0.1 to 1 Hz, the proposed harvester demonstrates the highest volumetric power (1.6-10.45 mW cm^-3 ) of all mechanical harvesters reported in the literature to the knowledge of the authors. Additionally, a high electrical peak power of 540 W kg^-1 and energy conversion efficiency of 2.15% are obtained from torsional and tensile mechanical energy.

Control of triboelectrification on Al-metal surfaces through microstructural design

The instantaneous discharge of accumulated static charge due to contact electrification can cause irreversible damage to electrostatic-sensitive systems. Despite major advances in reducing tribo-charges, the problem remains intractable. Here, four alumina microstructures are fabricated on aluminum (Al) by combining chemical etching and anodic oxidation, and the effects of surface composition and structure on the triboelectric performance are studied by assembling them with a polytetrafluoroethylene membrane into a solid-solid triboelectric nanogenerator. The results show that the short-circuit current of the hierarchical nanoporous anodic aluminum oxide (micro/nano-AAO) modified Al is 8.77 times smaller than that of pristine Al, which is attributed to the reduced contact area and presence of an oxide film on the surface of the modified metal. By regulating the diameter of alumina nanotubes, a positive correlation between the contact area and the measured charge density is theoretically demonstrated, which establishes the size of the contact area as the main factor affecting triboelectric outputs. In addition, the micro/nano-AAO based phone shell could provide more effective electrostatic protection than that based on an acrylic coating. This novel regulation of the triboelectric output by microstructural design provides a new direction for the development of antistatic materials in a vacuum and non-grounded environment.

Achieving ultra-stable and superior electricity generation by integrating transistor-like design with lubricant armor

Extensive work have been done to harvest untapped water energy in formats of raindrops, flows, waves, and others. However, attaining stable and efficient electricity generation from these low-frequency water kinetic energies at both individual device and large-scale system level remains challenging, partially owing to the difficulty in designing a unit that possesses stable liquid and charge transfer properties, and also can be seamlessly integrated to achieve preferential collective performances without the introduction of tortuous wiring and redundant node connection with external circuit. Here, we report the design of water electricity generators featuring the combination of lubricant layer and transistor-like electrode architecture that endows enhanced electrical performances in different working environments. Such a design is scalable in manufacturing and suitable for facile integration, characterized by significant reduction in the numbers of wiring and nodes and elimination of complex interfacing problems, and represents a significant step toward large-scale, real-life applications.

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A lubricant-armored transistor-like electricity generator is proposed

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The transistor-like electrode architecture causes high electrical output

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The lubricant armor ensures stable performance in extreme environments

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The design is scalable in manufacturing and suitable for facile integration

Liquid-liquid triboelectric nanogenerator based on the immiscible interface of an aqueous two-phase system

Solid nanogenerators often have limited charge transfer due to their low contact area. Liquid–liquid nanogenerators can transfer a charge better than the solid–solid and solid–liquid counterparts. However, the precise manipulation of the liquid morphology remains a challenge because of the fluidity limits of the liquid. In this work, using the surface tension of a droplet to fix its shape, a liquid-liquid triboelectric nanogenerator in Contact-Separation mode is designed using an immiscible aqueous-aqueous interface, achieving a contact surface charge transfer of 129 nC for a single droplet. The configuration is proven to be applicable in humid environments, and the two-phase materials have good biocompatibility and can be used as an effective drug carrier. Therefore, this nanogenerator is useful for designing future implantable devices. Meanwhile, this design also establishes the foundation of aqueous electronics, and additional applications can be achieved using this route.

While liquid-liquid interface offers better contact and charge transfer potential than solid-based counterparts, fluidity still poses challenges for their application. Here, authors show that charge transfer exists in aqueous two-phase systems and propose a nanogenerator design based on the immiscible aqueous-aqueous interface.

Self-Powered, Long-Durable, and Highly Selective Oil-Solid Triboelectric Nanogenerator for Energy Harvesting and Intelligent Monitoring

Triboelectric nanogenerators (TENGs) have potential to achieve energy harvesting and condition monitoring of oils, the "lifeblood" of industry. However, oil absorption on the solid surfaces is a great challenge for oil-solid TENG (O-TENG). Here, oleophobic/superamphiphobic O-TENGs are achieved via engineering of solid surface wetting properties. The designed O-TENG can generate an excellent electricity (with a charge density of 9.1 µC m^-2 and a power density of 1.23 mW m^-2), which is an order of magnitude higher than other O-TENGs made from polytetrafluoroethylene and polyimide. It also has a significant durability (30,000 cycles) and can power a digital thermometer for self-powered sensor applications. Further, a superhigh-sensitivity O-TENG monitoring system is successfully developed for real-time detecting particle/water contaminants in oils. The O-TENG can detect particle contaminants at least down to 0.01 wt% and water contaminants down to 100 ppm, which are much better than previous online monitoring methods (particle > 0.1 wt%; water > 1000 ppm). More interesting, the developed O-TENG can also distinguish water from other contaminants, which means the developed O-TENG has a highly water-selective performance. This work provides an ideal strategy for enhancing the output and durability of TENGs for oil-solid contact and opens new intelligent pathways for oil-solid energy harvesting and oil condition monitoring.

Triboelectric Nanogenerators for Harvesting Diverse Water Kinetic Energy

The water covering the Earth’s surface not only supports life but also contains a tremendous amount of energy. Water energy is the most important and widely used renewable energy source in the environment, and the ability to extract the mechanical energy of water is of particular interest since moving water is ubiquitous and abundant, from flowing rivers to falling rain drops. In recent years, triboelectric nanogenerators (TENGs) have been promising for applications in harvesting kinetic energy from water due to their merits of low cost, light weight, simple structure, and abundant choice of materials. Furthermore, TENGs can also be utilized as self-powered active sensors for monitoring water environments, which relies on the output signals of the TENGs caused by the movement and composition of water. Here, TENGs targeting the harvest of different water energy sources have been systematically summarized and analyzed. The TENGs for harvesting different forms of water energy are introduced and divided on the basis of their basic working principles and modes, i.e., in the cases of solid–solid and solid–liquid. A detailed review of recent important progress in TENG-based water energy harvesting is presented. At last, based on recent progresses, the existing challenges and future prospects for TENG-based water energy harvesting are also discussed.

Energy Conversion Analysis of Multilayered Triboelectric Nanogenerators for Synergistic Rain and Solar Energy Harvesting

The triboelectric nanogenerator (TENG) is an emerging technology that offers excellent potential for the conversion of mechanical energy from rain into electricity for hybrid energy applications. However, a high-performance TENG is yet to be achieved because a quantitative analysis method for the energy conversion process is still lacking. Herein, a quantitative analysis method, termed the "kinetic energy calculation and current integration" (KECCI) method, which significantly improves the understanding of the mechanical-to-electrical energy conversion process, is presented. Based on the KECCI method, a high-performance TENG is developed by systematically optimizing a biomimetic surface structure and instant switch design, with 1.25 mA short-circuit current (I_sc ), 150 V open-circuit voltage (V_oc ), and a high energy-conversion efficiency of 24.89%. Furthermore, a multilayered TENG device is proposed for continuously harvesting the kinetic energy of raindrops for further improvement in the energy-conversion efficiency. Finally, the multilayered TENGs are integrated with organic photovoltaics, achieving all-weather energy harvesting. This work presents a validated theoretical basis that will guide further development of TENGs toward higher performances, which will promote the commercialization of hybrid TENG systems for all-weather applications.

Sustainable power generation for at least one month from ambient humidity using unique nanofluidic diode

The continuous energy-harvesting in moisture environment is attractive for the development of clean energy source. Controlling the transport of ionized mobile charge in intelligent nanoporous membrane systems is a promising strategy to develop the moisture-enabled electric generator. However, existing designs still suffer from low output power density. Moreover, these devices can only produce short-term (mostly a few seconds or a few hours, rarely for a few days) voltage and current output in the ambient environment. Here, we show an ionic diode–type hybrid membrane capable of continuously generating energy in the ambient environment. The built-in electric field of the nanofluidic diode-type PN junction helps the selective ions separation and the steady-state one-way ion charge transfer. This directional ion migration is further converted to electron transportation at the surface of electrodes via oxidation-reduction reaction and charge adsorption, thus resulting in a continuous voltage and current with high energy conversion efficiency.

Energy harvesting of humidity present in air can be used for the development of clean energy sources and self-sustained systems. The authors propose a nanofluid energy conversion system with integrated ionic diode-type hybrid membrane for energy generation in environmental moisture.

Progress of Advanced Devices and Internet of Things Systems as Enabling Technologies for Smart Homes and Health Care

In the Internet of Things (IoT) era, various devices (e.g., sensors, actuators, energy harvesters, etc.) and systems have been developed toward the realization of smart homes/buildings and personal health care. These advanced devices can be categorized into ambient devices and wearable devices based on their usage scenarios, to enable motion tracking, health monitoring, daily care, home automation, fall detection, intelligent interaction, assistance, living convenience, and security in smart homes. With the rapidly increasing number of such advanced devices and IoT systems, achieving fully self-sustained and multimodal intelligent systems is becoming more and more important to realize a sustainable and all-in-one smart home platform. Hence, in this Review, we systematically present the recent progress of the development of advanced materials, fabrication techniques, devices, and systems for enabling smart home and health care applications. First, advanced polymer, fiber, and fabric materials as well as their respective fabrication techniques for large-scale manufacturing are discussed. After that, functional devices classified into ambient devices (at home ambiance such as door, floor, table, chair, bed, toilet, window, wall, etc.) and wearable devices (on body parts such as finger, wrist, arm, throat, face, back, etc.) are presented for diverse monitoring and auxiliary applications. Next, the current developments of self-sustained systems and intelligent systems are reviewed in detail, indicating two promising research directions in this field. Last, conclusions and outlook pinpointed on the existing challenges and opportunities are provided for the research community to consider.

Sustainable power generation via hydro-electrochemical effects

Recent efforts towards energy scavenging with eco-friendly methods and abundant water look very promising for powering wearables and distributed electronics. However, the time duration of electricity generation is typically too short, and the current level is not sufficient to meet the required threshold for the proper operation of electronics despite the relatively large voltage. This work newly introduced an electrochemical method in combination with hydro-effects in order to extend the energy scavenging time and boost the current. Our device consists of corroded porous steel electrodes whose corrosion overpotential was lowered when the water concentration was increased and vice versa. Then a potential difference was created between two electrodes, generating electricity via the hydro-electrochemical method up to an open-circuit voltage of 750 mV and a short-circuit current of 90 μA cm^-2. Furthermore, electricity was continuously generated for more than 1500 minutes by slow water diffusion against gravity from the bottom electrode. Lastly, we demonstrated that our hydro-electrochemical power generators successfully operated electronics, showing the feasibility of offering electrical power for sufficiently long time periods in practice.

Circadian humidity fluctuation induced capillary flow for sustainable mobile energy

Circadian humidity fluctuation is an important factor that affects human life all over the world. Here we show that spherical cap-shaped ionic liquid drops sitting on nanowire array are able to continuously output electricity when exposed to outdoor air, which we attribute to the daily humidity fluctuation induced directional capillary flow. Specifically, ionic liquid drops could absorb/desorb water around the liquid/vapor interface and swell/shrink depending on air humidity fluctuation. While pinning of the drop by nanowire array suppresses advancing/receding of triple-phase contact line. To maintain the surface tension-regulated spherical cap profile, inward/outward flow arises for removing excess fluid from the edge or filling the perimeter with fluid from center. This moisture absorption/desorption-caused capillary flow is confirmed by in-situ microscope imaging. We conduct further research to reveal how environmental humidity affects flow rate and power generation performance. To further illustrate feasibility of our strategy, we combine the generators to light up a red diode and LCD screen. All these results present the great potential of tiny humidity fluctuation as an easily accessible anytime-and-anywhere small-scale green energy resource.

Droplet generators convert mechanical movements of droplets into small-scale electricity. Here, Tang et al. report a humidity-driven power generator by utilizing daily humidity fluctuation in atmosphere enabling continuous generation of electricity upon moisture absorption and desorption cycles.

A Direct-Current Triboelectric Nanogenerator Energy Harvesting System Based on Water Electrification for Self-Powered Electronics

This study aimed to develop a simple but effective mechanical-to-electrical energy conversion for harvesting hydrokinetic energy based on triboelectric nanogenerator (TENG) technology. Here, a direct-current fluid-flow-based TENG is reported as a potential solution to solve the inconvenience of directly powering electronic devices where direct-current (DC) power is required. The falling of a water droplet (about 1.06 mL) from an elastomeric pipe can generate an open-circuit voltage of ~35 V, short-circuit current of 3.7 µA, and peak power of 57.6 µW by passing through a separated electrode. Notably, the electrical responses have the distinct characteristics of pulsed direct current. The ability to generate DC outputs enables the TENG to directly drive electronic devices. Our experimental results prove that this TENG can act as a power source to directly light up 50 light-emitting diodes without requiring a rectifier, and, also, the produced electric energy was demonstrated that can be stored directly in a capacitor to power commercial temperature and humidity IoT sensors. Furthermore, the device shows a greatly varied output voltage based on the droplet flow rate, with a linearity R2 = 0.998. This work highlights a promising potential for applications in harvesting hydrokinetic energy and self-powered sensors and systems.

Gridding Triboelectric Nanogenerator for Raindrop Energy Harvesting

Triboelectric nanogenerator (TENG) has the great potential to harvest the electrostatic energy and mechanical energy of raindrops. However, raindrops are small and scattered, and it is difficult to harvest their mechanical energy effectively. In this paper, a gridding triboelectric nanogenerator (G-TENG) with an area of 81 cm² is designed and developed to effectively harvest the mechanical energy of raindrops on a large scale. Its peak output power density is 8.56 mW/m², which is 245 times the value of 35 μW/m² of a general TENG without gridding. Each unit of the G-TENG can work independently, which can effectively decrease the mutual counteraction of elastic deformation among the adjacent positions of the raindrop impacting layer and avoid the accumulation of raindrops. Under the impact of simulated raindrops from a shower at a flow rate of 0.137 mL/(cm²·s), the open-circuit voltage (V_oc) and the short-circuit current density (J_sc) of the G-TENG reach 400 V and 2.5 mA/m², respectively. The peak output power density reaches 110 mW/m², which is 42 times the reported maximum value of 2.6 mW/m² of raindrop energy harvesting TENGs with the size larger than 10 cm². Moreover, the G-TENG can harvest the mechanical energy of raindrops at a wide range of raindrop flow rates from 0.055 to 0.219 mL/(cm²·s). This work contributes to the raindrop mechanical energy harvesting on a large scale.

Study of Contact Electrification at Liquid-Gas Interface

It is known that the suspended liquid droplets in clouds can generate electrostatic charges, which finally results in the lightning. However, the detailed mechanism related to the contact-electrification process on the liquid-gas (L-G) interfaces is still poorly understood. Here, by introducing an acoustic levitation method for levitating a liquid droplet, we have studied the electrification mechanism at the L-G interface. The tribo-motion between water droplets and air induced by the ultrasound wave leads to the generation of positive charges on the surface of the droplets, and the charge amount of water droplets (20 μL) gradually reaches saturation within 30 s. The mixed solid particles in droplets can increase the amount of transferred charge, whereas the increase of ion concentration in the droplet can suppress the charge generation. This charge transfer phenomenon at L-G interfaces and the related analysis can be a guidance for the study in many fields, including anti-static, harvesting rainy energy, micro/nano fluidics, triboelectric power generator, surface engineering, and so on. Moreover, the surface charge generation due to L-G electrification is an inevitable effect during ultrasonic levitation, and thus, this study can also work for the applications of the ultrasonic technique.

Droplet-based nanogenerators for energy harvesting and self-powered sensing

The energy crisis is a continuing topic for all human beings, threatening the development of human society. Accordingly, harvesting energy from the surrounding environment, such as wind, water flow and solar power, has become a promising direction for the research community. Water contains tremendous energy in a variety of forms, such as rivers, ocean waves, tides, and raindrops. Among them, raindrop energy is the most abundant. Raindrop energy not only can complement other forms of energy, such as solar energy, but also have potential applications in wearable and universal energy collectors. Over the past few years, droplet-based electricity nanogenerators (DENG) have attracted significant attention due to their advantages of small size and high power. To date, a variety of fundamental materials and ingenious structural designs have been proposed to achieve efficient droplet-based energy harvesting. The research and application of DENG in various fields have received widespread attention. In this review, we focus on the fundamental mechanism and recent progress of droplet-based nanogenerators in the following three aspects: droplet properties, energy harvesting and self-powered sensing. Finally, some challenges and further outlook for droplet-based nanogenerators are discussed to boost the future development of this promising field.

Quantifying Contact-Electrification Induced Charge Transfer on a Liquid Droplet after Contacting with a Liquid or Solid

Contact electrification (CE) is a common physical phenomenon, and its mechanisms for solid-solid and liquid-solid cases have been widely discussed. However, the studies about liquid-liquid CE are hindered by the lack of proper techniques. Here, a contactless method is proposed for quantifying the charges on a liquid droplet based on the combination of electric field and acoustic field. The liquid droplet is suspended in an acoustic field, and an electric field force is created on the droplet to balance the acoustic trap force. The amount of charges on the droplet is thus calculated based on the equilibrium of forces. Further, the liquid-solid and liquid-liquid CE are both studied by using the method, and the latter is focused. The behavior of negatively precharged liquid droplet in the liquid-liquid CE is found to be different from that of the positively precharged one. The results show that the silicone oil droplet prefers to receive negative charges from a negatively charged aqueous droplet rather than positive charges from a positively charged aqueous droplet, which provides a strong evidence about the dominant role played by electron transfer in the liquid-liquid CE.

Self-Healing and Elastic Triboelectric Nanogenerators for Muscle Motion Monitoring and Photothermal Treatment

Owing to wearing and unpredictable damage, the working lifetime of triboelectric nanogenerators (TENGs) is largely limited. In this work, we prepared a single-electrode multifunctional TENG (MF-TENG) that exhibits fast self-healing, human health monitoring capability, and photothermal properties. The device consists of a thin self-healing poly(vinyl alcohol)-based hydrogel sandwiched between two self-healing silicone elastomer films. The MF-TENG exhibits a short-circuit current, short-circuit transfer charge, and open-circuit voltage of 7.98 μA, 78.34 nC, and 38.57 V, respectively. Furthermore, owing to the repairable networks of the dynamic imine bonds in the charged layer and the borate ester bonds in the electrodes, the prepared device could recover its original state after mechanical damage within 10 min at room temperature. The MF-TENG can be attached to different human joints for self-powered monitoring of personal health information. Additionally, the MF-TENG under near-infrared laser irradiation can provide a photothermal therapy for assisting the recovery of human joints motion. It is envisaged that the proposed MF-TENG can be applied to the fields of wearable electronics and health-monitoring devices.

Light-Weight, Self-Powered Sensor Based on Triboelectric Nanogenerator for Big Data Analytics in Sports

With the rapid development of the Internet of Things (IoTs), big data analytics has been widely used in the sport field. In this paper, a light-weight, self-powered sensor based on a triboelectric nanogenerator for big data analytics in sports has been demonstrated. The weight of each sensing unit is ~0.4 g. The friction material consists of polyaniline (PANI) and polytetrafluoroethylene (PTFE). Based on the triboelectric nanogenerator (TENG), the device can convert small amounts of mechanical energy into the electrical signal, which contains information about the hitting position and hitting velocity of table tennis balls. By collecting data from daily table tennis training in real time, the personalized training program can be adjusted. A practical application has been exhibited for collecting table tennis information in real time and, according to these data, coaches can develop personalized training for an amateur to enhance the ability of hand control, which can improve their table tennis skills. This work opens up a new direction in intelligent athletic facilities and big data analytics.

Triboelectric Nanogenerators for Energy Harvesting in Ocean: A Review on Application and Hybridization

With recent advancements in technology, energy storage for gadgets and sensors has become a challenging task. Among several alternatives, the triboelectric nanogenerators (TENG) have been recognized as one of the most reliable methods to cure conventional battery innovation’s inadequacies. A TENG transfers mechanical energy from the surrounding environment into power. Natural energy resources can empower TENGs to create a clean and conveyed energy network, which can finally facilitate the development of different remote gadgets. In this review paper, TENGs targeting various environmental energy resources are systematically summarized. First, a brief introduction is given to the ocean waves’ principles, as well as the conventional energy harvesting devices. Next, different TENG systems are discussed in details. Furthermore, hybridization of TENGs with other energy innovations such as solar cells, electromagnetic generators, piezoelectric nanogenerators and magnetic intensity are investigated as an efficient technique to improve their performance. Advantages and disadvantages of different TENG structures are explored. A high level overview is provided on the connection of TENGs with structural health monitoring, artificial intelligence and the path forward.

From contact electrification to triboelectric nanogenerators

Although the contact electrification (CE) (or usually called 'triboelectrification') effect has been known for over 2600 years, its scientific mechanism still remains debated after decades. Interest in studying CE has been recently revisited due to the invention of triboelectric nanogenerators (TENGs), which are the most effective approach for converting random, low-frequency mechanical energy (called high entropy energy) into electric power for distributed energy applications. This review is composed of three parts that are coherently linked, ranging from basic physics, through classical electrodynamics, to technological advances and engineering applications. First, the mechanisms of CE are studied for general cases involving solids, liquids and gas phases. Various physics models are presented to explain the fundamentals of CE by illustrating that electron transfer is the dominant mechanism for CE for solid-solid interfaces. Electron transfer also occurs in the CE at liquid-solid and liquid-liquid interfaces. An electron-cloud overlap model is proposed to explain CE in general. This electron transfer model is extended to liquid-solid interfaces, leading to a revision of the formation mechanism of the electric double layer at liquid-solid interfaces. Second, by adding a time-dependent polarization termPscreated by the CE-induced surface electrostatic charges in the displacement fieldD, we expand Maxwell's equations to include both the medium polarizations due to electric field (P) and mechanical aggitation and medium boundary movement induced polarization term (Ps). From these, the output power, electromagnetic (EM) behaviour and current transport equation for a TENG are systematically derived from first principles. A general solution is presented for the modified Maxwell's equations, and analytical solutions for the output potential are provided for a few cases. The displacement current arising fromε∂E/∂t is responsible for EM waves, while the newly added term ∂Ps/∂t is responsible for energy and sensors. This work sets the standard theory for quantifying the performance and EM behaviour of TENGs in general. Finally, we review the applications of TENGs for harvesting all kinds of available mechanical energy that is wasted in our daily life, such as human motion, walking, vibration, mechanical triggering, rotating tires, wind, flowing water and more. A summary is provided about the applications of TENGs in energy science, environmental protection, wearable electronics, self-powered sensors, medical science, robotics and artificial intelligence.

Moisture-induced autonomous surface potential oscillations for energy harvesting

A variety of autonomous oscillations in nature such as heartbeats and some biochemical reactions have been widely studied and utilized for applications in the fields of bioscience and engineering. Here, we report a unique phenomenon of moisture-induced electrical potential oscillations on polymers, poly([2-(methacryloyloxy)ethyl] dimethyl-(3-sulfopropyl) ammonium hydroxide-co-acrylic acid), during the diffusion of water molecules. Chemical reactions are modeled by kinetic simulations while system dynamic equations and the stability matrix are analyzed to show the chaotic nature of the system which oscillates with hidden attractors to induce the autonomous surface potential oscillation. Using moisture in the ambient environment as the activation source, this self-excited chemoelectrical reaction could have broad influences and usages in surface-reaction based devices and systems. As a proof-of-concept demonstration, an energy harvester is constructed and achieved the continuous energy production for more than 15,000 seconds with an energy density of 16.8 mJ/cm2. A 2-Volts output voltage has been produced to power a liquid crystal display toward practical applications with five energy harvesters connected in series.

Triboelectric Nanogenerator-Based Sensor Systems for Chemical or Biological Detection

The rapid advances in the Internet of things and wearable devices have created a massive platform for sensor systems that detect chemical or biological agents. The accelerated development of these devices in recent years has simultaneously aggravated the power supply problems. Triboelectric nanogenerators (TENGs) represent a thriving renewable energy technology with the potential to revolutionize this field. In this review, the significance of TENG-based sensor systems in chemical or biological detection from the perspective of the development of power supply for biochemical sensors is discussed. Further, a range of TENGs are classified according to their roles as power supplies and/or self-powered active sensors. The TENG powered sensor systems are further discussed on the basis of their framework and applications. The working principles and structures of different TENG-based self-powered active sensors are presented, along with the classification of the sensors based on these factors. In addition, some representative applications are introduced, and the corresponding challenges are discussed. Finally, some perspectives for the future innovations of TENG-based sensor systems for chemical/biological detection are discussed.

All-Weather Droplet-Based Triboelectric Nanogenerator for Wave Energy Harvesting

The liquid-solid triboelectric nanogenerator (LS-TENG) has been demonstrated to harvest energy efficiently through the contact electrification effect between liquid and solid triboelectric materials, which can avoid the wear issue in solid-solid TENG. However, the droplet-based LS-TENG reveals the problems that it generally works with the continuous falling droplets or needs to be fully packaged, which greatly limit its practical application. Here, a droplet-based triboelectric nanogenerator (DB-TENG) with a simple open structure is designed to effectively solve these problems. The nonpackaged DB-TENG can work stably under extreme conditions with high humidity or high concentrations of salt, acid, or alkali solutions, showing the DB-TENGs can be flexibly utilized in all types of working environments with better reliability and lower maintenance costs. It is of great significance that the integrated DB-TENG network array can realize the all-weather ocean energy harvesting. Furthermore, under the simulated ocean wave, a scaled-up DB-TENG with considerable output performance can charge capacitors and drive electrical devices. Overall, the DB-TENG shows many advantages: simple open structure, all-weather working ability, timely supplement of water loss, no tight packaging, wear resistance, suitable for extreme working environments. This work provides a convenient and feasible way toward all-weather wave energy harvesting in real marine environments.

Functional liquid droplets for analyte sensing and energy harvesting

Over the past century, rapid miniaturization of technologies has helped in the development of efficient, flexible, portable, robust, and compact applications with minimal wastage of materials. In this direction, of late, the usage of mesoscale liquid droplets has emerged as an alternative platform because of the following advantages: (i) a droplet is incompressible and at the same time deformable, (ii) interfacial area of a spherical droplet is minimum for a given amount of mass; and (iii) a droplet interface allows facile mass, momentum, and energy transfer. Subsequently, such attributes have aided towards the design of diverse droplet-based microfluidic technologies. For example, the microdroplets have been utilized as micro-reactors, colorimetric or electrochemical (EC) sensors, drug-delivery vehicles, and energy harvesters. Further, a number of recently reported lab-on-a-chip technologies exploit the motility, storage, and mixing capacities of the microdroplets. In view of this background, the review initiates discussion by highlighting the different attributes of the microdroplets such as size, shape, surface to volume ratio, wettability, and contact line. Thereafter, the effects of the surface or body forces on the properties of the droplets have been elaborated. Finally, the different aspects of such liquid droplet systems towards technological adaptations in health care, sensing, and energy harvesting have been presented. The review concludes with a tight summary on the potential avenues for further developments.

Nanogenerator-based self-powered sensors for data collection

Self-powered sensors can provide energy and environmental data for applications regarding the Internet of Things, big data, and artificial intelligence. Nanogenerators provide excellent material compatibility, which also leads to a rich variety of nanogenerator-based self-powered sensors. This article reviews the development of nanogenerator-based self-powered sensors for the collection of human physiological data and external environmental data. Nanogenerator-based self-powered sensors can be designed to detect physiological data as wearable and implantable devices. Nanogenerator-based self-powered sensors are a solution for collecting data and expanding data dimensions in a future intelligent society. The future key challenges and potential solutions regarding nanogenerator-based self-powered sensors are discussed.

Contact Electrification at the Liquid-Solid Interface

Interfaces between a liquid and a solid (L-S) are the most important surface science in chemistry, catalysis, energy, and even biology. Formation of an electric double layer (EDL) at the L-S interface has been attributed due to the adsorption of a layer of ions at the solid surface, which causes the ions in the liquid to redistribute. Although the existence of a layer of charges on a solid surface is always assumed, the origin of the charges is not extensively explored. Recent studies of contact electrification (CE) between a liquid and a solid suggest that electron transfer plays a dominant role at the initial stage for forming the charge layer at the L-S interface. Here, we review the recent works about electron transfer in liquid-solid CE, including scenerios such as liquid-insulator, liquid-semiconductor, and liquid-metal. Formation of the EDL is revisited considering the existence of electron transfer at the L-S interface. Furthermore, the triboelectric nanogenerator (TENG) technique based on the liquid-solid CE is introduced, which can be used not only for harvesting mechanical energy from a liquid but also as a probe for probing the charge transfer at liquid-solid interfaces.

Vapor bubble induced electric current generation

Contact electrification (CE) has been utilized in various energy conversion systems in recent years. This work presents a constant electric energy output that was generated based on the CE at the water–metal interface. When a grounded Pt mesh is placed in water that is heated to boil, a continuous flow of electrons between the Pt mesh and the ground is generated. A possible mechanism for the generation of such electric current is based on the CE between the surface of the Pt mesh and water molecules. The local high-pressure thin liquid film regions between vapor bubbles and surface of Pt mesh promote this CE process. The constant water evaporation and bubble detachment enable the continuous electric current output. In this work, the impact of the heating temperature and the bias voltages on the generation of the current was also studied. This work provides an alternative approach to generate unidirectional current on the basis of CE at the water–metal interface, and it also offers new insights in the design of CE-based systems for the generation of electricity.

Energy Harvesting and Storage with Soft and Stretchable Materials

This review highlights various modes of converting ambient sources of energy into electricity using soft and stretchable materials. These mechanical properties are useful for emerging classes of stretchable electronics, e-skins, bio-integrated wearables, and soft robotics. The ability to harness energy from the environment allows these types of devices to be tetherless, thereby leading to a greater range of motion (in the case of robotics), better compliance (in the case of wearables and e-skins), and increased application space (in the case of electronics). A variety of energy sources are available including mechanical (vibrations, human motion, wind/fluid motion), electromagnetic (radio frequency (RF), solar), and thermodynamic (chemical or thermal energy). This review briefly summarizes harvesting mechanisms and focuses on the materials' strategies to render such devices into soft or stretchable embodiments.

Functional Fibers and Fabrics for Soft Robotics, Wearables, and Human-Robot Interface

Soft robotics inspired by the movement of living organisms, with excellent adaptability and accuracy for accomplishing tasks, are highly desirable for efficient operations and safe interactions with human. With the emerging wearable electronics, higher tactility and skin affinity are pursued for safe and user-friendly human-robot interactions. Fabrics interlocked by fibers perform traditional static functions such as warming, protection, and fashion. Recently, dynamic fibers and fabrics are favorable to deliver active stimulus responses such as sensing and actuating abilities for soft-robots and wearables. First, the responsive mechanisms of fiber/fabric actuators and their performances under various external stimuli are reviewed. Fiber/yarn-based artificial muscles for soft-robots manipulation and assistance in human motion are discussed, as well as smart clothes for improving human perception. Second, the geometric designs, fabrications, mechanisms, and functions of fibers/fabrics for sensing and energy harvesting from the human body and environments are summarized. Effective integration between the electronic components with garments, human skin, and living organisms is illustrated, presenting multifunctional platforms with self-powered potential for human-robot interactions and biomedicine. Lastly, the relationships between robotic/wearable fibers/fabrics and the external stimuli, together with the challenges and possible routes for revolutionizing the robotic fibers/fabrics and wearables in this new era are proposed.

Power Backpack for Energy Harvesting and Reduced Load Impact

Long-distance walking with heavy loads is often needed when going hiking or for field rescue, which is prone to cumulative fatigue. There is also a great need for labor-saving and biomechanical energy harvesting in daily life for extended security and communication needs. Here, we report a load-suspended backpack for harvesting the wasted energy of human motion based on a triboelectric nanogenerator (TENG). Two elastomers are incorporated into the backpack to decouple the synchronous movement of the load and the human body, which results in little or no extra accelerative force. With such a design, through theoretical analysis and field experiments, the backpack can realize a reduction of 28.75 % in the vertical oscillation of the load and 21.08 % in the vertical force on the wearer, respectively. Meanwhile, the mechanical-to-electric energy conversion efficiency is modeled and calculated to be 14.02 % under normal walking conditions. The designed backpack has the merits of labor-saving and shock absorption as well as electricity generation, which has the promising potential to be a power source for small-scale wearable and portable electronics, GPS systems, and other self-powered health care sensors.

A high-efficiency bioinspired photoelectric-electromechanical integrated nanogenerator

Currently, the key challenge in triboelectric nanogenerators (TENGs) is how to efficiently enhance the surface charge density. Here, a new strategy is proposed to increase the surface charge density by comprehensively utilizing solar energy and tidal energy, and a bioinspired photoelectric-electromechanical integrated TENG (Pem-iTENG) is developed. This enhancement of output performance is greatly attributed to the accumulation of photoelectrons from photocatalysis and the triboelectric negative charges from contact electrification. Pem-iTENG shows a maximal open-circuit voltage of 124.2 V and a maximal short-circuit current density of 221.6 μA cm⁻² under tidal wave and sunlight, an improvement by nearly a factor of 10 over that of reported TENGs based on solid-liquid contact electrification. More importantly, it exhibits a high energy conversion efficiency according to the evaluation method for solar cells. This work provides insights into development of high-performance TENGs by using different natural energy sources.

Increasing the surface charge density of a triboelectric nanogenerator (TENG) under a single energy source is of wide interest. Here, the authors increase the surface charge density by comprehensively utilizing solar energy and tidal energy to develop a bioinspired photoelectric-electromechanical integrated TENG.