Nanogenerators for Self-Powered Gas Sensing

Gel-Based Triboelectric Nanogenerators for Flexible Sensing: Principles, Properties, and Applications

The rapid development of the Internet of Things and artificial intelligence technologies has increased the need for wearable, portable, and self-powered flexible sensing devices. Triboelectric nanogenerators (TENGs) based on gel materials (with excellent conductivity, mechanical tunability, environmental adaptability, and biocompatibility) are considered an advanced approach for developing a new generation of flexible sensors. This review comprehensively summarizes the recent advances in gel-based TENGs for flexible sensors, covering their principles, properties, and applications. Based on the development requirements for flexible sensors, the working mechanism of gel-based TENGs and the characteristic advantages of gels are introduced. Design strategies for the performance optimization of hydrogel-, organogel-, and aerogel-based TENGs are systematically summarized. In addition, the applications of gel-based TENGs in human motion sensing, tactile sensing, health monitoring, environmental monitoring, human-machine interaction, and other related fields are summarized. Finally, the challenges of gel-based TENGs for flexible sensing are discussed, and feasible strategies are proposed to guide future research.

Ultrasensitive and Exclusive Chemiresistors with a ZIF-67-Derived Oxide Cage/Nanofiber Co3O4/In2O3 Heterostructure for Acetone Detection

Gas sensors for acetone detection have received considerable attention because acetone has a significant influence on both the environment and human health, e.g., it is flammable and toxic and may be related to blood glucose levels. However, achieving high sensitivity and selectivity at low concentrations is still a great challenge to date. Here, we report a unique chemiresistive gas sensor for acetone detection, which is composed of In2O3 nanofibers loaded with a porous Co-based zeolitic imidazolate framework (ZIF-67)-derived Co3O4 cage prepared by simple electrospinning and solvothermal methods. The ZIF-67-derived oxide cage/nanofiber Co3O4/In2O3 heterostructure has abundant reversible active adsorption/reaction sites and a type-I heterojunction, resulting in an ultrasensitive response of 954-50 ppm acetone at 300 °C. In addition, it demonstrates a low detection limit of 18.8 ppb, a fast response time of 4 s, good selectivity and repeatability, acceptable humidity interference, and long-term stability. With such excellent sensing performance to acetone, our chemiresistive gas sensor could be potentially applied for environmental monitoring and early diagnosis of diabetes.

Polypyrrole@CNT@PU Conductive Sponge-Based Triboelectric Nanogenerators for Human Motion Monitoring and Self-Powered Ammonia Sensing

Elastic sponges are ideal materials for triboelectric nanogenerators (TENGs) to harvest irregular and random mechanical energy from the environment. However, the conductive design of the elastic materials in TENGs often limits its applications. In this work, we have demonstrated that an elastic conductive sponge can be used as the triboelectric layer and electrode for TENGs. Such an elastic conductive sponge is prepared by a simple way of adsorbing multiwalled carbon nanotubes and monomers of pyrrole to grow conductive polypyrroles on the surface of an elastic polyurethane (PU) sponge. Due to the porous structure of the PU sponge and the conductive multiwalled carbon nanotubes (MWCNTs), PPy on the surface of PU could provide a large contact area to improve the output performance of TENGs, and the conductive sponge-based TENG could generate an output of open-circuit voltage of 110 V or a short-circuit current of 12 μA, respectively. The good flexibility of the conductive PU sponge makes the TENG harvest the kinetic energy of disordered motion with different amplitudes, allowing for human motion monitoring. Furthermore, the porous structure of PU and the synergistic effects of PPy and MWCNTs enable the conductive sponge to sense NH3 as a self-powered NH3 sensor. This work offers a simple way to construct a flexible TENG system for random mechanical energy harvesting, human motion monitoring, and self-powered NH3 sensing.

Design of stretchable and self-powered sensing device for portable and remote trace biomarkers detection

Timely and remote biomarker detection is highly desired in personalized medicine and health protection but presents great challenges in the devices reported so far. Here, we present a cost-effective, flexible and self-powered sensing device for H2S biomarker analysis in various application scenarios based on the structure of galvanic cells. The sensing mechanism is attributed to the change in electrode potential resulting from the chemical adsorption of gas molecules on the electrode surfaces. Intrinsically stretchable organohydrogels are used as solid-state electrolytes to enable stable and long-term operation of devices under stretching deformation or in various environments. The resulting open-circuit sensing device exhibits high sensitivity, low detection limit, and excellent selectivity for H2S. Its application in the non-invasive halitosis diagnosis and identification of meat spoilage is demonstrated, emerging great commercial value in portable medical electronics and food security. A wireless sensory system has also been developed for remote H2S monitoring with the participation of Bluetooth and cloud technologies. This work breaks through the shortcomings in the traditional chemiresistive sensors, offering a direction and theoretical foundation for designing wearable sensors catering to other stimulus detection requirements.

Metal-Organic Framework Based Triboelectric Nanogenerator for a Self-Powered Methanol Sensor with High Sensitivity and Selectivity

Triboelectric nanogenerators have shown great potential in the area of self-powered gas sensors in the past decade. In this paper, we developed a triboelectric nanogenerator (TENG) based on spiky structured ZIF-8@ZnO, which can harvest energy with high efficiency and act as a self-powered methanol sensor. The open-circuit voltage and short-circuit current generated by a ZIF-8@ZnO-based TENG is 58 V and 10 μA, achieving 2.4 times and 3.3 times enhancement compared to ZnO-based TENGs. The TENG can charge capacitors fast and light up at least 40 LEDs. ZIF-8@ZnO-based TENGs show good sensitivity and selectivity to methanol gas at room temperature due to the porous structure provided by ZIF-8 and the heterostructure of ZIF-8@ZnO. The response of ZIF-8@ZnO-based TENG to methanol reaches 30.35% at 100 ppm with excellent response (∼5.9 s) and recovery time (∼2.2 s). This work demonstrates the application of MOF-modified metal oxide semiconductors based on a self-powered gas sensor and proposes a promising solution to enhance the output performance and sensing properties of TENGs based on metal oxide semiconductors.

A holistic review on the recent trends, advances, and challenges for high-precision room temperature liquefied petroleum gas sensors

Liquefied petroleum gas (LPG), which is mainly composed of hydrocarbons, such as propane and butane, is a flammable gas that is considered a clean source of energy. Currently, the overwhelming use of LPG as fuel in vehicles, domestic settings, and industry has led to several incidents and deaths globally due to leakage. As a result, the appropriate detection of LPG is vital; thus, gas-sensing devices that can monitor this gas rapidly and accurately at room temperature, are required. This work reviews the current advances in LPG gas sensors, which operate at room temperature. The influences of the synthesis methods and parameters, doping, and use of catalysts on the sensing performance are discussed. The formation of heterostructures made from semiconducting metal oxides, polymers, and graphene-based materials, which enhance the sensor selectivity and sensitivity, is also discussed. The future trends and challenges confronted in the advancement of LPG room temperature operational gas sensors, and critical ideas concerning the future evolution of LPG gas sensors, are deliberated. Additionally, the advancements in the next-generation gas sensors, such as the wireless detection of LPG leakage, self-powered sensors driven by triboelectric/piezoelectric mechanisms, and artificial intelligent systems are also reviewed. This review further focuses on the use of smartphones to circumvent the use of costly instruments and paves the way for cost-efficient and portable monitoring of LPG. Finally, the approach of utilizing the Internet of Things (IoT) to detect/monitor the leakage of LPG has also been discussed, which will provide better alerts to the users and thus minimize the effects of leakages.

A Self-Powered Portable Nanowire Array Gas Sensor for Dynamic NO2 Monitoring at Room Temperature

The fast development of the Internet of Things (IoT) has driven an increasing consumer demand for self-powered gas sensors for real-time data collection and autonomous responses in industries such as environmental monitoring, workplace safety, smart cities, and personal healthcare. Despite intensive research and rapid progress in the field, most reported self-powered devices, specifically NO₂ sensors for air pollution monitoring, have limited sensitivity, selectivity, and scalability. Here, a novel photovoltaic self-powered NO₂ sensor is demonstrated based on axial p-i-n homojunction InP nanowire (NW) arrays, that overcome these limitations. The optimized innovative InP NW array device is designed by numerical simulation for insights into sensing mechanisms and performance enhancement. Without a power source, this InP NW sensor achieves an 84% sensing response to 1 ppm NO₂ and records a limit of detection down to the sub-ppb level, with little dependence on the incident light intensity, even under <5% of 1 sun illumination. Based on this great environmental fidelity, the sensor is integrated into a commercial microchip interface to evaluate its performance in the context of dynamic environmental monitoring of motor vehicle exhaust. The results show that compound semiconductor nanowires can form promising self-powered sensing platforms suitable for future mega-scale IoT systems.

Unveiling Evolutionary Path of Nanogenerator Technology: A Novel Method Based on Sentence-BERT

In recent years, nanogenerator technology has developed rapidly with the rise of cloud computing, artificial intelligence, and other fields. Therefore, the quick identification of the evolutionary path of nanogenerator technology from a large amount of data attracts much attention. It is of great significance in grasping technical trends and analyzing technical areas of interest. However, there are some limitations in previous studies. On the one hand, previous research on technological evolution has generally utilized bibliometrics, patent analysis, and citations between patents and papers, ignoring the rich semantic information contained therein; on the other hand, its evolution analysis perspective is single, and it is difficult to obtain accurate results. Therefore, this paper proposes a new framework based on the methods of Sentence-BERT and phrase mining, using multi-source data, such as papers and patents, to unveil the evolutionary path of nanogenerator technology. Firstly, using text vectorization, clustering algorithms, and the phrase mining method, current technical themes of significant interest to researchers can be obtained. Next, this paper correlates the multi-source fusion themes through semantic similarity calculation and demonstrates the multi-dimensional technology evolutionary path by using the “theme river map”. Finally, this paper presents an evolution analysis from the perspective of frontier research and technology research, so as to discover the development focus of nanogenerators and predict the future application prospects of nanogenerator technology.

Coupled Multiphysics Modelling of Sensors for Chemical, Biomedical, and Environmental Applications with Focus on Smart Materials and Low-Dimensional Nanostructures

Low-dimensional nanostructures have many advantages when used in sensors compared to the traditional bulk materials, in particular in their sensitivity and specificity. In such nanostructures, the motion of carriers can be confined from one, two, or all three spatial dimensions, leading to their unique properties. New advancements in nanosensors, based on low-dimensional nanostructures, permit their functioning at scales comparable with biological processes and natural systems, allowing their efficient functionalization with chemical and biological molecules. In this article, we provide details of such sensors, focusing on their several important classes, as well as the issues of their designs based on mathematical and computational models covering a range of scales. Such multiscale models require state-of-the-art techniques for their solutions, and we provide an overview of the associated numerical methodologies and approaches in this context. We emphasize the importance of accounting for coupling between different physical fields such as thermal, electromechanical, and magnetic, as well as of additional nonlinear and nonlocal effects which can be salient features of new applications and sensor designs. Our special attention is given to nanowires and nanotubes which are well suited for nanosensor designs and applications, being able to carry a double functionality, as transducers and the media to transmit the signal. One of the key properties of these nanostructures is an enhancement in sensitivity resulting from their high surface-to-volume ratio, which leads to their geometry-dependant properties. This dependency requires careful consideration at the modelling stage, and we provide further details on this issue. Another important class of sensors analyzed here is pertinent to sensor and actuator technologies based on smart materials. The modelling of such materials in their dynamics-enabled applications represents a significant challenge as we have to deal with strongly nonlinear coupled problems, accounting for dynamic interactions between different physical fields and microstructure evolution. Among other classes, important in novel sensor applications, we have given our special attention to heterostructures and nucleic acid based nanostructures. In terms of the application areas, we have focused on chemical and biomedical fields, as well as on green energy and environmentally-friendly technologies where the efficient designs and opportune deployments of sensors are both urgent and compelling.

Advances in Self-Powered Ultraviolet Photodetectors Based on P-N Heterojunction Low-Dimensional Nanostructures

Self-powered ultraviolet (UV) photodetectors have attracted considerable attention in recent years because of their vast applications in the military and civil fields. Among them, self-powered UV photodetectors based on p-n heterojunction low-dimensional nanostructures are a very attractive research field due to combining the advantages of low-dimensional semiconductor nanostructures (such as large specific surface area, excellent carrier transmission channel, and larger photoconductive gain) with the feature of working independently without an external power source. In this review, a selection of recent developments focused on improving the performance of self-powered UV photodetectors based on p-n heterojunction low-dimensional nanostructures from different aspects are summarized. It is expected that more novel, dexterous, and intelligent photodetectors will be developed as soon as possible on the basis of these works.

Battery-Sensor Hybrid: A New Gas Sensing Paradigm with Complete Energy Self-Sufficiency

Fully autonomous operation has long been an ultimate goal in environmental sensing. Although self-powered gas sensors based on energy harvesting have been widely reported to provide power for autonomous operation, these sensors rely on external sources of harvestable energy, thus are not completely self-sufficient. Herein, a battery-sensor hybrid device that can simultaneously function as both a power source and a gas sensor is presented. The battery-sensor consists of a cathode that reduces NO₂ to NO₂^- via a catalyst with Fe-N_x species distributed on highly graphitic porous nitrogen-doped carbon. On the basis of the efficient and selective electrocatalytic activity of the catalyst, the battery-sensor is capable of sensing NO₂ and does so without any external power, overcoming the long-standing grand challenge to achieve complete energy self-sufficiency. Furthermore, through controlling the working current the sensing range can be significantly expanded and electronically tuned, which is not only unprecedented for gas sensors but also of remarkable commercial practicality. The proposed battery-sensor hybrid architecture represents a new paradigm toward sensors with complete energy self-sufficiency.

A Photovoltaic Self-Powered Gas Sensor Based on All-Dry Transferred MoS2 /GaSe Heterojunction for ppb-Level NO2 Sensing at Room Temperature

Traditional gas sensors are facing the challenge of low power consumption for future application in smart phones and wireless sensor platforms. To solve this problem, self-powered gas sensors are rapidly developed in recent years. However, all reported self-powered gas sensors are suffering from high limit of detection (LOD) toward NO2 gas. In this work, a photovoltaic self-powered NO2 gas sensor based on n-MoS2 /p-GaSe heterojunction is successfully prepared by mechanical exfoliation and all-dry transfer method. Under 405 nm visible light illumination, the fabricated photovoltaic self-powered gas sensors show a significant response toward ppb-level NO2 with short response and recovery time and high selectivity at room temperature (25 °C). It is worth mentioning that the LOD toward NO2 of this device is 20 ppb, which is the lowest of the reported self-powered room-temperature gas sensors so far. The discussed devices can be used as building blocks to fabricate more functional Internet of things devices.

Gas sensing materials roadmap

Gas sensor technology is widely utilized in various areas ranging from home security, environment and air pollution, to industrial production. It also hold great promise in non-invasive exhaled breath detection and an essential device in future internet of things. The past decade has witnessed giant advance in both fundamental research and industrial development of gas sensors, yet current efforts are being explored to achieve better selectivity, higher sensitivity and lower power consumption. The sensing layer in gas sensors have attracted dominant attention in the past research. In addition to the conventional metal oxide semiconductors, emerging nanocomposites and graphene-like two-dimensional materials also have drawn considerable research interest. This inspires us to organize this comprehensive 2020 gas sensing materials roadmap to discuss the current status, state-of-the-art progress, and present and future challenges in various materials that is potentially useful for gas sensors.

Electrically Transduced Gas Sensors Based on Semiconducting Metal Oxide Nanowires

Semiconducting metal oxide-based nanowires (SMO-NWs) for gas sensors have been extensively studied for their extraordinary surface-to-volume ratio, high chemical and thermal stabilities, high sensitivity, and unique electronic, photonic and mechanical properties. In addition to improving the sensor response, vast developments have recently focused on the fundamental sensing mechanism, low power consumption, as well as novel applications. Herein, this review provides a state-of-art overview of electrically transduced gas sensors based on SMO-NWs. We first discuss the advanced synthesis and assembly techniques for high-quality SMO-NWs, the detailed sensor architectures, as well as the important gas-sensing performance. Relationships between the NWs structure and gas sensing performance are established by understanding general sensitization models related to size and shape, crystal defect, doped and loaded additive, and contact parameters. Moreover, major strategies for low-power gas sensors are proposed, including integrating NWs into microhotplates, self-heating operation, and designing room-temperature gas sensors. Emerging application areas of SMO-NWs-based gas sensors in disease diagnosis, environmental engineering, safety and security, flexible and wearable technology have also been studied. In the end, some insights into new challenges and future prospects for commercialization are highlighted.

Fabrication of flexible self-powered humidity sensor based on super-hydrophilic titanium oxide nanotube arrays

Stable and flexible super-hydrophilic nanotubular-based titanium oxide electrode has been utilized as the active electrode of self-powered humidity sensor. TiO₂ nanotubular electrodes fabricated through anodization method and utilized in combination with Kapton electrode as the triboelectric nanogenerator (TENG). Vertical contact-separation mode TENG performance has been examined in various range of frequencies and the maximum output voltage and current more than 300 V and 40 μA respectively with maximum power of 1.25 ± 0.67 mW has been achieved at 4 Hz. The fabricated TENG has been employed as the active self-powered humidity sensor. Super-hydrophilic feature of TiO₂ nanotubes resulted in full absorption of water molecules, and noticeable decrease in charge transfer across two triboelectric materials upon increasing humidity. The TiO₂-based TENG sensor was exposed to various relative humidity (RH) and the results showed that by increasing the humidity the output voltage and output current decreased from 162.24 ± 35.99 V and 20.4 ± 4.93 μA at RH = 20% to 37.92 ± 1.54 V at RH = 79% and 40.87 88 6.88 ± 1.7 μA at RH = 84%, respectively, Which shows the responsivity more than 300%. This method of measuring humidity has a simple and cost-effective fabrication that has various applications in many fields such as industry and medicine.

Flexible Self-Powered Real-Time Ultraviolet Photodetector by Coupling Triboelectric and Photoelectric Effects

The portable UV photodetector is used to timely remind humans of overexposure to UV radiation. However, the traditional UV photodetector cannot meet the practical demands, and the power supply problem hinders its further development. In this work, we demonstrated a flexible, transparent, and self-powered UV photodetector by coupling of triboelectric and photoelectric effects. The device integrates a flexible ZnO nanoparticle (NP) UV photodetector, a transparent- and flexible-film-based TENG (TFF-TENG), commercial chip resistors, and LEDs on the PET thin film. The TFF-TENG could harvest mechanical energy from finger tapping and sliding motion and power the ZnO NP UV photodetector to realize self-powered detection. The voltage of the constant resistors connected with the UV photodetector in series changes from 0.5 to 19 V under the UV light with power intensities increasing from 0.46 to 21.8 mW/cm², and the voltage variation is reflected by the number of LEDs directly. The excellent flexibility and transparency of the device could extend its application scenarios; for example, such a portable device could be applied to real-time monitoring of the UV radiation to remind humans of intense UV light.

Two-dimensional MoSe2 nanosheets via liquid-phase exfoliation for high-performance room temperature NO2 gas sensors

Molybdenum selenide (MoSe₂) has drawn significant interest due to its typical semiconductor properties. MoSe₂ is a relatively novel material in the field of gas sensors especially at room temperature. Herein, we utilize a facile and efficient synthetic method of liquid-phase exfoliation to exfoliate bulk MoSe₂ into nanosheets. Anhydrous ethanol is used as dispersant, so the low boiling point makes it easy to be removed from MoSe₂ nanosheets, which does not affect the subsequent sensing performance. The exfoliated few-layered MoSe₂ nanosheets shows significantly enhanced NO₂ gas response (1500% to 10 ppm NO₂ which is 18 times greater than pristine bulk MoSe₂), a low detection concentration (50 ppb), an outstanding repeatability, a remarkable selectivity, and a reliable long-term device durability (more than 60 d) at room temperature (25 °C). The reason of the significant improvement in gas sensing performance can be attributed mainly to the higher surface-to-volume ratio of exfoliated MoSe₂ nanosheets. It promotes the adsorption of gas molecules on the surface of the material, thereby facilitating the charge transfer process. The superior performance of this gas sensor makes MoSe₂ nanosheets a potential candidate for room temperature NO₂ detection.

A water-evaporation-induced self-charging hybrid power unit for application in the Internet of Things

A self-charging hybrid power unit has been developed by integrating a water-evaporation-induced nanogenerator with a flexible nano-patterned supercapacitor. The nanogenerator can harvest environmental thermal energy and mechanical energy through the water evaporation process, and the supercapacitor can be charged simultaneously. The former offers stable electrical power as output, whereas the Ppy-based supercapacitor shows a capacitance of 12.497 mF/cm² with 96.42% retention after 4,000 cycles. After filling the power unit with water as the fuel, it can be fully charged in about 20 min. The power unit can be flexibly integrated with electronic devices such as sensor nodes and wireless transmitters employing the Internet of Things. This new approach can offer new possibilities in continuous future operation of randomly distributed electronic devices incorporated in the Internet of Things.

A Shared-Electrode and Nested-Tube Structure Triboelectric Nanogenerator for Motion Energy Harvesting

Triboelectric nanogenerators with the function of harvesting human motion energy have attracted wide attention. Here, we demonstrate a shared-electrode and nested-tube structure triboelectric nanogenerator (SNTN) for harvesting human motion energy. The design of the SNTN employs flexible silicone rubber as the negative friction material and Ni-coated polyester conductive textile as the positive friction material and the electrode material. The entire structure consists of an inner triboelectric unit and an outer triboelectric unit. The inner triboelectric unit is formed by a hollow inner tube and a hollow middle tube, while the hollow middle tube and a hollow outer tube constitute the outer triboelectric unit. The hollow middle tube is used as the shared tube, and the electrode in the middle tube is used as the shared electrode of the two triboelectric units. Our research demonstrates that the output performance of the SNTN was improved significantly compared with a single triboelectric unit due to the cooperation of the two triboelectric units. When the SNTN is pressed by 300 N external force, output open-circuit voltage of 180 V and output short-circuit current of 8.5 μA can be obtained. The output electrical energy can light up 31 light-emitting diodes (LEDs) connected serially (displaying “XZTC”) and can drive a digital clock after rectifying storage, which shows application prospects in the field of illuminating devices and portable electronics.

Catalysis of Organic Pollutants Abatement Based on Pt-Decorated Ag@Cu2O Heterostructures

Pt-decorated Ag@Cu2O heterostructures were successfully synthesized using a simple and convenient method. The Pt nanoparticle density on the Ag@Cu2O can be controlled by changing the concentration of the Pt precursor. The synthesized Ag@Cu2O-Pt nanoparticles exhibited excellent catalytic performance, which was greatly affected by changes in the Ag@Cu2O-Pt structure. To optimize the material's properties, the synthesized Ag@Cu2O-Pt nanoparticles were used to catalyze toxic pollutants and methyl orange (MO), and nontoxic products were obtained by catalytic reduction. The Pt-decorated Ag@Cu2O nanoparticles showed excellent catalytic activity, which significantly decreased the pollutant concentration when the nanoparticles were used for catalytic reduction. The redistribution of charge transfer is the nanoparticles' main contribution to the catalytic degradation of an organic pollutant. This Pt-decorated Ag@Cu2O material has unique optical and structural characteristics that make it suitable for photocatalysis, local surface plasmon resonance, and peroxide catalysis.

Contact-Electrification between Two Identical Materials: Curvature Effect

It is known that contact-electrification (or triboelectrification) usually occurs between two different materials, which could be explained by several models for different materials systems ( Adv. Mater. 2018, 30, 1706790; Adv. Mater. 2018, 30, 1803968). But contact between two pieces of the chemically same material could also result in electrostatic charges, although the charge density is rather low, which is hard to understand from a physics point of view. In this paper, by preparing a contact-separation mode triboelectric nanogenerator using two pieces of an identical material, the direction of charge transfer during contact-electrification is studied regarding its dependence on curvatures of the sample surfaces. For materials such as polytetrafluoroethylene, fluorinated ethylene propylene, Kapton, polyester, and nylon, the positive curvature surfaces are net negatively charged, while the negative curvature surfaces tend to be net positively charged. Further verification of the above-mentioned trends was obtained under vacuum (∼1 Pa) and higher temperature (≤358 K) conditions. Based on the received data acquired for gentle contacting cases, we propose a curvature-dependent charge transfer model by introducing curvature-induced energy shifts of the surface states. However, this model is subject to be revised if the mutual contact mode turns into a sliding mode or more complicated hard-pressed contact mode, in which a rigorous contact between the two pieces of the same material could result in nanoscale damage/fracture and possible species transfer. Our study provides a primitive step toward understanding the basics of contact-electrification.

Coaxial Triboelectric Nanogenerator and Supercapacitor Fiber-Based Self-Charging Power Fabric

Although there has been rapid advancement in wearable electronics, challenges still remain in developing wearable and sustainable power sources with simple fabrication and low cost. In this work, we demonstrate a flexible coaxial fiber by fabricating a one-dimensional triboelectric nanogenerator (TENG) outside and a supercapacitor (SC) inside, which can not only harvest mechanical energy but also store energy in the all-in-one fiber. In such a coaxial fiber, carbon fiber bundles are utilized as the electrode material for the TENG as well as the active and electrode material for the SC. Meanwhile, silicone rubber serves as the separator between the SC and TENG, as the triboelectric material for the TENG, and as the encapsulation material for the whole fiber as well. Moreover, both SC and TENG exhibit good performance and stability, which ensures their long-term use in daily life. Because of the flexibility and durability of the carbon fiber and silicone rubber, the proposed coaxial fibers show great flexibility, which could be further knitted as cloth for sustainably powering wearable electronic devices. This work presents a promising platform for wearable electronics as well as smart textiles.

Triboelectric Nanogenerator Driven Self-Powered Photoelectrochemical Water Splitting Based on Hematite Photoanodes

Hematite is one of the most promising photoanodes for photoelectrochemical (PEC) solar water splitting. However, due to the low conduction band position for water reduction, an external bias is necessarily required with the consumption of extra power. In this work, a titanium modified hematite (Ti-Fe₂O₃) photoanode-based self-powered PEC water splitting system in tandem with a rotatory disc-shaped triboelectric nanogenerator (RD-TENG) has been developed. It is a fantastic strategy to effectively drive the hematite-based PEC water splitting by using the environmental mechanical energy through a TENG. When the rotation speed is 65 rpm (water flowing rate ∼0.61 m/s), the peak current reaches to 0.12 mA under illumination contrast to that in the dark with almost zero. As for 80 rpm, the peak currents are 0.17 and 0.33 mA in the dark or under illumination, respectively, indicating the simultaneous occurrence of electrolysis and PEC water splitting. When higher than 120 rpm, the peak current in the dark is nearly equal to that under illumination, which can be attributed to the high enough peak voltage for direct electrolysis of water. Such a self-powered PEC water splitting system provides an alternative strategy that enables to convert both solar and mechanical energies into chemical energies.

Triboelectric-Electromagnetic Hybrid Generator for Harvesting Blue Energy

Progress has been developed in harvesting low-frequency and irregular blue energy using a triboelectric-electromagnetic hybrid generator in recent years. However, the design of the high-efficiency, mechanically durable hybrid structure is still challenging. In this study, we report a fully packaged triboelectric-electromagnetic hybrid generator (TEHG), in which magnets were utilized as the trigger to drive contact-separation-mode triboelectric nanogenerators (CS-TENGs) and coupled with copper coils to operate rotary freestanding-mode electromagnetic generators (RF-EMGs). The magnet pairs that produce attraction were used to transfer the external mechanical energy to the CS-TENGs, and packaging of the CS-TENG part was achieved to protect it from the ambient environment. Under a rotatory speed of 100 rpm, the CS-TENGs enabled the TEHG to deliver an output voltage, current, and average power of 315.8 V, 44.6 μA, and ~ 90.7 μW, and the output of the RF-EMGs was 0.59 V, 1.78 mA, and 79.6 μW, respectively. The cylinder-like structure made the TEHG more easily driven by water flow and demonstrated to work as a practical power source to charge commercial capacitors. It can charge a 33 μF capacitor from 0 to 2.1 V in 84 s, and the stored energy in the capacitor can drive an electronic thermometer and form a self-powered water-temperature sensing system.

On the Electron-Transfer Mechanism in the Contact-Electrification Effect

A long debate on the charge identity and the associated mechanisms occurring in contact-electrification (CE) (or triboelectrification) has persisted for many decades, while a conclusive model has not yet been reached for explaining this phenomenon known for more than 2600 years! Here, a new method is reported to quantitatively investigate real-time charge transfer in CE via triboelectric nanogenerator as a function of temperature, which reveals that electron transfer is the dominant process for CE between two inorganic solids. A study on the surface charge density evolution with time at various high temperatures is consistent with the electron thermionic emission theory for triboelectric pairs composed of Ti-SiO₂ and Ti-Al₂ O₃ . Moreover, it is found that a potential barrier exists at the surface that prevents the charges generated by CE from flowing back to the solid where they are escaping from the surface after the contacting. This pinpoints the main reason why the charges generated in CE are readily retained by the material as electrostatic charges for hours at room temperature. Furthermore, an electron-cloud-potential-well model is proposed based on the electron-emission-dominatedcharge-transfer mechanism, which can be generally applied to explain all types of CE in conventional materials.

Triboelectric-Nanogenerator-Based Soft Energy-Harvesting Skin Enabled by Toughly Bonded Elastomer/Hydrogel Hybrids

A major challenge accompanying the booming next-generation soft electronics is providing correspondingly soft and sustainable power sources for driving such devices. Here, we report stretchable triboelectric nanogenerators (TENG) with dual working modes based on the soft hydrogel-elastomer hybrid as energy skins for harvesting biomechanical energies. The tough interfacial bonding between the hydrophilic hydrogel and hydrophobic elastomer, achieved by the interface modification, ensures the stable mechanical and electrical performances of the TENGs. Furthermore, the dehydration of this toughly bonded hydrogel-elastomer hybrid is significantly inhibited (the average dehydration decreases by over 73%). With PDMS as the electrification layer and hydrogel as the electrode, a stretchable, transparent (90% transmittance), and ultrathin (380 μm) single-electrode TENG was fabricated to conformally attach on human skin and deform as the body moves. The two-electrode mode TENG is capable of harvesting energy from arbitrary human motions (press, stretch, bend, and twist) to drive the self-powered electronics. This work provides a feasible technology to design soft power sources, which could potentially solve the energy issues of soft electronics.

Integrating a Silicon Solar Cell with a Triboelectric Nanogenerator via a Mutual Electrode for Harvesting Energy from Sunlight and Raindrops

Solar cells, as promising devices for converting light into electricity, have a dramatically reduced performance on rainy days. Here, an energy harvesting structure that integrates a solar cell and a triboelectric nanogenerator (TENG) device is built to realize power generation from both sunlight and raindrops. A heterojunction silicon (Si) solar cell is integrated with a TENG by a mutual electrode of a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) film. Regarding the solar cell, imprinted PEDOT:PSS is used to reduce light reflection, which leads to an enhanced short-circuit current density. A single-electrode-mode water-drop TENG on the solar cell is built by combining imprinted polydimethylsiloxane (PDMS) as a triboelectric material combined with a PEDOT:PSS layer as an electrode. The increasing contact area between the imprinted PDMS and water drops greatly improves the output of the TENG with a peak short-circuit current of ∼33.0 nA and a peak open-circuit voltage of ∼2.14 V, respectively. The hybrid energy harvesting system integrated electrode configuration can combine the advantages of high current level of a solar cell and high voltage of a TENG device, promising an efficient approach to collect energy from the environment in different weather conditions.

Liquid-Metal-Based Super-Stretchable and Structure-Designable Triboelectric Nanogenerator for Wearable Electronics

The rapid advancement of intelligent wearable electronics imposes the emergent requirement for power sources that are deformable, compliant, and stretchable. Power sources with these characteristics are difficult and challenging to achieve. The use of liquid metals as electrodes may provide a viable strategy to produce such power sources. In this work, we propose a liquid-metal-based triboelectric nanogenerator (LM-TENG) by employing Galinstan as the electrode and silicone rubber as the triboelectric and encapsulation layer. The small Young's modulus of the liquid metal ensures the electrode remains continuously conductive under deformations, stretching to a strain as large as ∼300%. The surface oxide layer of Galinstan effectively prevents the liquid Galinstan electrode from further oxidization and permeation into silicone rubber, yielding outstanding device stability. Operating in the single-electrode mode at 3 Hz, the LM-TENG with an area of 6 × 3 cm² produces an open-circuit voltage of 354.5 V, transferred short-circuit charge of 123.2 nC, short-circuit current of 15.6 μA, and average power density of 8.43 mW/m², which represent outstanding performance values for TENGs. Further, the LM-TENG maintains stable performance under various deformations, such as stretching, folding, and twisting. LM-TENGs in different forms, such as bulk-shaped, bracelet-like, and textile-like, are all able to harvest mechanical energy from human walking, arm shaking, or hand patting to sustainably drive wearable electronic devices.

A Review on Surface Stress-Based Miniaturized Piezoresistive SU-8 Polymeric Cantilever Sensors

In the last decade, microelectromechanical systems (MEMS) SU-8 polymeric cantilevers with piezoresistive readout combined with the advances in molecular recognition techniques have found versatile applications, especially in the field of chemical and biological sensing. Compared to conventional solid-state semiconductor-based piezoresistive cantilever sensors, SU-8 polymeric cantilevers have advantages in terms of better sensitivity along with reduced material and fabrication cost. In recent times, numerous researchers have investigated their potential as a sensing platform due to high performance-to-cost ratio of SU-8 polymer-based cantilever sensors. In this article, we critically review the design, fabrication, and performance aspects of surface stress-based piezoresistive SU-8 polymeric cantilever sensors. The evolution of surface stress-based piezoresistive cantilever sensors from solid-state semiconductor materials to polymers, especially SU-8 polymer, is discussed in detail. Theoretical principles of surface stress generation and their application in cantilever sensing technology are also devised. Variants of SU-8 polymeric cantilevers with different composition of materials in cantilever stacks are explained. Furthermore, the interdependence of the material selection, geometrical design parameters, and fabrication process of piezoresistive SU-8 polymeric cantilever sensors and their cumulative impact on the sensor response are also explained in detail. In addition to the design-, fabrication-, and performance-related factors, this article also describes various challenges in engineering SU-8 polymeric cantilevers as a universal sensing platform such as temperature and moisture vulnerability. This review article would serve as a guideline for researchers to understand specifics and functionality of surface stress-based piezoresistive SU-8 cantilever sensors.

A Self-Powered Breath Analyzer Based on PANI/PVDF Piezo-Gas-Sensing Arrays for Potential Diagnostics Application

The increasing morbidity of internal diseases poses serious threats to human health and quality of life. Exhaled breath analysis is a noninvasive and convenient diagnostic method to improve the cure rate of patients. In this study, a self-powered breath analyzer based on polyaniline/polyvinylidene fluoride (PANI/PVDF) piezo-gas-sensing arrays has been developed for potential detection of several internal diseases. The device works by converting exhaled breath energy into piezoelectric gas-sensing signals without any external power sources. The five sensing units in the device have different sensitivities to various gas markers with concentrations ranging from 0 to 600 ppm. The working principle can be attributed to the coupling of the in-pipe gas-flow-induced piezoelectric effect of PVDF and gas-sensing properties of PANI electrodes. In addition, the device demonstrates its use as an ethanol analyzer to roughly mimic fatty liver diagnosis. This new approach can be applied to fabricating new exhaled breath analyzers and promoting the development of self-powered systems.

Core–shell coaxially structured triboelectric nanogenerator for energy harvesting and motion sensing

Converting sustainable human motion energy into electric energy has become an urgent task for the advancement of next-generation wearable and portable electronics. Herein, a core–shell coaxially structured triboelectric nanogenerator (CSTN) was fabricated by inserting an inner hollow circular tube into an outer hollow circular tube, and a gasbag is constructed within the space between the inner and outer tubes. Both Ni-coated polyester conductive textile and the conductive silicone rubber were used as effective electrode materials. The CSTN has excellent properties, including flexibility, light weight, sustainability and biological compatibility due to its unique structural design and materials selection. The CSTN can convert various forms of human motion energy, such as pressing, bending and twisting motion, into electric energy. A high short-circuit current of 11 μA and an open-circuit voltage of 380 V can be obtained from a CSTN with a length of 6 cm, corresponding to a high peak power of 1.638 mW at a load resistance of about 10 MΩ. When six such CSTNs are connected in parallel and placed under shoes, the electric energy output by normal walking can light up 60 LEDs connected serially and power up a competition-timer. The device can also sense different bending angles or twisting angles according to its signal outputs under different deformation angles. This study indicates the promising application prospects of the CSTN for next-generation devices, including self-powered illuminating devices, portable electronics, body motion sensing and health monitoring.

A core–shell coaxially structured triboelectric nanogenerator (CSTN) is fabricated by inserting the inner hollow circular tube into the outer hollow circular tube, which can be used for energy harvesting and motion sensing.

Self-Powered Viscosity and Pressure Sensing in Microfluidic Systems Based on the Piezoelectric Energy Harvesting of Flowing Droplets

The rapid development of microscaled piezoelectric energy harvesters has provided a simple and highly efficient way for building self-powered sensor systems through harvesting the mechanical energy from the ambient environment. In this work, a self-powered microfluidic sensor that can harvest the mechanical energy of the fluid and simultaneously monitor their characteristics was fabricated by integrating the flexible piezoelectric poly(vinylidene fluoride) (PVDF) nanofibers with the well-designed microfluidic chips. Those devices could generate open-circuit high output voltage up to 1.8 V when a droplet of water is flowing past the suspended PVDF nanofibers and result in their periodical deformations. The impulsive output voltage signal allowed them to be utilized for droplets or bubbles counting in the microfluidic systems. Furthermore, the devices also exhibited self-powered sensing behavior due to the decreased voltage amplitude with increasing input pressure and liquid viscosity. The drop of output voltage could be attributed to the variation of flow condition and velocity of the droplets, leading to the reduced deformation of the piezoelectric PVDF layer and the decrease of the generated piezoelectric potential.