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Anthropogenetic environmental deterioration and climate change caused by energy production and consumption pose a significant threat to the future of humanity. Renewable, environmentally friendly, and cost-effective energy sources are becoming increasingly important for addressing future energy demands. Mechanical power is the most common type of external energy that can be converted into useful electric power. Because of its strong electromechanical coupling ability, the piezoelectric mechanism is a far more successful technique for converting mechanics energy to electrical energy when compared to electrostatic, electromagnetic, and triboelectric transduction systems. Currently, the scientific community has maintained a strong interest in piezoelectric micro-power generators because of their great potential for powering a sensor unit in the distributed network nodes. A national network usually has a large mass of sensor units distributed in each city, and a self-powered sensor network is eagerly required. This paper presents a comprehensive review of the development of piezoelectric micro-power generators. The fundamentals of piezoelectric energy conversion, including operational modes and working mechanisms, are introduced. Current research progress in piezoelectric materials including zinc oxide, ceramics, single crystals, organics, composite, bio-inspired and foam materials are reviewed. Piezoelectric energy harvesting at the nano- and microscales, and its applications in a variety of fields such as wind, liquid flow, body movement, implantable and sensing devices are discussed. Finally, the future development of multi-field coupled, hybrid piezoelectric micropower generators and their potential applications are discussed.

Piezoelectric energy harvester (PEH) arrays are promising in many application scenarios. However, few interface circuits have been developed to manage the multiple AC inputs from PEHs. This paper proposes an extensible P-SSHI (parallel synchronized switch harvesting on inductor) array interface scheme to realize a multi-input conversion from PEH arrays. We develop a split-inductor-capacitor topology that can extract electrical energy from multiple PEHs. The proposed circuit topology effectively handles the inductor access conflict in the cases of PEHs with close, identical, and opposite vibration phases, insensitive to AC input phase changes. For demonstrating the capability of dealing with multiple AC inputs from PEHs and the enhancement effect on power output, we construct and test a self-powered three-channel array circuit with the passive peak detection function. Excited by sinusoidal vibrations, our circuit effectively avoids the energy return and efficiently achieves the power combination of the three PEHs. Compared with the classic full-bridge circuit, the proposed circuit helps increase the power output by 4.8 times. Also, to show the universality of the circuit, the circuits operation under various excitations is tested and analyzed.

Inspired by hand push whisk blenders that are widely used in the kitchen, we here propose an energy harvester that enables conversion of low-frequency translational vibration to high-speed rotations and further generates electricity via the electromagnetic effect. The whisk-inspired energy harvester (W-EH) is composed of an internal thread sleeve, an external thread driving shaft, a spring, and a buoy. Compared with transmission approaches used by electromagnetic power generators, the proposed design has the superiority of low cost, ultra-low operation frequency (lower than 1 Hz), and high power output. Tests in water indicate that the normalized power density is 300% higher than the state of the art. Excited by mimicking ocean waves at 1.5 Hz, the W-EH prototype generates a maximum power of 64.4 milliwatts and succeeds in lighting up 115 LEDs simultaneously, which power is enough for continuous operations of most wireless sensors.

Abstract Most electronics such as sensors, actuators and energy harvesters need piezoceramic films to interconvert mechanical and electrical energy. Transferring the ceramic films from their growth substrates for assembling electronic devices commonly requires chemical or physical etching, which comes at the sacrifice of the substrate materials, film cracks, and environmental contamination. Here, we introduce a van der Waals stripping method to fabricate large-area and freestanding piezoceramic thin films in a simple, green, and cost-effective manner. The introduction of the quasi van der Waals epitaxial platinum layer enables the capillary force of water to drive the separation process of the film and substrate interface. The fabricated lead-free film, $${\text{Ba}}_{{{0}{\text{.85}}}} {\text{Ca}}_{{{0}{\text{.15}}}} {\text{Zr}}_{{{0}{\text{.1}}}} {\text{Ti}}_{{{0}{\text{.9}}}} {\text{O}}_{{3}}$$ Ba 0 .85 Ca 0 .15 Zr 0 .1 Ti 0 .9 O 3 (BCZT), shows a high piezoelectric coefficient d33 = 209 ± 10 pm V−1 and outstanding flexibility of maximum strain 2%. The freestanding feature enables a wide application scenario, including micro energy harvesting, and covid-19 spike protein detection. We further conduct a life cycle analysis and quantify the low energy consumption and low pollution of the water-based stripping film method.

Defect-enhanced selective ion transport within a generator made from an ionic liquid film supported by ZIF-8-based membranes enables high-performance moisture energy harvesting.

High-performance and fully transparent droplet energy harvesting panels for hybrid rain-solar energy harvesting and self-powered forest monitoring.

The human somatosensory system is capable of extracting features with millimeter-scale spatial resolution and submillisecond temporal precision. Current technologies that can render tactile stimuli with such high definition are neither portable nor easily accessible. Here, we present a wearable electrotactile rendering system that elicits tactile stimuli with both high spatial resolution (76 dots/cm 2 ) and rapid refresh rates (4 kHz), because of a previously unexplored current-steering super-resolution stimulation technique. For user safety, we present a high-frequency modulation method to reduce the stimulation voltage to as low as 13 V. The utility of our high spatiotemporal tactile rendering system is highlighted in applications such as braille display, virtual reality shopping, and digital virtual experiences. Furthermore, we integrate our setup with tactile sensors to transmit fine tactile features through thick gloves used by firefighters, allowing tiny objects to be localized based on tactile sensing alone.

To merge the gap between the low-power output of piezoelectric energy harvesters (PEHs) and the high-power demand of sensors of Internet of Things, researchers recently start to explore PEHs arrays. However, few circuits have been developed to manage the multiple ac inputs from PEHs arrays. This article presents a self-powered PEH array interface circuit based on the synchronized switching and discharging to a storage capacitor through an inductor (SSDCI) technique. The array interface can output a maximum total power greater than the sum of each individual peak power, and achieve a high efficiency when multiple PEHs connect to the SSDCI array circuit. Simulation and experiment are performed to demonstrate the advantages of the SSDCI array interface. The tested results show that the designed SSDCI array circuit achieves an efficiency of 82.3% with three input sources, and allows a gain up to 300% in terms of maximal output power compared to the full-bridge rectifier. In addition, only the peak detection is utilized for the switching control; therefore, the SSDCI array circuit is realized more easily and with fewer components compared to the existing synchronous electric charge extraction array interfaces.

This paper originally investigates the influence of frequency-up conversion effect on piezoelectric stack generators for high-performance energy harvesting. A compressive-mode piezoelectric generator is proposed comprised of a piezostack and a spring-mass system for electromechanical transduction and mechanical excitation, respectively. The frequency-up conversion effect is induced by amplitude truncations of the mass under external excitation. A control group, i.e. without truncation, is set for performance comparisons. Experimental results indicate that, with the frequency-up conversion, instantaneous power and average power are enhanced by over 1000 and 177 times, respectively. Each voltage response cycle consists of two stages: high-frequency high-amplitude oscillation and the rest of low-frequency low-amplitude harmonic vibration. The frequency of the former stage determines impedance matching. Due to the frequency converted up by 69 times, and intrinsic high capacitance of the stack, the matched resistance is significantly reduced from over 5 kΩ to 73.10 Ω, resulting much higher power response than that without conversion. Additionally, increase of spring stiffness causes the decrease of voltage responses and less effectiveness of frequency conversion effect. In this work we conclusively report that an instantaneous peak power output of 0.32 W can be generated from a millimeter-size piezoelectric generator, which can foster development of high-viability self-powered applications.

Bubbles have been extensively explored as energy carriers ranging from boiling heat transfer and targeted cancer diagnosis. Yet, despite notable progress, the kinetic energy inherent in small bubbles remains difficult to harvest. Here, we develop a transistor-inspired bubble energy generator for directly and efficiently harvesting energy from small bubbles. The key points lie in designing dielectric surface with high-density electric charges and tailored surface wettability as well as transistor-inspired electrode configuration. The synergy between these features facilitates fast bubble spreading and subsequent departure, transforms the initial liquid/solid interface into gas/solid interface under the gating of bubble, and yields an output at least one order of magnitude higher than existing studies. We also show that the output can be further enhanced through rapid bubble collapse at the air/liquid interface and multiple bubbles synchronization. We envision that our design will pave the way for small bubble-based energy harvesting in liquid media.

Current railway track condition monitoring relies on inefficient human inspectors and expensive inspection vehicles, where high-frequency inspection is unreachable since these methods occupy the tracks. This article proposes a batteryless railway monitoring system based on radio-frequency (RF) energy harvesting to detect early defects on rail tracks. The key part of the system is a batteryless wireless sensor tag (BLWST) installed on railway tracks. The BLWST can harvest RF energy from a reader installed on the train, and precisely measure and wirelessly transmit the vibration condition of tracks back to the reader. The proposed system eliminates the demands for cables and battery replacement, thus achieving low installation and maintenance costs. The high-frequency monitoring also provides a more reliable inspection than the existing methods. The BLWST is based on the 3-stage Dickson voltage multiplier (DVM) and can be activated by a dedicated RF power source at a maximum distance of 2.3 m. Experiments show that a maximum energy conversion efficiency of 25% and 500 working cycles per second are achieved. For demonstration, we construct a miniaturized railway system with the batteryless prototype and exhibit a reliable wireless power transfer and data communication.

In this paper, we present a simple metasurface based multiband reflective polarization converter for both linear and circular polarizations. We show that, on one hand, the proposed structure can convert the polarization of linearly polarized waves to the orthogonal direction at four frequency bands — 4.3 GHz, 7.2 GHz, 12.3 GHz, and 15.15 GHz. On the other hand, this metasurface can achieve linear to circular or circular to linear polarization conversion at 4 GHz, 4.75–5.95 GHz, 8.35–8.8 GHz and 14.35–14.6 GHz frequency bands. Such multiband operation originates from multiple resonances occurring in the structure based on meandered square ring and diagonal split strip resonator. Moreover, the polarization transforming capability is stable for oblique incident angle up to $15^{o}$ , for both transverse electric (TE) and transverse-magnetic (TM) polarizations. Furthermore, the proposed structure acts as a meta-reflector that maintains the handedness of the circular polarization upon reflection. Finally, a prototype of the proposed meta-structure is fabricated and measured for both normal and oblique incidence of electromagnetic waves. All the results present excellent resonant stability with respect to the different polarization and incident angles. In addition, we have also performed the tolerance analysis of different material parameters to understand the robustness of the structure. Due to the ability of the structure to perform three functionalities through a single layout, the proposed design can pave its way in different microwave applications such as satellite, radar, and 5G communications.

High-performance piezoelectric thin films generally contain toxic lead that limits the application scenarios especially in wearable and medical devices. Alternative lead-free piezoelectric materials such as Ba0.85Ca0.15Zr0.1Ti0.9O3 (BCZT) have been proved to have high biocompatibility and competitive piezoelectric performances. However, few have reported BCZT thin films integrated within flexible electronics. To fill this gap, we prepare BCZT thin films on flexible mica substrates and based on that fabricate piezoelectric nanogenerators (PENGs). The piezoelectric modulus d33 of the BCZT thin films evaluated from the established system reaches 150 pC N−1, which is comparable to those of films grown on single crystal substrates. The BCZT-based PENG can charge capacitors under mechanical excitation, indicating that it has broad application prospects in the field of flexible self-powered electronics.

This paper presents a modeling and dynamic analysis of a rotational high-efficiency compressive-mode piezoelectric energy harvester (HC-PEH) with partially thickened bow-shaped beams. Based on the Euler–Bernoulli beam theory and the extended Hamilton’s principle, the governing equations of the rotational HC-PEH system are formulated. The nonlinear electromechanical coupling partial differential equations of the system are transformed into ordinary differential equations in the truncated modal coordinates. The developed distributed-parameter model is validated against experimental data, and a good agreement is achieved. The stability and the nonlinear dynamic behavior of the rotational HC-PEH system in conditions of different offset distances and preloaded axial forces are investigated by numerical simulation results. A parametric study of the parameters directly related to the system design is also performed to provide fundamental guidance for understanding the electrical output performance and modulating the voltage–rotation speed responses of the harvester. The result shows that the design parameters of the bow-shaped beam and the PZT plate have apparent effects on the electrical output of the harvester system.

Bioelectronic devices implanted within the human body are increasingly used for diagnostic and therapeutic purposes, of which functions and lifespan could be significantly improved with the wireless energy transfer technology. However, limited by the electromagnetic radiation, low power output, high stiffness, short transfer distance and tissue attenuation, most wireless energy transfer systems cannot meet the requirements of implantable medical devices (IMDs). Here, inspired by the natural wood structures, we present a transmuscular ultrasonic wireless power transfer system based on a flexible wood-templated piezoelectric ultrasonic energy harvester (W-PUEH) in a unidirectional 3D interconnected ceramic-polymer topology. The developed flexible W-PUEH device demonstrates an output voltage of 21 V, an output current of 2 mA, and an average output power density of 304 μW cm-2, one order of magnitude higher than the state of the art. Further ex vivo and in vivo experiments demonstrate the sufficient power supply capacity of the W-PUEH and its potential applications in the implantable devices for the improvement of life quality and well-being of the recipients.

The lack of stable p-type van der Waals (vdW) semiconductors with high hole mobility severely impedes the step of low-dimensional materials entering the industrial circle. Although p-type black phosphorus (bP) and tellurium (Te) have shown promising hole mobilities, the instability under ambient conditions of bP and relatively low hole mobility of Te remain as daunting issues. Here we report the growth of high-quality Te nanobelts on atomically flat hexagonal boron nitride (h-BN) for high-performance p-type field-effect transistors (FETs). Importantly, the Te-based FET exhibits an ultrahigh hole mobility up to 1370 cm2 V−1 s−1 at room temperature, that may lay the foundation for the future high-performance p-type 2D FET and metal–oxide–semiconductor (p-MOS) inverter. The vdW h-BN dielectric substrate not only provides an ultra-flat surface without dangling bonds for growth of high-quality Te nanobelts, but also reduces the scattering centers at the interface between the channel material and the dielectric layer, thus resulting in the ultrahigh hole mobility "Image missing".

Piezoelectric biomaterials have attracted significant attention due to the potential effect of piezoelectricity on biological tissues and their versatile applications. However, the high cost and complexity of assembling and domain aligning biomolecules at a large scale, and the disordered arrangement of piezoelectric domains as well as the lack of ferroelectricity in natural biological tissues remain a roadblock toward practical applications. Here, utilizing the weak van der Waals interaction in the layered structure of small intestinal submucosa (SIS), a van der Waals exfoliation (vdWE) process is reported to fabricate ultrathin films down to the thickness of the effective piezoelectric domain. Based on that, the piezoelectric property is revealed of SIS stemming from the collagen fibril, with piezoelectric coefficients up to 4.1 pm V

Piezoceramic films are an essential class of energy-conversion materials that have been widely used in the electronics industry. Although current methods create a great freedom for fabricating high-quality piezoceramic films, it requires well-controlled synthesis conditions, including special high-cost equipment and planar substrates particularly. The limited substrate selections hinder the applications of piezoceramic films in 3D conformal structures where most objects possess complex curvilinear surfaces. To overcome such limitations, a fast, energy-efficient, and cost-effective approach, named flame treated spray (FTS) coating, is developed for preparing piezoceramic films on free-form surfaces. The flame treatment significantly enhances the hydrophilicity of a substrate, assisting in forming a uniform and continuous thin film. The followed spray coating deposits hundreds of nanometers to several micrometers thick films on 3D free-form surfaces. Given the size controllability and arbitrary surface compatibility of the FTS method, a highly conformal piezoelectric tactile sensor array (4 × 4) is assembled on a spherical surface for mimicking robot fingers and an on-site thin-film sensor on the wing of an aircraft model to monitor the vibration in real-time during flight. The FTS film deposition offers a highly promising methodology for the application of functional thin-film from micro- to marcoscale devices, regardless of conformal problems.

A global sentiment in early 2022 is that the COVID-19 virus could become endemic just like common cold flu viruses soon. The most optimistic view is that, with minimal precautions, such as vaccination, boosters and optional masking, life for most people will proceed as normal soon. However, as warned by A. Katzourakis of Oxford University recently [1], we must set aside lazy optimism, and must be realistic about the likely levels of death, disability and sickness that will be brought on by a ‘COVID-19’ endemic. Moreover, the world must also consider that continual circulation of the virus could give rise to new variants such as the new BA.2 variant (a subvariant of Omicron) continues to spread across the US and parts of Europe. Data from the CDC is already showing that BA.2 has been tripling in prevalence every two weeks [2]. Hence, globally, we must use available and proven weapons to continue to fight the COVID-19 viruses, i.e., effective vaccines, antiviral medications, diagnostic tests and stop an airborne virus transmission through social distancing, and mask wearing. For this work, we have demonstrated a smart mask with an optimally-coupled ultra-thin flexible soundwave sensors for tracking, classifying, and recognizing different respiratory activities, including breathing, speaking, and two-/tri-phase coughing; the mask’s functionality can also be augmented in the future to monitor other human physiological signals. Although researchers have integrated sensors into masks to detect respiratory activities in the past, they only based on measuring temperature and air flow during coughing, i.e., counting only the number of coughs. However, coughing is a process consisting of several phases, including an explosion of the air with glottal opening producing some noise-like waveform, a decrease of airflow to decrease sound amplitude, and a voiced stage which is the interruption of the air flow due to the closure of glottal and periodical vibration of partly glottis, which is not always present. Therefore, sensors used for cough detection should not be only sensitive to subtle air pressure but also the high-frequency vibrations, i.e., a pressure sensor that needs to be responsive to a wide input amplitude and bandwidth range, in order to detect air flows between hundreds of hertz from breath, and acoustic signals from voice that could reach ∼ 8000 Hz. Respiratory activities data from thirty-one (31) human subjects were collected. Machine learning methods such as Support Vector Machines and Convolutional Neural Networks were used to classify the collected sensor data from the smart mask, which show an overall macro-recall of about 93.88% for the three respiratory sounds among all 31 subjects. For individual subjects, the 31 human subjects have the average macro-recall of 95.23% (ranging from 90% to 100%) for these 3 respiratory activities. Our work bridges the technological gap between ultra-lightweight but high-frequency response sensor material fabrication, signal transduction and conditioning, and applying machining learning algorithms to demonstrate a reliable wearable device for potential applications in continual healthy monitoring of subjects with cough symptoms during the eventual COVID-19 endemic. The monitoring and analysis of cough sound should be highly beneficial for human health management. These health monitoring data could then be shared with doctors via cloud storage and transmission technique to help disease diagnosis more effectively. Also, communication barriers caused by wearing masks can be alleviated by combining with the speech recognition techniques. In general, this research helps to advance the wearable device technology for tracking respiratory activities, similar to an Apple Watch or a Fitbit smartwatch in tracking physical and physiological activities.

Extensive efforts have been made to harvest energy from water in the form of raindrops1–6, river and ocean waves7,8, tides9 and others10–17. However, achieving a high density of electrical power generation is challenging. Traditional hydraulic power generation mainly uses electromagnetic generators that are heavy, bulky, and become inefficient with low water supply. An alternative, the water-droplet/solid-based triboelectric nanogenerator, has so far generated peak power densities of less than one watt per square metre, owing to the limitations imposed by interfacial effects—as seen in characterizations of the charge generation and transfer that occur at solid–liquid1–4 or liquid–liquid5,18 interfaces. Here we develop a device to harvest energy from impinging water droplets by using an architecture that comprises a polytetrafluoroethylene film on an indium tin oxide substrate plus an aluminium electrode. We show that spreading of an impinged water droplet on the device bridges the originally disconnected components into a closed-loop electrical system, transforming the conventional interfacial effect into a bulk effect, and so enhancing the instantaneous power density by several orders of magnitude over equivalent devices that are limited by interfacial effects. A device involving a polytetrafluoroethylene film, an indium tin oxide substrate and an aluminium electrode allows improved electricity generation from water droplets, which bridge the previously disconnected circuit components.

Self-assembled monolayers (SAMs) have been widely employed as an effective way to modify interfaces of electronic/optoelectronic devices. To achieve a good control of the growth and molecular functionality of SAMs, we develop a co-assembled monolayer (co-SAM) for obtaining efficient hole selection and suppressed recombination at the hole-selective interface in inverted perovskite solar cells (PSCs). By engineering the position of methoxy substituents, an aligned energy level and favorable dipole moment can be obtained in our newly synthesized SAM, ((2,7-dimethoxy-9H-carbazol-9-yl) methyl) phosphonic acid (DC-PA). An alkyl ammonium containing SAM is co-assembled to further optimize the surface functionalization and interaction with perovskite layer on top. A champion device with an excellent power conversion efficiency (PCE) of 23.59 % and improved device stability are achieved. This work demonstrates the advantage of using co-SAM in improving performance and stability of PSCs.

The pyroelectric effect provides an efficient route to convert low-grade waste heat into electricity. (Bi0.5Na0.5)TiO3-based ceramic is one of the most attractive lead-free pyroelectric candidates due to its high polarization. However, the low breakdown strength (BDS) restricts its ability to achieve a high polarization, hindering the application for thermal energy harvesting based on the Olsen cycle. In this work, by incorporation of AlN into BNT-BZT ceramics to form the BNT-BZT: AlN composites, the BDS is enhanced from 160 kV cm−1 to 260 kV cm−1, leading to a high energy density of 1.54 J cm−3 which is ∼5 times that of the pristine BNT-BZT. The reason for the high energy harvesting performance is due to the addition of AlN that can depress the conductivity of the matrix, resulting in the high BDS that permits high electric fields to be applied on the samples to induce high polarization that changes pronouncedly with temperature.

Highly compact and geometrically complex piezoceramics are required by a variety of electromechanical devices owing to their outstanding piezoelectricity, mechanical stability and extended application scenarios. 3D printing is currently the mainstream technology for fabricating geometrically complex piezoceramic components. However, it is hard to print piezoceramics in a curve shape while also keeping its compactness due to restrictions on the ceramic loading and the viscosity of feedstocks. Here, we report a gravity-driven sintering (GDS) process to directly fabricate curved and compact piezoceramics by exploiting gravitational force and high-temperature viscous behavior of sintering ceramic specimens. The sintered lead zirconate titanate (PZT) ceramics possess curve geometries that can be facilely tuned via the initial mechanical boundary design, and exhibit high piezoelectric properties comparable to those of conventional-sintered compact PZT (d33 = 595 pC/N). In contrast to 3D printing technology, our GDS process is suitable for scale-up production and low-cost production of piezoceramics with diverse curved surfaces. Our GDS strategy is an universal and facile route to fabricate curved piezoceramics and other functional ceramics with no compromise of their functionalities., Fabricating geometrically complex piezoceramics in compact sintered bodies has been difficult to achieve. Here the authors demonstrate a gravity-driven sintering strategy where high-temperature viscous behavior of piezoceramic allows for forming of complex shaped sintered bodies.

In this paper, the effects of the coil thickness on output voltage and power of the electromagnetic energy harvesters are investigated. A configuration which is mainly comprised of two coil arrays, a pair of restoration springs and a magnet array is designed. The coil arrays are symmetrically distributed on both sides of the magnet array. Ten kinds of coil thicknesses are selected to examine how they influence the electric responses. Based on a series of fabricated prototypes, four experimental studies from the following aspects for comparison purposes: frequency sweep, constant frequency, impedance matching and charging capacitors are conducted. The results show that the resonant frequency of the prototypes is 20 Hz, well agreeing with the theoretical one of 19.7 Hz. The case of h = 5.7 mm (h is coil thickness) reaches the maximum voltage (26.61 V) at the excitation of 1.0 g, 20 Hz. The maximum peak-to-peak power (312.05 mW) and the power density (2.56 mW/cm3) come from the case of h = 4.7 mm at the excitation of 1.0 g, 20 Hz. The case of h = 4.7 mm displays the best charging performance for the capacitor of 220 μF. It is also one of the best cases regarding the charging rates for 680 µF, 1000 µF, 2200 µF and 6800 µF capacitors.

Human motion is a rich source of energy. However, the ultra-low frequency and the irregular feature of the motion make the above prospects still far from being effective. In this work, we present a translation-to-rotation converter for energy harvesting during human walking. We here propose the harvester that enables conversion of low-frequency translational vibration to high-speed rotations and further generates electricity via the electromagnetic effect. The transducer is composed of an internal thread sleeve, an external thread driving shaft, a spring, a gearbox, and a limit frame. Our design features low cost, low fabrication difficulty, and high power density. Experiments conducted under pseudo-walking conditions confirmed that the stiffness of the spring and the pitch of the sleeve are the two key parameters for the performance of the harvester. The harvester mounts in a shoe heel and engages power generation in the whole gait cycle. Test subjects walking with one device with optimal spring stiffness on each shoe produce a peak power of 3.7 watts at a pace frequency of 1 Hz, which is about one magnitude higher than that of shoe-mounted devices under pseudo-walking excitation. Moreover, the proposed device could open an alternative pathway for powering smart wearable or portable electronics.

In this paper, we design and experimentally validate an auxetic nonlinear piezoelectric energy harvester. This harvester combines a clamp-clamp beam and auxetic structures, which can improve the efficiency and bandwidth of the energy harvesting based on a pure mechanical structure without outer magnets or stoppers. Finite element analysis is performed to analyse the effects of the auxetic structures on the efficiency and nonlinearity of the energy harvester. The lumped parameter model is utilized to predict the performance of the energy harvester, which well matches the experimental results. In the experimental validation, under 0.1g base excitation, the power output of the two types of auxetic energy harvesters is 173% and 94% higher than the conventional nonlinear energy harvester. Besides, the bandwidths of the two types of auxetic energy harvesters are broadened by 1556% and 2142% compared with the linear systems.

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.

This paper compares the performance of three nonlinear compliant orthoplanar springs, namely, bi-leg, quad-leg and pent-leg designs to study the vibration mode interaction effect for widening the operational bandwidth of piezoelectric vibration energy harvester. All the designs have three or more vibration modes below 150 Hz with the addition of multiple masses. Finite element analysis results of spring designs and vibration mode shapes are presented and compared with experiments. The prototypes were manufactured and experimentally tested to analyze their dynamic voltage-frequency responses and operational bandwidths under sinusoidal and band-limited excitations. The experimental results demonstrate that the operational bandwidth is significantly broadened by the geometric nonlinearity of the structure. The addition of multiple masses brings the vibration modes of the system closer, and thus further helps to extend the operational bandwidth. In addition, we introduce multiple piezoelectric plates into harvesters to enable energy harvesting in all vibration modes. The quad-leg design shows the largest frequency bandwidth of 35 Hz, maintaining a peak-to-peak voltage of 4 V under the forward frequency-sweep excitations. Under the reverse frequency-sweep excitations, the voltage response and the operational bandwidth are comparable to those of the forward-sweep ones as a result of continuous overlapping of linear and nonlinear voltage peaks. Random excitation experiments demonstrate the effectiveness of the proposed mutli-leg structure in harvesting energy from wideband environmental vibrations.

Energy harvesting devices that prosper in harsh environments are highly demanded in a wide range of applications ranging from wearable and biomedical devices to self-powered and intelligent systems. Particularly, over the past several years, the innovation of triboelectric nanogenerators (TENGs) that efficiently convert ambient kinetic energy of water droplets or wave power to electricity has received growing attention. One of the main bottlenecks for the practical implications of such devices originates from the fast degradation of the physiochemical properties of interfacial materials under harsh environments. To overcome these challenges, here we report the design of a novel slippery lubricant-impregnated porous surface (SLIPS) based TENG, referred to as SLIPS-TENG, which exhibits many distinctive advantages over conventional design including optical transparency, configurability, self-cleaning, flexibility, and power generation stability, in a wide range of working environments. Unexpectedly, the slippery and configurable lubricant layer not only serves as a unique substrate for liquid/droplet transport and optical transmission, but also for efficient charge transfer. Moreover, we show that there exists a critical thickness in the liquid layer, below which the triboelectric effect is almost identical to that without the presence of such a liquid film. Such an intriguing charge transparency behavior is reminiscent of the wetting transparency and van der Waals potential transparency of graphene previously reported, though the fundamental mechanism remains to be elucidated. We envision that the marriage of these two seemingly totally different arenas (SLIPS and TENG) provides a paradigm shift in the design of robust and versatile energy devices that can be used as a clean and longer-lifetime alternative in various working environments.

Colloidal quantum dots (CQDs) have shown their advantages in gas-sensing applications due to their extremely small particle size and facile solution based processes. In this study, a high sensitivity of surface acoustic wave (SAW) NO2 sensor was demonstrated using SnS CQDs as the sensing layer. The delay line based SAW device with a resonant frequency of 200 MHz were fabricated on ST-cut quartz substrate. The SnS CQDs with average sizes of 5.0 nm were synthesized and deposited onto SAW sensors using a spin-coating method. The fabricated SAW sensor was capable of detecting a low concentration of NO2 gas at room temperature with a good efficiency and selectivity e.g., with a 1.8 kHz decrease of center frequency of the SAW delay line when exposed to 10 ppm NO2 at room temperature.