Utilizing a High-Performance Piezoelectric Nanocomposite as a Self-Activating Component in Piezotronic Artificial Mechanoreceptors

Challenges such as poor dispersion and insufficient polarization of BaTiO3 (BTO) nanoparticles (NPs) within poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)) composites have hindered their piezoelectricity, limiting their uses in pressure sensors, nanogenerators, and artificial sensory synapses. Here, we introduce a high-performance piezoelectric nanocomposite material consisting of P(VDF-TrFE)/modified-BTO (mBTO) NPs for use as a self-activating component in a piezotronic artificial mechanoreceptor. To generate high-performance piezoelectric nanocomposite materials, the surface of BTO is hydroxylated, followed by the covalent attachment of (3-aminopropyl)triethoxysilane to improve the dispersibility of mBTO NPs within the P(VDF-TrFE) matrix. We also aim to enhance the crystallization degree of P(VDF-TrFE), the efficiency characteristics of mBTO, and the poling efficiency, even when incorporating small amounts of mBTO NPs. The piezoelectric potential mechanically induced from the P(VDF-TrFE)/mBTO NPs nanocomposite was three times greater than that from P(VDF-TrFE) and twice as high as that from the P(VDF-TrFE)/BTO NPs nanocomposite. The piezoelectric potential generated by mechanical stimuli on the piezoelectric nanocomposite was utilized to activate the synaptic ionogel-gated field-effect transistor for the development of self-powered piezotronics artificial mechanoreceptors on a polyimide substrate. The device successfully emulated fast-adapting (FA) functions found in biological FA mechanoreceptors. This approach has great potential for applications to future intelligent tactile perception technology.

High-Performance Piezoelectric Nanogenerator of BTO-PVDF Nanofibers for Wearable Sensing

Piezoelectric nanogenerator (PENG) produces stable electrical signals in response to external mechanical stimuli and holds promise in the fields of flexible sensors and smart wearable devices. In practice, a high-performance PENG with a straightforward structure and exceptional reliability is deeply desired. This study optimally synthesizes piezoelectric composites comprising polyvinylidene fluoride (PVDF) incorporated with barium titanate (BTO) nanoparticles (NPs) and fabricated a PENG with heightened sensitivity by using the electrospinning technique. The polar β-phase content of the dual-optimized BTO-PVDF (barium titanate and polyvinylidene fluoride) electrospun fiber reaches up to 82.39%. In the bending mode, it achieves a remarkable maximum open-circuit voltage of 19.152 V, a transferred charge of 8.058 nC, and an output voltage per unit area of 2.128 V cm- 2. Under vertical pressure conditions, the BP-PENG exhibits an impressive voltage of 12.361 V while the force is 2.156 N, demonstrating a notable pressure sensing sensitivity of 5.159 V kPa-1, with an excellent linear relationship. Furthermore, the BP-PENG displays sensitive sensing features in monitoring hand movements. The sensitive response and high performance make it promising for applications in human motion monitoring and smart wearable devices.

Boosting the Output Performance of the MoS2 Monolayer-Based Piezoelectric Nanogenerator by Artificial Dual Strain Concentration

Piezoelectric nanogenerators (PENGs) with molybdenum disulfide (MoS2) monolayers have been intensively studied owing to their superior mechanical durability and stability. However, the limited output performance resulting from a small active area and low strain levels continues to pose a significant challenge that should be overcome. Herein, we report a novel strategy for the epoch-making output performance of a PENG with a MoS2 monolayer by adopting the additive strain concentration concept. The simulation study indicates that strain in the MoS2 monolayer can be initially augmented by the wavy structure resulting from the prestretched poly(dimethylsiloxane) (PDMS) and is further increased through flexural deformation (i.e., bending). Based on these studies, we have developed concentrated strain-applied PENGs with MoS2 monolayers. The wavy structures effectively applied strain to the MoS2 monolayer and generated a piezoelectric output voltage and current of around 580 mV and 47.5 nA, respectively. Our innovative approach to enhancing the performance of PENGs with MoS2 monolayers through the artificial dual strain concept has led to groundbreaking results, achieving the highest recorded output voltage and current for PENGs based on two-dimensional (2D) materials, which provides unique opportunities for the 2D-based energy harvesting field and structural insight into how to improve the net strain on 2D materials.

A 3D-Printed Piezoelectric Microdevice for Human Energy Harvesting for Wearable Biosensors

The human body is a source of multiple types of energy, such as mechanical, thermal and biochemical, which can be scavenged through appropriate technological means. Mechanical vibrations originating from contraction and expansion of the radial artery represent a reliable source of displacement to be picked up and exploited by a harvester. The continuous monitoring of physiological biomarkers is an essential part of the timely and accurate diagnosis of a disease with subsequent medical treatment, and wearable biosensors are increasingly utilized for biomedical data acquisition of important biomarkers. However, they rely on batteries and their replacement introduces a discontinuity in measured signals, which could be critical for the patients and also causes discomfort. In the present work, the research into a novel 3D-printed wearable energy harvesting platform for scavenging energy from arterial pulsations via a piezoelectric material is described. An elastic thermoplastic polyurethane (TPU) film, which forms an air chamber between the skin and the piezoelectric disc electrode, was introduced to provide better adsorption to the skin, prevent damage to the piezoelectric disc and electrically isolate components in the platform from the human body. Computational fluid dynamics in the framework of COMSOL Multiphysics 6.1 software was employed to perform a series of coupled time-varying simulations of the interaction among a number of associated physical phenomena. The mathematical model of the harvester was investigated computationally, and quantification of the output energy and power parameters was used for comparisons. A prototype wearable platform enclosure was designed and manufactured using fused filament fabrication (FFF). The influence of the piezoelectric disc material and its diameter on the electrical output were studied and various geometrical parameters of the enclosure and the TPU film were optimized based on theoretical and empirical data. Physiological data, such as interdependency between the harvester skin fit and voltage output, were obtained.

Built-In Piezoelectric Nanogenerators Promote Sustainable and Flexible Supercapacitors: A Review

Energy storage devices such as supercapacitors (SCs), if equipped with built-in energy harvesters such as piezoelectric nanogenerators, will continuously power wearable electronics and become important enablers of the future Internet of Things. As wearable gadgets become flexible, energy items that can be fabricated with greater compliance will be crucial, and designing them with sustainable and flexible strategies for future use will be important. In this review, flexible supercapacitors designed with built-in nanogenerators, mainly piezoelectric nanogenerators, are discussed in terms of their operational principles, device configuration, and material selection, with a focus on their application in flexible wearable electronics. While the structural design and materials selection are highlighted, the current shortcomings and challenges in the emerging field of nanogenerators that can be integrated into flexible supercapacitors are also discussed to make wearable devices more comfortable and sustainable. We hope this work may provide references, future directions, and new perspectives for the development of electrochemical power sources that can charge themselves by harvesting mechanical energy from the ambient environment.

Titania Nanoparticle-Stimulated Ultralow Frequency Detection and High-Pass Filter Behavior of a Flexible Piezoelectric Nanogenerator: A Self-Sustaining Energy Harvester for Active Motion Tracking

A significant driving force for the fabrication of IoT-compatible smart health gear integrated with multifunctional sensors is the growing trend in fitness and the overall wellness of the human body. In this work, we present an autonomous motion and activity-sensing device based on the efficacious nucleation of the polar β-phase in an electroactive polymer. Representatively, we investigate the nucleating effect of TiO2 nanoparticles on weight-modulated PVDF-HFP films (PT-5, PT-10, and PT-15) and subsequently prototype a sensing device with the film that demonstrates superior β-phase nucleation. The PT-10 film, with an optimal polar β-phase, shows the highest remnant polarization (2Pr) and energy density of 0.36 μC/cm2 and 22.3 mJ/cm3, respectively, at 60 kV/cm. The films mimic a high pass filter at frequencies above 10 KHz with very low impedance and high ac conductivity values. The frequency-dependent impedance studies reveal an effective interfacial polarization between TiO2 nanoparticles and PVDF-HFP, explicitly observed in the low-frequency region. Consequently, the sensor fabricated with PT-10 as the sensing layer exhibits ultralow frequency detection (25 Hz) resulting from the blood flow muscle oxygenation. The device successfully senses voluntary joint movements of the human body and actively tracks a range of motions, from brisk walking to running. Additionally, through repetitive human finger-tapping motion, the nanogenerator lights up multiple light-emitting diodes in series and charges capacitors of varying magnitudes under 50 s. The real-time human motion sensing and movement tracking modalities of the sensor hold promise in the arena of smart wearables, sports biomechanics, and contact-based medical devices.

Halide Tunablility Leads to Enhanced Biomechanical Energy Harvesting in Lead-Free Cs2SnX6-PVDF Composites

The main challenges impeding the widespread use of organic-inorganic lead halide perovskites in modern-day technological devices are their long-term instability and lead contamination. Among other environmentally convivial and sustainable alternatives, Cs₂SnX₆ (X = Cl, Br, and I) compounds have shown promise as ambient-stable, lead-free materials for energy harvesting, and optoelectronic applications. Additionally, they have demonstrated tremendous potential for the fabrication of self-powered nanogenerators in conjunction with piezoelectric polymers like polyvinylidene-fluoride (PVDF). We report on the fabrication of composites constituting solvothermally synthesized Cs₂SnX₆ nanostructures and PVDF. The electroactive phases in PVDF were boosted by the incorporation of Cs₂SnX₆, leading to enhanced piezoelectricity in the composites. First-principles density functional theory (DFT) studies were carried out to understand the interfacial interaction between the Cs₂SnX₆ and PVDF, which unravels the mechanism of physisorption between the perovskite and PVDF, leading to enhanced piezoresponse. The halide ions in the inorganic Cs₂SnX₆ perovskites were varied systematically, and the piezoelectric behaviors of the respective piezoelectric nanogenerators (PENGs) were investigated. Further, the dielectric properties of these halide perovskite-based hybrids are quantified, and their piezoresponse amplitude, piezoelectric output signals, and charging capacity are also evaluated. Out of the several films fabricated, the optimized Cs₂SnI₆_PVDF film shows a piezoelectric coefficient (d₃₃) value of ∼200 pm V^-1 and a remanent polarization of ∼0.74 μC cm^-2 estimated from piezoresponse force microscopy and polarization hysteresis loop measurement, respectively. The optimized Cs₂SnI₆_PVDF-based device produced an instantaneous output voltage of ∼167 V, a current of ∼5.0 μA, and a power of ∼835 μW across a 5 MΩ resistor when subjected to periodic vertical compression. The output voltage of this device is used to charge a capacitor with a 10 μF capacitance up to 2.2 V, which is then used to power some commercial LEDs. In addition to being used as a pressure sensor, the device is employed to monitor human physiological activities. The device demonstrates excellent operational durability over a span of several months in an ambient environment vouching for its exceptional potential in application to mechanical energy harvesting and pressure sensing applications.

Heterojunction Engineering Enhanced Self-Polarization of PVDF/CsPbBr3 /Ti3 C2 Tx Composite Fiber for Ultra-High Voltage Piezoelectric Nanogenerator

Piezoelectric nanogenerator (PENG) for practical application is constrained by low output and difficult polarization. In this work, a kind of flexible PENG with high output and self-polarization is fabricated by constructing CsPbBr₃ -Ti₃ C₂ T_x heterojunctions in PVDF fiber. The polarized charges rapidly migrate to the electrodes from the Ti₃ C₂ T_x nanosheets by forming heterojunctions, achieving the maximum utilization of polarized charges and leading to enhanced piezoelectric output macroscopically. Optimally, PVDF/4wt%CsPbBr₃ /0.6wt%Ti₃ C₂ T_x -PENG exhibits an excellent voltage output of 160 V under self-polarization conditions, which is higher than other self-polarized PENG previously. Further, the working principle and self-polarization mechanism are uncovered by calculating the interfacial charge and electric field using first-principles calculation. In addition, PVDF/4wt%CsPbBr₃ /0.6wt%Ti₃ C₂ T_x -PENG exhibits better water and thermal stability attributed to the protection of PVDF. It is also evaluated in practice by harvesting the energy from human palm taps and successfully lighting up 150 LEDs and an electronic watch. This work presents a new idea of design for high-performance self-polarization PENG.

Transition metal dichalcogenides nanomaterials based piezocatalytic activity: recent progresses and outlook

Piezoelectric materials have drawn significant attention from researchers in the recent past as the piezo-potential, induced by applied external stress, generates an electric field, which paves the way for the creation and transfer of electrons and holes. After the theoretical prediction of the existence of the piezoelectric effect in transition metal dichalcogenides (TMDCs) semiconductors, intense research efforts have been made by various researchers to demonstrate the effect experimentally. In addition 2D TMDCs exhibit layer-dependent tunable electronic structure, strongly bound excitons, enhanced catalytic activity at their edges, and novel spin/pseudospin degrees of freedom. The edge sites and activated basal planes of 2D TMDCs are shown to be highly active toward catalysis of the hydrogen evolution reaction (HER). However, as compared to electrocatalytic or even photocatalytic performances, TMDC materials exhibit poorer piezocatalytic activity, in general. Therefore, a numbers of research strategies have been made to intensify the piezoelectric effect by synthesizing different types of TMDC nanostructures, by coupling the piezoelectric effect with the photocatalytic effect, by doping with other materials, etc. This review discusses various techniques of synthesis of TMDCs nanostructures and the recent progresses in applications of TMDC nanomaterials in piezocatalysis. In the present article, the piezocatalytic dye degradation performances and HER activity using different TMDCs have been reviewed in detail. Different methods of increasing the piezocatalytic activity of various TMDCs nanostructures have been illustrated. Here, it has also been attempted to systematically summarize and provide an outlook of the charge transfer behaviour and catalytic mechanisms in large varieties of TMDC piezocatalysts and piezo-photocatalysts. In addition, advanced applications of TMDC piezocatalytic materials as piezoelectric nanogenerator, piezocatalytic dye degradation, piezo-phototronic dye degradation and HER studies have been highlighted.

Two-Dimensional Metal-Organic Framework Incorporated Highly Polar PVDF for Dielectric Energy Storage and Mechanical Energy Harvesting

Here, we introduce a 2D metal-organic framework (MOF) into the poly(vinylidene fluoride) (PVDF) matrix, which has been comparatively less explored in this field. Highly 2D Ni-MOF has been synthesized in this regard via hydrothermal route and has been incorporated into PVDF matrix via solvent casting technique with ultralow filler (0.5 wt%) loading. The polar phase percentage of 0.5 wt% Ni-MOF loaded PVDF film (NPVDF) has been found to be increased to ~85% from a value of ~55% for neat PVDF. The ultralow filler loading has inhibited the easy breakdown path along with increased dielectric permittivity and hence has enhanced the energy storage performance. On the other hand, significantly enriched polarity and Young's Modulus has helped in improving its mechanical energy harvesting performance, thereby enhancing the human motion interactive sensing activities. The piezoelectric and piezo-tribo hybrid devices made up of NPVDF film have shown improved output power density of ~3.26 and 31 μW/cm² compared to those of the piezoelectric and piezo-tribo hybrid devices comprising of neat PVDF (output power density ~0.6 and 17 μW/cm², respectively). The developed composite can thus be considered an excellent candidate for multifunctional applications.

Self-Charging Piezo-Supercapacitor: One-Step Mechanical Energy Conversion and Storage

With the contemplations of ecological and environmental issues related to energy harvesting, piezoelectric nanogenerators (PNGs) may be an accessible, sustainable, and abundant elective wellspring of energy in the future. The PNGs' power output, however, is dependent on the mechanical energy input, which will be intermittent if the mechanical energy is not continuous. This is a fatal flaw for electronics that need continuous power. Here, a self-charging flexible supercapacitor (PSCFS) is successfully realized that can harvest sporadic mechanical energy, convert it to electrical energy, and simultaneously store power. Initially, chemically processed multimetallic oxide, namely, copper cobalt nickel oxide (CuCoNiO₄) is amalgamated within the poly(vinylidene fluoride) (PVDF) framework in different wt % to realize high-performance PNGs. The combination of CuCoNiO₄ as filler creates a notable electroactive phase inside the PVDF matrix, and the composite realized by combining 1 wt % CuCoNiO₄ with PVDF, coined as PNCU 1, exhibits the highest electroactive phase (>86%). Under periodic hammering (∼100 kPa), PNGs fabricated with this optimized composite film deliver an instantaneous voltage of ∼67.9 V and a current of ∼4.15 μA. Furthermore, PNG 1 is ingeniously integrated into a supercapacitor to construct PSCFS, using PNCU 1 as a separator and CuCoNiO₄ nanowires on carbon cloth (CC) as the positive and negative electrodes. The self-charging behavior of the rectifier-free storage device was established under bending deformation. The PSCFS device exhibits ∼845 mV from its initial open-circuit potential ∼35 mV in ∼220 s under periodic bending of 180° at a frequency of 1 Hz. The PSCFS can power up various portable electronic appliances such as calculators, watches, and LEDs. This work offers a high-performance, self-powered device that can be used to replace bulky batteries in everyday electronic devices by harnessing mechanical energy, converting mechanical energy from its environment into electrical energy.

From Piezoelectric Nanogenerator to Non-Invasive Medical Sensor: A Review

Piezoelectric nanogenerators (PENGs) not only are able to harvest mechanical energy from the ambient environment or body and convert mechanical signals into electricity but can also inform us about pathophysiological changes and communicate this information using electrical signals, thus acting as medical sensors to provide personalized medical solutions to patients. In this review, we aim to present the latest advances in PENG-based non-invasive sensors for clinical diagnosis and medical treatment. While we begin with the basic principles of PENGs and their applications in energy harvesting, this review focuses on the medical sensing applications of PENGs, including detection mechanisms, material selection, and adaptive design, which are oriented toward disease diagnosis. Considering the non-invasive in vitro application scenario, discussions about the individualized designs that are intended to balance a high performance, durability, comfortability, and skin-friendliness are mainly divided into two types: mechanical sensors and biosensors, according to the key role of piezoelectric effects in disease diagnosis. The shortcomings, challenges, and possible corresponding solutions of PENG-based medical sensing devices are also highlighted, promoting the development of robust, reliable, scalable, and cost-effective medical systems that are helpful for the public.

Thin Film Piezoelectric Nanogenerator Based on (100)-Oriented Nanocrystalline AlN Grown by Pulsed Laser Deposition at Room Temperature

In wearable or implantable biomedical devices that typically rely on battery power for diagnostics or operation, the development of flexible piezoelectric nanogenerators (NGs) that enable mechanical-to-electrical energy harvesting is finding promising applications. Here, we present the construction of a flexible piezoelectric nanogenerator using a thin film of room temperature deposited nanocrystalline aluminium nitride (AlN). On a thin layer of aluminium (Al), the AlN thin film was grown using pulsed laser deposition (PLD). The room temperature grown AlN film was composed of crystalline columnar grains oriented in the (100)-direction, as revealed in images from transmission electron microscopy (TEM) and X-ray diffraction (XRD). Fundamental characterization of the AlN thin film by piezoresponse force microscopy (PFM) indicated that its electro-mechanical energy conversion metrics were comparable to those of c-axis oriented AlN and zinc oxide (ZnO) thin films. Additionally, the AlN-based flexible piezoelectric NG was encapsulated in polyimide to further strengthen its mechanical robustness and protect it from some corrosive chemicals.

Charge-Boosting Strategy for Wearable Nanogenerators Enabled by Integrated Piezoelectric/Conductive Nanofibers

The surface charge density enhancement by incorporating conductive paths into organic/inorganic piezoelectric composites is considered to be an effective way to achieve high-performance piezoelectric nanogenerators (PENGs). However, it is challenging to boost the charge density of aligned piezoelectric nanofibers due to the difficulty in efficiently building well-distributed conductive paths in their dense structure. In this work, a charge boosting strategy was proposed for enhancing the surface charge density of aligned piezoelectric nanofibers, that is, synchronously preparing piezoelectric/conductive hybrid nanofibers to realize the effective conductive paths for transferring the underlying charges to the surface of the PDMS/BaTiO₃ composites. To this end, antimony-doped tin oxide (ATO) conductive nanofibers and barium titanate (BaTiO₃) piezoelectric nanofibers with the same preparation conditions were selected and synchronously prepared by the polymer template electrospinning technology, followed by the calcination process. Benefiting from the well-distributed conductive paths for transferring the charges, the open-circuit voltage and short-circuit current of a PENG with 12 wt% ATO in hybrid nanofibers reached 46 V and 14.5 μA (30 kPa pressure), respectively, which were much higher than the pristine BaTiO₃-based PENG. The high piezoelectric performance of the developed PENGs guaranteed their great potential applications in powering wearable microelectronics and monitoring human activity. This charge boosting strategy via the piezoelectric/conductive hybrid nanofibers may inspire the further development of high-performance energy harvesting technology.

Triboelectric and Piezoelectric Nanogenerators for Self-Powered Healthcare Monitoring Devices: Operating Principles, Challenges, and Perspectives

The internet of medical things (IoMT) is used for the acquisition, processing, transmission, and storage of medical data of patients. The medical information of each patient can be monitored by hospitals, family members, or medical centers, providing real-time data on the health condition of patients. However, the IoMT requires monitoring healthcare devices with features such as being lightweight, having a long lifetime, wearability, flexibility, safe behavior, and a stable electrical performance. For the continuous monitoring of the medical signals of patients, these devices need energy sources with a long lifetime and stable response. For this challenge, conventional batteries have disadvantages due to their limited-service time, considerable weight, and toxic materials. A replacement alternative to conventional batteries can be achieved for piezoelectric and triboelectric nanogenerators. These nanogenerators can convert green energy from various environmental sources (e.g., biomechanical energy, wind, and mechanical vibrations) into electrical energy. Generally, these nanogenerators have simple transduction mechanisms, uncomplicated manufacturing processes, are lightweight, have a long lifetime, and provide high output electrical performance. Thus, the piezoelectric and triboelectric nanogenerators could power future medical devices that monitor and process vital signs of patients. Herein, we review the working principle, materials, fabrication processes, and signal processing components of piezoelectric and triboelectric nanogenerators with potential medical applications. In addition, we discuss the main components and output electrical performance of various nanogenerators applied to the medical sector. Finally, the challenges and perspectives of the design, materials and fabrication process, signal processing, and reliability of nanogenerators are included.

Enhanced piezoelectric response in BTO NWs-PVDF composite through tuning of polar phase content

We have fabricated a flexible, environment friendly piezoelectric nanogenerator (PENG) based on the ferroelectric Polyvinylidene fluoride (PVDF) composite incorporated with Barium titanate (BaTiO₃) nanowires (NWs) of piezoelectric coefficientd₃₃ = 308 pm V^-1. The single-layered PENG can deliver output power density of 10μW cm^-2and an output voltage of 2 V with a nominal mechanical load of 1 kPa. BaTiO₃(BTO) NWs of different concentrations were incorporated into PVDF to tune the polar phase content, internal resistance, and optimize the output power. We show that there exists a critical value of BTO NWs loading of 15 wt%, beyond which the piezoelectric energy harvesting characteristics of the PVDF nanocomposites decrease. The oxygen vacancies present in the BTO NWs surface attract the fluorine ions of PVDF chain and favour the formation ofβphase. The enhanced value of dielectric constant and dielectric loss of BTO-PVDF samples in the low frequency region suggest strong interfacial polarization in the composite system. The fabricated PENG can charge a super-capacitor up to 4 V within 35 s. The origin of the high power output from the BTO (15 wt%)-PVDF composite is attributed to the combined effect of enhanced polar phase content, strong interfacial polarization, and reduced internal resistance. This study provides an effective pathway in enhancing the performance of BTO-PVDF based piezoelectric energy harvesters.

Metal oxide nanocomposite based flexible nanogenerator: synergic effect of light and pressure

Here, we report the fabrication of nanocomposite comprising of CuO and poly (vinylidene fluoride-hexafluoro propylene) (PVDF-HFP) for application in flexible piezoelectric nanogenerators (PENG). The chemically grown CuO nanostructures have been characterized through electron microscopy, x-ray diffraction, and spectroscopic techniques. It has been found that the incorporation of optimal CuO nanostructures in PVDF-HFP can increase the output voltage of the PENG by 22 times and is assigned to the increment in the effective dielectric constant of host PVDF-HFP. Further, the nanogenerator exhibits a maximum power of ∼20μW cm^-2at 3 MΩ load and can charge a capacitor under continuous bio-mechanical impart. Further, upon slight alteration of the device configuration, the output of the nanocomposite-based nanogenerator can be enhanced under illumination condition. The increment in overall piezopotential through photoexcitation in optically active CuO nanostructures can be assigned to the increment in output voltage. The wavelength dependent output variation reveal the maximum output of the PENG under blue light. Further, under white light illumination, the nanogenerator exhibits a maximum power which is 3 times higher than in dark condition and can charge a capacitor 52 times faster. The development of such superior flexible and optically active nanogenerators are quite promising for futuristic self-powered devices operated under mechanical and solar energies.

High-Performance Flexible Piezoelectric Nanogenerator Based on Electrospun PVDF-BaTiO3 Nanofibers for Self-Powered Vibration Sensing Applications

In the present era of intelligent electronics and Internet of Things (IoT), the demand for flexible and wearable devices is very high. Here, we have developed a high-output flexible piezoelectric nanogenerator (PENG) based on electrospun poly(vinylidene fluoride) (PVDF)-barium titanate (BaTiO₃) (ES PVDF-BT) composite nanofibers with an enhanced electroactive phase. On addition of 10 wt % BaTiO₃ nanoparticles, the electroactive β-phase of the PVDF is found to be escalated to ∼91% as a result of the synergistic interfacial interaction between the tetragonal BaTiO₃ nanoparticles and the ferroelectric host polymer matrix on electrospinning. The fabricated PENG device delivered an open-circuit voltage of ∼50 V and short-circuit current density of ∼0.312 mA m^-2. Also, the PVDF-BT nanofiber-based PENG device showed an output power density of ∼4.07 mW m^-2, which is 10 times higher than that of a pristine PVDF nanofiber-based PENG device. Furthermore, the developed PENG has been newly demonstrated for self-powered real-time vibration sensing applications such as for mapping of mechanical vibrations from faulty CPU fans, hard disk drives, and electric sewing machines.

Mitigating the Negative Piezoelectricity in Organic/Inorganic Hybrid Materials for High-performance Piezoelectric Nanogenerators

The conversion of ecofriendly waste energy into useable electrical energy is of significant interest for energy harvesting technologies. Piezoelectric nanogenerators based on organic/inorganic hybrid materials are a key promising technology for harvesting mechanical energy due to their high piezoelectric coefficient and good mechanical flexibility. However, the negative piezoelectric effect of the polymer component in composite devices severely undermines its overall piezoelectricity, compromising the output performance of PVDF-based piezoelectric hybrid nanogenerators. Here, to conquer this, we report a two-step poling schedule to orient the dipoles of organic and inorganic components in the same direction. The optimized nanogenerator delivers a combination of high piezoelectric coefficient, great output performance, and remarkable stability. The isotropic piezoelectricity in the composite device collaborates to output a maximum voltage of 110 V and a power density of 7.8 μW cm^-2. This strategy is also applied to elevate the piezoelectricity of other organic/inorganic-hybrid-based nanogenerators, substantiating its universal applicability for composite piezoelectric nanogenerators. This study presents a feasible strategy for enhancing the effective output capability of composite nanogenerator technologies.

Recent Progress of Nanogenerators for Green Energy Harvesting: Performance, Applications, and Challenges

Natural sources of green energy include sunshine, water, biomass, geothermal heat, and wind. These energies are alternate forms of electrical energy that do not rely on fossil fuels. Green energy is environmentally benign, as it avoids the generation of greenhouse gases and pollutants. Various systems and equipment have been utilized to gather natural energy. However, most technologies need a huge amount of infrastructure and expensive equipment in order to power electronic gadgets, smart sensors, and wearable devices. Nanogenerators have recently emerged as an alternative technique for collecting energy from both natural and artificial sources, with significant benefits such as light weight, low-cost production, simple operation, easy signal processing, and low-cost materials. These nanogenerators might power electronic components and wearable devices used in a variety of applications such as telecommunications, the medical sector, the military and automotive industries, and internet of things (IoT) devices. We describe new research on the performance of nanogenerators employing several green energy acquisition processes such as piezoelectric, electromagnetic, thermoelectric, and triboelectric. Furthermore, the materials, applications, challenges, and future prospects of several nanogenerators are discussed.

Enhanced voltage response in TiO2nanoparticle-embedded piezoelectric nanogenerator

Poly (vinylidene fluoride) (PVDF) and its copolymers have piqued a substantial amount of research interest for its use in modern flexible electronics. The piezoelectricβ-phase of the polymers can be augmented with the addition of suitable fillers that promoteβ-phase nucleation. In this work, we report an improved output voltage response of poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) with the incorporation of 10 wt.% Titanium (IV) oxide nanoparticles into the polymer matrix. The nano-filler was dispersed in the polymer matrix to form nanocomposite films via the solution casting technique. X-ray Diffraction and Scanning Electron Microscopy measurements were performed to verify the structure and morphology of the films. Fourier Transform Infrared Spectroscopy revealed enhancement in theβ-phase nucleation from ∼15% to ∼36% with the addition of 10 wt.% titania nanoparticles. Thermogravimetric analysis and Differential Scanning Calorimetry results show improved thermal stability of the nanocomposite film, up to 345 °C, as compared to pristine PVDF-HFP. We also demonstrate a facile method for the fabrication of a piezoelectric nanogenerator withβ-PVDF-HFP/TiO₂nanocomposite as an active layer. The outputs from the fabricated nanogenerator reached up to 8.89 V through human finger tapping motions, paving way for its potential use in the field of sensors, actuators, and self-sustaining flexible devices.

An Artificial Intelligence-Enhanced Blood Pressure Monitor Wristband Based on Piezoelectric Nanogenerator

Hypertensive patients account for about 16% to 37% of the global population, and about 9.4 million people die each year from hypertension and its complications. Blood pressure is an important indicator for diagnosing hypertension. Currently, blood pressure measurement methods are mainly based on mercury sphygmomanometers in hospitals or electronic sphygmomanometers at home. However, people's blood pressure changes with time, and using only the blood pressure value at the current moment to judge hypertension may cause misdiagnosis. Continuous blood pressure measurement can monitor sudden increases in blood pressure, and can also provide physicians with long-term continuous blood pressure changes as a diagnostic reference. In this article, we design an artificial intelligence-enhanced blood pressure monitoring wristband. The wristband's sensors are based on piezoelectric nanogenerators, with a high signal-to-noise ratio of 29.7 dB. Through the transformer deep learning model, the wristband can predict blood pressure readings, and the loss value is lower than 4 mmHg. By wearing this blood pressure monitoring wristband, we realized three days of continuous blood pressure monitoring of the subjects. The blood pressure monitoring wristband is lightweight, has profound significance for the prevention and treatment of hypertension, and has wide application prospects in medical, military, aerospace and other fields.

Wearable Piezoelectric Nanogenerators Based on Core-Shell Ga-PZT@GaOx Nanorod-Enabled P(VDF-TrFE) Composites

High-output flexible piezoelectric nanogenerators (PENGs) have achieved great progress and are promising applications for harvesting mechanical energy and supplying power to flexible electronics. In this work, unique core-shell structured Ga-PbZr_xTi_1-xO₃ (PZT)@GaO_x nanorods were synthesized by a simple mechanical mixing method and then were applied as fillers in a poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) matrix to obtain highly efficient PENGs with excellent energy-harvesting properties. The decoration of gallium nanoparticles on PZT @GaO_x nanorods can amplify the local electric field, facilitate the increment of polar β-phase fraction in P(VDF-TrFE), and strengthen the polarizability of PZT and P(VDF-TrFE). The interfacial interactions of GaO_x and P(VDF-TrFE) are also in favor of an increased β-phase fraction, which results in a remarkable improvement of PENG performance. The optimized Ga-PZT@GaO_x/P(VDF-TrFE) PENG delivers a maximum open-circuit voltage of 98.6 V and a short-circuit current of 0.3 μA with 9.8 μW instantaneous power under a vertical force of 12 N at a frequency of 30 Hz. Such a PENG exhibits a stable output voltage after 6 000 cycles by the durability test. Moreover, the liquid gallium metal offers a mechanical matching interface between rigid PZT and the soft polymer matrix, which benefits the effective, durable mechanical energy-harvesting capability from the physical activities of elbow joint bending and walking. This research renders a deep association between a liquid metal and piezoelectric ceramics in the field of piezoelectric energy conversion, offering a promising approach toward self-powered smart wearable devices.

Piezoelectric Nanogenerators based on Lead Zirconate Titanate nanostructures: an insight into the effect of potential barrier and morphology on the output power generation

The high internal resistance of the perovskite materials used in Nanogenerators (NGs) lowers the power generation. It severely restricts their application for mechanical energy harvesting from the ambient source. In this work, we demonstrate a flexible Piezoelectric NG (PENG) with an improved device structure. Hydrothermally grown one-dimensional Lead Zirconate Titanate (Pb(ZrTi)O₃) of different morphologies are used as the generating material. The morphology of the PZT nanostructures, engineered from nanoparticles to needle-shaped nanowires to increase the surface to volume ratio, provides effective mechanical contact with the electrode. The reduction of the internal resistance of the PENG has been achieved by two ways: (i) fabrication of interdigitated electrodes (IDE) to increase the interfacial polarization and (ii) lowering of Schottky barrier height (SBH) at the junction of the PZT nanostructure and the metal electrode by varying the electrode materials of different work functions. We find that lowering of the SBH at the interface contributes to an increased piezo voltage generation. The flexible nano needles-based PENG can deliver output voltage 9.5 V and power density 615μW cm^-2on application low mechanical pressure (∼1 kPa) by tapping motion. The internal resistance of the device is ∼0.65 MΩ. It can charge a 35μF super-capacitor up to 5 V within 20 s. This study provides a systematic pathway to solve the bottlenecks in the piezoelectric nanogenerators due to the high internal resistance.

Piezoelectric Nanogenerator for Highly Sensitive and Synchronous Multi-Stimuli Sensing

Smart sensors are expected to be sustainable, stretchable, biocomfortable, and tactile over time, either in terms of mechanical performance, reconfigurability, or energy supply. Here, a biocompatible piezoelectric electronic skin (PENG) is demonstrated on the base of PZT-SEBS (lead zirconate titanate and styrene ethylene butylene styrene) composite elastomer. The highly elastic (with an elasticity of about 950%) PENG can not only harvest mechanical energy from ambient environment, but also show low toxicity and excellent sensing performance toward multiple external stimuli. The synchronous and independent sensing performance toward motion capture, temperature, voice identification, and especially the dual-dimensional force perception promotes its wide application in physiological, sound restoration, and other intelligent systems.

Piezoelectric Materials for Energy Harvesting and Sensing Applications: Roadmap for Future Smart Materials

Piezoelectric materials are widely referred to as "smart" materials because they can transduce mechanical pressure acting on them to electrical signals and vice versa. They are extensively utilized in harvesting mechanical energy from vibrations, human motion, mechanical loads, etc., and converting them into electrical energy for low power devices. Piezoelectric transduction offers high scalability, simple device designs, and high-power densities compared to electro-magnetic/static and triboelectric transducers. This review aims to give a holistic overview of recent developments in piezoelectric nanostructured materials, polymers, polymer nanocomposites, and piezoelectric films for implementation in energy harvesting. The progress in fabrication techniques, morphology, piezoelectric properties, energy harvesting performance, and underpinning fundamental mechanisms for each class of materials, including polymer nanocomposites using conducting, non-conducting, and hybrid fillers are discussed. The emergent application horizon of piezoelectric energy harvesters particularly for wireless devices and self-powered sensors is highlighted, and the current challenges and future prospects are critically discussed.

Enhanced Piezoelectric Performance of Various Electrospun PVDF Nanofibers and Related Self-Powered Device Applications

The piezoelectric polymer poly(vinylidene fluoride) (PVDF) has been widely employed for energy harvesting or sensors. Its piezoelectricity originates from the unique crystal structure and the oriented electric dipoles. Generally, electrospinning (ES) is able to improve the crystallinity and piezoelectricity of PVDF. In this investigation, three types of ES, including far-field ES with random (R-PVDF-FFES) or aligned distribution (A-PVDF-FFES) and near-field ES (PVDF-NFES), are applied to fabricate a one-step polarized PVDF membrane. To compare the piezoelectricity of separated PVDF-NFES fibers, the array of PVDF-NFES fibers is encapsulated in polylactic acid (PLA). The result shows that the piezoelectricity of PVDF is dramatically enhanced by NFES and FFES. In d₃₁ mode, the current density of PVDF-NFES-PLA is 75.63 pA cm^-2 with a 2 Hz shear force, which is four times larger than A-PVDF-FFES (17.62 pA cm^-2) or seven times larger than R-PVDF-FFES (10.63 pA cm^-2). Due to its outstanding property, the PVDF-NFES-PLA membrane is designed to be applied as a self-powered device. The unimorph cantilevers are prepared to harvest the vibration energy and their output power density reaches 95.3 μW m^-2 at a resonance frequency of 31 Hz. Meanwhile, the flexible bending sensor and pressure sensor are fabricated with a PVDF-NFES-PLA membrane, which shows the stable response for pressure with sensitivity values of 864.68 and 22.6 mV kPa^-1, respectively.

Polarization- and Electrode-Optimized Polyvinylidene Fluoride Films for Harsh Environmental Piezoelectric Nanogenerator Applications

While piezoelectric nanogenerators have demonstrated the effective conversion of tiny mechanical vibrations to electricity, their performances are rarely examined under harsh environmental conditions. Here, a multilayered polyvinylidene fluoride (PVDF) film-based piezoelectric nanogenerator (ML-PENG) is demonstrated to generate considerable and stable power outputs even at extremely low temperatures and pressures, and under strong UV. Up-/down-polarized PVDF films are alternately stacked, and Ag electrodes are intercalated between the two adjacent films. At -266 °C and 10^-5  Torr, the ML-PENG generates an open-circuit voltage of 1.1 V, a short-circuit current density of 8 nA cm^-2 , and a power density of 4.4 nW cm^-2 . The piezoelectric outputs are quite stable against prolonged illumination of UV, large temperature- and pressure-variations, and excessive mechanical vibrations. The piezoelectric power density is greatly enhanced above the freezing and glass transition temperatures of PVDF and recorded to be 10, 105, and 282 nW cm^-2 at -73, 0, and 77 °C, respectively. The ML-PENG generates sufficient power to operate five light-emitting diodes by harvesting biomechanical energy under simulated Martian conditions. This work suggests that polarization- and electrode-optimized ML-PENG can serve as a reliable and economic power source in harsh and inaccessible environments like polar areas of Earth and extraterrestrial Mars.

Enhanced Energy Harvesting Ability of ZnO/PAN Hybrid Piezoelectric Nanogenerators

Miniaturization of energy conversion and storage devices has attracted remarkable consideration in the application of wearable electronics. Compared with film-based flexible electronics, fiber-based wearable electronics (e.g., nanogenerators and sensors made from electrospun nanofibers) are more appealing and promising for wearables. However, there are two bottlenecks, a low power output and poor sensing capability, limiting the application of piezoelectric nanofibers. Herein, we integrated zinc oxide nanorods (ZnO NRs) to a less known piezoelectric polymer, polyacrylonitrile (PAN) nanofiber, forming a ZnO/PAN nanofabric, which significantly improved the pressure sensitivity and vibrational energy harvesting ability by about 2.7 times compared with those of the pristine PAN nanofiber, and the maximum output power density of ∼10.8 mW·m^-2 is achieved. Noteworthily, the ZnO/PAN nanofabric showed a power output about twice of the one made of ZnO and polyvinylidene fluoride. It was revealed that the integration of ZnO NRs clearly improved the planar zigzag conformation in microstructures of the PAN nanofiber. Further, successful demonstrations of a mechanically robust pressure sensor and wearable power source confirm the potential applications in human activity monitoring and personal thermal management, respectively.

Sustainable and Biodegradable Wood Sponge Piezoelectric Nanogenerator for Sensing and Energy Harvesting Applications

Developing low-cost and biodegradable piezoelectric nanogenerators is of great importance for a variety of applications, from harvesting low-grade mechanical energy to wearable sensors. Many of the most widely used piezoelectric materials, including lead zirconate titanate (PZT), suffer from serious drawbacks such as complicated synthesis, poor mechanical properties (e.g., brittleness), and toxic composition, limiting their development for biomedical applications and posing environmental problems for their disposal. Here, we report a low-cost, biodegradable, biocompatible, and highly compressible piezoelectric nanogenerator based on a wood sponge obtained with a simple delignification process. Thanks to the enhanced compressibility of the wood sponge, our wood nanogenerator (15 × 15 × 14 mm³, longitudinal × radial × tangential) can generate an output voltage of up to 0.69 V, 85 times higher than that generated by native (untreated) wood, and it shows stable performance under repeated cyclic compression (≥600 cycles). Our approach suggests the importance of increased compressibility of bulk materials for improving their piezoelectric output. We demonstrate the versatility of our nanogenerator by showing its application both as a wearable movement monitoring system (made with a single wood sponge) and as a large-scale prototype with increased output (made with 30 wood sponges) able to power simple electronic devices (a LED light, a LCD screen). Moreover, we demonstrate the biodegradability of our wood sponge piezoelectric nanogenerator by studying its decomposition with cellulose-degrading fungi. Our results showcase the potential application of a wood sponge as a sustainable energy source, as a wearable device for monitoring human motions, and its contribution to environmental sustainability by electronic waste reduction.

Nanofibers-Based Piezoelectric Energy Harvester for Self-Powered Wearable Technologies

The demands for wearable technologies continue to grow and novel approaches for powering these devices are being enabled by the advent of new energy materials and novel manufacturing strategies. In addition, decreasing the energy consumption of portable electronic devices has created a huge demand for the development of cost-effective and environment friendly alternate energy sources. Energy harvesting materials including piezoelectric polymer with its special properties make this demand possible. Herein, we develop a flexible and lightweight nanogenerator package based on polyvinyledene fluoride (PVDF)/LiCl electrospun nanofibers. The piezoelectric performance of the developed nanogenator is investigated to evaluate effect of the thickness of the as-spun mat on the output voltage using a vibration and impact test. It is found that the output voltage increases from 1.3 V to 5 V by adding LiCl as additive into the spinning solution compared with pure PVDF. The prepared PVDF/LiCl nanogenerator is able to generate voltage and current output of 3 V and 0.5 μA with a power density output of 0.3 μW cm⁻² at the frequency of 200 Hz. It is found also that the developed nanogenerator can be utilized as a sensor to measure temperature changes from 30 °C to 90 °C under static pressure. The developed electrospun temperature sensor showed sensitivity of 0.16%/°C under 100 Pa pressure and 0.06%/°C under 220 Pa pressure. The obtained results suggested the developed energy harvesting textiles have promising applications for various wearable self-powered electrical devices and systems.

Porosity Modulated High-Performance Piezoelectric Nanogenerator Based on Organic/Inorganic Nanomaterials for Self-Powered Structural Health Monitoring

In the modern era, structural health monitoring (SHM) is critically important and indispensable in the aerospace industry as an effective measure to enhance the safety and consistency of aircraft structures by deploying a reliable sensor network. The deployment of built-in sensor networks enables uninterrupted structural integrity monitoring of an aircraft, providing crucial information on operation condition, deformation, and potential damage to the structure. Sustainable and durable piezoelectric nanogenerators (PENGs) with good flexibility, high performance, and superior reliability are promising candidates for powering wireless sensor networks, particularly for aerospace SHM applications. This research demonstrates a self-powered wireless sensing system based on a porous polyvinylidene fluoride (PVDF)-based PENG, which is prominently anticipated for developing auto-operated sensor networks. Our reported porous PVDF film is made from a flexible piezoelectric polymer (PVDF) and inorganic zinc oxide (ZnO) nanoparticles. The fabricated porous PVDF-based PENG demonstrates ∼11 times and ∼8 times enhancement of output current and voltage, respectively, compared to a pure PVDF-based PENG. The porous PVDF-based PENG can produce a peak-to-peak short-circuit current of 22 μA, a peak-to-peak open-circuit voltage of 84.5 V, a peak output power of 0.46 mW (P=Voc2×Isc2), and a peak output power density of 41.02 μW/cm² (P/A). By harnessing energy from minute vibrations, the fabricated porous PVDF-based PENG device (area of A = 11.33 cm²) can generate sufficient electrical energy to power up a customized wireless sensing and communication unit and transfer sensor data every ∼4 min. The PENG can generate sufficient electrical energy from an automobile car vibration, which reflects the scenario of potential real-life SHM systems. We anticipate that this high-performance porous PVDF-based PENG can act as a reliable power source for the sensor networks in aircraft, which minimizes potential safety risks.

Electromechanical Modeling of Vibration-Based Piezoelectric Nanogenerator with Multilayered Cross-Section for Low-Power Consumption Devices

Piezoelectric nanogenerators can convert energy from ambient vibrations into electrical energy. In the future, these nanogenerators could substitute conventional electrochemical batteries to supply electrical energy to consumer electronics. The optimal design of nanogenerators is fundamental in order to achieve their best electromechanical behavior. We present the analytical electromechanical modeling of a vibration-based piezoelectric nanogenerator composed of a double-clamped beam with five multilayered cross-sections. This nanogenerator design has a central seismic mass (910 μm thickness) and substrate (125 μm thickness) of polyethylene terephthalate (PET) as well as a zinc oxide film (100 nm thickness) at the bottom of each end. The zinc oxide (ZnO) films have two aluminum electrodes (100 nm thickness) through which the generated electrical energy is extracted. The analytical electromechanical modeling is based on the Rayleigh method, Euler–Bernoulli beam theory and Macaulay method. In addition, finite element method (FEM) models are developed to estimate the electromechanical behavior of the nanogenerator. These FEM models consider air damping at atmospheric pressure and optimum load resistance. The analytical modeling results agree well with respect to those of FEM models. For applications under accelerations in y-direction of 2.50 m/s² and an optimal load resistance of 32,458 Ω, the maximum output power and output power density of the nanogenerator at resonance (119.9 Hz) are 50.44 μW and 82.36 W/m³, respectively. This nanogenerator could be used to convert the ambient mechanical vibrations into electrical energy and supply low-power consumption devices.

Self-Powered Sensors and Systems Based on Nanogenerators

Sensor networks are essential for the development of the Internet of Things and the smart city. A general sensor, especially a mobile sensor, has to be driven by a power unit. When considering the high mobility, wide distribution and wireless operation of the sensors, their sustainable operation remains a critical challenge owing to the limited lifetime of an energy storage unit. In 2006, Wang proposed the concept of self-powered sensors/system, which harvests ambient energy to continuously drive a sensor without the use of an external power source. Based on the piezoelectric nanogenerator (PENG) and triboelectric nanogenerator (TENG), extensive studies have focused on self-powered sensors. TENG and PENG, as effective mechanical-to-electricity energy conversion technologies, have been used not only as power sources but also as active sensing devices in many application fields, including physical sensors, wearable devices, biomedical and health care, human-machine interface, chemical and environmental monitoring, smart traffic, smart cities, robotics, and fiber and fabric sensors. In this review, we systematically summarize the progress made by TENG and PENG in those application fields. A perspective will be given about the future of self-powered sensors.

Controllable Core-Shell BaTiO3@Carbon Nanoparticle-Enabled P(VDF-TrFE) Composites: A Cost-Effective Approach to High-Performance Piezoelectric Nanogenerators

Piezoelectric nanogenerators (PENGs), as a promising solution to harvest mechanical energy from ambient environment, have attracted much attention over the past decade. Here, the core-shell structured BaTiO₃@Carbon (BT@C) nanoparticles (NPs) were synthesized by a simple surface-modifying method and then used to fabricate the efficient PENGs with poly[(vinylidene fluoride)-co-trifluoroethylene] (P(VDF-TrFE)). The carbon shell with the uniform thickness of 10-15 nm can increase the content of the polar β phase in P(VDF-TrFE) and significantly enhance the interfacial polarization between BT NPs and the polymer matrix during the poling process. Out of all compositions, the 15 wt % BT@C/P(VDF-TrFE) PENG exhibited the optimal piezoelectric performance with an output voltage of ∼17 V and a maximum power of 14.3 μW under bending-releasing mode. More importantly, the PENG can also efficiently harvest other types of mechanical energy from human activities and exhibits stable output after 1500 bending-releasing cycles. When the PENG was bent and beat by bicycle spokes, a peak voltage of 16 V was generated, which can light up 12 white LEDs directly and charge the commercial capacitors. Our research provides a new strategy to fabricate flexible and efficient PENGs from a nanoscale viewpoint; it can be hopefully applied in energy-harvesting systems and wearable electric sensors.

On the Maximal Output Energy Density of Nanogenerators

Nanogenerators have been demonstrated as a high-efficiency energy harvesting technology, while methods to evaluate merits of nanogenerators are being focused. Energy density, as a common way for evaluating energy devices, is believed to be strongly related to the output performance limit of nanogenerators. Hence, this report introduces an evaluation standard, the output energy density, to evaluate the performance of nanogenerators. With the sliding freestanding mode TENG as an example, theoretical simulations are conducted and experimental methods are developed to understand and optimize the maximal output energy density, with the breakdown effect considered. By comparing the output energy density of TENGs and other nanogenerators, sliding-triggered TENGs are demonstrated with the highest output energy density, which is approaching 1 × 10⁴ J/m³. This study demonstrates the advantages of sliding-triggered TENGs in output energy density due to the suppressed breakdown effect and further confirmed the "killer application" of TENGs in harvesting low-frequency and small-scale mechanical energy.

A Comparative Study on the Effects of Au, ZnO and AZO Seed Layers on the Performance of ZnO Nanowire-Based Piezoelectric Nanogenerators

In this study, different seed layers like gold (Au), zinc oxide (ZnO) and aluminum-doped ZnO (AZO) have been associated to ZnO nanowires (NWs) for the development of mechanical energy harvesters. ZnO NWs were grown by using a low temperature hydrothermal method. The morphological properties were investigated using Scanning Electron Microscopy (SEM) and the analysis of crystalline quality and growth orientation was studied using X-ray Diffraction (XRD). The obtained ZnO NWs are found to be highly dense, uniformly distributed and vertically well aligned on the ZnO and AZO seed layers, while ZnO NWs grown on Au possess a low density and follow a non-uniform distribution. Moreover, the NWs exhibited good crystal quality over the seed layers. The piezoelectric nanogenerator (PENG) consists of ZnO NWs grown on the three different seed layers, parylene-C matrix, Ti/Al top electrode and poly(dimethylsiloxane) (PDMS) encapsulated polymer composite. The measurements of the open circuit voltage (V_OC) were around 272 mV, 36 mV for ZnO, AZO seed layers while the PENG including Au seed layer presented a short-circuited state. This study is an important step in order to investigate the effect of different seed layers influencing the magnitude of the generated electrical performances under identical growth and measurement conditions. It will also help identify the most suitable seed layers for energy harvesting devices and their future integration in industrial applications.

Recent Progress in Self-Powered Skin Sensors

Self-powered skin sensors have attracted significant attention in recent years due to their great potential in medical care, robotics, prosthetics, and sports. More importantly, self-powered skin sensors do not need any energy-supply components like batteries, which allows them to work sustainably and saves them the trouble of replacement of batteries. The self-powered skin sensors are mainly based on energy harvesters, with the device itself generating electrical signals when triggered by the detected stimulus or analyte, such as body motion, touch/pressure, acoustic sound, and chemicals in sweat. Herein, the recent research achievements of self-powered skin sensors are comprehensively and systematically reviewed. According to the different monitoring signals, the self-powered skin sensors are summarized and discussed with a focus on the working mechanism, device structure, and the sensing principle. Based on the recent progress, the key challenges that exist and the opportunities that lie ahead are also discussed.

A Flexible Piezoelectric Nanogenerator Based on Aligned P(VDF-TrFE) Nanofibers

Aligned P(VDF-TrFE) nanofibers are successfully fabricated by advanced electrospinning. The aligned feature of the nanofibers is achieved by using parallel electrodes, which is fabricated by lithography and wet etching, and a rotating drum collector. Scanning electron microscope (SEM) images show that the nanofibers are highly ordered with a smooth surface and uniform diameter. X-ray diffraction (XRD) and Fourier Transform Infrared spectrum (FTIR) tests indicate that the fibers contain high β phase content. The nanogenerator based on aligned P(VDF-TrFE) nanofibers exhibits good electric performance with a maximum output voltage as high as 12 V and peak-peak short circuit current about 150 nA, highlighting the potential application of P(VDF-TrFE) on self-powered and wearable devices.

Effect of TC(002) on the Output Current of a ZnO Thin-Film Nanogenerator and a New Piezoelectricity Mechanism at the Atomic Level

Understanding the piezoelectricity mechanism is crucial for developing new materials for better performance. Here, we developed a nanogenerator based on the ZnO thin films having various TC(002) values. The output current well correlated to the magnitude of (002) texture coefficient (TC(002)). Additionally, the TC(002)-dependent photovoltaic and rectification properties are observed. When the film is subjected to persistent compression, the photovoltaic, rectification, and piezoelectric properties fade away. Based on our observation that the ZnO polar structure always shows a spontaneous electron field (SEF), we thus propose a new piezoelectricity mechanism. The [001]-orientated ZnO thin film with the SEF is equivalent to a capacitor, the compression functions as a discharging process, and the removal of the external stress serves as a charging process. The physical mechanism provides an insight into various energy conversion processes that will inspire advanced designs of high-performance nanogenerators, solar cells, and other optoelectronic devices.

Self-Powered Well-Aligned P(VDF-TrFE) Piezoelectric Nanofiber Nanogenerator for Modulating an Exact Electrical Stimulation and Enhancing the Proliferation of Preosteoblasts

Electric potential plays an indispensable role in tissue engineering and wound healing. Piezoelectric nanogenerators based on direct piezoelectric effects can be self-powered energy sources for electrical stimulation and have attracted extensive attention. However, the accuracy of piezoelectric stimuli on piezoelectric polymers membranes in vitro during the dynamic condition is rarely studied. Here, a self-powered tunable electrical stimulation system for assisting the proliferation of preosteoblasts was achieved by well-aligned P(VDF-TrFE) piezoelectric nanofiber membrane (NFM) both as a nanogenerator (NG) and as a scaffold. The effects of electrospinning and different post-treatments (annealing and poling) on the surface wettability, piezoelectric β phase, ferroelectric properties, and sensing performance of NFMs were evaluated here. The polarized P(VDF-TrFE) NFM offered an enhanced piezoelectric value (d31 of 22.88 pC/N) versus pristine P(VDF-TrFE) NFM (d31 of 0.03 pC/N) and exhibited good sensing performance. The maximum voltage and current output of the P(VDF-TrFE) piezoelectric nanofiber NGs reached -1.7 V and 41.5 nA, respectively. An accurate electrical response was obtained in real time under dynamic mechanical stimulation by immobilizing the NGs on the flexible bottom of the culture plate, thereby restoring the real scene of providing electrical stimulation to the cells in vitro. In addition, we simulated the interaction between the piezoelectric nanofiber NG and cells through an equivalent circuit model. To verify the feasibility of P(VDF-TrFE) nanofiber NGs as an exact electrical stimulation, the effects of different outputs of P(VDF-TrFE) nanofiber NGs on cell proliferation in vitro were compared. The study realized a significant enhancement of preosteoblasts proliferation. This work demonstrated the customizability of P(VDF-TrFE) piezoelectric nanofiber NG for self-powered electrical stimulation system application and suggested its significant potential application for tissue repair and regeneration.

Dominant Role of Young's Modulus for Electric Power Generation in PVDF⁻BaTiO₃ Composite-Based Piezoelectric Nanogenerator

The electric power output of a piezoelectric nanogenerator (PENG) depends on the various physical parameters of the constituent materials, including the piezoelectric coefficient, Young's modulus, and dielectric constant. Herein, we report the mechanical and electrical properties of a poly(vinylidene fluoride)⁻BaTiO₃ (PVDF⁻BTO) composite-based PENG. Variation of the BTO nanoparticle (NP) content enabled the systematic tuning of the physical parameters that are related to power generation in the composite. The Young's modulus of the PVDF⁻BTO composite initially increased, and then eventually decreased, with the increasing BTO content, which was probably due to the clustering effect of the high modulus BTO NPs. The dielectric constant of the composite continuously increased as the BaTiO₃ content increased. The piezoelectric outputs were greatly enhanced at 10 wt% of BTO, where the Young's modulus was the highest. These results indicate that the Young's modulus plays an important role in the piezoelectric power generation of the composite-based PENGs.

Biocompatible Collagen Nanofibrils: An Approach for Sustainable Energy Harvesting and Battery-Free Humidity Sensor Applications

In contrast with the conventional ceramic/oxide humidity sensors (HSs), a self-powered piezoelectric biopolymer HS with reasonable sensitivity, reliability, and a nontoxic and eco-friendly nature is highly desirable. A piezoelectric nanogenerator (PNG)-driven biopolymer-based HS provides a pathway toward a sustainable and greener environment in the field of smart sensors. For that, a piezoelectric collagen nanofibril biopolymer coated on to a cotton fabric has dual functionality (energy harvesting and sensor). Collagen PNG generates a maximum of 45 V/250 nA upon 5 N and can also work as a sensor to measure various percentages of relative humidity (% RH). The HS shows a linear response with a good sensitivity (0.1287 μA/% RH) in the range of 50-90% RH. These results open a field of eco-friendly multifunctional nanomaterials toward the development of noninvasive, implantable smart bio-medical systems.

Concurrent Harvesting of Ambient Energy by Hybrid Nanogenerators for Wearable Self-Powered Systems and Active Remote Sensing

Harvesting energy available from ambient environment is highly desirable for powering personal electronics and health applications. Due to natural process and human activities, steam can be produced by boilers, human perspiration, and the wind exists ubiquitously. In the outdoor environment, these two phenomena usually exist at the same place, which contain heat and mechanical energies simultaneously. However, previous studies have isolated them as separate sources of energy to harvest and hence failed to utilize them effectively. Herein, we present unique hybrid nanogenerators for individually/simultaneously harvesting thermal energy from water vapors and mechanical energy from intermittent wind blowing from the bottom side, which consist of a wind-driven triboelectric nanogenerator (TENG) and pyroelectric-piezoelectric nanogenerators (PPENGs). The output power of the PPENG and the TENG can be up to about 184.32 μW and 4.74 mW, respectively, indicating the TENG plays the dominant role. Our hybrid nanogenerators could provide different applications such as to power digital watch and enable self-powered sensing with wireless transmission. The device could also be further integrated into a face mask for potentially wearable applications. This work not only provides a promising approach for renewable energy harvesting but also enriches potential applications for self-powered systems and wireless sensors.

Flexible Piezoelectric Generators by Using the Bending Motion Method of Direct-Grown-PZT Nanoparticles on Carbon Nanotubes

Recently, composite-type nanogenerators (NGs) formed from piezoelectric nanostructures and multi-walled carbon nanotubes (CNTs), have become one of the excellent candidates for future energy harvesting because of their ability to apply the excellent electrical and mechanical properties of CNTs. However, the synthesis of NG devices with a high proportion of piezoelectric materials and a low polymer content, such as of polydimethylsiloxane (PDMS), continues to be problematic. In this work, high-piezoelectric-material-content flexible films produced from Pb(Zr,Ti)O₃ (PZT)-atomically-interconnected CNTs and polytetrafluoroethylene (PTFE) are presented. Various physical and chemical characterization techniques are employed to examine the morphology and structure of the materials. The direct growth of the piezoelectric material on the CNTs, by stirring the PZT and CNT mixed solution, results in various positive effects, such as a high-quality dispersion in the polymer matrix and addition of flexoelectricity to piezoelectricity, resulting in the enhancement of the output voltage by an external mechanical force. The NGs repeatedly generate an output voltage of 0.15 V. These results present a significant step toward the application of NGs using piezoelectric nanocomposite materials.

Er3+/Fe3+ Stimulated Electroactive, Visible Light Emitting, and High Dielectric Flexible PVDF Film Based Piezoelectric Nanogenerators: A Simple and Superior Self-Powered Energy Harvester with Remarkable Power Density

The design of an energy-harvesting unit with superior output characteristics, i.e., high power density, is a great technological challenge in the present time. Here, simple, lightweight, flexible, and cost-effective piezoelectric nanogenerators (PENGs) have been fabricated by integrating the aluminum electrodes onto Er³⁺/Fe³⁺ stimulated electroactive, visible-light-emitting, and large dielectric PVDF films in which ErCl₃·6H₂O and Fe(NO₃)₃·9H₂O act as the catalytic agents for electroactive β polymorph nucleation and the enhancement of dielectric properties. The developed PENGs exhibit excellent energy-harvesting performance with very high power density and very fast charging ability compared with the previously reported PVDF-assisted prototype nanogenerators. The PENGs lead to very large power density (∼160 and ∼55.34 mW cm^-3) under periodic finger imparting for Er³⁺- and Fe³⁺-stimulated PVDF-film-based energy-harvester units, respectively. The fabricated self-powered PENG is also able to light up 54 commercially available light-emitting diodes.

Theoretical Study of the BaTiO₃ Powder's Volume Ratio's Influence on the Output of Composite Piezoelectric Nanogenerator

The combination of the piezoelectric materials and polymer is an effective way to make the piezoelectric nanogenerator (PENG) possess both the polymer's good flexibility and ferroelectric material's high piezoelectric coefficient. The volume ratio of ferroelectric material in the composite is an important factor that determines the PENG's output performance. In this paper, the BaTiO₃/polydimethylsiloxane (PDMS) composite PENG was demonstrated as having an optimal volume ratio (46%) at which the PENG can output its highest voltage, and this phenomenon can be ascribed to the trade-off between the composite PENG's top electrode charge and its capacitance. These results are of practical importance for the composite PENG's performance optimization.

Direct Writing of Patterned, Lead-Free Nanowire Aligned Flexible Piezoelectric Device

A high-performance flexible piezoelectric nanogenerator (PNG) is fabricated by a direct writing method, which acquires both patterned piezoelectric structure and aligned piezoelectric nanowires simultaneously. The voltage output of the as-prepared PNG is nearly 400% compared with that of the traditional spin-coated device due to the effective utilization of stress. This facile printing approach provides an efficient strategy for significant improvement of the piezoresponse.

Lattice Strain Induced Remarkable Enhancement in Piezoelectric Performance of ZnO-Based Flexible Nanogenerators

In this work, by employing halogen elements (fluorine, chlorine, bromine, and iodine) as dopant we demonstrate a unique strategy to enhance the output performance of ZnO-based flexible piezoelectric nanogenerators. For a halogen-doped ZnO nanowire film, dopants and doping concentration dependent lattice strain along the ZnO c-axis are established and confirmed by the EDS, XRD, and HRTEM analysis. Although lattice strain induced charge separation was theoretically proposed, it has not been experimentally investigated for wurtzite structured ZnO nanomaterials. Tuning the lattice strain from compressive to tensile state along the ZnO c-axis can be achieved by a substitution of halogen dopant from fluorine to other halogen elements due to the ionic size difference between dopants and oxygen. With its focus on a group of nonmetal element induced lattice strain in ZnO-based nanomaterials, this work paves the way for enhancing the performance of wurtzite-type piezoelectric semiconductor nanomaterials via lattice strain strategy which can be employed to construct piezoelectric nanodevices with higher efficiency in a cost-effective manner.

Sponge-Templated Macroporous Graphene Network for Piezoelectric ZnO Nanogenerator

We report a simple approach to fabricate zinc oxide (ZnO) nanowire based electricity generators on three-dimensional (3D) graphene networks by utilizing a commercial polyurethane (PU) sponge as a structural template. Here, a 3D network of graphene oxide is deposited from solution on the template and then is chemically reduced. Following steps of ZnO nanowire growth, polydimethylsiloxane (PDMS) backfilling and electrode lamination completes the fabrication processes. When compared to conventional generators with 2D planar geometry, the sponge template provides a 3D structure that has a potential to increase power density per unit area. The modified one-pot ZnO synthesis method allows the whole process to be inexpensive and environmentally benign. The nanogenerator yields an open circuit voltage of ∼0.5 V and short circuit current density of ∼2 μA/cm(2), while the output was found to be consistent after ∼3000 cycles. Finite element analysis of stress distribution showed that external stress is concentrated to deform ZnO nanowires by orders of magnitude compared to surrounding PU and PDMS, in agreement with our experiment. It is shown that the backfilled PDMS plays a crucial role for the stress concentration, which leads to an efficient electricity generation.