Multiplying the Stable Electrostatic Field of Electret Based on the Heterocharge-Synergy and Superposition Effect.

Owing to magic charge storage behavior, an electret can exhibit an external electrostatic field, which is widely used in numerous domains such as electronics, energy, healthcare, and environment. However, the theory of the charge storage mechanism still needs further development to enhance the performance and stability of the electret. Herein, a stable charge storage model known as the heterocharge-synergy model (HSM) in electrets is proposed and verified, and the electrostatic field superposition effect of electrets is also proved. Based on the HSM and superposition effect, the stable electrostatic field intensity (average of ≈22.49 kV cm-1 and maximum of ≈29.58 kV cm-1 , which is close to the minimum air breakdown field intensity of ≈30 kV cm-1 ) of the composite electret film is multiplied by simple layer-by-layer stacking. Utilizing the multilayer composite electret films and designing a two-sided electrostatic induction structure, a two-sided bipolar single-electrode non-contact nanogenerator is constructed with transferred charge density up to ≈132.61 µC m-2 , which is twice as large as that of the non-contact nanogenerators with one-sided electrostatic induction structure. Clearing and utilizing the charge behaviors of the electret can boost the performance and enhance the stability of electret-based devices in various domains.

High-Performance Dielectric Elastomer Nanogenerator for Efficient Energy Harvesting and Sensing via Alternative Current Method.

Soft, low-cost, high-performance generators are highly desirable for harvesting ambient low frequency mechanical energy. Here, a dielectric elastomer nanogenerator (DENG) is reported, which consists of a dielectric elastomer capacitor, an electret electrostatic voltage source, and a charge pump circuit. Under biaxial stretching, DENG can convert tensile mechanical energy into electrical power without any external power supply. Different from traditional DEG with the charge outward transfer in direct current (DC), the DENG works based on shuttle movement of internal charges in an alternating current (AC). Through alternating current (AC) method, the charge density of the DENG can reach 26 mC m-2 per mechanical cycle, as well as energy density of up to 140 mJ g-1 . Due to the all-solid-state structure without air gap, the DENG is capable of working stably under different ambient humidity (20 RH%-100 RH%). To demonstrate the applications, a water wave harvester based on the DENG is constructed. The integrated device powers a sensing communication module for self-powered remote weather monitoring, showing the potential application in ocean wave energy harvesting.

Trap-Induced Dense Monocharged Perfluorinated Electret Nanofibers for Recyclable Multifunctional Healthcare Mask.

Recently, wearable and breathable healthcare devices for air filtering and real-time vital signs monitoring have become urgently needed since virus and particulate matter (PM) cause serious health issues. Herein, we present a trap-induced dense monocharged hybrid perfluorinated electret nanofibrous membrane (HPFM) for highly efficient ultrafine PM0.3 removal with an efficiency of 99.712% under low pressure drop (38.1 Pa) and high quality factor of 0.154 Pa-1. Furthermore, a recyclable multifunctional healthcare mask is constructed by integrating the HPFM-based nanogenerator, which realizes efficient PM0.3 filtering and wireless real-time human respiration monitoring simultaneously. More importantly, the performance of this mask is still relatively stable even at 100%RH humidity and 92 °C temperature conditions for 48 h, which infers that it can be reused after disinfection. The strategy of fabricating HPFM provides an approach to obtain charge-rich stable electret materials, and the design of multifunctional masks demonstrates their potential application for future personal protection and health monitoring devices.

Fiber-Based Energy Conversion Devices for Human-Body Energy Harvesting.

Following the rapid development of lightweight and flexible smart electronic products, providing energy for these electronics has become a hot research topic. The human body produces considerable mechanical and thermal energy during daily activities, which could be used to power most wearable electronics. In this context, fiber-based energy conversion devices (FBECD) are proposed as candidates for effective conversion of human-body energy into electricity for powering wearable electronics. Herein, functional materials, fiber fabrication techniques, and device design strategies for different classes of FBECD based on piezoelectricity, triboelectricity, electrostaticity, and thermoelectricity are comprehensively reviewed. An overview of fiber-based self-powered systems and sensors according to their superior flexibility and cost-effectiveness is also presented. Finally, the challenges and opportunities in the field of fiber-based energy conversion are discussed.

Electrospun Polytetrafluoroethylene Nanofibrous Membrane for High-Performance Self-Powered Sensors.

Polytetrafluoroethylene (PTFE) is a fascinating electret material widely used for energy harvesting and sensing, and an enhancement in the performance could be expected by reducing its size into nanoscale because of a higher surface charge density attained. Hence, the present study demonstrates the use of nanofibrous PTFE for high-performance self-powered wearable sensors. The nanofibrous PTFE is fabricated by electrospinning with a suspension of PTFE particles in dilute polyethylene oxide (PEO) aqueous solution, followed by a thermal treatment at 350 °C to remove the PEO component from the electrospun PTFE-PEO nanofibers. The obtained PTFE nanofibrous membrane exhibits good air permeability with pressure drop comparable to face masks, excellent mechanical property with tensile strength of 3.8 MPa, and stable surface potential of - 270 V. By simply sandwiching the PTFE nanofibrous membrane into two pieces of conducting carbon clothes, a breathable, flexible, and high-performance nanogenerator (NG) device with a peak power of 56.25 µW is constructed. Remarkably, this NG device can be directly used as a wearable self-powered sensor for detecting body motion and physiological signals. Small elbow joint bending of 30°, the rhythm of respiration, and typical cardiac cycle are clearly recorded by the output waveform of the NG device. This study demonstrates the use of electrospun PTFE nanofibrous membrane for the construction of high-performance self-powered wearable sensors.

Boosting the Efficient Energy Output of Electret Nanogenerators by Suppressing Air Breakdown under Ambient Conditions.

By virtue of simple fabrication, low cost, and high conversion efficiency, nanogenerators play a key role in promoting the development of self-powered systems and large-scale mechanical energy harvesting. Efforts have been ongoing for improving the output power of nanogenerators by maximizing their surface charge density via surface modification or structure optimization. Nevertheless, because of inevitable air breakdown during the operation process, enhancing charge density is not retainable, which is the most crucial limitation for the output performance of nanogenerators. Here, a suppressing breakdown strategy is developed to remarkably enhance the output charge density of the nanogenerator by embedding a dielectric film (polyvinylidene fluoride) with high permittivity into air gaps. Because of the air breakdown suppression and strongly field-induced dielectric polarization effect, the output charge density of ∼470 µC m-2 is obtained at ambient condition, which is ∼4 times larger than the value of the conventional nanogenerator with air breakdown. In addition, the effects of different dielectric materials and their different thicknesses are also studied for enhancing the output charge density of the nanogenerator. These results provide a guide to design the state-of-the-art nanogenerator for efficient mechanical energy harvesting.

Flexible THV/COC Piezoelectret Nanogenerator for Wide-Range Pressure Sensing.

Flexible pressure sensors possess promising applications in artificial electronic skin, intelligence robot, wearable health monitoring, flexible physiological signal sensing, etc. Herein, we design a flexible pressure sensor with robust stability, high sensitivity, and large linear pressure region on the basis of tetrafluoroethylene-hexafluoropropylene-vinylide (THV)/cyclic olefin copolymer (COC) piezoelectret nanogenerator. According to the theoretical analysis for piezoelectret nanogenerators with imbalanced charge distribution, THV and COC are utilized to promote the electric field inside the piezoelectret for output voltage enhancement. Meanwhile, the compression property of the piezoelectret nanogenerator is facilely tuned. Owing to high inner electric field and optimized compression property, the THV/COC piezoelectret nanogenerator exhibits a high sensitivity of 30 mV/kPa, which is 10 times higher than that of the traditional cellular polypropylene piezoelectret. Simultaneously, the linear pressure region reaches 150 kPa with excellent linearity ( R2 = 0.99963). The device is demonstrated to realize wearable pressure sensing with a wide pressure range from finger typing to fist hammering. This study presents a fabrication strategy for piezoelectret nanogenerators with high sensitivity and large linear pressure region, paving the way for development of wearable and flexible pressure sensing networks.

Noncontact Heartbeat and Respiration Monitoring Based on a Hollow Microstructured Self-Powered Pressure Sensor.

Advances in mobile networks and low-power electronics have driven smart mobile medical devices at a tremendous pace, evoking increased interest in household healthcare, especially for those with cardiovascular or respiratory disease. Thus, flexible battery-free pressure sensors, with great potential for monitoring respiration and heartbeat in a smart way, are urgently demanded. However, traditional flexible battery-free pressure sensors for subtle physiological signal detecting are mostly tightly adhered onto the skin instead of working under the pressure of body weight in a noncontact mode, as the low sensitivity in the high-pressure region can hardly meet the demands. Moreover, a hollow microstructure (HM) with higher deformation than solid microstructures and great potential for improving the pressure sensitivity of self-powered sensors has never been investigated. Here, for the first time, we demonstrated a noncontact heartbeat and respiration monitoring system based on a flexible HM-enhanced self-powered pressure sensor, which possesses the advantages of low cost, a high dynamic-pressure sensitivity of 18.98 V·kPa-1, and a wide working range of 40 kPa simultaneously. Specific superiority of physiological detection under a high pressure is also observed. Continuous reliable heartbeat and respiration information is successfully detected in a noncontact mode and transmitted to a mobile phone.

Highly Efficient Water Harvesting with Optimized Solar Thermal Membrane Distillation Device.

Water distillation with solar thermal technology could be one of the most promising way to address the global freshwater scarcity, with its low cost and minimum environmental impacts. However, the low liquid water productivity, which is caused by the heat loss and inadequate heat utilization in solar-thermal conversion process, hinders its practical application. Here, a compact solar-thermal membrane distillation system with three structure features: highly localized solar-thermal heating, effective cooling strategy, and recycling the latent heat, is proposed. The steam generation rate is 0.98 kg m-2 h-1 under solar illumination of 1 kW m-2 in the open system, while the liquid water productivity could be 1.02 kg m-2 h-1 with the solar efficiency up to 72% with a two-level device. The outdoor experiments show a water productivity of 3.67 kg m-2 with salt rejection over 99.75% in one cloudy day. These results demonstrate an easy and high-efficiency way for water distillation, especially suitable for household solar water purification.

Output optimized electret nanogenerators for self-powered long-distance optical communication systems.

High-speed optical communication systems are built for real-time, massive and remote information exchange. However, any power outage will paralyse the systems and cause a huge loss. Here we constructed a self-powered long-distance optical communication system (SLOCS) utilizing output enhanced parallel connected electret nanogenerators (NGs) as a backup power for the power outage. The output current of the electret NG was promoted from 1.45 µA to 8.14 µA through optimizing the thickness of the electret film. In the SLOCS, a coded message was successfully transmitted for 50 meters by pressing electret NGs. The as-fabricated SLOCS paves the way to a simple and cost-effective strategy for developing a reliable emergency communication system in case of power outage, simultaneously promoting the progress of self-powered electronics.

Sensitivity-Enhanced Wearable Active Voiceprint Sensor Based on Cellular Polypropylene Piezoelectret.

Wearable active sensors have extensive applications in mobile biosensing and human-machine interaction but require good flexibility, high sensitivity, excellent stability, and self-powered feature. In this work, cellular polypropylene (PP) piezoelectret was chosen as the core material of a sensitivity-enhanced wearable active voiceprint sensor (SWAVS) to realize voiceprint recognition. By virtue of the dipole orientation control method, the air layers in the piezoelectret were efficiently utilized, and the current sensitivity was enhanced (from 1.98 pA/Hz to 5.81 pA/Hz at 115 dB). The SWAVS exhibited the superiorities of high sensitivity, accurate frequency response, and excellent stability. The voiceprint recognition system could make correct reactions to human voices by judging both the password and speaker. This study presented a voiceprint sensor with potential applications in noncontact biometric recognition and safety guarantee systems, promoting the progress of wearable sensor networks.

Scalable salt-templated synthesis of two-dimensional transition metal oxides.

Two-dimensional atomic crystals, such as two-dimensional oxides, have attracted much attention in energy storage because nearly all of the atoms can be exposed to the electrolyte and involved in redox reactions. However, current strategies are largely limited to intrinsically layered compounds. Here we report a general strategy that uses the surfaces of water-soluble salt crystals as growth templates and is applicable to not only layered compounds but also various transition metal oxides, such as hexagonal-MoO3, MoO2, MnO and hexagonal-WO3. The planar growth is hypothesized to occur via a match between the crystal lattices of the salt and the growing oxide. Restacked two-dimensional hexagonal-MoO3 exhibits high pseudocapacitive performances (for example, 300 F cm(-3) in an Al2(SO4)3 electrolyte). The synthesis of various two-dimensional transition metal oxides and the demonstration of high capacitance are expected to enable fundamental studies of dimensionality effects on their properties and facilitate their use in energy storage and other applications.

Two-Dimensional Layered Heterostructures Synthesized from Core-Shell Nanowires.

Controlled stacking of different two-dimensional (2D) atomic layers will greatly expand the family of 2D materials and broaden their applications. A novel approach for synthesizing MoS2 /WS2 heterostructures by chemical vapor deposition has been developed. The successful synthesis of pristine MoS2 /WS2 heterostructures is attributed to using core-shell WO3-x /MoO3-x nanowires as a precursor, which naturally ensures the sequential growth of MoS2 and WS2 . The obtained heterostructures exhibited high crystallinity, strong interlayer interaction, and high mobility, suggesting their promising applications in nanoelectronics. The stacking orientations of the two layers were also explored from both experimental and theoretical aspects. It is elucidated that the rational design of precursors can accurately control the growth of high-quality 2D heterostructures. Moreover, this simple approach opens up a new way for creating various novel 2D heterostructures by using a large variety of heteronanomaterials as precursors.