Hongxia Wang

Supplementary figure legends and Supplementary Table S1.

Schottky DC generators from polypyrrole containing n-type inorganic semiconductors (SnO₂, WO₃, and CeO₂) show improved current outputs. The way to connect multiple generators significantly impacts overall electrical outputs.

This study demonstrates the important effect of water in polypyrrole (PPy) on the mechanical-to-electrical conversion of PPy-based Schottky DC generators. Increasing the water content in PPy significantly enhances device current outputs and slightly increases voltage outputs. When the water content in PPy was 10.49 wt%, the device's current output reached 262.60 μA, 31 times higher than that in the normal state (water content 6.67 wt% and current output 8.45 μA). However, current and voltage outputs decayed massively when the water content was reduced from 6.67 wt% to a lower level. The improved device performance caused by the high water content originated from the improved PPy conductivity and a larger reduction in the Schottky barrier height during device compression. We further showed that incorporating dimethyl sulfoxide (DMSO) into PPy stabilized the water content in PPy and enhanced the electrical outputs, especially when the water content was relatively low. 0.15 wt% DMSO in PPy containing 8.46 wt% water resulted in a device output of 105.90 μA cm⁻², 4.9 times higher than that of the DMSO-free PPy device. These novel findings may benefit the development of durable, high-performance Schottky DC generators.

This study reports a patterned nanofibrous membrane and its novel filtration performance. The nanofibers are prepared using an electrospinning technique, and the nanofibers are collected on a commercial nonwoven fabric covering a roller collector, which is prepared using a metal sheet with a round hole array. The nanofibers collected on the metal surface have a much denser structure than those in the hole areas. As a result, the patterned nanofibrous membrane has a different filtration performance (lower pressure drop and filtration efficiency) from the uniform nanofiber membrane, prepared under the same condition but using a smooth roller collector. When two layers of this patterned nanofibrous membrane are combined, the filtration performance depends on the layer arrangement. When the dual‐layer membranes are arranged with the dense and loose packing areas staggering between the layers, the filter shows lower filtration resistance but higher efficiency than the uniform dual‐layer filter. It is further shown that the staggered dual‐layer membranes (made of polyacrylonitrile nanofibers) maintain high efficiency after eliminating the electrostatic charges. The efficiency meets air filter F9, the highest requirement according to the air filter standard (EN 779:2012), enabling a long service life.

Polymer materials with mechanical energy-to-electricity conversion capability at high temperatures are highly desirable to develop energy harvesters, sensors, and other flexible devices for applications under harsh high-temperature conditions but remain an enormous challenge to develop. Herein, we report a polymer membrane that can convert mechanical energy into electricity at both room and high temperatures. The membranes were prepared by heat stabilization treatment of electrospun polyacrylonitrile (PAN) fibrous membranes to make PAN have a cyclized structure. The devices made of the treated PAN membranes can work at a temperature as high as 550 °C, which has never been reported on other polymer-based energy harvesting devices so far. A 2.5 × 2.5 cm² device at 450 °C can generate a 9.7 V voltage (up to 62 V, 4 μA current) with a maximum power density of 26.4 mW cm⁻². We further show that the oxidation cyclization of PAN due to stabilization treatment plays an essential role in achieving the high-temperature energy conversion. The energy conversion is mainly originated from a piezoelectric effect. We used the energy generated under high-temperature conditions to power commercial LEDs and charged capacitors to demonstrate the application. The electrical energy generated from a single device is sufficient to run a commercial LEDs. After rectification, it can also be stored in capacitors for further use. The PAN membranes are flexible and air permeable. They may form a novel polymer transducer candidate to develop flexible energy devices for various applications in high-temperature environments.

Acoustic sensors show a wide application in diverse fields. However, it is still a challenge to develop highly sensitive transducing materials for the detection of low‐to‐middle decibel sounds. Herein, for the first time the remarkable property of an acoustic sensor based on electrospun polyacrylonitrile (PAN) nanofibers in the recognition of human voices is reported. The nanofiber device shows a sensitivity as high as 23 401 mV P⁻¹ to 70 dB sound, which is much higher than most of the acoustic sensors reported in the literature. First the sounds from different musical instruments are used as a standard sound source to show the high accuracy of the nanofiber device to distinguish different sounds, e.g., from both the same and different instruments, and then people's voices are tested. Experimental results show that the device can distinguish people's voices with high resolution and the influence of background noise on speech recognition is very small. The acoustic sensor is stable and can be used in sound detection for a long time. Electrospun PAN membranes may be useful for the development of voice recognition systems for security, environmental protection, scientific research, and other high‐tech applications.

Three-dimensional (3D) microfibrous scaffolds hold great promise for biomedical applications due to their good mechanical properties and biomimetic structure similar to that of the fibrous natural extracellular matrix. However, the large diameter and smooth surface of microfibers provide limited cues for regulating cell activity and behaviors. In this work, we report a facile heat-welding-and-embossing strategy to develop 3D macroporous microfibrous scaffolds with a featured surface topography. Here, solid monosodium glutamate (MSG) particles with crystalline ridge-like surface features play a key role as templates in both the formation of scaffold pores and the surface embossing of scaffold fibers when short thermoplastic polypropylene microfibers were heat-welded. The embossing process can be programmed by adjusting heating temperatures and MSG/fiber ratios. Compared to traditional 3D microfibrous scaffolds, the as-welded 3D scaffolds show higher compressive strength and modulus. Taking mouse C2C12 myoblasts as a model cell line, the scaffolds with embossed surface features significantly promoted the growth of cells, interactions of cells and scaffolds, and formation of myotubes. The findings indicate that the as-prepared 3D scaffolds are a good platform for cell culture study. The facile strategy can be applied to fabricate different fibrous scaffolds by changing the combination of templates and thermoplastic polymer fibers with a melting temperature lower than that of the template. The obtained insights in this work could provide a guide and inspiration for the design and fabrication of functional 3D fibrous scaffolds.

The low‐latitude longitudinally periodic density structure with three or four peaks at a fixed local time is well known. Both harmonic analysis of observations and numerical simulations have been extensively performed to ascribe the periodic structures to different atmospheric tides. But to date, no research has been done to explain how the two structures transform between seasons. Besides, other different wave structures have also been reported. To clarify these uncertainties, the COSMIC electron density data during 2006–2019 are used to further examine the spatiotemporal variations of the longitudinal structure. The results show that a wavenumber‐4 structure is clearly distinguishable during equinoxes and the four peaks are basically the same size, but in two solstices, a wavenumber‐3 structure is finally observed and the three peaks vary greatly in size. The wavenumber‐4 is well established before noon while the stable wavenumber‐3 form cannot be reached until after noon. Due to the blurred boundary caused by plasma transportation, the density distribution can be characterized by other wave structures before reaching its stable form during solstices. Subdividing the data, we find that from vernal (autumn) equinox to June (December) solstice, the peak in American sector gradually merges with its west (east) side peak into a super‐large one, its east (west) side peak weakens into a small one, and the peak in Southeast Asian sector gradually shrinks (expands). We speculate that the transition from wavenumber‐4 to wavenumber‐3 is due to the modulation of transequatorial winds and magnetic declination.

Electrospun polyacrylonitrile membranes can be used to detect middle-intensity sound with high accuracy, sensitivity, and fidelity with overall performance higher than that of electrospun poly(vinylidene fluoride) in the same condition.

Energy generators that can directly convert mechanical forces into direct current (DC) power are highly promising for energy harvesting and the development of self-powered electronic technology. However, most DC generators developed so far produce a very low electrical output. The development of large output DC generators is still a challenge. In this study, a fabric-based DC generator that can produce a peak voltage output as high as 3.27 V and a current output up to 0.82 A m⁻² is developed for the first time. The device is composed of a Schottky diode formed by polypyrrole (PPy)-coated fabric and aluminum metal. Compared with the previously reported Al/PPy/Au Schottky generator that was prepared from a dense PPy plate, our fabric device has 4.6 times higher voltage output and 3.4 times higher power. Its energy conversion was found to come from both Schottky triboelectric and Schottky piezoelectric conversion principles, which is quite different from that of the previously reported Schottky DC generators. However, unlike conventional triboelectric generators that need the internal components to come into contact and separate during the energy conversion process, our device does not need to put the internal components in frequent contact and separation. All the components in this device maintain contact during the whole operation process. The electric energy generated can be used directly to power electronic equipment and be stored in a power bank for future use. Also, the device has good flexibility. It may have a great application prospect in the development of flexible power generation technology.