Organic-inorganic hybrid piezotronic bipolar junction transistor for pressure sensing.

With the rapid development of the Internet of Things (IoTs), wearable sensors are playing an increasingly important role in daily monitoring of personal health and wellness. The signal-to-noise-ratio has become the most critical performance factor to consider. To enhance it, on the one hand, good sensing materials/devices have been employed; on the other hand, signal amplification and noise reduction circuits have been used. However, most of these devices and circuits work in an active sampling mode, requiring frequent data acquisition and hence, entailing high-power consumption. In this scenario, a flexible and wearable event-triggered sensor with embedded signal amplification without an external power supply is of great interest. Here, we report a flexible two-terminal piezotronic n-p-n bipolar junction transistor (PBJT) that acts as an autonomous and highly sensitive, current- and/or voltage-mediated pressure sensor. The PBJT is formed by two back-to-back piezotronic diodes which are defined as emitter-base and collector-base diodes. Upon force exertion on the emitter side, as a result of the piezoelectric effect, the emitter-base diode is forward biased while the collector-base diode is reverse biased. Due to the inherent BJT amplification effect, the PBJT achieves record-high sensitivities of 139.7 kPa-1 (current-based) and 88.66 kPa-1 (voltage-based) in sensing mode. The PBJT also has a fast response time of <110 ms under exertion of dynamic stimuli ranging from a flying butterfly to a gentle finger touch. Therefore, the PBJT advances the state of the art not only in terms of sensitivity but also in regard to being self-driven and autonomous, making it promising for pressure sensing and other IoT applications.

Wearable Pressure Sensor Using Porous Natural Polymer Hydrogel Elastomers with High Sensitivity over a Wide Sensing Range.

Wearable pressure sensors capable of quantifying full-range human dynamic motionare are pivotal in wearable electronics and human activity monitoring. Since wearable pressure sensors directly or indirectly contact skin, selecting flexible soft and skin-friendly materials is important. Wearable pressure sensors with natural polymer-based hydrogels are extensively explored to enable safe contact with skin. Despite recent advances, most natural polymer-based hydrogel sensors suffer from low sensitivity at high-pressure ranges. Here, by using commercially available rosin particles as sacrificial templates, a cost-effective wide-range porous locust bean gum-based hydrogel pressure sensor is constructed. Due to the three-dimensional macroporous structure of the hydrogel, the constructed sensor exhibits high sensitivities (12.7, 5.0, and 3.2 kPa-1 under 0.1-20, 20-50, and 50-100 kPa) under a wide range of pressure. The sensor also offers a fast response time (263 ms) and good durability over 500 loading/unloading cycles. In addition, the sensor is successfully applied for monitoring human dynamic motion. This work provides a low-cost and easy fabrication strategy for fabricating high-performance natural polymer-based hydrogel piezoresistive sensors with a wide response range and high sensitivity.

A high-performance free-standing Zn anode for flexible zinc-ion batteries.

Zinc-ion batteries (ZIBs) have attracted significant attention owing to their high safety, high energy density, and low cost. ZIBs have been studied as a potential energy device for portable and flexible electronics. Here, a highly flexible free-standing Zn anode is fabricated using a simple spin-coating technique, and its application in ZIBs is demonstrated. The free-standing Zn anode precursor is formed by mixing Zn particles with carbon nanotubes and poly(vinylidene fluoride)-co-hexafluoropropylene (PVDF-HFP). The hexafluoropropylene group in PVDF-HFP improves the mechanical properties of the free-standing Zn anode, whereas the carbon nanotubes created percolation conduction in the composite electrode, leading to an increased electrical conductivity of the anode. Owing to the excellent electrical conductivity and high specific surface area of the free-standing Zn anode, ZIBs with high capacity, rate performance, and mechanical flexibility are achieved. The volumetric energy density of the ZIBs reaches 8.22 mW h cm-3 with a battery thickness of 0.4 mm. This work demonstrates that free-standing Zn anodes are promising anodes for flexible ZIBs.

Highly conductive locust bean gum bio-electrolyte for superior long-life quasi-solid-state zinc-ion batteries.

Rechargeable aqueous zinc-ion batteries (ZIBs) are promising wearable electronic power sources. However, solid-state electrolytes with high ionic conductivities and long-term stabilities are still challenging to fabricate for high-performance ZIBs. Herein, locust bean gum (LBG) was used as a natural bio-polymer to prepare a free-standing quasi-solid-state ZnSO4/MnSO4 electrolyte. The as-obtained LBG electrolyte showed high ionic conductivity reaching 33.57 mS cm-1 at room temperature. This value is so far the highest among the reported quasi-solid-state electrolytes. Besides, the as-obtained LBG electrolyte displayed excellent long-term stability toward a Zn anode. The application of the optimized LBG electrolyte in Zn-MnO2 batteries achieved a high specific capacity reaching up to 339.4 mA h g-1 at 0.15 A g-1, a superior rate performance of 143.3 mA h g-1 at 6 A g-1, an excellent capacity retention of 100% over 3300 cycles and 93% over 4000 cycles combined with a wide working temperature range (0-40 °C) and good mechanical flexibility (capacity retention of 80.74% after 1000 bending cycles at a bending angle of 90°). In sum, the proposed ZIBs-based LBG electrolyte with high electrochemical performance looks promising for the future development of bio-compatible and environmentally friendly solid-state energy storage devices.

A comparison study of MnO2 and Mn2O3 as zinc-ion battery cathodes: an experimental and computational investigation.

The high specific capacity, low cost and environmental friendliness make manganese dioxide materials promising cathode materials for zinc-ion batteries (ZIBs). In order to understand the difference between the electrochemical behavior of manganese dioxide materials with different valence states, i.e., Mn(iii) and Mn(iv), we investigated and compared the electrochemical properties of pure MnO2 and Mn2O3 as ZIB cathodes via a combined experimental and computational approach. The MnO2 electrode showed a higher discharging capacity (270.4 mA h g-1 at 0.1 A g-1) and a superior rate performance (125.7 mA h g-1 at 3 A g-1) than the Mn2O3 electrode (188.2 mA h g-1 at 0.1 A g-1 and 87 mA h g-1 at 3 A g-1, respectively). The superior performance of the MnO2 electrode was ascribed to its higher specific surface area, higher electronic conductivity and lower diffusion barrier of Zn2+ compared to the Mn2O3 electrode. This study provides a detailed picture of the diversity of manganese dioxide electrodes as ZIB cathodes.

Polyacrylic acid assisted synthesis of free-standing MnO2/CNTs cathode for Zinc-ion batteries.

Rechargeable aqueous zinc-ion batteries (ZIBs) have attracted significant attention due to the distinguishing characteristics of zinc metal, including its low price, abundance in earth, safety and high theoretical specific capacity of 820 mAh g-1. Manganese dioxide (MnO2) is a promising cathode for ZIBs due to high theoretical specific capacity, high discharge voltage plateau, cost-effectiveness and nontoxicity. However, the low electronic conductivity and volumetric changes during electrochemical cycling hinder its practical utilization. Herein, we demonstrate a polyacrylic acid (PAA)-assisted assembling strategy to fabricate freestanding and flexible MnO2/carbon nanotube/PAA (MnO2/CNT/PAA) cathodes for ZIBs. PAA plays an important role in providing excellent mechanical properties to the free-standing electrode. Moreover, the presence of CNT forms an electron conductive network, and the porous structure of MnO2/CNT/PAA electrode accommodates the volumetric variations of MnO2 during charge/discharge cycling. The as-fabricated quasi-solid-state Zn-MnO2/CNT/PAA battery delivers a high charge storage capacity of 302 mAh g-1 at 0.3 A g-1 and retains 82% of the initial capacity after 1000 charge/discharge cycles at 1.5 A g-1. The calculated volumetric energy density of Zn-MnO2/CNT/PAA battery is 8.5 mW h cm-3 (with a thickness of 0.08 cm), which is significantly higher than the reported alkali-ion batteries (1.3 mW h cm-3) and comparable to supercapacitors (6.8 mW h cm-3) and Ni-Zn batteries (7.76 mW h cm-3). The current work demonstrates that free-standing MnO2/CNT/PAA composite is a promising cathode for ZIBs.

A Hydrogenated Metal Oxide with Full Solar Spectrum Absorption for Highly Efficient Photothermal Water Evaporation.

Searching for cost-effective photothermal material that can harvest the full solar spectrum is critically important for solar-driven water evaporation. Metal oxides are cheap materials but cannot cover the full solar spectrum. Here we prepared a hydrogenated metal oxide (H1.68MoO3) material, in which H-doping causes the insulator-to-metal phase transition of the originally semiconductive MoO3. It offers a blackbody-like solar absorption of ≥95% over the entire visible-to-near-infrared solar spectrum, owing to its unusual quasi-metallic energy band, and high solar-to-heat conversion rate due to quick relaxation of excited electrons. Using a self-floating H1.68MoO3/airlaid paper photothermal film, we achieved a stable and high water vapor generation rate of 1.37 kg m-2 h-1, a superb solar-to-vapor efficiency of 84.8% under 1 sun illumination, and daily production of 12.4 L of sanitary water/m2 from seawater under natural sunlight. This thus opens a new avenue of designing cost-effective photothermal materials based on metal oxides.

One-step synthesis of MnOx/PPy nanocomposite as a high-performance cathode for a rechargeable zinc-ion battery and insight into its energy storage mechanism.

Rechargeable aqueous zinc-ion batteries (ZIBs) have attracted significant attention in the energy storage field. Manganese-based materials are the most promising cathode materials for ZIBs but they suffer from low electronic conductivity. Herein, a high-performance cathode for ZIBs based on nanocomposites consisting of mixed-valence manganese dioxide (Mn III and IV) and polypyrrole (MnOx/PPy) is prepared through an efficient one-step organic/inorganic interface redox reaction. The role of polypyrrole (PPy) in the MnOx/PPy cathode is elaborated. It not only provides an effective conductive network for MnOx but also contributes to the capacity of the composite. By optimizing the amount of PPy, the MnOx/PPy composite with 12 wt% PPy exhibits the highest capacity. As a result, the corresponding Zn-MnOx/PPy battery delivers a high capacity (302.0 mA h g-1 at 0.15 A g-1), excellent rate performance (159.9 mA h g-1 at 3 A g-1) and superior cycling stability. Furthermore, the results of ex situ characterization analysis reveal that H+ and Zn2+ insertion/extraction both occur in MnOx/PPy particles during the discharging/charging process, while only Zn2+ insertion/extraction occurs in the PPy electrode. This work develops an efficient one-step synthesis method for large scale production of manganese-based materials/conducting polymers as the cathode for ZIB application, and provides an insight into its energy storage mechanism.

Flexible quasi-solid-state zinc ion batteries enabled by highly conductive carrageenan bio-polymer electrolyte.

Flexible Zn-MnO2 batteries as wearable electronic power source have attracted much attention in recent years due to their low cost and high safety. To promote the practical application of flexible Zn-MnO2 batteries, it is imperative to develop flexible, mechanically robust and high performance solid state electrolyte. Herein, we construct a rechargeable quasi-solid-state zinc ion battery using kappa-carrageenan bio-polymer electrolyte. The kappa-carrageenan electrolyte is eco-friendly, low cost, and highly conductive (3.32 × 10-2 S cm-1 at room temperature). The mechanical robustness of kappa-carrageenan electrolyte is further reinforced by using a rice paper as scaffold. Benefiting from high ionic conductivity of the bio-polymer electrolyte, our zinc ion battery delivers a significant high energy density and power density (400 W h kg-1 and 7.9 kW kg-1, respectively), high specific capacity (291.5 mA h g-1 at 0.15 A g-1), fast charging and discharging capability (120.0 mA h g-1 at 6.0 A g-1). The zinc ion battery with bio-polymer electrolyte also shows excellent cycling stability and high bending durability. This work brings new research opportunities in developing low-cost flexible solid-state zinc ion batteries using green natural polymer.

Exploring the Intrinsic Piezofluorochromic Mechanism of TPE-An by STS Technique.

9,10-bis(4-(1,2,2-triphenylvinyl)styryl)anthracene (TPE-An) materials have attracted considerable attention in recent years because they have high luminescence efficiency and excellent piezofluorochromic properties, which have potential applications in organic light-emitting display (OLED) area. Scanning tunneling spectroscopy (STS) technique was used to study the piezofluorochromic mechanism of aggregation-induced emission (AIE) materials for the first time. Photoluminescence (PL) experiments revealed that the emission peak of TPE-An is observed to exhibit a red-shift with the increase of the grinding time. A theoretical calculation was carried out to find the relationship between the bandgap of TPE-An and the external force by combination of the classical tunneling theory and STS results. It is found that when the pressure variation on the surface of TPE-An film was increased to be over 4.38 × 10(4) Pa, the shrink of the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap can arrive at 1.1 eV. It is concluded that the piezofluorochromic behaviors of TPE-An should originate from the shrinking effect of the bandgap under external force. Moreover, this research method may shed light on comprehending and adjusting the piezofluorochromic characters of other AIE materials.