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  • 54

    Solvothermal-derived nanoscale spinel bimetallic oxide particles rationally bridged with conductive vapor-grown carbon fibers for hybrid supercapacitors

    Abstract

    Recently, bimetallic oxides with nanoscale morphology have emerged as promising and reliable electrode candidates for supercapacitors. Herein, we synthesized MnCo2O4 nanoparticles (NPs) (≤100 nm) via a facile one-step solvothermal method without further calcination. Thanks to the multi-valence states of manganese and cobalt elements as well as the structural characteristics of NPs, the MnCo2O4 NPs material delivered a maximum capacity of 44.8 mAh g−1 at a current density of 2 A g−1 in alkaline electrolyte. To improve the electrical conductivity and electrokinetics, vapor-grown carbon fibers (VCFs) were introduced into the MnCo2O4 (VCFs@MnCo2O4) material. Here, the VCFs connected to NPs can act as conductive bridges among the MnCo2O4 NPs and also transfer the generated charge promptly to the current collector. Consequently, the VCFs@MnCo2O4 composite demonstrated a higher specific capacity of 48.4 mAh g−1 (at 2 A g−1) than solitary MnCo2O4. Besides, the VCFs@MnCo2O4 composite demonstrated excellent cycling stability without degradation even after 2000 and 10000 charge-discharge cycles. Furthermore, the hybrid supercapacitor (HSC) was fabricated with VCFs@MnCo2O4 as a cathode and activated carbon as an anode, which showed a good specific capacitance of 63.8 F g−1 (2 mA cm−2). Also, this HSC device exhibited a considerable energy density of 20.6 Wh kg−1 and a power density of 2251.5 W kg−1. The efficiency of HSC was also tested by driving electronic components.

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  • 53

    LiTaO3-Based Flexible Piezoelectric Nanogenerators for Mechanical Energy Harvesting

    Abstract

    Mechanical energy is one of the freely available green energy sources that could be harvested to meet the small-scale energy demand. Piezoelectric nanogenerators can be used to harvest the biomechanical energy that is available in everyday human life and power various portable electronics. Herein, a ferroelectric material, i.e., lithium tantalate (LiTaO3), was synthesized and used to fabricate a flexible piezoelectric nanogenerator (FPNG). Generally, ferroelectric materials display a strong electrostatic dipole moment and high piezoelectric coefficient, thus resulting in enhanced electrical performance. First, LiTaO3 nanoparticles were synthesized and loaded into poly(vinylidene difluoride) (PVDF) to form a piezoelectric film and then, the piezoelectric composite film was sandwiched between two aluminum electrodes to fabricate an FPNG. The effect of the electrical performance of FPNG as a function of the concentration of LiTaO3 loaded into PVDF was systematically investigated and optimized. The 2.5 wt % FPNG exhibited open-circuit voltage, short-circuit current, and power density values of ∼18 V, ∼1.2 μA, and ∼25 mW/m2, respectively. Furthermore, the FPNG revealed good electrical stability and mechanical durability. Finally, the FPNG was employed as a weight sensor to harvest various biomechanical energies and operate low-power- electronics.

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  • 52

    CuCo LDHs Coated CuCoTe Honeycomb-Like Nanosheets as a Novel Anode Material for Hybrid Supercapacitors

    Abstract

    Metal–organic frameworks derived metal chalcogenides as a new class of active materials can abolish the existing challenges in supercapacitors with their large electroactive sites and enhanced electrochemical conductivities. With its adequate conductivity and electrochemical properties, tellurium based metal chalcogenide electrodes can deliver better electrochemical performances than other chalcogenides. Herein, CuCoTe honeycomb-like nanosheets are grown on nickel foam (CuCoTe HNSs/NF) and then CuCo layered double hydroxides are successively coated on them (CTC HLSs/NF). The CTC HLSs/NF electrode exhibits tremendous performance with its high specific capacity of 399 mAh g−1 at 7 A g−1 of current density and good capacity retention (81.3%) after 3000 cycles. Finally, CTC HLSs/NF electrode is utilized for the hybrid supercapacitor (HSC) assembly along with activated carbon coated nickel foam in an aqueous electrolyte. The fabricated HSC shows high energy density (214.7 Wh kg−1) and power density (40 kW kg−1). Moreover, the device retains 96.3% of its capacitance at the end of the 5000th cycle, showing its high stability. Owing to their unique morphology and superior electrochemical properties, the present method of fabrication and selected materials can address the issues faced by electrochemical capacitors.

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  • 51

    3D Printed Double Roller-Based Triboelectric Nanogenerator for Blue Energy Harvesting

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    3D Printed Double Roller-Based Triboelectric Nanogenerator for Blue Energy Harvesting

    by 
     1,2 and
     2,3,*
    1
    Department of Electronics and Information Convergence Engineering, Kyung Hee University, 1732 Deogyeong-daero, Giheung-gu, Yongin 17104, Korea
    2
    Institute for Wearable Convergence Electronics, Kyung Hee University, 1732 Deogyeong-daero, Giheung-gu, Yongin 17104, Korea
    3
    Department of Electronic Engineering, Kyung Hee University, 1732 Deogyeong-daero, Giheung-gu, Yongin 17104, Korea
    *
    Author to whom correspondence should be addressed.
    Micromachines 2021, 12(9), 1089; https://doi.org/10.3390/mi12091089
    Submission received: 23 August 2021 / Revised: 5 September 2021 / Accepted: 7 September 2021 / Published: 10 September 2021
    (This article belongs to the Special Issue Self-Powered Smart Systems)
     
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    Versions Notes

    Abstract

    The ocean covers 70% of the earth’s surface and is one of the largest uncultivated resources still available for harvesting energy. The triboelectric energy harvesting technology has the potential to effectively convert the ocean’s “blue energy” into electricity. A half-cylinder structure including rollers floating on the water has already been used, in which the pendulum motion of the rollers is driven by the waveform. For the stable motion of the rollers, the printed surface of the device was treated with acetone for attaining hydrophilicity. The electrical outputs with the proposed device were enhanced by increasing the contact surface area by simply implementing the double roller structure with double side-covered electrodes. With the optimized structure, the maximum power density reached a value of 69.34 µW m−2 at a load resistance of 200 MΩ with the device’s high output durability. Finally, the fabricated device was also applied to the artificial water waves to demonstrate the possibility of using this device in the ocean. By simply modifying the electrode structure and adding a roller, this device demonstrated the ability to generate over 160% of electrical output with the same covered area of the ocean by the triboelectric nanogenerators (TENGs) and potential ocean application.

    Graphical Abstract

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  • 50

    Chelate mediated synthesis of novel Mn2V2O7 and MnV2O6 materials with hierarchical morphological structures and improved redox behavior via multi-walled carbon nanotubes for asymmetric supercapacitors

    Abstract

    Designing different nano/microstructural geometries with porous properties is of great attention, allowing impressive electrochemical properties in energy storage technology. Herein, we report the manganese vanadium oxides of Mn2V2O7 and MnV2O6 with different morphologies by a most adaptable hydrothermal technique using three types of chelating agents (citric acid, ethylenediaminetetraacetic acid, and hexamethylenetetramine). The materials reveal a hierarchical thorny structure, i.e., an orientation form of microrods and nanorod bundles, and their comparative electrochemical analysis is performed. Furthermore, the thorny sphere morphology of MVO material is combined with the multi-walled carbon nanotubes and it reveals the enhancement in the specific capacity as compared to pristine. This electrode material exhibits superior capacity retention of 97% even after 4000 cycles at 5 A g−1 with its corresponding coulombic efficiency of 99%. Additionally, a pouch-like asymmetric supercapacitor device exhibits a high energy and power density values of 26.6 Wh kg−1 and 2875 W kg−1, respectively. Eventually, various low voltage electronic devices such as a mortar fan and blue light-emitting diodes are powered-up using the devices to test the real-time applications in the field of energy storage.

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  • 49

    Nano-Ag laminated ternary layered double hydroxides for hybrid supercapacitors

    Abstract

    Ternary layered double hydroxide (LDH)-based active materials can serve as potential electrode materials in the development of high-performance supercapacitors (SCs) due to their good structural features and high electrochemical activity. Herein, we report the rational design of conductive nano-silver (Ag) particles integrated hierarchical nickel-cobalt-molybdenum LDH (Ag@NCM LDH) using simple methods by excluding the non-conductive binders. The ternary NCM LDH material prepared with an equal volume ratio of water and ethanol displayed a hybrid morphology of micro flowers in situ grown on the nanosheet layer (i.e., MFs@NSs). The ternary NCM LDH MFs@NSs electrode demonstrated superior electrochemical properties to other NCM electrodes (prepared with different solvent volume ratios) and binary metal hydroxides as well. Thanks to the ternary metal hydroxides, nano-Ag particles, and MFs@NSs architecture, the prepared Ag@NCM LDH electrode achieved a high areal capacity of 890 μAh cm−2 at 5 mA cm−2. Moreover, the Ag@NCM LDH electrode showed good cycling stability with 88.8% retention after 15,000 cycles (measured at 30 mA cm−2). A hybrid SC (HSC) construction of Ag@NCM LDH and activated carbon electrodes revealed a high areal capacitance of 1752.2 mF cm−2 (at 5 mA cm−2) as well as superior energy and power densities of 0.569 mWh cm−2 and 3.82 mW cm−2, respectively. Especially, an HSC demonstrated long-life stability with 108.5% retention after 15,000 cycles (measured at 40 mA cm−2). Besides, HSC was tested to store the solar energy via a solar cell panel and subsequently powered different electronic components with its stored energy.

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  • 48

    Wireless Power Transfer and Telemetry for Implantable Bioelectronics

    Abstract

    Implantable bioelectronic devices are becoming useful and prospective solutions for various diseases owing to their ability to monitor or manipulate body functions. However, conventional implantable devices (e.g., pacemaker and neurostimulator) are still bulky and rigid, which is mostly due to the energy storage component. In addition to mechanical mismatch between the bulky and rigid implantable device and the soft human tissue, another significant drawback is that the entire device should be surgically replaced once the initially stored energy is exhausted. Besides, retrieving physiological information across a closed epidermis is a tricky procedure. However, wireless interfaces for power and data transfer utilizing radio frequency (RF) microwave offer a promising solution for resolving such issues. While the RF interfacing devices for power and data transfer are extensively investigated and developed using conventional electronics, their application to implantable bioelectronics is still a challenge owing to the constraints and requirements of in vivo environments, such as mechanical softness, small module size, tissue attenuation, and biocompatibility. This work elucidates the recent advances in RF-based power transfer and telemetry for implantable bioelectronics to tackle such challenges.

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  • 47

    Film-Sponge-Coupled Triboelectric Nanogenerator with Enhanced Contact Area Based on Direct Ultraviolet Laser Ablation

    Abstract

    Triboelectric nanogenerators (TENGs) recently have emerged as applicable and eco-friendly harvesting devices. Numerous studies have been actively conducted to fabricate a flexible and robust TENG with high-output performance. Herein, a film-sponge-coupled TENG (FS-TENG) is proposed using direct ultraviolet laser ablation, as a method for surface modification of a polyimide (PI) film. This state-of-the-art method has advantages of accuracy as well as time efficiency in creating the pattern on the surface; thus, the pre-designed patterns can be precisely constructed within only a minute. In the laser-ablated PI film, the structural design and chemical modification on the surface are investigated related to the triboelectric output performance. Thereafter, a sponge is fabricated based on non-woven polyamide and silicone rubber, which can fully contact with the micro-/nano-scaled structure on the surface of the PI film. After an optimization, the FS-TENG exhibits 48.19 V of open-circuit voltage and 1.243 μA of short-circuit current, which shows approximately 3 times enhanced electric performance compared to the FS-TENG using a pristine PI film. The FS-TENG device demonstrates its robustness through both mechanical stress and flexible stress by showing less than 5% degradation after 50,000 cycles. On the basis of the high flexibility and stability of the FS-TENG, a self-powered scoreboard is successfully developed for lighting a scoreboard in a soccer field. This feasible lighting system can be operated by harvesting the kinetic energy of a soccer player without an additional power source. The novel FS-TENG, thus, provides remarkable potential for a self-powered indoor harvesting system.

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  • 46

    Photopatterned microswimmers with programmable motion without external stimuli

    Abstract

    We introduce highly programmable microscale swimmers driven by the Marangoni effect (Marangoni microswimmers) that can self-propel on the surface of water. Previous studies on Marangoni swimmers have shown the advantage of self-propulsion without external energy source or mechanical systems, by taking advantage of direct conversion from power source materials to mechanical energy. However, current developments on Marangoni microswimmers have limitations in their fabrication, thereby hindering their programmability and precise mass production. By introducing a photopatterning method, we generated Marangoni microswimmers with multiple functional parts with distinct material properties in high throughput. Furthermore, various motions such as time-dependent direction change and disassembly of swimmers without external stimuli are programmed into the Marangoni microswimmers.

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  • 45

    Charge transfer band excitation of La3NbO7:Sm3+ phosphors induced abnormal thermal quenching toward high-sensitivity thermometers

    Abstract

    Different luminescent behaviors of La3NbO7:Sm3+ phosphors under the excitations of charge transfer band (CTB, 250 nm) and featured absorption peak (6H5/2 → 4H7/2, 405 nm) of Sm3+ ions were demonstrated. Under the excitation wavelength of 405 nm, the optimal La3NbO7:0.1Sm3+ phosphor exhibited an orange-red emission while the chromatic coordinate was found to be (0.609, 0.387), which also showed the excellent thermal performance, exhibiting its emission intensity of about 90.67% at 423 K with respect to 303 K. In the case of CTB excitation, the La3NbO7:0.1Sm3+ phosphor emitted an orange-yellow region with the chromaticity coordinate of (0.540, 0.443), and the emission intensity was stronger than the above one (λex =405 nm) even though the optimized sample would be changed to the La3NbO7:0.05Sm3+ phosphor. With the increase of temperature, the obtained sample revealed an abnormal thermal quenching phenomenon between the emission peak of the host material and the emission transition of 4G5/2 → 6H9/2 under the excitation wavelength of 250 nm, which could be suggested to turn into a pair of thermal-couple levels. Therefore, the sensing sensitivity of the obtained sample was further investigated based on the fluorescence intensity ratio theory. Eventually, the absolute and relative sensing sensitivities of the La3NbO7:0.01Sm3+ phosphor were estimated to be as high as 5.379 × 10−2 K−1 and 1.60% K−1, respectively.

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