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PUBLICATIONS

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Patents

Journals

  • 44

    Robust Wireless Power Transfer with Minimal Field Exposure Using Parity-Time Symmetric Microwave Cavities

    ABSTRACT

    This paper proposes cavity resonators under parity-time- (PT) symmetric conditions for robust wireless power transfer (WPT) with minimal exposure to electromagnetic (EM) fields. Transceiver cavities with the same resonant frequencies as that of the fundamental mode are coupled through very small evanescent waves leaking out of the lattices on the top surface of the cavities. This reduces the magnitude of the EM fields near the power-delivery system by 10 to 10000 times compared with conventional coil systems. Regardless of the small coupling factors between the cavities, the power-transfer efficiency is maintained at a high level due to the high quality factors of the cavities. According to coupled-mode theory, the resonant frequency of the coupled system changes sensitively with respect to the receiver position, making power delivery unstable. In this work, a feedback loop at the transmit cavity along with the saturated gain of the power amplifier achieves the PT-symmetric condition. The power-transfer efficiency is robust because the operating frequency is automatically locked to the resonant frequency, regardless of the operation conditions. The proposed cavity system is promising for supplementing the conventional coil system for WPT applications in which minimal field exposure is crucial. The paper is accompanied by videos demonstrating experiments.

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  • 43

    Unveiling redox-boosted mesoporous Co@NiO-SiO2 hybrid composite with hetero-morphologies as an electrode candidate for durable hybrid supercapacitors

    Abstract

    The nanoscale morphology and mesoporosity have a substantial effect on the energy storage properties because they offer a high surface area and porous nature. The former one bestows the accessibility of more redox-active sites, while the latter facilitates the easy entry of foreign atoms. In this report, we rationally synthesized the mesoporous NiO–SiO2 material with hetero-morphologies by a simple wet-chemical method, followed by calcination. The hetero-morphologies include nanospheres, nanoflakes, and nanoparticles collectively increased the surface area. To further increase the redox activity, the cobalt was hydrothermally doped to the NiO–SiO2 material (Co@NiO–SiO2). Consequently, the Co@NiO–SiO2 electrode demonstrated superior electrochemical response with a higher capacity of 41.7 μAh cm−2 compared to the NiO–SiO2 electrode (25 μAh cm−2) in a three-electrode system. Moreover, the Co@NiO–SiO2 electrode was sustained up to 10,000 cycles by retaining 95.5% of its initial capacity. The ability of the Co@NiO–SiO2 material towards practical applicability was also unveiled by fabricating a hybrid supercapacitor (HSC). The HSC delivered a notable energy density (42.3 μWh cm−2) and power density (10.2 mW cm−2). Furthermore, the HSC exhibited outstanding durability (10,000 cycles) without fading. The ability of HSC was also tested by energizing light-emitting diodes.

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  • 42

    Cerium vanadate/carbon nanotube hybrid composite nanostructures as a high-performance anode material for lithium-ion batteries

      Abstract

      The pristine CeVO4 and CeVO4/CNT hybrid composite nanostructured samples were facilely synthesized using a simple silicone oil-bath method. From the X-ray diffraction results, the formation of tetragonal CeVO4 with an additional minor phase of V2O5 was identified. When investigated as an anode material for lithium (Li)-ion batteries, the CeVO4/CNT hybrid composite nanostructure (HCNS) electrode demonstrated improved Li storage performance over the pristine CeVO4. The Li insertion/de-insertion electrochemical reaction with the CeVO4 was analyzed on the basis of cyclic voltammetry study. The cyclic voltammetry analysis revealed that the three-step reduction of V5+ to V3+, V3+ to V2+, and V2+ to V+ processes is involved and among them, only V5+ to V3+ is reversible during the Li-ion insertion into CeVO4. The CeVO4/CNT HCNS electrode exhibited a discharge capacity as high as 443 mA h g−1 (capacity retention of 96.3%) over 200 cycles at 100 mA g−1, whereas the pristine CeVO4 is limited to 138 mA h g−1 (capacity retention of 48%). Even at a high current density of 500 mA g−1, the CeVO4/CNT HCNS electrode delivered an excellent reversible capacity of 586.82 mA h g−1 after 1200 cycles.

      Graphical abstract

      Li+ insertion/de-insertion process into/out for the pristine CeVO4 and CeVO4/CNT HCNS electrodes.

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    • 41

      Binder-free preparation of bimetallic oxide vertical nanosheet arrays toward high-rate performance and energy density supercapacitors

      Fabrication of binder-free electrode materials with vertical nanoarchitecture is the prominent approach to achieve exalted energy storage performance. Herein, we synthesized NiCo2O4 (NCO) zigzag-patterned vertical nanosheet arrays (ZNSAs) directly over the nickel foam substrate by adopting a facile one-step solvothermal method, which is followed by a calcining process. The effect of ethanol and water solvents on the development of ZNSAs was studied by varying their volume concentration. Among the samples, the NCO ZNSA electrode prepared with an equal volume proportion of ethanol and water solvents revealed the highest electrochemical performance with the areal capacity of 146.1 μAh/cm2 at 4 mA/cm2 due to the well-developed NSAs with hierarchical and open-porous texture. Moreover, the NCO ZNSA electrode exhibited remarkable stability (97.6% retention) after the cycling process of 4000 cycles. A hybrid supercapacitor (HSC) device was also constructed with NCO ZNSAs and activated carbon as the cathode and anode, respectively. At 2 mA/cm2, the HSC showed a notable areal capacitance of 173.5 mF/cm2. Furthermore, the HSC exhibited a maximum energy density of 52.7 μWh/cm2 and power density of 10500 μW/cm2. With the energy storage capabilities, the HSC demonstrated its practical applicability by glowing light-emitting diodes and powering up a motor fan. 

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    • 40

      High capacity performance of NiCo2O4 nanostructures as a binder-free anode material for lithium-ion batteries

      Recently, binder-free nanostructured materials provide a great opportunity for advanced lithium-ion batteries (LIBs) owing to their improved electrical conductivity with good porous structure. NiCo2O4 (NCO) nanostructures were successfully deposited on copper foam (CF) substrate to form porous three-dimensional (3D) NCO@CF hybrid structures via a simple solvothermal synthesis, followed by further heat treatment at 300°C (designated as NCO@CF-300) and 400°C (NCO@CF-400). The as-prepared samples revealed distinctly mixed morphologies of 2D nanosphere and nanowire-like structures, tuned by the further heat treatment. Both the electrodes could be explored as a binder-free anode for next-generation LIBs. It is demonstrated that the good integration of 2D morphology of NCO with 3D architectured CF has a significant effect on its electrochemical results. For the first cycle, binder-free NCO@CF-300 and NCO@CF-400 electrodes delivered the discharge capacity values of 1946 and 2637 mA h g−1, respectively, at 500 mA g−1. Moreover, the NCO@CF-300 electrode exhibited stable reversible capacity and good rate capability. From these results, the growth of NCO nanostructures on the CF can be suggested as a potential anode material for high-performance Li-ion batteries. 

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    • 39

      Dual-functional platforms toward field emission displays and novel anti-counterfeiting strategy based on rare-earth activated materials

      Abstract

      Rare-earth (RE) ions-activated materials have been widely applied in various applications. In this report, the novel orange-red emitting Sr2YSbO6:Eu3+ phosphors with good photoluminescence and cathodoluminescence were successfully prepared. The obtained sample with good electron penetration depth could be proposed for field emission displays. Besides, a new anti-counterfeiting strategy based on the Sr2YSbO6:Eu3+ phosphors-polydimethylsiloxane (PDMS) film was also provided. Compared to conventional anti-counterfeiting techniques, the phosphors-PDMS film with lots of advantages of eco-friendly feature, stable physicochemical stability, and facile operation is potential for high-level anti-counterfeiting devices, which would be considered to open an avenue to explore novel practical applications.

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    • 38

      Disk Triboelectric Nanogenerator-Based Nonvolatile Memory toward Smart Identification System

      Abstract

      With the rapid advancement of the Internet of things (IoT), security issues of the IoT are emerging because the wireless networks for conventional IoT are easily exposed to hacking. By storing the critical data in a physically separate space, these issues can be suppressed. The nonvolatile memory (NVM) is an attractive solution because the stored data are not erased even after turning off the power. However, the NVM consumes the power for operating and remaining data are exposed to attack. Hence, NVM with high security and low power operation is highly required for IoT platforms. Herein, a disk triboelectric nanogenerator-based NVM (DTNVM) is developed. The DTNVM can be operated with low power because the reading process of stored data is conducted with triboelectricity. Since the ternary system is adopted, 23 to 119 trits can be stored at the DTNVM by changing the sampling time. The identification information is stored at the DTNVM and 91.3% of consistency of the data with a range of 10% tolerance is recorded as result of the reading. Based on the result, the DTNVM is expected to be utilized in the near future as a next-generation NVM and for safe identification systems at the IoT.

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    • 37

      Three-dimensional flower-like nickel doped cobalt phosphate hydrate microarchitectures for asymmetric supercapacitors

      Abstract

      Development of asymmetric supercapacitors (ASCs) using hierarchical three-dimensional (3D) morphologies is becoming crucial in energy storage applications due to the greater power density rather than batteries. Herein, 3D flower-like Co3(PO4)2·8H2O (CPH) and nickel doped CPH (Ni-CPH) microarchitectures were synthesized by a silicone oil bath method at low temperatures without calcination. The synthesized microarchitectures-based electrodes (bare CPH and Ni-CPH) revealed battery-like properties during the electrochemical study. Importantly, the Ni-CPH electrode showed improved electrochemical performance compared to the bare CPH electrode material. The specific capacity values of the CPH and Ni-CPH electrode materials were calculated to be 74 and 108 mAh g−1 at 0.5 A g−1, respectively. Furthermore, for the Ni-CPH electrode, 78% of capacity retention was obtained after 9000 cycles at 5 A g−1. Additionally, an ASC was developed while employing the optimized Ni-CPH electrode (positive-type) and activated carbon (negative-type) and it showed superior electrochemical results. The ASC device exhibited excellent capacity retention (94%) after 9000 cycles at 2 A g−1. Also, this device delivered a high energy density of 23.4 Wh kg−1 and a power density of 2103 W kg−1. Finally, several portable electronic devices were successfully tested using the obtained good energy and power density results from the ASC device for energy storage applications.

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    • 36

      Three-dimensional porous Co3O4 hexagonal plates grown on nickel foam as a high-capacity anode material for lithium-ion batteries

      Abstract

      Three-dimensional (3D) hierarchical hexagonal plate-like Co3O4 microstructure arrays were directly deposited on nickel (Ni) foam via a simple solvothermal process and consequent annealing treatment. The NH4F was employed as a template in the preparation of hexagonal plate-like Co3O4 microstructure arrays. The 3D hexagonal plate-like Co3O4 microstructure synthesized at 140 °C (Co3O4@NF-140) exhibited better morphology compared to the samples obtained at 160 °C (Co3O4@NF-160) and 180 °C (Co3O4@NF-180). Remarkably, as-synthesized materials were used as an anode material for lithium (Li)-ion batteries and the optimized electrode (Co3O4@NF-140) exhibited superior Li storage performance, i.e., noteworthy rate capability and good cycling performance. The outstanding capacity values of 500 and 390 mA h g−1 were obtained at 1 and 2 A g−1, respectively over 500 cycles. Additionally, the specific capacity of the optimized electrode (Co3O4@NF-140) could retain the 606 mA h g−1 at 500 mA g−1 after 100 cycles with less capacity fading and exhibited excellent rate performance. The superior performance is ascribed to the proficient charge transportation from the 3D hierarchical Co3O4 microstructures to the substrate, improved solid interface with the current collector, and strongly adhered microstructures related to the cyclic charge–discharge process.

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    • 35

      Natural silk-composite enabled versatile robust triboelectric nanogenerators for smart applications

      Abstract

      Strategies to maximize the surface charge density across triboelectric layers while protecting it from humidity are crucial in employing triboelectric nanogenerators (TENGs) for commercial/real-time applications. Herein, for the first time, we propose the utility of crystalline silk microparticles (SMPs) to improve the surface charge density in materials like polyvinyl alcohol to realise its applicability for TENG devices. Moreover, these SMPs are extracted from discarded Bombyx mori silkworm cocoons by facile, inexpensive, and single-step alkaline-hydrolysis treatment. We examine the performance of these composites with counter-materials composed of waste PTFE plastic cups to show reuse in recycled products. The processing cost of TENG developed from recycled materials is not only low but eco-friendly. The TENG performance as a function of the concentration of SMPs is investigated and compared with the composite's work-function and surface-potentials, with the distance-dependent electric field theoretical model employed to optimize the performance. Consequently, the optimized TENG exhibits maximum output voltage, current, charge, and power density of ∼280 V, 17.3 μA, 32.5 nC, and 14.4 W·m−2, respectively, creating a highly competitive energy harvester that can conform to the rigorous needs of wearables and mobile applications. Furthermore, the fully packaged silicone rubber device protects it from humidity and enables the device utility for practical applications with a soft, comfortable, and skin-friendly interface.

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