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The preparation of metal organic frameworks (MOFs) has come to the forefront in recent years because of their outstanding physical and chemical properties. Many MOFs such as Zn, Co, Ni, Fe, and Ag, etc., have been successfully synthesized. In this work, we followed the solvothermal assisted route to synthesize Ag-MOF (abbreviated as AMOF) nanosheets and then applied them as a photocatalyst to remove different organic pollutants, namely methyl orange (MO), crystal violet (CV), and methylene blue (MB). Chemical composition, optical properties, morphology, and microstructural analysis were analyzed using XPS, UV-visible spectrophotometer, FESEM, TEM, and EDS, respectively. The structural properties of AMOF nanosheets were studied by X-ray diffraction (XRD). Nitrogen adsorption and desorption isotherm analysis were utilized to evaluate the specific surface area and pore size of the AMOF nanosheets. Further, AMOF nanosheets showed notable photocatalytic performance for various dye pollutants degradation. The results confirmed 74.5, 85.5, and 90.7% of MO, CV, and MB dye pollutants removal after 120 min of irradiation with the rate constants (k) of 0.0123, 0.0153, and 0.0158 min−1, respectively. The effect of superoxide radicals (O2−) and photogenerated holes (h+) on the organic dye pollutants removal was investigated using radical scavenger trapping studies. Moreover, the stability study also confirmed the recyclability of the photocatalyst. Therefore, the findings of this research present a realizable method to grow AMOF photocatalyst for successful degradation of various dye pollutants.

Laser-induced breakdown spectroscopy (LIBS) is an analysis technique that determines the elemental composition of a target material. Metal scraps have a range of shapes and are contaminated with other substances such as paint or dirt. This makes it difficult to recognize each piece of metal scrap accurately and to obtain clear LIBS emission spectra of the target metals. In this study, two image processing algorithms are proposed to measure the three-dimensional shapes of metal scraps and to calculate the optimized (i.e., relatively clean and flat) surface areas of metal scraps. It was confirmed that 25% higher maximum classification accuracy was achieved when LIBS spectra were acquired from optimized rather than non-optimized (i.e., contaminated) surfaces.

According to energy crisis and increasing number of small electronics, energy harvesting is one of the most promising technologies for scavenging several types of wasted energy. Especially, with regard to vibrational energy harvesting, triboelectric nanogenerators and electromagnetic generators stand out due to their working mechanisms. Here, an oscillating charge pump-based hybrid generator with two triboelectric nanogenerators and an electromagnetic generator using a switching-free characteristic was fabricated with material optimization. To enhance the electrical output of the triboelectric nanogenerator, a concept of charge pumping with simply connecting an additional charge pump device and the nanostructure formation process on the contact metal layer were adopted. The electrical outputs were characterized as an output current of 9.17 μA with a current density of 83.2 mA m

A surface acoustic wave (SAW), device composed of polymer and ceramic fillers, exhibiting high piezoelectricity and flexibility, has a wide range of sensing applications in the aerospace field. The demand for flexible SAW sensors has been gradually increasing due to their small size, wireless capability, low fabrication cost, and fast response time. This paper discusses the structural, thermal, and electrical properties of the developed sensor, based on different micro- and nano-fillers, such as lead zirconate titanate (PZT), calcium copper titanate (CCTO), and carbon nanotubes (CNTs), along with polyvinylidene fluoride (PVDF) as a polymer matrix. The piezocomposite substrate of the SAW sensor is fabricated using a hot press, while interdigital transducers (IDTs) are deposited through 3D printing. The piezoelectric properties are also enhanced using a non-contact corona poling technique under a high electric field to align the dipoles. Results show that the developed passive strain sensor can measure mechanical strains by examining the frequency shifts of the detected wave signals.

Agglomeration is a common occurrence in polymer nanocomposites with carbon nanotubes (CNT) and graphene nanoplatelets (GNP) as fillers and usually leads to a reduction of electrical and mechanical properties of the composites. In this paper, we utilize a two-dimensional Monte Carlo percolation model for monofiller and hybrid nanocomposites to examine the effect of CNT agglomeration on electrical and electromechanical behavior of the nanocomposites. We generate microstructures of tunneling network with different agglomerate content, agglomerate morphology (equiaxed to rope-like), and agglomerate filler density to parametrically study the effects of agglomeration on percolation, conductivity and piezoresistivity. Electron tunneling in the network is modeled as the primary mechanism for electrical percolation. Our models indicate that high level of agglomeration leads to a systematic increase of the percolation threshold with a decrease of the conductivity, while low agglomeration level with low filler density within agglomerates improves the percolation and conductivity behavior. The addition of GNP as second filler in hybrid composites leads to an improvement in conductance and piezoresistivity despite the presence of CNT agglomeration.

A triboelectric nanogenerator (TENG) with an ultra-thin thickness of a tribo-dielectric layer (TDL) composed of poly heptadecafluorodecyl methacrylate (PFDMA) is demonstrated. The thickness of the PFDMA, which serves as the TDL, was precisely controlled within a range of 0.5 μm–12 μm through the use of initiated-chemical vapor deposition (i-CVD). The relationship between the TDL thickness and the output characteristics was analyzed by demonstrating the ultra-thin TDL thickness, which is the thinnest thickness polymer TDL ever reported. As the TDL thickness is increased, the short-circuit current (ISC) and the transferred charge (QTR) both increase while the open-circuit voltage (VOC) remains constant. Because a TENG with the ultra-thin TDL shows a different tendency between QTR and the total charge (QTDL) in the TDL, modeling is also carried out to gain a comprehensive understanding of the TDL thickness and output characteristics.

Sodium dodecylbenzenesulfonate (SDBS) is widely used as an efficient detergent in various fields. It is also used to remove radioactive atoms including Cs ions, and as a result, Cs-bound SDBS are generated during a nuclear decontamination process. A silica-based adsorbent with mesopores was prepared and surface-functionalized with amine groups to have a positively-charged pocket for SDBS with electrostatic attraction. The removal of SDBS using such an adsorbent was investigated under various conditions. The removal of high SDBS concentration (more than 2000 ppm) was successfully carried out using both a mesoporous structure and electrostatic attraction. The effects of the initial SDBS concentration, the concentration of adsorbent, the SDBS adsorption time, and the pH in the adsorption of SDBS with the mesoporous silica-based adsorbent were investigated in detail. In addition, the adsorption of Cs ion-bound SDBS was investigated for a practical decontamination process, implying that the adsorbent did not remove Cs ions together with SDBS.

Mixed metal chalcogenide nanoarchitectures have been attracting enormous attention as battery-type electrodes for hybrid supercapacitors (HSCs) owing to their enhanced electrochemical (EC) performance. Despite having high electrical conductivity and good EC properties, tellurium has not been fully utilized in metal chalcogenide electrodes as much as sulfur and selenium. Herein, a facile strategy for the fabrication of nickel and iron (NiFe) mixed metal telluride hierarchical nanorods (MMT HNRs) on nickel foam (NF) is proposed. Furthermore, conductive silver (Ag) is decorated on MMT HNRs (AMMT HNRs) to improve the conducting channels, thereby EC performance. Benefitting from the combined advantages of electroactive NiFe mixed metal, conductive tellurium and Ag, and hierarchical nanorod-like nanomorphology, the AMMT HNR electrode has delivered high areal capacity (1.1 mAh cm-2). Finally, the AMMT based HSC with activated carbon coated NF (AC/NF) as a negative electrode exhibited the highest areal capacitance (1176.5 mF cm-2) with high areal energy density (0.669 mWh cm-2) and power density (64 mW cm-2). Moreover, the HSC device has maintained good cycling stability (86% capacity retention) even after 5000 cycles. New findings of this study definitely shed light on the development of telluride-based mixed metal chalcogenide supercapacitors.

With the increased number of small electronics and demand for their energy source, renewable energy sources have received much attention. Above all, a triboelectric nanogenerator (TENG) based on the combination of contact electrification and electrostatic induction has been researched as a method of converting mechanical energy into electricity. In order to increase the electrical output of TENGs with raising the surface charge density, a lot of researchers have focused on the fabrication methods to employ micro-/nano-structures onto a contact surface of the TENG, but have been facing several issues regarding the degradation of the output performance from the iterative operation process. Hence, it is highly required to introduce an approach to enhancing the performance of TENG, while minimally degrading the output power during the long-term operation. In this paper, an air-gap embedded TENG (AE-TENG), which contains a microstructure on the non-contact surface by means of a sandpaper, is proposed. These small air-gaps between the spin-coated polydimethylsiloxane and the non-contact surface can significantly boost the total surface charge density of the dielectric layer. Thus, the electrical output performance of the AE-TENG is enhanced without any surface engineering on the contact surface. Furthermore, the effect of the air-gap induced surface charges on the electric potential is systematically analyzed by not only experimentally electrical outputs, but theoretical and computational modeling based on the V–Q–x relationship and simulation software tool. This air-gap induced triboelectric effect opens a new perspective of the development of electrical outputs by providing a structural/theoretical understanding for TENGs.

With aging, disability of the body can easily occur because the function of the body is degraded. Especially, swallowing disorder is regarded as a crucial issue because patients cannot obtain the nutrients from food by swallowing it. Hence, the rehabilitation of swallowing disorder is urgently required. However, the conventional device for swallowing rehabilitation has shown some limitations due to its external power source and internal circuit. Herein, a self-powered triboelectric nanogenerator for swallowing rehabilitation (TSR) is proposed. To increase the electrical output and pressure sensitivity of the TSR, the tilted reactive ion etching is conducted and the electrical output and pressure sensitivity are increased by 206% and 370%, respectively. The effect of the tilted reactive ion etching into the electrical output generated from the TSR is systematically analyzed. When the tongue is pressing, licking, and holding the TSR, each motion is successfully detected through the proposed TSR. Based on these results, the smart swallowing rehabilitation monitoring system (SSRMS) is implemented as the application and the SSRMS could successfully detect the pressing by the tongue. Considering these results, the SSRMS can be expected to be utilized as a promising smart swallowing rehabilitation monitoring system in near future.

An aluminum double-layered electrode (DE-Al) was successfully employed as two electrodes in a symmetrical supercapacitor (double-layered electrode symmetric SC (DE-SC)) and as a positive layer of a triboelectric nanogenerator (DE-TENG) with the aim of energy conversion and storage using a selfsame structured, self-powered flexible device. A facile water-assisted oxidation (WAO) process and metal sputtering after the WAO process can allow the electrodes to greatly improve the active surface area and the conductivity, leading to the enhancement of the electrochemical performances of a supercapacitor (SC). Particularly, this double-layered structure fabrication process is extremely less time-consuming and cost-effective. The electrochemical test of the proposed DE-Al was systematically conducted by cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS), along with the in-depth characterizations of the surface. From these studies, the DE-Al offers exceptional electrochemical properties compared with other structures, which were utilized as the electrodes in the polyvinyl alcohol/phosphoric acid (PVA/H3PO4) gel electrolyte. The improved performance apparently evidenced from the electrochemical tests of fabricated SC resulted from the enhanced electrical conductivity and large active surface area. The specific capacitance and cycle-life stability of the DE-SC were investigated by using a GCD analysis. Additionally, the EIS curves before and after stability test (for 3500 cycles) were obtained to prove the long-term endurance of DE-SC. A vertical contact and the separation mode of the TENG were also fabricated by using the same DE-Al as a positive layer and polydimethylsiloxane (PDMS) as a negative layer. Finally, the fabricated SC and TENG were successfully combined using a bridge rectifier to convert and store the mechanical energy as electrical energy. This simple design and facile fabrication of a double-layered-electrode-based structure is promising for the development of an energy conversion and storage device.

In Internet of Things (IoT) era, electronic textiles (E-textiles), which combine various functional devices on a fabric, have attracted attention. Among various components for E-textiles, the human-machine interface device, for example, a keyboard, is one of the most important parts. The triboelectric nanogenerator (TENG) can be a powerful sensing component for the interface device due to its cost-effectiveness, design flexibility and self-powered operation. Previous studies reported fabric-based sensor devices with TENG, but as yet, no device that harnesses commercial compatibility with the textile industry has been reported. It is timely to explore a low-cost TENG-based keyboard made completely of commercial fabric for early commercialization. This paper proposes a TENG-based wearable keyboard that uses only cheap commercial fabrics. Each cell in the proposed keyboard generates electrical signals according to an external touch without any power supply. After an appropriate filtering process, the proposed keyboard can detect a key stroke without any ambiguity. We verify the keyboard operation by typing a word and playing music. The material dependency of the proposed keyboard is experimentally validated with various material pairs. Finally, the endurance of the proposed keyboard against folding, repeated touches and washing is experimentally confirmed for actual applications in a real environment.

Cellulose is one of the emerging materials as a natural polymer. In this work, cellulose nanofiber is utilized as a dielectric layer in triboelectric nanogenerator. To retain the flexibility of the fabricated paper, Ag nanowires layer is employed for electrode material and counter triboelectric layer simultaneously. This triboelectric nanogenerator is fabricated by a filtration process with different cellulose nanofiber and Ag nanowire condition. This triboelectric nanogenerator operates by contacting and separating of only two same bilayer paper consisting of cellulose nanofiber and Ag nanowire. Cellulose nanofiber and Ag nanowires layer acting as not only triboelectric layer and counter-triboelectric layer but substrate and electrode respectively. The case of cellulose nanofiber solution homogenized with 20 passes in 1000 bar and Ag nanowire with 0.1 wt% solution shows the optimal peak corresponding to 21 V of open-circuit voltage and 2.5 µA of short-circuit current. The maximum power output represents the value of 693 mW/m2 for a 10 MΩ external resistance. Fabricated device is dispersed and completely dissolved in deionized water by using sonicator for 30 min and returns to original ingredient without producing any pollutant. This nanogenerator plays roles of not only harvesting ambient energy but folding sensor, humidity sensor, and e-paper-like device due to the paper-like characteristics and nano-porous of cellulose nanofiber and Ag nanowires.

Green energy techniques, such as harvesting wind energy, is gaining great attention as they can provide environmental-friendly and renewable power sources in this era of energy crisis. Here, we report a triboelectric nanogenerator (TENG) which harvest mechanical energy driven by wind that works as a sustainable power source as well as a self-powered wind speed sensor. By adopting a novel method that employs the rolling motion of polymer beads, which serve as freestanding layers, under the wind speed of 20 m/s, the TENG delivered output voltage up to 17.8 V, output current of 5.3 µA, and a corresponding output power density of 1.36 mW/cm2. The correlation between electrical outputs, wind velocity, width of electrode is systematically investigated. Furthermore, the fabricated device is able to generate an electrical energy regardless of the direction of the wind, and it works well even for long term in the surrounding environment. Additionally, the device is small in size and comparatively flat in its structure, further enhancing potential of its application, such as self-powered wind speed sensor, as an effective portable power source that is readily available to be used in our daily life.

Irrecoverable data destruction on a mobile device is important to prevent unintentional data disclosure. In this regard, transient electronics, a form of electronics that can be made to disappear or can be destroyed in a controllable manner, has been actively researched. To erase data completely, irreversible reactions such as physical or chemical destruction have been used. However, these techniques either require external voltage or destroy a memory device so that it cannot be reused. Here, we demonstrate a novel self-powered data-erasing method for nanoscale flash memory devices which uses triboelectricity via a kill switch, which consists of a nylon pad connected to a gate electrode of the flash memory. Through a one-time touch of the kill switch by a finger wearing a polytetrafluoroethylene (PTFE) glove, data stored in flash memory is set to the ‘1′ state on the chip scale simultaneously with low-level triboelectricity, allowing the memory to be reused afterward. Moreover, the memory can be permanently destroyed by a single touch of the kill switch with a finger without a glove that generates high-level triboelectricity. These erase methods provide a rapid and convenient means of self-powered irrecoverable data erasing in the era of the Internet of Things (IoT).

A triboelectric nanogenerator (TENG) is an energy harvester which converts ambient mechanical energy into electrical energy. A disk-based rotational TENG converts rotational mechanical energy into electrical energy with advantages over a vertical contact-separation based TENG. Since the disk-based TENG operates in grating-based sliding mode, its electrical output is advantageous in terms of prolonged continuous energy harvesting. However, rotational mechanical energy is less abundant than linear mechanical energy in typical environments. In this work, a new disk-based TENG which converts linear mechanical energy into electrical energy through the use of a gear system (linear-to-rotational TENG, LR-TENG) was fabricated as an experimental device. A TENG based on a vertical contact-separation mode (V-TENG), composed of identical triboelectric materials, was also fabricated as a control device. Here, it is experimentally demonstrated that the electrical output of the LR-TENG was superior to that of the V-TENG at the same magnitude of applied force and at an identical level of applied mechanical energy, as the surface charge density of the LR-TENG is much greater than that of the V-TENG. The LR-TENG can effectively provide power to numerous small electronic devices. Examples include charging a commercial capacitor and lighting commercial LEDs.

Tremendous energy is being wasted without any proper utilization despite the energy crisis and global warming. Especially, enormous thermal energy is wasted, which is generated from the industrial fields. To handle this issue, technology is highly desired to harvest this wasted thermal energy. Herein, a hybridized energy harvesting system based on Stirling engine (HESS) is proposed in order to harvest the wasted thermal energy in industrial fields. The developed HESS is composed of a disk triboelectric nanogenerator (disk TENG) and electromagnetic generator (EMG) based on Stirling engine that converts thermal energy into mechanical energy. Hence, the HESS can harvest the electricity from the mechanical energy converted from the thermal energy owing to the Stirling engine and shows its great stability of the performance by generating electrical output for more than 2 h without any specific degradation. Each part of the single EMG and the disk TENG in the HESS respectively generates an output power of 0.384 μW at 60 Ω and 1.2 μW at 30 MΩ, and the HESS can generate 1.4 μW at 9 MΩ by integrating each generated power. Moreover, the fabricated HESS generates 14.51 times more electrical energy than generated electrical energy through the single EMG. Considering these results, the next-generation heat recovery system can be implemented with the HESS by converting the wasted thermal energy into electrical energy in the industrial fields.

We prepared shape-controllable, sodium alginate-based hydrogels containing various particles inside the hydrogels. Centrifugal injection method was employed to obtain smaller sized gels, compared t...

A triboelectric nanogenerator (TENG) is a promising type of energy harvester which converts ambient mechanical energy into electrical energy efficiently as part of the effort to overcome the energy depletion issue. To enhance the electrical output of the TENGs, the surface morphology is intentionally created on the surface of a friction layer of these harvesters. In this work, the shape dependency of the geometric morphology of a TENG, in terms of the electrical output, force sensitivity, and durability, is investigated by measurements and simulations with the aid of ANSYS. A well-ordered pillar-shaped and dome-shaped nanostructure was created on polydimethylsiloxane (PDMS), which is commonly used as a friction layer in TENGs. It was found here that the TENG with dome-shaped PDMS (DP-TENG) showed greater force sensitivity than the TENG with pillar-shaped PDMS (PP-TENG). However, the PP-TENG was found to be more durable than the DP-TENG. These results stem from the fact that the dome-shaped nanostructure will deform more easily than the pillar-shaped nanostructure. This study suggests a guideline for the structural engineering of the surface morphology of the friction layer with the consideration of the geometric shape to maximize the important properties of the TENGs.

With the increasing number of devices that require electrical energy, various types of triboelectric nanogenerators (TENGs), a promising technology for energy harvesting, have been developed in recent years. However, conventional TENGs show relatively low mechanical flexibility because each layer of the conventional TENGs is composed of rigid materials. Low mechanical flexibility produces electrical energy in one-way operation and limits the utilization of TENGs in a variety of applications. In this work, we develop the liquid-metal embedded sponge-typed TENG (LMST). Liquid-metal and silicon rubber are utilized to fabricate the LMST with a peculiar sponge shape, which is inherently including the randomly distributed pores with dispersed liquid-metal droplets. Hence, the LMST can be bent 180° and stretched 300%, which indicates the great flexibility and stretchability of the LMST. The LMST with size of the 1.5 cm × 1.5 cm × 1.5 cm generated a short-circuit current (ISC) of 188 nA and an open-circuit voltage (VOC) of 24 V. Also, the 2.48 W/m2 of power density is generated from the proposed LMST by simply inserting wires into the LMST without any conventional processes for fabricating electrodes. Interestingly, the electrical power can be used simply by connecting wires to the LMS, and power can be greatly improved by increasing the number of wires connected to the LMST. Additionally, various shapes of the LMST are facilely designed using 3D printing technology for a wide spectrum of applications. To prove the applicability of the LMST with the unique porous structure, three types of self-powered sensor systems are demonstrated, which are detecting the pressure, the direction of a spinning ball, and real-time detecting malfunction of motor faults. The flexible advantages of the liquid-metal embedded sponge structure triboelectric nanogenerator device, allow us to extend its applicability to battery-free sensors that can be used in various locations.

Flexible and portable power sources are imperative chunks of wearable electronic devices. Fabric-based triboelectric nanogenerators (TENGs) have become most promising energy sources for wearable electronic devices as they can generate electrical energy from biomechanical movements with great flexibility. In this study, a highly flexible Co/Zn bimetal organic framework based TENG (BMOF TENG) is fabricated. Facilely prepared Co/Zn BMOF nanosheets are coated on flexible conductive fabric (BMOF/FCF) and utilized as the tribo-positive material against PTFE/Al based tribo-negative material for the fabrication of the BMOF TENG. The content of Zn is varied from 0% to 50% to achieve superior electrical output and observed that the 15% of Zn is appropriate for superior electrical output. The open circuit voltage, the short circuit current, and the charge density of the BMOF TENG are increased from 11 V to 47 V, from 1.06 µA to 7 µA, and from 4 nC/cm2 to ~17 nC/cm2, respectively when the Zn content is increased from 0% to 15%. Thus, the optimized content of Zn in Co/Zn BMOF helped to enhance the electrical output of the BMOF TENG by nearly 450%. The output power of the BMOF TENG is found to be 1.1 mW/m2 at a load resistance of 2 MΩ. As the phenomenon of gas sensing is largely associated to the surface of the material, the advantage of unique surface features of the BMOF/FCF is utilized in sensing hazardous gases. The device has shown variations in its electrical output in the presence of air and targeted gas and the good selectivity towards ammonia at room temperature. Ultimately, the BMOF TENG proposed in this study can convert mechanical energy into electrical energy and can be employed as an ammonia sensor in environmental monitoring and food quality assessment.

Constantly operated systems such as traffic and security systems that continually maintain the operation are continuously consuming tremendous energy as a standby power. The triboelectric nanogenerator (TENG) is a suitable alternative as the sustainable energy source of these systems because the TENG can be utilized as the energy source of these systems by generating the electricity from the wasted energy around the systems. The paint is an attractive material for the TENG because the paint is commonly utilized and the contact frequently occurs on the surface coated by the paint. Herein, the paint based TENG (PBT) is developed with the facile spray deposition to harvest this wasted mechanical energy generated from the paint coated surface due to the contact. The electrical outputs of the PBT are investigated and recorded maximum increased output of 280%. Furthermore, the applicability of the PBT is demonstrated by adopting the various materials for fabrication and the PBT shows great stability. Finally, intrusion detection system (IDS) and camouflaged keypad (CKP) are implemented as applications, respectively. The violation of traffic-norm at the stop line is successfully detected with the IDS, which is indicating the strong possibility for the traffic system. The CKP successfully replaces the keypad of the conventional keyboard with the guaranteeing high personal security during the login process. Considering good compatibility of the PBT, it can be expected to apply in the smart traffic system as well as advanced security system in near future.

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.

With the rapidly increasing number of older people in our societies, fall detection is becoming more important: Older adults may fall at home when they are alone and they may not be found in time for them to get help. In addition, a fall itself can cause serious injuries such as lacerations, fractures and hematomas. Although many previous studies have been reported on various fall detection technologies based on wearable sensors, the inconvenience of wearing them is problematic. Vision or ambient based methods may be alternatives, but high cost and complex installation process limit applicable areas. We propose a cost-effective, ambient-based fall detection system based on a pressure sensing triboelectric nanogenerator (TENG) array. Apart from simple observation of output signal waveforms according to different actions, key technologies, including appropriate filtering and distinguishing between falls and daily activities, are demonstrated with data acquisition from 48 daily activities and 48 falls by eight participants. The proposed system achieves a classification accuracy of 95.75% in identifying actual falls. Due to its low cost, easy installation and notable accuracy, the proposed system can be immediately applied to smart homes and smart hospitals to prevent additional injuries caused by falls.

A full-scale commercial aircraft morphing control surface using flexible matrix composite actuators was designed and demonstrated in this research. The muscle-like flexible matrix composite actuator is ideal for morphing structures as it deforms while actuating with the structure, and the hydraulically driven actuator can operate at pressures similar to existing aircraft hydraulic systems. Through a series of parameter studies performed with finite element models, an active spoiler was designed to control the gap between a spoiler and deployed Fowler flap. A full-scale prototype was then fabricated and tested under pseudo-aerodynamic loads. The prototype demonstrated that it can achieve the necessary deflections under the given loading condition. A closed-loop control system that allows the tip deflection of the spoiler to be precisely controlled under loading was designed.

Among the many types of wasted energy around us, mechanical energy has been considered to have a considerable amount of potential to be scavenged due to its abundance and ubiquity in our lives. To convert ambient mechanical energy into electrical energy efficiently, the triboelectric nanogenerator (TENG) has been intensively studied. Polydimethylsiloxane (PDMS), due to its superior mechanical and electrical properties, has commonly been selected as a friction layer in TENGs. Herein, it is newly discovered that the output power of a fabricated TENG is highly correlated with the Young's modulus of PDMS. An enhancement of the output power is achieved by the optimization of the PDMS mixture ratio. In addition, to improve the output power of the TENG further, a well-ordered microstructure was directly created on the surface of the PDMS by means of ultrafast laser irradiation. Direct patterning to create the surface morphology on the PDMS surface with the aid of laser irradiation is more efficient than conventional surface modification techniques such as replication and a few microfabrication steps. Compared to a control TENG using bare PDMS, an increase in the output power of more than twofold is achieved by an experimental TENG using patterned PDMS with a laser power of 29 mW. The TENG utilizing the patterned PDMS achieves a maximum output power density level of 107.3 μW/cm2.

A rotating-disk-based triboelectric nanogenerator (TENG) generating electrical energy from wind usually includes a propeller. TENGs are widely used because their high frequency of rotation allows them to generate a relatively high output current. Deep analysis of the gas flow in a TENG is essential to improve its energy conversion efficiency. However, previous studies have isolated the propeller and the TENG as separate entities that harvest wind energy and generate electrical energy, respectively. Most studies focused on each entity because considering both the dynamics and the TENG operation together is an intricate process. This paper introduces a dynamic analysis of the gas flow by dividing it into four vertical and horizontal directions and carrying out a COMSOL simulation to verify the pressure on the propeller and the flow of the gas. The electrical outputs are measured while varying the height and angle of the inlet and the number of wings on the propeller. After optimization, the P-TENG generated a high output power density of 283.95 mW/m2, which can light up 205 light-emitting diodes and drive a commercial small electronic appliance. In addition, optimizing the P-TENG through a variety of analyses allowed it to provide sustainable power to a self-powered wireless sensor system.

Thermal energy is generated from all equipment during operation. Likewise, thermal energy exists everywhere and dissipates uselessly. It is necessary to transform thermal energy into renewable electrical energy by utilizing materials and devices exhibiting unique features of thermal properties. Shape memory alloys (SMAs) consisting of a compound of nickel (Ni) and titanium (Ti), operate based on unique thermomechanical properties, such as shape memory effect (SME) and superelastic effect (SE). Due to these two effects, SMAs are possible to be deformed and recovered by external heat. One-dimensional SMA wire (SW) can effectively harvest wasted thermal energy by the phase change in SMA from a wrinkled state to a straight state in order to generate continuously rotating energy with two sheaves. In this paper, we propose a thermally-driven SMA-wire based hybrid generator (SW-HG), including a disk-TENG and thermoelectric generator (TEG) which lead to generate a high voltage and large current, respectively. In addition, the operation principle of rotating by the deformation and the recovery of the SW at the phase-change temperature are systematically investigated as well as analyzed using two theoretical models. The unique thermomechanical behavior of the SW-HG is applied to commercial vehicles using output signals for the real-time temperature monitoring inside the engine room. Considering these unique features of SW, the SW-HG is expected to be dependable devices for IoT applications as well as can effectively convert surrounding wasted heat energy into reusable electrical energy in real life.

Combining with an artificial intelligence and 5G technologies, the new era of Internet of Things (IoT) has been emerging. Hence, a human-machine interface (HMI) is becoming indispensable in IoT. However, conventional HMIs are suffering from limitations like, the requirement of power sources and structure-complexity. As a solution, triboelectric nanogenerator (TENG) based HMI can be considered as an attractive alternative due to the self-powered operation of TENG. Herein, we suggest the self-powered triboelectricity-based touchpad (TTP) which combines with artificial intelligence consisting of TENG array (49 pixels) constructed on thin, transparent, and flexible substrate, as well as this TTP works based on two major kinds of TENG mechanisms in tapping and sliding modes. The tracing ability of the TTP is demonstrated with high accuracy. Furthermore, the TTP recognizes the digit patterns from ‘0’ to ‘9’ with classification accuracy of 93.6%, 92.2%, and 91.8% at the bending angles of 0°, 119°, and 165° through the pre-trained neural network. With the remarkable applicability of the next-generation HMI in IoT, the TTP is implemented as the smart calculator by using graphical user interface modeling. Considering its good compatibility, it can be expected to be a promising HMI in near future with practical IoT towards artificial intelligence.

Many decentralized communities like rural communities of developing countries and remote communities, suffer from a scarcity of adequate water treatment because there is insufficient capital to construct relevant facilities, no experts to handle the required chemicals, insufficient electricity to operate them. Herein, we report a fully self-powered electro-coagulation (SPEC) system for de-centralized water treatment. For this, a triboelectric nanogenerator harvests ambient wind energy to provide the needed power. A simple electro-coagulation unit, composed of a simple pair of electrodes, successfully removes dissolved colloidal pollutants from water, while coagulating and precipitating pollutants, without any use of chemicals or an external power supply. The SPEC system removes 90% of algae and 97% of organic dye with self-powered treatment for 72 h. Due to its easy operation, and its low cost arising from its inherently simple structure, requirement for much smaller-scale facilities and low operating cost, the SPEC system could become a powerful candidate for appropriate water treatment in many regions lacking centralized water treatment.

A triboelectric nanogenerator (TENG) offers improved output power for enhanced performance and throughput at a low cost as a type of practical energy harvester. In this work, a novel method which enhances the output power with the aid of a nano-to-micro morphology and which reduces the fabrication cost via the simple process of the glass transition of a polystyrene (PS) substrate is introduced. The glass transition of the PS quickly produces the nano-to-micro morphology within 1 min with only a heating process in an air environment. The pristine PS substrate was crumpled in a uniaxial or biaxial direction when heated to a temperature of 170 °C. Thus, the throughput is notably increased without the use of complicated fabrication processes or high-level equipment. The power enhancement effects of the proposed TENG were characterized. Under the optimized condition, the fabricated nanogenerator showed a fourfold increase in both the open-circuit voltage and short-circuit current due to the fourfold enhancement of the surface charge density arising from the crumpled structure compared to a control device without a nano-to-micro structure on the PS substrate. In this case, 160 serially connected light-emitting diodes (LEDs) were successfully illuminated with the optimized device, which was 2 cm by 2 cm in size.

The performance of a triboelectric nanogenerator (TENG) was effectively enhanced by forming nanostructures at the contacting interface. The contacting interface usually consists of metal and polymer surface, but the formation of nanostructures have only been actively studied for the polymer so far. In this work, a simple and effective route to forming nanostructures on the metal surface is proposed, using a water-assisted oxidation (WAO) process. The one-step WAO process requires only hot water without any complicated equipment and treatment. Using the WAO process, densely packed micro and nanostructures were successfully formed on three target metal surfaces: aluminum, copper, and zinc. The output power of the TENG was enhanced after the nanostructure formation because of the increased contact area. The influence of the process conditions on the nanostructure morphology was additionally analyzed to maximize the output power. The simple and low-cost WAO process is advantageous in terms of practicality.

The sensitivity of the electronic properties of carbon nanotubes to gases, chemicals, temperature, and mechanical strain enables their use as fillers in nanocomposites for sensing applications. In this paper, the authors develop a low-cost and scalable process based on inkjet printing technology to fabricate printed flexible sensors used for strain and damage detection. A well-dispersed conductive water-based ink is fabricated with functionalized multiwall carbon nanotubes (MWCNT) and deposited onto paper and Kapton substrates to obtain a sheet resistance as low as 500 Ω/sq with about 30 printed layers. The number of printed layers, the direction of the electrical resistance measurement, and the type of substrate have clear effects on the sensor's electrical performances related to the detection of mechanical strain and impact damage. This work demonstrates the effectiveness of the printed sensors for micrometeoroid and orbital debris (MMOD) impact damage detection through hypervelocity testing.

Ever-rising skyscrapers and the growing number of densely populated large cities face an increased danger from earthquakes. To lessen the risk of this natural disaster, critical facilities must be equipped by a reliable warning system. However, conventional sensors which detect the velocity and acceleration of the seismic vibrations, consumes an external power, which cannot be placed in many buildings due to limitations in electricity supply. In this work, a self-powered seismic sensor is fabricated using the three-dimensional printing technology. The proposed device can be operated based on the triboelectricity, which converts an external mechanical energy into an electrical energy. The proposed device exhibits an exceptional behavior of a friction-free due to its sophisticated designed structure with a levitating oscillator. The fundamental parameters representing the device performance are systematically analyzed with various operation mode. Water-assisted oxidized Al electrodes allow full-contact with the polytetrafluoroethylene triboelectric oscillator film, subsequently, produce the highest values of output parameter as follows; an open-circuit voltage of 29.44 V, a short-circuit current of 204 nA, and a peak power density of 2.501 mW/m2 at a load resistance of 50 MΩ. Frequency reversibility is also demonstrated from the frequency response of the short-circuit current during the operation in lateral-contact mode. Because of inherently friction-free characteristics of the proposed device, this device shows high sensitivity, low sensing limit, and long residual signal, which is experimentally verified. In order to test the sensing ability of the device, artificial seismic vibrations are applied with vertical forces of 4 N and 2.4 N. An alert software program displays the sensing ability of the device with the signals over 1 V, which is the threshold voltage. The stability of the device in humid conditions and its durability for 3100 s in the output voltage measurement represent the possibility of this device as a reliable sensor. Due to the simple three-dimensional printing technology and its inherently friction-free characteristics, the proposed device can be employed as a self-powered sensor capable of warning about dangerous earthquake disasters.

Energy conversion cannot be sufficiently achieved from a single output device having a single input. Energy conversion efficiency and output power of triboelectric nanogenerator (TENG) can be enhanced by increasing the contact area or the number of devices. Especially, combining two or more of the four operational mode (contact-separation, lateral sliding, single electrode, freestanding) of triboelectric nanogenerator would be more efficient to harvest mechanical energy than with singleharvesting mode. In this paper, a hybrid coaxial triboelectric nanogenerator (HC-TENG) can generate electrical output from dual outputs. This HC-TENG consists of a disk-TENG and LS-TENG so that a large current can be harvested from the rotating movement of disk-TENG at a high frequency input and an additional output can be generated from the LS-TENG by adding one axis. The structure of device with one axis was inspired by the structure of crank engine, which can convert linear motion into rotational motion. When applying the structure of HC-TENG to an application, the input-output efficiency was calculated by using the rotational kinetic energy formula with the value of 16.66%. Moreover, the efficiency of HC-TENG was verified by comparing experimental values with theoretical calculations for the presence of the dual outputs. This combination of multiple TENGs can be adopted for enhancing efficiency from every ambient rotational movement in real life.

Energy harvesting technology, which collects natural energy and converts it into electrical energy, is a very important technology for the future energy industry. Solar cells are one of the most popular energy harvesting devices due to their high power and cost-effectiveness; however, because solar cells are highly influenced by weather conditions, they are not suitable for continuously generating electricity. In this paper we propose a new unified harvesting module (UHM), which is thin and lightweight, to harvest natural energy sources in various weather conditions. The UHM consists of two triboelectric nanogenerators (TENG), a transparent one for harvesting energy from raindrop impacts and one for harvesting wind energy, and a solar cell. This device can turn on 62 LEDs in real time without any energy storage device. Additionally, the UHM can be used as a self-powered weather sensor to detect rain drops, sunlight, and wind speed based on its electrical output. From this, the weather monitoring platform was subsequently demonstrated for use on a smart farm using a data acquisition system and LabVIEW software. This work proposes a new generator for not only harvesting natural energy, but also for broadening its application to the Internet of Things by developing practical usage of energy harvesting technology systems.

Among the surfactants used for removal of radioactive nuclides generated from nuclear decontamination, sodium dodecylbenzenesulfonate (SDBS) is frequently used. A potential environmental problem of incomplete elimination of SDBS from radioactive liquid wastes (RLWs), which contain a high concentration of SDBS and radioactive nuclides, still remains. Removal of SDBS was evaluated by adsorption using commercially available, macroporous polymer-based adsorbents, HP20 and HP2MGL, which are styrene (St)- and methyl methacrylate (MMA)-based crosslinked resin beads, respectively. The effect of the macroporosity and chemical structure of the macroporous adsorbent resins (MARs) on the adsorption behavior was investigated. HP20 did not have any functionality for adsorbing SDBS, but it showed effective adsorption toward SDBS (less than 20 min to reach 90% adsorption), because of the hydrophobic interaction between the phenyl groups in the St unit in HP20 and in the SDBS. The removal of SDBS from a mixed solution of SDBS and Cs ions was investigated to elucidate the adsorption process in an imitation of the sort of RLWs. This investigation suggests that MARs can potentially be used for the removal of SDBS not only from a SDBS solution but also from a mixed solution of SDBS and Cs ions.

A new form of generator known as the triboelectric nanogenerator (TENG) has recently been suggested as a simple and low-cost solution to scavenge ambient mechanical energy. Although there have been substantial advances in TENGs over the past few years, the power efficiency of TENGs must be enhanced further before they can be practically applied. In the present study, we report a ferromagnetic nanoparticle-embedded hybrid nanogenerator (FHNG) which operates based on both triboelectricity and electromagnetic induction. A TENG and an electromagnetic generator (EMG) efficiently cooperate to generate electrical energy from the same motion, i.e., the vibration of a synthesized nanoparticle. A surface-functionalized ferric oxide nanoparticle, which has strong ferromagnetism and high triboelectricity, was produced by a simple surface-coating process. The measured electrical characteristics revealed that the output voltage of both the TENG and the EMG components increased by approximately 50 times and by twofold, respectively, after the surface functionalization step. Moreover, when constant vibration of 3 Hz is applied to the fabricated FHNG, the TENG and EMG components correspondingly generated output power of 133.2 μW at a load resistance of 100 MΩ and 6.5 μW at a load resistance of 200 Ω. The output power per unit mass from the FHNG is greater than that according to the arithmetic sum of the individual TENG and EMG components, demonstrating synergy between the two components. Furthermore, the device can generate stable output under various vibration directions, amplitudes, and frequencies due to the fluid-like characteristics of the powder. The packaged structure also securely protects the device from external humidity and dust. Connected to a rationally designed power management circuit, a digital clock was turned on solely by the fabricated FHNG.

A micro- and nano-structure at the contact interface of triboelectric nanogenerator (TENG) is an essential element enabling a remarkable performance improvement. Despite the various innovative inventions of interfacial structures, there has been little understanding about the analytical interpretation of the contact interface. Herein, the deformation behavior of interfacial micro- and nano-structures during the contact process is systematically analyzed to clarify the origin of the contact pressure to output voltage relationship of TENG. The visualization experiment, simulation, theoretical modeling, and electrical measurements are conducted for the comprehensive analysis. The results confirm that the deformation of interfacial structures directly determines the pressure–voltage relationship of TENG. The surface enlargement effect by interfacial nanostructure formation becomes completely valid only when the provided contact pressure is stronger than the certain threshold pressure to make the full-contact condition. Due to the correlation between contact pressure and output voltage, interfacial structure of the TENG should be carefully customized depending on the application environment. Based on the analyzed pressure–voltage relationship of the TENG, a hybrid circuit for energy scavenging and digital pressure sensing is designed and experimentally demonstrated.

Energy harvesting techniques which convert ambient waste energy into usable electrical power have emerged with the increased energy demand and rapid growth of self-powered systems. Among them, the triboelectric nanogenerator (TENG), which utilizes contact-electrification, has attracted a great deal of attention owing to its high output energy, good cost-effectiveness, and simple fabrication process. Here, the TENG harvesting ambient mechanical energy based on internal hydrodynamic oscillation (Hy-TENG), is demonstrated. The Hy-TENG shows an output voltage and current of 22 V and 1.45 µA, respectively, under vibration of 5 Hz. The maximum instantaneous power density is 26.5 mW/m 2 . By utilizing water in the mechanical energy harvesting process, several powerful advantages are also guaranteed due to the shape adaptability of water. First, both a wide bandwidth and a low resonance frequency are verified, which are advantageous for applications based on human motion. These frequency response characteristics indicate the strong potential of the Hy-TENG in actual environments. Furthermore, the Hy-TENG is highly scalable, as water can easily be restored to its initial state without a mechanical conversion apparatus. Finally, Hy-TENG harvesting 3-dimensional random motion (3-D Hy-TENG) is developed with a simple design based on the easily shape-transformable characteristic of water. Additionally, enhanced endurance against environmental factors and mechanical damage can be expected based on the packaged structure and on the non-destructive water–solid contact.

Wind has come to be considered as an attractive energy source due to its abundance, ubiquity, and sustainability in nature. When an energy harvester driven by wind is miniaturized with highpower and robustness, it can be useful for self-powered systems of mobile electronic devices. In this work, a vertically stacked triboelectric nanogenerator (VS-TENG) is investigated. When wind is introduced into the air gap of the VS-TENG, a thin and flexible polymer membrane repeatedly comes in contact with and separates from upper and lower electrodes. This thin device structure makes the VS-TENG suitable for vertical stacking without bulky volume. Vertical stacking not only provides multiplied output power via parallel stacking but also enables utilization of bi-directional wind by a cross stack. & 2014 Published by Elsevier Ltd.

Large-area nanopatterning is introduced on a flexible gold substrate by block copolymer (BCP) lithography. We fabricate ultrahigh power energy harvesting nanogenerators based on contactelectrification principle exploiting the effective contact area enhancement by block copolymer lithography. Owing to the facile and effective surface area enhancement by BCP nanopatterning, significant enhancement of triboelectric charge induction is attained. Output currents from TENG increased at least 16 times after BCP nanopatterning. This nanogenerator can generate remarkably high output voltage and current, which can directly light up 504 serially connected LEDs. The resultant nanogenerator attains the maximum instantaneous current of 1.6 mA and output power of 93.2 W/m 2 . Electrical energy generated by single cycle of impact force is 0.54 mJ, and the average electrical energy is 114.89 mW. Those values indicate one of the highest device performance ever reported for the TENGs based on contact-separation mode thus far.