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.

Temperature is one of the fundamental parameters for thermodynamics and its accurate detection is necessary. The novel strategy of luminescent materials based on the fluorescence intensity ratio technique has been promised for thermometers in more practical environments to overcome the drawbacks of conventional thermometers in recent. Herein, the novel single-phase K3Gd(VO4)2:Tb3+/Sm3+ phosphors were successfully prepared, and the 4G5/2 → 6H9/2 (Sm3+) and 5D4 → 7F5 (Tb3+) transitions could be promised for luminescent thermometers. Besides, under the double-excited states of charge transfer band (CTB, 317 nm) and 4f-4f (478 nm) excitations, the K3Gd(VO4)2:Tb3+/Sm3+ phosphors exhibited different concentration quenching mechanisms in energy transfer processes and also showed superior absolute sensing sensitivity (Sa) and relative sensing sensitivity (Sr). Especially, the maximum Sa and Sr values reached up to 0.568 K−1 and 11.24% K−1, respectively and the temperature resolution of the KGV:0.2Tb3+/0.01Sm3+ phosphor was higher than 0.004 K under the excitation wavelength of CTB (317 nm), indicating that the double-excited states in the single-phase KGV:Tb3+/Sm3+ phosphors with superior sensing sensitivity could be a novel candidate for potential optical thermometers.

In the world of automotive technologies, a tremendous amount of small-scale rotational energy is wasted and not being converted into a usable energy form. A rotatory triboelectric nanogenerator can be a fortune technology to harvest such waste mechanical energy and convert it into electricity. In this regard, we developed a fully three-dimensional (3D) printed bidirectional rotatory hybrid nanogenerator (BR-HNG) to convert waste rotational energy into electricity, via a contact and separation mode mechanism. Initially, a single HNG was fabricated with porous sodium niobate and polydimethylsiloxane polymer (i.e., NaNbO3/PDMS) composite film, which was operated vertically against the aluminum to optimize a high and stable electrical output. After the robust and stability test of the single HNG, multiple HNGs were integrated into a 3D printed bidirectional rotatory frame to harvest the rotational energy. It was observed that the BR-HNG had a stable and high electrical direct current voltage of 37.5 V. Furthermore, the BR-HNG was attached to a bicycle to harvest the rotational energy while bicycling in everyday human life and power up various portable electronics.

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.