Two-dimensional graphitic carbon nitride nanosheets: a novel platform for flexible, robust and optically active triboelectric nanogenerators

Flexible Triboelectric Nanogenerators based on Hydrogel/g-C3N4 Composites for Biomechanical Energy Harvesting and Self-Powered Sensing

Flexible and stretchable triboelectric nanogenerators (TENGs) have been rapidly advanced owing to the demand for portable and wearable electronic devices that can work under universal or motional circumstances. While versatile materials can be applied in a TENG as dielectric materials, flexible and cost-effective electrodes are crucially important for the output performance of TENGs. Herein, we developed a poly(vinyl alcohol) (PVA) hydrogel TENG doped with a novel two-dimensional material, graphitic carbon nitride (g-C3N4), which could act as both a cost-effective flexible electrode and a positive dielectric for TENG with different morphologies. The measured peak-to-peak open-circuit voltage of the TENG reached 80 V at a dopant concentration of 2.7 wt % in single-electrode mode, which is far higher than that of the pristine PVA hydrogel TENG. As a demonstration of the application, the g-C3N4/PVA hydrogel TENG can be adopted as electronic skin to monitor the movement of the human body. Low-frequency mechanical energy-harvesting devices in different morphologies including discoid flake shape, tube shape, and spiral shape in the single-electrode mode or contact-separation mode have been designed, fabricated, and evaluated. All of these merits of the proposed hydrogel TENG after doping two-dimensional (2D) material g-C3N4 have demonstrated their promising potential for versatile applications in biomechanical energy harvesting and self-powered sensing.

Polyoxometalate-Modified g-C3N4 Composites with High Work Function for Triboelectric Nanogenerators

Designing friction materials with high electron storage capacity, high work function, low cost, and high stability is an important method to improve the output performance of a triboelectric nanogenerator (TENG). Here, we report two kinds of friction materials based on Keggin-type polyoxometalates (POMs)-modified graphite carbon nitride (g-C3N4), namely, g-C3N4@PMo12 and g-C3N4@PW12, and form TENG with commercial indium tin oxide/poly(ethylene terephthalate) (ITO/PET) electrodes. The performance test shows that the g-C3N4@PMo12 TENG device exhibits a high output voltage of about 78 V, a current of about 657 nA, and a transfer charge of about 15 nC, which is more than 3 times higher than that of unmodified TENG. This performance improvement is attributed to the fact that POM loaded on the surface of g-C3N4 can be used as a shallow electron trap to increase the electron storage capacity through electron interaction and to increase the charge density on the surface of the material by increasing the work function of the composite. This work not only broadens the choices of TENG friction materials but also offers a practical means of enhancing TENG's output performance.

TMDC ternary alloy-based triboelectric nanogenerators with giant photo-induced enhancement

Multifunctional self-powered energy harvesting devices have attracted significant attention for wearable, portable, IoT and healthcare devices. In this study, we report transition metal dichalcogenide (TMDC) ternary alloy (Mo0.5W0.5S2)-based self-powered photosensitive vertical triboelectric nanogenerator (TENG) devices, where the ternary alloy functions both as a triboelectric layer and as a photoabsorbing material. The scalable synthesis of the highly crystalline Mo0.5W0.5S2 ternary alloy can overcome the limitations of binary TMDCs (MoS2, WS2) by utilizing its superior optical characteristics, enabling this semiconductor-based TENG device to simultaneously exhibit photoelectric and triboelectric properties. Benefiting from visible light absorption, this vertical TENG device generates higher triboelectric outputs and exhibits excellent power harvesting properties under visible light illumination. The open circuit voltage and short circuit currents of the devices under illumination (410 nm, 525 μW cm-2) are enhanced by 62% and 253%, respectively, while in the darkness, a very high photoresponsivity of ∼45.5 V mW-1 (voltage mode) is exhibited, indicating the superior energy harvesting potential under ultralow illumination. Furthermore, the energy harvesting ability from regular human activities and the operation as artificial e-skin expands the multi-functionality of this TENG device, paving a pathway for simultaneous mechanical and photonic energy harvesting with self-powered sensing.

Flexible triboelectric nanogenerators of Au-g-C3N4/ZnO hierarchical nanostructures for machine learning enabled body movement detection

Here we report the development of triboelectric nanogenerator (TENG) based self-powered human motion detector with chemically developed Au-g-C3N4/ZnO based nanocomposite on common cellulose paper platform. Compared to bare g-C3N4, the nanocomposite in the form of hierarchical morphology is found to exhibit higher output voltage owing to the contribution of Au and ZnO in increasing the dielectric constant and surface roughness. While generating power ∼3.5μW cm-2and sensitivity ∼3.3 V N-1, the flexible TENG, is also functional under common biomechanical stimuli to operate as human body movement sensor. When attached to human body, the flexible TENG is found to be sensitive towards body movement as well as the frequency of movement. Finally upon attaching multiple TENG devices to human body, the nature of body movement has been traced precisely using machine learning (ML) techniques. The execution of the learning algorithms like artificial neural network and random forest classifier on the data generated from these multiple sensors can yield an accuracy of 99% and 100% respectively to predict body movement with great deal of precision. The exhibition of superior sensitivity and ML based biomechanical motion recognition accuracy by the hierarchical structure based flexible TENG sensor are the prime novelties of the work.

From Triboelectric Nanogenerator to Multifunctional Triboelectric Sensors: A Chemical Perspective toward the Interface Optimization and Device Integration

Triboelectric nanogenerators (TENGs) have intrigued scientists for their potential to alleviate the energy shortage crisis and facilitate self-powered sensors. Triboelectric interfaces containing triboelectric functionalized molecular groups and tunable surface charge densities are important for improving the electrical output capability of TENGs and the versatility of future electronics. In this review, following an introduction to the fundamental progress of TENG systems for mechanic energy harvesting, surface modifications that aim to increase the surface charge density and functionality are highlighted, with an emphasis on interfacial chemical modification and triboelectric energetics/dynamics optimization for efficient electrostatic induction and charge transfer. Recent advances in assemblies of multifunctional triboelectric sensing are briefly introduced, and future challenges and chemical perspectives in the field of TENG-based electronics are concisely reviewed. This review presents and advances the understanding of the state-of-the-art chemical strategies toward rational triboelectric interface engineering and system assembly and is expected to guide the rational design of highly efficient and versatile triboelectric sensing.

A Facile, Fabric Compatible, and Flexible Borophene Nanocomposites for Self-Powered Smart Assistive and Wound Healing Applications

Smart fabrics that can harvest ambient energy and provide diverse sensing functionality via triboelectric effects have evoked great interest for next-generation healthcare electronics. Herein, a novel borophene/ecoflex nanocomposite is developed as a promising triboelectric material with tailorability, durability, mechanical stability, and flexibility. The addition of borophene nanosheets enables the borophene/ecoflex nanocomposite to exhibit tunable surface triboelectricity investigated by Kelvin probe force microscopy. The borophene/ecoflex nanocomposite is further fabricated into a fabric-based triboelectric nanogenerator (B-TENG) for mechanical energy harvesting, medical assistive system, and wound healing applications. The durability of B-TENG provides consistent output performance even after severe deformation treatments, such as folding, stretching, twisting, and washing procedures. Moreover, the B-TENG is integrated into a smart keyboard configuration combined with a robotic system to perform an upper-limb medical assistive interface. Furthermore, the B-TENG is also applied as an active gait phase sensing system for instantaneous lower-limb gait phase visualization. Most importantly, the B-TENG can be regarded as a self-powered in vitro electrical stimulation device to conduct continuous wound monitoring and therapy. The as-designed B-TENG not only demonstrates great potential for multifunctional self-powered healthcare sensors, but also for the promising advancements toward wearable medical assistive and therapeutic systems.

Development of Triboelectroceutical Fabrics for Potential Applications in Self-Sanitizing Personal Protective Equipment

Attachment of microbial bodies including the corona virus on the surface of personal protective equipment (PPE) is found to be potential threat of spreading infection. Here, we report the development of a triboelectroceutical fabric (TECF) consisting of commonly available materials, namely, nylon and silicone rubber (SR), for the fabrication of protective gloves on the nitrile platform as model wearable PPE. A small triboelectric device (2 cm × 2 cm) consisting of SR and nylon on nitrile can generate more than 20 V transient or 41 μW output power, which is capable of charging a capacitor up to 65 V in only ∼50 s. The importance of the present work relies on the TECF-led antimicrobial activity through the generation of an electric current in saline water. The fabrication of TECF-based functional prototype gloves can generate hypochlorite ions through the formation of electrolyzed water upon rubbing them with saline water. Further, computational modelling has been employed to reveal the optimum structure and mechanistic pathway of antimicrobial hypochlorite generation. Detailed antimicrobial assays have been performed to establish effectiveness of such TECF-based gloves to reduce the risk from life-threatening pathogen spreading. The present work provides the rationale to consider the studied TECF, or other materials with comparable properties, as a material of choice for the development of self-sanitizing PPE in the fight against microbial infections including COVID-19.

Wearable Nanogenerators: Working Principle and Self-Powered Biosensors Applications

Wearable self-powered sensors represent a theme of interest in the literature due to the progress in the Internet of Things and implantable devices. The integration of different materials to harvest energy from body movement or the environment to power up sensors or act as an active component of the detection of analytes is a frontier to be explored. This review describes the most relevant studies of the integration of nanogenerators in wearables based on the interaction of piezoelectric and triboelectric devices into more efficient and low-cost harvesting systems to power up batteries or to use the generated power to identify multiple analytes in self-powered sensors and biosensors.