Crystalline Porous Material-Based Nanogenerators: Recent Progress, Applications, Challenges, and Opportunities.

Nanogenerator (NG) is a potential technology that allows to build self-powered systems, sensors, flexible and portable electronics in the current Internet of Things (IoT) generation. Nanogenerators include piezoelectric nanogenerators (PENGs) and triboelectric nanogenerators (TENGs), convert different forms of mechanical motion into useful electrical signals. They have evolved and expanded their applications in various fields since their discovery in 2006 and 2012. Material selection is crucial for designing efficient NGs, with high conversion efficiencies. In the recent past, crystalline porous mat erials (metal-organic frameworks (MOFs) and covalent organic frameworks (COFs)) have been widely reported as potential candidates for nanogenerators, owing to their special properties of large surface area, porosity tailoring, ease of surface, post-synthesis modification, and chemical stability. The present organized review provides a complete overview of all the crystalline porous materials (CPMs)-based nanogenerator devices reported in the literature, including synthesis, characterization, device fabrication, and potential applications. Additionally, this review article discusses current challenges, future directions, and perspectives in the field of CPMs-NGs.

Highly Anisotropic to Isotropic Nature and Ultralow Out-of-Plane Lattice Thermal Conductivity of Layered PbClF-Type Materials.

Materials with an extreme lattice thermal conductivity (κl) are indispensable for thermal energy management applications. Layered materials provide an avenue for designing such functional materials due to their intrinsic bonding heterogeneity. Therefore, a microscopic understanding of the crystal structure, bonding, anharmonic lattice dynamics, and phonon transport properties is critically important for layered materials. Alkaline-earth halofluorides exhibit anisotropy from their layered crystal structure, which is strongly determined by axial bond(s), and it is attributed to the large axial ratio (c/a > 2) for CaBrF, CaIF, and SrIF, in which Br/I acts as a rattler, as evidenced from potential energy curves and phonon density of states. The low axial (c/a) ratio leads to relatively isotropic κl values in the BaXF (X = Cl, Br, I) series. MXF (M = Ca, Sr, Ba) compounds exhibit highly anisotropic (a large phonon transport anisotropy ratio of 10.95 for CaIF) to isotropic (a small phonon transport anisotropy ratio of 1.49 for BaBrF) κl values despite their iso-structure. Moreover, ultralow κl (<1 W/m K) values have been predicted for CaBrF, CaIF, and SrIF in the out-of-plane direction due to weak van der Waals (vdWs) bonding. Overall, this comprehensive study on MXF compounds provides insights into designing low κl layered materials with a large axial ratio by fine-tuning out-of-plane bonding from ionic to vdWs bonding.

Electronically Robust Self-Assembled Supramolecular Gel as a Potential Material in Triboelectric Nanogenerators.

A series of self-assembling gluconamide conjugated naphthalimide amphiphiles (GCNA) was synthesized and the self-assembly of GCNA into gel rendered an increased electron density in naphthalimide moiety with an overall change in energy of 15.33×10-32  J via J-type aggregation. SEM analysis and X-ray diffraction underpinning the nanofibrillar formation, and rheological measurements confirmed the processablity and material fabrication. The enriched electron density in the aggregated GCNA4 via cooperative intermolecular non-covalent interactions makes it as effective electron donor in the fabrication of triboelectric nanogenerators (TENG). The TENG based on GCNA4-polydimethylsiloxane (PDMS) triboelectric pair generated an output voltage, current and power density of ∼250 V, 40 µA and ∼622 mW/m2 respectively, which is almost 2.4 times better in performance than the amorphous GCNA4 based TENG. The fabricated TENG can power-up 240 LEDs, wrist watch, thermometer, calculator and hygrometer.

Unique Contact Point Modification Technique for Boosting the Performance of a Triboelectric Nanogenerator and Its Application in Road Safety Sensing and Detection.

A triboelectric nanogenerator (TENG) is a potential technique that can convert waste kinetic energy to electrical energy by contact separation followed by electrostatic induction. Herein, a unique contact point modification technique has been reviewed carefully via the enlargement of the effective surface area of the tribo layer by using a simple and scalable printing method. In this study, the zinc sulfide (ZnS) nanostructure morphology has been introduced directly on an aluminum electrode (Al) as a tribo positive layer by a modified hydrothermal method and different line patterns directly printed on overhead projector (OHP) transparent sheets by a monochrome laser printer as a tribo negative layer to increase the effective contact area and work-function difference between two tribo layers. This dual parameter results in ∼11 times increment in the open-circuit output voltage (∼420 V) and ∼17 times increment in the short-circuit current density (∼83.33 mA m-2) compared to the normal one. Furthermore, with the proposed surface modification technique, an ultrahigh instantaneous output power density of ∼3.9 W m-2 at a load resistance of 2 MΩ was easily achieved. The direct energy conversion efficiency reached up to 66.67% at 2 MΩ load, which is very high compared to other traditional TENGs. Further, the fabricated TENG demonstrated efficacy in novel road safety sensing applications in hilly areas to control vehicle movement. Therefore, the current idea of surface engineering using a laser printer will be helpful for energy-harvesting enthusiasts to develop more efficient nanogenerators for higher energy conversions.