Enhanced Output Performance of Piezoelectric Nanogenerators by Tb-Modified (BaCa)(ZrTi)O3 and 3D Core/shell Structure Design with PVDF Composite Spinning for Microenergy Harvesting

External strategies for enhanced sensing performance of self-powered polyvinylidene fluoride-based sensors

The era of the Internet of Things has created an increasing demand for self-powered, flexible sensors. Among various intelligent materials, poly(vinylidene fluoride) (PVDF) has emerged as a promising candidate due to its flexibility, processability, biocompatibility, and unique electroactive properties. PVDF's distinctive piezoelectric, pyroelectric and triboelectric characteristics make it particularly suitable for self-powered flexible sensing applications. While research has primarily focused on enhancing the electroactive β phase, PVDF-based sensors still face limitations in their piezoelectric and pyroelectric performance. External strategies such as electrode design, stress/heat transfer improvements, microstructure optimization, and multifunctional synergy show great potential for improving sensing performance. Although numerous reviews address PVDF's polar phase enhancement, there is limited literature overviewing external strategies for performance optimization. This review focuses on external strategies for enhancing the sensing performance of PVDF-based sensors and their emerging applications. It also addresses practical challenges and future directions in PVDF-based sensor development.

Boosting Energy Harvesting Performance in Piezoelectric Composites with Aligned Porosity via a Dual Structure Design Strategy

Porous piezoelectric materials have attracted much interest in the fields of sensing and energy harvesting owing to their low dielectric constant, high piezoelectric voltage coefficient, and energy harvesting figure of merit. However, the introduction of porosity can decrease the piezoelectric coefficient, which restricts the enhancement of output current and power density. Herein, to overcome these challenges, an array-structured piezoelectric composite energy harvester with aligned porosity was constructed via a dual structure design strategy to enhance the output current and power density. Silver metal particles were introduced into a porous barium calcium zirconate titanate (BCZT) ceramic matrix as a secondary phase to regulate the dielectric, ferroelectric, and piezoelectric properties. The optimal addition of silver particles can reduce the size of ferroelectric domains, which leads to an enhanced piezoelectric coefficient and energy harvesting figure of merit. Combined with the design of the array structure, the maximum output voltage and current of the piezoelectric composite energy harvester can reach 76 V and 340 μA, respectively, with a peak power density of 0.97 mW/cm2. Furthermore, the array-structured piezoelectric energy harvester can light 40 blue LED bulbs and power commercial low-power electronic devices. This work provides a strategy to boost the output current and power density of porous piezoelectric materials via a micro-macro structure design strategy for energy harvesting applications.

Ingenious Structure Engineering to Enhance Piezoelectricity in Poly(vinylidene fluoride) for Biomedical Applications

The future development of wearable/implantable sensing and medical devices relies on substrates with excellent flexibility, stability, biocompatibility, and self-powered capabilities. Enhancing the energy efficiency and convenience is crucial, and converting external mechanical energy into electrical energy is a promising strategy for long-term advancement. Poly(vinylidene fluoride) (PVDF), known for its piezoelectricity, is an outstanding representative of an electroactive polymer. Ingeniously designed PVDF-based polymers have been fabricated as piezoelectric devices for various applications. Notably, the piezoelectric performance of PVDF-based platforms is determined by their structural characteristics at different scales. This Review highlights how researchers can strategically engineer structures on microscopic, mesoscopic, and macroscopic scales. We discuss advanced research on PVDF-based piezoelectric platforms with diverse structural designs in biomedical sensing, disease diagnosis, and treatment. Ultimately, we try to give perspectives for future development trends of PVDF-based piezoelectric platforms in biomedicine, providing valuable insights for further research.

High output flexible polyvinylidene fluoride based piezoelectric device incorporating cellulose nanofibers/BaTiO3@TiO2 piezoelectric core-shell structure

Flexible composite film has gained increasing attention in the fields of wearable devices and portable electronic products. In this work, a novel core-shell structure of cellulose nanofibers/BaTiO3@TiO2 (CNF/BTO@TiO2) was synthesized with the assistant of the biological macromolecule material of cellulose nanofiber (CNF), in which the CNF can improve the stability and dispersibility of BaTiO3 (BTO) in the aqueous phase and elevate the integrity of the core-shell structure. The core-shell structure can reduce the agglomeration of fillers in polyvinylidene fluoride (PVDF) and improve the structural defects of the composite film. Meanwhile, the core-shell structure can promote the polarization of the electric dipole and the formation of β phase in PVDF due to the generated interface spatial polarization between the shell of TiO2 and the core of BTO. When the content of the core-shell structure was 5 wt%, the β phase content reaches 61.89 %, and the piezoelectric coefficient of composite film reaches 84.29 pm/V. Thus the maximum output open-circuit voltage (VOC) and short-circuit current (ISC) of the piezoelectric composite film is as high as 13.10 V and 464.3 nA. In addition, its excellent pressure sensing capability allows for its application in various flexible electronic devices.

Giant piezoresponse in nanoporous (Ba,Ca)(Ti,Zr)O3 thin film

Lattice strain effects on the piezoelectric properties of crystalline ferroelectrics have been extensively studied for decades; however, the strain dependence of the piezoelectric properties at nano-level has yet to be investigated. Herein, a new overview of the super-strain of nanoporous polycrystalline ferroelectrics is reported for the first time using a nanoengineered barium calcium zirconium titanate composition (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 (BCZT). Atomic-level investigations show that the controlled pore wall thickness contributes to highly strained lattice structures that also retain the crystal size at the optimal value (<30 nm), which is the primary contributor to high piezoelectricity. The strain field derived from geometric phase analysis at the atomic level and aberration-corrected high-resolution scanning transmission electron microscopy (STEM) yields of over 30% clearly show theoretical agreement with high piezoelectric properties. The uniqueness of this work is the simplicity of the synthesis; moreover the piezoresponse d 33 becomes giant, at around 7500 pm V-1. This response is an order of magnitude greater than that of lead zirconate titanate (PZT), which is known to be the most successful ferroelectric over the past 50 years. This concept utilizing nanoporous BCZT will be highly useful for a promising high-density electrolyte-free dielectric capacitor and generator for energy harvesting in the future.

Highly Multifunctional Performances of Boron Nitride Nanosheets/Polydimethylsiloxane Composite Foams

Although this kind of hexagonal boron nitride (h-BN)-filled polydimethylsiloxane (PDMS) multifunctional composite foam has been greatly expected, its development is still relatively slow as a result of the limitation of synthetic challenge. In this work, a new foaming process of BNNSs-PDMS, alcohol, and water three-phase emulsion system is employed to synthesize a series of high-quality BNNSs/PDMS composite foams (BSFs) filled with highly functional and uniformly distributed BNNSs. As a result of well-bonded interfaces between the BNNSs and PDMS, enhanced multiple functions of BSFs appeared. The BSFs can show complete resilience at a compressive strain of 90% and only 3.99% irreversible deformation after 100,000 compressing-releasing hyperelastic cycles at a strain of 60%. On the basis of their outstanding shape-memory properties, the maximum voltage value of compression-driven piezo-triboelectric (CDPT) responses of the BSFs is up to ∼20 V. Depending on the remarkable super-elastic and CDPT performances, the BSFs can be used for sensitive sensing of temperature difference and electromechanical responses. Also, in the range of 12-40 GHz, the BSF materials display ultralow dielectric constants between 1.1 and 1.4 with proper dielectric loss tangent values of <0.3 and exhibit an enhanced and broadened sound adsorption capacity ranging from 500 to 6500 Hz. Although BSFs have high porosities of >65%, their thermal conductivities can still reach up to 0.407 ± 0.039 W m^-1 K^-1. Moreover, the BSF materials display favorable thermal stability, obviously reduced coefficient of thermal expansion, and good flame retardancy. All of these properties render the BSFs as a new category of excellent multifunctional material.

Structure design and wireless transmission application of hybrid nanogenerators for swinging mechanical energy and solar energy harvesting

With the rapid development of the Internet of Things, the maintenance-free and reliable power supply of widely distributed sensors is still a huge challenge, especially in wireless areas. Wireless power transmission is expected to alleviate the issue that the sensors must be connected by wire to power supply devices. Herein, a novel hybrid nanogenerator combining a triboelectric nanogenerator (TENG) and photovoltaic cell has been demonstrated, which can realize the simultaneous collection and wireless power transmission of swinging mechanical energy and solar energy. The wireless power transmission system based on the hybrid nanogenerator can be actualized through series connection of the TENG and photovoltaic cell with the aid of a specifically designed mechanical switch, enabling the system to generate DC pulses that favor transmitting energies through LC oscillation and a coupled receiver coil. At the receiver coil end, the open-circuit voltage (V_oc) of the hybrid nanogenerator can reach 80 V, showing excellent wireless output performance and the rationality of the wireless power transmission circuit. Moreover, the hybrid nanogenerator can wirelessly power a commercial temperature-humidity meter, which indicates the remarkable potential of improving the layout flexibility of sensor nodes. This work successfully realizes the wireless power transmission of hybrid nanogenerator-harvested swinging mechanical energy and solar energy by a simple and feasible circuit design, which can enrich the form of micro/nano energy adapted to wireless energy transmission.

Significant Electromechanical Characteristic Enhancement of Coaxial Electrospinning Core-Shell Fibers

Electrospinning is a low-cost and straightforward method for producing various types of polymers in micro/nanofiber form. Among the various types of polymers, electrospun piezoelectric polymers have many potential applications. In this study, a new type of functional microfiber composed of poly(γ-benzyl-α,L-glutamate) (PBLG) and poly(vinylidene fluoride) (PVDF) with significantly enhanced electromechanical properties has been reported. Recently reported electrospun PBLG fibers exhibit polarity along the axial direction, while electrospun PVDF fibers have the highest net dipole moment in the transverse direction. Hence, a combination of PBLG and PVDF as a core-shell structure has been investigated in the present work. On polarization under a high voltage, enhancement in the net dipole moment in each material and the intramolecular conformation was observed. The piezoelectric coefficient of the electrospun PBLG/PVDF core-shell fibers was measured to be up to 68 pC N^-1 (d₃₃), and the voltage generation under longitudinal extension was 400 mVpp (peak-to-peak) at a frequency of 60 Hz, which is better than that of the electrospun homopolymer fibers. Such new types of functional materials can be used in various applications, such as sensors, actuators, smart materials, implantable biosensors, biomedical engineering devices, and energy harvesting devices.