High-Performance Piezoelectric Nanogenerators Based on Hybrid Perovskite Nanomaterials for Energy-Harvesting

With the increasing drive to create self-powering nano/micro-electromechanical systems (NEMS/MEMS), maximizing the output performance of piezoelectric nanogenerators (PENGs) is essential. Organic-inorganic halide perovskites (OIHP) have unique advantages over century-old ceramic piezoelectrics, including mechanical flexibility, low-temperature processing, bio-friendliness, structural tunability and low cost, making them ideal candidates for electromechanical energy conversion devices and a wide range of applications. This research explores the piezoelectric properties of OIHPs and applies them to designing efficient piezoelectric nanogenerators. The novel perovskite materials which are studied here include cesium lead halide (CsPbX3), (4-aminotetrahydropyran)2 lead bromine chlorine ((ATHP)2PbBr2Cl2), (trimethylchloromethylammonium)2 tin chloride ((TMCM)2SnCl6), N-ethyl-1,4-diazoniabicyclo [2.2.2] octonium copper chloride (EDABCO-CuCl4), formamidinium (FA) lead bromine iodine (FAPbBr2I), and polystyrene (PS) functionalized FAPbBr2I. The perovskites are used to fabricate 0-3 type of piezoelectric composites (random distribution) and the PENG operation is assessed under 3-3 operational mode. The perovskite-polymer nanocomposites are employed to harvest mechanical energies from the vibration (< 50 Hz) to produce electricity and to use those nanogenerators as the power source to realize self-powered wireless sensing systems. \n \nFirstly, the dissertation discusses the piezoelectric and dielectric properties of different OIHP nanomaterials and the fundamentals of PENGs (Chapter 1). It also provides insights on halogen regulation, the synergistic effect of molecular dipole rotation with octahedral displacement, and quasi-spherical cation with Jahn-Teller distortion, thus transforming blind material search into a guided design strategy for piezoelectrics. The influence of nanomaterial distribution, geometry, porosity, dielectric constants, force, frequency, and ferroelectric manipulation of the composite film is then systematically analyzed (Chapter 2). Through a series of PENGs that are fabricated, characterized, and evaluated from halogen-tuned (I/Br) piezoelectric CsPbX3 (X=Br/Cl) (including piezo response force microscopy (PFM) and electric measurements), it is revealed that the highest piezoelectric charge constants (d33 ~ 47 pC/N) and output power density (24 μW/cm2.N) come from the CsPbBr2I PENG. This performance enhancement is attributed to the anisotropic lattice strain and elastic softness. The findings of this research emphasize the potential of inorganic CsPbX3-based PENGs in self-powered NEMS/MEMS (Chapter 3). \n \nThe second part of the dissertation outlines the use of novel OIHP nanorods of (ATHP)2PbBr2Cl2 to improve piezoelectric coefficients. These nanorods are synthesized through a ligand-assisted reprecipitation (LARP) method, which confers superior in-plane polarization, leading to d31 (transverse piezoelectric coefficient) of 3 times higher (~ 64 pC/N) than that of a commercial piezoelectric polymer of polyvinylidene fluoride (PVDF). Experiments also show that the nanorods can be dispersed in a flexible polydimethylsiloxane (PDMS) polymer, which leads to a large output voltage. The high output performance of the PENG is a result of the large surface area and bending of nanorods, which contribute to the d33 and d31 coefficients (Chapter 4). \n \nThe third part provides a design and synthesis route of a novel vacancy-ordered halide double perovskite of (TMCM)2SnCl6 to improve the d33 and g33 of piezoelectric materials simultaneously. The molecular dipole moment and atomic displacement in the inorganic [SnCl6]2- are influenced by the halogen bond between them, resulting in a large spontaneous polarization. Meanwhile, the presence of insulating molecules reduces the relative permittivity, leading to a high d33 of 137 pC/N, and g33 of 980×10-3 V•m/N. Experiments also demonstrate a high output voltage of 81 V at 40 Hz, 4.2 N in the PENG. This design showcases the potential of a heavy-metal free, highly piezoelectric materials in PENG design, which can potentially be integrated with batteries/capacitors to power an electrical circuit (Chapter 5). \n \nThe fourth part presents a green organic-inorganic metal halide EDABCO-CuCl4 to fabricate PENGs. This study on symmetry breaking highlights the independent roles of quasi-spherical molecules and spherical molecules with Jahn-Teller active and inactive metal ion complexes. With its highly confined 0D architecture and large tetrahedral distortion, it possesses a high d33 of 165 pC/N, and a massive g33 of 2,110 ×10-3 V•m/N, and its performance remains unchanged till ~ 180 oC. The high transduction coefficient produces an output power density of 43 μW/cm2 in a PENG at 50 kPa. The energy generation is also stably maintained up to ~ 160 oC. This study further demonstrates the design and fabrication of green PENGs capable of producing electrical energy in harsh environments (Chapter 6). \nThe fifth part presents a PENG design from the in-situ grown OIHP of FAPbBr2I nanoparticles (NP) that create a highly porous structure inside a piezoelectric polymer composite. The mechanism of pore formation, its characterization, and its implications on the PENG performance are explored. The porous composite increases the output piezo potential by 5-fold, while the OIHP nanoparticles enhance the relative permittivity by interfacial polarization, reduce the film conductivity and thus increase the output current by ~ 15-fold higher than the commercially available analog of PVDF. The PENG is used as a power source, successfully showcasing a self-powered wireless communication application between a PENG and smartphone, potentially realizing a proof-of-concept for future battery-independent IoT systems. The PENG is also capable of efficiently harvesting energy from the vibration of a car engine and storing it in capacitors- potentially applicable to other vibrating parts such as airframes, or the engine of an aircraft (Chapter 7). \nThe sixth part aims to enhance the output current by fabricating a cascade-type PENG structure. The research improves the OIHP's stability by functionalizing its surface with the polystyrene. As a result, the fundamental performance limiting factors of the perovskites, such as ion migration, crystallinity, and grain sizes are improved, all of which are conducive to PENG performance. The cascaded architecture by the newly designed piezoelectric composites results in a large current density of ~ 100 μA/cm2, an enhancement of more than an order of magnitude for the OIHP-based PENGs (Chapter 8). \nThis dissertation is devoted to deepening our understanding of the piezoelectric behaviour of the new perovskite nanomaterials in a soft and flexible polymer. The findings of exciting piezoelectric and dielectric properties in novel perovskite material will also have a substantial impact on many areas, including the field of energy harvesters, sensors, actuators, flexible optoelectronics, and soft robotics. At the art at which this research is developing, it is anticipated that PENG-based power sources will soon be driving portable electronics.

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