Polymer/Barium Titanate Composites Based Flexible Piezoelectric/Triboelectric Nanogenerators

Flexible piezoelectric nanogenerators（PENGs）and triboelectric nanogenerators（TENGs）can spontaneously convert various forms of mechanical energy in the surrounding environment into electrical energy,facilitating the realization of the miniaturization,integration and self-powered ability of microelectronic products and systems.Therefore,PENGs and TENGs have become a research hotspot in the field of renewable energy,and play a prominent role in the fields of driving micro-nano electronics and self-powered sensors.At present,the research of PENGs and TENGs has yielded some achievements.However,the materials and devices still have the problems:low piezoelectric and dielectric coeffecients of piezoelectric polymer,low loading contents,weak dispersion ability and surface stress of piezoelectric fillers in polymer matrix,complex structure of the hybrid device,and low surface charge density of the polymer based positive friction layer.The above problems can lead to low electrical output of the corresponding devices,which still limit their practical applications.Therefore,in this paper,barium titanate（BaTiO₃）ceramics with high piezoelectric and dielectric constants are used as fillers to construct a variety of new nanocomposites and flexible devices with different assembly structures.For different composite systems,different approaches for improving and optimizing the output performance of the flexible nanogenerators have been designed.The influence of the composite structure on mechanical and electrical properties of the material,and the relationship between the mechanical conditions on the output performance of the nanogenerator devices have been investigated.In addition,their applications in wearable devices,biomechanical energy harvesting and self-powered sensing have been fully explored.The main contents of this paper can be summarized as follows:Firstly,the flexible piezoelectric Poly（vinylidene fluoride）（PVDF）exhibits relatively low piezoelectric and dielectric coefficients,thus resulting weak piezoelectric outputs.Moreover,even though the addition of unitary filler in the polymer can improve the piezoelectric performance of the composites,but the improvement is still limited.Hence,in this work,graphene nanosheets and BaTiO₃ nanoparticals（NPs）were applied to blend with PVDF,and the ternary composite nanofibers with oriented structure were successfully prepared by electrospinning technology.Benefit from the hydrogen bond interaction between BaTiO₃ and PVDF,and the electrostatic interaction between graphene and PVDF,theβphase in PVDF can reach as high as 91.1%.In addition,the excellent mechanical and electrical properties of graphene can enhance the local stress and dielectric constant of PVDF.The BaTiO₃ NPs can provide the extra piezoelectric putputs.Therefore,the PENG with optimized composite nanofibers has an open circuit voltage of 11 V and an electrical power of 4.1μW.The voltage is 4.3 times higher than the output voltage of PVDF fibers.Due to the oriented structure,the anisotropic outputs can achieve a high ratio of 1:4.1.Under fast bending by fingers,the PENG can generate a peak voltage of 112 V,which can light up 15 LEDs and drive an electronic watch.The flexible PENG based on nanocomposite fibers can be easily fabricated and is expected to become an effective power source for portable electronics.Secondly,the intrinsic lowβphase proportion of PVDF leads to poor piezoelectric performance,and the realization of uniform dispersion of BaTiO₃ filler in the composite is difficult.In addition,limited by zero-dimensional structure,BaTiO₃ NPs exhibit low surface stress under the lateral bending deformation of the composite.Therefore,piezoelectric polymer PVDF-Tr FE with a high piezoelectricβphase proportion was applied as the polymer matrix.BaTiO₃ nanowires（NWs）with a high length-diameter ratio were synthesized by hydrothermal method,and then were grafted with poly（methyl methacrylate）（PMMA）by atom transfer radical polymerization technology,forming the PMMA@BaTiO₃ NWs with core-shell structure.The PMMA shell can not only reduce the surface activation energy of BaTiO₃ NWs and promote their uniform dispersion in PVDF-Tr FE matrix,but also strengthen the local stress on the BaTiO₃ NWs surface due to its high Young’s modulus.The output voltage and current of PMMA@BaTiO₃/PVDF-Tr FE composite fiber-based PENG device can be as high as 12.6 V and 1.30μA at the filler content of 10 wt%,which are 2.6 times and 2.8 times higher than the PVDF-Tr FE fibers based PENG,respectively.In addition,the PENG can output a power of 4.25μW,and there is no performance decline after 6000 mechanical cycles in the stability test.The PENG shows high mechanical sensitivity in the application fields of biomechanical energy harvesting and human motion monitoring,which demonstrates its great prospects in the application of smart wearable devices.Then,the content of the piezoelectric BaTiO₃ in the composite is a crucial role for the piezoelectric output performance;and the device based on single piezoelectric mechanism restricts the further improvement of its outputs.In this work,cellulose nanofibers and BaTiO₃ nanoparticles were used to prepare a porous composite aerogel by sol-gel method and freeze-drying technique,and then the flexible composite was obtained by pouring and curing polydimethylsiloxane（PDMS）silicone rubber.Due to the higher specific surface area of cellulose and the stronger hydrogen bond interaction with BaTiO₃,piezoelectric BaTiO₃ nanoparticles are uniformly dispersed in the composite aerogel,and the content of BaTiO₃ can be as high as 80.5 wt%（32.5 vol%）.Under the external stress of 80 k Pa,the maximum output voltage and power of the PENG can reach 15.5 V and 11.8μW,respectively.By sharing electrodes with piezoelectric devices,and applying PDMS and human skin as friction materials,a single-electrode triboelectric structure was introduced to form an integrated piezoelectric-triboelectric hybrid nanogenerator with a simple structure.Different poling directions in the PENG can induce different piezoelectric-triboelectric coupling effects.The positive coupling effect can significantly increase the output voltage and power of the hybrid device to 48V and 85μW,respectively.This research provides new methods and insights for the design and preparation of hybrid nanogenerators with simple structure,high flexibility,high output performance and easy operation.Finally,in terms of the triboelectric structure,polymers as the positive friction layers have relatively low surface charge density,which is the key factor in limiting the performance improvement of the TENGs.In response to the above problem,this paper applied degradable porous cellulose paper and PDMS silicone rubber as positive and negative friction materials to assemble triboelectric devices.Nanoparticles（i.e.,BaTiO₃,Ag）were used to modulate the permittivity and surface charge density of the two friction materials and improve the triboelectric outputs of the TENGs.After the dielectric modulation,the permittivity of the cellulose increases from 2.04 to 6.25,and the permittivity of PDMS increase from 2.45 to 2.95 at the frequency of 10³.The output voltage of the TENG can reach 88 V,and the current can reach 8.3μA with a transfer charge of 35 n C and an instantaneous power of 141μW.The performance of the device shows no decline after 5000 operating cycles.The TENG can be successfully applied as a sensor unit in a wireless transmission system,which can remotely monitor the operation of the machine and transmit messages from finger movements.In addition,the TENG can be also applied as an efficient power supply in the electro-polymerization system for the electropolymerization of polyaniline on the carbon nanotube electrode,which shows great potential in fabricating high-capacity electrodes for supercapacitors.This research provides a simple and effective method for constructing high-performance TENG,and promotes the practical applications of the TENG devices in the fields of wireless transmission and electro-polymerization system.