Since their initial development in 2012, triboelectric nanogenerators (TENGs) have gained popularity worldwide as a desired option for harnessing energy. The urgent demand for TENGs is attributed to their novel structural design, low cost, and use of large-scale materials. The output performance of a TENG depends on the surface charge density of the friction layers. Several recycled and biowaste materials have been explored as friction layers to enhance the output performance of TENGs. Natural and oceanic biomaterials have also been investigated as alternatives for improving the performance of TENG devices. Moreover, structural innovations have been made in TENGs to develop highly efficient devices. This review summarizes the recent developments in recycling and biowaste materials for TENG devices. The potential of natural and oceanic biowaste materials is also discussed. Finally, future outlooks for the structural developments in TENG devices are presented.

Recent developments in the field of energy harvesting technology that convert ambient energy resources into electricity enable the use of self-powered energy systems in wearable and portable electronic devices without the need for additional external power sources. In particular, piezoelectric-effect-based flexible energy harvesters have drawn much attention because they can guarantee power generation from ubiquitous mechanical and vibrational movements. In response to demand for sustainable, permanent, and remote use of real-life personal electronics, many research groups have investigated flexible piezoelectric energy harvesters (f-PEHs) that employ nanoscaled piezoelectric materials such as nanowires, nanoparticles, nanofibers, and nanotubes. In those attempts, they have proven the feasibility of energy harvesting from tiny periodic mechanical deformations and energy utilization of f-PEH in commercial electronic devices. This review paper provides a brief overview of f-PEH devices based on piezoelectric nanomaterials and summarizes the development history, output performance, and applications.

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• Development and Characterization of Hafnium-Doped BaTiO3 Nanoparticle-Based Flexible Piezoelectric Devices
  HakSu Jang, Hyeon Jun Park, Gwang Hyeon Kim, Gyoung-Ja Lee, Jae-Hoon Ji, Donghun Lee, Young Hwa Jung, Min-Ku Lee, Changyeon Baek, Kwi-Il Park
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  Dong Yeol Hyeon, Kwi-Il Park
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  Gyoung-Ja Lee, Min-Ku Lee, Jin-Ju Park, Dong Yeol Hyeon, Chang Kyu Jeong, Kwi-Il Park
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Porous polytetrafluoroethylene (PTFE) thin films are fabricated by spin-coating using a dispersion solution containing PTFE powders, and their crystalline properties are investigated after thermal annealing at various temperatures ranging from 300 to 500°C. Before thermal annealing, the film is densely packed and consists of many granular particles 200-300 nm in diameter. However, after thermal annealing, the film contains many voids and fibrous grains on the surface. In addition, the film thickness decreases after thermal annealing owing to evaporation of the surfactant, binder, and solvent composing the PTFE dispersion solution. The film thickness is systematically controlled from 2 to 6.5 μm by decreasing the spin speed from 1,500 to 500 rpm. A triboelectric nanogenerator is fabricated by spin-coating PTFE thin films onto polished Cu foils, where they act as an active layer to convert mechanical energy to electrical energy. A triboelectric nanogenerator consisting of a PTFE layer and Al metal foil pair shows typical output characteristics, exhibiting positive and negative peaks during applied strain and relief cycles due to charging and discharging of electrical charge carriers. Further, the voltage and current outputs increase with increasing strain cycle owing to accumulation of electrical charge carriers during charge-discharge.

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• Recent Development in Performance Enhancement of PVDF-Nanopowder Composite-based Energy Harvesting Devices
  Geon-Ju Choi, Il-Kyu Park
  Journal of Korean Powder Metallurgy Institute.2020; 27(3): 247.     CrossRef

We report on the succesful fabrication of ZnO nanorod (NR)-based robust piezoelectric nanogenerators (PNGs) by using Cu foil substrate. The ZnO NRs are successfully grown on the Cu foil substrate by using all solution based method, a two step hydrothermal synthesis. The ZnO NRs are grown along c-axis well with an average diameter of 75~80 nm and length of 1~1.5 μm. The ZnO NRs showed abnormal photoluminescence specrta which is attributed from surface plasmon resonance assistant enhancement at specific wavelength. The PNGs on the SUS substrates show typical piezoelectric output performance which showing a frequency dependent voltage enhancement and polarity dependent charging and discharging characteristics. The output voltage range is 0.79~2.28 V with variation of input strain frequency of 1.8~3.9 Hz. The PNG on Cu foil shows reliable output performance even at the operation over 200 times without showing degradation of output voltage. The current output from the PNG is 0.7 μA/cm² which is a typical output range from the ZnO NR-based PNGs. These performance enhancement is attributed from the high flexibility, high electrical conductivity and excellent heat dissipation properties of the Cu foil as a substrate.

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• Fabrication of a Graphene/ZnO based p-n junction device and its ultraviolet photoresponse properties
  Young-Tae Kwon, Sung-Oong Kang, Ji-Ae Cheon, Yoseb Song, Jong-Jin Lee, Yong-Ho Choa
  Applied Surface Science.2017; 415: 2.     CrossRef
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  Il-Kyu Park
  Journal of Korean Powder Metallurgy Institute.2017; 24(2): 102.     CrossRef
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  Il-Kyu Park
  Journal of Korean Powder Metallurgy Institute.2016; 23(4): 270.     CrossRef
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  Seong-Ho Baek, Il-Kyu Park
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