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.

1. Introduction
2. Powder and particle based TENG.
    2.1 Recycled bio-waste-based TENG
    2.2 Natural and Ocean materials based TENG
    2.3 Particle based TENG.
3. Conclusions
Acknowledgments

• Rayyan Ali Shaukat(Division of Advanced Materials Engineering, Jeonbuk National University, Jeonju, Jeonbuk 54896, Republic of Korea)
• Jihun Choi(Division of Advanced Materials Engineering, Jeonbuk National University, Jeonju, Jeonbuk 54896, Republic of Korea)
• Chang Kyu Jeong(Division of Advanced Materials Engineering, Jeonbuk National University, Jeonju, Jeonbuk 54896, Republic of Korea, Department of Energy Storage/Conversion Engineering of Graduate School (BK21 FOUR) & Hydrogen and Fuel Cell Research Center, Jeonbuk National University, Jeonju, Jeonbuk 54896, Republic of Korea, Department of JBNU-KIST Industry-Academia Convergence Research, Jeonju, Jeonbuk 54896, Republic of Korea) Corresponding author