Matryoshka-inspired hierarchically structured triboelectric nanogenerators for wave energy harvesting

There is a tremendous interest to harvest ocean wave energy due to its promising advantages of high-power density, wide distribution, independence of time of day, weather or seasons, and sustainability. However, most of the current energy harvesters or converters based on electromagnetic generators are suffering from unsatisfactory efficiency at low ocean wave triggering frequency and have the drawbacks of complex design, high cost and ease of corrosion in seawater. In this work, we report a matryoshka-inspired hierarchically structured triboelectric nanogenerator (HS-TENG) by nest-assembling multiple shells with decreasing sizes for effectively harvesting low-frequency wave energies with low-cost, high power density and high efficiency. As a proof-of-concept, we have fabricated a three-hierarchical-level HS-TENG by nesting three spherical acrylic shells filled with polytetrafluoroethylene (PTFE) balls into the gap spaces between the neighboring shells. The important factors that may affect the energy harvesting performance of the HS-TENG are studied, including the size and number of moving balls, the diameter of the holding shell, the amplitude and frequency of wave actuation, as well as the orientation angle between wave motion direction and electrode gap. It is demonstrated that the HS-TENG has superior output performance over the traditional single ball TENG (SB-TENG), with a maximum output peak power of 544 mu W at a low frequency of 2 Hz, more than 6.5 times of that obtained by a SB-TENG with the same size, capable of driving different wearable electronics. Based on the optimized design, we further demonstrate that a HS-TENG network formed by a 3 x 3 device array can directly lighten dozens of light-emitting diodes and power an electronic thermometer for monitoring water condition, indicating its capability in effectively harvesting water wave energy for potential large-scale deployment in the ocean. This study provides an innovative hierarchical structure that can effectively improve the output performance of TENGs and paves the way for developing next-generation high-performance TENGs for blue energy harvesting.