Triboelectric nanogenerators (TENGs) are attracting more and more attention since they can convert various mechanical energies into electric energy. However, in traditional TENGs for harvesting rotation energy, most of the contacts between two triboelectric materials are rigid-to-rigid contact with very large friction force, which limits their practical application. Here, we report an ultra-low-friction triboelectric-electromagnetic hybrid nanogenerator (NG). A freestanding mode TENG and a rotating electromagnetic generator (EMG) are integrated together to realize the complementary individual merits. The very soft and elastic contact between the two triboelectric materials in the TENG results into very small friction force. The influences of the type and the dimensions of the dielectric material on the performance of the TENG are studied systematically from theory to experiments. The results indicate that the open-circuit voltage and the transfer charge of the TENG increase with the rotation speed, which is very different from a traditional rotary TENG and is due to the increase of the contact area. The optimized TENG has a maximal load voltage of 65 V and maximal load power per unit mass of 438.9 mW/kg under a speed rotation of 1000 rpm, while the EMG has a maximal load voltage of 7 V and maximal load power density of 181 mW/kg. This demonstration shows that the hybrid NG can power a humidity/temperature sensor by converting wind energy into electric energy when the wind speed is 5.7 m/s. Meanwhile, it can be used as a self-powered wind speed sensor to detect wind speed as low as 3.5 m/s.

Wave energy is one of the most available energy sources in oceans. In this work, a design of high power density triboelectric nanogenerator (TENG) based on a tower structure is proposed for harvesting wave energy from arbitrary directions. Such tower-like TENG (T-TENG) consists of multiple units made of polytetrafluoroethylene balls and three-dimensional printed arc surface coated with melt adhesive reticulation nylon film. The power generation model coupled with the kinetic model for the T-TENG is proposed and discussed. The T-TENG can effectively convert arbitrary directional wave energy into electrical energy by utilizing charged balls rolling on an optimized arc surface due to ocean wave excitation. In addition, it is found that the power density of the present T-TENG increases linearly from 1.03 W/m3 to 10.6 W/m3 by increasing the units from 1 to 10 in one block. This supports that the power density of the T-TENG increases proportionally with the number of units connected in parallel without rectifiers due to its distinctive mechanism and structure. Therefore, the design of T-TENG provides an innovative and effective approach toward large-scale blue energy harvesting by connecting more blocks to form T-TENG networks.

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