Nanogenerators have been demonstrated as a high-efficiency energy harvesting technology, while methods to evaluate merits of nanogenerators are being focused. Energy density, as a common way for evaluating energy devices, is believed to be strongly related to the output performance limit of nanogenerators. Hence, this report introduces an evaluation standard, the output energy density, to evaluate the performance of nanogenerators. With the sliding freestanding mode TENG as an example, theoretical simulations are conducted and experimental methods are developed to understand and optimize the maximal output energy density, with the breakdown effect considered. By comparing the output energy density of TENGs and other nanogenerators, sliding-triggered TENGs are demonstrated with the highest output energy density, which is approaching 1 × 104 J/m3. This study demonstrates the advantages of sliding-triggered TENGs in output energy density due to the suppressed breakdown effect and further confirmed the "killer application" of TENGs in harvesting low-frequency and small-scale mechanical energy.
Keywords: breakdown effect; electromagnetic generator; maximal output energy density; piezoelectric nanogenerator; triboelectric nanogenerator.