Harvesters of waste mechanical energy as electricity are needed for diverse purposes, from using the energy of body motion to power wearable electronic devices to harvesting the energy of ocean waves to power cities. On very large scales, ordinary electromagnetic devices, which function as motors run in reverse, perform wonderfully in terms of energy conversion efficiency and the output power per harvester weight. However, their gravimetric energy density severely degrades when downsized to the scales needed for many important applications, and they perform best when constant mechanical energy input is provided. Hence, new or radically improved material-based technologies for harvesting mechanical energy use triboelectric, piezoelectric, dielectric elastomer, electrochemical capacitance, electrostatic induction, or moving ionic liquid effects.
The very recently developed twistron mechanical energy harvesters transformed the field by providing a peak output electrical power per harvester weight that is higher than for prior-art material-based technologies for frequencies from 0.1 Hz to 30 Hz (1, 2). In fact, the peak gravimetric power at 30 Hz (3.19 kW/kg) was over 12-fold that of prior-art material-based technologies for frequencies between 0.1 and 600 Hz (2).
These twistron mechanical harvesters are electrochemical devices that presently contain at least one carbon nanotube (CNT) yarn electrode. Either by directly changing the overall twist in the CNT yarn or by changing the length of a coiled yarn, which reversibly converts coiling twist to yarn twist, the resulting changes in yarn density produce the yarn capacitance changes that generate the voltage changes needed for energy harvesting. This mechanism explains why these harvesters are called twistrons. The needed charge for these voltage changes is automatically produced by charge injection from the electrolyte, so no voltage source is needed, unlike the case for dielectric elastomer harvesters where kilovolt power sources are typically used.
However, until our very recent work on a new type of CNT twistron harvester (3), these twistron harvesters had a major problem: their energy conversion efficiency did not exceed 7.6% (2). By changing twistron yarn structure, the energy conversion efficiency is now increased to 17.4% for elastically deforming the harvesting yarn electrode’s length and 22.4% for changing the total twist of this yarn electrode.
These newest twistrons contain plied yarns, like in ordinary polymer textiles, instead of the coiled CNT yarns that were previously used for twistrons. However, ordinary textiles have opposite chirality of yarn twist and plying twist (so they are heterochiral), while the improved twistrons have the same chirality yarn twist and plying twist (so they are homochiral). Our original coiled twistrons were self-coiled, meaning that a single CNT yarn was twisted and then coiled under a constant low tensile load. The tensile stresses reported here are normalized to the yarns cross-sectional area just below the onset of coiling, even if the yarns are eventually plied instead of coiled.
Scanning electron microscope images of these different types of CNT twistron harvester electrodes are shown in Fig. 1. These plied CNT twistrons use our tension optimization process (TOP), which previously helped increase the maximum energy conversion efficiency for coiled CNT twistrons from our initial 1.05% (1) to 4.66% (2). In contrast to our initial self-coiling process, where the stress applied during the entire self-coiling process was kept constant at about 35 MPa, the tensile stress applied during pre-plying twist insertion was about 42 MPa and the stress applied during yarn plying was much lower (about 22 MPa). Nevertheless, the output energy per cycle for the plied twistrons was 80 J kg-1 at 2 Hz, which is at least 6.7-fold that previously reported for non-twistron, material-based harvesters for this or higher frequencies. For higher frequencies than 2 Hz, the average power output of the plied twistrons substantially exceeds that of alternative material-based harvesters.
. Kim, S. H. et al. Harvesting electrical energy from carbon nanotube yarn twist. Science 357, 773-778 (2017).
. Wang, Z. et al. More powerful twistron carbon nanotube yarn mechanical energy harvesters. Adv. Mater., 2201826 (2022).
. Zhang, M. at al. Mechanical energy harvesters based on homochirally plied carbon nanotube yarns with tensile efficiency of 17.4% and torsional efficiency of 22.4%. Nat Energy (2023). https://doi.org/10.1038/s41560-022-01191-7