An Omnidirectionally Stretchable Piezoelectric Nanogenerator Based on Hybrid Nanofibers and Carbon Electrodes for Multimodal Straining and Human Kinematics Energy Harvesting

Stretchable piezoelectric nanogenerators (SPENGs) for human kinematics energy harvesting have limited use due to the low stretchability or mechanical robustness and the difficulty of structural design for omnidirectional stretchability. This study reports an efficient, omnidirectionally stretchable, and robust SPENG based on a stretchable graphite electrode on a 3D micropatterned stretchable substrate and a stacked mat of piezoelectric nanofibers. The stacked mat of free-standing nanofibers is alternatively composed of nanocomposite nanofibers of barium titanate nanoparticles embedded in polyurethane and poly(vinylidene fluoride-trifluoroethylene) nanofibers. The nanofiber SPENG (nf-SPENG) exhibits a high stretchability of 40% and high mechanical durability up to 9000 stretching cycles at 30% strain, which are attributed to the stress-relieving nature of the 3D micropattern on the substrate and the free-standing stacked hybrid nanofibers. The nf-SPENG produces a peak open circuit voltage (Voc) and short circuit current (Isc) of 9.3 V and 189 nA, respectively. The nf-SPENG is demonstrated to harvest the energy from human kinematics while walking when placed over the knee cap of a subject, generating a maximum Voc of 10.1 V. The omnidirectional stretchability, efficiency, facile fabrication process, mechanical durability, environmentally friendly lead-free components, and response to multimodal straining make this device suitable for self-powered wearable sensing systems.