Directional freezing-induced self-poled piezoelectric nylon 11 aerogels as high-performance mechanical energy harvesters

Polymeric piezoelectrics have been widely applied for energy harvesting applications due to their rapid processability, mechanical flexibility, and self-poling capability. Besides the dominantly used polyvinylidene fluoride and its copolymers, odd-nylons crystallized into the piezoelectric phase attracted interest in energy harvesting applications owing to their better thermal stability. However, realization of the piezoelectric phase with the preferred crystal orientation for self-poling in nylons has remained elusive under mild conditions. Herein, we demonstrate the facile preparation of unprecedented isotropic and anisotropic nylon-11 aerogels, and such aerogels are used to fabricate piezoelectric nanogenerators. The nylon-11 solution was frozen isotropically to yield isotropic aerogels, while unidirectional freezing resulted in anisotropic aerogels. The unidirectional freezing of nylon-11 solutions enabled the orientation of polymer crystals along the freezing direction. The as-fabricated aerogels crystallized predominantly into polar piezoelectric γ crystals and exhibit high thermal and mechanical stabilities. The mechanical energy harvesting performance of nylon-11 aerogels is studied by fabricating piezoelectric nanogenerators, among which the anisotropic aerogel delivers a higher peak-to-peak open circuit voltage of ∼30 V, which is 4-fold as compared to that of the isotropic aerogel. Furthermore, the developed piezoelectric nanogenerators with anisotropic aerogels delivered an excellent peak power density of 0.1 W m−3 and demonstrated for powering various electronic gadgets. This strategy provides a practical avenue for the self-poling of semicrystalline piezoelectric polymers, which is an efficient and readily scalable approach for energy-harvesting applications.