Anisotropic Poly(vinylidene fluoride-co-trifluoroethylene)/MXene Aerogel-Based Piezoelectric Nanogenerator for Efficient Kinetic Energy Harvesting and Self-Powered Force Sensing Applications

Lightweight flexible piezoelectric devices have garnered significant interest over the past few decades due to their applications as energy harvesters and wearable sensors. Among different piezoelectrically active polymers, poly(vinylidene fluoride) and its copolymers have attracted considerable attention for energy conversion due to their high flexibility, thermal stability, and biocompatibility. However, the orientation of polymer chains for self-poling under mild conditions is still a challenging task. Herein, anisotropic poly(vinylidene fluoride-co-trifluoroethylene) (PVDF-TrFE)/MXene aerogel-based piezoelectric generators with highly oriented MXene fillers are fabricated. The unidirectional freezing of a hybrid solution facilitates the strain-induced alignment of MXene nanosheets and polymer chains along the solvent crystal growth direction due to the robust interactions between the MXene nanosheets (O-H/F groups) and PVDF-TrFE chains (F-C/C-H groups). Consequently, this process fosters the development of abundant electroactive beta crystals with preferred alignment characteristics, leading to the formation of intrinsic self-oriented dipoles within the PVDF-TrFE aerogel. As a result, the piezoelectric properties of PVDF-TrFE are fully harnessed without any complex poling process, resulting in an open-circuit voltage of around 40 V with MXene loading of 3 wt % in anisotropic aerogel, which is 2-fold higher than that of the corresponding isotropic aerogel where the MXene nanosheets and polymer chains are randomly aligned. Furthermore, the developed piezoelectric nanogenerator was demonstrated as a tactile sensor which showed a high sensitivity of 9.6 V/N for lower forces (less than 2 N) and a sensitivity of 1.3 V/N in the higher force regime (2 N < force < 10 N). The strategy adopted here not only provides the enhancement of the piezoelectric crystalline form for self-poling but also paves an avenue toward developing self-powered energy harvesters using piezoelectric polymers.