The development of artificial intelligence and the Internet of things has motivated extensive research on self-powered flexible sensors. The conventional sensor must be powered by a battery device, while innovative self-powered sensors can provide power for the sensing device. Self-powered flexible sensors can have higher mobility, wider distribution, and even wireless operation, while solving the problem of the limited life of the battery so that it can be continuously operated and widely utilized. In recent years, the studies on piezoelectric nanogenerators (PENGs) and triboelectric nanogenerators (TENGs) have mainly concentrated on self-powered flexible sensors. Self-powered flexible sensors based on PENGs and TENGs have been reported as sensing devices in many application fields, such as human health monitoring, environmental monitoring, wearable devices, electronic skin, human–machine interfaces, robots, and intelligent transportation and cities. This review summarizes the development process of the sensor in terms of material design and structural optimization, as well as introduces its frontier applications in related fields. We also look forward to the development prospects and future of self-powered flexible sensors.

With the surge in demand for green energy, nanocomposites composed of piezoelectric polymers and nano-piezoelectric ceramics show great prospects in preparing excellent performance piezoelectric nanogenerators (PENGs). However, the agglomeration of the piezoelectric enhancement phase and the low degree of polarization significantly impair the piezoelectric properties of the nanofibers; thus, the output performance of PENGs is severely limited. In this work, barium titanate nanowires piezoelectric ceramics were synthesized by the hydrothermal method. By surface-initiated polymerization, the hyperbranched barium titanate nanowire was surface-grafted with poly­(methyl methacrylate) (PMMA). The high dielectric constant and low dielectric loss of hyperbranched polymers lead to higher β-phase content in electrospun nanofilms and suppress current leakage. The PMMA coating results in the uniform dispersion of BaTiO3 nanowires in poly­(vinylidene fluoride) (PVDF), thereby enhancing the piezoelectric performance of the fiber nanocomposite PENG. The open circuit voltage and short circuit current of the PENG composed of PVDF and core-double shell PMMA-coated hyperbranched BaTiO3 (BTO@HBP@PMMA) nanowires can reach 3.4 V and 0.32 μA, and the peak output power is 5.25 μW, which is significantly improved compared to that of the unmodified PENG. Furthermore, the flexible PENG has high durability and can continuously generate stable piezoelectric signals for 6000 cycles without decay. The prepared PENG can also efficiently harvest the energy generated by human daily activities. In general, this study provides a new microstructure design for the fabrication of high performance PENGs.

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