Functionalized Cu-Doped\nZnO/PVDF Composite: An Excellent\nEnergy Storage Material for Wearable Devices

Human health and well-being are major focuses of current\nresearch\nworldwide. Self-powered smart wearable technology holds great promise\nfor enhancing human life. However, developing materials with a high\nenergy storage capacity for powering sensors, wearables, and portable\nelectronics remains challenging. Here, we report on the design of\na composite material, PVDF/f-Zn<sub>1–<i>x</i></sub>Cu<sub><i>x</i></sub>O (<i>x</i> = 0, 0.01, 0.02,\n0.03), with high energy storage and energy- harvesting capacity. The\nmaterial was synthesized via a hydrothermal process, in which copper\n(Cu) was doped into zinc oxide (ZnO) and then amine-functionalized\nwith 3-aminopropyl triethoxysilane (APTES). Interestingly, the 2 wt\n% Cu-doped ZnO transformed from a nanoflake to a uniaxial nanorod\nmorphology during synthesis, a key factor for high-energy storage\nproperties. The modification of APTES facilitated the dispersion of\nuniaxial fillers within the polymer matrix. Adding f-Zn<sub>0.98</sub>Cu<sub>0.02</sub>O to polyvinylidene fluoride (PVDF) resulted in\na 154% increase in tensile strength and a 56% increase in Young’s\nmodulus compared with neat PVDF. Moreover, the PVDF/f-Zn<sub>0.98</sub>Cu<sub>0.02</sub>O nanocomposite exhibited excellent energy storage\ndensity (9 J/cm<sup>3</sup>) and efficiency (81%). Additionally, it\ndemonstrated an impressive piezoresponse, with an output voltage of\n∼12 V and a power density of approximately 21.17 μW/cm<sup>2</sup>, significantly higher than those of neat PVDF and other contemporary\ncomposites. The efficiency of the composite for wearable devices was\ntested through various biomechanical pressure applications such as\nfinger tapping, hand stomping, and finger bending, and it showed outstanding\nresponses.