Due to the impending of energy crisis, alternative energy sources, such as solar, wind, wave, and biomass, are developed to compensate for the high demand in conventional energy sources. Mechanical energy, as an energy source, is abundant and ubiquitous in our daily life. Based on the coupling of contact electrification and electrostatic induction, a triboelectric nanogenerator can convert mechanical energy to electricity, with the merits of light weight, simple design, low cost, and high-power output, rendering it an ideal power supply for next-generation wearable electronics. Great effort has been devoted to enhancing the output power and exploiting multi-functionalities to meet the various demands as well as developing generators that manage to harvest multiple mechanical energy forms.

In general, materials with high tendency of electron-donating and electron-accepting are ideal for generators fabrication. However, recent research has mostly focused on the latter, tribo-negative materials, such as polytetrafluoroethylene and polydimethylsiloxane, which have frequently been paired with metals (Cu/Al). This is mainly because most positive materials are natural or difficult to manipulate. Another reason of using metals as positive counterparts lies in the undetermined contact charging mechanism of insulator-insulator. Therefore, investigation of tribo-positive materials is essential and of significance to enhance the energy harvesting.

On the other hand, surface charge density generated upon contact separation is the key factor governing the output characteristics of the generator. Thus, the motivation of this study is to explore naturally occurring as well as synthetic polymeric materials as positive tribo-materials for mechanical energy harvesting. Subsequently, surface modification and bulk dielectric property engineering are employed as effective strategies to enhance the surface charge densities toward high-power output generators. In addition, a charge transfer mechanism during the contact electrification is proposed and discussed in detail.

Further, a skin-like motion harvester with high transparency, flexibility, and adaptability is conceived based on a polyionic material as an efficient current collector. A successful power delivery under tapping, bending, and curling is demonstrated, showing potential in harvesting human body motion energy.

Overall, this research work provides some insights on the approaches to boost the output power of the mechanical energy harvesters through materials aspects. The proposed mechanism can shed light on the understanding of charge transfer. The realization of multi-functional harvester takes a step forward to fulfil the versatile requirements of power sources. These wearable mechanical energy harvesters show promising application in future development of self-powered flexible electronics.