Performance Optimizations and Figure-of-Merits for Freestanding Triboelectric Nanogenerators

Energy harvesting is a burgeoning strategy to salvage mechanical energy by using triboelectric nanogenerators. Various working modes and structures were recently developed as portable self-power sources for next-generation electronics, especially freestanding configurations that imply higher power output. However, the optimization principles and ultimate performance with theoretical analysis are still vague. We set up a time-varying dynamic model to clarify the electric power generation mechanism due to electrostatic induction and indicate optimal matching between input mechanical frequency and output electrical characteristics. Multiple parameters were grouped into two dimensionless parameters, predicting the maximum power density limit. Derived materials and device figure-of-merit set the criteria for material selection and structure design, clarifying that the optimized performance only depends on material surface charge density and device vibration speed. Accordingly, a rolling charge pump is designed to provide enhanced and stable charge flow to the middle metal of a sandwich-structured dielectric layer. The induced charge density on encapsulated dielectric surfaces reaches up to 60 μC/m2, which is 30 times higher than that of common contact and separation mode. Furthermore, the inevitable parasitic capacitance is also revealed and quantified, suggesting optimal power output in practice. Experimental results prove the optimization strategies of theoretical frequency matching, charge pump design, and practical parasitic analysis, leading to a tremendous increase in power performance.