Optimization principles and the figure-of-merit for triboelectric generators

Energy harvesting with triboelectric nanogenerators is a burgeoning field, with a growing portfolio of creative application schemes attracting much interest. While power generation capabilities and its optimization is one of the most important subjects, a satisfactory elemental model that illustrates the basic principles and sets the optimization guideline remains elusive. We use a simple model to clarify how the energy generation mechanism is electrostatic induction, but with a time-varying character that makes the optimal matching for power generation more restrictive. By combining multiple parameters into dimensionless variables, we pinpoint the optimum condition with only two independent parameters, leading to predictions of the maximum limit of power density which allows us to derive the triboelectric material and device figure-of-merit. We reveal the importance to optimize device capacitance, not only load resistance, and to minimize the impact of parasitic capacitance. Optimized capacitances can lead to a more than ten-times overall increase in power density.