From contact-electrification to triboelectric nanogenerators.

Although the contact electrification(CE) (or usually called "triboelectrification(TE)") effect has been known for over 2600 years, its scientific mechanism still remains debated after decades. Interest in studying CE has been recently revisited due to the invention of the triboelectric nanogenerators(TENGs), which are the most effective approach for converting random, low-frequency mechanical energy into electric power for distributed energy applications. This review is composed of three parts that are coherently linked ranging from basic physics, through classical electrodynamics, and to technological advances and engineering applications. First, the mechanisms of CE are studied for general cases involving solids, liquids and gas phases. Various physics models are presented to explain the fundamentals of CE by illustrating that electron transfer is the dominant mechanism for CE for solid-solid interfaces. Electron transfer also occurs in the CE at liquid-solid and liquid-liquid interfaces. An electron-cloud overlap model is proposed to explain CE in general. This electron transfer model is extended to liquid-solid interfaces, leading to a revision of the formation mechanism of the electric-double-layer(EDL) at liquid-solid interfaces. Second, by adding a time-dependent polarization term Ps created by the CE induced surface electrostatic charges in the displacement field D, we expand Maxwell equations to include both the medium polarizations due to electric field (P) and non-electric field(such as strain) (Ps) induced polarization terms. From these, the output power, electromagnetic behaviour and current transport equation for a TENG are systematically derived from first principles. A general solution is presented for the modified Maxwell equations, and analytical solutions for the output potential are provided for a few cases. The displacement current arising from ε∂E/∂t is responsible for electromagnetic waves, while the newly added term ∂Ps/∂t is responsible for energy and sensors. This work sets the standard theory for quantifying the performance and electromagnetic behaviour of TENGs in general.