Journal Article triboelectric nanogenerators charge excitation energy harvesting air breakdown charge density charge hopping charge loss mechanisms charge self‐injection durability energy density intelligent monitoring liquid suspension mode switching non-contact quasi‐dipole potential relative permittivity secondary self‐charge excitation self-polarization solid-liquid triboelectrification triboelectric nanogenerator triboelectric nanogenerators (TENGs) triboelectric series ultrahigh power volume effect