Structures of freestanding triboelectric-layer-based TENGs: (a) An FTENG with contact mode for vibration sensing (Reproduced with permission [23]. Copyright 2014 American Chemical Society); (b) A FTENG with sliding mode for translational motion sensing (Reproduced with permission [51]. Copyright 2018 WILEY-VCH Verlag GmbH & Co., KGaA, Weinheim, Germany); (c) A non-contact TENG with a disk-like structure (Reproduced with permission [52]. Copyright 2014 American Chemical Society); (d) A non-contact TENG with a tube-like structure (Reproduced with permission [53]. Copyright 2017 WILEY-VCH Verlag GmbH & Co., KGaA, Weinheim, Germany).