Recent developments of hybrid piezo–triboelectric nanogenerators for flexible sensors and energy harvesters

Hybrid piezo-triboelectric nanogenerators constitute a new class of self-powered systems that exploit the synergy of piezoelectric and triboelectric mechanisms to improve energy harvesting efficencies and address the energy and power needs of portable and wearable electronic devices. The unique, synergistic electrical coupling mechanisms of piezoelectric and triboelectric effects increase the electric outputs and energy conversion efficiency of hybrid generators to beyond a linear summation of the contributions from individual triboelectric and piezoelectric mechanisms. Due to their large surface-area-to-volume ratios and outstanding mechanical, electronic and thermal properties, nanomaterials are favourable building blocks for constructing hybrid nanogenerators and represent a large family of flexible energy harvesting electronic structures and devices. Herein, we review the recent advances of hybrid piezo-triboelectric nanogenerators, with a particular focus on microstructure design, synergy mechanisms, and future research opportunities with significant potential for physiological monitoring, health care applications, transportation, and energy harvesting. The main strategies for improving electrical output performance are identified and examined, including novel nanostructures for increasing the contact area of the triboelectric pair, and nano-additives for enhancing the surface potential difference between the triboelectric pair and piezoelectric layers. Future applications and commercialization opportunities of these nanogenerators are also reviewed.