Triboelectric nanogenerator: from alternating current to direct current

Triboelectric nanogenerator (TENG) is considered as a potential solution to harvest distributed energy for the sustainable and reliable power supply of the internet of things. Although numerous researches on alternating current (AC) output TENG from fundamental physics to potential applications have been widely promoted in recent years, the studies about direct current (DC) output TENG is just beginning, especially for a constant current output. This work gives the summary of recent key researches from AC-TENG to DC-TENG, especially a constant current TENG, as well as the design of AC/DC-TENG. In addition, some new DC generators will also be summarized toward a wide range of readers. This study presents the similarities and differences between AC-TENG and DC-TENG, so that their impact and uniqueness can be clearly understood. Finally, the major challenges and the future outlooks in this rapidly emerging research field will be discussed as a guideline for future research.