Dear chioma ,

RF energy harvesting research mainly contains design of antenna and rectifier circuit.

If u take antenna, the research topics are

I) single or multi-band antenna

II) Different materials (substrates: solid and flexible)

iii) Circular polarization, dual polarization antennas

If u select the rectification

I) diode rectifier and voltage double

II) II) CMOS rectifier and charge pumps

Matching methods between antenna and rectifier

Multi source energy harvesting (RF + solar + piezoelectric etc)

RF energy can be harvested from electromagnetic waves at different different frequency. it can be from cell towers, mobile antennas etc. for more information refer IEEE, PIER, journals.

Thanks so much @ Naresh Bangari and Rahul Dev Mishra.

I appreciate your responses