"Nanoscale spin rectifiers for harvesting ambient radiofrequency energy" (2024) https://www.nature.com/articles/s41928-024-01212-1
From "NUS researchers develop new battery-free technology to power electronic devices using ambient radiofrequency signals" (2024) https://news.nus.edu.sg/nus-researchers-develop-new-battery-... :
> To address these challenges, a team of NUS researchers, working in collaboration with scientists from Tohoku University (TU) in Japan and University of Messina (UNIME) in Italy, has developed a compact and sensitive rectifier technology that uses nanoscale spin-rectifier (SR) to convert ambient wireless radio frequency signals at power less than -20 dBm to a DC voltage.
> The team optimised SR devices and designed two configurations: 1) a single SR-based rectenna operational between -62 dBm and -20 dBm, and 2) an array of 10 SRs in series achieving 7.8% efficiency and zero-bias sensitivity of approximately 34,500 mV/mW. Integrating the SR-array into an energy harvesting module, they successfully powered a commercial temperature sensor at -27 dBm.
Passive Wi-Fi or "backscatter redirection Wi-Fi": https://en.wikipedia.org/wiki/Passive_Wi-Fi :
> The system used tens of microwatts of power, [2] 10^−4 less energy than conventional Wi-fi devices, and one thousandth the energy of Bluetooth LE and Zigbee communications standards. [1]