back in 2015 some researchers came up with an e-ink display device which was powered entirely by harvesting joules from the NFC connection your phone used to talk to it, and I’m honestly kind of shocked these aren’t all over the place now? http://www.alansonsample.com/research/NFC-WISP.html
@Goldkin @noiob 100% absolutely (the linked article even demonstrates briefly charging its capacitor, before using it for temperature data logging!)