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
@ticky for what purpose? I guess they could be nifty rewritable labels on keys or sth but I don't see any super-obvious use case
@noiob @ticky Requires a source of power over USB as well. Battery on my solar graph was an extra $50, but provides its own solar power to the Pi independent of my power grid at home.