There is at least one gadget already in existence, an air pollution monitor, that does something like that, it charges up a battery by harvesting radio frequency energy from existing wireless and broadcast networks. The technology is called "Freevolt" and is described in this news story from 2015:
The gadget is manufactured by the British businessman Paul Drayson, who donated considerable sums to the Labour party when Tony Blair was in power, and as a result got made a Lord and subsequently did a few government minister jobs.
It sounds like Apple are attempting to do something similar with mobile phone charging, and as they are talking about their own patent applications, it looks like they're not using the Freevolt technology.
Drayson's air pollution monitor is on sale through Amazon UK:
It looks like the monitor is actually a carbon monoxide monitor, and it uses carbon monoxide levels as a proxy for pollution levels in general. Some of the customer reviews suggest that the monitor doesn't work very well in practice.