Im not sure if u can harvest electrons from air bcoz they are particles and for electrons to flow for electricity, u need to have a metal surface with sufficiently high positive potential difference to mobilize electrons. And i have never read anywhere if u can do that. May be there is some capacitive action in air where all electrons will try to come on metal plate. (kind of very hypothetical like comic book stuff).
On the other hand, the air is filled with very high concentration of electromagnetic energy which u can use to excite electrons in antenna like metal structure and then use some kind of voltage doubler, quadrupler etc to increase the voltage at expense of decreased current. U can also use the light i.e. photonic energy or sunlight to harvest electricity using solar panel or photovoltaic cell. Then there is free flowing wind for which wind turbines are used. And turbines in dam use flowing water. In a nutshell, u need some kind of energy available for free to excite electrons already existed in the metal circuit.
Hope that helps.