If you want electrons to flow, you need vacuum or gas at low pressure. this is because, electrons are extremely light, relative to air molecules. The air atoms or molecules stop them. The high positive potential attracts the electrons and accelerates them, causing a flow of current.
But why are you asking this under the title "thermionic emission"?
In thermionic emission, electrons gain energy due to heat. This energy is transformed into Kinetic energy, causing the electron flow.