As it is known, the thermionic emission (emission of electrons at high temperature) current density in vacuum depends on the temperature and the work function of the material. The work function is a material specific constant. Let's compare thermionic emission with thermal radiation. With thermal radiation, the amount of emitted photons per unit area for a black body depends only on the temperature. (It is proportional to T4). The black body absorbs all photons that fall on it. For non-black body, the emitted radiation is the black body radiation multiplied by a coefficient less than 1. The absorption is also proportional to the same coefficient. Because of this, at equal temperature, there is a balance between the emitted and absorbed radiation between objects. However, thermionic emission is very different. Here, the emission per unit area under equal temperature is larger for materials with lesser work function. However, the absorption doesn't depend on the work function - an object will absorb all electrons that hit it. So there is no balance between two objects with different work functions at the same temperature. If we place 2 such objects next to one another, more electrons will flow from the object with the smaller work function towards the one with larger work function than the other way round, since the object with smaller work function emits more. Therefore, a potential difference will be created between the 2 objects. If they are connected to an external circuit, a current will flow. This would be a thermionic generator with no need for cooling!