The transformation of thermal into electrical energy through thermionic energy conversion allows high conversion efficiencies as the hot and cold side are separated by a vacuum gap. Electrons released through thermal excitation from a cathode are collected by a counter-electrode with lower work function, ideally about 0.5eV, which can define the conversion efficiency. Converter electrodes based on diamond could achieve optimum work function values for emitter and collector by exploiting donor states introduced by nitrogen at 1.7eV and phosphorus at 0.6eV below the conduction band minimum, respectively. With the ability of diamond surfaces to attain a negative electron (NEA) affinity a surface barrier for electron emission can be mitigated and through single crystal material its stability at elevated temperatures enhanced.