There is a need to produce electric power by means that provide low pollution and high efficiency. Thermionic energy conversion (TEC) systems enable the direct conversion of energy from thermal to electric, based on the emission of electrons from a heated cathode, Diamond is an ideal material for the cathode because of its high temperature mechanical stability, its ability to be created with low resistivity, and its strong tendency to emit electrons. The efficiency of current TEC systems is not practical, as above approximately 700° C. the current produced decreases. The presence of hydrogen at the electron-emitting surface is required to enhance thermionic emission. The present invention provides a resupply of hydrogen to the emitting surface by diffusion of hydrogen through the diamond cathode, and enables efficient operation of TEC systems at temperatures well above the current limit; practical systems for direct conversion of heat to electricity are thus enabled.