Thermionic energy conversion is a process that allows direct conversion of heat into electrical energy without mechanically moving components. In a thermionic converter electrons from the emitter traverse a small gap, are collected by a counter-electrode, the collector, and a self generated voltage develops across the gap. We have prepared prepared an ultra-nanocrystalline diamond (UNCD) based thermionic electron emitter that exhibits a low effective work function of typically 1.4 eV. This was attributed in part to reduced band bending and to the negative electron affinity (NEA) surface. A thermionic energy converter comprised of 2 diamond electrodes were positioned to establish a 25 micron gap and the emitter which was operated at temperatures up to 700 Celsius with a self generated open circuit voltage of 0.35 V. The reduced power output of the device was in part attributed to space charge effects and diamond film resistivity. Utilizing surface ionization effects at the emitter by introducing atomic hydrogen into the converter gap resulted in significant power output increase. With atomic hydrogen in the gap, the converter was operated up to 750 Celsius indicative of efficient surface ionization for charge transfer as well as a stable NEA diamond surface.