A thermionic energy converter with a nickel collector and cesium vapor as a working gas was studied, and an abnormally low value of the surface work function of ≈1 eV was obtained if the collector was covered by a thin carbon layer. Scanning electron microscopy x-ray microanalysis data of the elemental composition of the collector's surface after its long exposure to plasma indicate that the carbon structure was intercalated with cesium atoms, and this change to surface structure can be a reason for the anomalously low work function ∼1 eV. The thermionic energy converter with such a collector demonstrated high heat-to-electric power conversion efficiency up to ∼20%.