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Planar electrolytic cells using partially stabilized tetragonal zirconia in polycrystalline form (TZP) were tested in vacuum and in a thermionic diode in cesium vapor. Oxygen transmission through TZP at temperatures from 800 to 1000 K and released as O2 was observed in a vacuum system with a mass spectrometer. The work function increase of a W emitter during an oxygen release was also easily demonstrated. The TZP cell, tested as an oxygen sensor, exhibited open circuit voltage changes, reversible with oxygen pressure variations. However, the data did not quite follow the theoretical predictions of Nernst's law. A similar TZP cell was then attached to the collector of a thermionic diode with a cesium pressure of 0.1 torr. Emission enhancement was observed following oxygen injection into the diode: the equivalent “bare” work function reached 5.7 eV after oxygen injection. Oxygen removal from the thermionic diode was achieved at very low cell currents.