We have examined electric field-assisted thermionic emission of oxygen radical anion (O-) and electron from a stoichiometric 12CaO7 Al 2O3 (C12A7). Lattice framework of C12A7 is composed of subnanometer-sized cages that can accommodate extra- framework species with negative charge, such as O-, O2- ions, and electrons. Absence of oxygen gas supply leads to a steady coemission of O- ion and electron with a ratio of ∼3:7. However, the oxygen gas supply markedly enhances the O- ion emission and suppresses the electron emission, realizing a sustainable and high-purity O- ion emission with a current density up to 11 nA cm-2 at 825°C. The O- ion emission accompanies a weak O2- ion emission, which is smaller by ∼130. TheO2 pressure at the rear side controls the concentrations of O- ion and electron on the emission surface as well as their concentrations in the bulk, and in turn, determines their emission intensities.
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