Abstract
Two-dimensional metal oxide pseudocapacitors are promising candidates for size-sensitive applications. However, they exhibit limited energy densities and inferior power densities. Here, we present an electrodeposition technique by which ultrathin CeO2−x films with controllable volumetric oxygen vacancy concentrations can be produced. This technique offers a layer-by-layer fabrication route for ultrathin CeO2−x films that render Ce3+ concentrations as high as ~60 at% and a volumetric capacitance of 1873 F cm−3, which is among the highest reported to the best of our knowledge. This exceptional behaviour originates from both volumetric oxygen vacancies, which enhance electron conduction, and intercrystallite water, which promotes proton conduction. Consequently, simultaneous charging on the surface and in the bulk occur, leading to the observation of redox pseudocapacitive behaviour in CeO2−x. Thermodynamic investigations reveal that the energy required for oxygen vacancy formation can be reduced significantly by proton-assisted reactions. This cyclic deposition technique represents an efficient method to fabricate metal oxides of precisely controlled defect concentrations and thicknesses.
Highlights
Two-dimensional metal oxide pseudocapacitors are promising candidates for size-sensitive applications
In order to clarify the mechanism of formation of such ultrathin CeO2−x films, electrodeposition data were deconvoluted in terms of the reactions over cycling
Electron micrographs of the thin-film nanostructures were obtained by high-resolution transmission electron microscopy (HRTEM; Philips CM 200, Eindhoven, the Netherlands) and field emission gun scanning/transmission electron microscopy (FEG-scanning transmission electron microscopy (STEM); JEOL JEM-F200 MultiPurpose FEG-STEM, Tokyo, Japan)
Summary
Two-dimensional metal oxide pseudocapacitors are promising candidates for size-sensitive applications They exhibit limited energy densities and inferior power densities. This technique offers a layer-by-layer fabrication route for ultrathin CeO2−x films that render Ce3+ concentrations as high as ~60 at% and a volumetric capacitance of 1873 F cm−3, which is among the highest reported to the best of our knowledge This exceptional behaviour originates from both volumetric oxygen vacancies, which enhance electron conduction, and intercrystallite water, which promotes proton conduction. Thermodynamic investigations reveal that the energy required for oxygen vacancy formation can be reduced significantly by proton-assisted reactions This cyclic deposition technique represents an efficient method to fabricate metal oxides of precisely controlled defect concentrations and thicknesses. To the best of our knowledge, intercalation pseudocapacitance has not been observed to date in CeO2, very high specific capacitances of this material have been reported previously[17,18], indicating its promise for energy storage applications
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