The Preparation And Electrochemical Property Adjustment Of Molybdenum Oxide Film

Micro-electrochemical capacitors (m-ECs), a potential energy storage devicewhich could be integrated on the chip, is under rapid development and could act animportant role in the micro-energy system. The future application of m-ECs requiresultra-high volumetric or areal capacitance, excellent rate capability and cycling stabilityof electrode materials. As a typical material being pseudocapacitive, metallic oxideexhibits huge specific capacitance. However, the poor electronic conductivity leads topoor rate capability and cycling stability. Herein, the fabrication and electrochemicalproperty of molybdenum oxide thin film have been studied. The composition,microstructure and morphology were controlled to enhance the electrochemicalperformance, opening a new way for molybdenum oxide to be applied as a superiorelectrode material in m-ECs in the future.Electronically conductive MoO2+xfilms have been successfully fabricated via RFmagnetron sputtering from a MoO3target. Multi-valence composition andmixed-phased microstructure, i.e., coexistence of MoO2nanocrystals and amorphousMoOx(2<x≤3), were achieved in these films, which exhibit excellent pseudocapacitancein Li2SO4electrolyte. An electrochemical mechanism was proposed after a systematicstudy with cyclic voltammetry. The pseudocapacitance originates from the cation (H+and Li+) insertion/extrusion in the amorphous MoOx, in which H+exhibits higheractivity. The MoO2grains could also catalyze the decomposition of water combined onthe surface, producing H atoms could be reversibly stored in amorphous MoOxthuspromote the pseudocapacitive process. Furthermore, the crystalline MoO2also improvesthe electronic conductivity and maintains a stable structure of the film. The MoO2+xfilmexhibit better electrochemical property than that of electrochemical double layermicro-capacitors with similar thicknesses. An asymmetric micro-device of MoO2+x(-)//2M Li2SO4//MnO2(+) was obtained, showing a high energy density and goodstability, verifying that the MoO2+xfilms could be practically applied as an anodematerial in m-ECs.Annealing of electrodeposited amorphous MoOxfilms in a reductive atmosphere iscarried out to study the effect of temperature and time on the electrochemical property.Optimal experimental parameters were determined to fabricate the film containing MoO2nanocrystallites and amorphous MoOx, which displays good rate capability,cycling stability and high specific capacitance. The method of electrodeposition allowsMoOxfilms to be grown on current collectors with complex morphology, thus could beemployed to develop micro-electrode with novel architectures. A three dimensionalmicro-electrode was prepared by electrodepositing MoOxon a Ti nanorod array, whichwas fabricated via electron beam evaporation employing glancing angle depositiontechnique. High areal capacitance and excellent cycling stability were obtained on suchmicro-electrode. Post-annealing in reductive atmosphere improves its rate capability andresponse speed. Hence, the electrochemical property of MoOxelectrode is furtherimproved with architecture design of employing current collectors with large specificarea, which promotes its application in m-ECs.