MoOx thin films deposited by magnetron sputtering as an anode for aqueous micro-supercapacitors

In order to examine the potential application of non-stoichiometric molybdenum oxide as anode materials for aqueous micro-supercapacitors, conductive MoOx films (2 <= x <= 2.3) deposited via RF magnetron sputtering at different temperatures were systematically studied for composition, structure and electrochemical properties in an aqueous solution of Li2SO4. The MoOx (x approximate to 2.3) film deposited at 150 degrees C exhibited a higher areal capacitance (31 mF cm(-2) measured at 5 mV s(-1)), best rate capability and excellent stability at potentials below -0.1V versus saturated calomel electrode, compared to the films deposited at room temperature and at higher temperatures. These superior properties were attributed to the multi-valence composition and mixed-phase microstructure, i.e., the coexistence of MoO2 nanocrystals and amorphous MoOx (2.3 < x <= 3). A mechanism combining Mo(IV) oxidation/reduction on the hydrated MoO2 grain surfaces and cation intercalation/extrusion is proposed to illustrate the pseudo-capacitive process.