MoO(x) 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 MoO(x) 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 Li(2)SO(4). The MoO(x) (x ≈ 2.3) film deposited at 150 °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 −0.1 V 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 MoO(2) nanocrystals and amorphous MoO(x) (2.3 < x ⩽ 3). A mechanism combining Mo(IV) oxidation/reduction on the hydrated MoO(2) grain surfaces and cation intercalation/extrusion is proposed to illustrate the pseudo-capacitive process.