MoO 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 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 MoO (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 MoO₂ nanocrystals and amorphous MoO (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.