Molybdenum oxide film with stable pseudocapacitive property for aqueous micro-scale electrochemical capacitor

• Conductive MoO2+x film composed by crystallized MoO2 grains and amorphous MoOx is prepared.
• Such film shows high volumetric capacitance and stable property.
• Good stability, high energy and power density are obtained from the asymmetric micro- capacitor of MoO2+x(-)//2 M Li2SO4//MnO2 (+).
• MoO2 grains are crucial in achieving superior property for the MoO2+x film electrode.

Conductive MoO2+x film deposited at 150 °C by magnetron sputtering is investigated as negative electrode material for micro-scale electrochemical capacitors. It is found that the film 860 nm thick exhibits high volumetric capacitance (385 F cm−3), good rate capability (47% capacitance retained at 500 mV s−1 relative to that at 20 mV s−1) and excellent cycling stability (100% retained after 5000 cycles). An asymmetric micro-device of MoO2+x(-)//2 M Li2SO4//MnO2(+) is obtained, showing a high energy density of 2.8 μWh cm−2 at a real power density of 0.35 mW cm−2, and good stability with no capacitance loss for 10000 cycles. It is revealed that the amorphous MoOx species with oxidation state of +5 to +6 provides pseudocapacitance through reversible insertion/extraction of H+ and Li+. More importantly, extra capacitance is contributed by catalytic decomposition of hydrated water on MoO2 grain surfaces, and the presence of large amounts of MoO2 grains ensures fast electronic transfer and structural stability. Thus the superior capacitive property of the MoO2+x film can be attributed to its multivalent and multi-phase structure with integrated functions.

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