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

Conductive MoO2+x film deposited at 150 degrees 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 mu Wh 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 MnO2 grain surfaces, and the presence of large amounts of MnO2 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. (C) 2014 Elsevier Ltd. All rights reserved.