Unveiling capacitive behaviors of MoO2 in different electrolytes and flexible MoO2-based asymmetric micro-supercapacitor

Unveiling of capacitive behaviors of a material is highly desirable to develop a supercapacitor with a high energy density. In this work, molybdenum dioxide (MoO2) is successfully prepared via a facile hydrothermal method and the different capacitive behaviors of MoO2 in different electrolytes are revealed. Compared to in neutral electrolyte (0.5 molL(-1) Na2SO4), MoO2@CF (carbon fibre) electrode has a higher discharge capacitance (509.8 F g(-1)) in an acidic electrolyte (0.5 molL(-1) H2SO4). This is because H+ ion is easier to adsorb on MoO2 than Na+ ion, and H+ ion has a higher diffusion rate than Na+ ion due to its smaller hydration radius. Besides, the charge storage mechanism of MoO2 is mainly composed of surface redox reaction (Faraday capacitance) and ion insertion/extraction reaction (battery capacitance) in acid electrolyte, while electric double-layer capacitors in neutral electrolyte. Nevertheless, the capacitive behaviors of MoO2 in different electrolytes have not been unveiled so far. Additionally, 64.5% of capacitance is still remained after 2500 cycles. Moreover, a flexible asymmetric micro-supercapacitor (ASC) is assembled with MoO2@CF positive electrode and activated carbon@CF negative electrode. The ASC can deliver an energy density of 22.90 Wh kg(-1) at a power density of 550.5 W kg(-1), which is higher than the relative results reported before. Furthermore, the ASC can drive a light-emitting diode (6 mm diameter, red) to work for 5 min after being charged for 5 s. The most importance is that the ASC is low-cost and flexible, which is handy to be embed in other power supply apparatuses.