Quantum‐Dot‐Mediated Controlled Synthesis of Dual Oxides of Molybdenum from MoS 2 : Quantification of Supercapacitor Efficacy

The versatile technological applications of molybdenum oxides requires the efficient synthesis of various stoichiometric molybdenum oxides. Thus, herein, a controlled method to synthesize both MoO 3 and MoO 2 from MoS 2 via quantum dot intermediates is reported. Microscopic, spectroscopic, and X‐ray studies corroborate the formation of orthorhombic α‐MoO 3 with a microbelt structure and monoclinic MoO 2 nanoparticles that self‐assemble into hollow tubes. Quantitative investigations into charge‐storage kinetics reveal that MoO 2 exhibits an excellent pseudocapacitive response up to a mass loading of 5 mg cm −2 with an areal capacity of 327.2 mC cm −2 at 5 mV s −1 , with 41.9 % retention at 100 mV s −1 . In contrast, above a mass loading of 0.5 mg cm −2 , the charge‐storage nature of MoO 3 electrodes switches from that of a supercapacitor to battery type. At a sweep rate of 50 mV s −1 , 87.2 % of the total charge is contributed by a capacitive response in a 1 mg cm −2 MoO 2 electrode. The charge‐storage kinetics of MoO 3 and MoO 2 reflect on the respective asymmetric supercapacitors. A MoO 2 //graphite asymmetric supercapacitor holds an outstanding energy density of 341 mW h m −2 at a power density of 4949 mW m −2 and delivers an ultrahigh power density of 28140 mW m −2 with an energy density 142 mW h m −2 and energy efficiency of 87 %.