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The present investigation elucidates a simple hydrothermal method for preparing nanostructured bismuth oxide (Bi 2 O 3 ) and carbon quantum dot (CQD) composite using spoiled (denatured) milk-derived CQDs. The formation of the CQD–Bi 2 O 3 composite was confirmed by UV–vis absorption, steady-state emission, and time-resolved fluorescence spectroscopy studies. The crystal structure and chemical composition of the composite were examined by X-ray diffraction, Fourier transform infrared spectroscopy, Raman spectroscopy, and thermogravimetric analysis. The surface morphology and the particle size distribution of the CQD–Bi 2 O 3 were examined using field emission scanning electron microscope and high-resolution transmission electron microscope observations. As an anode material in lithium-ion battery, the CQD–Bi 2 O 3 composite exhibited good electrochemical activity and delivered a discharge capacity as high as 1500 mA h g –1 at 0.2C rate. The supercapacitor properties of the CQD–Bi 2 O 3 composite electrode revealed good reversibility and a high specific capacity of 343 C g –1 at 0.5 A g –1 in 3 M KOH. The asymmetric device constructed using the CQD–Bi 2 O 3 and reduced graphene oxide delivered a maximum energy density of 88 Wh kg –1 at a power density of 2799 W kg –1 , while the power density reached a highest value of 8400 W kg –1 at the energy density of 32 Wh kg –1 . The practical viability of the fabricated device is demonstrated by glowing light-emitting diodes. It is inferred that the presence of conductive carbon network has significantly increased the conductivity of the oxide matrix, thereby reducing the interfacial resistance that resulted in excellent electrochemical performances.

We report here synthesis of nanostructured MnO 2 by using carbon-quantum dot (CQD) as reducing agent which is obtained by environmentally benign approach from waste material. The obtained carbon quantum dot-MnO 2 nanohybrid is characterized by various analytical techniques. The spectral studies reveal the characteristic absorption/emission features of nanostructured CQD-MnO 2 . The X-ray-diffraction and microscopic analysis indicate that the synthesized CQD-MnO 2 is crystalline in nature with the grain size in the range of 250 À 300 nm. Electrochemical studies reveals that the carbon quantum dot-MnO 2 nanohybrid electrode exhibits a high specific capacitance of 189 F g -1 at 0.14 A g -1 with an excellent stability over 1200 charge-discharge cycles. The excellent electrochemical performance is attributed to the large surface area and high conductivity due to the presence of conductive nanonetwork of the CQD in the carbon quantum dot-MnO 2 matrix. The symmetric supercapacitor (CQD-MnO 2 j Na 2 SO 4 j CQD-MnO 2 ) constructed using the carbon quantum dot-MnO 2 electrodes delivers high energy and power densities with a wide operating potential window of 0 -1.6 V.

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Acacia auriculiformis seedpod biomass-derived activated biocarbon was generated by carbonization followed by chemical activation using KOH. The formation of the biocarbon having hierarchical porous, pyrrolic nitrogen and high surface area has been confirmed using material characterization techniques. Then, sodium-ion energy storage performances of the biocarbon was examined in the half-cell that resulted in 255 mAh g −1 as the discharge capacity at 0.1 C rate with passable rate capability and cycling stability. Further, the activated biocarbon was also tested as the electrode material for symmetric sodium-ion ultracapacitors in aqueous and non-aqueous electrolytes. The aqueous ultracapacitor exhibited an energy density of nearly 62 Wh kg −1 , while the nonaqueous ultracapacitor resulted in a high specific energy of 138 Wh kg −1 . When assembled in a laboratory prototype pouch cell, the activated biocarbon electrode showed a high specific energy (150 Wh kg −1 ) at a specific power of 1495 W kg −1 . The disordered porous nitrogen-containing biocarbon associated with a high surface area leads to efficient sodium-ion storage as well as double-layer capacitance. The fabricated laboratory prototype sodium-ion ultracapacitor was practically tested to power a conventional red light-emitting diode for about 20 min on a single charge.