Carbon-Quantum-Dot-Derived Nanostructured MnO₂ and Its Symmetrical Supercapacitor Performances

We report here synthesis of nanostructured MnO₂ by using carbon-quantum dot (CQD) as reducing agent which is obtained by environmentally benign approach from waste material. The obtained carbon quantum dot-MnO₂ nanohybrid is characterized by various analytical techniques. The spectral studies reveal the characteristic absorption/emission features of nanostructured CQD-MnO₂. The X-ray-diffraction and microscopic analysis indicate that the synthesized CQD-MnO₂ is crystalline in nature with the grain size in the range of 250 − 300 nm. Electrochemical studies reveals that the carbon quantum dot-MnO₂ 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-MnO₂ matrix. The symmetric supercapacitor (CQD-MnO₂|Na₂SO₄|CQD-MnO₂) constructed using the carbon quantum dot-MnO₂ electrodes delivers high energy and power densities with a wide operating potential window of 0 - 1.6 V.