Green Strategy for the Synthesis of K<sup>+</sup> Pre-inserted MnO<sub>2</sub>/rGO and Its Electrochemical Conversion to Na-MnO<sub>2</sub>/rGO for High-Performance Supercapacitors
Amol S. Vedpathak, Mangesh A. Desai, Sunita Bhagwat, Shrikrishna D. Sartale
Abstract
An eco-friendly and low-cost green strategy has opted to synthesize K+ ion pre-inserted MnO2 (K-MnO2) and its hybrid with reduced graphene oxide (K-MnO2/rGO) nanostructures using aloe vera leaf extract (an effective reducing and capping agent). K-MnO2 and K-MnO2/rGO hybrid formations were confirmed by X-ray diffraction, Raman, and X-ray photoelectron spectroscopy measurements. The K-MnO2/rGO hybrid was found to be a better electrode material for supercapacitor application. By performing cyclic voltammetry measurements in 1 M Na2SO4 electrolyte solution, we found that the K+ ions present inside the MnO2 structure get replaced by the Na+ ions present in the electrolyte leading to Na+ ion inserted MnO2 (Na-MnO2) formation. The phase transformation of K-MnO2/rGO to Na-MnO2/rGO was confirmed using Raman and XPS analyses. This electrochemical phase transformation by Na+ ion intercalation improves the overall supercapacitive performances. It was revealed that, during the electrochemical cyclic process, the specific capacitance values of the K-MnO2/rGO hybrid hiked more than 4 times due to Na+ ion intercalation. The possible conversion mechanism of K-MnO2 to Na-MnO2 is provided. Fabrication of an asymmetric supercapacitor of Na-MnO2/rGO//activated carbon displays a high operating potential window of 2.2 V, a maximum energy density of 44.4 W h kg–1, and a power density of 8.79 kW kg–1.