One-Pot In Situ Synthesis of Mn3O4/N-rGO Nanohybrids for the Fabrication of High Cell Voltage Aqueous Symmetric Supercapacitors: An Analysis of Redox Activity of Mn3O4toward Stabilizing the High Potential Window in Salt-in-Water and Water-in-Salt Electrolytes

Abstract

The present work reports one-pot in situ synthesis of Mn3O4 nanocubes (average size = 33 nm) decorated on N-doped reduced graphene oxide to produce Mn3O4/N-rGO (N-GMn) nanohybrids by employing an aqueous reaction mixture of GO and greener reagent, glycine in the presence of MnCl2 under environmentally friendly mild conditions of temperature and near neutral pH ∼ 8. This process involves simultaneous reduction of GO and oxidation of Mn2+ species along with the doping of nitrogen forming Mn–O–C/O–Mn–N bonds to yield N-GMn nanohybrids, as confirmed by X-ray diffraction, Raman, Fourier transform infrared, and X-ray photoelectron spectroscopy studies. The N-GMn nanohybrids provide a high electrochemical potential window of −1.3 to +1.3 V, i.e., 2.6 V and −1.5 to +1.5 V, i.e., 3.0 V in neutral salt-in-water (0.5 M K2SO4) and water-in-salt (17 m NaClO4) electrolytes, respectively. Thus, the use of N-GMn as the electrode material, in which the integration of Mn3O4 and N-rGO takes place through covalent linkage (Mn–O–C/O–Mn–N), not only assisted in designing an aqueous symmetric supercapacitor in 17 m NaClO4 with high cell voltage (2.7 V) and energy density of 168 W h kg–1 at a power density of 675 W kg–1 but also contributed to achieve novel electrochemical attributes of a high cycling stability of 99.7% after 10,000 cycles. To the best of our knowledge, this is the first report on the Mn3O4/N-rGO-based symmetric supercapacitor, providing a high cell voltage of 2.7 V in an aqueous electrolyte comparable to the organic electrolyte-based commercial symmetric supercapacitor devices, thus demonstrating the importance of the present work for practical applications.