Supercapacitor Electrodes of MnO2 and MnO2/Graphene Nanosheets Synthesized by Liquid Phase Exfoliation

MnO₂ has been extensively investigated due to its high theoretical capacitance of  1100 to 1300 F.g^-11,  environmental friendly nature and low cost ².  The MnO₂ charge storage mechanism relies on the exchange of protons and/or cations with the electrolyte, redox activity involving a Mn(IV)/Mn(III) transition, and chemisorption of ions onto the MnO₂ surface ^1,3,4. As these are surface processes, it is paramount to design MnO₂ structures with an accessible high surface area. Recently, liquid phase exfoliation has become a powerful technique for the preparation of 2D nanosheets presenting a high surface area^5,6. Therefore this technique can be used to produce  nano layers of MnO₂, which will present an enhanced  utilization of the active material and a better electrochemical performance⁷. 

In the present work a high-surface area, porous MnO₂ powder was produced through the oxidation of Mn(NO3)₂ by KMnO₄. A poly(ethylene glycol)-poly(propylene glycol)-poly(ethylene glycol) triblock copolymer (PEG-PPO-PEG P123) was used as templating agent for the formation of a “flower-like” nanostructure (MOFN) with protruding 2D-nanostructures⁸. Subsequently, MOFN wére exfoliated in isopropanol at 37kHz for 3 hours, resulting in two types of materials: manganese oxide nanolayers (MOL) and a partially exfoliated material (PEMO). Following a novel approach, the MOFN were also exfoliated simultaneously with graphite resulting in a MnO₂layers/Graphene hybrid (GMOH).

The obtained dispersions were sprayed onto ITO electrodes and the electrochemical properties studied by cyclic voltammetry. By testing electrodes with different thicknesses it was found out that the electrochemical utilization is enhanced for GMOH (80.0 mF.cm^-2) at a thickness of 9700 nm). A capacitance as high as 300 F.cm^-3 was also achieved with GMOH thin electrodes followed by 225 F.cm^-3 for MOL and 100 F.cm^-3for PEMO.

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