Tuning the morphology of manganese oxide nanostructures for obtaining both high gravimetric and volumetric capacitance

Abstract

Obtaining both high gravimetric capacitance (C_{s_m}) and high volumetric capacitance (C_{s_V}) in supercapacitors is still a great challenge. We prepared manganese oxide (MO) nanostructures by pulsed laser deposition, using a metallic Mn target in an O₂ atmosphere with pressures ranging from 0.1 Torr to 2.0 Torr at room temperature. The morphology gradually changed from a dense film to nanofoam with different porosities and densities. Raman spectroscopy and X-ray photoelectron spectroscopy revealed a similar oxidation state despite distinct microstructures. C_{s_m} and C_{s_V} for the three typical nanostructures, namely thin films, perpendicular columnar structures and nanofoams, were compared. It was found that the highest C_{s_m} value was not obtained in the nanofoam sample with the highest porosity, but it was achieved in the sample with a perpendicular columnar structure with a C_{s_m} value of 976 F g⁻¹ at 5 mV s⁻¹. Such a configuration showed the highest C_{s_V} as well with a value of 830 F cm⁻³ at 5 mV s⁻¹. The best performance with voltage scan rates higher than 50 mV s⁻¹ was found in the nanofoam structures with the values of 612 F g⁻¹ at 100 mV s⁻¹ and 352 F g⁻¹ at 300 mV s⁻¹. Our research gives useful suggestions for material design in supercapacitor electrodes: a suitable microstructure can be used for applications focusing on different parameters of a supercapacitor. The results might be of general interest for the energy storage research community.