Correlation of the Graphene Fermi-Level Shift and the Enhanced Electrochemical Performance of Graphene-Manganese Phosphate for Hybrid Supercapacitors: Raman Spectroscopy Analysis.

A high structurally stable graphene-manganese phosphate (graphene-Mn3P2O8) composite with excellent cycling stability was prepared by the facile hydrothermal method. The correlation between the high electrochemical performance of graphene-Mn3P2O8 composite and the graphene Fermi-level shift was investigated using Raman spectroscopy by monitoring the disorder in the sp2 network of the composite graphene arising from the heterogeneous atoms doping during hydrothermal synthesis. The response of the Raman signatures of graphene to the chemical doping effect correlated to the electronic band structure in the vicinity of the Dirac point showed an upshift in the graphene Fermi level with an average value of about 190 meV, indicating that the composite graphene is n-type-doped. This was confirmed by X-ray photoelectron spectroscopy data, which showed a relatively high concentration of electron-donating heterogeneous atoms in the composite graphene. The electrochemical analysis confirmed that the n-type-doped composite graphene has enhanced the electrical conductivity of the Mn3P2O8 electrode and decreased the potential barriers between the electrode surface and electrolyte highest occupied molecular orbital (HOMO) for enhanced interfacial charge transfer between the electrode surface and the electrolyte; hence, the graphene-Mn3P2O8 composite electrode exhibited a high specific capacity of 38.4 mA h g-1 compared to the pristine Mn3P2O8 electrode (7.2 mA h g-1). Due to its excellent cycling stability (∼100% capacity retention over 5000 charge-discharge cycles at 5 A g-1), graphene-Mn3P2O8 composite is a promising electrode material for hybrid supercapacitors.