Correlation of the Graphene Fermi-Level Shift and the Enhanced Electrochemical Performance of Graphene-Manganese...

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.