Action mechanism of expanded graphite in the composite of expanded graphite/MnO₂ supercapacitor electrode materials

The pure MnO₂ and the composite of expanded graphite (EG)/MnO₂ were prepared by a very facile method. The experimental results show that the composite presents higher specific capacitance and more stable rate capacity. The action mechanism of EG in the composite was investigated by the measurements of XRD, TG, EIS, BET, etc. It is found that the δ-MnO₂ appears in the composite, whose mass percentage is approximately 30% compared with 3.4% of EG in the composite. The δ-MnO₂ has a special layered structure compared with the channel structure of α-MnO₂, which makes the ions diffuse and transport easier. The charge exchange resistance of the EG/MnO₂ composite is significantly lower than that of pure α-MnO₂ which may be caused by more tiny particles size of composite because the specific surface area of the composite being 38.7 m²·g⁻¹, with the pure α-MnO₂ of21.6 m²·g⁻¹. The larger specific surface area makes the contact between the material particles and the electrolyte solution more sufficient along with lower charge exchange resistance. In all, the action mechanism of EG in the composite can be summed as follow. The δ-MnO₂ with the layered structure is induced to grow on the surface of the EG which has the similar layered structure. At the same time, the grows-up of the MnO₂ particles is restrained by EG which makes the contact of particles with electrolyte solution more sufficient with the result of the reduce of charge exchange resistance. And, therefore, the specific capacitance is elevated accompanied with more stable rate capacity.