Asymmetric supercapacitor based on MnO2 and Fe2O3 nanotube active materials and graphene current collectors

Asymmetric electrochemical supercapacitors, containing MnO2 nanotube positive electrodes and Fe2O3 nanotube negative electrodes in 0.5 M Na2SO4 electrolyte were fabricated and tested. The nanotubular electrode materials were prepared by hydrothermal synthesis methods. A chelating polyelectrolyte was used for electrosteric dispersion and electrophoretic deposition (EPD) of MnO2 and Fe2O3 nanotubes. The individual chelating monomers of the polyelectrolyte created multiple bonds with Mn and Fe atoms on the nanotube surfaces and allowed for strong adsorption, efficient dispersion and EPD. Electrochemical testing results were compared for electrodes formed by EPD on reduced graphene oxide aerogel (rGO) and stainless steel (SS) current collectors. The MnO2/rGO and Fe2O3/rGO electrodes, showed significantly higher areal capacitances and gravimetric capacitances, normalized by active mass or by total mass of the electrode material and current collector, compared to MnO2/SS and Fe2O3/SS electrodes. The difference was attributed not only to capacitive properties and light weight of rGO, but also to better utilization of capacitive properties of the nanotubes and lower impedance. The use of rGO allowed for significant reduction of the ratio of active material mass to current collector mass, which offers benefits for the development of light weight devices. The capacitive properties of Fe2O3/rGO in the negative potential range closely matched the capacitive properties of MnO2/rGO in the positive potential range. Such matching was beneficial for the fabrication of asymmetric devices, which showed good electrochemical performance in a voltage window of 1.8 V.