In this report, nitrogen-, boron- and boron/nitrogen-doped graphene and nitrogen-doped carbon nanohorn were prepared. Electrochemical analysis has shown the higher capacitance performance of the nitrogen-doped graphene (NG) electrode, because nitrogen provides free valence electron to interact with electrolyte. Then magnetic metal oxides were in situ hybridized to a nitrogen-doped graphene to produce magnetic metal oxide/NG hybrid materials, and the electrochemical measurements of the prepared hybrid material electrodes were conducted without and with the external magnetic field (8.98 mT at the inflection point) of using a Helmholtz coil. The specific capacitance took an increasing order of NiO/NG (697 F/g, 747 F/g) < Co3O4/NG (963 F/g, 1092 F/g) < Fe3O4/NG (973 F/g, 1254 F/g) in an external magnetic field of (0 mT, 8.98 mT) at a scan rate of 5 mV/s. Although these electrodes displayed high capacitance and better charge/discharge profile, cycle retention (83 to 92% under no magnetic field) was not necessarily good or it fluctuated under 8.98 mT. These behaviours by the addition of magnetic metal oxides and external magnetic field are due to the electrical conductivity of metal oxides and the Lorentz force effect of the magnetic field, respectively. Thus, it can be confirmed that Fe3O4/NG hybrid has higher potential as a magnetic material electrode for supercapacitors and the magnetic field enhances the capacitance.

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To enhance the energy density, hybrid supercapacitors were fabricated, and their electrochemical features were investigated using a two-electrode configuration. By assembling nitrogen-doped graphene/magnetite (NG/Fe3O4) on indium tin oxide-coated (ITO) glass as a cathode and NG/carbon dots(Cdots)/Fe3O4 on ITO glass as an anode, a much higher gravimetric specific capacitance of 252.2 F/g, at a current density of 0.5 A/g, was obtained from this asymmetric supercapacitor compared with that (212.0 F/g) of a symmetric supercapacitor (NG/Cdots/Fe3O4)//(NG/Cdots/Fe3O4). A gravimetric energy density of 90.1 Wh/kg was obtained for an asymmetric ITO glass device at a specific power density of 400.0 W/kg. On the other hand, when an asymmetric two-electrode cell was fabricated with a Cdots/polypyrrole (PPy)/Fe3O4/TEMPO-oxidized cellulose nanofiber (TOCNF)-film electrode and a Cdots/PPy/TOCNF-film electrode, the specific capacitance (107.1 F/g) at a current density of 0.8 A/g was lower than that (456.4 F/g) of a symmetric (Cdots/PPy/Fe3O4/TOCNF)//(Cdots/PPy/Fe3O4/TOCNF)-film cell. Subsequently, a gravimetric energy density of 40.6 Wh/kg was achieved for a symmetric-film device at a specific power density of 320 W/kg. These results suggest that our method offers an efficient approach to developing symmetric and asymmetric devices consisting of hybrid materials for meeting the ever-increasing demands on energy-storage devices.