Effect of External Magnetic Field on Hybrid Supercapacitors of Nitrogen-Doped Graphene with Magnetic Metal Oxides

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.