Effect of low magnetic fields on the morphology and electrochemical properties of MnO2 films on nickel foams

Nanostructured MnO2 films have been deposited on nickel foams by a hydrothermal synthesis route under a low magnetic field. It was shown that even though the magnetic field was weak (at a level of several millitesla), it induced substantial change in the nanostructures and electrochemical properties of the MnO2 films. The as-prepared MnO2 thin films were largely amorphous and had a spherical superstructure composed of thin nanoflakes. The size of the microspheres decreased and, accordingly, the specific surface area increased with increasing magnetic field's strength. A probable formation mechanism for the nanostructures of MnO2 was proposed that the weak magnetic field enhanced the nucleation rate, and modified subsequent growth and aggregation processes, which all had a significant influence on the morphology and electrochemical performance of the MnO2 electrodes. The optimal MnO2 electrode synthesized under the low magnetic fields had a specific capacitance of 493.0 F g−1 at 2.0 A g−1 and a retention ratio of 95.6% over 1000 cycles. Therefore, low magnetic fields may be an economic and effective tool in hydrothermal synthesis of MnO2 electrodes for high-performance supercapacitors.