High-purity magnetite nanoparticles with a mixed quasi-spherical and cubic morphology have been successfully synthesized using a new alternating monopolar arrangement of multiple iron electrodes in water. The magnetite nanoparticles have a narrow size distribution with a mean size ranging from 20 to 30 nm, depending on the applied voltage and pH. The formation of magnetite nanoparticles is favored only in the pH range of 7-10; outside this range, magnetite nanoparticles cannot be produced. The magnetite nanoparticles exhibit ferromagnetic properties with saturation magnetization ranging from 16 to 75 emu/g. They demonstrated to be a promising electrocatalyst for oxygen reduction reactions in basic media. This facile method is a promising production route for magnetite nanoparticles and can be easily scaled up for large production. (C) 2020 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.