The electrochemical mechanism of the formation of magnetite nanoparticles is studied. The proposed mechanism suggests the formation of iron hydroxide Fe(OH) 2 in the presence of oxygen which produces lepidocrocite (γ-FeOOH) followed by its chemical dehydration. This is in contrast to other reported mechanisms that suggest the reduction of Fe(OH) 3 at the cathode. Video frames captured during the electrosynthesis of magnetite, in a typical two-electrode cell, indicate that the nanoparticles form in the region close to the anode. The pH value near the anode and cathode changes with time, indicating the formation of nanoparticles. Additional experiments in a two-compartment cell fitted with a cationic membrane, to avoid direct intermixing of Fe 2+ and OH − and possible oxide or oxyhydroxide reduction at the cathode, support this mechanism. The amount of dissolved oxygen in the electrolyte was found to be a key factor to produce magnetite by promoting the transformation of Fe(OH) 2 into (γ-FeOOH). Hydrogen bubbling during electrosynthesis does not contribute to the reduction of the oxyhydroxides, according to X-ray diffraction results. The paper presents a proposed mechanism for the formation of magnetite, based on previous and new evidence.

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In the search for a new energy strategy, metal–O2 batteries stand out as a potential alternative. To reduce losses due to high overpotential and to improve their performance, redox mediators emerge as a powerful strategy for the commercialization of these promising devices.