Size-controlled synthesis of spinel nickel ferrite nanoparticles by electrochemical method for metal-air batteries

An electrochemical processing route was developed to synthesize NiFe 2 O 4 spinel ferrite nanoparticles with controlled phase formation and morphology by electrooxidation of a sacrificial iron anode in aqueous NiSO 4 at alkaline pH under constant potentials. Applied voltage governed the phase evolution: particles produced at 3–6 V contained secondary phases (α-Fe 2 O 3 and/or β-Ni(OH) 2 ) with poor crystallinity, whereas 9–12 V yielded phase-pure cubic inverse-spinel NiFe 2 O 4 confirmed by XRD and electron diffraction. Increasing voltage increased throughput and crystal development, raising the production rate from 14.33 to 76.88 mg cm −2 h −1 and increasing crystallite size from 8.3 to 31.8 nm (3–12 V), while microscopy showed predominantly cubic particles with mean size up to ~56.8 nm at 12 V. The 12 V particles approached the target NiFe 2 O 4 stoichiometry (Ni:Fe:O ≈ 1:2:4) and exhibited the largest electrochemically active surface area (~240 cm 2 ) with reduced charge-transfer resistance. As a bifunctional oxygen electrocatalyst in alkaline media, the optimized sample delivered an OER onset potential of ~1.49 V (vs. RHE), an overpotential of 337 mV at 10 mA cm −2 , and a Tafel slope of 135 mV dec −1 , while ORR proceeded with n > 3.5 and