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

An electrochemical processing route was developed to synthesize NiFe₂O₄ spinel ferrite nanoparticles with controlled phase formation and morphology by electrooxidation of a sacrificial iron anode in aqueous NiSO₄ at alkaline pH under constant potentials. Applied voltage governed the phase evolution: particles produced at 3–6 V contained secondary phases (α-Fe₂O₃ and/or β-Ni(OH)₂) with poor crystallinity, whereas 9–12 V yielded phase-pure cubic inverse-spinel NiFe₂O₄ 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 NiFe₂O₄ stoichiometry (Ni:Fe:O ≈ 1:2:4) and exhibited the largest electrochemically active surface area (∼240 cm²) 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 <20% H₂O₂ selectivity. When implemented in rechargeable zinc–air batteries, the 12 V NiFe₂O₄ achieved a specific discharge capacity of ∼308.6 mAh g⁻¹ and stable cycling performance.