Magnetic-field-assisted self-assembly and oxygen-defect engineering in nickel ferrite oxide: structural evolution and electrochemical performance

Nickel ferrite (NiFe2O4) is an attractive pseudocapacitive electrode owing to its reversible Ni2+/Ni3+ and Fe2+/Fe3+ redox couples. However, its performance is hindered by nanoparticle agglomeration and limited conductivity that suppress interfacial charge storage. Here, a magnetic-field-assisted solvothermal strategy is used to direct NiFe2O4 precursors into colloidal bead-on-string one-dimensional assemblies, which enlarge the electrolyte-accessible surface and porosity while constructing continuous electron/ion transport pathways. Neutron powder diffraction combined with Rietveld refinement reveals a slight lattice expansion, a decrease in the oxygen positional parameter u, and B-site cation redistribution from Ni/Fe = 0.50/0.50 to 0.53/0.47, while the combined EPR, XPS, and refinement results support a higher relative concentration of oxygen-vacancy-related defects in the magnetic-field-assisted sample. In addition, low-angle peak broadening of the FWHM indicates refinement of the coherent domain size. The approximately eightfold conductivity enhancement is associated with oxygen-vacancy-related defect evolution, Fe2+/Fe3+ and Ni2+/Ni3+ valence redistribution, and mild B-site Ni enrichment. Meanwhile, the chain-like porous architecture improves electrolyte-accessible surface area and interparticle connectivity, which is consistent with the improved charge-transfer and pseudocapacitive response of NFO-G3. Consequently, the optimized sample (NFO-G3) delivers 370.14 F g−1 at 1.0 A g−1 in 2 M KOH and retains 96.16% of its capacitance after 10 000 cycles. This work establishes a guided, magnetic-field approach that couples self-assembly with defect engineering to tune structure and interfaces, offering a practical route to high-rate, durable pseudocapacitor electrodes.