Boosting the OER performance of NiFe₂O₄ through Cr and Mn doping via hydrothermal synthesis

The growing demand for green hydrogen requires efficient, cost-effective electrocatalysts for the oxygen evolution reaction (OER), a process currently hindered by sluggish kinetics. This study explores the optimisation of the spinel oxide NiFe₂O₄ through the partial Fe substitution with Cr and Mn, synthesised via a subcritical hydrothermal method, as an alternative to the standard Pt-group metals (PGM)-based electrocatalysts for the OER in alkaline environment. The work aims to establish a direct correlation between the chemical nature of the dopant, the resulting physicochemical properties, and the electrocatalytic performance. Detailed structural and surface characterisation, including XRD, TEM, and XPS, revealed distinct behaviours for the two dopants. Cr incorporation successfully produced phase-pure spinel nanoparticles with significantly reduced crystallite sizes and very high specific surface area (up to 226 m²/g). In contrast, high Mn substitution led to the formation of secondary phases (Ni(OH)₂) and nanoscale inhomogeneity, which persisted even after calcination, suggesting an incomplete inclusion of the three different metals in the same spinel lattice. Electrochemical investigations demonstrated that the nature of the dopant strongly influences OER activity. While Mn-doped samples showed higher apparent activity than pristine NiFe₂O₄, this improvement was attributed solely to an increased number of active sites (surface area) rather than improved intrinsic kinetics. Conversely, the Cr-substituted sample NiFeCrO₄ exhibited superior performance, surprisingly matching the OER performances of the benchmark IrO_X. This outstanding activity was ascribed to a synergistic effect: the material combines a high specific surface area with enhanced intrinsic kinetics, driven by an optimal composition rich in Cr³⁺ which is hypothesised to modulate the overall $e_g$ occupation to a favourable value for promoting the OER.