Effect of mixed metal ions and incorporation of activated carbon on the physiochemical and catalytic properties of spinel based (Ni,Fe)Co₂O₄@C nanoparticles

The challenge of producing cost-effective oxygen electrocatalysts for the oxygen reduction reaction (ORR) continues to impede the advancement of fuel cell technology. Herein, we report the effect of Ni and activated carbon on mixed metal (Ni,Fe)Co₂O₄ and (Ni,Fe)Co₂O₄@C spinel oxide nanoparticles synthesized by facile and versatile co-precipitation method followed by the physio-chemical, morphological properties and electrochemical analysis towards oxygen reduction reaction. The formation of FeCo₂O₄, (Ni,Fe)Co₂O₄ and (Ni,Fe)Co₂O₄@C nanoparticles was confirmed with X-ray diffraction and Raman analysis while D and G band revealed the presence of carbon in the (Ni,Fe)Co₂O₄@C nanoparticles. The Brunauer–Emmett–Teller analysis reveals the mesoporous behaviour of the synthesized multi-metal oxide nanoparticles. X-ray photoelectron spectroscopy analysis reveals the presence of Co³⁺/Co²⁺, Fe³⁺/Fe²⁺, Ni³⁺/Ni²⁺, O1s and C1s confirms the formation of complex (Fe³⁺Co²⁺)(Ni²⁺Ni³⁺Fe²⁺Fe³⁺Co³⁺)₂O₄ spinel structure. The ionic radii of the Co, Fe and Ni ions plays a vital role in the formation of complex spinel structures. The ORR analysis was studied by electrochemical analysis and observed (Ni,Fe)Co₂O₄ outperformed FeCo₂O₄ in terms of onset potential and current densities for the ORR. (Ni,Fe)Co₂O₄@C exhibited the highest catalytic activity with −2.18 mAcm⁻² at 0.1 V vs RHE, which is relatively comparable with that of commercial Pt/C. Furthermore, the electrocatalytic analysis revealed the ORR mechanism adheres to the direct "4e⁻" process. The intermetallic interactions between the Co, Fe and Ni ions along with high crystallinity, significant surface area, high porosity, and the influence of carbon incorporation that are all contribute to the improved electrocatalytic activity of the cobalite-based multi-metal spinel oxides.