Fe₃C/CoFe₂O₄ nanoparticles wrapped in one-dimensional MIL-53(Fe)-derived carbon nanofibers as efficient dual-function oxygen catalysts

Structure engineering for non-precious-metal catalyst plays the major roles in governing the activity and stability for oxygen evolution/reduction reactions (OER/ORR). Herein, via an electrospinning method, one-dimensional (1D) carbon fibers containing MIL-53 (Fe) and Co³⁺ are obtained as precursor for preparation of Fe₃C/CoFe₂O₄-embedded carbon nanofibers (Fe₃C/CoFe₂O₄@CNFs). Scanning electron microscopy image of Fe₃C/CoFe₂O₄@CNFs-1.5 (mass ratio of MIL-53(Fe) to CoCl₃·6H₂O is 1.5) shows that it has a well-formed 1D structure with diameters of 200–300 nm. Hydrophilic surface with a contact angle of 16.5° is detected for Fe₃C/CoFe₂O₄@CNFs-1.5 (250.1 m² g⁻¹), which shows a promising bifunctional activity with ΔE = 0.73 V (E_1/2 = 0.84 V (ORR) and E_j10 = 1.57 V (OER)). For ORR, E_1/2 of Fe₃C/CoFe₂O₄@CNFs-1.5 is close to that of commercial Pt/C (0.85 V), while its activity maintenance (92.5%) is much better than Pt/C (78.4%) after 30000 s, indicating that low-valence interfaces of CoFe₂O₄ (Co²⁺) and Fe₃C (Fe²⁺) can provide multiple active sites to stabilize ORR rate. For OER, Fe₃C/CoFe₂O₄@CNFs-1.5 has a low overpotential of 340 mV at 10 mA cm⁻² and a high Faraday efficiency of 92.13%. Co²⁺ acts as an initiator to boost the formation of active CoOOH for OER. Oxygen vacancies on the reverse spinel structure of CoFe₂O₄ make the surface Co²⁺ easier to be oxidized to Co³⁺ (CoOOH). Fe₃C/CoFe₂O₄ wrapped in carbon fiber lowers their aggregation and corrosion during ORR/OER. Networks constructed by the interconnected 1D nanofibers can provide 3D pathways for mass transfer. It indicates that bimetallic species (Fe/Co) can change the electronic and interface structures to promote bifunctional activity.