Fe2O3/spinel NiFe2O4 heterojunctions in-situ wrapped by one-dimensional porous carbon nanofibers for boosting oxygen evolution/reduction reactions

Abstract

One-dimensional (1D) nanofiber structure of electrocatalyst has attracted increasing attention in oxygen evolution/reduction reactions (OER/ORR) owing to its unique structural properties. Here, MIL-53(Fe) and Ni(NO3)2·6H2O are incorporated into the electrospun carbon nanofibers (CNFs) to prepare the nickel-iron spinel-based catalysts (Fe2O3/NiFe2O4@CNFs) with 1D and porous structure. The marked Fe2O3/NiFe2O4@CNFs-2 catalyst has a tube diameter of approximately 300 nm, a high surface area of 282.4 m2 g−1 and a hydrophilic surface (contact angle of 16.5°), which obtains a promising bifunctional activity with ΔE = 0.74 V (E1/2 = 0.84 V (ORR) and Ej10 = 1.58 V (OER)) in alkaline media. Fe2O3/NiFe2O4@CNFs-2 has a higher catalytic stability (93.35%) than Pt/C (89.36%) for 30,000 s tests via an efficient 4e− ORR pathway. For OER, Fe2O3/NiFe2O4@CNFs-2 obtains a low overpotential of 350 mV and a high Faraday efficiency of 92.7%. NiFe2O4 (Ni2+ in tetrahedral position) relies on its variable valence states (NiOOH and/or FeOOH) to obtain good catalytic activity and stability for OER, while CNFs wrap/protect the active components (Fe–N and graphic N) in the carbon skeleton to effectively improve the charge transfer (conductivity), activity and stability for ORR. Porous 1D nanofiber structure provides abundant smooth pathways for mass transfer. It indicates that the bimetallic active substances can promote bifunctional activity by synergistically changing the oxide/spinel interface structure.