MFe2O4 and MFe@Oxide Core–Shell Nanoparticles Anchored on N‐Doped Graphene Sheets for Synergistically Enhancing Lithium Storage Performance and Electrocatalytic Activity for Oxygen Reduction Reactions

The intrinsically low electric conductivity and self‐aggregation of MFe₂O₄ during charge/discharge affect their lithium storage performance and electrocatalytic activity. To mitigate these problems, it is shown that N‐doped graphene sheets (NGS), as a highly conductive platform, finely disperse the MFe₂O₄ nanoparticles and rapidly shuttle electrons to and from the MFe₂O₄ nanoparticles. Moreover, by forming a metal@oxide core–shell nanostructure, fast electron transfer from the exterior oxide layer to NGS is achieved. Introducing NGS into MFe₂O₄ allows the composites to exhibit the comparable specific capacity (based on the total mass) to MFe₂O₄, although over 10 wt% of NGS contributes a low specific capacity of around 320–400 mAh g^?1. More importantly, introducing NGS significantly increases the cycling stability performance: 97.5% (CoFe₂O₄/NGS) and ≈100% (NiFe₂O₄/NGS) of the specific capacities have been retained after 80 cycles, far higher than the capacity retentions of CoFe₂O₄ (35.3%) and NiFe₂O₄ (43.7%) tested under otherwise identical conditions. Also demonstrated are the excellent rate capabilities of the composites. For catalyzing the oxygen reduction reaction, the activity is significantly improved when the MFe₂O₄ nanoparticles are transformed into metal@oxide core–shell nanostructure, mainly because the core–shell nanostructure exhibits lower charge transfer resistance.