Boosting oxygen reduction activity of spinel CoFe₂O₄ by strong interaction with hierarchical nitrogen-doped carbon...

The unique hierarchical nitrogen-doped carbon nanocages (hNCNC) are used as a new support to homogeneously immobilize spinel CoFe₂O₄ nanoparticles by a facile solvothermal method. The so-constructed hierarchical CoFe₂O₄/hNCNC catalyst exhibits a high oxygen reduction activity with an onset potential of 0.966V and half-wave potential of 0.819V versus reversible hydrogen electrode, far superior to the corresponding 0.846 and 0.742V for its counterpart of CoFe₂O₄/hCNC with undoped hierarchical carbon nanocages (hCNC) as the support, which locates at the top level for spinel-based catalysts to date. Consequently, the CoFe₂O₄/hNCNC displays the superior performance to the CoFe₂O₄/hCNC, when used as the cathode catalysts in the home-made Al-air batteries. X-ray photoelectron spectroscopy characterizations reveal the more charge transfer from CoFe₂O₄ to hNCNC than to hCNC, indicating the stronger interaction between CoFe₂O₄ and hNCNC due to the nitrogen participation. The enhanced interaction and hierarchical morphology favor the high dispersion and modification of electronic states for the active species as well as the mass transport during the oxygen reduction process, which plays a significant role in boosting the electrocatalytic performances. In addition, we noticed the high sensitivity of O 1s spectrum to the particle size and chemical environment for spinel oxides, which is used as an indicator to understand the evolution of ORR activities for all the CoFe₂O₄-related contrast catalysts. Accordingly, the well-defined structure-performance relationship is demonstrated by the combination of experimental characterizations with theoretical calculations. This study provides a promising strategy to develop efficient, inexpensive and durable oxygen reduction electrocatalysts by tuning the interaction between spinel metal oxides and the carbon-based supports.