Bimetallic cobalt-based spinel is sparking much interest, most notably for its excellent bifunctional performance. However, the effect of Fe³⁺ doping in Co₃ O₄ spinel remains poorly understood, mainly because the surface state of a catalyst is difficult to characterize. Herein, a bifunctional oxygen electrode composed of spinel Co₂ FeO₄ /(Co0.72 Fe0.28 )Td (Co1.28 Fe0.72 )Oct O₄ nanoparticles grown on N-doped carbon nanotubes (NCNTs) is designed, which exhibits superior performance to state-of-the-art noble metal catalysts. Theoretical calculations and magnetic measurements reveal that the introduction of Fe³⁺ ions into the Co₃ O₄ network causes delocalization of the Co 3d electrons and spin-state transition. Fe³⁺ ions can effectively activate adjacent Co³⁺ ions under the action of both spin and charge effect, resulting in the enhanced intrinsic oxygen catalytic activity of the hybrid spinel Co₂ FeO₄ . This work provides not only a promising bifunctional electrode for zinc-air batteries, but also offers a new insight to understand the Co-Fe spinel oxides for oxygen electrocatalysis.
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Wang et al. (2019) studied this question.