Promoting oxygen reduction kinetics of single Fe sites for robust neutral Zn-air batteries via engineering synergistic Fe nanocluster as proton-feeding center

Constructing highly efficient and cost-effective catalysts for neutral oxygen reduction reaction (ORR) remains extremely challenging due to the sluggish reaction kinetics resulting from the low ionic conductivity and limited OH− concentration in the neutral electrolytes. Herein, we intentionally integrate the atomic Fe-N4 sites and Fe nanoclusters on N-doped multimodally porous carbon (FeSA+NC@NMPC) to achieve coherent optimization of rapid oxygen-containing intermediate conversions and fast water dissociation to provide abundant protons for boosting neutral ORR performance. As expected, the FeSA+NC@NMPC exhibits an excellent half-wave potential of 0.76 V in 0.1 M phosphate buffer solutions, outperforming that of commercial Pt/C (0.73 V). Theoretical calculations reveal the synergistic effect between atomic Fe-N4 sites and Fe nanoclusters, in which the former possess stable O2 adsorption and rapid intermediate conversion, while the latter facilitates fast water dissociation to supply protons for accelerating the proton-coupled electron transfer process. Moreover, the FeSA+NC@NMPC-based neutral zinc-air batteries afford a high open-circuit potential of 1.42 V and outstanding cycling stability at 5 mA cm−2 for 100 h. This work utilizes the advantages of both single sites and clusters of Fe to provide an in-depth understanding of the neutral ORR mechanism and advances the development of related energy storage and conversion technologies.