Stabilizing sub-3 nm NiFe hydroxides on atomic Fe–N4 moieties for efficient oxygen reactions in rechargeable Zn–air batteries

Abstract

NiFe hydroxides and atomic Fe–N₄ sites respectively represent one of the best noble-metal-free active components for oxygen evolution/reduction reactions (OER/ORR) in Zn–air batteries (ZABs), but their integration into one catalyst without a performance trade-off is still a great challenge. Here, we report a bifunctional OER/ORR catalyst in which ultrasmall NiFe hydroxides (average size: 2.1 nm) are integrated into graphene-supported porous carbon layers with abundant Fe–N₄ sites (NiFe&FeNCL/G) through a confining-coupling growth method. Experimental and theoretical results demonstrate that the NiFe hydroxides coupled with the Fe–N₄ sites enhance the intrinsic activity, resulting in much better OER performance than those loaded on other substrates. Meanwhile, some inactive Fe–N₄ ORR moieties are buried by ultrasmall NiFe hydroxide units, but the surrounding Fe–N₄ sites exhibit such a low energy barrier due to hydroxide modulation that the performance loss from the buried Fe–N₄ moieties can be overcome. As a result, NiFe&FeNCL/G displays a low potential gap of 0.68 V for OER/ORR. Furthermore, NiFe&FeNCL/G-based ZABs deliver excellent battery output performance, including high peak power densities of 404.9 mW cm⁻² under Zn–O₂ conditions and 242.2 mW cm⁻² in a Zn–air atmosphere system. More importantly, the charge–discharge voltage platform gap remains at 0.68 V at 10 mA cm⁻² for 700 hours, demonstrating the outstanding long-term cycling stability. These results indicate a significant advance in design principles to overcome the performance trade-off issue, paving the way for the future development of practical ZABs.