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 noblemetal-free active parts for oxygen evolution/reduction reactions (OER/ORR) in Zn-air batteries (ZABs), but integration of them into one catalyst without performance tradeoff is still a great challenge. Here we report a bifunctional OER/ORR catalyst that ultrasmall NiFe hydroxides (average size: 2.1 nm) are integrated in graphene-supported porous carbon layers with abundant Fe-N₄ sites (NiFe&FeNCl/G) through a confiningcoupling growth method. Experimental and theoretical research results demonstrate that the NiFe hydroxides coupled by the Fe-N₄ sites enhance the intrinsic activity, resulting in much better OER performance than those loading on other substrate.Meanwhile, there is a part of inactive Fe-N₄ ORR moieties buried by ultrasmall NiFe hydroxides, but the surrounding Fe-N₄ sites hold so low energy barrier due to hydroxide modulation that can overcome the performance loss from the buried Fe-N₄ moieties. 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 such as high peak power densities of 404.9 mW cm^-2 under Zn-O₂ conditions and 242.2 mW cm^-2 under Zn-air atmosphere. More importantly, chargedischarge voltage platform gap remains 0.68 V at 10 mA cm^-2 for 700 hours, demonstrating its outstanding long-term cycling stability. These results indicate a significant advance in design principle to overcome the performance trade-off issue, paving the way for future development of practical ZABs.