Core–Shell-Architected Active Sites from a Prussian Blue Analogue Enable Bifunctional Oxygen Electrocatalysis for High-Performance Rechargeable Zinc–Air Batteries

High Resolution Image Download MS PowerPoint Slide Rechargeable Zn–air batteries (RZABs) offer high energy and power densities along with superior safety and economic viability, making them ideal candidates for next-generation energy storage systems. However, their performance is heavily dependent on the development of efficient and durable bifunctional electrocatalysts for the oxygen reduction reaction (ORR) during discharging and the oxygen evolution reaction (OER) during charging, which have been major challenges for RZABs. Prussian blue analogues (PBA) have recently emerged as promising precursors for bifunctional oxygen electrocatalysts. Despite this, the precise engineering of active sites that selectively catalyze the ORR and the OER remains a challenge. In this study, we reported the successful transformation of NiFe PBA into a core–shell nanostructure, denoted as NGCA (nanographitic carbon shell/NiFe alloy core), via a facile annealing process at temperatures ranging from 500 to 900 °C. By fine-tuning the annealing temperatures, we optimized the formation of a transition metal–nitrogen-doped carbon (TM–N-C) shell, which acts as the primary ORR active site, while the NiFe alloy core selectively catalyzes the OER. This unique core–shell configuration delivered an exceptional bifunctional catalytic performance with a low OER/ORR potential gap of 0.87 V. The electrocatalyst’s efficacy was demonstrated in a liquid-state RZAB, exhibiting stable charge–discharge cycling for over 60 h without signs of overcharging. Electrochemical cycling tests and postmortem analyses revealed that the Ni-rich NiFe alloy core predominantly drives the OER. At the same time, ORR activity is retained by a synergistic interaction between Fe 5 C 2, which coexists with pyridinic-N and metal–N as the active sites. This study highlights the potential of PBA-derived materials as self-standing precursors for the scalable and cost-effective production of bifunctional oxygen electrocatalysts.