MOFs meet COFs: Ultra-low Ru loaded oxygen reduction reaction electrocatalyst with biomimetic branch-leaf structure for rechargeable Zn-air batteries

Developing innovative, efficient, and economical oxygen reduction reaction (ORR) electrocatalysts for zinc-air batteries (ZABs) is critical but challenging, and biomimetic-designed hybrid catalysts stand out. In this work, the hierarchical N-doped carbon nanofibers (Ru@FeZn-HNC/CNFs) anchored by ultra-small Ru nanoparticles (NPs) was achieved by covalent organic framework/metal organic framework (COF/MOF) template approach combined with electrospinning technology, which exhibits a unique and stable biomimetic “branch and leaf” morphology. This distinctive biomimetic architecture not only facilitates fast charge transfer and mass transport, but also exposes more active sites (Ru NPs, M-N x , and active N species). It is possible to remarkably reduce the Ru content without sacrificing ORR activity, thus ensuring cost-effectiveness. Specifically, the synthesized catalyst presents impressive limiting current density (J L  = 6.15 mA cm −2 ), outstanding half-wave potential (E 1/2  = 0.85 V), high 4e − selectivity (H 2 O 2 yield <2 %) and distinguished durability (94.7 % current retention after 20 h) at low Ru loading, surpassing the state-of-art Pt/C catalyst. Furthermore, the aqueous/solid-state ZABs assembled by Ru@FeZn-HNC/CNFs perform significantly better open-circuit voltage (1.48 V and 1.41 V), power density (175 mW cm −2 and 86.9 mW cm −2 ), specific capacity (810.2 mAh g Zn −1 and 720.2 mAh g Zn −1 ), and long-term cycling durability (> 300 h and 1000 min) than Pt/C-based counterparts. Intriguingly, Ru@FeZn-HNC/CNFs-based flexible ZABs demonstrate satisfactory foldability and mechanical stability. This study evidences the feasibility of Ru@FeZn-HNC/CNFs catalyst employed as air cathode of ZABs, and opens up a new avenue for the exploration of flexible wearable energy supply devices.