Confined-Space-Assisted Preparation of Fe<sub>3</sub>O<sub>4</sub>‑Nanoparticle-Modified Fe–N–C Catalysts Derived from a Covalent Organic Polymer for Oxygen Reduction

Iron–nitrogen–carbon (Fe–N–C) has been considered as one of the most promising nonprecious metal catalysts for the oxygen reduction reaction (ORR) in fuel cells and metal–air batteries. Herein, we prepare a highly active Fe<sub>3</sub>O<sub>4</sub>/Fe–N–C catalyst (named COP@K10-Fe-900), for the ORR from a layered tetraphenylporphyrin-based (TPP-based) covalent organic polymer (COP) grown in nanoconfined space as precursors, followed by iron ion incorporation and a pyrolysis process. The nanoconfined space, i.e., montmorillonite (K10) template, contributes to the unique layered structure of designed precursors and enables Fe<sub>3</sub>O<sub>4</sub> nanoparticles to disperse uniformly in the resulting layered Fe–N–C catalyst. The nanoconfined space reduces the iron-based particle size from ∼50–150 to ∼10 nm. An enhancement of 50 mV was obtained after using layer space confinement for half-wave potential. Moreover, the half-wave potential of the newly developed COP@K10-900 exceeds 20 mV as compared to the benchmark Pt/C in alkaline media. Therefore, we believe that this work can provide an important guideline for designing highly active M–N–C catalysts that can be widely used in energy conversion and storage devices.