π-Conjugation effect influenced the oxygen electrochemical reaction electrochemical efficiency in Fe–N–C catalysts

Abstract

Advancing cost-effective and efficient Fe–N–C catalysts is essential for expanding the commercial viability of Zn–air batteries. Covalent organic frameworks (COFs) have emerged as promising precursors owing to their tunable architecture and excellent chemical stability. However, their limited electrochemical activity and harsh synthesis conditions have hindered practical applications. In this work, macromolecular polyimide COFs (PI-COFs) were successfully synthesized through a high-temperature solid-phase method. The π-conjugation degree of the xCN/Fe catalysts was finely tuned by adjusting the size of the imide π-conjugation unit within the PI-COF framework. Structural characterization confirmed that variations in π-conjugation directly affected the density of delocalized π electrons. A volcano-type dependence was observed between π-electron delocalization and OER activity. Notably, the MNCN/Fe catalyst with moderate π conjugation demonstrated excellent bifunctional activity, with an ORR half-wave potential of 0.808 V (vs. RHE) and an OER overpotential of 322 mV at 10 mA cm⁻². The MNCN/Fe-based Zn–air battery could illuminate a small light bulb, verifying its practical feasibility. Overall, this work establishes an effective strategy to modulate π-conjugation in Fe–N–C catalysts for enhanced electrochemical energy conversion.