Dynamic evolution of self-renewal Fe–N–C catalysts for the acidic oxygen reduction reaction

Abstract

Heterogeneous molecular Fe–N–C catalysts hold promise for the oxygen reduction reaction (ORR), but their stability in acidic media remains a bottleneck. Here, we report the synthesis of a self-renewal Fe–N–C catalyst by uniformly polymerizing an iron polyphthalocyanine (FePPc) shell around carbon nanotubes (CNTs) via a microwave-assisted method. This FePPc/CNT catalyst achieves a much higher Fe mass loading (2.92 wt%) compared to directly depositing iron phthalocyanine (FePc) molecules on CNTs (FePc/CNT, 0.80 wt%) while maintaining a similar density of exposed Fe–N₄ sites to electrolytes. FePPc/CNT exhibits superior ORR activity in 0.1 M HClO₄ electrolyte with a half-wave potential (E_1/2) of 0.74 V (vs. reversible hydrogen electrode), a low Tafel slope of 51 mV dec⁻¹, and a high turnover frequency (TOF) of 0.98 site⁻¹ s⁻¹. Density functional theory (DFT) calculations attribute this enhanced activity to strong FePPc–CNT interactions that facilitate efficient electron transfer and favorable reaction energetics. Critically, FePPc/CNT demonstrates enhanced stability in the acidic electrolyte, retaining ∼80% of its initial current density after 24 h of the chronoamperometric test, outperforming FePc/CNT (42% after 5 h) and physically mixed FePPc and CNTs (49% after 24 h). Quantitative analysis reveals a unique self-renewal mechanism involving layer-by-layer shedding of FePPc, which exposes fresh active sites to sustain catalytic activity. At the same time, detached FePPc fragments sediment on CNTs. Furthermore, leached Fe ions migrate onto CNTs and aggregate into FeO_x nanoclusters, eventually leading to irreversible deactivation. These findings provide new insights for designing durable Fe–N–C catalysts for various reactions.