NH3-induced activation of hydrophilic Fe–N–C nanocages for enhanced oxygen reduction reaction

Abstract

Non-noble metal electrocatalysts for the oxygen reduction reaction (ORR) are urgently needed in metal–air batteries, seawater batteries and fuel cells. Fe–N–C materials are among the most active catalysts for the ORR. Fe–N–C synthesis usually requires post-heat treatment after pyrolysis which is time-consuming and inevitably triggers inactive aggregate Fe species due to difficulties in controllable atom-level modulation. Here, highly active Fe–N–C catalysts were prepared by a simple process involving an ammonia etching treatment by using ZIF-8 as a hard template and a mixture of FeSO₄ and 2-methylimidazole as the Fe, N and C precursors. The direct ammonia treatment modulates N and Fe active species and removes the unstable carbon framework to form pyrolyzed Fe–N–C nanocages with a well-dispersed pore structure. The obtained Fe–N–C exhibits a potential of 0.89 V vs. RHE at a kinetic current density of −1 mA cm⁻² (E₋₁) for the ORR, similar to commercial Pt/C, but outperforming it in terms of stability and methanol tolerance. In situ electrochemical Raman and density functional theory provide insights into the origin of the activity of Fe–N–C materials and the underlying ORR electrocatalytic mechanisms at the molecular level.