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

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.