Core–shell structured Fe–N–C wrapped by an ultrathin porous carbon shell as a robust electrocatalyst for the oxygen reduction reaction

Abstract

Porous carbon supported FeN_x catalysts have become one of the candidates to replace platinum group metal (PGM) catalysts for the oxygen reduction reaction (ORR) due to their high intrinsic reactivity and substantial accessible active sites. However, FeN_x sites suffer from insufficient long-term stability due to migration and aggregation of isolated Fe atoms or decomposition of FeN_x active moieties arising from undesired Fenton reactions. Herein, an effective core–shell structured Fe–N–C@NC catalyst composed of ultra-thin porous conductive carbon shells and carbon supported-FeN_x active cores was prepared by constructing a polyvinylpyrrolidone (PVP) molecular adsorbed Fe-doped ZIF-8 precursor followed by high temperature pyrolysis. Applied to the ORR, the Fe–N–C@NC catalyst displayed a much higher half-wave potential (0.876 V) than that of 20% Pt/C (0.854 V) and significantly increased durability in an alkaline electrolyte. In practical application, the Fe–N–C@NC-assembled Zn–air battery can reach a peak power density of 170.8 mW cm⁻², which is superior to that of 20% Pt/C (163.1 mW cm⁻²) and that of the pristine Fe–N–C (130.7 mW cm⁻²). More impressively, no apparent decay was observed after 150 h of charge and discharge testing. The XRD and XPS results after cycling revealed that the outer ultrathin carbon shells of Fe–N–C@NC could prevent the loss of the inner FeN_x active sites, maintaining stable and efficient ORR activity.