Deprotonation effect doubles active site density in Fe-N4-C catalyst for oxygen reduction electrocatalysis

Abstract

Iron-nitrogen-carbon (Fe-N-C) materials with Fe-N₄ structures have been considered as the most promising alternatives of scarce and precious platinum (Pt) for oxygen reduction reaction. Particularly, the high-temperature pyrolysis of a precursor mixture of N-containing amine polymers, Fe salts, and carbon supports, has become a popular method for the synthesis of high-performance Fe-N-C catalysts. The oxidative polymerization of amine monomers can usually proceed under acidic conditions, however, the acid-caused protonation of N-groups is not conducive to their coordination with Fe ions for the formation of high-density Fe-N₄ sites. Here, we propose a protonation elimination strategy of soaking the polymerization products in alkaline solutions to increase Fe-N₄ active sites. Theoretical calculations display that the Gibbs free energy change values of binding reactions between Fe ions and N-groups are -3.70 and -26.99 kcal/mol at pH 0 and 7, respectively, suggesting that the deprotonation can facilitate the Fe-N coordination. There is a two-fold increase in the number of Fe-N₄ active sites for final Fe-N-C catalyst which exhibits significantly enhanced ORR activity and excellent Zn-air battery performance. This deprotonation effect can be applied to different amine compounds and transition-metal ions as a universal strategy for the development of preeminent non-precious metal carbon catalysts.