Melamine-Sponge-Derived Fe-N/C Electrocatalyst with Tunable Pore Structure and Nitrogen Chemical State for Exceptional Oxygen Reduction Reaction

Abstract

Developing economic and highly active non-precious metal electrocatalysts for oxygen reduction reaction (ORR) is critical for the development of hydrogen fuel cells. While several nitrogen-carbon precursors have been explored, only limited systems such as phenanthroline-based catalyst after pyrolysis in NH3, polyaniline- and/or cyanimide-derived catalysts, Zn-based metal-organic frameworks (MOF), or a combination of the above precursors were reported to derive highly active Fe-N/C catalysts. It is still challenging to concurrently achieve high density Fe-N4 active sites with optimized local Fe-N/C environment, high microporous surface area, and rational macro-/meso-pore structures at an economic cost. For instance, although MOF-derived Fe-N/C catalysts have shown exclusive high density single Fe atoms confined in the N-doped microporous carbon, the lack of mesopores may limit their ORR performance. In addition, MOF-derived catalysts also suffered from its relatively high cost, hindering their large-scale applications. We report here a novel melamine-sponge-derived Fe-N/C catalyst with exceptional ORR electrocatalysis by virtue of their simultaneously tunable hierarchical micro-/meso-/macro-porous structure and nitrogen chemical state. The melamine sponge serves as a promising and low cost carbon precursor for hosting Fe-N active sites upon NH3 pyrolysis, showing (i) increasing mesopores for favorable mass transport while keeping a high microporous surface area and (ii) increasing pyridinic nitrogen surrounding the Fe center with increasing pyrolysis time. The optimum Fe-N/C catalyst shows high ORR activity particularly in alkaline media with a half-wave potential of 0.902 V/RHE and mass activity of 15.0 A/g at 0.9 V/RHE, markedly outperforming commercial Pt/C and most non-precious metal catalysts to date. Considering the low cost and industrial scalability of the MS precurosr, the MS-derived Fe-N/C catalysts are expected to be a promising candidate as non-precious fuel cell electrocatalyts.