Pyrolysis of Self-Assembled Iron Porphyrin Layers on Carbon Black As Core/Shell Structured Electrocatalysts for Highly Efficient Oxygen Reduction in Both Alkaline and Acidic Media

Abstract

Although significant progress has been achieved recently in the field of nonprecious metal electrocatalysts (NPMEs) for oxygen reduction reaction (ORR), yet highly active and durable NPMEs remained rare for dual applications to both alkaline and acidic conditions. In this study, we report simple carbonization of evaporation-induced self-assembled iron(III) porphyrin (FeP) layers uniformly coated on carbon black, leading to an unprecedented core/shell structured NPME composed of multi-layered N-doped graphite uniformly coated on carbon black. The thickness of graphite carbon shell can be readily adjusted up to about 7 nm by varying the amount of FeP loaded on carbon black. Interestingly, the obtained NPME exhibited a high ORR activity in both alkaline (half-wave potential of 0.87 V vs. RHE) and acidic (half-wave potential of 0.75 V vs. RHE) media, which is one of the highest ORR activities of NPMEs in document. In particular, the core/shell structured NPME demonstrated a remarkable durability in acidic conditions suprior to that of commerical Pt/C, which appears to be closely related to the multi-layered graphite shell. It is very likely that after the outermost graphite layer is consumed, the inner one becomes available and thus effectively maintains the original ORR activity. Furthermore, the core/shell NPME displayed nearly direct 4e and indirect 4e process toward ORR in alkaline and acidic media, respectively. This study pointed out a new avenue for the creation of high-performance NPMEs in both alkaline and acidic media by heat-treating self-assembled metallomacrocycles on various substrates, which may have potential applications in proton exchange membrane fuel cells (PEMFCs), metal-air batteries, and electrolyzers.