New strategies to improve two-electron oxygen reduction reaction selectivity of polypyrrole-based catalysts

Abstract

Direct electrochemical synthesis via the two-electron oxygen reduction reaction (ORR) is considered an environmentally friendly means to produce hydrogen peroxide (H₂O₂). Metal-free N-doped carbon materials are reported to display good electrocatalytic performance for H₂O₂ generation. Herein, we found that selectivity toward H₂O₂ production in the ORR can be further enhanced once the N atoms are saturated bonds in polypyrrole. Saturation can either be achieved by stabilizing the N–H bonds via hydrogen-bonding interactions during preparation or transforming the deprotonated nitrogen species into C–N bonds by calcining polypyrrole at appropriate temperatures. Eventually, our synthesized catalyst NPC-400 exhibits a 94% H₂O₂ selectivity and impressive stability. The mechanisms are elucidated by using control experiments and density-functional theory calculations. The α carbon atoms of pyrrolic-N are the active sites for selective H₂O₂ production. However, once the N atoms are not saturated, the OOH* intermediate species tend to bond with the N atoms, leading to relatively poor reaction selectivities. Our work provides new insight for designing high selectivity catalysts for H₂O₂ generation in the oxygen reduction reaction.