Insights into KMnO4 etched N-rich carbon nanotubes as advanced electrocatalysts for Zn-air batteries

[Display omitted] •Etching of KMnO4 could well tune the pyridinic-N content and defects of N-CNT.•Etching of KMnO4 significantly improved the ORR performance of N-CNT.•This catalyst even outperformed the commercial Pt/C in Zn-air battery.•The second nearest carbon atom to the pyridinic-N at the edge was the real active site. Increasing the number of active sites is critical for developing N-doped carbon electrocatalysts towards oxygen reduction reaction (ORR) in fuel cells and metal-air batteries applications. Herein, we prepared N-doped carbon nanotubes (N-CNT) with enriched pyridinic N and abundant defects resulted from the etching of KMnO4 of the precursor (polypyrrole). It was observed that the content of pyridinic N could be well controlled by regulating the etching time. The resultant catalyst displayed a superior ORR activity compared commercial Pt/C in an alkaline solution, which was further confirmed by home-made Zn-air batteries. Density functional theory (DFT) computations showed that the superior catalytic activity originated from the second nearest carbon atom to the pyridinic-N at the edge. This work provides a simple etching approach to alter the N configuration and the amount of defects in N-doped CNT, which can be extended to many other energy conversion materials.