Direct conversion of solid g-C3N4 into metal-ended N-doped carbon nanotubes for rechargeable Zn–air batteries

Abstract

Developing low-cost and bifunctional electrocatalysts with activity for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is highly desirable for metal–air batteries. Herein, we demonstrate an approach to realize the direct conversion of two-dimensional (2D) graphitic carbon nitride (g-C₃N₄) into one-dimensional (1D) metal-ended nitrogen doped carbon nanotubes (M–NCNTs). For Co–NCNTs, Co@CoO_x nanoblocks were found at the end of the nanotubes. The Co@CoO_x nanoparticles in Co–NCNTs can be chemically modulated by acid etching to endow them with catalytic activity towards the ORR and OER. After nitric acid (HNO₃) etching, the Co–NCNTs–N shows superior activity both in the ORR with an onset potential of 0.86 V and a half-wave potential of 0.80 V and in the OER with a potential of 1.58 V at 10 mA cm⁻². Assembled in Zn–air batteries, the Co–NCNTs–N electrode also presents an ultrahigh power density of 210 mW cm⁻², a small charge–discharge voltage gap of 0.80 V and a good stability of 130 hours, outperforming the commercial Pt/C–RuO₂ electrode.