Co-Based Single-Atom Catalysts on Multiwalled Carbon Nanotubes: Pyrolysis Temperature-Driven Optimization for Four-Electron Oxygen Reduction

A cobalt single-atom catalyst (SAC) sustained on multiwalled carbon nanotubes (MWCNTs), signified as CoNx@MWCNT800, was produced via pyrolysis. To study the impact of pyrolysis temperature on oxygen reduction reaction (ORR) performance, additional variants were prepared by varying the temperature. The structure and composition of the catalysts were confirmed through comprehensive characterization. Deconvoluted X-ray photoelectron spectroscopy established the existence of Co, N, and C, while high-angle annular dark field scanning transmission electron micrographs provided direct evidence of atomically dispersed cobalt. Inductively coupled plasma mass spectrometry was employed to evaluate the cobalt content before and after electrocatalysis, and energy dispersive X-ray for the compositional information. The ORR efficiency of the catalysts was assessed under both acidic and alkaline conditions. CoNx@MWCNT800 exhibited a high onset potential of 0.91 V vs. RHE and a half-wave potential (E1/2) of 0.81 V in alkaline media, and 0.81 V (onset) and 0.70 V (E1/2) in acidic media, delivering Tafel slopes of 79 and 139 mV dec–1, respectively. Its performance is comparable to commercial Pt/C, which exhibits E1/2 values of 0.87 V (alkaline) and 0.80 V (acidic) vs. RHE, with corresponding Tafel slopes of 108 and 125 mV dec–1. Electrochemical durability tests further demonstrate long-term stability over 25000 cyclic voltammetry cycles, along with a highly selective four-electron reaction pathway. These findings highlight the pivotal influence of pyrolysis temperature in modulating the structural and electronic configuration of the active Co–Nx sites, ultimately enabling superior intrinsic kinetics and one of the highest kinetic current densities reported among nonprecious metal catalysts. While alkaline ORR performance remained relatively stable across temperatures, a notable variation was observed in acidic media. Overall, CoNx@MWCNT800 shows strong potential as an effective electrocatalyst for fuel cell uses.