Influence of annealing temperature on oxygen reduction activity of sputtered Co catalysts on vertically-aligned carbon nanotubes

Cobalt and nitrogen were incorporated onto vertically-aligned carbon nanotubes (VACNTs) by a two-step process involving reactive sputtering and heat treatment to fabricate Co/N/CNT catalyst for oxygen reduction reaction (ORR). Both cyclic voltammetry (CV) and rotating-disk electrode (RDE) results demonstrate that annealing temperature plays an important role on improving catalyst activity, and the best ORR performance are obtained for the catalyst with Co loading of 34.3 μg/cm2 at 400 °C with peak potential of 0.778 V, and onset potential of 0.933 V in 0.1 M KOH. For the best catalyst of 400 °C treated, rotating-ring disk electrode (RRDE) measurement indicates that the number of electrons transferred is around 3.4 within the whole potential range. Effects of annealing temperature on surface morphology and chemical composition have been investigated using scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Based on XPS and electrochemical results, the enhanced ORR activity of the optimal Co/N/CNT catalyst is ascribed to the higher content of pyrrollic-N.