Switching the locus of oxygen reduction and evolution reactions between spinel active phase and carbon carrier upon heteroatoms doping

Abstract

Multi-walled carbon nanotubes (MWCNTs) doped with nitrogen and sulfur atoms and with deposited manganese–cobalt spinel catalysts were studied to reveal the role of active centers in the oxygen reduction and evolution reactions (ORR and OER) in alkaline media. Commercial MWCNTs were oxidized by boiling in concentrated nitric acid and then double-doped with N and S heteroatoms using thiourea precursor. Manganese-cobalt spinels supported on the MWCNTs were synthesized by a microwave-assisted solvothermal route. The catalysts were comprehensively characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy (RS), and X-ray fluorescence spectroscopy. The deposited mixed spinel active phase consists of well-dispersed nanocrystals with sizes in the range of 5–8 nm. The electrocatalytic activities of the synthesized catalysts in ORR (the onset potential and the number of transferred electrons) and OER (the overpotential required to reach the 10 mA cm−2 current density) were examined in alkaline media by rotating ring-disc electrode (RRDE) and rotating disc electrode (RDE) techniques, respectively. It was found that N and S doped MWCNT catalysts exhibit high activity in ORR with Eonset = 0.890 ± 0.013 V, and a number of electrons exchanged n = 3.83 ± 0.03, which remained essentially the same, within experimental error, upon spinel deposition. The addition of the spinel phase does not improve the electrocatalytic properties. The advantage of the dispersed spinel active phase was observed in OER, the activity of these samples was comparable to that of the RuO2 benchmark catalyst. In the case of ORR, the observed reactivity was accounted for by the incorporation of nitrogen and sulfur to the MWCNTs, whereas for OER the locus of the active centers is moved towards the manganese-cobalt spinel nanocrystals dispersed on the carbon nanotubes matrix. Functionalization of the latter with N and S atoms is then catalytically redundant.