Electrocatalysis of oxygen reduction on carbon nanotubes with different surface functional groups in acid and alkaline solutions

Electrochemical measurements, including cyclic voltammograms (CVs) and rotating ring disk electrode (RRDE) technique, are applied to determine the impact of surface functional groups on the electrocatalytic activity and H2O selectivity of multi-walled carbon nanotubes (MWCNTs) for the oxygen reduction reaction (ORR) both in acid and alkaline electrolytes. Here, three types of carbon nanotubes (CNTs) involving MWCNTs, MWCNTs-COOH (MWCNTs with carboxyl groups) and MWCNTs-OH (MWCNTs with hydroxyl groups) are investigated. Both CVs and RRDE results indicate that the order of ORR electrocatalytic activity and H2O selectivity is MWCNTs-COOH > MWCNTs-OH > MWCNTs in acid solution; while in alkaline medium, it is MWCNTs > MWCNTs-COOH > MWCNTs-OH. Furthermore, the ORR on these CNTs proceeds mainly through the two-electron reduction pathway followed by a gradual four-electron transfer reaction. Possible interpretation of the different ORR electrocatalytic activity and H2O selectivity in these two solutions is also discussed.