Redox-Active Copper-Benzotriazole Stacked Multiwalled Carbon Nanotubes for the Oxygen Reduction Reaction

A copper-benzotriazole metal-organic framework (MOF) was assembled on multiwalled carbon nanotubes (MWCNTs@BTAH-Cu2+) by π-π interaction, and the composite was used to catalyze the oxygen reduction reaction (ORR) in pH 7, 11, and 13. This electrode catalyzes the ORR through a surface-confined redox-mediated process of the Cu2+/Cu+ redox couple. BTAH adsorbed on MWCNTs, benzimidazole-Cu2+ system adsorbed on MWCNTs, and pristine MWCNTs were used as reference systems to illustrate the importance of the Cu2+/Cu+ redox couple and BTAH for ORR. ORR kinetics was characterized using the Koutecky-Levich analysis, rotating ring-disk electrode studies, and the Tafel analysis. From this analysis, the number of electrons transferred per O2, exchange current density, and Tafel slopes were estimated. The cyclic voltammetry studies performed for GCE/MWCNTs@BTAH-Cu2+ before and after chronoamperometry studies (for 10 h) in the respective O2-saturated electrolyte solution, indicate complete stability of the adsorbed BTAH-Cu2+ in pH 7 and 11. The surface excess values calculated from the cyclic voltammogram of MWCNTs@BTAH-Cu2+ were used for estimating the turnover frequency pertaining to 2-electron and 4-electron ORR. Based on these studies, pH 11 was identified as the optimum pH for carrying out ORR using MWCNTs@BTAH-Cu2+.