Tunable Metal-Free Imidazole-Benzimidazole Electrocatalysts for Oxygen Reduction in Aqueous Solutions.

A series of metal-free imidazole-benzimidazole catalysts (ImBenz-H, ImBenz-NO2, ImBenz-OCH3) for oxygen reduction reaction (ORR) were prepared. We demonstrate that the electrocatalytic O2 reduction by ImBenz-NO2 with the electron-withdrawing group showed high selectivity toward H2O with the number of electrons transferred (n=3.7) in a neutral aqueous solution. The highest ORR selectivity toward H2O2 was achieved using ImBenz-H (n=2.4) in an alkaline solution. Electrochemical studies of reaction kinetics disclosed that the highest turnover frequencies were obtained from ImBenz-H in both neutral and alkaline aqueous solutions. The results prove that the ORR selectivity is tunable by modulating the substituent of the ImBenz catalysts. Furthermore, DFT calculations suggested that the ORR mechanism of ImBenz-H involves the electron transfer from imidazole-benzimidazole to O2 resulting in the formation of H2O2 which supports the redox active properties of the catalysts ImBenz. Selectivity of electrocatalytic oxygen reduction in aqueous media is tuned by varying pH; redox-active metal-free imidazole-benzimidazole (ImBenz) catalyst with a NO2 substituent at pH 7 favors H2O production, and ImBenz without substituent at pH 13 favors H2O2 production.image