Design and synthesis of Co^IIHMTAA-14/16 macrocycles and their nano-composites for oxygen reduction electrocatalysis

The major concerns in the design of macrocycle based-ORR catalysts are: (i) understanding the macrocyclic, π-conjugation, central metal and substituent effects on ORR electrocatalysis; and (ii) the use of macrocycles on the electrode surface for the retention of ORR activity because of their poor stability. In this work, we demonstrated the aromaticity/π-electron conjugation effect on ORR activity by using the same macrocycles [HMTAA-14 and 16 (hexamethyltetraaza [14] and [16] annulenes)] with a difference in their macrocyclic cavity/π-electron conjugation. The macrocycles Co^IIHMTAA-14 and Co^IIHMTAA-16 and their nanocomposites with highly conductive carbon black were prepared by a microwave-assisted method and characterized by using multiple spectroscopy techniques. Comparative redox and oxygen reduction activity studies of Co^IIHMTAA-14 and Co^IIHMTAA-16 were undertaken by using cyclic voltammetry and linear sweep voltammetry in an alkaline medium. The composite Co^IIHMTAA-16@C showed good ORR activity compared to Co^IIHMTAA-14@C in O₂-saturated KOH electrolyte. Since the Co^IIHMTAA-14 and Co^IIHMTAA-16 systems have a similar central atom and substituents, the shift of the ORR peak position in the +ive potential region for HMTAA-16 can be attributed to the difference in the size of the macrocyclic cavity (macrocyclic effect) and the extra stability of HMTAA-16 annulene due to its aromaticity.