Role of macrocyclic salen-type Schiff base ligands in one-dimensional Co(II) complexes for superior activities toward oxygen reduction/evolution reactions

Developing high activities, stable, non-precious metal based bi-functional electrocatalysts oxygen evolution/reduction reactions (OER/ORR) in rechargeable metal-air batteries and regenerative fuel cell technologies is essential for future energy conversion and storage. In this work, the potential of utilizing the synthesized one-dimensional transition metal salen-type complexes (TM-SCs) as bi-functional electrocatalysts of ORR and OER is systematically explored by computational screening approach. The results demonstrate that different types of macrocyclic ligands, including N2O2, N3O and N4 as donor groups around the active sites, govern the OER/ORR catalytic performances. Co–SCs with N2O2 ligands exhibit the highest bifunctional catalytic activities. In particular, low limiting overpotentials of 0.22 V for OER and 0.33 V for ORR can be observed on Co sites, which are even superior to those of noble metal catalysts. Analyzing the linear relationships between the adsorption strength of intermediates and the overpotentials shows that the origin of excellent electrocatalytic performance is the smaller slope (0.86) for OOH∗ vs OH∗ on TM-SCs compared to metal surfaces, resulting in strengthened binding of the OOH∗ intermediate. Besides, the adsorption energies of the intermediates bound on Co–N2O2 are close to the ideal values, while too strong on the Co–N3O and Co–N4 catalysts. By applying external strains, the adsorption strengths of reaction intermediates can be further modulated due to the tunable d-band centers, and the resulting ORR/OER activities are further boosted. Considering that the Co salen-based chain has been synthesized experimentally, this work highlights the excellent electrocatalytic performances of this new material and devises novel strategy by straining for catalyst optimizations.