Developing Structurally Uniform Nitrogenous Model Systems to Interrogate the Metal-Free Oxygen Reduction Reaction

Abstract

Oxygen reduction reaction (ORR) is a kinetically sluggish reaction under uncatalyzed conditions. Numerous studies have revealed that mixtures of N-doped moieties in graphene improve ORR activity and selectivity. However, due to the surface heterogeneity of N-doped carbons, identifying the specific groups of the nitrogen moiety responsible for promoting ORR is challenging. In this study, we develop a structurally uniform electrochemical platform with well-defined nitrogen-containing terminal groups, including pyridazine, pyrimidine, pyridine, graphitic nitrogen, and amine. We uncover that pyridazine outperforms other nitrogen moieties in terms of ORR activity and product selectivity in a metal-free scenario, where pyridazine exhibits a 10-fold higher current density than the other nitrogen moieties. Our results demonstrate that ORR activity is greatly influenced by the solution pH and the steric and electronic environment of the nitrogen moiety, while ORR activity is unaffected by alkali and transition metal ions present in solution. This molecularly precise electrochemical model is envisioned to unveil unorthodox design principles that guide the development of new metal-free interfaces that catalyze ORR and other redox reactions involving proton-coupled electron transfer (PCET) steps that are instrumental to realizing a future sustainable society powered by alternative energy conversion schemes.