Pyrolytic Transformations and Kinetic Stabilization of Heteroatom Configurations in Nitrogen-Doped Carbon Electrocatalysts.

Abstract

Heteroatom-doped carbons are considered promising earth-abundant alternatives to traditional materials for electrochemical energy conversion and storage, with the N-doped carbons being the most widely studied. The heteroatom dopants, synthesized through pyrolysis, typically exhibit a mixture of configurations. This, combined with our limited understanding of how dopant configurations are determined and controlled, has hindered both fundamental and practical advancements in this area. Here, we elucidate the pyrolytic formation of N functionalities in nitrogen-doped carbons. This was accomplished using model platforms in which precursor molecules, containing either pyridinic, pyrrolic, or graphitic N, were uniformly adsorbed from the vapor phase onto preexisting carbon black. Tracking the pyrolysis of all three platforms revealed that the thermodynamically favorable state is characterized by a mixture of pyridinic, pyrrolic, and graphitic N functionalities. Achieving complete specificity in N configuration requires kinetically stabilizing the N states derived from the precursor. Furthermore, the transformation between pyrrolic and graphitic N is mediated by pyridinic N. The CO₂ electroconversion activity of individual N functionalities was evaluated with N-doped carbons containing exclusively pyridinic, pyrrolic, or graphitic N. We found that the intrinsic activity for the electrochemical CO₂ reduction reaction follows the pyrrolic > pyridinic > graphitic order on a per N basis. This study provides essential insights into the precision synthesis and design of heteroatom-doped carbons.