Selective nitrogen bonding states in nitrogen doped carbon via a solution plasma process for advanced oxygen reduction reaction

Nitrogen-containing carbon matrix is a promising candidate for the oxygen reduction reaction (ORR) in metal-air batteries. However, there are still remaining challenges in the modification of desired carbon-nitrogen bonding for further enhancement of ORR catalytic activity. Herein, we designed a simple and effective method to have selective C-N bonding in synthesized carbon nanoparticles via a liquid plasma process. The nitrogen-containing carbon was synthesized by pyrazine (an aromatic structure precursor) and acrylonitrile (a linear structure precursor). From elemental analysis, the atomic percentage of nitrogen content in carbon synthesized from pyrazine and acrylonitrile was 8.58 and 2.97 at%, respectively. The structural analyses from scanning electron microscopy (SEM), X-ray diffraction (XRD) and Raman spectroscopy showed slight differences in the carbon matrices. X-ray photoelectron spectroscopy (XPS) demonstrated that the chemical bonding states of nitrogen were different and dependent on the structure of the original precursor. The relative percentage of amino-N of nitrogen was significantly higher when the linear structured acrylonitrile was applied as a precursor. In contrast, pyridinic-type nitrogen bonding was more preferred in carbon synthesized from heterocyclic structural pyrazine. From electrochemical analyses, the key factor for high ORR performance was not related to the total nitrogen content. Instead, a more positive ORR onset and peak potential was observed when the ratio of amino-N and quaternary-N increased, where the content of pyridinic-N had a significant effect on current density. This work opens new directions in designing and synthesizing selective C-N bonding within a N-doped carbon matrix through a simple solution process operated at room temperature.