Space-confinement-induced synthesis of pyridinic- and pyrrolic-nitrogen-doped graphene for the catalysis of oxygen reduction

TL;DR: A novel strategy for the selective synthesis of pyridinic and pyrrolic N atoms and fewer quaternary N atoms is presented, which can enhance the activity of NC materials with sufficient active species that favor ORR and through an increase in electrical conductivity.

Abstract: The development of high-performance and low-cost catalytic materials for the oxygen reduction reaction (ORR) has been a major challenge for the large-scale application of fuel cells. Currently, platinum and platinum-based alloys are the most efficient ORR catalysts in fuel-cell cathodes; however, they cannot meet the demand for the widespread commercialization of fuel cells because of the scarcity of platinum. Thus, the ongoing search for platinum-free catalysts for the ORR has attracted much attention. Graphene, single-layer sheets of sp-hybridized carbon atoms, has attracted tremendous attention and research interest. The abundance of free-flowing p electrons in carbon materials composed of sp-hybridized carbon atoms makes these materials potential catalysts for reactions that require electrons, such as the ORR. However, these p electrons are too inert to be used directly in the ORR. In N-doped electron-rich carbon nanostructures, carbon p electrons have been shown to be activated through conjugation with lone-pair electrons from N dopants; thus, O2 molecules are reduced on the positively charged C atoms that neighbor N atoms. Recently, Hu and co-workers found that as long as the electroneutrality of the sp-hybridized carbon atoms is broken and charged sites that favor O2 adsorption are created, these materials will be transformed into active metal-free ORR electrocatalysts regardless of whether the dopants are electron-rich (e.g., N) or electrondeficient (e.g., B). Nitrogen-doped carbon (NC) materials are considered to be promising catalysts because of their acceptable ORR activity, low cost, good durability, and environmental friendliness. However, their ORR activity is less competitive, especially in acidic media. Relative to commercial Pt/C, the difference in the half-wave potential for ORR is within 25 mV in alkaline electrolytes but is greater than 200 mV in acidic electrolytes. The activity of NC materials can be enhanced through efficient N doping with sufficient active species that favor ORR and through an increase in electrical conductivity. The annealing of graphitized carbon materials, such as carbon nanotubes and microporous carbon black, in NH3 leads to insufficient substitution of nitrogen because of the well-ordered structure of the host materials. Alternatively, the direct pyrolysis of nitrogen-containing hydrocarbons or polymers produces NC materials with good incorporation of nitrogen. However, suitable pyrolysis temperatures are difficult to pinpoint; without optimization, temperatures that are excessively low or excessively high lead to low electronic conductivity or a remarkable loss of active N species, respectively. Recently, mesoporous-alumina-assisted and silica-template-assisted nitrogen incorporation, which can preserve a high content of N in synthesized NC materials, have been reported. However the activities of the resulting NC materials in the ORR were still significantly lower than that of Pt/C, even when the N content was as high as 10.7 atm%. Among three types of N atoms, that is, pyridinic, pyrrolic, and quaternary N, only the pyridinic and pyrrolic forms, which have planar structures, have been proven to be active in the ORR. In contrast, quaternary N atoms, which possess a 3D structure, are not active in the ORR. The low electrical conductivity of NC materials with quaternary N atoms results from the interruption of their p–p conjugation by the 3D structure and is thought to be predominantly responsible for the poor catalysis. Therefore, the synthesis of NC materials with more planar pyridinic and pyrrolic N atoms and fewer quaternary N atoms is important for the preparation of ORR-active catalysts. Herein, we present a novel strategy for the selective synthesis of pyridinicand pyrrolic-nitrogen-doped graphene (NG) by the use of layered montmorillonite (MMT) as a quasi-closed flat nanoreactor, which is open only along the perimeter to enable the entrance of aniline (AN) monomer molecules. The flat MMT nanoreactor, which is less than 1 nm thick, extensively constrains the formation of quaternary N because of its 3D structure but facilitates the formation of pyridinic and pyrrolic N. Nitrogen is well-known to be incorporated into quaternary N in tetrahedral sp hybridization but incorporated into pyridinic and pyrrolic N in planar sp hybridization. The confinement effect of MMT ensures that N is incorporated into the structure and that the graphitization is successful without significant loss of N species. Furthermore, planar pyridinic and pyrrolic N can be [*] Dr. W. Ding, Prof. Z.-D. Wei, Dr. S.-G. Chen, Dr. X.-Q. Qi, Dr. T. Yang, Dr. S. F. Alvi, Dr. L. Li The State Key Laboratory of Power Transmission Equipment and System Security and New Technology, College of Chemistry and Chemical Engineering, Chongqing University Shapingba 174, Chongqing (China) E-mail: zdwei@cqu.edu.cn