Scalable fabrication and active site identification of MOF shell-derived nitrogen-doped carbon hollow frameworks for oxygen reduction

Nitrogen-doped carbon materials as promising oxygen reduction reaction (ORR) electrocatalysts attract great interest in fuel cells and metal-air batteries because of their relatively high activity, high surface area, high conductivity and low cost. To maximize their catalytic efficiency, rational design of efficient electrocatalysts with rich exposed active sites is highly desired. Besides, due to the complexity of nitrogen species, the identification of active nitrogen sites for ORR remains challenging. Herein, we develop a facile and scalable template method to construct high-concentration nitrogen-doped carbon hollow frameworks (NC), and reveal the effect of different nitrogen species on their ORR activity on basis of experimental analysis and theoretical calculations. The formation mechanism is clearly revealed, including low-pressure vapor superassembly of thin zeolitic imidazolate framework (ZIF-8) shell on ZnO templates, in situ carbonization and template removal. The obtained NC-800 displays better ORR activity compared with other NC-700 and NC-900 samples. Our results indicate that the superior ORR activity of NC-800 is mainly attributed to its content balance of three nitrogen species. The graphitic N and pyrrolic N sites are responsible for lowering the working function, while the pyridinic N and pyrrolic N sites as possible active sites are beneficial for increasing the density of states.