N₈ ^- polynitrogen stabilized on boron-doped graphene as metal-free electrocatalysts for oxygen reduction reaction

Polynitrogen (PN) chain was predicted theoretically to be stable at ambient pressure by intercalating in multiple graphene layers. In this work, polynitrogen (N₈ ^-) deposited on boron-doped graphene (PN-BG) and graphene (PN-G) was synthesized experimentally by a facile cyclic voltammetry (CV) method. It was further used for oxygen reduction reaction (ORR), which showed superior activity via a four-electron pathway mechanism. BG was prepared by a one-pot hydrothermal method. Characterizations over BG substrate, including X-ray photoelectron spectroscopy, Raman specstroscopy, Brunauer-Emmett-Teller, scanning electron microscopy, and transmission electron microscopy, demonstrated that boron atoms were successfully doped into graphene matrix. The formation of polynitrogen (N₈ ^-) on BG was confirmed by attenuated total reflection Fourier transform infrared spectroscopy and temperature-programmed desorption. A larger amount of N₈ ^- was obtained on boron-doped graphene than that on graphene. Rotating disk electrode measurements indicated that PN-BG showed higher current density than PN-G catalyst because of the larger amount of N₈ ^- on BG. Compared to commercial Pt/C catalyst, PN-B1G has much better methanol tolerance. Kinetics study was also carried out to investigate the ORR pathway. Natural bonding orbital analysis confirmed the mechanism. This work provides a facile strategy to modify graphene structure and efficiently stabilize N₈ ^- on a graphene-based matrix.