Rational Synthesis of Polymeric Nitrogen N₈^-with Ultraviolet Irradiation and Its Oxygen Reduction Reaction Mechanism Study with in Situ Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy

Polynitrogen (PN) deposited on multiwalled carbon nanotubes was synthesized by cyclic voltammetry with ultraviolet (UV) irradiation. Compared to the sample formed without UV, a larger amount of N₈^-was synthesized and was found to distribute more uniformly on the MWNTs with 254 nm UV irradiation, indicating that the production of more azide (N₃)⁰radicals as the precursors for synthesis of N₈^-by photoexcitation of azide (N₃^-) ions is a rate-limiting step for PN synthesis. An oxygen reduction reaction kinetics study indicated a four-electron reaction pathway on N₈^-, whereas a two-electron process occurs on N₃^-. Analysis by in situ shell-isolated nanoparticle-enhanced Raman spectroscopy revealed that the side-on and end-on O₂adsorption occurred at N₈^-and N₃^-, respectively, confirming the electron transfer process. A full direct-methanol fuel cell study shows that methanol crossover typically reduces the current density of Pt/C by ∼40% but has very little effect on the performance of the PN-MWNT catalyst after testing for 120 h. Moreover, the power density from the PN-MWNT cathode is at least twice that from a Pt/C cathode.