Rational Synthesis of Polymeric Nitrogen N-8(-) 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 N-8(-) was synthesized and was found to distribute more uniformly on the MWNTs with 254 nm UV irradiation, indicating that the production of more azide (N-3)(0) radicals as the precursors for synthesis of N-8(-) by photoexcitation of azide (N-3(-)) ions is a rate-limiting step for PN synthesis. An oxygen reduction reaction kinetics study indicated a four-electron reaction pathway on N-8(-), whereas a two-electron process occurs on N-3(-). Analysis by in situ shell-isolated nanoparticle-enhanced Raman spectroscopy revealed that the side-on and end-on O-2 adsorption occurred at N-8(-) and N-3(-), 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 similar to 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.